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
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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
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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
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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
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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.
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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
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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.
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1.2 Typical Arrangement of CFB Boiler
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1.3 Advantage of CFB Boiler Fuel Flexibility
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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
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2. Hydrodynamic in CFB2.1 Regimes of Fluidization2.2 Fast Fluidized Bed2.3 Hydrodynamic Regimes in CFB2.4 Hydrodynamic Structure of Fast Beds
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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.
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2.1 Regimes of Fluidization Particle Classification
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2.1 Regimes of Fluidization Particle Classification
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2.1 Regimes of Fluidization Comparison of Principal Gas-Solid Contacting Processes
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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
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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.
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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
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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.
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2.1 Regimes of Fluidization
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2.1 Regimes of Fluidization Terminal Velocity
Terminal velocity is the particle velocity when the forces acting on particle is equilibrium
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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
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2.2 Fast Fluidization Definition
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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
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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
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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
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2.4 Hydrodynamic Structure of Fast Beds Velocity Profile in Fast Fluidized Bed
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2.4 Hydrodynamic Structure of Fast Beds Velocity Profile in Fast Fluidized Bed
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2.4 Hydrodynamic Structure of Fast Beds Particle Distribution Profile in Fast Fluidized Bed
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2.4 Hydrodynamic Structure of Fast Beds Particle Distribution Profile in Fast Fluidized Bed
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2.4 Hydrodynamic Structure of Fast Beds 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
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2.4 Hydrodynamic Structure of Fast Beds 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
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2.4 Hydrodynamic Structure of Fast Beds Effects of Circulation Rate on Voidage Profile
higher solid recirculation rate
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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
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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
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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
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2.4 Hydrodynamic Structure of Fast Beds Core-Annulus Model
core
annulus
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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
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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
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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
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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
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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)
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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)
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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
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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
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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.
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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
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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
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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
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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%.
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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
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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
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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
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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
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3.5 Biomass Combustion Agglomeration
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3.5 Biomass Combustion Fouling
- is sticky deposition of ash due to evaporation of alkali salt- result in low heat transfer to tube
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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
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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.
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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
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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 Fluidization Velocity
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4.1 Heat Transfer in CFB Boiler Effect of Fluidization Velocity
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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Start Fan
1.Start ID.Fan
2.Start HP Blower
3.Start SA.Fan
4.Start PA.Fan
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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
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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
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Purge
NFPA85: CFB Boiler purge logic
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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
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Start up burnerNFPA85 - Typical burner safety for CFB boiler
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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
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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
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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
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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
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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
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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)
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6.4 Normal Operation
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
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6.4 Normal Operation
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
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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
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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
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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
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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
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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
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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
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6.8 Malfunction and Emergency Bed pressure Bed temperature Circulation Tube leak Drum level
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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
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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
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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
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Bed Temperature Measured above grid nozzle about
20 cm Measured around the furnace cross
section It is the significant parameter to
operate CFB boiler
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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
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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
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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.
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7. Maintenance
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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
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6.3 Kick-Out Inspection Refractory
Wear Crack and fall down by
hammer(knocking)
Water tube Wear Thickness
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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
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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)
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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