Effects of Oxygen Concentrations and Heating Rates on Non-isothermal Combustion Properties of Jet Coal in East China

Abstract

By thermogravimetric (TG) experiments under 5 oxygen concentrations of 21.0, 40.0, 50.0, 60.0, 80.0% and 5 heating rates of 20.0, 30.0, 40.0, 50.0, 60.0 ℃/min, the combustion characteristics of jet coal and influences of oxygen concentrations and heating rates on them were investigated. Results show that the pyrolysis processes of jet coals consist of 3 segments of water evaporation, structure oxidation and mass-gaining, self-ignition and induced combustion and carbonization that lie in temperature ranges of 65.0~150.0 ℃, 150.0~290.0 ℃ and 290.0~650.0 ℃, respectively. Huainan jet coal samples burn more easily as universal ignition index increases. At constant heating rate, self-ignition temperature tends to decrease by 10.1~19.4 % with increasing oxygen concentration from 21.0 % to 80.0 %. However, the variation trend of oxidation process includes a slight rise when variation trend begins to fall. At constant oxygen concentration, the self-ignition temperature transformes into high temperature direction and lead to thermal lag effect. Values of Coal-oxidation time at various oxygen concentration levels decreased by 63.1~65.8 % at heating rates from 20.0 to 60.0 ℃/min. Accordingly, the coal-oxidation process is accelerated. A new index (R_S) named coal-oxidation difficulty level index to evaluate the lag effect of coal-oxidation process was proposed. Fitting results of three dimensionless parameters confirmed that the risk of spontaneous combustion determined by the coal-oxidation process still remains growing up although self-ignition temperature produces thermal lags with increasing heating rates.