Next Article in Journal
Advanced Technologies for Geosite Visualization and Valorization: A Review
Next Article in Special Issue
Dynamic Modelling and Energy, Economic, and Environmental Analysis of a Greenhouse Supplied by Renewable Sources
Previous Article in Journal
Case Study of Remodelling the As-Built Documentation of a Railway Construction into the BIM and GIS Environment
Previous Article in Special Issue
Novel Multibus Multivoltage Concept for DC-Microgrids in Buildings: Modeling, Design and Local Control
 
 
Search for Articles:
Title / Keyword
Author / Affiliation / Email
Journal
Article Type
 
 
Advanced Search
 
Section
Special Issue
Volume
Issue
Number
Page
 
Journals
Applied Sciences
Volume 13
Issue 9
10.3390/app13095597
applsci-logo
Submit to this Journal Review for this Journal Propose a Special Issue
► Article Menu

Article Menu

share Share announcement Help format_quote Cite question_answer Discuss in SciProfiles thumb_up
...
Endorse textsms
...
Comment
first_page
settings
Order Article Reprints
Font Type:
Arial Georgia Verdana
Font Size:
Aa Aa Aa
Line Spacing:
Column Width:
Background:
Review

Recent Advancements in Anaerobic Digestion and Gasification Technology

by
KeChrist Obileke
*,
Golden Makaka
and
Nwabunwanne Nwokolo
Department of Physics, Faculty of Science and Agriculture, University of Fort Hare, Private Bag X1314, Alice 5700, South Africa
*
Author to whom correspondence should be addressed.
Appl. Sci. 2023, 13(9), 5597; https://doi.org/10.3390/app13095597
Submission received: 10 April 2023 / Revised: 24 April 2023 / Accepted: 28 April 2023 / Published: 1 May 2023
(This article belongs to the Special Issue Renewable Energy Systems 2023)
Versions Notes

Abstract

:
In recent times, there has been a growing demand for the use of biomass as an alternative energy due to its sustainable nature. At present, anaerobic digestion and gasification has been proven as a promising technology for exploiting this energy from biomass. This review aims to provide a comprehensive and detailed analysis of the combination of anaerobic and gasification technology as a hybrid system for sustainable waste-to-energy generation. This review reveals that both anaerobic digestion and biomass gasification have been successfully demonstrated as technologies for energy recovery. However, to improve the conversion efficiency in both technologies, the utilization of an intensifier, additive, and enhancer will be required. Moreover, temperature has been identified as a major factor affecting the technologies and should be considered. The bibliometric study conducted revealed that China is the leading country and has set the pace for other countries to follow suit. Subsequently, waste-to-energy research could be easily implemented on a global scale. This study recommends an experimental study of anaerobic digestion and gasification as a hybrid system.

1. Introduction

According to the statistical review of world energy (2021), the COVID-19 pandemic was reported to have had a dramatic effect on the energy market. This contributed to the fastest rate of decline in primary energy and carbon emissions since the Second World War. Despite this report, the good news is that renewable energy continued to grow. For recording purposes, the current rate of primary energy consumption and carbon emission fell by 4.5% and 6.3%, respectively. This decline was the largest for primary oil since 1945 and the lowest level since 2011 for carbon emissions [1]. Renewable energy sources, such as wind, solar, and hydroelectricity, all grew despite the decline in overall energy demand. Statistically, renewable energy sources with the inclusion of biofuel and the exclusion of hydro rose by 9.7%, lower than the 10-year average of 13.4% per annum but with an increase to 2.9 Exajoules (EJ) in terms of energy. Solar and wind energy rose by 1.3 EJ and 1.5 EJ, respectively, thereby contributing to the largest renewable energy growth. Similarly, hydroelectricity in China improved by 0.4 EJ [1]. From the authors’ point of view, it is worrisome that biomass energy was not mentioned specifically, as it accounts for more than one-third of primary energy.
To this effect, biomass energy is regarded as renewable and sustainable energy. It is interesting that approximately 32% of the country’s overall energy supply comes from biomass, and 70% of the national population is said to depend on it. In addition to natural biomass, the use of conventional waste biomass as a source of energy is another promising option [2]. One major way and option for achieving waste-to-energy focusing on biogas and syngas is through anaerobic digestion and gasification technology [3]. These technologies are regarded as mature biochemical and thermochemical processes and are currently receiving research interest. Its efficiency in the substrate and product separation as well as the microbes’ ecology of anaerobes test contribute to its recent attention. Without a doubt, this review will promote and contribute to the development of the practical application of both technologies as a hybrid system.

2. Methodology

Amongst the existing types of literature reviews, which include systematic review, narrative review, traditional review, critical review, and state-of-the-art review, a conceptual review was employed [4]. The choice was based on the objective of the study, which aims to synthesize the area of conceptual knowledge, thereby contributing to a better understanding of a particular topic. Regarding the study, a conceptual review is most applicable, as it provides sensitive areas of the review based on recent advances in anaerobic digestion and biomass gasification. Furthermore, as a detailed and broad review containing a vast amount of information, the conceptual type of review was deemed to be the most suitable type to use. Findings were obtained from reputable databases, such as Springer, ScienceDirect, SCOPUS, Web of Science, and Google Scholar. In addition, substantial information from conference proceedings and industry reports was employed. The authors employed recent studies and literature in the review.

3. Anaerobic Digestion Technology

The term “anaerobic digestion” (AD) refers to the degradation of biomass under anoxic conditions in both natural and engineered systems [5]. Akhiar et al. [6] defined anaerobic digestion as a process wherein microorganisms degrade organic matter in the absence of oxygen to produce biogas. This technology is regarded as the earliest approach toward achieving a circular bioeconomy. Moreover, due to increasing industrial interest, studies on anaerobic microbial processes have greatly improved in identifying the important process parameter and factors that can promote methane production from all types of organic matter, such as manure and slurry [5]. Methane is said to substitute natural gas, especially as it tends to build up opportunities with regard to traditional petrochemistry [7]. Furthermore, anaerobic digestion has proven to be an ideal end-of-pipe technology for the treatment of waste. It serves as a process in the recovery of various biotechnological applications because of the wide substrate spectrum [5]. Such a biotechnological application, which includes the microbial electrochemical process and production of medium-chain fatty acid by anaerobic fermentation, can be coupled with anaerobic digestion. Anaerobic digestion has received immense interest in recent times because it addresses the major challenges faced by developing countries regarding environmental pollution resulting from the use of fossil fuels [8].

4. Recent Studies on Anaerobic Digestion Focusing on Biogas Production

Globally, approximately 50 million small-scale domestic digesters, 132,000 commercial digesters, and 700 upgrading biogas digesters have been installed. These digesters are fully functional and operate using anaerobic digestion technology. For instance, in Europe, to be specific, 15,000 commercial biogas digesters are in full operation [9]. In terms of biogas production, up to 31.24 billion cubic meters (bcm) were reported in 2015 from Europe. This number is predicted to increase by 70 bcm by 2030 because of the readily available resources [10]. On another note, the total global energy production resulting from biogas was approximately 2.8 × 105 TJ in 2000; fourteen years later, it increased to 1.3 × 106 TJ [NBD 2016]. According to the world biogas association report released in 2019, 10.5 GW of electricity generation came from the 17,783 biogas digesters in Europe. Although the specific time was not disclosed, it was indicated that Germany is leading with 10, 971 biogas digesters, followed by Italy (1655), France (742), Switzerland (632), and the United Kingdom 613 [11].
Pramanik et al. [12] provided a comprehensive review of anaerobic digestion as an effective technology for biogas. The review finds that anaerobic digestion achieves a higher energy production, especially the co-digestion process, whereas the two-stage anaerobic digestion process helps in increasing methane production. Hence, the authors recommended the use of food waste as a suitable substrate for biogas digester because of its excellent biodegradability and high-water content. In another study, Connor et al. [13] emphasized that the European agricultural sector is known for small-scale anaerobic digesters. This is because of the small-scale individual farm sizes and land productivity, which are insufficient to meet the feedstock requirement of large and medium-scale plants. The study reveals that there is a need for national support which will aid in adopting small-scale anaerobic digesters within the European countries. Nevertheless, there is still an overriding obstacle hindering the full commercialization of small-scale digesters, and this has to do with upfront investment costs. In addition to this, substrate type as well as a mode of utilization is another factor of consideration for efficient biogas production. Mono-digestion is characterized by imbalanced nutrients, lack of microbial communities, and fluctuation in operational parameters. In a bid to address these challenges, Ngan et al. [14] conducted a study focusing on the anaerobic digestion of rice straw with livestock manure. Rice straw is high in cellulose and, hence, requires pre-treatment before being fed into the biogas digester. Evidently from the study, it was revealed that rice straw when co-digested with biological waste (livestock manure) for anaerobic digestion showed a better degradability potential. Therefore, the condition for optimum pre-treatment, operational parameters, and mixture proportion with the substrate improves methane production, according to a study by Ngan et al. [14].
Various factors, such as anticipated toxicity, lower pH (6.6), and low carbon C:N ratio (12:2), are said to be responsible for the challenges faced in using only textile waste (sludge) for energy and nutrient recovery. To this effect, Kumar et al. [15] conducted a study on the anaerobic co-digestion of textile aerobic sludge with food waste and cow dung. This was assessed through a biochemical methane test under a mesophilic temperature of 35 °C. From the result, cow dung mixed with textile sludge in a ratio of 1:1 generated 524.4 mL/g of biogas, whereas using textile sludge only did not produce any biogas. In addition, the mixture of food waste and textile sludge resulted in biogas production of 288.3 mL/g, which is lower compared with that of cow dung with textile sludge. The findings from the study provided an efficient method for textile sludge utilization in biogas production.
The potential of using chicken manure as the sole feedstock for biogas production under anaerobic digestion was studied by Jurgutis et al. [16]. The choice of chicken manure in the study was because of its high biogas potential yield using a biochemical methane test. A methane yield of 508 mL CH4 g−1 VS was recorded throughout the 217 days of monitoring. The novelty of the study is associated with the adaptation of the same operational conditions (organic loading rate and retention time) during the fermentation process. To this end, approximately 93% of chicken manure was digested, with an organic loading rate of 3.14 kg VSL−1d−1 and total ammonium nitrogen of 5.5 g L−1. This proves that it is possible to digest chicken manure as a stand-alone feedstock as long as the ammonium concentration is carefully controlled, which is achieved via the freshwater dilution process.
In terms of the design parameters, Obileke et al. [17] mentioned that the biogas digester dimension and material of construction are important factors of consideration. The authors developed a design equation that considered the volume of the digester, inlet, and outlet chamber dimensions, as well as the shape of the digestion chamber. In terms of construction, the understudied biogas digester was fabricated using high-density polyethylene (HDPE) plastic, whereas the inlet and outlet chamber was constructed using bricks/cement. The developed plastic biogas digester and the appropriate dimension were found to be suitable for anaerobic digestion for biogas production (methane and carbon dioxide reported to be 60% and 30%, respectively) using cow dung as the substrate.
To stabilize organic waste while generating biogas, the anaerobic digestion of poultry litter was carried out by Chaump et al. [18]. The study originated as a result of technical challenges faced, which included seasonality of litter production, low carbon/nitrogen ratios, and limited digestibility of bedding. From the study, biomethane potential of 0.24–0.30 L/gVS and 0.15–0.16 L/gVS was obtained for litter leachate and whole litter, respectively, whereas the insoluble bedding material left after leaching was reported to be 0.08–0.13 L/gVS. The information from the results indicates that poultry litter leachate can be amended for digestion despite having a low carbon/nitrogen ratio. Hence, litter can be stored before digestion, although this leads to biomethane losses. The authors opined that the loss is because of the decomposition of organics during storage.

