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Content Day1
1. Introduction to CFB
2. Hydrodynamic of CFB
3. Combustion in CFB
4. Heat Transfer in CFB
5. Basic design of CFB
6. Operation
7. Maintenance
8. Basic Boiler Safety
9. Basic CFB control
3
Objectiveó To understand the typical arrangement in CFB
ó To understand the basic hydrodynamic of CFB
ó To understand the basic combustion in CFB
ó To understand the basic heat transfer in CFB
ó To understand basic design of CFB
ó To understand theory of cyclone separator
Know Principle Solve Everything
5
1.1 Development of CFB ó 1921, Fritz Winkler, Germany, Coal Gasification
ó 1938, Waren Lewis and Edwin Gilliland, USA, Fluid Catalytic Cracking, Fast Fluidized Bed
ó 1960, Douglas Elliott, England, Coal Combustion, BFB
ó 1960s, Ahlstrom Group, Finland, First commercial CFB boiler, 15 MWth, Peat
7
1.2 Typical Component of CFB Boiler
Wind 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
8
1.2 Typical Component of CFB Boiler
Bottom 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.
9
1.2 Typical Component of CFB Boiler
HP 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 Boiler
Cyclone 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 Boiler
Evaporative or Superheat Wing Wall
located on upper zone of furnace
it can be both of evaporative or SH panel
lower portion covered by erosion resistant materials
12
1.2 Typical Component of CFB Boiler
Fuel 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 Boiler
Refractory
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 Boiler
Solid 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 Boiler
Kick 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 Boiler
Lime 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.
19
1.3 Advantage of CFB Boiler
ó High Combustion Efficiency
- Good solid mixing
- Low unburned loss by cyclone, fly ash recirculation
- Long combustion zone
ó In situ sulfur removal
ó Low nitrogen oxide emission
20
2. Hydrodynamic in CFB2.1 Regimes of Fluidization
2.2 Fast Fluidized Bed
2.3 Hydrodynamic Regimes in CFB
2.4 Hydrodynamic Structure of Fast Beds
21
2.1 Regimes of Fluidization
ó 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
ó Particle Classification
<130
<180
<250
<600
CFB1
Size (micron)
<590<25025%
>420>100100%
<840<45050%
75%
100%
Distribution
<1190<550
<1680<1000
BFBCFB2
25
2.1 Regimes of Fluidization
ó 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
ó Bubbling Fluidization Beds
Minimum fluidization velocity is velocity where the fluid drag is equal to a particle’s weight less its buoyancy.
27
2.1 Regimes of Fluidization
ó Bubbling Fluidization Beds
For B and D particle, the bubble is started when superficial gas is higher than minimum fluidization velocity
But for group A particle the bubble is started when superficial velocity is higher than minimum bubbling velocity
28
2.1 Regimes of Fluidization
ó 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.
30
2.1 Regimes of Fluidization
ó Terminal Velocity
Terminal velocity is the particle velocity when the forces acting on particle is equilibrium
31
2.1 Regimes of Fluidization
ó 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
33
2.2 Fast Fluidization
ó 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 Bedsó Axial Voidage Profile
Bed Density Profile of 135 MWe CFB Boiler (Zhang et al., 2005)
Secondary air is fed
40
2.4 Hydrodynamic Structure of Fast Beds
ó Particle Distribution Profile in Fast Fluidized Bed
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
2.4 Hydrodynamic Structure of Fast Beds
ó Particle Distribution Profile in Fast Fluidized Bed
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
ó Effects of Circulation Rate on Voidage Profile
higher solid recirculation rate
43
2.4 Hydrodynamic Structure of Fast Beds
ó 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
ó 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
ó 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
47
2.4 Hydrodynamic Structure of Fast Beds
ó 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 boiler
3.2 Stage of Combustion
3.3 Factor Affecting Combustion Efficiency
3.4 Combustion in CFB
3.5 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 Combustion
A 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
ó 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 Combustion
Devolatilization 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
ó Char Combustion
2 step of char combustion
1. transportation of oxygen to carbon surface
2. 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
ó 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 àweak bridge connection of carbon until it can’t 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
ó 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
ó 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
ó 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
ó Operating condition (superficial velocity)
- high fluidizing velocity decrease combustion efficiency because
Increasing 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
ó 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 Efficiency
Operating 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.
