Question
Describe the formation of coal, starting from plant matter. What are the successive stages, and how does the carbon content change at each stage?
Solution — Step by Step
Coal formation begins with dead plant matter — trees, ferns, and vegetation that lived 300–360 million years ago during the Carboniferous period. When these plants died, they accumulated in swampy, waterlogged environments where oxygen was scarce. Low oxygen slowed down decomposition, so organic matter piled up instead of rotting fully.
Carbon content at this stage: roughly 50% in dry plant matter.
As more layers of plant debris piled on top, anaerobic bacteria partially decomposed the bottom layers. The product is peat — a soft, spongy, brown material that still looks organic. Peat has high moisture content and burns poorly.
Carbon content: ~55–60%. Peat is technically not coal yet — it’s the precursor. You can still see plant structures in it under a microscope.
Over millions of years, peat gets buried under sediment layers. The increasing pressure and heat (a process called coalification) drives out moisture and compresses the peat. It becomes lignite, also called brown coal.
Carbon content: ~60–70%. Lignite is the lowest-grade coal — it still has significant moisture and burns with lots of smoke. India has large lignite deposits in Tamil Nadu (Neyveli).
Further burial, more heat, more pressure. The carbon content rises as volatile compounds (moisture, methane, CO₂) are driven out. The result is bituminous coal — the most common type used in power plants and steel production (as coke).
Carbon content: ~70–86%. Bituminous coal burns hotter and cleaner than lignite. The sub-bituminous stage (70–76% carbon) is an intermediate step before full bituminous.
Under extreme geological pressure and temperature over very long time periods, bituminous coal becomes anthracite — the hardest, most carbon-rich coal. It has a shiny, glass-like appearance and burns with very little smoke.
Carbon content: 86–97%. Anthracite is the most efficient fuel but also the rarest. Most of India’s anthracite comes from Assam and Jammu & Kashmir.
Why This Works
The whole process is driven by increased pressure and temperature with time. As burial depth increases, two things happen: volatile compounds (water, methane, oxygen-containing groups) escape, and the carbon atoms rearrange into more ordered, graphite-like structures. This systematic loss of non-carbon elements is why carbon percentage keeps rising through each stage.
The technical term for this process is coalification or diagenesis → catagenesis. The rank of coal (peat → lignite → bituminous → anthracite) is essentially a measure of how far along this maturation process the coal has gone.
Alternative Method
A simpler way to remember the stages is by the mnemonic PLBA: Peat → Lignite → Bituminous → Anthracite. Each stage has progressively higher carbon content, lower moisture, lower volatile matter, and higher calorific value.
| Stage | Carbon % | Moisture | Calorific Value |
|---|---|---|---|
| Peat | 55–60% | Very high | Very low |
| Lignite | 60–70% | High | Low |
| Bituminous | 70–86% | Medium | High |
| Anthracite | 86–97% | Very low | Highest |
CBSE Class 8 and 10 ask specifically about the four stages and their carbon content. JEE questions focus more on the chemical products of coal (coke, coal tar, coal gas, ammonia liquor) obtained during destructive distillation. Know both angles.
Common Mistake
Students often list only three stages and forget that peat is technically the first stage. Also, many confuse the order — writing “anthracite comes before bituminous.” Remember: carbon content increases as you go from peat to anthracite, so anthracite is always the endpoint. The order Peat → Lignite → Bituminous → Anthracite never changes.