Page 373 - Air pollution and greenhouse gases from basic concepts to engineering applications for air emission control

352 12 Carbon Capture and Storage

As introduced in combustion chemistry above, it is very challenging in engi-
neering practices to achieve perfect mixing in the entire combustion device. Stoi-
chiometric, fuel lean, and fuel rich combustion take place at different spots in the
combustion device. As a result, the actual combustion formula is very complicated.
One example formula is

1 b
C a H b þ a þ ð O 2 þ3:76 N 2 Þ
/ 4

3:76 b
ð
½
! xCO 2 þ 1 xð½ ÞCOŠ þ yH 2 O+ 1 yÞH 2 Šþ a þ N 2 ð12:5Þ
/ 4

1 b 1 þ x þ y
þ a þ O 2
/ 4 2
where x and y in Eq. (12.5) can be determined by considering the chemical equi-
librium reactions. The general chemical equilibrium formula for the reaction aA þ
bB $ cC þ dD is described in Eq. (12.6)
c
P
n n d Dn
C D
K p ¼ a b ð12:6Þ
n n n
A B
where Dn ¼ c d a b: The chemical equilibrium constants based on partial
pressure for chemical reactions with CO 2 and CO, lnðK P Þ, can be found in
Table 12.2.
With the products described in Eq. (12.5), we can consider chemical equilibrium
reactions to solve the unknowns of x and y.
Example 12.1: CO 2 emission rate calculation
3
Natural gas is fed into a burner at a rate of 1,000 m /h at 1 atm and 25 °C.
Assuming that the air is premixed with / ¼ 1, and the final products at equilibrium
under 1,000 K and 1 atm contain O 2 ,N 2 ,CO 2 , CO, H 2 O, and H 2, determine the
emission rate of CO 2 generation.

Solution
Assuming the natural gas is pure methane, with / ¼ 1; a ¼ 1 and b ¼ 4 Eq. (12.5)
becomes

ð
CH 4 þ 2O 2 þ 3:76N 2 Þ
3 þ x þ y

ð
! xCO 2 þ 1 xð½ ÞCOŠ þ yH 2 O þ 1 yÞH 2 Š þ 7:52N 2 þ O 2
½
2