cfb boiler basic design, operation and maintenance

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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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Objectiveó To understand the typical arrangement in CFB

ó To understand the basic hydrodynamic of CFB

ó To understand the basic combustion in CFB

ó To understand the basic heat transfer in CFB

ó To understand basic design of CFB

ó To understand theory of cyclone separator

Know Principle Solve Everything

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1. Introduction to CFB1.1 Development of CFB

1.2 Typical equipment of CFB

1.3 Advantage of CFB

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1.1 Development of CFB ó 1921, Fritz Winkler, Germany, Coal Gasification

ó 1938, Waren Lewis and Edwin Gilliland, USA, Fluid Catalytic Cracking, Fast Fluidized Bed

ó 1960, Douglas Elliott, England, Coal Combustion, BFB

ó 1960s, Ahlstrom Group, Finland, First commercial CFB boiler, 15 MWth, Peat

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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

ó Fuel Flexibility

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1.3 Advantage of CFB Boiler

ó High Combustion Efficiency

- Good solid mixing

- Low unburned loss by cyclone, fly ash recirculation

- Long combustion zone

ó In situ sulfur removal

ó Low nitrogen oxide emission

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2. Hydrodynamic in CFB2.1 Regimes of Fluidization

2.2 Fast Fluidized Bed

2.3 Hydrodynamic Regimes in CFB

2.4 Hydrodynamic Structure of Fast Beds

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2.1 Regimes of Fluidization

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

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ó 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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ó Particle Classification

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ó Comparison of Principal Gas-Solid Contacting Processes

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ó Packed Bed

The pressure drop per unit height of a packed beds of a uniformly size

particles is correlated as (Ergun,1952)

Where U is gas flow rate per unit cross section of the bed called Superficial Gas Velocity

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ó 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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ó Bubbling Fluidization Beds

For B and D particle, the bubble is started when superficial gas is higher than minimum fluidization velocity

But for group A particle the bubble is started when superficial velocity is higher than minimum bubbling velocity

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ó Turbulent Beds

when the superficial is continually increased through a bubbling fluidization bed, the bed start expanding, then the new regime called turbulent bed is started.

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ó Terminal Velocity

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

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ó Freeboard and Furnace Height

- considered for design heating-surface area

- considered for design furnace height

- to minimize unburned carbon in bubbling bed

- the freeboard heights should be exceed or closed to the transport disengaging heights

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2.2 Fast Fluidizationó Definition

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2.2 Fast Fluidization

ó Characteristics of Fast Beds- non-uniform suspension of slender particle agglomerates or clusters moving up and down in a dilute

- excellent mixing are major characteristic

- low feed rate, particles are uniformly dispersed in gas stream

- high feed rate, particles enter the wake of the other, fluid drag on the leading particle decrease, fall under the gravity until it drops on to trailing particle

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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 Bedsó Axial Voidage Profile

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

Secondary air is fed

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ó Velocity Profile in Fast Fluidized Bed

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ó Velocity Profile in Fast Fluidized Bed

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ó 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 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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ó Effects of Circulation Rate on Voidage Profile

higher solid recirculation rate

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ó 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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ó Effect of Particle Size on Suspension Density Profile

- Fine particle - - > higher suspension density

- Higher suspension density - - > higher heat transfer

- Higher suspension density - - > lower bed temperature

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ó Core-Annulus Model

- the furnace may be spilt into two zones : core and annulus

Core

- Velocity is above superficial velocity

- Solid move upward

Annulus

- Velocity is low to negative

- Solids move downward

core

annulus

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ó Core-Annulus Model

core

annulus

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ó Core Annulus Model

- the up-and-down movement solids in the core and annulus sets up an internal circulation

- the uniform bed temperature is a direct result of internal circulation

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3. Combustion in CFB3.1 Coal properties for CFB boiler

3.2 Stage of Combustion

3.3 Factor Affecting Combustion Efficiency

3.4 Combustion in CFB

3.5 Biomass Combustion

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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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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

