Supercritical Water Gasification Process Research Articles

Prediction of Individual Gas Yields of Supercritical Water Gasification of Lignocellulosic Biomass by Machine Learning Models.

Supercritical water gasification (SCWG) of lignocellulosic biomass is a promising pathway for the production of hydrogen. However, SCWG is a complex thermochemical process, the modeling of which is challenging via conventional methodologies. Therefore, eight machine learning models (linear regression (LR), Gaussian process regression (GPR), artificial neural network (ANN), support vector machine (SVM), decision tree (DT), random forest (RF), extreme gradient boosting (XGB), and categorical boosting regressor (CatBoost)) with particle swarm optimization (PSO) and a genetic algorithm (GA) optimizer were developed and evaluated for prediction of H2, CO, CO2, and CH4 gas yields from SCWG of lignocellulosic biomass. A total of 12 input features of SCWG process conditions (temperature, time, concentration, pressure) and biomass properties (C, H, N, S, VM, moisture, ash, real feed) were utilized for the prediction of gas yields using 166 data points. Among machine learning models, boosting ensemble tree models such as XGB and CatBoost demonstrated the highest power for the prediction of gas yields. PSO-optimized XGB was the best performing model for H2 yield with a test R2 of 0.84 and PSO-optimized CatBoost was best for prediction of yields of CH4, CO, and CO2, with test R2 values of 0.83, 0.94, and 0.92, respectively. The effectiveness of the PSO optimizer in improving the prediction ability of the unoptimized machine learning model was higher compared to the GA optimizer for all gas yields. Feature analysis using Shapley additive explanation (SHAP) based on best performing models showed that (21.93%) temperature, (24.85%) C, (16.93%) ash, and (29.73%) C were the most dominant features for the prediction of H2, CH4, CO, and CO2 gas yields, respectively. Even though temperature was the most dominant feature, the cumulative feature importance of biomass characteristics variables (C, H, N, S, VM, moisture, ash, real feed) as a group was higher than that of the SCWG process condition variables (temperature, time, concentration, pressure) for the prediction of all gas yields. SHAP two-way analysis confirmed the strong interactive behavior of input features on the prediction of gas yields.

Experiment evaluation and thermodynamic analysis of pulping modification on ordered conversion of bark in supercritical water

Major obstacles of supercritical water gasification (SCWG) of lignocellulosic biomass, including ineffective pulping for energy self-sufficiency and a high conversion barrier significantly inhibited its large-scale development. In this study, the pulping effect of alkaline and salt pulping for high concentration bark slurry were compared systematically. The SCWG performances of bark (non-catalytic/catalytic) and bark slurry were evaluated. Based on energy self-sufficiency, a novel SCWG process of 50 wt% bark slurry was designated with a multistage reactor configuration and pulping process. The findings suggest that alkaline pulping had a wider applicability (reaching 50 wt%) and better rheological properties than salt pulping. Only 30 wt% bark slurry was capable for applying salt pulping because of solid residue with the excessive salt addition. Alkaline pulping resulted in an ordered conversion of 5 wt% and 10 wt% bark slurries during SCWG, with complete gasification of 5 wt% bark slurries prepared by KOH and NaOH at 650 and 680 °C, respectively. Besides, alkaline and salt pulping reduced the 26 % and 33 % carbon emissions, respectively. In the novel process, the optimal ratio of preheated water to bark slurry was 4 (water diversion to the primary and secondary gasification reactors of 1:3), which exhibits in energy and exergy efficiencies of 47.47 % and 48.63 %, respectively. This configuration reduces exergy losses in the gasification reactor and heat exchanger, aiding in the large-scale industrial output of SCWG.

