Gasification Biomass

1. Introduction

Since the Industrial Revolution, the fossil fuels that have fueled the global economy have also triggered climate change. Environmental hazards such as climate change and the evolution of greenhouse gases due to fossil fuels are the primary reasons behind the need for Renewable energy. Fossil fuels dominate the energy market, contributing to 81% of the total global energy supply, with coal being the highest contributor [1]. Another study [2] supported the fact stating the consumption of fossil fuels has contributed majorly to carbon emissions. It emphasizes the need to switch from non-renewable to renewable resources such as Biomass energy. Therefore, there is a strong urge for human existence to rely on renewable sources for sustainable development. Before using fossil fuels, lignocellulosic materials such as wood chips and wood pellets were in use and shared a significant part in contributing to cooking and heating applications [3], [4], [5], [6]. Biomass is a plant and animal-based product used for producing biopower, bioheat, liquid biofuels, and biogas [1]. During 2000–2018, the bioenergy sector has grown at an annual growth rate of 2% [3]. In 2019, wood fuel of 1.9 billion m3 of capacity and wood pellets of 38.9 million tons production were produced globally [3]. Different contributions of other products of biomass are shown in Fig. 1

Fig. 1. Data depicting the supply of biomass globally in 2018 [1].

Several methods are available for the conversion of biomass into valuable and justified products, such as thermal conversion, thermochemical conversion, biochemical conversion, and chemical conversion. Thermochemical conversion refers to the decomposition of biomass in an anoxygenic or oxygenic atmosphere at a temperature range of 523 to 1673 K [7], [8]. It includes different types of technology, as shown in Table 1. This review paper focuses on biomass gasification (a part of the thermochemical conversion), as it is believed to be the most attractive method for biomass conversion because of its ability to treat a variety of biomass and waste-derived feedstock such as wood, sludge, crop,, and agricultural residues [4], [9], [10]. According to Biomass Gasification Market Report (2021) [11], the prevailing gasification market attained $98.2 Billion in 2020 and is anticipated to display moderate growth throughout the forecast period (2021–2026). Therefore, vividly depicting the future scenario of biomass-derived products. The study [12] showed that a 50% CO2 reduction could be obtained by 2050, bringing biomass use to 26%.

Route	Temperature (K)	Pressure (MPa)	Main Products
Torrefaction	503–573	0.1	Solid fuels
Liquefaction	523–603	5–20	Bio-oils, Gases
Pyrolysis	573–873	0.1–0.5	Bio-oils, Transport Fuels
Gasification	773–1573	≥0.1	Syngas
Combustion	973–1673	≥0.1	Heat, Electricity

Table 1. Different types of thermochemical conversion processes [8], [13].

Gasification of biomass is a thermochemical process in which solid biomass is treated with a limited amount of gasifying medium to convert it into valuable gases such as Methane (CH4), Carbon Dioxide (CO2), Carbon Monoxide (CO), and Hydrogen (H2) and a solid residue named char. The gases can be further converted to liquid fuels via the F-T process or heat and energy for power generation units [14], [15], [16]. The gasifying medium could be a gas like air, oxygen, or supercritical water (SCW). The major obstacle faced in the gasification process is the formation of tar, which is difficult to purify and decreases the yield of H2 [17]. The primary focus of the current review paper is:

Introducing hydrothermal technology for biomass gasification
Studies of different types of gasifiers, gasification parameters
Economic analysis of gasifiers employed
Future directions for the readers

2. Gasifying medium

Biomass contains biopolymers such as Lignin (10–25 wt%), Cellulose (35–55 wt%), and Hemicellulose (20–40 wt%) [18], [19]. Generally, a high (cellulose + hemicellulose)/Lignin ratio can produce a high-content synthesis gas [14], [15]. However, for complete biomass combustion, the stoichiometric fuel/air ratio varies from 6:1 to 6.5:1. Nevertheless, for gasification, the required amount is 1.5:1 to 1.8:1, which signals incomplete/partial combustion [6], [20]. Biomass gasification requires a gasifying medium to form synthesis gas (CO and H2) that can be further processed into more valuable chemicals and biofuels. The gasifying medium can be air, steam, oxygen, or SCW.

