Coal is a fossil fuel and a combustible sedimentary rock formed through the process of carbonization. Over millions of years, dense vegetation buried under layers of sediment underwent anaerobic decomposition under conditions of intense pressure and high geological temperature.
The Carbonization Sequence
As geological time progresses, the moisture content and volatile matter within the organic debris decrease, while the relative percentage of fixed carbon increases. This chronological maturity transforms organic matter through distinct stages.
Types of Coal by Rank
- Peat: Regarded as the precursor to coal rather than true coal. It is a fibrous mass containing less than 40% carbon. It retains a high moisture content, burns with significant smoke, and possesses the lowest calorific value.
- Lignite: Commonly known as brown coal. It contains approximately 40% to 55% carbon. It is structural, soft, and exhibits relatively high moisture content. In India, significant deposits are mined at Neyveli, Tamil Nadu.
- Bituminous Coal: Frequently referred to as soft coal or commercial coal, holding 60% to 80% carbon. It is dense, black, and contains a high amount of volatile matter. It is the most abundant type globally and is heavily utilized in thermal power generation.
- Anthracite: The highest grade, ultra-metamorphic coal, containing 85% to 95% carbon. It is hard, jet-black, possesses a metallic luster, and contains minimal moisture. It burns slowly with a clean blue flame without emitting smoke. In India, its availability is limited primarily to the Jammu and Kashmir region.
Proximate vs. Ultimate Analysis of Coal
In chemical testing, the quality and industrial grade of coal are determined through two types of laboratory analysis. These metrics are critical for evaluating fuel efficiency and environmental impact.
Proximate Analysis
This process determines the physical characteristics of the coal sample by measuring its constituent fractions.
- Moisture Content: High moisture lowers the heating value and consumes heat during evaporation.
- Volatile Matter: Consists of organic compounds that turn into gas when heated. High volatile matter leads to long flames and high smoke production.
- Ash Content: The inorganic residue remaining after complete combustion. High ash content reduces the heat value and poses disposal challenges (such as fly ash).
- Fixed Carbon: The solid combustible residue left after moisture, volatile matter, and ash are accounted for. High fixed carbon denotes premium coal quality.
Ultimate Analysis
This process determines the elemental chemical composition of the coal mass.
- Carbon and Hydrogen: Primary elements that undergo exothermic oxidation to produce heat (CO2 and H2O).
- Sulfur: A deleterious impurity. Upon combustion, it converts to Sulfur Dioxide (SO2), a primary agent of acid rain and equipment corrosion.
- Nitrogen: Converts into Nitrogen Oxides (NOx) during combustion, acting as a major atmospheric pollutant.
Destructive Distillation of Coal
When coal is heated strongly at high temperatures (1000°C to 1400°C) in the total absence of air, it does not burn. Instead, it breaks down into solid, liquid, and gaseous fractions. This chemical process is known as destructive distillation or carbonization of coal.
Products of Destructive Distillation
| Product | Physical State | Primary Composition | Major Industrial Uses |
| Coke | Solid | 85% – 90% Fixed Carbon | Reducing agent in metallurgy (blast furnaces), manufacturing of water gas. |
| Coal Tar | Liquid | Complex mixture of aromatic hydrocarbons (benzene, toluene, naphthalene) | Synthesis of dyes, drugs, perfumes, explosives, and synthetic plastics. |
| Coal Gas | Gaseous | Methane (CH4), Hydrogen (H2), Carbon Monoxide (CO) | Illuminant and industrial fuel for heating. |
| Ammoniacal Liquor | Liquid | Ammonium Hydroxide (NH4OH) dissolved in water | Raw material for chemical fertilizers like Ammonium Sulfate. |
Coke: Production, Properties, and Types
Coke is the solid carbonaceous residue derived from the destructive distillation of low-ash, low-sulfur bituminous coal.
Properties of Coke
- It is a tough, porous, and greyish-black solid substance.
- It burns with practically no smoke and produces no soot, making it a cleaner fuel alternative than raw coal.
- It possesses a higher calorific value than the bituminous coal from which it is derived due to the removal of volatile matter and moisture.
Industrial Classification of Coke
- Metallurgical Coke: Hard, physically strong coke capable of supporting the heavy weight of iron ore layers inside a blast furnace without crushing. It must have very low phosphorus and sulfur content to avoid contaminating the metal.
- Petroleum Coke (Petcoke): A carbon-rich solid material derived specifically from the final cracking processes of crude oil refining rather than coal. It has a high calorific value but contains significant levels of sulfur and heavy metals, raising distinct environmental regulations regarding its combustion.
Key Fact-Sheet for UPSC Prelims
- Caking vs. Coking Coal: All coking coals are caking coals, but not all caking coals are coking coals. Caking coal softens and fuses into a plastic mass when heated. If this fused mass yields a strong, porous solid residue (coke) fit for metallurgical use, it is specifically designated as coking coal.
- Fly Ash: A byproduct of burning pulverized coal in electric power generating plants. It contains silicon dioxide (SiO2), aluminum oxide (Al2O3), and calcium oxide (CaO). It is widely utilized in India as a replacement for Portland cement and in manufacturing bricks.
- Syngas via Coal Gasification: A process where coal is reacted with oxygen and steam under high pressure to produce synthesis gas (Carbon Monoxide and Hydrogen). This clean-coal technology minimizes the emission of suspended particulate matter.
- Washing of Coal: A process of beneficiation where mined raw coal is separated from associated dense rocky impurities (shale, quartz) using gravity separation methods to lower its overall ash percentage before dispatch to power plants.