Advancing Municipal Solid Waste Management Through Gasification Technology

Abstract

This review thoroughly evaluates gasification as a transformative alternative to conventional methods for managing municipal solid waste (MSW), highlighting its potential to convert carbonaceous materials into syngas for energy and chemical synthesis. A comparative evaluation of more than 350 papers and documents demonstrated that gasification is superior to incineration and pyrolysis, resulting in lower harmful emissions and improved energy efficiency, which aligns with sustainability goals. Key operational findings indicate that adjusting the temperature to 800–900 °C leads to the consumption of CO₂ and the production of CO via the Boudouard reaction. Air gasification produces syngas yields of up to 76.99 wt% at 703 °C, while oxygen gasification demonstrates a carbon conversion efficiency of 80.2%. Steam and CO₂ gasification prove to be effective for producing H₂ and CO, respectively. Catalysts, especially nickel-based ones, are effective in reducing tar and enhancing syngas quality. Innovative approaches, such as co-gasification, plasma and solar-assisted gasification, chemical looping, and integration with carbon capture, artificial intelligence (AI), and the Internet of Things (IoT), show promise in improving process performance and reducing technical and economic hurdles. The review identifies research gaps in catalyst development, feedstock variability, and system integration, emphasizing the need for integrated research, policy, and investment to fully realize the potential of gasification in the clean energy transition and sustainable MSW management.

1. Introduction

Rapid urbanization, population growth, and industrialization have led to a global spike in the generation of municipal solid waste (MSW), which is one of the most urgent environmental and socio-economic issues of the twenty-first century [1]. The current waste-disposal infrastructure is under tremendous pressure due to the increase in waste associated with urbanization [2]. Not only have conventional waste-disposal methods such as landfilling and open dumping reached their limit in many areas, but they also significantly harm the environment by generating methane emissions, soil and groundwater pollution, and other greenhouse gas (GHG) emissions [3]. Furthermore, the amount of waste generated is equivalent to the amount of CO₂ and other GHG emissions in a certain location [4]. The major waste-producing countries in the world are shown in Figure 1 below [5,6].

It is anticipated that the amount of municipal solid waste generated will increase from 2.1 billion tonnes in 2023 to 3.8 billion tonnes by 2050 [6]. This challenge has triggered many policies and research aimed at reducing the negative impact of MSW. For example, in its communication of 11 December 2019, titled “The European Green Deal,” the EU Commission outlined a new growth strategy aimed at transforming the union into a fair and prosperous society with a modern, resource-efficient, and competitive economy where there are no net emissions of GHGs in 2050 and where economic growth is decoupled from resource use. By 2030, it aims to cut GHG emissions by 55%, advancing resource efficiency and a clean, circular economy.

According to the United Nations’ municipal solid waste generation statistics, nearly all the goods that consumers purchase are turned into waste within six months [7]. By 2050, it is anticipated that the overall amount of waste produced in low-income nations will have increased by more than three times due to economic development. In absolute terms, the Middle East and North African regions produce the least waste, whereas East Asia and the Pacific region produce the most. On the other hand, Sub-Saharan Africa, Latin America, the Caribbean, South Asia, and North America are the regions with the quickest growth rates, as shown in Table 1.

Over half of the waste in these areas is being disposed of in the open, and the future trends of waste growth will have significant effects on the environment, human health, and economic growth and development, necessitating immediate action [8]. The global percentage of MSW composition indicates that food and green waste represent 44% of the world’s waste dry recyclables, while paper and cardboard, plastic, metals, glass, wood, rubber and leather, and others constitute 17, 12, 5, 4, 2, 2, and 14%, respectively, of the waste composition [8]. These wastes are all generated from homes, schools, hospitals, and industries [9].

Table 1. Projected waste generation by region [9].

Region	2016 (Million Tonnes/Year)	2030 (Million Tonnes/Year)	2050 (Million Tonnes/Year)
Middle East and North Africa	129	177	255
Sub-Saharan Africa	174	269	516
Latin America and the Caribbean	231	290	396
North America	289	342	396
South Asia	334	466	661
Europe and Central Asia	392	440	490
East Asia and the Pacific	468	602	714

Table 1. Projected waste generation by region [9].

Region	2016 (Million Tonnes/Year)	2030 (Million Tonnes/Year)	2050 (Million Tonnes/Year)
Middle East and North Africa	129	177	255
Sub-Saharan Africa	174	269	516
Latin America and the Caribbean	231	290	396
North America	289	342	396
South Asia	334	466	661
Europe and Central Asia	392	440	490
East Asia and the Pacific	468	602	714

The EU’s directive for transitioning to a circular economy aims to ensure that waste, especially waste suitable for recycling or other recovery, is not disposed of in landfills. Additionally, through strict operational and technical requirements on waste and landfills, the directive provides for measures, procedures, and guidance to prevent adverse environmental effects, such as the pollution of soil, air, groundwater, and surface water, as well as the greenhouse effect and any resulting health risks, throughout the landfill’s entire lifecycle.

According to the directive, by 2016, and for some countries, 2020 and beyond, the amount of biodegradable waste that ends up in landfills must be cut by 35% from 1995 levels. This is one of the driving forces behind the processing of waste to energy (WtE), which aims to produce power, recover valuable products, and reduce the amount of waste through municipal waste management, which involves using the waste-management hierarchy. The American Environmental Protection Agency established the waste-management hierarchy as the best method to be employed in managing MSW. It includes source reduction, often known as waste prevention; it is the most environmentally friendly technique since it eliminates waste at the source. It can take many different forms, including reusing, decreasing packaging, redesigning products, and minimizing toxicity. Source reduction can minimize pollution and waste toxicity and reduce greenhouse gas emissions that affect the climate. Recycling and composting involve collecting used, reused, or unused material that would otherwise be discarded; sorting and processing recyclable products into raw materials; and reprocessing recycled raw materials into new products. Recycling has several benefits, including minimizing greenhouse gas emissions that cause climate change, preventing water and air pollution, and reducing the demand for new landfills. Energy recovery is a process that involves the recovery of energy by conversion of non-recyclable waste into usable heat, power, or fuel using a range of processes, such as anaerobic digestion, landfill gas recovery (LFG), combustion, pyrolysis, and gasification. This technique is commonly known as waste-to-energy (WtE), as shown in Figure 2. Converting non-recyclable waste into electricity and heat produces a sustainable energy source and minimizes carbon emissions by reducing the requirement for energy from fossil sources and decreasing methane output from landfills.

Treatment and disposal: Before disposal, waste can be treated to reduce its volume and toxicity. Physical therapies include shredding, chemical treatments like pyrolysis, and biological treatments like anaerobic digesters. Landfills are the most common method of waste disposal and play a significant role in an integrated waste-management system [10]. While incineration is successful in reducing waste volume, it has been criticized for consuming a lot of energy, having high operating costs, and emitting harmful pollutants like dioxins and furans [11].

To address these shortcomings, there is an urgent need for transformative waste-to-energy technologies that align with global sustainability goals, including the United Nations’ Sustainable Development Goals (SDGs 7) and the circular economy principle [12,13]. Among these technologies, gasification emerges as an auspicious and sustainable approach for advancing waste-management methods [14]. Unlike incineration, gasification converts MSW in a controlled, oxygen-limited environment by transforming waste into valuable resources (such as syngas) while reducing environmental impacts [15]. This clean energy carrier has a wide range of applications, from producing heat and electricity to hydrogen production and chemical synthesis, providing a sustainable method for lowering landfill dependency and significantly reducing greenhouse gas emissions [16,17]. By integrating gasification into municipal waste-management systems, cities can transition towards a cleaner, circular economy with greater energy security and reduced environmental impact [18]. What makes gasification unique among other thermal processes, such as incineration and pyrolysis, is its ability to operate in an oxygen-controlled environment, thereby significantly reducing the release of toxic pollutants [19]. Also, the high demand for sustainable waste-management methods has prompted compliance with environmental requirements such as the Medium Combustion Facilities Directive (MCPD) and Best Available Technologies (BAT) guidelines. These policies set strict limitations on emissions from waste-treatment plants and encourage the use of technologies such as gasification. Gasification reduces pollutants like NO_x and SO_x, aligning with modern environmental and energy recovery aims. Also, gasification facilities that comply with these policies emit substantially less NO_x than typical incineration plants, aligning with the EU’s Green Deal aims to cut pollution by 2050 [20]. Older facilities that do not adhere to the requirements of the MCPD and BAT will be replaced with new ones [21].

Despite the benefits of transforming waste into valuable products, as shown in the waste-to-energy conversion scheme, the wide-scale implementation of gasification technology for municipal solid waste (MSW) management still faces technical and economic constraints, such as tar formation, feedstock volatility, and high initial capital expenditures. This has motivated research interest from the scientific community focusing on overcoming these challenges through innovations such as plasma-assisted gasification, advanced catalysts, the use of hybrid system reactors, AI-based optimization processes such as the internet of things (IoT), chemical looping gasification, and co-gasification with biomass or industrial waste and integration with carbon capture technologies. By adopting these advancements, gasification enables the conversion of waste into valuable energy and chemical products while reducing its environmental impact.

Many reviews about gasification have been published, covering such topics as biomass gasification, gasifiers, gasifying media, and operational parameters [22]; the process and technological aspects of municipal solid waste gasification [23]; and recent advances in the development of biomass gasification technology [24] and prospects for gasification systems and their modeling. This indicates a research gap, since most reviews have not substantially or sufficiently covered gasification and its advancements in MSW management, including the effect of operating parameters and syngas cleaning strategies in recent years, which is the motivation behind this work.

In summary, integrating advanced waste-to-energy technologies has become imperative due to the significant challenges posed by increasing waste volumes, environmental pollution, and the need for cleaner energy sources. This review critically examines the potential of gasification technology for enhancing municipal solid waste management. Examining its operational principles, environmental benefits, and economic viability demonstrates the transformative role gasification could play in addressing concerns about waste. Furthermore, the study analyzes emerging trends and highlights research gaps, offering a basis for guiding future innovations and policy initiatives.

2. Methodology

A thorough evaluation of developments in municipal solid waste (MSW) gasification technology was conducted in this paper, using an integrated and thematic approach. To identify, select, and evaluate the most recent and relevant literature on the topic, the selection process was conducted in a well-organized and structured manner.

The review presents findings relevant to MSW management from over 378 reliable sources, aiming to provide a clear and essential understanding of municipal solid waste management through gasification. The introductory section includes comparative information on waste-management methods and the associated problems, highlighting gasification as the most advanced method that aligns with global policies and sustainability goals. Information for this section was obtained from credible sources, including the United Nations (UN), the World Bank, the Environmental Protection Agency (EPA), as well as websites, conferences, books, and articles with high impact factors. The literature was collected from highly reputable scientific databases, including Web of Science, Scopus, ScienceDirect, and Google Scholar, as well as various journal databases. These data were extracted from the databases by using key search terms such as “municipal solid waste,” “waste-to-energy conversion”, “gasification”, “effect of operating parameters in gasification”, “syngas,” “thermal conversion”, “co-gasification”, “solar assisted gasification”, “syngas cleaning techniques,” “AI and IoT integration”, etc.

To ensure that the information contained in the review is scholarly and meets a considerable standard, the following criteria were applied:

• Publication period

Generally, papers and legislative documents from various years were reviewed, but particular emphasis was placed on recent developments, specifically articles from 2018 to 2025.

• Publication type

Peer-reviewed articles, policy documents, and relevant conference materials were taken into consideration.

• Scope

Peer-reviewed papers focusing on the gasification of MSW, biomass, RDF, and process efficiency, as well as emissions, the effects of operating parameters, catalyst influence, and innovative integrations such as AI, IoT, and solar-assisted co-gasification, were included.

• Language

Articles published in English were included.

• Number of references

An initial number of 673 references were analyzed.

• Total references

In total, 378 references were selected and used in this paper.

3. Municipal Solid Waste Management

The term “municipal solid waste” (MSW) encompasses the diverse range of waste generated by urban dwellers or consumers of products [25]. It consists of a mixture of organic and inorganic materials discarded after use [26]. Sources of this waste include commercial, institutional, industrial, and residential activities, caused by factors such as economic activities, population growth, and cultural practices. MSW is not limited to solid materials but can also include liquids and gases, depending on the composition of the waste, its decomposition stage, and other external factors [26,27]. It typically contains furniture, appliances, food leftovers, and product packaging. The types of MSW generated, along with their sources and composition, are listed in

Table 2. The types of MSW, their composition, and examples.

MSW Type	Composition	Examples	References
Biodegradable Waste	Waste that decomposes naturally by microorganisms.	Food, yard waste, and paper.	[28,29]
Recyclable Waste	Waste materials that can be recovered, processed, and reused.	Plastic, glass, metals, and cardboard.	[30,31]
Inert Waste	Non-reactive, non-biodegradable, and has negligible or no negative environmental impacts.	Concrete, stones, ceramics, and construction debris.	[32]
Hazardous Waste	Materials causing a risk to human health or the environment.	Batteries, paints, chemicals, and e-waste.	[33,34]
E-Waste	Discarded electronic devices.	Computers, smartphones, and televisions.	[35]
Plastic Waste	Non-biodegradable synthetic polymers.	Plastic bottles, bags, and food packaging.	[36,37]
Biomedical Waste	Waste from healthcare facilities and activities.	Syringes, medical gloves, and pharmaceuticals.	[38,39]
Bulky Waste	Large items not suited for regular waste systems.	Furniture, appliances, and mattresses.	[40,41]
Liquid Waste	Waste in liquid form, often from households or businesses.	Used cooking oil, wastewater, and sludges.	[42,43]
Green Waste	Organic waste generated from gardening or agricultural activities.	Grass clippings, pruning waste, and weeds.	[44,45]

below.

This information is important because the physical attributes of solid wastes aid in the design and selection of suitable disposal or waste-management techniques, as well as methods of collection, transportation, recovery of valuable matter, and energy conversion [18].

Municipal solid waste (MSW) poses various environmental concerns, including pollution, health hazards, and/or inadequate waste-management systems resulting from improper waste disposal [46]. Jbara et al. [47] reported that in Diyala Governorate, Iraq, improper waste-disposal practices, such as burning, lead to air, soil, and water contamination with heavy metals, causing health issues in nearby communities. Abduwahab et al. [48], also mentioned that improper waste-disposal practices, such as open dumping, especially in Nigeria and some developing countries where these practices are standard, create favorable environments for the survival of disease-carrying vectors like mosquitoes, which can spread diseases such as dengue fever and malaria. This is particularly alarming given the high rate of child mortality linked to malarial infections in Nigeria. However, these reports do not compare the findings with those of other studies or regions or parts of the world, which could have helped in understanding whether the observed pollution levels or health concerns are unique or part of a broader trend.

Giusti mentioned that in Europe and America, landfilling and incineration are the most popular methods of MSW management. He further reported that France is the country with the highest number of incinerators in the EU, releasing a significant amount of dioxins. Zambon et al. [49] reported that in Venice, Italy, exposure to dioxins is linked to an increasing risk of developing sarcoma, a cancer or tumor that occurs in connective tissues.

Challenges are caused by increased urbanization, population growth, industrial development, etc., contributing to the worrisome rise in municipal solid waste generation and waste complexity [50]. The environmental impact of MSW is substantial, affecting air, soil, and water quality, and contributing to climate change through greenhouse gas emissions. The challenges associated with MSW can be managed effectively through innovative, eco-friendly, and sustainable waste-management solutions such as biological (landfill anaerobic digestion, etc.) and thermal methods (pyrolysis, incineration, gasification, etc.) [51,52].

During the WtE conversion of MSW, electricity, heat, and valuable gases are generated. MSW management is essential because it reduces the emission of GHG, serves as an alternative to fossil fuels for energy generation, and reduces the need for landfills. It also positively affects the economic growth of a society [53,54].

