STUDY OF ASH HANDLING SYSTEMS IN THERMAL POWER PLANTS

SUBMITTED BY

Ankit Sood

Staff No. – 6043593

BHARAT HEAVY ELECTRICALS LIMITED

MECHANICAL AUXILIARIES DEPARTMENT PS-PEM, NOIDA

i

Certificate

This is to certify that Ankit Sood has satisfactorily completed the proposed work as a part of his project entitled “STUDY OF ASH HANDLING SYSTEMS IN THERMAL POWER PLANTS” under my supervision in partial fulfillment of the requirements for absorption as an Engineer in Bharat Heavy Electricals Limited. Mr. Girish Bhagchandani (Controlling Officer) Dy. Manager MAUX PS-PEM, Noida BHEL

Mr. P.K. Das Mr. A.K. Rohatgi Mr. G.S. Mahal

(Sr. DGM) (AGM) (GM) MAUX MAUX MAUX PS-PEM, Noida PS-PEM, Noida PS-PEM, Noida BHEL BHEL BHEL

ii

Acknowledgements

The completion of any project is the endeavor of all the individuals that

support, inculcate and foster the much needed enthusiasm and confidence to

the doer of the project work, without which the whole task proves to be an

impossible mission.

Towards the successful completion of the project, I would like to

acknowledge my debt to my controlling officer, Mr. Girish Bhagchandani.

He was always present to guide me in my work. The project would have not

been possible without his inputs, guidance, support, encouragement and

constructive comments I received from him.

At the very outset, I wish my sincere gratitude to all the members of

Mechanical Auxiliaries Department for their cooperation during my work

especially Mr. P.K. Das (Sr. DGM), Mr. Mallik Moazzam (Dy. Manager)

and Mr. Rahul Nimesh (Sr. Engineer) who were always available to give

valuable input and advice.

Finally I would like to acknowledge my other colleagues in the department.

They were always around to help out or simply be good companions.

Ankit Sood

PS-PEM, MAUX

Staff No. - 6043593

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Abstract

Coal based thermal power plants have a share of nearly 54% of the total installed

capacity in the country. The coal used has high ash content; as a result the quantity of ash

generated from power plants is very high. Collection of ash from boiler and its auxiliaries

and disposing the same in a safe and planned forms a part of Ash Handling System. With

more stringent environmental norms coming into effect, the role of ash handling systems

has become even more critical.

Nowadays, ash is considered as a by-product of thermal power plants instead of

being termed as waste. Various ways of utilization of ash for commercial purposes are

being developed in an attempt to ensure a cleaner and healthier environment.

The aim of this project report is to cover the various aspects of ash handling

systems. An attempt has been made to cover the various ash handling systems presently

being used in the country.

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Contents

Certificate i

Acknowledgements ii

Abstract iii

Contents iv

List of Figures vi

List of Tables viii

Abbreviations viii

1. Thermal Power Plant 1

1.1 Fossil Fuels – Coal 3

1.1.1 Peat 3

1.1.2 Lignite 3

1.1.3 Sub-Bituminous 3

1.1.4 Bituminous 4

1.1.5 Anthracite 4

1.2 Boiler Ash 4

1.2.1 Coal Ash 5

1.2.1. a. Bottom Ash 5

1.2.1. b. Fly Ash 5

2. Ash Handling Plants 6

2.1 Bottom Ash Handling Systems 8

2.1.1 Hydraulic Bottom Ash Handling Systems 8

2.1.1. a. Water Impounded Bottom Ash Hopper 9

2.1.1. b. Crushers/Clinker Grinders 11

2.1.1. c. Jetpulsion Pump 11

2.1.2 Submerged Scraper Chain Conveyor 13

2.1.2. a. Trough 14

2.1.2. b. Scraper Bars and Connector 14

v

2.1.2. c. Chain and Connecting Link 15

2.1.2. d. Drive Sprocket 16

2.1.2. e. Idler Wheel 17

2.1.2. f. Drive Unit 17

2.1.2. g. Chain Tensioner 18

2.1.3 Dry Bottom Ash Extraction System 19

2.1.3. a. Hopper Bottom Door/Jaw Crushers 20

2.1.3. b. Extractor 20

2.1.3. c. Drag Chain 21

2.1.3. d. Pan Conveyor 21

2.1.3. e. Primary Crusher 22

2.1.3. f. Second Stage Extractor cum Cooler 22

2.1.3. g. Vibrating Feeder 23

2.1.3. h. Ash Conditioner 24

2.2 Fly Ash Handling System 25

2.2.1 Electrostatic Precipitator 25

2.2.2 Pneumatic Conveying Systems 27

2.2.2. a. Vacuum Pumps 29

2.2.2. b. Air Intake Valve 30

2.2.2. c. Ash Intake/Material Handling Valve 30

2.2.2. d. Air Lock Vessel 31

2.2.2. e. Bag/Separator Assembly 31

2.2.2. f. Branch Isolation Valve/Line Segregating Valve 32

2.2.2. g. Transport/Conveying Air Compressors 32

2.2.2. h. Vent Filter for Silo 33

2.2.2. i. Pressure-Vacuum Relief Valve 33

2.2.2. j. ESP/Silo Fluidizing Blowers and Heaters 34

2.2.2. k. Fluidizing Equipments 34

2.2.2. l. Rotary Unloader 35

2.2.2. m. Telescopic/Dry Unloading Spout 35

2.3 Ash Disposal System 37

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2.3.1 Slurry Disposal System 37

2.3.1. a. Ash Slurry Pumps 37

3 System Design Criteria 38

4 Utilization of Ash 41

4.1 Utilization of Bottom Ash 41

4.2 Utilization of Fly Ash 41

5 References 43

List of Figures 1.1 Schematic of Coal-Based Thermal Power Plant........................................... 1

