cfbboilerbasicdesignoperationandmaintenance-130421215807-phpapp02

1CFB Boiler Design, Operation and Maintenance

By Pichai Chaibamrung

2Content Day11. Introduction to CFB2. Hydrodynamic of CFB3. Combustion in CFB4. Heat Transfer in CFB5. Basic design of CFB6. Operation7. Maintenance8. Basic Boiler Safety9. Basic CFB control

3Objective To understand the typical arrangement in CFB To understand the basic hydrodynamic of CFB To understand the basic combustion in CFB To understand the basic heat transfer in CFB To understand basic design of CFB To understand theory of cyclone separator

Know Principle Solve Everything

41. Introduction to CFB1.1 Development of CFB 1.2 Typical equipment of CFB1.3 Advantage of CFB

51.1 Development of CFB 1921, Fritz Winkler, Germany, Coal Gasification 1938, Waren Lewis and Edwin Gilliland, USA, Fluid Catalytic Cracking,

Fast Fluidized Bed 1960, Douglas Elliott, England, Coal Combustion, BFB 1960s, Ahlstrom Group, Finland, First commercial CFB boiler, 15

MWth, Peat

61.2 Typical Component of CFB Boiler

71.2 Typical Component of CFB BoilerWind box and grid nozzle

primary air is fed into wind box. Air is equally distributed on furnace cross section by passing through the grid nozzle. This will help mixing of air and fuel for completed combustion

81.2 Typical Component of CFB BoilerBottom ash drain

coarse size of ash that is not take away from furnace by fluidizing air will be drain at bottom ash drain port locating on grid nozzle floor by gravity. bottom ash will be cooled and conveyed to silo by cooling conveyor.

91.2 Typical Component of CFB BoilerHP Blower

supply high pressure air to fluidize bed material in loop seal so that it can overflow to furnace

Rotameter

Supplying of HP blower to loop seal

10

1.2 Typical Component of CFB BoilerCyclone separator

located after furnace exit and before convective part.use to provide circulation by trapping coarse particle back to the furnace Fluidized boiler without this would be BFB not CFB

11

1.2 Typical Component of CFB BoilerEvaporative or Superheat Wing Wall

located on upper zone of furnaceit can be both of evaporative or SH panellower portion covered by erosion resistant materials

12

1.2 Typical Component of CFB BoilerFuel Feeding system

solid fuel is fed into the lower zone of furnace through the screw conveyor cooling with combustion air. Number of feeding port depend on the size of boiler

13

1.2 Typical Component of CFB BoilerRefractory

refractory is used to protect the pressure part from serious erosion zone such as lower bed, cyclone separator

14

1.2 Typical Component of CFB BoilerSolid recycle system (Loop seal)

loop seal is located between dip leg of separator and furnace. Its design physical is similar to furnace which have air box and nozzle to distribute air. Distributed air from HP blower initiate fluidization. Solid behave like a fluid then over flow back to the furnace.

15

1.2 Typical Component of CFB BoilerKick out

kick out is referred to interface zone between the end of lower zone refractory and water tube. It is design to protect the erosion by by-passing the interface from falling down bed materials

16

1.2 Typical Component of CFB BoilerLime stone and sand system

lime stone is pneumatically feed or gravitational feed into the furnace slightly above fuel feed port. the objective is to reduce SOx emission. Sand is normally fed by gravitation from silo in order to maintain bed pressure. Its flow control by speed of rotary screw.

17

1.2 Typical Arrangement of CFB Boiler

18

1.3 Advantage of CFB Boiler Fuel Flexibility

19

1.3 Advantage of CFB Boiler High Combustion Efficiency

- Good solid mixing- Low unburned loss by cyclone, fly ash recirculation- Long combustion zone In situ sulfur removal Low nitrogen oxide emission

20

2. Hydrodynamic in CFB2.1 Regimes of Fluidization2.2 Fast Fluidized Bed2.3 Hydrodynamic Regimes in CFB2.4 Hydrodynamic Structure of Fast Beds

21

2.1 Regimes of Fluidization Fluidization is defined as the operation through which fine

solid are transformed into a fluid like state through contact with a gas or liquid.

22

2.1 Regimes of Fluidization Particle Classification

23

2.1 Regimes of Fluidization Particle Classification

24

2.1 Regimes of Fluidization Comparison of Principal Gas-Solid Contacting Processes

25

2.1 Regimes of Fluidization Packed Bed

The pressure drop per unit height of a packed beds of a uniformly size particles is correlated as (Ergun,1952)

Where U is gas flow rate per unit cross section of the bed called Superficial Gas Velocity

26

2.1 Regimes of Fluidization Bubbling Fluidization Beds

Minimum fluidization velocity is velocity where the fluid drag is equal to a particles weight less its buoyancy.

