Landfill waste disposal

The landfill site—essentially an enormous hole in the ground (or more accurately a natural depression since sites are not generally excavated first)—is the main alternative to the technologies discussed in this chapter as a means of waste disposal. Though crude, its simplicity has led it to become the favored method of urban waste disposal across the globe. Waste that has been collected is simply off-loaded at the site until the depression or hollow is considered full. At that point earth is bulldozed over the deposited waste and the whole structure is left to settle. Over time the organic material within the site will decompose, producing carbon dioxide if there is air present but methane if the decomposition takes place in the absence of air. Methane is a common product in many landfill sites and its production can continue for one to three decades after the site has been closed and sealed.

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 that increasingly restricts that available for waste burial. More potent still are environmental concerns about the lack of recycling and the long-term effects of landfill disposal—effects resulting from the methane emissions from such sites and from the seepage of toxic residues into water supplies.

Such concerns have already led the EU to legislate¹ to restrict the use of landfill waste disposal. Where it does not already exist, similar legislation can be expected 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 specialized 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.

Sources of waste

There are two principles types of waste suitable for disposal in a power-from-waste plant: urban (primarily domestic) refuse, normally referred to as 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. In the developed world the waste from rural communities may be handled in a similar way but this is rare in the other parts of the world. 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 250–500%² and the trend is set to continue for perhaps another generation. In the United Kingdom, for example, 79% of the population already lived in cities by 1950 but this is expected to rise to 92% by 2030. In China only 13% of the population lived in cities in 1950 but by 2030 the proportion should reach 60%. 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. In 2008, for the first time, more than half the world’s population, or around 3.3 billion, lived in cities and towns. By 2030 the number of city dwellers is expected to reach 5 billion. These towns and cities constitute the source of MSW.

The amount of waste these populations produces varies from country to country and from continent to continent. In general, the city dweller in an industrialized 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.

According to the World Watch Institute, global municipal solid waste generation at the end of the first decade of the 21st century was around 1.3 billion tonnes. By 2025 the global annual production could double to 2.6 billion tonnes. The greatest producers were the 34 members of the Organization for Economic Cooperation and Development, which produced on average 1.6 million tonnes each day. Sub-Saharan Africa, by contrast, only produces 200,000 tonnes a day.

Waste composition

The composition of the waste varies from place to place. In general, the waste from the urban household in an industrialized country will contain 30–40% paper and cardboard and up to 10% plastic. The proportions of these in the waste from a household in, for example, the Dominican Republic will be much lower and the Dominican household’s waste will probably contain 80% food waste, whereas the proportion of food in the waste from a U.S. household may only be 26%.³ More generally, organic waste accounts for more than 60% of waste in low-income countries compared to 25% of the waste stream in richer counties.

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, then it cannot expect to generate any electricity.

Table 16.1 provides some figures for MSW energy content from different parts of the world. U.S. waste has the highest energy content of those listed. At 10,500 kJ/kg the value is approaching that of sub-bituminous coal. European cities and prosperous Asian cities such as Taipei generate waste with around 7500 kJ/kg. Meanwhile, the waste from typical midsize Indian cities contains roughly half this amount of energy. Some factors affecting the differing energy content are regional, and others are simply a matter of affluence.

In the case of Indian cities, for example, the low energy content may not be due entirely 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 the amount of noncombustible 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 handling and 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 organizations have to be examined. The issue will be particularly important for a private sector power-from-waste plant; it is less critical if the project is being built by a local municipality.

Waste collection and recycling

Urban refuse collection is organized 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 is 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.

Often, however, waste collection is carried out by private companies. The waste that these companies provides 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 neighborhoods tend to produce better-quality waste than poorer neighborhoods. 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.

Recycling is becoming an important part of municipal waste handling. This is partly a result of legislation such as that in the European Union and partly a matter of economics. World Bank figures have suggested that the global market for scrap metal and waste paper from municipal waste is worth $30 billion each year and the total waste management market may be valued at $400 billion annually. Even so, there is a long way to go before global waste management achieves adequate levels of recycling and reuse of material. However, the sorting of waste for recycling means that the best combustible waste that remains can also be separated, which can be valuable for a power-from-waste plant. Sorting can also help with production of RDF.

Waste power generation technologies

A power-from-waste plant (also commonly known as a waste-to-energy or WTE plant) is a power station fueled 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 fluidized-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 burned, 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 burned as received. Emission control systems will vary too, with toxic metals and dioxins a particular target, but nitrogen oxide, sulfur dioxide, other acidic gases, 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 burned, 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 and it continues to house the largest concentration of such plants. Altogether there are around 440 WTE plants in the EU producing 30 TWh of electricity and 55 TWh of heat in 2009.⁴ Japan has also made extensive use of waste combustion, though not always for power generation, with around 100 plants in operation, while the United States has a similar number. In 2011 there were about 800 WTE plants in operation in 40 countries around the world. These plants were estimated to have treated 11% of MSW generated globally.

