Power from waste

The generation of power from waste is a very specialised industry and its principal aim is not to produce electricity. Power-from-waste plants are combustion plants designed to destroy or reduce in volume municipal and in some cases industrial waste.1 As an incidental, but nevertheless valuable by-product, the processes adopted to manage these wastes may also be capable of generating electricity.

The level of exploitation of waste-to-energy plants varies from country to country. They have been used widely in parts of Europe, where waste has been burned since the end of the nineteenth century, and form a major part of Japan's waste disposal strategy. In contrast the USA has only adopted the technology patchily. In addition environmental concerns about the emissions from the plants has caused recent resistance to their construction both in the USA and elsewhere.

Where they are employed, these plants generally burn domestic and urban refuse - called in this context municipal solid waste (MSW) - using the resulting heat to generate steam to drive a conventional steam turbine. MSW can also be sorted and treated to produce a compacted fuel called refuse-derived fuel (RDF) which can be burned in a power station.

Some industrial waste may be treated in the same way. However industrial wastes are likely to contain toxic materials which have to be handled using special procedures. Where such care is not required, they can be dealt with in the same way as urban waste.

There are a number of other categories of waste, primarily resulting from the agricultural and forestry industries, that can be used to generate electricity. These have been dealt with under biomass in Chapter 15, which also dealt with the collection and use of methane produced in landfill refuse disposal sites. However we need to consider landfill briefly here since it offers the main alternative to waste combustion.

Landfill waste disposal

The landfill site - essentially and enormous hole in the ground where waste is dumped - is the main alternative to the technologies discussed in this chapter as a means of waste disposal. Though crude, its simplicity has led to it becoming the favoured method of urban waste disposal across the globe.

While landfill use remains popular in many countries, it is coming under pressure in others. This is partly a result of the demand for land which increasingly restricts that available for waste burial. More potent still are environmental concerns about the long-term effects of landfill disposal, effects resulting from the methane emissions from such sites (discussed in Chapter 15) and from the seepage of toxic residues into water supplies.

Such concerns have already led the European Union (EU) to legislate2 to restrict the use of landfill waste disposal. Similar legislation is bound to follow in other parts of the world. But waste will still be produced. This is where technological solutions, such as the power-from-waste plant, enter the equation.

Power-from-waste technology is not cheap. The specialised handling that waste requires, coupled with the need for extensive emission-control systems to prevent atmospheric pollution, make such plants much more expensive to build than any other type of combustion power plant. They are also expensive to operate.

If these plants had to survive on the revenue from power generation alone, they would never be built. Fortunately they have another source of income. Since waste has to be disposed of in a regulated manner, waste disposal plant operators can charge a fee - normally called the tipping fee -to take the waste. The tipping fee represents the main source of income for a power-from-waste plant. Any additional income derived from power generation will benefit the economics but the plant may well be able to survive without it.

Waste sources

There are two principle types of waste suitable for disposal in a power-from-waste plant: urban (primarily domestic) refuse, normally referred to a MSW, and industrial waste. Some industrial waste is broadly similar in content to MSW and this can be treated in the same way as the latter. Other industrial waste must be dealt with differently because of the hazardous or valuable materials it contains. This chapter is only concerned with MSW and it will not deal with industrial waste except where it can be burned with MSW.

The main source of MSW is an urban community.3 The quantity and size of such communities is growing rapidly. In the last two generations the number of people living in cities has increased by between 250% and 500%.4 This has been particularly notable in the developing world where the number of urban dwellers is expected to reach 2.71 billion by 2010. A further 1 billion live in the cities of the developed world. Thus, close to half the population of the world will be living in cities by 2010.

Urban dwelling has grown, particularly, rapidly in South America and the Caribbean where, by 2025, 80% of the populations will be living in towns.

But these regions are not unique. Urban communities are growing virtually everywhere. These towns and cities constitute the source of MSW.

The amount of waste these populations produce varies from country to country and from continent to continent. In general, the city dweller in an industrialised country produces far more waste than one in a developing country. Thus a typical Californian might produce 1.3-1.4 kg each day while a city dweller in Mexico City produces only half that. A Nigerian town dweller probably produces less than 200 g of waste each day.

In the mid-1990s the International Energy Agency estimated that developed countries alone produced an estimated 426 million tonnes of waste each year. If all this was used to generate electricity, potential output would be 191 TWh/year. Annual energy demand in 2001 was 13,290 TWh.5

Waste composition

The composition of the waste varies from place to place. In general the waste from the urban household in an industrialised country will contain 30-40% paper and cardboard and up to 10% plastic. The proportions of these in the waste from a household in the Dominican Republic will be much lower but the Dominican household's waste will probably contain 80% food waste whereas the proportion in a US household waste may only be 26%.6

There are other important differences. The waste from households in developing countries contains a high proportion of moisture, often as high as 50%, making it difficult to burn without first reducing the moisture content by drying. In contrast, the high proportions of paper and plastic in the waste from a household in the industrial world make it much easier to burn.

