Coal is one of the world’s most important sources of energy, fuelling almost 40% of electricity worldwide. In many countries this figure is much higher: Poland relies on coal for over 94% of its electricity; South Africa for 92%; China for 77%; and Australia for 76%. Coal has been the world’s fastest growing energy source in recent years – faster than gas, oil, nuclear, hydro and renewable. Coal has played this important role for centuries – not only providing electricity, but also an essential fuel for steel and cement production, and other industrial activities.
The build-up of silt and other sediments, together with movements in the earth’s crust (known as tectonic movements) buried these swamps and peat bogs, often to great depths. With burial, the plant material was subjected to high temperatures and pressures. This caused physical and chemical changes in the vegetation, transforming it into peat and then into coal.
The quality of each coal deposit is determined by temperature and pressure and by the length of time in formation, which is referred to as its ‘organic maturity’. Initially the peat is converted into lignite or ‘brown coal’ – these are coal types with low organic maturity. In comparison to other coals, lignite is quite soft and its color can range from dark black to various shades of brown.
Types of Coal
The degree of change undergone by a coal as it matures from peat to anthracite – known as coalification – has an important bearing on its physical and chemical properties and is referred to as the ‘rank’ of the coal. Low rank coals, such as lignite and sub bituminous coals are typically softer, friable materials with a dull, earthy appearance. They are characterized by high moisture levels and low carbon content, and therefore a low energy content.
Higher rank coals are generally harder and stronger and often have a black, vitreous lustre. They contain more carbon, have lower moisture content, and produce more energy. Anthracite is at the top of the rank scale and has a correspondingly higher carbon and energy content and a lower level of moisture.
The choice of mining method is largely determined by the geology of the coal deposit. Underground mining currently accounts for about 60% of world coal production; although in several important coal producing countries surface mining is more common. Surface mining accounts for around 80% of production in Australia, while in the USA it is used for about 67% of production.
Coal straight from the ground, known as run of-mine (ROM) coal, often contains unwanted impurities such as rock and dirt and comes in a mixture of different-sized fragments. However, coal users need coal of a consistent quality. Coal preparation – also known as coal beneficiation or coal washing – refers to the treatment of ROM coal to ensure a consistent quality and to enhance its suitability for particular end-uses
The treatment depends on the properties of the coal and its intended use. It may require only simple crushing or it may need to go through a complex treatment process to reduce impurities. To remove impurities, the raw run-of-mine coal is crushed and then separated into various size fractions. Larger material is usually treated using ‘dense medium separation’. In this process, the coal is separated from other impurities by being floated in a tank containing a liquid of specific gravity, usually a suspension of finely ground magnetite. As the coal is lighter, it floats and can be separated off, while heavier rock and other impurities sink and is removed as waste.
The way that coal is transported to where it will be used depends on the distance to be covered. Coal is generally transported by conveyor or truck over short distances. Trains and barges are used for longer distances within domestic markets, or alternatively coal can be mixed with water to form a coal slurry and transported through a pipeline.
Ships are commonly used for international transportation, in sizes ranging from Handymax (40-60,000 DWT), Panamax (about 60-80,000 DWT) to large Capesize vessels (about 80,000+ DWT). Around 700 million tonnes (Mt) of coal was traded internationally in 2003 and around 90% of this was seaborne trade. Coal transportation can be very expensive – in some instances it accounts for up to 70% of the delivered cost of coal.
Coal plays a vital role in power generation and this role is set to continue. Coal currently fuels 39% of the world’s electricity and this proportion is expected to remain at similar levels over the next 30 years. Consumption of steam coal is projected to grow by 1.5% per year over the period 2002-2030. Lignite, also used in power generation, will grow by 1% per year. Demand for coking coal in iron and steel production is set to increase by 0.9% per year over this period.
The biggest market for coal is Asia, which currently accounts for 54% of global coal consumption – although China is responsible for a significant proportion of this. Many countries do not have natural energy resources sufficient to cover their energy needs, and therefore need to import energy to help meet their requirements. Japan, Chinese Taipei and Korea, for example, import significant quantities of steam coal for electricity generation and coking coal for steel production.
Coal will continue to play a key role in the world’s energy mix, with demand in certain regions set to grow rapidly. Growth in both the steam and coking coal markets will be strongest in developing Asian countries, where demand for electricity and the need for steel in construction, car production, and demands for household appliances will increase as incomes rise.
