1 October 1997
(3) The amount of heat that would theoretically be released is the product of the
heating value (HHV) of the waste and the firing rate. In the above example, if the waste had a
heating value of 4,500 Btu/lb, the heat release rate would be calculated by the following:
(2,917) (4,500) = 13,126,500 Btu/h=13.1265 MBtu/h
b. Actual Heat Available for Recovery.
(1) Unfortunately, not all of the theoretically available heat can be recovered in a
(a) The combustion process may not consume all of the carbon.
(b) The basic laws of thermodynamics will not allow all of the heat to be recovered
as it passes through a heat exchanger.
(c) Large amounts of excess air will further reduce the amount of recoverable
(2) The amount of recoverable heat is determined by multiplying the theoretically
available heat by the thermal efficiency. Most incinerator/boiler combinations can be assumed
to have a thermal efficiency of 75%, but starved and controlled air should be assumed at
55% because of the large amounts of excess air used in the secondary combustion areas and
less complete burn-out of the carbon.
(3) Actual values should be obtained from typical manufacturers whenever possible.
(4) For the above example of a starved-air incinerator, the nominal amount of
recoverable heat is calculated using the following equation:
(13,126,500) (0.55) = 7,219,575 Btu/h
c. Efficiency of Energy Recovery Affected by the Temperature of the Media.
(1) If steam or hot water is the form of useful energy produced, the production rate will
also depend on the temperature of the water entering, and the enthalpy of the product leaving
(a) In the case of hot water, the heat transferred to the media will be based solely
on the difference between the inlet and outlet temperature of the water.
(b) If dry, saturated steam is to be produced, the enthalpy will be based upon the
exit pressure. The enthalpy of super-heated steam is based on both exit temperature and
pressure. Values may be found in most engineering handbooks.
(2) For the above example with a 55% overall thermal efficiency for a starved-air unit,
water of 190F (158.0 Btu/lb), and saturated steam at 150 Psig (1195.5 Btu/lb), the calculation
would be as follows: