Something you don't understand? Look it up on the Environment and Energy Wiki Encyclopaedia (beta version)

The Environment and Energy Portal is now operational 

Introduction

Let's start by defining fossil fuel; a fossil fuel is a hydrocarbon that can be extracted from the earth and is useable as an energy source. Generally speaking, they are formed by the aging of vegetable or animal organic matter over geological eras under conditions of heat and/or pressure. There are three basic families of fossil fuels, coal, oil and natural gas, with a certain amount of overlap between them. They all have one thing in common: the quantity available is limited and, once we have used them up, there will be no more. This is not theoretical, as may be seen by the fact that both the USA and the UK have used up most of their oil and much of their gas reserves already.

Coal

Coal is formed from mostly vegetable matter, but there are several types of coal. Going from the youngest to the oldest, the main categories are peat, lignite, bituminous coal and anthracite.

   Peat

Peat is fuel that has been formed by pressure from generations of sphagnum moss, reeds, detritus from tropical or deciduous forests etc. Most of the peat easily available as fuel is derived from moss in northern latitudes. It has been much exploited, in the past, as domestic fuel in Ireland, Scotland and the Nordic countries: some Scotch whisky owes its unique flavour to the very soft, acidic, water in peat bog regions and, some say, to the peat fires used for the distillation. Very little large-scale industrial use has been made of it because large volumes are required for a given amount of thermal energy, compared with other fossil fuels. Finland has peat-burning thermal electricity power generation. Peat is still being formed in sphagnum wetlands, but the quantities are small, typically a 1 mm thick layer being added each year. It is accompanied by considerable emissions of natural gas, some of which is trapped in the fibrous structure of the fuel.

Of all fossil fuels, peat is the worst for carbon dioxide emissions for a given amount of thermal energy produced. Although much peat has been produced over short geological time scales, typically 2 000 to 10 000 years, it is considered as fossil fuel although Finnish authorities have called it biomass. This is a specious argument because most exploited peat bogs have not been allowed to regenerate but have been turned into forest land. It cannot therefore be considered as a renewable resource, which the term biomass really implies.

Vast areas of peat bog within the Arctic circle are permanently frozen. They often contain enormous quantities of methane and carbon dioxide in the form of clathrates (in this context, a clathrate is a pseudo-compound where a gas is trapped within a lattice of ice molecules). It is feared that melting of the permafrost due to temperature increases will release large quantities of these gases, contributing to further temperature rises in a positive feedback loop.

   Lignite

Lignite is a very poor quality coal that some consider as being a young coal where the formation into a rock-like structure is incomplete. It is brown and fibrous with some of the original woody plant growth being apparent. Like peat, it has a very poor calorific value. It is used for generating electricity in Germany, Russia and elsewhere, but the quantity of ash, carbon dioxide and other pollutants emitted is very high. Reserves are small, compared to other fossil fuels.

   Bituminous coal

Bituminous coals are formed from ancient swamp vegetation, over millions of years. As the name implies, they contain considerable quantities of bitumen and many other organic compounds. The quality is very variable according to the ash content, age and other factors. The easiest coals to win are by open-cast mining where the top layers of rock are mechanically removed and the exposed seams are dug out. Drift mining is sometimes done where a coal seam is exposed in the side of a hill. However, most coal is mined from pits, where the miner descends, often to great depths and digs along the seams. The latter is a very dangerous occupation as pockets of highly inflammable gases ("fire damp"), mostly methane, can collect, causing frequent underground explosions. It is estimated that about 6 000 miners are killed annually in pit accidents, mostly from explosions.

Coal reserves are enormous, counting in hundreds of billions of tonnes. China, the USA and Australia are the largest producers but India is a larger consumer than Australia, while being the fourth largest producer. Poland is the largest European producer, the more easily exploited coal in Western Europe having been almost exhausted.

Bituminous coal is an important source of chemicals as well as being a fuel. It is possible to manufacture motor fuels from coal. However, being rich in chemicals, its combustion produces much pollution, including carbon dioxide, sulfur compounds, radioactive compounds and heavy metals, including mercury. It has the highest calorific value of all the coals.

   Anthracite

Anthracite is a very hard, high-carbon, low-ash, coal. It contains almost no volatile matter or bitumen. As a result, it burns cleanly, producing comparatively few pollutants, other than carbon dioxide. Its calorific value is slightly lower than that of most bituminous coals, so produces more carbon dioxide for a given thermal energy.

Known reserves of anthracite are small, probably less than one-tenth those of bituminous coal.

