The characterisation of coals for combustion
The use of coal in the production of energy, will continue around the world into the next century, and onwards. From an environmental perspective, as well as a financial one, man has attempted to increase the efficiency of energy production from initial raw materials. Environmentally, the poor conversion of coal to energy means a waste of earth's resources, as well as the production of more waste. From a financial viewpoint, poor combustion means less energy per tonne of coal, hence less profit. Poor combustion will also mean higher levels of carbon in ash. Increased carbon levels will change the physical properties of the ash, and therefore reduce the possible number of outlets for its disposal. It is essential, from a coal buyer’s point of view that he makes an 'informed' choice, as to the type of coal he is buying, especially if coals in question are imported. There is sufficient evidence to suggest that most techniques that currently exist are unable to characterise world coals successfully. The reason for this has been linked to the unusual petrographic nature of various world coals. It seems logical therefore, that an analytical technique based on the petrography of a coal, would be capable of providing a better characteristic assessment of any given coal type. The use of image analysis in providing petrographic information has been investigated, along with several different techniques for the characterisation of char and coal particles. Char production itself has been studied, mainly concerned with the production of representative char samples. A range of different operating conditions were used, including temperature ranges of 1000°C to 1400°C, oxygen contents of 0 to 3 %, and residence times of 100 to 200 milliseconds. From the experiments carried out, it was possible to correlate char structure to the initial coal using a reflectance/fluorescence program, specially designed for the measurement of a coals 'reactivity'. Other prediction systems, such Rank, Fuel Ratio and Reactive Macerals did not correlate as well to the combustion products from the Drop Tube Furnace.