Investigation of the conversion mechanism for hydrogen production by coal gasification in supercritical water

The technology of supercritical water gasification (SCWG) of coal has a great prospect because it converts coal into hydrogen-rich gas products efficiently and cleanly. However, there are bottlenecks affecting the complete gasification of coal in supercritical water (SCW) without catalyst under moderate conditions. This work is to explore the restricted factor for complete gasification of coal in SCW by investigating the conversion mechanism. The conversion mechanism of SCWG of coal with and without K₂CO₃ is proposed. Polycyclic aromatic hydrocarbons (PAHs) with graphite phase structures are formed by the condensation of aromatic structures at 550–750 °C. It is the restricted factor due to its characteristic of difficulty to be gasified. There is no condensation of aromatic structures in the process of SCWG of coal with K₂CO₃, which effectively inhibited the formation of PAHs with graphite phase structures. K₂CO₃ dramatically promoted the SCWG of coal, leading to carbon gasification efficiency (CE) reaching 98.43%.     

Catalytic supercritical water gasification mechanism of coal

Catalytic supercritical water gasification (SCWG) for H₂ production is a hopeful way of coal conversion to replace the traditional coal utilization mode. At present, the detailed catalytic mechanism in the process remains unknown. Herein, a comprehensive catalytic SCWG mechanism of coal is proposed by establishing a novel catalytic kinetic model. It shows that catalysts (K₂CO₃) break up the coal matrix by a cyclic redox reaction to produce plenty of mesopores, accelerating steam reforming of fixed carbon and coal pyrolysis. Water-gas shift reaction is facilitated by K₂CO₃ via formation of formate, which then promotes steam reforming of CH₄ at high temperature (≥700 °C) due to the decreasing CO. The proposed mechanism provides important insights in catalytic SCWG process of coal.     

Hydrogen production by catalytic gasification of coal in supercritical water with alkaline catalysts: Explore the way to complete gasification of coal

Supercritical water gasification (SCWG) of coal is a promising technology for clean coal utilization. In this paper, hydrogen production by catalytic gasification of coal in supercritical water (SCW) was carried out in a micro batch reactor with various alkaline catalysts: Na₂CO₃, K₂CO₃, Ca(OH)₂, NaOH and KOH. H₂ yield in relation to the alkaline catalyst was in the following order: K₂CO₃ ≈ KOH ≈ NaOH > Na₂CO₃ > Ca(OH)₂. Then, hydrogen production by catalytic gasification of coal with K₂CO₃ was systematically investigated in supercritical water. The influences of the main operating parameters including feed concentration, catalyst loading and reaction temperature on the gasification characteristics of coal were investigated. The experimental results showed that carbon gasification efficiency (CE, mass of carbon in gaseous product/mass of carbon in coal × 100%) and H₂ yield increased with increasing catalyst loading, increasing temperature, and decreasing coal concentration. In particular, coal was completely gasified at 700 °C when the weight ratio of K₂CO₃ to coal was 1, and it was encouraging that raw coal was converted into white residual. At last, a reaction mechanism based on oxygen transfer and intermediate hybrid mechanism was proposed to understand coal gasification in supercritical water.     

Hydrogen production by non-catalytic partial oxidation of coal in supercritical water: The study on reaction kinetics

Supercritical water gasification (SCWG) has attracted great attention for efficient and clean coal conversion recently. A novel kinetic model of non-catalytic partial oxidation of coal in supercritical water (SCW) that describes formation and consumption of gas products (H₂, CO, CH₄ and CO₂) is reported in this paper. The model comprises 7 reactions, and the reaction rate constants are obtained by fitting the experimental data. Activation energy analysis indicates that steam reforming of fixed carbon (FC) is the rate-determining step for the complete gasification of coal. Once CH₄ is produced by pyrolysis of coal, steam reforming of CH₄ will be the rate-determining step for directional hydrogen production.     

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 (C_ar-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.     

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. E_vdWaals 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.     

Gasification of coking wastewater in supercritical water adding alkali catalyst

In this study, the influence of alkali precipitation on the gasification of coking wastewater with KOH as catalyst at 540 °C, 25 MPa was investigated. Adding the KOH increased H₂ fraction and the gas yield. The alkali is accumulated in the reactor, and the catalytic effect was further improved with reaction time prolonging. The precipitated alkali in the reactor still showed high catalytic effect on the subsequent gasification of coking wastewater without further adding the alkali. The catalytic activity was slightly reduced owning to the transformation of KOH to K₂CO₃ and KHCO₃. A novel 4-lump kinetic model for coking wastewater in SCWG including feedstock, CH₄, CO, and CO₂ lumps is proposed. The kinetic parameters are estimated for each involved reaction.     

