Adsorption of Water Vapor from Ambient Atmosphere onto Coal Fines Leading to Spontaneous Heating of Coal Stockpile

Abstract

Low rank coals are susceptible to spontaneous combustion, because oxidation rate of low rank coals are higher than that of high rank coals. Even low rank coals, however, must be heated over a critical temperature before spontaneous combustion starts. This work assumed that rapid spontaneous combustion of coal in stockpile will start from the heat up of coal fines on the surface of the stockpile by the heat supplied from the sun and moist air. Then direct measurement of possible temperature increase by the adsorption of water vapor on coal from ambient atmosphere was undertaken by using three kinds of brown coals/lignites and a bituminous coal. When about 100 mg of each brown coal pre-dried at 80 °C and cooled to 28 °C were exposed to an ambient and stationary atmosphere of 28 °C with 67–77% relative humidity, the coal temperatures increased up to 40–43 °C in a minute or so. When the brown coals pre-dried and cooled to 38 °C were exposed to a stagnant saturated air at 38 °C, the coal temperatures increased up to over 60 °C in a minute or so. This was found to occur by rapid adsorption of water vapor from the ambient atmosphere onto the coal. Next, a simple model based on mass and enthalpy balances was successfully formulated to simulate the water adsorption and temperature change. The model simulation then clarified that the amount of heat generated by the adsorption of water vapor are much larger than the heat generated by initial coal oxidation and that most of heat generate…