Landfilling is the oldest, cheapest, and most practiced method of MSW management in developing countries [55]. Landfills are areas where waste is dumped or disposed of [56]. Biogas can be produced from landfill waste using biological waste-treatment techniques. This method primarily involves anaerobic digestion (AD), a process that converts organic waste, in the absence of oxygen, into landfill gas (LFG), which mainly consists of methane and carbon dioxide [57]. Takeda et al. [58] researched a type of MSW from landfills known as easily degradable groups (ED), which include materials like paper, cardboard, wood, putrescible organic matter, and pruning waste. The report demonstrated that these materials have higher methane generation potential than other types of MSW, making them essential for sustainable waste-management practices. They mentioned that ED MSW can produce substantial methane, with a theoretical methane yield (TMY) of 233.41 mL CH₄/g total volatile solids (TVS) over 24 years of landfilling through anaerobic digestion (AD); thus, there are considerable environmental advantages of using ED MSW for biogas production. The report noted that this technique can help reduce pollution and promote a circular economy by reintegrating waste into the production value chain, reducing the environmental concerns associated with landfill decomposition.

When MSW is disposed of in a landfill, the stabilization of solid waste produces leachate and landfill gas, which can be used to generate electricity and heat [26].

Moroase et al. [59] reported that landfill practices can capture the methane gas produced by decomposing waste to produce electricity. The methane collected can be used in small plants to produce energy; however, the report primarily relied on modeling and theoretical calculations to estimate the potential for electricity generation from landfills and could not experimentally validate its findings or use real-world data to validate the model’s predictions. This lack of empirical evidence raises questions about the reliability and viability of the findings and their applicability in practical scenarios, which led us to conclude that the report relied on assumptions.

Additionally, the continuous practice of landfilling raises some environmental and economic concerns, including groundwater contamination. The emission of GHGs such as methane resulting from the breakdown of waste poses various health risks and other challenges to society.

Research on mitigating methane emissions, capturing other harmful gases, and reducing emissions from landfills is underexplored and necessary, especially in areas where new WtE solutions are not practicable. Also, since waste in landfills is heterogeneous, particularly in most developing countries, which makes it challenging to sort during or before advanced treatment processes, we believe that it is possible to construct modern landfills with partitions to enable separation of each category of waste at the point of disposal for easy collection before advanced treatment processes. However, this will involve creating public awareness and education, especially among those with low literacy levels.

Furthermore, waste can be managed sustainably by converting it into energy, which also helps recover resources and mitigate the adverse effects on the environment and human health. Incineration is one of the most widely practiced and economically viable forms of WtE in developing countries, such as Malaysia, China, and Bangladesh, as an alternative to landfilling. Combustion or incineration involves burning the entire mass of waste in the incinerator. This process of incineration of MSW reduces the volume of MSW by up to 80–90%. During incineration, water is fed into a boiler, generating heat to boil water. The generated steam is used to turn a turbine generator, which produces energy. Many researchers have conducted studies on incineration as a significant method of waste-to-energy conversion, especially in terms of energy production. For example, Gu et al., in 2021 [60], investigated the energy recovery from incineration utilizing MSW in Beijing and reported that the volume of electricity generated per ton of waste ranged between 0.336 and 1.114 MWh/ton. Maria et al. [61] reported on the impact of MSW incineration on the environment and human health. As of the time of this report, only Zeng et al., in 2024 [62], have studied the environmental, energy, and techno-economic assessment of waste-to-energy by incineration, but their study was based solely on Java Island, Indonesia, thereby presenting a promising research opportunity for worldwide coverage. Their report established that the principal environmental repercussions of incineration activities include global warming, terrestrial ecotoxicity, eutrophication, and acidification. They also indicated that the cumulative energy output potential from incineration will reach 2,316,523 MWh/year by 2025 and will increase by 14.3% by 2050. However, since these data are limited to only a part of Indonesia, we are convinced that the scientific community can fully utilize this knowledge to provide better information on a global scale.

Overall, even though incineration is a robust method of WtE conversion, it comes with a few disadvantages, including high setup costs, the need for skilled labor, and environmental pollution if flue gas is not handled correctly. The method is mature and has been widely adopted, especially in countries like France, Japan, Singapore, the USA, and Denmark, among others [63].

Pyrolysis involves heating MSW above 850 °C in an oxygen-free environment, yielding bio-oil, syngas, and char [64]. Pyrolysis offers an eco-friendly alternative to incineration by converting hazardous organic solid waste into valuable fuels and chemicals, reducing environmental impact, minimizing ash production, and enabling energy and materials recovery, thus promoting sustainable waste-management practices [65]. Variability in feedstock has an impact on product uniformity and process stability. Scalability is also constrained by high energy consumption. The technique is emerging and commercially viable in some countries such as Japan, China, Germany, the UK, and Canada [64,66]. Gasification is a more advanced solution to the inefficiencies of incineration and pyrolysis and the environmental challenges of landfilling, providing cleaner energy recovery routes and resource efficiency. The next section goes into additional depth about the promise of gasification as the most preferred method of MSW management.

4. Gasification Process for the Conversion of MSW to Energy

Gasification is a modern technology used in industrialized countries to convert MSW into WtE [67]. It plays a pivotal role in power generation. Clean energy is obtained cheaply, and several forms of feedstock, such as MSW and biomass, can be employed. The countries that are currently using gasification technology commercially include China, Finland, Japan, the United States, Germany, South Africa, and the United Kingdom [68,69,70,71,72,73].

Gasification is a thermochemical process [74] that produces a variety of gases, known as producer gas, product gas, synthetic gas, or syngas [75], when carbon-rich materials (fuel) like coal, biomass, or MSW (refuse-derived fuel, waste sludge, polymeric material) react with a gasification agent like air, oxygen, steam, carbon dioxide, or a combination of these in the presence of a small amount of oxygen. A mixture of CO, H₂, N₂, CO₂, methane, and other low-molecular-weight hydrocarbons constitutes syngas. There may also be trace levels of tars, NH₃, and H₂S [76,77,78].

Depending on the reactor type and the feedstock’s chemistry, the operating temperature typically ranges between 800 and 1200 °C. Although the gasification process is primarily exothermic, heat is occasionally needed to initiate and maintain the process [79]. Exothermic and endothermic reactions, which react with partial oxidation below the stoichiometric level, are the foundation of the gasification process [80]. In the gasification chamber, both processes take place at the same time. While the material takes heat from the environment for burning in endothermic processes, the substance releases heat during combustion in exothermic reactions. The temperatures at which these complex reactions occur vary. Additionally, the Boudouard reaction, water–gas shift reaction, hydrogasification, methanation process, and steam reforming are the primary chemical processes of gasification [81,82]. The basic equations are as shown below [83]:

$$\begin{array}{l}\mathrm{C}+{\mathrm{H}}_2\mathrm{O}\to \mathrm{C}\mathrm{O}+{\mathrm{H}}_2\hspace{1em}\hspace{1em}\Delta \mathrm{H}=+131 \mathrm{k}\mathrm{J}/\mathrm{m}\mathrm{o}\mathrm{l}\\ \mathrm{W}\mathrm{a}\mathrm{t}\mathrm{e}\mathrm{r}\text{–}\mathrm{g}\mathrm{a}\mathrm{s} \mathrm{s}\mathrm{h}\mathrm{i}\mathrm{f}\mathrm{t} \mathrm{r}\mathrm{e}\mathrm{a}\mathrm{c}\mathrm{t}\mathrm{i}\mathrm{o}\mathrm{n}\end{array}$$

(1)

$$\begin{array}{l}\mathrm{C}+\mathrm{C}{\mathrm{O}}_2\to 2\mathrm{C}\mathrm{O}\hspace{1em}\hspace{1em}\Delta \mathrm{H}=+172 \mathrm{k}\mathrm{J}/\mathrm{m}\mathrm{o}\mathrm{l}\\ \mathrm{B}\mathrm{o}\mathrm{u}\mathrm{d}\mathrm{o}\mathrm{u}\mathrm{a}\mathrm{r}\mathrm{d} \mathrm{r}\mathrm{e}\mathrm{a}\mathrm{c}\mathrm{t}\mathrm{i}\mathrm{o}\mathrm{n}\end{array}$$

(2)

$$\begin{array}{l}\mathrm{C}+2{\mathrm{H}}_2\to \mathrm{C}{\mathrm{H}}_4\hspace{1em}\hspace{1em}\Delta \mathrm{H}=-74.8 \mathrm{k}\mathrm{J}/\mathrm{m}\mathrm{o}\mathrm{l}\\ \mathrm{H}\mathrm{y}\mathrm{d}\mathrm{r}\mathrm{o}\mathrm{g}\mathrm{a}\mathrm{s}\mathrm{i}\mathrm{f}\mathrm{i}\mathrm{c}\mathrm{a}\mathrm{t}\mathrm{i}\mathrm{o}\mathrm{n}\end{array}$$

(3)

$$\begin{array}{l}\mathrm{C}\mathrm{O}+{\mathrm{H}}_2\mathrm{O}\to {\mathrm{H}}_2+\mathrm{C}{\mathrm{O}}_2\hspace{1em}\hspace{1em}\Delta \mathrm{H}=-41.2 \mathrm{k}\mathrm{J}/\mathrm{m}\mathrm{o}\mathrm{l}\\ \mathrm{W}\mathrm{a}\mathrm{t}\mathrm{e}\mathrm{r}\text{–}\mathrm{g}\mathrm{a}\mathrm{s} \mathrm{s}\mathrm{h}\mathrm{i}\mathrm{f}\mathrm{t} \mathrm{r}\mathrm{e}\mathrm{a}\mathrm{c}\mathrm{t}\mathrm{i}\mathrm{o}\mathrm{n}\end{array}$$

(4)

$$\begin{array}{l}\mathrm{C}{\mathrm{H}}_4+{\mathrm{H}}_2\mathrm{O}\to \mathrm{C}\mathrm{O}+3{\mathrm{H}}_2\hspace{1em}\hspace{1em}\Delta \mathrm{H}=+206 \mathrm{k}\mathrm{J}/\mathrm{m}\mathrm{o}\mathrm{l}\\ \mathrm{S}\mathrm{t}\mathrm{e}\mathrm{a}\mathrm{m} \mathrm{r}\mathrm{e}\mathrm{f}\mathrm{o}\mathrm{r}\mathrm{m}\mathrm{i}\mathrm{n}\mathrm{g}\end{array}$$

(5)

$$\begin{array}{l}\mathrm{C}{\mathrm{H}}_4+2{\mathrm{H}}_2\mathrm{O}\to \mathrm{C}{\mathrm{O}}_2+4{\mathrm{H}}_2\hspace{1em}\hspace{1em}\Delta \mathrm{H}=+165 \mathrm{k}\mathrm{J}/\mathrm{m}\mathrm{o}\mathrm{l}\\ \mathrm{S}\mathrm{t}\mathrm{e}\mathrm{a}\mathrm{m} \mathrm{r}\mathrm{e}\mathrm{f}\mathrm{o}\mathrm{r}\mathrm{m}\mathrm{i}\mathrm{n}\mathrm{g}\end{array}$$

(6)

In chemical processes, the change in heat known as enthalpy (ΔH) is either an exothermic or an endothermic reaction [84]. When the exothermic reaction produces heat, the enthalpy change is often negative (−ΔH) [85]. A positive change of heat (+ΔH) is the enthalpy of an endothermic reaction, which removes heat from the system [86]. Many reactions go through both exothermic and endothermic stages during the gasification process. For instance, Equation (1) illustrates how C with steam is converted into H₂ and CO in a water–gas shift reaction, whereas Equation (4) shows how CO with steam is converted into H₂ and CO₂ [87]. As shown in Equation (2), the Boudouard reaction yields CO through the reaction of C and CO₂, whereas Equation (3) shows how hydrogasification, or the methanation process, transforms C and H₂ into CH₄. When steam methane reforming (steam reforming) occurs, CH₄ and H₂O are produced. Additionally, these can then react to produce CO and H₂, as shown in Equation (5), and CO₂ and H₂, as shown in Equation (6) above [83]. Figure 3 shows the flow chart of the gasification process.

The key steps in the gasification process are as follows:

• Drying Zone

The raw material’s moisture is removed here. The drying zone is typically at the top of the gasifier. This is the first area where the waste and the gasifier come into contact. This stage typically uses the heat from the conversion process to reach a temperature of approximately 160 °C [23]. Using the heat generated inside the regions or heat from the lower zones of the gasifier causes the water content in the feedstock to evaporate along with feed residues through a process known as the latent heat of vaporization [88]. This makes the material dry and more porous, facilitating efficient pyrolysis and gasification in subsequent stages [14]. The heat source can be conduction, convection, or radiation from the oxidation and reduction zones [89]. Baratieri et al. [90] experimented with the thermodynamic constraints on the performance of conversion processes and reported that moisture from woody biomass is transferred to the surrounding air during the gasifier’s drying process. The moisture layer around the biomass particles and the interior pores leading to the adsorption sites facilitate this transfer by diffusion. Since physical adsorption is thought to occur almost instantly, equilibrium between the fluid framework and the biomass surface can be assumed (for example, drying continues through various zones of the gasifier, including the pyrolysis, oxidation, and reduction zones, and the biomass particles do not shrink because of moisture evaporation). Zainal et al. [91] reported that the biomass particles’ size influences the drying rate. As might be predicted in drying operations, smaller particle sizes cause moisture to evaporate faster. When analyzing the gasifier model, they considered five distinct average particle sizes: 10, 20, 30, 40, and 50 mm. The findings support the anticipated pattern in drying behavior by showing that the rate of biomass drying dramatically increases as the particle diameter decreases, implying that improving particle size may enhance the gasifier’s overall performance. In addition to drying faster, smaller particles may help biomass undergo better thermochemical conversion into flammable gases, increasing power production and energy efficiency [92].

The rate of drying reaction (r_d) mol/(m³·s), and drying temperature (T_d) are related as follows [93,94,95]:

r_d = K_dC_H2O,l

(7)

$${\mathrm{K}}_{\mathrm{d}}={\mathrm{A}}_{\mathrm{d}}{\mathrm{e}}^{{\displaystyle \frac{{-\mathrm{E}}_{\mathrm{d}}}{{\mathrm{R}}_{\mathrm{u}}{\mathrm{T}}_{\mathrm{d}}}}}$$

(8)

where K_d is the drying process’s kinetics constant in 1/s, C_H2O,l is the concentration of moisture content in mol/m³, A_d is the pre-exponential factor = 5.13 × 106 s⁻¹ [96], E_d is the activation energy of the drying reaction = 88 kJ/mol [93], and R_u is the gas universal constant = 8.314 J/mol·K [81].

• Pyrolysis

The second stage of the gasification process, pyrolysis [97], comes after the drying stage. Waste is thermally broken down without oxygen during pyrolysis, producing volatile gases (like H₂, CO, CO₂, CH₄, H₂O, and NH₃), tar (condensable hydrocarbon vapors released from the solid matrix as gas and liquid in the form of mist, and char (the remaining de-volatilized carbonaceous solid waste residue); this process takes place at temperatures up to approximately 700 °C [98]. This step is crucial, as it transforms the waste into more reactive components that can be further processed in the gasification system [99,100,101]. When it comes to MSW, volatiles comprise a substantial portion of the carbonaceous fuel that surrounds the solid waste and contribute to the easily ignitable environment of fuel gases during the gasification process [23,89,102,103]. The main reaction is shown below:

Waste → Char + Tar + Volatiles

(9)

Kinetic considerations dominate the pyrolysis process for biomass particles smaller than 1 mm. This indicates that, rather than external variables like heat transfer, the rate at which the biomass decomposes is predominantly determined by the chemical reactions taking place within the particles [82].

The pyrolysis process, on the other hand, is affected by both kinetic variables and heat and mass transport phenomena for particles larger than 1 mm. This suggests that the reaction rate is impacted by the time needed for heat to enter the particle and for gases to exit, which rises with particle size. The reaction time, which is the amount of time required to achieve 95% weight loss, is strongly influenced by particle size [104,105,106]. Smaller particles have a larger surface area-to-volume ratio, which promotes faster heat transfer and reaction rates, thereby allowing them to lose weight more quickly [83].

