Although the combustion of fossil fuels is necessary to provide the power that drives our society and industries, it also presents the issue of acid rain. Acid rain is one of the severest consequences that our society faces due to engaging in heavy industry. Industrial processes relying on fossil fuels for power have led to an increase in acid rain due to greater emissions of gases such as sulfur dioxides (SO_x) and nitrogen oxides (NO_x). In the atmosphere these gases react with water to form acids which cause a downward shift in the pH of the atmospheric water. This water eventually condenses and falls to the ground as acid rain, where it has profound effects on the environment and its inhabitants.

A common misconception about SO₂ emissions is that cars and transportion are the primary sources. It is important to note that over 93% of all sulfur dioxide emissions are industral based (with only 3.7% transport based). However, cars and other forms of transportion (trucks, airplanes, boats, etc.) are responsible for large amounts of NO_x being released into the atmosphere ( creating smog and ozone pollution ). While some of their effects are similar, the large scale release of these two componds represent two different major enviornmental problems and should not be confused.

After SO₂ and NO₂ are released into the atmosphere they are not particularuly acidic or soluble in H₂O. However they are converted to H₂SO₄ and HNO₃ in the atmosphere (using cloud condensation nuclei as catalases¹) which are both acidic and soluble in H₂O. The process by which this occurs is :

The sulfuric acid produced in equation (2) is then released from the atmosphere in rain water. A simlar process occurs to produce HNO₃ and rid it from the atmosphere. The lifetime of SO₂ is only a few days and as a result rainfall containing high amounts of H₂SO₄ only occurs a few hundred miles from the source of emission. This effect can be seen in a recent pH map of the United States where the lowest pH readings are centered around the NorthEast (the country's industral backbone). Another important fact pertaining to this map is that rainfall is naturally acidic. Carbon dioxide found in the atmosphere is dissolved into the water droplets existing in clouds and effectly lowers the pH of rain to about 5.7 log units. Despite carbon dioxide's role in lowering the pH of rainfall, large increases in the amounts of carbon dioxide found in the atmosphere are not expected to cause a large decrease in the pH of rainfall. ( For instance, a doubling in CO₂ concerntrations results in only about a 0.2 unit decrese in pH)¹.

The SO₂ released be power plants can by reduced by appoximately 90% through the use of a scrubber or more offically known as flue gas desulfuriztion. This is achieved through the use of calcium carbonate.

In order to reduce the effects of acid rain, the EPA has recently initiated an Acid Rain Program aimed at "significantly reducing electric utilties emissions of sulfur dioxide and nitrogen oxides". The Clean Air Act's primary goal is reduction of annual SO₂ emissions by 10 million tons below 1980 levels (0.9 million tons). Legislation introduced by the EPA calls for a "two-phase tightening of the restrictions placed on fossil fuel-fired power plants." Meeting the new emission restrictions has forced companies to use existing techonoloies to reduce the level of emissions. A large number of emission reducing processes are availiable such as wet and dry flue gas desulfuriztion , dry acid gas scrubbing systems for solid waste incineration , electrostatic precipitators and fabric filters and are currently being empolyed by companies like General Electric.


The effectiveness of these processes can be easily seen by the almost 50% reduction of the total national amount of sulfer dioxide released from the 1980 emission level (about 40% below the EPA allowable emissions for 1995).

Despite the large decrease in SO₂ emissions by 1995, the annual heat input from utilities remained constant (even increasing in some states) as can be shown by examining figures² 2 and 3. This shows that current techonolgies are able to cause significant reduction of sulfur dioxide emissions without having an addverse effect on industral output.

figure 2

figure 3

The EPA has devised an Allowance Trading Plan to help ease the cost of installing clean air systems. Each year the allowable amount of emissions are are divided into credits and given to companies where the emissions of each company is not to exceed thier number of credits. Companies are allowed to sell their credits as they chose. By selling there credits to a larger company, smaller companies are able to afford the costs of installation. The volume of allowance trading shows that more and more companies are taking advantage of the EPA's system and are updating their plants. Since the second phase of the EPA's plan will increase the number of units involed from about 450 to 2000, this is a positive indicator that the future goals (for the year 2000) will be able to be met.





Other companies are using other approaches to help curve the cost of reduced emissions. Their solution, which is by far the best long term solution to pollution, is to recycle their waste products into other marketable goods.

There have been some companies that have taken advantage of the cleaning of the waste produced by coal-powered electric plants. The Tennessee Valley Authority's Cumberland Power plant is one such example. During the production of power, the sulfur gases produced are sent through electrostatic precipitators, which remove the fly ash, and then continue through the flue gas desulfurization scrubbers where the gas is sprayed with a scubber slurry that absorbs the SO₂. This SO₂ laden acidic liquid is then neutralized by the addition of crushed limestone which results in calcium sulfite. With a heavy blast of air, the material is oxidized and forms calcium sulfate- more commonly known as "gypsum." This Plant produced over a million tons of gypsum annually.

New techonologies are being developed to create more efficeint filtering systems. Companies are developing methods using activated coke/carbon to help reduce toxic gas emissions (claims a 98% sulfur removal rate). Other researchers³ report that vast improvements in the reaction rate of calcuim with SO₂ are possible by reducing the diameter of the calcium particles used in scubbers (thus increase their effectivness).

NO_x gasses have had recent solutions discovered. Ballard Power systems has designed a fuel cell that runs on external fuel. This process converts hydrogen and oxygen into water and produces electricity. (diagram) The only drawback to implementing this system is the cost. The fuel cells cost roughly ten times the amount of the internal combustion engine, running the price from near $3000 to $30,000. The fuel cells are hoped to be in cars within the next decade.