1.2 SEM Images of Bottom Ash......................................................................... 5

1.3 SEM Images of Fly Ash................................................................................ 5

2.1 Flow Scheme of Hydraulic Bottom Ash Handling System.......................... 8

2.2 Water Impounded Bottom Ash Hopper....................................................... 9

2.3 BA Hopper Discharge................................................................................ 10

2.4 Clinker Grinder............................................................................................. 11

2.5 Jetpulsion Pump............................................................................................ 12

2.6 General Arrangement of SSCC..................................................................... 13

2.7 SSCC Trough................................................................................................ 14

2.8 Scraper Bar and Connector........................................................................... 15

2.9 Chain and Connecting Link........................................................................... 16

2.10 Drive Sprocket........................................................................................... 16

2.11 Idler Wheel with Scraper.............................................................................. 17

2.12 Drive Unit..................................................................................................... 17

2.13 Chain Tensioner......................................................................................... 18

2.14 Schematic of Dry Bottom Ash Extraction System....................................... 19

2.15 Hopper Bottom Door.................................................................................... 20

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2.16 Dry Bottom Ash Extractor........................................................................... 21

2.17 Pan Conveyor............................................................................................ 22

2.18 Crusher ……………………………………………………………............. 22

2.19 Layout of Second Stage Cooler…………………………………………… 23

2.20 Vibrating Feeder ………………………………………………………...... 23

2.21 Working Principle of ESP (Corona Discharge)……………………............ 26

2.22 Electrostatic Precipitator………………………………….……………...... 26

2.23 Moving Bed/Dune Flow............................................................................. 27

2.24 Slug/Plug Flow ………….….……………………………………............... 27

2.25 Suspended/Dilute Phase…………………………………………................ 28

2.26 Vacuum-Pressure System for Fly Ash Disposal ………………………….. 29

2.27 Air Intake Valve......................................................................................... 30

2.28 Ash Intake Valve....................................................................................... 30

2.29 Air Lock Vessel …………………………………………………………… 31

2.30 Bag Filter/Separator …………………………………………………......... 32

2.31 Branch Isolation Valve ….…………………………………………........... 32

2.32 Vent Filter for Silo.…………………………….……….……………......... 33

2.33 Pressure-Vacuum Relief Valve ...…………………………………………. 34

2.34 Fluidizing Pads Arrangement.…………………………………….............. 34

2.35 Rotary Unloader ……………………………….……….……………........ 35

2.36 Telescopic Spout....................................................................................... 36

viii

List of Tables 1.1 Analysis of different types of Indian coals.................................................... 4

3.1 Flow Velocities in Pipes............................................................................. 39

Abbreviations

BMCR – Boiler Maximum Continuous Rating

GCV – Gross Calorific Value

HGI – Hardgrove Grindability Index

SEM – Scanning Electron Microscope

PF-Fired – Pulverized Fuel Fired Boiler

BHN – Brinell Hardness Number

 

1 Thermal Power Plant A thermal power station is a power plant in which heat energy (produced by combustion of fossil fuels

like coal, natural gas or oil in a boiler) is used to convert water into steam, which in turn is used to run

the prime mover (i.e. turbine) which then drives an electrical generator to produce electricity. After it

passes through the turbine, the steam is condensed in a condenser and pumped to boiler, where it was

heated. This whole cycle, which forms the basis of thermal power plant, is known as a Rankine cycle.

It is widely acknowledged that pulverized coal combustion will continue to be a major source of

electrical power generation for the foreseeable future. During combustion, the coal minerals transform to

ash particles which is the major byproduct of any coal based thermal power plant. Apart from ash, the

other major waste products of a thermal power station include the greenhouse gases, contaminated water

and coal mill rejects.

Typical diagram of a coal based thermal power station is as follows:

Fig. 1.1 Typical Schematic of Coal-Based Thermal Power Plant

1  

 

The major parts of Power Plant as indicated in the schematic above are as follows:

1. Cooling Tower

p

es

er

or

ne

p

er

ne

ve

ne

or

ter

or

er

lls

m

er

ter

an

ter

ke

er

ter

tor

an

ck

2. Cooling Water Pum

3. Transmission Lin

4. Transform

5. Electric Generat

6. Low Pressure Turbi

7. Condensate Pressure Pum

8. Condens

9. Intermediate Pressure Turbi

10. Steam Governor Val

11. High Pressure Turbi

12. De-aerat

13. Feed Hea

14. Coal Convey

15. Coal Hopp

16. Coal Mi

17. Boiler Dru

18. Bottom Ash Hopp

19. Super-hea

20. Forced Draft F

21. Re-hea

22. Air Inta

23. Economiz

24. Air Pre-Hea

25. Electrostatic Precipita

26. Induced Draft F

27. Chimney Sta

2  

 

1.1 Fossil Fuels - Coal

Fossil fuels are carbon based fuels – Coal, Petroleum and Natural Gas. Fuel selection plays an

important role in establishing design parameters of a power plant. Conventional thermal power plants

(coal based) utilize a combination of coal and fuel oil. Coal acts as the primary fuel while fuel oil is used

as the secondary fuel. Secondary fuel takes care of starting up of the boiler and then, along with the

primary fuel, stabilizes the flame till such a time (generally 30% BMCR) that the primary fuel alone can

take over the fuel requirement of the boiler.

Coal is a readily combustible sedimentary rock which is primarily composed of carbon along with

variable quantities of other elements including sulphur, hydrogen, nitrogen and oxygen.

There are various types/qualities of coal distinguished mainly by the ash, carbon and moisture

percentages present in unit quantity of coal. Coal analysis is carried out in order to ascertain the

composition of coal. There are two methods of coal analysis:

Proximate Analysis

Ultimate Analysis

On the basis of analysis, coal is generally classified into following categories:

1.1.1 Peat – It is a precursor of coal with high carbon content. It has little industrial importance and is

used as fuel in a few countries.

1.1.2 Lignite – It is geologically young coal which is considered the lowest rank of coal. It has carbon

content of around 25-35%, high inherent moisture of around 40% and an ash content ranging from 6 to

19%. The heat content of lignite ranges from 10 to 20MJ/kg. It is exclusively used as fuel for electric

power generation.

1.1.3 Sub-bituminous – It is a type of coal whose properties range from those of lignite to those of

bituminous coal. It is primarily used as fuel for steam-electric power generation. It has carbon content of

around 35-45%, around 20-45% ash and moisture content of 15-20%. Heat content ranges from 19 to

26MJ/kg. It has a lower sulfur content which makes it an attractive source of energy due to cleaner

burning.