• Oxidation/combustion

A limited amount of oxygen is introduced, and it reacts with the char and volatile gases [107,108,109]. The process by which solid carbonized waste reacts with atmospheric oxygen to produce CO₂ is called oxidation. Water is produced through the oxidation of the hydrogen found in waste. If carbon is partially oxidized and oxygen is present in sub-stoichiometric amounts, CO may be produced. Between 800 and 1000 °C, reduction takes place whether oxygen is present sub-stoichiometrically or not [110]. The reaction is exothermic, and the heat released during the reaction is used for the reduction reaction in the next step [79]. The following reactions occur during oxidation [111,112]:

Char_(s) + O₂→ CO₂   Carbon oxidation   ΔH = −394 kJ/mol

(10)

C_(s) + 0.5O₂ → CO₂   Carbon partial oxidation   ΔH = −110 kJ/mol

(11)

CO + 0.5O₂ → CO₂   Carbon monoxide oxidation   ΔH = −283 kJ/mol

(12)

H₂ + 0.5O₂ → H₂O   Hydrogen oxidation   ΔH = −242 kJ/mol

(13)

The volatile pyrolysis products are partially oxidized in highly exothermic reactions, resulting in a rapid temperature rise. The heat generated is used to drive the drying and pyrolysis of the fuel, as well as the gasification reactions. The oxidation reactions of the volatiles are very rapid, and the oxygen is consumed before it can diffuse to the surface of the char. Therefore, no combustion of the solid char can take place [113,114].

• Reduction

The main reactions that occur in the reduction stage are as described in Equations (1)–(6). CO, H₂, and CH₄ compose most of the synthetic gas (syngas) created by gasifying char, tar, and hydrocarbons with CO₂ and steam. Since these reactions are endothermic, the heat generated by the earlier oxidation reactions is required. Steam facilitates both water–gas shift processes (exothermic) and the steam reformation (endothermic) of char and tar [115,116]. The most efficient method of enhancing H₂ generation in steam gasification involves a reduction of water (H₂O). CO₂ drives the endothermic Boudouard reaction, which yields CO [105,117].

The advantage of gasification over other thermal waste-to-energy (WtE) conversion processes, as shown in

Table 3. Comparison of gasification with other thermal conversion technologies.

Aspect	Gasification [22,44,118,119]	Incineration [62,120]	Pyrolysis [120,121]
Primary Output	Syngas (H2, CO, CH4)	Heat and electricity	Bio-oil, biochar, syngas
Emission Levels	Low (with cleaning systems)	Higher NOx, SOx, dioxins, and furans	Minimal air emissions
Feedstock Flexibility	High	Moderate	Moderate
Energy Efficiency	High	Moderate	Moderate
By-product Usability	Recyclable slag or ash	Toxic fly ash requires treatment	Biochar (limited reuse potential)
Scalability	High	Large-scale only	Suitable for small- to medium-scale
Carbon Capture Feasibility	Feasible due to syngas use	Not feasible	Limited feasibility
Environmental Impact	Minimal with advanced technologies	Higher environmental burden	Lower than incineration

, has garnered widespread attention from the scientific community.

Numerous papers have been published that investigate the gasification characteristics of various types of waste to maximize energy recovery and address the global challenges of managing municipal solid waste (MSW) while providing sustainable energy solutions and reducing environmental impact. Notable among them are the works of Liu et al. [122], who reported the gasification of refuse-derived fuels in fluidized-bed gasifiers. Refuse-derived fuels (RDF) are fuels made from a variety of municipal solid wastes. They are produced by processing waste materials to eliminate non-combustible components and increase energy content, making them ideal for energy recovery techniques such as gasification. RDF is often composed of a mixture of organic and inorganic materials, including plastics, paper, textiles, and other combustible waste. In their research, they investigated various factors, such as temperature, oxygen concentration, steam-to-RDF ratio, and equivalence ratio, that influence the efficiency of gasification and the quality of the product gas. They concluded that optimizing temperature, oxygen concentration, and steam ratio, along with employing catalytic reforming, are essential strategies for enhancing the gasification process of refuse-derived fuels, ultimately leading to higher-quality syngas production.

Werle et al. [123] investigated the sewage sludge gasification in the fixed-bed gasifier, followed by characterization of the feedstock. The primary component of the system was an insulated, stainless-steel gasifier with an internal diameter of 150 mm and a total height of 300 mm. The sewage sludge was fed into the reactor from the top before the gasification processes, and the gasification product (syngas) was carried via a gas line and then cleaned by a cyclone, scrubber, and drop separator. Finally, the volumetric fractions of the main components of the generated gas were determined using the Fisher Rosemount and ABB integrated set of analyzers. They found that increasing the temperature and oxygen concentration of the gasification agent enhanced the production of primary gasification gas components, including CO, H₂, and CH₄.

However, some of the limitations of gasification include the insufficient yield of necessary products and incomplete gasification of plastics, which is caused by reducing fine dust and tar formation in the biomass gasification process. Motivated by these challenges, Zhu et al. [124] performed a synergistic co-gasification of polymeric materials and agri-waste (polyethylene and beech wood) in a lab-scale fluidized-bed reactor to optimize the yield of hydrogen in the presence of four different types of bed materials, such as silica sand, olivine, Na-Y zeolite, and zeolite Socony Mobil-5 (ZSM-5). The bed materials performed the role of a catalyst in the reaction process. Detailed explanations of these catalysts are not within the scope of this work. The investigation started with wood-only gasification, utilizing four different bed materials as a reference. Then, a 1:1 weight ratio of beech wood and PE was tested to see how the bed materials affected gaseous generation during the co-gasification process, utilizing various steam injection rates (0–400 g/h) and beech wood-to-PE ratios (4:0, 1:1, and 3:1). They discovered that the presence of a catalyst improves product output in variable degrees and that more significant amounts of steam fed into the fluidized-bed reactor, as well as more polyethylene, result in higher hydrogen production during the co-gasification process. However, their work primarily focused on the yield of hydrogen only. Research on syngas and other valuable products from the co-gasification of waste remains underexplored.

Also, Pinto et al. [125] studied the co-gasification of coal, biomass, and waste plastics using a fluidized-bed method. They observed that an improved gasification temperature boosts the subsequent breaking of produced hydrocarbons, releasing more hydrogen and reducing tar formation.

Several vital operating parameters, such as temperature, pressure, equivalence ratio, steam-to-carbon ratio, feedstock characteristics, gasifying agents, residence time, and catalyst usage, influence gasification efficiency and syngas quality while minimizing the negative environmental impact. The following section will detail these parameters and their implications for advanced gasification technology.

4.1. Effect of Operating Parameters

4.1.1. Effect of Temperature

Temperature plays a substantial role in gasification efficiency, product yield, and variability [88]. Table 4 presents a summary of the effects of these operating parameters. Research conducted on the gasification of WtE has proven that elevated temperatures, which are a key determinant of reaction kinetics and thermodynamics, enhance the thermochemical conversion of feedstock, resulting in greater carbon conversion efficiency, optimized syngas composition, and the minimization of byproducts like char and tar [126,127,128]. For example, Janajreh et al. [88] reported that increasing temperature causes endothermic processes to follow Le Chatelier’s model, resulting in CO production and CO₂ consumption via the Boudouard reaction (Equation (13)). Generally, optimal gasification temperatures usually range from 800 to 902 °C, where the production of hydrogen and other combustible gases is maximized [116].

Alouani et al. [129] performed research on municipal solid waste gasification using integrated simulation and performance analysis to produce hydrogen and other products. They found that 800 °C was the optimal temperature for efficient hydrogen generation. At this temperature, the production of hydrogen is significantly higher than at lower temperatures. For example, at 500 °C, the yield is 21.95 kg/h, which rises to 54.14 kg/h at 800 °C when employing an air–steam gasification system. However, subsequent temperature increases over 800 °C resulted in a drop in hydrogen output. This is due to the endothermic nature of the Boudouard process, which prefers higher temperatures, whereas the exothermic methanation reaction, which yields methane, prefers lower temperatures. They also discovered that the low heating value (LHV) of syngas rises with temperature, culminating at approximately 700 °C. LHV is influenced by the mole fractions of hydrogen, methane, and carbon monoxide in syngas. At higher temperatures, the syngas LHV is higher when employing air alone rather than a steam-air mix.

Although the paper discusses optimal hydrogen production at 800 °C, it does not thoroughly address the environmental impacts of the gasification process itself, such as emissions or the by-products generated during the process. This poses a question about the viability of their work and creates an interesting research opportunity.

Briesemeister et al. [130] reported that high temperatures promote tar cracking, yielding cleaner syngas and minimizing downstream tar-related complications. However, the advantages of elevated temperatures are mitigated by operational trade-offs, such as increased energy demand and reactor material restriction [131]. While moderate-temperature gasification (800–1000 °C) is commonly applied due to its balance between efficiency and operational costs, advanced systems have been developed to accommodate the requirements of high-temperature processes [88]. Such systems use efficient heat recovery units and strong refractory materials to offset the economic and material challenges associated with high operating temperatures [132]. These findings highlight the importance of determining temperature ranges based on feedstock qualities, anticipated syngas applications, and cost restrictions. Future research should focus on creating adaptive gasifiers capable of dynamically changing temperatures to boost performance.

4.1.2. Effect of Pressure

Various studies have demonstrated that pressure has a significant impact on the efficiency and product yield of the gasification of municipal solid waste (MSW) [133]. Higher pressures can be enhanced by altering the chemical equilibrium and improving feedstock conversion into syngas. By changing the chemical equilibrium and facilitating a better conversion of feedstock into syngas, higher pressures can improve the gasification process [134]. Recently, Szul et al. [135] reported a novel approach to determine the influence of pressure (0–2 bar_g) and CO₂ on biomass (bark and lignin) using an auto-thermal fluidized-bed gasification system. Wood pellet was used as the reference fuel, and a mixture of O₂ + CO₂ + H₂O was used as the gasification agent. It was observed that the increase in system pressure resulted in changes to the composition of the product gas, aligning with predictions based on Le Chatelier’s principle. Specifically, higher pressures were associated with increased yields of CH₄ and decreased product gas production. This is attributed to the thermodynamic principles governing gasification reactions, particularly the water–gas shift and Boudouard reactions, which are influenced by pressure. They added that adding CO₂ not only improves carbon conversion efficiency (CCE) but also helps contribute to a negative carbon footprint, which makes the process greener by possibly lowering total CO₂ emissions.

In another recent development, Krumins et al. [136] experimented with MSW using a MATLAB R2022b-based simulation model to analyze the performance, energy efficiency, and environmental impact of the waste obtained from the Daibe landfill near Cēsis, Latvia. The Genetic algorithm toolbox in MATLAB was used to enhance efficiency and minimize the system’s environmental impact. Higher pressures (1–5 bar) improve syngas production and energy efficiency by improving reaction kinetics and gas–solid interactions, which ultimately results in better conversion rates of municipal solid waste into valuable energy products with a lower environmental impact than incineration and landfills, according to their report. However, their report relied on MATLAB R2022b software and samples obtained from the Daibe landfill only. These limitations restrict the generalization and reproducibility of their findings.

Notable results indicate that, depending on process objectives, there is a trade-off between suppressed hydrogen output in high-pressure conditions and increased methane content for calorific enhancement [137]. These revelations highlight the necessity of creative strategies to utilize the advantages of pressurized gasification fully.

4.1.3. Effect of Feedstock

Several investigations have demonstrated that the type of feedstock has a significant impact on gasification efficiency and product yield [138]. The moisture content, ash composition, and volatile matter of various feedstocks are among the distinctive characteristics that influence the gasification process [139,140]. Therefore, understanding the energy content and creating product identities requires a thorough characterization and proximate analysis of the feedstock materials before introducing the feedstock to gasifiers for gasification. Gasification feedstock includes coke, biomass, MSW, and their various blends, among others [141].

The energy content, frequently referred to as the high or low heating value (H/LHV), is directly related to the high amounts of fixed carbon and volatile matter. While an increase in moisture or ash content lowers the HHV, an increase in the fraction of FC and volatile organic content (VOC) raises it [142].

As demonstrated by Equation (14), the feedstock’s HHV is typically proportional to the gasification efficiency (GE) metric, which is the ratio of the product’s (syngas) cumulative heating value over that of the feed material.

$$\mathrm{G}\mathrm{E}={\displaystyle \frac{{\mathrm{m}}_{\mathrm{c}\mathrm{o}}{\mathrm{H}\mathrm{H}\mathrm{V}}_{\mathrm{c}\mathrm{o}\mathrm{m}}+{\mathrm{m}}_{\mathrm{H}2}+{\mathrm{m}}_{\mathrm{C}\mathrm{H}4}+{\mathrm{H}\mathrm{H}\mathrm{V}}_{\mathrm{C}\mathrm{H}4}}{{\mathrm{m}}_{\mathrm{f}\mathrm{e}\mathrm{e}\mathrm{d}\mathrm{s}\mathrm{t}\mathrm{o}\mathrm{c}\mathrm{k}}+{\mathrm{H}\mathrm{H}\mathrm{V}}_{\mathrm{f}\mathrm{e}\mathrm{e}\mathrm{d}\mathrm{s}\mathrm{t}\mathrm{o}\mathrm{c}\mathrm{k}}}}$$

(14)

where m_co is the mass flow rate in kg/s, and HHV is the heating value in MJ/kg [142].

However, some of the literature uses cold gasification efficiency (CGE) instead of GE to indicate the absence of any sensible heat/enthalpy in the syngas product.

Basha et al. [143] reported the air co-gasification of palm kernel shells and polystyrene plastic. They reported that at 900 °C, and by increasing the PS content of the feedstock, a higher percentage of the gas was obtained with a higher HHV of the producer gas. They concluded that mixing PS and PKS is a promising approach to GE, as the high HHV of the gas makes it suitable to be used as fuel.

In another experiment, Pinto et al. examined the gas output from the gasification and co-gasification of plastic (PE) and biomass (pine wood) [144]. According to the data, the hydrogen content was 35% (v/v) when pine wood alone was used. Adding PE caused the H₂ concentration to rise to 52% (v/v). They stated that the breakdown of PE molecules was the cause of the rise in H₂ concentration. Using 60% PE in the waste mixture produced a maximum gas yield of 1.96 Nl/g daf, energy conversion (98%), and HHV (18.3 kJ/Nl) values. This mixture produced a C_nH_m concentration of approximately 13% (v/v) with a methane content of approximately 5% (v/v), despite the hydrogen concentration being approximately 50% (v/v). Zhuang et al. [145] conducted a gasification of textile waste using steam as a gasification agent, followed by catalytic reforming to enhance the syngas yield. They reported that the syngas yield increased from 20.86 to 80.97 mmol/g, and the hydrogen concentration increased from 17.79 to 50.91 vol%, representing an increase of 288.12% and 186.18%, respectively. This indicates that the choice of a suitable catalyst and gasification agent is a promising approach to optimizing syngas production.

Recently, Ishak et al., in 2024 [146], investigated the co-gasification of empty fruit bunch biomass (EFB), low-density polyethylene (LDPE) plastic waste, and polyethylene terephthalate (PET) as feedstock in a plasma–air co-gasification reactor. A reactor with an air-blown downdraft arrangement was used in the experiments. The air gasifying agent flow rate was adjusted between 10 and 22 Hz to achieve an equivalence ratio of 0.15 to 0.30. The study showed that the composition of syngas generated during gasification was significantly impacted by the type of feedstock, for example, when low-density polyethylene (LDPE) and empty fruit bunch (EFB) were mixed, the composition of hydrogen (H₂) and carbon monoxide (CO) typically decreased as the equivalence ratio (ER) increased. On the other hand, the EFB and polyethylene terephthalate (PET) mixture showed higher H₂ and CO with higher ER values, because the raw materials of EFB and LDPE contain more H, C, and O than PET. This underscores the role of co-gasification’s synergistic effect in optimizing gasification processes such as clean energy production and reduced environmental pollution.

However, the economic aspects of implementing co-gasification on a larger scale were not discussed in their report, which could have assisted in making a cost-effective comparison with other techniques for its application. The summary of the effect of these operating parameters is shown in Table 4 below.

Table 4. Effects of operating parameters (temperature, pressure, and feedstock).

Operating Parameter	Effect	Observation/Result	Benefit	References
Temperature	Influences the syngas composition and reaction kinetics Promotes CO yield through the Boudouard reaction Enhances tar cracking and production of H2	Optimal range: 800–900 °C	Improved carbon conversion efficiency Clean syngas with high energy content	[134,147,148]
Pressure	Influences chemical equilibrium and gas production	Reduces H2 yield and increases CH4 production Promotes feedstock conversion	Beneficial when CH4-rich syngas is required	[149,150]
Feedstock	Helps to determine syngas composition, yield, and reactor design Co-gasification of plastics with biomass results in higher CO and H2 yield	High volatile, low moisture, and ash lead to an improved gasification process	Improved synergies through co-gasification	[151,152]

Table 4. Effects of operating parameters (temperature, pressure, and feedstock).

