Pittsburgh, PA: U.S. Department of the Interior, Bureau of Mines, TN 395, 1992 Mar; :1-2
Objective: Predict the spontaneous combustion potential of a coal using an expert system computer program. Background: Approximately 15% of underground coal mine fires in the United States are attributed to spontaneous combustion. These spontaneous combustion tires frequently occur in worked-out or abandoned areas of a coal mine. This makes the control and extinguishment of the tire difficult and often requires large sections of the mine or even the entire mine to be sealed for long periods of time. A prior knowledge of the self-heating potential of the coal can be useful in preventing spontaneous combustion fires through the use of aggressive monitoring and control measures at existing coal mining operations, and in the planning of new mines. Many factors can affect the spontaneous combustion process in a mine, including the self-heating potential of the coal, ventilation, geologic factors, and mining conditions and practices. Various test methods have been used to evaluate the relative self-heating potential of coal, including crossing point temperature and adiabatic heating methods, and oxygen adsorption techniques. However, these tests are usuaJ1y time-consuming and difficult to conduct, limiting their use to laboratories. An adiabatic heating test has been used extensively by the Bureau of Mines to examine the self-heating potentials of coals. A ranking scheme was established to assess a coal's relative self-heating potential, based on the coal's minimum self-heating temperature (SHT). SHT is the minimum initial temperature from which a coal undergoes a sustained exothermic reaction in the adiabatic oven. Coals that have a minimum SHT < 70 degrees C are considered to have a high spontaneous combustion potential; those with minimum SHT's between 70 and 100 degrees C are a medium risk, and coals with minimum SHT's > 100 degrees C have a low spontaneous combustion potential. However, like other methods, this test is time consuming and difficult to conduct. A statistical analysis of the adiabatic oven results for coals showed that the minimum SHT of a bituminous coal was strongly dependent on its dry ash-free (DAF) oxygen content, a value readily obtainable from a routine ultimate analysis, by the expression: SHTmin' degrees C = 139.7 - 6.6 x [Oxygen, pct (DAF)]. Thus a bituminous coal's relative self heating potential could be determined by a simple empirical expression without the need for detailed and costly experiments. Approach: The results of the adiabatic heating tests were incorporated into an expert system computer program. An expert system is a computer program that uses available information to make decisions based on a series of rules provided by the programmer. It is an interactive, user friendly, and inexpensive method of conveying "expert" advice. In this case, the advice is about the self-heating potential of a coal sample, and requires only the input of data from a typical coal analysis. The program was designed to allow direct entry of data for a single sample or from an existing database. How It Works: The program requires the input of the coal's proximate and ultimate analyses and heating value. The program allows for the direct entry of sample data from the keyboard, or more typically for a mining operation, from an existing database. The program is compatible with several standard database programs. However, the program's access to a database requires that the existing database structure must match that required by the expert system program. Detailed instructions are included in the program to accomplish this. If an existing database of coal analyses is available, the user has the option of analyzing the self-heating potential of a single sample in the database, a range of specified samples, or the entire database. The results are then stored in the database, and can be viewed, edited, or printed out using the program's browse or report functions. The basis of the expert system is the empirical expression that predicts the minimum SHT, and the relative risk assignment based on the predicted SHT. However, this expression was found to be valid only for bituminous coals. Thus, the system first determines the rank of the coal sample. If the rank is bituminous, the risk is assigned based on the predicted SHT. If the predicted SHT is < 70 degrees C, the sample is assigned a high spontaneous combustion potential. If the SHT is between 70 degrees and 100 degrees C, a medium potential, and if the SHT is > 100 degrees C, the sample is assigned a low spontaneous combustion potential. If the rank is determined to be lignite or subbituminous, the coal is assigned a high self-heating potential. If the rank is anthracite, the sample is assigned a low self-heating potential. These assignments were based on laboratory results showing, in general, that subbituminous and lignite coals had minimum SHT's that fell in the high self-heating category, while anthracite coals showed very low self-heating potentials. The use of the expert system allows the assignment of the self-heating potential of a coal sample without the need for laboratory experiments. The program, however, evaluates only the relative self-heating potential of the sample, and does not take into account other important factors in the spontaneous combustion process, such as mining conditions and practices, and geologic factors. However, it is an important first step in determining the risk of a spontaneous combustion mine fire, and can have a significant impact in its prevention.