In one line
Proximate analysis finds the percentages of moisture, volatile matter, ash and fixed carbon in coal by simple weighing and heating, and tells us the practical quality of the coal.
Explained simply
It is a quick "health check" of coal. You heat a small sample under different conditions and see how much weight it loses each time. Water leaves first, then gases (volatile matter), then the remaining carbon burns off leaving ash. What is left after removing the other three is fixed carbon, the part that really gives heat.
How it works
- Moisture: heat about 1 g of powdered air-dried coal in a silica crucible at 105–110 °C for 1 hour in an oven. Cool in a desiccator and weigh.
\( \%M = \dfrac{\text{loss in weight}}{\text{wt. of coal}} \times 100 \) - Volatile matter: heat the moisture-free sample in a covered crucible at 950 ± 20 °C for exactly 7 minutes in a muffle furnace. Cool and weigh.
\( \%VM = \dfrac{\text{loss in weight}}{\text{wt. of coal taken}} \times 100 \) - Ash: heat a known weight of coal in an open crucible at 700–750 °C for about 30 min (until constant weight). Weigh the residue.
\( \%A = \dfrac{\text{wt. of residue}}{\text{wt. of coal}} \times 100 \) - Fixed carbon: \( \%FC = 100 - (\%M + \%VM + \%A) \)
Key formula
Significance of each result:
- Moisture – lowers calorific value, wastes heat in evaporation, increases transport cost. Lower is better.
- Volatile matter – high VM coal burns with a long smoky flame and gives low calorific value; it is preferred for making coal gas, but low VM is better for furnaces and metallurgical coke.
- Ash – non-combustible; lowers calorific value, causes clinkers that block air supply, and creates disposal cost. Lower is better.
- Fixed carbon – higher FC means higher calorific value and better coal; it helps in designing furnaces because fixed carbon burns in the solid state.
Quick example
2.0 g coal lost 0.06 g at 110 °C: %M = 3%. In the VM test it lost a further 0.56 g: %VM = 0.56/2.0 × 100 = 28%. Separately, 2.0 g left 0.16 g ash: %A = 8%. %FC = 100 − (3 + 28 + 8) = 61%.
Common mistakes
- Using an open crucible for VM (the coal would burn) or a covered one for ash.
- Calculating %VM on the moisture-free mass when the question expects it on the original sample mass. Read the data carefully and state the basis.
- Saying high VM is always good. It depends on use.
How MU asks it
"Explain proximate analysis of coal and give its significance." (5–6 marks) and numericals (3–5 marks). For full marks: four tests with temperature, time, covered/open crucible, formula for each, and the significance of each parameter. In numericals, show each percentage separately with the formula before substituting.