Questions & Answers on Ash handling system

1.What is Ash?

Ash is the remaining product of solid or liquid fuel after burning

2.What are the various components of Ash?

Ash has following components

• Silica (SiO2)
• Alumina (AlO3)
• Iron Oxide (Fe2O3)
• Sodium Oxide (Na2O)
• Potassium Oxide (K2O)
• Calcium Oxide (CaO2)
• Magnesia (MgO)

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3.Which fuel has more ash Liquid, solid or Gaseous fuel?

Solid, Liquid & Gaseous fuels are having more ash consecutively

4.What are the various types of ash produced in Boilers?

Bottom ash & Fly ash are generated in Boilers

5.Which ash is more in quantity?

Generally Fly ash is more around 70-80% & bottom ash is around 20-30%

6.What do you mean by Fly ash?

Ash which is carried out by flue gas is called fly ash.

7.What can fly ash could cause in downstream system of The Boiler?

• If ash is more, it creates following problems in downstream of the Boiler
• Improper heat transfer in Super heaters, economizers & APH
• Erosion of pressure parts & flue gas ducts
• If there is low velocity, ash deposits in ducts, APH ESP etc

8.Which type of Ash removal is more dangerous & why?

• Bottom ash removal is more dangerous, because;
• Bottom Ash is at higher temperature
• Ash is high Abrasive & Corrosive in Nature
• When it comes in contact with water high hot fumes are formed
• Risk of frequent clinker formation

9.What are the different devices or systems used to separate Fly ash from flue gas before letting it into atmosphere?

• Electrostatic Precipitator
• Bag filter (Fabric separators)
• Wet scrubber
• Inertial separators (Settling chamber, Baffle chambers, Cyclone separator)
• Fabric hybrid filter

10.Briefly explain the Fabric Separator type bag filters

In this system, fabric bags are used to filter the flue gas to separate the dust. Dust laden gases enter the bag house and passes through fabric bags which act as filter.The bags are woven with material nylon, fiber glass etc. Each bag is externally supported by steel/metal cage. The bag filter house is provided with an explosion vent to avoid explosion during abnormal operation conditions.

Further, the bag filter house consists of hoppers & ash handling system to remove fly ash separated in bag filters.

In bag filters, the dust collects at the outer surface of the bag since flue gas flow from out side to inside of the bag.

Mechanism of dust collection:

Gravity: Due to gravitational force & sudden lower velocity large sized dust/ash particles fall down into hopper due to Gravitational force.

Inertial collection: Due to inertial, heavy dust particles strike the bag filters placed in the flue gas path & fall down into the hopper, since they do not change their flow direction due to inertia.

Interception: Due to the fine mesh or size of the bag filters, dust or ash particles cannot cross the filters. Instead they hit filters & fall down into the hopper.

Electrostatic effect: Electrostatic force between dust particles & bag filter causes the dust to capture.

11.How do you remove dust particles from Bag filters?

• Mechanical Shaker
• Reverse air
• Reverse Jet

12.What are the various materials of composition (MOC) of Bag filters?

Sl No.	Bag filter material	Operating temperature (0C)
1	Nylon	85-90
2	Polyester	130-140
3	Polyphenylene  sulphide or Ryton	180-190
4	Fibre glass	250-260
5	Fibre glass fabric coated with PTFE	250-260

13.What are the various factors considered for selection of Bag filters?

• Flue gas temperature
• Moisture level in flue gas
• Dust or ash particles size
• O2% in flue gas
• Flue gas velocity
• Dust or ash particles abrasiveness
• Air to cloth ratio

14.What are the main functions of Ash handling system?

• To remove the ash from Boiler furnace & other various ash discharge points
• To convey this ash to nearby storage area like ash silo
• Ash disposing

15.What are the various types of Ash handling systems used in Boilers?

Mechanical ash handling system: In this system chain, belt & screw conveyors are used to convey the ash from various ash termination points to ash silo.

Pneumatic ash handling system:

Pneumatic ash handling system is used widely in most of the Power plants. High pressure air is used to convey the ash to the suitable location.

16.What are the various types of Pneumatic ash handling systems used in Boilers/power plants?

• Lean phase ash handling system
• Medium phase
• Dense phase

17.Why dense phase ash handling system is used in almost all Boilers Ash handling plant?

Because it has less air consumption due to volumetric ration of air & ash is more. Sometimes instead of pressurized air vacuum system is used to convey the ash.

Briefly explain the dense phase Ash handling system

In this system, Ash conveying system is placed just below the ash hopper. This system consists of Main ash hopper

• Surge hopper with electromagnetic or Mechanical vibrators
• Knife edge gate valve
• SS expansion bellow
• Dome valve assembly & operating mechanism
• Ash & air conveying valves, solenoid valves
• Pressure switches & limit switches
• Conveying pipelines

If the temperature of the ash is more (Economiser & APH) surge hopper is made with water jacket for continuous circulation of water.

The system can be operated from local & remote in probe mode or timer mode.

Calculation part:

1. A Boiler is consuming 72 TPH an imported coal having ash % 8, calculate the total ash generated in a complete month. Assume there is no stoppages or load fluctuation

Total coal consumed in a month = 72 X 24 X 30 = 51840 MT

Total ash generated in a month = 51840 X 8 / 100 = 4147.2 MT

2. A boiler consumes 7 TPH of coal, calculate the total fly ash generated in a day if coal has 35% ash.

Total ash generated = 7 X 24 X 35 / 100 = 58.8 MT

We know that, fly ash is around 80% of total ash.

So total fly ash generated is 58.8 X 80 / 100 = 47.04 MT

3. A Boiler generates 20 MT of ash in a day, calculate total coal consumed in a day if coal has 5% of ash in it

Total ash generated =20 MT/day

Ash % in coal = 5%

Therefore total coal consumed = 20  / 5% = 400 MT

 

 For related articles read Power plant & Calculations

 

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