Impact of biomass char on coal char burn-out under air and oxy-fuel conditions

Abstract

Although biomass co-firing is now well established in pulverised fuel (PF) combustion under conventional air-fired conditions, there is little information available on how biomass will behave in oxy-fuel firing. Using thermogravimetric analysis (TGA) and a drop tube furnace (DTF), this study examines the impact of co-firing biomass and coal under oxy-fuel conditions compared to normal air firing, with the emphasis on the potential catalytic effect of biomass-contained alkali and alkaline metals on coal char burnout. Individual chars and their blends prepared from sawdust, pinewood and a South African coal in a DTF and normal tube furnace under slow-heating conditions have been used in TGA char burnout tests. The results demonstrate that the coal/biomass char blends burned off significantly faster than predicted under both oxy-fuel and air-firing conditions, and this synergistic catalytic effect was found to be considerably more pronounced in oxy-fuel conditions. In particular, the DTF biomass/coal char blends from devolatilisation in CO2 burn off approximately two times faster than those prepared in nitrogen. To further examine the catalytic effect, the raw sawdust sample was first extracted with 5M hydrochloric (HCl) acid to remove its contained alkali and alkaline metals before the char preparation and subsequent char burnout tests. It was found that the removal of the alkali and alkaline metals led to almost complete loss of the catalytic effect as observed with the untreated sawdust derived char samples. The results indicate that biomass having relatively high contents of alkali and alkaline metals can serve as effective combustion catalysts to improve coal combustion efficiency.