A Scoping Review on Environmental, Economic, and Social Impacts of the Gasiﬁcation Processes

: In recent years, computer-based simulations have been used to enhance production processes, and sustainable industrial strategies are increasingly being considered in the manufacturing industry. In order to evaluate the performance of a gasiﬁcation process, the Life Cycle Thinking (LCT) technique gathers relevant impact assessment tools to offer quantitative indications across different domains. Following the PRISMA guidelines, the present paper undertakes a scoping review of gasiﬁcation processes’ environmental, economic, and social impacts to reveal how LCT approaches coping with sustainability. This report categorizes the examined studies on the gasiﬁcation process (from 2017 to 2022) through the lens of LCT, discussing the challenges and opportunities. These studies have investigated a variety of biomass feedstock, assessment strategies and tools, geographical span, bioproducts, and databases. The results show that among LCT approaches, by far, the highest interest belonged to life cycle assessment (LCA), followed by life cycle cost (LCC). Only a few studies have addressed exergetic life cycle assessment (ELCA), impact assessment (SIA), consequential life cycle assessment (CLCA), and water footprint (WLCA). SimaPro ® (PR é Consultants, Netherlands), GaBi ® (sphere, USA), and OpenLCA (GreenDelta, Germany) demonstrated the greatest contribution. Uncertainty analysis (Monte Carlo approach and sensitivity analysis) was conducted in almost half of the investigations. Most importantly, the results conﬁrm that it is challenging or impossible to compare the environmental impacts of the gasiﬁcation process with other alternatives since the results may differ based on the methodology, criteria, or presumptions. While gasiﬁcation performed well in mitigating negative environmental consequences, it is not always the greatest solution compared to other technologies.


Introduction
After coal, petroleum, and natural gas, biomass is the world's fourth-largest energy source, accounting for a considerable amount of global primary energy consumption [1]. Biomass presently contributes roughly 14% of the world's yearly energy consumption in all forms [2]. As an alternative, biomasses, such as agricultural waste, forestry waste, municipal solid, and industrial waste, are renewable energy resources used for producing either solid or liquid fuels [3,4]. There are different processes to produce biomass energy, such as thermochemical, biological, and physical conversion (oilseed extraction). Thermochemical conversions can be categorized into combustion, pyrolysis, and gasification. Biological conversion can be achieved by fermentation or anaerobic digestion [5][6][7][8]. Moreover, there are some novel approaches to merging microbiology, electrochemistry, and electronics, such as microbial electrochemical technologies (METs) [9]. Converting organic sources into electricity and treating organic waste stream in microbial fuel cells (MFCs) [10], hydrogen or methane generation in microbial electrolysis cells (MEC) [11], CO2 elongation to volatile fatty acids (VFAs) in microbial electro-synthesis (MES) cells [12], low-cost desalination in microbial desalination cells (MDCs) [13], and microbial reverse electrodialysis cells (MRCs) However, replacing fossil fuels with biobased fuels can positively impact the environment; since biomass is considered a renewable resource, every technology has its limitations, and biomass gasification is no exception.
Unless suitable and efficient preventive measures are implemented and consistently enforced, biomass gasification plants result in environmental pollution, occupational health, and safety risks [22]. For example, the produced gas in its normal state is highly contaminated with condensable hydrocarbons, soot, char particles, and ash [26]. Gasification plants have many environmental issues, such as mass-burn incinerators, water, air pollution, ash, and other by-product disposals [27]. Economy, society, and the environment are the three elements of sustainability [28]. The LCT broadens the idea of cleaner production to include the product's complete life cycle and sustainability [29]. The term "life cycle thinking" Environments 2022, 9,92 3 of 20 refers to how a product's life cycle assessment (LCA), life cycle cost (LCC), and social impact assessment (SIA) are considered over its entire life cycle [30].
More precisely, LCT is a theoretical approach that studies improvements and reductions in all mentioned impacts at all processing stages (cradle-to-grave). These stages include extraction, conversion, transformation, distribution, use, demolition, and end-oflife treatment [31]. Nevertheless, it is not clear what kind of information is available in the literature about the scopes and challenges of assessing the environmental impacts of the biomass gasification process. Therefore, the present study aims to conduct a systematic review of biomass gasification processes' environmental, economic, and social impacts through a scoping review to discover how much LCT research has been undertaken. This study follows the PRISMA guidelines [32]. The problem is addressed in this study by answering the following four research questions: • What are the significant interests in the most recent investigations on life cycle thinking of gasification processes? • Which dimension (environmental, economic, and social) is these studies' most frequently used aspect? • What are the main life cycle assessment tools, methodologies, and impact categories?
The research focuses on the challenges associated with the gasification process. However, the question remains whether or not this process has a lower environmental impact than commercial processes for producing chemicals and fuels from fossil sources. The remainder of the article is organized as follows. Section 2 provides a background to gasification process technology and its environmental impacts; Section 3 describes the research methodology; Section 4 gives research results; Section 5 discusses them; and Section 6 concludes the review.

