Energy Recovery Options

Landfill gas recovery. Landfill gas is produced by the decomposition of organic wastes in the airless conditions of a landfill site. Landfill gas typically contains around 55% methane and 40% carbon dioxide together with small amounts of nitrogen, hydrogen and water. These gases can be collected using a network of horizontal pipes and wells, laid prior to and during filling of the site with waste. Beneficial use of landfill gas for energy evolved as a solution to the problem of potentially explosive gas leaking from landfill sites. Since methane is a greenhouse gas with a higher global warming potential than its combustion product carbon dioxide, using methane for energy recovery has the further benefit of reducing the net potential for global warming.

18 Residua, Ash handling from waste combustion: technical brief, 1st Floor, The British School, Otley Street, Skipton, North Yorkshire BD23 1EP, UK, 1996.

Anaerobic digestion. Organic waste can be broken down using anaerobic digestion (AD), and the methane gas generated can be recovered. Anaerobic digestion has been used extensively for sewage sludge and for agricultural wastes. Its use to treat MS W, often with sewage sludge, provides a fuel which can be used, like landfill gas, to directly fire burners, to generate electricity or which can be cleaned and added to gas supplies. A major advantage of AD is that all the gas produced can be collected and used, unlike the gas in a landfill where collection efficiencies are relatively low (50% or less). AD also produces a solid residue or digestate which can be cured and used as a fertiliser.

Combustion. The conventional waste combustion technique is mass burn incineration, which involves the waste being burned as delivered, after removal of bulky items. To aid combustion, there is usually some mixing of the waste. In the past incineration plants were mainly designed purely to process waste, but today's plants are usually designed to recover energy (as steam, hot water or electricity) from the waste.

Refuse-derived fuel (RDF). The manufacture of refuse-derived fuel (RDF) is not new. It was originally devised as a means of avoiding the need to burn MSW immediately, and instead to turn it into a transportable, storable fuel. RDF production enables the subsequent thermal conversion of combustible portions of waste. While mass burning demands little sorting or processing of the waste, in RDF production the waste may undergo a number of pre-processing stages. At its simplest, RDF may be a coarse, fluff-like material produced from mixed MSW by a series of screening stages, plus magnetic removal of ferrous and non-ferrous metals. Alternatively additional processing may turn it into a densified, pelletised (or cubed) fuel, for ease of transport and storage. Turning waste into coarse or pelletised RDF differs from mass burn in being two-stage, where the first processing stage can be conducted completely separately from the second burning stage, and may be at a different site and time.

Fluidised bed combustion. Fluidised bed combustion technology is based on a system where, instead of the waste being burned on a grate (as in mass burn processes), the fire bed is composed of inert particles such as sand or ash.

When air is blown through the bed, the bed material behaves as a fluid. There are several different designs of fluidised bed (FB) combustors, for example circulating and bubbling beds. All need waste of uniform size. Compared to mass burn, fluidised bed combustion systems have reduced emissions, partly because of the process itself and partly because it is possible to add lime to the bed. Since as much as a third of the cost of mass burn plants is spent on the air pollution control (APC) system, there are savings to be made as fluidised bed systems have smaller APC needs. On the other hand, mass burn plants have no need for front-end processing of the waste. Also, as they are typically larger, and so benefit from economies of scale, the cost per tonne of waste processing in the two systems may not be markedly different.

Because FB systems are typically smaller, they can be more appropriate for smaller communities. The need to pre-process waste prior to combustion in an

FB combustion plant, in order to reduce its size and make it uniform, provides an opportunity to maximise materials recycling. However, while metals can be separated from the waste when it is being shredded and reduced in size, for successful recycling of most materials they must be kept clean and this requires them to be separated at source, not mixed with other wastes. While FBs have been used in industrial applications for a number of years, and to burn wood chips and similar single-material fuels, their use for mixed waste is more recent. Mixed MSW is not an easy fuel to burn, because of its variability, and maintenance costs of FB combustors used for MSW are likely to be much higher than those for a single, predictable waste stream like wood chips.

USA. There were 122 EfW plants in America in 1999, an increase of three on the previous year. Overall, the proportion of waste incinerated declined from 9% in 1997 to 7.5% in 1998, largely as a result of increased recycling and composting. Data from the US Environmental Protection Agency show that in relative and absolute terms the most recent data indicate that both materials recycling and energy recovery have reached a peak, together.

Europe. Brussels-based ASSURRE profiled incineration in Europe (2000), identifying 304 incineration facilities in 18 European countries, 96% of which recover energy. The average unit capacity is 177000 tonnes per year. Units vary in size from an average of 83 000 tonnes per year per site in Norway to 488 000 tonnes per site in the Netherlands. On an annual basis, Europe has 50.2 million tonnes of capacity to treat household and related waste. 49.6 terawatt hours (TWh) of energy are recovered from MSW each year. 70% of this is used for district heating and 30% for electricity generation.

Types of energy produced vary between countries, depending on optimum technology and local demand. The annual amount of energy generated from incineration is equivalent to the electricity demand of Switzerland. Per capita, energy recovered from incineration is highest in Denmark, Sweden and Switzerland. It is lowest in Spain, UK, Italy and Finland.

Treatment costs vary by country, ranging from €25-30 per tonne in Spain and Denmark to €160 per tonne in Germany.

