Catalytic gasification of wheat straw in hot compressed (subcritical and supercritical) water for hydrogen production

To supplement the increasing energy demands and cope with the greenhouse gas emissions, biofuels generated from lignocellulosic biomass are gaining widespread attention. In this study, wheat straw was used as a candidate lignocellulosic biomass to produce hydrogen fuel through hydrothermal gasification. The fluid phases of water investigated for gasification included subcritical (300 and 370°C) and supercritical (450 and 550°C) phases. Along with the effects of temperature (300‐550°C), the influences of feed concentration (20‐35 wt%) and reaction time (40‐70 minutes) were comprehensively studied for wheat straw gasification in subcritical and supercritical water. To maximize hydrogen and total gas yields, the effects of two metal catalysts (eg, Ru/Al2O3 and Ni/Si‐Al2O3) were examined. Hydrogen and total gas yields, as well as lower heating values of the gas products, were comparatively evaluated during the subcritical and supercritical water gasification of wheat straw. Supercritical water gasification of wheat straw at 550°C with 20 wt% feed concentration for 60 minutes of reaction time resulted in higher yields of hydrogen (2.98 mmol/g) and total gases (10.6 mmol/g). When compared to noncatalytic gasification, catalytic gasification using 5 wt% loading of Ru/Al2O3 and Ni/Si‐Al2O3 enhanced the hydrogen yields up to 4.18 and 5.1 mmol/g, respectively, along with respective total gas yields of 15 and 18.2 mmol/g. Nonetheless, wheat straw‐derived biochar produced at high supercritical water temperatures also retained high carbon content and calorific value.

