Study on spontaneous combustion of lignite based on the theory of equivalent oxidation exposure time

Spontaneous combustion of stockpiled coal can occur under some appropriate circumstances. The open-en- ded isothermal method was used to investigate the characteristic parameters of lignite,such as the critical self-ignition temperature (Tc ) and apparent activation energy of the cube lignite sample of different size (0. 05,0. 1,0. 15 and 0． 25 m in side length). Multi-fields coupling mathematical model of homogeneous porous media was developed based on the theory of equivalent oxidation exposure time ( EOET) and applied to simulate the self-combustion process of lignite. Thereby the dynamic distribution of temperature,oxygen concentration and seepage field were analyzed system- atically. Heating process of lignite sample in the experiment can be divided into four stages,which are rapid heating stage,slow heating stage,temperature rebounding stage and self-heating stage respectively. Coupling mechanism of heat generation transfer,water evaporation and migration,oxygen consumption and migration and natural convection of coal sample was investigated through combined experiment and simulation. Relationship between relative EOET and heat generation acceleration rate was demonstrated to be a negative correlation. Critical self-ignition temperatures of large- scale coal stockpiles for numerical simulation were in close agreement with the prediction values of empirical function.