Modeling Test of Combustion Cavity Growth during Underground Coal Gasification in the Early Stage of Ignition.

The growth parameters of the underground coal gasification (UCG) combustion cavity are important for the regulation of its gasification process. The irregular cavities formed in the early stages of ignition can affect the stability of the gasification process. In this study, a heat-solid coupling model is used to determine the combustion cavity boundary at the early stage of coal seam ignition to simulate the movement of the combustion cavity boundary by indirectly inheriting the coal seam temperature. It reveals the evolution of the temperature field, stress field, and plasticity zone at the combustion cavity boundary at the early stage of ignition in the UCG process and compares with the ex situ small-scale experiments. The simulation results show that in the early stage of ignition, the temperature transfer to the top of the coal seam and the direction of the gasification agent outlet pipeline is faster, while the transfer rate to the direction of the gasification agent inlet pipeline is slower. The main stresses are mainly distributed in the left and right sides of the combustion cavity and gradually increase directly above. The plastic zone is mainly distributed directly above the combustion cavity and arc-shaped plastic zones. The experimental results show that the temperature directly above the combustion cavity is higher than in the other directions, and the ash layer hinders the temperature transfer to the bottom. Therefore, the combustion cavity has a longer elliptical shape in the upper part, which is consistent with the simulation results. The model better reveals the extension law of the combustion cavity at the early stage of UCG ignition and provides theoretical guidance for the study of combustion cavity formation.

Effects of Pressurized Pyrolysis on the Chemical and Porous Structure Evolution of Coal Core during Deep Underground Coal Gasification.

During deep underground coal gasification, the semicoke produced by the pyrolysis of dense coal cores is an important material for its gasification and combustion. In this paper, pressurized pyrolysis experiments were carried out on dense coal cores at 700 °C and pressures of 1, 2, and 3 MPa using a shaft furnace. The resulting semicoke and raw coal were analyzed using the characterization methods such as the N2 isothermal adsorption/desorption and scanning electron microscopy, Fourier transform infrared spectrometry (FTIR), and a pressurized thermogravimetric analyzer coupled with a FTIR spectrometer. The pyrolysis gas generation characteristics during pressurized pyrolysis were studied. The mechanisms of evolution of aliphatic functional groups and pore structures in semicoke during pressurized pyrolysis were revealed. The results indicate that the increase in pressure obviously changed the gas composition, most notably, the relative content of CH4 and H2 in the pyrolysis gas. The methane in the pyrolysis gas during pressurized pyrolysis of dense coal cores is mainly from the secondary reaction. As the pyrolysis pressure increased, the ratio of -CH2-/-CH3 became smaller, indicating that the pressure promoted the breakage of the long fat chains. With the increase of the pyrolysis pressure, the surface deformation of pressurized pyrolysis semicoke increases, and the pore structure becomes more abundant.

Mineralogical Characterization of Gasification Ash with Different Particle Sizes from Lurgi Gasifier in the Coal-to-Synthetic Natural Gas Plant.

The Lurgi gasifier in China is one of the most suitable technologies to produce synthetic natural gas (SNG) from coal; however, a large amount of byproduct ash is discharged during the Lurgi gasification process, causing many environmental problems. Based on ash samples collected from a commercial Lurgi gasifier in a Chinese coal-to-SNG plant, this paper studied the mineral composition and microscopic appearance of gasification ash with different particle sizes. The typical minerals were identified and investigated by comparing them with the ash from a laboratory fixed-bed reactor. The results showed that the main high-temperature minerals in the Lurgi gasification ash with different particle sizes under the gasification condition of 4 MPa and 1100 °C were anorthite (CaAl2Si2O8), augite (CaFeSi2O6), hematite (Fe2O3), and gehlenite (Ca2Al2SiO7). As the particle size of the Lurgi gasification ash increased, the quartz content increased but the residual carbon content decreased. Additionally, the high-temperature minerals were more likely to agglomerate with fine particles of the ash. The FactSage modeling showed that calcium-bearing minerals were formed earlier than iron-bearing minerals. The high Fe2O3 content in ash hindered the transformation of calcium-bearing minerals into the high-melting-point mullite, resulting in a low ash flow temperature. Additionally, the fine ash had a relatively high content of calcium-bearing minerals which was not conducive to its utilization as an additive in cement and concrete.