Gasification Technology
Biomass gasification for energy generation may appear to be a new technique, although it has been around for over a century [33]. Even though gasification technology has been around for decades, it has yet to reach its full potential. The fundamental principles governing its operation, notably feedstock variability and the type of gasification system, are still ambiguous [34]. Gasification technology is a thermochemical process used to convert organic substances into valuable gas (so-called syngas, a mixture of CO and H 2 ). Temperature, equivalent ratio, and pressure impact the syngas composition [35]. The gasifier (reactor) and its configuration are the most critical factors affecting the reactions and products [36]. Generally, gasifiers are classified based on their fluidization regime (gas-solid contacting mode) and gasifying medium [37,38]. Based on the gas-solid contacting mode, fixed bed gasifiers (also known as the moving bed (a moving bed is also known as this type of gasifier since the fuel moves downward in the gasifier)), fluidized bed gasifiers, and entrained flow gasifiers are the three main types of gasifiers with commercial or nearcommercial applications [34,39]. However, there are some other uses that employ specific gasifier types or gasification processes.
These technologies are usually targeted at utilizing a wider variety of feedstock than only coal and demonstrate innovative applications of gasification [40]. As illustrated in Figure 1, each type can be further subdivided into specific commercial types. In all gasification processes, however, the phenomena of pyrolysis followed by partial oxidation of the residual carbon are prevalent [41]. In general, due to the wide range of raw materials available, developing a valid theory to describe the entire gasification process is quite challenging [42]. Over the years, different suppliers have developed gasifiers commercially. Table 2 summarizes the technological development of the gasification process during the past decades [40,[42][43][44][45][46][47][48][49].

Process Challenges
The gasification process still has to be optimized to reduce the energy loss caused by pretreatment of the biomass prior to the conversion process, optimizing the carbon con version efficiency in the reactor, reducing tar production, and cleaning the syngas for fur ther processing [16].
Both the gasifier's performance and the composition of syngas are affected by the moisture content of the biomass. Brammer and Bridgwater showed that high moisture content in the biomass has a negative impact on the quality of the produced syngas and the system's overall performance [50].
Although a high moisture content might not be a big problem in a fluidized bed due to using steam as the fluidizing agent, the entrained gasifier is more sensitive to the mois ture. A downdraft gasifier's maximum moisture content is typically 25%, whereas an up draft gasifier's maximum moisture content is often 50% [51]. Drying biomass before gasi fication might result in high capital and energy expenditures in small-and medium-scale gasification plants [16].
The contaminants within the biomass might reduce the efficiency of the thermochem ical conversion process [52]. The most significant challenge for chemical production and energy generation using biomass gasification may be the high cost of auxiliary equipmen required to produce clean contaminant-free syngas. Consequently, the overall cost of the process increases significantly, accounting for more than half of the ultimate price of bio fuel produced [53].
One of the most severe problems encountered throughout the various biomass gasi fication methods is tar formation [54]. Condensable hydrocarbons, with or without addi tional oxygen-containing hydrocarbons, and more complex polycyclic aromatic hydrocar bons make up the tars formed during gasification [55]. Tar formation results in the deac tivation of catalysts, the halting of the downstream operations, and the generation of car cinogenic compounds [56].

