Supercritical Water Gasification Of Coal Research Articles

Reactive particle-laden flow in industrial-scale supercritical water gasification reactor: Modeling and study on flow patterns

Coal gasification in supercritical water (SCW) is a highly complicated reactive particle-laden flow process, which couples multiphase flow hydrodynamics, conjugate heat transfers, and complex chemical reactions. A quantitative understanding is the key issue for reactor design and optimization. In this study, the Eulerian-Eulerian model is used to describe the particles-SCW flow in a novel industrial-scale thermally integrated supercritical water gasification (TISCWG) reactor with conjugate heat transfer boundary conditions and porous media model. The sophisticated chemical reactions are using a species transport model with a seven-step lumped kinetics. Four zones of the TISCWG reactor and four typical flow patterns in the gasification chamber are firstly revealed as 1) the bottom accumulation region with the nonhomogeneous solids distribution and back-mixing of particles, 2) the upper developed fluidization region with the SCW and coal particles moving in a plug-flow regime in the center and a downward annular gravity-driven flow near the wall, 3) the jet-affected region strongly affected by the vortexes near the nozzle and presented as a trans-critical jet regime and 4) the dilute region where dilute solids slow down and fall back into the gasification zone. Flow in the gasification chamber is strongly affected by the heat transfer with the preheat zone, as reflected by an annular gravity-driven flow film flowing downwards near the gasification wall. A funnel-shaped structure promotes a more uniform distribution of solids in the accumulation region rather than a cylindrical structure. The effect of flow patterns on coal gasification processes is also studied. Volatile pyrohydrolysis reactions depend on the time-varying solids distribution and the gaseous products at the system outlet with a typical H2 mole fraction of 61.42% and CO2 of 34.98%.

Intrapore water–gas shift reaction inhibits coal gasification in supercritical water

Water-gas shift reaction (WGSR) is key to the supercritical water gasification (SCWG) of coal in hydrogen production, but it has not been considered in studies inside pores where coal conversion occurs. Here, to investigate the impact of intrapore WGSR on the SCWG of single irregular-shaped coal particle, a three-dimensional multi-species reaction–diffusion model is proposed and validated through experiments. We observe that the intrapore WGSR inhibits the SCWG of coal by slowing down the fluid diffusion and retarding the fixed-carbon steam reforming heterogeneous reaction (FSRR). Increasing gasification temperature enhances the inhibition by WGSR while increasing mole fraction of H2O weakens the inhibition. The area and volume of pores have little effect on the inhibition because increasing area enhances the FSRR but retards the diffusion while increasing volume facilitates the WGSR but promotes the diffusion. An analytical model is thereby proposed to quantify the inhibition, and to characterize the prolonged gasification time.

Catalytic gasification of anthracene in supercritical water with Ni/TiO2 catalyst: Products distribution and kinetics

Supercritical water gasification (SCWG) is a clean and efficient utilization way of coal to produce hydrogen-rich gas. As the consumption of PAHs is the rate-determining step to the rapid gasification of coal in SCW, anthracene was used as the simplest model of PAHs to investigate the ring-opening and gasification mechanism of PAH compounds in supercritical water (SCW). The SCWG experiments of 7.5 wt% anthracene were carried out in SCW under various temperatures (400–550 °C) and residence times (5–30 min). Ni/TiO2 catalysts were added to achieve the fast and complete gasification of anthracene, and a carbon gasification efficiency of 95.73% was achieved at 500 °C, which was 100 times higher than 0.95% of the non-catalytic condition. A kinetic model including a detailed reaction pathway for anthracene was successfully proposed to reveal the catalytic mechanism of anthracene gasification in SCW. The results indicated that the addition of Ni/TiO2 effectively reduced the activation energy of anthracene hydrogenation to generate CH4, and this reaction was most critical for the generation of CH4. This work guided the determination of key parameters for industrial-scale reactors to achieve an optimal design, providing a promising method for in-situ elimination of char during SCWG of coal.

Study of hydrogen production from coal gasification in supercritical water

The coal supercritical water gasification technology is a new technology that directly converts coal into hydrogen, with advantages such as cleanliness, high efficiency, and low cost. With the global trend towards hydrogen energy development, the coal supercritical water gasification technology has broad application prospects. Research on this technology is significant for providing a new path for China's clean energy development, promoting sustainable development in the energy field, and contributing to global emission reduction and climate change response. This article analyzes the current development status of coal gasification technology, summarizes the related categories in four aspects: raw material selection, catalysts, reactors, and heating methods, and analyzes the unique advantages of each method. Based on this, a relatively efficient hydrogen production scheme is briefly selected. In addition, the future development trend of this technology is discussed. This article may provide a reference for clean energy development and the efficient utilization of coal resources.

