NO emission during co-firing coal and biomass in an oxy-fuel circulating fluidized bed combustor

Abstract

Co-firing biomass and coal under oxy-fuel combustion in a circulating fluidized bed combustor (CFBC) holds the potential for negative CO2 emission into the atmosphere and is a promising technology to realized atmospheric CO2 reduction. Experiments co-firing coal and three kinds of Chinese biomass, i.e. rice husk (RH), wood chips (WC) and dry wood flour (WF) under oxy-fuel condition were carried out in a 10kWt CFBC. Results show that burning biomass separately produces higher NO emissions and a higher fuel nitrogen conversion ratio than burning coal without biomass addition due to the higher volatile matter content of the biomass; the fuel nitrogen conversion ratio is in agreement with the H/N ratio in the fuel under both air and oxy-fuel atmosphere. In oxy-fuel combustion, lower NO emission is observed than in air combustion, because CO2 replacing N2 reduces the yield of NO precursors like NH3 during the devolatilization process and enhances NO reduction via char/NO/CO reaction. NO emission increases as temperature, overall oxygen concentration and primary oxidant fraction increase during oxy-fuel combustion. Oxygen staging succeeds in controlling NO emission in a comparatively low level at high overall oxygen concentration condition. The results can be helpful for the design and operation of the oxy-fuel fluidized bed combustor.