Gasification's Environmental Impacts
The environmental impact of biomass gasification is related to input and output val ues of material flows, energy flows, emissions to air and water, and by-products. The in put material composition depends on the type of biomass used and its origin. The gasifi cation process is robust, and mixtures of biomasses can be used, which challenges the evaluation of the biomass feed. The contaminants in the material will vary and affect the environmental impact assessment. Other input flows related to water resources, the en ergy sources for heating the reactor, and catalytic compounds used in the reactor must be considered in the assessment. The output of emissions to air and water needs to be care fully monitored. Fly ash generation, dust, gaseous emissions, and water pollution are sig nificant adverse environmental impacts [57]. Moreover, combustible gases, vapors, dust fire risks, carbon monoxide poisoning, and gas leaks are the primary hazards of gasifier

Process Challenges
The gasification process still has to be optimized to reduce the energy loss caused by pretreatment of the biomass prior to the conversion process, optimizing the carbon conversion efficiency in the reactor, reducing tar production, and cleaning the syngas for further processing [16].
Both the gasifier's performance and the composition of syngas are affected by the moisture content of the biomass. Brammer and Bridgwater showed that high moisture content in the biomass has a negative impact on the quality of the produced syngas and the system's overall performance [50].
Although a high moisture content might not be a big problem in a fluidized bed due to using steam as the fluidizing agent, the entrained gasifier is more sensitive to the moisture. A downdraft gasifier's maximum moisture content is typically 25%, whereas an updraft gasifier's maximum moisture content is often 50% [51]. Drying biomass before gasification might result in high capital and energy expenditures in small-and mediumscale gasification plants [16].
The contaminants within the biomass might reduce the efficiency of the thermochemical conversion process [52]. The most significant challenge for chemical production and energy generation using biomass gasification may be the high cost of auxiliary equipment required to produce clean contaminant-free syngas. Consequently, the overall cost of the process increases significantly, accounting for more than half of the ultimate price of biofuel produced [53].
One of the most severe problems encountered throughout the various biomass gasification methods is tar formation [54]. Condensable hydrocarbons, with or without additional oxygen-containing hydrocarbons, and more complex polycyclic aromatic hydrocarbons make up the tars formed during gasification [55]. Tar formation results in the deactivation of catalysts, the halting of the downstream operations, and the generation of carcinogenic compounds [56].

Gasification's Environmental Impacts
The environmental impact of biomass gasification is related to input and output values of material flows, energy flows, emissions to air and water, and by-products. The input material composition depends on the type of biomass used and its origin. The gasification process is robust, and mixtures of biomasses can be used, which challenges the evaluation of the biomass feed. The contaminants in the material will vary and affect the environmental impact assessment. Other input flows related to water resources, the energy sources for heating the reactor, and catalytic compounds used in the reactor must be considered in the assessment. The output of emissions to air and water needs to be carefully monitored. Fly ash generation, dust, gaseous emissions, and water pollution are significant adverse environmental impacts [57]. Moreover, combustible gases, vapors, dust, fire risks, carbon monoxide poisoning, and gas leaks are the primary hazards of gasifier operation [58].
Dust is created during storage, handling, feeding, feedstock preparation, and fly ash removal [59]. Because of the acidic conditions in landfills, the ash that remains after gasification is hazardous and poses particular problems [60]. The gasification process produces many tiny solid particles, mostly fly ash and char (unburned carbons). These cause a similar issue as dust and biomass ash. Ash may also constitute a fire hazard, demonstrating the need to keep it wet and sealed [22]. During the cooling and cleaning of produced syngas, wastewater is produced as an effluent [61]. The disposal of some contaminates in effluents, such as phenolic and terry components, reveals severe environmental problems and requires adequate pretreatment before discharging into the environment [26].

Research Design
The present study adopts a scoping review methodology to summarize and analyze the history and status of life cycle thinking in the gasification technology context and indicate related challenges and limitations. In addition, the possible promising areas for Environments 2022, 9, 92 6 of 20 improvement and knowledge gaps were identified. A scoping review, at a general level, aims to map the key concepts rapidly underpinning a research area and the main sources and types of evidence available which can be undertaken as stand-alone projects in their own right, especially where an area is complex or has not been reviewed comprehensively before [62]. At least four frequent reasons exist for conducting a scoping study: to evaluate the study's scope, range, and nature; to assess the practicality of conducting a comprehensive systematic review study; to summarize and share findings; and to explore knowledge gaps in the literature [63]. This technique is chosen because it is much more rigorous than a simple search and requires multiple and systematic searches [64].
There is a contrast between systematic and scoping reviews [65]. In the systematic review, the main concern is based on a well-defined research question with a relatively narrow range for answers, while a scoping review addresses broader questions and topics [63].

Searching Procedure
The following steps were conducted under the scoping review protocol illustrated in Four main research questions were defined.