Understanding the conversion mechanisms in supercritical water conditions of PET as a model compound of plastic wastes

In this work, polyethylene terephthalate (PET) was used as a model compound for understanding the conversion mechanism of plastic wastes through supercritical water gasification (SCWG) for producing: i) a gas rich in methane/hydrogen; and ii) for obtaining valuable molecules to be recycled in chemical processes (e.g., benzoic and acetic acids).The experiments were conducted at two different temperatures (above the critical point of water): 450 °C and 600 °C, in order to study the influence of this parameter on the conversion yield. The holding time in the reactor was set at 30 min for all trials. The experimental data were compared to the thermodynamic equilibrium calculations and the results showed that increasing the temperature from 450 °C to 600 °C, the total gas yield is highly improved (from 8.6 mol gas/kg of dried feedstock to 22 mol gas/kg of dried feedstock). The experiments with Raney-nickel catalyst indicated a significant improvement of the carbon conversion yield, which led to a higher gas production from mild to higher temperatures. The maximum total gas yield obtained with this catalyst was 63.4 mol gas/kg of dried feedstock at 600 °C, which is almost 3 times higher than that obtained at the same operating conditions without catalyst. The HHV of the obtained gas at 600 °C with the catalyst was 64.7 MJ/kg gas and 57.9 MJ/kg gas at 450 °C. Additionally at mild operating temperatures (450 °C), the obtained performance with the catalyst was even higher than that obtained at 600 °C without catalyst (50 mol gas/kg dried feedstock and 22 mol gas/kg dried feedstock, respectively), which is the major contribution of this work. At mild temperatures (450 °C) the methane production is highly favored, meanwhile at 600 °C the hydrogen production is enhanced. The methane yield obtained at 450 °C was 21.3 mol CH4/kg dried feedstock and the hydrogen yield obtained at 600 °C was 20.6 mol H2/kg dried feedstock. In the aqueous phases obtained from the experiments performed without catalyst at 450 °C, it was found valuable intermediate species such as benzoic and acetic acids (at a concentration of 4462 mg/L and 946 mg/L, respectively). This study represents a sustainable approach for valorizing plastic wastes by catalytic SCWG process.

Comparative Assessment of Hydrothermal Gasification and Anaerobic Digestion using Aspen Plus and SuperPro

Wet biomass is gaining increasing attention as an energy source globally. Various wet biomass materials like macroalgae, microalgae, sewage sludge, cattle manures, and food waste, are of high moisture contents, typically around 70% or more. There are two routes of high potential for converting these wet biomasss resoures to gaeous fuel, which are biomethanation and supercritical water gasification (SCWG). Biomethanation is conversion of wet-organic residues to biogas using via anaerobic digestion process. SCWG is a thermochemical conversion process, taking place in supercritical water producing enriched CH4 and H2 gases. In this study, the two processes are comperatively investigated using different process simulation softwares. Aspen Plus was employed for SCWG, while the biomethanation process was simulated with SuperPro. Various raw materials were used as input. We calculated the energy efficiency of the biomethanation process using experimental yields from the literature. For the SCWG process, we determined the higher heating value based on the simulated composition of the methane-enriched gas. Additionally, we conducted an economic analysis to compare the two processes, taking into consideration specific criteria relevant to the Norwegian context.

In-situ hydrogen production and battery electrode materials from metal effluent and biomass

Supercritical water gasification (SCWG) is an effective technique for the conversion of biomass into hydrogen-rich fuel gas. This work addresses two important aspects of the SCWG process by employing the concept of waste to wealth: first, the requirement of heterogeneous catalysts for better efficiency, and second, the safe disposal of byproducts of the process. Metal-contaminated aqueous effluent (Zn-EPW) is used as the reaction medium with the catalyst for single-step hydrothermal conversion of banana pseudo-stem (BPS) to synthesize multi-dimensional value-added products such as H2-rich fuel gas, nano metal–carbon hybrids (NCH), and treated aqueous phase. This study demonstrates the enhanced conversion of biomass into hydrogen-rich gas and recovery of heavy metals (HMs) in the form of nanometal carbon hybrids, which can be utilized to fabricate carbon-based electrodes in Li-S batteries. A comparative study of feedstock, such as a combination of Zn-EPW adsorbed on BPS (MA-BPS) with Millipore water, direct treatment Zn-EPW with BPS, and Millipore water with BPS (without HMs), was performed. The results inferred the catalytic effect of heavy metals increased the yield of hydrogen about ∼2.9 times with Zn-EPW and BPS and ∼1.7 times with MA-BPS and Millipore water over the non-catalytic operation. Furthermore, the obtained NCH were characterized to evaluate their performance for electrodes in energy storage devices. The electrochemical performance of NCH obtained after multi-step SCWG of MA-BPS with Millipore water and single-step treatment of Zn-EPW with BPS at 600 °C, have specific capacity of 610 and 615 mAh.g−1 at 0.1C respectively.

