Effects of O2 Feeding Strategy and Over-Fire Air Configuration on Oxy-Fuel Combustion Characteristics in an Opposed Wall-Fired Utility Boiler

Abstract

Much attention has been focused on the flue gas recycle mode and O2 concentration in previous numerical studies on oxy-fuel utility boilers, while the effects of O2 feeding strategy and OFA configuration on oxy-fuel combustion characteristics in a 600 MW wall-fired utility boiler were mainly investigated in the present study. The results showed that the optimal O2 volume fraction for the oxy-fuel combustion of the wall-fired boiler was approximately 30%. The O2 feeding strategy of decreasing the O2 concentration in inner second air (denoted by [O2]ISA) could effectively reduce NOx formation without affecting the flame stability. In addition, staged oxy-fuel combustion could be achieved with the burner configuration of closing inner second air (ISA) nozzles meanwhile opening over fire air (OFA) nozzles. The NOx emission decreased significantly from 149.05 to 97.83 mg/MJ when the staged oxy-fuel combustion was employed. Furthermore, the burnout of char and CO under staged oxy-fuel condition was greatly affected by the velocity of OFA. Hence, a modified OFA configuration was proposed to promote the turbulent mixing condition in the upper furnace after the staged oxy-fuel combustion was adopted. With the application of the modified OFA configuration, the unburned carbon in fly ash decreased to the same level as that in air combustion (oxy-fuel combustion of 4.83% and air combustion of 4.67%), while the NOx emission remained at 96.48 mg/MJ. The present work will be helpful for the retrofitting of the existing utility boiler with oxy-fuel combustion technology.