61
3.4 Combustion in CFB Boiler
ó 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
ó 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
ó 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
ó Fuel Characteristics
- high volatile content (60-80%)
- high alkali content à sintering, slagging, and fouling
- high chlorine content à corrosion
65
3.5 Biomass Combustion
ó Agglomeration
SiO2 melts at 1450 C
Eutectic Mixture melts at 874 C
Sintering tendency of fuel is indicated by the following
(Hulkkonen et al., 2003)
66
3.5 Biomass Combustion
Options 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
68
3.5 Biomass Combustion
ó Fouling
- is sticky deposition of ash due to evaporation of alkali salt
- result in low heat transfer to tube
69
3.5 Biomass Combustion
ó Corrosion Potential in Biomass Firing
- hot corrosion
- chlorine reacts with alkali metal à from low temperature melting alkali chlorides
- reduce heat transfer and causing high temperature corrosion
71
4.1 Gas to Particle Heat Transfer
ó 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
ó 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
ó Effect of Fluidization Velocity
No effect from fluidization velocity when leave the suspension density constant
79
4.1 Heat Transfer in CFB Boiler
ó Heat transfer to the walls of commercial-size
Low suspension density low heat transfer to the wall.
81
5 Design of CFB Boiler
ó 5.1 Design and Required Data
ó 5.2 Combustion Calculation
ó 5.3 Heat and Mass Balance
ó 5.4 Furnace Design
ó 5.5 Cyclone Separator
82
5.1 Design and Required Data
The 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
ó 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
ó 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 Designó The furnace design include:
1. Furnace cross section
2. Furnace height
3. Furnace opening
1. Furnace cross section
Criteria
- 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 opening
Criteria
- Fuel feed ports
- Sorbent feed ports
- Bed drain ports
- Furnace exit section
89
5.5 Cyclone Separator
ó The centrifugal force on the particle entering the cyclone is
ó The drag force on the particle can be written as
ó Under steady state drag force = centrifugal force
90
5.5 Cyclone Separator
ó 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
ó 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
ó 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.
93
Content
6.1 Before start
6.2 Grid pressure drop test
6.3 Cold Start
6.4 Normal Operation
6.5 Normal Shutdown
6.6 Hot Shutdown
6.7 Hot Restart
6.8 Malfunction and Emergency
94
6.1 Before Start
ó all maintenance work have been completely done
ó All function test have been checked
ó cooling water system is operating
ó compressed air system is operating
ó Make up water system
ó Deaerator system
ó Boiler feed water pump
ó Condensate system
ó Oil and gas system
ó Drain and vent valves
ó Air duct, flue gas duct system
95
6.1 Before Start
ó Blow down system
ó Sand feeding system
ó Lime stone feeding system
ó Solid fuel system
ó Ash drainage system
ó Control and safety interlock system
96
6.2 Grid Pressure Drop Testó For check blockage of grid
nozzle
ó Furnace set point = 0
ó Test at every PA. load
ó Compare to clean data or design data
ó Shall not exceed 10% from design data
ó 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
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
Purgeó To carry out combustible gases
ó To assure all fuel are isolated from furnace
ó Before starting first burner for cold start
ó If bed temp < 600 C or OEM recommend and no burner in service
ó Total air flow > 50%
ó 300 sec for purging time
103
Start up burner
ó Help to heat up bed temp to allowable temperature for feeding solid fuel
ó Will be stopped if bed temp > 850 C
ó Before starting, all interlock have to passed
ó Main interlock
ó Oil pressure > minimum
ó Control air pressure > minimum
ó Atomizing air pressure > minimum
105
Drum and DA low level cut-offó Test for safety
ó During burner are operating
ó Open drain until low level
ó Signal feeding are not allow
ó Steam drum low level = chance to overheating of water tube
ó DA low level = danger for BFWP
106
Boiler warm up
ó Gradually heating the boiler to reduce the effect of thermal stress on pressure part, refractory and drum swell
ó Increase bed temp 60-80 C/hr by adjusting SUB
ó Control flue gas temperature <470 C until steam flow > 10% MCR
ó Close vent valves at drum and SH when pressure > 2 bar
ó Continue to increase firing rate according to recommended start up curve