Thermodynamic and environmental analysis of an integrated multi-effect evaporation and organic wastewater supercritical water gasification system for hydrogen production

Organic wastewater poses significant environmental and human health risks. Supercritical water gasification (SCWG) presents a promising approach for converting organic waste into hydrogen-rich mixed gases. Nevertheless, the high moisture content present in organic wastewater hampers the energy efficiency of the SCWG process. To tackle this challenge, we have designed and developed an organic wastewater SCWG hydrogen production system incorporating a multi-effect evaporator. The developed system demonstrates the ability to achieve autothermal conditions for the treatment of swine wastewater with a solids concentration of 4.3%. Notably, this system exhibits impressive energy efficiency, reaching a maximum value of 44.88%, along with an exergy efficiency of 33.90% when processing 7% swine wastewater. Exergy analysis further reveals that the incorporation of a multi-effect evaporator leads to a reduction in exergy destruction within the gasification unit, oxidation unit, and heat exchanger. These improvements highlight the positive impact of the multi-effect evaporator on the overall performance of the supercritical water gasification system. Sensitivity analysis indicated that suitable concentrated concentration and an appropriate amount of preheated water can improve hydrogen production. The system minimum GWP reaches 5.27 kgCO2-eq/kgH2 after adding the CCUS. This work expands the range of applicable feedstocks for the autothermal SCWG process.

Supercritical water gasification of waste R410A refrigerant mixture for the resource utilization: ReaxFF reactive molecular dynamic simulation and density functional theory calculation study

Harmless treatment and resource utilization of the waste HFC refrigerants have gradually become the focus of attention. As a clean waste resource utilization technology, supercritical water gasification has good potential in converting waste refrigerant into high value-added chemicals and fuels. In this study, the supercritical water gasification mechanism of R410A, and the interaction mechanism between R125 and R32, and between H2O and R410A during the gasification process are investigated by using ReaxFF reactive molecular dynamic simulation and density functional theory calculation study. The results show that the main products obtained from the supercritical water gasification of R410A are HF, H2, CO and CO2. More than 90 % of F atoms in R410A can be mineralized to form HF molecules. H2 and CO are two important syngas products during the supercritical water gasification process. Hydrogen bonds are generated between R125 and H2O molecules and between R125 and R32 molecules, which leads to the decomposition of R125 molecules more easily, and the decomposition of R32 molecules more difficult. This work provides a possible way to convert waste HFC refrigerant mixture into high value-added chemicals and syngas.

Comparison study of supercritical water gasification for hydrogen production on a continuous flow versus a batch reactor

This study compares batch and continuous supercritical water gasification (SCWG) processes for green hydrogen production from biomass. It offers insights for optimizing processes, enhancing yields, quality, and energy efficiency, assessing scale-up feasibility, and supporting techno-economic analyses. Glucose, glycerol, and black liquor were SCWG-treated at 500 °C with K2CO3 catalyst in a self-built continuous-flow reactor (150 g/h) and a batch reactor (75 mL). Comparisons primarily focused on gas product yields. Batch reactors outperformed continuous-flow reactors in hydrogen (glucose: 1.53 to 0.9 mmol/g, glycerol: 7.22 to 1.14 mmol/g, black liquor: 2.88 to 1.74 mmol/g) and total gas yields due to differences in reaction time and heating behavior. Temperature effects on continuous operation (450–600 °C) were studied, with glycerol showing the highest hydrogen yield increase (from 1.21 to 4.30 mmol/g). The study discusses the applicability of both reactors for biomass SCWG processes and their implications for sustainable green hydrogen production from renewable feedstocks.

