Coal Gasification Process Research Articles

Experimental simulations of methane-oriented underground coal gasification using hydrogen - The effect of coal rank and gasification pressure on the hydrogasification process

This paper presents a series of surface experimental simulations of methane-oriented underground coal gasification using hydrogen as gasification medium. The main aim of the experiments conducted was to evaluate the feasibility of methane-rich gas production through the in situ coal hydrogasification process. Two multi-day trials were carried out using large scale gasification facilities designed for ex situ experimental simulations of the underground coal gasification (UCG) process. Two different coals were investigated: the “Six Feet” semi-anthracite (Wales) and the “Wesoła" hard coal (Poland). The coal samples were extracted directly from the respective coal seams in the form of large blocks. The gasification tests were conducted in the artificial coal seams (0.41 × 0.41 × 3.05 m) under two distinct pressure regimes - 20 and 40 bar. The series of experiments conducted demonstrated that the physicochemical properties of coal (coal rank) considerably affect the hydrogasification process. For both gasification pressures applied, gas from “Six Feet” semi-anthracite was characterized by a higher content of methane. The average CH4 concentration for “Six Feet” experiment during the H2 stage was 24.12% at 20 bar and 27.03% at 40 bar. During the hydrogasification of “Wesoła" coal, CH4 concentration was 19.28% and 21.71% at 20 and 40 bar, respectively. The process was characterized by high stability and reproducibility of conditions favorable for methane formation in the whole sequence of gasification cycles. Although the feasibility of methane-rich gas production by underground hydrogasification was initially demonstrated, further techno-economic studies are necessary to assess the economic feasibility of methane production using this process.

Sorption-enhanced steam gasification of fine coal waste for fuel producing

Improving the quality of syngas from fine coal waste using the sorption-enhanced gasification process is a novel technology in the production of H2. The effect of CaO on CO2 absorption and H2 increase in the steam fine coal gasification process was determined in a fixed bed gasifier. The steam gasification process took place at 650 °C using bentonite and CaO as catalysts and absorbents. Steam increased the H2 concentration in the syngas to 58 vol%. In-situ CO2 absorption is more effective with the addition of CaO. The maximum percentage of CO2 was absorbed when the Ca/C ratio 2 was 78.33 %. The H2 content in the syngas after the CO2 was absorbed increased rapidly to 75.80 vol% at a Ca-to-carbon-mole ratio (Ca/C) of 1.5 and a steam-to-feedstock ratio (S/F) of 1.5. CaO did not produce significant results for low heating value (LHV) or cold gas efficiency (CGE), with results of 12 MJ/Nm3 and 44.53 %. The dominant water gas shift reaction due to the influence of steam and CaO increased H2/CO up to 9.11, which made the syngas from this work suitable for Fischer–Tropsch synthesis.

Parametric trend life cycle assessment for hydrogen fuel cell towards cleaner shipping

Given the rapidly increasing concern of the climate change, this paper is aimed to answer whether hydrogen fuel cells can truly be a green solution in the shipping sector from a life cycle perspective. To achieve this goal, the parametric trend life cycle assessment which is LCA-based methodology was applied for around 2000 ships presently engaged in international and domestic services. The lifecycle environmental impacts of various hydrogen production methods were evaluated, including steam methane reforming, coal gasification, methanol cracking, and electrolysis via wind energy. The performance of three representative types of fuel cell systems, proton exchange membrane fuel, molten carbonate fuel cell, and solid oxide fuel cell were taken into account. The steam methane reforming and coal gasification processes were found to have the greatest environmental potentials across their lifetime. However, this paper points out that steam methane reforming could make better lifecycle merits than conventional diesel or LNG products, if production pathways are properly proposed. Additionally, when using LNG as the primary fuel source for fuel cells, it was found that the LNG upstream phase would produce about 100 times more emissions than the downstream phase. The research findings were summarized and condensed into a form of lifetime environmental indicators which enable us to understand/evaluate the quantified correlations between holistic environmental impacts of fuel cells and ship characteristics. The research findings are expected to assist stakeholders in making informed decisions, while also providing an insight into near-future regulatory frameworks and policy making for a green hydrogen maritime economy.

