Modern Engineering Thermodynamics

612 CHAPTER 15: Chemical Thermodynamics
EXAMPLE 15.9
Compute the heat of reaction of methane when the reactants are at the standard reference state but the products are at 500.°C.
Assume that the product gases can be treated as ideal gases with constant specific heats and the combustion water is in its
vapor state.
Solution
Here, q r = h P − h R °, where, from Example 15.8, h R °= −74:873 MJ/kgmole CH 4 : Assuming all the components of the products
behave as ideal gases with constant specific heats, we can use Eq. (15.15) in Eq. (15.10) to find
h P = ∑
P
ðn i /n fuel Þ h°+c f p ðT − T° Þ i
= h° f + 2:00 h f° CO2 H2OðÞ g
+ 7:52 h f° N2
+ c p
CO2 + 2:00 c
p H2OðÞ g
+ 7:52 c
p N2
ðT
− T° Þ
= −393:522 + 2:00ð−241:827Þ + 0 + ½0:03719 + 2ð0:03364Þ
+ 7:52ð0:02908ÞŠð500 − 25Þ = −723:680 MJ/kgmole CH 4
where the molar specific heats are obtained from Table C.13b (note that they are converted from kJ/kgmole to MJ/kgmole
for use here). Then,
q r = −723:680 − ð−74:870Þ = −648:810 MJ/kgmole CH 4
Exercises
25. Repeat Example 15.9 for a products temperature of 800.°C rather than 500.°C. Assume all other variables are
unchanged. Answer: q r = −552 MJ=kgmole CH 4 .
26. Rework Example 15.9 for the case where the combustion products have been cooled to 30.0°C and the water in the
products is in the liquid state. Answer: q r = −889 MJ=kgmole CH 4 .
27. Use the method of Example 15.9 to determine the heat of reaction of acetylene when the reactants are at the standard
reference state and the products are at 1000.°C. Answer: q r = −990 MJ=kgmole C 2 H 2 .
In Example 15.9, note that, even though the nitrogen does not enter into the chemistry of the reaction, it does
enter into the thermodynamics of the reaction, because a significant portion of the heat of combustion went
into heating the nitrogen from 25.0°C to500.°C. Thus, it is easy to see why the use of too much excess air can
reduce the net heat production of a combustion reaction and cause significant energy losses to the environment
via hot exhaust gases.
EXPLOSION LIMITS AND IGNITION TEMPERATURES
OF COMMON FUELS
Fuel burns continuously only when the amount of fuel and air present are within the explosion (or flammability) limits of the
reaction. A fuel−air mixture does not ignite when the mixture is below the lower explosion limit (LEL) or when it is above
the upper explosion limit (UEL). Both temperature and pressure affect these limits. For example, as the pressure of the mixture
decreases below atmospheric pressure, the UEL decreases and the LEL increases. However, as the pressure increases
above atmospheric pressure, the UEL increases but the LEL stays nearly constant. Also, as the mixture temperature increases,
the UEL increases and the LEL decreases.
The ignition temperature is the lowest combustion temperature at which more heat is generated by the combustion process
than is lost to the surroundings. A fuel−air mixture does not burn continuously if the combustion temperature is below
the ignition temperature, unless heat is supplied to the reaction from the surroundings.
Table 15.4 lists explosion limits and ignition temperatures for some common fuel−air mixtures. These LEL and UEL values
are lower and upper fuel to air ratio explosion limit percentages on a molar (or volume) basis.
Note that the LEL for hydrogen is higher than almost all the hydrocarbon fuels shown (including gasoline), making it less
dangerous from a LEL explosion point of view.