Influence of inherent moisture on the ignition and combustion of wet Victorian brown coal in air-firing and oxy-fuel modes: Part 1: The volatile ignition and flame propagation

Abstract

This study for the first time reported the volatile ignition and oxidation for wet Victorian brown coal in both air-firing and oxy-fuel modes. The aim is to quantitatively understand the evaporation of inherent moisture and its overlapping with the subsequent volatile ignition and oxidation steps during pulverised coal combustion. A wet Victorian brown coal sample with the moisture content up to 30wt.% and a size range of 63–104μm has been tested. Its ignition, volatile flame propagation and char oxidation have been diagnosed optically in a laboratory-scale flat-flame burner reactor (FFBR) coupled with a high-speed camera. 1-D modelling was also conducted to predict coal ignition based on the classic coal drying theory and coal devolatilisation rate. As has been confirmed, the release of moisture on the initial drying stage is incomplete prior to the subsequent volatile ignition. The fraction of moisture released upon drying only accounted for 10% of its original amount in wet brown coal prior to ignition. This is due to a firm residence of the moisture within coal capillaries, and a rapid heat feedback from hot gas to the dried particle surface that was ignited instantaneously prior to the release of the remaining moisture. The unevaporated moisture was observed to be released with the volatiles together to form a thick cloud layer in the vicinity of the coal particle, thereby enlarging the sizes of the volatile flame remarkably, which are approximately twice the size of the dried coal flame. However, the intensity of the wet coal flame was much weaker than dried coal, due to the dilution effect of the inherent moisture on the volatile cloud. In the oxy-fuel combustion mode, the inherent moisture interrelated with CO2, a tri-atomic gas with a larger specific heat capacity than N2 to significantly delay the ignition of the wet coal particle and its flame intensity and propagation velocity.