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

15–2 
 THEORETICAL AND ACTUAL

COMBUSTION PROCESSES

It is often instructive to study the combustion of a fuel by assuming that the

combustion is complete. A combustion process is completeif all the carbon

in the fuel burns to CO 2 , all the hydrogen burns to H 2 O, and all the sulfur (if

any) burns to SO 2. That is, all the combustible components of a fuel are

burned to completion during a complete combustion process (Fig. 15–8).

Conversely, the combustion process is incompleteif the combustion prod-

ucts contain any unburned fuel or components such as C, H 2 , CO, or OH.

Insufficient oxygenis an obvious reason for incomplete combustion, but it

is not the only one. Incomplete combustion occurs even when more oxygen

is present in the combustion chamber than is needed for complete combus-

tion. This may be attributed to insufficient mixing in the combustion cham-

ber during the limited time that the fuel and the oxygen are in contact.

Another cause of incomplete combustion is dissociation, which becomes

important at high temperatures.

Oxygen has a much greater tendency to combine with hydrogen than it

does with carbon. Therefore, the hydrogen in the fuel normally burns to

completion, forming H 2 O, even when there is less oxygen than needed for

complete combustion. Some of the carbon, however, ends up as CO or just

as plain C particles (soot) in the products.

The minimum amount of air needed for the complete combustion of a fuel

is called the stoichiometricor theoretical air.Thus, when a fuel is com-

pletely burned with theoretical air, no uncombined oxygen is present in the

product gases. The theoretical air is also referred to as the chemically cor-

rect amount of air,or 100 percent theoretical air.A combustion process

with less than the theoretical air is bound to be incomplete. The ideal com-

bustion process during which a fuel is burned completely with theoretical

air is called the stoichiometricor theoretical combustionof that fuel (Fig.

15–9). For example, the theoretical combustion of methane is

Notice that the products of the theoretical combustion contain no unburned

methane and no C, H 2 , CO, OH, or free O 2.

CH 4  21 O 2 3.76N 22 SCO 2 2H 2 O7.52N 2

756 | Thermodynamics

oxygen, giving a total of 95.2 moles of air. The air–fuel ratio (AF) is deter-

mined from Eq. 15–3 by taking the ratio of the mass of the air and the mass

of the fuel,

That is, 24.2 kg of air is used to burn each kilogram of fuel during this

combustion process.

24.2 kg air/kg fuel



120 4.76 kmol 21 29 kg>kmol 2

1 8 kmol 21 12 kg>kmol 2  1 9 kmol 21 2 kg>kmol 2

AF

m (^) air
m (^) fuel

1 NM (^2) air
1 NM (^2) C 1 NM (^2) H 2
CH 4 + 2(O 2 + 3.76N 2 ) →

• no unburned fuel


• no free oxygen in products

CO 2 + 2H 2 O + 7.52N 2

FIGURE 15–9

The complete combustion process

with no free oxygen in the products is

called theoretical combustion.

Combustion

AIR chamber

CnHm

Fuel n CO 2

m H

2 O

Excess O 2

N 2

2

FIGURE 15–8

A combustion process is complete if
all the combustible components of the
fuel are burned to completion.