422 POWER PLANT ENGINEERING13.10.3 Alkanes
When Alkanes (CnH2n+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 (CnH2n), 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:
CnH2n+2 → CnH2n + H 2
CnH2n + 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)13.11 POLUTION DUE TO GAS COMBUSTION
13.11.1 Unburned Hydrocarbons (UHCS)
Any fuel entering a flame will be reacted. Thus, when unburned fuel is emitted from a combus-
tor, the emission is caused by fuel ‘avoiding’ the flame zones. For example, in piston engines, fuel-air
mixture ‘hides’ from the flame in the crevices provided by the piston ring grooves. Further, some re-
gions of the combustion chamber may have a very weak flame, that is, they have either very fuel-lean or
very fuel-rich conditions and consequently they have a low combustion temperature. These regions will
cause intermediate species such as formaldehyde and alkenes to be emitted. Sometimes the term ‘prod-
ucts of incomplete combustion,’ or PICs is used to describe such species. The term UHC represents the
sum of all hydrocarbon species emitted.13.11.2 Carbon Monoxide (CO)
Carbon monoxide is emitted because the temperature is too low to effect complete oxidation of
the CO to CO 2 , because the time (i.e., the residence time) available in the combustion chamber is too
short, or because there is insufficient oxygen present. Usually, it is more difficult to design and operate
a combustor for very low CO than for very low unburned hydrocarbons. Exhaust emissions of CO are
controlled by providing the combustor with sufficient air to assure oxidation of the CO. However, too
much air is ‘bad,’ since then the post-flame zone will be too cool to oxidize the CO. Catalysts in the
exhaust stream are also used to control CO. These provide about a 90% conversion of the CO to CO 2 ,
and typically use platinum (or a mixture of platinum group metals) as the active sites (on a ceramic or
metal substrate) to oxidize the CO. Almost all automobiles sold today in the US and in many other
countries are equipped with three-way catalysts. By running the engine at stoichiometric fuel-air ratio,
there is enough O 2 left in the exhaust to effect oxidation of the CO and UHCs in the catalyst, and there
is a sufficient quantity of reducing species in the exhaust to effect chemical reduction of the NOx to N 2.
Thus, the three ‘ways’ are CO, UHCs, and NOx.
13.11.3 Nitric Oxide (NOx)
Nitric oxide forms by attack of O-atoms on N 2. The predominant mechanism is the extended
Zeldovich mechanism: