Page 87 - Wastewater Solids Incineration Systems

56 Wastewater Solids Incineration Systems

TABLE 5.1 Oxidation reactions.

Reactions Heat released, kJ/kg (Btu/lb)
C O CO 33 700 (14 500)
2 2
2H O 2H O 144 000 (62 000)
2 2 2
S O SO 10 500 (4 500)
2 2
2CO O 2CO 10 200 (4 400)
2 2
CH 2O CO 2H O 55 600 (23 900)
4 2 2 2

3.1.1 Heating Value of Combustible Materials
The heating value is the quantity of energy (heat) that can be released per unit mass
of solid material being fed to the system and is the prime indicator of combustion
potential. The conventional unit of measurement is kilojoules per kilogram (kJ/kg)
or British thermal units per pound (Btu/lb). The heating value can be determined by
an oxygen bomb calorimeter. For wastewater solids, the heating value is never the
same at each test. It varies from plant to plant and from sample to sample and
depends on its chemical oxygen demand or on the treatment process used. As an
example, primary sludge has higher volatile contents and higher heating values than
digested biosolids. Some waste products will digest and evolve organic gases if they
are allowed to be stored for an extended period of time, thereby reducing their
heating value. Inert solids in process feed streams also will have an effect on their
heating value.
Typically, heating values for WWTP solids are in the range of 12 800 to 19 750
kJ/kg (5500 to 8500 Btu/lb) of dry solids. In general, a decrease in heating value and
a decrease in total combustible solids content will decrease the capacity of the unit.
An increase in heating value and increase in total combustible solids will increase the
capacity of the unit.
3.1.2 Water Content of Combustible Materials
The quantity of water in the feed material has a significant effect on the operation of
the incineration system. Water has no heating value but requires a large amount of
heat to be vaporized and heated to the operating temperature of the incinerator. The
heat required to vaporize this water must be supplied by the combustible materials
in the feed stream or by auxiliary fuel.