27

2.1 Regimes of Fluidization Bubbling Fluidization Beds

For B and D particle, the bubble is started when superficial gas is higher than minimum fluidization velocityBut for group A particle the bubble is started when superficial velocity is higher than minimum bubbling velocity

28

2.1 Regimes of Fluidization Turbulent Beds

when the superficial is continually increased through a bubbling fluidization bed, the bed start expanding, then the new regime called turbulent bed is started.

29

2.1 Regimes of Fluidization

30

2.1 Regimes of Fluidization Terminal Velocity

Terminal velocity is the particle velocity when the forces acting on particle is equilibrium

31

2.1 Regimes of Fluidization Freeboard and Furnace Height

- considered for design heating-surface area- considered for design furnace height- to minimize unburned carbon in bubbling bed - the freeboard heights should be exceed or

closed to the transport disengaging heights

32

2.2 Fast Fluidization Definition

33

2.2 Fast Fluidization Characteristics of Fast Beds

- non-uniform suspension of slender particle agglomerates or clusters moving up and down in a dilute- excellent mixing are major characteristic- low feed rate, particles are uniformly dispersed in gas stream- high feed rate, particles enter the wake of the other, fluid drag on the leading particle decrease, fall under the gravity until it drops on to trailing particle

34

2.3 Hydrodynamic regimes in a CFB

Lower Furnace below SA: Turbulent or bubbling

fluidized bed

Furnace Upper SA: Fast Fluidized Bed

Cyclone Separator :Swirl Flow

Return leg and lift leg : Pack bed and Bubbling Bed

Back Pass:Pneumatic Transport

35

2.4 Hydrodynamic Structure of Fast Beds Axial Voidage Profile

Bed Density Profile of 135 MWe CFB Boiler (Zhang et al., 2005)

Secondary air is fed

36

2.4 Hydrodynamic Structure of Fast Beds Velocity Profile in Fast Fluidized Bed

37

2.4 Hydrodynamic Structure of Fast Beds Velocity Profile in Fast Fluidized Bed

38

2.4 Hydrodynamic Structure of Fast Beds Particle Distribution Profile in Fast Fluidized Bed

39


40


Effect of SA injection on particle distribution by M.Koksal and F.Hamdullahpur (2004). The experimental CFB is pilot scale CFB. There are three orientations of SA injection; radial, tangential, and mixed

41


No SA, the suspension density is proportional

l to solid circulation rate

With SA 20% of PA, the solid particle is hold up

when compare to no SA

Increasing SA to 40%does not significant on

suspension density aboveSA injection point but the low zone is

denser than low SA ratio

Increasing solid circulationrate effect to both

lower and upper zoneof SA injection pointwhich both zone is

denser than lowsolid circulation rate

42

2.4 Hydrodynamic Structure of Fast Beds Effects of Circulation Rate on Voidage Profile

higher solid recirculation rate

43

2.4 Hydrodynamic Structure of Fast Beds Effects of Circulation Rate on Voidage Profile

higher solid recirculation rate

Pressure drop across the L-valve is proportional to solid recirculation rate

44

2.4 Hydrodynamic Structure of Fast Beds Effect of Particle Size on Suspension Density Profile

- Fine particle - - > higher suspension density- Higher suspension density - - > higher heat transfer- Higher suspension density - - > lower bed temperature

45

2.4 Hydrodynamic Structure of Fast Beds Core-Annulus Model

- the furnace may be spilt into two zones : core and annulus

Core - Velocity is above superficial velocity- Solid move upward

Annulus- Velocity is low to negative- Solids move downward

core

annulus

46

2.4 Hydrodynamic Structure of Fast Beds Core-Annulus Model

core

annulus

47

2.4 Hydrodynamic Structure of Fast Beds Core Annulus Model

- the up-and-down movement solids in the core and annulus sets up an internal circulation- the uniform bed temperature is a direct result of internal circulation

48

3. Combustion in CFB3.1 Coal properties for CFB boiler3.2 Stage of Combustion3.3 Factor Affecting Combustion Efficiency3.4 Combustion in CFB3.5 Biomass Combustion

49

3.1 Coal properties for CFB BoilerProperties- coarse size coal shall be crushed by coal crusher

- sizing is an importance parameter for CFB boiler improper size might result in combustion loss

- normal size shall be < 8 mm

50

3.2 Stage of CombustionA particle of solid fuel is injected into an FB undergoes the

following sequence of events:- Heating and drying- Devolatilization and volatile combustion- Swelling and primary fragmentation (for some types of coal)- Combustion of char with secondary fragmentation and attrition

51

3.2 Stages of Combustion Heating and Drying

- Combustible materials constitutes around 0.5-5.0% by weight

of total solids in combustor- Rate of heating 100 C/sec 1000 C/sec- Heat transfer to a fuel particle (Halder 1989)