5. Intensification of the Anaerobic Digestion Process

As stated by Sathyan et al. [8], the need to improve anaerobic digestion has been of great interest in recent years. The use of different types of additives or accelerants has been explored in several synergies, thereby enhancing the methane yield and quality of digestate, which are products of anaerobic digestion. It is important to know that the use of the additive in enhancing the performance of anaerobic digestion reduces the challenges associated with slow start-up, substrate-induced inhibition, and low energy output [8]. Some of these additives include microbes, enzymes, microelements, biological metabolites, adsorbents, deforming agents, and chelating agents. Therefore, Table 1 presents these additives, their substances, and the effects from various studies.
Some of these additives are cost-effective and have been widely studied to test their efficacy. In this case, their findings and results were successful in enhancing gas production, favourable digestion condition, and superior organic degradation [21,33].
Carbon-based materials intensify the process of anaerobic digestion by promoting the growth of microorganisms, providing required nutrients to microorganisms and improving the activities of enzymes. These results in quick methane production and content as well as improved quality digestate [31]. Previous studies conducted by Capson et al. [29], Lin et al. [30], Mostafa et al. [31], and Yang et al. [32] revealed that carbon material is necessary for anaerobic digestion because it enhances its efficiency and stability, thereby increasing the methane yield.
The properties of graphene in relation to anaerobic digestion include electric conductivity, high mechanical strength, and large surface area. To improve the stability and efficiency of biogas digester through anaerobic digestion, graphene is required and recommended. The introduction of graphene during anaerobic digestion improves the methanogens, which is the microorganism responsible for methane production in the low-temperature range of 10–20 °C [27,34]. Furthermore, graphene results in the change in syntrophic microbes between methane production archaea and acidogenic microbes, which increases the rate of methane production by 28% to be specified through 71.1% Methanobacterium and Methanosarcina (11.3%). Graphene indeed intensifies and influences microbial community dynamics during anaerobic digestion.

6. Feedstock for Anaerobic Digestion

The nutritional composition is regarded as a major factor that determines the suitability of any feedstock or substrate for biogas production. This nutritional composition affects the biogas yield, methane content, biodegradability, and degradation of the biomass kinetic involved [35]. These nutrients include carbohydrates, protein, and fats, as shown in Table 2. Interestingly, any feedstock for the anaerobic digestion process should be readily biodegradable and should not contain any toxic components that would impair the activities of microbes [36]. Table 2 shows the nutrient compositions of feedstocks with their methane yields and methane compositions.
From Table 2, it is seen that lipids produced the highest methane yield of 1.01 m3/kgVS as well as a methane percentage composition of 70% compared with protein and carbohydrates. This implies that substrates that are rich in lipids hold great potential for methane production. Nevertheless, the use of such substrate should be closely monitored, as a drop in pH can be toxic to the microbial community because of degradation that releases long-chain fatty acids. Considering the feedstocks that are protein-rich, Nwokolo et al. [39] mentioned that these feedstocks have a high potential for methane yield. However, they increase the alkalinity of anaerobic digestion because of the degradation process that releases ammonium. This process enhances the digestate and inhibits the activities of the methanogens. Further, Nwokolo et al. [39] stated that this inhibition occurs because of the shift in equilibrium from ammonium to ammonia, usually in a concentration value of 53–1450 mg/L. On a broader scale, substrates or biogas production are classified into agricultural waste, industrial organic waste, and municipal waste.

6.1. Agricultural Waste (AW)

Agricultural wastes are widely used due to their availability, and they include animal waste and lignocellulosic biomass in the form of residue from crops and forest and energy crops. However, the authors chose to focus on animal waste as a major feedstock for biogas production. This is driven by its availability, biodegradability, and global usage as compared with others [39]. These animal wastes are characterized by high nutrient content and organic matter concentration, neutral pH, different microbes, etc. Hence, they produce a lower rate of biogas production because of their pre-digestibility by the animal intestine. The conversion of animal waste using anaerobic digestion provides benefits such as effective waste management techniques and sustainable biogas energy generation. Mostly, animal wastes, such those from pigs, goats, sheep, cattle, and poultry, are widely studied as viable substrates to produce biogas. Regarding characteristics of the animal wastes, parameters such as total solids (TS), volatile solids (VS), pH, and carbon-to-nitrogen ratio (C/N) play a vital role in the enhancement of biogas yield using animal waste. To this effect, the recommended ranges of these parameters for the different types of animal waste are summarised in Table 3.
From Table 3, it can be observed that different animal wastes have different parameter conditions (TS, VS, pH, and C/N ratio). The discrepancies in the physiochemical characteristics of these animal wastes can be attributed to their origin, the difference in their diet, variation in digestion, and intestinal micro-organism [48]. Depending on the pH, the buffering capacity protects the process of digestion against acidification linked with high total solids. Furthermore, animal waste with the required pH provides a suitable environment for the optimum functioning of the microorganism during anaerobic digestion. Total solids are responsible for the contribution of water towards the growth of microorganisms, thereby facilitating the dissolution and transport of nutrients. As the amount of water decreases or increases, it reduces the activities of microorganisms because of the total solids [49].

6.2. Industrial Waste (IW)

Industrial organic waste for anaerobic digestion consists of by-products, residues, and waste from numerous activities in the industries. Some of these industries include the food and beverages industry, pulp and paper industry, petrochemical industry, and textile industry. Selected wastes from industries include food and beverage waste, sugar, starch, and fruit processing wastes produced in large quantities. According to Lup et al. [50], industrial organic wastes are characterized by a high content of hydrocarbons, fats, and protein. Industrial wastes are usually not used as feedstock for anaerobic digestion, especially for biogas production, because of their low degradability (30–50%) and recalcitrant chemical properties [39]. This particular property does not affect food waste, which is an example of industrial waste. Hence, they are the most widely used substrate among industrial waste for anaerobic digestion. The pulp and paper industry generates waste of high biological oxygen demand (BOD), high dye content of 300–2800 m/g, and 1200–6500 colour units [20]. To improve the biodegradability of industrial waste for anaerobic digestion, it is recommended that they are co-digested with other substrates because of their low total solid content of <1%. However, industrial waste for anaerobic digestion employed various technologies which are specific for biogas production via anaerobic digestion. Some of these technologies are shown in Table 4.

6.3. Municipal Solid Waste (MSW)

Municipal solid waste is waste generated from household waste, commercial waste, and waste from cleaning. Precisely, it consists of a fraction of household and garden waste. The composition of MSW includes paper waste, plastic waste, glass waste, metal waste, organic waste, and textile waste. For efficient digestion of these solid wastes, impurities, such as plastics, glass, and metals, need to be removed, as they affect the operation of the process, especially for biogas production [51,52,53,54]. To lower greenhouse gas emissions and lessen the dependency on the use of external energy, the utilization of source-separated municipal organic waste is needed [55]. In considering the anaerobic digestion of MSW, particle size is one factor that must be considered. The particle size impacts the surface area, which increases and influences the hydrolysis rate during anaerobic digestion and lag phases. These are known as the limiting stage of adaptation of microorganisms to the substrate. [56]. Towards this, Parra-Orobio et al. [56] mentioned that a particle size of <2 mm is recommended and effective for methane production, having an electrical energy of 2960.4 kWh per week. The recommended particle size of <2 mm is 19% higher than in reactors with larger particles. Thus, the <2 mm particle size indicates lower design cost and maintenance. This is because the smallest particle enables a shorter retention time. Despite the recommended value of the particle size, an optimal range for the particle size still has not been defined. Notwithstanding, the size variability (from the crushing process) is another parameter capable of affecting the kinetics of the anaerobic digestion of MSW. Approximately1.3 tonnes of MSW are generated globally every year. This MSW consists of 46% organic waste, 17% paper waste, 10% plastic waste, 5% glass waste, 4% metal waste, 3% textile waste, 13% inert waste, and 2% miscellaneous waste [57].
At the continental level, Africa generates the highest organic composition of municipal waste at 66%, followed by Europe (54%), South America (53%), and then Asia (47%). North America and Australia are regarded as the countries with the least composition of municipal waste generation, producing 26% and 25%, respectively. The variation in the organic composition of this waste by continent might be attributed to the rates of the organic portion of waste from low-income countries (50–70%) versus high-income countries (20–40%) [58]. Interestingly, anaerobic digestion of municipal waste provides many benefits, such as the separation of organic biodegradable fractions from total municipal waste, generation of renewable energy, reduction of landfilling, and mitigation of pollution.
A grouping of the waste category of the feedstock required for anaerobic digestion was discussed earlier, and this is presented in Table 5.

