Spontaneous combustion of stored coals poses a serious problem for both coal producers and users. The problem assumes even greater significance in attempts to prepare a product of higher calorific value through thermal drying of low-rank coals since removal of moisture enhances the potential for spontaneous ignition and combustion. Low-temperature oxidation of coal is the primary source of heat leading to spontaneous combustion; its understanding is clearly important for prevention of fires in coal stockpiles. Most experimental data on low-temperature oxidation reported in the literature have been obtained for dry coals of higher rank, and at relatively higher temperatures. Theoretical understanding of the influence of particle size on the overall oxidation rank is also not complete.; A kinetic model for low-temperature oxidation is developed for a single isothermal particle. This formulation through the inclusion of transport resistances, accounts for the dependence of the observed reaction rate on particle size and specific external surface area over the entire particle size range with smooth transition between different reaction regimes. Earlier studies on the effect of particle size on the reaction rate have found a dependence of S{dollar}sp{lcub}rm n{rcub}sb{lcub}rm ex{rcub}{dollar} for the oxygen consumption rate. The result from this investigation shows that this is a consequence of intraparticle resistance. It is also shown that in situ moisture in fresh coal exerts a very significant retarding influence on the rate of low-temperature oxidation at constant temperature. The influence of particle size is pronounced even at low temperatures and small particle sizes for fresh wet coal. For dried coal on the other hand, the influence of particle size becomes important only at significantly higher temperatures or for considerably larger particles.; The single particle reaction-diffusion model has been extended to interpret results from an isothermal fixed bed reactor. The results indicate that the model provides a satisfactory explanation for the particle size and temperature dependence of low-temperature oxidation of coal.; An examination of the driving forces resulting from natural convection and forced convection (a result of wind velocity) establishes the importance of forced convection in the initial heatup of the coal stockpile. A model for self-heating in coal stockpiles has been developed with both wind driven convection and natural convection as the driving forces for mass transfer within the coal stockpile. The importance of wind driven convection in the self-heating process leading to spontaneous combustion (which until now was ignored) has been established. Qualitative agreement between predictions from the above model and actual large scale coal stockpile experiments lend validity to the above model. Due to a lack of experimental data, quantitative comparisons are not possible.
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机译：储煤的自燃对煤炭生产者和使用者都构成了严重的问题。在试图通过低阶煤的热干燥制备更高热值产品的过程中，该问题具有更大的意义，因为水分的去除会增加自燃和燃烧的可能性。煤的低温氧化是导致自燃的主要热源。它的理解对于防止煤堆起火显然很重要。文献中报道的大多数关于低温氧化的实验数据都是在较高温度下和相对较高温度下获得的干煤。粒度对总体氧化等级的影响的理论理解也不完整。针对单个等温粒子建立了低温氧化的动力学模型。通过包含运输阻力的这种配方，说明了观察到的反应速率对整个粒径范围内粒径和比外表面积的依赖性，并且在不同反应方案之间进行了平稳过渡。较早的关于粒度对反应速率影响的研究已经发现S {dollar} sp {lcub} rm n {rcub} sb {lcub} rm ex {rcub} {dollar}与氧气消耗速率有关。该研究的结果表明，这是颗粒内抗性的结果。还显示出新鲜煤中的原位水分对恒温下的低温氧化速率具有非常显着的延迟影响。对于新鲜的湿煤，即使在低温和小粒径下，粒径的影响也很明显。另一方面，对于干煤，只有在明显更高的温度下或对于更大的颗粒，颗粒大小的影响才变得重要。单颗粒反应扩散模型已扩展为解释等温固定床反应器的结果。结果表明，该模型为煤的低温氧化的粒径和温度依赖性提供了令人满意的解释。对自然对流和强制对流（风速的结果）所产生的驱动力的研究确定了强制对流在煤堆初始加热中的重要性。已经开发了一种以风对流和自然对流为动力的煤堆自热模型，作为驱动力在煤堆内进行传质。风力对流在导致自燃的自热过程中的重要性已经确立（迄今为止一直被忽略）。上述模型的预测与实际大规模煤炭储备实验之间的定性一致，使上述模型具有有效性。由于缺乏实验数据，无法进行定量比较。
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