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

Posted in Industrial Boilers and Heat Recovery Steam


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