A Brief Overview of Coal: Part Two – Rank & Uses
Coal rank is effectively a measure of the maturity of coal. The first stage of the coalification of peat forms brown coal, or lignite, followed by subbituminous coal, bituminous coal and finally anthracite. Before discussing each coal rank in a little more detail, it is important to note that there are other variables that can affect the value and use of coal.
Inorganic impurities result from clay and other minerals that are deposited at the same time as the plant matter. When burnt, most of this inorganic matter remains as an ”ash” residue, but some of it is ejected to the atmosphere.
During coalification, iron within the peat is converted to iron carbonate (siderite), which is typically deposited in fractures (“cleats”) within the coal bed. When burnt the carbonate is vented to the atmosphere as carbon dioxide leaving behind the iron as metal.
Nitrogen and sulphur are notorious contaminants in coal. When burnt they form oxides (sulphur dioxide and the three oxides of nitrogen) that vent to the atmosphere. These combine with water in the atmosphere to form the pollutant “acid rain”. It is worth noting that most modern coal fired power stations use a variety of methods to remove pollutants before the gas is released to atmosphere.
Coal also contains numerous trace elements such as zinc and silver. Occasionally a coal bed may be rich enough in these elements for them to be a valuable by-product. Coal can also contain elevated levels of hazardous elements such cadmium, selenium, arsenic and uranium.
The rank of coal is determined by the percentage of fixed carbon, moisture, volatile matter and energy content per unit. The four ranks mentioned above are often further divided, often based upon volatile content.
Lignite, also known as brown coal, is soft and friable and often contains recognisable plant fragments. It is defined as having a calorific or heating value of less than 8,300 BTU/lb (British Thermal Units per pound, see below for more detail). Fixed carbon (dry and free of “ash”) is typically less than 60%.
Lignite has a high water content, up to 75%, and a high ash content of up to 50% by weight. It can spontaneously combust and disintegrates when heated. It is a very dirty fuel because of the high ash content and high volatiles. It is difficult to store and uneconomic to transport over any distance.
Lignite is typically used in power stations to produce steam (which is then used to drive turbines to produce electricity), when superior coal is not available and where the power stations are located adjacent to the coal resource.
Sub-bituminous coal is dark brown to black and has a heating value up to 13,000 BTU/lb and fixed carbon up to around 60%. Water or moisture can be up to 45%, but is typically less than 25%, and ash up to 10%. Sulphur and nitrogen can each range up to 2% leading to acid rain forming emissions, especially if burnt at lower temperatures.
Its main use is in power stations to produce steam. It produces an alkaline ash that, when burnt together with bituminous coal in power stations, can help lower sulphur emissions.
Bituminous coal is dark brown to black and has a heating value of up to 16,000 BTU/lb and fixed carbon of up to 85%. Water or moisture can be up to 17% but is typically much lower and ash content can be up to 12%. It often has a higher sulphur content than sub-bituminous coal and commonly contains pyrite (iron sulphide), which in turn can contain hazardous metals such as lead and mercury.
Bituminous coal has two main end uses; thermal coal is used in power stations to produce steam; and metallurgical coal is used to produce coke and PCI (pulverised direct injection coal, used in steelmaking). These are also referred as steaming coal and coking coal, respectively.
Coke is produced by heating bituminous coal in the absence of oxygen. This drives off volatile hydrocarbons, some sulphur and most of the water. Coke is a hard, strong, porous rock comprised almost entirely of carbon. It is used for a variety of industrial processes but mainly as a reductant in blast furnaces to reduce iron oxide to metallic iron. By-products formed during the manufacture of coke include coal tar, oil, ammonia and gas.
Anthracite is the top ranked and most expensive coal. It has been buried the deepest and the hottest and is often the oldest. It is hard, black and brittle. With further pressure and temperature anthracite is converted to graphite.
Anthracite has a heating value of 13,000 to 16,000 BTU/lb and fixed carbon of 85-98%. Moisture content ranges between 5-15% and ash is up to 20% but it is typically much lower. It has very volatiles and very low sulphur and nitrogen. It is very difficult to ignite, but once alight, burns hot and clean and virtually smoke free.
Depending upon its grade, anthracite is used as a domestic fuel and in power stations. Higher grades are used in steel-making and other metallurgical industries. High quality anthracite is increasingly used to partially replace coke in blast furnaces, and elsewhere in the steelmaking process. It provides significant cost saving because of the high carbon content, low contaminants and low volatiles.
Coal is used as the feedstock to produce synthetic liquid fuel by a proprietary process. This process was used by Germany during WWII and has been developed and refined in South Africa for over 50 years. Coal seams are of course the source of coal seam gas, an increasingly important source of methane.
Coal, or by-products of coal treatment, is used in a wide range of industrial processes to produce pharmaceuticals, dyes, preservative and other products. The various ashes” produced by the burning of coal have a range of uses from road base, to concrete, to wallboard.
Part Three – Reserves & Production, Mining & Processing
The British Thermal Unit is a traditional unit of energy that approximates to the amount of energy required to raise a pound of water from 39oF to 40oF. It is equivalent of approximately 1.055 kjoules. 1 BTU/Lb is equivalent to 2.3 kJ/Kg.