Supercritical Water Gasification Research Articles

Hydrogen production through catalytic supercritical water gasification: Energy and char formation assessment

Biomass gasification has a great potential for sustainable energy production, effectively satisfying the growing energy demand. A promising technology for sustainable hydrogen production is the supercritical water gasification (SCWG) of biomass. This study proposes a new model to assess gas yields and char formation in the SCWG process. This research employs a thermodynamic approach to model the reactor, assuming the equilibrium condition. The presence of catalyst on the SCWG is also incorporated in the proposed model, considering a deviation term for the Gibbs free energy of solid char. In addition, a process simulation is conducted to analyze the energy performance of the SCWG unit. Four different feedstocks, including sunflower, corncob, leather waste, and coal, are considered in the SCWG process. The newly developed model considerably improves the prediction of the gas yields and solid char formation, based on the experimental data. Compared to the basic model, the gas yield and formed char from the sunflower, corncob, leather waste, and coal are increased by 85.37, 62.52, 89.30, and 34.05%, respectively. It is found that the basic model fails to obtain the solid char content for coal at 750 °C and 24.55 MPa, while the new model offers an accurate estimation of the coal’s char formation. According to the sensitivity results, the temperature and feed concentration substantially influence the gas yields and char formation, while pressure is less impactful. Moreover, the carbon gasification efficiency (CGE) increases at higher temperatures and smaller biomass concentrations. The outcomes of energy analysis reveal the positive impact of higher temperature, pressure, and biomass concentration. Also, the energy consumption per hydrogen production holds a minimum value at 828.6 °C. The specific energy consumption per hydrogen production unit increases at higher pressures and reduces as the biomass concentration lowers.

Partial oxidation gasification kinetics of indole in supercritical water for hydrogen production

Indole is one of refractory intermediate products in hydrothermal conversion of biomass, and its decomposition significantly restricts the improvement of gasification efficiency and hydrogen production in biomass supercritical water gasification (SCWG). In order to improve gasification efficiency of indole and moderate reaction conditions for hydrogen production in biomass SCWG, indole (0.25 mol/L) was gasified in supercritical water at 540–600 °C, 25 MPa, 0–0.8 of oxidation coefficients (OC) in min-batch reactors. Corresponding pathways and kinetics of partial oxidation of indole in supercritical water for hydrogen production were explored. The results showed that the addition of oxidant (hydrogen peroxide) promoted steam reform and decomposition, especially ring-opening reactions, so increasing the gasification efficiency of indole and the yields of H2 and CO2 in gaseous products. The maximum hydrogen gasification efficiency could reach up to 60.3% at OC = 0.6, which was about twice that of non-oxidant. Moreover, 29 groups of reaction rate constants, activation energies and Arrhenius constants were obtained in the established kinetic model.

Predicting gas production by supercritical water gasification of coal using machine learning

Supercritical water gasification of coal is a potential clean conversion technology. Applying machine learning (ML) methods can reduce costs and avoid the distortion of mechanism models, which has attracted increasing attention. This paper collected 208 experimental samples, including a total of 3536 data points used as a data set to investigate six independent ML models. A 5-fold cross-validation method combined with grid search was used to obtain the optimal hyperparameter combination. The overall performance ranking of the six developed models is GBR > RF > SVR > DT > ANN > ABR. The features were analyzed using the interpretable model with SHAP values, which showed that the contribution of operating conditions to the gas yield reached 88.55 %, and coal properties to gas yield was only 11.45 %. The top three models with the best prediction performance of each gas were weighted and combined to establish a hybrid model. The performance of the hybrid model on the test set is improved compared with the original GBR model. The carbon gasification efficiency of 17 supplementary experimental samples outside the dataset was predicted using the hybrid model. The MRE of 17.92 % and the R2 of 0.920 were obtained, showing a solid generalization ability.

Influence of reaction parameters on the fate of nitrogen during the supercritical water gasification of dewatered sewage sludge

This study investigated the effects of different parameters on the fate of nitrogen (N) in products after supercritical water gasification (SCWG) of dewatered sewage sludge (DSS). N distribution and morphology were most affected by temperature, followed by reaction time and heating rate, while reaction pressure had little effect on them. In terms of specific performance, higher temperature, longer reaction time, and slower heating rate were beneficial to the increase of NH4+-N content in the liquid phase. Compared with raw sludge, after SCWG, the solid phase contained more inorganic-N and less protein-N, a certain proportion of quaternary-N and nitrile-N. The proportion of N-containing compounds in the biocrude phase was between 0.26%–20.34%, suggesting the importance of more research on N in the biocrude phase. The recovery rate of N in all samples was between 64.34%–93.82%. The major proportion of N (42.27%–60.91%) was transformed into the liquid phase, while the remaining entered the solid phase (10.54%–21.45%) and the biocrude phase (6.18%–15.78%). These findings are helpful to better understand the principle of N distribution in products of DSS after SCWG and provide some new ideas for reducing N-containing by-product formation in the future.

