Coal Gasification Process Research Articles

Experimental Investigation on the Pore Structure Evolution of Coal in Underground Coal Gasification Process

Underground coal gasification (UCG) has been shown to be a promising method for deep coal resources. A series of complicated chemical reactions can induce a considerable change in the pore structure of coal and thus promote the UCG process in turn. Currently, most studies on the effect of elevated temperature on the pore structure of coal were not involved in an air atmosphere, bringing a series of difficulties to understanding the pore structure evolution of gasified coal in the UCG process. The objective of this work was to investigate the pore structure evolution of coal heated in nitrogen and air atmospheres at elevated temperatures. Thermogravimetry tests were first conducted to gasify coal samples, and then, the method scanning electron microscopy was used to observe the microscopic morphology of the pore structure. Besides, the effect of final temperature, atmosphere, pressure, and residence time on the thermal dynamics of coal at elevated temperatures was comprehensively discussed. Results indicated that the temperature range of a heating process of coal can be classified into three stages, 25–320 °C, 320–750 °C, and 750–1000 °C. For the three temperature ranges, drying, primary pyrolysis, and secondary pyrolysis can dominate, respectively, under a nitrogen atmosphere, while the combustion and gasification process will prevail at a high temperature under an air atmosphere. Because of mass loss, the coal was becoming porous during the heating process. Compared with the intact structure of coal when the temperature is lower than 300 °C, the pore space can interconnect under moderate-temperature conditions (500 °C) like a honeycomb, and then, only an ash framework remained under a higher-temperature condition (700 °C) under an air atmosphere. In comparison, the coal heated in a nitrogen atmosphere can gradually turn into a porous char. This investigation can provide some new insights into the pore structure evolution of gasified coal and contribute to the mechanisms of a UCG process.

In-situ Raman spectral investigation into hydrogen bonding characteristics of supercritical water

Supercritical water is widely used as a superior reactant or solvent in the gasification of biomass and coal. Research shows that hydrogen bonding could reduce the potential energy of chemical reaction between water molecules and organic molecules. However, it is still uncertain whether hydrogen bonds exist in supercritical water at the temperature of coal gasification process (above 873 K). In this paper, a high temperature and pressure visualization platform (Tmax = 1013 K) was built. The hydrogen bonding degree of water from liquid to supercritical is obtained in a wider temperature range. It is a remarkable fact that hydrogen bonds still exist in supercritical water even at temperatures as high as 1013 K. These hydrogen bonds mainly exist in the form of dimer and trimer, which is a significant factor that can’t be ignored in the study of gasification mechanism of coal and organic matter in supercritical water.

Inerting mechanism of magnesium carbonate hydroxide pentahydrate for coal dust deflagration under coal gasification

To reduce the risk of coal dust deflagration caused by the leakage in the coal gasification process, a semi-closed vertical combustion tube was built with a high speed camera and micro-thermocouples. And the inerting effect of magnesium carbonate hydroxide pentahydrate (MCHP) with different particle sizes was investigated on the coal dust deflagration. The results showed that the initial flame became very faint and the brightness of the flame was greatly weakened after the addition of MCHP powder. And as the inerting ratio increased, both of the flame velocity and temperature decreased. When the particle size of MCHP powder decreased, the flame propagation time extended from 132 to 196, 223, and 228 ms respectively. And the residue analysis revealed that the decomposition of MCHP not only absorbed the highly reactive radicals of combustion, but also formed inert gases and heat-resistant protective film, which greatly reduced the deflagration intensity of coal dust.

Density functional theory investigation of As4 adsorption on Ti, V, Cr, Mn-doped graphene

Coal gasification technology can effectively solve the problem of clean coal utilization, but the problem of volatilization of harmful metals such as arsenic in the coal gasification process cannot be underestimated. In this paper, based on DFT (density functional theory), the adsorption characteristics of As4 on the surface of different carbon-based single-atom Ti/V/Cr/Mn adsorbents (nitrogen atom-doped, single vacancy and double vacancy) were studied. The single-atom adsorbent geometry, adsorption configuration, adsorption energy, electronic structure, electron density and other data were analysed. By comparing and analysing the calculation results of sixteen adsorbents, it is found that the catalyst with the double-deficient nitrogen-doped substrate has higher stability. Through the analysis of charge transfer and density of state, it was found that the adsorption of a single-atom adsorbent catalyst to As4 is related to charge transfer and hybridization. In particular, doped nitrogen atoms can enhance the charge transfer and hybridization of the system. amongst the sixteen adsorbents, the double-vacancy nitrogen-doped Ti catalyst is a promising As4 adsorbent, which provides a reference for further research on the adsorption of toxic gases on the surface of single-atom catalysts and the design of new nonnoble metal adsorbents.

