Catalytic supercritical water gasification of black liquor along with lignocellulosic biomass

Supercritical water gasification (SCWG) is a novel technology for environmental pollution management and hydrogen production from biomass and wastes. In this study, the SCWG of black liquor (BL) which is high-potential biomass and rich in alkalis was investigated. The experiments were conducted in a batch reactor at 350–400 °C, reaction time of 1–60 min, and constant concentration of 9 wt% of BL in the absence and presence of heterogeneous catalysts (3–5 wt%), lignocellulosic biomass, and formic acid (5 and 7 wt %) in three parts. First, the SCWG of BL was performed without any additive. The experimental results showed that the maximum production of H₂, CO₂, and CH₄ was obtained at the highest temperature and reaction time; 400 °C and 60 min. The hydrogen yield was also enhanced by increasing the temperature, and reached 3.51 mol H₂/kg dry ash free-black liquor (DAF-BL) at 400 °C. Reaction time increment improved the gas product and gasification efficiency up to 28.03 mmol and 21.73%, respectively. Subsequently, three heterogeneous catalysts (MnO₂, CuO, and TiO₂) were used, however 5 wt% of MnO₂ was the best catalyst, significantly improving the hydrogen yield compared to the same condition of BL gasification without a catalyst. Hydrogen yield reached 5.09 mol H₂/kg (DAF-BL) at 400 °C and the reaction time of 10 min. Finally, BL with poplar wood residue as a lignocellulosic biomass and formic acid was gasified separately and the highest hydrogen yield was obtained in the case of 5 wt% of formic acid (10.79 mol H₂/kg (DAF-BL)). Overally, SCWG dramatically reduced the chemical oxygen demand of BL to 76% using 5 wt% of formic acid.     

Hydrogen production from glycerol by supercritical water gasification in a continuous flow tubular reactor

In this work, glycerol was used for hydrogen production by supercritical water gasification. Experiments were conducted in a continuous flow tubular reactor at 445∼600 °C, 25 MPa, with a short residence time of 3.9∼9.0 s. The effects of reaction temperature, residence time, glycerol concentration and alkali catalysts on gasification were systematically studied. The results showed that the gasification efficiency increased sharply with increasing temperature above 487 °C. A short residence time of 7.0 s was enough for 10 wt% glycerol gasification at 567 °C. With the increase of glycerol concentration from 10 to 50 wt%, the gasification efficiency decreased from 88% to 71% at 567 °C. The alkali catalysts greatly enhanced water-gas shift reaction and the hydrogen yield in relation to catalysts was in the following order: NaOH > Na₂CO₃>KOH > K₂CO₃. The hydrogen yield of 4.93 mol/mol was achieved at 526 °C with 0.1 wt% NaOH. No char or tar was observed in all experiments. The apparent activation energy and apparent pre-exponential factor for glycerol carbon gasification were obtained by assuming pseudo first-order kinetics.     

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.     

Hydrogen production from low temperature supercritical water Co-Gasification of low rank lignites with biomass

Co–gasification of low rank lignite (Çan) with sorghum energy crop was investigated under low temperature conditions with supercritical water (773 K, 26.9 MPa). The effects of the water volume in the reactor, blending ratios of the coal/sorghum mixtures, the use of different catalysts, and the variation of feedstock concentrations on the gasification efficiency, product distribution, and hydrogen yields were evaluated. Synergistic effects were observed for both the gasification efficiency and the hydrogen yield with a coal content of 25 wt% in the coal/biomass mixture. Increasing the initial water volume, decreasing the feedstock concentration, and using the alkali metal catalysts Na₂CO₃ and K₂CO₃ significantly increased the gasification efficiency and the hydrogen yield. In experiments with CaO, almost all the carbon dioxide formed was isolated from the gas product during gasification, and the hydrogen yield was more than 70%. The liquid products were mainly composed of alkylphenols and their derivatives.     