Operating Parameter	Effect	Observation/Result	Benefit	References
Temperature	Influences the syngas composition and reaction kinetics Promotes CO yield through the Boudouard reaction Enhances tar cracking and production of H2	Optimal range: 800–900 °C	Improved carbon conversion efficiency Clean syngas with high energy content	[134,147,148]
Pressure	Influences chemical equilibrium and gas production	Reduces H2 yield and increases CH4 production Promotes feedstock conversion	Beneficial when CH4-rich syngas is required	[149,150]
Feedstock	Helps to determine syngas composition, yield, and reactor design Co-gasification of plastics with biomass results in higher CO and H2 yield	High volatile, low moisture, and ash lead to an improved gasification process	Improved synergies through co-gasification	[151,152]

4.2. Effect of Gasification Agents

Gasification agents significantly impact gasification efficiency, affecting the yield and quality of syngas generated. Air, oxygen, steam, and CO₂ are examples of different agents that have different effects on the gasification process, and they can be tailored for different feedstocks and operating circumstances [138,153].

Table 5. Effect of gasification agent on gasification.

Gasification Agent	Effect on Gasification	Advantages	Limitations	References
Air	Low calorific value of 3–5 MJ/Nm3, low syngas yield due to nitrogen dilution	Cost-effective The need for an external oxidant is eliminated	Due to N2 contamination, the syngas heating value is low	[154,155]
Oxygen	High CO and H2 yield	No nitrogen contamination High calorific value	Higher heating value than air gasification Higher syngas quality	[153,156]
Steam	Enhances H2 through the water–gas shift and steam reforming reactions	Promotes syngas yield and tar cracking	May require heat input Excess steam may affect the CO and CH4 yield	[157,158]
CO2	Promotes CO yield via the Boudouard reaction	Supports systems with a low carbon footprint and promotes carbon recycling	Reduces H2 yield Promotes CO yield	[158,159,160]

summarizes these agents and their characteristics to enable the proper choice of agent based on the desired product.

4.2.1. Air

Since air is easily accessible and air gasification can generate sufficient heat without the need for external heat sources, it is the most straightforward and cost-effective method of achieving gasification [154]. However, because the significant amount of nitrogen in air (79%) dilutes the content of syngas and does not participate in the gasification reaction, the produced gas has a comparatively low calorific value of 3–5 MJ/Nm³ [155]. Jamro et al. [161], successfully achieved a 45.79 mol% of H₂ from the gasification of MSW in a tubular batch reactor using air as the gasification agent at a temperature of 703.49 °C. The syngas composition stood at 76.99 wt.%, with a tar content as low as 6.80 wt.%, demonstrating the potential of air as the gasification agent and MSW as a promising method for H₂ production.

On the contrary, a study by Wei et al. [162] revealed that the addition of air as a gasification agent during the gasification of chili straw waste in a fixed bed at a temperature of 750 °C and pressure of 1 atm reactor leads to a decrease in the concentration of H₂ up to 33.33% at an equivalent ratio of 0.39. CO concentration also decreased from 53.34 to 39.13%. CO₂ concentration, however, increased to a maximum of 28.51% at an ER of 0.39 while CH₄ also decreased. They further stated that the oxidation reactions that occur during the gasification process can be used to study the observed changes. As CO and H₂ react with O₂ in oxidation processes, carbon and oxygen undergo a carbon (char) oxidation reaction when air is introduced as the gasifying agent. While the partial oxidation of reaction char is significantly decreased, the full oxidation of carbon increases as the ER increases. This causes the CO content to decrease as it reacts with O₂ to generate CO₂, which causes the CO₂ content to rise sharply and surpass the CO reduction. Furthermore, the process of H₂ oxidation decreases with the increasing air supply, resulting in the consumption of H₂. This indicates that certain optimizations, such as oxygen enrichment, are needed to support air gasification and produce high-quality syngas.

Using oxygen-enriched air as a gasification agent, Liu et al. [163] recently studied biomass gasification. They found that, while the gas production increased by 1.0%, the tar yield declined from 107 to 94.21 g/Nm³ when the air temperature rose from 25 to 250 °C, because of increased tar cracking. The highest air temperature of 250 °C was optimal for H₂ generation, which was 12.26%. A new finding by Restrepo et al. [164] using an air-blown, moving-bed downdraft gasifier (MBDG) demonstrated that the removal of impurities from raw producer gas (RPG) produced during biomass gasification is greatly improved by using a two-stage air system or by varying the gasifier’s total airflow and air ratio. The second air supply step is very effective at decreasing pollutants, such as tars, particulates, and other gases like H₂S, NH₃, and HCL, by 60% while maintaining efficiency.

Another study by Li et al. [165] on the gasification of woody biomass in a fixed-bed gasifier using air as the gasifying agent witnessed a rise in H₂ and CO concentrations. When the flow rate of the air was increased from 10 L/min to 45 L/min and the ER from 0.22 to 0.29, the H₂ and CO concentrations increased from 8.26% to 12.30% and 14.76% to 16.96%, respectively. However, other gases such as CO₂, CH₄, and C_mH_n witnessed a decline in concentration. The addition of air accelerated the release of moisture and volatiles from the biomass, leading to an increased oxidation reaction that produced a significant amount of heat. According to Le Chatelier’s principle, the moisture released during the pyrolysis and drying stages increases the concentration of H₂ while decreasing that of CH₄ due to the methanation and water–gas shift processes. To optimize the process for high syngas yield, they enriched the air gasifying agent with O₂ at a flow rate of 35 L/min. Increasing the concentration of O₂ from 21 to 28% led to an increase in CO from 15.42 to 20.35%, CH₄ from 1.77 to 4.05%, CO₂ from 15.16% to 25.15%, and C_mH_n from 0.70 to 2.06%, at a constant gasifying agent flow rate and an increase in O₂ concentration. The oxidation process intensified, leading to a more intense forward reaction, and, as in the methanation reaction, a dynamic equilibrium between CO and CO₂ is achieved when higher O₂ concentrations drive the conversion of C and CO to CO₂. The effect of O₂ enrichment on H₂ concentrations was not clearly stated.

4.2.2. Oxygen

The produced gas generated by employing oxygen as a gasification agent has a higher HHV than its counterpart produced by air gasification because of the absence of nitrogen in the syngas [166]. Oxygen gasification has seen tremendous development with recent breakthroughs. For instance, to optimize the quality of the produced gas, Gomez et al. [167] investigated the impact of rising O₂ concentration during direct air gasification of leftover eucalyptus biomass. The outcome demonstrated that adding more O₂ as a gasification agent increases CO₂ and flammable species. However, at higher O₂ concentrations, gas output drops dramatically, by 33%. They reported that this improvement was brought about by a decrease in the amount of N₂ in the air gasification, which in turn caused the concentration of N₂ in the producer gas to decrease. This resulted in a 57% improvement in the lower heating value and the gas quality for energetic applications.

Raibhole et al. [168] developed an ASPEN Plus model for gasifying charcoal, rice husk, and wood pellets combined with a cryogenic oxygen plant using air and oxygen as gasification agents. Interestingly, oxygen as a gasifying agent produced higher-quality syngas yields with high calorific values (CV) across all samples compared to air, as shown in

Table 6. Syngas composition from the feedstock.

Fuel	Gasifying Agent	Syngas Composition
		H2	CO	CO2	CH4	CV (MJ/kg)
Indian coal	Air	8.8	41.8	0.623	17.3	12.59
	Oxygen	15.3	60.1	0.003	0.23	19.55
Rice husk	Air	22.9	18.4	13.0	0.8	5.49
	Oxygen	36.5	21.8	20.2	0.6	9.14
Wood pellets	Air	32.1	29.8	7.9	0.9	9.22
	oxygen	4.07	37.8	11.3	1.7	13.19

below.

Park et al. [169] performed an oxygen enrichment gasification on hardwood sawdust in a dual-tube type gasifier. Using oxygen conditions of 21, 25, 30, and 35%, respectively, they found that, at 30% enrichment, H₂ and CO concentrations reached a maximum of 47.64%, and a tar reduction efficiency of 72.46%, due to improved partial oxidation, a cold gas efficiency of 78.00%, and carbon conversion efficiency of 80.24% were all achieved. This indicates that 30% enrichment is the optimal oxygen condition for tar reduction and improved syngas yield.

Another study by Liu and co-workers [170] to investigate the effect of oxygen on the air gasification of rice straw in a two-stage pilot fluidized-bed gasifier found that increasing the oxygen concentration from 21% to 45% led to an increase in the volume of combustible gases, particularly CO and H₂. CO increased from 13.09 to 19.34%, while H₂ rose from 3.26 to 5.38%. However, the total gas yield decreased from 1.20 to 1.04 Nm³/kg, representing a 13.19% reduction compared to normal air gasification. Additionally, the heating value of the gas, carbon conversion efficiency, and gasification efficiency increased by 26.11, 43.94, and 9.47%, respectively. The changes were attributed to the decrease in N₂ concentration as the oxygen concentration increased. Finally, the addition of secondary oxygen (33%) led to an increase in the concentration of CO₂ and CO but a decrease in the concentration of H₂ and CH₄ (due to the promotion of more oxidation reaction, Boudouard and water–gas shift reaction), demonstrating the role of secondary oxygen in enhancing gas calorific value with an adverse effect in the yield of some combustible gases. While this work has highlighted the importance of oxygen addition in a two-stage gasifier, the result is limited to only a specific amount of secondary oxygen (33%). The potential effect of oxygen concentration beyond 33% has not been stated, which could have provided more insights into the effect of oxygen concentration on the gasification process. Additionally, conducting a similar experiment using other biomass and co-gasification processes presents a research opportunity for further studies.

A novel approach in oxygen gasification involves the use of oxygen carriers through chemical looping gasification (CLG). In this process, the lattice oxygen of the oxygen carrier supplies the oxygen required for the gasification reaction. Solid fuel is gasified and transformed into syngas by regulating the lattice oxygen-to-fuel ratio. The following are some benefits of the CLG technique [171]: (1) recycling of the oxygen carrier provides oxygen for fuel gasification without the need for additional oxygen; (2) heat from the oxygen carrier in the air reactor can be used as heat for fuel gasification; (3) the oxygen carrier catalyzes the gasification reaction; and (4) CLG can lower the emission of harmful gases like sulfur compounds and nitrogen oxides. The method was adopted by Luo et al. [172], who used iron ore as an oxygen carrier in a fixed-bed reactor for the gasification of rice husk and achieved a low heating value of 12.25 MJ/Nm³, gasification efficiency of 26.88%, a syngas yield of 56.40%, a hydrogen yield of 26.88%, H₂/CO of 1.72 at an optimum temperature of 800 °C, and a residence time of 30 min O/C of 1.5. They concluded that, out of several methods of preparing oxygen carriers, the co-precipitation method yielded the best results.

4.2.3. Steam

Steam is one of the most popular and effective gasification agents due to its high hydrogen content and increased synthetic gas output, without dilution with N₂. In addition to facilitating tar cracking, steam gasification offers other advantages, including a significantly higher synthetic gas enthalpy value and an ideal residence period [173]. Through several methods, steam acts as an efficient gasification agent, significantly increasing gasification efficiency. Numerous studies have effectively documented its relevance in promoting higher hydrogen yields and improving the quality of syngas. In comparison to other agents, such as air or CO₂, Khan et al. [174] noted that the benefits of using steam as a gasification agent include increased hydrogen enrichment, reduced sulfur and nitrogen pollution, and a higher heating value of producer gas due to the reduction of non-combustible diluents.

In their study, Shafiq et al. [175] used Aspen Plus^® modeling to investigate the steam gasification of MSW in a dual-fluidized gasifier using catalytic coal bottom ash and dolomite as the adsorbents. At 650 °C, a 5.15 kg/h MSW input rate, a 1.4 CaO-MSW ratio, and 0.07% catalytic coal bottom ash were maintained. When the steam-to-feedstock (S/F) ratio was 0.5, the hydrogen yield in synthetic gas was 58.9%; at an S/F ratio of 1, it rose to 71.2%. At an S/F ratio of 1.9, the highest hydrogen yield of 74.9% was attained. This hydrogen (H₂) fraction rise was due to the methane reformation, char gasification, and water–gas shift reaction. The HHV syngas decreased from 18.1 to 14.4 MJ/Nm³ due to the increase in CO₂ yield and the presence of H₂O in the product gas, whereas the yields of methane (CH₄), carbon monoxide (CO), and carbon dioxide (CO₂) declined as the S/F ratio increased. High CO levels of approximately 15.9% were noted at an S/F ratio of 2. The reform of steam methane explains why the CO component was so high. They mentioned that technical difficulties related to tar formation and environmental concerns have emerged as significant obstacles to the commercialization of biomass gasification.

To address this, Fu et al. [176] reported an ASPEN plus^® modeling in which steam was used as a gasification agent to transform municipal solid waste (MSW) into H₂-rich syngas. Steam flow rates (SFRs) of 0.138–0.312 kg/h and temperatures of 750–900 °C were employed. The findings showed that raising the gasification temperature raised the H₂ content. The tar production decreased due to the tar cracking reaction, whereas the gas yield increased by 29.8% as a result of the accelerated endothermic reactions when the temperature was elevated from 750 to 900 °C. They concluded that by concurrently fostering the tar-cracking and water–gas shift reactions, the addition of steam significantly increased H₂ production. Another study by Singh et al. [177] revealed that an optimal steam-to-biomass ratio of 0.5 mL/min and temperature of 800 °C were responsible for tar reduction, and the highest syngas yield of 1.2 m³/kg against the STR of 0.625 mL/min and temperature of 700 °C produced 0.95 m³/kg of syngas. A combination of steam and another gasification agent is an advanced approach for improving the process efficiency [153].

To investigate the influence of steam on air gasification and gas composition, as well as on tar and particulate matter reduction, Elbl et al. [178] recently conducted a gasification experiment of SS and digested it in a fluidized-bed gasifier at 750 °C. They found that, due to the reforming reactions in which H₂ is the major product, adding steam to the air gasification of SS increased the H₂ content in the gas composition by 10% and resulted in a high LHV of 4.21 MJ/Nm³. However, the opposite was discovered when the gasification was conducted using air only as the gasifying agent, with the highest LHV being 4.06 MJ/Nm³. They mentioned that low temperatures may be responsible for the opposite trend. Additionally, due to the predominant water–gas shift reactions upon the addition of steam, the CO₂ concentration also increased for both the SS and the digested samples: the addition of steam resulted in a significant reduction in tar of 54.5% and 29%, respectively. However, the addition of air, or steam with air, did not affect particulate matter reduction.

These findings are crucial for guiding further research on the tested fuels by adjusting operating parameters and informing the research community on the role of steam in hydrogen production during the gasification of MSW. Future research should explore the steam-and-air gasification of the SS and digestate or a combination of both, utilizing higher temperatures and varying parameters.

4.2.4. CO₂

CO₂ gasification has emerged as a promising route for carbon utilization with breakthrough research. The use of CO₂ as a gasification agent significantly impacts gasification efficiency by influencing the composition of the produced gas and the overall energy recovery [110,179]. In gasification processes, CO₂, a greenhouse gas that contributes to global warming, can be utilized as a gasifying agent to reduce atmospheric CO₂ accumulation and the production of CO as an industrial feedstock [180].

From the standpoint of CO₂ consumption and CO yield enhancement, using CO₂ as a gasification agent is preferable due to the Boudouard reaction [180,181,182]. According to Butterman et al. [183], the benefits of using CO₂ gasification include the following: no energy is needed for vaporization; the H₂/CO ratio in the producer gas may be adjusted readily to meet the specific needs of the process; CO₂ can increase the amount of volatiles in reactive char, leading to an enhanced gasification efficiency; and gasifying CO₂ rather than nitrogen can produce flue gas with a high CO₂ content that is appropriate for CO₂ recovery and recycling directly. One of the primary disadvantages of carbon dioxide gasification is that it requires an external heat source to sustain the gasification temperature, because the heat from partially burning biomass in oxygen or air is insufficient [184,185]. The expense of capturing and using CO₂ is one of the factors preventing wider utilization. It is necessary to optimize feedstock properties, pressure, and temperature [160]. A few breakthroughs in the realm of CO₂ gasification include the works of Islam [153], who developed an Aspen Plus simulation prediction model for the gasification of biomass (pine wood chips) using different gasifying agents, such as CO₂, steam, air, and H₂O_2, which was further validated experimentally in a bubbling fluidized-bed gasifier. The findings indicate that using CO₂ as a gasification agent resulted in the production of CO gas, compared to other agents.