3  

 

1.1.4 Bituminous – It is high carbon coal with carbon content ranging from 60 to 80%, high calorific

value of around 24 to 33MJ/kg and a low moisture content of just around 3%. Ash content varies

between 15 and 35% Due to these characteristics it is widely used in power generation plants and also in

making metallurgical coke.

1.1.5 Anthracite – It is the highest rank coal. With nearly 92 to 98% carbon, it has a high heat content

of nearly 35MJ/kg.

S. No. Name of Coal Field/Colliery

GCV of Coal

Proximate Analysis HGI

Kcal/kg Moisture (%) Ash (%) Volatile (%)

1 Raniganj 4900 - 5300 4 - 9 10 - 25 30 - 40 40 - 50

2 Talcher 4600 - 4900 7 - 9 28 - 38 27 - 31 60 - 66

3 Singrauli 4450 - 4800 9 - 10 21 - 24 28 - 29 50 - 65

4 Singareni 4000 - 4400 7 - 10 28 - 42 25 - 35 44 - 60

5 Neyveli 2900 - 3300 30 - 60 2 - 15 20 - 26

Table 1.1 Analysis of different types of Indian coals

1.2 Boiler Ash

Boiler ash is a generic term applied to many types of ash produced by the burning of various materials.

They are 4 general types of boiler ash commonly available, each with its own chemical and

environmental characteristics:

Incinerator Ash – produced from burning MSW (Municipal Solid Waste, i.e. Garbage) as a

waste disposal method

Tire Ash – produced from burning shredded tires for fuel in power generating plants

Wood Ash – from boilers where wood (or bark) is used as a heating source.

Coal Ash – from coal powered electrical generating power plants, actually two forms,

bottom ash and fly ash

4  

 

1.2.1 Coal Ash – This type of ash is produced from burning coal for electrical power generation and

is the waste product that results. There are two primary forms, bottom ash and fly ash. Bottom ash

accumulates at the bottom of the burner while fly ash is collected in Air pre-heater hoppers, Duct

Hoppers, Electrostatic Precipitator Hoppers and stack hoppers. They have very different chemical and

physical properties and are the inorganic constituents of the burned fuel that are not completely

combusted.

For coal based thermal power plant, the quantity of ash varies from 12% to 46% of the total coal burned

depending largely on the quality of coal used. 1.2.1. a. Bottom Ash – It is the non combustible components of coal which end up as ash or form

clinkers which stick to the hot walls of coal burning furnace during its operation. Bottom ash

accounts for nearly 12 to 15% of the total ash generated.

Fig. 1.2 SEM Images of Bottom Ash

1.2.1. b. Fly Ash – The finer particles of ash which are carried away with the flue gases and

collected by Electrostatic Precipitators is termed as Fly Ash. Fly ash accounts for nearly 75 to 80%

of the total ash generated. The particle size varies between 10 to 76 microns.

Fig. 1.3 SEM Images of Fly Ash

Rest of the ash is collected in economizer, air pre-heater, duct and stack hoppers.

5  

 

2 Ash Handling Plants (AHP) Coal used in thermal power plants contains two things; organic matter and mineral matter. The organic

matter produces heat when burnt while the mineral matter cannot be burned and therefore leaves the

residue called ash. Ash handling refers to the method of collection, conveying, interim storage and load

out of various types of ash residue left over from solid fuel combustion processes. The quantity of ash is

very high, thereby requiring proper arrangement to handle the same. It is an integral part for successful

operation of thermal power plants.

The most common types of ash include bottom ash, bed ash, fly ash and ash clinkers. There are two

distinct aspects of ash handling; ‘Ash Collection’ and ‘Ash Disposal’. Scope of ash handling system

starts from the Bottom Ash Hoppers (for Bottom Ash) and Electrostatic Precipitators Hoppers (for Fly

Ash) and terminates at the ash storage silos or ash dykes/ponds.

AHP’s are custom built systems and are different for every power plant. There are various factors which

are considered while selecting and designing AHP for any power plant. Some of the critical inputs

required for design process include:

Properties of ash being handled - Ash types vary as a result of different fuels, combustion

methods, flue gas treatments and various boiler designs. Chemical composition, particle size,

particle density, shapes and size distribution can also vary.

Elevation and ambient conditions of site

Mode of ash conveying (Wet or Dry)

Distance over which ash has to be transferred

Ultimate Disposal (Land fill, truck disposal, ash ponds etc. or commercial utilisation)

Separate systems are required to be installed for Bottom and Fly Ash handling. Some of the systems

available are as follows:

6  

 

Bottom Ash Handling Systems a. Hydraulic Systems (Intermittent system) – Ash is collected in the water impounded

bottom ash hopper and is transported by jet pumping to a /de-watering bin//ash slurry

sump/ash pond.

b. Mechanical Systems(Continuous system) – It is a continuous removal system from

conventional PF-fired boilers and has two alternatives, either a Submerged Scraper Chain

Conveyor (Wet System) or a Dry Bottom Ash Extraction System

Fly Ash Handling Systems a. Vacuum Pneumatic Conveying Systems

b. Pressure Pneumatic Conveying Systems

c. Vacuum-Pressure Conveying Systems

7  

 

2.1 Bottom Ash Handling Systems In a conventional PF-fired boiler, around 12 to 15% of the total ash produced is in the form of bottom

ash, which is a dark grey, granular, porous and sand size material that is collected in the bottom ash

hopper installed at the bottom of the furnace and then conveyed to the ash pond/silos/de-watering bins.

Generally, the ash collected in the Economizer Hoppers is also transferred to the Bottom Ash Hopper for

further disposal along with the bottom ash.

2.1.1 Hydraulic Bottom Ash System In case of hydraulic systems, the bottom ash is collected in water filled hopper at the bottom of the

furnace. Bottom ash hopper is generally designed for effective storage capacity of 6 hours operation in a

500/600 MW plant. Twice in a shift of 8 hours, the stored bottom ash is removed intermittently by

means of high pressure water jets and transported as a slurry through a pipeline system either to a

disposal pond or slurry sump or to a decant basin for dewatering and stockpiling for disposal or for

further use.

