煤自燃初期低温氧化产热特性及关键结构演变特征

Characteristics of heat production and key structure evolution in the initial stage of coal spontaneous combustion

  • 摘要: 煤自燃初期氧化产热是导致煤自燃的根源,也是衡量煤自燃危险性最直接的评价指标。为了测定煤在自燃初期消耗单位氧气产生的氧化热,创新性地通过六通阀与定量环组合,精确控制向煤样罐中通入的空气量,并基于煤氧化热理论构建计算模型,测定煤在自燃初期不同条件下的单位耗氧氧化热。结果表明:在50 ℃恒温氧化条件下,LJ、WD、ZZ、XT 4种煤样消耗单位氧气产生的氧化热分别为66.94、40.76、42.82、23.31 kJ/mol。这一发现表明,煤自燃初期产生的氧化热远低于数值模拟中常用的氧化热数值300 kJ/mol,且变质程度最低的LJ煤氧化放热量最大,4种煤样都在氧化时间为40 s左右时达到了最大产热速率。为了分析自燃初期不同煤样的产热量呈现差异性的原因,通过低温氮气吸附试验测试煤样的孔隙结构,结果发现煤的孔隙结构不是决定煤氧化放热量差异的主导性因素,孔隙结构更发育只是有效促进了煤氧复合反应。而电子顺磁共振试验结果表明煤自燃初期氧化产热是由反应位点控制的动力学行为,煤样原生反应位点的数量是决定产热量的主导因素。初始氧化温度的升高使得煤中初始反应位点增多,也使得煤产生反应位点的速率加快。反应位点的本质为自由基,自由基浓度的增加速率决定了煤氧化反应的速率。

     

    Abstract: The heat production during the initial stage of coal spontaneous combustion is the root cause of coal spontaneous ignition and serves as the most direct indicator for assessing the risk of coal spontaneous combustion. To determine the oxidation heat produced per unit of oxygen consumed by coal in the initial stage of spontaneous combustion., an innovative method was employed, utilizing a six-port valve combined with a quantitative loop to precisely control the amount of air introduced into the coal sample tank Based on the theory of coal oxidation heat, a calculation model was constructed, enabling the successful determination of the unit oxygen consumption oxidation heat of coal under different latent period conditions. Experimental results revealed that under constant temperature oxidation at 50℃, the oxidation heat generated by the four coal samples, namely LJ, WD, ZZ, and XT, was 66.94 kJ/mol 40.76 kJ/mol, 42.82 kJ/mol, and 23.31 kJ/mol, respectively. This discovery indicates that the oxidation heat produced during the initial stage of coal spontaneous combustion is significantly lower than the commonly used oxidation heat value of 300 kJ/mol in numerical simulations, and that LJ coal, with the lowest degree of metamorphism, produced the highest heat. Four coal samples all reached the maximum heat production rate when the oxidation time proceeded to about 40 s. To analyze the reasons for the differences in heat production under various temperature conditions during the initial stage of coal spontaneous combustion, the pore structure of the coal samples was tested through low-temperature nitrogen adsorption experiments. The results found that the pore structure of coal is not the dominant factor determining the differences in coal oxidation heat release; rather, a more developed pore structure merely effectively promotes the coal-oxygen complex reaction. Electron paramagnetic resonance (EPR) experimental results demonstrate that the oxidation heat production during the initial stage of coal spontaneous combustion is a kinetic behavior controlled by reaction sites. An increase in temperature leads to an increase in the number of primary reaction sites in coal and accelerates the rate of secondary reaction site generation during the coal oxidation process. The essence of these reaction sites is free radicals, and the rate of increase in free radical concentration determines the rate of coal oxidation reactions.

     

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