Experimental and Numerical Simulation Study of CO2-Oxy Underground Coal Gasification Process

Author:Liu Hong Tao

Supervisor:yao hong


Degree Year:2019





Underground coal gasification technology is a clean,safe and efficient energy utilization technology.After years of development,the technology has made considerable progress and development.However,the traditional oxygen-enriched air gasification process and oxygenenriched steam gasification process still have some problems,such as nitrogen content of gas,difficulty in water vapor transportation to coal surface,simple process,poor gas quality,unstable operation,low level of process control,unclear understanding of distribution state and expansion law of gasification reaction area.CO2-Oxy underground coal gasification technology is a technology with great potential.It is not only conducive to overcome the problems of nitrogen content in gas and water vapor transportation to gasification surface in traditional gasification process,but also has great value in carbon capture and storage.This thesis systematically studied the channel reaction state,long-term stability and characteristics of the cavity in CO2-Oxy underground coal gasification process by means of experiments and numerical simulation,so as to guide the method of industrial process control.Based on the mechanism analysis of CO2-Oxy underground coal gasification process,the reaction characteristics of CO2-Oxy underground coal gasification under different oxygen concentration were studied by using model test method.The results showed that when the oxygen concentration is lower than 60%,CO in the gas continues to increase with the increase of oxygen concentration,while when the oxygen concentration is higher than 60%,the volume fraction of CO in the gas component decreases.This shows that the proportion of CO2 reduction reaction in coal seam increases with the increase of CO2 volume fraction,which affects the temperature field and the formation of reaction products.Carbon emission reduction caused by CO2 recycling in CO2-Oxy underground coal gasification process was analyzed by material balance method.The results show that with the increase of oxygen concentration,carbon emission reduction in CO2-Oxy underground coal gasification process first increases and then decreases.The most carbon emission reduction is about 0.04 kg/kgce under the condition of about 50% oxygen concentration.Moreover,the long-term stability characteristics of CO2-Oxy underground coal gasification process at 60% oxygen concentration were studied by model test.The results show that there is a “dessert” gasification stage with higher gas composition and higher gas flow rate in the gasification process,which is about 21.43% and 50.80% of the total coal consumption from a single working face.The average cold gas efficiency in this stage is 72.23%,accounting for 38.15% of the total reaction time in the whole life cycle of a single gasifier.The characteristics of the variation of the cavity parameters under different conditions were studied with different oxygen concentration in the thesis.For the first time,the threedimensional shapes of the cavity under different oxygen concentration,different flow velocity and different retrogression distance were obtained.The expanding speed of the cavity affected by the retrogression distance was compared under 40%,60% and 80% oxygen concentration.The results show that the cavity expands fastest when the oxygen-enriched concentration is 60% and the retreat distance is 15 cm,and the coal seam utilization efficiency is high.Under the experimental conditions,high oxygen concentration(80%)leads to coking in the cavity,and the expansion of the cavity is affected by coking.The polar coordinate equation of the shape trajectory of the cavity is established by comparing the angle the bottom area of cavities,and the calculation method and the value table of the expanding of the cavities are given,which can provide a scientific reference for industrial control of the cavity.In order to further verify the experimental results,a two-dimensional mathematical model based on the channel gasification model was established by analyzing the process characteristics of underground coal gasification,and the axial and radial gasification mechanisms of CO2-Oxy underground coal gasification process were studied.By computer simulation,the calculation results of molar(volume)composition of outlet gas,radial and axial temperature distribution,thickness changes of radial reaction zone,oxygen concentration distribution with time and initial conditions are obtained,and compared with the experimental results.It was verified that the initial component of the reaction increased during the single gasification cycle,and then decreased after reaching a certain value,which was close to the “dessert” stage phenomenon in the previous experiment results.With the increase of reaction time,the inner and outer boundaries of coal seams expand radially,and the thickness of coal seams involved in the reaction increases(about 0.2 meters).The radial expansion of coal seam mainly occurs in high temperature zone,which is consistent with the change trend of temperature field.Along the axis of the gasification channel,the temperature decreases slowly over a long distance,which provides conditions for the reaction of the gas in the output process.When oxygen enters the gasification channel,some react with coal and some react with combustible gas.The larger the oxygen fraction in gasifying agent,the shorter the oxygen distribution along the axis channel,that is,the shorter the length of the oxidation zone.When the oxygen fraction in gasifying agent is 60%,oxygen is consumed within 18 meters of the axial gasification channel;when the oxygen fraction in gasifying agent is 80%,it is consumed within 15 meters.The simulation results are consistent with the phenomena of laboratory test and field test,which indicates that the model and solution method based on the above analysis are reliable and hopeful to provide technical support for CO2-Oxy underground coal gasification process in industrial environment.