A typical flow scheme depicting hydraulic type bottom ash system is as follows:

Fig. 2.1 Flow Scheme of Hydraulic Bottom Ash Handling System

8  

 

The major components/equipments which form a part of hydraulic bottom ash system include:

2.1.1. a. Water Impounded Bottom Ash (BA) Hopper – Water impounded bottom ash hopper

is the heart of bottom ash handling system. BA hoppers are made of refractory lined heavy steel

plates and are generally of ‘V’, ‘double V/W’ or ‘triple V’ shape. The hoppers are generally sized

for 6 to 8 hours active storage capacity.

The hot, burning bottom ash clinkers that fall from the furnace are quenched and fractured when

they come in contact with the cool water. Twice in a shift of every 8 hours, the bottom ash discharge

system is put into operation for typical 500/600 MW plant. The collected ash is discharge through a

feed gate located at the sloping portion of each ‘V’ section. Below each feed gate, a crusher is

provided to grind the oversize clinkers prior to entering the jet pump. A jetpulsion pump is used to

convey the resulting slurry of ash and water through pipeline to slurry sump for further conveying to

a distant pond.

Fig. 2.2 Water Impounded Bottom Ash Hopper

The major components/systems which form a part of BA Hopper are:

9  

 

Seal Water Trough – Conventional boilers operate at a certain negative pressure. In order

to maintain the negative pressure, BA hoppers are provided with a seal water trough which

acts as a sealing arrangement for the boiler. It is provided at the top of BA hopper to prevent

ingress of air. A typical view of seal water trough is shown in above picture.

Fig. 2.3 BA Hopper Discharge

Poke Holes & Inspection Windows – Poke holes are provided on the side of each ‘V’

section of the hopper. It facilitates in removal of the erstwhile hot clinkers which get stuck to

the sides of hopper. Inspection windows enable to monitor the interior of bottom ash hopper.

10  

 

Refractory Cooling Water System – Cooling water is continuously circulated around the

refractory lining provided inside the bottom ash hopper to maintain its temperature in order

to ensure proper functioning and enhance the operation life of lining.

Make Up Water Connection – The bottom ash hoppers are provided with a continuous

source of water in order to maintain the temperature of water (generally at 60 °C) and also to

cool the ash. As cooling of ash takes place, the water temperature increases. Therefore, in

order to ensure effective cooling of further ash being collected, water at a lower temperature

is continuously fed into BA hopper.

BA Overflow System – As make up water is continuously fed into the BA hopper to ensure

the permissible temperature, continuous overflow of water shall take place in the BA hopper.

This water is collected in an overflow weir box, which also acts as a sealing arrangement for

hopper (similar to the seal water trough). The water is collected in BA overflow sump and

further transferred to the clarifier/tube-settler by centrifugal pumps for clarification and

reuse. Generally 15 minutes storage capacity is provided in the overflow sump.

Jetting and Slope Nozzles – Water nozzles are provided along the walls of BA hopper in

order to prevent the ash from sticking onto them.

2.1.1. b. Crushers/Clinker Grinders – A clinker grinder is provided at the bottom of BA hopper

in order to break the oversized ash clinkers to a pre-defined value (generally less than 25mm) to

ensure easy handling and conveying of ash slurry. Crushers can be of two types – single roll crusher

and double roll crusher.

Fig. 2.4 Clinker Grinder

2.1.1. c. Jetpulsion Pump – It is a venturi type device which utilizes high pressure water supply

to convey abrasive solids. It is provided below the clinker grinders for conveying bottom ash slurry

11  

 

to the ash pond/dewatering bins. It is a ‘convergent-divergent nozzle with ash slurry fed from top

through bottom ash hopper and high pressure water for transporting slurry from other side. .

Fig. 2.5 Jetpulsion Pump

12  

 

2.1.2 Submerged Scraper Chain Conveyor (SSCC) It is a wet type of continuous bottom ash removal system. The SSCC system starts from the bottom of

BA hopper. In contrast to hydraulic systems, BA hopper in case of SSCC is not of water impounded

type. Ash from the furnace is collected and partially quenched through jetting nozzles in BA hopper. BA

hoppers are provided with sliding or swing type bottom doors which are normally open and allow ash to

fall down from the hopper into the water impounded SSCC trough. Subsequent quenching of ash takes

place in the trough and ash settles down and collects between scraper bars. The scraper bars are

connected with chain links which in turn are mounted on drive sockets being driven by a driving unit

(generally an electric/ hydraulic motor and reduction gearbox). Scraper bars move slowly and push the

accumulated ash towards an inclined dewatering ramp. As the flight bars travel up the inclined ramp,

compaction of ash takes place. Also, the water entrained in ash gets drained. Only a small amount of

residue moisture remains in the ash which is discharged from the edge of dewatering ramp. The

relatively dry ash can now be conveyed with standard mechanical belt conveyors/centrifugal

pumping/hydraulic sluicing system to a disposal point.

General arrangement of SSCC system is given below:

Fig. 2.6 General Arrangement of SSCC

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Major components of SSCC System include:

2.1.2. a. Trough – The main body of SSCC system, i.e. trough, is of double chamber type. The

upper chamber is water impounded, where ash is collected and removed. The upper chamber is

formed from a single piece of steel. The bottom of portion of upper chamber is lined with an

abrasion resistant steel plate or basalt tiles to sustain the impact of heavy clinkers falling from BA

hopper. Also, abrasion resistant wear liners are also placed along the sides to protect the side walls.

The lower chamber on the other hand stays dry, providing the return path for scraper bars. Access

ports are provided along the length of the chamber for servicing of chain and scraper bars. Abrasion

resistant steel rails are also provided along the bottom of lower chamber. Scraper bars move over

these rails, instead of bottom of trough, thereby preventing abrasive wear.

Fig. 2.7 SSCC Trough

2.1.2. b. Scraper Bars and Connector – Scraper bars are made of structural beam (conventional

‘I’ beam is used). Abrasion resistant wear plates are welded to the top and bottom of each scraper

14  

 

bar. Scraper bar chain connector is an open type horn type which surrounds the horizontal link of

drive chain. The chain and scraper bar connectors are protected by a chain shield. Shield is an angled

steel fabrication welded to the side of trough and protruding above chain. It deflects large pieces of

material away from the chain and also helps to prevent ash from accumulation around the chain

thereby reducing wearing of the chain and ensuring smoother operation.