Europe has also developed the most widely used waste combustion technology based on waste incineration. Two companies, Martin GmbH based in Munich, Germany, and Swiss company Von Roll, accounted for close to 70% of the market for the dominant technology called mass-burn at the end of the 20th century.⁵ The rest of the market is divided among a number of smaller companies, mostly based in either Europe, the United States, or Japan. The dominant European technology has also been widely licensed. It was the source of the technology used in most U.S. 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, for example.

At the same time 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 petrochemicals.

Traditional waste incineration plants

The traditional method of converting waste to energy is by burning it directly in a special combustion chamber and grate, a process that is often called mass burning (Figure 16.2). The dominant European technologies use this system. These involve specially developed moving grates, often inclined to control the transfer of the waste, and long combustion times to ensure that the waste is completely destroyed. Designs have evolved over 20–30 years and are generally conservative.

More recently, fluidized-bed combustion systems have sometimes been used in place of traditional grates. Such systems are good at burning heterogeneous fuel but require it to be reduced to small particles first.

The actual grate, be it conventional or fluidized bed, forms only a part of a waste treatment plant. A typical solid waste combustion facility is integrated into a waste collection infrastructure. Waste is delivered by the collecting trucks to a handling (and possibly a sorting) facility where it must be stored in a controlled environment to prevent pollution. Recyclable materials may be removed at this stage, though metallic material may be recovered after combustion. Grabs and conveyors will then be used to transfer the combustible waste from the store to the combustor.

Plant components, and particularly the grates, must be made of special corrosion-resistant materials. The grate must also include a sophisticated combustion control system to ensure steady and reliable combustion, while the quality and energy content of the refuse fuel varies. In some more modern systems oxygen is fed into the grate to help control combustion. The temperature at which the combustion takes place must usually be above 1000 °C to destroy chemicals such as dioxins, but must not exceed 1300 °C as this can affect the way ash is formed and its content.

Hot combustion gases from the grate flow vertically into a boiler where the heat is captured to generate steam. The combustion process in the grate and the temperature profiles within the boiler have to be maintained carefully to control the destruction of toxic chemicals. Most of the residual material after combustion is removed from the bottom of the combustion chamber as slag. However, there may be further solid particles in the flue gases, some of which can be recycled into the furnace.

Upon exiting the combustion and boiler system, the exhaust gases have to be treated extensively. While the combustion chamber may utilize techniques to minimize nitrogen oxide emissions—though further reduction may prove necessary—a system to capture sulfur will be required. This will probably be designed to capture other acidic gases such as hydrogen-chloride too. There may be a further capture system based on active carbon that will absorb a variety of metallic and organic residues in the flue gases. Then some sort of particle filter will be needed to remove solids. By this stage the exhaust gases should be sufficiently clean to release into the atmosphere, but continuous monitoring systems are required to make sure emission standards are maintained.

Dust from the flue-gas filters is normally toxic and must be disposed of in a landfill. Other flue-gas treatment residues will probably need to be buried too. The slag from the combustor may, however, be clean enough to exploit for road construction. Modern mass burn plants aim to generate slag that can be utilized in this way.

Mass burn plants may burn up to 2000 tonnes/day of MSW. Where a smaller capacity is required, a different type of combustion system, called a rotary kiln, can be employed. As its name suggests, this system uses a rotating combustion chamber that ensures that all the waste is burned. The chamber is inclined so that the material rolls from one end to the other as it burns. Such combustors are capable of burning waste with a high moisture content, perhaps up to 65%. Capacities of rotary kilns are up to 200 tonnes/day of refuse, suitable to meet the needs of small urban communities.

Gasification and pyrolysis

In recent years a number of companies have developed new WTE technologies based on both gasification and pyrolysis. These technologies are derived from the power and the petrochemicals industries. Pyrolysis is a partial combustion process carried out at moderate to high temperatures in the absence of oxygen and it can produce a mixture of gaseous, liquid, and solid residues. The traditional method of producing charcoal is a form of pyrolysis, as is the production of gas from coal which leaves a residue of coke. Gasification, meanwhile, involves heating solid material at high temperatures in a limited amount of air or oxygen to produce a char waste and a combustible gas. In both cases the gas will normally be burned to generate heat and steam.

