Sources Of Energy – The Fossil Fuels

There are three basic forms of fossil fuel:

• Solid: coal and its products
• Liquid: oil and its products
• Gas: natural gas and liquified petroleum gas (LPG)

All fossil fuels are organic in origin and thus contain carbon to a greater or lesser degree. This means that there is no way of avoiding emitting CO: when we burn fossil fuels. However, there are considerable differences in the proportion of carbon in each of these different fuels and thus in the amount of CO, emitted. When we compare the burning of coal with the burning of oil or gas we arrive at a very interesting and relevant comparison. (It is assumed that coal is pure carbon, oil is represented by approximately one carbon atom to two hydrogen, and methane is CH4, a carbon-hydrogen ratio of 1 to 4):

• Coal/coke (carbon): C + 02 = C02
• Heating oil: C:H4 + 302 = 2C02 +2H20
• Gas (methane): CH4 + 202 = CO, + 2H20

If we compare the above chemical reactions we can see that the main product of combustion of coal is C02, whereas with both oil and gas, water (which is ecologically neutral) is produced as well, indicating a less polluting burn. In fact, as a rule of thumb, the higher the proportion of hydrogen (a high energy burner) in a fuel, the less harmful it is. The ultimate ecolog¬ical fuel is hydrogen (in which the only product of combustion is water); there is a growing lobby for its development linked to solar power. We thus have a range of fuels with carbon (coal) at one end and hydrogen at the other.

Solid fuels (coal and coke)
The use of coal presents us with another problem. Besides burning with the highest proportion of C02, coal burning can be polluting in other ways. The constituents of coal vary enormously from anthracite, which is around 94% pure carbon, to brown coal or lignite, which contains many other compounds including sulphur. Most of the coal in Britain lies somewhere between these two extremes. When burnt, not only C02 but also sulphur dioxide is produced; this combines with water to form sulphurous acid—the main cause of both smog and acid rain. In Britain, the clean air acts of the 1950s and 1960s, which introduced smokeless fuel zones, reduced the sulphur pollu¬tion in the big cities; but this pollution was transferred to the upper levels of the troposphere (the lowest layer of the atmosphere up to 18 kilometres) via the tall chimneys of coal-fired power stations and coking plants. From being a local problem of smog in large cities, it became the regional/global problem of acid rain.

Fewer and fewer people are now burning solid fuel in their houses, for reasons of dirt, smell, pollution, inefficiency, time and cost. This trend is likely to continue, and from an ecological point of view should not be discour¬aged. Burning coal in an open hearth, where 90% of the heat goes up the chimney, belongs to a bygone age. If, however, you are heating your house with one of the latest high-efficiency solid fuel boilers, you are doing less damage to the environment than if you used electricity for the same purpose.

Liquid fuel (oil and its derivatives)
In Britain, oil had its heyday as fuel for heating during the 50’s and 60’s; a combination of the discovery of North Sea gas and then the ’73 energy crisis put an end to its popularity. If we look at the chemical reaction, we can see that when oil is burnt approximately one molecule of water is produced for every carbon dioxide molecule. This points to oil being a better fuel to burn than coal in terms of carbon dioxide emissions. On average the sulphur content is also considerably less. This makes oil a more benign fuel than coal to burn in terms of acid rain and other pollutants; but we need also to take into account the other environmental costs of extrac¬tion and transportation. The main environmental cost is that of marine pollu-tion, which is a growing concern.

Liquefied petroleum gas (propane and butane)
LPG is a product of oil; it exists in two forms, propane (C3H«) and butane (C4H,(I). These are both members of the paraffin series of which methane is the first member and heating oil is about the sixteenth. Both have a carbon content closer to oil than to natural gas. Energy costs for its distribution are higher than for natural gas, and similar comments can be made about its polluting effects on the sea as with heating oil. The most interesting recent development in the use of LPGs is as a substitute for the chlorofluorocar-bons (CFCs) in refrigerators.

Natural gas (methane)
Methane is the cleanest of all the fossil fuels to burn as there are two mole¬cules of water produced for every one of carbon dioxide. It has very few-impurities and its cost in terms of extraction and distribution is only 7%, which makes it the most efficiently distributed of all the fossil fuels. This is because of the huge network of gas mains that crosses Britain. The efficiency of appliances that use natural gas can be of a very high order (condensing gas boilers can extract up to 95% of the available fuel heat in peak conditions). However there are also gas appliances with very low efficiencies, such as the mock coal fires with open fireplaces, where most of the heat goes straight up the chimney.

Natural gas is therefore the most ecological choice amongst fossil fuels, but we should treat it very sparingly as we may have little more than 30 years accessible reserves. However it could become a renewable source for the future: in China methane gas is commonly manufactured from agricultural biomass. We could do the same.

The other side of the coin is that methane is also an increasingly serious greenhouse gas: a thousand million tons is released into the atmosphere every year from rubbish tips and agriculture, contributing nearly 20% of global warming. If a higher proportion of this gas could only be collected and used before escaping into the atmosphere we would be solving two problems at once.

Tagged as: Energy, Fossil, Fuels, Sources