7. Gasification Technology

The thermochemical conversion of biomass materials to produce syngas, coal, tar, and some low-molecular-weight hydrocarbons in the presence of the oxidizing agent is known as gasification [59]. Similar to anaerobic digestion (biogas), the major end-product of gasification is syngas, which consists of carbon monoxide, hydrogen, methane, carbon dioxide, and traces of other gases (flammable gases). Although the end application of gasification products is mainly for electricity and heat generation, it can be used as transportation fuel [60]. The biomass gasification system consists of the following individual units: a feeding system, gasifier, cyclone, and gas scrubber. These units are said to contribute to the overall efficiency of the gasification process [61]. During gasification, the oxidizing agent, consisting of oxygen, air, steam, or mixtures thereof, is usually provided in stoichiometric amounts to maintain partial oxidation of the biomass material. The choice of any of the oxidizing agents is guided by the expected quality of the syngas in terms of the heating value and cost [62]. To lower the cost, the air is usually used as an oxidizing agent; however, it results in syngas with low heating values (4–6 MJ/Nm3) [63]. This is said to be attributed to the higher concentration of nitrogen present in syngas. Using oxygen and steam as oxidizing agents increases the heating value of syngas. It was reported that the heating value of 9–10 MJ/Nm3 and 17–18 MJ/Nm3 was obtained for oxygen and steam, respectively. To maintain the partial oxidation of biomass material during gasification, there is usually the provision of sub-stoichiometric quantities [59].

8. Recent Studies on Gasification Focusing on Syngas Production

This section presents the selected recent studies conducted by various scholars and researchers on biomass gasification. This is necessary to have an overview and track the trend of research in this regard. On the positive side, the development of the biomass gasification process has contributed to high potential energy efficiency that is environmentally friendly and inexpensive, mostly as a waste-to-energy technology. Therefore, its contributions cannot be neglected, especially for the production of syngas. Currently, on the gradual progress made so far on biomass gasification, it is interesting to mention that some high-profile gasifier units have been installed and are operating in a different part of the world. Table 6 shows a few of the gasification plants installed in selected parts of the world.
Reviewing the recent studies on biomass gasification from the literature, the gasification of eucalyptus wood chips in a downdraft gasifier to produce syngas in South Africa was conducted by Nwokolo et al. [59]. The authors sourced the eucalyptus wood chip from the sawmill industry. The eucalyptus wood was gasified using air as the oxidizing agent, and the produced syngas were measured. With the aid of a non-dispersive infra-red sensor and palladium/nickel (Pd/Ni) gas sensor., the volumetric composition of the syngas produced was as follows; hydrogen (22.3–22.5%), carbon monoxide (22.3–24.3%), methane (1.9–2.1%), carbon dioxide (9.8–10.7%), and nitrogen (41.5–42.9%). These syngas produced a higher heating value (HHV) of 6.08 MJ/Nm3, obtained from hydrogen, carbon monoxide, and methane composition. The study provided evidence that gasification is an alternative for the waste-to-energy conversion.
To predict and optimise the rate of syngas production, Dang et al. [65] developed a robust biomass gasification process model. This was possible by incorporating updated biomass reaction kinetics and dense bed hydrodynamics into the Aspen Plus platform. The model was developed using 864 datasets of four input parameters and demonstrated good agreement and best line of fit under a range of operating conditions. This was achieved when comparing the model prediction with the experimental result. An optimal syngas yield from 61.4% to 78.5% by volume was obtained on a dry N2-free basis. According to the optimization model result, a maximum syngas production of 78.6 vol% occurred at a temperature of 900 °C, equivalent ratio (ER) of 0.23, steam-to-biomass (S/B) ratio of 0.21, and moisture content (MC) at 30 wt%.
Li et al. [26] carried out an experimental study on the gasification of cotton stalks to enhance the Boudouard reaction and carbon monoxide production. The parameters studied included carbon dioxide flow rate (50–400 mL/min), carbon dioxide concentration (0–100%), temperature (750–950 °C), and feedstock particle size of 0.1–8 mm. From the study, it was revealed that high temperature had a positive effect on gas yield and gasification efficiency (GE) due to the Boudouard reaction. As the particle sizes increased, GE first increased and then decreased due to the different heating conditions of the particle, with the highest GE of 88% at 0.8–1 mm obtained. The yield of CO and GE improved with an increase in the flow rate of CO2 and CO2 concentration. However, the conversion rate of CO2 decreased at a large CO2 flow rate, and the growth rate of GE decreased for CO2 concentrations greater than 50%. Therefore, the maximum gasification efficiency (GE) reached up to 125% at the recommended optimum temperature, particle size, and CO2 concentration of 850–900 °C, 50–100%, and 1mm, respectively. Gasification efficiency (GE) is defined as the ratio of the low heating value (LHV) of gases to that of biomass feedstock.
Synthesis gas composition was obtained from the gasification of biomass involving a mixture of carbon black, resulting from waste tires and sewage sludge. Varank et al. [66] modelled the process via the response surface method, while optimization involved the use of Box–Behnken design. For the syngas composition, the determination coefficient obtained for H2, CO, CH4, and heating values was 92.86%, 95.40%, 96.15%, and 96.80%, respectively. These were the indicators used for the modelling using a second-degree polynomial equation. The high determination coefficient reported was an indication that the polynomial equation conformed to a sufficient level with the experimental data. Based on the validation result, the percentages of the H2, CH4, and CO were 12.75%, 8.07%, and 7.87%, respectively, whereas the heating value was 1420.3 kcal/m3. The findings from the study confirmed that the gasification process was suitable for the production of high-quality syngas from waste material and recommended the use of the Box–Behnkan design for the optimization of the process.
To produce syngas, Peng et al. [67] employed the use of a catalyst during the gasification of wood residue in a fluidized bed gasifier while using a combination of air and steam as an oxidizing agent. During the study, the following parameters were considered; catalyst loadings (20%, 30%, and 40%) at residence time (20, 40, and 60 min) and temperature (750, 825, and 900 °C). The authors revealed that a high temperature of 900 °C, catalyst loading of 40%, and residence time of 60 min was found most suitable to produce syngas of high quality.
To facilitate the optimization of energy efficiency and economic viability of biomass gasification, Yao et al. [68] developed a gasification model. The model aimed to predict the rate of production and quality of syngas. It was mentioned that the three regions of the velocity profiles (natural convection region, mixed convection region, and forced convection region) were used for the development and building of the model. From the study, it was observed that the developed model provided accurate predictions against experimental data; nonetheless, there was a deviation of approximately 10%. Moreover, an equivalence ratio of 0.25 was found suitable for maximizing the economic benefits of biomass gasification.
Air steam gasification of pine sawdust with calcined cement as a catalyst was investigated for the production of syngas. The study was motivated by the operational problems relating to the formation of tar during gasification. As a result, tar was minimized in the producer gas due to catalyst cracking. According to Sui et al. [69], the employment of cement as a catalyst enhanced the hydrogen concentration and lowered the tar yield.

