Effect of FGR position on the characteristics of combustion, emission and flue gas temperature deviation in a 1000 MW tower-type double-reheat boiler with deep-air-staging

•Injection positions affect the flow characteristics of RFG in the furnace.•Higher temperature produce lower NO emission under fuel-rich condition.•NO emission reduces 10–30% with RFG introduced from L-nozzle to H-nozzle.•The temperature deviation of tower-type boiler is small.•The nozzle above the burner is preferable for RFG introduction. Flue gas recirculation (FGR), which is widely adopted to control the steam temperature of ultra-supercritical (USC) double-reheat boiler, significantly affects the temperature distribution, NOx generation, and flue gas temperature deviation in furnace. This study presents a numerical study on the combustion process in a 1000 MW tower-type double-reheat boiler with deep-air-staging. The effect of FGR injection position on the temperature distribution, NOx emission, and gas temperature deviation was studied and discussed under 100% boiler maximum continuous rating (BMCR), 75% turbine heat acceptance (THA), and 50% THA loads. The numerical model was validated by the in-house thermal calculation code and design parameters provided by the boiler manufacturer. The results show that, under the same load, when recirculating flue gas (RFG) is injected into the furnace from the nozzle blow the burner, the burner zone temperature is lower but the NO generation is higher than the case in which RFG is injected from the nozzle above the burner. Compared to the former, NOx emission of the latter reduced by 12.3%, 31.2%, and 33.8% under 100% BMCR, 75% THA, and 50% THA load, respectively. Both the inlet flue gas temperature and the heat flux of the primary superheater decrease when RFG was introduced from the nozzle above the burner. The case in which RFG is injected from the nozzle above the burner was found to be an applicable and effective method to control the NO generation and regulate the steam temperature of the double-reheat boiler.