Producing hydrogen-rich syngas via microwave heating and co-gasification: a systematic review

Co-gasification contributes significantly to the generation of hydrogen-rich syngas since it not only addresses the issue of feedstock variation but also has synergistic benefits. In this article, recent research on hydrogen concentration and yield, tar content, gasification efficiency, and carbon conversion efficiency is explored systematically. In feedstocks with high water content, steam gasification and supercritical hydrothermal gasification technologies are ideal for producing hydrogen at a concentration of 57%, which can be increased to 82.9% using purification technology. Carbonized coals, chars, and cokes have high microwave absorption when used as feedstocks. Moreover, coconut activated carbon contains elements that provide a high tan δ value and are worthy of further development as feedstocks, adsorbents or catalysts. Meanwhile, the FeSO4 catalyst has the greatest capacity for storing microwave energy and producing dielectric losses; therefore, it can serve as both a catalyst and microwave absorber. Although microwave heating is preferable to conventional heating, the amount of hydrogen it generates remains modest, at 60% and 32.75% in single-feeding and co-feeding modes, respectively. The heating value of syngas produced using microwaves is 17.44 MJ/m³, much more than that produced via conventional heating. Thus, despite a lack of research on hydrogen-rich syngas generation based on co-gasification and microwave heating, such techniques have the potential to be developed at both laboratory and industrial scales. In addition, the dielectric characteristics of feedstocks, beds, adsorbents, and catalysts must be further investigated to optimize the performance of microwave heating processes.


Introduction
Future hydrogen consumption is estimated to reach 2.3 Gt/yr in industries that include synthetic chemistry, transportation, buildings, heating, and power storage. Hydrogen can decarbonize approximately 18% of energy-related industries and complement renewable electricity to serve as an essential component of a 100% renewable energy society (Oliveira et al., 2021). At the moment, more than 98% of hydrogen is generated from fossil fuels, namely, 76% from the wet reforming of hydrocarbons and 22% via coal gasification (Lepage et al., 2021). The continuous increases in energy demand, environmental concerns and resource depletion, and fluctuating fossil fuel prices (Okolie et al., 2020a), have generated interest in producing hydrogen from sustainable energy sources such as biomass, municipal solid waste (MSW), and sewage sludge.
Numerous processes have been developed for hydrogen production from renewable feedstocks, including electrolysis, pyrolysis, hydrothermal, gasification, and reforming. Currently, the primary technique for hydrogen generation is steam reforming. However, this method has several problems, including thermal inefficiency, mechanical and maintenance concerns, difficult management, and significant capital investment requirements. There are two primary techniques for producing hydrogen from solid fuels: thermochemical and biological. Pyrolysis, liquefaction, wet reforming, gasification, and supercritical water gasification (SCWG) are plausible thermochemical routes (Shahbaz et al., 2020a). The main advantage of pyrolysis and gasification technologies is the capacity to treat lower-quality raw materials under highefficiency settings (Cormos, 2013). However, both pyrolysis and gasification frequently require external heat, and the hydrogen produced contains various impurities, such as tar, coke, CH₄, CO₂, N₂, O₂, CO, NH₃, and HCl. As a result, technologies for producing hydrogen-rich syngas from solid fuels and MSW are critical for the future. Without neglecting the importance of other factors, there are at least four areas with the potential for continued development: technology, raw resources, economics, and the environment.
Several technologies and methods for converting biomass to syngas with a high concentration of hydrogen have been thoroughly studied. Biomass can be converted to hydrogen chemically by examining the effects of reaction factors such as conversion techniques, influencing variables, and catalyst types on hydrogen generation and carbon conversion efficiency (CCE) . The CCE and hydrogen production are influenced by feedstocks, reactors, temperatures, catalysts and agent/feedstock ratios. Natural catalysts present in ash, particularly alkaline earth metal compounds, facilitate CCE and boost tar cracking to create hydrogen from biomass via environmentally friendly routes. Metal catalysts and oxides increase hydrogen production by steam-reforming techniques and C-O and C-C bond splitting . Several other studies have examined the synthesis of hydrogen from biomass by thermochemical processes, most notably pyrolysis and gasification (Pandey et al., 2019). Meanwhile, CO-rich syngas that can particularly be obtained using CO₂ as gasifying agent has been evaluated (Chan et al., 2021). Hydrogen-rich syngas production from biogenic, polymeric, and petroleum wastes has been extensively studied using sub-and super-critical hydrothermal gasification. This method is particularly well-suited to raw materials with high water contents because it eliminates the need for drying and dewatering, thereby lowering operating costs (Okolie et al., 2020a). Along with the conversion technology, hydrogen-rich syngas can be generated by integrating equipment and purification technologies such as membranes. These are widely available at lower operating costs than other gas-purifying technologies but still require further development in terms of selectivity and permeability and to minimize constraints to achieve practical feasibility (Shahbaz et al., 2020a). Additional options include the use of a gasifier in conjunction with reforming (Demirbas, 2016;Shahabuddin et al., 2020) or the co-gasification of glycerol and lignocellulosic biomass (Đurišić-Mladenović et al., 2016).
Although the potential for raw material usage is considerable, biomass is available in various forms, which imposes limits on its homogenous and steady supply. To compensate for variations in raw materials, a co-feeding technique has been introduced for use with pyrolysis, gasification or hydrothermal processes that combine several types of feedstocks. A cogasification review addressed syngas produced by the co-feeding of petroleum coke and coal , coal and biomass (Valero and Usón, 2006 (Inayat et al., 2019). It was observed that while the routes from unusual solid fuels to hydrogen-rich syngas have been extensively explored, they have seldom been reviewed from the perspective of microwave heating. Microwave gasification has been studied, emphasizing catalyst roles, syngas generation, CCE elimination, and suggestions for ideal settings (Arpia et al., 2022).
Thus, the present study's objectives were to 1) provide a detailed bibliographic analysis of gasification and co-gasification technology for producing hydrogen-rich syngas and 2) compare the use of conventional and microwave heating in syngas production according to studies published between 2012 and 2022. Thus, this study is intended to provide an understanding of current knowledge to direct future research, particularly regarding technology for hydrogen production from gasification and cogasification utilizing conventional or microwave heating. During the systematic review, a bibliographic coupling analysis and text mining analysis were performed using VOSviewer software. A time overlay and clustering visualization of keywords, countries and authors making significant contributions to the field of hydrogen-rich syngas from gasification are systematically reviewed in this study. Table 1 compares the current review's findings on many elements of hydrogen-rich syngas production from gasification to those of earlier review articles published between 2012 and 2022. To our knowledge, there are relatively few studies of hydrogen-rich syngas generation that incorporate the notion of cofeeding with microwave heaters.
The following ten groups of research questions explain why in-depth investigations should be conducted: ➢ What function does microwave technology play in gasification and co-gasification? ➢ What factors must be addressed in order to maximize the effect of microwave heating on gasification and co-gasification? ➢ What effect do feedstocks have on the hydrogen content of syngas? ➢ Which feedstocks have been investigated in terms of gasification and co-gasification using microwave or conventional heating to produce hydrogen-rich syngas? ➢ How can we identify co-feeding configurations that are ideal for hydrogen-rich syngas produced by microwave-heated co-gasification? ➢ Is there a synergistic effect when co-gasification is used? ➢ What effect does the gasification agent have on the hydrogen content of syngas? ➢ Which gasification agents have been investigated in connection to gasification and co-gasification using microwave or conventional heating to produce hydrogen-rich syngas? ➢ What function do catalysts play in the gasification and co-gasification of hydrogen-rich syngas under conventional and microwave heating? ➢ What research gaps remain, and do they need further investigation to guide future studies?