52

3.2 Stages of CombustionDevolatilization and volatile combustion

- first steady release 500-600 C- second release 800-1000C- slowest species is CO (Keairns et al., 1984)- 3 mm coal take 14 sec to devolatilze

at 850 C (Basu and Fraser, 1991)

53

3.2 Stages of Combustion Char Combustion

2 step of char combustion1. transportation of oxygen to carbon surface2. Reaction of carbon with oxygen on the carbon surface

3 regimes of char combustion- Regime I: mass transfer is higher than kinetic rate- Regime II: mass transfer is comparable to kinetic rate- Regime III: mass transfer is very slow compared to kinetic rate

54

3.2 Stage of Combustion Communition Phenomena During Combustion

Volatile release cause the particle swell

Volatile release in non-porous particle cause the high internal pressure result in break a coal particle into fragmentation

Char burn under regime I, II, the pores increases in size weak bridge connection of carbon until it cant withstand the hydrodynamic force. It will fragment again call secondary fragmentation

Attrition, Fine particles from coarse particles through mechanical contract like abrasion with other particles

Char burn under regime I which is mass transfer is higher than kinetic trasfer. The sudden collapse or other type of second fragmentation call percolative fragmentationoccurs

55

3.3 Factor Affecting Combustion Efficiency Fuel Characteristics

the lower ratio of FC/VM result in higher combustion efficiency (Makansi, 1990), (Yoshioka and Ikeda,1990), (Oka, 2004) but the improper mixing could result in lower combustion efficiency due to prompting escape of volatile gas from furnace.

56

3.3 Factor Affecting Combustion Efficiency Operating condition (Bed Temperature)

- higher combustion temperature --- > high combustion efficiency

High combustion temperature result in high oxidation reaction, then burn out time decrease. So the combustion efficiency increase.

Limit of Bed temp

-Sulfur capture

-Bed melting

-Water tube failure

57

3.3 Factor Affecting Combustion Efficiency Fuel Characteristic (Particle size)

-The effect of this particle size is not clear

-Fine particle, low burn out time but the probability to be dispersed from cyclone the high

-Coarse size, need long time to burn out.

-Both increases and decreases are possible when particle size decrease

58

3.3 Factor Affecting Combustion Efficiency Operating condition (superficial velocity)

- high fluidizing velocity decrease combustion efficiency becauseIncreasing probability of small char particle be elutriated from circulation loop

- low fluidizing velocity cause defluidization, hot spot and sintering

59

3.3 Factor Affecting Combustion Efficiency Operating condition (excess air)

- combustion efficiency improve which excess air < 20%

Excess air >20% less significant improve combustion efficiency.

Combustion loss decrease significantly when excess air < 20%.

60

3.3 Factor Affecting Combustion EfficiencyOperating Condition

The highest loss of combustion result from elutriation of char particle from circulation loop. Especially, low reactive coal size smaller than 1 mm it can not achieve complete combustion efficiency with out fly ash recirculation system.However, the significant efficiency improve is in range 0.0-2.0 fly ash recirculation ratio.

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3.4 Combustion in CFB Boiler Lower Zone Properties

- This zone is fluidized by primary air constituting about 40-80% of total air.- This zone receives fresh coal from coal feeder and unburned coal from cyclone though return valve- Oxygen deficient zone, lined with refractory to protect corrosion- Denser than upper zone

62

3.4 Combustion in CFB Boiler Upper Zone Properties

- Secondary is added at interface between lower and upper zone- Oxygen-rich zone- Most of char combustion occurs- Char particle could make many trips around the furnace before they are finally entrained out through the top of furnace

63

3.4 Combustion in CFB Boiler Cyclone Zone Properties

- Normally, the combustion is small when compare to in furnace- Some boiler may experience the strong combustion in this zone which can be observe by rising temperature in the cyclone exit and loop seal

64

3.5 Biomass Combustion Fuel Characteristics

- high volatile content (60-80%)- high alkali content sintering, slagging, and fouling- high chlorine content corrosion

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3.5 Biomass Combustion Agglomeration

SiO2 melts at 1450 CEutectic Mixture melts at 874 C

Sintering tendency of fuel is indicated by the following (Hulkkonen et al., 2003)

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3.5 Biomass CombustionOptions for Avoiding the Agglomeration Problem

- Use of additives - china clay, dolomite, kaolin soil

- Preprocessing of fuels- water leaching

- Use of alternative bed materials- dolomite, magnesite, and alumina

- Reduction in bed temperature

67

3.5 Biomass Combustion Agglomeration

68

3.5 Biomass Combustion Fouling

- is sticky deposition of ash due to evaporation of alkali salt- result in low heat transfer to tube

69

3.5 Biomass Combustion Corrosion Potential in Biomass Firing

- hot corrosion - chlorine reacts with alkali metal from low temperature melting alkali chlorides- reduce heat transfer and causing high temperature corrosion

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4. Heat Transfer in CFB4.1 Gas to Particle Heat Transfer4.2 Heat Transfer in CFB

71

4.1 Gas to Particle Heat Transfer Mechanism of Heat Transfer

In a CFB boiler, fine solid particles agglomerate and form clusters or stand in a continuum of generally up-flowing gas containing sparsely dispersed solids. The continuum is called the dispersed phase, while the agglomerates are called the cluster phase.

