Page 223 - Advanced Thermodynamics for Engineers, Second Edition

10.2 COMBUSTION OF SIMPLE HYDROCARBON FUELS 211

availability, supply logistics and the need for trouble-free operation over a wide range of conditions.
Industrial and marine gas turbines can compete effectively with the diesel engine only through the use
of very cheap fuels, such as residual oil or surplus gas. As a rule, gaseous fuels present no special
problems, but residual oils produce a highly destructive ash and sometimes copious amounts of
exhaust smoke. Attempts to burn pulverized solid fuels in open-cycle gas turbines have generally
proved unsuccessful. However, developments in fluidised-bed combustion could dramatically change
that situation.
One fuel which approaches single component composition is ‘natural gas’ which consists largely of
methane (CH 4 ). Methane is the simplest member of a family of hydrocarbons referred to as paraffins
or, more recently, alkanes which have a general formula C n H 2nþ2 . The lower alkanes are methane
(CH 4 ), ethane (C 2 H 6 ), propane (C 3 H 8 ) and butane (C 4 H 10 ) etc. Two other alkanes that occur in dis-
cussion of liquid fuels are heptane (C 7 H 16 ) and octane (C 8 H 18 ). The alkanes are referred to as saturated
hydrocarbons because it is not physically possible to add more hydrogen atoms to them. However, it is
possible to find hydrocarbons with less than 2n þ 2 hydrogen atoms and these are referred to as un-
saturated hydrocarbons: a simple unsaturated hydrocarbon is acetylene (C 2 H 2 ), which belongs to a
chemical family called alkenes. Some fuels contain other constituents in addition to carbon and
hydrogen. For example, the alcohols contain oxygen in the form of an OH radical. The chemical
symbol for methanol is CH 3 OH, and that for ethanol is C 2 H 5 OH: these are the alcohol equivalents of
methane and ethane.
Often fuels are described by a mass analysis which defines the proportion by mass of the carbon
and hydrogen e.g. a typical hydrocarbon fuel might be defined as 87% C and 13% H without specifying
the actual components of the liquid. Solid fuels, such as various coals, have a much higher carbon/
hydrogen ratio but contain other constituents including oxygen and ash.
The molecular weights (or relative molecular masses) of fuels can be evaluated by adding together
the molecular (or atomic) weights of their constituents. Three examples are given below.

¼ 12 þ 4 1 ¼ 16
ðm w Þ
Methane ðCH 4 Þ
CH 4
¼ 8 12 þ 18 1 ¼ 114
ðm w Þ
C 8 H 18
OctaneðC 8 H 18 Þ
¼ 12 þ 3 1 þ 16 þ 1 ¼ 32
ðm w Þ CH 3 OH
Methanol ðCH 3 OHÞ
10.2 COMBUSTION OF SIMPLE HYDROCARBON FUELS
The combustion of a hydrocarbon fuel takes place according to the constraints of chemistry. The
combustion of methane with oxygen is defined by:
/ 2H 2 O
CH 4 þ 2O 2 CO 2 þ
1 kmol 2 kmol 1 kmol 2 kmol
(10.2)
12 þ 4 2 32 12 þ 32 2 ð2 þ 16Þ
16 kg 64 kg 44 kg 36 kg
In this particular case, there is both a molar balance and a mass balance: the latter is essential but
the former is not. Usually combustion takes place between a fuel and air (a mixture of oxygen and