Fig. 2.8 Scraper Bar and Connector

2.1.2. c. Chain and Connecting Link – Depending upon the quantity of ash to be handled,

chain sizes are decided. The chains are case hardened can withstand high shock loads. Chain

connecting links are of the same size as chain links. The connectors are secured onto the chain with

the help of roll pins.

15  

 

Fig. 2.9 Chain and Connecting Link

2.1.2. d. Drive Sprocket – Chain is driven through the drive sprockets. The sprocket teeth are

adjustable and can be adjusted to match with the worn out chain thereby ensuring smooth operation

of unit.

Fig. 2.10 Drive Sprocket

16  

 

2.1.2. e. Idler Wheel – They are used to support and guide the chain where the chain changes its

direction. Idlers at the base of dewatering ramp also include a scraper which removes the ash from

idler grooves before the chain comes into contact with the wheel. This prevents the chain from

slipping off. The top of scraper is sloped inward so as to prevent ash from collecting on it.

Fig. 2.11 Idler Wheel with scraper

2.1.2. f. Drive Unit – Electric or hydraulic motors are used to drive the conveyor through a

helical/planetary-gear reducer. The speed and torque of the motor are adjustable in order to meet the

boiler output.

Fig. 2.12 Drive Unit

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2.1.2. g. Chain Tensioner – It gives an indication of the level of chain wear. It is also provided

with an arrangement for manual adjustment of chain tension in order to compensate for elongation of

the chain. Chain tensioner can be manual as well as hydraulic type depending on conveyor capacity

handled.

Fig. 2.13 Chain Tensioner

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2.1.3 Dry Bottom Ash Extraction System It is a system used for collection of bottom ash. In this system, the bottom ash hopper is connected to the

boiler via a mechanical seal. The seal absorbs boilers thermal expansions. Similar to the SSCC systems,

bottom doors are provided below the hopper which, apart from crushing the occasional huge lumps of

ash, provides a means to isolate the bottom ash extractor from boiler and converts it into a temporary

storage facility. The bottom ash extractor is positioned below the BA hopper. The main component of

extractor is the pan conveyor which collects and conveys the ash. This pan conveyor is sealed inside the

extractor. Ash is cooled through a co-current flow of ambient air. Pan conveyor discharges the ash into a

(primary) crusher to reduce its size. A pre-crusher can also be provided above the primary crusher to

take care of any large ash clinkers which the primary crusher might not be able to crush.

In order to carry out further cooling of ash, a post cooler (similar in construction as the BA extractor)

may be employed. As per the system requirement, a secondary crusher (to further reduce ash size) along

with a contact cooler (for additional cooling of ash) may be provided after the post-cooler.

General Layout of Dry Bottom Ash Extraction System is as follows:

Fig. 2.14 Schematic of Dry Bottom Ash Extraction System

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The major components/equipments of Dry Bottom Ash Extraction System include:

2.1.3. a. Hopper Bottom Door/Jaw Crushers – The bottom doors/crushers are operated with a

hydraulic power unit with separate actuation cylinder for both gates. The bottom doors provide

multiple uses which include crushing of oversized ash clinkers, isolation of hopper to enable

temporary storage facility to enable hopper maintenance and also to facilitate the extraction of

accumulated ash at system restart. In case of failure of a component downstream of the dry extractor

cum cooler, the doors are simply closed, and the necessary intervention can be done to remove the

failure while ash is stored inside the hopper. The doors are refractory lined on the upper part that

comes in contact with the hot ash.

In case of discontinuous operation, if there is a built up of ash inside the hopper on the top of bottom

doors, after restarting the extractor, the stored ash is extracted gradually from the hopper by opening

the bottom doors sequentially (pair by pair).

Fig. 2.15 Hopper Bottom Door

2.1.3. b. Extractor – It is the key component of the dry bottom ash extraction system. It is

designed to withstand the arduous operating conditions under the boiler throat, characterized by high

temperatures and shock impacts by large ash clinkers falling from the boiler. The primary function

of the extractor is to extract the ash from the boiler and cool it to a sufficiently low temperature that

allows the discharge into the downstream processing equipment. The extractor is a completely sealed

20  

 

enclosure and ash is cooled, while conveying, by a small controlled amount of ambient air that flows

by natural draft into the extractor’s casing through the inlet valves.

The chief component of extractor is the pan conveyor which receives and extracts the ash falling

from the boiler. The pan conveyor is enclosed inside the extractor’s external casing that is

completely sealed, thus preventing any uncontrolled infiltration of ambient air, and in the same time

preventing any leakage of ash or gas to the outside atmosphere. From the pan conveyor the ash is

discharged onto a primary crusher for size reduction and is then conveyed and processed further to a

final destination in accordance with the specific needs of the user.

Fig. 2.16 Dry Bottom Ash Extractor

2.1.3. c. Drag Chain – This is an additional feature which prevents the build-up of fine ash/dust

on the bottom floor of extractor casing. The Drag Chain consists of two lateral chains connected by

scraper bars that sweep the accumulated dust over the extractor floor to the extractors’ head section,

where it is discharged into the primary crusher. Among the Dry Bottom Ash Handling System

vendors, this feature is offered by only a few vendors. It is indicated in the Fig. 2.14.

2.1.3.d. Pan Conveyor – It is made up of stainless steel mesh belt, which carries partially

overlapping steel pans forming a virtually sealed belt conveyor. The basic concept of the pan

conveyor is similar to a traditional belt conveyor. The conveyor is supported by carrying idlers

across its entire width, in order to withstand heavy mechanical impacts. All bearings are fitted

outside the casing to protect them from heat and to allow easy maintenance. The driving force is

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transmitted by friction between the head pulley and the belt, while a pneumatic take-up device on the

tail pulley supplies a constant tension.

Fig. 2.17 Pan Conveyor 2.1.3. e. Primary Crusher – A single-roller primary crusher serves to reduce the size of big ash

lumps to a maximum outlet size of approx. 80mm. The single-roller crusher is designed to handle the

largest possible range of friable materials. As ash falls onto the crusher, the rotating cam teeth shear

and split the ash clinkers against an anvil plate. The curved anvil plate acts to produce perpendicular

forces at all angles of contact between the cams, ash and plate. The crushed ash then drops through

the clearance between the rotor and the anvil plate into the downstream equipment. As the cams

rotate and mesh with the combing plate, crushed ash is efficiently cleared from the cams. The wear

parts (teeth, anvil plate etc.) are made of materials with a high temperature and abrasion resistance,

which guarantees a long life.