Experimental investigation on the supercritical water gasification of coal by integrating the hydrogen oxidation process into a supercritical water fluidized bed

The autothermal gasification process is crucial for the industrial application of supercritical water gasification (SCWG) in a supercritical water (SCW) fluidized bed. However, researchers often consider providing external heat to investigate the gasification process and seldom achieve autothermal gasification. Herein, the hydrogen oxidation process was integrated into an SCW fluidized bed to provide heat for the gasification process and to investigate gasification results under various experimental conditions. Using this method, a carbon gasification efficiency of 98.6% and a H2 yield of 68.03 mol/kg were obtained at an experimental temperature of 690 °C. After the gasification products were partially oxidized in the oxidation zone, a H2 yield of 13.6 mol/kg and a total carbon content of <10 ppm in the liquid products were obtained. Compared with the uniform electrical heating method, the heating method based on the hydrogen oxidation process in SCW has obvious advantages, can improve the coal slurry concentration and mass ratio of coal to SCW in the reactor, and maintain a milder temperature under near-complete gasification conditions. This work provides a basis for heat supply in the industrial application of SCWG technology.

New insight into Allam cycle combined with coal gasification in supercritical water

In this work, we proposed and investigated the power generation system where coal gasification in supercritical water is combined with Allam cycle (CAllam cycle). This CAllam cycle features zero pollutant emission, easy CO2 collection and no extra heating. Our simulations demonstrated the differences between the natural gas Allam cycle and our proposed CAllam cycle, and also elucidated the mechanisms underlying these differences. Results show that recycling CO2 not only reduces the temperature of gas turbine blades but also further improves the system efficiency. Results also show that the gas turbine outlet pressure corresponding to the maximum efficiency is smaller and away from the gas-liquid line. Furthermore, we graphically identified the combination of CO2 recycling ratio and gas turbine outlet pressure, that the CAllam cycle allowed. Built on these findings, our proposed CAllam cycle was optimized graphically. Taking 40 wt% Hongliulin coal as an example, the maximum system efficiency is 53.19% and the corresponding CO2 recycling ratio and gas turbine outlet pressure are 1.158 and 2.43 MPa, respectively.

Numerical investigation on the heat transfer of particle arrays in supercritical water

The heat transfer characteristics of particles in supercritical water (SCW) are the research foundation for the development of the technology of coal gasification in SCW. The study encounters two challenging problems: the complexity of particle spatial distribution and the special variation of SCW properties. Therefore, this work focuses on contrasting the particle interaction characteristics between in SCW and conventional fluid, instead of the direct study of particle clusters in SCW. Research is conducted by the particle-resolved numerical simulation method. In the simulations, the SCW inflow is 973 K and 25 MPa which is a usual operating condition, and all particle surface temperatures are constant and uniform with the values of 300 K, 672 K and 973 K which are respectively below, at and beyond the pseudocritical temperature zone. The side-by-side and streamwise distributed arrays, which are simple but representative, are adopted as the research objects firstly. The effects of particle number and inter-particle distance on heat transfer are studied. Results indicate that the effects of particle interaction on heat transfer under all heat transfer states are similar in the side-by-side array; while only the effects under the pseudocritical and supercritical states are similar in the streamwise array. Then the structured packed array is adopted, and the effects of void fraction (ε) under different heat transfer states are studied. Moreover, a new correlation dependent on ε and Re is built for predicting the ratio of the Nu of a particle in the structured packed array and that of a single particle.

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.

Experimental study on optimizing reaction path for promoting the gasification of coal in supercritical water

Coal gasification in supercritical water (SCW) has great potential in industrial application for converting coal into hydrogen-rich gas in a clean and efficient manner. However, when the reaction conditions are improperly coupled with reaction path, undesirable side reactions would take place, which is prone to reduce the efficiency of converting coal into hydrogen-rich gas. It is revealed that the condensation of aromatic structures and the formation of aromatic ether (Car-O) groups are the restrained reactions according to the reaction path obtained by studying the variation of gas–liquid-solid products during supercritical water gasification (SCWG) of coal. Methanol, Fe2O3 and Ni(CH3COO)2·4H2O are added to inhibit the restrained reactions. It is found that these all facilitate the cracking and gasification of aromatic structures. Fe2O3 and Ni(CH3COO)2·4H2O except methanol, reduce the formation of Car-O groups effectively. The optimal concentration of these corresponding to the greatest promotion is 7.5 wt% (methanol), 2.5 wt% (Fe2O3) and 2.5 wt% (Ni(CH3COO)2·4H2O), respectively. Ni(CH3COO)2·4H2O (2.5 wt%) has the highest promotion on the production of H2 and CO, as well as the cracking and gasification of aromatic structures. The corresponding maximum yields of H2 and CO increase by 0.29 and 1.85 times, respectively. Fe2O3 (2.5 wt%) has the greatest promotion on the decomposition of Car-O groups, and the relative content of Car-O groups decreases by 80.86%. This research provides the theoretical basis for regulating the ordered conversion of coal into hydrogen-rich gas during SCWG of coal.