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 by non-catalytic partial oxidation of coal in supercritical water: Explore the way to complete gasification of lignite and bituminous coal

Supercritical water gasification (SCWG) of coal is a promising technology for clean coal utilization. In this paper, hydrogen production by non-catalytic partial oxidation of coal was systematically investigated in supercritical water (SCW) with quartz batch reactors for the first time. The influences of the main operating parameters including residence time, temperature, oxidant equivalent ratio (ER) and feed concentration on the gasification characteristics of coal were investigated. The experimental results showed that H₂ yield and carbon gasification efficiency (CE) increased with increasing temperature and decreasing feed concentration. CE increased with increasing ER, and H₂ yield peaked when ER equaled 0.1. CE increased quickly within 1 min and then tended to be stable between 2 and 3 min. In particular, complete gasification of lignite was obtained at 950 °C when ER equaled 0.1, as for bituminous coal, at a higher temperature of 980 °C when ER equaled 0.2.     

Kinetics study for sodium transformation in supercritical water gasification of Zhundong coal

Zhundong coal (ZDC) has attracted much attention due to its high alkali metal content which can lead to a series of problems such as furnace slagging and ash fouling. Supercritical water gasification (SCWG) become a better choice for ZDC coal utilization because of its unique chemical and physical properties. The transformation mechanism of alkali metals during SCWG process was different from conventional ways of coal utilization. Systematic research about it could hardly be found. In this study, ZDC was used to explore sodium transformation mechanism and kinetics during supercritical water gasification under typical conditions. We got four kinds of sodium including the water-soluble fraction (L1), the carboxylic matrix-associated fraction (L2), the macromolecular organic group-associated fraction (L3), and the inorganic silicate mineral fraction (L4) through sequential extraction method after SCWG. A reaction pathway of sodium transformation in supercritical water gasification was proposed. A quantitative kinetic model for describing sodium transformation mechanism was developed. Finally, it was found that, L1 played an important role in catalytic process and mineral in coal weaken the catalytic process by combining with L1. L2 and L3 served as the two important intermediate products in the coal gasification, which explained the catalytic mechanism of sodium. L3 showed better reactivity. Sodium finally tended to deposit in the form of NaSiAlO₄ (L4) which was stable and environmentally friendly. All of these could provide basis for high-efficiency utilization of ZDC and the design of a reactor.     

Supercritical water gasification of fuel gas production from waste lignin: The effect mechanism of different oxidized iron-based catalysts

Iron and iron oxides have been employed to catalyze supercritical water gasification (SCWG) of lignin, a typical component of pulp and paper mill wastewater. To investigate the effects of different oxidation sates of Fe-based catalysts during SCWG process, all simulations were carried out through ReaxFF molecular dynamics method. During the catalytic SCWG process, the degradation rate of guaiacyl dimer lignin (GDL) molecule was inversely proportional to the valence state of iron, the higher oxidation state of Fe in iron-based catalyst was, the lower the catalytic degradation ability would be, and then GDL molecule underwent a series of reactions, accompanying with the generation of small molecules, among most of them were fuel gas products. In terms of gas products, Fe catalyst had a unique advantage in catalytic hydrogen production. Moreover, it is found that iron with low oxidation state was beneficial to the formation of CO, while iron with high oxidation state was CO₂. Our simulation results further revealed the formation mechanisms of CO, CO₂ and CH₄. Migration of lattice oxygen in iron oxides was also visualized through figure, and spent catalyst showed different sources in the final, demonstrating that SCW participates in the entire reaction providing not only H but also O free radicals.     

Experimental study on gasification and kinetic characteristics of inferior coal with high ash content under CO2 atmosphere

The effect of heating rate and particle size on gasification of one inferior coal was experimentally studied. A homogeneous reaction model was used to calculate kinetic parameters with the Freeman–Carroll method. The results show that gasification reactivity can be enhanced by reduction of coal particle size and increase of heating rate. Additionally, coal ash plays a catalytic role to a certain extent on gasification. It was also found that the reaction rate can be enhanced significantly, when increasing the ash-coal weight ratio from 1:2 to 2:1. The gasification order under CO₂ atmosphere is close to 1. There is a kinetic compensation effect between activation energy and frequency factor for the gasification of the inferior coal investigated.     

