Formulation of the Heat Generation Rate of Low-Temperature Oxidation of Coal by Measuring Heat Flow and Weight Change at Constant Temperatures Using Thermogravimetry–Differential Scanning Calorimetry

This work successfully measured heat generation rates accompanying the air oxidation of three low-rank coals at seven constant temperatures ranging from 50 to 150 °C using a thermogravimetry–differential scanning calorimetry analyzer. The heat generation rates were well-expressed by the sum of three parallel first-order reactions, which are expressed by dQj/dt = kjQj0 exp(−kjt) (j = 1, 2, and 3), where Qj is the amount of heat generated, t is the time, kj is the first-order rate constant, Qj0 is the maximum amount of heat generated, and kjQj0 gives the maximum heat generation rate. The rate constants k1, k2, and k3 were well-represented by the Arrhenius equations. The activation energies were 2.78–7.58 kJ/mol for k1, 9.29–13.48 kJ/mol for k2, and 55.5 kJ/mol for k3. The maximum amounts of heat generated by reaction 3, Q30, were constants, irrespective of the temperature ranging from 340 to 671 kJ/kg of coal, as expected. On the other hand, Q10 and Q20 had to be regarded as variables increasing with the temperature. The calculated dQ/dt versus t relationships and Q versus t relationships using the estimated kj and Qj0 showed excellent agreements at all temperatures for all of the coals. This shows that the rate parameters obtained can well be used to represent the heat generation rate at temperatures between 50 and 150 °C. It was also found that the H2O-forming reaction with simultaneous formation of coal–oxygen complexes is the dominant process of the oxygen–coal interaction in the temperature range examined on the basis of the measurements of weight changes and formation rates of H2O, CO2, and CO.