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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 ÒcogenerationÓ 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.

Cogeneration

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 plantÕs 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.

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