The heat transfer to furnace wall occurs through conduction from particle clusters, convection from dispersed phase, and radiation from both phase.

72

4.1 Heat Transfer in CFB Boiler Effect of Suspension Density and particle size

Heat transfer coefficient is proportional to the square root of suspension density

73

4.1 Heat Transfer in CFB Boiler Effect of Fluidization Velocity

No effect from fluidization velocity when leave the suspension density constant

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4.1 Heat Transfer in CFB Boiler Effect of Vertical Length of Heat Transfer Surface

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4.1 Heat Transfer in CFB Boiler Effect of Bed Temperature

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4.1 Heat Transfer in CFB Boiler Heat Flux on 300 MW CFB Boiler (Z. Man, et. al)

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4.1 Heat Transfer in CFB Boiler Heat transfer to the walls of commercial-size

Low suspension density low heat transfer to the wall.

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4.1 Heat Transfer in CFB Boiler Circumferential Distribution of Heat Transfer Coefficient

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5 Design of CFB Boiler 5.1 Design and Required Data 5.2 Combustion Calculation 5.3 Heat and Mass Balance 5.4 Furnace Design 5.5 Cyclone Separator

82

5.1 Design and Required DataThe design and required data normally will be specify by owner or client. The basic design data and required data are;

Design Data :- Fuel ultimate analysis - Weather condition- Feed water quality - Feed water properties

Required Data :- Main steam properties - Flue gas temperature- Flue gas emission - Boiler efficiency

83

5.2 Combustion Calculation Base on the design and required data the following data

can be calculated in this stage :- Fuel flow rate - Combustion air flow rate- Fan capacity - Fuel and ash handling capacity- Sorbent flow rate

84

5.3 Heat and Mass Balance

Fuel and sorbent

Unburned in bottom ash

Feed water

Combustion air

Main steam

Blow down

Flue gas

Moisture in fuel and sorbent

Unburned in fly ash

Moisture in combustion air

Radiation

Heat input

Heat output

85

5.3 Heat and Mass Balance Mass Balance

Fuel and sorbent

bottom ash

Solid Flue gas

Moisture in fuel and sorbent

fly ash

Make up bed material

bottom ash

Fuel and sorbent

Make up bed material

Solid in Flue gas

fly ash

Mass output

Mass input

86

5.4 Furnace Design The furnace design include:1. Furnace cross section2. Furnace height3. Furnace opening

1. Furnace cross sectionCriteria- moisture in fuel- ash in fuel- fluidization velocity- SA penetration- maintain fluidization in lower zone at part load

87

5.4 Furnace Design2. Furnace height

Criteria- Heating surface- Residual time for sulfur capture

3. Furnace openingCriteria- Fuel feed ports- Sorbent feed ports- Bed drain ports- Furnace exit section

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5.5 Cyclone Separator 6.1 Theory 6.2 Critical size of particle

89

5.5 Cyclone Separator The centrifugal force on the particle entering the cyclone

is

The drag force on the particle can be written as

Under steady state drag force = centrifugal force

90

5.5 Cyclone Separator

Vr can be considered as index of cyclone efficiency, from above equation the cyclone efficiency will increase for :

- Higher entry velocity- Large size of solid - Higher density of particle- Small radius of cyclone- low value of viscosity of gas

91

5.5 Cyclone Separator The particle with a diameter larger than theoretical cut-

size of cyclone will be collected or trapped by cyclone while the small size will be entrained or leave a cyclone

Actual operation, the cut-off size diameter will be defined as d50 that mean 50% of the particle which have a diameter more than d50 will be collected or captured.

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6. Operation

93

Content6.1 Before start6.2 Grid pressure drop test6.3 Cold Start6.4 Normal Operation6.5 Normal Shutdown6.6 Hot Shutdown6.7 Hot Restart6.8 Malfunction and Emergency

94

6.1 Before Start all maintenance work have been completely done All function test have been checked cooling water system is operating compressed air system is operating Make up water system Deaerator system Boiler feed water pump Condensate system Oil and gas system Drain and vent valves Air duct, flue gas duct system

95

6.1 Before Start Blow down system Sand feeding system Lime stone feeding system Solid fuel system Ash drainage system Control and safety interlock system