Lahijani et al. [182] achieved a 45% tar reduction and a syngas 70% yield during the co-gasification of rice husk, rice straw, and PE in a bubbling fluidized-bed gasifier. Mauerhofer et al. [186] studied the utilization of CO₂ as a gasification agent by producing high-value product gas from olivine as the bed material and softwood as fuel. They claimed that using pure CO₂ as a gasification agent produced promising results between 827 and 838 °C. As the amount of CO₂ employed as a gasification agent increased, so did CO generation, carbon utilization efficiency, and overall cold gas efficiency.

Tamaja et al. [187] used a fluidized-bed dual reactor to study the gasification of MSW. To assess the impact of CO₂ as a gasification agent, temperatures ranging from 400 to 700 °C were employed, with silica sand serving as a heat conduction medium between the coupled combustion and gasification reactors. They discovered that raising CO₂ levels significantly increased CO composition, reaching 22.73%. However, steam, acting as a gasification agent, is linked to a rise in H₂ rather than an increase in CO₂. They concluded that combining the two agents at different temperatures is a viable strategy to maximize syngas production with a high calorific value. Their work, however, is limited to specific temperatures, raising questions about the efficiency of the process at higher temperatures. Given the disadvantages of using CO₂ as a gasification agent mentioned above, it is necessary to develop an optimization method to overcome the demand for external energy during its utilization, which should be the focus of future research.

4.3. Effect of the Catalyst

The efficiency of the gasification process often faces limitations due to tar formation, char reactivity, and product gas composition [188]. A catalyst can be employed to address this. By lowering activation energies, catalysts enhance the conversion of biomass into syngas by increasing reaction rates, reducing operational temperatures, and improving product yields [189]. The choice and characteristics of catalysts significantly influence the efficiency of gasification processes [190].

Numerous studies have been conducted to investigate the impact of catalysts on gasification efficiency, focusing on various catalyst categories, including mineral-based catalysts, transition metal-based catalysts, and alkali and alkaline earth metal-based (AAEM) catalysts.

4.3.1. Akali and Alkaline Earth-Based Metal (AAEM) Catalysts

AAEMs are highly reactive elements that belong to groups 1 and 2 of the periodic table. They are frequently used in catalytic applications as oxides, carbonates, or salts, such as magnesium oxide (MgO), calcium oxide (CaO), potassium carbonate (K₂CO₃), or sodium bicarbonate (NaHCO₃) [191,192]. By promoting carbon conversion, speeding up reaction kinetics (such as the water–gas and Boudouard reactions), and lowering tar formation, these catalysts have shown great promise in increasing gasification efficiency [193], with alkaline earth metals offering more advantages like acid gas capture and reduced environmental risks, which makes them suitable for a variety of applications in biomass and municipal solid waste gasification, whereas alkali metals exhibit superior catalytic properties but face difficulties like leaching and deactivation at high temperatures [193,194]. However, compared to alkali metals, alkaline earth metal catalysts require higher loading rates to have noticeable effects [195]. Several authors have reported research on utilizing these catalysts to optimize gasification efficiency.

Ren et al. [196] reported a study in which a fixed-bed reactor was employed to assess the role of inherent alkali and alkaline earth metals (AAEMs) in promoting the gasification of candlenut wood with large particle sizes under oxygen–steam conditions. They demonstrated that AAEMs, particularly potassium (K) and sodium (Na), significantly boost the char–steam gasification, water–gas shift, and steam–tar reforming reactions. This catalytic effect led to an increase in H₂ production and overall gas yield during the gasification process.

Lv et al. [197] investigated the interaction between waste cypress sawdust (WCS) and MSW coal liquefaction residue (CLR), an industrial municipal solid waste. They reported that the AAEMs in WCS and iron minerals in CLR synergistically promoted char gasification, enhancing gasification efficiency.

Furthermore, Wu et al. [198] recently reported the fixed-bed steam gasification of guaiac-lignin (G-type lignin) in the form of brown powder, which is mostly obtained from coniferous woods and primarily made up of β-O-4 monomers. An amount of 5 mmol of each of six distinct AAEM species (Na₂CO₃, K₂CO₃, and CaCO₃) and sodium salts (NaHCO₃, NaSO₄, and NaAlO₂) were impregnated into the lignin complex to determine their influence on gasification efficiency. They demonstrated that all six catalysts could improve lignin gasification performance by increasing total gas yields, syngas yields, H₂/CO ratios, and carbon conversion rates. The order of catalytic strength was NaCO₃ > NaHCO₃ > K₂CO₃, CaCO₃, Na₂SO₄, and NaAlO₂, which exhibited poor catalytic abilities. Na₂CO₃ and K₂CO₃ impregnation considerably reduced the liquid and solid yields, particularly for the almost entirely gasified coke, while increasing the gas yield to approximately 70% and 58%, respectively. Although the increase was small, adding CaCO₃ also raised the gas output by almost 43%.

Overall, alkali metal salts were beneficial to coke gasification and tar reforming. Interestingly, these findings were consistent with the report by Huang and Wang et al. [199,200], which mentioned that alkali metals can weaken the C–C bonds and increase the interlayer distance by generating intercalation compounds with carbon to improve char gasification. According to the authors, Figure 4 shows the carbon conversion rates, gas distribution, and three-phase product yields of lignin under the catalysis of various carbonates, both with and without a catalyst.

Focusing on the conversion of lignin, a byproduct of biomass processing, the study contributes to the development of renewable energy sources and addresses environmental challenges related to lignin disposal. However, the inability to generalize their findings to other waste types opens a vast opportunity for further research.

4.3.2. Effect of Transition-Metal-Based Catalysts

Nickel-Based and Other Metal Oxide Catalysts

Nickel-based catalysts are catalyst materials that utilize nickel (Ni) as the active ingredient to accelerate chemical processes. High activity in hydrogenation, steam reforming, and tar cracking, and their low cost are a few exceptional properties of nickel-based catalysts that make them useful in a variety of industrial processes [201,202]. Examples of these metal oxide catalysts include NiO, V₂O₅, Cr₂O₃, MnO₂, Fe₂O₃, Co₂O₃, CuO, ZnO, etc.

Preparation methods include the impregnation method, in which a support material, such as silicon or alumina (SiO₂, Al₂O₃), is impregnated with a solution of nickel salts and sodium silicate (Na₂SiO₃) as a binder. The impregnated material is then dried at approximately 105 °C overnight to remove moisture and ensure stability, and calcined to anchor nickel on the support [153,203,204]. Other methods for preparing nickel-based catalysts include sol–gel, co-precipitation, and hydrothermal techniques, etc. [205,206].

The primary purposes of nickel-based catalysts and other metal oxides in gasification are to improve reaction efficiency, minimize the production of unwanted byproducts such as tar, and optimize syngas composition, particularly by increasing the hydrogen yield [202]. In addition, their catalytic reformation of syngas properties aids in the removal of impurities such as HCl and H₂S, thereby improving the gasification efficiency [203]. They are crucial for clean and effective energy conversion because of their excellent thermal stability and robust interaction with hydrocarbon feedstocks [207]. To overcome issues like deactivation brought on by sintering, poisoning, or carbon deposition, advanced formulations and doping with promoters are frequently used [208,209]. Due to their high hydrogen yield potential, nickel-based catalysts are the most widely researched transition metals in gasification [210].

Recently, Ngo et al. [203] investigated the fluidized-bed gasification of hydrogen yield generation and tar yield reduction in rice straw using nickel and zeolite catalysts and a hot gas cleaning (HGC) temperature of 250 °C. They reported a slight increase in H₂, CO, and CH₄ yields, from 7.31 to 8.03%, from 14.57 to 17.34%, and from 3.97 to 4.41%, respectively, when the zeolite catalyst was used. However, when the zeolite was replaced with the Ni-based catalyst (NiO/SiO₂-Al₂O₃), H₂, CO, and CH₄ yields increased from 7.31 to 14.57%, 12.24 to 24.63%, and 3.48 to 3.55%, respectively, and tar removal increased from 50 to 70%. They further mentioned that the Ni-based catalyst can extend the reaction time and CH₄, CO, and H₂ production, indicating that it is crucial in promoting the water–gas shift reaction and methane reforming.

In 2024, Wu et al. [211] investigated the performance of four different metal oxide catalysts (NiO, MnO₂, Al₂O₃, and Fe₂O₃) in the gasification of spent coffee grounds collected from various restaurants in Tokyo, Japan. The samples, along with the catalysts, were prepared by simple physical mixing, followed by gasification in a fixed-bed reactor at temperatures of 800, 900, and 1000 °C, respectively, to further understand the effect of temperature on syngas production. Their findings revealed that the MnO₂ catalyst performed better than other catalysts at 900 °C, indicating that higher temperatures are effective for an improved gas yield. They stated that the high performance of Mn was due to its better thermal conductivity and improved activity. However, their result only showed the production of CO and CH₄ in the gas mixture, excluding H₂, which is a major component of the syngas. This presents a valuable research opportunity for further work, as understanding the performance of catalysts in hydrogen production is crucial, as it will guide gasification industries and the research community toward the full potential of these catalysts. Further research should focus on the production of other valuable gases and utilize temperature conditions outside those used in this report to fully understand how temperature variations affect catalyst performance and syngas yield.

In another study using high-density polyethylene (HDPE) as feedstock and a U-type quartz lab-scale reactor, Farooq et al. [212] investigated the effect of Ni loading on different types of alumina (γ-Al₂O₃, mesoporous Al₂O₃, 13 nm-sized Al₂O₃, and <50 nm-sized Al₂O₃) for enhanced hydrogen production. The incipient wet impregnation method was used to prepare the loaded catalyst. γ-Al₂O₃, m-Al₂O₃, 13 nm Al₂O₃, and <50 nm-sized Al₂O₃ were impregnated with 30% Ni using Ni (NO₃)₂·6H₂O as the precursor. The result of the hydrogen gas yield, as shown in Figure 5, was found to be in the order of Ni/<50 nm Al₂O₃ > Ni/g-Al₂O₃ > Ni/m-Al₂O₃ > 13 nm Ni/Al₂O₃. The highest dispersion of nickel on the nano-sized alumina (<50 nm) and surface area was responsible for maximum efficiency for H₂ production (24.05% by volume), indicating that Ni/<50 nm is a better catalyst for H₂ generation (13 nm Al₂O₃ and Ni/m-Al₂O₃) with comparatively less Ni dispersion, showing a positive trend towards more CO and CO₂ generation. Additionally, the C1–C4 generation exhibited a decline with decreasing support size. These findings are crucial in selecting the type of Ni catalyst support to produce hydrogen and other gases, depending on the specific gas required.

Olivine-Based Catalysts

The chemical formula for olivine, a naturally occurring magnesium–iron silicate mineral, is (Mg, Fe)₂SiO₄ [213]. Olivine is widely utilized as a catalyst or catalyst support to enhance MSW gasification processes because of its mechanical strength, thermal stability, and affordability [191]. It is sometimes incorporated with nickel or other active metals to optimize its catalytic properties, thereby reducing tar formation and improving syngas quality [214]. Olivine has also been reported to have a dual function, acting as both bed material and catalyst during gasification due to its unique physical, chemical, and thermal properties [215]. It can be employed in large-scale gasification processes due to the combination of its natural and catalytic properties [216]. While olivine has a unique advantage, its catalytic strength is lower than that of specialized metal-based catalysts, which is why it is sometimes modified with active metals like nickel and potassium.

Various studies have demonstrated that olivine can convert MSW into valuable gases while minimizing undesirable byproducts. To eliminate tar production and produce high-quality syngas, Tian et al. [216] investigated the air-gasification process of pine sawdust using a fluidized-bed unit with Ni/olivine as an in situ tar destruction catalyst. The findings show that when Ni/olivine is used as the bed material, cold gas efficiency (GE) reaches its maximum value of 72.4% at an equivalency ratio (ER) of 0.21. Then, because of the intensified char combustion reaction, GE decreases further with an increase in ER to 0.39. While H₂ concentration (14.1–29.8%), char conversion efficiency (CCE) (51.3–9.1%), cold gas efficiency (GE) (44.8–86.1%), and gas low heating value (5.13–7.19 MJ/Nm³) rise at high temperatures, the tar yield (1.3–6.8 g/Nm³) dramatically declines with temperature.

Xiao et al. [217] reported the co-gasification of pine sawdust and Shenmu bituminous coal in a dual-looping gasification (DLG) process with olivine as bed material and in situ tar destruction catalysts. They found that tar is efficiently destroyed in the DLG at an elevated reformer temperature and with an olivine catalyst. Cao et al. [218] experimented with the gasification of pine sawdust in a small-scale fluidized-bed gasifier using olivine as an in-bed material. The aim was to study the effect of adding olivine on the product gas composition and tar concentration. They concluded that steam gasification of biomass with olivine can be a promising option for producing H₂-rich syngas with reduced tar formation and lower CO₂ emissions. This is based on their findings, in which the CO and H₂ contents rose by 15.5% and 11%, respectively, when the temperature was raised from 700 to 850 °C. As the steam/biomass (S/B) ratio rose from 0.3 to 1.2, the H₂ yield rose from 35.4 to 57.2 g/kg. At 800 °C, an S/B ratio of 1.2, and a 50% olivine content, the maximum H₂ output of 71.4 g/kg and the lowest tar concentration of 1.3 g/Nm³ were obtained. This indicates that optimizing operating conditions is a promising way to enhance the catalytic properties of the olivine catalyst.

Advancements in utilizing olivine catalysts include calcination and doping with some metals and alkali metals. Calcination is known to improve catalytic performance by increasing surface reactivity and further eliminating contaminants, thereby minimizing environmental impact and generating clean gas [219]. The addition of or doping with alkali-based metal has been mentioned to increase the basicity of the catalysts, improving their activity in tar cracking and CO₂ capture [193,220]. Considering this, Cortazar et al. [221] investigated the synergistic effect of Fe (5 wt.%) doping on an olivine catalyst in the continuous steam gasification of pine wood sawdust in a novel bench plant fitted with a fountain-confined spotted-bed reactor at 850 °C. The effect of the calcined olivine catalyst on the gasification process was compared with that of the impregnated Fe/olivine catalyst. Their findings revealed encouraging results, as all gasification products (gas composition, H₂ yield, carbon conversion efficiency, and gas production) increased significantly with the addition of Fe/olivine; meanwhile, tar formation was also reduced. Gas production increased from 1.30 to 1.46 Nm³/kg, and the Fe/olivine catalyst increased hydrogen generation from approximately 5 to 6.25 wt.%. Figure 6 illustrates the composition of gases from their results. The H₂/CO ratio increased from 1.41 for olivine to 3.26 for the iron catalyst because iron impregnation increased the H₂ concentration from 43.2 vol% for olivine to 48.2 vol% and decreased the CO concentration. As a result, the CO₂ level increased to 28.2 vol%.

These findings suggest that adding iron to olivine enhances the water–gas shift reaction and the cracking and steam reforming reactions of light hydrocarbons. The tar concentration decreased by approximately half, from 20.6 to 10.4 g/Nm³, and the carbon conversion efficiency was responsible for 87.6% of the reduction. Their XPS study revealed that Fe was primarily located on the exterior of the catalyst, making it readily accessible to the volatiles and facilitating tar cracking and reforming processes.

While olivine has proven to be a promising catalyst for gasification process optimization, it has been reported that its mining and processing have been associated with negative environmental impacts, such as land degradation and air pollution through dust and particulate matter generated during drilling [222]. This calls for an alternative method of olivine synthesis. Even though a few studies were reported on the laboratory production of synthetic olivine [223,224], further research is needed on the synthesis of the catalyst.