Regulation mechanism of coal gasification in supercritical water for hydrogen production: A ReaxFF-MD simulation

The regulation study on coal gasification process in supercritical water (SCW) can promote the hydrogen production and upgrading of coal utilization. ReaxFF molecular dynamics simulation integration with representative coal model was first introduced to investigate the regulation mechanism of liquid organics on coal gasification in SCW. Hongliulin coal model was constructed and verified its rationality and accuracy. Among the liquid organics, phenols exhibit a positive effect with the H2 number increasing above 34%. The regulation mechanism is dug deeper into from the perspective of the intermolecular interaction and reactive sites. EvdWaals is the main driving force and a maximum of reaction capability of coal molecules reaches 11.01 kJ/mol. The key reaction process in which the hydrogen is greatly improved is the degradation of heavy components under the regulatory effect of phenol. Moreover, the reactive sites of aromatic structure also change from the side chain to conjugated rings. Degradation mechanism of heavy components in the SCWG of coal is summarized. The experimental results verify that H2 yield is increased by 59% and the solid mass is reduced to 0.72 mg with phenol. This conclusion demonstrated the feasibility of the ReaxFF MD simulation method to guide the clean utilization and industrial application of coal gasification in SCW.

Thermodynamic, environmental analysis and comprehensive evaluation of supercritical water gasification of biomass fermentation residue

In this paper, a residue from a simultaneous saccharification and fermentation (SSF) process using corn stover as the feedstock was taken as the material to study the thermodynamic and environmental performance of supercritical water gasification for hydrogen production . The supercritical water gasification system of the SSF residue (SSF-SCWG) was simulated by Aspen Plus. Taking temperature (600 °C, 650 °C, 700 °C) and residue concentration (5 wt%, 10 wt%, 15 wt%) as independent variables, the gasification yield and thermodynamic performance of the system under different working conditions were studied, and two methods to improve energy efficiency were proposed, including recycling waste heat by organic Rankine cycle and reheating cycle. The results show that increasing the temperature and decreasing the residue concentration are beneficial to increasing the hydrogen production rate. Among all the working conditions, the system has the highest energy efficiency of 36.8% and exergy efficiency of 39.5%, when the reaction temperature is 700 °C and the mass concentration is 15 wt%. On this basis, the energy efficiency was further improved by 5.68% by integrating the organic Rankine cycle and 9.2% by integrating the reheat cycle . The life cycle assessment of the system was carried out by SimaPro . The scope of life cycle study is the operation stage of the SSF-SCWG system and the functional unit is 1 ton of SSF residue. The environmental burden of the system was quantified with the impact assessment methods of ReCipe Endpoint and ReCipe Midpoint . The life cycle results show that the system is environmentally friendly, with global warming potential of −216.6 kg CO 2 -eq/functional unit and acidification potential of −8.9 kg SO 2 -eq/functional unit. Finally, considering thermodynamic and environmental indexes comprehensively, entropy weight method and technique for order preference by similarity to ideal solution model were adopted to make multi-attribute decision for all working conditions, and the optimal conditions for the SSF-SCWG system were obtained. • A novel system of supercritical water gasification of a fermentation residue is designed. • The organic Rankine cycle and reheat cycle are used to increase energy efficiency. • The life cycle environmental burden of the system is quantified. • A multi-attribute decision making method is used for the optimal operation conditions.

Machine learning aided multi-objective optimization and multi-criteria decision making: Framework and two applications in chemical engineering

To accelerate data-driven studies for various optimization applications in chemical engineering, a comprehensive machine learning aided multi-objective optimization and multi-criteria decision making (abbreviated as ML aided MOO-MCDM) framework is proposed in the present paper. The framework comprises a total of seven steps; firstly, study the application and its input-output datasets to identify objectives, constraints and required ML models; secondly, select ML model(s) for some or all objectives and constraints; thirdly, train the chosen ML model(s), including finding optimal hyperparameter values in each of them using an advanced/global optimization algorithm; fourthly, formulate the MOO problem for the application; fifthly, select a MOO method and develop/test the program; sixthly, solve the formulated MOO problem with the developed/tested MOO program many times and review the Pareto-optimal solutions obtained; lastly, perform MCDM using several methods and choose one Pareto-optimal solution for implementation. The proposed ML aided MOO-MCDM framework is useful for process design and operation of chemical and related processes. It is shown to be beneficial for the optimization of two complex chemical processes, which are supercritical water gasification process aiming for H2-rich syngas with lower greenhouse gas emissions, and combustion process in a power plant targeting for higher energy output and lower pollution of the environment.

