Thermochemical Conversion of Biomass for Syngas Production: Current Status and Future Trends

: The thermochemical conversion of different feedstocks is a technology capable of reducing the amount of biowaste materials produced. In addition, the gasiﬁcation of feedstock using steam as a gasifying agent also produces hydrogen, which is a clean energy fuel. This article aimed to encapsulate the current status of biowaste gasiﬁcation and to explain, in detail, the advantages and limitations of gasiﬁcation technologies. In this review paper, different gasifying agents such as steam, air, and oxygen, as well as their effects on the quality of syngas production, are discussed. In addition, the effects of reactor conﬁguration and different operating parameters, such as temperature, pressure, equivalence ratio, and incorporation of a catalyst, as well as their effects on the ratio of H 2 /CO, LHV, syngas yield, and tar production, were critically evaluated. Although gasiﬁcation is a sustainable and ecologically sound biomass utilization technology, tar formation is the main problem in the biomass gasiﬁcation process. Tar can condense in the reactor, and clog and contaminate equipment. It has been shown that an optimized gasiﬁer and a high-activity catalyst can effectively reduce tar formation. However, key biowaste treatment technologies and concepts must ﬁrst be improved and demonstrated at the market level to increase stakeholder conﬁdence. Gasiﬁcation can be the driving force of this integration, effectively replacing fossil fuels with produced gas. In addition, support policies are usually needed to make the integration of biomass gasiﬁcation technology into the industry proﬁtable with fully functional gasiﬁcation plants. Therefore, to address such issues, this study focused on addressing these issues and an overview of gasiﬁcation concepts.


Introduction
Global warming and environmental problems linked to the extensive use of fossil fuels have diverted research toward renewable energy resources. Furthermore, nonrenewable energy resources are being depleted while the world's energy demand continues to grow tremendously [1][2][3][4][5]. Renewable options include solar, wind, tidal, wave, hydro, and biomass energy. Among all existing renewable energy resources, the conversion of biomass for energy production contributes to over 70% of all renewable energy generation. The participation of biomass in the world's primary energy supply accounts for over 10%. The

Gasification History and Reactions
Gasification history can be traced back to the 16 th century. The idea of gasification went through several stages of development. Among the various thermochemical processes for converting biomass into products, gasification is widely applied using a limited supply of oxygen or any other oxidizing agents including carbon dioxide, steam, or a mixture of both to transform solid biomass into liquid or gaseous biofuel. The gasification reactor is also called a gasifier [16,17]. Compared to combustion, the efficiency of converting biomass to biofuel through gasification is higher. Consequently, the biofuel achieved using the gasification process is a promising technology and provides better efficiency in energy generation. Biomass gasification plants range from a few kilowatts up to several megawatts of power generation [18,19]. Furthermore, the efficiency of the gasification plant ranges from 70% to 80% [20], depending on the gasification agent used. The main product obtained from biomass gasification is syngas, commonly known as synthesis gas. The resulting synthesis gas composition contains CO, H2, CO2, and CH4. These gases are burned in a gas turbine to generate electricity. Biochar is a by-product of biomass gasification that is utilized in the agriculture field to improve soil fertility [16,17]. The composition of the syngas changes with the gasification agent used during the gasification of biomass. The most frequently used gasifying agents are carbon dioxide, oxygen, steam, air, a mixture of air-steam, or a mixture of air and carbon dioxide streams in different proportions. Among various gasifying agents, the air is extensively applied due to its high availability and zero purchase cost [16]. Gasifiers are also designed to convert other biomass raw materials and the dry organic part of municipal solid waste into energy [21,22]. The types of gasifiers used for energy recovery from biomass [17] are mainly fixed bed, EFG, and FBG. Table S1 (Supplementary Information) shows a detailed review of biomass gasification. Table S1 presents a short review regarding the kinetic parameter estimations for different feedstocks. It has been observed that different mathematical models, including the volume reaction model, shrinking core model, random pore model, etc., were applied to estimate the kinetic parameters, i.e., pre-exponential factor or Arrhenius constant (A) and activation energy (E). Both isothermal and non-isothermal techniques were applied in the experiments. Due to the inconsistency of the dimension of A (change in order and mathematical expression used), the comparison of A seems to mean less. However, activation energy was found in the range of 30 to 243 KJ/mole for all the used feedstocks except the mixture of scrap tire powder, industrial waste, and sewage sludge for which the activation energy was estimated in the range of 65.7 × 10 3 to 185.4 × 10 3 KJ/mol. The possible reason for this high activation energy is the presence of higher hydrocarbons in scrap tires. It is a common understanding that a high amount of activation energy has a negative effect on combustion/gasification. Table 1 represents different fuels, heating values, and a comparison of prices. The major reactions during biomass gasification include

Gasification History and Reactions
Gasification history can be traced back to the 16th century. The idea of gasification went through several stages of development. Among the various thermochemical processes for converting biomass into products, gasification is widely applied using a limited supply of oxygen or any other oxidizing agents including carbon dioxide, steam, or a mixture of both to transform solid biomass into liquid or gaseous biofuel. The gasification reactor is also called a gasifier [16,17]. Compared to combustion, the efficiency of converting biomass to biofuel through gasification is higher. Consequently, the biofuel achieved using the gasification process is a promising technology and provides better efficiency in energy generation. Biomass gasification plants range from a few kilowatts up to several megawatts of power generation [18,19]. Furthermore, the efficiency of the gasification plant ranges from 70% to 80% [20], depending on the gasification agent used. The main product obtained from biomass gasification is syngas, commonly known as synthesis gas. The resulting synthesis gas composition contains CO, H 2 , CO 2, and CH 4 . These gases are burned in a gas turbine to generate electricity. Biochar is a by-product of biomass gasification that is utilized in the agriculture field to improve soil fertility [16,17]. The composition of the syngas changes with the gasification agent used during the gasification of biomass. The most frequently used gasifying agents are carbon dioxide, oxygen, steam, air, a mixture of air-steam, or a mixture of air and carbon dioxide streams in different proportions. Among various gasifying agents, the air is extensively applied due to its high availability and zero purchase cost [16]. Gasifiers are also designed to convert other biomass raw materials and the dry organic part of municipal solid waste into energy [21,22]. The types of gasifiers used for energy recovery from biomass [17] are mainly fixed bed, EFG, and FBG. Table S1 (Supplementary Information) shows a detailed review of biomass gasification. Table S1 presents a short review regarding the kinetic parameter estimations for different feedstocks. It has been observed that different mathematical models, including the volume reaction model, shrinking core model, random pore model, etc., were applied to estimate the kinetic parameters, i.e., pre-exponential factor or Arrhenius constant (A) and activation energy (E). Both isothermal and non-isothermal techniques were applied in the experiments. Due to the inconsistency of the dimension of A (change in order and mathematical expression used), the comparison of A seems to mean less. However, activation energy was found in the range of 30 to 243 KJ/mole for all the used feedstocks except the mixture of scrap tire powder, industrial waste, and sewage sludge for which the activation energy was estimated in the range of 65.7 × 10 3 to 185.4 × 10 3 KJ/mol. The possible reason for this high activation energy is the presence of higher hydrocarbons in scrap tires. It is a common understanding that a high amount of activation energy has a negative effect on combustion/gasification. Table 1 represents different fuels, heating values, and a comparison of prices. The major reactions during biomass gasification include endothermic and exothermic, such as partial oxidation, steam methane reforming, water gas shift reaction, Boudouard reaction, etc., as explained in Table 2.   Notes: coefficients of soot and gaseous products are represented by n1, n2, n3, n4, n5, n6, and n7. While w shows the quantity of water per kmol of feedstock used, a represents a quantity of oxygen per kmol of feedstock.

