Proximate Analysis
The proximate analysis determines the moisture, volatile matter, fixed carbon, and ash in the coal (see Figure 4-1). The analysis is an essentially practical tool providing an initial indication of the coal’s quality and type. The methods for performing these analyses have been standardized by all the major standards institutions (e. g., ASTM, ISO, DIN, BS, and others). These standards, though similar in nature, are different from one another in, for example, the temperature specified for determining the volatiles content, so it is important when providing data to specify the method used.
Moisture is determined by drying the coal under standard conditions for 1 h at 104-110°C. The method determines the sum of all moisture; that is, both the surface moisture caused by rain and so on, and the inherent moisture. The inherent moisture is the water that is very loosely bound in the coal. It can vary from a few percent in anthracite to 60-70% in browncoal. (Equilibrium moisture is defined by ASTM D1412, BS1016 part 21; bed/inherent moisture by ASTM D3173, DIN 51 748).
Volatile matter is determined by heating the coal in a covered crucible for a defined time at a defined temperature (e. g., 7 min at 950°C to ASTM). The loss in mass, minus the mass of the moisture, represents the mass of the gaseous constituents formed by the pyrolysis under the conditions mentioned (ASTM D3175, BS 1016 part 104, DIN 51720).
Ash is the inorganic residue that remains after combustion of the coal. It consists mainly of silica, alumina, ferric oxide, lime, and of smaller amounts of magnesia, titanium-oxide, and alkali and sulfur compounds (ASTM D3174, BS 1016 part 104, DIN 51719).
Fixed carbon is determined by subtracting from 100 the mass percentages of moisture, volatile matter, and ash (ASTM D3172). It should be remarked that fixed carbon is an artificial concept and does not mean that this material was present in the coal as pure C in the beginning.
Although the proximate analysis already tells the expert a lot about the coal, for gasification it is mandatory to have also the ultimate analysis at our disposal, which tells us more about the elemental composition of the hydrocarbonaceous part of the coal.