9. Intensification of Gasification Process

As mentioned earlier, in anaerobic digestion, intensification focuses on ways to enhance and improve biomass gasification. To optimize the process of gasification, Paulauskas et al. [70] investigated the efficacy of residual char gasification in a gasifier. The study used char made from wood and sewage sludge under different operating conditions in the gasification set-up. The findings showed that the intensifier and additive caused an increase in air flux from 0.11 kg/m2s to 0.32 kg/m2s as well as in the gasification rate from 0.8 to 2.61 g/min. Consequently, a 72% decrease in the duration of wood char gasification was recorded. This implies that wood char and sewage sludge char are suitable for the intensification of biomass gasification.
A continuous fly ash circulation was used as an intensifier or enhancer in industrial dual-fluidized gasification systems. This was evidenced in the study conducted on the effect of ash circulation on the performance of a dual fluidized bed gasification system. In the study, Pissot et al. [71] mentioned that ash-forming elements are released from fuel during the gasification of biomass. Hence, these elements are known to have a positive impact on the quality of the gas produced. To assess the impact on the gas quality, the coarse ash was intensified by an injection of untreated olivine in fine particle size and then recirculated. This resulted in a decrease in the tar concentration. From the study, it was revealed that the effect of ash circulation increased with time at a higher rate than in the control experiment.
In another study, Chen et al. [72] explored the use of calcium oxide in the steam gasification of sewage sludge to enhance the quality of syngas and hydrogen yield. The calcium oxide (CaO) was in situ to capture CO2 released directly from the biomass matrix, thereby causing a steam reformation reaction. This process drives the conversion of biomass process in the production of more hydrogen. Thus, the quality of hydrogen in the gas stream was enhanced as high as 70–80% due to CO2 sorption.
Similarly, Yang et al. [73] developed a two-stage sorption that relies on the integration of a CaO-based CO2-carrying cycle in the gasification of sewage sludge, thereby intensifying the sludge carbon. During this process, the CO2 was first captured and stored in the form of CaCO3 at a higher temperature to gasify the sludge char for carbon monoxide (CO) production. The proposed intensification process resulted in 284.7 NmL of syngas per gram of dry sludge, gross CO/H2 molar ratio of 2.3, and hydrogen and carbon-rich gas steam at a temperature of 550 °C and 750 °C, respectively. The study recommends CaO-based CO2 as an efficient option for improving carbon during the gasification of sewage sludge. On a similar note, the use of calcium oxide improved the mole ratio of hydrogen from 0.0 to 0.70 and the yield of syngas from 10.70 vol% to 49.40 vol% [74].
As observed, the catalyst enhanced syngas yield because of its effect on the activation energy [21]. To improve the formation of hydrogen, the study used dolomite (MgCO3·CaCO3) as a catalyst, which was calcinated for 4 h at a temperature of 900 °C. Dolomite, being a natural mineral-based catalyst, enhanced the formation of hydrogen. One advantage of dolomite is that it is inexpensive, abundantly available, and can aid in the removal of tar from producer gas [75]. Findings from a study by Guan et al. [76] revealed that the application of dolomite in MSW gasification reduced the content of tar from 9.71 weight% to 0.0 weight% and increased the syngas production from 0.620 to 1.470 m3/kg feedstock. Regarding the content of hydrogen in the syngas, this also increased from 30.6 to 50.2%. In a similar development, transition-metal-based catalysts (copper, iron, and cobalt) are said to be efficient and effective in the steam reformation of tar. For high tar content in syngas, these transition-based catalysts are easily deactivated during carbon deposition [74]. Nickel is another suitable catalyst that aids in deactivation and sintering during gasification reactions and reducing fouling. For instance, Wang et al. [77], mentioned that the addition of nickel oxide (NiO) in MSW gasification caused an increase in the rate of syngas production from 0.78 to 1 m3/kg MSW and in hydrogen production from 21.9 to 80.7 g H2/kg MSW.

10. Feedstock for Gasification Technology

Biomass gasification entails the use of various types of feedstocks, which include but are not limited to, woody biomass (chips, wood pellet, sawdust, and tree bunches); agricultural and food biomass (rapeseed, jatropha, miscanthus, sugar cane bagasse, nutshell, rice husk, wheat straw, and oil production waste); and industrial and animal biomass [64]. Before gasification, feedstocks are characterized to determine their suitability; Table 7 presents the characterization of some selected feedstocks for gasification.
For a successful study of the gasification process, the feedstock and its properties are necessary for syngas production [90]. These properties (moisture content, heating value, content of ash, alkali metals, and lignin ratio) are briefly discussed, as they affect the composition and yield of the syngas [91,92]. A study conducted by Tian et al. [34] revealed that feedstock with a high composition of cellulose and hemicellulose resulted in the production of syngas with a higher concentration of carbon monoxide and methane, whereas feedstock with a high composition of lignin generated syngas with a higher concentration of hydrogen.
Having provided the summary of the properties of some selected feedstock for gasification in Table 7, it is important to state the optimum ranges for these feedstocks according to their classifications. Hence, Table 8 presents the summary of the recommended operating parameter of biomass waste from the literature.

11. Bibliometric Analysis of the Study

In this section, the data employed were obtained from the Web of Science (WoS) core collection. In total, 207 and 801 results from the WoS the core collection were obtained for anaerobic digestion and biomass gasification, respectively. The choice of WoS was to maintain consistency in the study and avoid any overlap in the data collection. The period considered for the search was set to be 10 years from 2011 to 2021, focusing on journal types, publishers, and countries. Original articles, review papers, and proceeding papers were the document types considered; these are presented in Table 9a,b. This search was carried out to investigate the trend in research on anaerobic digestion and biomass gasification. For the successful analysis of the bibliometric, the keywords criteria used were “anaerobic digestion”, “biogas digester”, and “biogas” for anaerobic digestion, while for biomass gasification, the keywords used were “biomass gasification”, “gasifier”, and “syngas”.
Although the entire period set for bibliometric analysis was 2011 to 2021, the authors also considered the recent years from 2017 to 2021. It was observed that there is a fluctuation in the total number of publications in the two research fields, implying that the research in the two fields has not been consistent. Hence, future study is recommended. Regarding the journals, Energy and Fuel are the most active journals in terms of the number of publications in anaerobic digestion and biomass gasification, with 83 and 587, respectively. The Energy and Fuel journals are published by Elsevier. For the anaerobic digestion and biomass gasification research, Elsevier published a total number of 80 and 498 publications (original article, review paper, and proceeding papers). Moreover, it can be deduced that Energy, Fuel and Green Sustainable Science Technology are present and common in research or study. In addition, in terms of publishers, Elsevier, Springer Nature, and Taylor and Francis are known as the popular publishers present in both journals. For countries researching anaerobic digestion and biomass gasification, China is regarded as the leading country (See Table 9a,b). This might be because of the country’s interest in funding renewable energy and alternative energy research, especially as it relates to anaerobic digestion for biogas production and biomass gasification for syngas production.

12. Challenges Facing Anaerobic Digestion and Biomass Gasification

The recent challenges and future concerning anaerobic digestion and biomass gasification are discussed. For anaerobic digestion, though additives play an essential role during the intensification and enhancement of the anaerobic digestion process as earlier seen, not all of them (microbes) can stimulate complex organics, such as municipal sludges. Therefore, the need for favourable conditions capable of enhancing healthy growth over competing microorganisms should be developed. To achieve this, a biogas digester with two stages of anaerobic digestion has the tendency and capability to enrich the bacteria. To meet the energy demands with fewer carbon emissions than conventional energy systems, new technologies for generating this must be flexible. In this regard, carbon material as an enhancer or additive plays a critical role in this area. However, continued research in this direction is necessary in enhancing methane production in the future. Therefore, on the basis of the experimental studies, it can be seen that more attention and interest should be given to additives and their effect on continuous biogas digesters. [93,94,95]. Furthermore, regarding feedstock variability, Uddin and Wright [36] mentioned that the wide variability in the properties (physical and chemical) is a problem in choosing the proper technology. That is, the physical properties (feedstock size and moisture content) need to be compatible with the gasification technology. In the Akinbomi et al. [96] study, the challenges relating to anaerobic digestion focus on environmental and operational factors. The study revealed anaerobic digestion was inhibited by fluctuations in temperature, pH, and ammonia concentration. Nonetheless, the composition of the feedstock, position, handling, pumping, and mixing of the high-solids waste also play a role. According to Akinbomi et al. [96], anaerobic digestion is affected by temperature, pH, and feedstock composition (with regard to C/N). These are referred to as environmental factors. The authors recommended that the condition of the parameter of these environmental factors must be met for a stable, optimal, and efficient anaerobic digestion system. Accumulating intermediate products, such as ammonia, volatile fatty acid, and long-chain fatty acid, during anaerobic digestion is usually detrimental for the microbes, thereby causing process inhibition. For instance, NH3 consumes VFA, which is responsible for the lowering of methane production via methanogens. As a result of this, the accumulation of VFA reduces the pH of the biomass digester. The higher organic loading rate is caused as a result of an accumulation of volatile fatty acids [37].
For biomass gasification, the thermodynamics of its operation is not well understood. The limitation affecting biomass gasification can simply be understood on the basis of a technical issue that the gasification technology is facing with poor mixing and heat transfer within the gasifier [97]. According to the authors, this makes it a bit challenging to achieve an even distribution of fuel as well as temperature across the geometry of the gasifier. Therefore, it is reported that the scaling-up of the gasifier is difficult. This is attributed to the characteristic of the design (gasifier geometry and air inlet velocity) [97]. Furthermore, the operation of the gasification process is said to be complex. This is a result of the need to simultaneously control air supply, bed material, and feedstock during the operation of the gasifier [98]. This factor prompts the high particulate observed, which circulates and causes equipment erosion during the gasification of syngas. Therefore, the gasifier requires high-pressure conditions for successful operation. Hence, this leads to low volumetric gas flow rate, condensation during compression, and other complications (defluidization). Although the process of biomass gasification has been in existence all this time, the fundamental principles underpinning its operation are still vague. This is especially true concerning the type of material suitable as feedstock. Due to the high temperature and pressure of the gasification process, there is deposited liquid ash in the gasifier wall because of the turning of fuel oxygen mixtures into the turbulent flame of the dust. This development brings about a technical issue when analysing the ash melting behaviours of the material used as feedstock in the gasification technology. As a result of its operating conditions, a specific type of material is used as a feedstock because of its complexity [97].

13. Conclusions and Recommendation

Among the various sources of renewable energy, biomass energy is regarded as the most widespread and readily available renewable energy source globally. Anaerobic digestion and biomass gasification have been identified as suiting technologies for their conversion to valuable products. Therefore, this research study reviewed recent advances in these technologies (2017–2021): intensification, feedstock, bibliometric analysis of anaerobic digestion, and biomass gasification. The review concluded by providing the challenges affecting both technologies. Having successfully established these headings under the present review, the following key findings in the field of anaerobic digestion and biomass gasification have been highlighted.
The system design for the biogas digester and gasifier depends on the dimension and material used for construction and fabrication. In terms of the performance of both technology, temperature affects both anaerobic digestion and biomass gasification. Therefore, the employment of intensifiers, additives, and enhancers is a promising approach to use and is required for improving the process of anaerobic digestion and biomass gasification, although co-digestion is recommended for anaerobic digestion due to its contribution to enhancing methane production. From the review, despite the various feedstock or substrates used in both technologies (agricultural and food waste and industrial waste), it was observed that animal waste and woody biomass are used mostly as feedstock or substrate for anaerobic digestion and biomass gasification, respectively. Considering the academic trend of the study, China is seen as the leader in anaerobic digestion and biomass gasification research. Based on this, the country will see a growth of biogas and syngas production in the coming years. Energy, Fuel, and Green Sustainable Science Technology are the most active journals on anaerobic digestion and biomass gasification. The review recommends the necessity to conduct a comprehensive and detailed bibliometric analysis of the status and trend of research in anaerobic digestion and biomass gasification technologies. To widen the scope of the technologies, more experimental studies are needed, focusing on intensification to enhance and improve both anaerobic digestion and biomass gasification. Future works are required, focusing on the development of a hybrid system in combining anaerobic digestion and gasification for the recovery of energy from animal waste and woody biomass.

