Characterization of temperature criteria using gas-phase fuel streams for MILD coal combustion

Abstract

MILD combustion is achieved when the reactants’ inlet temperature (Tinlet) is higher than the self-ignition temperature of the reactants (Tself-ignition) and the temperature increase upon reaction (ΔT=Tmax-Tinlet) is lower than Tself-ignition. The method to get Tinlet and Tself-ignition is ambiguous for coal combustion because the multiple fuel streams are released from coal particles through vaporization, devolatilization, tar/soot reaction and char oxidation/gasification at different temperatures and different times as the particles heat up. We propose a method to determine the gas-phase fuel streams in coal combustion using the reaction rate of each subprocess and gas and particle temperature profiles in the reactor before ignition. The mixture of fuel streams with oxidizer is used to get Tinlet and Tself-ignition for the temperature criteria for MILD coal combustion. By way of example, this method is applied to identify the required dilution rate needed to reach MILD coal combustion. The MILD coal combustion regime does not shift significantly for the two types of coal used in this work (Illinois #6 and Guizhou), even with the temperature and composition of the gas fuel mixture affected by the rank of coal. It is observed that the required dilution rate, Kv, to reach MILD regime increases for lower oxidizer temperature, with the decrease of both inlet and maximum temperatures. The required dilution rate to achieve MILD coal combustion is not sensitive to the model for tar/soot reactions, even for a simplified model that uses C2H2 for tar while neglecting the formation of soot.