A novel hydrogen production system based on the three-step coal gasification technology thermally coupled with the chemical looping combustion process

Coal gasification technology is a significant process for the coal-based hydrogen production system and is considered as a key technology in the transition to “Hydrogen Economy”. To decrease the exergy destruction and enhance the cold gas efficiency of the coal gasification process, a novel three-step gasification technology thermally coupled with the chemical looping combustion process is proposed. And the hydrogen production system with CO2 recovery is integrated based on the three-step gasification technology. Results indicated that the cold gas efficiency of the three-step coal gasification technology is 86.9%, which is 10.1% points enhanced compared with GE gasification technology. Besides, the novel system has an energy efficiency of 62.3%, which is 3.1% higher than that of the reference system. Exergy analysis presented that the employment of the three-step gasification technology contributed to the reduction of system exergy destruction by 4.2%. Furthermore, the energy utilization diagram (EUD) suggested that matching between endothermic reactions and exothermic reactions plays important role in the enhancement of cold gas efficiency.

Optimization of coal gasification process based on a dynamic model management strategy

Owing to the complex physical and chemical reactions, optimization of the coal gasification process requires a large computational cost. In chemical engineering design and optimization, the surrogate model is often used to assist the evolutionary algorithm (EA) in solving computationally expensive problems. In this paper, a dynamic model management strategy based on adaptive surrogate selection (ASS) is proposed to identify an appropriate surrogate model to assist the particle swarm optimization (PSO) algorithm faced with complex problems. In ASS, based on the given dataset, a right surrogate model was selected from the diversity model library by adopting the minimum root mean square error and K-fold cross-validation before assisting PSO to identify an optimal solution, and reselected constantly with the addition of a new sample. Based on the ASS, in addition to the adaptive global model, the local model is introduced and selected adaptively to refine the optimal solution more rapidly, then switched dynamically with the global model. The most uncertain sample was also considered for searching the unexplored region and escaping from the local optimum. Comparison of the specific surrogate EAs and three other state-of-the-art surrogate-assisted EAs revealed that the proposed strategy significantly improved the optimization performance of PSO and the effective syngas yield of the coal gasification process.

Parameter Fluctuation Modeling and Extreme Value Prediction of Coal Gasification Based on Jump Diffusion Model

In order to accurately predict the random fluctuation of uncontrollable process parameters in coal gasification process and optimize operation parameters, a stochastic differential equation with jump was proposed to study the random dynamic sequence of process parameters changing with time. Considering the effect of emergencies, the jump process was introduced into the geometric Brownian motion model to simulate the jump caused by emergencies, and the stochastic differential equation with jump was established to simulate and predict the parameter fluctuation sequence. The results show that the method can accurately simulate and predict the random fluctuation sequence of uncontrollable process parameters and describe the random fluctuation process of parameters, which lays a foundation for the study of its influence on the gas production performance of coal gasification.

USE OF MAGNETIC FIELDS FOR INTENSIFICATION OF COAL GASIFICATION PROCESS

Underground coal gasification is an alternative method for mining coal from thin and ultra-thin seams, which enables conversion of solid fossil fuels into combustible gases at the site of coal occurrence. At the same time, in the case when the coal seam thickness is critically small for the effective course of thermochemical reactions, it is necessary to intensify the gasification process. This paper studies one of the possible methods to intensify the process of underground coal gasification due to the influence of magnetic fields on the injected blast supplied into the gas generator gasification channel. Research tests conducted on a bench setup confirm the effectiveness of injected blast activation in a magnetic field by creating magnetic field inhomogeneity by placing permanent magnets and a discrete solid magnetized phase in a special device. For the first time, the dependence of changing growth of carbon participation during the solid fuel gasification process on changing magnetic field strength in the range of 0-600 E has been determined. It has been proven that the injected blast magnetization can significantly intensify the underground gasification process by increasing the carbon participation share in the fuel, which may be of practical importance for increasing the yield of combustible components.

