2.1. Fossil Fuels (Petroleum, Natural Gas and Coal)

Coal, oil and gas are three major forms of fossil fuels non-renewable sources of energy formed from the organic remains of plants and animals buried under earth for millions of years. Fossil fuels consist largely of carbon and hydrogen. The burning process actually is chemical reactions with oxygen in the air. For the most part, the carbon combines with oxygen (O) to form CO2, and the hydrogen (H) combines with oxygen to form water vapor. The released CO2 is the cause of the greenhouse effect.

Coal is a solid combustible material consisting of organic matter and minor amounts of inorganic materials. There are basically four types of coal, each of which varies in terms of its heating value, chemical composition, ash content, and geological origin. The four types of coal are anthracite, bituminous, sub-bituminous, and lignite. Many hazardous pollutants are released when coal is burned. Major pollutants are: SO2, nitrogen oxides (NOx), carbon monoxide (CO), particulates, hydrocarbons, ozone (O3), volatile organic compounds, toxic metals (cadmium (Cd), arsenic (As), nickel (Ni), chromium (Cr), and beryllium (Be). As coal is burned, several pollutants linked to the environmental problems of acid rain, urban ozone, and global climate changes are realesed. Noncombustible mineral content of coal is partitioned into bottom ash and fly ash. Flue gases are also released from the combustion of this fossil fuel.

Bottom ash and boiler slag are composed principally of silica, alumina, and iron (Fe), with smaller content of calcium (Ca), magnesium (Mg), sulphates, and other compounds. Due to the inherent salt and heavy metal content and in some cases low pH, this material may exhibit corrosive and toxic properties. Elements showing enrichment in bottom ash or slag include Barium (Ba), Be, Cobalt (Co), Manganese (Mn), Cesium (Cs), Copper (Cu), Nickel (Ni), Strontium (Sr), Tantalum (Ta), Vanadium (V), Tungsten (W), Europium (Eu), Hafnium (Hf), and Zirconium (Zr) concentrated in part by density segregation effects. Oxides of slicon (SiO), aluminum (Al), Fe, and Ca comprise more than 90% of the mineral component of typical fly ash. Mg, potassium (K), sodium (Na), titanium (Ti), and sulphur (S) are minor contituents. They account for about 8% of the mineral component, while trace constituents such as As, Cd, lead (Pb), mercury (Hg), and selenium (Se) together make up less than 1% of the total composition of coal. Flue gases from coal combustion consist of mainly uncombusted nitrogen, carbon dioxide, and water wapor.

The discovery of crude oil, in the 19th century, produced an inexpensive liquid fuel source that aided in the industrialization of the world and the improvement of living standards. The petrochemical industry is a major source of hazardous organic wates, produced during the manufacture or use of hazardous substances. The recovery, transportation, and storage of raw oil or petrochemicals are major sources of hazardous wastes, often produced as the consequence of technological accidents. Sea water and fresh water pollution due to oil and oil-product spills, underground or soil pollution due to land spills or leakage from pipelines or tanks, and air pollution due to incineration of oil or oil sludge, are major cases of environmental pollution. Gasoline is the main product in the petrochemical industry and consist of ~70% aliphatic linear and branched hydrocarbons, and 30% aromatic hydrocarbons, including xylenes, toluene, di- and tri-methylbenzenes, ethylbenzenes, benzene, and others. Other pure bulk chemicals used for chemical synthesis include formaldehyde, methanol, acetic acid, ethylene, polyethylenes, ethylene glycol, polyethylene glycols, propylene, propylene glycol, polypropylene glycols, and such aromatic hydrocarbons as benzene, toluene, xylenes, styrene, aniline, phthalates, naphthalene, and others.

Natural Gas is a combustible mixture of gaseous hydrocarbons that accumulates in porous sedimentary rocks, especially those yielding petroleum. Natural gas is primarily composed of metane, but also contains ethane, propane, butane and heavier hydrocarbons. It also contains small amounts of nitrogen (N), CO2, hydrogen sulphide (H2S) and trace amounts of water. The most toxic components (propane and butane) in the mixture are present in small amounts. The leakage of methane, a potent global warming gas, during the drilling and extraction of natural gas from wells and its transportation in pipelines is an important environmental hazard. It burns cleaner than other fossil fuels. The combustion of natural gas produces negligible amounts of S, Hg, and particulates

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