Portfolio articles
The PRISMA method was used in this systematic review (Moher et al., 2009), as shown in Figure 1. The initial step was to identify a research topic on hydrogen-rich syngas by searching Google Scholar, which generated 2,610 articles. If only publications published after 2012 are included in searches, the number of research articles on hydrogen-rich syngas themes drops to 1,510 on Google Scholar and 664 on ScienceDirect.

Data synthesis and analysis
A total of 76 high-impact publications were meticulously compiled and analyzed in accordance with the study's objectives. Some secondary data were retained, particularly those published before 2012 that were relevant to the issue under discussion, bringing the total to 140 articles. To begin, an overview of hydrogen-rich gasification and co-gasification is provided. Second, focus research was undertaken on microwave gasification and co-gasification to determine their current research status, including any shortcomings. In this second emphasis, data on the major factors affecting the synthesis of hydrogen-rich syngas via microwave and conventional heating were gathered and analyzed. Third, the function and method of co-gasification for generating hydrogen-rich syngas via microwave heating was explored and analyzed. This procedure yielded a summary of the fundamentals of microwave co-gasification, the main findings, many factors affecting hydrogen-rich syngas, and critical information on microwave cogasification. Figure 2 depicts an analysis of keyword co-occurrences conducted using VOSviewer software. There were 402 thresholds of 11,288 keywords and four clusters with a minimum occurrence of 20. Biomass had the most occurrences (842), followed by 15 additional keywords in the following order: gasification, pyrolysis, hydrogen, hydrogen generation, catalysts, carbon dioxide, synthesis gas, tar, steam reforming, carbon, steam, biomass gasification, chemical reactors, fluidized beds, and temperature. It has been observed that biomass feedstocks are used more frequently in the production of hydrogen-rich syngas. Steam is the most extensively examined gasification agent. Carbon dioxide, catalyst, and tar were also among the issues that received much attention from researchers between 2012 and 2022. Meanwhile, the numbers of occurrences of hydrogen-rich gas, microwaves, co-gasification, and economy were 56, 86, 61, and 50, respectively. Moreover, there were 504, 142,831,61,22,81,15,86, and 13 occurrences of hydrogen, syngas, gasification, co-gasification, hydrothermal gasification, supercritical water gasification, plasma gasification, microwave, and dielectric, respectively.