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.

Coal gasification process driven by concentrated solar radiation for carbon neutralization: Reaction and energy characteristics

Solar-driven gasification products for chemical feedstock are one of the effective means to utilize coal in a low-carbon and resourceful way. However, few studies on the reaction and energy characteristics are based on experimental data of concentrated solar coal gasification. This study designed a novel experimental solar radiation gasification thermogravimetric device. An energy model of a solar radiation dish thermochemical conversion system was also developed. Compared to indirect radiation, direct radiation has a 10 % higher carbon conversion rate, an increased energy upgrade factor (up to 0.86), and a 38.5 % increase in energy conversion efficiency. Furthermore, we investigate direct radiation-catalyzed gasification to assess the effects of different types and ratios of catalysts. The results showed that the catalytic effect of K2CO3 was better than that of Na2CO3, which would improve the energy conversion efficiency by 4.8 %. For K2CO3, the efficiency was increased by 14.1 % through increasing the doping ratio from 5 % to 10 %. Meanwhile, this study analyzed the reaction kinetics of direct radiation-catalyzed gasification. Finally, we constructed a solar concentrating radiation dish thermochemical conversion system model based on the experimental data. We found that the system energy efficiency in the direct radiation form was 15.3 % higher than that in the indirect radiation form; besides, adding the catalyst in the direct radiation form increased the energy efficiency by 23.8 %. We also found that the gasifier exergy efficiency in direct radiation catalyst gasification was 29.7 %, and that of indirect radiation gasification was 7.23 %. The monthly solar exergy distribution follows the solar radiation closely. The results guide the chemical process of solar thermal conversion.

Quantifying the energy saving potential and environmental benefit of hydrogen-based steelmaking process: Status and future prospect

The application of hydrogen is considered to be an effectiveg way for decarbonising the iron and steel industry. This research aims to quantifying the energy saving potential and environmental benefit of two hydrogen-based steelmaking processes through exergy analysis and life cycle assessment: hydrogen-enriched shaft furnace-electric furnace (HESE) process based on coal gasification and hydrogen shaft furnace-electric furnace (HSE) process based on water electrolysis. It is estimated that the energy consumption and CO2 emission of the HESE process are 11.82 GJ/t and 1121.28 kg/t, achieving a 39.79% of energy-saving and a 45.42% of CO2 emission-reduction when compared with the blast furnace-basic oxygen furnace (BF-BOF) process. The research goes further to evaluate the future HSE process of its energy saving potential and environmental benefit. When renewable electricity is applied, the HSE process will have relatively low carbon emission (i.e. 120.92 kg). However, the energy consumption reaches 13.42 GJ with an exergy efficiency of 33.91%. A key feature of the process lies in the power consumption, which can be balanced through the share of scrap used, and its future links with the prosperity of hydrogen economy. The present work should do helpful effort to the carbon neutral goal of iron and steel industry.

Online temperature estimation of Shell coal gasification process based on extended Kalman filter

Obtaining the temperature inside the gasifier of a Shell coal gasification process (SCGP) in real-time is very important for safe process operation. However, this temperature cannot be measured directly due to the harsh operating condition. Estimating this temperature using the extended Kalman filter (EKF) based on a simplified mechanistic model is proposed in this paper. The gasifier is partitioned into three zones. The quench pipe and the transfer duct are seen as two additional zones. A simplified mechanistic model is developed in each zone and formulated as a state-space representation. The temperature in each zone is estimated by the EKF in real-time. The proposed method is applied to an industrial SCGP and the effectiveness of the estimated temperatures is verified by a process variable both qualitatively and quantitatively. The prediction capability of the simplified mechanistic model is validated. The effectiveness of the proposed method is further verified by comparing it to a Kalman filter-based single-zone temperature estimation method.