96

6.2 Grid Pressure Drop Test For check blockage of grid

nozzle Furnace set point = 0 Test at every PA. load Compare to clean data or design

data Shall not exceed 10% from

design data Perform in cold condition

Pw

Pb

FI

Pf= 0

97

6.3 Cold Start

Fill boiler

Boiler Interlock

Start up Burner

Feed Solid Fuel

Boiler Warm Up

Purge

Start Fan

Feed Bed Material

Raise to MCR

-100 mm normal level

ID,HP,SA,PA

Low level cut off

300 S

Tb 150-200 C

30-50 mbar, Tb 550-600 C

98

Fill Boiler

-Close all water side drain valve

-Open all air vent valve at drum and superheat

-Open start up vent valve 10-15%

-Slowly feed water to drum until level 1/3 of sigh glass

99

Start Fan

1.Start ID.Fan

2.Start HP Blower

3.Start SA.Fan

4.Start PA.Fan

100

Boiler Interlock

Emergency stop in order

Furnace P. < Max (2/3)

ID. Fan running

HP Blower start

Drum level > min (2/3)

SA. Fan running

PA. Fan running

HP. Blower P. > min

PA. Flow to grid > min

Trip Solid Fuel

Flue gas T after Furnace < max

Trip Soot Blower

Trip Oil

Trip Sand

Trip Lime Stone

Trip Bottom Ash

101

Purge To carry out combustible gases To assure all fuel are isolated

from furnace Before starting first burner for

cold start If bed temp < 600 C or OEM

recommend and no burner in service

Total air flow > 50% 300 sec for purging time

102

Purge

NFPA85: CFB Boiler purge logic

103

Start up burner Help to heat up bed temp to allowable temperature for

feeding solid fuel Will be stopped if bed temp > 850 C Before starting, all interlock have to passed Main interlock Oil pressure > minimum Control air pressure > minimum Atomizing air pressure > minimum

104

Start up burnerNFPA85 - Typical burner safety for CFB boiler

105

Drum and DA low level cut-off Test for safety During burner are operating Open drain until low level Signal feeding are not allow Steam drum low level = chance

to overheating of water tube DA low level = danger for BFWP

106

Boiler warm up Gradually heating the boiler to reduce the effect of

thermal stress on pressure part, refractory and drum swell Increase bed temp 60-80 C/hr by adjusting SUB Control flue gas temperature

10% MCR Close vent valves at drum and SH when pressure > 2 bar Continue to increase firing rate according to

recommended start up curve Operate desuperheater when steam temperature are with

in 30 C of design point Slowly close start up and drain valve while maintain steam

flow > 10% MCR

107

Feed bed material Bed material should be sand which size is according to

recommended size Start feed sand when bed temp >150 C Do not exceed firing rate >30% if bed pressure

108

Feed solid fuel Must have enough bed material Bed temperature > 600 C or manufacturer

recommendation or refer to NFPA85 Appendix H Pulse feed every 90 s Placing lime stone feeding, ash removal system

simultaneously Slowly decrease SUB firing rate while increasing solid fuel

feed rate Stop SUB one by one, observe bed temperature increasing Turn to auto mode control

109

Rise to MCR Continue rise pressure and temperature according to

recommended curve until reach design point Drain bottom ash when bed pressure >45-55 mbar Slowly close start up valve Monitor concerning parameters

110

6.4 Normal Operation

Increasing- manual increase air flow- manual increase fuel flow- monitor excess oxygen- monitor steam pressure

Decreasing- manual decrease air flow- manual decrease fuel flow- monitor excess oxygen- monitor steam pressure

Changing Boiler load (manual)

111


Furnace and emssion- monitor fluidization in hot loop- monitor gas side pressure drop- monitor bed pressure- monitor bed temperature-monitor wind box pressure- monitor SOx, Nox, CO

Furnace and Emission Monitoring

112


Bottom ash drain- automatic or manual draining of bottom ash shall be judged by commissioning engineer for the design fuel. - when fuel is deviated from the design, operator can be judge by themselves that draining need to perform or not.- bed pressure is the main parameter to start draining

Soot blower- initiate soot blower to clean the heat exchanger surface in convective part- frequent of soot blowing depend on the degradation of heat transfer coefficient.- normally 10 C higher than normal value of exhaust temperature

Bottom ash and Soot Blower

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6.4 Normal Operation Boiler Walk Down

- boiler expansion joint- Boiler steam drum- Boiler penthouse- Safety valve- Boiler lagging- Spring hanger- Valve and piping- Damper position- Loop seal - Bottom screw- Combustion chamber- Fuel conveyor

114

6.4 Normal Operation Sizing Quality

- crushed coal, bed material, lime stone and bottom ash sizing shall be periodically checked by the operator- sieve sizing shall be performed regularly to make sure that their sizing is in range of recommendation

115

6.5 Normal Shut Down1. Reduce boiler load to 50% MCR2. Place O2 control in manual mode3. Monitor bed temperature4. Continue reducing load according to shut down curve5. Maintain SH steam >20 C of saturation temperature6. Start burner when bed temperature 650 C8. Decrease SUB firing rate according to suggestion curve9. Maintain drum level in manual mode10. Stop solid fuel, line stone, sand feeding system