Bibliometric Analysis
The sizes and colors of the keywords in Figure 2 correspond to their occurrence and clusters encountered. The wider the circle, the more common the occurrence. Meanwhile, the tighter the link between the keywords depicted in the figure, the closer they are in relation to each other. To better understand the investigation's quantitative findings, a qualitative content analysis using VOSviewer based on four clusters was done. Cluster 1 is red and is dominated by research on exergy, energy efficiency, fossil  fuels, investment, supercritical water, raw materials, and life cycles, as determined by text mining analysis. Cluster 1's keyword distribution appears to be more spread out than those of the other clusters. Cluster 2 is green, with a higher frequency of occurrence than cluster 1, and is associated with studies on pyrolysis, catalysis, bio-oil, and biofuels. Additionally, Cluster 2 demonstrates that bio-oils and biofuels are extensively explored in relation to the catalytic pyrolysis process. Cluster 3 (blue) also includes a high prevalence of issues related to hydrogen production and catalyst research. Cluster 4 (yellow) has a high number of references to biomass, gasification, and steam gasification. Figure 3 depicts a chronological overlay of keywords from publications with the keywords gasification and hydrogen-rich. The colors in Figure 3 represent the average date of publication. Blue represents earlier publications, whereas yellow represents more current publications. Yellow phrases, such as investment, economy, life cycle, and sustainable development, with an average date of publication of 2017, indicate that this research topic is relatively new and under-researched. The two primary keywords employed in this review, microwaves and co-gasification, had average publication dates of 2016. It can also be noted that the term microwaves is explored together with biofuels, heating, bio-oil, pyrolysis, catalysts, biomass, energy efficiency, carbon dioxide, and gas synthesis. Meanwhile, co-gasification is extensively researched together with gasification, carbon dioxide, biomass, coal, and hydrogen. It may also be noted from Figure 3 that microwaves and co-gasification are still used and studied separately.
Time overlay visualization of countries that published hydrogen-richand gasification-related papers between 2012 and 2022 is shown in Figure 4. China appears to have the most publications, followed by India, Malaysia, Iran, Canada, the United States, Germany, and South Korea. With an average publication date of 2019, Canada, Saudi Arabia, and Qatar are the nations that are pursuing intense gasification and hydrogen-rich research.    Table 2 Figure 7. New issues, such as synthesis gas, catalysts, hydrogen generation, supercritical water, thermochemistry, and gasification efficiency, are highlighted in yellow. As seen in Figure 8, the 216 papers found with Google Scholar may then be classified using the primary keywords hydrogen-rich, gasification, cogasification, and microwave published after 2012. The vast majority of researchers (> 54%) are interested in microwave gasification, whereas just a few (1%) are interested in microwave co-gasification. There are only two articles containing the keywords microwave, gasification, and hydrogenrich. In addition, there are no articles containing the keywords microwave, co-gasification, and hydrogen-rich. In publications with a high impact, 76 were gathered and utilized as a portfolio in this study's systematic review, which is anticipated to offer a full grasp of the study's breadth and uncover knowledge gaps (Xiao and Watson, 2019). Figure 9 illustrates the word connections that were often referenced in titles, abstracts, and keywords in   The data for the most cited articles in the field of hydrogen-rich syngas generated by gasification. the 76 publications. As can be seen, syngas, co-gasification, waste, catalyst, hydrogen production, and steam gasification were more often referenced while fewer references were given to coal, plasma, activated carbon, CH₄, CaO, hydrogen yield, and supercritical water gasification. In the meanwhile, greenyellow keywords such as co-gasification, catalyst, hydrogen-rich syngas, microwave, and supercritical water gasification have been researched in recent years.

Microwave-assisted gasification and co-gasification
Co-gasification is a method of producing synthetic gas from materials or fuels. H2, CO, CO2, and N2 are the major mass fractions of syngas generated by this process. This is accomplished by heating feedstocks to temperatures > 700 °C and regulating the oxygen and/or steam quantity present in the reaction. Syngas comprises a variety of components, most notably hydrogen, and the amount of gas produced is determined by heat, which can be internal or external in nature.
Internal heating is generated by the exothermic reaction of some gasification processes. Heating from the outside is often accomplished by using an electric heater, hot gas or hot steam. Plasma technology has also been used for heating during gasification and for tar cleaning throughout its development. Microwave heating is beginning to draw the attention of many academics and is also frequently employed in the industry due to its capacity to permeate and be absorbed by radiation-exposed materials. Compared to traditional heating technologies, such as electric or gas heaters, microwaves are capable of more rapid and uniform heating of materials (Ahmad et al., 2001). Overall, microwaves have started to be widely employed in the area of gasification due to their exceptional properties (Ke et al., 2019).
Microwaves are a form of electromagnetic radiation with relatively high frequencies of 0.3-300 GHz (Suard et al., 1996). In practice, the most commonly used frequency is 2.45 GHz (Hrycak et al., 2014), with wavelengths ranging from 1 to 1000 mm (Beneroso and Fidalgo, 2016). Microwave generators typically comprise a high-voltage source, a control unit, and a magnetron head (Hrycak et al., 2014). The magnetron head is cooled by air or water, safeguarding it against the harmful effects of reflected microwave radiation.
Microwaves used in gasification have been demonstrated to increase gas output. Numerous feedstocks have been investigated for use in microwave gasification in order to generate syngas, most notably, coffee hulls In the case of microwave heating, a one-or two-dimensional model is sufficient to provide an accurate simulation. This can incorporate mass and heat transfer models and consider volume changes in the affected material (Zhang et al., 2020a). Microwave gasification principles, methodologies, relevant factors, advantages, and downsides have been studied in order to create syngas with high hydrogen concentrations (Ho et al., 2017). However, there are no studies or microwave gasification reviews on cogasification, particularly techniques capable of producing high hydrogen syngas concentrations. While microwaves have been frequently used to generate hydrogen-rich syngas, the process is not yet well understood, necessitating a search for the state of the art.
Apart from being used for homogeneous heating processes, microwaves are now being extensively studied for heating heterogeneous reactions (Hunt et al., 2013;Guler et al., 2017;Bai et al., 2018). Microwaves radiate into a solid's core in heterogeneous processes. The presence of an electric field, on the other hand, leads to the dipoles (δ) being more unidirectional with respect to the solid, as seen in Figure 10. Tan δ quantifies the microwave power attenuation in a material that results in heating (Omar et al., 2011). Solids with a low tan δ value (< 0.1) have low microwave absorbance, while solids with a high tan δ (> 0.5) have a high capacity for microwave absorption (Beneroso and Fidalgo, 2016). It is critical at this step to select feedstocks with a high, medium, or low tan δ value. Materials with a low tan δ value should not be excluded from use in microwave heating systems because they can be driven by a high-intensity electric field.
The primary distinction between traditional and microwave heating is the manner in which heat is produced. In conventional heaters, heat is transmitted by conduction from the surface to the core, resulting in a significant temperature differential for solids with a Biot number > 0.1. As a result, the reaction begins on the surface of the solid and progresses inwards. Microwave heating creates heat at the molecular level and starts exactly from the material's interior. Microwave radiation is absorbed by substances with a tan δ > 0.5 and is converted to heat, resulting in consistent interior heating. Additionally, microwave radiation has a high rate of penetration into materials, allowing it to generate heat more quickly than traditional heating. Conventional heating and microwaves are mainly different in terms of the way heating occurs. In conventional heating, heat is transported from the solid surface to the core by conduction so that the solid with a Biot number > 0.1 will have a considerable temperature gradient. As a result, the gasification reaction begins on the surface of the solid and gradually works inward. In microwave heating, microwave energy generates heat at the molecular level, which starts precisely from the inside of the solid. Microwave energy is absorbed by solids (tan δ > 0.5) and converted into heat, consequently producing uniform internal heating. In addition, microwave radiation also gets into solids rapidly so that it can produce heat faster than conventional heating (Xie et al., 2014). When microwaves are absorbed by the solids' interior, it generates hot spots that enhance heterogeneous reaction rates.
Heterogeneous reactions involve a gas phase and solid phase. Some heterogeneous reactions that are important in gasification include the Boudouard reaction (Eq. 1) and the water-gas reaction (Eq. 2).