Characteristics of Lignocellulosic Biomass
The generic term for non-fossilized organic matter and biodegradable material is biomass. It is produced directly through photosynthesis and is usually applied for the manufacturing of chemicals and fuels. Moreover, the most widely available biomass resources for energy production are wood, starch, and sugarcane. Generally, wheat, corn, cereals, oil crops (canola, sunflower, palm, rapeseed), and nonfood crops include eucalyptus, miscanthus, willow, etc. Non-lignocellulosic biomass residues from agricultural and forest residues include wet organic materials such as animal wastes, sewage sludge, and organic waste fractions from municipal solid wastes, and are commonly used for bioenergy production [23]. In bioenergy production, the more commonly used feedstocks are lignocellulosic material due to their low cost and abundance [25]. Table 3 shows the main components of biomass. The composition of lignocellulosic biomass comprises proteins, lipids, extractives, and carbohydrates [7,27]. The carbohydrate in lignocellulosic biomass is up to 75%, which is particularly prominent in the form of hemicellulose (23-32%), cellulose (38-50%), and lignin (15-25%), [28]. Lignin, cellulose, and hemicellulose are intermeshed and physically connected by covalent and noncovalent bonds [28,29]. Figure 2 shows the intermeshed structure of lignocellulosic biomass having cellulose as its main component [27,30]. Cellulose is not soluble in most solvents and is not prone to enzymatic hydrolyzes and acids [8,25]. In comparison to cellulose, hemicellulose has some distinctions, as hemicellulose comprises polysaccharides other than glucose such as pentoses and hexoses [9,25]. The structure of hemicellulose is smaller in length, marginally branched, and not as crystalline as the structure of cellulose. Lignin has a three-dimensional configuration comprising aromatic monomer units, and it is a high molar mass molecule [27]. Lignin is composed of several types of arrangements that repeat unsystematically over cross-linked covalent bonds, and is a relatively hydrophobic polymer.
Sustainability 2022, 14, x FOR PEER REVIEW Figure 2. Structure and characteristics of biomass concerning lignin, cellulose, and hemi Reprinted with permission from ref. [23]. Copyright 2018 Elsevier.

Composition of Feedstock
The potential of feedstock is determined through the investigation of proxim ultimate analysis to produce valuable syngas. The results of different biomasses c Reprinted with permission from ref. [23]. Copyright 2018 Elsevier.

Proximate and Ultimate Analysis
High ash-containing feedstock leads to difficulties during gasification including the plugging of the reactor, catalyst sintering, and appropriate ash disposal. Furthermore, by increasing the ash content of poultry manure from 17.2% to 25.1%, the gasification efficiency declined from 63% to 33%, and the concentration of H 2 and CO decreased meaningfully. Furthermore, the decrease in the HHV of the syngas reduced from 4.3 MJ/m 3 to 2.6 MJ/m 3 . The value of the volatile matter in biomass is another component taken into consideration. The increase in volatile matter results in rises in tar formation during the gasification of feedstock. The composition of tar primarily consists of aromatic compounds such as naphthalene, benzene, phenols, etc. The primarily aromatic compounds evaporate and then condense under gasification conditions. Moreover, shorter chain organics lead to the generation of tar at low temperatures such as aldehydes, ketones, and acids. Municipal solid waste 77-86 wt% and agricultural waste 73 to 83 wt% result in the production of a large amount of tar due to the higher value of volatile matter. During the gasification of biomass, fixed carbon controls the rate of gasification and the production of syngas [49]. Furthermore, volatile matter, fixed carbon, and ash content directly impact the gasification efficiency, gas yield, and inhibitory compound production. Therefore, the proximate analysis of the feedstocks is performed to thoroughly examine and select appropriate reaction conditions, reactor configurations, and catalysts.
During the gasification of feedstock, the higher concentration of oxygen and carbon tends to the higher amounts of CO and CO 2 in syngas composition. While temperatures below 600 • C may cause CH 4 production, the concentration of CH 4 decreases, and CO and CO 2 formation increase because of the methane decomposition and reforming reactions. Therefore, the gasification of municipal solid waste and agricultural residues produce more CO and CO 2 as compared with other waste feedstocks. The greater amounts of sulfur and nitrogen in the feedstock tend to produce a greater amount of Nox and SOx poisoning in the catalyst. The presence of nitrogen is mostly in the form of organic complexes, and it reacts with hydrogen during gasification to generate hydrogen cyanide and ammonia. In addition, nitrogen is released in small quantities in the molecular nitrogen form such as aromatic organic compounds and nitrogen oxides, whereas a trivial quantity of N 2 is retained as unreacted in the feedstock solids [49,50]. Under gasification conditions, sulfur is released in the form of H 2 S, which creates problems during the gas separation and treatment. In biomass feedstocks, the sulfur content is less than 1.5 weight %. Furthermore, animal waste products and sewage sludge contain sulfur~1 wt. % and 0.5 wt. %. Higher amounts of sulfur lead to gas treatment issues and catalyst poisoning during the gasification of feedstock [50]. Maitlo et al. [51] investigated rice husk, cotton stalks, sugarcane bagasse, and saw dust. The results are given in Table 4.

Gasification Agents
The process of biowaste gasification can be categorized as exothermic or endothermic. In addition, gasification may be classified by the gasifying agent, such as oxygen, steam or air, used during the gasification of biowaste. The application of different gasifying agents yields different syngas compositions, by-products, and heating values. The applications of different gasifying agents and their impact on syngas composition are summarized in Table 5 [31,53].

Gasifying Agent (Steam)
The data given in Table 5, comparing with the gasification of air and oxygen, shows that the amount of H 2 produced by steam gasification is larger and the calorific value is higher [54]. This trend is caused by the role of steam in stimulating water-gas reactions, WGSR, methanation reaction, and steam reforming reactions. The primary reactions for example water-gas reactions and steam decomposition are the main factors that lead to the oxidation of feedstock. The change of organic carbon and hydrogen from the decomposition of steam into H 2 and CO is primarily endorsed by the above reactions. Once the raw material is burned, the WGSR is liable for the transformation of gaseous H 2 O and CO into H 2 and CO 2 . Garcia et al. [55] found that, as the S/B ratio was enhanced, the H 2 and CO 2 concentration in syngas composition was also increased, whereas the content of CO and CH 4 in syngas composition reduced. Hernandez et al. also confirmed these trends [56] by gasifying the dealcoholized marc of grape. The study found that, as S/B increased, the H 2 /CO and H 2 /CO 2 ratios increased, while the CH 4 /H 2 ratio decreased. This change in reaction mechanism is the effect of the steam reforming of tars and the steam reforming of methane. During the gasification process, the reforming reactions of carbon and methane are intensified, resulting in a reduction in CH 4 and a rise in H 2 formation. Furthermore, the rise in S/B leads to a decrease in the CO/CO 2 ratio, indicating that the WGSR has become dominant. In addition, steam gasification leads to a reduced content of recalcitrant by-products during the gasification of biowaste. Lapania et al. [57] observed that, when the S/B ratio was enhanced from 0.5 to 1.0, the tar formation lessened by 49%, and the char concentration reduced by 76%. This is because the steam initiates various condensation reactions with carbon, which decrease the carbon content [58]. Roche et al. [34] also found that steam reduces the amount of tar from 17% to 24%, compared to air. Hence, it was concluded that steam plays a vital role in making more hydrogen and methane during its gasification process.