ó Operate desuperheater when steam temperature are with in 30 C of design point
ó Slowly close start up and drain valve while maintain steam flow > 10% MCR
107
Feed bed material
ó Bed material should be sand which size is according to recommended size
ó Start feed sand when bed temp >150 C
ó Do not exceed firing rate >30% if bed pressure <20 mbar otherwise overheating may occur for refractory and nozzle
ó Continue feed bed material unit it reach 30 mbar
108
Feed solid fuel
ó Must have enough bed material
ó Bed temperature > 600 C or manufacturer recommendation or refer to NFPA85 Appendix H
ó Pulse feed every 90 s
ó Placing lime stone feeding, ash removal system simultaneously
ó Slowly decrease SUB firing rate while increasing solid fuel feed rate
ó Stop SUB one by one, observe bed temperature increasing
ó Turn to auto mode control
109
Rise to MCR
ó Continue rise pressure and temperature according to recommended curve until reach design point
ó Drain bottom ash when bed pressure >45-55 mbar
ó Slowly close start up valve
ó Monitor concerning parameters
110
6.4 Normal Operation
ó Increasing
- manual increase air flow
- manual increase fuel flow
- monitor excess oxygen
- monitor steam pressure
ó Decreasing
- manual decrease air flow
- manual decrease fuel flow
- monitor excess oxygen
- monitor steam pressure
Changing Boiler load (manual)
111
6.4 Normal Operation
ó 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
6.4 Normal Operation
ó 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
ó 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
113
6.4 Normal Operation
ó 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
ó 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 Down
1. Reduce boiler load to 50% MCR
2. Place O2 control in manual mode
3. Monitor bed temperature
4. Continue reducing load according to shut down curve
5. Maintain SH steam >20 C of saturation temperature
6. Start burner when bed temperature <750 C
7. Empty solid fuel and lime stone with bed material >650 C
8. Decrease SUB firing rate according to suggestion curve
9. Maintain drum level in manual mode
10. Stop solid fuel, line stone, sand feeding system
116
6.5 Normal Shut Down
11. Maintain drum level near upper limit
12. Continue fluidizing the bed to cool down the system at 2 C/min by reducing SUB firing rate
13. Stop SUB at bed temperature 350 C
14. Continue fluidizing until bed temperature reach 300 C
15. Slowly close inlet damper of PAF and SAF so that IDF can control furnace pressure in automatic mode
16. Stop all fan after damper completely closed
17. Stop HP blower 30 S after IDF stopped
18. Stop chemical feeding system when BFWP stop
19. Continue operate ash removal system until it empty
117
6.5 Normal Shut Down
20. Open vent valve at drum and SH when drum pressure reach 1.5-2 bar
21. Open manhole around furnace when bed temp < 300 C
118
6.6 Emergency Shut down
ó Boiler can be held in hot stand by condition about 8 hrs
ó Hot condition is bed temp >650 C otherwise follow cold star up procedure
ó Boiler load should be brought to minimum
ó Stop fuel feeding
ó Wait O2 increase 2 time of normal operation
ó Stop air to combustion chamber to minimize heat loss
119
6.7 Hot restart
ó Purge boiler if bed temperature < 600 C
ó Start SUBs if bed temperature > 500 C
ó Monitor bed temperature rise
ó If bed temperature does not rise after pulse feeding solid fuel. stop feeding and start purge
120
6.8 Malfunction and Emergency
ó Bed pressure
ó Bed temperature
ó Circulation
ó Tube leak
ó Drum level
121
Bed Pressure
Bed 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
ó Effect of low bed pressure
- poor heat transfer
- boiler responds
- high bed temperature
- damage of air nozzle and refractory
ó Effect of high bed pressure
- increase heat transfer
- more efficient sulfur capture
- more power consumption of fan
123
Bed Pressure
ó Cause of low bed pressure
- loss of bed material
- too fine of bed materials
- high bed temperature
ó Cause of high bed pressure
- agglomeration
- too coarse of bed material
124
Bed Temperatureó Measured above grid nozzle about
20 cm
ó Measured around the furnace cross section
ó It is the significant parameter to operate CFB boiler
125
Bed temperature
ó Effect of high bed temperature
- ineffective sulfur capture
- chance of ash melting
- chance of agglomeration
- chance to damage of air nozzle
126
Bed temperature
ó 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
ó Circulation is particular phenomena of CFB boiler.