Experimental investigation on kinetic model with reaction heat for coal gasification in supercritical water

An accurate kinetic model, including the reaction pathway and reaction heat, is crucial to reactor scale-up and process optimization for the supercritical water gasification (SCWG) of coal. However, the reported kinetic models seldom satisfy the mass and energy conservation law due to the proposed inappropriate reaction pathway and difficult determination of reaction heat in supercritical water (SCW). This work focuses on determining the reaction heat and establishing a kinetic model based on mass and energy conservation law. A reaction pathway combined with reaction heat determination by a lumped method was proposed to establish a kinetic model and to quantify the reaction heat of each reaction in the coal gasification process. The steam reforming reaction I and II in the model have the highest activation energy, 59.629 ± 10.281 kJ/mol and 63.249 ± 3.937 kJ/mol, respectively. Reaction heats of the reaction between volatile and SCW and the steam reforming reactions exceed + 600 kJ/mol. Moreover, due to the combined influence of endothermic and exothermic reactions, the entire gasification process shift from endothermic to exothermic as the gasification proceeds. This work provides a more accurate mathematical model for the SCW reactor.

Energy, exergy and economic (3E) evaluations of a novel power generation system combining supercritical water gasification of coal with chemical heat recovery

Supercritical water gasification (SCWG) is a promising clean coal utilization technology. A proposed power system with integrated coal SCWG, supercritical turbine, gas turbine and chemical heat recovery is proposed in this paper. And the power system with integrated combined cycle and parallel heat recovery is re-proposed for structure simplification and efficiency enhancement. With a gasification condition of 660℃/250 bar, and a coal-water-slurry concentration of 11.3 %, the efficiencies of the proposed system and the system with parallel heat recovery are 49.06 % and 47.93 %, respectively. The power efficiency of the proposed system is greatly improved compared with the traditional systems. This is mainly due to the reasonable use of the pressure energy of the gasification product through the supercritical turbine, the release and use of high level chemical energy stored in the syngas in the gas turbine, and chemical heat recovery of the sensible heat of the flue gas from the gas turbine. Exergy and economic analyses are also performed. The levelized cost of electricity (LCOE) of the proposed power system is 0.2963 ¥/kWh. The power system proposed herein can be considered as one of the suitable options to achieve the carbon peak and carbon neutrality goals.

Experimental investigation on organic functional groups evolution in hydrogen production process by coal gasification in supercritical water

Supercritical water gasification (SCWG) of coal has great application prospect for converting coal into hydrogen-rich gas efficiently and cleanly. However, the previous study on the reaction mechanism for SCWG of coal is relatively macroscopic rather than reflects the reaction essence deeply. The evolution of organic functional groups in Zhundong lignite (ZD), Hongliulin bitumite (HLL) and Ningxia anthracite (NX) during SCWG, as well as the correlation with gaseous products were analyzed quantitatively in this paper. It was found that the lower rank coal contained more free radicals and produced more H2 with SCW. H2 yield of the three types of coal exceeded 2 times the hydrogen content in coal at 800 °C. The organic functional groups evolve in 2–4 stages during SCWG process. The decomposition and gasification of organic functional groups mainly took place in low or medium temperature range. About 95% of C=O groups and 90% of aromatic C=C groups cracked and were gasified. Aromatic ether (Car-O) groups were formed in high temperature range. The reasonable functional relationship between the parameters of gaseous products and organic functional groups was established, providing a new approach to predict organic functional groups through gaseous products. This research may lay the foundation for further optimization design of reactor.

Insight into the internal relevance of three-phase products in supercritical water gasification of coal

• Internal relevance of three-phase products of coal gasification was investigated. • Insight into the formation mechanism of products in the micro dimension. • Solid-liquid reaction model in coal particle surface was established. • Control steps for decreasing the irreversibility of reaction process was summarized. Supercritical water gasification (SCWG) is a promising technology for coal utilization and has attracted more attentions. This paper is aimed to obtain coal gasification behavior and the internal relevance of products from the perspective of micro level. Evolution characteristics of three-phase products were employed in high heating rate batch reactor. As a result, removal of functional groups in solid particles led to the decrease of the gasification reactivity of char (R coal > R 20s > R 60s > R 100s > R 1200s ). The hydrogen bonds formed by water forced the coal structure decomposition, in which the CO and CO 2 gas were generated from the deetherization and decarboxylation reaction. H 2 and CH 4 primarily came from the collision of free radicals and polycondensation reaction. Liquid intermediate components were detected by high-performance liquid chromatography quantitatively. The content of phenol and naphthalene was up to 280.8 ppm and 3647.4 ppm, and the mutual conversion mechanism of organic matters and polycyclic aromatic hydrocarbons (PAHs) was represented. Effectively controlling the ring growth of PAHs and the polycondensation of solid fragments is the primary goal to inhibit char formation, which is of great significance to decrease the irreversibility of chemical reaction process and realize the large-scale industrial production.

