The competitive reaction mechanism between oxidation and pyrolysis consumption during low-rank coal combustion at lean-oxygen conditions: A quantitative calculation based on thermogravimetric analyses

The evolution of the oxidation-pyrolysis competitive mechanism during low-rank coal combustion at lean-oxygen conditions determines the development of the coalfield fires and the burning loss rate of the coal resource. In the present work, the mass increments of oxygen-chemisorption were measured and calculated firstly. An abstract shrinking core model was built for distinguishing the oxidation and pyrolysis pathways. On this basis, the consumption rates of oxidation and pyrolysis were respectively calculated with the TGA data obtained under the lean-oxygen gradient of 21-0 %. The results demonstrate that the oxidation path gradually delays in temperature while the pyrolysis exhibits great temperature stability as the oxygen concentration decreases. Besides, the significant transition of the competitive consumption mechanism exists below 9 % for the oxidation phase before ignition and 5 % for the combustion phase after ignition, respectively. Two temperature indicators of T-C1 and T-C2 for guiding the optimal fire extinguishing schemes based on the actual oxygen concentrations and temperatures are proposed. Furthermore, 1 % is identified as the limiting oxygen concentration for the development of the coalfield fires where the ignition mechanisms of both the HST and WD coal transform into the homogeneous ignition. The WD lignite, which always maintains the coke state as the burnout mechanism, holds higher reactivity but wider dispersion degrees of combustion compared with the HST sub-bituminous coal.