Comprehensive experimental study on supercritical water gasification of oilfield sludge: Effect of operation parameters,and catalysts on the products

Supercritical water gasification (SCWG) was adopted to treat oilfield sludge and produce syngas. The effect of temperature (400–450 °C), reaction time (30–90 min) and catalyst addition on syngas production and residual products during SCWG of oilfield sludge was studied. When increasing SCWG temperature from 400 to 450 °C with reaction time of 60 min, the H₂ yield and the selectivity of H₂ increased significantly from 0.53 mol/kg and 75.53% to 0.98 mol/kg and 78.09%, respectively. It is noteworthy that when the reaction time was too long, CO₂ and CO were converted to CH₄ with the consumption of H₂ via methanation reaction. The addition of Ni/Al₂O₃ catalyst can substantially promote the production of high-quality syngas from SCWG of oilfield sludge. The H₂ yield and its selectivity at 450 °C and 60 min were as high as 1.37 mol/kg and 84.05% with 10Ni/Al catalyst. Moreover, the catalysts with bimetal loading (Fe–Ni, Rb–Ni or Ce–Ni) were found to be beneficial for improving gasification efficiency, H₂ yield, and the degradation of organic compounds. Among them, 5 wt% Rb on 10Ni/Al catalyst performed the best catalytic activity for SCWG at 450 °C and 60 min, which had the highest H₂ yield of 1.67 mol/kg and selectivity of 86.09%. More than 90% of total organic carbon in sludge was decomposed after the SCWG with all the catalysts. These findings indicated that catalytic SCWG is a promising alternative for efficiently dealing with oilfield sludge.     

Investigation of the interaction between intermediates from gasification of biomass in supercritical water: Formaldehyde/formic acid mixtures

Supercritical water gasification (SCWG) is a promising technology for converting wet biomass and waste into renewable energy. While the fundamental mechanism involved in SCWG of biomass is not completely understood, especially hydrogen (H₂) production produced from the interaction among key intermediates. In the present study, formaldehyde mixed with formic acid as model intermediates were tested in a batch reactor at 400 °C and 25 MPa for 30 min. The gas and liquid phases were collected and analyzed to determine a possible mechanism for H₂ production. Results clearly showed that both gasification efficiency (GE) and hydrogen efficiency (HE) increased with addition of formic acid, and the maximum H₂ yield reached 17.92 mol/kg with a relative formic acid content of 66.67% in the mixtures. The total organic carbon removal rate and formaldehyde conversion rate also increased to 67.33% and 89.81% respectively. The reaction pathways for H₂ formation form mixtures was proposed and evaluated, formic acid promoted self-decomposition of formaldehyde to generate H₂, and induced a radical reaction of generated methanol to produce more H₂.     

Effects of Lewis acid on catalyzing gasification of sewage sludge and model compounds in supercritical water

In this work, we investigated how different types and concentrations of Lewis acids effect gas yield and composition from supercritical water gasification (SCWG) of dewatered sewage sludge (DSS). Furthermore, the catalytic mechanism was investigated using AlCl₃ as a representative Lewis acid catalyst. Results showed that non-catalytic gasification of DSS produced a hydrogen yield of 0.13 mol/kg organic matter (OM) and total gas yield of 4.82 mol/kg OM; while the addition of 10 wt% Lewis acid resulted in a significantly improved H₂ and total gas yield of 0.77–7.76 mol/kg OM and 7.57–22.88 mol/kg OM, respectively. In addition, the Lewis acids tested herein depicted the following activity trend: AlCl₃＞FeCl₃＞NiCl₂＞ZnCl₂. The required temperature for generating hydrogen from SCWG of DSS was greatly reduced when AlCl₃ was present during the heating process. Furthermore, AlCl₃ significantly increased the hydrogen yield from SCWG of the model compounds. Finally, AlCl₃ catalyzed SCWG of guaiacol and glycerol showed the best hydrogen selectivity.     

