Stage development characteristics of oxygen-lean combustion of coal in fire zone. Part I: The evolution law of pyrolysis and combustion stage characteristics

Coal fire oxygen-lean combustion is a global catastrophe, well known and difficult to describe. To deepen the understanding of the stage characteristics under the competition between pyrolysis and oxidation in coal oxygen-lean combustion. In this study, a TA-Q600 simultaneous thermal analyzer was used to investigate the macroscopic mass characteristics of three typical low-rank coals during oxygen-lean combustion processes under different time-scale effects. Through a coupled competitive comparison of pyrolysis and combustion characteristic temperature points, the stage characteristics and evolution laws of coal in the oxygen-lean combustion process were comprehensively analyzed. The results showed that the stage development of coal pyrolysis can be divided into four stages; under different time-scale effect, the higher the coal rank, the better the separation between the thermal decomposition and the thermal polycondensation processes. The stage development patterns of coal structure conversion combustion were divided into three categories, and the stage development types were divided into six categories. The difference in the burnout state caused by the decrease in oxygen concentration includes to 4–7 different combustion progressions. When the oxygen concentration falls within the range of 5–1%, the coal combustion stage transitions and delays from semi-coke burnout to coal coke burnout. The evolution of the burnout state, induced by the oxygen concentration, remained unaffected by the coal rank but was relatively less influenced by the time-scale effect. With an increase in the coal rank under a 1% oxygen concentration, the stage progression total gradually diminishes. This characteristic remains unaffected by the time-scale effect. As the coal rank increased, the influence of the time-scale effect on the oxygen concentration of the stage development pattern evolution became increasingly evident. The results of the study guided the identification of the development progression in different areas of the fire zone and provided safer temperature and oxygen concentration indicators for fire suppression work and unsealing of the fire zone.