Mathematical model and methods for solving heat-transfer problem during underground coal gasification

Purpose. A mathematical model development for heat transfer during underground coal gasification based on the transcendental equation solution by the Newton-Raphson method. Methods. The heat-transfer model development is based on the research into a temperature field with a variable size of the gasification zone when passing through the phase transformation boundary, which changes abruptly. The research on the coal seam T(x, t) temperature field and the displacement length of the phase transition boundary S(t) is based on the integration of the differential heat-transfer equation with the fulfillment of one-phase Stefan problem conditions. The proportionality factor (β), characterizing the ratio of the displacement length of the “generator gas – coal” phase transition boundary to the time of coal seam gasification, is determined by substituting the Boltzmann equation and using the Newton-Raphson method based on solving the obtained transcendental equation. Findings. The main problems related to laboratory research on the coal gasification process have been identified. A mathematical model of heat transfer during underground coal gasification for a closed georeactor system has been developed, taking into account the effective change in its active zones. Originality. A mathematical model of heat transfer during underground coal gasification at the phase transition boundary has been developed, under which the one-phase Stefan problem conditions are fulfilled. Dependences of the change in the underground gas generator temperature, taking into account the change in the active zones of chemical reactions along the length of the combustion face and the gasification column, have been revealed. In addition, the dependences of the change in the phase transition boundary of a “generator gas – coal” heterogeneous system have been determined, which characterize the displacement length of the phase transition boundary on time and reveal the relationship between the thermal conductivity coefficient, specific heat capacity, as well as bulk density of coal and its calorific value. Practical implications. A method has been developed to determine the displacement length of the phase transition boundary of a “generator gas – coal” heterogeneous system and its relationship between the time and temperature of gasification process. This makes it possible to predict in the future the change in the active zones of the underground gas generator along the length of the gasification column.

A new separation flowsheet for resources recovery from waste coal gasification fine slag black water and its benefits analysis

The gasification fine slag black water (GFSBW) is a kind of slurry including fine gasification fine slag particles produced in the coal gasification process and ineffective disposal process causes environmental pollution and resources waste. The aim of this study is to recover combustible matters, high silica-aluminum minerals and water resources via reprocessing GFSBW by innovative separation process of “flotation-agitator-ceramic membrane vacuum (CMV) dewatering”. The physicochemical properties of samples are characterized and at least 6 kinds of carbon-ash occurrence modes of gasification fine slag were found. The flotation process could be a useful method to separate residual carbon and tailing ash. The dewatering processes of flotation slurries are then achieved by the ceramic membrane vacuum filtration, followed by dewatering combustible matters, high silica-aluminum minerals and industrial-water-grade filtrate will be recovered. It is worth noting that the components separation of GFS eliminates their synergistic water holding effect and the average dewatering effect after flotation is improved by 11.67%, and the fixed carbon content of combustible matters is elevated from 22.74% to 44.64%, while the tailing ash including high silica-aluminum minerals is purified to 97.25%. A novel flotation-agitator-ceramic membrane vacuum (CMV) dewatering flowsheet is developed, and useful resources could be recovered to realize waste comprehensive utilization. The proposed GFSBW processing flowsheet could bring 9.82 million US$ per year which is benefit.

Are Wetlands as an Integrated Bioremediation System Applicable for the Treatment of Wastewater from Underground Coal Gasification Processes?

Underground coal gasification (UCG) can be considered as one of the clean coal technologies. During the process, the gas of industrial value is produced, which can be used to produce heat and electricity, liquid fuels or can replace natural gas in chemistry. However, UCG does carry some environmental risks, mainly related to potential negative impacts on surface and groundwater. Wastewater and sludge from UCG contain significant amounts of aliphatic and aromatic hydrocarbons, phenols, ammonia, cyanides and hazardous metals such as arsenic. This complicated matrix containing high concentrations of hazardous pollutants is similar to wastewater from the coke industry and, similarly to them, requires complex mechanical, chemical and biological treatment. The focus of the review is to explain how the wetlands systems, described as one of bioremediation methods, work and whether these systems are suitable for removing organic and inorganic contaminants from heavily contaminated industrial wastewater, of which underground coal gasification wastewater is a particularly challenging example. Wetlands appear to be suitable systems for the treatment of UCG wastewater and can provide the benefits of nature-based solutions. This review explains the principles of constructed wetlands (CWs) and provides examples of industrial wastewater treated by various wetland systems along with their operating principles. In addition, the physicochemical characteristics of the wastewater from different coal gasifications under various conditions, obtained from UCG’s own experiments, are presented.

