Pulverized Coal Ignition in Oxyfuel Combustion with CO2 Recirculation: Group of Particles Mode

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 ex-periments. The experiments were conducted with a U.S. high-volatile bituminous coal under 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 location 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. A simple model that considers the presence of a stable vola-tile flame was developed to understand the effect of CO2 as balance gas in group-particle combustion. The model shows that the difference in thermodynamic properties of N2 and CO2 and the synergistic effect observed when groups of coal particles devolatilize in a hot oxidizing environment are responsible for the observed results. The paper analyzes the implication that these results may have in industrial pulverized-coal burners.