Author Contributions

K.O. contributed to the conception of the study; K.O. and N.N. organized the database and performed the analysis of the review; G.M. supervised and provided resources for the study; K.O. wrote the first draft of the manuscript; K.O., N.N. and G.M. wrote various sections of the manuscript. All authors contributed to the manuscript revision, read, and approved the submitted version. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The data that support the findings of this study are available from the corresponding author upon reasonable request.

Acknowledgments

The authors would like to acknowledge the financial support received from the Govan Mbeki Research Development Centre (GMRDC), University of Fort Hare.

Conflicts of Interest

The authors declare no conflict of interest.

References

  1. Dudley, B. Statistical Review of World Energy, 70th ed.; Whitehouse Associates Press: London, UK, 2021. [Google Scholar]
  2. Benti, N.E.; Gurmesa, G.S.; Argaw, T.; Aneseyee, A.; Gunta, S.; Kassahun, G.S.; Aga, G.S.; Asfaw, A. The current status challenges and prospects of using biomass energy in Ethiopia. Biotechnol. Biofuels 2021, 14, 209. [Google Scholar] [CrossRef]
  3. Victorin, M.; Davidsson, Å.; Wallberg, O. Characterization of mechanically pre-treated wheat straw for biogas production. BioEnergy Res. 2020, 133, 833–844. [Google Scholar] [CrossRef]
  4. Votteler, R.G.; Brent, A.C. A literature review on the potential of renewable electricity sources for mining operations in South Africa. J. Energy S. Afr. 2016, 27, 1–21. [Google Scholar] [CrossRef]
  5. Nikolausz, M.; Kretzschmar, J. Anaerobic digestion in the 21st century. Bioengineering 2020, 7, 157. [Google Scholar] [CrossRef] [PubMed]
  6. Akhiar, A.; Zamri, M.F.; Torrijos, M.; Shamsuddin, A.; Battimelli, A.; Roslan, E.; Mohamad, H.M.; Carrere, H. Anaerobic digestion industries progress throughout the world. In IOP Conference Series: Earth and Environmental Science; IOP Publishing: Bristol, UK, 2020; Volume 476, p. 012074. [Google Scholar] [CrossRef]
  7. Adnan, A.I.; Ong, M.Y.; Nomanbhay, S.; Chew, K.W.; Show, P.L. Technologies for Biogas Upgrading to Biomethane: A Review. Bioengineering 2019, 6, 92. [Google Scholar] [CrossRef] [PubMed]
  8. Sathyan, A.; Haq, I.; Kalamdhad, A.; Khwaraikpam, M. Recent advancements in anaerobic digestion: A novel approach for waste to energy. Adv. Organic. Waste Manag. 2022, 233–246. [Google Scholar] [CrossRef]
  9. Zheng, L.; Chen, J.; Zhao, M.; Cheng, S.; Wang, L.P.; Mang, H.P.; Li, Z. What could China give to and take from other countries in terms of the development of the biogas industry? Sustainability 2020, 12, 1490. [Google Scholar] [CrossRef]
  10. Xue, S.; Song, J.; Wang, X.; Shang, Z.; Sheng, C.; Li, C.; Zhu, Y.; Liu, J. A systematic comparison of biogas development and related policies between China and Europe and corresponding insights. Renew. Sustain. Energy Rev. 2020, 117, 109474. [Google Scholar] [CrossRef]
  11. World Biogas Association. 2019. Available online: https://www.worldbiogasassociation.org/wp-content/uploads/2019/09/WBA-globalreport-56ppa4_digital-Sept-2019.pdf (accessed on 11 November 2022).
  12. Pramanik, S.; Suja, F.; Md Zain, S.; Pramanik, B. The anaerobic digestion process of biogas production from food waste: Prospects and constraints. Bioresour. Technol. Rep. 2019, 8, 100310. [Google Scholar] [CrossRef]
  13. Connor, S.O.; Ehimen, E.; Pillai, S.C.; Black, A.; Tormey, D.; Bartlett, J. Biogas production from small-scale anaerobic digestion plants on European farms. Renew. Sustain. Energy Rev. 2021, 139, 110580. [Google Scholar] [CrossRef]
  14. Ngan, V.; Mervin, F.S.; Nam, T.; Van Theo, H.; Detras, M.; Hung, D.V.; Cuong, D.; Hung, N.V. Anaerobic digestion of rice straw for biogas production. Sustain. Rice Straw Manag. 2020, 65–92. [Google Scholar] [CrossRef]
  15. Kumar, P.; Samuchiwal, S.; Malik, A. Anaerobic digestion of textile industries wastes for biogas production. Biomass Convers. Biorefinery 2020, 10, 715–724. [Google Scholar] [CrossRef]
  16. Jurgutis, L.; Slepetiene, A.; Volungevicius, J.; Amaleviciute-Volunge, K. Biogas production from chicken manure at different organic loading rates in a mesophilic full scale anaerobic digestion plant. Biomass Bioenergy 2020, 141, 105693. [Google Scholar] [CrossRef]
  17. Obileke, K.; Mamphweli, S.; Meyer, E.; Makaka, G.; Nwokolo, N. Design, and fabrication of a plastic biogas digester for the production of biogas from cow dung. J. Eng. 2020, 2020, 1848714. [Google Scholar] [CrossRef]
  18. Chaump, K.; Preisser, M.; Saravanan, R.; Prasad, R.; Adhikari, S.; Higgins, B. Leaching, and anaerobic digestion of poultry litter for biogas production and nutrient transformation. Waste Manag. 2019, 84, 413–422. [Google Scholar] [CrossRef]
  19. Parawira, W. Enzyme research and applications in biotechnological intensification of biogas production. Crit. Rev. Biotechnol. 2012, 32, 172–186. [Google Scholar] [CrossRef]
  20. Zhang, J.; Zhao, W.; Zhang, H.; Wang, Z.; Fan, C.; Zang, L. Recent achievement in enhancing anaerobic digestion with carbon-based functional materials. Bioresour. Technol. 2018, 266, 555–567. [Google Scholar] [CrossRef]
  21. Yun, S.; Zhang, C.; Wang, Y.; Zhu, J.; Wei, Y. Synergistic effects of Fe salts and composite additives on anaerobic digestion of dairy manure. Int. Biodeterior. Biodegrad. 2019, 136, 82–90. [Google Scholar] [CrossRef]
  22. Li, X.; Yun, S.; Zhang, C.; Fang, W.; Huang, X.; Du, T. Application of nano-scale transition metal carbides as accelerants in anaerobic digestion. Int. J. Hydro. Energy 2018, 43, 1926–1936. [Google Scholar] [CrossRef]
  23. Liu, W.; Zheng, H.; Wu, Z.; Wang, Y. Effects of pH profiles on nisin fermentation coupling with foam separation. Appl. Microbiol. Biotechnol. 2010, 85, 1401–1407. [Google Scholar] [CrossRef]
  24. Patel, V.; Patel, A.; Datta, M. Effects of adsorbents on anaerobic digestion of water hyacinth-cattle dung. Bioresour. Technol. 1992, 40, 179–181. [Google Scholar] [CrossRef]
  25. Gaddy, J.L. Performance of Anaerobic Digesters. U.S. Patent US5783081A, 21 July 1998. [Google Scholar]
  26. Li, J.; Xie, Y.; Zeng, K.; Flamant, G.; Yang, H.; Yang, X.; Zhong, D.; Du, Z.; Chen, H. Biomass gasification in molten salt for syngas production. Energy 2020, 210, 118563. [Google Scholar] [CrossRef]
  27. Wu, B.; Lin, R.; Kang, X.; Deng, C.; Xia, A.; Dobson, A.D.; Murphy, J.D. Graphene addition to digestion of thin stillage can alleviate acidic shock and improve biomethane production. ACS Sustain. Chem. Eng. 2020, 8, 13248–13260. [Google Scholar] [CrossRef]
  28. Abbas, Y.; Yun, S.; Wang, Z.; Zhang, Y.; Zhang, X.; Wang, K. Recent advances in bio-based carbon material for anaerobic digestion. Renew. Sustain. Energy Rev. 2021, 135, 110378. [Google Scholar] [CrossRef]
  29. Capson-Tojo, G.; Moscoviz, R.; Ruiz, D.; Santa-Catalina, G.; Trably, E.; Rouez, M.; Crest, M.; Steyer, J.P.; Bernet, N.; Delgenès, J.P.; et al. Addition of granular activated carbon and trace elements to favour volatile fatty acid consumption during anaerobic digestion of food waste. Bioresour. Technol. 2018, 260, 157–168. [Google Scholar] [CrossRef]
  30. Lin, R.; Cheng, J.; Zhang, J.; Zhou, J.; Cen, K.; Murphy, J.D. Boosting biomethane yield and production rate with graphene: The potential of direct interspecies electron transfer in anaerobic digestion. Bioresour. Technol. 2017, 239, 345–352. [Google Scholar] [CrossRef]
  31. Mostafa, A.; El-Dissouky, A.; Fawzy, A.; Farghaly, A.; Peu, P.; Dabert, P.; Le Roux, S.; Tawfik, A. Magnetite/ graphene oxide nanocomposite for enhancement of hydrogen production from gelatinaceous wastewater. Bioresour. Technol. 2016, 216, 520–528. [Google Scholar] [CrossRef]
  32. Yang, Y.; Zhang, Y.; Li, Z.; Zhao, Z.; Quan, X.; Zhao, Z. Adding granular activated carbon into anaerobic sludge digestion to promote methane production and sludge decomposition. J. Clean. Prod. 2017, 149, 1101–1108. [Google Scholar] [CrossRef]
  33. De Vrieze, J.; Regueiro, L.; Props, R.; Vilchez-Vargas, R.; Jáuregui, R.; Pieper, D.H.; Lema, J.M.; Carballa, M. Presence does not imply activity: DNA and RNA patterns differ in response to salt perturbation in anaerobic digestion. Biotechnol. Biofuels 2016, 9, 244. [Google Scholar] [CrossRef]
  34. Tian, T.; Qiao, S.; Li, X.; Zhang, M.; Zhou, J. Nano-graphene induced positive effects on methanogenesis in anaerobic digestion. Bioresour. Technol. 2017, 224, 41–47. [Google Scholar] [CrossRef]
  35. Atelge, M.; Krisa, D.; Kumar, G.; Eskicioglu, C.; Nguyen, D.D.; Chang, S.W.; Atabani, A.; Al-Muhtaseb, A.H.; Unalan, S. Biogas production from organic waste: Recent progress and perspectives. Waste Biomass Valor. 2020, 11, 1019–1040. [Google Scholar] [CrossRef]
  36. Uddin, M.; Mba, W.M. Anaerobic digestion fundamentals, challenges, and technological advances. Phys. Sci. Rev. 2022, 19, 1–19. [Google Scholar] [CrossRef]
  37. Schnürer, A. Biogas production: Microbiology and technology. In Anaerobes in Biotechnology; Springer: Berlin/Heidelberg, Germany, 2016; pp. 195–234. [Google Scholar]
  38. McGenity, T.J.; Timmis, K.N.; Fernández, B.N. Hydrocarbon and Lipid Microbiology Protocols; Springer: Berlin/Heidelberg, Germany, 2016. [Google Scholar]
  39. Nwokolo, N.; Mukumba, P.; Obileke, K.; Enebe, M. Waste to energy: A focus on the impact of substrate type in biogas production. Processes 2020, 8, 1224. [Google Scholar] [CrossRef]
  40. Shen, J.; Zhao, C.; Liu, Y.; Zhang, R.; Liu, G.; Chen, C. Biogas production from anaerobic co-digestion of durian shell with chicken, dairy, and pig manures. Energy Convers. Manag. 2019, 198, 110535. [Google Scholar] [CrossRef]