Contaminant Formation and Release During Underground Coal Gasification Process at High Pressure Regime

Contaminant Formation and Release During Underground Coal Gasification Process at High Pressure Regime

Basic intensity regulation of layered double oxide for CO2 adsorption process at medium temperature in coal gasification

High purity hydrogen for integrated gasification combined cycle power generation (IGCC) can be produced by efficient adsorption of CO2 after water gas shift reaction in coal gasification. Based on adjustability of composition in layered double hydroxides (LDH) precursor, basicity intensity was modified and exhibited different adsorption capacity. Relationship among basic sites, CO2-bound modes, and adsorption capacity were established. Furthermore, by utilizing LDH anionic exchangeability, we introduced laurate ion into LDH interlamellar, and further mediated surface structure using methyl cellulose with long carbon chain. As-obtained Mg3Al-LDO-C12-MC effectively increased the number of medium/strong basic sites. Consequently, CO2-adsorption capacity at medium temperature of 300 °C was further increased to 0.85 mmol/g with the maximum rate of 0.37 mmol/g·min. This work develops a strategy to regulate basic site of adsorbents according to structural mechanism for CO2 elimination under medium temperature, and thus improve adsorption performance in industrial coal gasification process.

Comprehensive optimization of coal chemical looping gasification process for low CO2 emission based on multi-scale simulation coupled experiment

Carbon dioxide (CO2) emission reduction has become an urgent topic to be studied and solved with the in-depth understanding of global warming and frequent occurrence of extreme climate. At present, the application of chemical looping technology in coal gasification to produce high purity synthesis gas is an important way to achieve CO2 emission reduction, ensure energy security, and promote ecological civilization. In this paper, a new fusion research method combining multi-scale modeling and experimental testing is adopted to comprehensively optimize the coal chemical looping gasification (CCLG) process, aiming at ruducing CO2 emissions in the synthesis gas production of CCLG. In order to investigate the effect of main reaction conditions on CCLG process, the pyrolysis and gasification experiments of Meihuajing coal are carried out in a tubular furnace reactor with Fe2O3 and CuO as oxygen carriers respectively. Subsequently, the multi-scale simulation of CCLG process including molecular dynamics (MD) and computational fluid dynamics (CFD) simulations is performed to validate the experimental results and supplement important reaction kinetics data. Both of multi-scale modeling and experimental testing suggest that the optimum pyrolysis temperature range, gasification temperature range, char/oxygen carrier mass ratio (C/O), and steam flow are below 900 ℃, 900–950 ℃, 1:1.5, and 0.15 g/min respectively, providing an effective guidance for the optimal design of practical CCLG pilot plant.

In-situ study of fractal properties of coal char particles during catalytic gasification

Interactions of potassium-based catalysts with Shenfu (SF) char during catalytic gasification was observed by an in-situ heating stage microscope. The effects of the gasification temperature (800–900 °C) and the catalyst loading (4.4%, 10%) on the reactivity of coal char were investigated. The heating stage microscopy was used to visualize the catalytic gasification process of coal char particles and the fractal theory was introduced to analyze the surface structure of coal char particles to reveal the gasification reactivity. The experimental results show that the fractal dimension of coal char particles is positively correlated with the carbon conversion rate, and the fractal dimension increases by increasing the gasification temperature and the catalyst loading. The relationship between the initial gasification reaction rate and the fractal dimension of coal char particles is consistent with that between the carbon conversion rate and the fractal dimension of coal char particles. There is an exponential relation between the fractal dimension of coal char and the char angle; the fractal dimension increases with the increase of coal char particle angle; and the fractal dimension of coal char particles can be used in the study of coal char catalytic gasification process.

