Characteristics Study On Dynamic Temperature Field And The Combustion Cavity Growth During Underground Coal Gasification

Underground coal gasification is a process of converting coal resources into combustible gas in situ,and is a clean,safe and efficient coal utilization technology.Underground coal gasification not only improves the utilization rate of coal resources,but also reduces environmental pollution and damage,which has obvious economic and social benefits.Due to the complexity and invisibility of the underground coal gasification process,the realization of stable production and commercial development still requires further research and improvement.The dynamic evolution of the coal seam temperature field and the combustion cavity growth are the key to the stability control of the underground coal gasification process.This study selected Mengdong lignite as the experimental coal to study the evolution of the temperature field of coal seam pyrolysis and the combustion cavity growth,in order to provide a data reference for the industrial application of underground coal gasification.The main research contents and conclusions are as follows:(1)Through the analysis of the properties of Mengdong lignite,it was concluded that Mengdong lignite has high volatile content,low nitrogen,and low coal metamorphism.It was a high-quality coal for underground coal gasification.The influence of different heating rate and particle size on coal pyrolysis process was investigated by thermogravimetric analysis.The research results showed that as the heating rate increased,the total weight loss of Mengdong lignite decreased,and the maximum weight loss rate increased.Affected by heat transfer and the diffusion of volatiles,the weight loss of large-particle coal was slightly less than that of small-particle coal.However,when the coal particle size was less than0.2mm,the weight loss rate decreased slightly due to the influence of the inert components of the coal.The phenomenological model was used to study the kinetics of coal pyrolysis.The kinetic parameters truly reflected the process of coal pyrolysis.Under the same pyrolysis conditions,the activation energy in the low temperature section was greater than the activation energy in the high temperature section.The activation energy gradually increased with the increased of the heating rate.Due to the different particle size of coal,the activation energy and pre-exponential factor required for pyrolysis were also different.(2)The dynamic temperature field evolution and heat transfer law of underground coal gasification were studied by heated large pieces of raw coal and artificial coal on one side.The research results showed that in the horizontal section of the coal seam,the heat transfer rate inside the coal seam was higher than that at the edge of the coal seam,and the temperature field is generally distributed in a ring shape.In the vertical section of the coal seam,the heat transfer along the coal seam showed a temperature gradient distribution.The farther away from the heat source,the lower the coal seam temperature.The heat transfer rate of large raw coal was higher than that of artificial coal,but the heat transfer uniformity was poor.The distribution of both sides of the drying temperature line was uneven,and the300?characteristic temperature line was arc-shaped.Due to the existence of cracks in a large piece of raw coal,the heat is mainly carried out in the coal seam by heat conduction,and the convective heat transfer in the coal seam cracks accelerated the heat transfer rate,which was beneficial to improve the heat transfer efficiency of underground coal gasification.(3)A high-temperature heat pipe was used as a heat source to heat the center of the coal pillar to study the dynamic temperature field evolution and the law of the distribution of pyrolysis products during the pyrolysis of the coal pillar.The research results showed that the coal seam near the heat source heated up faster and the temperature was higher.Due to the poor heat transfer performance of the coal pillar,the increase in the distance of the heat source caused the coal seam temperature to drop rapidly.The temperature field of the coal pillar had obvious distribution characteristics in each temperature zone.The temperature in the middle of the coal seam was relatively high,and the two sides were relatively low,and the temperature zone was distributed in an arched shape.The 200°C characteristic temperature line moved faster than the 600°C characteristic temperature line,which makes the area of the dry distillation and drying zone continue to increase.The H₂ content of coal pillar pyrolysis was the highest,followed by CH₄,and the content of CO₂ and CO was relatively low.In the early stage of pyrolysis,the content of CH₄ was higher,and the gas calorific value was higher.With the increase of pyrolysis time,the proportion of CO₂ volume increased,which made the calorific value of coal gas gradually decrease.(4)Through the combustion of artificial coal pillars under different working conditions,the influence of different combustion methods,gasification agent flow rates and oxygen concentration on the combustion cavity growth was investigated.The research results showed that the chemical reaction of forward combustion and reverse combustion were similar,but the physical processes were different.Due to the different gas phase flow types,the geometry and volume of the combustion cavity were quite different.Regardless of whether it was forward combustion or reverse combustion,when the gasification agent flow rate and oxygen concentration increased,the coal consumption per unit time increased,and the volume of the combustion cavity increased.The increase of oxygen concentration was beneficial to the radial growth of the combustion cavity,but when the oxygen concentration was high,the coal ash slagged in the combustion cavity,which affected the law of the combustion cavity growth.