In several countries, the energy supply has been discussed in terms of feasible options for the medium and long terms. Hydroelectric power-stations cannot be considered in some countries because there are insufficient river falls or flows to warrant either being financially attractive or providing an economic return on the investment. Also nuclear generation is being rejected by many environmentalists.

Natural gas has been claimed to be the best solution for power generation because of its low pollutant emissions and its relatively large availability for the next few decades. Its use in industry as a substitute for fuel oil in heating processes and for electricity generation in cogeneration systems is becoming increasingly popular.

Solid wastes burning in municipal power-stations is being proposed as a possible solution, not only for solving the problem of disposing of the wastes but also for producing electricity for export to the grid: this paper discusses this question in the Brazilian context and proposes some solutions to be considered in a real case-study according to an international experience review.

The economic attractiveness of an enterprise is dependent on parameters such as the interest rate considered and the loan period; however, when different and/or competing technologies are under analysis, the differential cost should be evaluated to determine the economic gap between them. Fig. 1 presents an illustration of competing technologies with their corresponding investment costs; capacity is the available quantity of each technology at a certain cost.

Technology C is marginal because it has the highest cost among the possibilities considered; the least cost technology A will be recommended until its capacity Q_a is fully reached. When it occurs, technology B is the next possibility to be explored until its capacity Q_b is reached; differential costs A and B are the capacities Q_a and (Q_b?Q_a) multiplied by the difference between the costs of marginal technology and the corresponding one.

Nowadays, the solid-waste power-stations have higher costs if compared with the natural gas or the hydroelectric power-stations: the same steam thermal-cycle has an investment cost of 850–1000 US$/kW if burning fuel oil, 1300–1800 US$/kW for coal and around 4500 US$/kW for urban solid-wastes. Hence, at present, the last one will only be considered in the absence of the others or if there are sufficient incentives for its use. Also the environmental impacts of burning municipal solid-wastes are not comparable with the adverse impacts of burning natural gas. However, in the long term, because of the urgency of introducing new power-stations to satisfy the continuously growing electricity demand, it is desirable to analyse the feasibility of burning municipal (and some other) wastes, which are largely generated by human activities. The useful disposal of such wastes in several countries is a problem that demands urgent decisions.

More electricity is required in the developed and developing countries because of its convenience and the intensive use of electronic systems in the industrial, residential and commercial sectors. However, only part of the energy of the input fuel is converted into useful energy (e.g. as electricity or mechanical work) and the remaining part is disposed of as heat and losses.

Cogeneration is defined as the combined production of more than one form of energy as a result of the combustion of a fuel. The major difference between a conventional power-station and a cogeneration system is that the latter is designed to produce the electricity as well as to recover the exhaust heat for district heating or for a refrigeration cycle, whereas, in the former, the aim is only to produce electricity. In a steam cycle, for example, a cogeneration system would have a back-pressure steam-turbine and a power-station would have a condensing one: Fig. 2 illustrates the differences.

Latin America has only recently become concerned with the economic exploitation of refuse. In the United States, waste recycling and the associated energy generation are well established. Pollutant emissions as a result of solid-waste power generation have been studied extensively. Typical USA municipal solid-waste plant characteristics are: combustion capacity from 13 to 4000 ton/day (mean: 786 ton/day); electricity produced from 0.5 to 935 MW (mean: 176 MW); steam production from 1.25 to 1159 ton/h (mean: 115 ton/h).

In Europe, there is an agreement that MSW should be burned as a desirable option — see Fig. 3.

In Japan, it is desirable to burn MSW because of the lack of space; estimates are that 82% of the 50×10⁶ tons/year of MSW generated are related to the energy production; 253×10⁶ tons/year of industrial wastes are also discarded in which 39% are organic wastes and 6% are some different wastes of considerable heating value (as oils, plastics, etc.). District-heating systems receive the hot water produced by the MSW incineration and the steam also obtained is used to produce electricity in multiple units; 108 incineration units in 1991 were responsible for generating 320 MW(e).

The population of more than one billion inhabitants of China generates 200×10⁶ tons/year, that is equivalent to 0.55 kg/person/day; from this quantity, only a little part is incinerated. In India more than 90% of the MSW is disposed of in embankments.

Brazil’s most important pertinent development was undertaken by São Paulo State Energetic Company — CESP, in 1989 to construct two MSW power plants for São Paulo — Brazil’s biggest city: 12000 tons/day of wastes were intended to be burned to generate electricity to supply about 400 000 residences. São Paulo City burned at that time no more than 500 tons/day.

Electricity generated by burning municipal solid-wastes incurs an investment cost higher than those of other competing technologies, although it can be an important contribution to environmental and social improvements.

Guaratinguetá Region (GR), a group of eight cities in the Southeast region of São Paulo State, Brazil, consists of the following cities: Guaratinguetá, Cachoeira Paulista, Aparecida, Cunha, Lorena, Piquete, Roseira and Potim. It is located in the most industrialised region of São Paulo State: important chemical industries exist in Guaratinguetá and Piquete; some heavy industries (paper and allied products; fabricated metals industries) in Cruzeiro, and some new food industries are now being built in Lorena. In 1993, German researchers analysed the structure of wastes generated in the region: they concluded that 66500 tons/year were produced in the GR as a result of commercial, residential and public contributions, including the wastes from hospitals.

The MSW heating value and moisture content for the Guaratinguetá Region ranged from 4000 to 5000 kJ/kg and from 40 to 50%, respectively.

In the same report it was also concluded prematurely that incineration is not appropriate for the Guaratinguetá Region, because of the composition and low heating value of the local MSW. Nevertheless, it is important to investigate more thoroughly the feasibility of producing electricity and recovering heat in a cogeneration system that can burn, total or partially, the solid wastes generated locally.

An interesting possibility is the use of a combined system to generate energy; the incineration equipment, that burns the MSW, produces steam and is connected to a gas-turbine system burning a gaseous fuel such as natural gas. International experience suggests incineration (associated with the conventional steam cycle) and gasification (in a gas or combined cycle) as two possibilities for converting the solid wastes energy content into useful energy.

The objective of this report is to present technical and economic feasibility analyses for two cogeneration schemes to be located in an industrial district at Guaratinguetá City.