Sophisticated interplay of operating conditions governs flow field transition and optimal conversion inside tangentially fired gasifiers

A computational model is developed to simulate reacting environment inside the pilot-scale, tangentially-fired, Mitsubishi Heavy Industry entrained-flow coal gasifier. Using this comprehensive model, the impact of three operating parameters, namely, swirl number (SN), reactor pressure and mass throughput on the gasifier flow field and coal gasification process is investigated. We demonstrate that for given values of any two of these parameters, it requires a threshold value of the third parameter to form the central recirculation zone (CRZ), the absence of which lowers char conversion. Therefore, a modified SN incorporating the combined effect of conventional SN, reactor pressure and mass throughput on the onset of CRZ formation is proposed for tangentially-fired gasifiers. Furthermore, the impact of variation in reactor pressure and mass throughput on char conversion is shown to be mediated through the corresponding change in particle residence time and pressure effect on kinetics. The results demonstrate that the operating parameters interact in a complex state space that governs the gasifier flow physics and optimal operation.

Study on the characteristics of self-developed pulverized coal flow regulation valve for coal gasification process

Pulverized coal flow regulation valve is one of the key equipment of pulverized coal gasification process, which plays an essential role in regulating coal flow rate. However, due to complex valve flow channels and gas-solid interactions, there were few reports on the characteristics of this valve. In this paper, experiments of dense-phase pneumatic conveying were carried out with a self-developed pulverized coal flow regulation valve installed in the pipeline. Firstly, the effect of the valve opening on the pressure distribution of the conveying system was analyzed. Then, the flow patterns of gas-solid two-phase flow inside the vertical pipe and inside the horizontal pipe were investigated by the Electrical Capacitance Tomography (ECT) system, respectively. Finally, after analyzing the influence of the regulation valve on the gas-solid flow parameters and based on data regression analysis, the relationship between valve opening and solid velocity and concentration were fitted, and a semi-empirical model was established for pulverized coal flow rate prediction. The model can effectively predict the pulverized coal flow rates under different valve openings with deviations mostly less than± 10%, which is of great significance for flow rate regulation in practical applications.

Experimental simulate on hydrogen production of different coals in underground coal gasification

Research on hydrogen production from coal gasification is mainly focused on the formation of CO and H2 from coal and water vapor in high-temperature environments. However, in the process of underground coal gasification, the water gas shift reaction of low-temperature steam will absorb a lot of heat, which makes it difficult to maintain the combustion of coal seams in the process of underground coal gasification. In order to obtain high-quality hydrogen, a pure oxygen-steam gasification process is used to improve the gasification efficiency. And as the gasification surface continues to recede, the drying, pyrolysis, gasification and combustion reactions of underground coal seams gradually occur. Direct coal gasification can't truly reflect the process of underground coal gasification. In order to simulate the hydrogen production laws of different coal types in the underground gasification process realistically, a two-step gasification process (pyrolysis of coal followed by gasification of the char) was proposed to process coal to produce hydrogen-rich gas. First, the effects of temperature and coal rank on product distribution were studied in the pyrolysis process. Then, the coal char at the final pyrolysis temperature of 900 °C was gasified with pure oxygen-steam. The results showed that, the hydrogen production of the three coal chars increased with the increase of temperature during the pyrolysis process, the hydrogen release from Inner Mongolia lignite and Xinjiang long flame coal have the same trend, and the bimodality is obvious. The hydrogen release in the first stage mainly comes from the dehydrogenation of the fat side chain, and the hydrogen release in the second stage mainly comes from the polycondensation reaction in the later stage of pyrolysis, and the pyrolysis process of coal contributes 15.81%–43.33% of hydrogen, as the coal rank increases, the hydrogen production rate gradually decreases. In the gasification process, the release of hydrogen mainly comes from the water gas shift reaction, the hydrogen output is mainly affected by the quality and carbon content of coal char. With the increase of coal rank, the hydrogen output gradually increases, mainly due to the increasing of coal coke yield and carbon content, The gasification process of coal char contributes 56.67–84.19% of hydrogen, in contrast, coal char gasification provides more hydrogen. The total effective gas output of the three coal chars is 0.53–0.81 m3/kg, the hydrogen output is 0.3–0.43 m3/kg, and the percentage of hydrogen is 53.08–56.60%. This study shows that two-step gasification under the condition of pure oxygen-steam gasification agent is an efficient energy process for hydrogen production from underground coal gasification.

