Single-particle combustion of two biomass chars

Abstract

In order to assess the combustion reactivities of chars produced from the pyrolysis of woody and herbaceous biomass, we have subjected particles of Southern pine and switchgrass chars to two sets of combustion experiments. In the first, a dilute stream of biomass char particles (nominal size, 75–106 μm) is burned in a laminar flow reactor at 12 mole % O 2 and a mean gas temperature of ∼1600 K. In situ optical measurements reveal that at a given residence time in the early stages of char conversion, biomass char particles burn over a much wider temperature range (∼450 K) than coal particles (∼150 K) and that the biomass char particle temperatures span the entire range of the theoretical limits (from the slowest burning inert particles to the fastest burning diffusion-controlled particles). As biomass char conversion proceeds, however, mean particle temperatures decrease, and particle temperature distributions narrow-consequences of the preferential removal of more reactive carbon as well as a number of physical and chemical transformations of the inorganic constituents of the chars (vaporization, surface migration and coalescence, and incorporation into silicate structures). Kinetic parameters for median particles taken at the early stages of char conversion indicate that biomass chars are somewhat less reactive than low-rank lignites and subbituminous coals, somewhat more reactive than high-rank low-volatile bituminous coals, and comparable in reactivity to high-volatile bituminous coals. In the second set of combustion experiments, individual biomass char particles are suspended on an inert mesh and suddenly subjected to a hot 6 mole % O 2 environment. Video images of reflected light and near-infrared emission for a number of pine and switchgrass char particles demonstrate the heterogeneity of the biomass chars in terms of both the sequence of morphological changes and the temperature-time histories of the particles as they undergo combustion.