ó Heating and Drying

- Combustible materials constitutes around 0.5-5.0% by weight

of total solids in combustor

- Rate of heating 100 °C/sec – 1000 °C/sec

- Heat transfer to a fuel particle (Halder 1989)

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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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ó Char Combustion

2 step of char combustion

1. transportation of oxygen to carbon surface

2. Reaction of carbon with oxygen on the carbon surface

3 regimes of char combustion

- Regime I: mass transfer is higher than kinetic rate

- Regime II: mass transfer is comparable to kinetic rate

- Regime III: mass transfer is very slow compared to kinetic rate

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ó Communition Phenomena During Combustion

Volatile release cause the particle swell

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

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

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

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

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ó Fuel Characteristics

the lower ratio of FC/VM result in higher combustion efficiency (Makansi, 1990), (Yoshioka and Ikeda,1990), (Oka, 2004) but the improper mixing could result in lower combustion efficiency due to prompting escape of volatile gas from furnace.

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ó Operating condition (Bed Temperature)

- higher combustion temperature --- > high combustion efficiency

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

Limit of Bed temp

-Sulfur capture

-Bed melting

-Water tube failure

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ó Fuel Characteristic (Particle size)

-The effect of this particle size is not clear

-Fine particle, low burn out time but the probability to be dispersed from cyclone the high

-Coarse size, need long time to burn out.

-Both increases and decreases are possible when particle size decrease

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ó Operating condition (superficial velocity)

- high fluidizing velocity decrease combustion efficiency because

Increasing probability of small char particle be elutriated from

circulation loop

- low fluidizing velocity cause defluidization, hot spot and sintering

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ó Operating condition (excess air)

- combustion efficiency improve which excess air < 20%

Excess air >20% less significant improve combustion efficiency.

Combustion loss decrease significantly when excess air < 20%.

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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

ó Lower Zone Properties

- This zone is fluidized by primary air constituting about 40-80% of total air.

- This zone receives fresh coal from coal feeder and unburned coal from cyclone though return valve

- Oxygen deficient zone, lined with refractory to protect corrosion

- Denser than upper zone

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ó Upper Zone Properties

- Secondary is added at interface between lower and upper zone

- Oxygen-rich zone

- Most of char combustion occurs

- Char particle could make many trips around the furnace before they are finally entrained out through the top of furnace

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ó Cyclone Zone Properties

- Normally, the combustion is small when compare to in furnace

- Some boiler may experience the strong combustion in this zone which can be observe by rising temperature in the cyclone exit and loop seal

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ó Fuel Characteristics

- high volatile content (60-80%)

- high alkali content à sintering, slagging, and fouling

- high chlorine content à corrosion

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ó Agglomeration

SiO2 melts at 1450 C

Eutectic Mixture melts at 874 C

Sintering tendency of fuel is indicated by the following

(Hulkkonen et al., 2003)

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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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ó Agglomeration

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ó Fouling

- is sticky deposition of ash due to evaporation of alkali salt

- result in low heat transfer to tube

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ó Corrosion Potential in Biomass Firing

- hot corrosion

- chlorine reacts with alkali metal à from low temperature melting alkali chlorides

- reduce heat transfer and causing high temperature corrosion

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4. Heat Transfer in CFB4.1 Gas to Particle Heat Transfer

4.2 Heat Transfer in CFB

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4.1 Gas to Particle Heat Transfer

ó Mechanism of Heat Transfer

In a CFB boiler, fine solid particles agglomerate and form clusters or stand in a continuum of generally up-flowing gas containing sparsely dispersed solids. The continuum is called the dispersed phase, while the agglomerates are called the cluster phase.

The heat transfer to furnace wall occurs through conduction from particle clusters, convection from dispersed phase, and radiation from both phase.

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4.1 Heat Transfer in CFB Boiler

ó Effect of Suspension Density and particle size

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

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ó Effect of Fluidization Velocity

No effect from fluidization velocity when leave the suspension density constant

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ó Effect of Vertical Length of Heat Transfer Surface

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ó Effect of Bed Temperature

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

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ó Heat transfer to the walls of commercial-size

Low suspension density low heat transfer to the wall.