forestry biomass can potentially replace fossil fuels partially or completely when used in pure-form, blended-form, or as synthetic fuels. 2 Biofuels are considered carbon-neutral as the plants recycle the CO 2 released from their combustion during photosynthesis. 3,4 Biofuels are produced from renewable bioresources such as lignocellulosic biomass (eg, agricultural crop residues, forestry biomass, and energy crops), algae, and other organic wastes such as municipal solid wastes, cattle manure, sewage sludge, and industrial effluents. 5 These organic wastes can be transformed to biofuels (eg, bio-oil, syngas, biogas, hydrogen, bioethanol, and biobutanol) through appropriate thermochemical technologies (eg, liquefaction, pyrolysis, and gasification), and biochemical technologies (fermentation and anaerobic digestion). 6 Typically, lignocellulosic biomass contains cellulose (35-55 wt%), hemicellulose (20-40 wt%), and lignin (10-25 wt%). 7 Enormous amounts of agricultural crop residues are accessible throughout the world in the form of straw, stalks, fibers, shells, husks, pits, de-oiled cakes, pulps, etc. Among all, straws comprise of nearly 15-40% of the total agricultural crop residues, which have tremendous possibilities to be converted to biofuels. 8 Hydrogen is a clean fuel with a high-energy content of 120 MJ/kg compared to any other conventional fuel. 9 It is a clean fuel because it results in water upon combustion. Although traditionally generated through the reformation of natural gas, H 2 can be produced through gasification, biophotolysis, photo-fermentation, dark fermentation, and electrolysis. 9,10 Hydrogen can be generated from lignocellulosic biomass through hydrothermal gasification. Organic materials including lignocellulose materials (cellulose, hemicellulose, and lignin) degrade into simple molecules during gasification in subcritical water (SbCW) and supercritical water (SCW) to produce syngas. Syngas, the main gas product of hydrothermal gasification process, is a mixture of H 2 , CO, CO 2 , CH 4 , C 2 H 6 , and C 2+ gases. 11,12 Through gas-to-liquid technologies such as Fischer-Tropsch catalysis and syngas fermentation, syngas can be converted to green diesel, light and heavy synthetic petroleum, ethanol, and other hydrocarbons. 13 Freshly harvested agricultural biomass usually contains high water content. Prior to their thermochemical conversion (pyrolysis, gasification, or liquefaction), it is essential to remove moisture through drying or preheating, which adds cost and energy to the overall process. 11,14,15 However, subcritical and supercritical water gasification eliminates the requirement of additional preheating steps because of the use of water as the aqueous reaction medium. Because water is the only medium in hydrothermal gasification, its fluid phases affect the overall process. The critical temperature (T C ) and critical pressure (P C ) of water are 374°C and 22.1 MPa, respectively. 16 Subcritical water gasification is characterized with temperatures and pressures lower than the critical points of water (T < 374°C and P < 22.1 MPa). In contrast, supercritical water gasification employs temperatures and pressures higher than the water's critical points (T > 374°C and P > 22.1 MPa). Supercritical water is known to exhibit superior mass and heat transfer properties with a combination of gas and liquid-like properties.
Hydrothermal gasification has been investigated for H 2 -rich syngas production from several varieties of lignocellulosic biomass including agricultural and forestry residues as well as energy crops. Nanda et al 17 performed SbCW and SCW gasification of pinewood and wheat straw impregnated with catalytic nickel nanoparticles at 300-500°C and 23-25 MPa pressure for 15-45 minutes. Compared to noncatalytic gasification, higher yields of H 2 (2.8-5.78 mmol/g) and total gases (9.5-16.2 mmol/g) were obtained from SCW gasification of Ni-impregnated biomasses under optimal conditions (temperature: 500°C; biomass-to-water ratio: 1:10; residence time: 45 minutes; and pressure: 25 MPa). SCW gasification of timothy grass (an energy crop) at 650°C with 1:8 biomass-to-water ratio for 45 minutes led to higher yields of H 2 (5.15 mol/kg) and total gases (17.2 mol/kg). 18 However, addition of 3% KOH as catalyst to the gasification process maximized H 2 and total gas yields up to 8.91 and 30.6 mol/kg, respectively. Safari et al 19 reported maximum H 2 yields of 4.1, 4.63, and 7.25 mmol/g from noncatalytic SCW gasification of almond shell, walnut shell and wheat straw, respectively, at 440°C and 10-20 minutes of reaction time. In another study, SCW gasification of 2.5 wt% of wheat straw black liquor at 750°C for 50 minutes resulted in maximum H 2 yields of 62.4 mol/kg and carbon gasification efficiency of 98.2%. 20 The use of homogeneous catalysts (eg, alkalis, carbonates, and hydroxides) and heterogeneous catalysts (eg, metals) in hydrothermal gasification has also found to enhance the gas yields. The catalysts have the potential to crack the biomass and its fragmented derivatives (eg, intermediates, tars, and char) by several mechanisms, especially by: (a) the cleavage of C-C bonds; (b) dehydration of oxygenated compounds via C-O bond cleavage; and (c) suppression of re-polymerization of formed intermediates to tar and char. In addition, metal catalysts can also degrade the organic acids and complex phenolic compounds, which are key intermediates during the fragmentation of lignocellulosic biomass. 21,22 Transitions metal catalysts have proved to be active for the degradation of biomass at supercritical conditions. [23][24][25] However, the harsh operating conditions (high temperatures and high pressures) during SCW gasification often result in the sintering of metallic catalysts, deactivation and nondurable repeat cycles. In addition, to make the metallic catalysts more selective toward H 2 production rather than CH 4 or any other gases, it is important to modify the catalyst surface with a stable support for activity toward dehydrogenation reactions. This directs toward the implication of the metallic supports for the active metals to make them more stable and durable without losing activity, selectivity, and stability.
Reports suggest that the use of Ni and Ru-based catalysts in SCW gasification of biomass has shown superior performances in gas yields. 11,[26][27][28] The performances of Nisupported catalysts were tested for gasification of different feedstocks with results inferring that Ni/α-Al 2 O 3 and Ni/ hydrotalcite enhanced H 2 selectivity and yields. 29 In another study by Kang et al, 30 the screening of Ni-based catalysts onto different supports with various promoters was tested for the SCW gasification of lignin. They found that the activity of Ni-based catalysts with Al 2 O 3 support and cerium as a promoter improved H 2 yields and selectivity. Afif et al 31 employed Raney nickel catalyst to gasify activated sludge at 380°C and 3 wt% feed concentration for 15 minutes. A gas product constituting 46% H 2 , 25% CH 4 , and 29% CO 2 was reported.
In a study on the SCW gasification of lignin at 400°C, Osada et al 32 reported that the stability of different Rusupported catalysts varied as Ru/TiO 2 > Ru/γ-A 2 O 3 > Ru/C. While Ru/TiO 2 maintained its high gasification activity for three subsequent cycles, Ru/C lost its activity after the first run due to the decrease in its surface area. Although Ru/γ-A 2 O 3 showed good catalytic performance at the initial stage, its activity gradually reduced during repetitive cycles. This was due its structural transition from gammastate to alphaphase and dissolution of active Ru species in SCW. Nevertheless, alumina (Al 2 O 3 ) has shown high performance toward their stability and activity than other catalyst supports. 24 Moreover, depending on chosen biomass species and the operating conditions, the metallic catalysts may show variable selectivity to a particular gas.
One of the main objectives of the current study was to investigate the catalytic performances of commercial ruthenium (Ru/Al 2 O 3 ) and nickel (Ni/Si-Al 2 O 3 ) catalysts in SbCW and SCW gasification of wheat straw, which is a model agricultural crop residue representing lignocellulosic biomass. There is lack of literature on the comparative SbCW and SCW gasification of wheat straw using metallic catalysts. To fill this knowledge gap, the current study aimed at examining the aqueous decomposition behavior of wheat straw at variable process parameters such as temperature, feed concentration, reaction time, and catalysts.

