1 августа, 2013 
doctype 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. |
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
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