4.4. Effect of Residence Time

One important factor affecting the conversion of MSW or feedstock into syngas is the impact of residence time during the gasification process. Many researchers have established that longer residence times increase gasification efficiency by enabling more thorough reactions and improving carbon and hydrogen conversion yields. The amount of time biomass or carbonaceous material stays in the gasification reactor or gasifier during the gasification process is known as residence time in gasification [225,226].

Research conducted by Chen et al. [227] on sewage sludge in supercritical water with a high-heating-rate batch reactor revealed an increase in gas distribution as the residence time increased from 3 to 20 min, and then 30 min further. H₂ yield reached its maximum of 20.66 mol/kg at 750 °C and 30 min. However, CH₄ and CO yields increased, but not significantly. Martinez et al. [228] mentioned that the increase in residence time allows for more thorough reactions to occur, which can lead to a higher yield of syngas. This is particularly important in optimizing the gasification process to maximize energy output. The residence also affects the char conversion rate in the gasifier, ultimately enhancing the overall product yield from the gasification process.

This is consistent with the report by Chanthakett et al. [83], who reported that the residence times of gasification technologies, like fluidized-bed reactors and plasma, can be several hours. This allows municipal solid waste (MSW) to decompose more thoroughly over an extended period, leaving less residual material after gasification. This is beneficial because it increases waste-to-syngas conversion and improves gasification efficiency.

Cerone et al. [229] experimented on a pilot plant running continuously (20–30 kg/h of biomass feed) using an updraft gasifier to investigate the evolution of syngas composition at various heights of the reactive biomass bed. It was shown that the residence time of syngas had a direct correlation with the synthesis of light hydrocarbons. The results supported the hypothesis that longer residence times promote the conversion of tar into lighter products by demonstrating that the output of light hydrocarbons increased as residence time increased.

4.5. Effect of Equivalent Ratio

The ratio between the actual and stoichiometric fuel-to-air ratios is known as the equivalence ratio (ER), and it is a crucial parameter in gasification operations. The lowest ratio of gasifying agent to fuel or biomass that is precisely sufficient for heating the fuel entirely is known as the stoichiometric ratio. It has a significant influence on the overall energy yield and the syngas composition, which in turn affects gasification efficiency [230]. While deviations can result in inefficiencies, a balanced ER can maximize syngas production [231].

In addition to indirectly affecting the temperature, pressure, calorific value, and gas components of the gasification reactor, the ER also affects the interaction between the MSW feed rate and the gasifier supply rate. To produce hydrogen-rich syngas with a 50% yield, the ideal ER range is 0.2–0.32 [232]. Hang et al. [233] investigated the effect of the equivalence ratio on the gasification of solid refuse fuel (SRF), which is made up of 80% various plastic packaging, 10% HDPE, and 10% wastepaper and chopstick wood in a 1 kg/h scale lab-scale bubble fluidized-bed reactor using air as the gasifying agent. The operating conditions included a 0.15–0.3 equivalence ratio, a 600–900 °C gasifier temperature, an air velocity of 0.12 m/s (3.3 U_mf) for silica sand, and a 9.26–18.51 g/min feeding rate; silica sand was used as the bed material. Their findings reveal that by increasing the ER, the gas composition of CH₄ and H₂ decreased, whereas that of CO₂, CO, and C₂H₂ increased. A higher ER tends to favor oxidation reactions, leading to the formation of CO₂. Additionally, the tar content decreases when the ER rises, and the CCE tends to increase while the calorific value of the produced gas decreases. This is because plastic compounds combust completely as the ER rises, which increases CO₂ and lowers high-calorific-value C2–C3 hydrocarbon gases. Therefore, if the gasification operating conditions are maintained, the ER must be as low as possible to improve the product gas quality.

A similar trend was observed when Zin et al. [234] studied the air gasification of pelletized empty fruit bunch in a thermal arc plasma suction downdraft gasifier. The ER range was 0.18, 0.27, 0.36, 0.42, and 0.46 for the syngas yield, as shown in Figure 7 below.

When the equivalent ratio was raised from 0.18 to 0.46, the concentration of CO in the syngas increased from 9.92 to 13.08 vol%, while the H₂ decreased from 10.29 to 8.59 vol% as the ER increased from 0.18 to 0.27. At equivalence ratios of 0.36, 0.42, and 0.46, the H₂ content notably increased to 6.31, 6.77, and 6.12 vol%, respectively.

The presence of the H₂O component from the oxidation zone and the feedstock’s moisture content, which promoted the water–gas reaction, caused the H₂ reduction. With a value of 1.69 vol%, the gas composition of CH₄ peaked at an ER of 0.36. Additionally, the HHV decreased with an increase in the ER due to the contamination of the syngas by N₂ from the air [235], resulting in a lower energy value.

Recently, Ishak et al. [146] examined the effects of various equivalence and blending ratios of empty fruit bunch biomass (EFB), low-density polyethylene (LDPE) plastic waste, and polyethylene terephthalate (PET) as feedstock on the composition of syngas (H₂, CO, CO₂, and CH₄), as well as on the cold gas efficiency (CGE), higher heating value (HHV), lower heating value (LHV), and carbon conversion efficiency (CCE) through the plasma co-gasification method using an air-blown downdraft gasifier. The ER was between 0.15 and 0.3, and the blending ratios were E90:P10 (90:10%), E80:P20 (80:20%), and E70:P30 (70:30%). A cyclone filter was used to separate solid particles from the synthetic gas for gas cleaning. Using a peristaltic pump, the syngas was divided into two streams and delivered to the flare and gas cleaning units.

To determine whether the syngas in the flare unit was flammable, it was ignited using a gas lighter gun. The gas cleaning unit’s peristaltic pump reduced the risk of contamination and ensured a steady, regulated syngas flow. Except for a BR of E90:P10, where the composition of CO was increased with an increase in the ER, blended mixtures of EFB-LDPE employing all types of BRs (E90:P30, E80:P20, and E70:P30) showed a decrease in the composition of H₂ concentration as the equivalence ratio increased from 0.15 to 0.21. This is because a high air supply in the gasifier tends to cause complete combustion, which reduces the quality of the syngas. The increase in the composition of CO for a BR of E90:P10 was due to the low amount of LDPE, which facilitates a complete combustion rate. At an ER of 0.17, the highest H₂ and CO composition, HHV, and LHV were generated with a BR of E70:P30 for the mixture of EFB and LDPE, indicating that better composition of syngas production is achieved with a higher amount of LDPE in the feedstock blends.

The maximum values for gas yield, CGE, and CCE were obtained by gasifying the EFB-PET mixture with a BR of E90:P10 at an ER of 0.27; these values were 1.18 m³/kg, 27.07%, and 63.13%, respectively. The EFB-PET mixture’s higher carbon content mainly contributed to the higher CGE, CCE, and gas yield value.

The ER is determined by dividing the mass-based actual air-to-feedstock ratio by the stoichiometric amount of air-to-feedstock ratio, as found in Equation (15).

$$\mathrm{E}\mathrm{R}={\displaystyle \frac{\left(\frac{\mathrm{F}\mathrm{u}\mathrm{e}\mathrm{l}}{\mathrm{A}\mathrm{i}\mathrm{r}}\right)\mathrm{s}\mathrm{t}\mathrm{o}\mathrm{i}\mathrm{c}\mathrm{h}\mathrm{i}\mathrm{o}\mathrm{m}\mathrm{e}\mathrm{t}\mathrm{r}\mathrm{i}\mathrm{c}}{\left(\frac{\mathrm{F}\mathrm{u}\mathrm{e}\mathrm{l}}{\mathrm{A}\mathrm{i}\mathrm{r}}\right)\mathrm{A}\mathrm{c}\mathrm{t}\mathrm{u}\mathrm{a}\mathrm{l}}}$$

(15)

where AF actual is the actual air-fuel ratio, which is calculated by dividing the amount of feedstock by the oxidizer flow rate, and AF stoichiometric is the ratio of air required for the feedstock to burn to the amount of feedstock completely.

Although these researchers’ contribution was valuable for understanding the effect of ER on gasification efficiency, a research gap appears to exist regarding the utilization of other gasifying agents to assess the impact of the ER, as they primarily relied on air as the gasification agent in their studies.

5. Syngas Cleaning Strategies

In gasification technology, gas cleaning is an essential process that guarantees the generated syngas (synthesis gas) meets the quality requirements for its intended uses [236]. Particulates, tar, alkali metals, sulfur, and nitrogen compounds—illustrated in Figure 8 below—are contaminants that must be removed from syngas to prevent damage to downstream machinery or performance issues [237].

Among the constituents of syngas, H₂S, COS, and CO₂ are referred to as acid gases. These gases are corrosive in moist environments because they dissolve in water and form an acidic solution. Additionally, global warming is attributed to the presence of CO₂. These gases must be removed to lessen the corrosiveness of syngas and the CO₂ emissions from downstream units like turbines [238]. The most widely used gas cleaning types employed in gasification technologies include particulate (char/soot) removal, tar removal, acid gas (CO₂, H₂S) removal, nitrogen compound and ammonia removal, alkali metal and trace contaminant removal, moisture removal, and hot and cold gas cleaning methods.

5.1. Particulate Removal

Particulates are solid or semi-solid impurities that are inevitably generated during the gasification of carbonaceous materials [152,239]. They may occur in the form of ash particles and metallic residues (Na, K, Ca, Mg, and trace metals such as Se, As, Sb, Cr, Zn, Pb, etc.), and unreacted carbon (char), soot, fly ash, or dust, along with the produced gas [240]. The gasification process requires removing particulates from gases, as they impact the lifespan and efficiency of equipment and the ability to produce clean syngas for chemical synthesis and energy production [238]. Methods employed for their removal are classified as low- to mid-high-temperature methods (bag and sand filters, electrostatic precipitation) and high-temperature methods (ceramic filters and cyclones) [241].

Research utilizing each of these methods has been reported with the aim of substantially reducing particulate matter to produce clean gas. On the low-temperature methods, Morselli et al. [242] reported particulate matter removal efficiency of 50–60% from biomass using fabric filter bags from syngas produced by commercial 25 KW_e downdraft gasifiers. Through medium temperatures, studies have shown that high-efficiency cyclones can remove particles of 5 microns and larger [213,243]. Scrubbers can remove particulate matter of 1 micron and larger with 80% effectiveness [244]. For particulate matter smaller than 1 micron, separation efficiencies of approximately 99% and 95% are known to be achieved by tube filters and electrostatic precipitators, respectively [245].

Relying on this information, Parihar et al. [246] developed a prototype tube-type wet electrostatic precipitator (ESP) for cleaning syngas by gasifying coconut shells using an open top-down draft gasifier. The gasifier produces hot syngas, consisting of CO (21%), H₂ (16%), CH₄ (1.5%), CO₂ (12%), and N₂ (balance) with a maximum particulate matter content of 700 mg/Nm³, at a temperature of approximately 550–600 °C. Their findings revealed a removal of 83% of the particulate matter from the syngas, which is higher than that of fabric filter bags. This is an encouraging result, because the achieved value falls below the permissible particulate matter for syngas, which is 50 mg/Nm³ for gas engine operations.

High concentration levels of particulate matter in syngas decrease an engine’s lifespan and increase maintenance costs by causing operational problems such as nozzle deposits and obstructions in the gas injection system [247]. The high-temperature particulate matter removal methods can be employed and have proven to be a promising alternative to low- and mid-temperature methods. It removes particulate matter typically by employing cyclones and ceramic filters at temperatures between 900 and 1000 °C to remove particulate matter (PM) from syngas, achieving up to 98% removal efficiency.

The advantage of this technique is that it does not necessitate cooling, thereby promoting energy efficiency [248]. Using this technique, Wang et al. [249] recently investigated the filtration of particulate matter using a ZnO sorbent dispersed on a ceramic filter (ZnO/CF) at temperatures ranging from 873 to 1073 K for one hour. They achieved a particulate matter removal efficiency of (>96%), and from their findings concluded that utilizing this method can reach both the goal of high-temperature desulfurization (HTDS) and high-temperature particulate filtration (HTPF) in a reactor. This underscores the possibility of ZnO/CF being adopted for the syngas clean-up process, offering high energy efficiency, a lower price, and a reduced carbon footprint for cleaner power production.

However, the limited surface area of the ceramic filter media and the considerable pressure drop across the filter media are significant obstacles that can be addressed with appropriate porosity and wash coating or dip coating techniques [250], which require further investigation.

5.2. Tar Removal

Tar is a mixture of complex condensable hydrocarbons produced along with syngas during the gasification of MSW or other carbonaceous materials. These hydrocarbons include aromatics and polyaromatics. Tar may result in operating problems for downstream equipment, such as fouling, clogs, corrosion, and catalyst deactivation. It is regarded as an undesired byproduct [218]. Tar can result in several problems when it is introduced to an internal combustion engine. Particles of tar can clog injectors and fuel filters, preventing fuel from reaching the engine cylinders. This may result in poor combustion, misfiring, and fuel starvation. Additionally, unequal fuel distribution across the cylinders due to injector obstruction can lead to engine imbalance and decreased performance [251,252,253,254].

Studies employing various strategies to remove tar from syngas have been reported by Pranolo et al. [255]. The research examined the gasification of cocoa pod husks in a downdraft gasifier, utilizing a cyclone as a dust collector and air as a gasifying agent at temperatures between 491 and 940 °C. The aim was to minimize the tar content in the syngas by adjusting operating parameters, such as temperature, flow rate, and ER, while recycling the syngas from the gasifier. They reported that this process resulted in a 97.19% reduction in tar. Recently, Khajeh et al. [256] investigated the catalytic tar reduction efficiency of an iron catalyst prepared by impregnating and calcinating iron on various biomass, biochar, and activated carbon (AC), with and without a potassium promoter (K₂CO₃ as biochar activator and K source) in a fixed-bed reactor.

Their findings indicated that the catalyst synthesized by co-impregnating and calcining Fe and K (B-Fe-K) on raw biochar resulted in a tar reduction of 80.23%, which is higher than the reduction without the K promoter at 73.73% and other modifications. Maulana et al. [257] experimented to reduce the production of tar during the gasification process; wood pellets were used as a feedstock in a downdraft gasifier. A wet scrubber was employed, and the absorbent was made using food-grade vegetable glycerin as the solvent and coconut shell charcoal with a mesh size of 10–20 as the packing material. The aim was to study how altering the two absorbent types in the scrubber affected the reduction of tar. Their research showed promising results, with 100% tar removal rates for acids, 51.88% for furans, 65.04% for alcohols, and 35.12% for ketones, achieved by varying the properties of vegetable food-grade glycerin and coconut shell charcoal as absorbents. Multistage catalytic gasification (MCG), which increases syngas production and reduces tar concentration by encouraging stable hydrocarbon breakdown and enhancing oxygen supply for oxidation, is a new strategy for syngas cleaning from tar contamination. The method involves utilizing thermal energy and catalysts, such as dolomite, kaolin, and zeolite, and blending and varying them in stages to enhance syngas production and reduce tar.

Using a batch-type downdraft gasifier, Wiyono et al. [258] investigated the MCG of coconut shells with 50:50 zeolite–dolomite, 50:50 zeolite–kaolin, and 50:50 kaolin–dolomite blends. They demonstrated that multistage catalytic gasification raises syngas yield while simultaneously reducing the tar content during the gasification of coconut shells. However, the report did not provide values to support their claims. In another study aimed at optimizing tar reduction, Putro et al. [235] proposed an innovative strategy that combined using a wet scrubber after the reactor with the influence of operating parameters like temperatures. The scrubbers were set up in sequence and held a variety of solvents, including water, lubricating oil, isopropyl alcohol, and cooking oil. The gasification temperature was between 500 and 600 °C and 700 and 800 °C, while the feedstock was palm kernel shell. Compared to other solvents, the study found that isopropyl alcohol reduced tar by 99.25% at 800 °C. They also stated that 0.31 g/Nm³ is the lowest concentration of tar syngas. As a result, a gas compressor can receive syngas from PKS gasification with a maximum allowable tar content of 0.5 g/Nm³.