Gasifying Agent (Air)
The most common gasifying agent is air, as it is copious and easy to use. Air-based gasification performance is highly dependent on temperature and equivalent ratio (ER). In particular, the higher the temperature of the air presented into the gasifier, the greater the heating value of the dry syngas produced [59]. Since air comprises up to 79% nitrogen, the generated gas is extremely diluted, which upsurges the treatment cost of gas separation [60]; thus, air gasification has serious flaws. Furthermore, the gaseous products formed by air gasification have a low calorific value of 3.50-7.80 MJ/m 3 . Hence, the use of air as a gasifying medium is generally limited to low heating and power production [61]. Mohammed et al. [62] observed that when the ER was enhanced from 0.150 to 0.350, for the gasification of empty fruit clusters, the production of tar decreased from 78% to 62% and for coal from 13% to 3%. Adding oxygen to the air will cause oxidization of the tar and its compounds, thus decreasing the tar and tar content during CO and CO 2 production. Other research studies have confirmed the influence of ER on tar production [53,63,64]. With the increase in the ER, the gasification efficiency (GE) and the CCE incline to rise and then decrease.
Gao et al. [65] found that when ER was raised from 0.180 to 0.220, GE enhanced from 61.43% to 68.15%, then reduced to 59.56% as ER was raised to 0.280 in pinewood chip gasification. Zhao et al. [66] observed a similar inclination in the CCE value of wood chip gasification. When ER was enhanced from 0.220 to 0.310, CCE improved from 81% to 91.5%, but when ER increased to 0.34, CCE decreased to 88.5%. This trend of GE and CCE can be elucidated by suppressing the heat transfer between the solid particles. With the addition of more air to the gasifier, the inhibited heat transfer between solid particles and the air was observed which may result in a reduction in GE and CCE. In the process of air gasification, the H 2 and CO concentration will first rise to the maximum value with the increase in ER, and then decline. With the increase in ER, the mole fractions of CH 4 will continue to decrease, while the content of CO 2 will continue to increase [63,67,68]. Zhao et al. [66] confirmed these trends, which showed that with an increase in ER from 0.22 to 0.34, the CO concentration reduced from 25.7 vol% to 21.5% vol, while the CH 4 concentration in syngas lessened from 2.45 vol% to 0.87 vol%, and H 2 concentration declined from 14.6 vol% to 10.2 vol%. At the same time, the CO 2 concentration slowly improved from 11.70% to 12.30%, while LHV dropped from 6.67 MJ/m 3 to 4.650 MJ/m 3 with an increase in ER [66].

Gasifying Agent (Oxygen)
Oxygen is often used as a gasification agent, because it can produce syngas with a medium heating value. In this regard, the air-to-fuel ratio (AF) and ER are very important parameters to be calculated using Equations (17) and (18), respectively. The precise trend of increase in ER and its effects on gas characteristics under oxygen and air is identical. Niu et al. [53] observed that when the ER changes between 0.18 and 0.230, the calorific value of the generated gas can reach 8-10 MJ/m 3 . Moreover, an ER higher than 0.230 will cause the HHV of generated gas to drop sharply. The reason for this is that, with the increase in ER, the gasification reaction starts to favor the oxidation reactions, which will greatly increase the CO 2 content in syngas composition [53]. Gao et al. [65] also confirmed the same and reported that H 2 and CO content decreased, and CO 2 content increased when ER was enhanced from 0.050 to 0.30. As compared with the gasification of air, the gasification of oxygen results in higher CCE. Nonetheless, two disadvantages of gasification using oxygen as a gasifying agent are: (1) the purchasing of pure oxygen is costly, affecting the overall operating cost of the plant, and (2) the separation of oxygen from synthesis gas is difficult and costly. Hence, oxygen is usually used with steam as a gasifying agent. m f = Mass flowrate of fuel From the above discussion, it has been established that air has serious disadvantages over the rest of the gasifying agents, due to the higher costs for the post-separation of high nitrogen. Oxygen alone is an expansive choice for the generation of pure oxygen and the separation of oxygen from syngas. Steam produces a good quantity of H 2 and CH 4 in syngas, and with less CO in syngas steam alone has few disadvantages. So, the most feasible gasifying agent is an appropriate mixture of oxygen and steam. This will improve the performance of the gasifier as well as enhance the overall economics of the process.

Catalyst Selection
Gasification reactions are exothermic, and therefore different types of catalysts are used for reducing the activation energy of reactions involved in the gasification process. The application of catalysts improves the syngas yield, CCE, and H 2 content in syngas. Besides that, the incorporation of catalysts removes recalcitrant gasification by-products, including tar and char. The main types of catalyst are homogenous and heterogeneous catalysts. Homogeneous catalysts are those which occur in the same phase (gas or liquid) as the reactants, or have the same phase as the reaction mixture, while heterogeneous catalysts have a different phase. The selection of catalyst incorporation depends upon the preferred gaseous product [35]. The different types of catalyst, such as homogenous and heterogeneous, and their gasification performance are given in Table 6.

