TITLE.PM5

506 ENGINEERING THERMODYNAMICS

dharm

\M-therm\Th11-1.pm5

i.e., V 2 =

75 882 0 08 288

286 76

..××

× = 0.0804 m

3

Heat received by water = 28 × 4.18 × (23.5 – 10) = 1580 kJ

Higher calorific value of fuel =

1580

008.

= 19750 kJ/m^3. (Ans.)

Amount of water vapour formed (i.e., steam condensed) per m^3 of gas burnt =

006

008

. = 0.75 kg
Lower calorific value, L.C.V. = H.C.V. – 2465 × 0.75
= 19750 – 1848.7 = 17901.3 kJ/kg. (Ans.)

11.20.Adiabatic Flame Temperature

In a given combustion process, that takes place adiabatically and with no work or changes
in kinetic or potential energy involved, the temperature of the products is referred to as the
‘adiabatic flame temperature’. With the assumptions of no work and no changes in kinetic or
potential energy, this is the maximum temperature that can be achieved for the given reactants
because any heat transfer from the reacting substances and any incomplete combustion would
tend to lower the temperature of the products.
The following points are worthnoting :
(i)The maximum temperature achieved through adiabatic complete combustion varies
with the type of reaction and per cent of theoretical air supplied.
An increase in the air-fuel ratio will effect a decrease in the maximum temperature.
(ii) For a given fuel and given pressure and temperature of the reactants, the maximum
adiabatic flame temperature that can be achieved is with a ‘stoichiometric’ mixture.
(iii) The adiabatic flame temperature can be controlled by the amount of excess air that is
used. This is important, for example, in gas turbines, where the maximum permissible tempera-
ture is determined by metallurgical considerations in the turbine, and close control of the tem-
perature of the products is essential.

11.21. Chemical Equilibrium

The calculation of the adiabatic flame temperature is based, in part, on the assumption that
the reaction goes to completion. Owing to dissociation, complete conversion of the reactants to the
products is not accomplished. As a consequence of the failure to achieve complete conversion of the
reactants, the maximum reaction temperature cannot attain the level of the theoretical adiabatic
flame temperature.
The combination of CO and O 2 produces CO 2 together with a release of energy. In an adi-
abatic system no heat is transferred to the surroundings, hence the temperature of the mixture of
the products and reacting substances rises rapidly. As the mixture temperature increases to higher
levels the rate of dissociation of the CO 2 becomes increasingly more pronounced. Since the dissocia-
tion of CO 2 requires absorption of energy, a condition is reached where the rate of evolution and
the rate of absorption of energy are in balance. At this point no further increase in temperature
can be observed and the reaction is in chemical equilibrium. For this condition
C + O 2 CO 2
At each temperature of the equilibrium mixture the substances participating in the reac-
tion exist in unique proportions. For the combustion of CO the right-hand side of the equation

CO +^12 O 2 = (1 – x) CO 2 + x CO + x 2 O 2 ...(11.30)