Development of a Partitioning Kinetic Model of Biomass Gasification in Supercritical Water with a Fluidized Bed Reactor

Biomass supercritical water gasification technology is considered an effective way to realize the utilization of biomass, and further development of this technology depends on an accurate kinetic model. In this work, the reactor was distributed into a nozzle affected zone, a dense phase zone, and a freeboard zone according to the characteristic of the physical and chemical field, and a reaction kinetic model suitable for each zone was developed, which could be used for dynamic and scale-up studying of the reactor. First, three groups of gasification experiments (feedstock and biochar with low and high concentrations) were conducted to obtain the gas yield for calculation of the kinetic parameter, and obtain the evolution of the specific area of biochar for the calculation of the porous structure constant ψ. Then, detailed reaction pathways for biomass and biochar were proposed and a random pore kinetic model taking specific surface area into account was developed based on the first order of each reactant.

Supercritical water gasification of oil-containing wastewater with a homogeneous catalyst: Detailed reaction kinetic study

Supercritical water gasification (SCWG) is a promising technology for oil-containing wastewater treatment. This paper aims to establish a reaction kinetic model to provide better guidance for optimal industrial reactor design. The model is developed based on the experimental results obtained from K2CO3-catalyzed SCWG of diesel (the model compound of oil containment in wastewater) at various conditions of 500–650 °C and 15.23–64.45 s. Then the model validation by using the experimental data from other conditions. The validation results showed that the kinetic model can predict the gas concentration with an acceptable deviation. Afterward, the indicators of carbon gasification efficiency and gas yield versus residence time are predicted. The results show that the required residence time for the complete gasification is varied from 214.2 to 2150.8 s when the temperature changes from 500 to 650 °C. Moreover, the reaction rate analysis result indicates that the two reactions contributed most to the hydrogen production are the forward water-gas shift and the reverse CO methanation reaction. Additionally, the sensitivity analysis result reveals that the hydrolysis reaction of diesel has a significant influence at the initial stage, while the CO and CO2 methane reactions play a vital role at the late stage for gas production.

Hydrogen production optimization from sewage sludge supercritical gasification process using machine learning methods integrated with genetic algorithm

Hydrogen production from the supercritical water gasification (SCWG) of sewage sludge (SS) is a sustainable and efficient process. However, the challenging and intricate task for the experimental technique is to find out the correlation between proximate, ultimate analysis and gasification conditions with H2 production. This process is complicated, expensive and requires many experimental techniques. To accurately predict and analyze the effect of input parameters on SCWG of SS process economically, an efficient model must be developed. The novelty of this study includes the consideration of four different machine learning (ML) methods integrated with Genetic Algorithm for the prediction, analysis, and evaluation of Hydrogen yield from the supercritical water gasification of sewage sludge. The ML methods included Support Vector Machine, Ensembled Tree, Gaussian Process Regression, and Artificial Neural Network. The results suggests that GPR is favored for predicting Hydrogen yield (Coefficient of determination (R2) = 0.997, Root Mean Square Error (RMSE) = 0.093, and is highly recommended for dealing with complex variable-target correlation. On the other hand, the performance of Support Vector Machine (SVM) was poor with R2 = 0.761 and RMSE = 2.479. The R2 and RMSE for Ensembled Tree (ET) and Artificial Neural Network (ANN) was 0.994, 0.560 and 0.943, 1.521 respectively. The partial dependence plot shows that temperature, moisture content and pressure are among the effective parameters of SCWG. Furthermore, optimization techniques such as genetic algorithms are incorporated to optimize H2 production by tuning the ML hyperparameters. Additionally, a Graphical User Interface was developed by utilizing the optimized GPR model for ease in computing H2 yield.The optimum ML method integrated with GA will be beneficial for researcher to predict the H2 yield for the experimental work.

