Simultaneous investigation of coal ignition and soot formation in two-stage O2/N2 and O2/CO2 atmospheres

Due to the intermittent nature of most renewable energy sources, the flexible oxy-coal combustion technology equipped with carbon capture, utilization, and storage (CCUS) will play an indispensable role in achieving carbon neutrality. In this work, we fundamentally investigated coal stream ignition and soot evolution in a series of staged O-2/N-2 and O-2/CO2 atmospheres for two kinds of coal samples. A novel two-stage flat-flame burner can provide a controlled temperature (1500 K) and atmosphere that are comparable to those in practical boilers. We incorporated in-situ Mie scattering of coal particles, visible light of coal flame, laser-induced incandescence (LII) of soot, as well as online soot particle sampling (in combustion) and gas analysis (in pyrolysis). Compared with conventional O-2/N-2 atmosphere, the O-2/CO2 case under the same oxygen fraction (X-O2 = 0.3) substantially delays the ignition of coal streams (i.e., 10.1 ms for lignite and 8.9 ms for bituminous coal) on the more representative two-stage burner, and reduces soot volume fraction in the coal flame (i.e., by 26% for lignite and 48% for bituminous coal). The growth and oxidation of primary soot are retarded by the O-2/CO2 ambience, which might be attributed to the lowered coal/char surface temperature and the smaller O-2 diffusion coefficient in oxy-mode. In particular, we find that elevating oxygen fraction reduces soot volume fraction in the O-2/N-2 case, but exceptionally promotes soot formation in the O-2/CO2 case. When the effect of coal rank is concerned, the bituminous coal ignites much later than the lignite due to less amount of combustible light gases released during pyrolysis. The lowered local temperatures in bituminous coal combustion lead to reduced soot forming ability in the O-2/CO2 ambience, but in O-2/N-2 conditions, burning bituminous coal is sootier.