Air is the most common gasifying medium used for gasification because of its easy availability and low cost [21]. However, if steam acts as a gasifying medium, then product gas contains a higher H/C ratio [22]. Using air and steam, McCaffrey et al. [23] studied the gasification of almond biomass on a laboratory-scale fluidized bed gasifier. High hydrogen production and heating value were found in the case of steam over the air. Lv et al. [24] also supported the improvement in hydrogen yield by applying oxygen/steam gasification over air gasification using a downdraft gasifier. Oxygen is fed with steam as a gasifying agent to overcome the dilution of syngas by nitrogen commonly incorporated during air gasification. Oxygen can also serve as a gasification agent, as shown by Zhou et al. [25] in their study of biomass gasification on rice husk, sawdust, and camphor wood in an entrained-flow gasifier. However, oxygen-blown gasification is an energy-intensive and expensive route due to the separation of oxygen from the air via compression and refrigeration [26].

Supercritical water (not to be confused with subcritical) is a promising gasifying medium that has attracted much attention due to its numerous advantages over conventional gasifying mediums [26], [27]. Water is present at its supercritical conditions of 647 K temperature and 221.2 bar pressure above its critical point, where the distinction between liquid and gas phases does not exist [19], [28]. As the two phases co-exist, surface tension vanishes as the supercritical point is reached [29]. The process that uses supercritical water as a gasifying medium is called supercritical water gasification (SCWG).

One of the most significant advantages of employing SCWG is treating humid biomass feedstock that surpasses every conventional process [30]. Depending on the biomass feedstock, the moisture content varies from 5% to 35%, which consumes a lot of evaporation heat and overshadows the combustion heat obtained from gasification [5], [19]. In that case, the conventional gasification process requires pre-treatment of the feedstock such, as biomass drying. This unnecessarily increases the process’s cost [14], [31]. Unlike other biomass conversion processes (air/steam/oxygen gasification), SCWG is a hydrothermal process,, and therefore, high water content does not pose a problem [32]. In addition to hydrogen contribution from biomass, water from SCW also serves as a hydrogen donor and a gasifying agent [32], [33], [34]. Compared with other gasification processes, such as air/steam/oxygen gasification, the SCWG has high gasification efficiency at lower temperatures, as found in comparing two studies by Shayan et al. [35] and Yan et al. [36]. Table 2 depicts the heating values of different gasifying agents. Air has the lowest heating value accounted for due to dilution (Nitrogen) in it. Although, heating values also depend on the type of biomass taken and other parameters discussed in section.

Route	Medium	Type of biomass	Heating Value (MJ/kg biomass)	Reference
Gasification	Oxygen	–	15–17	[105]
	Air	Sewage sludge	8.02	[37]
	Steam	Bamboo	18	[104]
	SCW	Multiple feedstocks	41.6–61.6	[33]

Table 2. Heating value of syngas produced through biomass gasification for different gasifying medium.

3. Gasifiers

3.1. Gasifier operation

The gasifier is divided into different zones: drying, pyrolysis, combustion, and reduction zones [5], [8], [13]. Tar reforming is also an active step to convert large hydrocarbon molecules into smaller hydrocarbons [10]. Different zones surrounding the gasifier are described below:

3.1.1. Drying zone

Biomass contents are varied in terms of moisture present [5]. Table 3 shows different types of biomasses with varying moisture content in them. Depending upon the biomass used for gasification, the quality of the product is primarily determined. Biomass with a moisture content of 10% to 20% is typically recommended to produce syngas with a high heating value [14], [15]. High moisture-content biomass is usually dried in the drying zone before it can be used for gasification. However, high moisture content causes energy loss, and the quality of the product is degraded [33], [34]. The bounded water with the biomass is converted into steam above 373 K, and the process continues till 473 K [8], [13].

Biomass Feedstock	Moisture Content on Wet Basis (%)
Corn Stalks	40–60
Wheat Straw	8–20
Rice Straw	50–80
Rice Husk	7–10
Dairy Cattle Manure	88
Wood Bark	30–60
Sawdust	25–55
Food Waste	70
RDF Pellets	25–35
Water Hyacinth	95.3

Table 3. Moisture content in different biomass feedstock [8], [13], [34].

