Power Plant Engineering

13.10.2 METHANE
If the fuel is methane (the primary constituent of natural gas), the following reaction steps occur.
Due to the heat and attack by the active species, the methane reacts to a methyl radical (CH
3
),
which reacts to formaldehyde (HCHO). The formaldehyde reacts to a formal radical (HCO), which then
forms carbon monoxide (CO). Through these steps, the active species are used up and H
2
and H
2
O are
formed in addition to the CO. Overall, the process is as follows:
CH
4
+ O
2
→
CO + H
2
+ H
2
O
Thus, the original fuel is converted into two new fuels (CO and H
2
O) and into one product
(H
2
O). The process occurs very quickly, within a fraction of a millisecond to a few milliseconds, de-
pending on the flame temperature, pressure, and fuel-air ratio. (Actually, methane is one of the slower
burning hydrocarbon gases.) The process is called Oxidative Pyrolysis.
Following oxidative pyrolysis, the H
2
oxidizes, forming H
2
O, replenishing the active species,
and releasing heat. This occurs very quickly, usually in less than a millisecond.
H
2
+ (1/2) O
2
→
H
2
O
Finally, the CO oxidizes, forming CO
2
and releasing more heat. This process is generally slower
than the other chemical steps, and typically requires a few to several milliseconds to occur.
CO + (1/2) O
2
→
CO
2
The combustor may be thought of as having flame zones, in which the free radical activity is
high, the fuel undergoes oxidative pyrolysis, the hydrogen oxidizes, and the CO begins to oxidize, and
a post-flame zone, in which the CO continues to oxidize and reaches its final exhaust concentration.

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POWER PLANT ENGINEERING
13.10.3 ALKANES
When Alkanes (C
n
H
2n+2
) of n = 2 and above are burned, the reaction chemistry is different from
that of the methane. First, the Alkanes undergo conversion to one or more alkenes (C
n
H
2n
), especially to
ethylene (C
2
H
4
) and pyropylene (C
3
H
6
). The alkenes then undergo oxidative pyrolysis reaction to CO,
H
2
, and H
2
O, but though a different chemical mechanism than given above for methane. The overall
reactions are:
C
n
H
2n+2
→
C
n
H
2n
+ H
2
C
n
H
2n
+ O
2
→
CO + H
2
+ H
2
O (not balanced)
H
2
+ (1/2) O
2
→
H
2
O (+ Active Species + Heat)
CO + (1/2) O
2
→
CO
2
(+ Heat)