programs

Waste to Energy Program 2

Advanced Air Pollution Control But the furnace is only half the story. The really advanced technology in trash burning is the air pollution controls. If you've ever seen leaves burning, or rubber burning, or even just wood burning on an open fire, it may be hard to picture burning garbage cleanly. But waste-to-energy plants do not use an open fire. They use a very carefully controlled fire with several different kinds of air-cleaning equipment. As a result, waste-to-energy plants have less pollution coming out of their smokestacks than most fossil fuel power plants in the U.S. Pollution controls can respond quickly and automatically to changes in the fuel and the furnace. Computer-controlled monitors sample the air continuously at several places in the furnace, the exhaust system, and in the smokestack. The computers adjust the air feeding the fire. They also adjust the addition of lime and other chemicals used to remove pollutants. Of course, human operators in the control room monitor the computers. Plant operators want to keep emissions as low as possible, because this improves the plant's efficiency, reduces operating costs and protects the environment. In many plants today, the computers also send their information over the telephone lines directly to the local environment protection authorities. Every plant must report its pollution control data to environmental regulators and health authorities on a daily, weekly, monthly or quarterly basis. Waste-to-energy plants are among the most closely regulated power plants in the country. Like coal, oil, and natural gas, burning trash produces various gases that must be controlled to protect human health and the environment. These are:

• carbon monoxide
• nitrogen oxides
• the acid gases, sulfur dioxide and hydrogen chloride
• ordinary smoke and soot, known as particulates, which can contain dioxins and solid metals
• mercury

Good air pollution control begins in the furnace. High furnace temperature and proper air control prevent the formation of dioxins and carbon monoxide, which are created when organic matter does not burn completely.⁶ Good furnace control also limits the formation of nitrogen oxides. Nitrogen oxides that do form are reduced by spraying ammonia or urea into the hot exhaust (a technology called Selective Non-Catalytic Reduction), which converts nitrogen oxides to harmless nitrogen and water. Acid gases are removed by a "dry scrubber." This is a device that typically sprays wetted lime powder into the hot exhaust. The scrubber uses lime to neutralize acid gases, just as a gardener uses lime to neutralize acid soil. Dry scrubber systems trap up to half the mercury. The rest can be controlled by blowing activated charcoal into the exhaust gas to adsorb mercury. Currently, waste-to-energy is the only garbage management method that permanently removes significant quantities of mercury from the environment. The greatest advances in mercury control, however, have come from the reduction of mercury in batteries, paint, and other consumer products, an effort manufacturers, states and the U. S. Environmental Protection Agency began a few years ago. Due to the removal of mercury from consumer products, Tampa, Florida's waste-to-energy plant has recorded an 85 percent drop in mercury emissions from 1985 to 1991, with no change in pollution controls.⁷ The final step in pollution control is to remove all these chemicals the lime salts, the activated charcoal, ordinary smoke and soot, dioxins, and metals like lead, cadmium and nickel which are found in many consumer products. These particles are called "fly ash" because they are light and tend to fly around in the hot flue gas. Fly ash is usually removed by a "bag house," which works like a giant vacuum cleaner with hundreds of fabric filter bags. Some plants use a different device, called an electrostatic precipitator, which uses electrically-charged plates to capture the small particles of fly ash, much like a television screen attracts house dust. The U. S. Environmental Protection Agency has done field research that shows that lime-rich ash hardens into a concrete-like substance when it is handled properly. Harmless carbon dioxide and water are essentially the only remaining combustion gases emitted from the "smoke" stack.

Cleaner, Healthier Air

Waste-to-energy plants make steam and electricity. Compared to other fuels burned to make steam and electricity, they are very clean. In most cases, building a waste-to-energy plant improves air quality by replacing other fuels. Oil is a good comparison. Eleven million American homes burn Number 2 fuel oil, also known as home heating oil, each winter.⁸ A modern waste-to-energy plant, with its advanced pollution controls, is cleaner than a home oil furnace on an equivalent energy basis. Waste-to-energy actually replaces fuel burned at utility power plants. Utilities do not normally burn home heating oil because it is too expensive. Utility power plants usually burn Number 6 fuel oil (known as residual oil) or coal. As the graphs show, waste-to-energy plants are much cleaner than power plants burning residual oil or coal. In some ways, they are even cleaner than power plants that burn natural gas.

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©From America's Newest Energy Source and Making a Clean Energy Source Cleaner. Call (202) 659-3819 for samples. Volume discounts available for quantities from 100 to 20,000 copies. All material Copyright 1994 by the AIMS Coalition [American Society of Mechanical Engineers (ASME), Integrated Waste Services Association (IWSA), Municipal Waste Management Association of the U.S. Conference of Mayors (MWMA), and Solid Waste Association of North America (SWANA)].