Heterogeneous Catalysts
Olivine (2MgO·SiO 2 ), dolomite (MgCO 3 ·CaCO 3 ), and shells are natural mineral-based catalysts that are used directly or with some pretreatment techniques including calcination. Natural catalysts are abundantly available and cost-effective, and are widely used for tar removal from producer gas [69]. Moreover, the catalytic efficiency of natural catalysts can be increased through calcinating for 4 h at a temperature of 900 • C. The application of dolomite as a catalyst enhances H 2 formation during the gasification of biomass.
Noble metal-based catalysts involve Pt, Rh, and Ru that possess high catalytic activity, having high sulfur resistance and high stability in the steam reforming of tar. Among the noble metal-based catalysts, Rh was observed as more significant compared with other catalysts. Noble catalysts are expensive compared to other catalysts.
Furthermore, besides noble catalysts, transition metal-based catalysts are also used for the reforming of tar. Transition metal-based catalysts include Cu, Co, and Fe. Transition metal-based catalysts are efficient in the steam reforming of tar, and are deactivated easily during carbon deposition in the case of high tar content in syngas.
The leading attraction of the above catalyst is that once the gasification reactions are quenched, the used catalyst can be recovered easily. Since heterogeneous natural catalysts are abundantly available, low cost, and can reduce tar content in the obtained syngas, they are most commonly used. Guan et al. [70] showed that in the application of dolomite to gasify municipal solid waste, the reduction in tar content was from 9.71 weight% to 0.0 weight%, thereby increasing the gas production from 0.620 to 1.470 m 3 /kg feedstock, while the H 2 content increased from 30.6 to 50.2%. With the porous nature and higher content of alkaline components in MgO, Al 2 O 3 , CaO, etc., dolomite helps the cracking of tar compounds, which can participate in tar reduction. Heterogeneous catalysts are recognized to decompose tar to yield gaseous products.
Wu et al. [44], observed that, using a nickel catalyst, the H 2 content touched 52% with a volume at 750 • C, reached 950 • C under non-catalytic gasification, and the tar content decreased from 27.30 g/m 3 to 8.30 g/m 3 in syngas with no catalyst in it. Although, it is well known that Ni deactivates and sinters during the reaction, so catalyst support is used to reduce fouling. Wang et al. [32] found that by using NiO/MD for MSW gasification, the syngas production was increased from 0.78 to 1.62 m 3 /kg MSW, and the H 2 production was increased from 21.9 to 80.7 g H 2 /kg MSW. Compared with gasification using catalysts, tar production was increased from 38.7 to 0.23 g/m 3 [32]. NiO/MD is not only conducive in the cracking of tar and the generation of H 2 , but also for alkaline oxides including MgO, CaO, etc., on dolomite support, which can prolong the service life of the catalyst without affecting the results when used for 10 h. Ni-Al 2 O 3 is recognized as an effective catalyst for the decomposition of tar. The comparison of results when not using a Ni-Al 2 O 3 catalyst in AWP gasification observed that H 2 production improved from 3.90 to 16.40 mmol/g of manure, and CCE was improved from 23.5 to 40.6%. The comparison of gasification results under non-catalytic conditions showed that the tar transformation rate improved from 27.70 to 1.50 weight% [37].

Homogeneous Catalysts
The application of homogenous catalysts leads to higher mole fractions of H 2 in producer gas composition. These catalysts are low-cost as compared with heterogeneous catalysts; however, homogenous catalysts are challenging to reclaim from reactions. Homogenous catalysts result in corrosive conditions. Compared with heterogeneous catalysts, homogenous catalysts are not frequently employed because of some drawbacks.
Hu et al. [71] used a CaO catalyst and inspected that when the ratio of the catalyst to biowaste improved from 0.0 to 0.70, the H 2 mole fraction and gas production improved from 10.70 vol% to 64.70 mL/g MSW at 49.40 vol% and 277.70 mL/g MSW. When the ratio of catalyst to raw material exceeded 0.70, the H 2 content began to decline. This increase can be described by the fact that a large amount of CaO makes it difficult to transfer heat. CaO is a tremendous catalyst for the selective separation of H 2 , because it is a CO 2 adsorbent. In the steam gasification process of biological waste, NaOH is also used as a catalyst to promote H 2 production. Gai et al. [36] performed gasification using SS and stated that NaOH stimulated the WGSR and improved the H 2 mole fractions in syngas composition to 32.40 vol%, as compared to 24.0 vol% without a catalyst. Furthermore, it has been witnessed that the use of NaOH and other alkaline catalysts, such as KOH, reduces the mole fractions of CO and CO 2 in syngas composition [36].
The application of catalysts decreases the reaction temperature and enhances the carbon conversion rate; moreover, it increases the hydrogen selectivity from biomass for both steam gasification and supercritical water gasification. The addition of catalyst promotes tar cracking and the reforming of condensable fractions. Furthermore, the introduction of steam increases syngas production, particularly increasing hydrogen production. Therefore, catalytic steam reforming is a commonly used technique, as it removes tar effectively and simultaneously increases the selectivity of H 2 . Hence, the syngas contains lower CO and CH 4 because of the steam reforming of methane and the water-gas shift of CO. Moreover, H 2 formation increases during the catalytic gasification of biomass.

Types of Gasifiers
Many types of gasifiers are commonly used for the gasification of biomass through thermochemical conversion technologies. Depending upon the moisture content, ash content, shape, size, type of raw material, and requirements of the users, gasifiers are mainly divided into two categories, i.e., fixed bed and FBG [87,88]. A few other types of gasifiers are also employed for the gasification of biomass, including rotary kiln, plasma, and EFG [89,90]. Brief schematic diagrams of biomass gasifiers are given in Figure 3, keeping in view the advantages and disadvantages of reactors for the catalytic gasification of biomass. Biomass gasification is performed in various gasifier configurations such as FBG, fixed bed, EFG, and plasma gasifiers. EFG is widely engaged in biomass gasification due to good mixing capabilities, higher heating and mass transfer rates, scalability, feed flexibility, high carbon conversion efficiency, and high reaction rates.

Fixed-Bed Gasifiers
Fixed-bed gasifiers are further divided into three categories, i.e., updraft, downdraft, and cross draft gasifiers. These categories of fixed-bed gasifiers are because of the interaction between biomass and gasifying agent [93].

Updraft Gasifier
The oldest and simplest gasifier used for synthesis gas production is an updraft gasifier. A gasifying agent is introduced from the inlet point located at the bottom of the gasifier and moves upward in an updraft reactor, whereas the raw feedstock is introduced from the top of the gasifier and travels downward through gravity [94]. Thus, gasifying agents and biomass create a counter-current flow within the gasifier. Biomass and gasifying agents pass through all the zones of the gasifier, such as the drying, pyrolysis, reduction, and oxidation zones, and different types of reactions occur in all zones of the updraft gasifier. The main strength of the updraft gasifier includes a low-pressure drop, high thermal efficiency, and low slag formation. At high flame temperature, the updraft gasifier tends to produce low dust content in syngas. The major drawbacks of updraft gasifiers are long engine startup times, low syngas production, and high tar sensitivity [95]. The schematic diagram of the updraft gasifier is shown in Figure 3a.

Downdraft Gasifier
The downdraft reactor is commonly called a concurrent gasifier in which the feedstock and reacting gas passage together downward in the same direction. Comprising gas and tar, all decomposition products move through the oxidation zone of the downdraft gasifier, and the cracking of feedstock occurs continually. In a downdraft reactor, high-quality synthesis gas with low tar content can be achieved compared with an updraft gasifier. The fixed-bed downdraft reactor is commonly used for small-scale power generation [96,97]. The key constituents of biomass (lignin, hemicellulose, and cellulose) decompose, producing H 2 , CO 2 , CH 4, and H 2 O by the reaction of hydroxyl and methoxy. The schematic diagram of the downdraft gasifier is shown in Figure 3b.

Cross Draft Gasifier
Compared to "updraft" and "downdraft" fixed-bed reactors, the cross draft reactor is the simplest type of feedstock gasification reactor. The feedstock is introduced from top to bottom and the thermochemical transformation of the biomass takes place continuously within the reactor. As shown in Figure 3c, the downdraft reactor is configured to have an ash zone, combustion, and a reduction section. In comparison with the "updraft" and "downdraft" types, the air is injected into the descending reactor from the side. The cross draft reactor has excellent flexibility in synthesis gas production. However, biomass fuel content is high, volume is small, and gasification is difficult [98].