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ó Circumferential Distribution of Heat Transfer Coefficient

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5 Design of CFB Boiler

ó 5.1 Design and Required Data

ó 5.2 Combustion Calculation

ó 5.3 Heat and Mass Balance

ó 5.4 Furnace Design

ó 5.5 Cyclone Separator

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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

ó Base on the design and required data the following data can be calculated in this stage :

- Fuel flow rate - Combustion air flow rate

- Fan capacity - Fuel and ash handling capacity

- Sorbent flow rate

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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

ó Mass Balance

Fuel and sorbent

bottom ash

Solid Flue gas

Moisture in fuel and sorbent

fly ash

Make up bed material

bottom ash

Fuel and sorbent

Make up bed material

Solid in Flue gas

fly ash

Mass output

Mass input

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5.4 Furnace Designó The furnace design include:

1. Furnace cross section

2. Furnace height

3. Furnace opening

1. Furnace cross section

Criteria

- moisture in fuel

- ash in fuel

- fluidization velocity

- SA penetration

- maintain fluidization in lower zone at part load

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5.4 Furnace Design2. Furnace height

Criteria

- Heating surface

- Residual time for sulfur capture

3. Furnace opening

Criteria

- Fuel feed ports

- Sorbent feed ports

- Bed drain ports

- Furnace exit section

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5.5 Cyclone Separator

ó 6.1 Theory

ó 6.2 Critical size of particle

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ó The centrifugal force on the particle entering the cyclone is

ó The drag force on the particle can be written as

ó Under steady state drag force = centrifugal force

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ó Vr can be considered as index of cyclone efficiency, from above equation the cyclone efficiency will increase for :

- Higher entry velocity

- Large size of solid

- Higher density of particle

- Small radius of cyclone

- low value of viscosity of gas

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ó The particle with a diameter larger than theoretical cut-size of cyclone will be collected or trapped by cyclone while the small size will be entrained or leave a cyclone

ó Actual operation, the cut-off size diameter will be defined as d50 that mean 50% of the particle which have a diameter more than d50 will be collected or captured.

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6. Operation

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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

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6.1 Before Start

ó all maintenance work have been completely done

ó All function test have been checked

ó cooling water system is operating

ó compressed air system is operating

ó Make up water system

ó Deaerator system

ó Boiler feed water pump

ó Condensate system

ó Oil and gas system

ó Drain and vent valves

ó Air duct, flue gas duct system

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6.1 Before Start

ó Blow down system

ó Sand feeding system

ó Lime stone feeding system

ó Solid fuel system

ó Ash drainage system

ó Control and safety interlock system

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6.2 Grid Pressure Drop Testó For check blockage of grid

nozzle

ó Furnace set point = 0

ó Test at every PA. load

ó Compare to clean data or design data

ó Shall not exceed 10% from design data

ó Perform in cold condition

Pw

Pb

FI

Pf= 0

97

6.3 Cold Start

Fill boiler

Boiler Interlock

Start up Burner

Feed Solid Fuel

Boiler Warm Up

Purge

Start Fan

Feed Bed Material

Raise to MCR

-100 mm normal level

ID,HP,SA,PA

Low level cut off

300 S

Tb 150-200 C

30-50 mbar, Tb 550-600 C

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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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Purgeó To carry out combustible gases

ó To assure all fuel are isolated from furnace

ó Before starting first burner for cold start

ó If bed temp < 600 C or OEM recommend and no burner in service

ó Total air flow > 50%

ó 300 sec for purging time

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Purge

NFPA85: CFB Boiler purge logic

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Start up burner

ó Help to heat up bed temp to allowable temperature for feeding solid fuel

ó Will be stopped if bed temp > 850 C

ó Before starting, all interlock have to passed

ó Main interlock

ó Oil pressure > minimum

ó Control air pressure > minimum

ó Atomizing air pressure > minimum

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Start up burnerNFPA85 - Typical burner safety for CFB boiler

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Drum and DA low level cut-offó Test for safety

ó During burner are operating

ó Open drain until low level

ó Signal feeding are not allow

ó Steam drum low level = chance to overheating of water tube

ó DA low level = danger for BFWP

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Boiler warm up

ó Gradually heating the boiler to reduce the effect of thermal stress on pressure part, refractory and drum swell