An experimental and thermodynamic equilibrium investigation of heavy metals transformation in supercritical water gasification of oily sludge

Supercritical water gasification (SCWG) is an advanced and highly efficient method for treating oily sludge. However, it is crucial to consider the transformation characteristics of heavy metals (HMs) during the SCWG process to prevent potential secondary pollution. This work studied the transformation and distribution characteristics of Cu, Cr and Zn after SCWG of oily sludge in a batch reactor at temperatures ranging from 550 to 700 °C. Additionally, thermodynamic equilibrium analysis was conducted to assess the distribution of HMs based on the minimization of Gibbs free energy. Experimental results indicated that higher temperatures led to the conversion of HMs into more stable forms, effectively immobilizing them within solid products. Furthermore, the addition of Na2CO3 enhanced this process and contributed to a reduction in HMs pollution in the effluent. Thermodynamic equilibrium results were consistent with our experimental data, indicating that the molar fraction of stable HMs forms followed the order: Cr > Cu > Zn. Besides, it is worth noting that Na2CO3 had a limited impact on the distribution of Cu and Cr. However, it played a significant role in inhibiting the formation of silicate Zn at lower temperatures, promoting the decomposition of ZnO*Al2O3 into unstable Zn. This may explain the higher presence of unstable Zn when Na2CO3 was introduced. In summary, this study offers valuable insights into the transformation characteristics of heavy metals and strategies for pollution control during SCWG of oily sludge.

A large-eddy simulation study of the transcritical mixing process in coaxial jet flow under supercritical condition

Supercritical water gasification (SCWG) is a promising clean energy technology for utilizing fossil fuels or organic solid waste with the challenge of delivering and dispersing liquid feedstock into the reactor. This paper investigates the transcritical mixing process in coaxial injections using large-eddy simulation. The results reveal that large temperature and density gradients exist between the inner injected water and the ambient fluid in the near field of transcritical water injection. Structure comparison demonstrates that the cross-section of the inner potential core shrinks more quickly in the non-premixing coaxial injection than the single injection, which can facilitate the dispersion and dilution of injected water during the heating process due to the shear mixing with the coaxial high-velocity outer injection. In addition, in the premixing coaxial injections, the substantial radial velocity fluctuation induced by the contracting cross-section leads to a much shorter inner potential core and more intensive heat and mass transfer. Swirling inlets can significantly strengthen the radial velocity fluctuation, thereby accelerating the temperature increase and species blending. Coaxial injections can effectively be used for dispersing feedstock in the SCWG process, particularly for high-viscosity materials.

Molecular Dynamics Investigation of the Gasification and Hydrogen Production Mechanism of Phenol in Supercritical Water

Supercritical water gasification is an efficient and clean method for converting biomass into hydrogen-rich gas. Phenol plays a crucial role as an intermediate product in biomass supercritical water gasification, and studying its reaction pathway in supercritical water is essential for understanding the chemical reaction mechanism and optimizing biomass energy conversion processes. In this paper, we investigated the conversion mechanism of phenol gasification and hydrogen production in supercritical water using a combined approach of reactive force field (ReaxFF) and density functional theory (DFT). We determined the decomposition pathways and product distribution of phenol in supercritical water. The calculation results demonstrate that in the supercritical water system, the efficiency of phenol conversion for hydrogen production is approximately 27 times higher than that of hydrogen production through gasification in the pyrolysis state. Moreover, both the carbon conversion rate and hydrogenation rate in the supercritical water system are significantly higher compared to those in the pyrolysis system. Furthermore, we found that the energy in the supercritical system is approximately half that of the pyrolysis system, favoring the ring-opening reactions of phenol and promoting hydrogen production. In contrast, the pyrolysis system produces a greater quantity of aromatic compounds, leading to tar formation and having significant implications for both the reaction process and reactor design. Additionally, we conducted comparative experiments between the supercritical water gasification process and the pyrolysis process to explore the advantages of supercritical water gasification.