116

6.5 Normal Shut Down11. Maintain drum level near upper limit12. Continue fluidizing the bed to cool down the system at 2

C/min by reducing SUB firing rate13. Stop SUB at bed temperature 350 C14. Continue fluidizing until bed temperature reach 300 C15. Slowly close inlet damper of PAF and SAF so that IDF

can control furnace pressure in automatic mode16. Stop all fan after damper completely closed17. Stop HP blower 30 S after IDF stopped18. Stop chemical feeding system when BFWP stop19. Continue operate ash removal system until it empty

117

6.5 Normal Shut Down20. Open vent valve at drum and SH when drum pressure

reach 1.5-2 bar21. Open manhole around furnace when bed temp < 300 C

118

6.6 Emergency Shut down Boiler can be held in hot stand by condition about 8 hrs Hot condition is bed temp >650 C otherwise follow cold

star up procedure Boiler load should be brought to minimum Stop fuel feeding Wait O2 increase 2 time of normal operation Stop air to combustion chamber to minimize heat loss

119

6.7 Hot restart Purge boiler if bed temperature < 600 C Start SUBs if bed temperature > 500 C Monitor bed temperature rise If bed temperature does not rise after pulse feeding solid

fuel. stop feeding and start purge

120

6.8 Malfunction and Emergency Bed pressure Bed temperature Circulation Tube leak Drum level

121

Bed PressureBed pressure is an one of importance parameter that effect on boiler efficiency and reliability.

Measured above grid nozzle about 20 cm.

Pw

Pb

FI

Pf= 0

122

Bed Pressure Effect of low bed pressure

- poor heat transfer- boiler responds- high bed temperature- damage of air nozzle and refractory Effect of high bed pressure

- increase heat transfer- more efficient sulfur capture- more power consumption of fan

123

Bed Pressure Cause of low bed pressure

- loss of bed material- too fine of bed materials- high bed temperature

Cause of high bed pressure- agglomeration - too coarse of bed material

124

Bed Temperature Measured above grid nozzle about

20 cm Measured around the furnace cross

section It is the significant parameter to

operate CFB boiler

125

Bed temperature Effect of high bed temperature

- ineffective sulfur capture- chance of ash melting- chance of agglomeration- chance to damage of air nozzle

126

Bed temperature Cause of high bed temperature

- low bed pressure- too coarse bed material- too coarse solid fuel- improper drain bed material- low volatile fuel- improper air flow adjustment

127

Circulation Circulation is particular

phenomena of CFB boiler. Bed material and fuel are

collected at cyclone separator Return to the furnace via loop

seal HP blower supply HP air to

fluidize collected materials to return to furnace

128

Circulation Effect of malfunction circulation

- No circulation result in forced shut down- high rate of circulation - high circulation rate need more power of blower- low rate of circulation

129

Circulation Cause of malfunction circulation

- insufficiency air flow to loop seal nozzle- insufficient air pressure to loop seal- plugging of HP blower inlet filter- blocking or plugging of loop seal nozzle-

130

Tube leak Water tube leak

- furnace pressure rise- bed temperature reduce- stop fuel feeding- open start up valve- dont left low level of drum- continue feed water until flue gas temp < 400 C- continue combustion until complete- small leak follow normal shut down

131

Drum levelSudden loss of drum level

- when the cause is known and immediately correctable before level reach minimum allowable. Reestablish steam drum level to its normal value and continue boiler operation-if the cause is not known. Start immediate shut down according to emergency shut down procedure

132

Drum levelGradual loss of drum level

- boiler load shall be reduced to low load- find out and correct the problem as soon as possible- if can not maintain level and correct the problem, boiler must be taken out of service and normal shut down procedure shall be applied.

133

7. Maintenance

134

Before maintenance work Make sure that all staff are understand about safety

instruction for doing CFB boiler maintenance work Make sure that all maintenance and safety equipments

shall be a first class

135

Overview Boiler Maintenance

Refractory and tube are the main area that need to be checked

136

6.1 Windbox Inspection Inspect sand inside windbox

after shutdown Drain pipe

Crack Air gun pipe

Refractory Crack, wear and fall down inspect

by hammer(knocking) if burner is under bed design

Drain pipe

137

6.2 Furnace Inspection Nozzle :

Wear Fall-off

Refractory Crack, wear and fall down inspect

by hammer knocking if burner is under bed design

Feed fuel port Wear Crack

Burner

Refractory

Burner Feed FuelNozzle

138

6.2 Furnace Inspection Limestone port

Crack Deform Refractory damage at connection

between port and refractory

Secondary & Recirculation Air port Crack Deform Refractory damage at connection

between port and refractory

Bed Temperature Check thermo well deformation Check wear

Secondary & Recirculation Air port

139

6.3 Kick-Out Inspection Refractory

Wear Crack and fall down by

hammer(knocking)