Hydrogen production through biomass gasification in supercritical water: A review from exergy aspect

Hydrogen production through supercritical water gasification (SWG) of biomass has been widely studied. This study reviews the main factors from exergy aspect, and these include feedstock characteristics, biomass concentration, gasification temperature, residence time, reaction catalyst, and reactor pressure. The results show that the exergy efficiencies of hydrogen production are mainly in the range of 0.04–42.05%. Biomass feedstock may affect hydrogen production by changing the H₂ yield and the heating value of biomass. Increases in biomass concentrations decrease the exergy efficiencies, increases in gasification temperatures generally increase the exergy efficiencies, and increases in residence times may initially increase and finally decrease the exergy efficiencies. Reaction catalysts also have positive effects on the exergy efficiencies, and the reviewed results show that the effects are followed KOH > K₂CO₃ > NaOH > Na₂CO₃. Reactor pressure may have positive, negative or negligible effects on the exergy efficiencies.     

Effect of Ni ratio on mesoporous Ni/MgO nanocatalyst synthesized by one-step hydrothermal method for thermal catalytic decomposition of CH4 to H2

The Ni/MgO catalysts were synthesized by hydrothermal method and the effect of changes in nickel ratio on the physicochemical properties and catalytic performance were investigated for TCD of CH₄ to H₂. In all catalysts, the only NiO–MgO phase as solid solution was formed, and crystals size were in the nanoparticles range. The results confirmed that the catalysts with mesoporous structure and high porosity were successfully synthesized by one step in the absence of surfactants. The increasing nickel ratio from 10 to 40 wt% increases the methane conversion from 28.3 to 48.6% and hydrogen yield from 33.2 to 53.2%, respectively. Also, the stability of catalysts depends on the amount and size of nickel particles in the structure of the catalysts. Among the catalysts, 40 and 30 wt% showed the highest initial activity (48.6% methane conversion) and the highest stability (above 45% hydrogen yield for 180 min), respectively.     

Hydrogen production by sewage sludge gasification in supercritical water with a fluidized bed reactor

In this work, gasification of sewage sludge in supercritical water was investigated in a fluidized bed reactor. Effect of operating parameters such as temperature, concentration of the feedstock, alkali catalysts and catalyst loading on gaseous products and carbon distribution were systematically studied. The results showed that the increase of temperature and the decrease of feedstock concentration were both favorable for gasification, and the addition of catalyst enhanced the formation of hydrogen better. The K₂CO₃ catalyst could better enhance gasification efficiency and the catalytic activity of different catalysts for hydrogen production was in the following order: KOH > K₂CO₃ > NaOH > Na₂CO_3. The maximum molar fraction and yield of hydrogen reached to 55.96% and 15.49 mol/kg respectively with KOH at 540 °C. Most carbon in feedstock existed in gaseous and liquid products, and alkali catalysts mainly promoted the water-gas shift reaction rather than steam reforming.     

Supercritical water gasification of timothy grass as an energy crop in the presence of alkali carbonate and hydroxide catalysts

This study is focused on identifying the candidature of timothy grass as an energy crop for hydrogen-rich syngas production through supercritical water gasification. Timothy grass was gasified in supercritical water to investigate the impacts of temperature (450–650 °C), biomass-to-water ratio (1:4 and 1:8) and reaction time (15–45 min) in the pressure range of 23–25 MPa. The impacts of carbonate catalysts (e.g., Na₂CO₃ and K₂CO₃) and hydroxide catalysts (e.g., NaOH and KOH) at variable mass fractions (1–3%) were examined to maximize hydrogen yields. In the non-catalytic gasification of timothy grass, highest hydrogen (5.15 mol kg⁻¹) and total gas yields (17.2 mol kg⁻¹) with greater carbon gasification efficiency (33%) and lower heating value (2.21 MJ m⁻³) of the gas products were obtained at 650 °C with 1:8 biomass-to-water ratio for 45 min. However, KOH at 3% mass fraction maximized hydrogen and total gas yields up to 8.91 and 30.6 mol kg⁻¹, respectively. Nevertheless, NaOH demonstrated highest carbon gasification efficiency (61.3%) and enhanced lower heating value of the gas products (4.68 MJ m⁻³). Timothy grass biochars were characterized through Fourier transform infrared spectroscopy, Raman spectroscopy and scanning electron microscopy to understand the behavior of the feedstock to rising temperature and reaction time. The overall findings suggest that timothy grass is a promising feedstock for hydrogen production via supercritical water gasification.     

