Study On The Process Of Heat And Mass Transfer In Loose Coal With Phase Transition For Liquid Carbon Dioxide

For a long period,the self-ignition was a primal problem which has troubled the safety mining of coal.The key teclhnologies for inhibiting the spontaneous combustion of coal and its correlative disasters included two aspects:cooling and inerting.In recent years,with the fast development of coal chemical industry,considering the production of liquid carbon dioxide has increased greatly and its better cooling,inerting,and explosive-resisted capacity,the liquid carbon dioxide gradually played a more important role in the process of prevention and extinguishment of various coal fires caused by spontaneous combustion.However,the scientific application and effect prediction of liquid carbon dioxide cannot be achieved due to the problems involving phase change,multicomponent transport,and heat transfer in loose coal have not been solved.Accordingly,this paper conducted researches on the process of heat and mass transfer in loose coal with phase change for liquid carbon dioxideAccording to the seepage characteristics of composite superheated and saturated gas after the phase transition of liquid carbon dioxide,the seepage pressure drop characteristics of carbon dioxide was studied under different pressure and flow rate using the experimental apparatus for permeability coefficient of loose coal.As results,the calculated method of permeability and non-Darcy factor based on the porosity and granularity was proposed,thus the nonlinear seepage mechanism of mixed carbon dioxide gas was obtained.The temperature variation characteristics during the process of liquid carbon dioxide injection were experimental studied.The endothermic phase transition region,convective heat transfer region and conductive heat transfer region were classified according to the reduction of coal temperature as well as the transmission regime of liquid carbon dioxide.Thus,effects of initial pressure,out-let diameter,and flow rate on the radius of the temperature-reduction by phase transition were evaluated.Experimental results indicated that the increments of initial pressure,out-let diameter,and flow rate could certainly enlarge the temperature-reduction region with a relative scope.Subsequently,the mathematical model based on the radius of temperature-reduction region versus multi-influence factors was proposed.Finally,the differences in the temperature-reduction regimes and characteristics of liquid nitrogen and liquid carbon dioxide were analysed.Based on the coupling mechanisms of heat and mass transfer with phase transition for liquid carbon dioxide,the method of calculating the physical parameters of the actual state equation of phase transition point heat source coupling was proposed which to simulate the liquid carbon dioxide phase transition and heat and mass transfer process in loose coal.By comparing with the experimental data,the validity of the model and simulation method was verified and the process of carbon dioxide diffusion showed the three stage characteristics of the breakthrough,the rapid growth and the stability,which the pressure and the flow were identified as key factors of the above process.Through numerical analysis,the evolution of temperature,concentration distribution law of the carbon dioxide and oxygen when liquid carbon dioxide was injected to open mined area,was studied.Based on the simulation results,the relationship between the pouring time and the cooling range and the diffusion region of carbon dioxide was obtained.From the perspective of the inerting effect,proposed the the key development strategy for parameter of"position" should be prior to the parameter of "flow",simultaneously for different injection position and flow,proposed the theoretical method of analyzing the inerting effect at different times of pouring.Finally,the liquid carbon dioxide direct injection for extinguishing coal fires through long distance pipeline transportation was developed,which solved the problem of line clogging by dry ice formed by phase change.Subsequently,a successful application had been achieved in the process of extinguishing sealed fire in II020210 long-wall face and prevention of coal self-heating in ?020207 gob for Yangchangwan colliery.