WTE Technology Overview:
Technology
WTE facilities are designed to burn 24 hours a day, 7 days a week, at high
temperatures and a utilization rate or capacity factor around 85
percent.(52) On average, a ton of garbage produces 500 to 600
kilowatt-hours of electricity, or 4,000 to 6,000 pounds of steam. WTE
facilities generally fall into four categories: mass burn, refuse-derived
fuel, modular controlled air, and pyrolysis.
WTE combustion is similar to conventional combustion of solid fuels such as
coal. The MSW fuel is either burned in its original form with little
preprocessing (mass burn) or, after the extraction of recyclable materials,
converted to refuse-derived fuel (RDF) for more efficient combustion. The
fuel handling equipment, boiler, ash disposal, emissions control, and power
plant controls are similar to those for coal-fired power plants. The most
important differences between the two arise from the much greater
variability of the MSW and its much higher proportion of compounds that
adversely affect boiler and emissions control operations. The net effect of
the fuel variability is that operating and maintenance costs tend to be
high and performance tends to be uneven. The use of RDF instead of
unprocessed MSW improves boiler performance, but at a significant fuel
preparation cost. Additionally, the low heat content of MSW (roughly
one-half that of coal), the high proportion of non-combustible materials,
and potentially harmful compounds mean that twice the mass of material must
be handled, combusted, and environmentally controlled than with coal. This
further increases costs.
The major components of a typical mass burn power plant are shown in Figure
10. In a typical 40 megawatt power plant, the charging chutes of each of
the two mass burn boilers receive mixed waste via an overhead crane and
bucket. A hydraulic ram pushes the waste onto the sloping grate of the
furnace, where it is passed through three zones: drying, combustion, and
burnout. Air is injected above and below the grate. The heat transfer
surface is located in the waterwalls and convective pass, where superheated
steam (900 psi and 830 oF) is generated. The steam from the two
incinerators is used to drive a 40 megawatt steam turbine generator (the
gross capacity is 45.5 megawatts, with 5.5 megawatts used for auxiliary
power). Assuming a 24.8 percent moisture content and 4,900 Btu per pound, a
40 megawatt mass burn power plant can consume 1,606 tons of waste per day.
The average facility has a thermal efficiency of 20.8 percent and a net
heat rate of 16,377 Btu per kilowatt-hour.
A WTE facility has many environmental controls. Ammonia is injected into
the boiler convection pass to control nitrogen oxide emissions. A lime
spray dry scrubber removes sulfur dioxide, hydrochloric acid, and other
acid gases and a baghouse removes lime solids and fly ash, which may
contain heavy metals, dioxins, furans, and other toxic substances. Bottom
ash and fly ash are land-filled. Combusting the waste reduces the amount
that has to be land-filled by about 90 percent. Thus, WTE facilities may be
justified not on electricity generation costs alone but as a means to
eliminate a major social problem and a growing expense.
A second type of WTE facility is a modular controlled-air incineration
system, generally prefabricated and shipped to the site, with a capacity of
less than 50 tons per day. Modular systems feed MSW into a primary chamber
where incomplete combustion produces a combustible gas that is burned in a
second chamber, usually in conjunction with oil or gas. This technology
produces very low particulate emissions, but its low pressure steam is not
suitable for the generation of electricity for sale to utilities.
Refuse-Derived Fuel (RDF) facilities consist of an RDF processing area and
an RDF-fired stoker boiler. RDF processing includes flail milling, trommel
screening, magnetic separation, and size reduction. The resulting fuel,
with a heat content of 5,900 Btu per pound, is transported by conveyor to
the power plant, where it is injected by the spreader stroker and combusted
in suspension and on the grate. The other parts of the plant are similar to
those of a mass burn plant. Assuming a moisture content of 28.2 percent and
heat value of 5,663 Btu per pound, a 40 megawatt RDF plant can consume
1,396 tons of fuel per day. The plant has a thermal efficiency of 20.7
percent, gross capacity of 46 megawatts, and a heat rate of 16,464 Btu per
kilowatt-hour.
A fourth WTE technology is pyrolysis. A pyrolysis system decomposes organic
waste in a high-temperature, oxygen-deficient chamber. Efforts to continue
to commercialize this technology have declined, and operating facilities
using this technology have closed down.
The Bristol Resource Recovery Facility, which began commercial operation in
May 1968, converts up to 650 tons of solid waste to saleable energy each
day. At maximum output, the plant generates over 16 megawatts; remaining
electricity is sold to a local utility and used to power area homes and
businesses. (Source: Ogden Martin Systems of Bristol, Inc.) |