Hydrogen production from dairy wastewater using catalytic supercritical water gasification: Mechanism and reaction pathway

The supercritical water gasification (SCWG) of real dairy wastewater (cheese-based or whey) was performed in a batch reactor in presence of two catalysts (MnO₂, MgO) and one additive (formic acid). The operational conditions of this work were at a temperature range of 350–400 C and the residence time of 30–60 min. The catalysts and formic acid were applied in 1 wt%, 3 wt%, and 5 wt% to determine their effect on hydrogen production. The concentrations of catalysts and formic acid were calculated based on the weight of feedstock without ash. The results showed that increased temperature and prolonged residence time contributed to the hydrogen production (HP) and gasification efficiency (GE). The gas yield of hydrogen in the optimum condition (400 C and 60 min) was achieved as 1.36 mmol/gr DAF (dry ash free). Formic acid addition was favored towards enhancing hydrogen content while the addition of metal oxides (MnO₂ and MgO) had an apex in their hydrogen production and they reached the highest hydrogen in 1 wt% concentration then ebbed. Moreover, GE was increased by the addition of the catalysts and formic acid concentrations. The highest hydrogen content (35.4%) was obtained in 1 wt% MnO₂ and the highest GE (32.22%) was attained in the 5 wt% formic acid concentration. A reaction pathway was proposed based on the GC-MS data of feedstock and produced liquid phase at different condition as well as similar studies.     

Supercritical water gasification of black liquor with wheat straw as the supplementary energy resource

Supercritical water gasification (SCWG) is a new treatment of black liquor (BL) for both energy recovery and pollution management. To provide more energy for the pulp mill, it is proposed to use the pulping raw material as supplementary energy source because it is readily available, inexpensive and renewable. In this study, co-gasification of BL and wheat straw (WS) in supercritical water was investigated. The synergistic effect was observed in the co-gasification because the addition of wheat straw can make better use of the alkali in BL. The maximum improvement of the gasification by the synergistic effect was obtained with the mixing ratio of 1:1. The influences of the temperature (500–750 °C), reaction time (5–40 min), mixture concentration (5.0–19.1 wt%), mixing ratio (0–100%) and the wheat straw particle diameter (74–150 μm) were studied. It was found that the increase of temperature and reaction time, and the decrease of concentration and wheat straw particle size favored the gasification by improving the hydrogen production and gasification efficiency. The highest carbon gasification efficiency of 97.87% was obtained at 750 °C. Meanwhile, the H₂ yield increased from 12.29  mol/kg at 500 °C to 46.02  mol/kg. This study can help to develop a distributed energy system based on SCWG of BL and raw biomass to supply energy for the pulp mill and surrounding communities.     

Supercritical water gasification of food waste: Effect of parameters on hydrogen production

Food waste is a type of municipal solid waste with abundant organic matter. Hydrogen contains high energy and can be produced by supercritical water gasification (SCWG) of organic waste. In this study, food waste was gasified at various reaction times (20–60 min) and temperatures (400 °C-450 °C) and with different food additives (NaOH, NaHCO₃, and NaCl) to investigate the effects of these factors on syngas yield and composition. The results showed that the increase in gasification temperature and time improved gasification efficiency. Also, the addition of food additives with Na⁺ promoted the SCWG of food waste. The highest H₂ yield obtained through non-catalytic experiments was 2.0 mol/kg, and the total gas yield was 7.89 mol/kg. NaOH demonstrated the best catalytic performance in SCWG of food waste, and the highest hydrogen production was 12.73 mol/kg. The results propose that supercritical water gasification could be a proficient technology for food waste to generate hydrogen-rich gas products.     

Supercritical water gasification of sewage sludge and combined cycle for H2 and power production – A thermodynamic study

An integrated system of supercritical water gasification (SCWG) and combined cycle has been developed for H₂ production and power generation. Sewage sludge and lignite coal were selected as raw material in this simulation. The effects of feed concentration (10–30 wt%) and lignite coal addition (0–50 wt%) on syngas yield and H₂ yield were also investigated in the temperature range of 500 °C–700 °C. Several heat exchangers were considered in the proposed integrated system to minimize energy loss. High pressure syngas was expanded by using turbo-expander to produce electricity, resulting in the improvement of the total efficiency. The results showed that the minimum feed concentrations of 14.25 wt%, 18.75 wt%, and 25.50 wt% were required to achieve self-sufficient energy at 500 °C, 600 °C, and 700 °C, respectively. However, the lower feed concentration and higher temperature were preferable for syngas production. The highest syngas and H₂ yield were obtained at 700 °C and 10 wt% feed concentration. The SCWG could produce 178.08 kg syngas from 100 kg feed and 9.06 kg H₂ were obtained after H₂ separation. The total power generation from turbo-expander and combined cycle module was 48.37 kW. By combining SCWG and combined cycle, the total efficiency could reach 63.48%. It worth mentioning that the addition of lignite coal could help reduce the minimum feed concentration to achieve autothermal condition, but did not have significant improvement on H₂ production.     