2.
After multiple tries and errors, an initial search was undertaken utilizing available scientific databases (Scopus, ScienceDirect, and Web of Science (WoS)). The search strings are provided in Table 3. At this level, no limitations were set to the initial search. The search was applied to the title, abstract, and keywords in Scopus and ScienceDirect and all WoS categories. As a result, 6682, 9755, and 2460 documents (in all categories) were listed in Scopus, WoS, and ScienceDirect, respectively. Because of the number of AND/OR operator limitations, the string was divided into three strings. The asterisk (*) is a regularly employed symbol that broadens a search by finding terms with identical initial letters. It may be used in conjunction with distinctive word stems to obtain variants of a phrase with less keystrokes. For example assess* can find assess, assessing, assessment, assessed, etc.

3.
Since life cycle studies on gasification technologies have mainly gained prominence over the past two decades, this study focused on published literature (2017-2022).
Applying this limit, the number of documents dropped to 2363, 5515, and 1310 for Scopus, WoS, and ScienceDirect, respectively. 4.
As another limitation, the language of the studies was limited to English. As a result, only a few documents were eliminated. The remaining studies became 2275, 5480, and 1310 for Scopus, WoS, and ScienceDirect, respectively. 5.
By applying the search strings to only the title, a significant reduction in the number of documents was observed. The listed studies experienced a significant drop to 144, 116, and 91 for Scopus, WoS, and ScienceDirect, respectively. 6.
For the final step at the screening stage, by tailoring the string and eliminating "OR environmental," more accurate results were achieved, and the number of documents was reduced to 40, 43, and 35 for Scopus, WoS, and ScienceDirect, respectively (118 studies in total). 7.
There were many duplicates in the list. Therefore, in this stage (step 3 in Figure 2), by trimming the list and removing duplicates, 48 documents remained. These were listed in Excel to perform the necessary investigation. 8.
The eligibility of the studies was assessed by a full-text screening. As a result, six studies were considered non-relevant and were eliminated from the list. All in all, the final list consisted of 42 publications. 9.
The bolographic information of the results, such as the title, the country of origin, the technology, the year of publication, the aim of the study and scope, the methodology, and the barriers and challenges, was extracted.   Applying this limit, the number of documents dropped to 2363, 5515, and 1310 for Scopus, WoS, and ScienceDirect, respectively. 4. As another limitation, the language of the studies was limited to English. As a result, only a few documents were eliminated. The remaining studies became 2275, 5480, and 1310 for Scopus, WoS, and ScienceDirect, respectively. 5. By applying the search strings to only the title, a significant reduction in the number of documents was observed. The listed studies experienced a significant drop to 144, 116, and 91 for Scopus, WoS, and ScienceDirect, respectively. 6. For the final step at the screening stage, by tailoring the string and eliminating "OR environmental," more accurate results were achieved, and the number of documents was reduced to 40, 43, and 35 for Scopus, WoS, and ScienceDirect, respectively (118 studies in total). 7. There were many duplicates in the list. Therefore, in this stage (step 3 in Figure 2), by trimming the list and removing duplicates, 48 documents remained. These were listed in Excel to perform the necessary investigation. 8. The eligibility of the studies was assessed by a full-text screening. As a result, six studies were considered non-relevant and were eliminated from the list. All in all, the final list consisted of 42 publications.    Figure 3 illustrates the PRISMA flow diagram of the present study. PRISMA methodology is a well-established reporting template for scoping reviews. It illustrates the screening processes' results to report the remaining studies at each stage.
Environments 2022, 9, x FOR PEER REVIEW 8 of 22 9. The bolographic information of the results, such as the title, the country of origin, the technology, the year of publication, the aim of the study and scope, the methodology, and the barriers and challenges, was extracted. Figure 3 illustrates the PRISMA flow diagram of the present study. PRISMA methodology is a well-established reporting template for scoping reviews. It illustrates the screening processes' results to report the remaining studies at each stage.

Limitations
The present study is limited to English language studies and the literature published after 2017. Furthermore, although it covers conference papers and proceedings, this study did not cover grey literature such as publicly accessible records and reports.

Limitations
The present study is limited to English language studies and the literature published after 2017. Furthermore, although it covers conference papers and proceedings, this study did not cover grey literature such as publicly accessible records and reports.

Results
This section provides the descriptive information associated with the latest studies on life cycle assessment (LCA), life cycle cost (LCC), and social impact assessment (SIA) of the gasification process.

Number of Publications
The year-wise analysis gives a picture of the research progress. It may be challenging to discern a clear trend based on recent studies. To better understand how interest has grown in this topic, the years 2000-2016 were added to the research period.

