Page 330 - Wastewater Solids Incineration Systems

Appendix A Combustion Fundamentals 291

heat loss is a unique function of the combined effects of air preheat temperature, fuel
temperature, XS air (oxygen in flue gas), and the moisture content of the fuel.

4.3 Availability of Heat
In the previous sections, the gross HHV of a few different fuels were identified. The
next section will deal with the subject of available heat. Available heat can be defined
as the HHV of the fuel or waste being combusted minus heat losses associated with
the flue gas, including the heat of vaporization associated with the flue gas moisture.
The percent available heat is the available heat divided by the heat input. Available
heat must always be defined at specific inlet and outlet temperatures and excess air
level. When associated with auxiliary fuels, it generally represents the heat available
for transfer to another body.
The concept of available heat is also useful to characterize wet sludge cake. If the
available heat (temperatures and XS air level specified) from the flue gas resulting
from the combustion of sludge cake is negative, then addition of heat to the process
is required. If the available heat is zero, then the sludge cake is autogenous at speci-
fied conditions. If the available heat is a positive number, then either heat must be
extracted to meet the specified conditions or the conditions must be changed so as to
result in zero availability. This may involve reducing the inlet temperature,
increasing the outlet temperature, increasing the XS air level, or some combination of
these three methods.
The calculation of the available heat from common auxiliary fuels will serve as
an introduction to the broader concept of heat and material balances for sludge incin-
eration systems. For this, it is necessary to introduce the concept of enthalpy. The
enthalpy of a flue gas represents the total heat content, expressed as Btu/lb, of a gas
above a baseline or reference temperature which, for this text, is 15°C (60°F).
Enthalpy is the mathematical product of the difference between the actual gas tem-
perature and the base temperature times the average specific heat between those two
temperatures:

h C (T T )
p a b
Where
h enthalpy, Btu/lb;
C average specific heat at constant pressure (between T and T ), Btu/(lb-°F);
p a b
T actual temperature, °F; and
a
T base temperature, °F.
b