Study On Gas Mass Transfer And Reaction In Ash Layer Of Underground Coal Gasification

Coal gasification is a key process in many technologies for clean and effective utilization of coal.It is particularly important to China due to its coal based energy structure.Different from the traditional coal gasification technologies,the underground coal gasification takes place in coal bed and is operated by introducing gasification agents to predrilled channels to support the in-situ gasification reactions for syngas production while keeping the ash remains underground.However,after more than hundred years study and testing no large-scale underground coal gasification plant has been successively operated for a commercially meaningful time.Most of the research and developments(R&D)as well as the field testing were focused mainly on the manipulating of operating conditions with limited ability to understand the mechanism of mass transfer and reaction.The computer simulation models established therefore are lacking sufficient details to reveal the black box of underground gasification and the key parameters used for the transport and reaction processes are not sufficiently sound.It is recognized by many that the ash layer remained on the coal surface(more accurately on the char surface)during gasification has a great influence on the mass transfer and reaction of gasification agent even when the ash layer is only a few millimeters in thickness.This ash layer effect is generally small in traditional coal gasification technologies due to the continued ash discharging,but is large in underground coal gasification because the presence of thick ash layer all the time.Researches in porous medium materials showed that the diffusion behavior of gas in pores,similar to that in the ash layer in coal gasification,is remarkably different from that in the bulk gas phase,which alters the mass transfer behavior and product distribution.Besides,syngas produced in gasification reactions at the char surface may react with O2 in a high temperature ash layer which also influences the mass transfer behavior and product distribution.These phenomena,however,have not been well addressed with sufficient quantification in the literature.Under these circumstances,this thesis studies the behaviors of mass transfer and reaction of gases,including the gasification agent and the gasification product,in the ash layer quantitatively via specially designed experimental setups at the lab scale(tube furnace and TGA)and flow field simulation by CFD.The detailed studies include the followings:(1)CFD simulation of mass transfer of gas in millimeter-scale ash layer.It is found that the countercurrent flow of CO,produced at and diffused away from the char surface,and the gasification agent O2,diffused from the bulk gas to the char surface,in the ash layer leads to combustion of CO to CO2 and an obvious temperature increase in ash.The combustion zone shifts away from the char surface with increasing CO2 and H2O contents in gasification agent.(2)Experimental evidence on syngas combustion in the ash layer.One-dimensional gasification experiments are designed,based on a fixed-bed tubular reactor and a thermal balance(TGA),to investigate the morphology and composition of ash layer formed when O2 is the gasification agent.It is observed that the ash of Luxian coal with a deformation temperature of higher than 1733 K melts at the nominal reaction temperature of 1573 K.This difference in temperature clearly indicates combustion of CO to form CO2 in the ash layer,which raises the local temperature in the ash layer leading to local ash melting.The product distribution and selectivity change consequently.The results show that CO is mainly generated by the reaction of CO2 and C,and the CO content in the product decreases with an increase in ash layer thickness.(3)The main mass transfer behaviors of gases.The experimental data obtained involve complex interaction of external diffusion in the bulk gas,internal diffusion in the ash layer and the surface reaction.These individual steps are decoupled and a method determining the effective diffusion coefficient of O2 and CO2 is established.It shows that the pore diameter of ash layer and the effective diffusion coefficient of gas increase over time.The combustion of CO in the ash layer enhances the mass transfer of O2 and thus the effective diffusion coefficient of O2.The effective diffusion coefficient of O2 is larger than that of CO2 under the same conditions.(4)Establishment of a method quantifying the effective diffusion coefficients of gases in none-melting ash layer based on the thickness of ash layer and temperature.It shows that the underground coal gasification rate is controlled mainly by the diffusion resistance in the ash layer.A simple version of the method ignores the external diffusion in the bulk gas is still sufficiently accurate.The contribution of interal resistance to overall resistance is higher than 65%for O-2 and higher than 85%for CO2 when the ash layer is thicker than 3 mm.(5)Establishment of a percolation channel model for CFD simulation of underground coal gasification.The model quantitatively correlates the thickness of ash layer,the feeding rate of gasification agent,the effective diffusion coefficients of the gases and the surface reaction.