Combustion of Pulverized Size Fuel Particles under Oxygen-Enriched Conditions with Overlap of Devolatilization and Char Oxidation

摘要

The combustion characteristics of pulverized biomass particles, produced at a gas temperature of 1350 K and at discrete oxygen concentrations in the range of ppm levels - 48 percent, by volume, were investigated in an entrained laminar flow reactor. The particles experienced high heating rates during devolatilization, on the order 1 centre dot 105 K/s, and during char oxidation, particle burned at rates limited by the combined effects of pore diffusion and chemical reaction, i.e., under zone II burning conditions. In order to quantify reactivities and possible carbon deactivation during the combustion of biomass chars under zone II conditions, chars extracted at successive residence times from the flow reactor were subjected to oxygen reactivity tests in a pressurized thermogravimetric analyzer (PTGA). Oxygen levels in the TGA were maintained at a constant value of 6 percent, by volume, and temperatures were limited to about 723 K to ensure that reactivities were measured under kinetically-controlled (zone I) conditions. Measured conversion rates during oxidation tests under zone I burning conditions were combined with surface area measurements to yield intrinsic reactivities as a function of char conversion. The intrinsic char reactivities were characterized using a four-step heterogeneous reaction mechanism, a modification of a previously published reaction mechanism for carbon oxidation based on a density functional study of the chemisorption of oxygen on graphite surfaces. The Arrhenius parameters determined for the reactions in the mechanism yielded predicted mass loss rates at 1350 K that explained the mass loss rates in the laminar flow reactor tests.