Australia's oldest deposits of black coal, found in NSW and Queensland, were formed between 225 and 180 million years ago. Our Gondwana Coals are of permo-carboniferous age and were discovered by the Geological Survey of Bangladesh in 1985. Coal is composed mainly of carbon (50-98%), hydrogen (3-13%) and oxygen with smaller amounts of nitrogen, sulphur and other elements. It also contains a little water and inorganic matter that remain as a residue known as ash when coal is burnt.

Coal is divided into four classes: lignite, sub-bituminous, bituminous and anthracite. Initially peat, the precursor of coal, was converted into lignite or brown coal. Over many more millions of years under the effects of temperature and pressure the maturity of the lignite progressively increased and transformed into sub-bituminous coals. As this process continued, further chemical and physical changes occurred until these coals became harder and more mature, at which point they are classified as bituminous or hard coals. The progressive increase in the organic maturity continues further to form anthracite.

Comparison of the data obtained from the thermal analysis of Barapukuria coal samples with other internationally-traded coals indicated that its ash content is substantially higher than of those coals used for both coking and steam generation, although fixed carbon and volatiles are broadly similar. Ash content is an important consideration. As environmental legislation globally becomes more rigorous, the scope of materials classed as pollutants is likely to widen considerably.

Trace elements in coal may be potentially liberated and become active in the environment, either by mobilisation and concentration in emitted particles or in the gaseous phase, or, in the case of involatiles, remain as bottom ash, grits or fly ash. The latter materials are usually disposed of to landfill, or used as construction materials. In both cases, pollutants may be eventually leached from them into groundwater. Thus trace element analysis of the unburned coal may be of importance in the estimation of the eventual extent of pollution from its combustion.

Research has suggested that relative volatility of each element has a bearing on its eventual fate after combustion. Where trace elements are involatile, they tend to have equal concentration in all the ash streams, which may be estimated by the determination of the ash concentration in the original coal. Where trace elements are only partially volatile, their behaviour on combustion is more complex. Condensation on to fly ash particles with a high specific surface area in cooler parts of the furnace system may occur, resulting in these particles becoming more enriched in certain elements and with a consequent reduction in concentration in the bottom ash. Those trace elements which are readily volatile are concentrated in the vapour or gas phase and are depleted in most solid phases.

Pollution by trace elements may, to some degree, be mitigated by the cleaning of coal prior to combustion. Cleaning is used to control the ash and sulphur contents of coals but can also have a significant impact on the removal of trace elements. If various mineral fractions are removed from the coal during cleaning, then the trace metals associated with those minerals may also be removed.

Trace elements are generally washed out of the 'cleaned' coal to some degree, although, in general, they are not washed out to the same degree as the bulk ash, ie there is a degree of concentration of the trace elements in the washed coals. This is particularly true of some of the more volatile and toxic elements such as selenium (Se), mercury (Hg), boron (B) and cadmium (Cd). By coal cleaning, thus removing the fine fraction, it is possible to reduce the concentration of a range of trace elements, and also the coals' ash contents. Sulphur and chlorine contents may also be reduced.

The overall effect on bulk ash composition is to wash out the inert elements of the ash (alumino-silicates) and to tend to concentrate those elements which promote ash deposition problems (specifically, iron, calcium and sodium). Thus, overall, although the amount of ash is significantly reduced, it should be expected to be much more pernicious in nature. Trace elements that occur in appreciable amounts in the gaseous phase include As, B, Hg, Se and Sb which may be enriched in fly ash. Analyses of the furnace bottom ash and convective section ash show broadly similar trace element concentrations from tests involving unwashed and washed coals.

However, the trace element concentrations in the fly ash, i.e. ash which could ultimately be emitted to the atmosphere or disposed of on land, are considerably higher from the washed coal. Washing of coals is successful in removing a significant proportion of the bulk ash material, and to wash out the inert (alumino-silicate) fractions of the mineral matter, but with the effect of rendering the remaining material more pernicious (in terms of slagging potential).

In terms of the effect of trace elements in coal on the operation of combustion equipment, the most important are sodium and potassium. They are undesirable in steaming coals because they can give rise to fouling and slagging problems in boilers, although small additions can improve electrostatic precipitation performance. They are also undesirable in coking coals since they tend to increase coke reactivity in the blast furnace. In addition, chlorine may cause corrosion and fouling problems in boilers.

Less than 0.2 percent chlorine in coal is considered low, over 0.5 percent is considered high. Sodium is often associated with chlorine, and can itself cause slagging and fouling problems during combustion.

