Coal composition and types

Coal is classified by rank according to their percentage of fixed carbon and heating value. Fixed carbon is the carbon residue that remains when coal is heated-without combustion- to drive off volatile matter. The volatile matter includes gases and vapours released by coal when heated. Usually, a coal’s heating value and percentage of carbon (except in anthracite) have an inverse relationship with the moisture and volatile matter content. A Btu (British thermal unit) is the standard unit of measurement for heating value and is defined as the amount of heat required to raise the temperature of 1 pound of water 1 degree Fahrenheit (ºF). There are basically four types of coal, each of which varies in terms of its heating value, its chemical composition, ash content, and geological origin. The four types of coal are lignite, sub bituminous, bituminous, and

anthracite is the highest rank of coal. Hendrickson (1975) provides the following description of some of these coal types: Lignite, the lowest rank of coal, was formed from peat which was compacted and altered. Its colour has become brown to black, has a fibrous, earthy texture and is commonly has a high moisture content, low heating value(less than 8,300 Btus). And it is composed of recognizable woody materials imbedded in pulverized (macerated) and partially decomposed vegetable matter. Lignite displays jointing, banding, a high compared with the higher coals. Sub bituminous coal is difficult to distinguish from bituminous and is dull, black colored, shows little woody materials, is banded, and has developed bedding planes. Sub bituminous coal has moisture content less than that of lignite, and a heating value that ranges from 8,300 to 11,500 Btus. The coal usually splits parallel to the bedding. It has lost some moisture content, but is still of relatively low heating value. Bituminous coal is dense, compacted, banded, brittle, and displays columnar cleavage and a dark black colour. It is more resistant to disintegration in air than are sub bituminous and lignite coals. It has low moisture and S content (Liu et al., 2005), carbon content that ranges from 69 to 86 %, volatile matter content range from 15 to 50 %, and its heating value that ranges from 10,500 to 15,500 Btus. The principal components of bituminous coal fly ash are silica, alumina, iron oxide, and calcium, with varying amounts of carbon, as measured by the loss on ignition (LOI) (Adriano et al., 1980). Several varieties of bituminous coal are recognizable. Anthracite is the highly metamorphosed coal, is jet black in colour, is hard and brittle, breaks with a conchoidal fracture, and displays a high lustre. It has a carbon content that ranges from 86 to 98 % and a slightly lower heating value than bituminous coal due to a low volatile matter content(less than 7 %). Neither peat nor graphite is coal, but they are the initial and end products of the progressive coalification process. Coals vary greatly in their composition. Many researches have been done on coal deposits throughout the world because of the environmental interest in the trace elements. Studies on occurrence and distribution of trace elements in coal showed that their affinities differ from one deposit to another (Querol et al., 1996). Of 1200 coals categorized by the Bituminous Coal Research Institute, no two had exactly the same composition (Hendrikson, 1975).

Trace elements are concentrated mainly in the heavy accessory minerals and organic matter in coal. In decreasing order of significance, the trace elements in coal may occur as: element-

major and impurity components in the inorganic amorphous matter; and elements in the fluid constituent (Vassilev et al., 1997a). Typical compositions of dry, ash-free coal by mass include 65-95 % carbon, 2-7 % hydrogen, up to 25 % oxygen and 10 % sulphur, and 1-2 % nitrogen (Essenhigh, 1976). Inorganic mineral matters as high as 50 % has been observed, but 5-15 % is more typical. Moisture levels commonly vary from 2-20 %, but values as high as 70 % have been observed (Hobbs et al., 1993). Coal occurs in association with diverse types of inorganic minerals such as alumino-silicates (clay minerals), carbonates (calcite and dolomite), sulphides (pyrite), chlorides, and silica (quartz). Some elements such as sulphur occur in both the organic and inorganic coal fractions. The inorganic minerals, deposited along with the plant material, are inherent and make up 5 to 10 % of the coal. The inorganic constituents in coal are responsible for a series of technological and environmental problems related to coal mining, coal processing such as preparation, combustion, pyrolysis, gasification and liquefaction, and utilization of coal wastes (Vassilev, 1994, Querol et al., 1995). The sulphur and nitrogen content are important as emissions of their chemical oxides during coal burning can cause acid rain. Unrestrained emissions from coal combustion resulted in widespread damage to forests and lakes in Europe, the USA and Canada. There exist some relationship between the coal ranks and chemical and mineral composition. The low-rank coals (Cdaf <75wt %) are relatively rich in moisture, volatile matter, ash, H, N, O and S, and their ashes are abundant in MgO, CaO and SO3. The high-rank

coals demonstrate increased contents of SiO2, A12O3, Fe2O3, K2O, Na2O and TiO2. (Vassilev et

al., 1996b). The coal ash is abundant in aluminosilicates as the contents of the Fe oxides are relatively similar to the sum of the alkaline earth oxides. The minerals identified in the coal are mainly quartz, kaolinite, pyrite and calcite and, to lesser extents, dolomite, ankerite, illite, chlorite, opal, feldspars, marcasite, gypsum, melanterite and hematite (Liu et al., 2005; Vassilev and Vassileva, 1998).