Pyrolysis can produce a range of products from waste, depending on the temperature and the time the waste spends in the pyrolysis reactor. At lower temperatures with short residence times, more oils and tars are produced. Longer residence times lead to more solid residue (char). When the temperature is low, these solid and liquid products can contain complex and sometimes toxic organic molecules and must generally then be combusted at a controlled high temperature to generate power in a conventional boiler system.

Many WTE pyrolysis plants operate at relatively high temperatures so that they do not produce any liquid or tar residues, only combustible gas and a solid residue. Waste is normally sorted first, removing and recycling as much metal, glass, and plastics as possible. The remainder is then shredded and reduced to small particles of perhaps 2 mm in size before exposing it to a high temperature of around 800 ºC to convert it very quickly into a combustible gas and solid ash. The gas, which has a calorific value up to 22 MJ/kg, is cleaned and can then be used in a gas engine to generate power. Alternatively, the gas can be used in a conventional boiler.

One of the main advantages touted for high-temperature pyrolysis over incineration is that production of toxic organic compounds, such as dioxins and furans, is minimized. One company’s pyrolysis WTE plant is designed to treat around 90 tonnes a day of waste with a calorific value of 8.4 MJ/kg and a moisture content of 40%, generating 2 MW of power in the process.⁶

Another, lower-temperature pyrolysis system, developed in the 1990s in Japan,⁷ employs an initial pyrolysis process followed by combustion to generate heat. Waste delivered to the plant is first shredded and then fed into a rotating pyrolysis drum where it is heated to around 450 ºC. The heat, provided by hot air generated at a later stage in the process, pyrolyses the waste, converting it into a combustible gas and a solid residue.

The solid residue contains any metal that entered with the waste. This can be removed at this stage for recycling. Both iron and aluminum can be segregated in this way. The remaining solid slag is crushed. The gas and the crushed residue are then fed into a high-temperature combustion chamber operating at 1300 °C where it is completely burned. Combustion is controlled to limit nitrogen oxide formation. Incombustible material adheres to the walls of the combustion chamber where it flows, in liquid form, to the bottom. From here it is led out of the bottom of the furnace and immediately quenched, creating an inert granular material suitable for road building.

Hot flue gases from the combustion chamber are used to generate steam to drive a turbine. Dust is then removed from the exhaust gases and returned to the combustion system. Following this, a flue-gas treatment system removes any remaining acid gases. Only this material, around 1% of the original volume of the MSW, needs to be disposed of in a landfill. This system also claims to keep residual levels of dioxins extremely low.

Waste gasification (Figure 16.3) is similar to pyrolysis but it generally takes place at a higher temperature, typically 1000 °C, to produce a combustible, low calorific–value gas that can be burned either in a gas engine or in a conventional boiler system. As with pyrolysis, the products of gasification depend on the temperature used and lower temperatures can lead to more contaminants in the gas. The synthetic gas produced during gasification is generally a mixture of carbon monoxide and hydrogen but the carbon monoxide can be converted into more hydrogen by using a shift reaction in which the gas is mixed with water vapor and passed over a catalyst at high temperatures. Provided it is clean enough, this gas can be used either for power generation or as a feedstock for industrial processes.

A new version of the gasification process that is being developed for MSW processing is plasma gasification (Figure 16.4). This involves burning the waste in a plasma arc at temperatures that can reach in excess of 10,000 °C, although the actual gasification will generally take place at slightly lower temperatures. Even so, the temperature is so high that all the components of the waste are broken down to atomic constituents and the product is expected to be a relatively pure syngas.

A plasma gasification reactor consists of a gasification chamber with a plasma torch generating a high-temperature arc close to the bottom. Solid waste is introduced above the torch, and as it falls through the high-temperature region, it is gasified in the presence of a controlled amount of air introduced from the bottom of the chamber. Any metal and slag is removed from the bottom of the reactor while syngas exits from the top. The high temperatures reduce the levels of chemicals such as dioxins to a very low level, often much lower than the levels produced in a conventional incineration plant.

The syngas from a plasma gasifier can be burned in an engine or conventional combustion plant. It is also possible to feed the gas directly into a coal-fired power plant in the same way as was discussed for biomass gasification in Chapter 15.

