Abstract
Low-rank coal contains abundant hydroxyl, carboxyl and alkyl side chains, and reactions related to these groups are the main reason for the spontaneous combustion of low-rank coal. Here, two different low-rank coals (BRXL, YJL52) are selected. Firstly, the ignition temperatures of the coals are determined by thermogravimetric method. Secondly, the coals are heated at 100°C temperature intervals before the ignition temperature in the thermogravimetry, and infrared measurement is performed to explore the changes of these groups. Combining previous studies in the literatures with infrared analysis, it is found that reactions related are as follows: phenolic hydroxyl converting into alcoholic hydroxyl, alcoholic hydroxyl further oxidizing to carboxyl, and carboxyl decarboxylating into alkyl side chains. After that, the changes of phenolic hydroxyl and carboxyl on the surface of the coal at 100°C temperature intervals are determined by titration, which further reveal the main reactions occurred in every temperature interval. Additionally, the actual heat release in different temperature ranges is discussed with the reaction enthalpies of the above-mentioned main reactions. As a result, the key temperature stage that causes spontaneous combustion is found. The screening study in this paper on the reaction of low-rank coal before spontaneous combustion provides a theoretical basis for the control of spontaneous combustion of low-rank coal.
Highlights
In-depth study of the spontaneous combustion mechanism of low-rank coal can effectively prevent and control its spontaneous combustion [1]
3.2.The change of alkyl side chain, hydroxyl and carboxyl As shown in Fig. 2(a), at the stage of 20-100 °C, three functional groups of YJL52 slightly decrease in nitrogen atmosphere and air atmosphere
The changes of hydroxyl, carboxyl, and alkyl side chains at different temperature stages before ignition of two coals are studied by IR
Summary
In-depth study of the spontaneous combustion mechanism of low-rank coal can effectively prevent and control its spontaneous combustion [1]. The indicator gas method [4] revealed the oxygen reaction of coal before spontaneous combustion by analyzing the gas emitted during low-temperature oxidation of coal. These studies only speculated on the causes of spontaneous combustion of low-rank coal from macroscopic phenomena and could not explain the mechanism of spontaneous combustion from a microscopic perspective. Wang [5] conducted adiabatic oxidation experiments to study the conversion of functional groups in coal samples of different ranks, and found that in the slow low-temperature accumulation process, alkyl side chains combine with oxygen atoms to form unstable intermediates, which decomposed into gaseous products and stable solid complexes. The heat release of low-rank coal before spontaneous combustion and the reaction enthalpy changes of the reactions are correlated to find the key temperature stage of sufficient heat storage which causes the spontaneous combustion of low-rank coal
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