Investigation of Indonesian low rank coals gasification in a fixed bed reactor with K2CO3 catalyst loading

Catalytic steam gasification is considered one of promising technologies for converting solid carbonaceous feedstocks into hydrogen-rich syngas, which is an important source of hydrogen for various industrial sectors. The K₂CO₃-catalyzed steam gasification of low rank coals (LRCs) was conducted in a fixed bed reactor for elucidating the effects of gasifying temperature and catalyst loading amount on hydrogen yield. Hydrogen-rich syngas can be obtained at gasifying temperature of 800 °C and loading amount of 10 wt% K₂CO₃. The loading amount of 10 wt% K₂CO₃ was the saturation point and provided a good gasification reactivity in catalytic steam gasification of three LRCs. The experimental data of these three LRCs were well described by the random pore model (RPM). The RPM fitted the experimental data at 800 °C better than the experimental data obtained at 700 °C and 600 °C. Reactivity index (R_0.5), activation energy (Ea) and reaction rate constant (k) were also used to predict the characteristics of the K₂CO₃-catalyzed steam gasification process. Catalytic steam gasification utilizing the mixture of three LRCs as a feedstock was also investigated and displayed X_C of 86.22% and 0.95 mol mol^?1-C, indicating a good feasibility and potential industrial applications.     

Gasification of horse dung in supercritical water

Horse dung naturally contains phosphorus and nitrogen which affect the environment negatively, if the nutrients are not recovered and utilized. In this paper, the influence of alkali on the gasification of horse dung at 560 °C, 25 MPa was investigated. The results show that LiOH addition increased H₂ fraction and the gas yield. The precipitated alkali in the reactor still showed high catalytic effect on the subsequent gasification of horse dung without further adding the alkali. A novel 4-lump kinetic model for horse dung in SCWG including feedstock, CH₄, CO, and CO₂ lumps is proposed.     

Sustainable hydrogen production options from food wastes

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

Molecular dynamic investigation on sulfur migration during hydrogen production by benzothiophene gasification in supercritical water

Benzothiophene (BT) is a key sulfur-containing intermediate product in the thermal conversion process of coal and heavy oil. The migration process of the sulfur element may affect the thermal utilization design of BT. In this paper, BT was used as a model compound to simulate the supercritical water gasification (SCWG) process by molecular dynamics with a reactive force field (ReaxFF) method, and the laws of hydrogen production and sulfur migration mechanisms were obtained. Increasing the molecule number of supercritical water (SCW) and increasing the reaction temperature can enhance the generation of hydrogen and promote the conversion of organic sulfur to inorganic sulfur. Water was the main source of H₂, and H₂S was the main gaseous sulfur-containing product. SCW had a certain degree of oxidation due to a large number of hydroxyl radicals, which could increase the valence of sulfur. The conversion process of BT in SCW was mainly divided into four stages, including thiophene ring-opening; sulfur separation or carbon chain broke with sulfur retention; carbon chain cleaved, and gas generation. The lumped kinetic parameters of the conversion of sulfur in BT to inorganic sulfur were calculated, and the activation energy was 369.98 kJ/mol, which was much lower than those under pyrolysis conditions. This article aims to clarify the synergistic characteristics of hydrogen production and sulfur migration in the SCWG process of BT from the molecular perspective, which is expected to provide a theoretical basis for pollutant directional removal during hydrogen production by sulfur-containing organic matters in SCW.     

Kinetic analysis of catalytic coal gasification process in fixed bed condition using Aspen Plus

Coal is one of the energy resources useful for solving the energy crisis. It has met nearly half of the rise in global energy demand over the last decade, growing even faster than total renewables. Catalytic coal gasification is useful technology in SNG (Substitute Natural Gas) and IGFC (Integrated Gasification Fuel Cell) plants that use coal. The Catalytic Coal Gasification Process developed by Exxon in 1978 was simulated with Aspen Plus in the fixed bed type reactor. The purpose of this study is to derive kinetic parameters from experimental results in literature and compare them using the catalytic coal gasification model in Aspen Plus. Carbon–Steam reaction is an important reaction in catalytic gasification reaction since steam is only an oxidant feeding in the system. Mainly, alkali metal gasification catalysts like potassium carbonate increase the rate of steam gasification. The kinetic values calculated from the experimental data are 0.30126, 0.09204, and 0.076995 (cc mol⁻¹ h⁻¹). Obtained kinetic value k_f determines k_o and E values compared with Arrhenius equation to input Aspen Plus simulation. Another major focus is on low-rank coal because upgrading low-rank coal is very useful for energy efficiency and environmental aspects. Upgrading coal means removing moisture from low-rank coal. Boiler efficiency is decreased because a lot of moisture content and CO₂ emissions are increased. Carbon dioxide and the flue gas emissions for the same energy level can be reduced by about 30%. Low-rank coal will be increased energy requirement for removing carbon dioxide. The investigation of the drying characteristics of low-rank coal is performed in our laboratory. The experimental results based on the drying characteristics are reflected in this simulation process.     

