Combustion and NOx emissions characteristics of a down-fired 660-MWe utility boiler retro-fitted with air-surrounding-fuel concept

Air-surrounding-fuel is a well-known concept used within tangential and wall-fired boilers. Here, we report for the first time on industrial experiments performed to study the effects of this concept on a 660 MWe full-scale down-fired boiler. Data are reported for the gas temperature distributions along the primary air and coal-mixed flows, furnace temperatures, gas compositions, for example O2, CO and NOx, and gas temperatures in the near-wall region. The influence of concentration control valve (CCV) opening on combustion and NOx emission in the furnace were determined. The results show that the flame stability, temperature distribution, unburnt carbon are influenced by both concentration ratios and fuel-rich flow velocities. As CCV opening increases, NOx emissions decrease from 2594 mg/m3 to 1895 mg/m3. Considering altogether economic benefits and environmental protection issues, 30% is the optimal value for the CCV opening.

Research highlights
► The application of air-surrounding-fuel concept on down-fired boiler.
► Experiments on a 660 MWe down-fired boiler, of the largest capacity in the world.
► The concentration ratio of the fuel-rich flow should be neither too large nor too small in down-fired boiler.
► A 30% CCV opening appears to be the better option for this boiler retro-fitted with air-surrounding-fuel concept.