Supercritical water gasification and partial oxidation of municipal sewage sludge: An experimental and thermodynamic study

Supercritical gasification (SCWG) and supercritical partial oxidation (SCWPO) technologies have emerged as preferred means of converting wet biomass to hydrogen-rich gases. We experimentally investigated the effects of moisture content, pressure and oxidation coefficient (n) on mole fraction, yield, gasification efficiency and energy recovery of gaseous products from SCWG or SCWPO of municipal sewage sludge, as well as on the carbon and nitrogen contents in liquid products. Potential of sludge for producing gaseous products was thermodynamically analyzed by an Aspen Plus model. The results show that 87 wt%, 25 MPa and n = 0 were optimum conditions for sludge gasification. Sludge with 87 wt% moisture content was pumpable at 75 °C, and further increasing the moisture content decreased the heating value and energy recovery of gaseous products. Pressure played little role in both the experimental and equilibrium gas yields. Highest mole fractions and yields of H₂ and CH₄ were achieved at n = 0.     

Influence of AlCl3 and oxidant catalysts on hydrogen production from the supercritical water gasification of dewatered sewage sludge and model compounds

Hydrogen has attracted significant attention as a clean energy source. Supercritical water gasification (SCWG) technology can produce hydrogen-rich gas while also disposing of sludge. The hydrogen yield from the SCWG of sludge is greatly increased when catalyzed by AlCl₃. In this paper, a combined catalyst based on AlCl₃ was proposed to further increase the hydrogen yield of SCWG of dewatered sewage sludge (DSS). Analysis of the products from catalytic gasified of DSS and its model compounds were used to propose a catalytic mechanism and reaction pathway of the catalytic SCWG of DSS. Among the combined catalysts used for the SCWG of DSS, 10 wt% AlCl₃–H₂O₂ (mass ratio 8:2) had the best hydrogen production effect, and the hydrogen yield reached 8.88 mol/kg organic matter. This was 14% higher than when catalyzed by 10 wt% AlCl₃. During catalysis with AlCl₃, Al³⁺ reacted with OH⁻ in water and precipitated as Al(OH)₃, which produced an acidic environment in the liquid product. Al(OH)₃ dehydrated to form an AlO(OH) and deposited in the solid product. A small amount of H₂O₂ promoted the steam reforming reaction of organic matter in DSS, which increased the hydrogen yield. H₂O₂ further promoted the hydrogen yield in an acidic environment. The catalytic effect of AlCl₃ was unaffected by H₂O₂. The H⁺ generated by AlCl₃ during catalysis promoted H₂O₂ to further depolymerized organic matter (such as humic substances) in DSS, so that AlCl₃–H₂O₂ catalyzed the SCWG of DSS to further increase the hydrogen yield. The order of hydrogen yield catalyzed by AlCl₃–H₂O₂ was guaiacol > humic acid > glycerol > alanine > glucose. Compared with AlCl₃, AlCl₃–H₂O₂ reduced the hydrogen yield of glucose by nearly 20% and increased the hydrogen yield of humic acid by about 17% (25.81 mol/kg feed).     

Valorisation of vegetable market wastes to gas fuel via catalytic hydrothermal processing

Residues of leek, cabbage and cauliflower from the market places as representatives of lignocellulosic biomass were processed via hydrothermal gasification to produce energy fuel. The experiments were carried out in a batch reactor at temperatures 300, 400, 500 and 600 °C and corresponding pressures varying in the range of 7.5–43 MPa. Natural mineral additives trona, dolomite and borax were used as homogenous catalysts to determine their effects on the gasification. More than 70 wt% of carbon in vegetable residue samples were detected in the gas phase after the hydrothermal gasification process at 600 °C. The addition of trona mineral further promoted the gasification reactions and as a result, less than 5 wt% carbon remained in the solid residue at the same temperature, degrading the biomass samples into gas and liquid products. The fuel gas with the highest calorific value was recorded to be 25.6 MJ/Nm³, from the hydrothermal gasification of cabbage at 600 °C, when dolomite was used as the homogeneous catalyst. The liquid products obtained in the aqueous phase were detected as organic acids, aldehydes, ketones, furfurals and phenols. The gas products were consisted of hydrogen, carbon dioxide, methane, and as minors; carbon monoxide and low molecular weight hydrocarbons (ethane, propane, etc.). Above 500 °C, all biomass samples yielded 50–55 vol% of CH₄ and H₂ while the CO₂ composition was around 40 vol% as the gas product.     

