Ash Handling Systems

Types of Ash• Burning of Coal leads to generation of Ash• Can be the non- combustible part of fuel, or

generated after the burning of the coal• Two Types are :-• Bottom Ash• Fly Ash

Bottom Ash

• Non- Combustible Part of Combustion• Comprises traces of combustibles embedded in

forming clinkers and sticking to hot side walls of a coal-burning furnace

• The clinkers fall by themselves into the water or sometimes by poking manually, and get cooled

• May be used as an aggregate in road construction and concrete

Fly Ash

• Residual of Coal Combustion• Comprises of fine particles that rises with Flue

Gases• All types of fly ash includes substantial

amounts of silicon dioxide and Calcium Oxide• Size range – from 0.5 µm to 100 µm

Challenges of Ash handling

• Indian coal presents high ash content generally which tends to be inconsistent. Design of the system has to adequately cover anticipated variations and be capable of handling the worst scenario.

• System has to be environmentally friendly. • System has to be reliable with least maintenance

problems. • System has to be energy efficient

Ash Handling SystemsAsh

Bottom Ash

Wet Handling System

W Shaped System

Scrapper System

Dry Handling System

Pneumatic Convey

Megaldi System

Dry Chain Conveyer

System

Fly Ash

Lean Conveying

Dense Phase Conveying

Wet Ash Handling System

• Wet type systems have water impounded hopper and jet pumps for intermittent removal

• The bottom ash fall into a W-type water impounded (water filled) ash storage hopper

• The stored ash is removed by means of jet pumps and transported in slurry pipe lines to the ash slurry sump for its further disposal

W shaped System

• Water Consumption is very high in Water Impounded Hopper Type System

• Generally Water requirement is 3 times that of Ash Removed

• The water requirement is reduced greatly in Scrapper Chain type System (Water:Ash = 10:90)

Scrapper Chain Conveying System• Belt conveyors are used to dispose the bottom ash from scraper

chain conveyor to bottom ash silo. • Employ continuously operating or submerged belt conveyor

running below boiler furnace.• SCC system receives hot clinker, slag and ash falling from the

boiler, through a transition chute to water filled trough. • The trough containing the conveyor is filled with water for

quenching of bottom ash.• Bottom ash discharged onto scraper chain conveyor , where it is

fed to grinder that crushes it to 25mm size.• The crushed bottom ash conveyed to bottom ash silo by belt

conveyors.

Recirculation System

Submerged Flight Conveyor

Pneumatic Ash Extractor

Vibratory Ash Extractor

Fly Ash Handling

• Lean Phase Conveying• Dense Phase Conveying

Lean Conveying

• This system operates on the lean phase principle with a high air to ash ratio and high conveying velocities.

• The velocity is in the range of 15 m/sec to 30 m/sec.• Operating Vacuums (negative pressure) upto 530

mm of mercury. • The material/air ratio in the lean phase is in the

range of 20:1.• Short conveying distance upto 200 meters.

Dense Phase Conveying

• The Dense Phase Pneumatic Pressure Conveying Systems uses low volume, medium pressure air stream.

• Operating pressure upto 5 kg./cm2 high capacity conveying line.

• Conveying distance upto 1500 meters. • The material air ratio is in the range of 20 -

100 to 1.

OPERATIONAL PRINCIPLE• As soon as the ash level reaches a fixed level in the collecting

hopper, the level probe senses its presence, it allows the system to initiate a conveying cycle.

• The inlet valve opens to allow the ash to gravitate into the conveying vessel, till it closes automatically.

• On closure of the valve, the conveying vessel gets pressurized and the material resistance helps pressure build up which conveys the material through pipe in the destination silo.

• When conveying is complete which is sensed by the control system, air supply to the system is stopped and system is ready for the next cycle

ComparisonS.N. Lean Conveying Dense Phase Conveying1. Storage of ash in hopper for 8-12

hours causes chocking, compacting, etc.

Ash is removed as soon as collected.

2. Due to limitation of distance in Lean System, location of silos has too be close to ESP.

No distance restriction.

3. Due to high velocities, air separation is inefficient.High velocities result in high wear rates of associated equipments.

Lower wear.

4. High maintenance prone. Low maintenance.5. Choking of hopper No such problem.6. High power consumption. Low power consumption.

Design and reliability of pneumatic conveyer

Depends on:• The accuracy of the input parameters and the

degree in which they reflect the reality.• The accuracy of the performance data of the

pneumatic installation components.• The completeness of the theory on which the

calculation algorithm is built.• The degree of approximations in the calculation

algorithm.

For a pressure pneumatic conveying system, the intake conditions are important as they determine the mass flow of gas.Ambient conditions Intake conditions

The intake conditions are: – Temperature– Pressure– Relative Humidity (RH)

The mass flow of gas determines the Solid Loading Ratio (SLR) and this ratio determines the pressure drop for material collision and friction losses.In addition the pressure drop for gas resistance is influenced by the mass flow of gas.Ambient conditions.

Ambient conditions are not necessarily the same as the intake conditions. The intake conditions can be indoor and the pipe routing can be outdoor with completely different ambient conditions

The ambient conditions are: Temperature Pressure Relative Humidity (RH)

Material properties:In the calculation, the basic pneumatic conveying parameters are the suspension velocity and the Solid Loss Factor (SLF).The pneumatic conveying properties of the material can have a great influence on the created pressure drop in combination with the Solid Loading Ratio (SLR).

Material properties are: Particle size distribution Particle shape Particle density

Mathematical description of the theory of pneumatic conveying:

Although the basic principle of pneumatic conveying is simply the transfer of impulse from a moving gas to a moving solid, the involved physical processes are quite complex.

The calculation model is based on the law of conservation of energy, Newton’s laws and thermodynamic laws for gases and heat exchanges.

• Involved physical processes are – gas compressing– gas expansion– condensation of water vapor– heat exchange with material– heat exchange with surroundings– acceleration by impulse transfer between gas and material particle.– deceleration by inter particle- and wall collisions– Gas density changes by condensation of water vapor in the conveying

gas.– pressure drop for keeping the particle in suspension– extra pressure drop for sedimentation (vertical, slope and horizontal)– pressure drop for gas resistance– pressure drop for acceleration– pressure drop for elevation– pressure drop for solid collision- and friction losses.– tank pressurizing– pipe line purging

Computer programs or algorithms that calculate a pneumatic conveying installation with a minimum of input variables, neglecting complex heat exchanges, condensation, the occurring of sedimentation, the dependency of product losses of the SLR and the turbulence (Re-number) and the calculation of the slip velocity are much less accurate than a computer program that accounts for all these effects.

Fly Ash Reuse• Concrete production, as a substitute material for Portland cement • sand Embankments and other structural fills • Grout and Flowable Fill production • Waste stabilization and solidification• Cement clinkers production - (as a substitute material for clay)• Mine reclamation • Stabilization of soft soils • As Aggregate substitute material (e.g. for brick production) • Mineral filler in asphaltic concrete • Agricultural uses: soil amendment, fertilizer, cattle feeders, soil stabilization

in stock feed yards, and agricultural stakes Loose application on rivers to melt ice

• Loose application on roads and parking lots for ice control

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