Recent studies for coal ignition in oxyfuel combustion have been conducted as single- particle experiments or in pilot-scale burners. While single-particle experiments have the advantage of well-controlled atmospheres in which it is possible to infer a good under- standing of the phenomena related to pulverized-coal ignition, they raise the question of the effect that burning coal in a multi-particle mode, such as exists in industrial burners, may have on particle ignition. On the other hand, pilot-scale experiments are often diffi- cult to control and may not allow a simple understanding of the ignition phenomenon. This paper presents experimental results obtained under group-combustion regime in a laminar flow environment using coal flows larger than those observed in single-particle experiments but smaller than those of industrial burners. We expect the results exhibit some of the synergistic effects observed in industrial-size coal boiler flames. Further- more, combustion of groups of coal particles can be used to reasonably infer characteris- tics of flame stability - something difficult to achieve in single-particle combustion expe- riments. The experiments were conducted with a U.S. high-volatile bituminous coal un- der oxygen concentrations ranging from 12% to 48%, with N2 or CO2 as balance gas, at two gas temperatures (1130 K and 1650 K). The standoff distance from the coal flame to the burner was used as a metric of ignition delay and the variation in time of the flame lo- cation as an indication of flame stability. The results showed that oxygen concentration has a strong effect on ignition delay at 1130 K. The presence of CO2 as balance gas sig- nificantly changed how the standoff distance responded to changes in oxygen concentra- tion, particularly at 1130 K. The theory developed for drop group combustion was re- viewed for coal particles combustion. The analysis shows that traditional understanding of group combustion based on the G number needs to be revised to take into account a balance between the fast devolatilization process and the laminar flame speed that control group combustion.
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