Newsletter and Technical Publications
<Municipal Solid Waste Management>
1.5.3 Energy production
In waste-to-energy plants, heat from the burning waste
is absorbed by water in the wall of the furnace chamber, or in separate boilers.
Water is heated to the boiling point and changes to steam. At that point, either
the steam is used for heating, or it is used to turn turbines to generate
electricity. The amount of energy recovered from waste is a function of the
amount of waste combusted, the energy value of the waste stream, and the
efficiency of the combustion process.
Most of the MSW incineration currently practiced in industrialized countries
incorporates energy recovery in the form of steam, which is used either to drive
a turbine to generate electricity or directly for heating or cooling. In past
years it was common to simply burn MSW in incinerators to reduce its volume and
weight, but energy recovery has become more prevalent since the 1980s.
The three basic types of waste-to-energy incineration involve the generation
of electricity, steam, or the “cogeneration” of both electricity and steam. In
North America, communities are generally moving away from developing steam
facilities and toward producing electricity; about 90% of operating mass-burn
facilities generate electricity. This trend is due partly to a preference for
relatively stable electricity markets, such as utilities, as compared to
industrial customers of steam, who are perceived as less reliable purchasers.
However, deregulation of electricity markets (e.g., in Britain and the US) may
change the relative value of steam production vs. electricity production.
In Europe, steam generation for heating and cooling has always been the
primary means of waste-to-energy production. In fact, a key factor to consider
in evaluating the practicality of MSW incineration is the presence of an
existing infrastructure for steam district heating. In Japan, energy produced by
incinerators in large cities is widely used for heating community swimming pools
or air-conditioning, sometimes as compensation to nearby communities for having
the incinerator close to them.
Electricity production and use
Electricity-producing incineration facilities use steam to drive a turbine
connected to an electric generator. Of the electricity produced in incineration
facilities, about one-fifth is used at the facility for general operations. The
remaining electricity is sold to public and private utilities or nearby
industries. In many countries utilities provide a stable market for electricity
generated from incinerators. The availability of purchasers and rates for
electricity sales will, however, vary by region.
Steam production and use
The energy generated by European waste-to-energy plants typically goes to
supply steam to district heating loops; the heavy reliance on district heating,
and the ready market for steam that it provides, is part of what makes
incineration so attractive in European cities. The coupling of incineration with
electricity generation, which contributes substantially to the capital costs of
incineration, is quite rare in Europe, in part because European countries do
not, in general, have utility rate structures that allow non-utility-generated
electricity to be sold to the grid.
Steam generated in incineration facilities can also be used directly by a
customer for manufacturing operations. Steam generated in an incinerator is
supplied to a customer through a steam line, and condensed steam is sometimes
returned by a separate line.
Marketing steam to end users requires (a) identifying industries and
institutions (e.g., hospitals, colleges, public buildings, and factories) that
use steam in the vicinity of the facility or (b) purposely siting the facility
near potential steam purchasers. Some cities may also have commercial steam
distribution utilities, which may facilitate steam sales. To ensure a consistent
supply of steam to end users, incineration facilities sometimes have a back-up
boiler. Likewise, to adjust for variations in demand for steam (including
seasonal variation), facilities may also need to be equipped with a by-pass to
allow temporary halts in steam generation and/or steam delivery.
Combined production of steam and electricity is referred to as cogeneration
and can occur in two ways. If the energy cus-tomer requires steam conditions
(pressure and temperature) that are less than the incineration plant’s design
specifications, a turbine-generator is used to produce electricity and thus
reduce steam conditions to appropriate levels for the customer. Or, if the steam
purchaser cannot accept all the steam produced by the facility, the excess can
be converted to electricity.