C + CO2  2CO
H298K = 173 kJ/mol Eq. 1 C + H2O  CO + H2 H298K = 132 kJ/mol Eq. 2 The presence of carbon atoms causes microwave heating to be more efficient than conventional heating. Along with material carbon, the presence of potassium-containing ash contributes significantly to the heating and acceleration of homogeneous and heterogeneous processes . Additionally, potassium-containing ash functions as a catalyst. A similar phenomenon occurs when a CaO-Zr/H-ZSM-5 catalyst is employed to increase hydrogen production by up to 61% . This has been demonstrated to accelerate the following homogenous reaction, called dry reforming of methane (Eq. 3).
The hydrogen content of syngas is seen in Table 3. It is clear that steam is the preferred gasification agent for the formation of hydrogen-rich syngas. The greatest hydrogen levels of 82.9% are observed under conventional heating when the gasification reactor is paired with a syngas purification reactor. Hydrogen levels are greatest at 57% during co-gasification. Microwave heating produces the greatest hydrogen levels under single-feeding and co-feeding, at 60% and 32.75%, respectively. While the systems and heating methods are rather fascinating, it is clear that there is still a lot of unexplored territory in terms of producing hydrogen-rich syngas, particularly via co-gasification with microwave heating.
A high hydrogen yield does not always imply a high hydrogen concentration. As demonstrated in Table 4, high hydrogen yields of > 50 g H2/kg feedstock may be produced by co-gasification of sludge and biomass, supercritical biomass gasification, microwave-pyrolysis of gumwood, and steam gasification of lignite. This indicates that microwave technology and steam agents have a substantial influence on the creation of high hydrogen yields. Organic non-coal materials produce a large amount of hydrogen, 120 g/kg (Parvez et al., 2019), compared to 130.87 g/kg from coal (Zhao et al., 2021a).

Effect of feedstock and blending ratio on hydrogen-rich syngas
One concern of the public is MSW. Implementing gasification power Table 3. Hydrogen content in syngas from gasification using conventional and microwave heating.

Conventional
Microwave Ref.

