Principle of CFB Boiler , 30 April 2012, Presented at SCGBKK ,TH

BASIC DESIGN OF
CIRCULATING FLUIDIZED BED
BOILER
30 APIRL 2012, Bangkok, Thailand
Pichai Chaibamrung
Asset Optimization Engineer
Asset Optimization and Reliability Section
Energy Division
Thai Kraft Paper Industry Co.,Ltd.

Biography
Name :Pichai Chaibamrung

Education
2009-2011, Ms.c, Thai-German Graduate School of Engineering
2002-2006, B.E, Kasetsart Univesity

Work Experience
Jul 11- present : Asset Optimization Engineer, TKIC
May 11- Jun 11 : Sr. Mechanical Design Engineer, Poyry Energy
Sep 06-May 09 : Engineer, Energy Department, TKIC

Email: pichacha@scg.co.th

By Chakraphong Phurngyai :: Engineer, TKIC

Content
1. Introduction to CFB
2. Hydrodynamic of CFB
3. Combustion in CFB
4. Heat Transfer in CFB
5. Basic design of CFB
6. Cyclone Separator
7. Operation Optimization


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
• To have awareness on operation optimization


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


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


1.2 Typical Arrangement of CFB Boiler


1.3 Advantage of CFB Boiler
• Fuel Flexibility


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


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


• Fluidization is defined as the operation through which fine solid are
transformed into a fluid like state through contact with a gas or
liquid.


• Particle Classification

Distribution Size (micron)
Foster HGB PB#15
100% <600 <1000 <1680

75% <250 <550 <1190

50% <180 <450 <840

25% <130 <250 <590

100% >100 >420


• Particle Classification


• Comparison of Principal Gas-Solid Contacting Processes


• 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


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


• 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


• 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.


• Terminal Velocity

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


• 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


2.2 Fast Fluidization
• Definition


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


2.3 Hydrodynamic regimes in a CFB
Cyclone Separator :
Swirl Flow
Back Pass:
Pneumatic Transport

Furnace Upper SA:
Fast Fluidized Bed
Lower Furnace below SA:
Turbulent or bubbling
fluidized bed

Return leg and lift leg :
Pack bed and Bubbling Bed


• Axial Voidage Profile
Secondary air is fed

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


• Velocity Profile in Fast Fluidized Bed


• 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



Increasing solid circulation
Increasing SA to 40%
rate effect to both
does not significant on
lower and upper zone
suspension density above
of SA injection point
SA injection point
which both zone is
but the low zone is
denser than low
denser than low SA ratio
solid circulation rate

No SA, the suspension With SA 20% of PA,
density is proportional the solid particle is hold up
l to solid circulation rate when compare to no SA


• Effects of Circulation Rate on Voidage Profile

higher solid recirculation rate


• Effects of Circulation Rate on Voidage Profile

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

higher solid recirculation rate


• 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


• Core-Annulus Model
- the furnace may be spilt into two zones : core and annulus

Core
- Velocity is above superficial velocity
core
- Solid move upward

Annulus
- Velocity is low to negative annulus

- Solids move downward


• Core-Annulus Model

core

annulus


• 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


3. Combustion in CFB
3.1 Stage of Combustion
3.2 Factor Affecting Combustion Efficiency
3.3 Combustion in CFB
3.4 Biomass Combustion


• A particle of solid fuel 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


3.1 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)


• 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)


• 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


• Communition Phenomena During Combustion

Attrition, Fine particles from
coarse particles through
mechanical contract like
Volatile release in non-porous abrasion with other particles
particle cause the high
internal pressure result in
break a coal particle into
fragmentation

Char burn under regime I
which is mass transfer is
higher than kinetic trasfer.
The sudden collapse or other
Volatile release cause the type of second fragmentation
particle swell call percolative fragmentation
occurs

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”


• 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.


• Operating condition (Bed Temperature)
- higher combustion temperature --- > high combustion efficiency

Limit of Bed temp
-Sulfur capture
-Bed melting
-Water tube failure

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


• 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


• 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


• Operating condition (excess air)
- combustion efficiency improve which excess air < 20%

Combustion loss
decrease
significantly
when excess air
< 20%. Excess air >20% less
significant improve
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.


3.3 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


• 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


• 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


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


• 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)


• 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


• Agglomeration


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


PB#11 : Fouling Problem (7 Aug 2010) August 2010

1.Front water wall upper
opening inlet 4. Screen tube & SH#3
- Slag
- Overlay tube (26Tubes)
- Replace refractory

2.Right water wall
- Change new
tubes (4 Tubes)
May 2010 Aug 2010

5.Roof water wall
-Change new tubes (4 Tubes)
- Overlay tube
- More erosion rate
1.5 mm/2.5 months
3.Front water wall
- Add refractory 2 m.
(Height) above kick-out


PB11 Fouling

May2010 Aug2010 Oct2010
6 months 2 months 2 months

Severe problem in Superheat tube fouling
•Waste reject fuel (Hi Chloride content)
•Only PB11 has this problems
•this problems also found on PB15 (SD
for Cleaning every 3 months)


