Ash Behavior & Ash Handling in Boilers

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Ash Behavior & Ash Handling in Boilers

1. Effect of Ash on Boiler Performance

Ash present in coal significantly affects boiler efficiency and operation:

  • Reduces GCV (Gross Calorific Value) of coal

  • Low fusion ash forms deposits on tube walls → reduces heat transfer → affects steam generation

  • Clinker formation:

    • Low fusible ash forms clinkers

    • Causes clogging of furnace

    • Leads to severe corrosion of grate bars in stoker-fired boilers

2. Bonded Ash Deposit

Definition

A bonded ash deposit is a dense slag layer strongly attached to boiler heat transfer surfaces.

Formation Mechanism

  • At high flue gas temperatures, ash particles become plastic/sticky

  • These particles stick to:

    • Water wall tubes

    • Superheater tubes

  • Deposits grow by capturing more fly ash particles

Further Strengthening Processes

  • Cracking

  • Crystallization

  • Sulphation

  • Other physicochemical reactions

3. Factors Affecting Bonded Ash Formation

  1. Mineral matter in flue gas

  2. Exposure time at high temperature

  3. Furnace temperature

  4. Heating rate

  5. Flue gas composition

  6. Temperature of:

    • Flue gas

    • Fly ash

    • Heating surfaces

  7. Physicochemical reactions inside ash deposits

4. Ash Fusion Temperature (AFT)

Definition

Temperature at which ash starts melting and forms clinker.

Stages of Ash Fusion

  1. Initial Deformation Temperature (IDT)

    • Ash begins softening

    • Solid–solid phase change

  2. Hemispherical Temperature (HT)

    • Partial melting occurs

    • Forms hemisphere shape due to viscous melt

  3. Flow Temperature (FT)

    • Ash becomes highly fluid

    • Melt starts flowing

5. Classification of Coal Based on Fusion Behavior

TypeHemispherical Temperature
Refractory> 1400°C
Medium Refractory1300–1400°C
Fusible< 1350°C

6. Ash Fusion Based on Chemical Composition

(a) Ash Fusion Index (R)

R=SiO2+Al2O3Fe2O3+CaO+MgO+AlkaliesR = \frac{SiO_2 + Al_2O_3}{Fe_2O_3 + CaO + MgO + Alkalies}

  • Low R → Coal is clinker forming (fusible)

  • High R → Coal is refractory

(b) Hemispherical Temperature Calculation

HT(°C)=1030+73.3×RmHT (°C) = 1030 + 73.3 \times R_m Rm=3.33SiO2+1.97Al2O32.5Fe2O3+3.57CaO+5MgO+3.22(Na2O+K2O)R_m = \frac{3.33SiO_2 + 1.97Al_2O_3}{2.5Fe_2O_3 + 3.57CaO + 5MgO + 3.22(Na_2O + K_2O)}

7. Slagging and Fouling

(a) Slagging

  • Occurs in radiation zone

  • Forms deposits on furnace walls

Rs=(Fe2O3+CaO+MgO+Na2O+K2OSiO2+Al2O3+TiO2)×SR_s = \left(\frac{Fe_2O_3 + CaO + MgO + Na_2O + K_2O}{SiO_2 + Al_2O_3 + TiO_2}\right) \times S

Slagging Classification

Rs ValueCategory
< 0.6Low
0.6–2.0Medium
2.0–2.6High
> 2.6Very High

(b) Fouling

  • Occurs in convection zone

Rf=(Fe2O3+CaO+MgO+Na2O+K2OSiO2+Al2O3+TiO2)×Na2OR_f = \left(\frac{Fe_2O_3 + CaO + MgO + Na_2O + K_2O}{SiO_2 + Al_2O_3 + TiO_2}\right) \times Na_2O

Fouling Classification

Rf ValueCategory
< 0.2Low
0.2–0.5Medium
0.5–1.0High
> 1.0Severe

8. Ash Handling Systems

Types

  1. Mechanical

  2. Hydraulic

  3. Pneumatic

  4. Steam Jet

9. Fly Ash Collection Systems

(A) Mechanical Collectors

  • Gravitational separator

  • Cyclone separator

  • Impingement separator

(B) Electrical Collectors

  • Electrostatic Precipitator (ESP)

(C) Wet Collectors

  • Spray scrubber

  • Packed bed scrubber

  • Cyclone scrubber

10. Electrostatic Precipitator (ESP)

Working Principle

  • Flue gas passes through electric field

  • Dust particles are ionized

  • Collected on oppositely charged plates

Key Feature

  • Very high efficiency for fine particles

11. Cyclone Separator

Working

  • Gas enters tangentially

  • Creates swirling motion

  • Centrifugal force separates heavier particles

Advantages

  • Low maintenance

  • Efficient for large particles

  • Works better at high loads

Disadvantages

  • Poor efficiency for fine particles

  • Pressure loss

  • High power consumption

12. Bag Filter (Baghouse)

Working

  • Gas passes through fabric bags

  • Dust is trapped

  • Clean gas exits

  • Bags cleaned by reverse air flow

Merits

  • Up to 99.9% efficiency

  • Works for particles ≥ 1 micron

Demerits

  • Suitable for low sulphur coal (<1%)

13. Cyclone Scrubber (Wet Type)

  • Combines cyclone action + water spray

  • Dust absorbs water → forms slurry

  • Collected in bottom tank

14. Pneumatic Ash Handling System

Working

  • High velocity air carries ash

  • Cyclone separator removes particles

  • Final filtration before release

Advantages

  • Dust-free operation

  • No spillage

  • Flexible system

  • Low installation cost

Disadvantages

  • Noisy

  • High maintenance due to abrasion

15. Hydraulic Ash Handling System

(a) Low Velocity System

  • Water carries ash slowly

  • Velocity: 3–5 m/s

  • Capacity: 50 TPH, distance up to 500 m

(b) High Velocity System

  • High-pressure water jets break and transport ash

  • Capacity: 120 TPH, distance up to 1 km

Conclusion

Understanding ash behavior is essential for:

  • Improving boiler efficiency

  • Reducing maintenance issues

  • Preventing slagging and fouling

  • Designing effective ash handling systems

Efficient ash management ensures safe, reliable, and economical operation of thermal power plants.


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