EVALUATION AND PREVENTION OF SPONTANEOUS COMBUSTION DURING HANDLING AND STORAGE OF COAL

Spontaneous combustion of coal has historically been a major problem for the coal industry, predominantly during storage and transportation. Various methods have been used in the laboratory for evaluating the propensity of different coal sources to self-heat. However, the heterogeneity of coal and the complexity of the system has resulted in inconsistencies and sometimes conflicting results as indicated by the findings reported in several publications. The primary objective of the current study was to build a laboratory scale apparatus that simulates the condition of a coal stockpile to evaluate the events leading to spontaneous combustion and develop potential remedies. As such, the influential factors can be identified with confidence, thereby providing an improved understanding of the spontaneous combustion. An adiabatic heating apparatus was designed and constructed which included instrumentation to closely monitor the oxidation process and the stages leading to spontaneous combustion under various conditions. The device was equipped with thermocouples which measured the temperature rise as a function of time leading to the determination of an index value that indicated the propensity of a given coal source to spontaneously combustion. The index was referred to as the R70 value which was measured as the temperature was increased during the period of rapid oxidation. The units for the index was degrees Celsius per hour. As such, a high index value reflected the likelihood of spontaneous combustion for a given coal source. To standardize the test procedure, a detailed three-level statistical experimental design was conducted involving three critical parameters, i.e., particle size, oxygen flow rate and the duration of the drying period prior to feeding oxygen to the system. Using empirical models describing the R70 value as a function of the parameter values developed from the test data, it was determined that R70 was sensitive to the sample particle size and drying time. A decrease in particle size and drying time significantly increased the R70 value while the oxygen rate did not have a significant impact over the range of values tested. Based on the results of the test program, a standard test procedure was established to evaluate various coal sources and identify chemicals that could be used to remediate the spontaneous combustion issue. Several sub-bituminous coal sources collected from the Powder River Basin were tested in the apparatus and found to be prone to spontaneous combustion as indicated by R70 values that approached 50oC per hour. Several chemicals were evaluated as a means of eliminating or slowing the spontaneous combustion process. These agents included anti-oxidants, binders and humectants. Organic binders were used to agglomerate the fine coal particles which limited surface area exposure. The effect significantly reduced the oxidation rate as indicated by a reduction in the R70 index from 44.07oC/hr to 5.71oC/hr. However, after entering the latent heat stage, the temperature increased rapidly at a rate of 27.58oC/hr. Humectants were evaluated which contained several hydrophilic groups, mainly hydroxyl groups, and thus have an affinity for water. As a result, when the coals were treated with humectant, the latent heat rate was reduced to 4.24oC/hr although the R70 remained relatively high. By using a combination of humectant and binder, the optimum result was obtained with an R70 value of 5.04oC/hr and a latent heat rate of 11.06oC/hr. These findings were successfully implemented into industrial practice for significantly delaying the spontaneous combustion event