| Coalbed methane is one of the major disaster sources in the field of coal mine,it is also a greenhouse gas that is harmful to the environment.But if it can be puted in proper use,it is an efficient and clean resource.Coal seam in China has the characteristic of high gas and low permeability,so it has great significance to discuss the effect of pore structure modification on degassing coalbed methane.In this paper,based on the engineering background of degassing coalbed methane by hot steam heating,thermal response characteristics,gas desorption characteristics and pore structure development of coal during hot steam heating was studied by laboratory experiments,numerical simulation and theoretical analysis.Besides,the engineering application of this technology is prospected.Several experiments were carried out and achieved some results as following:A hot steam heating system was designed and set up to do experiments.Based on the experimental system,thermal response characteristics of coal during hot steam heating was studied.The coal shows the characteristics of stage,similarity,fluctuation,inhomogeneity,hysteresis and attenuation.In comparison,anthracite has better thermal conductivity,and hot steam has better heating effect on anthracite.The influence of temperature on coal gas adsorption was studied.It is found that the adsorption of coal gas can be significantly reduced by increasing the temperature.At the same time,the effect of water molecules on the desorption behavior of methane molecules was studied,it is found that when water molecules coexist with methane molecules,water molecules are more likely to adsorb on the surface of higher-order coal than methane molecules,when methane molecules are adsorbed on the surface of coal,the external water molecules can promote the desorption of methane molecules.A coal gas adsorption and desorption experiment system during hot steam heating was designed and set up to do experiments.The cumulative desorption amount and desorption rate of coal gas were measured.Under the conditions of different gas pressure and axial confining pressure,when there is hot steam,the cumulative desorption amount and desorption rate of the two coal samples increase obviously.Under the condition of lower gas pressure and external axial confining pressure,the promoting effect of hot steam on coal gas desorption is more obvious.In contrast,the promotion effect of hot steam on the gas desorption of anthracite is better.Based on the characteristic values,the macroscopic cracks of coal are analyzed qualitatively and quantitatively.It is found that the macroscopic fracture of coal body develops and expands obviously after the hot steam heating,especially in the place where the hot steam directly impacts.Micropores and fractures of coal after hot steam were studied,it is found that the role of hot steam can effectively promote the fracture and penetration of pores in the coal body,and help to transform the closed pores into open or semi-open pores in the coal body.It can significantly reduce the proportion of micro pores and small pores,and increase the proportion of medium pores and large pores in the coal body,which also promotes the development and expansion of micro fractures in the coal body to a certain extent.Based on the previous conclusions and the research in this paper,the engineering application assumption of degassing CBM by hot steam heating is put forward.The prevention and control methods of coal spontaneous combustion disaster caused by the application of this technology are discussed,several key factors controlling coal spontaneous combustion are described. |
