Kinetic Reaction Mechanism And Extinguishing Conditions Of Smoldering Coal In Sealed Fire Area Of Coal Mine

The rapid recovery of fire in closed fire area after opening is a general problem in most mining area of China,leading to re-closed and even gas explosion,causing major casualties.In the oxygen-poor conditions of closed fire area,the coal pile could maintain smoldering.Due to the low combustion temperature and small gas production rate of smoldering coal,it is difficult to be found out.Smoldering coal pile exists in the fire area which was judged to have been completely extinguished.For the hidden fire,at present,unremitting efforts and exploration has been put on the coal spontaneous combustion mechanism and prevention means.However,coal spontaneous combustion is only the cause of the smoldering coal piles,there is a lack of systematic study on the combustion mechanism and characteristics of the smoldering coal piles,which hinders the research of high efficiency fire fighting technology.Theoretical analysis,laboratory experiments and numerical simulation were used in this paper,and the dynamic reaction mechanism and the extinguishing conditions of the smoldering coal was systematically studied.The main results and innovations are as follows:The evolution characteristics of the pyrolysis and oxidation reaction of the smoldering coal in the different oxygen concentration were studied.The difference of solid-phase carbonization and liquid-phase carbonization of different coal samples in inert atmosphere was analyzed,and the "1+2" and "1+3" step reaction models of coal pyrolysis were constructed.A Newtonian kinematic analysis method for studying the thermal behavior of coal was proposed,which solves the problem that the development characteristics of coal under high oxygen concentration can not be analyzed based on thermogravimetric characteristics.The results show that the oxidation heat intensity of the smoldering coal in the low temperature region is greater than the pyrolysis endotherm,and in the high temperature region it is the pyrolysis control interval.According to the characteristics of the thermal decomposition process of the smoldering coal,based on the differences in the physical and chemical properties of the coke,the "3+3","3+4" and "3+5" steps kinetic reaction model of the smoldering coal are constructed.The kinetic reaction mechanism of the smoldering coal in different oxygen concentration was studied.The Kissinger-Genetic Algorithm method was used to calculate the kinetic parameters of the pyrolysis and oxidation reaction of the smoldering coal,which solved the problem that the traditional kinetic analysis method could not separate the sub-reactions.The results show that only the kinetic parameters under the condition of low oxygen concentration can accurately reflect the stage evolution characteristics of the smoldering reaction of coal.The optimal kinetic reaction mechanism of the smoldering coal is composed of 3-step pyrolysis and 5-step oxidation reaction.It contains three reaction sequences: coal oxidation,semi-coke pyrolysis and semi-coke oxidation.When the thermal behavior of the sub-reaction is neglected,the coal oxidation sequence is the main reaction in the whole oxygen scale(1-21%).The proportion of the semi-coke oxidation and the half-coke pyrolysis sequence gradually increase when the oxygen concentration is less than 10% and 5% respectively.The dynamic development characteristics and control mechanism of self-sustaining smoldering coal were studied.A thermo-flow-chemical coupled one-dimensional transient mathematical model for the dynamic evolution of the smoldering coal was established,and the dynamic development process of horizontal,vertical,forward and reverse smoldering coal piles and the physical and chemical composition of smoldering front was analyzed.The results show that the reverse smoldering combustion temperature is higher than the forward smoldering.There is a secondary drying process in the forward smoldering,which increases the pyrolysis endotherm.In the forward smoldering the pyrolysis and oxidation front exist at the same time,but there is only the oxidation front in the reverse phase.The vertical smoldering combustion temperature is higher than the horizontal smoldering.The natural convection is formed in the smolder coal piles driven by the density difference between the hot flue gas in the furnace and the outside gas of low temperature.And the mass flow rate of the natural convection in vertical smoldering is much larger than the horizontal smoldering.The critical flow rate and oxygen concentration for extinguishing smoldering coal pile were studied.The dynamic development process of the smoldering coal under different oxygen-deficient conditions was tested and simulated.The variation law of the sub-reaction with the oxygen concentration and its influencing factors were analyzed.The results show that the critical air supply of the smoldering coal(the long flame coal sample)with temperature 500?,porosity 0.54 and coke layer thickness 10 mm is 2-3g/(m~2·s)in the adiabatic environment.When only the pyrolysis endotherm is taken into account,the critical oxygen concentration(volume percentage)is 1-2% while the temperature of lignite and long flame coal samples is greater than 280 ° C and 200 ° C,respectively.The influence of buoyancy can not be neglected,and the mass flow of natural convection is more than 45% of the critical air supply.The thicker the coke layer is,the lower the critical oxygen supply condition is,and the effect of the coke layer size increases with the decrease of oxygen concentration.The higher the moisture content,the lower the intensity of the smoldering,and the critical moisture content of the long flame coal is 40-50% when the forced air supply is 5 g/(m~2·s).Under different oxygen-deficient conditions,the reaction rate of semi-coke pyrolysis sequence is always more than a thousand times of that of coal oxidation and semi-coke oxidation,and it is a key reaction sequence to control the dynamic development of smoldering coal.