  41. Wang, Z.; Jiang, Y.; Wang, S.; Zhang, Y.; Hu, Y.; Hu, Z.-H.; Wu, G.; Zhan, X. Impact of total solids content on anaerobic co-digestion of pig manure and food waste: Insights into shifting of the methanogenic pathway. Waste Manag. 2020, 114, 96–106. [Google Scholar] [CrossRef]
  42. Ning, J.; Zhou, M.; Pan, X.; Li, C.; Lv, N.; Wang, T.; Cai, G.; Wang, R.; Li, J.; Zhu, G. Simultaneous biogas, and biogas slurry production from co-digestion of pig manure and corn straw: Performance optimization and microbial community shift. Bioresourc. Technol. 2019, 282, 37–47. [Google Scholar] [CrossRef]
  43. Duan, N.; Zhang, D.; Lin, C.; Zhang, Y.; Zhao, L.; Liu, H.; Liu, Z. Effect of organic loading rate on anaerobic digestion of pig manure: Methane production, mass flow, reactor scale and heating scenarios. J. Environ. Manag. 2019, 231, 646–652. [Google Scholar] [CrossRef]
  44. Achinas, S.; Li, Y.; Achinas, V.; Euverink, G.J.W. Influence of sheep manure addition on biogas potential and methanogenic communities during cow dung digestion under mesophilic conditions. Sustain. Environ. Res. 2018, 28, 240–246. [Google Scholar] [CrossRef]
  45. Imeni, S.M.; Pelaz, L.; Corchado-Lopo, C.; Busquets, A.M.; Ponsá, S.; Colón, J. Techno-economic assessment of anaerobic co-digestion of livestock manure and cheese whey (Cow, Goat & Sheep) at small to medium dairy farms. Bioresourc. Technol. 2019, 291, 121872. [Google Scholar]
  46. Scarlat, N.; Fahl, F.; Dallemand, J.F.; Monforti, F.; Motola, V. A spatial analysis of biogas potential from manure in Europe. Renew. Sustain. Energy Rev. 2018, 94, 915–930. [Google Scholar] [CrossRef]
  47. Cheong, D.Y.; Harvey, J.T.; Kim, J.; Lee, C. Improving Biomethanation of Chicken Manure by Co-Digestion with Ethanol Plant Effluent. Int. J. Environ. Res. Public Health 2019, 16, 5023. [Google Scholar] [CrossRef] [PubMed]
  48. Caruso, M.C.; Braghieri, A.; Capece, A.; Napolitano, F.; Romano, P.; Galgano, F.; Altieri, G.; Genovese, F. Recent updates on the use of agro-food waste for biogas production. Appl. Sci. 2019, 9, 1217. [Google Scholar] [CrossRef]
  49. Orhorhoro, E.K.; Ebunilo, P.O.; Sadjere, E. Experimental Determination of Effect of Total Solid (TS) and Volatile Solid (VS) on Biogas Yield. Am. J. Mod. Energy 2017, 3, 131–135. [Google Scholar] [CrossRef]
  50. Lup, D.T.; Stroe, A.M.; Chezan, P.M.; Pica, E.M. The Importance of Waste Incineration. Stud. Univ. Babes-Bolyai Ambient. 2018, 63, 43–48. [Google Scholar] [CrossRef]
  51. Deng, L.; Liu, Y.; Wang, W. Biogas Technology, 1st ed.; Springer: Singapore, 2020. [Google Scholar]
  52. Singh, A. Managing the uncertainty problems of municipal solid waste disposal. J. Environ. Manag. 2019, 240, 259–265. [Google Scholar] [CrossRef]
  53. Rao, M.N.; Sultana, R.; Kota, S.H. Chapter 2—Municipal Solid Waste BT—Solid and Hazardous Waste Management, 1st ed.; Elsevier: Amsterdam, The Netherlands, 2017. [Google Scholar]
  54. Salomone, R.; Saija, G.; Mondello, G.; Giannetto, A.; Fasulo, S.; Savastano, D. Environmental impact of food waste bioconversion by insects: Application of Life Cycle Assessment to process using Hermetia Illucens. J. Clean. Prod. 2017, 140, 890–905. [Google Scholar] [CrossRef]
  55. Goossensen, M. Anaerobic Digestion of Municipal Organic Waste in Amsterdam; An Explorative Technical Feasibility Study and Scale Comparison. Master’s Thesis, Wageningen University and Research, Wageningen, The Netherlands, 2017. [Google Scholar]
  56. Parra-Orobio, B.; Torres-Lozade, P.; Marmolejo-Rebellon, F. Anaerobic digestion of municipal biowaste for the production of renewable energy: Effect of particle size. Braz. J. Chem Eng. 2017, 34, 481–491. [Google Scholar] [CrossRef]
  57. Asamoah, B.; Nikiema, J.; Gebrezgabher, S.; Odonkor, E.; Njenga, M. A Review on Production, Marketing and Use of Fuel Briquettes; CGIAR Research Program on International Water Management Institute (IWMI): Anand, India, 2016. [Google Scholar]
  58. Dlamini, S.; Simatele, M.D.; Serge Kubanza, N. Municipal solid waste management South Africa: From waste to energy recovery through waste-to-energy technologies in Johannesburg. Local Environ. 2019, 24, 249–257. [Google Scholar] [CrossRef]
  59. Nwokolo, N.; Mukumba, P.; Obileke, K. Gasification of eucalyptus wood chips in a downdraft for syngas production in South Africa. Int. J. Renew. Energy Res. 2020, 10, 663–668. [Google Scholar]
  60. Luo, X.; Wu, T.; Shi, K.; Song, M.; Rao, Y. Biomass Gasification: An Overview of Technological Barriers and Socio-Environmental Impact. In Gasification for Low-Grade Feedstock; Yun, Y., Ed.; InTechOpen: London, UK, 2018. [Google Scholar]
  61. Kirsanovs, V.; Blumberga, D.; Karklina, K.; Veidenbergs, I.; Rochas, C.; Vigants, E.; Vigants, G. Biomass Gasification for district heating. Energy Procedia 2017, 113, 217–223. [Google Scholar] [CrossRef]
  62. Solarte-Toro, J.; Chacón-Pérez, Y.; CardonaAlzate, C. Evaluation of biogas and syngas as energy vectors for heat and power generation using lignocellulosic biomass as raw material. Electron. J. Biotech. 2018, 33, 52–62. [Google Scholar] [CrossRef]
  63. Gallucci, F.; Liberatore, R.; Sapengo, L.; Volponi, E.; Venturini, P.; Rispoli, F.; Pris, E.; Carnevale, M.; Colanttoni, A. Influence of Oxidant Agent on Syngas Composition: Gasification of Hazelnut Shells through an Updraft Reactor. Energies 2020, 13, 102. [Google Scholar] [CrossRef]
  64. Glushkov, D.; Nyashina, G.; Shvets, A.; Pereira, A.; Ramanathan, A. Current Status of the Pyrolysis and Gasification Mechanism of Biomass. Energies 2021, 14, 7541. [Google Scholar] [CrossRef]
  65. Dang, Q.; Zhang, X.; Zhou, Y.; Jia, X. Prediction, and optimization of syngas production from a kinetic based biomass gasification process model. Fuel Process. Technol. 2020, 212, 106604. [Google Scholar] [CrossRef]
  66. Varank, G.; Ongen, A.; Guvenc, S.; Ozcan, H.; Can-Guven, E. Modelling, and optimization of syngas production from biomass gasification. Int. J. Environ. Sci. Technol. 2022, 19, 3345–3358. [Google Scholar] [CrossRef]
  67. Peng, W.; Wang, L.; Mirzaee, M.; Ahmadi, H.; Esfahani, M.; Fremaux, S. Hydrogen and syngas production by catalytic biomass gasification. Energy Convers. Manag. 2017, 135, 270–273. [Google Scholar] [CrossRef]
  68. Yao, Z.; You, S.; Ge, T.; Wang, C.W. Biomass gasification for syngas and biochar co-production: Energy application and economic evaluation. Appl. Energy 2018, 209, 43–55. [Google Scholar] [CrossRef]
  69. Sui, M.; Li, G.; Guan, Y. Hydrogen, and syngas production from steam gasification of biomass using cement as catalyst. Biomass Convers. Biorefinery 2020, 10, 119–124. [Google Scholar] [CrossRef]
  70. Paulauskas, R.; Zakarauskas, K.; Striūgas, N. An Intensification of Biomass and Waste Char Gasification in a Gasifier. Energies 2021, 14, 1983. [Google Scholar] [CrossRef]
  71. Pissot, S.; Vilches, T.; Thunman, H.; Seemann, M. Effect of ash circulation on the performance of a dual fluidized bed gasification system. Biomass Bioenergy 2018, 115, 45–55. [Google Scholar] [CrossRef]
  72. Chen, S.; Sun, Z.; Zhang, Q.; Hu, J.; Xiang, W. Steam gasification of sewage sludge with CaO as CO2 sorbent for hydrogen-rich syngas production. Biomass Bioenergy 2017, 107, 52–62. [Google Scholar] [CrossRef]
  73. Yang, X.; Tian, S.; Kan, T.; Zhu, Y.; Xu, H.; Strezov, V.; Nelson, P.; Jiang, Y. Sorption-enhanced thermochemical conversion of sewage sludge to syngas with intensified carbon utilization. Appl. Energy 2019, 254, 113663. [Google Scholar] [CrossRef]
  74. Maitlo, G.; Ali, I.; Mangi, K.H.; Ali, S.; Maitlo, H.A.; Unar, I.N.; Pirzada, A.M. Thermochemical Conversion of Biomass for Syngas Production: Current Status and Future Trends. Sustainability 2022, 14, 2596. [Google Scholar] [CrossRef]
  75. Islam, M.W. A review of dolomite catalyst for biomass gasification tar removal. Fuel 2020, 267, 117095. [Google Scholar] [CrossRef]
  76. Guan, Y.; Luo, S.; Liu, S.; Xiao, B.; Cai, L. Steam catalytic gasification of municipal solid waste for producing tar-free fuel gas. Int. J. Hydrogen Energy 2009, 34, 9341–9346. [Google Scholar] [CrossRef]
  77. Wang, J.; Cheng, G.; You, Y.; Xiao, B.; Liu, S.; He, P.; Guo, D.; Guo, X.; Zhang, G. Hydrogen-rich gas production by steam gasification of municipal solid waste (MSW) using NiO supported on modified dolomite. Int. J. Hydrogen Energy 2012, 37, 6503–6510. [Google Scholar] [CrossRef]
  78. Riaza, J.; Mason, P.; Jones, J.M.; Gibbins, J.; Chalmers, H. High temperature volatile yield and nitrogen partitioning during pyrolysis of coal and biomass fuels. Fuel 2019, 248, 215–220. [Google Scholar] [CrossRef]
  79. Schneider, J.K.; da Cunha, M.E.; dos Santos, A.L.; Maciel, G.P.; Brasil, M.C.; Pinho, A.R.; Caramão, E.B. Comprehensive two-dimensional gas chromatography with fast-quadrupole mass spectrometry detector analysis of polar compounds extracted from the bio-oil from the pyrolysis of sawdust. J. Chromatogr. A 2014, 1356, 236–240. [Google Scholar] [CrossRef]
  80. Nguyen, T.S.; He, S.; Raman, G.; Seshan, K. Catalytic hydro-pyrolysis of lignocellulosic biomass over dual Na2CO3/Al2O3 and Pt/Al2O3 catalysts using n-butane at ambient pressure. Chem. Eng. J. 2016, 299, 415–419. [Google Scholar] [CrossRef]
  81. He, J.; Strezov, V.; Kumar, R.; Weldekidan, H.; Jahan, S.; Dastjerdi, B.H.; Zhou, X.; Kan, T. Pyrolysis of heavy metal contaminated Avicennia marina biomass from phytoremediation: Characterisation of biomass and pyrolysis products. J. Clean. Prod. 2019, 234, 1235–1245. [Google Scholar] [CrossRef]