5.3. Metal and Trace Contaminants Removal

Trace and heavy metals are naturally occurring elements found in small amounts in a variety of waste feedstocks, including biomass and MSW [259]. Elements such as arsenic (As), cadmium (Cd), and selenium (Se) are examples of such trace metals, which are primarily found in very small amounts. The large atomic weights and densities of heavy metals, including chromium (Cr), lead (Pb), and mercury (Hg), are what distinguish them [224]. These metals can significantly impact downstream applications and syngas quality even at low concentrations. Low-quality syngas releases these metals during combustion or utilization in gas engines into the environment, thereby affecting the air, water, and soil, and posing significant risks to the ecosystem and human health [260,261,262].

Because of this, removing heavy and trace metals from syngas during gasification is essential to guaranteeing the gas’s quality and suitability for use in downstream processes. Adsorption employing activated carbon, zinc oxide, manganese-based oxides, new sorbents such as metal-organic frameworks (MOFs), wet scrubbing, and ceramic filters are some of the techniques developed [263]. Ngoc Lan Thao and co-workers [264] successfully developed a hot gas cleaning system using activated carbon as an adsorbent. Their findings revealed that the system has a strong capacity for metal removal, ranked in the following order: K > Cr > Ca > Pb > Mg > Cd > Na > Zn > Cu. Liu et al. [265] reviewed the performance of metal oxide-based adsorbent (MOBA) on removing mercury (Hg) from syngas by classifying the performance of MOBA as physisorption (or CeO₂-based, Fe₂O₃-based, CuO-based, and V₂O₅-based) and chemisorption (for MnO₂ and Co₃O). They reported that the mercury removal capacity of these metal-based oxides follows a decreasing order of V₂O₅ > MnO₂ > Co₃O₄ > Fe₂O₃ > CuO(CeO₂).

Novel hybrid solutions that integrate filters and sorbents are gaining traction due to their synergistic efficiencies and reduced environmental footprint. These combinations simultaneously reduce tar formation, H₂S, and heavy metals, and further prevent catalyst deactivation [237]. Szul et al. [266] studied the high-temperature dry scrubbing of syngas using ceramic filters and sorbents. They demonstrated that integrating the filters and sorbents could substantially reduce heavy metal contamination from syngas. Further research on trace and heavy metal removal should focus more on developing high-surface-area and porous materials, such as MOFs, to serve as adsorbents for these metals.

5.4. Acid Gas Removal (CO₂, H₂S)

Syngas, which is made up of CO, H₂, CO₂, and H₂S, is essential for producing other chemicals and electricity [127]. However, there are several difficulties when CO₂ and H₂S are present. As a diluent, CO₂ reduces the calorific value of syngas and affects the catalytic processes that produce methanol and Fischer–Tropsch synthesis. Furthermore, CO₂ causes global warming; hence, efficient capture methods are required [267,268]. H₂S is highly poisonous and corrosive, causing SO₂ emissions, equipment deterioration, and catalyst poisoning, contributing to acid rain and environmental contamination. Therefore, strict removal techniques are necessary for optimizing syngas [269].

The need for mitigating these gases for ecological protection purposes has gained so much attention from scientists that is has led to the development of conventional techniques for CO₂ removal, including physical absorption strategies using solvents like dimethyl ether of polyethylene glycol (DEPG, selexol) and chilled methanol (rectisol), both of which use different solvents to achieve their acid gas removal efficiency, and chemical absorption using methylamine and alkanol amine, etc. Since CO₂ capture usually includes co-occurring acid gas (H₂S), the H₂S can also be removed physically using Selexol and Rectisol [270]. However, these processes operate in a multi-column complicated scheme; therefore, high energy requirements are the main drawbacks. Gatti et al. adopted a systematic procedure [270] to absorb CO₂ and H₂S from syngas in a designed absorption column and divide it into two. Before entering the column, the raw syngas is initially compressed to an absorption pressure of 60 bar and cooled to 253 K. The two compartments of the absorption column (tray column) are designated for the absorption of CO₂ and H₂S, respectively.

Cold methanol (Rectisol) is introduced from the top of the CO₂ absorber section, where it absorbs CO₂ from the rising syngas stream from the bottom of the column. After absorbing CO₂, the cold methanol is separated into two fractions: 50% is transferred to the CO₂ desorption section, while the other 50% is directed to the H₂S absorber section, where it extracts H₂S from the syngas. The recovery of CO₂ and H₂S is subsequently performed using a desorption process. Their findings demonstrated that the syngas had CO₂ and H₂S removed at 98% and 50 ppb (parts per billion) by volume, respectively. This high capture efficiency is adequate to satisfy the requirements for Fischer–Tropsch synthesis and other downstream applications. Additionally, the concentration of H₂S is low enough to meet the Fischer–Tropsch catalyst’s tolerance requirements.

Another study by Maneeintr et al. [271] using Selexol to absorb H₂S and CO₂ from biomass syngas resulted in a product gas with 7.4% CO₂ and 1.12 × 10⁻⁶ ppb of H₂S. These results exceed the acceptable requirements for CO₂ and H₂S in a product gas (permissible levels: less than 5% for CO₂ and 10 ppb for H₂S). However, to obtain clean syngas within acceptable acid gas limits, they used methyl diethanolamine (MDEA) as the absorption solvent, which resulted in 3.64 vol% of CO₂ and 0.02 ppb of H₂S.

Castrillon et al. [272] investigated the H₂S and CO₂ adsorption efficiency of a commercial activated carbon (Desorex K43) impregnated with K₂CO₃, NaOH, and Fe₂O₃. They mentioned that this combination followed the mechanism of physical adsorption and chemical reaction processes. Their findings revealed that the material impregnated with Fe₂O₃ and NaOH had the highest adsorption capacity of 0.85–4.58 mmol/g for H₂S removal and 1.61–1.88 mmol/g for CO₂ capture under dry conditions at a pressure of 1 bar. To address the challenges of using the conventional acid gas removal (AGR) solvents used during the physical removal process, Taheri et al. [273] recently reported the CO₂ and H₂S removal efficiency of a novel ionic liquid (ILS) known for its acid gas removal efficiency, non-volatility, non-corrosivity, and ease of process setup. They found that the removal efficiency of ILS doubled that of Selexol and Rectisol.

Chemically, amines, especially alkanol amines, are commonly used in the absorption process to remove CO₂ and H₂S from syngas produced by gasification. These substances react reversibly with acid gases, making it possible to separate and purify syngas components effectively [274,275]. However, research utilizing chemicals used for CO₂ and H₂S is limited at the time of this report; therefore, this requires more attention. As for the H₂S, it can be removed chemically and industrially using the Claus process. The method converts H₂S to elemental sulfur and sulfuric acid, occurring in two steps: thermal and catalytic reactions. During the thermal reaction step, SO₂ and H₂O are produced when H₂S is partially oxidized with oxygen or air at high temperatures, while the catalytic process occurs at lower temperatures, where the SO₂ generated in the first step reacts with the remaining H₂S over a catalyst (such as alumina or titanium dioxide) to produce elemental sulfur [276].

6. Recent Innovations in Gasification Technology

Innovations in MSW gasification technology are focused on addressing the challenges of conventional gasification processes while improving process efficiency, syngas quality, yield, environmental protection, and sustainability [277]. Innovative approaches include chemical looping gasification, plasma gasification, integrated gasification combined cycle, solar-assisted gasification, advanced catalytic reforming, and AI-based optimization processes such as the Internet of Things (IoT). Integration with carbon capture technologies is also gaining traction, as it enhances the potential of gasification for achieving negative carbon emissions [119,278,279,280,281,282,283].

6.1. Chemical Looping Gasification (CLG)

Chemical looping gasification (CLG) is a novel process that utilizes a dual reactor (fuel and air reactors) or oxidation and reduction reactors to transform solid feedstock into nitrogen-free syngas without direct air contact, using oxygen carriers (OCs) to deliver oxygen to the fuel to enhance the gasification process [284] The principle is based on molecular oxygen being substituted with an oxygen carrier (OC) in CLG. In the fuel reactor, the OC oxidizes biomass to syngas, and in the air reactor, the air re-oxidizes the reduced OC, as shown in Figure 9. By substituting inexpensive OCs for molecular oxygen, the cost of the oxygen source can be significantly decreased [285].

Additionally, the heat generated during OC restoration can be recycled back into the fuel reactor with the OC, meeting the waste gasification process’s energy requirements. Moreover, the reduced OC can enhance the gasification efficiency by acting as a suitable catalyst for the cracking of biomass tar and char [286].

Examples of such oxygen carriers include those that are iron-based (Fe₂O₃/Fe₃O₄) [287], nickel-based (NiO/Ni) [288], and copper-based (CuO/Cu) [289]; perovskite (LaFeO₃) [290]; and spinels (MnFe₂O₄). Methods of preparation include co-precipitation, impregnation [291], sol–gel [292], mechanical mixing [293], and spray drying [294].

The primary requirements that oxygen carriers must meet include having various oxide states to facilitate redox reactions, low activation energies for association and dissociation processes, strong abrasion resistance, high redundancy for prolonged use, non-toxicity, and cost-effectiveness [295]. Additionally, in contrast to conventional gasification, CLG generates high-quality syngas and releases fewer pollutants [296,297]. Notable achievements in CLG include the works of Huang et al. [287], who conducted a chemical looping gasification of rice husk using an Fe-based oxygen carrier in a fixed-bed reactor to examine the influence of operating parameters such as the oxygen carrier-to-rice husk ratio, residence time of the feedstock, method of preparation, and temperature. Interestingly, they found that the highest gasification efficiency occurred when the OC-to-rice husk ratio was 1.5, and the optimal yield was found at a residence time of 30 min. They further reported that co-precipitation outperformed other preparation methods at 800 °C. A 26.88% gasification efficiency, 35.64% H₂ content, 56.40% syngas content, 1.72 H₂/CO ratio, and 12.25 MJ/Nm³ LHV were obtained from their work, indicating a positive outcome across all indices to produce clean syngas with the lowest possible tar content and high gas yield, and to determine the three-phase product of gasification.

Wang et al. [298] performed CLG of commercial sawdust in a bubbling fluidized-bed gasifier using a natural manganese iron ore as the OC. They found that increasing the gasification temperature from 750 to 950 °C led to reduced char, increased syngas production, and reduced solid and liquid formation. Additionally, the polyaromatic hydrocarbons (PAHs), part of the tar, decreased from 55.1% to 13.7%. In comparison, a small portion of the monoaromatic hydrocarbons increased from 8.8 to 52.6%, indicating that high temperatures are responsible for the cracking of more PAHs, which eventually decompose into more syngas and CH₄.

However, these reports focused exclusively on CLG without a clear comparison to the conventional gasification process, failing to demonstrate any advantages of CLG over the traditional gasification method. This deficiency prompted Wang et al. [299], who conducted a comparative study between CLG of rapeseed straw at 650–800 °C using lean iron ore as the OC and traditional biomass gasification as a blank experiment using quartz sand as bed material. They reported that the carbon conversion rate and carbon conversion of biomass increased with increasing temperature. Adding an OC accelerates the conversion of carbon in biomass, resulting in a higher total gas yield compared to traditional gasification (blank experiment), with the carbon conversion rate reaching its peak at a residence time of 1 to 2 min. They further mentioned that adding an OC lowers the activation energy of the gasification process, which promotes the reaction and offers a theoretical justification for using low-grade iron ore in the CLG process. They concluded that the performance of the OC needs to be further tested, both before and after the reaction. The highly active OC and the environmentally friendly CLG technology require further investigation. This report highlights the consistency of the OC’s effectiveness in CLG, as the comparative studies are similar to those found in the previous literature.

6.2. Plasma Gasification

Plasma gasification is a cutting-edge waste-treatment technique that uses a plasma torch (an electrically and highly ionized conductive condition) at high temperatures between 1500 and 3000 °C to transform various materials into energy and valuable byproducts [279,300]. The plasma torch generates intense heat by ionizing inert gases such as nitrogen or argon, breaking down feedstock into syngas. Plasma produces a highly reactive atmosphere in gasifiers due to its high concentration of electrons, ions, and radicals, which accelerates chemical processes and enhances gasification efficiency.

The system comprises a gasification chamber where waste interacts with plasma, a feed system for introducing materials, a refractory lining for heat protection, a cooling system, syngas clean-up mechanisms, and slag handling for solid residues, as illustrated in Figure 10. The process begins with feedstock preparation, where municipal solid waste (MSW) or other organic materials are sorted, shredded, and fed into the gasification chamber. A plasma arc, with temperatures exceeding 5000 °C, gasifies the material into syngas and minor gases. Inert residues, such as slag, are removed and potentially repurposed [279,301]. Before use, the syngas undergoes cleaning and refining processes such as removing particulates, wet scrubbing, using solid absorbents for heavy metals, performing water–gas shift reactions, and removing acid gases to eliminate contaminants and ensure its suitability for electricity generation, heating, or conversion into fuels [269].

What distinguishes plasma gasification from conventional gasification processes is its ability to handle hazardous materials, where high temperatures neutralize toxic substances, ensuring environmental protection. The syngas composition typically consists of 30–60% CO, 25–30% H₂, up to 15% CO₂, and small amounts of methane. Plasma gasification offers numerous opportunities for converting waste into energy because of its eco-friendliness and its advantage of producing H₂-rich syngas. Various studies have been conducted to produce quality syngas from tires [302], hazardous waste [301], MSW [303], kitchen waste [304], and plastics [305]. Notable breakthroughs come from the works of Messerle et al. [306], who conducted steam– and air–plasma gasification on biomedical waste (BMW), bony tissue from animals (bones of animal origin), and mixed waste from health care facilities, also referred to as household waste (HW). Based on thermodynamic calculations using the TERRA code [307], developed for calculations involving high-temperature processes, they reported a total syngas concentration of 53.4 vol% and 84.9 vol% with the heat of combustion of 3510 and 5664 kJ/kg for air– and steam–plasma gasification, respectively, for BMW at temperatures of 1600 K. HW plasma gasification showed a total syngas concentration of 82.4 and 94.5 vol% with the heat of combustion of 13,620 and 18,697 kJ/kg for air and steam PG, respectively. Experimental results on air–plasma gasification of both BMW and HW revealed a total syngas concentration of 69.6% and 71.1% vol., with impressive carbon conversion efficiencies of 79.3% and 91.8%, respectively. They further reported that neutral slag was obtained from the mineral mass of the BMW, but no harmful impurities were found in the syngas during their experiments.

However, a neutral slag was obtained. Further experiments using other gasification agents besides air and steam are required to validate their findings across all gasification agents. To produce high hydrogen syngas, Dong et al. [308] experimented on MSW (a mixture of paper, textile, wood, and plastic). They reported 60% of H₂ in the produced syngas. Another plasma gasification experiment was reported for the first time on electrical switches (a source of e-waste) consisting of Bakelite and polyamide, using CO₂ as a gasification agent by Mallick et al. [309]. Interestingly, they obtained a syngas yield of approximately 85 with 23.36 and 43.61 vol% of hydrogen and CO, respectively. They, however, found a significant amount of residue of 24.58%, which can lead to operational challenges. Using catalysts may be an effective way to minimize residue formation. Advancements in plasma gasification involve the integration of carbon fuel cells (CFCs) with plasma gasification to enhance efficiency. Overall efficiencies of up to 79.5% can be achieved when plasma gasification and DCFCs are integrated under optimal conditions.

According to the literature studies, plasma gasification is an effective method for generating high-quality syngas with a low tar content (less than 10 mg/Nm³). This enables the syngas to be used directly in solid oxide fuel cells and internal combustion engines, eliminating the need for additional tar removal procedures [310]. While plasma gasification offers advantages, its limitations include the high energy requirements to generate plasma. This technique also involves significant capital expenses, which may pose a barrier to its large-scale implementation [311]. Future research should concentrate on reducing costs and optimizing process parameters.

6.3. Integration with a Carbon Capture System

Gasification integrated with CO₂ capture is a viable technique for producing quality syngas. When gasification and CO₂ capture are integrated, greenhouse gas emissions are effectively reduced while energy from various feedstocks is recovered. CO₂ capture involves the removal of carbon dioxide from the gas stream produced during MSW gasification. The technique has been employed in CO₂ capture from plastic and biomass gasification, achieving an encouraging capture efficiency of 90% with 99.9% purity for the produced hydrogen [312,313].

