Oxy-combustion behavior of torrefied biomass particles

Abstract

A fundamental investigation was conducted on the combustion characteristics of torrefied biomass in both air and simulated oxy-combustion conditions containing 21% or 30% oxygen mole fractions in a carbon dioxide balance. The targeted torrefied biomass types were herbaceous, waste crop, and woody. The experimental setup that was used in this investigation consisted of a droptube furnace, operated at a wall temperature of 1400 K coupled to an optical pyrometer and a highspeed high-resolution camera. Entire luminous particle combustion profiles of single particles were recorded pyrometrically and cinematographically. Combustion of these particles took place in two phases. Initially, volatiles evolved and burned in spherical envelope flames; then, upon extinction of these flames, char residues ignited and burned. Replacing the background gas from air to an oxy-fuel atmosphere, at 21% O2 reduced the luminosity of flame; reduced the particle combustion temperatures and prolonged their burnout times. Increasing the oxygen mole fraction in CO2 to 30% increased the luminosity of the flame. For each biomass sample, the necessary oxygen concentration that would produce combustion temperature and time parameters most similar to those encountered in air combustion was determined through experiments at lower and upper thresholds. A correlation was derived for exploratory predictions of the oxygen level requirement for effective oxy-combustion of a fuel, based on its physical and chemical properties.Accurate experimental determination of this level is still necessary to minimize the amount of the oxygen that needs to be supplied by an air-separation unit in order to reduce the energy consumption penalty associated with this process.