Spontaneous combustion coal parameters for the Crossing-Point Temperature (CPT) method in a Temperature-Programmed System (TPS)

Coal fires originate from coal spontaneous combustion caused by oxidation starting even at low temperatures. In order to evaluate the heating and oxidation of coal placed inside a container, a Temperature-Programmed System (TPS) is used under various flow and oxidation conditions for temperatures ranging from 40 to 250 °C. As the coal samples are heated in the container within the TPS, the concentration histories of the gaseous products are measured as well as the temperature history at the center of the container. The Crossing-point Temperature (CPT) is the temperature (temperature and corresponding time) at which the increasing coal temperature is equal to the increasing oven temperature within the TPS. We have developed energy and oxygen mass conservation equations for the coal pile in a container inside the TPS oven assuming uniform conditions for temperature and oxygen concentrations, both assumptions partially justified because the heating rate in the oven is very low (1 °C/min). By subtracting the convective heat from the thermal inertia of the coal pile in the energy equation, we have obtained from the experimental data the heat owing to oxygen reaction or moisture evaporation. From the oxygen conservation and measurements, we have determined apparent activation energy and pre-exponential factor for oxidation assuming that oxidation is proportional to oxygen concentration. This information is useful for the mathematical modeling of oxidation and heating in the present experiments. Subsequently, the energy and mass conservation equations were solved after being transformed to non-dimensional form, which shows that four dimensionless parameters control the heating of coal in the present experiments. Only three of these parameters are examined in detail, namely a dimensionless flow time, a heat release parameter B, and a Damkohler number Da whereas a moisture parameter is discussed qualitatively. Based on the present analysis and data, four stages are identified: (I) initial heating with essentially no reaction or evaporation, (II) evaporation period with essentially no reaction, (III) an unsteady accelerated oxidation period during which crossover may occur and (IV) a heating late period where all the oxygen reacts inside the container.