Assessment of the catalytic effect of various biomass ashes on CO2 gasification of tire char

The aim of this study was to determine the effect of various biomass ashes, comprising catalytically active components, on tire char reactivity during the CO₂ gasification process. Ashes from the combustion of corn cobs, beet pulp, sunflower husks and beech chips were selected for the research. Moreover, industrial fly ash from a coal-fired power plant was used as a reference. The tire char-ash blends with different ash contents (0–15 wt%) were gasified in the CO₂ atmosphere under non-isothermal conditions using dynTHERM Rubotherm thermobalance. Based on the n-order Coats and Redfern method, gasification reactivity indicators and kinetics parameters were calculated. The results showed that the addition of biomass ashes enhanced reactivity of tire char, and the magnitude of these changes depended on both the quantity and type of the additive. With the increase in the amount of added biomass ashes, the catalytic effect increased, and their efficiency can be ranked as follows: sunflower husk ash > corn cobs ash ≅ beet pulp ash > beech chips ash. In turn, reference fly ash from a power plant slightly affected the CO₂ gasification of tire char, regardless of its amount. Moreover, a statistically significant correlation between the reactivity indicator and the amount of K₂O, MgO and P₂O₅ in ashes analysed has been proved (reactivity indicator improved with an increase in these components amount). The performed analysis provides valuable information regarding the composition of catalysts characterised by high catalytic activity in the tire char gasification process.     

Preparation and evaluation of Ni/γ-Al2O3 catalysts promoted by alkaline earth metals in glycerol reforming with carbon dioxide

Glycerol is the main by-product in the biodiesel process and can be considered as a promising and renewable source for hydrogen generation through the reforming process. In this work, catalysts with 15 wt% Ni supported on 3 wt% M ? Al₂O₃ (M = MgO, CaO, SrO, and BaO) were prepared and employed in the glycerol dry reforming (GDR) reaction to produce hydrogen and carbon monoxide. The textural characteristics of the fresh and spent catalysts were determined using the ICP, BET, TPR, TPO, and SEM analyses. Based on the obtained results, the catalyst promoted by SrO had the highest catalytic activity. The results indicated that adding various alkaline-earth oxides into the catalyst support decreased the Ni crystalline size from 17.2 nm to 7.4–10.9 nm. Moreover, all promoted catalysts showed better catalytic performance and the promoted sample with 3 wt% SrO possessed higher stability than unpromoted catalyst during 20 h on stream.     

Evaluation of catalytic subcritical water gasification of food waste for hydrogen production: Effect of process conditions and different types of catalyst loading

Food waste is a kind of wet bio-waste which has been a challenge for the ecological environment and disposal. In this paper, hydrogen production from subcritical water gasification (SbWG) of food waste with and without catalyst loading was systematically investigated. The effects of reaction temperature (300–360 °C), residence time (30–90 min), food waste concentration (10–30 wt%) and catalysts (Ni/γ-Al₂O₃, Ni/ZrO₂, NaOH, KOH, and FeCl₃) were studied within a pressure range of 10.5–20 MPa. The optimal process condition for SbWG of food waste without catalysts loading was determined to be 360 °C and 90 min with 10 wt% food waste. The liquid products and hydrochar were characterized by TOC, TGA/DTG, and SEM. The TOC concentration of liquid products decreased vastly with increasing reaction temperature. The highest H₂ yield (1.88 mol/kg), H₂ mole fraction (35.01%), and H₂ selectivity (53.86%) were achieved at 360 °C for 90 min with 5 wt% loading of KOH. It can be concluded that the performance of the catalysts for improving hydrogen production in SbWG of food waste was in the following order: KOH > NaOH > Ni/γ-Al₂O₃ > Ni/ZrO₂ > FeCl₃. The catalytic SbWG can be a potential alternative for energy conversion of food waste and hydrogen production.     

