Comparative radiative property measurements of single biomass and coal particles burning at high reactor temperatures

Abstract

This research measured the emissivity, temperature and soot loading of pulverized bituminous coal and three types of renewable biomass. Combustion occurred in a laboratory-scale drop tube furnace (DTF), where the fuels experienced high heating rates and ignited and burned in two distinct phases: devolatilization accompanied by volatile matter combustion in envelope flames, and sequential combustion of the char residues. Multi-wavelength spectrometry was used to concurrently determine the emissivity and the temperature of burning particles at discrete points in their history. Three-wavelength pyrometry was used to obtain entire temperature-time histories, and high-speed photography was used to provide both visual observations of phenomena and to obtain two-dimensional temperature distributions. Results showed that soot in volatile matter flames of burning single particles of the bituminous coal and of the three types of torrefied biomass had spectral emissivities in the range of 0.03–0.10, in the wavelength domain of 600–1000 nm. Emissivities were nearly constant with wavelength, i.e., soot mantles in these flames radiated as graybodies. Spectral emissivities of the chars of the bituminous coal and the three types of torrefied biomass were in the range of 0.29–0.5 and varied with wavelength, i.e., they radiated as non-graybodies. The highest measured temperatures of soot in volatile flames for all fuels were between 2150 and 2360 K and maximum temperatures of chars were between 1700 and 2000 K, based on the three measurement techniques. The graybody assumption caused negligible (± 10 K) deviations in volatile matter flame temperatures; however, it caused significant deviations (± 20–100 K) in char temperatures. Envelope flame diameters of torrefied biomass particles were larger than those of coal. To the contrary, soot volume fractions in the envelope flames of coal particles were found to be higher than those in biomass particle flames. Finally, the soot volume fractions in the flames of these fuels were inversely proportional to particle size.