Fig. 2.18 Crusher

2.1.3. f. Second stage cooler cum extractor – The second stage coolers are totally enclosed

mechanical conveyors, generally very similar in construction as the bottom ash extractor, which are

utilized in case there is further cooling and conveying requirement.

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Fig. 2.19 Layout of Second Stage Cooler

2.1.3. g. Vibrating Feeder – The vibrating feeder is a machine with free oscillation, driven by

electromagnetic vibrators. The vibrating feeder is used for metered withdrawing of bulk material

from silos, hoppers etc., especially in all those applications where very precise volumetric or weight

metering is required.

The movement induced in the vibratory feeders is of unidirectional type with amplitude of

oscillation in relation to the weight of the vibrating feeder and the supply voltage of the

electromagnetic vibrator coming from the electronic controllers. The capacity can be remote-

adjusted instantaneously during operation by means of an electronic thyristor control board,

equipped with a voltage stabilizer.

Flow rate is adjusted merely by varying the amplitude of oscillation of the feeder by means of

relative potentiometer.

Fig. 2.20 Vibrating Feeder

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2.1.3. h. Ash Conditioner – The mixer-unloader is a totally enclosed, paddle-type mixer

designed to condition, bottom ash, and other abrasive or dusty materials. The unit features the ability

to accurately control both the material feed rate and the water supply flow rate, as well as insure a

complete and controlled mixing prior to discharge. In this manner, the ash or other material can be

precisely conditioned with minimum water usage, no excess water or dust, and minimum operator

involvement.

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2.2 Fly Ash Handling System

Fly ash forms a major part of the total ash produced in any power plant. Approximately 80% of the total

ash is carried away by the flue gases and collected in the Electrostatic Precipitator. The ash collected in

ESP’s is then generally conveyed by suitable pneumatic systems.

2.2.1 Electrostatic Precipitator (ESP)

ESP is a particulate collection device that removes particles from a flowing gas using an induced

electrostatic charge. This mechanism is called Corona Discharge. The basic working principle of ESP is

as follows:

It contains a row of thin vertical wires, and followed by a stack of large flat metal plates oriented

vertically, with the plates typically spaced about 1 cm to 18 cm apart, depending on the application. The

air or gas stream flows horizontally through the spaces between the wires, and then passes through the

stack of plates.

A negative voltage of several thousand volts is applied between wire and plate. If the applied voltage is

high enough an electric (corona) discharge ionizes the gas around the electrodes. Negative ions flow to

the plates and charge the gas-flow particles.

The ionized particles, following the negative electric field created by the power supply, move to the

grounded plates.

Particles build up on the collection plates and form a layer. The layer does not collapse, thanks to

electrostatic pressure (given from layer resistivity, electric field, and current flowing in the collected

layer).

The automatic plate-rapping systems and hopper-evacuation systems remove the collected particulate

matter while on line, theoretically allowing ESPs to stay in operation for years at a time.

An electrostatic precipitator can consistently provide 99% removal reducing emissions levels to 0.002 -

0.015 grains per dry standard cubic foot of exhaust gas.

ESP used in thermal power plants has a number of fields and each field has a number of collection

hoppers.

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Fig. 2.21 Working Principle of ESP (Corona Discharge, Smoke ≈ Ash)

Fig. 2.22 Electrostatic Precipitator

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Fly ash handling and evacuation systems play a very important role in ensuring the efficient

performance of ESP’s.

2.2.2 Pneumatic Conveying Systems Pneumatic conveying is basically an arrangement which enables the movement of dry material through

an enclosed pipeline by utilizing moving air. There are basically two modes of pneumatic conveying:

Dense Phase Conveying – In this mode, material is conveyed in such a way that the ratio of mass of

material being transported and mass of air being utilized is very high. In such cases, the material

conveying takes place at lower velocities, thereby reducing the amount of erosion in the enclosing

vessel (generally pipes). Dense phase mode can further be classified as follows:

Moving Bed/Dune Flow – This type of flow is possible in case the material being

conveyed has good air retention properties. In this case, the material is conveyed in the

form of dunes on the bottom of pipeline or as a pulsatile moving bed.

Fig. 2.23 Moving Bed/Dune Flow

Slug/Plug Flow – This type of flow is possible in case the material being conveyed has

good permeability. Here, the material is conveyed as full bore plugs, separated by air

gaps.

Fig. 2.24 Slug/Plug Flow

Suspended/Dilute Phase Conveying – In this mode, the material is conveyed while being completely

suspended in the conveying air. Very high conveying velocities are possible in this mode of

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conveying. The ratio of mass of material being conveyed and the quantity of air being utilized is

very low. Practically any material can be conveyed in dilute phase, provided that requisite amount of

conveying air is available.

Fig. 2.25 Suspended/Dilute Phase

Pneumatic conveying systems are the best solution for conveying of fly ash in a thermal power plant and

are widely adopted nowadays.

There are mainly two types of Pneumatic Conveying Systems:

Vacuum Systems (Negative pressure) – In this system, the ash is transported in an air stream at less

than atmospheric pressure. Vacuum pumps are used to create the required negative pressure. The

major limitation of this system is the vacuum levels. High vacuum levels cannot be generated by

vacuum pumps. Thereby, there is a limitation of conveying distance and conveying capacity in

vacuum systems.

Pressure Systems – In this system, a positive displacement blower/compressor is used to generate air

at high pressures for pushing of material through pipelines. These types of systems can be used to

convey large quantities of materials over long distances.

Thermal power plants utilize a combination of both above systems for conveying of fly ash. The fly ash

collected in ESP, Air Pre Heater and Duct hoppers is evacuated through a vacuum system and conveyed

to an intermediate surge hopper. A vacuum pump is utilized for this purpose. There after ash is separated

from conveying air through bag filters and collected in intermediate surge hoppers from where it is

subsequently conveyed under pressure to the storage silos for final disposal. The conveying air pressure

is generated by Transport Air Compressors/Blowers. From the storage silos, ash can be loaded onto

trucks and sent for final disposal or reuse in cement or other industries.