Comparative study between supported and doped MgO catalysts in supercritical water gasification for hydrogen production

Different catalyst structures may influence the catalytic performance of catalysts in supercritical water gasification (SCWG). This study reports the catalytic activity of supported (SP) and doped (DP) MgO catalysts in catalyzing the gasification of oil palm frond (OPF) biomass in supercritical water to produce hydrogen. Two types of supported catalysts, labelled as Ni-SP (nickel supported MgO) and Zn-SP (zinc supported MgO), were synthesized via impregnation method. Another two types of doped catalysts, labelled as Ni-DP (nickel doped MgO) and Zn-DP (zinc doped MgO), were synthesized by using the self-propagating combustion method. All the synthesized catalysts were found to be pure with the doped catalysts exhibited small crystallites, in comparison to that produced by the supported catalysts. The specific surface area increased in the order of Ni-DP (67.9 m² g⁻¹) > Zn-DP (36.3 m² g⁻¹) > Ni-SP (30.1 m² g⁻¹) > Zn-SP (13.1 m² g⁻¹). Regardless of supported or doped, the Ni-based catalysts always had larger specific surface area than that in the Zn-based catalysts. Unexpectedly, the Zn-based catalysts with smaller surface area for SCWG produced higher hydrogen (H₂) yield from the OPF biomass. When compared to the non-catalytic reaction, the H₂ yield increased by 187.2% for Ni-SP, 269.0% for Zn-SP, 361.7% for Ni-DP, and 438.1% for Zn-DP. Among the studied catalysts, the Zn-DP displayed the highest H₂ yield because it had the highest number of basic sites; approximately twenty-fold higher than that of the Zn-SP catalyst. The Zn-DP also proved to be the most stable catalyst, as verified from the X-Ray photoelectron spectroscopy (XPS) results. As such, this study concludes that the catalytic performances of the synthesized catalysts do not only depend on the specific surface area, but they are also influenced by the number of basic sites and the catalyst stability. It is trustworthy to note that this is the initial study that associated SCWG with doped catalysts. The doped catalysts, hence, may serve as a new catalyst system to generate SCWG reactions.     

Hydrogen production via steam gasification of ash free coals

The kinetics of the coal to hydrogen conversion can be significantly enhanced by introducing catalysts. The catalysts are, however, commonly deactivated by irreversible interaction with mineral matters in coal. This work addresses hydrogen production via steam gasification of ash free coals. Following the production of ash free coals (AFCs) derived from various raw coals (brown, bituminous, and coking coal), fixed-bed steam gasification of the AFCs was performed as a function of temperature and which was compared with one another and also with that of the matching raw coals. In the absence of a catalyst, AFCs produced from different parent coals exhibited similarly low gasification reactivity, comparable to a high rank coal (coking coal) at 700 °C. As expected, the reaction became faster with increasing temperature in the range, 700–900 °C. The steam gasification of AFCs was highly activated by K₂CO₃ above 700 °C. It was very likely that water–gas shift reaction associated with the gasification of AFCs was also catalyzed.     

Hydrogen production by supercritical water gasification of biomass: Explore the way to maximum hydrogen yield and high carbon gasification efficiency

Supercritical water gasification (SCWG) is a promising technology for wet biomass utilization. In this paper, orthogonal experimental design method, which can minimize the number of experiments compared with the full factorial experiments, was used to optimize the operation parameters of SCWG with a tubular reactor system. Using this method, the influences of the main parameters including pressure, temperature, residence time and solution concentration on biomass gasification were also investigated. Simultaneously, in order to further improve the gasification efficiency of biomass, acid hydrolysis pretreatment of feedstock, oxidizers addition and increasing reaction temperature were employed. Results from the experiments show that in the range of experimental parameters, the order of the effects of the factors on H₂ yield of corn cob gasification in SCW is temperature > pressure > feedstock concentration > residence time. Temperature and pressure have a significant and complicated effect on biomass gasification. Hydrogen yield increases by the acid hydrolysis pretreatment of feedstock, and oxidizer addition reduces the hydrogen yield but it promotes the increase in carbon gasification efficiency. Biomass feedstock with high concentration was gasified successfully at high reaction temperature.     

