Waste to Energy

views updated

Waste to Energy

Waste to energy (WTE) is the term used to describe the conversion of waste by-products into useful steam or steam-generated electricity. Typically, WTE is produced by converting municipal solid waste (MSW), which is defined as residential and commercial refuse, and makes up the largest source of waste in industrialized countries. This industry has been producing heat and power in the United States for a century, and there are currently more than one hundred WTE plants nationwide. Recently, however, the definition of waste has been expanded from MSW to include wastes such as wood, wood waste, peat, wood sludge, agricultural waste, straw, tires, landfill gases, fish oils, paper industry liquors, railroad ties, and utility poles. In 1999 these by-products produced approximately 3.2 quadrillion BTUs (i.e., 1 × 1015 British thermal units, which is also known as a quad) of energy out of approximately 97.0 quads of energy consumed in the United States.

Nearly thirty million tons of trash are processed each year in WTE facilities to generate steam and electricity. The benefits to society include the following: preventing the release of greenhouse gases such as methane into the atmosphere if the trash were landfilled; reducing the impact on landfills by reducing the volume of the waste 80 to 90 percent; providing an alternative to coal use, which prevents the release of emissions such as nitrogen oxides into the atmosphere; and saving the earth's natural resources by using less oil, coal, or natural gas for electricity generation.

The Process of Converting Waste to Energy

Generally, WTE facilities can be divided into two process types: mass burn and refuse-derived fuel (RDF). Mass burn facilities process raw waste that has not been shredded, sized, or separated before combustion, although large items such as appliances and hazardous waste materials and batteries are removed before combustion. In mass burn systems, untreated MSW is simply burned, with the heat produced converted into steam, which can then be passed through a steam turbine to generate electricity or used directly to supply heat to nearby industries or buildings.

RDF is a result of processing MSW to separate the combustible fraction from the noncombustibles, such as metals and glass. RDF is mainly composed of paper, plastic, wood, and kitchen or yard wastes, and has a higher energy content than untreated MSW. Like MSW, RDF is then burned to produce steam and/or electricity. A benefit of using RDF is that it can be shredded into uniformly sized particles or compressed into briquettes, both of which facilitate handling, transportation, and combustion. Another benefit of RDF rather than raw MSW is that fewer noncombustibles such as heavy metals are burned.

Energy Production from Waste in the United States and South America

South America, with its agrarian societies, surprisingly consumes very few wastes for the production of steam or electricity. Brazil is the largest country in South America and is also the largest energy consumer, consuming about 8.5 quads of energy each year as compared to 6.1 quads for Mexico, 12.5 quads for Canada, and 97.0 quads for the United States. Due to the large size of Brazil's agricultural sector, biomass is seen as the best future alternative energy source. Currently, Brazil produces about 4,000 gigawatt (1 × 109) hours annually (i.e., 0.1 quads equivalent) in the sugar industry to run its own refineries and distilleries. At the same time, Brazil produces up to 3.9 billion gallons of ethanol (i.e., 0.5 quads equivalent) for automobiles each year, although it is manufactured from sugar and not waste materials. No other South American countries produce significant quantities of energy from waste; however, Argentina's biomass energy use, like Brazil's, is expected to grow in the coming years.

In the United States, corn is the primary feedstock along with barley and wheat that is currently being used to produce ethanol, although neither corn or grains are considered wastes. Considerable ongoing research is exploring the use of true biomass wastes such as corn stover or wood chips and sawdust for ethanol production. One project at the U.S. Department of Energy involves the cofiring of sawdust and tires with coal in utility boilers.