The various parameters of proximate analysis define properties of given coal and coke:
Fix carbon: Fix carbon acts as the main heat generator during burning. Fix carbon gives a rough estimate of the heating value of coal.
Volatile matter: Volatile matter contributes to the heating value of coal. Increase in percentage of volatile matter in
coal proportionately increases the flame length and helps in easier ignition of coal and coke. Sets of minimum limit of furnace height and volume. Influences secondary oil support.
Ash: Ash is an impurity which will not burn. Ash content is important in the design of furnace grate, combustion volume, pollution control equipment (ESP) and ash handling plant. Ash increases transportation, handling, storage cost. Ash affects combustion efficiency and boiler efficiency. Ash causes clinkering and slagging problems in a boiler.
Moisture: Moisture increases transportation, handling, storage cost. Moisture decreases the heat content per kg of power plant coal. Moisture increases heat loss due to evaporation and superheating of vapour. Moisture helps in radiation heat transfer.
Significance of Ultimate Analysis Parameters
The ultimate analysis is useful in determining the quantity of air required for combustion and volume and composition of combustion gases. This information like H/C or O/C ratios is required for calculation of flame temperature and flue gas duct design.
Effect of sulphur content: Affects clinkering and slagging tendencies. Corrodes chimney and other equipment. Limits flue gas exit temperature.
Effect of hydrogen content: H/C ratio is directly related to aromaticity of coal.
Higher is the H/C ratio, higher will be the heating value and lower carbon dioxide emission during combustion.
Significance of Gross Calorific Value
Calorific value (CV): Calculation of CV of coal-based on proximate analysis gives a very rough estimate of calorific value of coal but is used for deciding the grade of coal and formula adopted by Coal India is as below:
Useful heat value of coal = 8900 – (M + Ash)
The ultimate analysis gives a fairly correct calorific value of coal and formula to be used is as below
GCV of coal in Kcal/Kg = (355 x C + 1148 x H + 95 x S – 145 X O) / 4.1868
C, H, S, O are % figures obtained in the ultimate analysis of coal. A caloric value obtained by calorimeter is the only correct value and gives GCV. Calorific value decreases with increase in moisture content and increases with increase in fuel ratio (FC/VM).
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