Circulating and Bubbling Fluidized Bed Gasifiers
The bubbling fluidized bed gasifier is kept fluidized by inert materials such as dolomite, olivine, sand, SiO 2 , etc. The agent used for gasification is located from the bottom of the bed through the distribution network at varying flow rates. The inner bed is kept fluidized to achieve an appropriate reaction with raw material. The inert liquid-like material is constantly agitated in the existence of gas bubbles to provide uniform heat transfer conditions for gas and biomass. The circulating bed gasifier at this point is alike to the BFBG, as shown in Figure 3d,e. The separator cyclone is installed at the reactor outlet to bring the biomass to the bubbling reactor. The high-velocity gas energy, usually 5-10 m/s, can be provided from combustion to drive the pyrolysis and gasification of the biomass. Presently, the FBG is a capable reactor for achieving tar-free syngas [99,100]. It is characterized by feed flexibility, higher mass and heat transfer efficiency, a uniform temperature within the gasifier, and high syngas yield. Biomass such as rice straw, grass, and wheat straw, having a high content of ash, can damage equipment and reduce the quality of syngas production [101]. The temperature at which FBG is operated is usually maintained from 800 • C-900 • C, which is determined according to the melting point. Since the gasification reaction usually takes place at high temperatures, high activity catalysts must be planned during the gasification of feedstock.

Entrained Flow Reactor
Feedstock with a size less than 1 mm along with gasifying agents are fed in cocurrent EFG. The usual operating pressure for EFG is controlled from 25 to 30 bar and the temperature of the reactor is normally maintained from 1300 • C to 1500 • C. The size of the feedstock is reduced to make its slurry easy, then the slurry is introduced into the EFG. More specifically, EFG is employed for the treatment of feedstocks including biomass, coal, mixed plastic waste, forest residues, sewage sludge, and agricultural residues for syngas production. The removal of moisture through heat treatment is necessary before the application of selected raw material for efficient gasification [102,103]. Figure 3f shows the arrangement of the gasifier for biomass gasification.

Plasma Reactor
In a plasma gasifier, two electrodes are commonly installed such as carbon electrodes or Cu electrodes. Furthermore, when the internal temperature of the reactor approaches up to 10,000 • C, the plasma gasifier will provide an electric charge commonly known as "arc". The atomic degradation of feedstock takes place in gasifiers during gasification under different atmospheres. In the absence of the oxidizing agent, the energy for biomass temperature rise is provided through the plasma process. In the following two situations, the plasma reactor for the gasification of biomass may be applied. (1) Plasma gasifiers can make gaseous products clean during the upstream process of biomass gasification. Moreover, tar degradation occurs in light components. (2) The temperature of the gasifier can be controlled at varying moisture content, flowrate, size of feedstock, gasifying agent, and syngas quality, independently. The plasma reactor is shown in Figure 3g. The reactor can treat general, medical, and hazardous waste, and can achieve full carbon conversion regardless of the type of raw material. This plasma technology can reach very high temperatures and decompose raw materials completely into CO and H 2 , producing clean synthesis gas [104]. The use of higher plasma power rises the H 2 /CO ratio, resulting in higher temperatures and greater cracking of the tar. Under different operating conditions, the HHV of syngas produced is between 6 and 7 MJ/Nm 3 . It has been determined that the H 2 /CO ratio produced is much higher than that of conventional reactors using air, oxygen, and steam as gasifying agents [105]. During the gasification of hazardous industrial waste under plasma gasification conditions, the H 2 /CO ratio is observed to be approximately 1.0. A study conducted by Mountouris et al. [106] found that the use of sewage sludge under plasma gasification conditions had a similar gaseous composition.

Rotary Kiln
The rotary kiln gasifier is an important type of reactor, and it is extensively used on industrial scale waste gasification. The rotary furnace gasifier is usually a steel shell having a wear-resistant refractory structure that can decrease the temperature of the metal shell. As shown in Figure 3h, dry and fixed-size feedstock materials are organized and fed by maintaining an appropriate feeding rate. The generated gas and slag are commonly discharged from the top and bottom of the gasifier outlet. Due to the continuous rotation of the reactor, exposure to the surface of the biomass and gasification agent can be fully contacted. Nevertheless, the flue gases formed by gasification can take away a lot of heat. Therefore, unless a barrier is installed in the reactor, the heat interchange between biomass and reaction gas is ineffective [87].
Fixed-bed gasifiers, such as updraft and downdraft gasifiers, are less efficient during gasification in terms of carbon conversion efficiency, cold gas efficiency, and higher heating value of syngas produced. Furthermore, fluidized bed gasifiers are efficient and produce high-quality syngas. Particularly, entrained flow gasifiers are most efficient in terms of operation, H 2 production, and product purity.

Performance of a Gasifier
The performance of a gasifier is evaluated through the consideration of various parameters such as cold gas efficiency, CCE, gas production, HHV, LHV, exergy efficiency, and equivalence ratio. Syngas production (G V , m 3 /kg) is the ratio between the volume of syngas under standard condition (V g , m 3 ) and the feeding of biomass into the gasifier (M biomass , kg). The gas production is further demonstrated in Equation (19).
The syngas lower heating value is the chemical energy contained in the tar-free cold syngas. The total calorific value of syngas is given in Equation (20). It is the sum of the LHV of syngas individual components (LHV i ) multiplied by the vol.% of each component gas i (x i ): (20) while (HHV syngas ) is the sum of the LHV of syngas and the latent heat of water vapor formed during combustion of feedstock. ER is equal to the amount of oxygen introduced into the reactor in comparison to the amount of oxygen required for the complete combustion of feedstock. The generic formula of biomass is assumed to be CH x O y , for defining the stoichiometric reaction of biomass with oxygen. Equation (21) is the stoichiometric reaction of biomass from which the amount of oxygen is calculated with a fixed amount of fuel. Then, the calculated values are used in Equations (17) and (18) for the estimation of equivalence ratios (ER).
In the gasification process, the CCE is the % of carbon in feedstock converted into syngas. Equation (22) represents carbon conversion efficiency [20,48].

CCE =
Carbon in syngas × syngas flow rate carbon content in biomass × biomass flow rate × 100 (22) In the gasification process, CGE is the ratio between the chemical energy in syngas and the chemical energy contained within the feedstock, and is demonstrated in Equation (23).

CGE =
LHV syngas × syngas flow rate LHV biomass × biomass flow rate × 100 (23) The exergy efficiency is the last parameter calculated during the efficiency measurement of the gasifier. The exergy is the estimation of the actual potential of the system to perform work. It is the ratio of energy that flows out and into a system. In exergy efficiencies, both physical and chemical exergy contributions are taken into consideration at high operational temperatures. For the adiabatic gasifier, Equation (24) demonstrates the exergy efficiency. ψ = n syngas e ch, syngas + e ph, syngas M biomass × e ch, biomass + n agent × e agent (24) In Equation (6), n syngas is syngas molar amount (kmol), e ch, syngas + shows the chemical exergy of syngas (kJ/kmol), M biomass represents the feeding of biomass (kg), e ph, syngas shows the physical exergy of syngas (kJ/kmol), e ch, biomass is biomass chemical exergy (kJ/kg), n agent is the gasifying agent molar amount (kmol), and e agent shows gasifying agent chemical exergy (kJ/kmol).

