Wet oxy-combustion of blends of coal and biomass

Abstract

Oxy-fuel combustion of blends of coal and biomass has been experimentally investigated, largely replacing CO2 with H2O as temperature moderator in the firing atmosphere. The novel concept of the so-called oxy-steam combustion, i.e. combustion in O2/H2O atmospheres, merges the advantages of reducing auxiliary consumptions and lowering air in-leakages. Two blends of coal and biomass have been tested, in 80:20 mass ratio. Forestry wood residues from pine and raw residues from vineyard pruning have been selected. The experiments have been carried out in lab-scale electrically-heated entrained flow reactor for a set of O2/CO2 and O2/CO2/H2O atmospheres, with steam contents up to 40 % and oxygen contents up to 35%. When 20% of coal is substituted with biomass, fuel conversion rates increase and NO formation rates decrease, with a different extent depending on the biomass type. The higher volatile and lower nitrogen contents of the biomass determine that observation. As regards the effect of CO2 replacement by H2O, it can be observed that particular steam concentrations lead to the maximum fuel conversion and the minimum NO formation rates. This behavior is related to the effects of steam when replacing carbon dioxide: increase of flame temperature, oxidant diffusivity and char gasification, as well as active participation in NOx reduction mechanisms in the gas-phase.