CFB Boiler Design, Operation and Maintenance Guide

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CFB Boiler Design, Operation and
Maintenance
By Pichai Chaibamrung

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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

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Objective
— 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

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1. Introduction to CFB
1.1 Development of CFB
1.2 Typical equipment of CFB
1.3 Advantage of CFB

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1.1 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

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1.2 Typical Component of CFB Boiler

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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

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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.

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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

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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

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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

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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

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

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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.

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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

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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.

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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 CFB
2.1 Regimes of Fluidization
2.2 Fast Fluidized Bed
2.3 Hydrodynamic Regimes in CFB
2.4 Hydrodynamic Structure of Fast Beds

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— 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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— Particle Classification
<130
<180
<250
<600
CFB1
Size (micron)
<590<25025%
>420>100100%
<840<45050%
75%
100%
Distribution
<1190<550
<1680<1000
BFBCFB2

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— Particle Classification

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— Comparison of Principal Gas-Solid Contacting Processes

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— 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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— Bubbling Fluidization Beds
Minimum fluidization velocity is velocity where the fluid
drag is equal to a particle’s weight less its buoyancy.

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— 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

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

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— 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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— Axial Voidage Profile
Bed Density Profile of 135 MWe CFB Boiler (Zhang et al., 2005)
Secondary air is fed

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— Velocity Profile in Fast Fluidized Bed

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— Velocity Profile in Fast Fluidized Bed

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— Particle Distribution Profile in Fast Fluidized Bed

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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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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 above
SA injection point
but the low zone is
denser than low SA ratio
Increasing solid circulation
rate effect to both
lower and upper zone
of SA injection point
which both zone is
denser than low
solid circulation rate

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— Effects of Circulation Rate on Voidage Profile
higher solid recirculation rate

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— 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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— 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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— 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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— Core-Annulus Model
core
annulus

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— 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 CFB
3.1 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

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3.1 Coal properties for CFB Boiler
Properties
- 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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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

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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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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)

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— 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

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— 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 à
weak 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 fragmentation
occurs

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— 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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— 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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— 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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— 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

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— 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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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.

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

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— 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)

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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

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— Agglomeration

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

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— 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

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4. Heat Transfer in CFB
4.1 Gas to Particle Heat Transfer
4.2 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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— Effect of Fluidization Velocity
No effect from fluidization velocity when leave the suspension density constant

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— Effect of Vertical Length of Heat Transfer Surface

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— Effect of Bed Temperature

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— Heat Flux on 300 MW CFB Boiler (Z. Man, et. al)

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— Heat transfer to the walls of commercial-size
Low suspension density low
heat transfer to the wall.

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— 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 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

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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

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

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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 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 Design
2. 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

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5.5 Cyclone Separator
— 6.1 Theory
— 6.2 Critical size of particle

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— 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
— 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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— 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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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
— 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

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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

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

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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

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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

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

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Start up burner
NFPA85 - 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

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 <470 C until steam flow >
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 <20 mbar
otherwise overheating may occur for refractory and nozzle
— Continue feed bed material unit it reach 30 mbar

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
— 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

113
— 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
— 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
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
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
— 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 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
— 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
- 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
— 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 Fuel
Nozzle

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:
— Erosion at tube connection
— Refractory
hammer(knocking)
— Guard
— Crack
— fall down

142
6.4 Superheat I (Omega Tube)
— Offset Water Tube:
— Erosion at offset tube
— SH tube
— Omega Guard
— Crack
— fall down
Omega Guard
Offset Water
Tube

143
6.5 Roof
— Water Tube:
— 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
hammer(knocking)

147
6.9 Outlet Separator
— Water Tube
— Tube Thickness
— Erosion
— Outlet Central Pipe:
—Support or Hook
— Refractory
—Crack 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
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
— High erosion between SH tube and
wall
— Steam erosion due to improper soot
blower
— Guard
— Fall down
— Crack

150
6.12 Economizer
— Water Tube
— High erosion between economizer
tube and wall
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
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
— 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 precautions
CFB 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
— 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
Refractory
— 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
Solid 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 POINT
PROCESS
MANIPULATED VARIABLE

165
Basic control
— Simple feed forward plus feedback control
PR IM ARY VARIABLE
XT
YT
SECO NDARY
VARIABLE
A T A
f(x)
PROCESS
SET POINT
K

166
Basic control
— Simple cascade control
PRIMARY VARIABLE
XT
ZT
K
K
SET POINT
A AT
PROCESS
f(x)
SECONDARY
VARIABLE

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 flow
Main 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
Auto
Cascade
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
PID
Auto
Cascade
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

CFB Boiler Design, Operation and Maintenance Guide

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