Page 281 - Materials Chemistry, Second Edition

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252 Waste Management Practices: Municipal, Hazardous, and Industrial

FIGURE 9.2 Common grate types for a MSW combustor. Underfire air is forced upwards through the grates,
and overfire air is passed over the top of the burning MSW. (From Vesilind, P. A., Solid Waste Engineering, 1st
ed., 2002. Reproduced with kind permission of Brooks/Cole, a division of Thomson Learning: www.thom-
sonrights.com.)

MSW can be combusted for the production of steam, which is useful for driving turbines and
subsequent electricity generation. The remaining steam has little industrial use, however, unless it
is located sufficiently close to other buildings to be applied for space and water heating. Often the
residual steam is condensed to liquid water which is either cooled and used again in the power plant
or released into the local environment. Usually, boiler water is treated and reused because it is too
expensive to be used only once. Small amounts, less than 10%, are blowdown (i.e., fresh water
added to the system) to minimize the concentration of dissolved solids (Vesilind et al., 2002).
If hot water is discharged directly into a body of surface water, it will create adverse effects in
streams, rivers, and estuaries; as a result, heat discharges are regulated by federal and state codes.
The typical limit on heat discharges is that the temperature of the receiving water cannot be raised
o
by more than 1 C. Given this restriction on the temperature of water returned to local bodies of
water, the heated water must be cooled prior to discharge. Various means are used for dissipating