Combustion of wood-chips in circulating fluidized bed boilers - NO and CO emissions as functions of temperature and air-staging

Reduction of NO emissions during combustion of wood-chips in a 12 MW circul ating fluidized bed boiler was investigated by lowering the air-ratio in th e combustor through secondary air addition in the cyclone outlet. This mode of operation was studied at three loads and also in combination with secon dary air addition in the cyclone inlet and/or at 2.2 m height. A significan t reduction in NO emissions was obtained without high CO emissions. Importa nt for the success of NO abating measures was that the CO emissions should not increase to high levels. Therefore the conditions for CO burnout were s tudied. The temperature in the duct downstream of the cyclone outlet, the " exit chamber", was found to be important, as the burnout of CO was not comp leted in the cyclone, not even under normal operating conditions. The expla nation seems to be incomplete mixing of the gases entering the cyclone. The re was a clear correlation between the temperature in the exit chamber and the CO emissions, and various measures taken to increase the temperature in the exit chamber were effective in reducing CO emissions. (C) 1999 Elsevie r Science Ltd. All rights reserved.