Duct burners used in HRSGs also generate NOx and CO, adding to the emissions from the turbine exhaust gases. The calculation procedure for estimating the NOx and CO in ppmv after combustion is shown in Q6.26e. It may be noted that the values of NOx and CO in lb/h are always higher after combustion; however, the values in ppmv may or may not be, depending on the initial ppmv values of NOx and CO and the contribution by the burner. Typical NOx and CO emissions from duct burners are listed in Table 4.1.
With distillate oils containing fuel-bound nitrogen in the range of 0.05%, nearly 80-90% of it is converted to NOx, whereas with heavy fuel oils with 0.3% nitrogen, about 50% of it is converted to NOx. In the case of packaged boiler burners, the emissions depend on burner design, on whether fuel is premixed with air, on whether fuel or air is staged, and on the combustion temperature as discussed below. NOx emission ranges from 0.04 to 0.1 lb/Mm Btu for natural gas firing and increases if hydrogen or a fuel with a high combustion temperature
TAble 4.1 Typical Emissions from Various Fuels
Is fired. Typical CO emissions range from 30 to 100ppmv. Combustion technology is improving day by day. Readers should note that significant changes in burner design or combustion techniques could be made available to the industry before this book is even published!