Influence of steam on ignition of Victorian brown coal particle stream in oxy-fuel combustion: In-situ diagnosis and transient ignition modelling

Abstract

In this paper, we have investigated the ignition behaviour of Victorian brown coal under the steam-rich oxy-fuel combustion conditions. Two differently ranked coal, sub-bituminous and bituminous coal, were also tested for comparison. The ignition experiments were conducted in O2/N2/H2O and O2/CO2/H2O atmospheres, in an entrained flow reactor coupled with flat flame burner and in-situ optical diagnosis tools. A transient ignition model and 1-D single film model were also developed and used to predict particle ignition and the extent of char-steam gasification reaction, respectively. As has been confirmed, the injection of external steam to the furnace is beneficial in accelerating the ignition of Victorian brown coal in both air and oxy-firing modes with 21% O2. This effect is even more pronounced for the combustion of dense particle stream. In the oxygen-enriched condition, the ignition time was similar regardless of the steam concentration in the bulk gas. The transient modelling prediction confirmed that the homogeneous water–gas shift reaction (CO+H2O↔CO2+H2) was the main cause for the accelerated ignition for brown coal. It produced a highly concentrated flammable gas mixture of CO and H2 in coal particle cloud, which in turn accelerated the homogenous ignition of volatiles. This promotion effect is the most dominant for brown coal, because its volatile is rich in light hydrocarbons including gaseous CO that can readily react with the external steam via water–gas shift reaction. Moreover, the external steam triggered the heterogeneous char-steam gasification reaction for the brown coal, which further improved the overall char burnout rate. Regarding another two high-rank coals, their changes on both ignition and char burnout rate are however marginal upon the introduction of external steam, due to the shortage of gaseous volatiles (thus no water–gas shift reaction triggered) and the difficulty for their char to undergo steam gasification reaction under the experimental conditions observed here.