| Biomass and catalysts
Wheat straw (Triticum aestivum) used in this study was obtained as straw bales from a local farm in Saskatoon, Saskatchewan, Canada. After collection, wheat straw was roughly chopped, mechanically dusted to remove any soil particles and air dried for a week. Prior to gasification, the chopped wheat straw was ground to an average particle size of <1 mm using an IKA MF10 Basic Microfine S1 grinder (ThermoFisher Scientific Inc., Mississauga, ON, Canada). The pulverized wheat straw was stored in airtight containers at room temperature prior to its gasification.
Two metal catalysts such as Ru/Al 2 O 3 and Ni/Si-Al 2 O 3 used in gasification experiments were procured from Sigma-Aldrich Canada Co., Oakville, ON, Canada. The two commercial metal catalysts such as Ru/Al 2 O 3 (PubChem SID: 24867548) and Ni/Si-Al 2 O 3 (PubChem SID: 24852528) had the reported surface areas of 90 and 175 m 2 /g, respectively.

| Hydrothermal gasification
Hydrothermal gasification of wheat straw was performed under a pressure range of 21-23 MPa to examine the effects of temperature (300-550°C), feed concentration (20-35 wt%) and reaction time (40-70 minutes). For all experiments, deionized water served as the medium of hydrothermal gasification. In a typical experiment, a calculated amount of wheat straw (eg, 2 g at 20 wt%) was loaded into the tubular reactor along with 8 mL of deionized water. After determining the optimal temperature, feed concentration and reaction time, the effects of Ru/Al 2 O 3 and Ni/Si-Al 2 O 3 at a constant loading of 5 wt% were studied for catalyzing the hydrothermal gasification.
Hydrothermal gasification was conducted in a tubular batch reactor made of stainless steel SS316 (length: 10 in., outer diameter: 0.5 in. and inner diameter: 0.37 in.). The schematics, geometry and design of the gasification reactor have been mentioned in Figure 1. The reactor assembly was custom-made using the tubing and fittings of SS316 grade procured from Swagelok (Swagelok Central Ontario, Mississauga, ON, Canada). The tubular reactor was electronically heated using an ATS split furnace, controlled by ATS temperature controller and monitored by Omega Type-K thermocouple (Spectris Canada, Inc., Laval, QC, Canada).
The inert gas nitrogen was used to create an initial reactor pressure of 10-15 MPa that expanded to the desired subcritical and supercritical pressure of 21-23 MPa depending on set gasification temperature following the equation of state. Furthermore, as the reactor was heated to the desired temperature, the initial reactor pressure increased from 10-15 MPa up to 21-23 MPa following the ideal gas law. Depending on the heating rate of the furnace and the final test temperature, the reactor took nearly 30-40 minutes to reach water's critical state. After the desired test conditions were attained (ie, pressure: 21-23 MPa and temperature: 300-550°C), the reaction was started by monitoring the reaction time (40-70 minutes). The pressure gauges monitored the pressure inside the reactor assembly and pressure relief valves aided in releasing any extra pressure beyond 27 MPa (safety limit for the reactor assembly).
After the completion of the gasification experiments, the hot gaseous products passed through the 2-μm filter to entrap any char fines. The gas products then passed through the gas-liquid separator where they were cooled by the cold-water spray. The condensed liquid effluent was collected gravimetrically from the bottom of the separator, whereas the noncondensable gas products passed through the desiccant column (LabClear, Oakland, CA, USA) to trap the moisture after being collected in gas sampling bags. After the collection of gases, the solid products (biochar) were allowed to cool inside the reactor. The cooled biochar was collected, weighed, and stored in dry and airtight glass jars placed inside a desiccator.

