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WASTE TO ENERGY

Many different types of waste can be converted into useful energy. In advanced countries,the forest products industry makes extensive use of wood wastes to generate heat and electricity. In other countries agricultural wastes are a significant source of energy. But by far the largest source of waste in the industrialised advanced countries or in the urban areas of other countries is municipal waste.

Over the ages and throughout the world, many different methods have been used to manage municipal solid waste (MSW). Two of the oldest methods are landfilling and burning. Burning became popular in many areas of the world because it reduces the volume of MSW up to 90 percent, helps to destroy bacteria and germs, and reduces the amount of waste to be landfilled. After efficient combustion, only residual ash remains for disposal.

Uncontrolled combustion, however, has the potential to produce considerable air pollution. It is for this reason that the modern waste-to-energy (WTE) plant evolved which is characterized by highly controlled combustion supported by extensive air pollution control and ash management systems. Such systems allow MSW to be processed by combustion in strict compliance with government regulations for air, water, and solid waste emissions protective of human health and the environment.

Waste-to-energy turns trash into steam or electricity to heat, cool, light and/or otherwise power homes and industry through the process of combustion. Just as coal, oil or natural gas is burned in boilers to generate electricity, household trash is used as a fuel to generate power.

Waste-to-energy facilities process nearly 30 million tons of trash each year and generate enough power to meet the needs of 2.4 million homes. More than 37 million people in 31 states rely on the 102 waste-to-energy plants nationwide.

Waste-to-energy facilities generate about 2,800 megawatts, or about one-quarter of total biomass generation. Biomass accounts for about 1.4 % of the total electricity generated. Renewable energy totals slightly more than 2% of the electricity market.

Waste-to-energy prevents the release of greenhouse gases such as methane, carbon dioxide, nitrogen oxides, and volatile organic compounds.

Waste-to-energy prevents the release of more than a million tons of methane into our atmosphere, assuming the same amount of trash now processed at waste-to-energy facilities is disposed in a landfill without methane recovery.

Waste-to-energy prevents the release of more than 10 million tons of carbon dioxide into our atmosphere, assuming the same amount of trash is disposed in landfills with methane recovery.

Waste-to-energy power as an alternative to coal prevents the release of nearly 25,000 tons of nitrogen oxides and 5,000 tons of volatile organic compounds.

Waste-to-energy also depletes less of the earth's natural resources than oil, coal or natural gas-powered electricity generation.

Modern waste-to-energy facilities differ significantly from old fashioned municipal incinerators. The waste-to-energy process recovers the heat value of combusted trash to generate steam and electricity to power homes and industry. Modern pollution control systems ensure a cleaner-burning power plant. Combustion reduces the volume of trash by about 90 percent and the remaining ash may be recycled in landfills as daily cover or used in road building materials.

Waste-to-energy technology is divided into three basic types:

Mass burn facilities generate energy by feeding mixed municipal waste into large furnaces dedicated solely to burning trash. The resulting energy produces steam or electricity. Many mass burn facilities have nearby material recovery facilities, or MRFs, that separate and recycle trash prior to processing.

Mass burning is the oldest, simplest and most popular method of recovering energy from municipal solid waste. In mass burn systems, untreated MSW is simply incinerated. The heat given off is converted into steam, which can then be passed through a turbine to generate electricity, used directly to supply heat to nearby industries or buildings, or to produce both electricity and low temperature heat suitable for space heating. Producing electricity from high temperature steam, and usable heat as a byproduct, is called co-generation or "combined heat and power." This results in more efficient use of fuel. Incinerating MSW generally results in a volume reduction of 80 - 90%, therefore reducing landfill space requirements.

There are several types of MSW mass burn combustion systems for recovering energy from waste, including refractory and modular furnaces; but "waterwall" furnaces are the most popular at the present time. Waterwall technology is similar to the furnaces used at coal burning power plants. Exhaust gases are generally passed through a "scrubber", an "electrostatic precipitator" or a "fabric filter baghouse" in order to remove particulates such as "fly ash", and acid gases, before they are released through the stack.

Refuse-derived fuel or RDF plants remove recyclable or unburnable materials and shred or process the rest of the trash into a uniform fuel. Sometimes, RDF powers a generating plant on site, and sometimes the fuel is burned off site for energy.

Refuse derived fuel (RDF) is a result of processing MSW to separate the combustible fraction from the non-combustibles, such as metals, glass and cinders. RDF is predominantly composed of paper, plastic, wood and kitchen or yard wastes, and has a higher energy content than untreated MSW, typically in the range of 12,000 - 13,000 kJ/kg. This figure will vary, depending upon local paper and plastic recycling programs. Like MSW, RDF can be burned to produce electricity and/or heat. RDF processing is often combined with recovery of metals, glass and other recyclable materials in a resource recovery facility. At the present time, RDF combustion is less common than mass burning; but this may change in the future as recovery of recyclable materials and environmental concerns over incinerator emissions become more important.