Supercritical water synthesized Ni/ZrO2 catalyst for hydrogen production from supercritical water gasification of glycerol

Catalysts are crucial to promote the technical feasibility of supercritical water gasification (SCWG) for H₂ production from wet biomass, yet catalysts prepared by conventional methods normally encounter sintering problems in supercritical water. Herein, a series of ZrO₂-supported Ni catalysts were tried to be prepared by supercritical water synthesis (SCWS) and evaluated for SCWG in terms of activity and property stability. The SCWS was conducted at 500 °C and 23 MPa using metal nitrates as starting materials. Effect of precursor concentration on property and catalytic performance of the SCWS-prepared catalysts for SCWG of 20 wt% glycerol were systematically studied. XRD, SEM-EDS, TEM and TGA were applied for catalyst characterization. Results verified the successful obtaining of Ni/ZrO₂ nanocatalysts with Ni crystals of 30–70 nm and ZrO₂ crystals of ~11 nm by the SCWS process, which were found to be active on the WGSR for SCWG to increase the H₂ yield as high as 155%. Importantly, the SCWS-prepared Ni/ZrO₂ catalysts exhibited excellent property stability and anti-coking ability for SCWG of glycerol.     

Sustainable hydrogen production options from food wastes

In this study, two thermochemical processes, namely steam gasification and supercritical water gasification (SCWG), were comparatively studied to produce hydrogen from food wastes containing about 90% water. The SCWG experiments were performed at 400 and 450 °C in presence of catalyst (Trona, K₂CO₃ and seaweed ash). The maximum hydrogen yield was obtained at 450 °C in presence of K₂CO₃ catalyst. In second process, hydrothermal carbonization was used to convert food wastes into a high-quality solid fuel (hydrochar) that was further gasified in a dual-bed reactor in presence of steam. The steam gasification of hydrochar was carried out with and without catalysts (iron?ceria catalyst and dolomite). The maximum hydrogen yield obtained from steam gasification process was 28.08 mmol/g dry waste, about 7.7 times of that from SCWG. This study proposed a new concept for hydrogen production from wet biomass, combination of hydrothermal carbonization following steam gasification.     

Catalytic gasification of food waste in supercritical water over La promoted Ni/Al2O3 catalysts for enhancing H2 production

Ni/Al₂O₃ catalyst is the one of promising catalysts for enhancing H₂ production from supercritical water gasification (SCWG) of biomass. However, due to carbon deposition, the deactivation of Ni/Al₂O₃ catalyst is still a serious issue. In this work, the effects of lanthanum (La) as promoter on the properties and catalytic performance of Ni/Al₂O₃ in SCWG of food waste were investigated. La promoted Ni/Al₂O₃ catalysts with different La loading content (3–15 wt%) were prepared via impregnation method. The catalysts were characterized using XRD, SEM, BET techniques. The SCWG experiments were carried out in a Hastelloy batch reactor in the operating temperature range of 420–480 °C, and evaluated based on H₂ production. The stability of the catalysts was assessed by the amount of carbon deposition on catalyst surface and their catalytic activity after reuse cycles. The results showed that 9 wt% La promoter is the optimal loading as Ni/9La–Al₂O₃ catalyst performed best performance with the highest H₂ yield of 8.03 mol/kg, and H₂ mole fraction of 42.46% at 480 °C. La promoted Ni/Al₂O₃ catalysts have better anti-carbon deposition properties than bare Ni/Al₂O₃ catalyst, resulting in better gasification efficiency after reuse cycles. Ni/9La–Al₂O₃ catalyst showed high catalytic activity in SCWG of food waste and had good stability as it was still active for enhancing H₂ production when used in SCWG for the third time, which indicated that La promoted Ni/Al₂O₃ catalysts are potential additive to improve the SCWG of food waste.     