A typical flow scheme of Fly Ash Handling System is as given below:

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Fig. 2.26 Vacuum-Pressure System for Fly Ash Disposal

The major components of Fly Ash Handling System are as follows:

2.2.2. a. Vacuum Pumps – It is utilized to generate a vacuum which is then utilized to suck the

fly ash collected below ESP hoppers. Conventionally liquid ring type vacuum pumps are employed

for vacuum conveying as they are more apt to handle carry over ash with air sucked.

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2.2.2. b. Air Intake Valve – It is a spring loaded valve which can be adjusted to control the air

flow in vacuum system conveying line.

Fig. 2.27 Air Intake Valve

2.2.2. c. Ash Intake/Material Handling Valve – It is a metallic seated valve used to feed the

ash in the conveyor line. It can also control the ash feed rates.

Fig. 2.28 Ash Intake Valve

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2.2.2. d. Air Lock Vessel – Once the ash is collected in the intermediate surge hopper, it is

transferred to the air lock vessel. Once the air lock vessel is filled, the vessel inlet is closed and

pressurized by compressed air for conveying.

Fig. 2.29 Air Lock Vessel

2.2.2. e. Bag Filter/Separator Assembly – It is equipment utilized to remove coarse ash from

the conveying air and then filter out the remaining fine ash. Ash laden air enters the bag filter and is

collected on bag filter due to adsorption phenomenon. The ash gets accumulated on the bags and is

dislodged through intermittent reverse pulse jetting of instrument air and is collected below.

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Fig. 2.30 Bag Filter/Separator

2.2.2. f. Branch Isolation/Line Segregating Valve – It is a slide valve with a metallic slide

gate used for isolation of vacuum extraction line.

Fig. 2.31 Branch Isolation Valve

2.2.2. g. Transport/Conveying Air Compressors – Oil free rotary screw type compressors are

used for generating high pressure conveying air for ash transportation to silo(s).

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2.2.2. h. Vent Filter for Silo – They are used for venting silos into which material is conveyed.

Conveying air, fluidizing air and air displaced by ash is vented through the vent filter. As the ash

fills the silo, it displaces air. The air, carrying the finer ash particles, rises in the bag area. As the air

passes through the bags, dust and ash is captured and gets collected on the outer surface of the bags.

The air is vented into the atmosphere. Ash collected on the filter bags is returned to the silo by using

pulses of high pressure air.

Fig. 2.32Vent Filter for Silo

2.2.2. i. Pressure-Vacuum Relief Valve – Fly ash storage silos are provided with a pressure-

vacuum relief valve on the top to prevent built-up of excessive pressure or vacuum in the silo. Such

a situation may cause damage to the storage silo. Due to some fault in the unloading equipment, a

negative pressure may be built-up in the silo and on exceeding a pre-set value, the relief valve allows

air to enter the silo and thereby negate the negative pressure built. Similarly, if pressure exceeds a

pre-set limit, relief valve allows air to escape from the silo.

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Fig. 2.33 Pressure-Vacuum Relief Valve

2.2.2. j. ESP//Silo Fluidizing Blowers and Heaters – Fluidization is a process similar

to liquefaction whereby a granular material is converted from a static solid-like state to a dynamic

fluid-like state. This process occurs when a fluid (in this case warm air) is passed up through the

granular material. Centrifugal blowers are used to generate low pressure air for fluidization purpose.

The heater unit connected at the discharge end of blower heats up the air to around 140°C in order to

enhance the fluidization process.

2.2.2. k. Fluidizing Equipments – They are provided at the bottom of storage silos in order to

assist the flow of material from silo outlet. Fluidizing media is diffuser stones/ceramic tiles with

pores for air entrance. They are arranged in such a way that it causes the aerated ash to move

towards the outlet.

Fig. 2.34 Fluidizing Pads Arrangement

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2.2.2.l. Rotary Unloader – It is fitted at the bottom of fly ash silos and is utilized for

conditioning of ash before it is being unloaded onto open trucks for final removal. The dry ash is

mixed with water in order to prevent ash from escaping into the atmosphere while unloading onto

trucks. The equipment carries out mixing of ash and water through a tumbling action. It consists of a

rotary drum and a stationary scraper bar which removes the ash from surface of the drum causing

tumbling action and thereby mixing.

Fig. 2.35 Rotary Unloader

2.2.2. m. Telescopic/Dry Unloading Spout – While unloading dry ash into closed trucks, an

unloading spout is utilized. It consists of a telescopic tube inside flexible, retractable outer tube. The

outer tube connects to the inlet flange of the closed tank. The telescopic tube is raised and lowered

through an electric winch. Ash discharges from the bin through the inner telescopic tube into the

truck. The displaced air from the truck enters the open space between the concentric tubes thereby

minimizing fugitive dust. Venting action can be assisted by an electric fan. High unloading rates are

possible with such arrangement.

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Fig. 2.36 Telescopic Spout

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2.3 Ash Disposal System

The method of ash from power plants may be, either in wet or dry conditions, by roads or rail vehicles or

pumped in the form of slurry to a disposal area or by a combination of both the systems. In case fly ash is to

be commercially used, then it can be conveyed pneumatically (in dry state) and then taken via transport

trucks/train. Alternatively, fly ash is combined with bottom ash and disposed to low lying areas/ash ponds

through ash slurry pumps in case disposal through trucks/tankers is not logistically possible. This system,

though environmentally not very safe, is the most economical method of ash disposal and was widely

followed. However MOEF (Ministry of Environment and Forest) has stated guidelines for maximum ash

utilization and instructed plant owners to install AHP system ensuring same. High Concentration slurry

density system has been recommended by MOEF wherever logistic do not permit dry disposal of fly ash.

2.3.1 Slurry Disposal System

The ash collected in various hoppers at different collection point of boiler and ESP is mixed with water, in

hopper/flushing apparatus/collector tank and taken to ash slurry sump by pumping/gravity for further

disposal to ash pond. . The slurry collected in the sump is continuously pumped to the ash ponds through

the ash slurry pumps. .