Effect of alkaline earth oxides on nickel catalysts supported over γ-alumina for butanol steam reforming: Coke formation and deactivation process

Nickel catalysts were synthesized by the wet impregnation of three different supports: γ-Al₂O₃ and alumina promoted with either 10 wt % of MgO or 10 wt% of CaO. The catalysts were evaluated in butanol steam reforming at 500 °C, atmospheric pressure, GHSV of 500,000 h⁻¹ and 10% v/v butanol in the feed. Both promoters decreased catalyst acidity and increased basicity. The catalyst promoted with MgO exhibited the lowest acidity (1.1 μmolNH₃ m⁻²), whereas that promoted with CaO, the highest basicity (870.7 μmolCO₂ m⁻²). The promotion with MgO led to the highest hydrogen yield (66%) and stability, associated with its highest nickel dispersion (3.4%), lowest acidity and lowest coke formation normalized by carbon converted (3.0 mmol L mol⁻¹). The catalyst promoted with CaO presented the most severe deactivation, associated with its lowest dispersion (1.0%) and the highest amount of encapsulated coke (3.5 mmol L mol⁻¹).     

Copper promoted Co/MgO: A stable and efficient catalyst for glycerol steam reforming

Different types of cobalt-based mixed oxide catalysts (20 wt%Co/MgO, 5 wt%Cu-20 wt% Co/MgO, 20 wt%Co/50%MgO–50%Al₂O₃) were synthesized by the co-precipitation method and applied for hydrogen production from glycerol steam reforming. The catalysts were characterized using X-ray diffraction (XRD), H₂-Temperature-programmed reduction (H₂-TPR), CO₂-Temperature Programmed desorption, CO-Chemisorption, and CHN techniques. The H₂-TPR analysis showed the reducibility of cobalt-oxide (5Cu20CM; 5 wt%Cu-20 wt% Co/MgO) was enhanced by the copper, and reduction profiles of cobalt oxide shifted to a lower temperature (<450 °C). Among the catalysts, 5Cu20CM showed a maximum yield of hydrogen (74.6%) with 100% conversion of glycerol to the gaseous phase. The superior catalytic performance of 5Cu20CM for glycerol conversion was attributed to the smaller particle size (7 nm), higher dispersion of cobalt (35.0%), and the higher surface area (56 m²/g) of cobalt metal. Furthermore, Raman spectroscopy of the spent catalysts confirmed that the copper promoted cobalt-magnesium catalyst suppressed the carbon formation, consequently, 5Cu20CM catalyst showed a stable performance up to 30 h.     

Experimental analysis of air-steam gasification of biomass with coal-bottom ash

Biomass gasification is an attractive option for producing high-quality syngas (H₂+CO) due to its environmental advantages and economic benefits. However, due to some technical problems such as tar formation, biomass gasification has not yet been able to achieve its purpose. The purpose of this work was to study the catalytic activity of coal-bottom ash for fuel gas production and tar elimination. Effect of gasification parameters including reaction temperature (700–900 °C), equivalence ratio, EQR (0.15–0.3) and steam-to-biomass ratio, SBR (0.34–1.02) and catalyst loading (5.0–13 wt %) on gas distribution, lower heating value (LHV) of gas steam, tar content, gas yield and H₂/CO ratio was studied. The tar content remarkably decreased from 3.81 g/Nm³ to 0.97 g/Nm³ by increasing char-bottom ash from 5.0 wt% to 13.0 wt%. H₂/CO significantly increased from 1.12 to 1.54 as the char-bottom ash content in the fuel increased from 5.0 wt% to 13.0 wt%.     

