| China is the largest coal consumer and producer in the world.Coal is the major fossil fuel in China.However,the traditional utilizations of coal face with many problems such as environmental pollution.Underground coal gasification technology integrates the three major processes of well construction,coal mining and transformation,and it is a very potential clean coal production technology in the future.However,the gasification process takes place underground,and various phenomena and parameters change with time and space.It is too expensive to conduct laboratory experiments and field trials directly,and it is very difficult to control and monitor the underground gasification process.Therefore,in order to understand the physical and chemical phenomena in the gasification process and the factors affecting them,understand the mechanism of the gasification process,and select the optimal gasification parameters,modeling is particularly important.In this thesis,a mathematical model of underground coal gasification based on computational fluid dynamics is established,and numerical simulation of the oxygen-enriched air and oxygen-enriched CO2 underground coal gasification process is carried out to predict the composition of the syngas,the growth rate of the cavity and other parameters.In this thesis,coal seams are described as porous media with initial porosity and permeability.Three-dimensional transient simulation of underground coal gasification is realized in fluent software by using mass conservation,momentum conservation,component transport equation,combined with uer-defined function.The simulated values are compared with the literature values,and they are in good agreement,which shows that the model is accurate and reliable.Numerical simulation of the underground coal gasification process of oxygen-enriched air and oxygen-enriched CO2 is carried out in this thesis,and the influence of different oxygen concentrations on the composition and heating value of syngas is obtained.The result shows that when the oxygen concentration is 70%,the syngas quality and operating parameters in the two gasification processes reach the optimal gasification conditions,and the result is explained with the contour of temperature and reaction rate.When the oxygen concentration is in the range of 65%to 80%,it is beneficial to the oxygen-enriched CO2 gasification process,while in the remaining oxygen concentration range,it is beneficial to the oxygen-enriched air gasification process.When the gasification process is in a stable stage,the cavity growth rate in the two gasification processes is in the range of 0.55～0.7 m/d,and due to the presence of CO2,the cavity growth rate of the oxygen-enriched CO2 gasification process is faster than that of the oxygen-enriched air gasification process.In addition,the water-gas shift reaction is the main chemical reaction to adjust the content of H2 and CO in the syngas,which determines the ratio of H2 and CO in the syngas.In this thesis,a small-scale three-dimensional transient underground coal gasification model is established,which lays the foundation for expanding the size and coupling various physical processes,and has certain significance for the research progress of underground coal gasification. |
