Simulation of Spontaneous Heating of a Small Fixed Bed of Dried Coal Exposed to a Flowing Wet Air Stream

Abstract

First, less than 30 g of brown coal packed in a sphere stainless net and placed in an air naturally circulating temperature-programmed oven was found to start spontaneous combustion from the center of the coal sphere when the oven temperature was raised to 135 °C. This was because the heat generation rate by the oxidation of coal was larger than the rate of heat dissipation rate by natural convection at 135 °C. Next, to clarify the mechanism by which coal is heated to 135 °C, the temperature increase of less than 10 g of the fixed bed of dried brown coal was measured by contacting the coal with flowing dry air or wet air at 50, 70, 95, and 107 °C. It was found that the maximum temperature increase in the dry air was almost null at 70 °C and only 5 °C or so even at 107 °C. On the other hand, the maximum temperature increases were respectively 10, 25, 46, and 40 °C at 50, 70, 95, and 107 °C in the wet air, suggesting that the adsorption of water vapor plays a crucial role in increasing the coal temperature high enough for the spontaneous combustion to start. A simple simulation program was developed for expressing the temperature change observed by the above fixed bed experiment by formulating the mass and enthalpy balance equations with the separately measured heat generation rate by the coal–oxygen interaction at 50–150 °C and the adsorption isotherm and the heat of adsorption of water vapor on coal at 70 °C. The simulation program well expressed the temperature change for the experiments performed at 70 °C in both the wet air and the dry air, indicating that the spontaneous heating of coal is brought about by the coupled effect of the adsorption of water vapor and the coal oxygen interaction. In addition, the simulation showed how the spontaneous combustion of coal starts in the wet air.

Improved Calculation Model for the Shortest Spontaneous Combustion Period

“…The numerical model was established based on the actual environment in the field to forecast the spontaneous combustion process by data-computing programs. 7 , 26 − 29 However, the applicability of the model is poor, and the results are uncertain and difficult to verify owing to the complexity of actual environmental conditions.…”

“…To predict the spontaneous combustion locations and ignition period of coal more accurately, the model experiment on spontaneous combustion has been developed with a higher degree of simulation and more accurate and reliable data, which has successfully solved the key problems of on-site simulation of coal spontaneous combustion. − However, this method is more time-consuming and has a higher cost and poorer repeatability. The numerical model was established based on the actual environment in the field to forecast the spontaneous combustion process by data-computing programs. ,− However, the applicability of the model is poor, and the results are uncertain and difficult to verify owing to the complexity of actual environmental conditions.…”