| Gas phase analysis
An Agilent 7820A gas chromatography (Agilent Technologies, Santa Clara, CA, USA) was used to analyze the gaseous products. The gas chromatography was equipped with a thermal conductivity detector (TCD) consisting of three columns, namely Ultimetal HayesepQ T 80/100 mesh column (H 2 , CO, and CH 4 ), Ultimetal Hayesep T 80/100 mesh column (CO 2 ) and Ultimetal molsieve13 80/100 mesh column (N 2 and O 2 ). All the columns were maintained at 60°C using argon as the carrier gas.
The individual gas yield (in mmol/g) and total gas yield (in mmol/g) were calculated as the moles of individual gases and total gases, respectively, per gram of biomass. Carbon gasification efficiency or CGE (in %) and lower heating value or LHV (in kJ/Nm 3 ) of the gas phase were calculated using the following equations.

| Biomass and biochar characterization
Proximate analysis of wheat straw and its biochars was performed to determine the moisture, ash, volatile matter and fixed carbon using the standard ASTM protocols. [35][36][37] To determine the moisture content, about 1 g of sample was taken in a crucible and heated to 105°C for 2 hours in a muffle furnace. The difference in the weight of the crucible with Total number of carbon moles in CO, CO 2 , CH 4 and C 2 H 6 Number of carbon moles in the feedstock × 100.
the tubular batch reactor used for hydrothermal gasification of wheat straw the sample after drying was attributed to moisture. Ash and volatile matter content were determined as the difference in the weight of the crucible with 1 g of sample after heating to 575 ± 10°C for 4 hours and 950 ± 10°C for 7 minutes, respectively. The fixed carbon in the biomass and biochar was calculated from the difference of moisture, volatile matter, and ash.
Ultimate analysis of wheat straw and its biochars was performed using an Elementar vario EL III CHNOS analyzer (Elementar Analysensysteme, Hanau, Germany) to determine carbon, hydrogen, nitrogen, sulfur, and oxygen contents. Oxygen was determined as the difference of carbon, hydrogen, nitrogen, sulfur, and ash. The higher heating value (HHV) of wheat straw and its biochars was measured using a 6400 Automatic Isoperibol Calorimeter (Parr Instrument Company, Moline, IL, USA).

| Product distribution
Wheat straw was gasified at temperatures of 300-550°C with feed concentrations of 20-35 wt% for reaction times of 40-70 minutes under a pressure range of 21-23 MPa to understand their individual and combined effects. The yield of products from the hydrothermal gasification of wheat straw is given in Table 1. The yields of gases increased with the rise in temperature (from 300 to 550°C), decreased with the upsurge in feed concentration (from 20 to 35 wt%) and increased with the elevation in reaction time (from 40 to 70 minutes). On the contrary, the solid product yield (ie, biochar) was at the expense of gas yields. The yield of liquid products was relatively higher in the downstream always due to the medium of gasification being water.
The estimated mass balance for solid, liquid and gas products was up to a maximum of 83 wt% while the unaccounted portion was attributed to the constraints in product collection. Most researchers have also reported a total assessed mass balance up to 60, 66 and 85% from SCW gasification of cellulose, 38 glucose, 39 and lignocellulosic biomass, 17 respectively. As reported from the studies, several reasons owing to the discrepancy in mass balance from hydrothermal gasification systems are related to the entrainment of fine char particles in the liquid phase and the presence of condensable fractions in the gas phase during the time of product collection.