One of the benefits of RDF is that it can be shredded into uniformly sized particles or densified into "briquets". Both of these possibilities facilitate handling, transportation and combustion. Easily handled, RDF can often be burned or "co-fired" with another fuel such as wood or coal in an existing facility. RDF is thus valuable as a low cost additive which can reduce costs of generating heat or electricity in a variety of applications. Mass burning of MSW requires specially designed boilers to handle the uneven composition of MSW.

Another benefit of burning RDF rather than raw MSW is that fewer non-combustibles such as heavy metals are incinerated. Although metals are inert, and give off no energy when they are incinerated, the high temperatures of a MSW furnace causes metals to be partially "volatized", resulting in release of toxic fumes and fly ash. The composition of RDF is more uniform and well understood than that of MSW; therefore fewer combustion controls are required for RDF combustion facilities than for facilities burning untreated MSW. Plasma and thermal pyrolysis are technologies currently under development that can break down RDF to release combustible gases, which can either be burned directly to produce heat and/or electricity or condensed into oil.

Modular facilities are similar to mass burn plants, but these smaller plants are prefabricated and can be quickly assembled where they are needed.

Public officials who are responsible for local solid waste management agree that waste-to-energy and recycling are compatible, and they are mutually supportive. Waste-to-energy is a proven, locally generated source of energy that uses garbage as fuel to produce clean electricity or steam. This technology provides a safe means for recovering the inherent energy value of non-recyclable materials.

More than 80 percent of waste-to-energy plants operating in the U.S. recycle 773,000 tons annually of ferrous metals on-site. Waste-to-energy plants recycle an additional 460,000 tons of glass, metals, paper, plastic, batteries, ash and yard wastes. More than 75% of these facilities are located in communities with off-site recycling programs.

Waste-to-energy provides a safe and effective method for destroying organisms found in garbage that could become a serious health threat. It also reduces the volume of municipal solid waste by as much as 90 percent, leaving only 10 percent to be reused or landfilled. More than 300,000 tons of ash is being used annually as an aggregate in material for roads and as landfill cover.

Waste-to-energy plants dispose of approximately 30 million tons of trash each day while generating 2,800 megawatts of electricity. The technology of today's waste-to-energy plants is based on sound science and testing, making waste-to-energy a viable method for the disposal and recycling of solid waste.

Today, modern WTE plants not only destroy garbage; they produce valuable renewable energy in the forms of steam, hot water, and electricity. A single WTE plant can meet the MSW management needs of numerous towns and cities.

Landfill Gas

A recent alternative to the two methods of deriving energy from waste through the direct combustion of the organic material is the collection and combustion of "landfill gas". Landfill gas is produced by the natural anaerobic (oxygen free) decomposition of organic matter in landfills. Composed of 40-60% methane (CH₄), with the remainder being largely carbon dioxide. Landfill gas has a gross heating value of about 17,000 kilojoules per cubic metre, about half that of traditional natural gas. Each tonne of MSW produces about 70 cubic metres of landfill gas. Methane is a more effective greenhouse gas than the carbon dioxide that is produced when it is burned; therefore using landfill gas to produce energy has a positive impact on the greenhouse effect. The Brock West landfill east of Toronto has received waste from about one million people for the past 15 years and is now producing enough landfill gas to generate 21 MW of electric power.

Landfill gas is naturally produced by anaerobic processes inside a landfill. Methods of collecting landfill gas depend upon the design of a particular landfill. Methane is a potentially explosive gas, so many landfills designed in the past twenty years have been built with a system of pipes which is used to collect methane. This gas has traditionally been simply flared (burned) off, but is now in many cases collected as a useful fuel. Landfills not built with methane collection systems can have small diameter wells drilled and perforated plastic pipes installed to provide a collection and recovery system. Pumps are required to remove the gas, and necessitate increased maintenance costs. Slightly larger landfills can use gas turbines which are similar to jet engines to generate electricity and heat. Gas turbines are more efficient than reciprocating engines and are generally more tolerant of impurities in the gas. Finally, large volumes of gas can be used to power a traditional thermal steam generating station. This set up can handle most impurities and can also mix landfill gas with natural gas to increase generating capacity when needed.

According to theUS Department of Energy:
"Biomass is a term that includes all energy materials that emanate from biological sources, whether they are wood or wood wastes, residue of wood processing industries, food industry waste products, or municipal solid waste. Unlike the burning of fossil fuels, combustion of biomass merely recycles the carbon fixed by photosynthesis in the growth phase.

"On the average, about 80 percent of the dry weight of municipal solid waste is organic materials..."

Waste-to-energy is the only renewable energy technology that provides communities with dual environmental benefits: a clean source of electricity and clean trash disposal.

The fuel used in waste-to-energy plants to produce clean electricity is municipal solid waste. Trash is both "sustainable" and "indigenous" - two basic criteria for establishing what is a renewable energy source. Society will continue to generate waste and no one can reasonably claim that waste will ever be 100 percent eliminated.