Effect of Temperature
The temperature of gasification is one of the most significant parameters that affect the quality of the gas produced and the efficiency of the process. As most of the gasification reactions are endothermic, the temperature rise will increase the endothermic cracking reaction of tar, and result in a reduction in the tar content producing H 2 , CO, and CO 2 in synthesis gas composition. Methane decomposition reactions are highly endothermic, so CH 4 can be maintained at lower temperatures, i.e., 300-600 • C. Furthermore, when the temperature of the reactor is maintained higher than 600 • C, CH 4 will be decomposed through the reforming reaction of methane and hydrocarbons [107,108]. On the other hand, H 2 increases with the increase in overall temperature, mainly due to the steam reforming, hydrocarbon reforming, and methane decomposition reactions. Although the production trends of H 2 and CH 4 are well represented in the literature, there is still no clear trend for CO and CO 2 . The effect of temperature on the non-catalytic steam gasification of organic waste was investigated by different researchers [44,[109][110][111]. With the increase in gasification temperature, all biowaste materials follow the theoretical trend of H 2 and CH 4 . For SS gasification (700-1000 • C), H 2 increased by 7.0% and CH 4 decreased by 4.0% [109]; H 2 of SS and pine chips (600-1000 • C) increased by 11.0%, and CH 4 decreased by 2.5% (900 • C) [110]; for wood waste (750-950 • C), H 2 increased by 19.7% and CH 4 decreased by 5.1% [51]; legume straw H 2 increased by 15.0% and CH 4 decreased by 4.0% (750-850 • C) [111]. Though, as anticipated, there was no clear trend in the composition of CO 2 and CO. With the gasification using steam as a gasifying agent for the mixture of SS and pine sawdust, and SS alone, only SS displayed a decrease in CO 2 and an increase in CO within the temperature range, but the gasification of wood residues and legume straws presented an increase and decrease in CO 2 . Some studies have shown that, as CO content increases, temperature also increases, which reduces CO 2 content [112,113], while others have shown a contrary tendency [114]. The upsurge in CO at the cost of CO 2 is endorsed to the Boudouard reaction; though, because this reaction directly competes with the WGSR, which is supposed to be dominant above 750 • C, these trends remain ambiguous.

Effect of Pressure
Considering the downstream use of the produced gas, biomass gasification is usually carried out under high atmospheric pressure. Some applications of product gas are performed through Fischer-Tropsch synthesis to convert gas into methanol or synthetic diesel, requiring high-pressure syngas production where the gasification of biomass under atmospheric pressure is advantageous. Moreover, the rise in gasifier operating pressure decreases the production of tar in the produced gas. Furthermore, some studies performed in an FBG show that the content of tar mostly increases naphthalene as the pressure of the gasifier increases from 0.1 MPa to 0.5 MPa, so the content of CO decreases, whereas the increase in the concentration of CH 4 and CO 2 was observed. A gasification model coupled with SOFC and gas turbines was carried out to prove that moderate pressures, for example, up to 4 bar, will not have a significant effect on the process of gasification. Interestingly, this affected the efficiency of the turbine, so the overall efficiency of the unit improved from 23% to 35% [115].

Effect S/B Ratio
Among various operating parameters for syngas yield, the S/B ratio is the most significant operating parameter during the gasification of biowaste. Moreover, the main reaction for H 2 production is the steam reforming reaction. Other major reactions that are involved in making H 2 include: cracking of tar, water-gas shift reaction, and water-gas reactions. The effect of the S/B ratio on syngas composition was investigated by Guan et al. and Tursun et al. [70,116]. When the S/B ratio was enhanced, it resulted in a decrease in the mole fractions of CH 4 and CO in syngas composition, while the rise in mole fraction of CO 2 was increased. Guo et al. [117] stated, regarding the S/B ratio, that from 0.4 to 2.0, the mole fractions of CO lessened from 36.5 to 22.5%, the mole fractions of CH 4 reduced from 2, 4% to 0.6%, and the mole fractions of CO 2 improved from 13% to 20%. However, the H 2 inclination is not monoatomic, as with other gas yield tendencies. The H 2 mole fractions first increase as the S/B ratio increases and then drop after reaching a certain maximum H 2 yield. Chen et al. [107] observed that when the acid hydrolyzed residue of corncob was gasified with steam when the S/B ratio increased from 1.3 to 5.3, H 2 concentration increased from 38.4% to 45.6%. By further enhancing the S/B ratio ranging from 5.3 to 7.5, the H 2 concentration dropped from 45.6% to 42.5%. The impact of the S/B ratio on tar yield, syngas production, CCE, and HHV was estimated by Guan et al. and Pfeifer et al. [70,118]. There is a critical S/B ratio that depends on the reactor configuration and application of the catalyst.
Many research investigations have shown that, by enhancing the S/B ratio greater than 1.0, the calorific value of produced gas is considerably reduced, because of the sharp reduction in the mole fractions of CO and CH 4 [119]. In addition, previously conducted research studies have observed that CCE [70] and gas production rate [70] will first rise and then decline after the critical S/B value. In addition, tar production will decline and then rise after this value [99]. Consequently, knowing the critical value of S/B for particular gasification is essential for optimizing gas production. The increase in the S/B ratio reduces tar formation, and makes tar more phenolic, with more C-O-C bonds, and easier catalytic conversion [120]. Though, some other research studies disagree with this trend. Fuentes-Cano et al. [121] observed that steam cannot decompose light hydrocarbons and aromatic hydrocarbons, and has merely a trivial effect on the disintegration of non-aromatic tar compounds, which is consistent with earlier studies [122,123]. Fuentes-Cano et al. [121] found that when steam was added, the yield of gravimetric tar dropped from about 285 g/kg SS to about 250 g/kg SS, and from 110 g/kg at 900 • C. Commonly, the molar ratio of H 2 /CO produced by gasification is nearly 0.7. The effect of S/B ratio on H 2 /CO ratio of the syngas for biomass feedstocks was inspected by Ge et al. [45]. The type of biomass and S/B value have a substantial effect on the H 2 /CO composition of synthesis gas. Between S/B ratios of 1 and 2, an extensive variety of H 2 /CO ratios were witnessed. This may be caused by numerous aspects. For instance, different feedstocks, catalysts, and reaction environments will produce diverse H 2 /CO ratios, resulting in great variability. Within this S/B range, syngas may be used in diverse ways to obtain various types of end products, including diesel, gasoline, and methanol, through the FT process. When the S/B ratio is between 2 and 3, the syngas will have an H 2 /CO ratio more than 2, thereby producing high-value synthetic gaseous products H 2 and ammonia, etc. This means that when S/B ratio (>2) rises, the role of the catalyst and reaction conditions in determining the H 2 /CO ratio will weaken. Figure 4 shows the effect of S/B ratio on syngas composition during the gasification of biomass. While Figure 5 shows the effect of S/B ratio on syngas production, LHV, CCE, and tar production for biowaste gasification. about 285 g/kg SS to about 250 g/kg SS, and from 110 g/kg at 900 °C. Commonly, the molar ratio of H2/CO produced by gasification is nearly 0.7. The effect of S/B ratio on H2/CO ratio of the syngas for biomass feedstocks was inspected by Ge et al. [45]. The type of biomass and S/B value have a substantial effect on the H2/CO composition of synthesis gas. Between S/B ratios of 1 and 2, an extensive variety of H2/CO ratios were witnessed. This may be caused by numerous aspects. For instance, different feedstocks, catalysts, and reaction environments will produce diverse H2/CO ratios, resulting in great variability. Within this S/B range, syngas may be used in diverse ways to obtain various types of end products, including diesel, gasoline, and methanol, through the FT process. When the S/B ratio is between 2 and 3, the syngas will have an H2/CO ratio more than 2, thereby producing high-value synthetic gaseous products H2 and ammonia, etc. This means that when S/B ratio (>2) rises, the role of the catalyst and reaction conditions in determining the H2/CO ratio will weaken. Figure 4 shows the effect of S/B ratio on syngas composition during the gasification of biomass. While Figure 5 shows the effect of S/B ratio on syngas production, LHV, CCE, and tar production for biowaste gasification.