Water tube Wear Thickness

140

6.3 Kick-Out Inspection Water Tube:

Thickness measuring Erosion at corner CO Corrosion due to incomplete

combustion at fuel feed side. Defect from weld build up Water tube sampling for internal

check every 3 years

Inside water tube inspect by borescope

welded build up excessive metal because use welding rod size bigger than tube thickness

141

6.4 Superheat I (Wingwall) Water Tube:

Thickness measuring Erosion at tube connection

Refractory Crack and fall down by

hammer(knocking)

Guard Crack fall down

142

6.4 Superheat I (Omega Tube) Offset Water Tube:

Thickness measuring Erosion at offset tube

SH tube Thickness measuring

Omega Guard Crack fall down

Omega Guard

Offset Water Tube

143

6.5 Roof Water Tube:

Thickness measuring Erosion

Refractory Crack, wear and fall down by

hammer(knocking)

144

6.6 Inlet Separator Water Tube:

Thickness measuring near opening have more erosion than another tube because of high velocity of flue gas

Refractory Crack, wear and fall down by

hammer(knocking)

145

6.7 Steam Drum Surface :

Surface were black by magnetite

Deposits Deposits at bottom drum need to

check chemical analysis

Cyclone Separator Loose

Demister Blowdown hole

Plugging

U-Clamp Loose

Deposits at bottom drum

146

6.8 Separator Central Pipe:

Deformation Crack

Refractory Wear at impact zone due to high

impact velocity Crack and fall down by

hammer(knocking)

147

6.9 Outlet Separator Water Tube

Tube Thickness Erosion

Outlet Central Pipe: Support or Hook

RefractoryCrack and fall down by hammer(knocking)

148

6.10 Screen Tube Water Tube

Thickness measuring upper part of screen tube at corner have more erosion than another area because of high velocity of flue gas

Guard Loose

Refractory Crack and fall down by

hammer(knocking)

Weld build up or install guard to prevent tube erosion

upper part of screen tube at corner have more erosion

149

6.11 Superheat Tube Tube

Thickness measuring High erosion between SH tube and

wall Steam erosion due to improper soot

blower

Guard Fall down Crack

150

6.12 Economizer Water Tube

Thickness measuring High erosion between economizer

tube and wall Steam erosion due to improper soot

blower

Guard Fall down Crack

Guard

Install guard to prevent tube erosion

151

6.13 Air Heater Tube

Cold end corrosion due to high concentrate SO3 in flue gas

Steam erosion due to improper soot blower

Inlet air heater

Cold end corrosion due to SO3 in fluegas

152

8. Basic Boiler Safety

153

Warning

Operating or maintenance procedure which, if not as described could result in injured death

or damage of equipment

154

General safety precaution Electrical power shall be turned off before performing

installation or maintenance work. Lock out, tag out shall be indicated All personal safety equipment shall be suit for each work Never direct air water stream into accumulation bed

material or fly ash. This will become breathing hazard Always provide safe access to all equipment ( plant from,

ladders, stair way, hand rail Post appropriate caution, warning or danger sign and

barrier for alerting non-working person Only qualify and authorized person should service

equipment or maintenance work

155

General safety precaution Do not by-pass any boiler interlocks Use an filtering dust mask when entering dust zone Do not disconnect hoist unless you have made sure that

the source is isolated

156

Equipment entry Never entry confine space until is has been cooled, purged

and properly vented When entering confine space such as separator, loop seal

furnace be prepared for falling material Always lock the damper, gate or door before passing

through them Never step on accumulation of bottom ash or fly ash. Its

underneath still hot Never use toxic fluid in confine space Use only appropriate lifting equipment when lift or move

equipment

157

Equipment entry Stand by personnel shall be positioned outside a confine

space to help inside person incase of emergency Be carefully aware the chance of falling down when enter

cyclone inlet or outlet. Don not wear contact lens with out protective eye near

boiler, fuel handing, ash handing system. Airborne particle can cause eye damage Don not enter loop seal with out installing of cover over

loop seal downcomer to prevent falling material from cyclone

158

Operating precautionsCFB boiler process Use planks on top of bed materials after boiler is cooled

down. This will prevent the chance of nozzle plugging Do not open any water valve when boiler is in service Do not operate boiler with out O2 analyzer Do not use downcomer blown donw when pressure > 7

bar otherwise loss of circulation may occure Do not operate CFB boiler without bed material When PA is started. PA flow to grid must be increase to

above minimum limit to fully fluidized bed maerial Do not operate CFB boiler with bed pressure > 80mbar.