Sulphur in coal is considered to be present in three forms -- pyritic sulphur, sulphate sulphur and organic sulphur. Where the sulphur is pyritic or sulphatic it is part of the mineral matter and its content can be lowered by washing the coal. Organic sulphur is distributed through the carbonaceous part of the coal and cannot be washed out by conventional techniques. The presence of sulphate sulphur often indicates the coal has experienced some oxidation and pyritic sulphur is often the cause for spontaneous combustion problems. The presence of sulphur will cause the formation, during combustion, of sulphur dioxide, which is a serious pollutant. Most countries have in place regulations regarding emissions of this gas to the atmosphere. Sulphur is also undesirable in coking coal since it accumulates in the hot metal which may require desulphurisation if greater than 0.002 percent.

Other elements which may be present in coal and which may exacerbate both operational and environmental problems are nitrogen and phosphorus. Nitrogen is part of the organic material in coal. Under certain combustion conditions a portion will be emitted to the atmosphere as polluting nitrogen oxides unless removed from flue gas. Phosphorus is to be avoided in coking coal because it accumulates in the hot metal and gives undesirable properties to the resultant steel. It can also create problems during combustion by the formation of hard phosphatic deposits inside boilers.

Two kinds of mineral matter are generally found in all coals. The first is called "inherent mineral matter" and refers to the portion organically combined with the coal. It cannot be determined petrographically and includes those elements which have been assimilated by plants for nutritive purposes. The second is known as "extraneous mineral matter" and includes that portion which is foreign to the plant material. Extraneous mineral matter is the larger contributor to the total ash content. It consists of detrital minerals deposited during coal accumulation and minerals deposited from solution or suspension during and after coal accumulation.

Ash is composed of a complex mixture of oxides, but its analysis generally does not indicate the composition of the minerals in the parent coal. Ash consists mostly of silica (SiO2) and alumina (Al2O3). The presence of large amounts of the oxides of iron (Fe2O3), calcium (CaO), sodium (Na2O) and/or potassium (K2O) generally indicates an ash with low fusion temperatures. However, the ash value contributes little to an understanding of the identity and distribution of the mineral matter which is responsible for the ash.

Such information is important for several reasons: (1) it aids in determining the extent of economical coal beneficiation; (2) the chemical and physical properties of coal are influenced by the character of the mineral matter; (3) it is a reflection of the geologic history of the coal bed. Barapukuria ashes consisted of between 50 and 60m percent silica, with 25-40 m percent alumina.

Although coal is cheaper than natural gas and oil but its CO2 emission factor is highest among fossil fuels when it is used as fuel especially in power plants thus causing global warming significantly. There is a long history of efforts to turn coal into either a gaseous or a liquid fuel. In 1800, coal gas was made by heating coal in the absence of air. Coal gas or town gas, as it was also known, became so popular that many of the major cities and towns had a local gas house in which it was generated. Gas lanterns, of course, were eventually replaced by electric lights. But coal gas was still used for cooking and heating until the more efficient natural gas became readily available. A slightly less efficient fuel known as water gas can be made by reacting the carbon in coal with steam.

Water gas from which CO2 has been removed is called synthesis gas because it can be used as a starting material for a variety of organic and inorganic compounds. It can be used as the source of H2 for the synthesis of ammonia, It can also be used to make methyl alcohol, or methanol. Methanol can then be used as a starting material for the synthesis of alkenes, aromatic compounds, acetic acid, formaldehyde, and ethyl alcohol (ethanol). Synthesis gas can also be used to produce methane, or synthetic natural gas (SNG).

The first step toward making liquid fuels from coal involves the manufacture of synthesis gas (CO and H2) from coal. In 1925, Franz Fischer and Hans Tropsch developed a catalyst that converted CO and H2 at 1 atm and 250 to 300C into liquid hydrocarbons. By 1941, 740,000 tons of petroleum products per year were produced in Germany, using Fischer-Tropsch technology. Fischer-Tropsch technology is based on a complex series of reactions that use H2 to reduce CO to CH2 groups linked to form long-chain hydrocarbons.

At the end of World War II, Fischer-Tropsch technology was under study in most industrial nations. The low cost and high availability of petroleum, however, led to a decline in interest in liquid fuels made from coal. The only commercial plants using this technology today are in the Sasol complex in South Africa, which uses 30.3 million tons of coal per year. China now wants to cut down its oil import dependence by building a commercial scale direct coal liquefaction plant in Inner Mongolia adopting this technology, which will produce around 50,000 barrels a day of finished gasoline and diesel fuel.

Another approach to liquid fuels from coal is based on the reaction between CO and H2 to form methanol, CH3OH.

Methanol can be used directly as a fuel, or it can be converted into gasoline with catalysts such as the ZSM-5 zeolite catalyst developed by Mobil Oil Company. As the supply of petroleum and gas becomes smaller and their cost continues to rise, a gradual shift may be observed toward liquid or gaseous fuels made from coal.

Dr. Rafiqul Islam is Professor, Dept. of Applied Chemistry & Chemical Technology, Dhaka University.