| Effect of gasification temperature
Temperature is the prime operating parameter that decides the quantity of product gas yields. To detail the impact of temperature on product gas yields from wheat straw, SbCW (300 and 370°C at 21 MPa) and SCW (450 and 550°C at 23 MPa) conditions were comparatively studied at a fixed feed concentration of 20 wt% and reaction time of 60 minutes. The increase in the temperature from 300 to 550°C resulted in around 10-fold rise in H 2 yield, that is, from 0.26 to 2.98 mmol/g (Figure 2). The yields of CO 2 (5.30 mmol/g), CH 4 (1.42 mmol/g), and C 2 H 6 (0.69 mmol/g) also increased at 550°C compared to lower temperatures. At SCW temperatures (450 and 550°C), steam reforming and water-gas shift reaction become predominant contributing to the yields of H 2 and CO 2 . 40 Since the reforming of biomass is usually endothermic in nature and requires high amount of energy, the temperature has a positive influence on their conversion. 41,42 Biomass undergoes complex degradation pathways with many concurrent thermal cracking and reforming reactions to produce product gas mixture. Thermal cracking reactions majorly produce hydrocarbons, whereas the reforming reactions mostly result in the formation of H 2 . The pyrolytic and cracking reactions become significant to supplement the yield of hydrocarbons (ie, CH 4 and C 2 H 6 ) at high SCW temperatures. Figure 2 also reveals an increasing trend of CO yield up to 450°C following which it decreases at 550°C. A slight decline in CO yield at high SCW temperature (550°C) indicates its consumption in water-gas shift reaction as the combination of water and CO is termed as "water-gas". Water-gas shift reaction is exothermic; hence, the active participation of CO as a reactant produces H 2 and CO 2 . 43 It has been underlined that the conversion of CO decreases and that of H 2 increases at high temperatures since the reaction is exothermic. 42,44 In accordance to our findings, Resende and Savage 45 highlighted contrasting differences in CO yields at high temperatures. During SCW gasification of 9 wt% lignin for 45 minutes, Resende and Savage 45 reported an increase in the yields of H 2 , CO 2 , and CH 4 with increase in temperature from 365°C to 725°C. In contrast, the yield of CO varied as 365°C (0.9 mmol/g) < 500°C (2.0 mmol/g) > 600°C (1.5 mmol/g) > 725°C (0.45 mmol/g). This varying trend in CO yield was due to inconsistent gasification behavior of certain intermediate biomass degradation products at high temperatures. As evident from Figure 2, a very similar trend in CO yield was noticed in the current study 300°C (0.08 mmol/g) < 370°C (0.15 mmol/g) < 450°C (0.29 mmol/g) > 550°C (0.25 mmol/g). Although water-gas shift reaction is exothermic, it becomes active at excess water concentrations and high water densities. 42,45 Therefore, the yields of H 2 and CO 2 consistently increases with rising temperature and water density, whereas the yield of CO decreases because of its consumption in watergas shift reaction promoted at higher water densities, which is in turn determined by elevated temperatures. Moreover, in our parametric study on the influence of gasification temperature, the feed concentration of wheat straw used was 20 wt% with the remainder being water (ie, 80 wt%). This high concentration of water in the reaction medium favored water-gas shift reaction at high temperatures of 550°C despite its exothermic nature.
The overall rise in the individual gas yields also led to an increment in the total gas yields from 1.7 to 10.6 mmol/g ( Table 2). The CGE also heightened from 4% (at 300°C) to 22.7% (at 550°C). High-temperature cracking and reforming reactions are a few key factors in determining the gas yields along with the conversion efficiencies of the organic materials. 46,47 The LHV of the gas products also improved from 167 to 1301 kJ/Nm 3 with the elevation in temperature from 300 to 550°C. The superior calorific value of the product gases from gasification of waste biomass in the presence of highpressure steam or oxygen over the air gasification opts for its degradation via thermochemical routes. 48 The improved gas yields at higher temperatures indicate the efficiency of SCW over SbCW reaction conditions. At SbCW temperatures, where hydronium and hydroxide ions prevail, the conversion of the biomass happens through ionic mechanisms. 24,40 In contrast, at SCW conditions, the highly active free radicals are generated from water that attacks the long-chain molecules and branched-ring molecules into simple hydrocarbons and oxygenated molecules. 11 The thermodynamics of degradation of organic compounds at lower temperatures (SbCW) convey that CH 4 dominates in the gas mixture while H 2 is favored at high temperatures (SCW). 40,49 The proximate and ultimate characterization of wheat straw and its biochars derived at SbCW (300 and 370°C) and SCW (450 and 550°C) temperatures are presented in Table 3. Wheat straw contained 44.1 wt% carbon, 6 wt% hydrogen, 0.4 wt% nitrogen, 0.01 wt% sulfur and 45.1 wt% oxygen. The rise in gasification temperature resulted in the decrease in the biochar yields (Table 1), as well as gradual lowering of the T A B L E 2 Total gas yields, carbon gasification efficiency and lower heating value of gas products obtained from hydrothermal gasification of wheat straw

Operating conditions Parameters
Total gas yield (mmol/g)

