# Relating FGR and Oxygen in the Wind-Box

Flue gas recirculation affects the oxygen in the wind-box by diluting it. One may measure the oxygen values to evaluate the FGR rate used.

TABLE 3.8 Effect of FGR on Boiler Performance

 Load (%) 100 100 50 50
 Excess air, % 15 15 15 15 FGR, % 0 15 0 15 Combustion, temp, °F 3,230 2,880 3,230 2,880 Furnace exit temp, °F 2,350 2,188 2,007 1,956 Gas temp to superheater, °F 1,695 1,630 1,323 1,334 Gas temp to evaporator, °F 1,250 1,240 944 973 Gas temp to economizer, °F 630 645 543 555 Gas temp leaving 300 315 263 270 Economizer, °F Flue gas flow, lb/h 185,500 215,000 88,900 104,000 Efficiency, % HHV 84.26 83.9 85.1 84.9 Steam flow, lb/h 150,000 150,000 75,000 75,000 Steam temp, °F 748 756 686 711 Economizer exit water 338 355 318 333 3 P F Boiler backpressure, 6.2 7.8 2.0 2.5 In. WC Feedwater temp, °F 228 228 228 228
 Fuel: standard natural gas; 1% blowdown; steam pressure = 650psig.

Example 8

A boiler firing natural gas at 15% excess air uses 119,275 lb/h of combustion air, and about 14,000 lb/h of flue gases is recirculated. Determine the oxygen levels in the wind-box. Let us assume that the air is dry and is 77% by weight nitrogen and 23% oxygen. Then the amount of nitrogen in air = 0.77 x 119,275 = 91,842 lb/h, and that of oxygen = 27,433 lb/h.

The flue gas analysis (vol%) is CO2 = 8.29, H2O = 18.17, N2 = 71.07, and O2 = 2.47.

To convert to percent by weight (wt%) basis, first obtain the molecular weight:

MW = (8.29 x 44 + 18.17 x 18 + 71.07 x 28 + 2.47 x 32)/100 = 27.61

% CO2 = 8.29 x 44/27.61 = 13.21 Similarly, H2O = 11.84 wt%, N2 = 72.07, and O2 = 2.88.

The individual constituents in the mixture of 14,000 + 119,275 = 133,275 lb/h of gases are

CO2 = 0.1321 x 14,000 = 1849.4 lb/h H2O = 0.1184 x 14,000 = 1658 lb/h N2 = 91,843 + 0.7207 x 14,000 = 101,922 lb/h

O2 = 27,433 + 0.0288 x 14,000 = 27,836 lb/h

Converting this to percent by volume basis as we did earlier, we have CO2 = 0.9 vol %, H2O = 1.98, N2 = 78.37, and O2 = 18.75