Influence Of Metamorphism Degree And Pretreatment On Spontaneous Combustion Tendency Of Coal

Spontaneous combustion of coal is a slow and complicated process involving series of physicochemical reactions,which are affected by lots of factors.The nature of coal oxidation and spontaneous combustion is that the active groups in coal react with oxygen to release heat.Aliphatic hydrocarbon side chains and oxygen-containing functional groups are the main participants,and the pore structure has a direct effect on the internal diffusion of oxygen.In this thesis,the wire basket method was used to determine the critical ignition temperature of different coal samples,and the spontaneous combustion characteristics of coal with different metamorphism and pre-treatment Shengli(SL)brown coal were studied by combining CO₂ emission and characteristic temperature in the process.Firstly,four kinds of coals with different metamorphism degree are selected:Zhaotong(ZT)lignite,Shengli(SL)lignite,Mahuangliang(MHL)coal and Malan(ML)coal to investigate the effect of metamorphism on spontaneous combustion tendency of coal.Four coals with different metamorphic levels and hydrothermally upgraded lignite with large differences in pore structure are selected to study the correlation between physical and chemical structure and spontaneous combustion tendency.(1)Critical ignition temperature,apparent activation energy,CO₂ release amount,and characteristic temperature at which CO₂ transitions from slow release to rapid increase can be used as indicators to evaluate the spontaneous combustion tendency of coal.The lower the critical spontaneous ignition temperature and the apparent activation energy,the greater the CO₂ emission,and the smaller the characteristic temperature,result in the smaller the spontaneous combustion tendency.(2)The spontaneous oxidation of coal is a complicated heating process.Compared with the dehydrated pretreated lignite,the lignite without dehydration pretreatment undergoes the stages of moisture endothermic and evaporation at the initial stage during the oxidation heating process.In the initial stage of coal temperature rise,the moisture in the coal body absorbs heat,so that the temperature of the coal pile increases rapidly.As the temperature rises,the heating rate of the coal body slows down significantly,because the energy provided by the outside world and the heat released by its own oxidation are mostly used for water evaporation.(3)As the degree of coalification deepens,the pore structure and active groups show a decreasing trend.For coals with low to medium metamorphism,methylene,oxygen-containing functional groups and pore structure are the dominant factors that affect their tendency to spontaneous combustion.For low-metamorphic coals,the spontaneous combustion tendency is controlled by the number of aliphatic hydrocarbons,especially methylene,and pore structure,followed by the number of oxygen-containing functional groups.(4)The spontaneous combustion tendency of hydrothermally upgraded lignite has obvious segmental correlation between aliphatic hydrocarbons and oxygen-containing functional groups.The hydrothermal upgrading temperature of 230 ^oC is the cut-off point.In the low-temperature upgrading stage(<230 ^oC),the specific surface area is the influencing factor of its spontaneous combustion tendency.In the high-temperature upgrading stage(>230 ^oC),the active groups and specific surface area affect their spontaneous combustion tendency.(5)Under the nitrogen atmosphere,using fixed bed and microwave drying technology,the low-intensity drying temperature or power can increase the spontaneous combustion tendency of the upgraded lignite;at the same upgrading temperature or power,compared with the nitrogen atmosphere,the critical temperature of spontaneous combustion of upgraded lignite obtained by pretreatment under air atmosphere is higher.In the case of no spontaneous combustion,a fixed bed device can be selected to pretreat lignite at 160 ^oC;If the energy consumption of upgrading is considered at the same time,microwave drying technology can be used to obtain upgraded lignite with reduced tendency to spontaneous combustion at 700 W upgrading power.