ó Bed material and fuel are collected at cyclone separator
ó Return to the furnace via loop seal
ó HP blower supply HP air to fluidize collected materials to return to furnace
128
Circulation
ó 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
ó 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
ó Water tube leak
- furnace pressure rise
- bed temperature reduce
- stop fuel feeding
- open start up valve
- don’t 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 level
Sudden 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 level
Gradual 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.
134
Before maintenance work
ó Make sure that all staff are understand about safety instruction for doing CFB boiler maintenance work
ó Make sure that all maintenance and safety equipments shall be a first class
136
6.1 Windbox Inspectionó Inspect sand inside windbox
after shutdown
ó Drain pipeó Crack
ó Air gun pipe
ó Refractory ó Crack, wear and fall down inspect
by hammer(knocking) if burner is under bed design
Drain pipe
137
6.2 Furnace Inspectionó Nozzle :
ó Wear
ó Fall-off
ó Refractoryó Crack, wear and fall down inspect
by hammer knocking if burner is under bed design
ó Feed fuel portó Wear
ó Crack
ó Burner
Refractory
Burner Feed FuelNozzle
138
6.2 Furnace Inspectionó Limestone port
ó Crack
ó Deform
ó Refractory damage at connection between port and refractory
ó Secondary & Recirculation Air portó Crack
ó Deform
ó Refractory damage at connection between port and refractory
ó Bed Temperatureó Check thermo well deformation
ó Check wear
Secondary & Recirculation Air port
139
6.3 Kick-Out Inspectionó Refractory
ó Wear
ó Crack and fall down by hammer(knocking)
ó Water tubeó Wear
ó Thickness
140
6.3 Kick-Out Inspectionó Water Tube:
ó Thickness measuring
ó Erosion at corner
ó CO Corrosion due to incomplete combustion at fuel feed side.
ó Defect from weld build up
ó 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)ó Water Tube:
ó Thickness measuring
ó Erosion at tube connection
ó Refractoryó Crack and fall down by
hammer(knocking)
ó Guardó Crack
ó fall down
142
6.4 Superheat I (Omega Tube)ó Offset Water Tube:
ó Thickness measuring
ó Erosion at offset tube
ó SH tubeó Thickness measuring
ó Omega Guardó Crack
ó fall down
Omega Guard
Offset Water Tube
143
6.5 Roofó Water Tube:
ó Thickness measuring
ó Erosion
ó Refractoryó Crack, wear and fall down by
hammer(knocking)
144
6.6 Inlet Separatoró Water Tube:
ó Thickness measuring near opening have more erosion than another tube because of high velocity of flue gas
ó Refractoryó Crack, wear and fall down by
hammer(knocking)
145
6.7 Steam Drumó Surface :
ó Surface were black by magnetite
ó Depositsó Deposits at bottom drum need to
check chemical analysis
ó Cyclone Separatoró Loose
ó Demister
ó Blowdown holeó Plugging
ó U-Clamp ó Loose
Deposits at bottom drum
146
6.8 Separatoró Central Pipe:
ó Deformation
ó Crack
ó Refractoryó Wear at impact zone due to high
impact velocity
ó Crack and fall down by hammer(knocking)
147
6.9 Outlet Separatoró Water Tube
ó Tube Thickness
ó Erosion
ó Outlet Central Pipe: óSupport or Hook
ó RefractoryóCrack and fall down by hammer(knocking)
148
6.10 Screen Tubeó Water Tube
ó Thickness measuring upper part of screen tube at corner have more erosion than another area because of high velocity of flue gas
ó Guardó Loose
ó Refractoryó 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 Tubeó Tube
ó Thickness measuring
ó High erosion between SH tube and wall
ó Steam erosion due to improper soot blower
ó Guardó Fall down
ó Crack
150
6.12 Economizeró Water Tube
ó Thickness measuring
ó High erosion between economizer tube and wall
ó Steam erosion due to improper soot blower
ó Guardó Fall down
ó Crack
Guard
Install guard to prevent tube erosion
151
6.13 Air Heateró Tube
ó Cold end corrosion due to high concentrate SO3 in flue gas
ó Steam erosion due to improper soot blower
Inlet air heater
Cold end corrosion due to SO3 in fluegas
153
Warning
Operating or maintenance procedure which, if not as described could result in injured death
or damage of equipment
154
General safety precaution
ó Electrical power shall be turned off before performing installation or maintenance work. Lock out, tag out shall be indicated
ó All personal safety equipment shall be suit for each work
ó Never direct air water stream into accumulation bed material or fly ash. This will become breathing hazard
