Mathematical Modeling of the Underground Coal Gasification Process in One Gasification Cycle

Abstract

Underground coal gasification (UCG) has its own reaction characteristics with the growth of the combustion cavity because the coal seam is fixed while the gasification face moves. The theoretical understanding of UCG requires continuous and deep research. In order to explain the UCG mechanism by means of simulation calculation, based on the analysis of the reaction process of UCG, radial and axial reaction models are established. The comprehensive heat transfer coefficient of the coal seam is determined by comparing the calculation results with the field test results. The differential method is used to calculate variations in the composition of the outlet gas, the temperature field distribution in the channel, the thickness of the radial reaction zone, and the influence of the oxygen fraction on the length of the oxidation zone. The results indicate that in one gasification cycle, the changes of gas composition is close to that of the field test, which initially increases and then decreases with increasing gasification time. The radial expansion of the coal seam mainly occurs in the high-temperature zone, consistent with the trend of the channel temperature field. The radial expansion thickness in the coal seam is between 0.2 and 0.8 m. The higher the oxygen fraction in the gasifying agent is, the shorter the range of the oxygen distribution along the axial direction will be, that is, the shorter the length of the oxidation zone.

Numerical Simulation of the Temperature Distribution and Evolution Law of Underground Lignite Gasification

“…The second stage is the gasification reaction stage, when the axial distance is about 10–12 m and the temperature increases to the highest point. 22 With an increase in the axial distance, the temperature decreases gradually. This is mainly due to the decrease in the pyrolysis gas and oxygen concentrations with the increase in the axial distance of the gasification channel and the reduced heat released by combustion.…”

“…As the reaction progressed, the oxygen concentration in the gasification channel gradually decreased, as shown in Figure 2. Liu et al found that the oxygen concentration decreased to zero when the gasification channel was approximately 30 m. 22 Therefore, based on the need for control variables, the corresponding gasification channel length was set to 30 m when the oxygen concentration was zero. 2.2.…”

“… 20 , 21 Owing to limitations of the model size, it is difficult to obtain the gasification parameters of thick and superthick coal seams, but a numerical simulation method can be realized. 22 Kinetic and thermodynamic equilibrium models are commonly used numerical simulation models of UCG. 23 , 24 Most of the attention is on the composition of the produced gas (H 2 , CO, CH 4 , etc.)…”

“…and the temperature distribution in the gasifier under the condition of single coal thickness. 22 , 25 In addition, the temperature field under the condition of single coal seam thickness is studied by means of mathematical analysis or numerical simulation, and the corresponding analytical solution is obtained. 26 − 30 Xin uses the physical envelope curve method to study the temperature field under fixed boundary conditions and determines the influence range of the temperature field.…”

Pore Structure and Compressibility Characteristics of Heat-Treated Coals by N2 Adsorption/Desorption and Mercury Intrusion Porosimetry

“…As a clean and highly efficient hydrocarbon source, coalbed methane (CBM) has achieved extensive commercial development in the Unites States, Canada, and China in recent years. − Compared with conventional natural gas, CBM is mainly present in an absorbed form in the coal matrix, and the production process of CBM involves the desorption of the gas from the pore surface by reducing the reservoir pressure and the subsequent promotion of gas migration from the matrix pore system to the cleat/fracture system. , In addition to the hydraulic fracturing and injection of CO 2 or other displacement gases, the in situ heat treatment of coals currently provides a feasible alternative for enhancing the CBM recovery. , Previous investigations have mostly focused on the structural behavior of coal at elevated temperature and the chemical aspect of coal pyrolysis/gasification, − while there is a limited amount of research on improving coal reservoir properties to facilitate CBM recovery after heat treatment . Therefore, accurate characterization of the pore structure evolution at elevated temperatures and the thermomechanical response of coals are of great significance for enhancing the CBM flow capacity and maximizing the recovery.…”