Oil

Like coal, there are several types of crude oil or petroleum. Petroleum is a mixture of hundreds of types of organic compounds, but the main components are known as alkanes which have the general formula H₃C-(CH₂)_n-CH₃. The three main categories, judged by the average value of n are Light, Intermediate and Heavy. Further classification is Sweet and Sour, depending on the quantity of sulfur it contains. Crude oil, as it comes out of the ground must be refined. This is done partly by fractional distillation but, for a given type of petroleum, this alone would give fixed ratios of, for example, petrol (gasoline), kerosene, light fuel oil, heavy fuel oil etc. The demand for petrol is much higher than some of the other components, so the heavier molecules are "cracked" into lighter ones, so that each product from the refinery is produced according to the demand. Nevertheless, light crude oil fetches a higher price than heavy, because it is cheaper to produce light components from them, such as petrol or diesel fuel. 

There are many impurities in crude oil that must be removed from the finished products. These include sand and other insoluble products which generally end up in the still bottoms, mixed with the heaviest bitumens. These have little or no commercial value and may even be landfilled. Even the "sweetest" oil has some sulfur which must be partially or quasi-totally removed. Other molecules may be useful in the petrochemical industry.

The calorific value of oil varies with the fraction. On an average, the higher hydrogen content confers typical values about 50% higher than that of coal per kg.

Burning oil products produces many pollutants including carbon dioxide. If we take an average diesel fuel oil with n=15, 1 kg of oil will produce about 3¼ kg of CO₂. However, to produce that oil from wellhead to car will produce a lot more than that. Furthermore, there is quite an important emission of methane from crude oils. With some light oils, it can bubble out like the gas in soda water; in these cases, it is worthwhile to collect it and sell it off as natural gas. However, the quantity in heavy petroleum is insufficient to justify the collection and it is vented as the storage tank is filled, adding to the greenhouse gas emissions.

Known and proven reserves of oil are a contentious issue, but there is another such issue which may be of more economic importance, the rate at which it can be extracted. It is known that oil companies tend to exaggerate the reserves under their control, not because the oil isn't there but because it may be uneconomical to extract them. If you have a glass of Coca Cola with a curved bottom, you cannot suck up the last drops with a straw; the drink is there but it would not matter if it weren't. As oil is held in porous rock, there is also the question as to how fast it can flow to the well bottom; if a near-depleted well takes a day to fill the well to a single barrel-full, it may no longer be viable to pump it up. Generally speaking, the rate at which oil can be extracted from a well drops dramatically as it nears depletion. As we draw more and more oil out of the ground, so the rate drops and this is why we are approaching "peak-oil". In a few years, the demand will outstrip the supply. To some extent, this is being somewhat offset by new discoveries, but these do not offer sufficient reserves to do more than put back the peak by a year or two. Also, new drilling and extraction methods tend to better reach "the last drop".

While on the subject, there are two sources of oil which are somewhat special. The first is oil shale. This is a slate-like rock which is mined, like coal, in a number of places. It is imbibed with a light petroleum which can be distilled out fairly easily. As a general rule, it is not economically viable at current oil prices but some of it may be if prices rise to more than, say, $100/bbl. Much was mined in Scotland during the 1939-1945 war but this was stopped as soon as tankers were able to bring oil without risk of being sunk by enemy activity because of the high extraction cost of shale oil and the danger involved in extracting an unstable rock from deep pits. The other special case is the Athabasca oil or tar sands in Alberta, Canada. This is an enormous deposit of a mixture of sand and bitumen-like crude oil. To extract this and refine it into useful fractions is only just economically viable with oil prices higher than about $60/bbl, because it is an enormously energy-intensive operation. It requires shovelling out two tonnes of oil-saturated sand to obtain a single net useful barrel of oil (the rest is used to drive the machinery and heat the stills in the refinery!) However, the deposits are so vast that this certainly constitutes North America's most important reserves of oil.

Natural Gas

I have devoted a whole essay to this subject and I don't wish to add anything else.

The following table gives an idea of the amount of energy that can be obtained from one kilogram of each of different types of fuel, the amount of carbon dioxide produced by the combustion of one kilogram of fuel, in kilograms and the weight of carbon dioxide emitted by burning sufficient fuel to produce a gigajoule of energy.

It is therefore clear why producers of natural gas state that it is a clean fuel. What is less evident, as explained in the essay on natural gas, is that, of all the fuels, natural gas requires considerable energy to transport, especially if liquified, and emissions of the gas itself can produce equivalent carbon dioxide levels exceeding that of coal, making it the highest global greenhouse gas of all fossil fuels.

Discuss anything related to this page at this forum.

The opinions on this site are personal and do not necessarily reflect those of any third party. All information is given in good faith but no responsibility is taken for such information; any person or organisation using such information should ascertain that it is suitable for his/her/their conditions of use.

No reproduction of the contents of part or the whole of this site may be made in any form without the written permission of the owner. An exception is made that a print-out may be made for one individual's private use without seeking permission; it is forbidden to make multiple copies or to photocopy a print-out. Links may be made to this site.

Contact us

Copyright © CypEnv 2004/2008