Iron ore as oxygen carrier improved with potassium for chemical looping combustion of anthracite coal

Chemical looping combustion (CLC) is an innovative combustion technology with inherent separation of CO₂ without energy penalty. When solid fuel is applied in CLC, the gasification of solid fuel is the rate-limiting process for in situ gasification of coal and reduction of oxygen carrier. The K₂CO₃-decorated iron ore after calcinations was used as oxygen carrier in CLC of anthracite coal, and potassium ferrites were formed during the calcinations process. The experiments were performed in a laboratory fluidized bed reactor with steam as a gasification medium. Effects of reaction temperature, K₂CO₃ loading in iron ore and cycle on the gas concentration, carbon conversion, gasification rate and yields of carbonaceous gases were investigated. The carbon gasification was accelerated during the fast reaction stage between 860 °C and 920 °C, and the water–gas shift reaction was significantly enhanced in a wider temperature range of 800 °C to 920 °C. With the K₂CO₃ loading in iron ore increasing from 0% to 20% at 920 °C, the carbon conversion was accelerated in the fast reaction stage, and the fast reaction stage became shorter. The yield of CO₂ reached a maximum of 94.4% and the yield of CO reached a minimum of 3.4% when use the iron ore loaded with 6% K₂CO₃. SEM analysis showed that the K₂CO₃-decorating in iron ore would cause a sintering on the particle surface of oxygen carrier, and the K₂CO₃ loading in iron ore should not be too high. Cycle experiments indicate that the K₂CO₃-decorated iron ore has a relative stable catalytic effect in the CLC process.     

Detailed kinetic analysis and modelling of the dry gasification reaction of olive kernel and lignite coal chars

The dry gasification process of solid fuels is a promising pathway to mitigate and utilize captured CO₂ emissions toward syngas generation with tailored composition for several downstream energy conversion and chemical production processes. In the present work, comprehensive kinetic analysis and reaction modelling studies were carried out for olive kernel and lignite coal chars gasification reaction using pure CO₂ as gasifying agent. Chars reactivity and kinetics of the gasification reactions were thoroughly examined by thermogravimetric analysis at three different heating rates and correlated with their physicochemical properties. The reactivity of olive kernel char, as determined by the mean gasification reactivity and the comprehensive gasification characteristic index, S, was almost three times higher compared to that of the lignite coal char. It was disclosed that the fixed carbon content and alkali index (AI) have a major impact on the reactivity of chars. The activation energy, E_a, estimated by three different model-free kinetic methods was ranged between 140 and 170 kJ/mol and 250–350 kJ/mol for the olive kernel and lignite coal chars, respectively. The activation energy values for the lignite coal char significantly varied with carbon conversion degree, whereas this was not the case for olive kernel char, where the activation energy remained essentially unmodified throughout the whole carbon conversion range. Finally, the combined Malek and Coats-Rendfrem method was applied to unravel the mechanism of chars-CO₂ gasification reaction. It was found that the olive kernel char-CO₂ gasification can be described with a 2D-diffusion mechanism function (D2) whereas the lignite coal char-CO₂ gasification follows a second order chemical reaction mechanism function (F2).     

Experimental study on oil-containing wastewater gasification in supercritical water in a continuous system

Recently, promising multi-fluid thermal stimulation technology has been studied and applied successfully to the offshore heavy oil recovery process. However, there are still some shortcomings, including heavy reliance on diesel, heavy and bulky water pretreatment plant, etc., which hinders the further industrialization process of the current generation system of thermal multi-fluid. A novel thermal multi-fluid generation technology to decrease reliance on diesel was presented in this paper. In this technology, oil-containing wastewater can be used directly as the material and energy source. Increased attention should be given to the supercritical water gasification (SCWG) process as the core of this novel technology. An experimental study on SCWG of diesel, a representative of oil-containing wastewater, was described detailly in this paper. Experiments were conducted in a continuous SCWG system under different experimental conditions, as follows: pressure of 23 MPa; reaction temperature of 600 °C–680 °C; mass ratios of water to diesel in emulsification of 1:1, 1:2.5, and 1:3.5; and reactor diesel concentration (R_DC) of 4.70 wt% to 7.70 wt%. The effects of alkali catalyst (K₂CO₃) on gaseous and liquid products were also investigated. A simplified gasification mechanism in SCW was obtained, which may provide a theory basis for the novel thermal multi-fluid generation technology based on oil-containing wastewater gasification in supercritical water.