ó Increase bed temp 60-80 C/hr by adjusting SUB

ó Control flue gas temperature <470 C until steam flow > 10% MCR

ó Close vent valves at drum and SH when pressure > 2 bar

ó Continue to increase firing rate according to recommended start up curve

ó Operate desuperheater when steam temperature are with in 30 C of design point

ó Slowly close start up and drain valve while maintain steam flow > 10% MCR

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Feed bed material

ó Bed material should be sand which size is according to recommended size

ó Start feed sand when bed temp >150 C

ó Do not exceed firing rate >30% if bed pressure <20 mbar otherwise overheating may occur for refractory and nozzle

ó Continue feed bed material unit it reach 30 mbar

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Feed solid fuel

ó Must have enough bed material

ó Bed temperature > 600 C or manufacturer recommendation or refer to NFPA85 Appendix H

ó Pulse feed every 90 s

ó Placing lime stone feeding, ash removal system simultaneously

ó Slowly decrease SUB firing rate while increasing solid fuel feed rate

ó Stop SUB one by one, observe bed temperature increasing

ó Turn to auto mode control

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Rise to MCR

ó Continue rise pressure and temperature according to recommended curve until reach design point

ó Drain bottom ash when bed pressure >45-55 mbar

ó Slowly close start up valve

ó Monitor concerning parameters

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ó Increasing

- manual increase air flow

- manual increase fuel flow

- monitor excess oxygen

- monitor steam pressure

ó Decreasing

- manual decrease air flow

- manual decrease fuel flow

- monitor excess oxygen

- monitor steam pressure

Changing Boiler load (manual)

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ó Furnace and emssion

- monitor fluidization in hot loop

- monitor gas side pressure drop

- monitor bed pressure

- monitor bed temperature

-monitor wind box pressure

- monitor SOx, Nox, CO

Furnace and Emission Monitoring

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ó Bottom ash drain

- automatic or manual draining of bottom ash shall be judged by commissioning engineer for the design fuel.

- when fuel is deviated from the design, operator can be judge by themselves that draining need to perform or not.

- bed pressure is the main parameter to start draining

ó Soot blower

- initiate soot blower to clean the heat exchanger surface in convective part

- frequent of soot blowing depend on the degradation of heat transfer coefficient.

- normally 10 C higher than normal value of exhaust temperature

Bottom ash and Soot Blower

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ó Boiler Walk Down

- boiler expansion joint

- Boiler steam drum

- Boiler penthouse

- Safety valve

- Boiler lagging

- Spring hanger

- Valve and piping

- Damper position

- Loop seal

- Bottom screw

- Combustion chamber

- Fuel conveyor

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ó Sizing Quality

- crushed coal, bed material, lime stone and bottom ash sizing shall be periodically checked by the operator

- sieve sizing shall be performed regularly to make sure that their sizing is in range of recommendation

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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

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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

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20. Open vent valve at drum and SH when drum pressure reach 1.5-2 bar

21. Open manhole around furnace when bed temp < 300 C

118

6.6 Emergency Shut down

ó Boiler can be held in hot stand by condition about 8 hrs

ó Hot condition is bed temp >650 C otherwise follow cold star up procedure

ó Boiler load should be brought to minimum

ó Stop fuel feeding

ó Wait O2 increase 2 time of normal operation

ó Stop air to combustion chamber to minimize heat loss

119

6.7 Hot restart

ó Purge boiler if bed temperature < 600 C

ó Start SUBs if bed temperature > 500 C

ó Monitor bed temperature rise

ó If bed temperature does not rise after pulse feeding solid fuel. stop feeding and start purge

120

6.8 Malfunction and Emergency

ó Bed pressure

ó Bed temperature

ó Circulation

ó Tube leak

ó Drum level

121

Bed Pressure

Bed pressure is an one of importance parameter that effect on boiler efficiency and reliability.

Measured above grid nozzle about 20 cm.

