principle of cfb boiler , 30 april 2012, presented at scgbkk ,th

BASIC DESIGN OFCIRCULATING 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.

By Chakraphong Phurngyai :: Engineer, TKIC

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: [email protected]


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

<130

<180

<250

<600

Foster

Size (micron)

<590<25025%

>420>100100%

<840<45050%

75%

100%

Distribution

<1190<550

<1680<1000

PB#15HGB



• 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 bubblingfluidization 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

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



• Axial Voidage Profile

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

Secondary air is fed



• 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




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



• Effects of Circulation Rate on Voidage Profile

higher solid recirculation rate



• Effects of Circulation Rate on Voidage Profile

higher solid recirculation rate

Pressure drop across the L-valve is proportional to 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

- Solid move upward

Annulus

- Velocity is low to negative

- Solids move downward

core

annulus



• 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

Volatile release cause the particle swell

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

Char burn under regime I, II, the pores increases in size àweak bridge connection of carbon until it can’t withstand the hydrodynamic force. It will fragment again call “secondary fragmentation”

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

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



• 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

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



• 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%

Excess air >20% less significant improve combustion efficiency.

Combustion loss decrease significantly when excess air < 20%.



• 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


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

3.Front water wall- Add refractory 2 m. (Height) above kick-out

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

5.Roof water wall-Change new tubes (4 Tubes)- Overlay tube - More erosion rate 1.5 mm/2.5 months

4. Screen tube & SH#3 - พบ Slag ทีเ่กาะจาํนวนมาก

1.Front water wall upper opening inlet

- Overlay tube (26Tubes)- Replace refractory

May 2010 Aug 2010


PB11 Fouling

May20106 months

Aug20102 months

Oct20102 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 theheight 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 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


5.3 Heat and Mass Balance

• Mass Balance

Fuel and sorbent

bottom ash

Solid Flue gas

Moisture in fuel and sorbent

fly ash

Mass input

Make up bed material

bottom ash

Fuel and sorbent

Make up bed material

Solid in Flue gas

fly ash

Mass output


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


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


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 oneoperating variable

• Optimizationobjective 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 airinjection, 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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