High Dimensional Model Representation Approach for Prediction and Optimization of the Supercritical Water Gasification System Coupled with Photothermal Energy Storage

Supercritical water gasification (SCWG) coupled with solar energy systems is a new biomass gasification technology developed in recent decades. However, conventional solar-powered biomass gasification technology has intermittent operation issues and involves multi-variable characteristics, strong coupling, and nonlinearity. To solve the above problems, firstly, a solar-driven biomass supercritical water gasification technology combined with a molten salt energy storage system is proposed in this paper. This system effectively overcomes the intermittent problem of solar energy and provides a new method for the carbon-neutral process of hydrogen production. Secondly, the high dimensional model representation (HDMR) approach, as a surrogate model, was used to predict the production and lower heating value of syngas developed in Aspen Plus, which were validated using experimental data obtained from the literature. The ultimate analysis of biomass, temperature, pressure, and biomass-to-water ratio (BWR) were selected as input variables for the model. The non-dominated sorted genetic algorithm II (NSGA II) was considered to maximize the gasification yield of H2 and the LHV of syngas in the SCWG process for five different types of biomass. Firstly, the results showed that HDMR models demonstrated high performance in predicting the mole fraction of H2, CH4, CO, CO2, gasification yield of H2, and lower heating value (LHV) with R2 of 0.995, 0.996, 0.997, 0.996, 0.999, and 0.995, respectively. Secondly, temperature and BWR were found to have significant effects on SCWG compared to pressure. Finally, the multi-objective optimization results for five different types of biomass are discussed in this paper. Therefore, these operating parameters can provide an optimal solution for increasing the economics and characteristics of syngas, thus keeping the process energy efficient.

Predictive control of reactor network model using machine learning for hydrogen-rich gas and biochar poly-generation by biomass waste gasification in supercritical water

Supercritical water gasification (SCWG) technology can convert biomass into hydrogen rich gas and biochar. Fluidized bed reactor is promising for the industrialization of this technology, and the reactor dynamic performance study is of great significance for its scaling up. However, current simulation studies mainly focus on steady-state analysis using Computational Fluid Dynamics (CFD) software, it is difficult to conduct dynamic study due to its high computational costs. To this end, a reactor network model (RNM) that accounts for the flow, heat transfer, and kinetic dynamics for fluidized bed reactor is firstly developed based on the partitioning theory, which can reduce the computation time of dynamic simulation from several days to seconds. Additionally, extensive open-loop simulations of the RNM are carried out to generate the dataset for the development of a recurrent neural network (RNN) model. Additionally, current studies mainly focus on open-loop simulation, closed loop optimization is absent. To this end, a framework for building a machine learning (ML) model and a ML-based nonlinear predictive control scheme is developed. Model predictive control (MPC) schemes based on the RNN model is used to optimize the SCWG process to achieve multiple objectives, such as maximizing hydrogen yield and carbon yield. The open loop simulation results demonstrate that H2 yield decreases from 0.00022 to 0.0002 kg s−1 with temperature reducing from 700 to 600 °C. To increase H2 yield to the setpoint, MPC increases temperature and mass flowrate to 708 °C and 417.57 kg h−1. Additionally, MPC increases carbon yield and minimizing CO2 yield by decreasing temperature to 475 °C and increasing mass flowrate to 407.38 kg h−1.

Direct numerical simulations of polypropylene gasification in supercritical water

In order to reduce environmental pollution by plastic wastes, supercritical water gasification (SCWG) appears as a promising technology. The present study investigates the SCWG process of polypropylene (PP) plastic waste using particle-resolved direct numerical simulations (PR-DNS). A directional ghost-cell immersed boundary method has been used to solve the reacting boundary condition, including detailed molecular diffusion models. To validate the procedure, SCWG of a coal particle has been first investigated as a benchmark, analyzing in detail interphase momentum and heat and mass transfer, and chemical reactions are analyzed. Surface reactions and the resulting Stefan flow expand the boundary layer around the particle, impacting the efficiency of heat and mass transfer. Adding then a suitable reaction mechanism, SCWG of PP plastic wastes leading to combustible gases is analyzed by PR-DNS and found to be very efficient. The gasification temperature is an important parameter to control SCWG efficiency. To the authors' knowledge, this is the first PR-DNS study investigating the SCWG process for plastic wastes, and it provides interesting information regarding transfer processes and their limitations.