The Origin of Studies
Country-wise analysis of the selected publications shows that twenty-eigh contributed to this topic. As seen in Figure 5, the highest contribution belong with 15 studies, followed by the United States and Spain with seven publica and Italy with six. Fifteen countries were involved in only a single study categ der "Other Countries." Austria, Chile, Colombia, Denmark, Iran, Ireland, Mal ippines, Qatar, Romania, Saudi Arabia, Singapore, South Africa, Switzerland land belong to the group with one publication. In another classification, over se cent of the contribution belongs to the developed countries.

The Origin of Studies
Country-wise analysis of the selected publications shows that twenty-eight countries contributed to this topic. As seen in Figure 5, the highest contribution belongs to China with 15 studies, followed by the United States and Spain with seven publications each, and Italy with six. Fifteen countries were involved in only a single study categorized under "Other Countries." Austria, Chile, Colombia, Denmark, Iran, Ireland, Malaysia, Philippines, Qatar, Romania, Saudi Arabia, Singapore, South Africa, Switzerland, and Thailand belong to the group with one publication. In another classification, over seventy percent of the contribution belongs to the developed countries. with 15 studies, followed by the United States and Spain with seven publicatio and Italy with six. Fifteen countries were involved in only a single study categor der "Other Countries." Austria, Chile, Colombia, Denmark, Iran, Ireland, Malay ippines, Qatar, Romania, Saudi Arabia, Singapore, South Africa, Switzerland, a land belong to the group with one publication. In another classification, over seve cent of the contribution belongs to the developed countries.

Publications by Document Type
As discussed earlier, all types of publications were considered in this revie eighty percent of selected documents were articles, followed by conference paper percent) and book chapters and reviews (two percent each). As seen in Figure 6, o review articles demonstrated the study's significance. Ramos and Rouboa [66] r different aspects of life cycle thinking (environmental, social, and economic) on

Publications by Document Type
As discussed earlier, all types of publications were considered in this review. Over eighty percent of selected documents were articles, followed by conference papers (fifteen percent) and book chapters and reviews (two percent each). As seen in Figure 6, only two review articles demonstrated the study's significance. Ramos and Rouboa [66]

Publications by Subject Area
Fourteen studies on gasification processes addressed the life cycle thinking ap proaches (based on extracted data from Scopus). As seen in Table 4, energy, environmen tal science, and engineering have the highest contribution with 30, 27, and 17 percen respectively, followed by chemical engineering.

Publications by Subject Area
Fourteen studies on gasification processes addressed the life cycle thinking approaches (based on extracted data from Scopus). As seen in Table 4, energy, environmental science, and engineering have the highest contribution with 30, 27, and 17 percent, respectively, followed by chemical engineering.