Single feeding
Co-feeding Single feeding Co-feeding Aspects of raw materials, particularly changes in their composition in terms of both quality and quantity, provide several challenges to researchers, one of which is handled through the notion of co-feeding. Materials of low quality, such as MSW and biomass, as well as low-grade sludge, can be combined with high-grade fuels such as coal or polyethylene. Co-gasification of MSW or biomass with high-grade coal is an exciting possibility for zero-emissions fossil-fuel power plants while also contributing to energy security (Cormos, 2013).The next commonly explored feedstock is sludge, which is incorporated in low-grade fuels. Sludge and coal co-gasification has been studied ( . The consensus is that adding wet sludge to coal boosts its catalytic reactivity. This is most likely due to the high mineral content of wet sludge. However, adding wet sludge to coal increases the release of contaminants such as H2S, NH3, and HCl (Akkache et al., 2016). Additionally, the co-gasification of sewage and textile sludge has been studied and demonstrates synergistic benefits (Nguyen and Chiang, 2021). Abundant biomass is the potential feedstock for renewable solids. Cofeeding low-grade biomass with plastic has been proposed to avoid operational issues associated with plastic gasification, i.e., plastic feeding and minimization of the quantity of fine dust produced due to the incomplete gasification of waste plastics (Pinto et al., 2002). The improved results of co-gasification processes can be attributed to an improvement in the pre-gasification plastic pyrolysis stage. Alkali metals present in biomass ash have a catalytic influence on the cogasification of biomass and plastic. Along with plastic, biomass co-gasification with pet coke positively impacts the mixture's reactivity, lowering both the emissions and reaction times of NOx, CO2 and SOx Microalgae are a form of biomass that draws much research interest as they do not compete with foodstuffs. They have also been researched in relation to cogasification with bituminous coal (Fermoso et al., 2018). Biomass can first be treated into char or activated carbon and then co-gasified with coal to improve the microwave heating performance (Wang et al., 2022). The development of co-gasification has demonstrated the synergistic impact of using two raw materials and demonstrates, in several ways, that cogasification may overcome some of the challenges associated with individual gasification and increase the quality of syngas. There is a very limited number of studies in the existing literature on microwave co-gasification such as the work on palm kernel shells and coal by Ahmad et al. (2019), indicating that microwave co-gasification is still a relatively new field of study.
The major components of lignocellulosic biomass and coal are cellulose, hemicellulose, and lignin. Dry biomass generally comprises 40-50% cellulose, 25-35% hemicellulose, and 10-30% lignin. Carbohydrates hydrolyze under hydrothermal conditions to produce monomers, which are then further degraded at higher temperatures. It has been established that cellulose begins to degrade at 180 °C (Yang et al., 2020) and that its decomposition rate increases with temperature. The temperature of 180 °C has also been discovered as a critical turning point in the breakdown of cellulose when using microwave heating (Budarin et al., 2010). The reaction path for the thermal degradation of cellulose in SCWG, hydrothermal gasification, and microwave-assisted gasification is shown in Figures 11a, b, and c, respectively. Meanwhile, hemicellulose is a biopolymer that surrounds a cellulose skeleton but is different from cellulose in content and structure. Hemicellulose decomposes more quickly in water due to the presence of side chains and less homogenous structure, and hemicellulose degradation occurs between 180 °C and 340 °C (Yang et al., 2020). Unlike cellulose and hemicellulose, lignin has an aromatic heteropolymer matrix that adds to the strength and rigidity of cell walls.
The first stage in developing a co-gasification plan for hydrogen-rich syngas is to identify source materials readily available locally in significant quantities. The following stage is to choose raw materials with a considerable amount of volatile or fixed carbon, as shown in Table 5. This stage is critical because selecting the appropriate feedstocks for co-feeding is a multi-pronged strategy toward building large plants while lowering unit operating costs, increasing efficiency, and improving emissions control (Lopez et al., 2015). However, feedstocks selected only based on their volatile or fixed carbon content may still cause problems during cogasification since high operating temperatures, such as 800-900 °C, can result in the occurrence of ash slagging and fouling (Akkache et al., 2016). To address slagging and fouling, further research on slagging and fouling indices is required. For example, for olive pomace and wood scraps, the computed indexes of slagging and fouling are high. Wet sludge has moderate fouling and slagging values. On the other hand, Plastic has a low slagging and fouling index.
A critical stage in co-gasification under microwave heating is the selection of feedstocks with high dielectric characteristics. This is because microwave energy does not travel mostly by convection or conduction, as is the case with conventional heating, but rather through dielectric loss. Thus, relying solely on proximate and ultimate analyses to determine the characteristics of feedstocks might result in failures in co-gasification systems aimed at yielding hydrogen-rich syngas. As a result, multiple researchers have undertaken experiments to assess the dielectric   characteristics of various feedstocks, as well as the parameters of beds and catalysts, as illustrated in Table 6. According to Table 6, the poor dielectric characteristics of empty fruit bunches with an 18% moisture content, oil palm fibre, char from oil palm fibre, and coal necessitate the inclusion of microwave absorbers to assist in heating (Omar et al., 2011;Salema et al., 2013;Zhang et al., 2020 b). Coals, in general, have low microwave absorber contents because they lack graphene lattices that allow delocalized π-electrons to travel and pair with the microwave electromagnetic field. Thus, only carbonized coals, chars and cokes display strong microwave absorbance qualities due to the presence of a significant number of delocalized electrons moving on the graphene structures (Beneroso and Fidalgo, 2016). The conversion of biomass to activated carbon is a more environmentally friendly method of producing hydrogen, which also produces a valuable co-product (Demirbas, 2016). Interestingly, coconut activated carbon contains elements that provide a high tan δ (Omar et al., 2011); as such, they are worth further development as feedstocks, adsorbents, or catalysts.
Apart from feedstocks, the conversion of microwave energy to heat can be accomplished by selecting appropriate beds, adsorbents, or catalysts. The inclusion of a high concentration of silica and a moderate concentration of calcium oxide may contribute to a low loss value since these materials are known to be inert under microwave heating (Haque, 1999). Another study showed that FeSO4 has the greatest capacity for storing microwave energy and exhibiting dielectric loss, attenuation speed and microwave intensity, followed by CaCl2 and K2CO3. As a result, FeSO4 and CaCl2 can serve as both catalysts and microwave absorbers (Zhang et al., 2020 b).
It is reasonable to predict that microwave heating results in high radiation penetration into a substance but a low conversion to energy. The polarization factor related to free and bound water relaxation may be the major determinant of the intermediate dielectric constant value. A low dielectric loss value might result from extremely little conduction and the Maxwell-Wagner effect (Metaxas and Meredith, 1983). Due to the fact that a molecular arrangement is dependent on the physical state of a material, whether solid, liquid or gas, dielectric characteristics may also vary. As a result, the dielectric polarization of compounds is dependent on their dipole moment. Due to fast molecular rotation, high dielectric constants are seen in gases and liquids. In contrast, because molecular rotation in solids is constrained, an electric field has a smaller effect on their dielectric characteristics. It is critical to note that the dielectric properties of materials, particularly the tan δ value, are affected by the amount of water present, the temperature, and the microwave frequency used, implying that microwave gasification and co-gasification remain viable for hydrogen-rich syngas research.

