Atmospheric Effects of Burning Fossil Fuels

 

 

The main source of heat and electrical energy, particularly in developing countries, are the fossil fuels which include coal, natural gas, petroleum, shale oil, and bitumen. Besides containing large quantities of carbon, hydrogen and oxygen, all fossil fuels contain other materials including metal, sulfur, and nitrogen compounds. During the combustion of fossil fuels different pollutants like fly ash (from coal), sulfur oxides (SO2 and SO3), nitrogen oxides (NO2 and NO), and volatile organic compounds are released into the atmosphere. Fly ash contains numerous heavy metals in trace amounts. Gross emission of pollutants is tremendous all over the world. These pollutants are present in the atmosphere in such forms and quantities that they affect man and his environment.

Air pollution caused by gaseous and particulate matter produced by combustion of fossil fuels not only acts directly on the environment but by contamination of water and soil leads to their degradation. Sulfur and nitrogen oxides in the atmosphere are transformed to sulfuric and nitric acids in the presence of water vapors, fog, and water droplets. Wet and dry deposition of inorganic pollutants leads to acidification of environment. These phenomena affect the health of the people, increase corrosion, and destroy cultivated soil and forests. Most of the plants, especially coniferous trees are not resistant to the harmful effects of these pollutants, and on prolonged exposure their leaves wither and fall. Widespread forest damage has been reported around the world. Many cultivated plants are not resistant to these pollutants, especially in the early period of vegetation.

Other problems of burning fossil fuels are the emission of volatile organic compounds into the atmosphere. These emissions cause stratospheric ozone depletion, ground level photochemical ozone formation, toxic or carcinogenic human health effects, exacerbation of the global greenhouse effect, accumulation and persistence in the environment.

The principal air pollutants resulting from fossil fuel combustion are the following:

a)      Carbon monoxide

b)      Oxides of sulfur, SO2 and SO3 (represented as SOx)

c)      Oxides of nitrogen, NO and NO2 (NOx); and

d)     Particulates, consisting primarily of very fine soot and ash particles.

Air pollution may result also from unburned hydrocarbons. These either pass through energy conversion devices without burning or escape into the air by evaporation before they can be burnt. For many years, lead compounds contributed to air pollution, but the nearly complete elimination of ‘leaded’ gasoline has reduced this problem significantly.

These primary pollutants can further interact with the environment to generate additional deleterious effects. Examples of these effects (secondary pollutants) are acid rain and smog, the greenhouse effect and the high ozone levels in the air we breathe. (This last effect should not be confused with the ozone layer depletion, which is also becoming an environmental problem but has no direct relationship with fossil fuel utilization.)

Primary Air Pollutants

Carbon Monoxide: Carbon monoxide (CO) is a product of incomplete combustion of any fuel. It is both a highly poisonous gas and the principal constituent of photochemical smog. Health effects associated with human exposure to carbon monoxide include headache, nausea, unconsciousness and even death. The main culprits of CO pollution are urban automobiles and transportation vehicles. It has been estimated that some 100 million tons of CO are emitted every year in the United States alone.

Sulfur Oxides: Sulfur oxides arise during combustion from oxidation of sulfur in sulfur containing fuels (some coals and some petroleum-based products). The principal producs are SO2 and SO3. Sulfur dioxide has an annoying odor and irritates eyes and the respiratory tract. When released to the atmosphere, it can react with oxygen in the air to form sulfur trioxide. Sulfur trioxide irritates the mucous membranes of the respiratory tract. A concentration of 1 ppm is enough to cause coughing and choking. Sulfur trioxide dissolves in water to form sulfuric acid, which is a strong acid capable of corroding or destroying many materials. Sulfur trioxide can absorb moisture from the atmosphere to form very fine droplets of sulfuric acid. Inhalation of these droplets can harm the respiratory system. Chronic exposure leads to a much greater likelihood of suffering from bronchitis. Sulfur trioxide can also dissolve readily in rain drops, and fall to the earth as acid rain.

Nitrogen Oxides: Nitrogen oxides have two sources. Fuel NOx is produced when nitrogen atoms chemically combine with the molecules of the fuel and are oxidized during the combustion process to form nitric oxide:

2 N (in fuel) + O2 → 2 NO

In addition, thermal NOx is produced in some combustion processes that operate at such high temperatures that nitrogen molecules in the air are oxidized to nitric oxide:

N2 (in air) + O2 → 2 NO

(Remember that air is 79% N2 and 21% O2.) When the nitric oxide is emitted to the environment, it readily reacts with oxygen in the air to form nitrogen dioxide:

2 NO + O2 → 2 NO2

Nitrogen dioxide is a noxious gas that can cause inflammation of the lungs and, at high concentrations, even death. In addition, nitrogen oxides will react further with water and oxygen to form nitric acid:

4 NO2 + 2 H2O + O2 → 4 HNO3

Like sulfuric acid, nitric acid is a very strong acid that easily corrodes or attacks many materials. Nitric acid is also a component of acid rain. Unlike the case of SOx and CO, no particular source is to blame, because much of the emissions are thermal NOx from the air and not from the fuel, as discussed above. For this reason, it is much more difficult to reduce NOx emissions than SOx emissions.

