Experimental Study On Character And Prevention Of Coal Reignition In Closed Fire Area Of Anthracite Mine

Coal is an important energy source in China,which plays an irreplaceable role in national economic development.With the massive exploitation of coal resources,coal spontaneous fires occur frequently.It affects the mine production operation,causes the waste of resources,poses the great threat to the worker's life safety.Due to complex environmental conditions in some mines,the direct fire extinguishing effect is not good,and the method of sealing fire area is often used to extinguish the fire.However,when the fire area is unsealed,reburning accidents often occur,causing casualties and property losses.Therefore,the study of reburning in fire areas has become a major topic that many scholars need to study and solve urgently.In this paper,a self-made physical simulation test platform was used to simulate the extinguishment and re-ignition process of closed fire area in underground coal mine.The change law of the temperature at the center of the fire source is monitored,and it is concluded that the temperature is negatively correlated with time.The cooling process can be divided into two stages,the rapid cooling stage(950?200?)and the slow cooling stage(200?30?).The changing laws of the four groups of coal samples of different quality from 0.25-1.00kg are compared.It can be seen that as the quality of coal samples increases,the cooling rate slows down,and the time required for extinguishment increases.The coal sample with the mass of1.00kg is about 3 hours longer than that of the coal sample with the mass of 0.25kg.The experiment compares the change law of typical gas indicators in the attenuation process of a closed fire zone.It is found that after the fire zone is closed,CO and H₂ have a good correlation with the coal temperature attenuation law.But after considering the sample quality as a variable,the regularity of H₂ is more obvious.Before the coal temperature drops to 200°C,the H₂concentration produced by a 1.00kg coal sample is about 3-9 times that of a 0.25kg coal sample.Obviously,in the initial stage of cooling,the concentration of H₂ released by samples of different quality varies greatly.In the process of coal self-extinguishing,five sets of temperature nodes between 500-100°C during the cooling and attenuation process are selected,and samples are taken for microstructure research.Fourier infrared spectroscopy FTIR showed that the content of representative alcohol phenol ether(C-O)functional groups of the coal sample in the closed fire area is about 73%.And the aromatic ring substitution(C-H)structure was relatively less is about 15%.The peak areas are compared,and both show an upward trend during the process of extinguishing the fire area and cooling down.The content of C-O and C-H structures increases at a rate of about 0.15%/?and 0.025%/?,respectively.It can be seen that the increase of the C-O structure is more significant;The observation of the surface pore structure shows that the pore distribution on the coal surface gradually changes from complex to simple as the temperature decreases.Combined specific surface area and pore size distribution test experiment.It was found that during the process from 500 to 100?,the specific surface area decreased from 23.76 m²/g to 10.52 m²/g,the pore volume decreased from 0.063 m³/g to0.033m³/g,and the micropores decreased from 1.18nm to 1.04nm.The hole increased from6.13nm to 9.29nm.It can be seen that during the quenching and cooling process,the proportions of other pore parameters except mesopores are all decreasing.The pore connectivity of coal samples gradually weakened.Through the EDS energy spectrum experiment,it is found that the changes in the content of C and O elements in the coal sample are reversely correlated.C dropped from 87.56%to 17.35%,and O increased from 7.91%to 52.21%.The ratio of the two also reflects the risk of re-ignition of coal samples.A self-made experimental device with a capacity of 1.5t was used to simulate the re-ignition experiment in the fire area.Select multiple nodes behind the closed fire area for ventilation and oxygen supply.It is found that the critical point of the re-ignition temperature of Shanxi anthracite is between 250-300?.The CO₂ and N₂inert gases used for cooling and extinguishing are compared.The cooling rates of the two are about 1.22?/min and 6.01?/min,respectively.Obviously,the CO₂ cooling rate is faster.According to the observation results of infrared thermal imaging,the fire spread from the center to the surroundings.After closing the simulated fire area,the coal continued to spread in the form of smoldering,and the temperature was maintained at about 900-1000°C,which tended to be on the air supply side.Through TG-DTG experiment comparison,it is found that the maximum weight loss rate of coal sample corresponding to 500?is 0.58%/min.After the temperature decays,the weight loss rate of the sample decreases successively.Two models,Coats-Redfern and Achar,are used for kinetic calculations.The reaction mechanism of the coal sample conforms to the three-dimensional diffusion Jander equation(n=2).When the temperature of the fire zone drops from 500°C to100°C,the activation energy value increased from 99.56 k J/mol to 244.66 k J/mol.It shows that the difficulty of reaction increases and the risk of re-ignition decreases.Three fire extinguishing materials:gel,aerosol and heptafluoropropane are used to carry out fire extinguishing experiments.It was found that the temperature dropped from 980°C to room temperature,the aerosol fire extinguishing only takes 150 min,the heptafluoropropane fire extinguisher takes about 200 min,and the gel is somewhere in between.The initial cooling effect of gel and aerosol is obvious.It only took 3-5min for the surface temperature to drop from 680?to 200?.It is inferred that the solid and liquid extinguishing materials can be evenly distributed for local fires.The temperature drops quickly and can effectively prevent re-ignition.The gas fire extinguishing agent is easy to disperse when the space is relatively open.It is not suitable to extinguish fire,and the effect of preventing re-ignition is poor.A small experimental device is used as a physical model.According to the theory of heat and mass transfer,the energy balance equation is derived.A mathematical model of the temperature field is established.And the time required for the fire zone to be safely unsealed is deduced,that is,t=Cm·ln[T₁-T_?)/(T₂-T_?)].The model calculation result is compared with the temperature change experiment result.The model calculation results fit well with the experimental results,which can provide a theoretical reference for the prediction of mine fire zone unsealing.