All these factors affect the energy content of waste, and energy content is a crucial factor in determining the viability of a power-from-waste plant. Unless the plant can produce enough excess heat from waste combustion to raise steam, it cannot expect to generate any electricity.

Table 16.1 provides some figures for MSW energy content from different parts of the world. US waste has the highest-energy content, 10,500 kJ/kg, approaching that of sub-bituminous coal (see Table 16.1). European cities

Table 16.1 Energy content of urban wastes from different regions

Energy content (kJ/kg)

10,500 7500 7500

Western Europe Taiwan (Taipei) Mid-sized Indian cities Sub-bituminous coal

3300-4600 10,700-14,900

Source: United States Agency for International Development.7

and prosperous Asian cities such as Taipei generate waste with around 7500kJ/kW. The waste from typical mid-sized Indian cities contains roughly half this amount of energy.

In the latter case the low-energy content may not be entirely due to the quality of waste. In cities in India - but not them alone - much of the urban waste is collected by city sweepers. Such waste is contaminated with considerable quantities of stone, earth and sand. In Bombay, for example, the amount of non-combustible material of this type in waste may reach 30%. Not only does this reduce the energy content of the waste, it could also damage a combustion system so the design of a waste disposal plant has to take its presence into account.

Given such local variations in waste content it is vitally important, before a power-from-waste plant is built, that the waste available be carefully assessed. For that, local waste-collection procedures and organisations have to be examined.

Waste collection

Urban refuse collection is organised in different ways in different parts of the world. In some countries it is run by municipalities, in others it is provided by private operators. Where a municipality run waste collection as a service, the same city might build and operate its own power-from-waste plant. Under these circumstances the composition of the waste can be readily assessed and controlled if necessary.

More often waste collection is carried out by private companies. The waste that these companies provide will vary in quality. In some cases it will contain the whole range of waste, but in others it will have been sorted to remove the more valuable material. Some countries now require that glass, metal, plastic and paper be recycled. This too will affect the quality of the MSW available.

Inevitably the quality of waste will vary by season. Economic factors are also important. Waste will be poorer in a recession than in a boom. Local variations can also be significant. Richer neighbourhoods tend to produce better quality waste than poorer neighbourhoods. This has led to the suggestion that the quality of waste for a power-from-waste plant might be maintained by collecting only from prosperous areas of a city.

Whatever the strategy, knowledge of the waste, its source and its variations will form a necessary part of the management of a waste-to-energy plant. That information can only be gained with practical experience, by analysis of waste collected by the contractor that will provide waste for the plant. Even with this knowledge, it may be impossible to maintain an adequate energy content in the waste throughout the year. Then the only solution may be to add some higher-energy content fuel to the waste. Biomass waste from local sources will often be the most economical solution in this situation.

Waste power generation technologies

A power-from-waste plant is a power station fuelled with urban waste. As already indicated, such a facility may have as its primary function, waste disposal. Nevertheless the technologies employed will be traditional power generation technologies as used in combustion plants. Combustion systems include grate burners, some fluidised-bed burners, and more recently gasification and pyrolysis. Heat generated in these combustion systems is used to raise steam and drive a steam generator.

Within the broad outline above, power-from-waste plants vary enormously. Much depends on the waste to be burnt, its energy content, the amount of recyclable material or metal it contains and its moisture content. Waste may be sorted before combustion or it may be burnt as received. Emission-control systems will vary too, with toxic metals and dioxins a particular target, but nitrogen oxides, sulphur dioxide and carbon monoxide emissions must all fall below local limits. Carbon dioxide emissions may need monitoring to comply with greenhouse gas emission regulations.

Once the waste has been burnt, residues remain. Power-from-waste plants will generally reduce the volume of waste to around 10% of its original. A way must then be found to dispose of this residual ash. If it is sufficiently benign, it may be used as aggregate for road construction. Otherwise it will probably be buried in a landfill. Other residues from emission-control systems will have to be buried in controlled landfill sites too.

Northern Europe has been the traditional home of waste incineration plants for power generation. Altogether there were around 250 municipal waste combustion plants in the EU in the late 1990s, most in the northern countries of the Union. Between them they had a generating capacity of around 1500MW, almost half the global total of 3200MW in 1997.8 Japan has also made extensive use of waste combustion, though not always for power generation. In 1999 there were about 600 waste-to-energy plants in operation worldwide.

Europe has also developed the most widely used waste combustion technology. Two companies, Martin GmbH based in Munich and the Zurich company Von Roll, accounted for close to 70% of the market for the dominant technology, called mass burn, at the turn of the century.9 The rest of the market is divided among a number of smaller companies, most based in either Europe, the USA or Japan.

The dominant European technology has been widely licensed. It was the source of the technology used in most US power-from-waste plants built in the late 1970s and early 1980s. More recently several developing countries of Asia have taken interest in power from waste and European technology has been modified for use in China.

Newer technologies based on gasification and pyrolysis are being developed by a variety of companies. These are based on technologies from other industries such as power generation and petrochemicals.

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