Modern Engineering Thermodynamics

15.7 Heat of Reaction 609
CRITICAL THINKING
In the calculation of the higher and lower heating values, HHV and LHV, both the reactants and the products are at the
SRS. In the HHV calculation, all the water in the combustion products is condensed into a liquid, but in the LHV calculation,
all this water is in the vapor phase. Wait a minute, how can you have all the LHV water in the vapor phase at the SRS
temperature of 25.0°C, which is clearly below the dew point temperature of water at the SRS pressure of 0.100 MPa (T DP =
T sat (0.100 MPa) = 100°C)? The answer is that you cannot, so the LHV state does not really exist. But, if the LHV can never
be achieved, then what good is it? The answer is that the LHV is merely a benchmark value used for evaluating combustion
processes. For example, the HHV and LHV are most commonly used in the calculation of combustion efficiency η combustion ,
defined as
η combustion =
Actual heat produced by a combustion process
HHV or LHV
In this calculation, the HHV is used if the temperature of the combustion products can be reasonably lowered below 100.°C
(as in industrial power plant boilers or domestic home furnaces), and the LHV is used when this is not possible (as in internal
combustion engines). When the temperatures of the combustion products are necessarily above 100.°C, the LHV makes a
more reasonable benchmark for the calculation of a combustion efficiency (note that, since the LHV is less than the HHV, the
combustion efficiency is larger when the LHV is used).
enthalpy of water at the standard reference state temperature (25.0°C or 77.0°F). Note that both the HHV and
the LHV are negative for heat-producing combustion reactions.
If the reactants and the products are both at the standard reference state, then Eq. (15.11) gives the standard reference
state heat of reaction, q r °,as
q r °=∑
P
ðn i /n fuel
Þ h f ° i −∑ R
ðn i /n fuel Þ h° f (15.13)
i
This equation can be used to determine the HHV and LHV heats of combustion tabulated in Tables 15.2
and 15.3, as illustrated in the following example.
Table 15.2 Molar Based Higher Heating Values (HHV) for Common Fuels Where Both
the Reactants and the Products Are at the SRS and the Water in the Combustion Products
Is in the Liquid Phase
(HHV) molar
Fuel
MJ/kgmole
Btu/lbmole
Hydrogen, H 2 (g) −285.84 −122,970
Carbon, C(s) −393.52 −169,290
Carbon monoxide, CO(g) −282.99 −121,750
Methane, CH 4 (g) −890.36 −383,040
Acetylene, C 2 H 2 (g) −1299.60 −559,120
Ethylene, C 2 H 4 (g) −1410.97 −607,010
Ethane, C 2 H 6 (g) −1559.90 −671,080
Propylene, C 3 H 6 (g) −2058.50 −885,580
Propane, C 3 H 8 (g) −2220.00 −955,070
n-Butane, C 4 H 10 (g) −2877.10 −1,237,800
Benzene, C 6 H 6 (g) −3270.0 −1,406,000
n-Octane, C 8 H 18 (g) −5454.5 −2,345,000
n-Decane, C 10 H 22 (g) −6754.7 −2,904,000
Methyl alcohol, CH 3 OH(g) −764.54 −328,700
Ethyl alcohol, C 2 H 5 OH(g) −1409.30 −606,280
Source: Reprinted by permission of the publisher from Holman, J. P., 1980. Thermodynamics, third ed. McGraw-Hill, New York, p. 466
(Table 11-1).