The global coal market is large and diverse, with many different producers and consumers from every continent. Coal supplies do not come from one specific area, which would make consumers dependent on the security of supplies and stability of only one region. They are spread out worldwide and coal is traded internationally.
Coal Is Used For Generating Electricity
Steam coal, also known as thermal coal, is used in power stations to generate electricity. The earliest conventional coal-fired power stations used lump coal which was burnt on a grate in boilers to raise steam. Nowadays, the coal is first milled to a fine powder, which increases the surface area and allows it to burn more quickly. In these pulverized coal combustion (PCC) systems, the powdered coal is blown into the combustion chamber of a boiler where it is burnt at high temperature. The hot gases and heat energy produced converts water – in tubes lining the boiler – into steam.
The high pressure steam is passed into a turbine containing thousands of propeller-like blades. The steam pushes these blades causing the turbine shaft to rotate at high speed. A generator is mounted at one end of the turbine shaft and consists of carefully wound wire coils. Electricity is generated when these are rapidly rotated in a strong magnetic field. After passing through the turbine, the steam is condensed and returned to the boiler to be heated once again
The electricity generated is transformed into the higher voltages – up to 400,000 volts –used for economic, efficient transmission via power line grids. When it nears the point of consumption, such as our homes, the electricity is transformed down to the safer 100-250 voltage systems used in the domestic market. Modern PCC technology is well-developed and accounts for over 90% of coal-fired capacity worldwide. Improvements continue to be made in conventional PCC power station design and new combustion techniques are being developed. These developments allow more electricity to be produced from less coal – this is known as improving the thermal efficiency of the power station.
Coal In Iron & Steel Production
Steel is essential to everyday life – cars, trains, buildings, ships, bridges, refrigerators, medical equipment, for example, are all made with steel. It is vital for the machines which make nearly every product we use today. Coal is essential for iron and steel production; some 64% of steel production worldwide comes from iron made in blast furnaces which use coal.
Coke is made from coking coals, which have certain physical properties that cause them to soften, liquefy and then re-solidify into hard but porous lumps when heated in the absence of air. Coking coals must also have low sulphur and phosphorous contents and, being relatively scarce, are more expensive than the steam coals used in electricity generation.
The coking coal is crushed and washed. It is then ‘purified’ or ‘carbonized’ in a series of coke ovens, known as batteries. During this process, by-products are removed and coke is produced.
Coal In Cement Production
Coal is used as an energy source in cement production. Large amounts of energy are required to produce cement. Kilns usually burn coal in the form of powder and consume around 450g of coal for about 900g of cement produced. Coal is likely to remain an important input for the global cement industry for many years to come.
Coal combustion products (CCPs) can also play an important role in concrete production. CCPs are the by-products generated from burning coal in coal-fired power plants. These byproducts include fly ash, bottom ash, and boiler slag and flue gas desulphurization gypsum. Fly ash, for example, can be used to replace or supplement cement in concrete. Recycling coal combustion products in this way is beneficial to the environment, acting as a replacement for primary raw materials.
Other uses of Coal
Other important users of coal include alumina refineries, paper manufacturers, and the chemical and pharmaceutical industries. Several chemical products can be produced from the by-products of coal. Refined coal tar is used in the manufacture of chemicals, such as creosote oil, naphthalene, phenol, and benzene. Ammonia gas recovered from coke ovens is used to manufacture ammonia salts, nitric acid and agricultural fertilisers. Thousands of different products have coal or coal by-products as components: soap, aspirins, solvents, dyes, plastics and fibres, such as rayon and nylon.
Coal is also an essential ingredient in the production of specialist products
* Activated carbon - used in filters for water and air purification and in kidney dialysis machines.
* Carbon fiber – an extremely strong but light weight reinforcement material used in construction, mountain bikes and tennis rackets.
* Silicon metal – used to produce silicones and silanes, which are in turn used to make lubricants, water repellents, resins, cosmetics, hair shampoos and toothpastes.
Coal cleaning, also known as coal beneficiation or coal preparation, increases the heating value and the quality of the coal by lowering levels of sulphur and mineral matter (see Section 2 for a description of coal preparation techniques). The ash content of coal can be reduced by over 50%, helping to cut waste from coal combustion. This is particularly important in countries where coal is transported long distances prior to use, since it improves the economics of transportation by removing most of the noncombustible material. Coal cleaning can also improve the efficiency of coal-fired power stations, which leads to a reduction in emissions of carbon dioxide.