This might be grid nozzle plugging

159

Operating precautions on cold start up the rate of chance in saturated steam shall

not exceed 2 C/min On cold start up the change of flue gas temp at cyclone

inlet shall not exceed 70 C/min Do not add feed water to empty steam drum with

different temperature between drum metal and feed water greater than 50 C All fan must be operated when add bed material

160

Operating precautionsRefractory When entering cyclone be aware a chance of falling down Refractory retain heat for long period. Be prepared for hot

surface when enter this area An excessive thermal cycle will reduce the life cycle of

refractory After refractory repair, air cure need to apply about 24 hr

or depend on manufacturer before heating cure Heating cure shall be done carefully otherwise refractory

life will be reduced

161

Operating precautionsSolid Fuel Chemical analysis of all solid fuel shall be determined for

first time and compared with OEM standard Sizing is important Burp feeding shall be performed during starting feeding

solid fuel instead of continuous feeding

162

9. Basic CFB Boiler Control

163

Basic control Furnace control Main pressure control Main steam pressure control Drum level control Feed tank control Solid fuel control Primary air control Secondary air control Oxygen control

164

Basic control Simple feedback control

PRIMARY VARIABLE

XT

K

A T A

f(x)

SET POINTPROCESS

MANIPULATED VARIABLE

165

Basic control Simple feed forward plus feedback control

PR IM ARY VARIABLE

XT

YT

SECO NDARYVARIABLE

A T A

f(x)

MA NIP ULATED VARIAB LE

P ROCE SS

S ET POINT

K

166

Basic control Simple cascade control

PRIMARY VARIABLE

XT

ZT

K

K

SET POINTA AT

PROCESS

f(x)

MANIPULATED VARIABLE

SECONDARYVARIABLE

167

Basic control

CO

SP

PV

PID

Control Mode of PID-MAN (Manual)-AUT (Automatic)-CAS (Cascade)

Signal to open0-15 m3/h

0-100% (closed open)

4-20 mAElectrical signal 4-20 mA

Eng. Unit 0-15 m3/h

Percent 0-100 % 0-100%

168

Feed water control

LT

PT

PIDPID

Make up water

Heating steam

Pressure

-Manual mode 0-100% heating steam valve position

-Auto mode, specify pressure set point

-Temperature compensation

Level

-Manual mode 0-100% make up water valve

-Auto mode, specify level set point

-Temperature compensation

-Protection, high level over flow

169

Drum Level control

DP feed water pump

Control valve

A, SP

M, 0-100%

Main steam flowMain steam Pressure

Manual mode, 0-100% control valve

Auto mode, specify drum level. Automatically adjust valve

Protection

-lower limit

-2/3 principle

- 10 s delay

-Close steam valve for low level

170

Main steam pressure control

SP

PV

FF

CO

171

Combustion Calculation

SA SPPA SP

Total air SP Total Fuel SP

Fuel1 SP Fuel3 SPFuel2 SP

PA.Fan Conveyor1 Conveyor2 Conveyor3SA.Fan

X -

Main steam Pressure

172

Solid Fuel Control

M

WT

PIDCascade

Auto

Manual

Manual : speed of coal conveyor is specified by operator

Auto : operator specify fuel flow load

Cascade: fuel flow set point calculated by main steam pressure control

173

Primary air control

M

PID

FT

AutoCascade

PVManual

Manual: position of damper is specified

Auto: desired air flow is specified by operator

Cascade: set point is calculated from master combustion

Flow (interlock) > minimum

PA wind box P > minimum

PA running

174

Secondary air control

M

PIDAutoCascade

Manual

FT

FT

PT

Manual

Manual

PID Auto

Cascade

Lower SA

Upper SA

FTPV

175

HP Blower Control Pressure is controlled by control valve Control valve is connected to primary air It will release the air to primary air duct if pressure higher

than set point If operating unit stop due to disturbance or pressure fall

down, stand by unit shall be automatically started Pressure should be higher than 300 mbar, boiler interlock Pressure < 350 mbar parallel operation start

176

Furnace Pressure control

M

PID

PT

Auto

Furnace pressure

Manual

PID

Manual

Auto

2/3 furnace P < max (35 mbar)

177

Lime stone control Lime stone can be control by lime stone/ fuel flow ratio SO2 feed back control Manual feed rate

178

Fuel oil control

M

A

Pressure control

Pressure control valve

Flow control valve

Auto

Manual

179

Referenced Prabir Basu , Combustion and gasification in fluidized bed, 2006 Fluidized bed combustion, Simeon N. Oka, 2004 Nan Zh., et al, 3D CFD simulation of hydrodynamics of a 150 MWe circulating fluidized bed

boiler, Chemical Engineering Journal, 162, 2010, 821-828 Zhang M., et al, Heat Flux profile of the furnace wall of 300 MWe CFB Boiler, powder

technology, 203, 2010, 548-554 Foster Wheeler, TKIC refresh training, 2008 M. Koksal and F. Humdullahper , Gas Mixing in circulating fluidized beds with secondary

air injection, Chemical engineering research and design, 82 (8A), 2004, 979-992

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