Excess Air from Flue Gas Analysis

In operating plants, data on flue gas analysis will be available using which the plant engineer may arrive at the excess air used. This will help the plant engineer evaluate the boiler efficiency, and air and flue gas quantities. A formula that is widely used to obtain excess air E in % is [2]

(1.2) where O 2 , CO, and N 2 are % volume of oxygen, carbon monoxide, and nitrogen on dry flue

E = 100 × (O 2 – CO/2)/[0.264N 2 – (O 2 – CO/2)]

gas basis. Another formula is used when an Orsat-type analyzer is used for analyzing the flue gases; SO 2 will be absorbed with CO 2. The oxygen is on dry volumetric basis.

E=K 1 O 2 /(21 – O 2 ) (1.3) Constant K 1 = (100C + 237H 2 + 37.5S + 9N 2 – 29.6O 2 )/(C + 3H 2 + 0.375S – 0.375O 2 ) (1.4)

where C, H 2 ,N 2 , S, and O 2 are fraction by weight of carbon, hydrogen, nitrogen, sulfur, and oxygen in fuel.

Combustion Calculations

Let us check the value of constant K 1 for Example 1.1.

% weight of CH 4 in fuel = 83.4 × 16/(83.4 × 16 + 15.8 × 30 + 0.8 × 28) = 0.729. % weight of C 2 H 6 in fuel = 15.8 × 30/(83.4 × 16 + 15.8 × 30 + 0.8 × 28) = 0.259

%N 2 by weight in fuel = 0.012

Carbon C in CH 4 = 0.75 × 0.729 = 0.5467 and fraction hydrogen = 0.1823 Carbon in C 2 H 6 = (24/30) × 0.259 = 0.2072 and fraction hydrogen = 0.0518

Total C fraction by weight = 0.5467 + 0.2072 = 0.7539, and total hydrogen by weight = 0.0518 + 0.1823 = 0.2341. Hence,

K 1 = [100 × 0.7539 + 237 × 0.2341 + 9 × 0.012]/[0.7539 + 3 × 0.2341] = 89.9 Let us compute the heating value of the fuel from the weight fractions. Lower heat-

ing value (LHV) = 0.729 × 21,529 + 0.259 × 20,432 = 20,980 Btu/lb = 11,655 kcal/kg = 48,800 kJ/kg (where 21,529 and 20,432 in Btu/lb are the LHV of methane and ethane from Tables 1.1 and 1.2).

HHV = 0.729 × 23,879 + 0.259 × 22,320 = 23,188 Btu/lb = 12,882 kcal/kg

= 53,940 kJ/kg = 53.93 × 10 6 J/kg.

K 1 may also be obtained from Table 1.4 for various fuels. If oxygen is measured on true volume basis (wet basis), then one uses constant K 2 for excess air evaluation as shown:

K 2 = (100C + 363H 2 + 37.5S + 9N 2 – 29.6O 2 )/(C + 3H 2 + 0.375S – 0.375O 2 ) (1.5) Many modern analytical techniques, such as those employing infrared or paramagnetic

principles, also measure on a dry gas basis because they require moisture-free samples to avoid damage to the detection cells. These analyzers are set up with a sample condition- ing system that removes moisture from the gas sample. However, some analyzers, such as in situ oxygen detectors employing a zirconium oxide cell, measure on the wet gas basis. Results from such equipment need to be corrected to a dry gas basis before they are used in the ASME equations.

These values of K 1 and K 2 have been arrived at after performing calculations on several fuels with different fuel analysis and hence give a good working average value.

TABLE 1.4

Constants K 1 and K 2 for Excess Air Evaluation

Carbon monoxide

Blast furnace gas

Coke over gas

10 Steam Generators and Waste Heat Boilers: For Process and Plant Engineers

TABLE 1.5

Combustion Constants A (Air Required for 1 MM kJ fuel), kg

Fuel

A, kg/GJ

Max CO 2 (Dry Flue Gas) A, lb/MM Btu

575 Bagasse

Blast furnace gas

20 650 Coke oven gas

670 Refinery and oil gas

13.3 720 Natural gas

727–733 Fuel oils, furnace oil, and lignite

740–750 Bituminous coals

Using Table 1.4, for methane (natural gas), K 1 = 90. O 2 on dry flue gas basis is 3%. Hence, excess air = 90 × 3/(21 – 3) = 15%. If wet basis is used, then K 2 = 110.5. Then, excess air = 110.5 × 2.47/(21 – 2.47) = 14.7—close to 15%. These constants are good estimates for a type of fuel, and so some minor variations may be expected depending on actual fuel analysis.

There is another approximate method to get the excess air from CO 2 values, but the accu- racy is not good; see Table 1.5.

E = max CO 2 on dry flue gas basis/%CO 2 measured. In our example, from Table 1.5, max CO 2 = 12%, while actual is 10.3%. Hence, E = 12/10.3 = 1.165 or 16.5% excess air. This is only an estimate. The O 2 basis is more accurate.

Excess Air from Flue Gas Analysis