Catalytic poppy seed gasification by lanthanum-doped cobalt supported on sepiolite

In this study, sepiolite as support material, was firstly used to synthesize a series of Co/xLa-SEP catalysts. Besides, a newly proposed biomass, poppy seed, was utilized in gasification for assessment of catalyst activity under various La loadings (0–10 wt% with a 2 wt% increment for each) and temperatures (550, 650, 750, 850 °C). The non-catalytic gasification resulted in the highest H₂ concentration of 3.89 mol/kg poppy seed at 850 °C, whereas, the catalyst with 6 wt% La loading showed superior catalytic performance even at the much lower temperature of 650 °C, yielding 4.75 mol H₂/kg poppy seed. Based on the results, char formation was almost utterly hindered in presence of 10 wt% La laoding at 850 °C, whereas a significant reduction of tar to 3.83 wt% was attained in corporation of 4 wt% La at 750 °C. The prepared catalysts were characterized by X-Ray Fluorescence (XRF), X-Ray Diffraction (XRD), Thermogravimetric Analysis (TGA), Fourier Transform Infrared Spectroscopy (FT-IR), Brunauer-Emmett-Teller (BET) and Scanning Electron Microscopy (SEM). SEM images and BET data presented that the Co/6La-SEP catalyst exhibited smallest particle size and the highest surface area (251.05 m²/g).     

Noncatalytic gasification of isooctane in supercritical water: A Strategy for high-yield hydrogen production

Continuous supercritical water gasification of isooctane, a model gasoline compound, is investigated using an updraft gasification system. A new reactor material, Haynes^® 230^® alloy, is employed to run gasification reactions at high temperature and pressure (763 ± 2 °C; 25 MPa). A large-volume reactor is used (170 mL) to enable the gasification to be run at a long residence time, up to 120 s. Various gasification experiments are performed by changing the residence time (60-120 s), the isooctane concentration (6.3-14.7 wt%), and the oxidant concentration (equivalent oxidant ratio 0-0.3). The total gas yield and the hydrogen gas yield increase with increasing residence time. At 106 s and an isooctane concentration of 6.3 wt%, a very high hydrogen gas yield of 12.4 mol/mol isooctane, which is 50% of the theoretical maximum hydrogen gas yield and 92% of the equilibrium hydrogen gas yield under the given conditions, is achieved. Under these conditions, supercritical water partial oxidation does not increase the hydrogen gas yield significantly. The produced gases are hydrogen (68 mol%), carbon dioxide (20 mol%), methane (9.8 mol%), carbon monoxide (1.3 mol%), and ethane (0.9 mol%). The carbon gasification efficiency is in the range 75-91%, depending on the oxidant concentration. A comparison of supercritical water gasification with other conventional methods, including steam reforming, autothermal reforming, and partial oxidation, is also presented.     

High-yield hydrogen production from glucose by supercritical water gasification without added catalyst

Continuous supercritical water gasification of glucose is investigated with a recently developed updraft gasification apparatus under various conditions: temperatures of 600–767 °C, residence times of 15–60 s, glucose concentrations of 1.8–15 wt% and without added a catalyst. The experimental gas yields are compared with predicted values at equilibrium that are estimated via Gibbs free energy minimization. Total gas yields and hydrogen gas yield increase with temperature. At 740 °C and 1.8 wt%, hydrogen gas yields become very high (10.5–11.2 mol/mol glucose). The hydrogen gas yields do not vary significantly with different residence times. The hydrogen gas yield decreases to 5.7 mol/mol glucose at 15 wt%, a value very close to the predicted value (6.3 mol/mol glucose). Only acetic acid is detected in the liquid effluents at temperatures above 740 °C, while 42 products are detected at 600 °C. The highest hydrogen gas yield obtained in this study is 11.5 mol/mol glucose at 25 MPa, 767 °C, and 1.8 wt%, for 60 s; this value is very close to the theoretical equilibrium hydrogen yield of 11.9 mol/mol glucose. Under these conditions, the carbon efficiency is very high (91%) and total organic carbon (TOC) in the liquid product is very low (23 ppm), indicating that glucose is almost completely converted to gaseous products. Comparison with other work under similar operating conditions shows that the current reactor is capable of attaining higher hydrogen gas yields at temperatures above 650 °C. Possible explanations for the higher hydrogen gas yields are presented.