Lower heating value (kJ/N 3 )
Effect of temperature ( biochars' moisture and volatile matter contents. In contrast, the rise in temperature from 300 to 550°C also led to the increase in the ash and fixed carbon contents. Because of gasification at higher SCW temperatures, the organic components in wheat straw underwent thermal cracking, decomposition, and dehydration leading to the removal of volatile matter and development of fixed carbon moieties. In our study, SCW at 550°C acted as a suitable reaction medium with enhanced hydrothermal degradation biomass leading to high gas yields and low biochar yield with large carbon content. Moreover, biochar produced at 550°C (76.4 wt%) demonstrated the highest carbon content when compared to biochars generated at 300°C (55.8 wt%), 370°C (61.1 wt%) and 450°C (68.2 wt%) ( Table 3). This indicates that biochar produced at high temperatures (and SCW conditions) is more carbonaceous and stable due to the removal of volatile matter. [50][51][52][53] Like carbon, the nitrogen content in biochars was also found to amplify with gasification temperature. For example, wheat straw-derived biochar produced at 550°C (1.92 wt%) had greater nitrogen content than its precursor biomass (0.4 wt%) and biochars generated at 300-450°C (0.82-1.77 wt%).
A van Krevelen diagram was plotted to evaluate the carbon content and degree of aromaticity in wheat straw and its biochar produced at SbCW and SCW temperatures (Figure 3). The diagram demonstrated the significance of atomic ratios (H/C and O/C) on the calorific value of biomass and biochars. Wheat straw and its biochars produced at low SbCW temperatures (300 and 370°C) revealed relatively higher atomic H/C and O/C ratios than those of the biochars generated at SCW temperatures (450 and 550°C) ( Table 3). With the lowering of atomic H/C and O/C ratios in high SCW temperature biochars, an increase in their HHV was illustrated in the van Krevelen diagram ( Figure 3). As determined experimentally, the HHV elevated in the following order: wheat straw (15.6 MJ/ kg) < 300°C (22.6 MJ/kg) < 370°C (23.4 MJ/kg) < 450°C (25.8 MJ/kg) < 550°C (29.5 MJ/kg) owing to the amplification of the carbon content in the biochars (Table 3).

| Effects of feed concentration
The effect of variable feed concentrations of 20-35 wt% on wheat straw gasification at 550°C, 23 MPa for 60 minutes is shown in Figure 4. Prior to understanding the impacts of feed concentration, the role of water needs to be interpreted during the gasification. The increase in organic feed concentration leads to a decrement in the water content lowering its activity during the degradation process. 24 During SCW gasification, water plays important roles both as a reactant and as a medium in many vital reactions such as hydrolysis, steam reforming, and water-gas shift reaction, all leading to the main product of interest, that is, H 2 . 9,11 Hence, a decrease in water content in experiments with high feed concentration indicates the suppression of the above-mentioned reactions, which could inhibit H 2 yields.
At a constant temperature (550°C), pressure (23 MPa), and reaction time (60 minutes), the H 2 yields decreased from 2.98 mmol/g (at 20 wt% feed concentration) to 1.12 mmol/g (at 35 wt% feed concentration) (Figure 4). The  yields of CO 2 also reduced from 5.3 to 2.38 mmol/g with the rise in feed concentration, thus indicating the suppression of reforming and water-gas shift reactions. The reforming reactions are retarded largely at high feed concentrations, thus lowering the yields of H 2 and CO 2 . However, the lower gas products of reforming reactions are compensated by the gas yields from secondary cracking reactions including methanation of CO 2 that become significant at high feed concentrations. Therefore, the yields of CO, CH 4 , and C 2 H 6 increased up to 0.84, 2.3, and 1.08 mmol/g, respectively, at 35 wt% feed concentration. With the conquest of major gas products, that is, H 2 and CO 2 , a little decline in the total gas yields was observed with the rising feed concentration ( Table 2). The total gas yields from SCW gasification wheat straw decreased from 10.6 mmol/g (at 20 wt% feed concentration) to 7.7 mmol/g (at 35 wt% feed concentration). Consequently, a reduction in the CGE was found with 35 wt% feed concentration (20.9%) compared to 20 wt% feed concentration (22.7%). Similarly, the increase in the concentration of combustible gas products (ie, CO, CH 4 , and C 2 H 6 ) at higher feed concentration resulted in greater LHV of the gas phase. For example, the LHV of the gas products improved from 1301 to 1739 kJ/Nm 3 as the feed concentration increased from 20 to 35 wt%, respectively. The greater LHV of the product gas mixture at higher feed concentrations was also due to heightened concentrations of hydrocarbons from the predominant secondary cracking reactions and methanation. Since higher gas yields of 10.6 mmol/g were obtained at a low feed concentration of 20 wt%, it was selected as the optimal condition along with 550°C as the prime temperature for further parametric studies. It should be noted that higher temperatures and lower feed concentration when combined improve free-radical mechanism and reforming reaction in supercritical water gasification, thereby increasing H 2 yields. 11,24