3.1.2. Pyrolysis zone

In this zone, several large molecular groups are converted into smaller hydrocarbons to form biochar, liquid products, and gaseous molecules [38]. Hemicellulose, being the least-stable compound, starts decomposing within 423 to 623 K, forming vapours, char, and tar. Hence, if char making is an objective, then the temperature of 573 K is sufficient. Cellulose present in biomass decomposes at 548 to 623 K, forming tar, char, and gaseous products. However, the quantity of tar is released more by cellulose than by hemicellulose. The lignin part of lignocellulosic biomass gets converted into aromatics. Also, lignin produces more char than cellulosic material. The lignin decomposition occurs at a temperature range of 523 to 773 K [8], [13]. Hence, the pyrolysis temperature predominantly affects the selectivity of products, and above 773 K, negligible tar is formed and yielding mostly gaseous products and bio-oils [39], [40]. Therefore, biomass pyrolysis occurs at a temperature range of 398 to 773 K, forming different products depending on temperature selection [5].

3.1.3. Combustion/oxidation zone

Technically, the combustion process is the easiest and most direct route for the processing of biomass decomposition [41], [42]. However, the overall heat liberated from biomass constituents in the combustion zone is low than gasification [42]. Inside the combustion zone, exothermic chemical reactions occur, increasing the temperature within the range of 1373 to 1773 K [5], [8], [13]. Also, the amount of gasifying agent is usually controlled so that it does not reach the ash’s slagging temperature, causing problems in its operation [40]. In this zone, the final products formed are CO, CO2, H2, and H2O, and the heat liberated is used for partially drying the constituents and partially in the pyrolysis process [5], [43].C+O2→CO2ΔH=-406kJg.moleH2+12O2→H2OΔH=-242MJKg.mole

3.1.4. Reduction zone

As mentioned earlier, tar is the biggest problem generated in biomass gasification. Excessive tar content in the fuel gas will reduce the overall efficiency of biomass and increase the overall separating cost of the plant [13], [17]. If tar is left untreated, it can clog the filters and can even polymerize into complex molecules [44]. Table 4 shows the limit of tar to various end uses of biomass gasification. The reduction zone is named so because it reduces the tar particles present in the produced gas by bringing them to a high temperature of around 1273 K [45].

Application	Tar (g/N.m3)
Syngas Production	0.1
Gas Turbine	0.05–5
IC Engine	50–100
Fuel Cells	<1.0

Table 4. Maximum limits of biomass tar production [8], [12].

Tar can be reduced in two ways: (i) In-situ Tar Reduction (Primary Process) and (ii) Secondary Process [46]. In the primary process, the tar is removed by optimizing and designing the gasifier, varying the operating conditions of a gasification process, and incorporating catalysts bed to ease the process further. Post-gasification equipment is employed during the secondary process, such as wet scrubbers, wet electrostatic precipitation, or other filters. In this review paper, the in-situ or primary process is discussed.

Following reactions take place during in-situ tar reduction [5], [20]:

Tar Reforming: CxHy+xH2O->x+y2H2+xCO
Bouduard Reaction: C+O2->2CO ΔH=-172kJmole
Water Gas Reaction: C+H2O->CO+H2ΔH=-131kJmole
Water Gas Shift Reaction: CO+H2O->CO2+H2ΔH=41.2kJmole
Methanation Reaction: C+2H2->CH4 ΔH=74.8kJmole

3.2. Types of gasifiers

The size and type of gasifier depend on several parameters, including the demand of the products, moisture content, and fuel availability. They are mentioned as:

3.3. Fixed bed and moving bed gasifier

Fixed bed gasifiers are the most commercially equipped gasifiers because of their simple design and easy operation. These are termed moving bed because the fuel moves down the gasifier in the form of a plug. The operating pressure range of the fixed bed gasifier lies within 0 to 70 bars. The gas composition for fixed bed gasifier is 3–5% CH4, 10–15% CO2, 10–15% CO, 15–20% H2, and 40–50% N2 [47], [48]. Depending upon the contact between the gasifying agent and biomass fuel, fixed bed gasifiers are further classified as:

3.3.1. Updraft gasifiers

In updraft gasifiers, biomass flows from the top of the reactor, and the gasifying agent enters through the bottom, giving the feel of counter-current operation. Hence, updraft gasifiers are also termed counter-current gasifiers. As shown in Fig. 2(a), the lowest portion of the reactor is a “combustion” zone, where the products formed due to drying and pyrolysis are combusted. This helps to raise the temperature of the combustion zone, and to that of up-flowing gas to about 1000 K. The hot gases that flow upward are immediately reduced in the vicinity of the reduction zone and serve as a medium for biomass drying and steam generation. Updraft gasifiers possess the highest thermal efficiency because of the low temperature of gases exiting the gasifier unit [5], [14], [49], [50].

