Page 135 - Wastewater Solids Incineration Systems

102 Wastewater Solids Incineration Systems

1.0 INTRODUCTION

Heat recovery and reuse is an integral part of a modern incineration system. In most
cases, feed cake to the incinerator is not sufficiently dry to be fully autogenous or
thermally self-supporting during combustion. Therefore, it is necessary to provide
additional energy to the process, either using auxiliary fuel (natural gas, fuel oil, etc.),
thermal energy recovered downstream of the incinerator, or both, to properly sustain
the combustion process.
Most high-temperature thermal processes, such as incineration, offer a variety of
opportunities for energy recovery. Every stack that exhausts hot flue gases to the
atmosphere represents irretrievable thermal energy. Similarly, quenching of hot flue
gases in the wet scrubber represents energy “down the drain,” not to mention the
demands this places on plant water systems. In a typical incinerator operating at 760
to 870°C (1400 to 1600°F), heat loss to the stack or scrubber drain can be significant.
The incinerator exhaust gases represent a valuable energy resource that, when effec-
tively recovered, can have a positive effect on operating costs. If this energy is recov-
ered as useful heat, then it translates to energy cost savings and possible reduction in
capital costs. Heat recovery from hot flue gases will reduce spray-cooling require-
ments and reduce volume flow through downstream gas cleaning systems. However,
a careful technical and economic evaluation must be conducted before embarking on
expensive and complex schemes. Although inclusion of heat recovery systems does
require some capital investment and routine operation and maintenance effort and
cost, this approach can often offset capital and operation and maintenance cost sav-
ings of downstream equipment.
Recovered energy may or may not suffice to completely eliminate the need to
purchase auxiliary fuel for incineration; however, it can greatly reduce the fuel
requirements and is typically cost-effective. Moreover, recovered energy can be used
to serve other portions of the plant through the generation of steam, hot water or
heated thermal fluids or, taken a step further, for the generation of electrical power.
Whenever there is a market for energy recovered in the wastewater treatment plant
(WWTP), there is an opportunity for savings. Taking advantage of the energy prop-
erties of feed solids is what makes incineration with energy recovery a true form of
beneficial use.
Waste heat recovery can take many different forms. In general terms, it can be
classified as primary recovery and secondary recovery, based on ultimate the use of the
recovered energy. Primary recovery refers to energy recovery that is used specifically