CEMP-ET
EI 11C302
1 October 1997
(1) Typically the steam or hot water produced in the energy recovery system is matched to
the existing space-heating power plant conditions. The incinerator is used as a base-load
generator due to its need to burn waste at a constant rate of throughput. This inflexibility in
operating the incinerator means that the existing fossil-fueled, space-heating plant will be swing-
loaded to follow changes in the daily demands.
(2) During the summer months, steam or high-temperature hot water produced by the boiler
may be used to produce chilled water for air conditioning. Large, multi-stage, high-pressure (125
psig steam) absorption chillers are available from several manufacturers. Steam turbines running a
mechanical chiller and exhausting into low-pressure (12 psig) absorption units are available. It is
also possible to purchase ammonia refrigeration units that work off of low-quality waste heat;
however, low-pressure absorption units should never be used by themselves because of their low
thermal efficiency.
d. Co-generation. It may be desirable to equip the incinerator plant with a steam turbine to
produce electricity or do other mechanical work (e.g., pump water, run air compressors) if the
incinerator plant is larger than 50-tpd capacity. It is also possible to generate both electricity and
district heating steamin small steam generation facilities.
(1) Steam turbines are available that use steam at pressures as low as 300 psig, perform
mechanical work, and exhaust the steam at 150 psig or lower.
(2) Electricity Generation. The designer can expect electrical generation rates of 400 kWh
from a 50-tpd incinerator plant and more than 1,600 kWh from a 200-tpd plant, despite the relatively
low overall plant efficiencies (i.e., 10-12%).
(3) Operation on Demand. Co-generation is often used when high-pressure steam can be
delivered to the district heating system via a high-pressure heat exchanger to produce low-pressure
steam when the demand for district heating is high. It can also be used to send the high-pressure
steam directly to the turbine when the demand for district heating is low.
7-4. ASH SYSTEM.
.
a. The hot ash from an incinerator is usually dumped directly into a water-filled quench tank at
the discharge end of the primary combustion chamber. The water is maintained at a level that
provides a seal at the exit of the incinerator and thereby prevents unwanted air from entering the
primary combustion chamber.
b. Removal Methods. Several methods of ash removal may be used, each with their
advantages and disadvantages. Problems with ash system design and optimization are reasons
why it is vital to hire a contractor with extensive experience in the design of waste incineration
facilities. Experience has shown that plants have more trouble with the ash removal system than
with any other support system. Two causes stand out: (1) under design, and (2) failure to remove
wire-containing products, cable, and band strapping from the incinerator feed stock.
(1) Drag-Chain Conveyor. Drag-chain conveyors are the most common device for removal
of ash. Water is allowed to drain from the ash as it is carried up a chute and out of the plant. Due
to the gritty environment, the drag-chain conveyor is normally a high-maintenance item. The plant
7-4
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