| Effects of reaction time
The impacts of reaction time were studied for wheat straw gasification at an optimal SCW temperature (550°C) and pressure (23 MPa) with optimal feed concentration (20 wt%). From Figure 5, it is evident that H 2 yield increased exponentially from 40 minutes (1.68 mmol/g) to 60 minutes (2.98 mmol/g) and thereby decreased at 70 minutes (2.06 mmol/g). A gradual increase in CO 2 yield from 2.53 mmol/g (at 40 minutes) to 7.48 mmol/g (at 70 minutes) indicates the oxidation of intermediate products of wheat straw gasification. Like CO 2 yields, there was steady improvement in CH 4 yields from 0.42 to 2.52 mmol/g with the increase in reaction time from 40 to 70 minutes. The yield of C 2 H 6 , although mediocre than the major gases, showed a stable amplification from 0.59 to 0.82 mmol/g as the reaction time increased to 70 minutes. Interestingly, CO yield followed a reverse trend to H 2 , CO 2 , and CH 4 but its maximum yield was achieved within a least reaction time of 40 minutes, after which it declined at longer time intervals. The main gas products at longer reaction times were CO 2 and CH 4 , which resulted in an increase in the total gas yields from 5.8 to 12.9 mmol/g ( Table 2). Owing to the high concentration of CH 4 , the LHV of the gas phase increased from 782 kJ/Nm 3 (in 40 minutes) to 1655 kJ/Nm 3 (in 70 minutes). The CGE increased from an initial 12.8% (in 40 minutes) to 31.9% (in 70 minutes) of SCW gasification of 20 wt% wheat straw at 550°C under 23 MPa pressure.
Thermal cracking of biomass is favored at longer reaction times and high temperatures. 54 However, an optimal reaction time is essential for superior H 2 yields. The yield F I G U R E 3 van Krevelen diagram for wheat straw and biochars produced from hydrothermal gasification at 300-550°C for 60 min with 20 wt% feed concentration F I G U R E 4 Effect of feed concentration on hydrothermal gasification of wheat straw at 550°C and 23 MPa for 60 min of H 2 was maximum at 60 minutes (2.98 mmol/g) and then decreased at 70 minutes (2.06 mmol/g) because of many secondary reactions such as hydrogenation, methanation (or Sabatier reaction) and Boudouard reaction. Water is a major product of methanation and hydrogenation reactions that consume H 2 and CO. Moreover, along with water another major product of hydrogenation and Boudouard reaction is carbon (or char). 12 Boudouard reaction and water-gas shift reaction primarily consumes CO to produce char and CO 2 ; hence a decrement in CO yield and increment in CO 2 yield were noticed after 70 minutes of reaction time ( Figure 5). Longer residence time also favor water-gas shift reaction that produces CO 2 and H 2 . 42 Although CO 2 accumulated in the system due to its additional supply from Boudouard reaction, the H 2 was consumed during hydrogenation and methanation to produce CH 4 .