  82. Pighinelli, A.L.M.T.; Boateng, A.A.; Mullen, C.A.; Elkasabi, Y. Evaluation of Brazilian biomasses as feedstocks for fuel production via fast pyrolysis. Energy Sustain. Dev. 2014, 21, 42–50. [Google Scholar] [CrossRef]
  83. Yi, L.; Liu, H.; Li, S.; Li, M.; Wang, G.; Man, G.; Yao, H. Catalytic pyrolysis of biomass wastes over Org-CaO/Nano-ZSM-5 to produce aromatics: Influence of catalyst properties. Bioresour. Technol. 2019, 294, 122186. [Google Scholar] [CrossRef] [PubMed]
  84. Akubo, K.; Nahil, M.A.; Williams, P.T. Pyrolysis-catalytic steam reforming of agricultural biomass wastes and biomass components for production of hydrogen/syngas. J. Energy Inst. 2019, 92, 1987–1996. [Google Scholar] [CrossRef]
  85. Liu, X.; Luo, Z.; Yu, C.; Xie, G. Conversion mechanism of fuel-N during pyrolysis of biomass wastes. Fuel 2019, 246, 42–50. [Google Scholar] [CrossRef]
  86. Chang, Z.; Duan, P.; Xu, Y. Catalytic hydro-pyrolysis of microalgae: Influence of operating variables on the formation and composition of bio-oil. Bioresourc. Technol. 2015, 184, 349–354. [Google Scholar] [CrossRef]
  87. Chernova, N.I.; Kiseleva, S.V.; Larina, O.M.; Sytchev, G.A. Manufacturing gaseous products by pyrolysis of microalgal biomass. Int. J. Hydrogen Energy 2020, 45, 1569–1577. [Google Scholar] [CrossRef]
  88. Borel, L.D.M.S.; Lira, T.S.; Ribeiro, J.A.; Ataíde, C.H.; Barrozo, M.A.S. Pyrolysis of brewer’s spent grain: Kinetic study and products identification. Ind. Crop. Prod. 2018, 121, 388–395. [Google Scholar] [CrossRef]
  89. Ahmad, N.; Abnisa, F.; Wan Daud, W.M.A. Liquefaction of natural rubber to liquid fuels via hydrous pyrolysis. Fuel 2018, 218, 227–235. [Google Scholar] [CrossRef]
  90. Alptekin, F.; Celiktas, M. Review on catalytic biomass gasification for hydrogen production as a sustainable energy form and social, technological, economic, environmental, and political analysis of catalysts. ACS Omega 2022, 7, 24918–24941. [Google Scholar] [CrossRef]
  91. Chuayboon, S.; Abanades, S.; Rodat, S. Insights into the Influence of Biomass Feedstock Type, Particle Size and Feeding Rate on Thermochemical Performances of a Continuous Solar Gasification Reactor. Renew. Energy 2019, 130, 360–370. [Google Scholar] [CrossRef]
  92. Yu, H.; Wu, Z.; Chen, G. Catalytic Gasification Characteristics of Cellulose, Hemicellulose and Lignin. Renew. Energy 2018, 121, 559–567. [Google Scholar] [CrossRef]
  93. Liu, Y.; Yun, S.; Zhou, X.; Hou, Y.; Zhang, T.; Li, J.; Hagfeldt, A. Intrinsic origin of superior catalytic properties of tungsten-based catalysts in dye-sensitized solar cells. Electrochim. Acta 2017, 242, 390–399. [Google Scholar] [CrossRef]
  94. Fan, Q.; Zhang, W.; Yun, S.; Xu, J.; Song, Y. III-Nitride polymorphs: XN (X=Al, Ga, In) in the Pnma phase. Chem. Eur. J. 2018, 24, 17280–17287. [Google Scholar] [CrossRef] [PubMed]
  95. Liu, Y.; Yun, S.; Zhou, X.; Hou, Y.; Zhang, T.; Li, J.; Hagfeldt, A. The role of conductive materials in the start-up period of thermophilic anaerobic system. Bioresour. Technol. 2017, 239, 336–344. [Google Scholar]
  96. Akinbomi, J.G.; Patinvoh, R.J.; Taherzadeh, M.J. Current challenges of high-solid anaerobic digestion and possible measures for its effective applications: A review. Biotechnol. Biofuels 2022, 15, 52. [Google Scholar] [CrossRef]
  97. Mohammadi, A.; Anukam, A. The Technical Challenges of the Gasification Technologies Currently in Use and Ways of Optimizing Them: A. Review. In Energy Recovery; IntechOpen: London, UK, 2022. [Google Scholar] [CrossRef]
  98. Basu, P. Biomass Gasification, Pyrolysis and Torrefaction: Practical Design and Theory; Academic Press: London, UK, 2018. [Google Scholar]
Table
Table 1. Additives used in anaerobic digestion enhancement.
Table 1. Additives used in anaerobic digestion enhancement.
Types of AdditivesFunctionsReferences
Enzymes (Cellulose, Amylase, Lipase, etc.)Facilitate the activities of hydrolysis of the particulate compounds[19]
Nutrients (Nitrogen, Phosphorus, Potassium, etc.)Essential for the growth of biogas fermentation microorganisms[20]
Microelements (Iron, Cobalt, Nickel, etc.)Essential for methanogens activities[21]
Inhibitors (Heavy-metal ions)Promote the biological activity of the system at a low concentration[22]
Forming agent (Polyester deformer)Help to eliminate formation during anaerobic digestion[23]
Absorbent (Aluminium powder)Help to increase biogas content and methane content to a different degree; decrease BOD and COD content[24]
Chelating agentIncrease the growth rate of methanogens and gas production of anaerobic digestion[25]
AccelerantsIncrease biogas yield or production, COD, and buffer capacity as well as degradation rate[20,21,26,27]
Carbon-based materials (Carbon nanotube, Graphene, and Carbon cloth)Promote microbial growth and microbial immobilization; provide nutrients to microbes, resulting in faster methane production; stimulate the mechanism of methanogens as well as improve the efficiency and stability of the anaerobic process, thereby increasing methane yield[28,29,30,31,32]
Table
Table 2. Reported studies on nutritional composition for AD with methane yield and composition [37,38].
Table 2. Reported studies on nutritional composition for AD with methane yield and composition [37,38].
Composition of NutrientMethane Yield (m3/kgVS)Methane Composition (%)
Carbohydrate0.4250
Protein0.5050
Lipids1.0170
Table
Table 3. Optimum ranges of substrate parameters for enhanced biogas production.
Table 3. Optimum ranges of substrate parameters for enhanced biogas production.
Animal WasteTotal Solids (TS)Volatile Solids (VS)pHCarbon/Nitrogen RatioReferences
Pig manure6.4–7.56.2–82.86.4–7.55.7–13.5[40,41,42,43]
Goat manure33.7–55.527.7–89.47.918.0[44,45]
Sheep manure22.3–40.018.7–72.77.16–8.111.3–14.7[26,44]
Cattle manure14.5–22.711.9–72.07.1–8.614.59–18.9[26,40,44]
Poultry manure20.0–92.618.3–84.16.9–7.47.5–9.75[22,40,46,47]
Table
Table 4. Various technologies employed in industrial waste for anaerobic digestion [36].
Table 4. Various technologies employed in industrial waste for anaerobic digestion [36].
Technologies for Industrial WasteDescription
Up-flow anaerobic sludge blanket (UASB)This involves the pumping of waste into the sludge bed, consisting of granular bacteria and a sludge blanket. The UASB essentially separates gas and prevents bacteria sludge from flowing into the waste.
Anaerobic fixed-film (AFF) systemThe AFF employs the use of a fermentation tank to provide a biological firm as an intermediary on which to fix bacteria, thereby reducing bacteria losses from the system.
Continuous stirred tank reactor (CSTR) systemThis technology uses a stirring machine characterized by high efficiency of digestion to reduce the hold-up time within the anaerobic digester.
Modified covered lagoon (MCL) systemThe MCL system is covered by high-density polyethylene (HDPE) plastic sheet as a container for biogas production. This system increases the contact area of bacteria sludge with the waste.
Table
Table 5. Feedstock of anaerobic digestion by waste classification.
Table 5. Feedstock of anaerobic digestion by waste classification.
Feedstock for Anaerobic DigestionSource of Waste
Animal wasteEnergy crop, livestock manure, remains from harvest and farm mortality, etc.
Industrial wasteDairy product waste, processing of food/beverage, pharmaceutical industries, waste from a slaughterhouse, residues from agro-processing, etc.
Municipal wasteSewage sludge, yard trimmings, food waste from restaurants, and organic fraction of MSW.
Table
Table 6. The recent operating gasifier units with their capacity and locations [64].
Table 6. The recent operating gasifier units with their capacity and locations [64].
Gasifier UnitLocationCapacity
TCG global steam reformingUSA136,000 t/year
Two-stage thermal processNorway78,000 t/year
Fixed-bed dry bottomUSA16,000 t/day
General ElectricChina1500 t/day
Table
Table 7. Characteristics of feedstock for gasification.
Table 7. Characteristics of feedstock for gasification.
Woody Biomass
Types of FeedstocksMoisture Content (wt%)Volatile Matter Content (wt%)Ash Content
(wt%)		Heat of Combustion (MJ/kg)References
Wood pellet6.784.30.820.8[78]
Saw dust7.4-0.6-[79]
Chips (pine wood)4.578.42.616.1[80]
Root of mango tree (tree bunches)5.7367.873.9118.52[81]
Eucalyptus prophyll11.3775.340.2717.16[82]
Agricultural and Food Biomass
Miscanthus10.6765.655.3417.00[81]
Oil production waste5.980.17.620.1[78]
Sugar cane bagasse-87.42.9-[83]
Wheat straw2.50–7.0060.0–75.05.64–15.016.12 ± 0.19[78,84,85]
Rice husk8.0261.4318.02-[84]
Pea nutshell-84.11.4-[83]
Cocoa nutshell7.1668.582.26-[84]
Jatropha seed cake-73.57.3-[83]
Animal and Industrial Biomass
Microalgae3.0–10.070–815.0–10.0 20.0–25.0[86,87]
Brewer’s spent grain3.9783.33.2221.6[88]
Spent coffee grounds2.6680.441.25-[85]
Natural rubber1.7189.983.6045[89]
Table
Table 8. Summary of recommended operating parameters of biomass waste for gasification technology [64].
Table 8. Summary of recommended operating parameters of biomass waste for gasification technology [64].
Classification of BiomassMoisture Content (wt%)Volatile Matter (wt%)Ash Content (wt%)Heat of Combustion (MJ/kg)
Woody biomass4.0–12.060.0–85.0 0–5 16.0–20.0
Agricultural and food biomass2.0–12.064.0–85.00–18 15–17
Animal and industrial biomass3.0–10.0 70–81 5–10 24–25
Table
Table 9. (a) Bibliometric analysis of anaerobic digestion research. (b) Bibliometric analysis of biomass gasification research.
Table 9. (a) Bibliometric analysis of anaerobic digestion research. (b) Bibliometric analysis of biomass gasification research.
(a)
Anaerobic digestionPublication year consideredDocument type consideredJournalsPublishersCountries
2017 (18)Original articles (169)
 
