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

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 10 kWt 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 N-2 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. (C) 2015 Elsevier Ltd. All rights reserved.