Change pattern of acoustic emission signal and generation mechanism during coal heating and combustion process

The early warning of coal spontaneous combustion is difficult to solve the perception of fire information. The acoustic emission (AE) signal generated by thermal damage and destruction on coal rock can perceive the temperature, and then issue a warning on the evolutionary state of coal spontaneous combustion. In order to study the change of AE signal in the process of coal heating and combustion, we explore the difference of and the time and frequency changes of the AE signals released by coal in different temperatures in the process of coal spontaneous combustion, and study the generation mechanism of the AE signal, so as to provide a theoretical basis for the inversion of coal temperature and for monitoring coal spontaneous combustion. The AE signal test system is constructed. By choosing the bituminous coal from Dongtan coal mine as the research subject, we test and analyze the change of the AE signal during coal heating process, analyze the evolutionary process of coal deformation and cracking under the influence of temperature and the change of coal composition, and examine the generation mechanism of AE signal in the process of coal heating. The results show that the ringing counts of coal increase sporadically from heating to combustion, and the correlation coefficient exceeds 0.87 by linear fitting, indicating that the frequency of coal deformation and cracking increases during the heating process. With the increase of temperature, the growth rate of ringing counts and the average frequency (AF value) of AE signal generated by coal are lower in the initial heating stage, but are higher when the temperature rises rapidly. The cumulative damage parameters show an inflection point when the coal temperature exceeds 80°C, and the main damage mode changes from rapid expansion damage to progressive damage. By analyzing the macro- and microscopic changes of coal structure and composition at different temperatures, we find that the mineral conversion rate in coal affects the main damage mode inside the coal body with the increase of temperature, and that the high mineral conversion rate corresponds to the rapid expansion damage inside the coal body, which promotes the derivative expansion of coal pores and cracks, and produces gradually increasing AE signals.