Pw

Pb

FI

Pf= 0

122

Bed Pressure

ó Effect of low bed pressure

- poor heat transfer

- boiler responds

- high bed temperature

- damage of air nozzle and refractory

ó Effect of high bed pressure

- increase heat transfer

- more efficient sulfur capture

- more power consumption of fan

123

Bed Pressure

ó Cause of low bed pressure

- loss of bed material

- too fine of bed materials

- high bed temperature

ó Cause of high bed pressure

- agglomeration

- too coarse of bed material

124

Bed Temperatureó Measured above grid nozzle about

20 cm

ó Measured around the furnace cross section

ó It is the significant parameter to operate CFB boiler

125

Bed temperature

ó Effect of high bed temperature

- ineffective sulfur capture

- chance of ash melting

- chance of agglomeration

- chance to damage of air nozzle

126

Bed temperature

ó Cause of high bed temperature

- low bed pressure

- too coarse bed material

- too coarse solid fuel

- improper drain bed material

- low volatile fuel

- improper air flow adjustment

127

Circulation

ó Circulation is particular phenomena of CFB boiler.

ó Bed material and fuel are collected at cyclone separator

ó Return to the furnace via loop seal

ó HP blower supply HP air to fluidize collected materials to return to furnace

128

Circulation

ó Effect of malfunction circulation

- No circulation result in forced shut down

- high rate of circulation

- high circulation rate need more power of blower

- low rate of circulation

129

Circulation

ó Cause of malfunction circulation

- insufficiency air flow to loop seal nozzle

- insufficient air pressure to loop seal

- plugging of HP blower inlet filter

- blocking or plugging of loop seal nozzle

-

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Tube leak

ó Water tube leak

- furnace pressure rise

- bed temperature reduce

- stop fuel feeding

- open start up valve

- don’t left low level of drum

- continue feed water until flue gas temp < 400 C

- continue combustion until complete

- small leak follow normal shut down

131

Drum level

Sudden loss of drum level

- when the cause is known and immediately correctable before level reach minimum allowable. Reestablish steam drum level to its normal value and continue boiler operation

-if the cause is not known. Start immediate shut down according to emergency shut down procedure

132

Drum level

Gradual loss of drum level

- boiler load shall be reduced to low load

- find out and correct the problem as soon as possible

- if can not maintain level and correct the problem, boiler must be taken out of service and normal shut down procedure shall be applied.

133

7. Maintenance

134

Before maintenance work

ó Make sure that all staff are understand about safety instruction for doing CFB boiler maintenance work

ó Make sure that all maintenance and safety equipments shall be a first class

135

Overview Boiler Maintenance

Refractory and tube are the main area that need to be checked

136

6.1 Windbox Inspectionó Inspect sand inside windbox

after shutdown

ó Drain pipeó Crack

ó Air gun pipe

ó Refractory ó Crack, wear and fall down inspect

by hammer(knocking) if burner is under bed design

Drain pipe

137

6.2 Furnace Inspectionó Nozzle :

ó Wear

ó Fall-off

ó Refractoryó Crack, wear and fall down inspect

by hammer knocking if burner is under bed design

ó Feed fuel portó Wear

ó Crack

ó Burner

Refractory

Burner Feed FuelNozzle

138

6.2 Furnace Inspectionó Limestone port

ó Crack

ó Deform

ó Refractory damage at connection between port and refractory

ó Secondary & Recirculation Air portó Crack

ó Deform

ó Refractory damage at connection between port and refractory

ó Bed Temperatureó Check thermo well deformation

ó Check wear

Secondary & Recirculation Air port

139

6.3 Kick-Out Inspectionó Refractory

ó Wear

ó Crack and fall down by hammer(knocking)

ó Water tubeó Wear

ó Thickness

140

6.3 Kick-Out Inspectionó Water Tube:

ó Thickness measuring

ó Erosion at corner

ó CO Corrosion due to incomplete combustion at fuel feed side.