Evolution Analysis of Functional Groups of Coal Gasification Process in Supercritical Water

Evolution Analysis of Functional Groups of Coal Gasification Process in Supercritical Water

Proposal and analysis of a poly-generation process for coal gasification and ironmaking

The novel gasification-reduction coupling process (GRCP) was proposed to obtain higher coal gas/hot metal poly-generation system efficiency. The hematite particles replaced previous moderators (H2O/CO2) in the entrained-flow gasifiers, and direct reduced iron (DRI) can be obtained simultaneously. In this study, the thermodynamic model was established to predicate material streams in GRCP. Numerous cases with different material ratios were also examined to investigate the relevant effects, such as effective gas ratio ([CO] + [H2]), metallization ratio (Fe wt.%), and reduction degree (R). To further quantify energy streams, the exergetic analysis was also conducted according to different definitions. The results indicated that both chemical and physical exergy of high temperature reducing gas transferred into DRIs through the coupling process. Therefore, a significant decrease was observed in cold gas efficiency (72.7%). The exergetic evaluation also shows that GRCP still kept a high-level (83.6%) based on all substances. Since the chemical exergy of DRI was stable exergy storage during the subsequent utilization, higher system efficiency can be expected in the further development of GRCP.

Inhibition evaluation of magnesium hydroxide, aluminum hydroxide, and hydrotalcite on the flame propagation of coal dust

Coal dust suspended in a syngas atmosphere may result in a dust explosion during the coal gasification process. A vertical tube with a high-speed camera and thermocouple was built to investigate the flame propagation characteristics of coal dust. Magnesium hydroxide (MH), aluminum hydroxide (ATH), and hydrotalcite (HT) were selected as the inhibitors to compare their inhibition effect. The results showed that the effects of the three inhibitors differed greatly. The addition of MH prolonged the flame propagation time, but its inhibition of flame temperature was the worst. The peak temperature and velocity of the flame after the addition of HT were the smallest compared to that after the addition of MH and ATH. Besides, the analysis of residues proved that the unburned coal dust was completely encapsulated by the high-temperature resistant oxides from HT so that the coal dust was most effectively protected, which showed the best heat absorption properties and the lowest decomposition temperature compared with that of MH and ATH.

Modification of Cu2+ in polyphenol oxidase extract from purple eggplant for phenol degradation in coal wastewater treatment

Cracking coal-forming organic compounds during the gasification process produces liquid waste containing phenolic compounds that require special handling based on their toxicity. As one of the components, there is liquid waste resulting from the coal gasification process. The purpose of this research was to study the activity of polyphenol oxidase (PPO) on phenol after the addition of Cu2+ to purple eggplant (Solanum melongena L.) extract and its potential to work more effectively in phenol biodegradation for coal wastewater containing phenol. Enzyme activity and phenol determination were carried out spectrophotometrically. The results showed PPO activity of 25.90-38.10 U/mL; 4.0 mM phenol and the activity of PPO-Cu2+ was 21.58-46.32 U/mL; 2-10 mM CuSO4; 2.0-4.0 mM phenol. Based on Michaelis Menten’s graph, the initial rate of PPO-Cu2+ was 0.015 mM/min and the initial rate of PPO was 0.15 mM/min using 2 mM phenol as a substrate. Lineweaver-Burk’s graph shows the KM of PPO-Cu2+ = 6.92 mM, which is lower than KM of PPO = 13.05 mM. Its means that the phenol response has a higher affinity for PPO-Cu2+ than PPO. The application of PPO-Cu2+ in purple eggplant extract works effectively as much as 46.7% for artificial coal liquid waste containing phenol.

Effects of typical inhibition materials on the flame propagation of pulverized coal mixture

Inert powders played an important role in the explosion inhibition of dust removal equipment and coal gasification process. In this paper, SiO2, dry water, carrageenan dry water, and NaHCO3 were adopted to explore the influences of inert powders on the deflagration flame propagation characteristics of mixed pulverized coal by a vertical pipeline. The results showed that the flame brightness of the mixed pulverized coal decreased with the irregularly discrete of flame front after the addition of inert powders. As the increasing of pulverized coal mesh number, the peak flame temperature first increased and then decreased, while the drop value of peak flame velocity and temperature increased with the increasing of mass concentration. In addition, the inerting rate affected the propagation of pulverized coal composite flame. When the inerting rate exceeded 1.0, the peak temperature drop increased rapidly and dry water exhibited the best inhibition performance.

