List of five important fossil fuels:- 1. Oil 2. Coal 3. Natural Gas 4. Uranium 5. Nuclear Energy.
Fossil Fuel # 1. Oil:
Crude oil is a mixture of liquid hydrocarbon compounds sometimes found permeating sedimentary rocks. Its elements are, by weight, carbon 82.2-87.1 per cent, hydrogen 11.7-14.7 per cent, oxygen 0.1 -4.5 per cent, nitrogen 0.1-1.5 per cent and sulphur 0.1-5.5 per cent.
Different names, based on the number of carbon atoms in their compounds, are given to products derived from crude oil. They are called gasoline (C4 to C10), kerosene (C11 to C13), diesel fuel (C14 to C18), heavy gas oil (C19 to C23), lubricating oil (C26 to C40) and waxes (over C40).
Oil is generated from organic matter in sedimentary rocks at depths of about 800-5,000 m at temperatures between 66°C and 150°C. Its predominant source material is probably marine organisms. Three steps are involved in the conversion of organic matter to petroleum.
Fossil Fuel # 2. Coal:
Coal is a complex organic material consisting of fused carbon rings held together by assorted hydrocarbon and other atomic (O, N, S) linkages. Its average composition is something like C10H8O (this ratio of 10 carbon atoms to 8 of hydrogen can be contrasted with the ratio of 10 carbons to 17.5 hydrogen’s in crude oil).
It is formed from dead plant material which has accumulated in swamps—usually in estuarine deltaic deposits—and then has been consolidated and altered by increasing temperature and pressure.
In a similar evolutionary pattern to oil, the first stage in the conversion process is an anaerobic breakdown of the plant material which causes volatile products to be liberated and lost to give a compacted structure less mass of compounds enriched in carbon.
The second stage is the process of coalification which proceeds through the ranks of peat, lignite, sub-bituminous coal, bituminous coal and anthracite to graphite. The proportion of carbon is gradually increased in this progression. Calorific values of the various ranks range from 15-26 kJ g-1 for low rank lignite’s, through 31-35 kJ g-1 for bituminous coals to 30-33 kJ g-1 for anthracite.
Coal deposits are not found before the lower Carboniferous age (about 400 million years ago) and the most important and widespread date from the Carboniferous to the early Triassic age (345 to 200 million years ago) and from the Jurassic to the early Tertiary age (150 to 50 million years ago). In general, the older coals have the highest rank but, depending on the geological history of the deposits this is not necessarily so.
Although coal is widespread, the major deposits are unevenly distributed (Fig. 24.3). Ten countries account for 92 per cent of the currently estimated reserves and resources. Three countries alone, the US, the former Soviet Union and China, have 83 per cent of the resources and 60 per cent of the reserves.
All three could become exporters of coal and in any case, compared with rates of use, the total reserves are so large that the uneven spread does not, at the moment, mean supply difficulties.
Fossil Fuel # 3. Natural Gas:
Increasing interest is being shown in natural gas and there are two broad reasons for this. First, it has recently been realized that its resource base is much larger than had previously been thought. Second, it is environmentally more benign than other fuels, particularly coal.
Generating electricity from advanced turbines, using natural gas instead of coal, can reduce carbon emissions by 50 per cent for each unit of production and by a further 20 per cent because of their increased efficiency.
However, it would be important to ensure that such machinery was largely proof against leaks because the reductions in greenhouse gas emissions could be wiped out, even if only 3 to 4 per cent of the methane finds its way into the atmosphere (methane is 30 times more powerful as a greenhouse absorber than CO2).
Burning methane does not release SO2, because the source of sulphur, H2S, is removed before the gas is distributed.
Occurrence and global distribution of gas:
Natural gas consists of hydrocarbons with one to five carbon atoms, together with small amounts of other gases as impurities. It was formed under essentially the same kind of conditions as oil, anaerobic decomposition of organic matter under heat and pressure assisted by bacteria. Marine organisms are the primary source material for oil, but natural gas can be formed both from land plants and from marine organic material.
Gas can be formed in very young deposits as, for example, marsh gas in swamps; it can also be formed in association with coal deposits, particularly those of the Permo-Carboniferous, with crude oil and as ‘thermal’ gas below the oil window.
Therefore the depths and areas of sedimentary basins which may hold gas far exceed those of oil. Gas fields, like oil fields, are not distributed uniformly and differ in size and geographical concentration, but because of its more diverse origins gas is more widespread.
Gas, found on its own in ‘dry’ wells, is called ‘non-associated gas’. It is also found dissolved under pressure in oil in a reservoir or as a ‘gas cap’ over an oil pool; in these cases it is called ‘associated gas’. It has been estimated that about 72 per cent of world reserves are non- associated, 17 per cent are dissolved and 11 per cent are gas caps.
Natural gas fields are classified in terms of their size as supergiant’s with 10 x 1012 ft3, (2.83 x 109 m3), or more, of recoverable gas and giant (world class) gas fields with between 3 x 1012 ft3 (8.5 x 1010 m3) and 10 x 1012 ft3 (2.8 x 1011 m3) of recoverable gas.
Fossil Fuel # 4. Uranium:
Uranium, in a variety of reservoirs with widely differing concentrations, occurs extensively in the earth’s crust, with an average presence of about 2 ppm. Granites contain up to 20 ppm, currently exploited ores typically have over 350 ppm, coals commonly have around 20 ppm, (some are known with 500-2000 ppm). Sea water contains 0.0005 ppm. The estimate for the earth’s crust as a whole is 2.5 x 1013 tonnes.
Uranium increases in quantity as the ore grade decreases, thus every tenfold decrease in ore grade leads to a three-hundredfold increase in the recoverable amount. The grade of ore mined at any particular time is determined by the price of the open market. Currently the price is not too high because, following the world-wide slow down in nuclear power plant construction, there is very little demand for it.
World distribution of uranium ore:
Primary uranium ores are derived from Precambrian sources which have been buried and submitted to high temperatures and pressures. This results in the creation of uranium-rich magmas, or uranium-rich solutions, which give rise to the primary ore bodies. These, in addition to magmatic deposits, comprise pegmatite’s, vein deposits and veins associated with unconformities.
Secondary ore bodies are derived from primary ores by sedimentary processes. Most of them are to be found in sandstones, conglomerate placers, phosphates and shale’s. Many more deposits must exist in numerous unexplored areas, particularly in remote shield regions.
Fossil Fuel # 5. Nuclear Energy:
When coal, oil and natural gas are burned, their hydrocarbons react with the oxygen in air to yield carbon dioxide and water. In the process, energy is released. The source of this energy is the rearrangement of the chemical bonds of the various compounds involved in the reactions. Chemical bonds involve the outermost electrons of atoms.
Nuclear energy results from rearrangements of the components of the nuclei of atoms and because the forces between these are very much greater than those between the outer electrons of the atom the energy released in nuclear reactions is very much greater than that in chemical reactions.
Typically, 100 million times more energy is released in a nuclear reaction than in a chemical reaction.
There are two possible ways by which energy is released from radioactive materials:
1. By fission — whereby a nucleus with A greater than 120 is split into two nuclei which have A (A denotes atomic weight) around 60 and because the new nuclei have greater binding energies, energy must be released in the process.
2. By fusion — whereby two light nuclei (A below about 20) fuse together to form a nucleus with A up to around 60. As the binding energy curve is much steeper for nuclei with low values of A than for those with values of A above 120, the energy released in the process of fusing together two light nuclei is much greater than that in the fissioning of a heavy nucleus.
The steps involved in the energy generation from enriched uranium shown in Fig. 24.4: