| Conveyor belt fire is associated with random locations of occurrence,rapid spread,highly toxic and high-temperature smoke,and difficulty in emergency rescue,thus severely threatening people’s life and health and the safe operation of mines.Thus,this paper aimed to investigate the disaster-causing factors of conveyor belt fires,divide the types of conveyor belt fires and study the pyrolysis characteristics of different conveyor belts and co-combustion characteristics of coal and conveyor belts.This paper also analyses the gas escape pattern of conveyor belt fire,illustrates heat transfer between coal and conveyor belt during co-combustion,and examines the characteristics of the disaster regularity of conveyor belt fire under a ventilation network.Based on the analysis of previous research on conveyor belt fire,this paper considered lignite,plastic whole-core flame retardant conveyor belts(PVC),rubber whole-core flame retardant conveyor belts(PVG),steel wire core conveyor belts(SC)and non-flame retardant conveyor belts(NFR)as research objects.First,conveyor belt fire is divided into two types:pyrolysis of conveyor belts and co-combustion of coal and conveyor belts.Pyrolysis of conveyor belts is the first stage of conveyor belt fire that continues till completion when the conveyor belts have no surrounding coal.When there is coal surrounding conveyor belts,the conveyor belts are ignited immediately when coal is ignited upon reaching its ignition point,thus resulting in the co-combustion of coal and conveyor belts.Second,regarding conveyor belt fire under the condition of pyrolysis of conveyor belts,the microstructure of the conveyor belts is tested,and the pyrolysis characteristics and pyrolysis gas of conveyor belts are studied and analysed.Mesoporous pore volume and internal loose and porous structure play a positive role in the pyrolysis of conveyor belts.PVC,PVG and SC exhibit two weight-loss peaks at 280℃-300℃ and 420℃-460℃.The weight loss in the first zone is due to the break of the C-Cl key,evolving a large amount of HCl gas,with the remaining skeleton forming conjugated polyene.The weight loss in the second zone is due to the break of the C-C key,resulting in polyene fragments.NFR exhibits only one weight-loss peak at 422.12℃ that is located at the dividing point between the NFR pyrolysis crosslinking condensation reaction and depolymerisation reaction.Regarding the pyrolysis gas of conveyor belts,the existence of the Cl element is the biggest difference between conveyor belts and coal.HCl gas escapes from the conveyor belts at a temperature of around 170℃,reaching the maximum escape amount at a temperature of 270℃-290℃,while the maximum escape of CO and CO2 comes after the temperature of 460℃.Therefore,HCl gas is considered a significant factor in conveyor belt fire.Third,for the conveyor belt fire under co-combustion of coal and conveyor belts,the co-combustion characteristics are studied,the heat transfer process is revealed,the impact of different heating rates on the combustion performance is explored,and the change pattern of combustion kinetics parameters is analysed.The comprehensive effect of both coal and conveyor belts causes co-combustion,with coal combustion taking the dominant role in the preliminary phase and combustion of conveyor belts taking the dominant role in the later phase.At the beginning and end of the co-combustion,coal transfers heat to conveyor belts.In the middle of the co-combustion,conveyor belts transfer heat to coal.Ultimately,when the temperature of the coal and the conveyor belts is the same,the heat transfer stops and co-combustion is completed.With the increase in the heating rate,the combustion TG-DTG curve moves towards the high-temperature area,increasing the maximum mass loss rate.The activating energy of coal takes the leading role in the phase of low conversion rate,while the activating energy of conveyor belts takes the leading role in the phase of high conversion rate.Finally,the characteristics of the disaster regularity of conveyor belt fire under a ventilation network are mastered.Considering the major fire accident at Songzao Coal Mine,the change in wind flow and air volume near the disaster source area,the working face and the main air inlet and outlet shafts are given.As indicated,the time interval is less than 3 minutes from the beginning of 1 ppm CO gas found in 2324-1 working face to the increase in its concentration to 1500 ppm,where the maximum CO concentration was more than 1800 ppm.Meanwhile,HCl gas arising out of the fire may have caused injuries and death at a possible earlier time.Thus,the paper concludes that bidirectional ventilation doors can play a significantly positive role in mitigating the spread of conveyor belt fire accidents. |
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