Energy and exergy analyses of an intensified char gasification process

Conventional coal gasification technologies have been widely used in chemical synthesis and power generation industries, and are regarded as clean technologies for utilising fossil fuel resources. Although it has not yet realised widespread commercial application, fluidised bed gasification technology has the potential for high energy utilisation efficiency if the high carbon content in discharged ash can be decreased. This article introduces a novel concept for an intensified gasification process, and provides a comprehensive energy and exergy analyses comparing this with a conventional fluidised bed gasification process, to demonstrate potential improvements in terms of energy and exergy efficiencies. The results show that the intensified gasification process can increase the energy and exergy efficiencies by 8.1% and 7.1%, respectively, in the gasification temperature range from 850 to 1100 °C, compared with the conventional process. Increasing gasification temperature improves the energy utilisation efficiency, and consumes more oxygen and less steam. It also increases CO yield from 0.957 to 0.997 and lowers H2 yield from 0.498 to 0.442. The exergy analysis shows that irreversible gasification reactions are the major source of exergy destruction. Hence, this should be the focus for improvements in energy utilisation of coal gasification processes.

Characteristics of Water Contaminants from Underground Coal Gasification (UCG) Process—Effect of Coal Properties and Gasification Pressure

One of the most important issues during UCG process is wastewater production and treatment. Condensed gasification wastewater is contaminated by many hazardous compounds. The composition of the generated UCG-derived wastewater may vary depending on the type of gasified coal and conditions of the gasification process. The main purpose of this study was a qualitative and quantitative characterization of the UCG wastewater produced during four different UCG experiments. Experiments were conducted using semi-anthracite and bituminous coal samples at two distinct pressures, i.e., 20 and 40 bar. The conducted studies revealed significant relationships between the physicochemical composition of the wastewater and the coal properties as well as the gasification pressure. The strongest impact is noticeable in the case of organic pollutants, especially phenols, BTEX and PAH’s. The most abundant group of pollutants were phenols. Conducted studies showed significantly higher concentration levels for bituminous coal: 29.25–49.5 mg/L whereas for semi-anthracite effluents these concentrations were in much lower range 2.1–29.7 mg/L. The opposite situation occurs for BTEX, higher concentrations were in wastewater from semi-anthracite gasification: 5483.1–1496.7 µg/L, while in samples from bituminous coal gasification average BTEX concentrations were: 2514.3–1354.4 µg/L. A similar relationship occurs for the PAH’s concentrations. The higher values were in case of wastewater from semi-anthracite coal experiments and were in range 362–1658 µg/L while from bituminous coal gasification PAH’s values are in lower ranges 407–1090 µg/L. The studies conducted have shown that concentrations of phenols, BTEX and PAH’s decrease with increasing pressure. Pearson’s correlation analysis was performed to enhance the interpretation of the obtained experimental data and showed a very strong relationship between three parameters: phenols, volatile phenols and CODcr.

Determination of the Extent of the Rock Destruction Zones around a Gasification Channel on the Basis of Strength Tests of Sandstone and Claystone Samples Heated at High Temperatures up to 1200 °C and Exposed to Water

This article presents the results of laboratory tests regarding the influence of high temperatures on changes in the strength and structural parameters of rocks that are present in the immediate vicinity of a gasification channel. Sandstone and claystone samples were heated at 300 °C, 600 °C, 900 °C and 1200 °C. Additionally, the heated samples were placed in water for 24 h. The results of the laboratory tests were used in the numerical simulation using RS2 software. The main goal of modeling was to determine the extent of the rock destruction zone around the gasification channel for dry and wet rock masses. In the numerical simulations, three widths of the gasification channel and three ranges of high-temperature impact were modeled. On the basis of the obtained results, it was found that the extent of rock destruction, both in the roof and in the floor, is greater by several percent for a wet rock mass. For the first time, this research presents the effect of water on heated rock samples in terms of the underground coal gasification process. The results of laboratory tests and numerical simulations clearly indicate a reduction in strength, deformation and structural parameters for the temperature of 1200 °C.