2.3.1. a. Ash Slurry Pumps

These are special centrifugal pumps designed for pumping a mixture of abrasive ash and water at a

reasonable efficiency. They are made of abrasion resistant alloy cast iron/Ni-hard material with high

hardness of nearly 550BHN to ensure long service life.

Clear water sealing arrangement is provided for gland sealing in order to protect the shaft stuffing box

from abrasive ash laden water. Depending upon location of ash pond, multiple pumps are provided in

series to generate the required head for disposal along long distances. .

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3 System Design Criteria A typical Ash Handling System (AHS) for a 500/600 MW plant is designed as per the following design

criteria.

Maximum ash generation at 100% BMCR with worst coal firing considered

for system design.

Ash Generation and Distribution

a. Bottom ash : 20 percent of the total ash production per unit.

b. Economizer ash : 5 percent of the total ash production per unit.

c. Air heater ash : 5 percent of the total ash production per unit.

d. Duct hopper : 2.5 percent of the total ash production per unit(if applicable).

e. Fly ash (ESP) ash : 85 percent of the total ash production per unit.

System Operating Time

AHS shall normally operate continuously. However, the rated capacity of sub-systems shall be

designed considering following.

a. Bottom Ash System: Removed twice in a shift of eight hours – ash generation of four

hours extracted in maximum 90 minutes including line flushing.

b. Economizer Ash System: it is continuously removed to bottom ash hopper for further

disposal along with bottom ash.

c. Fly (Air Heater & ESP) Ash System: To remove entire fly ash generated per shift of 8

hours per unit in maximum 360 minutes.

Density Considered for Bottom Ash & Fly Ash a. Density for Bottom Ash for Volumetric Sizing : 650 kg/m3

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39  

b. Density for Fly Ash for Volumetric Sizing : 750 kg/m3

c. Density for Bottom Ash for Structural Sizing : 1600 kg/m3

d. Density of Fly Ash for Structural Sizing : 1600 kg/m3

e. Density of Ash Slurry for Bottom Ash slurry : up to 25 % (w/w)

d. Density of Ash Slurry for Fly Ash slurry : up to 30 % (w/w)

e. Density of combined Bottom Ash & Fly Ash Slurry : up to 25 % (w/w)

f. Particle Density of Fly Ash : 2000 kg/ m3

Velocity Considered in Pipes for Various Duties Maximum Velocity in Air & Water Lines shall be as per the table below

Table 3.1 Flow Velocities in Pipes

Bottom Ash Slurry Lines : 2.3 / 2.8 m/sec.

Fly Ash slurry lines : 1.5 – 2.6 m/sec

Combined Ash Slurry (minimum/maximum) : 2.3 / 2.8 m/sec.

Tip Speed for Ash Disposal Pump (Max) : 1676 m/min

Ash Disposal Pump Speed (Max.) : 1000 rpm

Initial pick up velocity in air/ash mixture for : 9-20 m/sec (For both vacuum & conveying pressure)

Service

Velocity (m/sec)

Pipe size

below 50 mm

Pipe size of 50

mm to 150 mm

Pipe size of 200

mm & above

Water Pump Suction 0.6 – 0.9 1.2 – 1.5 1.2 – 1.5

Water Pump Discharge 0.9 – 1.8 1.5 – 2.4 1.5 – 2.5

Compressed Air 15 15 15

 

Friction factor ‘C’ as per William Hazan’s

Friction factor ‘C’ for Ash Slurry Lines : 140.

Friction factor ‘C’ for Drain / Sludge Lines : 130

Friction factor ‘C’ for Recovery Water Lines : 120

Friction factor ‘C’ for Bottom Ash Over-flow Lines : 120

Friction factor ‘C’ for Water Lines : 100

Capacities of Various Tanks

Bottom Ash Over flow Tank : for 15 min

Slurry Sump (each compartment) : for 5 min

Ash Water Sump : for 30 min

Drain Sumps : for 10 min

Recovery Water Sump : for 30 min

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4 Utilization of Ash Disposal of ash is one of the most serious environmental issues of present time. Huge quantities of ash

are generated in thermal power plants around the globe. Till around 1980’s and 1990’s, disposal of ash

in a low lying area (ash ponds) was generally followed. Large areas of lands, spanning thousands of

acres, were required for this purpose. Most of these ash ponds are already filled up by now. In addition

to the non-availability of land now, another problem which has come up is the contamination of ground

water and rivers due to the leaching of toxic minerals from these ash dumps. Small quantities of ash

have been found to be helpful to improve certain properties of soil increase its productivity. But, large

quantities of ash dumps results in permanent destruction of land resources.

In order to counter these problems, stricter environmental norms have been put into effect which calls

for treatment and recycling of ash instead of disposal. Various methods have been developed to which

are being used to make ash commercial use of ash possible.

4.1 Utilization of Bottom Ash Bottom ash is a coarse gritty material which has a few possibilities for utilization which are as follows:

As a component in compost for improving the properties of soil. It has been found to be a

satisfactory substrate for growing ornamentals.

As an aggregate in road construction.

For making of concrete blocks used in construction work.

In cement gravel

In paving bricks

4.2 Utilization of Fly Ash Recycling of fly ash has become an increasingly popular proposition mainly due to unavailability of

landfills and also the pollution control norms. The properties of fly ash like pozzolanic nature, spherical

shape and relative uniformity makes it an excellent engineering material.

41  

 

Other beneficial uses of fly ash include:

As a replacement of Portland cement content of concrete

As a grouting material

For embankment construction and as structural fill

As waste stabilization and solidification

For stabilization of soft soils

As a flowable fill

In paints, roofing tiles

As a geo-polymer (a binding material)

In mine reclamation

Making concrete bricks

As mineral filler in asphaltic concrete

42  

 

43  

5 References

Training modules from National Power Training Institute

http://en.wikipedia.org/

Article on ”Pneumatic Conveying of Fly Ash in Thermal Power Plants” by Sh. Girish

Bhagchandani, Dy. Manager, Mechanical Auxiliaries, PS-PEM, BHEL

Equipment Brochures from M/s United Conveyor Corporation (USA), M/s DCIPS (India), M/s

Magaldi Power SpA (Italy)