Physicochemical structure, combustion characteristics and SiO2 properties of entrained flow gasification ash

During the gasification process of coal, fine ash (FA) and coarse slag (CS) solid waste are produced, which contain a large amount of residual carbon and ash (mainly SiO2), resulting in energy waste and environmental pollution. Therefore the main goal of this research is to recover the wasted energy from FA and CS. The investigation of the physicochemical structure, combustion characteristics and SiO2 properties of entrained flow gasification ash (EFGA) is an important basis for the valorization of EFGA. The physicochemical, combustion characteristics and SiO2 properties of EFGA in a entrained flow bed of 112,000 Nm3/h gas-forming capacity were investigated, showing that FA and CS undergo different gasification processes, with FA having a more developed pore structure and high specific surface area. FA and CS have different degrees of graphitization, with defects or heteroatoms in FA causing a scattered carbon layer structure with an irregular graphitic carbon crystal structure, and organic molecular functional group fragments forming an indeterminate structure and active sites. The organic molecule functional group fragments constitute an indeterminate structure and active sites. Kinetic analysis shows that the influence of chemical structure on the combustion characteristics of EFGA is higher than that of physical structure. Compared to CS, FA has fewer active sites and a small number of oxygen-containing functional groups, resulting in a weaker combustion reactivity of FA. The combustion activity of EFGA is lower than that of circulating fluidized bed gasification ash and anthracite, making it difficult to be eliminated by direct combustion, although co-combustion is feasible. The specific surface area of SiO2 prepared by EFGA after acid treatment and calcination is small (<39.7086 m2/g), SiO2 exists as crystals and the elemental silicon exists mainly in the form of Si–O, O–Si–O and hydrated silicon dioxide Si–O2-nH2O, which can be used as a raw material for the preparation of silica.

The Layout of the Combustion Cavity and the Fracture Evolution of the Overlying Rock during the Process of Underground Coal Gasification

Based on the thermodynamic and elastodynamic theories, the controlling equation of temperature-stress coupling action on rocks containing random damage units is established by combining the Mogi-Coulomb damage criterion. And the numerical calculation model of combustion cavity expansion under temperature-stress coupling condition is established by using ABAQUS secondary development program. The fracture field evolution law during the expansion of the gasification cavity was studied under two conditions: perpendicular to (condition 1) and along the intermediate principal stress (condition 2). It is found that under the condition 1, the gasification cavity gradually forms a smaller fracture circle, while the surrounding rock at the floor of the gasification cavity generates a wide range of equivalent damage areas under the condition 2, which is unfavorable for practical engineering. The condition 1 deployment scheme is more practical in terms of the degree of rupture and the subsequent gasification process. The gasification cavity is obviously affected by the horizontal stress. When the horizontal stress is small, the stability of the surrounding rock is seriously damaged. It is necessary to fully consider the influence of in situ stress in the layout of the gasification cavity; at the same time, measures are added to the process to reduce the degree of rupture of the surrounding rock. The evolution law of the temperature field and rupture field of the surrounding pressure in the combustion cavity during the whole process of UCG was numerically simulated. The floor rupture zone develops gradually with the advancement of the working face of the combustion cavity, and deeper rupture zones appear in several areas, which need special attention in the engineering.

Modeling and Control of Energy Conversion during Underground Coal Gasification Process

The underground coal gasification (UCG) technology is an unconventional method of coal mining, and its approaches represent new scientific knowledge [...]