Effects of gasification agents and equivalence ratio on hydrogen-rich syngas
Numerous gasification agents have been reported for the production of hydrogen-rich syngas, including air, oxygen (Xu et  The equivalence ratio (ER) is the ratio between the actual gasification agent-fuel mixture and the stoichiometric air-fuel mixture required for complete combustion. Of pyrolysis and air vapor gasification, the latter produces significantly more hydrogen from biomass, since the combination of water and char results in the decomposition of medium biomass products, resulting in the production of hydrogen-rich gases during the subsequent water gas shift reaction (WGSR), as shown in Eq. 4.

CO + H2O  CO2 + H2
H298K = −41.5 kJ/mol Eq. 4 Compared to air or steam gasification, SCWG can process wet biomass directly without drying and achieves high efficiency at relatively low temperatures. SCWG gasifies biomass at a succession of temperatures and pressures over a range of residence durations, producing a mixture of H2, CO2, CO, and CH4. However, the cost of hydrogen synthesis from wet biomass via SCWG is many times that of hydrogen produced by steam methane reforming (SMR) (Demirbaş, 2005).  Gasification agents that have been tested with microwave gasification technology include CO2 (Lahijani et al., 2014;Chun and Song, 2020), steam (Djebabra et al., 1991;Sekiguchi and Orimo, 2004;Yoon and Lee, 2012;Shin et al., 2013;Liu et al., 2019), and air (Yoon and Lee, 2012;Yoon et al., 2013a). Steam gasification agents continue to outperform air and CO2. This is in addition to the fact that a steam agent is capable of producing hydrogen-rich syngas.
An ER of 0.35 results in a high lower heating value (LHV) of 12.2 MJ/m 3 during coal gasification using air agents. LHV concentrations decrease to 10.9 and 10.5 MJ/m 3 at ER = 0.25 and 0.45, respectively. Low ER values indicate a lack of air, which may result in a lack of CO2 and H2O reactants. On the other hand, if an overabundance of oxygen leads to an increase in CO2 in syngas, the LHV decreases. Due to the increased oxidation of hydrocarbons, particularly tar micromolecular hydrocarbons, an improved ER appears to result in an increase in gas production. Additionally, in addition to LHV, changes in ER have a major influence on cold gas efficiency (CGE) and CCE . The ER is typically chosen as 0.1-0.5, with 0.2-0.3 being the best range for producing hydrogen-rich syngas (Schmid et al., 2021). Alternatively, it is also common to use steam-to-carbon ratio (S/C) in steam gasification instead of ER. A low S/C can lower char conversion and increase tar yields. As H2O encourages the balanced creation of hydrogen and carbon dioxide, the shifting reaction of water gas is significantly impacted by the S/C. As a consequence, raising the S/C ratio by 1.5-2.5 can increase H2 outcomes as well as H2/CO ratios, which are critical for downstream synthesis processes (Schmid et al., 2021).

Effects of temperature and pressure on hydrogen-rich syngas
High temperatures are observed to improve hydrogen and carbon monoxide concentrations, carbon conversion, and CGE during gasification and cogasification processes (Emami-Taba et al., 2012). The fundamental reason for this is that the key processes in gasification ( Eqs. 1 and 2) are endothermic, which means that the rate of the gasification reactions increases with temperature. As a result, temperatures > 700 °C are widely utilized. In gasification and co-gasification processes involving some types of biomass, too-high temperatures are undesirable due to the presence of ash, which may melt at high temperatures.
When biomass or coal solids are heated traditionally, the gasification process commonly results in the melting of chlorides (CaCl2 and MgCl2), formed when metal oxides react with HCl in char. Slagging develops due to the melting of this ash, which significantly lowers the activity of heterogeneous processes and tar-cracking reactions . CaCl2 and MgCl2 have melting temperatures of 782 °C and 714 °C and boiling points of 1600 °C and 1412 °C, respectively. As a result, the working temperatures of gasification and co-gasification are between 700 °C and 1000 °C, which are between the melting and the boiling points of CaCl2 and MgCl2, resulting in coal char slagging and agglomeration. Owing to the existence of slagging and agglomeration, the Brunauer-Emmett-Teller (BET) specific surface area of coal char of 127.6 m 2 /g is reduced to 74.9 m 2 /g (Kai et al., 2017).
If the goal is to optimize the output of hydrogen-rich syngas produced by gasification or co-gasification, a moderate temperature, slow heating rate, and long residence time are recommended. The most significant impact is temperature, which affects most of the response variables assessed during gasification and co-gasification processes. Increased temperatures are beneficial in lowering the amount of tar in the syngas and maximizing gas production, efficiency, and carbon conversion (Gil-Lalaguna et al., 2014). Research indicates that the energy rates of heated gases are 23% greater using microwaves than with conventional heating (Parvez et al., 2019).
Pressure, on the other hand, plays a crucial role in conventionally-heated gasification (Wolfesberger et al., 2009), microwave-assisted gasification (Wnukowski et al., 2021), hydrothermal gasification, and SCWG (Sheikhdavoodi et al., 2015). In conventionally-heated gasification, high temperatures of up to 950 °C and pressures of up to 10 bar result in lower tar concentrations (Wolfesberger et al., 2009). Gasification of supercritical water at 25 MPa and temperatures of 400-800 °C with a sugarcane bagasse raw material has been investigated in batch reactors at a constant pressure of 25 MPa, with the largest quantity of hydrogen (75.6 mol kg -1 ) obtained at 800 °C (Sheikhdavoodi et al., 2015). In practice, temperature, pressure, feedstock type (coal, biomass or their blends), and ER all affect hydrogen-rich syngas production. Furthermore, while all of these factors are important and interconnected during gasification, temperature is most vital.

