Environmental Engineering Reference
In-Depth Information
products, but all of the fuel's chemical energy will have been released in the combustion of this
“lean” mixture.
3.9.1
Fuel Heating Value
When a mixture of fuel and air is burned, the temperature of the combustion products formed is
much higher than that of the fuel-air mixture. In some instances, heat may be transferred from the
hot combustion products to a colder fluid; for example, in a steam boiler, this heat causes the water
to warm and then boil to steam. The amount of heat available for this purpose is called the fuel
heating value and is usually expressed in energy units per unit mass of fuel.
Consider a combustion chamber that is supplied with a steady flow of a fuel-air mixture (the
reactants) at a pressure
p
r
and temperature
T
r
. If the fuel is burned at constant pressure
p
r
and
if no heat is lost from the combustion chamber (
Q
=
0), then the product gas temperature
T
p
will be higher than
T
r
, but the product gas enthalpy
h
p
{
T
p
,
p
r
}
will exactly equal the reactant
stream enthalpy
h
r
{
, by equation (3.20). This process may be illustrated by identifying the
reactant and product states as points in the enthalpy-temperature diagram of Figure 3.1, in which
the enthalpies of the reactants (
h
r
) and products (
h
p
) are shown as functions of temperature, at
the pressure
p
r
, as the upper and lower curve, respectively. The reactant enthalpy can be identified
as the point
R
at the intersection of the upper (reactant) curve and the vertical line at the reactant
temperature
T
r
. The horizontal line through this point then intersects the lower (product) curve
at the point
P
, where the product temperature is
T
p
, assuring that
h
p
{
T
r
,
p
r
}
T
p
,
p
r
}=
h
r
{
T
r
,
p
r
}
.
T
p
is
called the
adiabatic combustion temperature
.
We are now in a position to determine the fuel heating value. If the hot product gases are
subsequently cooled at constant pressure to the reactant temperature
T
r
at the point
P
, then the
heat removed per unit mass of product gas will be equal in magnitude to the reduction in enthalpy
of the product gas between
T
p
and
T
r
,or
h
p
{
T
p
,
p
r
}−
h
p
{
T
r
,
p
r
}=
h
r
{
T
r
,
p
r
}−
h
p
{
T
r
,
p
r
}
.
p
r
h
h
r
p
p
=p
r
R
P
h
p
P
T
T
r
T
p
Figure 3.1
The enthalpy
h
of the reactants (upper curve) and the products (lower curve) of a combustion
process, as functions of the temperature
T
, are related by the fuel heating value. For adiabatic, constant
pressure combustion, the products temperature
T
p
is greater than the reactant temperature
T
r
.
