Coal is a generic term which covers a set of solid fuels with very different compositions and specific heats. Coal was almost the only fuel of XIXth century’s industrial revolution with its development of steam engines and metallurgy. The largest source of electricity in the world, coal is indispensable for iron ore processing and carbon-chemistry. Though it lost its prominence to oil during the second half of the XXth century, coal is now back with a vengeance, notably in China, with worrying potential consequences on our planetary climate.
Coal is a rock containing a large amount of carbon, the formation of which began at the end of the primary era therefore called carboniferous era (360 to 290 million years BC). Coal comes from the accumulation of vegetal materials either in situ or after transportation. Accumulation basins have often subsided deeper and deeper, allowing the forest to keep growing over long periods of time. Sediments ended up buried at various depths, from a few hundreds meters for lignite to several kilometres for anthracite. All along the subsidence process, organic carbon turned into mineral carbon, under the action of micro organisms, most of the water was expelled and the rock hardened. Coal can be divided into several categories according to the degree of mineralization.
Coal stricto sensu is the part which is black and of organic origin. Its chemical composition is characterized by an hydrogen content (4%) half of oil’s. When the product is dry, le carbon content varies from 55% for lignite to 90% for anthracite.
The combustion of coal generates water vapour. It is sometimes possible to possible to recover the quantity of heat contained in this water One thus distinguishes two calorific values: The Net calorific value (NCV) which supposes that the heat contained in the water vapour is not recovered. The Gross calorific value (GCV) which supposes that the heat contained in this vapour is recovered. Those values are expressed in kilocalories per kilogramme. The average calorific value of coal is the « tce »: tonne of coal equivalent, with a NCV of 7000 kcal/Kg. By convention, 1 tce = 07 toe. Taking into account the various calorific values and water contents, the actual calorific value of a given coal varies considerably. For a given weight, lignite will release two to three times less heat than anthracite.
In practice, we shall use only the distinction between "coal" and "lignite".
Ultimate coal resources are probably in the neighbourhood of 5 000 billion tonnes, out of which 900 qualify as "reserves". The table below describes the geographic repartition of these coal reserves. Every continent has some, but for Middle East which does not lack other energy reserves!
These coal reserves are equivalent to 375 billion toe.
World coal production has drastically increased since 2000 and exceeds now 5 billion metric tons, not including some 900 million tons of lignite. Coal is durably back, in view of the prices and limited reserves of oil and gas. 50% coal is used for electricity generation, 16% for steelmaking and 5% for cement fabrication. The remaining 29% is used for heating and other industrial uses including carbon chemistry. Contrary to oil and gas, coal is mostly burnt on the spot (and lignite, entirely). Imports/Exports account only for 15% of total coal production.
Despite its nuisances, coal is by far the largest source of electricity in the world, supplying 40% of the total power production. Solutions exist and are being implemented to reduce drastically polluting releases from coal plants, but not yet their CO2 releases. There is therefore cause for worry in the fact that China, for the last three years, has been ordering two coal plants every week !
Grisou and Black Lung
The coal mining industry suffers from a bad image because of the grisou and coal dust explosions which still cause numerous victims, and because of Black Lung diseases.
Methane (grisou) makes an explosive mixture with air as soon as its concentration lies between 6 and 16%. In most of the mines, efficient air blowing is enough to eliminate all explosion risk, but there are still hazardous mines in that respect. Worldwide, explosions claim every year thousands of victims.
Black lung and silicosis used to plague all the French miners before 1945. Since then, dust prevention measures have allowed to eradicate this professional disease from the French mines – now shut down anyway. However, elsewhere in the world the disease remains a plague: between 1991 and 1995 China recorded 500.000 silicosis cases and 24000 deaths from it.
“Conventional” atmospheric Pollution
It is during its final uses, notably power generation, that coal causes atmospheric pollutions: releases of sulphur and nitrogen oxides, hydrocarbons and fly ahes. In the presence of water and ashes, some of the sulphur oxides turn into sulphuric acid, the cause of the “acid rains” which affect the environment. The other gaseous effluents and aerosols are toxic and irritate the lung while hydrocarbons can cause cancers.
All this "conventional" pollution is almost eliminated in modern coal power plants when they are equipped with so-called "scrubbers", but there are still many older plants still in operation around the world. Of course, measures to reduce or eliminate pollution have a cost per tonne of coal burnt, as illustrated above:
Toward "Clean Coal "