Keywords

Fossil fuel; photosynthesis; anthracite; bituminous coal; lignite; peat; heat content; coal reserves; coal production; coal trade; coal cleaning; physical cleaning; moisture reduction; ash removal; fluid-based separation; chemical cleaning

Coal Resources and the Coal Trade

Coal is one of the most widely available fuels. It is found in more than 70 countries according to the World Energy Council in its 2013 Survey of World Energy Resources and is actively mined in 50 countries. This means that most nations either have access to domestic supplies or can purchase it from a close neighbor. Even so, there is an active global trade in coal and this has expanded significantly over the last 20 years.

The amount of coal available for human exploitation is usually quantified in terms of the proven reserves from different countries and regions. This figure represents the known accessible reserves. The total quantities within the earth may be significantly higher. At the end of 2013, the total proven reserves were 891,531 Mtonnes as shown in Table 2.2.

Table 2.2 also shows a breakdown of proven reserves by region. The largest reserves, 310,538 Mtonnes, are found in Europe and Eurasia. However, 70% of this coal is sub-bituminous and lignite and only 30% is harder coals such as anthracite and bituminous coal. The second-largest regional reserves, 288,328 Mtonnes are found in the Asia Pacific region, which includes China and India, both of which have large deposits. The coal in this region is roughly 55% anthracite and bituminous coal and 45% sub-bituminous and lignite. North America has total reserves of 245,088 Mtonnes, with 54% lignite and sub-bituminous and 46% harder coals. The two other regions in Table 2.2, Central and South America, and the Middle East and Africa, both have much smaller reserves.

The current level of coal global reserves is predicted to last for 113 years at the rate of extraction found across the world today. However, there are large regional variations. European and North American reserves are expected to last for around 250 years, while those in the Asia Pacific region are only expected to last for 54 years, a reflection of the vastly differing rates of consumption.

While coal reserves are widely scattered, five countries hold between them 75% of all the deposits. These are the USA (28%), Russia (18%), China (13%), Australia (9%), and India (7%).

Part of the reason for the rapid decline in Asian Pacific coal reserves can be seen in Table 2.3 which lists the top coal-producing nations of the world in 2013. At the top of the list, with by a wide margin the largest global production, is China with 3561 Mtonnes or close to 46% of the global total. The second-largest producer, the USA, mined only 904 Mtonnes or roughly one quarter of that mined by China. India, with production of 613 Mtonnes, has the third-largest industry.

Much of the production listed in Table 2.3 is for steam coal but several countries also produce significant quantities of lignite which is used exclusively in local power station. The top lignite producers are Germany, Russia, the USA, Poland, Turkey, and Australia.

The global trade in coal accounts for about 15% of the total coal consumed according to the World Energy Council, with most coal being used in the country where it is produced. Coal has a lower energy density that either oil or natural gas, as shown in Table 2.4. The average energy density of coal is around 24 MJ/kg (although as seen above some coals can have an energy density of up to 33 MJ/kg). In contrast crude oil has an average energy density of 46 MJ/kg and natural gas 54 MJ/kg. This means that coal is more costly to transport than either of the other two fossil fuels. Transportation costs therefore account for a large part of the cost of coal so that according to BP, in 2013, imported Japanese steam coal was 56% more expensive than US Central Appalachian coal.

As a consequence of the cost, the steam coal market (coal for power generation) is divided into two regional markets. One, the Atlantic market, comprises importing nations of Western Europe, principally the UK, Germany, and Spain. The second, the Pacific market, serves the major Asian importers, Japan, South Korea, Taiwan, India, and China.

The largest exporters of coal are Australia and Indonesia. Both are well placed to serve this Pacific market. Russia is another large coal exporter. Its traditional market has been Europe but the country is trying to win a greater share of the Pacific market. The USA, Colombia, and Canada all export coal, much of it to Europe. South Africa is also a major exporter. It is well placed to export to both the Pacific and the Atlantic markets.