3.3.2. Downdraft gasifiers

Contrary to updraft gasifiers, the gasifying agent is fed at the combustion zone, middle section, as shown in Fig. 2(b). Biomass flows from the top to the drying section, where the moisture is removed and then further into the pyrolysis zone, where the solid fuel is converted into char and gases. Now, as the motion continued, the gasifying agent is fed, and the gases are combusted. The principal purpose of maintaining the gasification temperature is met at this zone, and then the biomass further moves into the reduction zone. Since the direction of biomass flow and gas is co-current in this gasifier, downdraft gasifiers are called “Co-Current Gasifiers [5], [13], [48], [49]”.

3.3.3. Cross-draft gasifiers

In cross-draft gasifiers, the gasifying agent is fed from the side nozzles, and the fuel is fed from the topmost position. Opposite to updraft and downdraft gasifiers where the product gas releases from the top or bottom position, the product gas in case of cross-draft gasifiers releases from the opposite channel to where the gasification agent enters, as shown in Fig. 2(c). Because of this configuration, cross-draft gasifiers are also called “Side-draft gasifiers.” Since the gasification agent facilitates the combustion process, the temperature of the combustion zone reaches nearly 1773 K [5], [14], [47]. The advantages and disadvantages of all three fixed/moving bed gasifiers are shown in Table 5.

Fig. 2. Schematic diagram of (a) Updraft gasifier, (b) Downdraft gasifier, (c) Cross-draft gasifier [5], [13], [14], [49].

Type of Gasifier	Advantages	Disadvantages
Updraft gasifier	- Can handle biomass with high moisture (<60%) and ash content (<25%) - Overall good thermal efficiency - Utilizes heat of combustion effectively because of counter-current operation - Less pressure drop - Slight tendency to forming slag	- Ideal only for small scale uses - Highest tar yield (30–150 g/N.m3) - Not suitable for high-volatility fuels - Takes long time to start the engine - Low production of syngas - Low reaction capability
Downdraft gasifier	- Low tar production rate than updraft gasifiers (0.015–3 g/N.m3) - Takes less time to ignite	- Induces low thermal efficiency because of the high outlet temperature of a gas - Particulate content is high
Cross-draft gasifier	- Lowest tar production (0.01–0.1 g/N.m3) - TGood permeability of bed offered - Offer faster response time - Start-up time for an engine is relatively low - Can handle high-moisture biomass only if the top part of the gasifier remains open for escape	- Suitable for small scale units - Not suitable for a high ash and tar content

Table 5. Advantage and disadvantages of different packed bed gasifier [13], [14], [15], [20], [49], [50], [51], [52], [53].

3.4. Fluidized bed gasifiersr

The fluidized bed gasifiers offer uniform temperature distribution, a well-mixing platform for gas and solid. That, in turn, reduces the risk of agglomerating the fuel stack [51]. Silica, sand, dolomite, glass beads are commonly used as bed material for fluidized bed gasifiers. However, it is reported that the use of magnesite can further enhance the production of H2 as compared to sand as a bed material [52]. The operating temperature of fluidized bed reactors largely depends on the melting point of bed material and ash and hence, is restricted within 923 to 1223 K [54], and the pressure is in the range of 0 to 70 bar [48]. Tar formation is intermediate between updraft and downdraft gasifier, with an average of 10 g/N.m3 [8], [13]. Because of the temperature at which this gasifier operates, the reactions cannot move near equilibrium, reducing the formation of hydrocarbon in outlet gas [5]. Instead, the carbon conversion efficiency of these gasifiers is significant and attainable to 95% [55].

Fluidized bed gasifiers are further divided into two main categories based on the velocity of the gasifying medium: (i) Bubbling bed gasifier, (ii) Circulating bed gasifier.

