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 O-2, 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/m(3) to 1895 mg/m(3). Considering altogether economic benefits and environmental protection issues, 30% is the optimal value for the CCV opening. (C) 2010 Elsevier Ltd. All rights reserved.