Combustion and Flame, Vol.213, 426-440, 2020
On the minimum oxygen requirements for oxy-combustion of single particles of torrefied biomass
A fundamental investigation was conducted on the combustion characteristics of pulverized torrefied biomass particles both in air and in simulated oxy-combustion gases, containing oxygen and carbon dioxide. Power generation by oxy-combustion of renewable biomass, coupled with carbon-dioxide capture and utilization or sequestration, can contribute to a reduction of the atmospheric concentration of carbon dioxide. This is because renewable biomass absorbs carbon dioxide during its growth, but this method will potentially release almost none to the atmosphere. The goal of this research is to determine the minimum oxygen mole fraction in the furnace input gases, which still induces single particle combustion characteristics that are similar to those encountered in air. Torrefied biomass types were herbaceous, waste crop, and woody. Entire luminous combustion profiles of single particles, burning in a drop-tube furnace, were recorded pyrometrically and cinematographically from ignition to extinction. Combustion took place in two phases: volatiles evolved and burned in spherical envelope flames and, upon extinction of these flames, the residual chars ignited and burned. Replacing the air background gas with 21%O-2/79%CO2 prolonged the ignition delay and the burnout time of biomass particles and reduced their temperature. To the contrary, further increasing the oxygen mole fraction in CO2 to 30% caused opposite trends. For each biomass type, the oxygen fraction in CO2 that would produce combustion temperature and time parameters most similar to those of air combustion was projected to be between those two oxygen concentrations (21-30%), and this was verified experimentally. An empirical correlation was formulated to make initial estimates of oxy-combustion concentrations needed for each fuel based on their physical and chemical properties. Reduction of the amount of oxygen in oxy-combustion is important to curtail the energy penalty inflicted by the operation of the air separation unit. (C) 2019 The Combustion Institute. Published by Elsevier Inc. All rights reserved.