3.4.1. Bubbling bed gasifier

Bubbling bed gasifier is classified as the oldest reactor in fluidized bed gasifiers [56], [57]. The beds are majorly made up of inert particles such as sand or silica. Moreover, it is observed that imparting catalytic activity to the bed material shows a significant enhancement in the conversion rate of tar [57]. In these reactors, gasifying medium ascends the bed with a low gas velocity of<1 ms−1 to carry the fluidization process and maintain the state of the gas bubble mixture. When particles reach the top of the bed, the cross-sectional area increases, and hence, the velocity, in turn, decreases, causing the particles to descend to the bed. Since this favors the formation of particulates, a bubbling bed reactor needs an army of cyclone separators at the exit of the reactor [58]. Bubbling bed reactors can convert biomass feedstocks with high ash contents and better flexibility over fuel processing and loading. However, they are less efficient for converting carbon as compare to circulating bed reactors [59]. Fig. 3 shows the functional diagram of Bubbling bed reactors.

Fig. 3. Schematic diagram of bubbling bed gasifier [6], [49].

Fig. 4. Schematic diagram of Circulating Bed Gasifier [6], [59].

3.4.2. Circulating bed gasifier

Circulating bed gasifiers/reactors are attractive because of the ample residence time they provide. The fluidization velocity in the circulating bed reactor (3.5–5.5 ms−1) is much more than that of the Bubbling bed reactor. Here, the solids flow outside the reactor and are recycled back into the reactor, as shown in Fig. 4. Due to the high inlet gasification medium velocity, the solids are pushed towards the walls. This can lead to severe back mixing if not operated at optimal conditions. Further, the accumulation of solids in the recycle zone can also create a problem in free-flowing and can cause agglomeration if the moisture content is high. However, these reactors can provide a high gas yield and increase the heating value of the gasifier products [5], [8], [13].

Circulating bed reactors offer many other advantages over all other types of gasifiers, such as being suitable for large-scale applications, low tar production, higher carbon conversion efficiency, and optimal residence time. However, the overall cost of the system is high [51], [58], [59].

3.5. Advancements in gasifiers

Fluidized bed gasifiers were the promising technology for gasification and tar reforming, but still, they possess limited scale-up due to their difficult operating and high initial and operating cost [50], [54], [60], [61]. Hence, many new reactors are designed and optimized, which are presented here:

3.5.1. Entrained flow gasifier

One of the most successful gasifiers is an entrained flow reactor operating at a 20–70 bar pressure. The carbon conversion efficiency of almost 100% is achievable in an entrained flow reactor, and hence, they are also suitable for large-scale applications [47], [60]. The operating temperature of these gasifiers is above or close to 1273 K, suitable for tar cracking [62], [63]. However, the temperature attained is determined mainly by the type of biomass fuel-fed [64], [65]. Moreover, the residence time in these reactors is also low, owing to their high efficiency and low methane content at the outlet gas [65]. Entrained flow gasifier works in a fashion of co-current reactors where biomass fuel, either in minute particles or slurry and a gasifying agent is fed to the reactor. This results in entraining the particles as they travel through the dense cloud. The gas is taken out from the top/bottom part, and the slag or liquid ash gets deposited at the bottom [66]. However, operability at the high temperature of these gasifiers creates problems for material selection and ash melting [67]. Fig. 5 shows the schematic diagram of the entrained flow gasifier.

Fig. 5. Schematic diagram of entrained flow gasifier [12], [14], [62].

3.5.2. Dual fluidized bed gasifiers

Dual fluidized bed reactors have come into existence to overcome the drawbacks of fluidized bed reactors. This system either connects the circulating and bubbling reactors or operates as a single unit. It primarily consists of a gasification unit and a combustion unit. The two units are connected thermally that transports the heat from combustion to the gasification chamber [50], [68]. A gasification agent is fed into the first unit, raising the bed’s temperature to over or around 1073 K [69]. Hence, tar production remains a problem in this gasifier. Other types of dual fluidized bed gasifiers are, (i) Transport Gasifier which combines entrained-flow and fluidized bed gasifier properties, (ii) Twin Reactor System where the two units, gasification, and combustion are separated from each other, (iii) Chemical Looping Gasifier, where two different streams of CO2 and H2 rich are separated and hence, CO2 can easily sequestrate [8], [13], [58].