Catalytic effect on hydrogen-rich syngas
Microwaves are inextricably related to catalysts, just as they are with feedstock. Catalysts speed-up reactions in a unique way. The catalyst's role is to lower the activation energy sufficiently to allow the majority of particles to react. Catalysts can reduce a reaction's activation energy by guiding the interacting particles in a way that maximizes the probability of successful contact. The second technique entails interacting with reactants in order to develop intermediates that require less energy to convert to products.
K2CO3 is the preferred alkali catalyst for coal SCWG due to its high catalytic activity and low cost . The yield of H2 improves when K2CO3 is used as a catalyst. However, the addition of K2CO3 introduces ash aggregation. The formation of a K-bearing eutectic leads to a reduction in carbon gasification efficiency and long-term shutdown of the reaction system (Mastuli et al., 2019). It has also been discovered that when NaOH and KOH are used as catalysts for SCWG of bagasse, a significant rise in H2 generation occurs (Sheikhdavoodi et al., 2015). Nonetheless, K2CO3 is less appropriate for gasification and cogasification by microwave heating due to its limited capacity to convert microwave energy to heat, as measured by tan δ = 0.031 (Zhang et al., 2020b).
When applied to flue gas streams from solid fuel combustion, calcium refining is believed to be an effective technology for absorbing CO2 and eliminating contaminants (Dashtestani et al., 2021). CaO facilitates the development of low-melting-point albite and anorthite. However, more Al2O3 addition leads to the formation of high-melting-point mullite, which is why alkaline materials derived from Al2O3 are not extensively utilized as catalysts in co-gasification processes (Shi et al., 2021). The key drawback of CaO sorbents is that their ability to absorb CO2 is readily diminished by up to 80% after many cycles, leading to a shift in the CO2 reaction's equilibrium (Granados-Pichardo et al., 2020). This is because of the sintering and clumping of CaO particles, which results in a decrease in surface area (Gao et al., 2017). Meanwhile, magnesium oxide (MgO) can operate as a framework and limit the sintering and agglomeration caused by the interaction of CO2 with CaO-based sorbents at elevated temperatures (Papalas et al., 2021). CaO and MgO are added to WGSR to alter the thermodynamic equilibrium by collecting CO2 and increasing H2 generation (Bunma and Kuchonthara, 2018).
Noble metal-based catalysts are generally claimed to be less susceptible to coking than catalysts based on transition metals (e.g., Ni, Fe, Co, Pd, etc.) (Rostrupnielsen and Hansen, 1993;Effendi et al., 2003). While some noble metals exhibit excellent selectivity, their high cost and scarcity preclude their practical usage. As a result, different catalysts have been utilized to produce hydrogen-rich syngas, such as K/Ca (Koido et  However, because the dielectric characteristics of these catalysts are unknown, there is still huge potential to investigate them further for use in microwave heating. Along with catalysts, solid materials, such as bed and adsorbent, are employed to absorb heat during gasification. Calcination dolomite is utilized as bed material. Meanwhile, activated carbon is used as a tarcracking ingredient in syngas production with a low-tar and high-hydrogen content . The quantity of activated carbon adsorbent seems to have a significant effect on tar removal and H₂ generation. When calcined dolomite and activated carbon are used as the fluidizing bed material, the H₂ concentration is 26.1 vol%, and the total tar content is almost 96% less than when silica sand is used (Cho et al., 2013).

Heating value of hydrogen-rich syngas
Meanwhile, previous research has established that extremely high temperatures reduce the concentrations of combustible gases, such as CO, CH4, and H2, resulting in a decrease in LHV. At temperatures between 800 and 1000 °C, the energy content of syngas decreases from 12.2 to 10.6 MJ/m 3 . Syngas LHV of 12.2 MJ/m 3 is attained at ER 0.35. Moreover, the cold gas conversion efficiency is 63.4%, and the CCE is 66.0% at a gasification temperature of 1000 °C . At elevated temperatures, the Boudouard reaction is the dominant reaction, followed by the WGSR . A high temperature must be addressed to avoid the slagging and blockage of chlorides in the coal char . High LHV values of syngas correlate with the gasification reactor's efficiency. The efficiency of cold gasification is defined as the ratio of energy contained in the gas products to that in the raw materials. Experimental values for CGE range from 39% to 66%. As shown in Table 7, microwave heating enables high LHV values of up to 17.44 MJ/m 3 (Lupa et al., 2013), indicating that this technique is worth developing to create syngas rich in hydrogen with a high hydrogen yield, high heat value, and improved CGE and CCE. Table 7.
Heating value of syngas produced from gasification.