Energy and Exergy Efficiency
Exergy and energy efficacy are the two main performance gauges that are commonly used to assess dissimilar processes in the chemical engineering, transportation, and agricultural sectors. Energy is the ability of a system to perform work, while exergy is the maximum valuable work that can be achieved when the scheme changes from one state to equilibrium. The exergy energy in the reaction process is the total energy in the synthesis gas divided by the sum of the energy in raw material and processing energy. S/B is a parameter that strongly affects exergy efficiency. Husseini et al. [125] reported that in sawdust gasification when S/B was enhanced from 0.15 to 0.20, the exergy efficiency dropped from 28.0% to 17.0%. However, further research has observed that there is an optimal S/B value that can maximize energy efficiency. Figure 6 shows the effect of S/B on the exergy efficiency of biowaste using a mixture of wood pellets (1000 °C) and eighty-six varieties of biomass at 700 °C [126]. Wu et al. [126] found that, at 1000 °C, when S/B reaches 0.53, energy and exergy value increase to about 83% and 75%, respectively. Although, when S/B increases, both values drop to 71% when S/B is enhanced to 2.50. Heat recovery is another parameter that affects exergy efficiency.
Song et al. [127] observed that the exergy efficiency, devoid of heat reclamation, extended the highest value of 52.1, 49.9, 48.5, and 46.4% at 700, 750, 800, and 850 °C, and at these temperatures, the heat recovery rate of steam reached 65.7, 63.9, 63.3, and 61.7% respectively. Compared with the reaction without heat recovery, heat recovery resulted in a 14% escalation in exergy efficiency. The recovery of heat has significant consideration for making most of the energy efficiency and exergy of the gasification process. The suitable S/B value must be determined experimentally to maximize the energy exergy/efficiency, H2 selectivity, HV, and the resulting syngas yield.

Energy and Exergy Efficiency
Exergy and energy efficacy are the two main performance gauges that are commonly used to assess dissimilar processes in the chemical engineering, transportation, and agricultural sectors. Energy is the ability of a system to perform work, while exergy is the maximum valuable work that can be achieved when the scheme changes from one state to equilibrium. The exergy energy in the reaction process is the total energy in the synthesis gas divided by the sum of the energy in raw material and processing energy. S/B is a parameter that strongly affects exergy efficiency. Husseini et al. [125] reported that in sawdust gasification when S/B was enhanced from 0.15 to 0.20, the exergy efficiency dropped from 28.0% to 17.0%. However, further research has observed that there is an optimal S/B value that can maximize energy efficiency. Figure 6 shows the effect of S/B on the exergy efficiency of biowaste using a mixture of wood pellets (1000 • C) and eighty-six varieties of biomass at 700 • C [126]. Wu et al. [126] found that, at 1000 • C, when S/B reaches 0.53, energy and exergy value increase to about 83% and 75%, respectively. Although, when S/B increases, both values drop to 71% when S/B is enhanced to 2.50. Heat recovery is another parameter that affects exergy efficiency. Figure 6. S/B ratio and its effect on exergy efficiency during biowaste gasification. Reprinted with permission from ref. [124]. Copyright 2018 Elsevier.

Formation of Tar and Removal Techniques
Producer gas (PG) tar is very common in the biowaste gasification process, and it is considered to be the main technical obstacle for the expansion and application of industrial-scale gasification technology [24,[128][129][130]. Tar is a combination of highly aromatic condensable organic compounds which are made during the partial oxidation of feedstock [120]. The composition of tar largely depends on the temperature of the reactor and is mainly divided into primary products such as phenol, secondary products such as benzene, toluene, and xylene, and tertiary products such as pyrene, indene, and naphthalene at 200-500 °C, 500-1000 °C, and over 700 °C, respectively [120,131]. Primary tar and tertiary tar are commonly exclusive, and before tertiary products appear the primary products are destroyed [120]. While the PG temperature is less than 400 °C, the tar condenses downstream of the reactor, causing various complications including catalyst deactivation, blockage, fouling, corrosion, and general failures of equipment [130]. Therefore, it is technically impractical to use crude PG with high tar content in certain applications, such as fuel and chemical catalytic synthesis, before the main refining steps. Condensation of tar in pipes and other process equipment also reduces the efficiency of the plant and increases operating costs. In addition, these composites can develop into intricate molecular activities through polymerization, which increases the exertion of removing them. Tar compounds comprise a substantial portion of energy, which is lost as they are removed from PG [132]. Therefore, the tar concentration in PG should be decreased to a value well-suited with the required downstream application. This may necessitate costly gas conditioning equipment, which makes the process economically unappealing. Furthermore, for direct application such as fuel in the combustion process of furnaces, tar is not a problem if condensation is evaded in the transportation pipeline that feeds the burner. Nevertheless, more innovative applications, for instance in internal combustion engines or secondary fuel synthesis, have strict limits on the tar concentration in PG. Therefore, for biomass gasification technology to take a profitable step forward and be integrated into different biorefining projects, it is imperative to improve cost-effective processes to eliminate or reduce tar formation. Besides that, tar compounds can also be altered into lighter gaseous substances including CO, CH4, and H2, meeting strict tar concentration limits. Various tar removal processes and technologies are being studied [133,134]. Song et al. [127] observed that the exergy efficiency, devoid of heat reclamation, extended the highest value of 52.1, 49.9, 48.5, and 46.4% at 700, 750, 800, and 850 • C, and at these temperatures, the heat recovery rate of steam reached 65.7, 63.9, 63.3, and 61.7% respectively. Compared with the reaction without heat recovery, heat recovery resulted in a 14% escalation in exergy efficiency. The recovery of heat has significant consideration for making most of the energy efficiency and exergy of the gasification process. The suitable S/B value must be determined experimentally to maximize the energy exergy/efficiency, H 2 selectivity, HV, and the resulting syngas yield.