3.5.3. Supercritical water gasifier

Supercritical Water Gasification is a featured technology that can convert wet biomass into respective syngas. Biomass drying is not required when operating an SCW reactor, and hence, a large amount of energy cost can be saved [19]. Matsumura [32] compared SCWG with biomethanation from an economic perspective. The studies revealed that the hydrogen production cost was 0.01 $/MJ lower than biomethanation under similar conditions. Kamler et al. [70] performed the SCWG of municipal sludge for energy recovery. The paper showed that with SCWG, 150 kWh of electricity each year can be saved, accounting for 6 $ billion. More economic studies are presented in Table 6. However, some of them signify the inability to reduce the production costs of feedstocks, which in turn, make the operation unprofitable.

Feed stock	Major Product(s)	Gasification Temperature (°C), Pressure (MPa)	Product Cost	Major Key Points	Reference
Black liquor	Pulp	450–600, 25	-	- Pulp production capacity can increase up to 75% - Depicting high thermal efficiency - Minimum selling price of pulp can be reduced to 22%, leading to more demand	[74]
Sewage sludge	H2	700, 35	3 $GJ−1	- SCWG found to be a cost-effective method	[75]
Sewage sludge	Bio heavy-oil	375, 23	1.82 $/kg	- High heating value of 36 MJ/kg obtained - High selling profit obtained for supercritical water	[76]
Algae slurry	syngas	605, 24	2.5 $/L	High cost of feedstock such as algae, hydrogen makes the process unprofitable with respect to fossil-fuel based operations for low production of syngas	[77]
Microalgae	Biofuels	400	3.2 $/kg	High energy requirements for the feedstock – microalgae (194–484 €/GJ) in compare to natural gas (17.3 €/GJ) makes the current process highly unfavorable	[78]

Table 6. Economic analysis of various literature studies conducted via SCWG.

In an SCW reactor, supercritical water is used as a solvent and has immense benefits such as low dielectric constant and enhanced solubility [19], [25], [71], [72]. However, when operating SCW at high temperature and pressure, there may be concern over corrosion and durability of the reactor [19], [71]. Moreover, the flow and operational parameters largely influence the type of reactor used discussed in section 3. Water present at 647.12 K temperature and 221.2 bar pressure above its critical point is termed to be supercritical [19], [25]. SCW gasification is generally performed at a pressure of about 30 MPa and a temperature of between 873 and 923 K [4], [73]. When exceeding the temperature of 873 K, Carbon gets converted to CO2 as water starts acting as an oxidant [13]. When catalysts are employed, the gasification could be done at a temperature range of about 623–773 K [15]. When no heat losses are considered, the energy efficiency of SCWG is found to be 30%, where 70% methane is obtained in the product gas [25]. However, methane can be converted to hydrogen via a reforming reaction. The schematic diagram of SCWG is shown in Fig. 6.

Fig. 6. Schematic diagram of SCW reactor [72].

4. Operating parameters

4.1. Gasification temperature

The gasification temperature largely determines the product gas quality, tar formation, reactor requirement, and capital costs [[79], [80]]. Numerous studies have been done to support the fact [[32], [81], [82], [83], [84], [85], [86]]. Dung et al. [87] investigated Ni/BCC and Ni/Al2O3 catalysts for decomposition of tar from woody biomass gasification, resulting in catalytic tar reforming at a low temperature of 923 K. Wongsiriamnuay et al. [88] performed the catalytic gasification of bamboo in a fluidized bed. Using calcined dolomite as a catalyst, a tar-reforming reaction was promoted when the temperature was increased from 400 to 600 °C. In addition, CO2 contents were decreased during the reaction.

4.2. Gasification pressure

The gasification performance and the product gas quality are greatly influenced by the partial pressure of gasifying agents and gasification pressure [79]. In the case of SCW, pressure dependency is quite critical. With an increase in pressure, the dielectric constant and density of SCW increase, and hence, the rate of biomass disruption increases [79]. The increase in density of SCW also causes a cage formation around polymerization or coking and thus, decreases the rate of those reactions. At the same time, this property caused at high pressure favors reforming and water–gas shift reactions.

Moreover, the increase in density also favors the lignin decomposition present in biomass [89]. Basu et al. [73] investigated SCWG of rice husk at different pressures (23–34 MPa) over a temperature range of 500-700OC. It was found that pressure has a more pronounced effect on the SCWG of biomass at high temperatures. On the contrary, Kruse [31] studied SCWG of glucose and glycerol over the pressure range [5–45 MPa]. It was concluded that the pressure had a negligible effect on the yield of product gases.

