Microsoft Word - Cengel and Boles TOC _2-03-05_.doc

Chapter 15 | 777

Adiabatic

combustion

chamber

25 °C, 1 atm

CO 2

T 0 = 25°C

AIR

CH 4

25 °C, 1 atm H

2 O

O 2

N 2

FIGURE 15–33

Schematic for Example 15–10.

EXAMPLE 15–10 Second-Law Analysis of Adiabatic Combustion

Methane (CH 4 ) gas enters a steady-flow adiabatic combustion chamber at
25°C and 1 atm. It is burned with 50 percent excess air that also enters at
25°C and 1 atm, as shown in Fig. 15–33. Assuming complete combustion,
determine (a) the temperature of the products, (b) the entropy generation,
and (c) the reversible work and exergy destruction. Assume that T 0 298 K
and the products leave the combustion chamber at 1 atm pressure.

Solution Methane is burned with excess air in a steady-flow combustion
chamber. The product temperature, entropy generated, reversible work, and
exergy destroyed are to be determined.
Assumptions 1 Steady-flow conditions exist during combustion. 2 Air and
the combustion gases are ideal gases. 3 Changes in kinetic and potential
energies are negligible. 4 The combustion chamber is adiabatic and thus
there is no heat transfer. 5 Combustion is complete.
Analysis (a) The balanced equation for the complete combustion process
with 50 percent excess air is

Under steady-flow conditions, the adiabatic flame temperature is determined
from HprodHreact, which reduces to

since all the reactants are at the standard reference state and h

• f°O for O 2
  and N 2. Assuming ideal-gas behavior for air and for the products, the h


• 
  

  f° and
  hvalues of various components at 298 K can be listed as

  
  

h

• f° h
  
  
  • 298 K
    Substance kJ/kmol kJ/kmol
  
  
  
  

CH 4 (g) 
74,850 —

O 2 0 8682

N 2 0 8669

H 2 O(g) 
241,820 9904

CO 2 
393,520 9364

Substituting, we have

which yields

By trial and error, the temperature of the products is found to be

Tprod1789 K

hCO 2  2 hH 2 OhO 2 11.28hN 2 937,950 kJ

 1 1 kmol CH 421 
74,850 kJ>kmol CH 42

¬ 1 1 kmol O 22310 hO 2
86822 kJ>kmol O 24

¬ 1 11.28 kmol N 22310 hN 2
86692 kJ>kmol N 24

¬ 1 2 kmol H 2 O 231
241,820hH 2 O
99042 kJ>kmol H 2 O 4

¬ 1 1 kmol CO 2231
393,520hCO 2
93642 kJ>kmol CO 24

aNp^1 h°fh
h°^2 paNrh°f,r^1 Nh°f^2 CH 4

CH 41 g 2  31 O 2 3.76N 22 SCO 2 2H 2 OO 2 11.28N 2