Coal Processing and Cleaning

Processing coal before combustion offers a way of improving the quality of a coal, both economically and environmentally. Processing can be carried out physically or chemically but the only type of cleaning exploited commercially is physical cleaning. Chemical cleaning processes are under development but have not so far proved cost-effective.

The most well-established methods of coal-cleaning focus on removing excess moisture from the coal, reducing the amount of incombustible material which will remain as ash after combustion and reducing the sulfur content of the coal. Moisture removal reduces the weight and volume of the coal, rendering it more economical to transport and easier to burn. Ash removal also reduces the mass and volume of coal and improves its combustion properties and aids power plant performance.

Moisture is removed from coal by drying. This can simply be solar drying, leaving the coal in the open before transporting it. Drying coal in this way can reduce its mass and increase its energy density, making it relatively cheaper to transport.

The alternative, drying coal by heating, is most often carried out at the power station utilizing waste heat in the plant flue gases. Such a procedure is absolutely essential when burning high-moisture lignites such as brown coal. In this case drying does not affect transportation costs because the fuel has, by this stage, already reached the power station.

Physical separation of ash from coal relies on a difference in density between coal and its common impurities. This is applied quite widely during coal preparation, both before and at a power station.

Ash removal is carried out by crushing the coal into small particles. Incombustible mineral particles are more dense than the coal and can be separated using a gravity-based method, often with air or water as the separation medium. Fluid-based separation may exploit up-currents of air that pass through the crushed coal, sometimes in a fluidized bed. The lighter coal particles are carried away by the fluid for recovery at a later stage, leaving the impurities behind in the fluidized bed. Mixing the crushed coal with water also offers an effective means of separation as the coal particles float and the heavier mineral particles sink. Such treatment will remove some minerals containing sulfur, and can result in a reduction of up to 40% in sulfur dioxide emissions during combustion. (Some sulfur is bound to the carbon in the coal. Such sulfur is not affected by this type of cleaning.) Coal that is cleaned using a wet cleaning process (coal washing) must then be dried before combustion.

The severity of the physical processing is associated with the fineness of the coal. Coal crushing or grinding is needed to release the impurity particles from the fuel. Treatment of fine coal particles tends to be more intense than that of coarse particles. Both processes involve some coal losses, which can vary from between 2% and 15% of the total coal. Rejected coal can still be used in a plant capable of burning low-quality coal such as a fluidized bed furnace.

In addition to the common physical techniques outlined above there are some more specialized techniques being developed. These including using liquid carbon dioxide to selectively trap coal particles from a slurry of coal particles in water, and an electrostatic process in which the differing electrical insulation properties of coal and mineral impurities allow them to be separated after a charge has been applied to both. Cyclones that can selectively separate coal from pyrites, one of the main sources of sulfur in coal, have also been tested, as have magnetic techniques for separation.

There have been attempts to develop more advanced methods for coal treatment employing either higher-temperature processing of the coal or chemical rather than physical processes. These processes, which are aimed at removing polluting impurities in the coal to make it a cleaner fuel to burn, have not so far found commercial application. Coal cleaning processes are also being developed to treat coal wastes that have previously been discarded to make them suitable for combustion.

Chemical cleaning processes often target specific impurities, particularly sulfur. Selective oxidation of the sulfur using specific reagents has been tested, as has a process in which the ground coal is fed into a bath of molten sodium and potassium hydroxide. The latter dissolves much of the impurity material, leaving the coal which can be separated physically from the mixture. Chemical processes such as these and others are feasible but complex and currently appear to be too costly to implement. However they may become cost-effective if they can clean the coal to a sufficient level of purity that no further cleanup to remove sulfur – for example – is required.

Dry coal cleaning was popular in the USA from the 1930s until 1990 but declined after that date. However, there has been a recent revival in interest in coal cleaning as a way of reducing transportation costs. There is also interest in India where some coals are transported up to 1000 km or more. Dry cleaning processes can reduce the ash content in some of these coals from 40% to 30%.