Formation of Tar and Removal Techniques
Producer gas (PG) tar is very common in the biowaste gasification process, and it is considered to be the main technical obstacle for the expansion and application of industrial-scale gasification technology [24,[128][129][130]. Tar is a combination of highly aromatic condensable organic compounds which are made during the partial oxidation of feedstock [120]. The composition of tar largely depends on the temperature of the reactor and is mainly divided into primary products such as phenol, secondary products such as benzene, toluene, and xylene, and tertiary products such as pyrene, indene, and naphthalene at 200-500 • C, 500-1000 • C, and over 700 • C, respectively [120,131]. Primary tar and tertiary tar are commonly exclusive, and before tertiary products appear the primary products are destroyed [120]. While the PG temperature is less than 400 • C, the tar condenses downstream of the reactor, causing various complications including catalyst deactivation, blockage, fouling, corrosion, and general failures of equipment [130]. Therefore, it is technically impractical to use crude PG with high tar content in certain applications, such as fuel and chemical catalytic synthesis, before the main refining steps. Condensation of tar in pipes and other process equipment also reduces the efficiency of the plant and increases operating costs. In addition, these composites can develop into intricate molecular activities through polymerization, which increases the exertion of removing them. Tar compounds comprise a substantial portion of energy, which is lost as they are removed from PG [132]. Therefore, the tar concentration in PG should be decreased to a value well-suited with the required downstream application. This may necessitate costly gas conditioning equipment, which makes the process economically unappealing. Furthermore, for direct application such as fuel in the combustion process of furnaces, tar is not a problem if condensation is evaded in the transportation pipeline that feeds the burner. Nevertheless, more innovative applications, for instance in internal combustion engines or secondary fuel synthesis, have strict limits on the tar concentration in PG. Therefore, for biomass gasification technology to take a profitable step forward and be integrated into different biorefining projects, it is imperative to improve cost-effective processes to eliminate or reduce tar formation. Besides that, tar compounds can also be altered into lighter gaseous substances including CO, CH 4 , and H 2 , meeting strict tar concentration limits. Various tar removal processes and technologies are being studied [133,134].
Mechanical treatments can achieve high tar elimination efficiency, but can considerably decrease the energy efficiency of a plant, generate hazardous waste, lessen gas production, and reduce profitability [135]. Nevertheless, even major processes are not adequate to attain complete tar elimination [136]. Furthermore, tar removal reactions are considered kinetically limited reactions. Hence, the rate of reaction can be increased by enhancing the temperature or by the application of catalysts. The catalytic decomposition of tar is a complex mix of equilibrium reactions and hydrocarbon decomposition. When the molecular weight of tar increases, the dew point also increases causing serious operating problems. Dry reforming reactions are catalyzed by metal catalysts. Furthermore, dry reforming is more favored as compared with steam reforming reactions above 830 • C. Moreover, catalytic filters have recently been developed for the elimination of tar from producer gas. In this method, the combination of filtration for particle elimination and catalytic tar cracking from producer gas is achieved in one step. The experimental results revealed that this method is efficient for the catalytic elimination of tar and particles from producer gas. Different operating parameters for gasifiers and their effects on syngas production are summarized in Table 7.

Carbon Conversion and Cold Gas Efficiency
Kuo et al. [97] inspected the gasification performance of three biomasses, including raw bamboo, torrefied bamboo at 250 • C, and torrefied bamboo at 300 • C using a downdraft gasifier, which was assessed by thermodynamic investigation. Two factors-modified Equivalence Ratio (ERm) and Steam Supply Ratio (SSR)-were considered to regarding their impact on biomass gasification. CGE, CCE, and HHV were used as indicators to detect gasification performance. Analysis showed that torrefied bamboo was beneficial to increase the production of syngas. The greater the torrefied temperature, the greater the production of syngas, but the lower the ERm value of TB at 300 • C. As the high calorific value of TB at 300 • C was considerably improved over that of raw bamboo and TB at 250 • C, the former possesses the minimum CGE amongst the three fuels. In the research scope of ERm and SSR, the CC value of raw bamboo and TB at 250 • C was always greater than 90%, but with the increase in ERm, more CO 2 was produced, thereby reducing CGE. The maximum raw bamboo synthesis gas yield value and CGE, with TB at 250 • C and TB at 300 • C, were found for ERm: SSR was (0.2, 0.9), (0.22, 0.9) and (0, 28, 0.9) respectively. After considering the yields of syngas, CGE, and CC at the same time, predictions showed that TB at 250 • C was a more feasible gasification fuel.
Teh et al. [159] discussed that biomass gasifiers should be operated above 900 • C for the efficient conversion of carbon into syngas. Hoque et al. [160] achieved cold gas efficiency >60% for rice husk, and >70% for both sawdust and coconut shells. Konda et al. [161] worked on oil palm fronds, and obtained carbon conversion efficiency of 95% and cold gas efficiency of 59.1%, respectively. Figure 7 shows CGE, CCE, and LHV of (a) raw bamboo, (b) torrefied bamboo at 250 • C, and (c) torrefied bamboo at different temperatures. Teh et al. [159] discussed that biomass gasifiers should be operated above 900 °C for the efficient conversion of carbon into syngas. Hoque et al. [160] achieved cold gas efficiency >60% for rice husk, and >70% for both sawdust and coconut shells. Konda et al. [161] worked on oil palm fronds, and obtained carbon conversion efficiency of 95% and cold gas efficiency of 59.1%, respectively. Figure 7 shows CGE, CCE, and LHV of (a) raw bamboo, (b) torrefied bamboo at 250 °C, and (c) torrefied bamboo at different temperatures.

Conclusions
The following conclusions can be drawn from the current review:

•
It is established that an appropriate mixture of pure oxygen with steam has advantages over other options for biomass gasification in terms of technical and economical perspectives; • The chemical composition of the biomass, reactor design, gasifier temperature, and catalyst work together to selectively separate H 2 and improve gasification efficiency; • The commercial application of biomass gasification is presently restricted to municipal solid waste and agricultural waste; • The main problems facing the commercial prospects of bio-waste gasification include large amounts of tar in the gas produced, difficulty in separating individual gaseous compounds, and the deactivation of gasification catalysts due to compounds containing nitrogen and sulfur; • There are several challenges related to plasma gasification, and attention should be paid to the success and future commercialization of plasma gasification; • The catalytic gasification of biomass is a key technology for environmental and chemical production.
Future sustainable biomass availability is one of the key reservations when considering large-scale biomass usage. However, the increased demand for limited biomass resources raises the requirement for monitoring biomass supply. Policies to encourage the further replacement of fossil fuels with biomass in the industry should be designed. The following are important research criteria for further biomass usage on an industrial scale:

•
Develop and demonstrate advanced technologies to transform biomass into fuels and energy; • Pursue and develop techniques to further incorporate the use of biomass where it is technically viable; • Develop and optimize sustainable and cost-effective biomass supply; • Design market-based solutions to foster investments in biomass.