Page 231 - Mechanical Engineers' Handbook (Volume 4)

220 Furnaces

The heat released in a combustion reaction is
Total heats of formation of combustion products Total heats of formation of reactants
Heats of formation can be conveniently expressed in terms of Btu per pound mol, with
the pound mol for any substance equal to a weight in pounds equal to its molecular weight.
The heat of formation for elemental materials is zero. For compounds involved in common
combustion reactions, values are shown in Table 4.
Data in Table 4 can be used to calculate the higher and lower heating values of fuels.
For methane:
CH 2O CO 2H O
4
2
2
2
HHV
169,290 (2 122,976) 32,200 383,042 Btu/lb mol
383,042/385 995 Btu/scf
LHV
169,290 (2 104,040) 32,200 345,170 Btu/lb mol
345,170/385 897 Btu/scf
Available heats from combustion of fuels, as a function of flue gas and preheated air
temperatures, can be calculated as a fraction of the HHV. The net ratio is one plus the
fraction added by preheated air less the fraction lost as sensible heat and latent heat of water
vapor, from combustion of hydrogen, in flue gas leaving the system.
Available heats can be shown in chart form, as in the following figures for common
fuels. On each chart, the curve on the right is the fraction of HHV available for combustion
with 110% cold air, while the curve on the left is the fraction added by preheated air, as
functions of air or flue gas temperatures. For example, the available heat fraction for methane
burned with 110% air preheated to 1000 F, and with flue gas out at 2000 F, is shown in Fig.
1: 0.41 0.18 0.59 HHV.
Values for other fuels are shown in charts that follow:
Fig. 2, fuel oils with air or steam atomization
Fig. 3, by-product coke oven gas
Fig. 4, blast furnace gas
Fig. 5, methane

Table 4 Heats of Formation
Molecular Heats of Formation
a
Material Formula Weight (Btu/lb mol )
Methane CH 4 16 32,200
Ethane C 2 H 6 30 36,425
Propane C 3 H 8 44 44,676
Butane C 4 H 10 58 53,662
Carbon monoxide CO 28 47,556
Carbon dioxide CO 2 44 169,290
Water vapor H 2 O 18 104,040
Liquid water 122,976
a The volume of 1 lb mol, for any gas, is 385 scf.