One step methane production based on catalytic pressurized calcium looping gasification with in-situ CO2 capture and self-sustained heat supply

Catalytic steam hydrogasification of coal is a direct method for methane production. Calcium looping concept is usually used in coal gasification process for in-situ carbon dioxide removal and heat supply. In this paper, a new process combining catalytic steam hydrogasification and calcium looping was proposed and investigated using a self designed instantaneously feeding reactor under high-temperature and pressurized conditions. The effects of operation conditions (including hydrogen concentration with a range of 0–50 vol%, gasification pressure with a range of 0.1–3.5 MPa, gasification temperature with a range of 700–800 °C, and gasification-calcination cycle number up to six) on the performance of the new process have been studied. The results show that: (i) increasing H2 concentration is beneficial to methane products; (ii) high temperature and low pressure are not conducive to methane production and carbon dioxide capture as well as the self-sustained heat supply in gasifier; (iii) the methane content and carbon conversion can be maintained at 30–40 vol% and 75–80% for the durability tests. According to the performance of gas products, 750 °C 3.5 MPa and Ca/C = 0.5 are suggested for the new process. In addition, the gasification reactivity can be affected by the Ca–K-Char interaction as indicated by the XRD, FT-IR and SEM-EDX analysis.

CFD modelling of coal gasification in a fluidized bed with the effects of calcination under different operating conditions

This paper investigates the thermochemical and physical conversion processes of coal gasification numerically with particular interest on calcination in a bubbling fluidized bed furnace. A comprehensive Eulerian-Eulerian three-dimensional model is developed for studying the gasification process. Three calcination cases are carried out under different operating conditions while one inert case is conducted to evaluate the effect of calcination. The presented numerical results aim at determining the mechanism of coal gasification in an air-steam environment with different flowrates. Evidence of particle segregation is found in the bed of coal and limestone due to density reduction and diameter shrinkage. Char conversion is investigated for different air-coal and steam-coal ratios, also the effect of bed temperature, fluid flowrate and fuel feeding rate on the carbon conversion is studied comprehensively. The highest char conversion rate is observed in the airflow rate of 17.0 kg/h where the bed temperature is found to be maximum. A noticeable impact of calcination is found in the gaseous emission while increasing CO2 concentration. Time averaged solid and gas temperature and species concentration profiles indicate the steady-state condition of numerical simulation.

Optimization study on improving energy efficiency of power cycle system of staged coal gasification coupled with supercritical carbon dioxide

This paper designs and studies a staged coal gasification coupled S–CO2 power cycle system. The three-stage pulverized coal gasifier of a coal gasification plant is used as the research object for basic research. The calculation is carried out in the Fluent software in a finite rate mode to make the results closer to reality. Through the analysis of the results, it is found that the structure of coal gasifier and coal gasification process have optimization space. By adding distributed nozzles and reflux plates, the effect of pulverized coal gasification is improved. In the process, adopt staged coal gasification to separate the decomposition of volatile matter and the gasification of coke, and the gasification agent is introduced under the optimum gasification agent ratio to enhance the efficiency of cold gas. In addition, this paper also carried out two local improvements and optimizations in the power cycle process. The first is to use the interstage heat of the compressor for thermochemical regeneration and S–CO2 power cycle. After the improvement and optimization, the exergy loss of the system and the consumption of heating coal have been reduced, and the net power generation efficiency of the system increased to 43.2%. The second is to improve the flue gas waste heat utilization system. A multi-split Brayton cycle replaces the steam Rankine cycle, and the cycle parameters are optimized. When the outlet pressure of the compressor is 28 MPa, the exergy efficiency of the bottom cycle is 41.9%. After all improvements are completed, the net efficiency of the whole system reaches 45.7%.

Mineralogical examination of the entrained-flow coal gasification residues and the feed coals from northwest China

Minerals are the dominant impurities in coal and their changes in gasifiers could cause many problems such as fouling, slagging, and ash deposition. The mineralogy of the coal gasification residues derived from two commercial entrained-flow coal gasification plants from the Ningdong Energy and Chemical Industry Base were determined. A range of minerals were detected by the analyses of X-ray diffraction and scanning electron microscope with associated energy-dispersive X-ray spectrometer (SEM–EDS). The mineral species formed in General Electric (GE) gasification were more abundant than that in the Gaskombinat Schwarze Pumpe (GSP) gasification. The neoformed minerals such as grossular and hedenbergite detected in the residues indicated that the gasification processes share some similarities with magmatism in geological system. The existence of wustite in the GE coal gasification residues indicated the reductive environment in the GE gasifier. Compared to the GSP feed coals, the higher proportions of calcite, pyrite, and siderite and the higher Na contents in the GE feed coals indicated that the ash problems probably happen in the coal-water slurry gasification. Reducing the proportions of those Na-, Ca-, and Fe-containing mineral from the feed coal is necessary for the improvement of the coal gasification process.