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Acid Rain in New England

Reducing Acid Rain

Little Squam Lake, Ashland, NH - Photographed by Ian Cohen  - Click for a larger image.

Reducing emissions of SO2 and NOx is necessary if we are to reduce acid deposition. The first attempts at reducing SO2 took place in 1936 at the Battersea Plant in London, England . In recent years, we have made considerable progress in finding ways to reduce emissions of both SO2 and NOx.

There are two principal methods for reducing sulfur emissions at power plants and other facilities that burn coal or oil: fuel switching and scrubbing. Fuel switching means using coal or oil which contains less sulfur, or switching to fuels such as natural gas which have little or no sulfur. Scrubbing means removing sulfur by electrostatic, or chemical (wet or dry scrubbing) means. Electrostatic involves placing electrically charged plates, called electrostatic precipitators, inside the industry’s stack. These attract the positively charged sulfur particles to the surface. The surface is periodically cleaned, removing the sulfur before it gets into the air. Wet scrubbing means injecting water or a chemical solution into the exhaust gases. Dry scrubbing involves a chemical such as lime, which reacts with the gases without the use of water. The sulfur will react with the water or chemical and fall out. All types of scrubbing do pose a problem; we must find an environmentally acceptable way of disposing of the sulfur after we have removed it.

There are several methods of reducing NOx emissions. Some are mechanical: changing the ratio of air to fuel, or changing the temperature of the combustion. The cooler the flame is, the less NOx the furnace gives off. Others are chemical: injecting chemicals such as ammonia, which will react with the NOx and convert it back into nitrogen and oxygen.

EPA's Acid Rain Program has given the Utility Industry a reason to reduce SO2 and NOx emissions. This program was established by Title IV of the 1990 Clean Air Act Amendments. It set a cap on the amount of SO2 power plants can emit. The program also addressed NOx emissions, but only set maximum emission rates based on the type of boiler.

The Acid Rain Program focused on power plants, the largest single source of SO2 emissions, and a major source of NOx emissions. The plants affected by the program submitted permit applications explaining how they planned to comply with the program. EPA issues permits to each facility. The program also requires the use of continuous emissions monitors (CEMs) which measure their emissions and transmit the information directly to EPA.

A unique element of the program is its use of emissions trading as a compliance option. Although the national cap on emissions limits the total SO2 released into the air, companies may decide the most cost effective method. Each plant is assigned a number of "allowances," based on their average annual SO2 emissions during the period from 1985 to 1987. These could be bought, traded, sold, or held. At the end of the year, each facility has to surrender one allowance for each ton of SO2 it emitted. EPA set up an allowance tracking system, and also set up auctions and direct sales to enable plants which were not assigned allowances to obtain those they needed to operate. In 1990, the 263 units designated as part of the Phase I program emitted 10.0 million tons of SO2. In 1995, the first year in which the units were required to comply with Phase I of the program, they reduced their emissions to 5.3 million tons. This is a 47% reduction in emissions over 5 years.

Phase II of the program began in 2000. It affects more power plants, and gives all of the plants fewer allowances. Merrimack Station in New Hampshire had 31343 allowances per year in Phase I. They have 13530 allowances in Phase II. This will lead to further reductions in SO2 emissions.

The SO2 cap and allowance trading system proved successful, and is now being used or considered for use to regulate several other pollutants, including NOx and carbon dioxide.

The Acid Rain Program also monitors and limits emission of NOx. In New England, the Acid Rain NOx rules have not had a noticeable effect, because other programs have imposed more stringent limits.

The Acid Rain Program also promotes the use of renewable energy and energy conservation. Some allowances were set aside to award to companies which encouraged residential, commercial, and industrial conservation or used certain forms of renewable energy. Six New England companies were awarded 4186 allowances for energy conservation and for using biomass and landfill gas as a source of energy. Allowances are also available for solar, wind, and geothermal energy.

New England States have also been active in the fight against acid rain. Prior to 1990, Massachusetts and New Hampshire had passed laws limiting the emissions from power plants. In 2001, Massachusetts adopted regulations that will further limit emissions from large power plants by as much as 75% of SO2 and 50% of NOx. Connecticut has adopted regulations that will further limit emissions by as much as 50% of the SO2 and 30% of the NOx currently being emitted.

The United States has been working with Canada to reduce transboundary effects of Acid Deposition. A bilateral Air Quality Agreement was signed in March 1991. The third biennial report, published in 1996, focused on the progress made by the United States and Canada in achieving emissions reduction goals. Their 2006 Annual Progress Report (PDF) (84 pp, 2.7 MB, about PDF) reported that both countries have made progress in reducing emissions which lead to acid rain. The Integrated Atmospheric Deposition Network (IADN) was established in 1990 to collect data that can be useful in assessing the relative importance of atmospheric deposition. The IADN program will establish a database for atmospheric deposition data in both countries.

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