 
 
Review papers (21)
 
 
 
Proceedings papers (18)		Energy and Fuel (83)Elsevier (80)India (30)
2018 (21)Environmental Science (56)Springer Nature (34)China (28)
2019 (17)Biotechnology Applied Microbiology (48)Wiley (11)Germany (24)
2020 (15)Green Sustainable Science Technology (30)Taylor and Francis (10)Denmark (20)
2021 (23)Agriculture Engineering (28)MDPI (8)South Africa (16)
(b)
Biomass gasificationNumber of publications per yearDocument TypeJournalsPublishersCountries
2017 (18)Original articles (764)
 
 
 
Proceedings papers (47)
 
 
 
Review papers (37)		Energy and Fuel (587)Elsevier (498)China (147)
2018 (21)Engineering Chemical (293)American Chemical Society (58)USA (100)
2019 (17)Thermodynamic (150)Springer Nature (50)Italy (93)
2020 (15)Green Sustainable Science Technology (83)MDPI (47)Malaysia (43)
2021 (23)Environmental Science (77)Taylor and Francis (29)Sweden (43)
(a) Source: https://www.webofscience.com/wos/woscc/summary/211f5dbe-3eb0-4982-b0eb-72df5c6c1513-569f8899/relevance/1 (accessed on 30 June 2022); (b) Source: https://www.webofscience.com/wos/woscc/summary/df29a3ed-2d94-4f12-9c07-ae972f7233b5-3d84b141/relevance/1 (accessed on 30 June 2022).
Disclaimer/Publisher’s Note: The statements, opinions and data contained in all publications are solely those of the individual author(s) and contributor(s) and not of MDPI and/or the editor(s). MDPI and/or the editor(s) disclaim responsibility for any injury to people or property resulting from any ideas, methods, instructions or products referred to in the content.

Share and Cite

MDPI and ACS Style

Obileke, K.; Makaka, G.; Nwokolo, N. Recent Advancements in Anaerobic Digestion and Gasification Technology. Appl. Sci. 2023, 13, 5597. https://doi.org/10.3390/app13095597

AMA Style

Obileke K, Makaka G, Nwokolo N. Recent Advancements in Anaerobic Digestion and Gasification Technology. Applied Sciences. 2023; 13(9):5597. https://doi.org/10.3390/app13095597

Chicago/Turabian Style

Obileke, KeChrist, Golden Makaka, and Nwabunwanne Nwokolo. 2023. "Recent Advancements in Anaerobic Digestion and Gasification Technology" Applied Sciences 13, no. 9: 5597. https://doi.org/10.3390/app13095597

Note that from the first issue of 2016, this journal uses article numbers instead of page numbers. See further details here.

Article Metrics

Back to TopTop