Fly ash is a by-product of:
Correct Answer: B. Coal
📚 Detailed Explanation: Fly Ash Is a By-Product of Coal Combustion
Why B (Coal) is correct: Fly ash is generated as a residue from the combustion of pulverised coal in thermal power plant boilers. Fine ash particles are carried upward by the flue gases, then captured by pollution-control equipment (electrostatic precipitators) before the gases are discharged through the chimney. These captured fine particles are fly ash.
Production of Fly Ash
| Step | Process |
|---|---|
| 1. Coal pulverisation | Coal is ground to fine powder (<75 µm) to enable complete combustion |
| 2. Combustion | Pulverised coal burned at 1200–1700°C in boiler furnace |
| 3. Ash formation | Mineral matter in coal melts, spherodises, and solidifies into glassy spheres as it cools |
| 4. Transport by flue gas | Fine particles (fly ash) carried upward with exhaust gases; coarser particles fall as bottom ash |
| 5. Collection | Electrostatic precipitators or bag filters capture fly ash before gas exits chimney |
Types of Ash from Coal Combustion
| Ash Type | How Produced | Particle Size | Use |
|---|---|---|---|
| Fly ash | Carried by flue gas; collected by precipitators | 1–100 µm (mostly <45 µm) | SCM in concrete; PPC manufacture |
| Bottom ash | Coarser particles fall to base of boiler | 0.1–10 mm | Fill material; road sub-base |
| Pond ash | Fly + bottom ash mixed with water; pumped to ash ponds | Mixed | Land filling; brick making |
| Mound ash | Fly/bottom ash deposited dry (IS 3812) | Mixed | Engineered fill |
Properties of Fly Ash as a Concrete Material
| Property | Value / Range |
|---|---|
| Specific gravity | 1.9–2.6 (lower than OPC at 3.15) |
| Particle shape | Spherical glassy particles (beneficial for workability) |
| SiO2 content | 45–65% (reactive silica drives pozzolanic reaction) |
| Indian standard | IS 3812 Part 1: 2013 — Fly ash for use as pozzolana and admixture |
- Fly ash is a by-product of burning pulverised coal in thermal power plants.
- Collected by electrostatic precipitators; consists of fine glassy spheres.
- IS 3812 Part 1 classifies fly ash for use in concrete and governs its quality standards.