ó Defect from weld build up

ó Water tube sampling for internal check every 3 years

Inside water tube inspect by borescope

welded build up excessive metal because use welding rod size bigger than tube thickness

141

6.4 Superheat I (Wingwall)ó Water Tube:


ó Erosion at tube connection

ó Refractoryó Crack and fall down by

hammer(knocking)

ó Guardó Crack

ó fall down

142

6.4 Superheat I (Omega Tube)ó Offset Water Tube:


ó Erosion at offset tube

ó SH tubeó Thickness measuring

ó Omega Guardó Crack

ó fall down

Omega Guard

Offset Water Tube

143

6.5 Roofó Water Tube:


ó Erosion

ó Refractoryó Crack, wear and fall down by

hammer(knocking)

144

6.6 Inlet Separatoró Water Tube:

ó Thickness measuring near opening have more erosion than another tube because of high velocity of flue gas

ó Refractoryó Crack, wear and fall down by

hammer(knocking)

145

6.7 Steam Drumó Surface :

ó Surface were black by magnetite

ó Depositsó Deposits at bottom drum need to

check chemical analysis

ó Cyclone Separatoró Loose

ó Demister

ó Blowdown holeó Plugging

ó U-Clamp ó Loose

Deposits at bottom drum

146

6.8 Separatoró Central Pipe:

ó Deformation

ó Crack

ó Refractoryó Wear at impact zone due to high

impact velocity

ó Crack and fall down by hammer(knocking)

147

6.9 Outlet Separatoró Water Tube

ó Tube Thickness

ó Erosion

ó Outlet Central Pipe: óSupport or Hook

ó RefractoryóCrack and fall down by hammer(knocking)

148

6.10 Screen Tubeó Water Tube

ó Thickness measuring upper part of screen tube at corner have more erosion than another area because of high velocity of flue gas

ó Guardó Loose

ó Refractoryó Crack and fall down by

hammer(knocking)

Weld build up or install guard to prevent tube erosion

upper part of screen tube at corner have more erosion

149

6.11 Superheat Tubeó Tube


ó High erosion between SH tube and wall

ó Steam erosion due to improper soot blower

ó Guardó Fall down

ó Crack

150

6.12 Economizeró Water Tube


ó High erosion between economizer tube and wall


ó Guardó Fall down

ó Crack

Guard

Install guard to prevent tube erosion

151

6.13 Air Heateró Tube

ó Cold end corrosion due to high concentrate SO3 in flue gas


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

ó Electrical power shall be turned off before performing installation or maintenance work. Lock out, tag out shall be indicated

ó All personal safety equipment shall be suit for each work

ó Never direct air water stream into accumulation bed material or fly ash. This will become breathing hazard

ó Always provide safe access to all equipment ( plant from, ladders, stair way, hand rail

ó Post appropriate caution, warning or danger sign and barrier for alerting non-working person

ó Only qualify and authorized person should service equipment or maintenance work

155

General safety precaution

ó Do not by-pass any boiler interlocks

ó Use an filtering dust mask when entering dust zone

ó Do not disconnect hoist unless you have made sure that the source is isolated

156

Equipment entry

ó Never entry confine space until is has been cooled, purged and properly vented

ó When entering confine space such as separator, loop seal furnace be prepared for falling material

ó Always lock the damper, gate or door before passing through them

ó Never step on accumulation of bottom ash or fly ash. Its underneath still hot

ó Never use toxic fluid in confine space

ó Use only appropriate lifting equipment when lift or move equipment

157

Equipment entry

ó Stand by personnel shall be positioned outside a confine space to help inside person incase of emergency

ó Be carefully aware the chance of falling down when enter cyclone inlet or outlet.

ó Don not wear contact lens with out protective eye near boiler, fuel handing, ash handing system. Airborne particle can cause eye damage

ó Don not enter loop seal with out installing of cover over loop seal downcomer to prevent falling material from cyclone

158

Operating precautions

CFB boiler process

ó Use planks on top of bed materials after boiler is cooled down. This will prevent the chance of nozzle plugging

ó Do not open any water valve when boiler is in service

ó Do not operate boiler with out O2 analyzer

ó Do not use downcomer blown donw when pressure > 7 bar otherwise loss of circulation may occure