Corrosion behaviors of Ni-based and Fe-based alloys in supercritical water gasification with inorganic chloride salt

Supercritical water gasification (SCWG) technology is considered the most promising technology for treating oily wastewater. However, the impact of mixed inorganic salts on the reactor during the SCWG process of oily wastewater still lacks research. In this study, Fe-based alloys and Ni-based alloys were exposed to an environment of inorganic chloride salt in supercritical water. The analysis found that while Ni-based alloys have higher corrosion resistance than Fe-based alloys, in an environment of supercritical water containing inorganic chlorides, the surface corrosion of Fe-based alloys was significantly suppressed, while the surface corrosion of Ni-based alloys was exacerbated.

Comparative economic, environmental and exergy analysis of power generation technologies from the waste sludge treatment

As a growing amount of sewage sludge is being produced from wastewater treatment plants, there is an urgency to develop innovative processes to dispose of such municipal waste well. One popular and widely acceptable way of utilizing waste sludge is for power generation. Although there is a good implementation of power generation plants from sewage sludge in Hong Kong, only a few published works have compared the conventional incineration-based process with chemical looping combustion (CLC) technology and the new supercritical water gasification (SCWG) process in detail. In this work, we performed a comprehensive energy, environmental and economic assessment of three power generation processes from the three thermochemical conversions of sewage sludge: i) conventional direct incineration + carbon capture (DI-CC), ii) steam gasification + CLC (G-CLC), and iii) SCWG + gas turbines (SCWG-GT). The final comparative results showed that the G-CLC scheme performed the best from the perspective of economic cost and benefits as well as the environmental global-warming potential (GWP) indicator, although the net energy and exergy efficiencies were not the largest among the three schemes. Notably, the SCWG-GT process for power generation showed its energy superiority with energy and exergy efficiencies of 20.70% and 16.69% consequently. Overall, CLC-based technology can be a promising process for power plant development in the future.

Hydrogen production via supercritical water gasification of glycerol enhanced by simple structured catalysts

The supercritical water gasification (ScWG) technology is a promising alternative for H2-rich gas production from renewable sources, such as residual glycerol from biodiesel manufacture. Combined with heterogeneous catalysts, the ScWG process can achieve improved selectivity towards the desired products and high conversion efficiency in short reaction times. In this work, the efficiency of a synthesized Ni-based catalyst supported in cordierite (CRD) honeycomb structure on the ScWG of glycerol was evaluated and compared with two commercial automotive catalysts. Initially, to determine the best experimental conditions, the ScWG experiments were conducted in the absence of catalysts at constant conditions pressure (25 Mpa) and volumetric flow rate (10 mL min−1). The temperature range of 400–700 °C and glycerol feed composition between 10 and 34 wt% were evaluated. The catalysts evaluated were characterized by SEM-EDS, XRD, N2 adsorption/desorption, XRF, WDS and TGA. The liquid and gaseous products were analyzed by TOC and gas chromatography, respectively. Results indicated that Ni/CRD catalyst showed the highest H2 yield (5.38 mol H2 per mol of glycerol fed) and long-term stability. Additionally, a comparison between the experimental results on the ScWG of glycerol and simulated thermodynamic equilibrium data was also reported. Thus, results demonstrated the great potential of the prepared catalyst to improve H2-rich gas production from glycerol gasification.

Hydrogen Production by Catalytic Supercritical Water Gasification of Black Liquor-Based Wastewater

In this work, the wastewater obtained from the hydrothermal liquefaction of black liquor was treated and valorized for hydrogen production by supercritical water gasification (SCWG). The influence of the main process parameters on the conversion yield was studied. The experiments were conducted at three different temperatures (below and above the critical point of water): 350 °C, 450 °C and 600 °C. The results showed that by increasing the temperature from 350 °C to 600 °C, the total gas yield was highly improved (from 1.9 mol gas/kg of dried feedstock to 13.1 mol gas/kg of dried feedstock). The H2 composition was higher than that of CH4 and CO2 at 600 °C, and the HHV of the obtained gas was 61.2 MJ/kg. The total organic carbon (TOC) removal efficiency was also improved by increasing the temperature, indicating that the SCWG process could be used for both applications: (i) for wastewater treatment; (ii) for producing a high calorific gas. The experiments with the Raney-nickel catalyst were performed in order to study the catalyst’s influence on the conversion yield. The results indicated that the catalyst enhances carbon conversion and gas production from mild to higher temperatures. The maximum total gas yield obtained with this catalyst was 32.4 mol gas/kg of dried feedstock at 600 °C, which is 2.5 times higher than that obtained at the same operating conditions without a catalyst. The H2 yield and the HHV of the obtained gas with the catalyst were 20.98 mol gas/kg dried feedstock and 80.2 MJ/kg, respectively. However, the major contribution of the catalytic SCWG process was the improvement of the total gas yield at mild operating temperatures (450 °C), and the obtained performance was even higher than that obtained at 600 °C without catalyst (17.81 mol gas/kg dried feedstock and 13.1 mol gas/kg dried feedstock, respectively). This is a sustainable approach for treating wastewater at mild temperatures by catalytic SCWG.

