Clarifying the influence of moisture on the ignition and combustion of wet Victorian brown coal in air-firing and oxy-fuel modes: Part 2: Contribution of gasification reaction to char oxidation rate

Abstract

This paper is the second part of the study to clarify the influence of moisture in Victorian brown coal oxy-fuel combustion, with a focus on char oxidation and gasification reaction through experimental and modelling efforts. An in-situ high-speed two-colour pyrometer with the wavelength band of 0.85–1.05μm was employed to measure particle temperature in the flat flame burner reactor. The combustion stage of carbon particle was simulated for the transient phenomena with particle heating and radiative heat transfer. The multiple surface reaction single-film approach, including char–O2, char–CO2 and char–steam reactions, was employed and the contribution of individual reactions to carbon consumption rate was determined via matching with the measured particle temperature. Irrespective of the initial moisture content, the extent of char–steam gasification reaction was found to account for ~15% in the air-firing case. This reaction was mainly triggered by the external steam in the reactor, rather than the inherent moisture that resided preferentially as volatile cloud on char particle surface. The combined effect of both char–CO2 and char–steam gasification was significant in oxy-fuel combustion mode, especially for the wet coal. In the oxy-21 case, these two reactions have a total extent of around 8% and 18% on the burning char surface of dried and wet coal, respectively. The char–CO2 gasification is insignificant, because the char particle temperature was low. Increasing the oxygen percentage to 31% in CO2 enhanced the total extent of these two gasification reactions to 28%, based on the mass of total carbon. Such an extent is comparable with the literature. However, the steam gasification rate for brown coal char was far higher, ~26% relative to ~10% for high-rank bituminous coal reported in the literature. This substantiates the strong steam gasification reactivity of Victorian brown coal char. The contribution of inherent moisture to char–steam gasification reaction is crucial in the combustion of wet coal in the oxy-21 case, accounting for ~10%. This is due to the long residence of the unevaporated steam as a thick cloud on the char surface. Increasing the oxygen concentration in CO2 enhanced the char–O2 reaction, the release rate of volatiles and inherent moisture. Therefore, the char–steam reaction caused by the inherent moisture within coal matrix was minimised and eventually diminished in the oxy-31 case.