Discussion
A comprehensive content-based analysis was performed to answer several research questions. This section focuses on recent research conducted during the previous five years. Among 48 selected studies belonging to the period between 2017 to February 2022, 42 studies were considered relevant to the topic. Except for review articles by Ramos and Rouboa [66] and Michaga et al. [67], other studies focused on a specific aspect of life cycle thinking in the gasification process. Table 5 lists different life cycle thinking aspects and their frequencies in the selected publications. Among seven different approaches, LCA was dominating, followed by LCC. Most of the studies (over seventy percent) studied a single aspect. Almost twenty percent studied two different aspects, and ten percent studied three aspects. For example, Korre et al. [68] performed a life cycle environmental impact assessment on the underground coal gasification process, including CO 2 capture and storage. Li et al. [69] assessed ELCA and LCA of hydrogen production from biomass-staged gasification, and Li and Cheng [70] compared hydrogen production from coke oven gas and coal gasification from three different points of view (life cycle energy assessment, carbon emissions, and life cycle costs).
Different software and databases were employed to carry out the life cycle assessment. The software SimaPro ® (PRé Consultants, Netherlands) and GaBi ® (sphere, USA) showed the highest contribution, followed by OpenLCA. Ecoinvent and ELCD were at the top of the list of employed databases. Table 6 provides an overview of the selected articles' life cycle methods and different approaches using software and databases. The cradle-to-grave approach encompasses the whole life cycle of a resource, from its extraction ('cradle') to its use and disposal ('grave') [71]. Cradle-to-gate is another approach studied by different researchers [70,[72][73][74]. Cradle-to-gate examines a product's partial life cycle, beginning with resource extraction (cradle) and ending at the factory gate before transporting to the consumer [75]. Cradle-to-gate evaluations are occasionally used to develop environmental product declarations (EPDs), referred to as business-to-business EDPs [71]. As mentioned earlier, all the assessments were performed based on process simulations. Among fourteen studies that referred to their process software, eleven simulations were conducted using Aspen Plus ® (Aspen Technology, Inc., USA) versions 8.8, 11, 9 [21,68,69,73,76-82]. The other three software were EASETECH [83], the integrated environmental control model (IECM) [84], and DeST [85]. Uncertainty analysis was considered in fifty percent (22 out of 42) studies. Only sensitivity analysis and Monte Carlo simulation were employed among many methods and tools to model and analyze uncertainty in a system. Sixteen studies only Environments 2022, 9, 92 11 of 20 used sensitivity analysis, three applied Monte Carlo simulation to cope with uncertainty, and the remaining three employed both methods. More information is given in Table 6.  Table 7 summarizes the different processes and their used feedstock, the number of scenarios in the analysis, and the year of publication in 42 recent articles. As seen, a wide range of raw materials has been covered, such as municipal solid wastes [78,86,[89][90][91]111], wheat straw [69,92,93], biomass, water for supercritical water gasification processes [81,94], pinewood [80], etc. The majority of the articles used scenario-based analysis to compare different alternatives. Among selected articles, fourteen studies compared gasification and another method for biomass conversion or disposal, such as incineration, pyrolysis and fast pyrolysis, landfilling, hydrothermal carbonization, combustion, fermentation, and steam reforming. Keller et al. [83] conducted a comparative life cycle analysis of two feedstock recycling technologies: waste gasification and pyrolysis. Although both feedstock recycling paths decrease greenhouse gas emissions under similar production system assumptions, gasification resulted in a greater reduction than pyrolysis. Similarly, Alcazar-Ruiz et al. [76] conducted a comparative life cycle study to measure the sustainability of two processes (gasification and fast pyrolysis) for bio-oil production from agricultural wastes. Separation stages were the primary contributors to all mid-point impact categories in the fast pyrolysis. Finally, contrary to the results reported in [83], the most ecologically beneficial method of creating one MJ bio-oil was not the gasification process. Bianco et al. [86] focused on the environmental consequences of generating power from the incineration and gasification of municipal solid waste. The study revealed that, depending on the accounting rules, the effect outcomes might vary greatly and can lead to opposing conclusions for some impact categories. Corvalán et al. [100] performed a comparative LCA analysis on the hydrothermal carbonization (HTC) of urban organic solid waste and gasification process. Upon evaluating the conversion of 1 ton of organic fraction USW, the results indicated that gasification performed better than HTC. Considering the generation of 1 MWh, HTC has a lower environmental effect than gasification because of its better energy efficiency. Similarly, Parascanu et al. [73] compared the LCA of four scenarios of gasification and combustion with two feedstock each (agave bagasse and sugarcane bagasse) in Mexico.
The results indicated that, environmentally, agave bagasse combustion is the best option, followed by agave bagasse gasification, sugarcane bagasse gasification, and sugarcane bagasse combustion. A comprehensive LCA was conducted by Sun et al. [77] to compare the environmental performance of converting corn stover to biofuels in fermentation, pyrolysis, and gasification processes. They conclude that the total environmental performance of the system for producing high-grade jet fuel from maize stover by gasification synthesis is optimum. Moreover, fermentation scores poorly in almost all environmental effect categories for 1 GJ of biofuel, whereas pyrolysis has the greatest comparable CO2 emission. Similarly, Tang et al. [91] found that, in comparison with incineration, although gasification-based systems were excellent in mitigating environmental impacts, they had a greater impact on global warming. Muthudineshkumar and Anand [97] reported that for biofuel production from biomass, between gasification and syngas fermentation, gasification reduced pollution emissions and was an ecologically friendly method of fuel use. Nevertheless, in contrast, due to economic and societal problems, Valente et al. [103,104] found that hydrogen from biomass gasification cannot currently be regarded as a viable alternative to conventional hydrogen. On the other hand, considering economic and economic performances separately, environmentally, hydrogen from biomass gasification performs substantially better than hydrogen from steam methane reforming, although the opposite result was reached in economics. Zang et al. [80] examined the technological alternatives of biomass gasification, syngas combustion, and CO 2 emission control in the LCA of eight biomass-integrated gasification combined cycles (BIGCCs). Results showed that the GWP of BIGCC systems is less than 240 kg CO 2 -equivalent/MWh, which is negative when BIGCC systems are integrated with CO 2 capture and storage technology. In addition, the exterior syngas combustion technique has a lower GWP, human toxicity potential, and ozone depletion potential than the internal syngas combustion technology, and the Selexol CO 2 capture [112] method is more environmentally friendly than the MEA CO 2 capture [113] method.
In another approach, two studies addressed by Ouedraogo et al. [90] compared LCA of gasification and landfilling for the disposal of MSW. The LCA found that, in comparison with gasification, landfilling is a significant contributor to global warming, ecotoxicity, eutrophication, acidification, smog formation, and cancer and non-cancer human health outcomes. Finally, Demetrious and Crossin [99] assessed the environmental performance of mixed paper and mixed plastic waste management in landfills, incineration, and combined gasification-pyrolysis using LCA for impacts mentioned in Table 8. According to the data, mixed paper handled with incineration or gasification-pyrolysis created fewer greenhouse gas emissions than mixed plastic managed in landfill. The studies above confirm that it is Environments 2022, 9, 92 14 of 20 impossible to make conclusions about the gasification process because the studies could have opposite results under different methodology, boundaries, or assumptions.
Six studies have investigated a combined process (gasification combined with one or more processes). Through LCA, Reaño et al. [74] evaluated the environmental performance and energy efficiency of rice straw power generation utilizing a combination of gasification and an internal combustion engine (G/ICE). The results showed that the GWP of this process was 27% lower than the GWP of rice straw on-site burning, and that biogenic methane emissions from flooded rice fields may be mitigated to lower the system's GWP by 34%. Using energy generated by the G/ICE system to supply farm and plant activities might reduce the environmental impact and increase the effectiveness of the process. Iannotta et al. [78] investigated the environmental performance of a novel integrated process based on supercritical water gasification and oxidation for treating carbon black and used oil as model wastes. It is demonstrated that this process decreases effects in several categories and results in a positive energy balance during the life cycle, ensuring good environmental performance. Moretti et al. [72] offered the LCA of novel high-efficiency bio-based power technology that combines biomass gasification with a 199 kW solid oxide fuel cell to generate heat and electricity.  [110] It demonstrated superior environmental performance compared to natural gas and the German/European grid. The other two studies were also discussed above [80,99].
In another approach, Li et al. [107] performed a multi-criteria optimization model (TOPSIS) based on LCA for a biomass gasification-integrated combined cooling, heating, Environments 2022, 9,92 15 of 20 and power system to study the overall performance criterion, the primary energy saving ratio, the total cost saving ratio, and the CO 2 emission reduction ratio. It is concluded that the system fueled by biomass greatly differs from that fueled by fossil fuels in energetic, economic, and environmental aspects. Consequently, exclusive assessments and optimizations are required.
The remaining 23 studies have addressed different aspects of LCT (mostly LCA) for a single gasification process in different impact categories. Table 8 gives an overview of covered different impact categories and the life cycle methodologies employed by these articles.

