Chemical, thermal and dilution effects of carbon dioxide in oxy-fuel combustion of wood in a fixed bed

Experimental and numerical modeling was performed on eucalyptus wood combustion under oxy-fuel conditions using a fixed bed reactor in order to isolate the role of various carbon dioxide effects on the burning rate. Wood combustion was investigated under four different mixtures of O-2 and Ar/CO2/N-2: 21 % O-2/79 % N-2; 21 % O-2/22.5 % CO2/56.5 % Ar; 40 % O-2/60 % CO2; and 40 % O-2/47 % CO2/13 % Ar. The first three mixtures were designed to have the same peak temperatures in order to isolate chemical and dilution effects of CO2. This was achieved by substituting some percentage of CO2 with Ar in O-2/CO2 mixture while maintaining a constant concentration of O-2. The fourth mixture was meant to isolate the thermal effect of CO2. The results were obtained from both the experimental rig and numerical simulation for a fixed bed configuration. Wood combustion in the fixed bed was modeled using Lagrange-Euler method, where gas-phase was calculated using computational fluid dynamics (CFD), that is Euler phase, while solid-phase was tracked in Lagrange phase using discrete element method (DEM). The results show that ignition time in CO2 environment decreases gradually as O-2 concentration is increased. On the other hand, burning rate and flame front speed increase as O-2 concentration is increased. It was established that dilution effect is the most influential parameter on the burning rate of wood combustion in an oxy-fuel system.