ó Always provide safe access to all equipment ( plant from, ladders, stair way, hand rail
ó Post appropriate caution, warning or danger sign and barrier for alerting non-working person
ó Only qualify and authorized person should service equipment or maintenance work
155
General safety precaution
ó Do not by-pass any boiler interlocks
ó Use an filtering dust mask when entering dust zone
ó Do not disconnect hoist unless you have made sure that the source is isolated
156
Equipment entry
ó Never entry confine space until is has been cooled, purged and properly vented
ó When entering confine space such as separator, loop seal furnace be prepared for falling material
ó Always lock the damper, gate or door before passing through them
ó Never step on accumulation of bottom ash or fly ash. Its underneath still hot
ó Never use toxic fluid in confine space
ó Use only appropriate lifting equipment when lift or move equipment
157
Equipment entry
ó Stand by personnel shall be positioned outside a confine space to help inside person incase of emergency
ó Be carefully aware the chance of falling down when enter cyclone inlet or outlet.
ó Don not wear contact lens with out protective eye near boiler, fuel handing, ash handing system. Airborne particle can cause eye damage
ó Don not enter loop seal with out installing of cover over loop seal downcomer to prevent falling material from cyclone
158
Operating precautions
CFB boiler process
ó Use planks on top of bed materials after boiler is cooled down. This will prevent the chance of nozzle plugging
ó Do not open any water valve when boiler is in service
ó Do not operate boiler with out O2 analyzer
ó Do not use downcomer blown donw when pressure > 7 bar otherwise loss of circulation may occure
ó Do not operate CFB boiler without bed material
ó When PA is started. PA flow to grid must be increase to above minimum limit to fully fluidized bed maerial
ó Do not operate CFB boiler with bed pressure > 80mbar. This might be grid nozzle plugging
159
Operating precautions
ó on cold start up the rate of chance in saturated steam shall not exceed 2 C/min
ó On cold start up the change of flue gas temp at cyclone inlet shall not exceed 70 C/min
ó Do not add feed water to empty steam drum with different temperature between drum metal and feed water greater than 50 C
ó All fan must be operated when add bed material
160
Operating precautions
Refractory
ó When entering cyclone be aware a chance of falling down
ó Refractory retain heat for long period. Be prepared for hot surface when enter this area
ó An excessive thermal cycle will reduce the life cycle of refractory
ó After refractory repair, air cure need to apply about 24 hr or depend on manufacturer before heating cure
ó Heating cure shall be done carefully otherwise refractory life will be reduced
161
Operating precautions
Solid Fuel
ó Chemical analysis of all solid fuel shall be determined for first time and compared with OEM standard
ó Sizing is important
ó Burp feeding shall be performed during starting feeding solid fuel instead of continuous feeding
163
ó Basic control
ó Furnace control
ó Main pressure control
ó Main steam pressure control
ó Drum level control
ó Feed tank control
ó Solid fuel control
ó Primary air control
ó Secondary air control
ó Oxygen control
164
Basic control
ó Simple feedback control
PRIMARY VARIABLE
XT
K
A T A
f(x)
SET POINTPROCESS
MANIPULATED VARIABLE
165
Basic control
ó 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
ó Simple cascade control
PRIMARY VARIABLE
XT
ZT
K
K
SET POINT
A 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 à 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
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
PV
Manual
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
ó Pressure is controlled by control valve
ó Control valve is connected to primary air
ó It will release the air to primary air duct if pressure higher than set point
ó If operating unit stop due to disturbance or pressure fall down, stand by unit shall be automatically started
ó Pressure should be higher than 300 mbar, boiler interlock
ó 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
ó Lime stone can be control by ó lime stone/ fuel flow ratio
ó SO2 feed back control
ó Manual feed rate
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