Coal particle ignition in a combustion environment with a reducing-to-oxidizing transition

A transition from a reducing-to-oxidizing (R-O) environment is characteristic of what a coal particle experiences in the near-burner region of pulverized coal (pc) furnaces. The R-O transition can influence early-stage coal combustion processes such as ignition, aerosol formation, and char burnout. However, fundamental studies have focused on either oxidizing conditions (mimicking the post-flame region) or reducing conditions (mimicking the devolatilization region). The effect of this transition on early-stage coal combustion has, until now, not been considered. Here, the role of this transition on single-particle ignition is evaluated. PRB sub-bituminous coal was used, with a particle size of 125-149 μm and two nominal gas temperatures of 1300 K and 1800 K. The experimental findings for purely-oxidizing conditions with 20 vol. % oxygen are compared with those of the R-O environment. Single particles were tracked using high-speed, highresolution videography. Emission intensities of the particles were used to evaluate the prevailing ignition modes in both oxidizing and R-O environments. Homogeneous-to-heterogeneous mode of ignition is prevalent for purely oxidizing conditions for both nominal gas temperatures of 1300 K and 1800 K. However, hetero-homogeneous ignition is favored in R-O environment at 1800 K and heterogeneous ignition at 1300 K gas flame temperature.