Interface-resolved direct numerical simulations of interphase momentum, heat, and mass transfer in supercritical water gasification of coal

Interactions between a reacting particle and the surrounding fluid are complex due to the interplay between flow dynamics, heat and mass transfer, and chemical reactions. In the present work, particle–fluid transport processes in supercritical water gasification of coal are studied using high-fidelity interface-resolved direct numerical simulations. The impact of different factors on the particle–fluid interactions are evaluated by performing simulations of the flow around two-dimensional particles considering different numerical configurations. The outgoing Stefan flow from the particle surface is found to cause expanded boundary layers for velocity, heat, and species. The temperature-induced changes in transport properties around a heated particle lead to a higher drag force and decreased heat/mass transport; those differences are further enlarged when taking into account the volumetric expansion of the fluid. Transport limitation for coal gasification in a realistic configuration is finally investigated. Temperature-induced fluid dilatation is then the major factor affecting drag force and heat transfer around the reacting particle, and mass transport is significantly impacted by species production or consumption in the boundary layer. Reaction heat release and variations in fluid composition within the thermal boundary layer lead to a slight enhancement of heat transfer. This work reveals and quantifies the main mechanisms affecting the exchanges between a reacting coal particle and surrounding supercritical water regarding both thermal and chemical aspects. It also provides high-fidelity data to later fit the reduced models needed for simulations of large-scale supercritical water gasification installations.

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.

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.

A perspective on multi-component resistance analogy analysis for process intensification: Taking supercritical water gasification of coal as an example

Resistance analysis widely focuses on many aspects of scientific research such as flow, heat transfer, and electrical conductivity. However, resistance analysis is often limited to a specific field, and engineering applications (such as supercritical water gasification process) often require the coupling of multiple resistances. Supercritical water gasification (SCWG) provides a clean and low-carbon way of coal utilization for hydrogen production. Realizing the complete gasification of coal under mild temperature conditions is the prerequisite for the industrialization of this technology. Therefore, measures must be taken to reduce the process resistance and accomplish the process intensification. In this paper, a novel idea of multi-component resistance analogy analysis was applied to solve this problem from the perspective of intensification of interface reaction, boundary layer, and vertex diffusion, respectively. Based on the current research progress and outstanding issues, the further research will focus on the application of resistance analogy analysis to general multiphase flow and reactor optimization design and scale-up.

Deep Integration for Efficiency Enhancement of the Power Generation System Combined Supercritical Water Gasification of Coal with Chemical Heat Recovery

Deep Integration for Efficiency Enhancement of the Power Generation System Combined Supercritical Water Gasification of Coal with Chemical Heat Recovery

A New Cleaner Power Generation System Based on Self-Sustaining Supercritical Water Gasification of Coal

A New Cleaner Power Generation System Based on Self-Sustaining Supercritical Water Gasification of Coal

Optimal design and thermodynamic analysis on the hydrogen oxidation reactor in a combined hydrogen production and power generation system based on coal gasification in supercritical water

The novel power generation system based on coal gasification in supercritical water (SCW) holds great promise to meet the increasing energy demands in China, with less emissions in comparison to the traditional coal-fired power generation. The stable and complete oxidation of hydrogen occurring in the supercritical water oxidation (SCWO) reactor is paramount to the safety and overall energy efficiency of the system. In order to better understand and improve the performance of this reactor, optimal design and thermodynamic analysis were conducted using Aspen Plus. The outlet hydrogen concentration and the internal temperature of the reactor were taken as the two key indicators of the optimal design. The optimized results for the reactor were found to be the reactor diameter of 90 mm, axial length of 10.75 m, oxygen inlet number of 3 with the oxygen ratio of 11.0%/39.9%/49.1%, under the given inlet working fluid components and flow rates. The three-stage reactor design was confirmed to have good robustness when different types of coals are adopted for generating syngas. In addition, by analyzing the enthalpy and exergy flow distributions in the reactor, it was revealed that the total exergy destruction for the whole three-stage reactor is only about 6.29%, and that the exergy destruction is consistent with the oxidation process. These findings could be used as guidelines for the design and implementation of the SCWO reactor towards higher safety margin and energy efficiency for this new coal-boiled power generation technique.