Flue Gas Recirculation and Excess Air
Present-day packaged steam generators operate at high excess air (15-20%) with flue gas recirculation (FGR) rates ranging from 0% to 30% to limit CO and NOx. Flue gas recirculation refers to the admission of flue gases from the boiler exit back into the burner region in order to lower the combustion temperature, as shown in Fig. 4.6, which in turn lowers NOx. See Table 4.2 for the effect of FGR on combustion temperature.
The reason for the use of high excess air can be seen from Fig. 4.10, which shows that as the excess air is increased, the NOx level increases and then drops off. At substoichiometric conditions, the combustion temperature is not high and hence the NOx formation is less; however, as the excess air increases, the combustion temperature increases, which results in higher NOx. Further increase in excess air (or FGR) lowers the flame temperature and hence NOx decreases. Also, at a low excess air rate, the CO generation is high due to poor mixing between fuel and air. Hence to meet both CO and NOx levels, 15% excess air and 15% FGR rates are not unusual today in oil — and gas-fired steam generators. Some burner suppliers recommend 15% excess air and 30% FGR rates to limit the NOx to less than 9 ppmv on natural gas firing. The FGR system naturally adds
Table 4.2 Effect of FGR on Combustion Temperatures with 15% Excess Air
To both the initial and operating costs of the boiler. Hence excess air on the order of 5%, which was typical decades ago, is not adequate to limit CO, though efficiencywise it makes sense. The combination of high FGR rate and excess air factor increases the mass flow of flue gases through the boiler, though the steam generation may be unchanged, making it necessary to use a larger boiler for the same duty. If the same boiler (designed several decades ago) were used, the flue gas mass flow through the boiler could be 20-25% higher, resulting in significant pressure drop across the heating surfaces and consequently higher fan power consumption.
Table 4.3 shows the effect of different excess air and FGR rates on the performance of a boiler of 100,000 lb/h capacity generating steam at 300 psig using feedwater at 230°F. Cases 1 and 2 use an economizer. Cases 3 and 4 show the results without the economizer. In all these calculations the boiler is assumed to be the same and the burner is changed to handle the higher excess air and FGR rate. The new burner is assumed to have the same pressure drop as the earlier one. The pressure drop differences shown are due to the difference in the flue gas flow rates through the boiler.
Using an electricity cost of 7 cents/kWh and fuel cost of $3/MM Btu, the additional fuel and electricity costs due to the lower efficiency and higher gas pressure drop were computed and are shown below in Table 4.3. Due to the higher excess air and FGR rate, the annual operating cost increases by $43,400 in case 2 over case 1. This does not include the cost of the bypass system, damper, and controls. When the economizer is not present, the differential operating cost is even more, $69,000 per year. Two conclusions may be drawn from this study:
Table 4.3 Effect of Excess Air and Flue Gas Recirculation on Boiler Operating Costs
A With an economizer. b Without an economizer.
1. Modifying an existing boiler to handle new emission levels will be expensive in terms of operating costs.
2. Operating a boiler without the economizer results in a higher gas pressure drop even for the same excess air. Case 3 shows an increase of 1.6in. WC over case 1. This is due to the larger flue gas flow in case 3 arising out of lower boiler efficiency.
As shown in Fig. 4.13, the effect of FGR on NOx reduction gradually decreases as the FGR rate increases; that is, NOx reduction is very high at low FGR rates and as the FGR rate increases the incremental NOx reduction becomes smaller. On oil firing, the effect of FGR is less significant. Operators must consider the risk of operating a boiler near the limits of inflammability when using high amounts of FGR. Figure 4.14 Shows the narrowing between the upper flammability limit and the lower ignition limit as FGR increases. Integrating control systems to maintain fuel/air ratios at high FGR rates is difficult because FGR dampens the combustion process to the ragged edges of flammability— flame-outs and flame instability. Full metering combustion control systems with good safety measures are necessary in such cases.
As the FGR rate increases, the gas pressure drop across the boiler increases, and the boiler must be made larger with wider tube spacing or the fan power consumption can be significant as shown iN Table 4.3. A boiler using 20% FGR is equivalent to a 20% increase in its size compared to a boiler of the same capacity not using FGR. One has to be concerned about the flame stability at low loads and also the excess CO formed. Generally, in packaged boilers the FGR duct is connected to the fan inlet duct and a separate FGR fan is not required. Large
Figure 4.13 NOx versus flue gas recirculation.
1 H——————————— 1—————————- !—————————— 1—————————— 1—————————— 1—————————- 1
0 10 20 30 40 50
FIgure 4.14 Flue gas recirculation and limits of inflammability. (Adapted from newsletter of Coen Co., Spring 1996, Burlingame, CA.)
Industrial boilers use separate FGR fans. If the flue gases contain oxides of sulfur, then mixing the flue gases at the fan inlet may lower the temperature below the acid vapor point and risk potential corrosion at the fan and inlet ductwork; in such cases, a separate fan may be used to admit the flue gases directly near the burner throat. With induced FGR, the inlet temperature to the fan increases. With 80°F ambient temperature and 15% FGR at 320°F flue gas temperature, the mixed air temperature at the fan inlet is about 112°F. The air density decreases, which results in a slightly larger volume of air to be handled by the fan. FGR also affects the performance of steam generators because it affects the gas temperature profile throughout the boiler. This is illustrated in Chapter 3.