Catalytic supercritical water gasification of aqueous phase directly derived from microalgae hydrothermal liquefaction

Microalgae (N. chlorella) hydrothermal liquefaction (HTL) was conducted at 320 °C for 30 min to directly obtain original aqueous phase with a solvent-free separation method, and then the supercritical water gasification (SCWG) experiments of the aqueous phase were performed at 450 and 500 °C for 10 min with different catalysts (i.e., Pt-Pd/C, Ru/C, Pd/C, Na₂CO₃ and NaOH). The results show that increasing temperature from 450 to 500 °C could improve H₂ yield and TGE (total gasification efficiency), CGE (carbon gasification efficiency), HGE (hydrogen gasification efficiency), TOC (total organic carbon) removal efficiency and tar removal efficiency. The catalytic activity order in improving the H₂ yield was NaOH > Na₂CO₃ > None > Pd/C > Pt-Pd/C > Ru/C. Ru/C produced the highest CH₄ mole fraction, TGE, CGE, TOC removal efficiency and tar removal efficiency, while NaOH led to the highest H₂ mole fraction, H₂ yield and HGE at 500 °C. Increasing temperature and adding proper catalyst could remarkably improve the SCWG process above, but some N-containing compounds were difficult to be gasified. This information is valuable for guiding the treatment of the aqueous phase derived from microalgae HTL.     

Characteristics and prediction model of hydrogen production of oily sludge by supercritical water gasification

Harmless treatment and resource utilization of oily sludge are urgent and related to the sustainable green, and low-carbon development of the petroleum industry. Aiming to the supercritical water gasification (SCWG) of oily sludge for hydrogen production, this paper investigated the effects of critical factors, including reaction temperature, initial pressure, retention time, and feed concentration, on the mole fraction, the gas yield, the gasification efficiency, and the hydrogen yield potential. The interaction mechanisms among these four factors were discussed and revealed with a reasonable prediction model of hydrogen production. Results showed that the longer retention time, higher temperature, and lower feed concentration could accelerate hydrogen production from oily sludge by SCWG. The synthetic promotion of the hydrogen yield exists between the temperature and the retention time, while the temperature predominates. A 2.63-fold increase in the H₂ yield was obtained when the condition changed from 135 min to 380 °C to 10 min and 555 °C. The hydrogen production of oily sludge by SCWG, at lower temperature and higher pressure was worse than that at higher temperature and lower pressure.     

Experimental study on the energy conversion of food waste via supercritical water gasification: Improvement of hydrogen production

In this study, the model food waste was gasified to hydrogen-rich syngas in a batch reactor under supercritical water condition. The model food consisted of rice, chicken, cabbage, and cooking oil. The effects of the main operating parameters including temperature (420–500 °C), residence time (20–60 min) and feedstock concentration (2–10 wt%) were investigated. Under the optimal condition at 500 °C, 2 wt% feedstock and 60 min residence time, the highest H₂ yield of 13.34 mol/kg and total gas yield of 28.27 mol/kg were obtained from non-catalytic experiments. In addition, four commercial catalysts namely FeCl₃, K₂CO₃, activated carbon, and KOH were employed to investigate the catalytic effect of additives at the optimal condition. The results showed that the highest hydrogen yield of 20.37 mol/kg with H₂ selectivity of 113.19%, and the total gas yield of 38.36 mol/kg were achieved with 5 wt% KOH addition Moreover, the low heating value of gas products from catalytic experiments with KOH increased by 32.21% compared to the non-catalytic experiment. The catalytic performance of the catalysts can be ranked in descending order as KOH > activated carbon > FeCl₃ > K₂CO₃. The supercritical water gasification (SCWG) with KOH addition can be a potential applied technology for food waste treatment with production of hydrogen-rich gases.     

Assessment of supercritical water gasification of food waste under the background of waste sorting: Influences of plastic waste contents

Waste sorting is being gradually implemented as a key measure for circular and sustainable development in China, food waste will be separately collected and separated from municipal solid waste (MSW), thus the plastic content in food waste also will be reduced. In this study, supercritical water gasification (SCWG) of food waste with different contents of plastic (0–3.5 wt%) was experimentally investigated to simulate the influence of waste sorting on the food waste treatment. The results showed that lower plastic content in food waste favored higher gas yield and gasification efficiencies. The highest H₂ yield and total gas yield were 3.11 mol/kg and 8.41 mol/kg in the plastic-free case, respectively. When the plastic content decreased from 3.5 wt% to 0 wt%, the cold gas efficiency (CGE), carbon conversion efficiency (CE) and hydrogen gasification efficiency (HE) increased by 125.97%, 173.48% and 94.09%, respectively. However, lower plastic content negatively affected the quality of produced syngas through decreasing H₂ mole fraction and LHV. The solid residues from SCWG of food waste with lower plastic content had higher ratio of fixed carbon to volatile matter (FC/VM). Based on the analysis of pyrolysis properties and combustion behavior, decreasing the plastic content in food waste helped to improve the thermal stability of solid residues. Moreover, lower plastic content resulted in a decrease of total organic carbon (TOC) concentration in liquid effluent, which is favorable for further treatment of liquid effluent.     

