Investigation of the Thermokinetic Characteristics during Combustion of Pyrolysis Semi-Coke from Non-Stick Coal

Pyrolysis semi-coke plays a significant role during the spread of subsurface coal fires and the reignition of extinguished fire areas. In order to explore the combustive properties of pyrolysis semi-coke from non-stick coal, its combustion process was experimentally investigated. The kinetic characteristics of raw coal and semi-coke, along with the effect of semi-coke pyrolysis temperature, were analyzed. Meanwhile, the correlation analysis between reaction kinetic parameters and main microscopic functional groups was conducted to identify the transformation of critical functional groups during semi-coke combustion. The primary findings are summarized as follows: As the pyrolysis temperature increases, the oxygen-absorption mass-gain stage gradually expands, and the thermal decomposition/combustion mass-loss stage presents a shortening trend. The quantity of heat release increases initially and then decreases, reaching its maximum in the 400-500 °C range. The apparent activation energy (E) during semi-coke decomposition/combustion at high-temperature decreases progressively with the increase of conversion rate. The average value of E for semi-coke initially decreases and subsequently increases with rising pyrolysis temperature. It reaches the minimum at 500 °C. The reaction mechanisms of raw coal and semi-cokes pyrolyzed at 300 and 400 °C respectively correspond to spherically symmetrical phase boundary reactions, spherically symmetrical three-dimensional diffusion, and reaction orders. The semi-coke pyrolyzed at 500 and 600 °C belongs to the random nucleation and subsequent growth. From the microscopic perspective, as semi-coke pyrolysis temperature increases, the critical functional groups affecting the decomposition/combustion reaction transform from aliphatic hydrocarbons to oxygen-containing functional groups and then to aromatic hydrocarbons. These results offer theoretical groundwork for in-depth comprehension of the dynamic process associated with underground coal fire spread and the reignition characteristics in extinguished fire zones.