Economic and environmental sustainability aspects
Depending on the designation, air gasification that produces low-heatingvalue syngas of 4-6 MJ/m 3 and 8-14 vol% H2 can be improved in quality by using a pure oxygen supply, which has cost and safety consequences. Biomass steam gasification can generate gas with a medium caloric value (10-16 MJ/m 3 ) and, in particular, gas with a high hydrogen content (30-60 vol%) (Xiao et al., 2011). Therefore, the most expensive method is oxygen gasification, followed by steam and air gasification in terms of expense (Hosseinzadeh et al., 2022). Biomass gasification, a keyword that frequently appears in cluster analysis on VOSviewer, has feedstock prices, feedstock costs, and production costs of US$2-5/GJ, US$2.9-7.1/GJ H2, and US$5-6/GJ H2, respectively. Hydrogen produced through the biomass gasification and coal gasification processes is still more expensive than that produced through SMR (Parthasarathy and Narayanan, 2014).
Keywords plasma gasification and supercritical water gasification have attracted much study, which has revealed certain issues, as shown in Figure 6. The cost of creating hydrogen is several times greater than with steam gasification or SMR (Parthasarathy and Narayanan, 2014; Udomsirichakorn and Salam, 2014). Nonetheless, SCWG promises to convert wet biowaste directly into hydrogen without the need for an energy-intensive pre-drying step (Su et al., 2022). As shown in VOSviewer (Fig. 1), the keywords economic analysis and hydrogen production have different clusters. This implies that the cost of creating hydrogen gas at the capacity of the fluidized beds used in basic research is now uncertain because little data is available in the literature. However, without such economic figures, full techno-economic analyses are not yet conceivable (Fremaux et al., 2015). Meanwhile, it is expected that hydrogen-rich syngas will be cost-competitive in the long run. Fluctuation in feedstock prices and the projected usage of CO2 capture and storage (CCS) in fossil fuel-based processes must be considered. As a result, the development of co-gasification solutions with diversified feedstocks and competitive costs may become very appealing, particularly at hydrogen-production scales > 100 t/d (Udomsirichakorn and Salam, 2014).
From an environmental standpoint, the tar and CO2 produced during the gasification process are unavoidably problematic. When the hydrogen extracted from a stream of hydrogen-rich gases includes impurities such as CO2, CH4, CO, and tar, this might cause large expenditures in commercial applications. Thus, if CO2 and tar produced during gasification can be caught and cracked simultaneously, these technologies become substantially more desirable and cost-effective for hydrogen generation (Udomsirichakorn and Salam, 2014). However, capturing the CO2 created adds roughly 25-30% to the original cost of hydrogen synthesis.
In terms of environmental sustainability, conventional gasification produces less CO2 than plasma gasification. Coal gasification generates 18.0 kg CO2eq/kg H2 (Chen et al., 2019), whereas plasma gasification generates 31 kg CO2eq/kg MSW (Ramos et al., 2019). Furthermore, a comparison of various research indicates that the results of the life cycle analysis of biomass-tohydrogen processes are superior to those of the life cycle analysis of coal-tohydrogen production. According to this research, the life cycle energy consumption of the biomass-based hydrogen-production process is roughly one-fourth that of the coal-based process.
Finally, attention must be paid to the existence of tar, in addition to the hydrogen concentration, hydrogen yield, LHV, CGE, and CCE. Tar is one of the difficulties that must be solved in syngas production since it can clog gasifier equipment, piping, and power plant systems. The tar content of raw gas can be decreased from around 40 g/m 3 to 10 g/m 3 by utilizing a two-stage gasification technique in conjunction with Ca impregnation. Tar with a concentration of 17.6 g/Nm 3 is capable of co-gasifying MSW and coal char in a three-stage gasification process . Co-gasification may also be utilized to reduce tar from 58.23 to 9.74 g/Nm 3 when 50% high-density polyethylene (HDPE) is used as a feedstock (Lopez et al., 2015). A reduced tar concentration of 5.8 g/Nm 3 was successfully generated from three decoupled reactors co-gasifying biomass/coal feedstock at 850 °C utilizing steam agents (Tursun et al., 2016). Thus, it is worthwhile to continue developing cogasification systems with microwave heating that generate syngas with a high hydrogen content, high hydrogen yield, high CGE/CCE ratio, and low tar level.

Conclusions and future works
We conducted a thorough assessment of gasification and co-gasification systems that use either conventional or microwave heating to produce hydrogen-rich syngas. The factors of hydrogen concentration, hydrogen production, CGE, CCE, and tar content were considered. The following are the key findings:

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Microwaves may convert energy into heat within solids, thereby improving heat dispersion and increasing the quantity of syngas. However, the processes and mechanisms that can boost the hydrogen concentration of syngas must be investigated further. ▪ Coconut activated carbon comprises components that give high tan δ values and so merit further investigation as raw materials, adsorbents or catalysts. Materials with a tan δ > 0.5 are required for microwave-assisted gasification. ▪ Co-gasification is critical for the production of hydrogen-rich syngas because it tackles the issue of raw material unpredictability while delivering synergistic benefits. The blending ratios suitable for microwave-assisted co-gasification must be thoroughly investigated in order to create syngas with high hydrogen content. ▪ While microwave heating is superior to conventional heating, the quantity of hydrogen produced remains small, at 60% and 32.75% for single-feeding and co-feeding processes, respectively. Furthermore, syngas generated via microwave heating has a heating value of 17.44 MJ/m³, which is significantly higher than that of conventional heating. ▪ Steam gasification agents can generate hydrogen-rich syngas. Steam gasification, hydrothermal gasification, and SCWG are necessary to create hydrogen at a rate of 57%, which can be enhanced to 82.9% by employing purification technology. Such an increase is critical so that hydrogen can be utilized immediately in the industrial sector. ▪ Gasification, in conjunction with microwave heating, necessitates the careful selection of source materials with high dielectric constants. The dielectric characteristics of raw materials, beds, adsorbents, and catalysts should be investigated further to improve the performance of microwave-heated, hydrogen-rich syngasgeneration technology. ▪ Hydrogen-rich syngas generation strategies based on shared gasification and microwave heating are currently underutilized and have the potential to be expanded at both the laboratory and commercial scales.