Researchers have employed various methods to effectively capture CO₂, including calcium looping gasification and the use of calcium-based solvents in both combustion and gasification, achieving a CO₂ capture efficiency of 90% [314]. The technique involves carbonation and calcination, which can be accomplished by alternating gas streams or altering the operating temperatures [314]. While the calcination reaction is endothermic, operating at high temperatures of 850–950 °C, the carbonation reaction in the calcium looping process is exothermic at temperatures between 600 and 650 °C. The carbonation and calcination reactions have been described below [315]:

$$\begin{array}{l}\mathrm{Carbonation}\\ \mathrm{C}\mathrm{a}\mathrm{O}+\mathrm{C}{\mathrm{O}}_2\to \mathrm{C}\mathrm{a}\mathrm{C}{\mathrm{O}}_3\hspace{1em}\Delta {\mathrm{H}}_{298}=-178 \mathrm{k}\mathrm{J}/\mathrm{m}\mathrm{o}\mathrm{l}\end{array}$$

(16)

$$\begin{array}{l}\mathrm{Calcination}\\ \mathrm{C}\mathrm{a}\mathrm{C}{\mathrm{O}}_3\to \mathrm{C}\mathrm{a}\mathrm{O}+\mathrm{C}{\mathrm{O}}_2\hspace{1em}\Delta {\mathrm{H}}_{298}=178 \mathrm{k}\mathrm{J}/\mathrm{m}\mathrm{o}\mathrm{l}\end{array}$$

(17)

Acharya et al. [278] reported that one special aspect of the technique is the internal regeneration of sorbent from the gasifier, in situ CO₂ capture, and the production of hydrogen-rich syngas. They conducted chemical looping gasification of sawdust integrated with carbon capture using CaO as the solvent in a fluidized-bed gasifier. Using steam as the gasification agent and a temperature of 580 °C, they obtained a 71% H₂ concentration and almost 0% CO₂ content in the gas product, with 40% of the CaO regenerated. Recently, Dashtestani et al. [315] investigated the carbon capture efficiency of a novel CaO-Fe₂O₃ sorbent material in biomass gasification via carbonation–calcination. They reported a carbon capture efficiency of 90.4% after three looping cycles with an optimum temperature of 620 °C. They deduced that a balance between the equilibrium gas composition, kinetic parameters, and the contribution of CaO/Fe active sites is responsible for the high CO₂ capture efficiency.

Chai et al. [313] mentioned that carbon capture and utilization (CCU) can recycle captured CO₂ from gasification to the reforming stage, serving as a secondary gasification agent. It can also be used to gasify MSW through the integrated system. Detchusananard et al. [316] investigated the effectiveness of the CO₂ recirculation method for enhanced H₂-rich gas using CaO as the sorbent material. Using an ASPEN Plus model, they reported an energy conversion efficiency of 88.09% with a specific CO₂ emission of 4.4 g CO_2eq/MJ. Gasification integrated with fuel cells, such as solid oxide electrolytic cells (SOEC), has also achieved 87.88% carbon capture efficiency [317].

Integrated gasification and combined cycle (IGCC) with carbon capture is another interesting innovation in integrated gasification solutions that has attracted widespread attention across industries and academia. According to Cormos et al. [318], the method is promising because when it comes to plant energy efficiency, the process offers the best chance of capturing CO₂ with minimal fines, capital, and operational costs. The IGCC is a power generation method that produces syngas by partially oxidizing solid fuel with oxygen and steam, which can capture approximately 90% of CO₂, contributing to a significant reduction in greenhouse gas emissions. The setup is coupled with a carbon capture and storage (CCS) material. The concentration of CO₂ is first increased through the water–gas shift reaction, which is then accompanied by an acid gas removal (AGR) system that captures both CO₂ and H₂S. The captured CO₂ is then dried, compressed, and stored for use. Utilizing this technique with novel modifications in the system’s design, Shi et al. [319] achieved 99.9% CO₂ capture, demonstrating the technology’s potential to reduce greenhouse gas emissions, contributing to cleaner energy. However, this approach still requires more attention to maximize its potential regarding gas yield, as previous research mainly focused on CO₂ capture.

6.4. Solar-Assisted Gasification

Solar-assisted gasification is a novel and environmentally friendly process that uses concentrated solar energy as a heat source to drive the gasification of carbonaceous materials such as biomass, plastics, or municipal solid waste (MSW) to produce syngas and other useful products [320]. Integrating solar systems into the gasification process is a promising approach to producing clean syngas with reduced emissions compared to traditional gasification processes. During the process, mirrors, lenses, or other optical systems are used to concentrate solar energy to reach the high temperatures (typically above 1000 °C) required for gasification [321,322]. Solar energy is transmitted to the gasifier through a solar receiver that may be exposed to direct or indirect radiation. Heat transfer is then sufficiently supplied by the receiver to the reacting particles, enabling the production of syngas through endothermic reactions, as shown in Figure 11 below [323,324].

Because of their unique designs that promote heat transfer, reaction kinetics, and mass flow rate, fluidized-bed reactors [325] and internally circulating fluidized-bed reactors are used [322]. Packed-bed reactors have also been widely used for this process by various researchers [323,324]. According to Müller et al. [326], the advantages of solar gasification over conventional gasification include the production of more syngas per unit of feedstock, because no feedstock is burned to supply the reaction heat; the energy content of the feedstock is improved by the solar energy’s input to produce syngas with higher calorific value and reduced CO₂ intensity, which makes it possible for the temperature of the reaction to rise, resulting in faster reaction kinetics and fewer by-products.

Bellouar et al. [321] performed a solar gasification of biomass (wood consisting of pine and spruce) in a high-temperature tubular solar reactor with a packed bed and drop tube focus of a 2 kW thermal solar concentrator from the PROMES-CNRS laboratory and achieved a 93.5% carbon conversion rate. Kalinci et al. [327] studied the solar-assisted gasification of biomass (sewage sludge, fluff, and beech wood charcoal) for improved hydrogen production in a packed-bed solar reactor, using steam as the gasification medium. Their result revealed an increase in hydrogen content of the product syngas from 48% to 57.6%, with an improved calorific value of the biomass equal to the heat change of the process by solar input. Gómez-Barea et al. [325] performed a thermodynamic and kinetic analysis of solar–steam gasification of biomass under fluidized-bed conditions. They reported an 80% carbon conversion efficiency corresponding to an average char residence time of 28 min. The analysis revealed that the technique can produce syngas with a solar share of 12%, indicating that approximately 12% of the total energy in the syngas was generated from solar energy. However, the analysis revealed that due to kinetic limitations, not all biomass was converted to syngas, resulting in a significant amount of unconverted carbon that could impact the quality and overall yield of the syngas.

Deshmukh et al. [281] reported a similar carbon conversion efficiency (80%) when they conducted a solar-assisted steam–air gasification of agricultural residues (bagasse, rice straw, and wheat straw) using Aspen Plus simulation, with chemical reactions enhanced with response surface methodology (RSM). Solar energy was used as a heat source for steam generation, which would later produce green hydrogen. The optimal temperature, equivalence ratio, and steam-to-air ratio of 800–900 °C, 0.2, and 1.8–2.2, respectively, yielded a 40% molar hydrogen content (MHC) and a high heating value of 6 MJ/kg, along with an 80% carbon conversion efficiency. Sun et al. [328] developed a novel allothermal chemical looping–solar gasification system that consists of fuel and steam reactors using magnetite (Fe₂O₃) as the oxygen carrier (OC). This system achieved lower energy and higher exergy efficiencies of 66.16% and 77.68%, respectively. This is an encouraging result compared to auto-thermal CLG, which achieved energy and exergy efficiencies of 86.19% and 74.43%, respectively.

As it is known, the function of the OC is to deliver free oxygen to the fuel and to enhance the heat for the endothermic reactions in the fuel reactor. They further demonstrated that since the regeneration of the OC produces hydrogen in the steam reactor but releases low heat, an external solar system is used to heat the regenerated OC carrier to a specific temperature, ensuring allothermal operation (utilizing external heat to drive the operation). OC storage tanks are coupled with the solar receiver to store extra solar energy during a power outage. At the outlet of the steam reactor, the thermal energy of the OC is further used to generate power. However, this technique did not provide details about the production of CO and other valuable gases, limiting its industrial and laboratory viability. Overall, the issue of utilizing solar-assisted gasification in cloudy or nighttime conditions has not been adequately addressed. This raises questions about the general applicability of the method in areas where solar energy availability is limited, thereby opening a research opportunity for future studies.

6.5. AI-Based (IoT) Assisted Gasification Technologies

The adoption and subsequent integration of artificial intelligence (AI) in gasification technology has become a revolutionary approach to promoting the gasification of MSW [329]. AI- and IoT-assisted gasification facilitate intelligent control, real-time decision-making, predictive management, and fault detection, effectively overcoming the limitations of traditional gasification processes, including process instability, environmental issues, and variations in feedstock composition [330]. The benefits of using AI and IoT are shown in

Table 7. Benefits of AI and IoT use in gasification.

Benefits	Description	References
Process efficiency	Efficiency is enhanced through real-time process parameter monitoring and control.	[331,332]
Minimization of emissions	Precise control of parameters reduces GHG and pollutant emissions.	[333,334]
Improved safety	Safety is enhanced by monitoring harmful gases and activating safety measures.	[335,336]
Cost-effectiveness	Low-cost IoT-based solutions lower operating and implementation expenses.	[336,337]

below.

Algorithms utilizing artificial intelligence (AI) were created to effectively predict system output by learning from and analyzing system characteristics with minimal experimental data points to optimize process conditions.

Thus, to achieve the desired results, AI-based solutions can assist in understanding the basic concepts and characteristics of gasification [338,339]. AI machine-learning algorithms such as Support Vector Machines (SVM) [340], Gaussian Process Regression [341], and Non-Linear Response Quadratic algorithms (NLRQM) [342] have been used to identify relationships between operational conditions, thereby improving process efficiency, optimizing operational parameters, and predicting syngas composition.

Ayodele et al. [343] conducted a performance analysis of twelve machine-learning algorithms based on Support Vector Machines (SVM), Gaussian Process Regression (GPR), and Non-Linear Response Quadratic algorithms (NLRQM) using Sequential Quadratic modeling (SQP) (an optimization algorithm that helps optimize syngas yield by modifying process parameters to produce accurate predictions) and Levenberg–Marquardt (LM) (an algorithm that enhances the learning process in machine-learning models, which helps in the prediction of syngas composition) during the co-gasification of biomass waste (coconut shell and palm fronds) in a downdraft gasifier catalyzed by Portland cement, dolomite, and limestone for hydrogen-rich syngas production. The aim was to develop precise forecasting models and to understand their effectiveness in modeling the intricate relationships between gasification parameters and syngas yield. They found that GPR models outperformed SVM models in predicting syngas production, while NLRQM-LM (the Levenberg–Marquardt model) and NLRQM-SQP demonstrated the best forecasting capabilities. The effect of the catalyst produced 42, 38, and 45 mol% for Portland cement, dolomite, and limestone, respectively, based on the model. These findings underscore the potential of machine learning in optimizing the gasification process.

AI-driven fault detection reduces downtime by predicting equipment breakdowns using deep-learning models such as convolutional neural networks (CNNs) [344]. CNNs have been successfully applied in several industrial processes. For example, studies have shown that CCNs are effective in fault detection and the real-time prediction of quality variables in gasification processes [345,346]. Recently, by combining mechanistic analysis and convolutional neural networks (CNNs), an operation optimization model for the shell coal gasification process (SCGP) was developed by Wang et al. [347]. This model uses the correlations between process variables to improve prediction accuracy and operational efficiency. An industrial case study was utilized to validate the model’s efficacy.

They discovered that the operational conditions had significantly improved, leading to an 8.9443% increase in syngas production. This indicates that the optimization technique successfully optimized the gasification process. A different study by Aentung et al. [348] employed a multi-objective optimization approach integrating a genetic algorithm (GA) with response surface methodology (RSM) to identify the Pareto frontier, which represents the best balance between competing goals, and to simultaneously improve performance indicators of gasifiers, particularly cold gas efficiency (CGE) and carbon conversion efficiency in the co-gasification of plastic waste blended with biomass in a downdraft fixed-bed gasifier. The adiabatic RGibbs model in Aspen Plus, a process simulation program that helps to confirm that the designed model accurately predicts the performance of the gasifier under various conditions, was used to validate the stoichiometry modeling approach used for the co-gasification process. The best outcomes for the target parameter were found using the Order of Preference by Similarity to Ideal Solution (TOPSIS) method. Using this method, they reported that the optimal temperature of 967.89 °C, a steam-to-feed ratio of 1.40, and a plastic-to-biomass ratio of 72.23 yielded a CGE of 91.78% and a CCE of 83.77%, respectively. IoT technology has emerged as an advancement in AI. It is becoming popular in environmental monitoring, owing to its potential to revolutionize the energy sector and promote ecological protection technologies [349]. By leveraging wireless sensor networks (WSNs) and cloud computing, IoT enables remote monitoring and the collection of real-time data on gasification parameters, including temperature, pressure, and gas composition [350,351]. For example, a study by Raj et al. [349] utilized a novel IoT-integrated sensor fusion (an environmental sensor for measuring different GHGs) to assess greenhouse gas emissions, applicable in industries and urban areas for real-time monitoring and management of these gases’ environmental impacts. Machine-learning algorithms generate predictive models in the system by analyzing fused sensor data. These models provide the system with a comprehensive understanding of GHG emissions across regions by allowing it to predict emission estimates beyond sensor locations, facilitating emission reduction actions. These findings highlight the potential application of the proposed technique in gasification plants to control emissions, which requires further study in future research.

Additionally, intelligent control solutions based on the Internet of Things optimize process stability while enabling remote operations [352]. However, this comes with the risk of cyber threats, which need to be addressed to ensure the process’s efficiency and safety [353].

7. Prospective and Future Research Directions

Due to its ability to convert various carbonaceous materials and waste streams into useful energy and chemical products, gasification is becoming an increasingly popular and adopted method for managing municipal solid waste (MSW), especially in developed countries such as Germany, the USA, Finland, and the UK. The yield, quality, composition, and process efficiency of syngas have been improved with the use of gasification agents, such as steam and CO₂, and with advancements in catalysis. By combining cutting-edge technologies like chemical looping, solar-assisted systems, plasma-assisted gasification, and digital tools like artificial intelligence (AI) and the Internet of Things (IoT), gasification is evolving from a traditional thermal process into a smart, adaptable energy platform.

However, widespread application is still constrained by technical obstacles such as tar formation, feedstock unpredictability, system scalability, and economic viability. Future research should focus on optimizing catalysts for heterogeneous waste and examining the techno-economic and environmental performance of integrated systems. Lifecycle assessments (LCA) and carbon footprint evaluations under different operational conditions and feedstock blends should receive special attention.

Furthermore, translating laboratory breakthroughs into commercially viable solutions necessitates interdisciplinary collaboration. Gasification can develop into a key component of circular economy plans and a practical means of accomplishing global waste reduction and climate goals by promoting collaboration among material science, data analytics, policy, and environmental engineering.

8. Conclusions

This review critically examines gasification as an innovative and sustainable method for managing municipal solid waste (MSW) and producing syngas from carbon-rich materials. The goal is to reduce environmental impacts while recovering valuable materials and energy. The study demonstrated that, in terms of emission reduction, energy efficiency, and syngas quality, gasification outperforms traditional methods such as pyrolysis and incineration by critically evaluating operational factors, gasification agents, and the role of catalysts.

Significant findings indicate that optimizing temperature (800–900 °C), pressure, and feedstock properties significantly improves syngas composition and process efficiency. While nickel-based and alkali/alkaline earth catalysts enhanced tar reforming and conversion rates, advanced gasification agents, particularly steam and oxygen, were demonstrated to increase H₂ and CO yields. Furthermore, new technologies such as solar-assisted gasification, chemical looping, and plasma-assisted gasification, combined with AI and IoT integration, have shown the potential to address current economic and technological hurdles.

Despite these advancements, drawbacks still exist, such as high capital costs, feedstock heterogeneity, and tar formation. The optimization of catalyst compositions for various waste streams, enhancing system integration, and expanding hybrid gasification technologies should be the main areas of future research.

Collaboration among various sectors, investment, and integrated policy support are essential to fully leverage gasification and transition to a circular, low-carbon economy.