FUELS AND COMBUSTION 507dharm
\M-therm\Th11-1.pm5represents the distribution of the equilibrium products resulting from a reaction between CO and
O 2. In this equation x denotes the fraction of dissociated CO 2. At low temperatures the
fraction (1 – x) approaches unity while at high temperatures (1 – x) shows a substantial reduction in
magnitude.
For the combustion of H 2 with O 2
H 2 +^12 O 2 H 2 O
and H 2 +^12 O 2 = (1 – x) H 2 O + x H 2 +x 2 O 2 ...(11.31)
It is essential to distinguish between the effects of dissociation and the losses resulting from
incomplete combustion of fuel. Incomplete combustion, which may be attributed to a number of
factors, results in a discharge from the system of combustible substances. Dissociation, on the
other hand, is of transient nature. Usually any appreciable degree of dissociation extends over a
very short time interval at the highest level of temperature attained in the reaction. The gaseous
products are likely to be discharged from the system at a temperature that is indicative of a low
degree of dissociation. For example, dissociation does not influence the heating value determined
in a fuel calorimeter. Although the maximum temperature attained in the calorimeter is limited
by chemical equilibrium, the combustion process moves to completion with the decrease in the
temperature of the products. The reduction in temperature is a result of heat transfer to the jacket
water. Dissociation of the products is negligible at room temperature, which is essentially the
calorimeter reaction temperature.
The temperature of the products discharged from the combustion chamber of the gas tur-
bine power plant is limited to approximately 870°C by introduction of a large quantity of excess
air. Absorption of energy in the water walls of a boiler furnace limits the outlet gas temperature to
approximately 1100°C. The quantity of dissociated products at temperatures ranging upward to
1100°C is not appreciable. In the cylinder of I.C. engine, considerably higher maximum tempera-
tures—that is, in excess of 1100°C are attained, hence in the analysis of this thermal power
system consideration must be given to the effects of dissociation. Of particular significance is the
effect of reduced maximum temperature on the system availability. As a result of heat transfer
and work performed by the gaseous medium the products are discharged from the system at a
temperature below the level at which an appreciable degree of dissociation is observed.
The proportions of the dissociated products in chemical equilibrium at temperature T are
established from the equilibrium constant. The evaluation of the equilibrium constant is achieved
in accordance with the analysis presented by Van’t Hoff.
11.22.Actual Combustion Analysis
In evaluating the performance of an actual combustion process a number of different pa-
rameters can be defined depending on the nature of the process and the system considered. The
combustion efficiency in a gas turbine for instance can be defined as
ηcombustion ideal
actual(F/A)
(F/A)= ...(11.32)where (F/A)ideal = Fuel-air ratio required for adiabatic and complete combustion and in which the
products would attain the adiabatic flame temperature.
In case of a steam generator (boiler)
ηsteam generator =
Heat transferred to steam / kg fuel
Higher heating value of the fuel
...(11.33)