Particulate Matter: Particulate matter emissions (soot and fly ash) are a concern because they can contribute to long-term respiratory problems. Many of these particles are extremely small, of the order of 10 micrometer or less, and they are thus suspended in the air we breathe. After inhaling them, they get trapped in the very thin air passages inside the lungs. Over a period of years this reduces the air capacity of the lungs. Reduced air capacity leads in turn to severe breathing and respiratory problems. Chronic asthma or emphysema can result, as well as increased general susceptibility to respiratory diseases. To make things worse, these particles may carry along small amounts of hazardous trace elements or potentially carcinogenic organic molecules. Particulate matter is also an aesthetic nuisance. Areas with high concentrations of air-borne particulate matter are more likely to experience fogs, because these particles are preferred nucleation sites for water droplets. Smoke and soot are also very undesirable aesthetically.

Unburned Hydrocarbons: Unburned hydrocarbons represent another source of air pollution associated with the use of fossil fuels (especially gasoline), even though they are not a result of combustion. Much of the emission of unburned hydrocarbons to the air occurs as a result of evaporation from fuel tanks (remember the smell of gasoline during your last fill-up?) and as a result of leaks or spills. Taken individually, these events seem trivially small. But on any given day millions of vehicles are being refilled with gasoline. In addition, if you drive a car whose engine is poorly tuned, a significant fraction of gasoline sweeps right through the engine and ends up unburned in the exhaust system.

Secondary Air Pollutants

Acid Rain: Sulfur and nitrogen oxides combine with water to form acid rain. Sulfur oxides are primarily responsible for acid rain. Increases in acid rain correlate with increases in SOx emissions: the highest acidity of acid rain is found in areas having the highest concentrations of SOx. Acid rain falling on land can acidify the soil, harming crops and forests. It has been estimated that more than 50% of the red spruce in the Adirondacks, the Green Mountains in Vermont and the White Mountains in New Hampshire have died in the past 25 years. In Europe, the estimated forest damage due to acid rain ranges from less than 10% in Spain and France to more than 50% in the U.K.

Smog: Smog is another secondary pollutant. This term was developed to describe a substance that is a hybrid of smoke and fog. The SOx aerosols are one source of smog formation. As discussed earlier, sulfuric acid droplets, or sulfuric acid absorbed on the surface of soot and fly ash particles, can attract moisture from the air to form what is often referred to as conventional or ‘classical’ smog. Such smog, whose principal components are NOx, Sox and particulates, was prevalent in the heyday of the coal-fueled Industrial Revolution, before the transportation revolution of the 20th century.

Modern-day smog is often referred to as ‘photochemical’ smog. It is produced by complex, sunlight-stimulated chemical reactions among the components of automobile exhaust. It is responsible for much of today's air pollution in cities such as Los Angeles and Denver. Carbon monoxide from incomplete combustion of automobile engines, particulate matter and NOx all react to generate the noxious brown haze.

An estimated 80% of smog today arises from vehicle exhausts. Not only does smog smell bad and obstructs vision, but both short-term and long-term exposure to it may be hazardous. Eye irritation develops upon short-term exposure. Chronic pulmonary diseases, asthma, bronchitis and even lung cancer may result from longer-term exposure; in addition, paint and fabrics slowly deteriorate during long-term exposure.

Ground Level Ozone: Ground-level ozone (O3) is a secondary air pollutant and an important smog constituent. It is formed by complex chemical reactions of primary pollutants with oxygen (O2). Its effect depends on its concentration in the air. At low concentrations, it can be beneficial, as in fresh air after a storm. At higher concentrations, it is an irritant. Its concentration rises proportionately with that of primary pollutants and it is often reported as an indicator of smog accumulation in a city. The energy and fuels industry (primarily vehicles and fuel filling stations) accounts for about 50% of ground level ozone; the rest comes from other industrial and nonindustrial uses.

(Much of the information contained in this article is from “Fossil Fuels: Environmental Effects” accessed from http://www.ems.psu.edu/~radovic/Chapter11.pdf)

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