• 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


Foster Wheeler experience Wood/Forest Residual

Straw,Rice husk

Waste Reject


3.5 Performance Modeling
• Performance of Combustion
- Unburned carbon loss
- Distribution and mixing of volatiles, char and oxygen along the
height and cross section of furnace
- Flue gas composition at the exit of the cyclone separator
(NOx,SOx)
- Heat release and absoption pattern in the furnace
- Solid waste generation


4. Heat Transfer in CFB
4.1 Gas to Particle Heat Transfer
4.2 Heat Transfer in CFB


4.1 Heat Transfer in CFB Boiler
• 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.


• Effect of Suspension Density and particle size

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


• Effect of Fluidization Velocity

No effect from fluidization velocity when leave the suspension density constant


• Effect of Fluidization Velocity


• Effect of Vertical Length of Heat Transfer Surface


• Effect of Bed Temperature


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


• Heat transfer to the walls of commercial-size

Low suspension density low heat
transfer to the wall.


• Circumferential Distribution of Heat Transfer Coefficient


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 Heat Absorption


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


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


5.3 Heat and Mass Balance
Heat input
• Heat Balance Main steam

Heat output
Radiation

Feed water
Blow down

Flue gas

Moisture in fuel Unburned in fly ash
and sorbent

Fuel and
sorbent
Combustion air

Unburned in Moisture in
bottom ash combustion air


5.3 Heat and Mass Balance
Mass input
• Mass Balance
Mass output

Solid Fluein Flue gas
Make up Solid gas
bed material

Fuel and
sorbent

Moisture in fuel fly ash
and sorbent

Fuel and
sorbent

Make up
bed material
fly ash
bottom ash
bottom ash


5.4 Furnace Design
• The furnace design include: 1. Furnace cross section
1. Furnace cross section Criteria
2. Furnace height - moisture in fuel
3. Furnace opening - ash in fuel
- fluidization velocity
- SA penetration
- maintain fluidization in lower
zone at part load


5.4 Furnace Design
2. Furnace height 3. Furnace opening
Criteria Criteria
- Heating surface - Fuel feed ports
- Residual time for sulfur capture - Sorbent feed ports
- Bed drain ports
- Furnace exit section


6. Cyclone Separator
• 6.1 Theory
• 6.2 Critical size of particle


6.1 Theory
• 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


6.1 Theory

• 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
- Higher value of viscosity of gas


6.2 Critical size of particle
• 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.


6.2 Critical size of particle

Effective number

Ideal and operation efficiency


7. Operation Optimization
7.1 Maximization vs. Optimization
7.2 Choice for Optimization
7.3 Case Study


7.1 Maximization vs Optimizaiton
• Maximization
objective is to get the highest performance considering only one
operating variable

• Optimization
objective is to get the best performance considering many
operating variables. Many of these operating variables have
exactly the opposite effect, making it impossible to get the highest
performance from each of these variables


7.2 Choices for Optimization

• Combustion efficiency
• Boiler efficiency
• Unburned carbon content in fly ash
• Wear of tubes as result of erosion and reducing conditions
• Fuel mix (percentage of different fuels) at different operation
conditions
• Power consumption (net output of plant)
• SO2 emissions
• NOx emissions
• Air split (split between PA, and SA)
• Sootblowing cycle
• Excess air (related to boiler efficiency)
• Bed inventory


7.3 Case Study
Case I PB#16 High bed Temperature

Advantage
- higher combustion efficiency.
Concerning parameter
- high bed temp mean higher flue gas volume
- high flue gas volume higher fluidization velocity
- high fluidization velocity, higher erosion
- high bed temp., higher probability for NOx emission
- higher lime stone consumption
- ash Sintering
- materials break up due to over heating


7.3 Case Study
• Operation Survey
- Average bed temp 920-935 C (some point >970 C)
- Bed pressure 30-40 mbar
- PA/SA ratio 0.68 : 0.32
- Boiler load > 90%
- SA upper / lower ratio 0.75 : 0.25
- O2 4%
- CO 0.04 ppm


7.3 Case Study
• What we found. What we have done.
Found : Bed pressure is lower when comparing to other unit.
Typically, 50 mbar.
Done : increasing bed pressure to 50 mbar.
Result : bed temperature dramatically decrease.
Concerning : power consumption of PA is slightly increased

Learning point ?


7.3 Case Study
Found : ratio of PA/TA is low
Done : increasing PA ratio from 68% to 70-71%
Result : bed temperature dramatically decrease.
Concerning : Power consumption of PA increase, high DP over
grid nozzle

Learning point ?


7.3 Case Study
Found : ratio of SA lower/ upper is low
Done : increasing SA flow by partial close SA upper valve
Result : bed temperature dramatically decrease. SA pressure
increase
Concerning : SA power consumption is increased

Learning point ?


References
• 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


THANK YOU FOR YOUR ATTENTION


Principle of CFB Boiler , 30 April 2012, Presented at SCGBKK ,TH

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