Assessment of rice husk biomass from different agro-ecological zones of Tanzania for biofuels feasibility via supercritical water gasification

In this work, rice husks (RH) from 4 agro-ecological zones of Tanzania were characterised for their potential use via supercritical water gasification (SCWG). The physical and chemical characteristics of different varieties of RH were evaluated, and the results were correlated with their geographical factors. The SCWG was conducted at a temperature of 492 °C, feed concentration of 9.5 wt%, residence time of 120 min, and catalyst loading of 15%. The catalyst was prepared by impregnating calcined limestone with 5% iron from the iron (III) ammonium citrate precursor. The RH samples exhibit a moisture content of 6.3–6.9 wt%, volatile matter 56.8–62.8% dry basis (%db), fixed carbon 11.64–19.5%db, ash content 22.2–27.3%db, and carbon content 29.97–3.4%db. The higher heating value ranged between 12.0 and 12.8 MJkg−1, extractives from 10.93 to 18.0%db, cellulose from 34.4 to 38.5%db, lignin from 16.3 to 17.5%db, and hemicellulose from 17.0 to 21.0%db. The results from noncatalytic SCWG showed that the gasification efficiency (GE) varied between 59.3 and 62.7%, gas volume between 278.5 and 305.2 mLg−1 biomass, gravimetric tar between 24.3 and 31.6 mgg−1 biomass, and char yield between 34.6 and 38.6 wt%. Phase characterisation of the fabricated catalyst showed that the main reactive sites are Ca2Fe2O5, CaO and Fe2O3, which promote char gasification and gas and tar formation reactions. When 5%Fe/limestone is employed, the GE ranges between 69.3 and 73.6%, gas volume is between 401.8 and 427.3 mLg−1, tar yield ranges between 57.5 and 62.8 mgg−1, and char between 20.1 and 24.9 wt%. The findings show that the fabricated catalyst is suitable for the simultaneous production of gaseous and liquid biofuels from rice husks via the SCWG process.

Advanced exergy analysis on supercritical water gasification of coal compared with conventional O2-H2O and chemical looping coal gasification

Supercritical water gasification (SCWG) that converting coal into hydrogen-rich syngas is a promising cascade utilization technique of coal. Advanced exergy analysis of SCWG is conducted to evaluate avoidable exergy destructions, and results are compared with conventional (CG) and chemical looping gasification (CLG). In the benchmark condition, the exergy efficiency of SCWG process is 68.63%, which is higher than CG and CLG. The biggest irreversibility of gasification process occurs in gasifier caused by gasification reaction, and SCWG gasifier has high avoidable exergy destruction potential. The modified exergy efficiency of SCWG process is 82.38% under unavoidable conditions. However, avoidable exergy destruction portion of syngas self-heating recovery is only 3.55% due to heat transfer mismatch between supercritical water and syngas. Suitable external heat sources should be adopted, such as the variable heat source with constant cp, which can reduce exergy destruction of supercritical water heating to 44.86%. The effects of parameters on SCWG is performed with single factor analysis method. Coal water slurry concentration (CWSC) is the key factor affecting the performance of SCWG. With gasification temperature < 750 °C, gasification pressure > 23 MPa, CWSC > 17% and oxygen-coal ratio < 0.46 kg kg−1, the exergy efficiency of SCWG process is higher than CG and CLG.