Experimental Study On The Subsection Characteristic And Mechanism Of Spontaneous Combustion Of Coal

Spontaneous combustion of coal is a main natural disaster during exploitation,storing, and transportation of coal. Present study has revealed that there is subsectioncharacteristics during self-heating process of coal, but the mechanism of thischaracteristic is unknown. Since subsection characteristics of coal is very important inguiding the prevention and control of spontaneous combustion of coal, thisdissertation focuses on the mechanism of subsection characteristics by studying thetemperature rising, heat release, oxygen consumption, gas generation, variation offunctional groups, heat effect of reactions, activation energy characteristics.Adiabatic oxidation testing method was used to study the temperature risingcharacteristics during self-heating process of coal. Approximate critical temperatureof the two stages was obtained by using linear fitting method and the temperaturerising characteristics in the two respective stages and their differences were alsostudied. It was found that slow temperature rising stage of all coal samples lasts quitea long time which account for90%of total time of the whole self-heating process.However, when coal is in the rapid temperature rising stage, the temperature of thecoal rises quite quickly and reaches200℃in a short time. Besides, for different coalsamples, the coal with higher spontaneous combustion tendency experiences shortslow temperature rising stage. All of these experiments results show that there is agreat difference of the reaction characteristics in the two stages respectively, and theslow temperature rising stage is important in determining the spontaneous combustiontendency of coal.It was found that there is subsection characteristics in the heat release of coal bystudying heat release characteristics. When coal temperature is low, the heat releaseamount is quite little. The total heat release in the slow temperature rising stage is lessthan20%the heat release in the whole oxidation process. It can be concluded that thesubsection characteristics of coal is mainly caused by great difference of quantity ofheat release in the two stages. Hence, to find the mechanism of subsectioncharacteristics, the reaction characteristics of functional groups should be studied.Furthermore, it was found that the coal with higher spontaneous combustion tendencyreleased more heat during oxidation process which made the rising the temperaturemore quickly and the functional groups in coal more easy to be activated.To find the mechanism of the subsection characteristics and different coal has different spontaneous combustion tendency, in-situ FTIR was adopted to study thedistribution and variation of functional groups in coal and quantum chemistrycalculation method and Differential Scanning Calorimetry method were used tocalculate the enthalpy of reactions and activation energy respectively. It was foundthat there are less aromatic, less aliphatic hydrocarbons, and more oxygen-containingfunctional groups in coal with higher spontaneous combustion tendency. During thereaction process of coal,-CH₂-/-CH₃/-OH/-COOH/C=O are the main functionalgroups reacts. All these functional groups decrease at the beginning of the coaloxidation process, then increase with the rising of the temperature. And the coal withhigher spontaneous combustion tendency, the decrease of the functional groups'amount is larger. It also was found that oxidation of-CH₃/-CH₂-and C=O can releasegreat heat which is about400⁶00kJ/mol while the heat released from thedecomposition reactions of–OH/-COOH and C=O is quite little. Besides, theactivation energy increases with the rising of the temperature of the coal which meanscertain reactions can occur only in certain temperature.As the process of coal reaction accompanies with oxygen consumption and gasgeneration, the amount of oxygen consumption and gas generation was tested. It wasfound that during the initial stage of coal oxidation, the oxygen consumption is quitefew which infer that oxidation is not the main reaction occurred in this stage. CO andCO₂are the main gas and as the coal temperature rises, hydrocarbons start to bereleased. The amount of the gas is larger in the same temperature in the coal withhigher spontaneous combustion tendency. By comprehensive analysing the oxygenconsumption, gas generation characteristics, variation of functional groups and theheat effect of different kinds of reactions in coal, it can be concluded that in the lowtemperature rising stage, the reactions are mainly–COOH producing CO₂, C=Oproducing CO and-OH producing H₂O. The heat release of all these reactions is quitefew which makes the heat is difficult to generate in the low temperature rising stage.In the rapid temperature rising stage,-CH₃/-CH₂-/C=O begins to oxidation andreleases a lot of heat. The difference in the spontaneous combustibility of differentcoal samples is due to the difference of the distribution of functional groups in coal,the coal contains more oxygen-containing functional groups has a higher spontaneouscombustion tendency. The research results are important in improving the efficient ofthe prevention and controlling of spontaneous combustion of coal.