1 October 1997
(b) Loss of MRF Capacity. If there is an MRF, it will probably have sufficient tipping floor
capacity to store 1 to 2 days' collection. Such storage capacity will have to be in the building in
order to protect the materials from the elements. Once the storage capacity in both facilities is filled,
the processed burnable waste would have to be sent to the landfill.
7-3. ENERGY RECOVERY. Energy recovery will normally be in the form of steam and/or hot
water. Typical efficiency of modular unit facility heat recovery is a nominal 65%. The more efficient
field-erected facilities typically achieve 80-85% recovery of the energy generated.
a. Heat Exchanger System.
(1) Modular Unit Systems. In a modular incinerator facility (i.e., less than 200 tpd), the
energy recovery heat exchanger will usually be a packaged convective heat exchanger that serves
all the incinerator units in the facility. The heat exchanger may be of the firetube or watertube type.
Firetube construction is more common in small units since they are generally the least expensive. If
a pressure above 300 psig is required (e.g., for a steam turbine or for steam central heating), a
watertube heat exchanger is needed.
(2) Field-Erected Units. Large, field-erected incinerators use a combination of an integral,
radiant-heated water-wall boiler that forms the walls of the secondary combustion chamber and
convection-heated super heaters and economizers. Each unit has a complete heat recovery system
for the gases generated in the incinerator, including its own steam drum.
(3) Heat Transfer Surface Cleaning. Soot blowers must be provided for both types of
boilers since cleaning of fly ash from a heat exchanger is frequently the limiting factor on how long
the incinerator can continue to operate. Firetube boilers are multi-pass to minimize length and
permit cleaning. The thermal efficiency will also be lower if soot is not blown out regularly.
(4) Other Types of Soot Removal. "Hot cyclones" (refractory-lined) have been used
between modular incinerator units and the associated heat-recovery system to reduce the dust
loading on heat-transfer surfaces. These have seen limited use and then only in small units.
(5) Economizers. Part of the reason for higher efficiencies in the large field-erected
incinerator facilities is economizers, which are usually not cost effective in small units. The
economizer is designed to remove the heat in the gases leaving the steam and boiler water
convection heaters (shown on figure 4-4). As much as 10% of the total recoverable heat in the
gases may be removed in the economizer to preheat boiler feedwater or combustion air. Care
must be taken to ensure that economizer tube surface temperatures remain in excess of acid gas
condensation temperatures. The exit gas temperature from the heat exchanger system will usually
be limited to above 450oF to prevent tube metal corrosion. Specially designed equipment may
allow a reduction in temperature to 300oF. Acid gas corrosion appears at temperatures below
b. Operating Mode. The waste-heat boiler and incinerator must be sized and operated to be
base loaded. Thermal cycling (kept warm and on standby) for this type of application is not feasible
and will result in damage (e.g., tube buckling and refractory loss) to the equipment.
c. Energy Use.