4.3. Bed material

Bed material possesses a prominent place when deciding on a gasifier. It can be inert or catalytic based on other process conditions and demand [79]. Silica, Dolomite, Limestone, Olivine, and Alkaline metal oxides are amongst the inert materials. However, Ni and K-based catalysts in bed can influence the operating parameters (pressure, temperature) and product gas composition [14], [90]. An experiment was conducted where K, Ca, and Fe materials were added into bed and analyzed for char conversion, increasing oxygen transport through bed material, decreasing tar production, and altering gas composition [91]. It was found that bauxite leaves an excellent impression for fuel conversion, and olivine ensures good quality producer gas production when a balance between the three materials is taken. Another paper studied the addition of K and S into the ash-coated olivine [90]. Hence, alkali metal catalysts such as KOH, K2CO3, Na2CO3, KHCO3, and NaOH are found effective in literature for the gasification of biomass [79].

The use of Ni and Ru-based catalysts have also been studied extensively. A study was done where Ni/Al2O3 and Ru-doped Ni/Al2O3 catalysts were analyzed for biomass gasification [92]. It was concluded that Ru doping only reduces coke formation, but no change in H2, CO, and CO2 yields were observed. Different catalysts support also play a significant role in deciding the yields. A report shows that incorporating catalytic support enhances the gasification efficiency and increases the yields of H2 [93]. The choice of activated carbon as a catalyst can be a promising step for SCW gasification [79].

Hence, alkali metal catalysts support increasing hydrogen yield but lag on operating at high temperatures because of the deactivation of other metals and enhanced plugging. On the other hand, Ni and Ru-based catalysts are free from the shortcomings of alkali metal catalysts but are expensive, less resistant to coke formation, and require hot water-resistant materials for support [16].

4.4. Type of biomass

Biomass primarily consists of cellulose, hemicellulose, and lignin. High (cellulose + hemicellulose)/lignin ratio is necessary to produce high content syngas [14], [15]. Lv et al. [94] compared the effect of cellulose, lignin on biomass gasification with six types of biomasses whose cellulose content varied from 55% to 85% and lignin content from 10% to 35%. The tar and gas yields were observed to be dependent on the cellulose content and increased with increasing cellulose in biomass.

Qian et al. [95] investigated gasification on switchgrass, sorghum straw, and red cedar as different biomasses with different ERs. Due to the high BET surface area, red cedar was found to be a better feedstock. Also, when the ER ratio was low, red cedar provides HHV of 9.09 MJ/kg, but it decreases with an increase in ER. Yanik et al. [96] performed the gasification of lignocellulosic materials and tannery wastes via SCW mechanism and found that fibrous structure, amount of coke in the feedstock, and organic materials affect the yield and composition of gaseous products. While dealing with biomass, moisture content also plays a significant role. Low-moisture content biomass (<15 wt%) are suitable for the gasifiers. With an increase in moisture content, the energy requirements vary proportionately [63]. Basu [13] compared different biomass feedstocks based on moisture content, and it was found that wheat straw, rice husk are considered the better feedstocks due to their low moisture content.

4.4.1. Equivalence ratio (ER)

The equivalence ratio is the ratio of actual air to fuel required to the stoichiometric requirement of air to fuel. Increasing the ER introduces more air into the gasifier, thereby increasing the oxidation reaction rate forming more CO2. Jayathilake [97] confirmed the statement by performing gasification on birchwood feed. Syngas yield and CGE have shown an increasing trend as ER increases. Jangsawang et al. [98] performed biomass gasification on cellulose and concluded that the product composition strongly depends on ER. There’s a strong correlation found between ER and gasification temperature. For gasification between 600 and 900 K, the optimum ER was found to be 3. For a moderate temperature range of 1000–1500 K, the ratio decreases to 2. Thereby increasing the temperature results in low ER.

4.4.2. Residence time

Residence time is defined as the average time the molecules spent in the reactor. The gasification efficiency increases to a certain extent when increasing residence time but remains unchanged after a particular value of residence time increment [79]. Hernandez et al. [99] performed the gasification upon three types of biomass fuels, including grapevine pruning, sawdust, and marc of grape. Longer residence time increased the syngas yield and, in turn, increases the H2/CO ratio. But remains constant after the temperature moved to 1050 °C. Latifi et al. [100] experimented on sawdust for the production of bio-oil. The group found that longer residence times and high gasification temperature helped increase the syngas yield and quality.