Characteristic of food waste gasification in supercritical water for hydrogen production

In this work, an experimental study was done in an autoclave reactor to evaluate the gasification characteristics of food waste in supercritical water. The effects of reaction temperature (550–700 °C), residence time (0–30 min), feedstock concentration (5 wt%-9 wt.%), catalyst type (K2CO3, Na2CO3, and Raney-Ni), and catalyst loading (Catalyst/dry feedstock 0.5–2) on gas production and liquid products were investigated. The results indicated that higher reaction temperature and longer residence time positively promoted food waste gasification. The organic compound species in liquid products decreased quickly to form gas products with the increased temperature, and the aromatic compounds were the key organic matter for the complete gasification of food waste. The addition of catalysts could significantly convert more liquid intermediates into gaseous products, and improve the gasification performance of food waste. The catalytic performance of catalysts can be ranked as K2CO3> Raney-Ni > Na2CO3. H2 yield and carbon gasification efficiency increased with the increase of K2CO3 loading, reaching the highest values of 38.29 mol kg−1 and 95.84% with the addition of 14 wt% K2CO3, respectively. This work indicated that food waste could be well treated and utilized as an energy resource to produce H2 by SCWG technology.

Renewable butadiene: A case for hybrid processing via bio- and chemo-catalysis

1,3-butadiene (butadiene) is a by-product produced during naphtha steam cracking, predominantly used in tyre manufacturing. Recently, steam crackers have converted to using more cost effective, lighter feedstocks such as shale gas, yielding less butadiene. The potential shortfall, coupled with concerns around increasing greenhouse gas emissions, provides a unique opportunity for renewable production. This study investigated the techno-economics and greenhouse gas emissions associated with renewable butadiene production routes within the context of a China located pulp mill. A hybrid bio-catalytic route, utilising black liquor, was compared against two chemo-catalytic routes using forestry residues and pulpwood. The hybrid bio-catalytic route uses a novel aerobic gas fermentation platform, employing heat integrated supercritical water gasification and aerobic gas fermentation to produce acetaldehyde, followed by chemo-catalytic upgrading (Acet-BD). The two chemo-catalytic routes catalytically upgrade biomass derived syngas; where one route (Eth-BD) passes through an ethanol intermediate, and the other (Syn-BD) utilises a series of commercialised catalytic technologies with propene as an intermediate. The hybrid bio/chemo-catalytic route, Acet-BD, was the only route profitable using the nominal techno-economic inputs, producing a Net Present Value of $2.8 million and Minimum Selling Price of $1367 tn−1. In contrast, the two chemo-catalytic routes produced Minimum Selling Prices of $1954 tn−1 (Eth-BD) and $2196 tn−1 (Syn-BD), demonstrating the competitiveness of this novel platform. Sensitivity analyses highlighted the equipment capital as the main contributor to increased Minimum Selling Price for all cases, and the Acet-BD route presented a 19% probability of achieving a positive net present value. Moreover, owed to the low process emissions and sequestration of biogenic carbon, all routes produced net negative emissions within a cradle-to-gate framework. As such, renewable butadiene production has potential as a net carbon sink for pulp mill residues conventionally destined for energy recovery.

Catalytic gasification of phenol in supercritical water with different metal cations

Phenol is a representative compound in coal industrial wastewater. Metal cations are the main harmful components in the wastewater of the electroplating and dyeing industries. Supercritical water gasification (SCWG) of phenol with metal cations could provide a promising way to degrade coal chemical wastewater and metal-containing wastewater simultaneously. In this study, SCWG of phenol with Mg2+, Mn2+, and Ni2+ was carried out to investigate the effect of metal cations on gasification results. Results showed that the gaseous product was mainly composed of H2, CO, CH4 and CO2, and CO2 was dominant. Prolonging residence time and increasing the concentration of metal cations increased the gas yield of CO2 and carbon gasification efficiency. Higher concentrations of metal cations facilitated methane-reforming reaction and water–gas shift reaction. Metal cations could enhance the ring-opening reaction of phenolic compounds and facilitate the conversion of phenol and liquid intermediates to minor molecule gaseous products. The order of catalytic effect was ranked as Ni2+>Mn2+>Mg2+. Mg2+, Mn2+ and Ni2+ were finally transformed into the corresponding metal hydroxides and metal oxides which could be recycled for reutilization. NiO could be reduced into metallic Ni, which consumed the H2 but effectively catalyzed SCWG of phenol.

Energy Hybridization with Combined Heat and Power Technologies in Supercritical Water Gasification Processes

Supercritical water gasification (SCWG) has been shown to be an effective technology to valorize a wide range of organic waste by transforming them into gases with high energy potential, such as hydrogen and methane. However, the industrial implementation of these processes is rarely extended due to the huge energy requirements during plant start-up and operation. The purpose of this study is to explore feasible ways of energy integration by hybridizing SCWG processes with combined heat and power technologies, such as exhaust gases coming from (i) internal combustion engines or (ii) gas turbines. The analysis focuses on energy consumption with the aim of optimizing the operation and design of plants. System configurations are simulated with Aspen Plus considering data from the literature for the gasification of glycerol and using typical plant capacities on an industrial scale. Results show the thermal power required in heat exchangers and the electricity generation from residual energy in hot effluents as a needed step to optimize the plant configuration and boost energy synergies with other technologies.

