A study on pulverized coal ignition using a two-stage flat-flame burner with a transition from a reducing to oxidizing environment

Coal particles experience a transition from a reducing to oxidizing environment in the near-burner region of pulverized coal (pc) burners. For the first time, we report a fundamental study of ignition of a coal-particle stream experiencing a flame environment that transitions from a reducing to an oxidizing environment (termed reducing-to-oxidizing transition). High-speed videography is used to observe the particles in situ, and scanning electron microscopy is used to characterize the sampled particles. The effects of particle size on ignition are presented for four size bins for PRB subbituminous coal at nominal gas temperatures of 1800 K. The results show that the ignition behavior of particles in reducing-to-oxidizing conditions is different from that in purely oxidizing conditions. The reducing-to-oxidizing transition increases ignition delay time, on average, by more than 100% over those of oxidizing conditions. However, for particles below 150 μm diameter, ignition delay time is relatively independent of particle size in either oxidizing or reducing-to-oxidizing conditions due to high heating rate. Hetero-homogeneous ignition mode is dominant in both environment, except for the 125-149 μm particle size range in oxidizing environment, where homogeneous-to-heterogeneous ignition occurs.