Flue gas recirculation is widely used as a method of NOx control because it reduces the flame temperature and thus lowers NOx formation as discussed in Chapter 4. The effect of FGR on boiler performance is quite significant. Not only is the gas temperature profile across the boiler different, but the steam tempera­ture and gas pressure drop are also affected.

Table 3.8 Shows the performance of a 150,000 lb/h boiler with and without FGR. The following points may be noted:

1. The flue gas quantity increases with FGR; hence the backpressure increases at all loads.

2. The steam temperature is higher with FGR in both 100% and 50% load cases, but the difference is greater at low loads.

3. The furnace exit gas temperature is lower with FGR, and the gas temperature across the superheater is higher at 50% load than at 100%. Thus load plays a big role in the temperature profiles.

4. The efficiency naturally drops due to the higher stack gas temperature at both 100% and 50% loads.