Page 136 - Wastewater Solids Incineration Systems

Heat Recovery and Reuse 103

to improve performance of the combustion process by reducing or eliminating the
need for auxiliary fuel. This process can include use of recovered energy to preheat
combustion air to the incinerator or use of recovered energy to thermally dewater or
dry incoming feed cake, or both. Thermal dewatering or drying systems may use
energy in the flue gases either by direct contact or indirect, where steam or thermal
fluids heated by energy in the flue gases is then used to thermally treat incoming feed
cake prior to incineration. Either of these approaches can be used to reduce the need
for auxiliary fuel.
Secondary recovery uses heat in the flue gases for beneficial purposes outside of
the combustion envelope. Heat recovery could be in the form of preheating an
external medium or generation of power. For seasonal space heating, an economizer
may be added as a source of hot water for space heating or in-plant process purposes.
Steam generated in a boiler can be used throughout the WWTP, sold to neighboring
users, used for production of electricity in a cogeneration system, or used directly in
steam turbine drives. The exhaust flue gases can also be used, directly or indirectly,
to preheat flue gases downstream of the scrubbing system before they are exhausted
to the atmosphere to provide plume suppression. When recovered energy is benefi-
cially used, energy is used that would otherwise be wasted to displace an equivalent
(or greater) amount of energy that would have been required from other sources.
From a practical perspective, heat transfer technology can be classified as either
direct or indirect. Direct heat transfer refers to a process in which the heat source
comes in direct contact with the material or flowing stream being heated. An
example would be direct injection of a stream of hot air to the exhaust stack to reheat
flue gases before discharge for plume suppression. Another example would be a
process by which feed cake is preheated or dried by direct contact with flue gases.
This approach has gained limited acceptance as a means of preheating or thermally
dewatering the feed cake. Most municipal WWTP incineration systems use indirect
heat transfer.
Indirect heat transfer refers to a process in which a physical barrier separates the
heat source from the material or flowing stream being heated. For example, indirect
heat transfer occurs when incinerator flue gases are used to preheat combustion air
in a heat exchanger. This example is referred to as recuperation and is the most
common and economical approach. In a fluid bed incinerator, the fluidizing air is
typically preheated to temperatures from 200 to 650°C (400 to 1200°F). In a multiple-
hearth furnace, the utility of combustion air preheat is practically limited to a max-
imum of approximately 200°C (400°F), as the nature of the process and equipment