Gasification and pyrolysis. Gasification is the process of reacting carbon with steam to produce hydrogen and carbon monoxide. Gasification converts a solid or liquid feedstock into gas by partial oxidation under the application of heat.

Pyrolysis is a complex series of reactions initiated when material is heated (to around 400-800 °C), in the absence of oxygen, to produce condensable and non-condensable vapour streams and solid residues. Heat breaks down the molecular structure of waste, yielding gas, liquid and a solid char, all of which can be used as fuels.

Both technologies have primarily been used for specific and generally single, unmixed waste streams such as tyres and plastics, or to process RDF. However, one German pyrolysis plant has been processing MSW since 1985.19 German

19 K. Strange, Advanced thermal treatment techniques - an overview, IBC conference, The Future of

Waste Management and Minimisation, Regents College, London, September 21-22, 1999.

waste company Deutsche Babcock's plant was commissioned in Giinzburg, Bavaria in 1983. Since 1985 the plant has been in permanent operation. The shredded waste is fed into a gas-fired rotary drum where it is pyrolysed at temperatures of 400-500 °C. The gas passes through a cyclone for the removal of coarse particulates and is then directly burnt in a post-combustion chamber at temperatures of around 1200 °C. Despite the above example, neither pyrolysis nor gasification is generally considered suitable for handling mixed, untreated MSW in large volumes at present.

The most popular application today is apparently to displace conventional MSW incineration, although Juniper has also identified specific niche opportunities for refineries, the paper and pulp industry and waste tyre disposal. However, the lion's share of the burgeoning market will be in the conversion of agricultural residues into a renewable energy resource, either by direct combustion for power generation, or through the creation of bio-oils (synthetic fuels for vehicles).

These advanced thermal treatment (ATT) technologies are very varied, with more than 60 different systems deemed technically and commercially interesting. Applying ATT systems to MSW streams will increase in future mainly because these technologies are perceived to represent cleaner, more socially acceptable waste management systems than conventional incineration with energy recovery. Gasification and pyrolytic process can convert waste into molecular building blocks, to generate new feedstock compounds for the petrochemical sector: waste tyres and plastics can be recovered for material re-use and energy recovery. However, Juniper believes that the economic climate presently means that most applications will focus on combustion and energy recovery from the products recovered.

The key drivers encouraging the adoption of ATT systems are:

• reduction in the volume of waste for final disposal

• rendering the waste for final disposal inert

• recovering value from the waste (usually as energy)

• pressure for more sustainable waste management systems

• the complementary nature of materials and energy recovery

• diversion of biodegradable materials from landfill

• chronic present or anticipated future shortage of landfill capacity

• economic instruments, such as landfill taxes and alternative energy subsidies

It seems clear that the pyrolysis and gasification business is staking a claim to territory occupied by the conventional EfW incineration sector. Nowadays, a new EfW plant can cost between USS30-100 million to build, with operating costs of USS50-100 per tonne. A key factor is the increased pressure to reprocess materials within the policy context of waste material flow management. This leads to increased waste collection and source separation, which also tends towards the isolation of high calorific value fraction (and separately, a smaller quantity of hazardous materials).

K. Strange Landfill

Landfill is often regarded as the last resort waste management option, an 'out of sight, out of mind' solution. While this may be partly true, modern landfilling is an active treatment process applied to most solid wastes. An engineered landfill is designed to contain waste and its decomposition products until they are sufficiently stable and inert to present no significant risks to health or the environment. Other benefits, such as material and energy recovery or land reclamation, may also be derived from properly designed facilities.

Municipal solid wastes became a problem with increased urbanisation. Waste disposal became a priority, not only because of the nuisance of waste dumped in the streets, but because of very real health risks. Epidemics of yellow fever, cholera, smallpox and typhus were not unknown. For a time, cities disposed of their garbage in rivers or lakes. Others simply dumped garbage in open pits on unused land. In many of the world's poorer countries, conditions for waste disposal are still rudimentary.

Modern landfills. A common landfill classification system for reflects the type of waste each receives. There are landfills for hazardous wastes, municipal wastes and inert wastes. In practice, these are not exclusive definitions. Other variants include mono-fills, in which single waste types are allowed, and co-disposal sites, in which municipal and hazardous wastes may be combined.

Landfills can also be classed by the management strategy employed:

Total containment. Virtually all movement of water through the landfill is prevented. Total containment imposes a long-term responsibility for monitoring and supervision. This strategy is often used with hazardous wastes, less frequently for municipal solid waste.

Containment and collection of leachate. Leakage of water from the landfill is controlled (but not eliminated) by using a low-permeability liner beneath the wastes, and by collection, removal and external treatment of liquid decomposition product (leachate). Risks of leachate pollution depend on the extent to which the containment barrier integrity is maintained and on the efficiency of leachate management. This strategy demands expensive, active systems, and research is underway into accelerated leaching - the flushing bio-reactor concept - to speed stabilisation, from perhaps centuries to just a few decades.

Controlled contaminant release. In this approach, the base liner is made of natural, often local, materials. While sumps for collection and removal of leachate are sometimes provided, leachate levels are permitted to rise within the waste, permitting gradual migration through the liner into the ground. Naturally, this approach is not suited to every location and geological setting, so full risk and environmental impact assessment approaches should be carried out before development.

Unrestricted contaminant release. Here, no control is used for water infiltration or leachate escape. This occurs, by default, in waste dumps, particularly in poorer countries.

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