Refuse-derived fuel

RDF is the product of the treatment of MSW to create a fuel that can be burned easily in a combustion boiler. To produce RDF, waste must first be shredded and then carefully sorted to remove all noncombustible material such as glass, metal, and stone. Shredding and separating is carried out using a series of mechanical processes that are energy intensive. The World Bank has estimated that it requires 80–100 kWh to process one tonne of MSW and a further 110–130 kWh to dry the waste.⁸

After the waste has been shredded and separated, the combustible portion is formed into pellets that can be sold as fuel. The original intention of this process was to generate a fuel suitable for mixing with coal in coal-fired power plants. This, however, led to system problems and the modern strategy is to burn the fuel in specially designed power plants. An alternative is to mix the RDF with biomass waste and then burn the mixture in a power plant. Since RDF production must be preceded by careful sorting, this type of procedure is best suited to situations where extensive recycling is planned.

Environmental issues

Urban waste, its production, and its fate are major environmental issues. As already noted, modern urban living produces enormous quantities of waste in the form of paper, plastic, metals, and glass, as well as organic materials. How these wastes are processed is a matter of increasing global concern. Wastes such as paper, glass, and metal can be recycled, as—in theory—can plastics. From an environmental perspective it makes sense to reuse as much waste as possible, so environmentalists generally favor maximum recycling. Many governments now promote recycling too. However, the economics of recycling are not clear-cut and there are critics who consider it economically ineffective. Since such debates pitch sustainability against economics, the issue is not easily resolved.

While recycling offers the ideal solution, in practice there are frequently neither the facilities nor the infrastructure to recycle effectively. Even where recycling is employed there is still a residue of waste that cannot be reused. Thus, there remains a considerable volume of waste for which an alternative means of disposal is required. The only options currently available are burial in a landfill site or combustion.

The combustion of waste would seem initially to be the ideal solution. Combustion reduces the quantity of waste to 10% or less of its original volume. At the same time it produces energy as a by-product and this energy can be used to generate electricity, heating, or both. Unfortunately, waste often contains traces of undesirable substances that may emerge into the atmosphere as a result of combustion. Other hazardous products may result from the combustion itself, with the waste providing the chemical precursors. So, while solving one environmental problem, waste combustion can generate others.

In the face of this, the combustion of waste has become subject to strict legislation. This sets limits on amounts of different hazardous materials that can be released as a result of the process. Chief among these are heavy metals, such as mercury, cadmium, and others, and potent organic compounds, such as dioxins and furans.

Modern WTE plants aim to meet the regulatory requirements imposed on them. In spite of this they are often extremely unpopular and even when the technology appears capable of limiting emissions to extremely low levels, it can be difficult gain permission to build plants. New waste conversion technologies such as gasification and pyrolysis may be able to help overcome popular objections if they can be shown to generate negligible toxic emissions.

Waste plant emissions

A plant burning waste produces four major types of product. First, there is the solid residue from the grate itself, normally termed slag or ash. Second, there is the chemical product resulting from flue-gas treatment systems. Third, there is a quantity of dust in the flue gases emerging from the plant boiler; this is normally captured with filters or an electrostatic precipitator. Finally, there is the flue gas itself.

Cost of energy from waste plants

The capital cost of equipment to generate electricity from waste is generally much higher than for conventional power generation equipment to burn fossil fuel. Plant design is specialized and must include refinements for emission control that are not necessary in a fossil fuel plant. Grate design is unique too.

Against this must be offset the revenue of the plant, not only from the electricity generated but also from the fuel itself, the waste. Industry and municipalities expect to pay to dispose of their waste. Consequently, the economics of a project should be designed so that the revenue from the waste disposal contracts are adequate to enable the power from the plant to be sold competitively. It should be remembered, however, that a waste to energy plant has as its primary purpose the treatment and elimination of MSW. Electricity is a useful by-product of this process but generation is not the main function of the plant.

A study carried out for the Mayor of London and published in 2008 looked at the cost of the principle waste combustion technologies. The main findings are shown in Table 16.2. The study concluded that a conventional incineration facility would cost around £45 m for a plant with the capacity to treat 100,000 tonne/y of MSW while for a 200,000 tonne/y plant the cost would be £76 m. With the maximum power output from the smaller plant put at 6 MW, this equates to £7500/kW while the larger plant has a maximum output of 12 MW, equating to 6300/kW.

Advanced thermal treatment plants such as gasifiers and pyrolysis plants have slightly higher costs, as shown in the table. Their potential power outputs are also slightly lower. As a consequence the unit cost of a 100,000 tonne/y advanced plant is £9100/kW while for the 200,000 tonne/y plant the unit cost is £7700/kW. Operating costs for the plants are broadly similar at between £40/tonne and £70/tonne depending upon plant size.

U.K. costs are similar to estimates for plants in the United States where the cost of a typical municipal waste combustion plant was put at $5,000–10,000/kW during the middle of the first decade of the 21st century. Again smaller plants are relatively more expensive than larger plants.