Conclusions
The current research addresses a need left by the absence of thorough reviews on life cycle thinking approaches for gasification processes. Even though the gasification process's environmental and techno-economic aspects are well recognized, measuring their social impacts is still infrequent. Following the PRISMA methodology and a scoping review, 42 studies between 2017 and 2022 were selected. Among different LCT approaches, LCA received the most attention, followed by LCC. In a limited number of studies, exergetic life cycle assessment (ELCA), life cycle energy assessment (LCEA), social impact assessment (SIA), consequential life cycle assessment (CLCA), and water footprint (WLCA) were investigated. It can be concluded that the life cycle impact and cost assessments have received the most attention since 2017. SimaPro ® , GaBi ® , and OpenLCA were employed significantly. The uncertainty analysis was performed in more than half of the studies using sensitivity analysis and Monte Carlo simulation.
Moreover, the results indicate that the recent studies were interested in adopting scenario-based and comparative life cycle assessments. The results confirm that it is hard to draw conclusions regarding the environmental impacts of the gasification process since findings may vary depending on the technique, parameters, or assumptions. Although the gasification process significantly reduces negative environmental impacts, it is not always the best alternative compared to different processes. While these studies suffer greatly from the uncertainties, in future works, it is suggested that uncertainty analysis should be considered in all the investigations.