ó Do not operate CFB boiler without bed material

ó When PA is started. PA flow to grid must be increase to above minimum limit to fully fluidized bed maerial

ó Do not operate CFB boiler with bed pressure > 80mbar. This might be grid nozzle plugging

159


ó on cold start up the rate of chance in saturated steam shall not exceed 2 C/min

ó On cold start up the change of flue gas temp at cyclone inlet shall not exceed 70 C/min

ó Do not add feed water to empty steam drum with different temperature between drum metal and feed water greater than 50 C

ó All fan must be operated when add bed material

160


Refractory

ó When entering cyclone be aware a chance of falling down

ó Refractory retain heat for long period. Be prepared for hot surface when enter this area

ó An excessive thermal cycle will reduce the life cycle of refractory

ó After refractory repair, air cure need to apply about 24 hr or depend on manufacturer before heating cure

ó Heating cure shall be done carefully otherwise refractory life will be reduced

161


Solid Fuel

ó Chemical analysis of all solid fuel shall be determined for first time and compared with OEM standard

ó Sizing is important

ó Burp feeding shall be performed during starting feeding solid fuel instead of continuous feeding

162

9. Basic CFB Boiler Control

163

ó Basic control

ó Furnace control

ó Main pressure control

ó Main steam pressure control

ó Drum level control

ó Feed tank control

ó Solid fuel control

ó Primary air control

ó Secondary air control

ó Oxygen control

164

Basic control

ó Simple feedback control

PRIMARY VARIABLE

XT

K

A T A

f(x)

SET POINTPROCESS

MANIPULATED VARIABLE

165

Basic control

ó Simple feed forward plus feedback control

PR IM ARY VARIABLE

XT

YT

SECO NDARYVARIABLE

A T A

f(x)

MA NIP ULATED VARIAB LE

P ROCE SS

S ET POINT

K

166

Basic control

ó Simple cascade control

PRIMARY VARIABLE

XT

ZT

K

K

SET POINT

A AT

PROCESS

f(x)

MANIPULATED VARIABLE

SECONDARYVARIABLE

167

Basic control

CO

SP

PV

PID

Control Mode of PID

-MAN (Manual)

-AUT (Automatic)

-CAS (Cascade)

Signal to open0-15 m3/h

0-100% (closed à open)

4-20 mAElectrical signal 4-20 mA

Eng. Unit 0-15 m3/h

Percent 0-100 % 0-100%

168

Feed water control

LT

PT

PIDPID

Make up water

Heating steam

Pressure

-Manual mode 0-100% heating steam valve position

-Auto mode, specify pressure set point

-Temperature compensation

Level

-Manual mode 0-100% make up water valve

-Auto mode, specify level set point

-Temperature compensation

-Protection, high level over flow

169

Drum Level control

DP feed water pump

Control valve

A, SP

M, 0-100%

Main steam flowMain steam Pressure

Manual mode, 0-100% control valve

Auto mode, specify drum level. Automatically adjust valve

Protection

-lower limit

-2/3 principle

- 10 s delay

-Close steam valve for low level

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

PV

Manual

Manual: position of damper is specified

Auto: desired air flow is specified by operator

Cascade: set point is calculated from master combustion

Flow (interlock) > minimum

PA wind box P > minimum

PA running

174

Secondary air control

M

PIDAutoCascade

Manual

FT

FT

PT

Manual

Manual

PID Auto

Cascade

Lower SA

Upper SA

FTPV

175

HP Blower Control

ó Pressure is controlled by control valve

ó Control valve is connected to primary air

ó It will release the air to primary air duct if pressure higher than set point

ó If operating unit stop due to disturbance or pressure fall down, stand by unit shall be automatically started

ó Pressure should be higher than 300 mbar, boiler interlock

ó Pressure < 350 mbar parallel operation start

176

Furnace Pressure control

M

PID

PT

Auto

Furnace pressure

Manual

PID

Manual

Auto

2/3 furnace P < max (35 mbar)

177

Lime stone control

ó Lime stone can be control by ó lime stone/ fuel flow ratio

ó SO2 feed back control

ó Manual feed rate

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 basic design, operation and maintenance

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