Page 55 - Handbook of Energy Engineering Calculations

Note that the percent weight in the above list is calculated after obtaining the
sum under Column 2 × Column 3. Thus, the percent methane =
(1334.4)/(1334.4 + 474 + 22.4) = 72.89 percent.
From a standard reference, such as Ganapathy, Steam Plant Calculations
Manual, Marcel Dekker, Inc., find the combustion constants, K, for various

fuels and use them thus: For the air required for combustion, A = (K for
c
methane)(percent by weight methane from above list) + (K for ethane)

(percent by weight ethane); or A = (17.265)(0.7289) + (16.119)(0.2589) =
c
16.76 lb/lb (16.76 kg/kg).
Next, compute the higher heating value of the fuel (HHV) using the air

constants from the same reference mentioned above. Or HHV = (heat of
combustion for methane)(percent by weight methane) + (heat of combustion
of ethane)(percent by weight ethane) = (23,879)(0.7289) + (22,320)(0.2589)

= 23,184 Btu/lb (54,018.7 kJ/kg). Then, the amount of fuel equivalent to
1,000,000 Btu (1,055,000 kJ) = (1,000,000)/23,184 = 43.1 lb (19.56 kg),
which requires, as computed above, (43.1)(16.76) = 722.3-lb dry air (327.9
kg), which agrees closely with the value given in step 1, above.
Similarly, if the fuel were 100 percent methane, using the steps given

above, and suitable constants from the same reference work, the air required
for combustion is 17.265 lb/lb (7.838 kg/kg) of fuel. HHV = 23,879 Btu/lb
(55,638 kJ/kg). Hence, the fuel in 1,000,000 Btu (1,055,000 kJ) =

(1,000,000)/(23,879) = 41.88 lb (19.01 kg). Then, the dry air per million Btu
(1.055 kg) fired = (17.265) (41.88) = 723 lb (328.3 kg).
Likewise, for propane, using the same procedure, 1 lb (0.454 kg) requires
15.703-lb (7.129-kg) air and 1 million Btu (1,055,000 kJ) has
(1,000,000)/21,661 = 46.17-lb (20.95-kg) fuel. Then, 1 million Btu

(1,055,000 kJ) requires (15.703)(46.17) = 725-lb (329.2-kg) air. This general
approach can be used for various fuel oils and solid fuels—coal, coke, etc.
Good estimates of excess air used in combustion processes may be

obtained if the oxygen and nitrogen in dry flue gases are measured.
Knowledge of excess-air amounts helps in performing detailed combustion
and boiler efficiency calculations. Percent excess air, EA = 100(O −
2
CO )/[0.264 × N – (O – CO/2)], where O = oxygen in the dry flue gas,
2
2
2
2
percent volume; CO = percent volume carbon monoxide; N = percent
2
volume nitrogen.