Ignition and combustion of single pulverized biomass and coal particles in N2/O2 and CO2/O2 environments

The ignition and combustion characteristics of single particles of two biomass residues and three coals were evaluated in the hot combustion products of a CH4 fired Hencken burner. Particles of corn straw and wheat straw residues, and lignite, bituminous coal and anthracite, all in the size range 125-150 were injected into the flue gas with a mean temperature of 1550 K and a mean O-2 concentration of 15 mol.%. The experiments were carried out in N-2/O-2 and CO2/O-2 environments. Temporally resolved CH* chemiluminescence images of the single burning particles were captured by an ICCD camera equipped with a 430 nm band-pass filter. The results show that the biomass and lignite particles ignited homogeneously, while the bituminous coal and anthracite particles ignited heterogeneously in both N-2/O-2 and CO2/O-2 environments. For the biomass and lignite particles, the gas-phase combustion of the volatiles took place in an enveloping flame surrounding the particle, followed by the heterogeneous oxidation of the char. The biomass particles ignited earlier than the coal particles. For the three coals, the ignition delay time decreased almost linearly with the increase of their volatiles content. When N-2 was replaced by CO2, the ignition of all solid fuels was delayed, and the burnout time of volatiles was lengthened. The combustion of all solid fuels was markedly less intense in the CO2/O-2 environment than in the N-2/O-2 environment, due to the lower binary diffusivity of the O-2 in CO2 and the higher volumetric heat capacity of the CO2, as compared with N-2.