| Effects of metal catalysts
The effects of two metal catalysts, that is, Ru/Al 2 O 3 and Ni/Si-Al 2 O 3 at 5 wt% loading were studied to maximize H 2 yields at optimal SCW gasification conditions of 550°C and 23 MPa pressure with 20 wt% feed concentration for 60 minutes of reaction time ( Figure 6). Ni/Si-Al 2 O 3 was superior to Ru/Al 2 O 3 in increasing the yields of H 2 , CO 2 , CH 4 , and C 2 H 6 . The H 2 yield reached its maximum with 5 wt% Ni/Si-Al 2 O 3 (5.1 mmol/g) when compared to Ru/Al 2 O 3 (4.18 mmol/g) and noncatalytic gasification (2.98 mmol/g). CO formation was not detected in the gas phase with the application of catalysts confirming the activation of water-gas shift and methanation of CO resulting in the overall contribution of H 2 (4.18-5.1 mmol/g) and CH 4 (2.99-3.84 mmol/g). Nickel-and ruthenium-based catalysts are capable of depolymerizing lignocellulosic biomasses with the potential to improve methanation and water-gas shift reactions even at near-critical conditions. 23 Nickel has high selectivity for CH 4 even at their low loading concentrations. 17,[55][56][57] Ruthenium also exhibits some catalytic activity toward the formation of CH 4 via methanation of CO and CO 2 , 29,58-60 although it performance is relatively lower than Ni.
With catalyst application, the total gas yields and carbon gasification efficiency were highest in the case of 5 wt% Ni/ Si-Al 2 O 3 (18.2 mmol/g and 38.2%) followed by 5 wt% Ru/ Al 2 O 3 (15 mmol/g and 31.7%) and noncatalytic gasification (10.6 mmol/g and 22.7%) ( Table 2). The LHV of the gas products also increased in the order: noncatalytic gasification (1301 kJ/Nm 3 ) < 5 wt% Ru/Al 2 O 3 (2038 kJ/Nm 3 ) < 5 wt% Ni/Si-Al 2 O 3 (2544 kJ/Nm 3 ). The higher total gas yields were attained with Ni/SiO 2 -Al 2 O 3 compared to Ru/Al 2 O 3 , which might be attributed to the higher surface area resulting in the adsorption and catalytic action of Ni on aqueous intermediates. The CGE was also higher in the case of Ni/ Si-Al 2 O 3 (38.2%) compared to that of Ru/Al 2 O 3 (31.7%).
At low SbCW temperatures, the formation of tar from biomass is quite evident, which could hinder the gas yields. This directs to look out for possible alternates to reduce tar and improve the gas yields. With the capability to cleave O-H, C-H, and C-C bonds, nickel can effectively decompose recalcitrant tars as well as long-chain and oxygenated compounds into gases. 16,61,62 The catalytic activity of nickel compared to other noble metals is significant higher toward H 2 and CH 4 production through hydrogenation and methanation, making it a suitable catalyst for SCW gasification of biomass. 27

| Overall reaction mechanisms
Based on the overall experimental observations, the following reactions are proposed for the gas yields. A mechanistic representation of the overall reaction pathways involved in SbCW and SCW gasification of wheat straw is shown in Figure 7. Initially, the lignocellulosic biomass (wheat straw) undergoes depolymerization and hydrolysis to form soluble carbohydrates and organic acids at SbCW temperatures. These intermediates approach the active sites of the catalysts, where they are further degraded by either dehydrogenation or secondary hydrolysis into simpler C 1 to C 2 compounds (eg, aldehydes, ketones, alcohols, aromatics, and phenolics) and a small amount of gases products. 64 The formed aqueous products are easily catalyzed into permanent fuel gases at higher SCW temperatures. Depending on the gasification temperature, reaction time, water content, the gas products may undergo water-gas shift reaction, hydrogenation, methanation or Sabatier reaction and Boudouard reaction as represented in the following equations. 12,14,40 The role of water in hydrothermal gasification is not only as a gasifying medium but also as a reactant, which results in the higher carbon gasification efficiencies. 24,42 Considering water as a medium with its reaction conditions beyond its critical point enables to degrade waste biomass and organics with higher conversion efficiencies yielding a high quantity of H 2 rich fuel gas mixture. Moreover, the exothermic reactions during SCW gasification can supplement additional heat energy to lower the activation energy, aid in the participation of most reactant molecules for near-complete gasification and maximize the syngas yields. 65,66 Nevertheless, the presence of catalysts can lead to the degradation of intermediate components contributing to gas yields, improved gasification efficiency and superior heating value of the syngas. 27

| CONCLUSIONS
This study investigated the hydrothermal gasification of wheat straw, which is abundantly found agricultural crop residue at a global scale. The study investigated the influence of temperature, feed concentration, reaction time, and catalysts on H 2 and total gas yields along with carbon gasification efficiency and lower heating value of the gas products. SCW conditions were found to be more favorable for wheat straw gasification than SbCW conditions. The optimal temperature, feed concentration and reaction time were found to be 550°C, 20 wt% and 60 minutes, respectively, at 23 MPa pressure. The noncatalytic SCW gasification at the above-mentioned optimal reaction conditions resulted in high yields of H 2 (2.98 mmol/g), total gases (10.7 mmol/g) with an LHV of 1301 kJ/Nm 3 . Furthermore, 5 wt% Ni/Si-Al 2 O 3 maximized the H 2 yield to 5.1 mmol/g compared to Ru/Al 2 O 3 (4.18 mmol/g). Due to high H 2 and CH 4 yields, the LHV of the gas phase from catalytic SCW gasification of wheat straw also increased up to 2544 kJ/Nm 3 with Ni/ Si-Al 2 O 3 . The overall results indicate that wheat straw could be a promising agricultural crop residue through hydrothermal gasification with Ni and Ru-based catalysts to produce H 2 -rich syngas.