4.4.3. Particle size of biomass

The lower the particle size of biomass, the higher the surface area that it offers. Hence, this property can increase or decrease the solubility depending upon the particle size of feedstock. It can also actively participate in enhanced heat and mass transfer operations and offers better exposure to reaction vicinity, which can improve the product yield [19]. Hernandez et al. [99] concluded that the decrease in particle size could improve the syngas quality, CGE, and H2/CO ratio. Luo et al. [85] showed an increase in carbon conversion efficiency from 58.0% to 99.87% for the small particle size. In addition, char and tar formation depends on particle size and gasification temperature. However, negligible tar formed when a particle (<0.075 mm) was chosen.

The particle size can vary depending upon the type of reactor chosen for the gasification operation. Fixed bed reactors can tolerate particles of up to 51 mm, and on the other hand, entrained flow reactors cannot cooperate with particle sizes greater than 0.15 mm. Bubbling bed reactors from fluidized bed reactors can only digest particle sizes up to 6 mm [79]. Not much study has been conducted finding the variation of particle size on SCWG [19].

4.4.4. Steam to biomass ratio

Steam to Biomass ratio is defined as the amount of steam to biomass fed in the gasifier. For the biomass gasification process, the optimal value of the steam to biomass ratio varies from 0.3 to 1.0 [79]. Different biomass feedstocks such as coffee bean husks, green wastes, food wastes, municipal solid wastes, pine sawdust, wood residue, and wood chips were analyzed [29]. It was concluded that the H2 production increased with increasing steam to biomass ratio. Higher H2 concentrations were obtained for steam to biomass ratio of 1.35–4.04 [79].

5. Future prospects

Converting biomass feedstocks into energy production is a promising field because of the increasing shortage of fossil fuels and enhanced global problems. However, the researchers are still facing significant problems in anti-coking, anti-sintering, and anti-poisoning catalyst development. Catalysts can alter the temperature, product yields, and tar formation for gasification. Then, after catalyst manufacturing, a successful scale-up is another problem that needs to be addressed. The impact of various catalyst designs on different reactors and subjected to various parameters can be studied [[101], [102], [103], [104]]. Still, there is room for better and cost-effective development of catalysts in the field of biomass gasification. Another critical factor to be taken into account is the kind of gasifier/reactor used to produce maximum H2. In section 3.2, various kinds of gasifiers are discussed, but each one faces specific issues and is limited to either small- or large-scale operations. However, many advancements, including downdraft gasifiers with throat, dual fluidized bed gasifiers, staged reactors, will affect significantly different gasification parameters [[3], [101], [102], [103], [104], [105], [106]].

The field of hydrothermal gasification still needs much attention because of its potential benefits. Moreover, data processing and machine learning methods need to be incorporated for better processing of gasification data. Such methods will outpower the conventional data collection methods and facilitate understanding various gasification parameters for future researchers.

6. Conclusion

In our research , different gasification technologies are presented, along with an emphasis on SCWG. The primary technologies such as fixed bed, fluidized bed, some advanced gasifiers such as entrained flow, dual fluidized bed and SCW reactor are also considered. Biomass conversion is a promising technology that encapsulates much potential to replace the use of fossil fuels. With an increase in global warming and health-related issues, it is vital to harness bio-energy for day-to-day applications. Different parameters such as feedstock type, operating conditions (pressure and temperature), residence time, bed material etc., for the formation of biofuels using F-T synthesis are discussed. As depicted, syngas quality of biomass strongly depends on these factors. However, for some conditions discussed, biomass gasification cannot overcome the high energy costs, capital expenditure, and total product costs. Therefore, the scalability of biomass energy to large areas remains a challenging task for the renewable energy field. SCWG is a superior technology that can be cost-effective compared to various other gasifiers but lacks plugging, efficiency, corrosion, and high hydrogen production cost.

Future research, including process optimization, reactor selection, low-cost catalyst preparation, and better selection of gasifying medium and technology, can exponentially increase the growth of the biomass era. Ni-based catalysts are extensively researched in the past years because of economic viability and activity. However, they are easily poisoned and needs improvement. Then comes transition and rare-earth metals, which are doped into Ni-based catalysts to improve the performance of conventional catalysts. Hence, novel catalysts are still to be discovered to enhance the selectivity, activity, and productivity of biomass gasification.