Optimization and modeling of process parameters during hydrothermal gasification of biomass model compounds to generate hydrogen-rich gas products

Hydrothermal gasification in subcritical and supercritical water is gaining attention as an attractive option to produce hydrogen from lignocellulosic biomass. However, for process optimization, it is important to understand the fundamental phenomenon involved in hydrothermal gasification of synthetic biomass or biomass model compounds, namely cellulose, hemicellulose and lignin. In this study, the response surface methodology using the Box-Behnken design was applied for the first time to optimize the process parameters during hydrothermal (subcritical and supercritical water) gasification of cellulose. The process parameters investigated include temperature (300–500 °C), reaction time (30–60 min) and feedstock concentration (10–30 wt%). Temperature was found to be the most significant factor that influenced the yields of hydrogen and total gases. Furthermore, negligible interaction was found between lower temperatures and reaction time while the interaction became dominant at higher temperatures. Hydrogen yield remained at about 0.8 mmol/g with an increase in the reaction time from 30 min to 60 min at the temperature range of 300–400 °C. When the temperature was raised to 500 °C, hydrogen yield started to elevate at longer reaction time. Maximum hydrogen yield of 1.95 mmol/g was obtained from supercritical water gasification of cellulose alone at 500 °C with 12.5 wt% feedstock concentration in 60 min. Using these optimal reaction conditions, a comparative evaluation of the gas yields and product distribution of cellulose, hemicellulose (xylose) and lignin was performed. Among the three model compounds, hydrogen yields increased in the order of lignin (0.73 mmol/g) < cellulose (1.95 mmol/g) < xylose (2.26 mmol/g). Based on the gas yields from these model compounds, a possible reaction pathway of model lignocellulosic biomass decomposition in supercritical water was proposed.     

Thermodynamic analysis of hydrogen production via supercritical water gasification of coal,sewage sludge,microalga, and sawdust

This study investigates the thermodynamic equilibrium analysis of the supercritical water gasification (SCWG) involving several typical feedstocks (coal, sewage sludge, microalga, and sawdust). The effects of various parameters including feed concentrations, temperatures, and pressures are analyzed. It is observed that temperature and feed concentration play a determining role in the yield of hydrogen, while the effect of pressure is very limited. Results show that the feed concentration of 15–20 wt% is optimal for hydrogen production. Furthermore, the effects of composition (hydrogen to carbon ratio and oxygen to carbon ratio) of the feedstock on the yield of product gases are investigated, which is useful for screening potential feedstock for SCWG. The results show that maximum H₂ and CH₄ molar yields are achieved at a low O/C and a high H/C ratio.     

Evaluation on hydrothermal gasification of waste tires based on chemical equilibrium analysis

Massive amounts of waste tires are produced globally, which brings great challenges to the disposal and recycling of used tires. Hydrothermal gasification is a promising option for recycling waste tires. The hydrothermal gasification of waste tires was evaluated based on the chemical equilibrium analysis along with the response surface methodology (RSM) in terms of subcritical temperature range (250–300 °C), transition temperature range (350–400 °C), supercritical temperature range (550–600 °C), supercritical pressure (22.5–30.5 MPa) and feedstock concentration (5–20 wt%). CH₄ yield at 350 °C reached a maximum, 41.575 mmol/g. H₂ yield increased from 0.0283 to 53.602 mmol/g with increasing the temperature from 250 °C to 600 °C. CH₄ yield at the supercritical temperature increased with lifting the feedstock concentration, while H₂ yield decreased. The optimal parameters regarding maximum H₂ and CH₄ yields in the subcritical temperature range were 300 °C, 22.5 MPa and 12.5 wt%, respectively, while they in the supercritical temperature range were 550 °C, 30.5 MPa and 5.4 wt%, respectively. RSM was more suitable for predicting H₂ yield in the hydrothermal gasification of waste tires at subcritical and supercritical temperature ranges, but it was available for predicting CH₄ yield in three temperature ranges. This study can provide basic data for the hydrothermal treatment of waste tires.