Catalytic supercritical water gasification of biomass waste using iron-doped alkaline earth catalysts

Catalytic supercritical water gasification of biomass waste using iron-doped alkaline earth catalysts

Biofuel production from supercritical water gasification of sustainable biomass

• Biofuel production by supercritical water gasification (SCW) is increasingly being developed. • A comprehensive overview of the SCWG technology is provided to produce biofuels in a sustainable way. • Some misleading concepts applied to SCWG processes are explained. • Several improvements to be made and examined in future research and development are proposed. A review of biofuel production from supercritical water gasification (SCWG) of sustainable biomass has been performed, mainly organic waste, following a critical thinking in this field of knowledge. Thus, sub- and super- critical water properties and hydrothermal processing are briefly commented on. Then, the feedstocks usable in SCWG are fully reviewed and a brief description of the studies on the kinetics and mechanisms of reactions is carried out. Next, thermodynamic and process simulation are discussed, aimed at producing liquid and gas biofuels. After that, a brief comment on the viability of SCWG processes to produce biofuels is provided based on techno-economic and lifecycle assessments. Finally, some remarks on where we are and where we should go are given in order to advance this technology towards its maturity. This review explains some misleading concepts applied to SCWG processes, provides a brief but comprehensive overview of the technology focused on producing biofuels in a sustainable way, allows a better understanding of the SCWG of biomass for biofuel production, and proposes a series of improvements to be made and examined in the future research.

The methods of biofuel generation by using microalgae: A review

Biofuels researchers extended to fit the growing fuel requirements and protected regional energy. Because of renewable and no impact on carbon biomass is preferred to replace fossil fuels. 2nd generation biofuels generated from micro-algae and lignocellulosic biomass (3rd generation biofuels) have become an essential raw material for the generation of renewable biofuels. Microalgae are rapidly multiplying microorganisms seen in the water, are the essential raw material for bio-fuel generation for the features like huge growth ratio, yield per area, potency in CO2 capture, and solar energy transformation. Photobioreactors on marginal lands, ponds, and seawater is the best growing area for algae, industries also cultivate the algae. Many methods have been used for the generation of biofuels from algae includes anaerobic digestion, hydrothermal liquefication, supercritical water gasification, and pyrolysis. This review article in brief explains the process of biofuel generation by using microalgae.

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.

Hybridization of supercritical water oxidation and gasification processes at pilot plant scale

The combination of SCWO with SCWG forms a unique system that has been tested in the present study at pilot plant scale using a single reactor with a flowrate of up to 150 kg/h. SCWO of a highly concentrated ethanol solution takes place in the first section of the tubular reactor, rising temperature. Then, an isopropanol solution is gasified into the second section to prevent any further increments of the temperature and producing hydrogen. The effect of different flowrates, the addition of different oxidant amounts and the preheating temperature of the gasification feed have been evaluated obtaining a gas effluent containing 10.7% H2 and 17.2% CH4 in the best conditions that have could tested, although better results would be obtained if processes could be carried out at more extreme conditions based on literature evidences.

Coupling of hydrothermal pretreatment and supercritical water gasification of sewage sludge for hydrogen production

In this study, hydrothermal pretreatment and supercritical water gasification were coupled to form a combined process for the treatment of dewatered sludge for hydrogen production. First, the effects of varying hydrothermal pretreatment conditions on the transformation of organic matter in sludge were studied. Results showed that about 31% of the carbon in sludge was transferred into liquid products at 250 °C for 60 min, which were considered to be the optimal pretreatment conditions considering both the hydrothermal pretreatment effects and the energy consumption requirements. The organic matter components were determined, showing that 87% of the carbohydrate components in sludge were transformed during the process of hydrothermal pretreatment, with 49% of crude proteins and 62% of humus remaining in the solid phase products. During the subsequent process of supercritical water gasification, AlCl3, KOH, K2CO3 and CaO were selected as catalysts. Compared with directly catalyzed supercritical water gasification of sludge, the integrated process was found to improve H2 selectivity, H2 yield and energy recovery. Moreover, the use of AlCl3 as a catalyst showed the highest H2 yield and energy recovery. The H2 yield and the energy recovery increased by 45.1% and 13.2%, respectively.