Experimental Study On Main Controlling Factors And Thermal Resistance Prevention Of Underground Coal Smoldering Propagation

In view of the increasing risk of smoldering fires in coal mines,in order to deeply study the main control factors of coal smoldering propagation and the effect of thermal resistance to prevent smoldering.In this paper,Yunnan lignite（YNL）,Shanxi bituminous coal（SXB）and Shanxi anthracite（SXA）are taken as experimental coal samples,and the effect of particle size,ventilation rate,moisture content and filling thermal resistance materials on the forward smoldering spread characteristics are systematically studied through synchronous thermal analysis,microscopic test of specific surface area and pore analyzer and physical simulation experiment.The experimental results of particle size show that the reduction of particle size changes the pore structure of coal samples,facilitates both diffusion and mass transfer during the combustion process.The increase of these characteristics is responsible for the strong smoldering capability of coal.There are five distinct combustion stages for the different coal samples in the temperature range between 25.0 and 800.0℃.In addition,the coal-oxygen composite reaction is consistent with a first-order chemical reaction.With the decrease of particle size,the characteristic temperature point of coal samples decreased by 13.6-84.8℃,and the apparent activation energy decreased by 5.7-28.9%in each stage.The critical minimum air-supply for smoldering coal is 0.13 m³·h^-1·kg^-1 to 0.20 m³·h^-1·kg^-1 The critical quantity increases with increasing particle size.The maximum smoldering temperatures peak at 608.0℃to 696.0℃for SXB,at 543.1℃to 675℃for SXA,and at 427.0℃to 489.7℃for YNL.The average smoldering spread rates for each stage can be ranked as follows:average moisture-evaporation-front spreading rate>average pyrolysis-front spreading rate>average char-oxidation-front spreading rate>average peak-temperature-front spreading rate.The smolder reaction occurs in the following sequence:The moisture evaporation occurs first,which is followed by the pyrolysis reaction and the char-oxidation reaction.Then,the temperature gradually decreases after reaching its maximum.Key parameters such as average peak smoldering-temperature,average peak smoldering-temperature rise rate,spread rate of the average moisture-evaporation front,average pyrolysis-front spreading rate,average char-oxidation-front spreading rate,average peak-temperature front,and the risk of smoldering propagation also increase with decreasing particle size.The mathematical model of smoldering propagation is established,the expression describing the forward smoldering propagation rate is derived,and it is verified that the reciprocal of the smoldering propagation rate is proportional to the porosity of the material.With increasing ventilation rates,both the characteristic temperature and the activation energy of the studied coal at each stage decrease first before they increase.The heat release,however,occurs in reverse order（all samples reach a turning point at 75.0 m L/min）.The critical ventilation range for smoldering propagation of coal is 0.18-0.27 m³·h^-1·kg^-1.With increasing ventilation rates,key parameters and the risk of smoldering propagation increase before they decrease,reaching their maxima at 22.5 L/min.The critical air-flow rate（0.01242 m/s）was calculated using a theoretical air-volume calculation.The smoldering spread rate is proportional to the ventilation rate when it is below the critical air-supply.It is also inversely proportional to the ventilation rate when it exceeds a critical parameter,which was verified for different ventilation conditions of SXA coal.With increasing moisture content,the end temperature of moisture evaporation and ignition temperature of coal samples increased by 79.4℃and 16.9℃respectively,and the activation energy of pyrolysis stage and char oxidation stage increased by 33.9 k J/mol and 27.0k J/mol respectively,and the total heat release decreased by 4200.3 J/g.The critical moisture content range of coal sample smoldering extinguish is 29%-33%.Key parameters and the risk of smoldering propagation decrease with increasing moisture content（1%to 33%）.It is verified that the reciprocal of smoldering spread rate is proportional to the moisture content.Compared with smoldering of raw coal,the temperature of burnt zone after filling with water increases by 4.7-109.0℃and can promote smoldering spread of coal samples,while the temperature of burnt zone after filling with other thermal resistance materials decreases by 30.7-422.1℃under the comprehensive effects of oxygen insulation,temperature reduction and heat absorption,which makes smoldering difficult to maintain spread or even extinguish.With the backward propagation of smoldering,the smoldering peak temperature and the smoldering peak heating rate show a downward trend,with the smoldering peak temperature decreasing by 13.2-70.6%and smoldering peak heating rate decreasing by 71.2-88.7%.The spread rate of fil water at that rear position increase by 0.3-1.1 cm/h,while the spreading rate decreased by 0.1-12.9cm/h by filling other thermal resistance material.The evaluation results of reducing temperature effect,reducing heating rate effect,reducing smoldering spread rate effect after filling thermal resistance materials and defining weight and calculating by entropy method are in the following order:gel>metal mesh>inhibitor>sand>rock powder>refractory soil>mud>water.