DESIGN THEORY OF CIRCULATING FLUIDIZED BED.pdf

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DESIGN THEORY OF CIRCULATING FLUIDIZED BED

BOILERS

Guangxi YUE, Junfu LU, Hai ZHANG, Hairui YANG, Jiansheng

ZHANG, Qing LIU, Zheng LI, Eric JOOS*, Philippe JAUD*

Department of Thermal Engineering, Tsinghua University, Beijing100084, China; * EDF France Paris 78401, France

ABSTRACT

Studies on circulating fluidized bed (CFB) boilers have being conducted at the Tsinghua

University (TH) for about two decades and much of works are done to link the fundamentals with practical application. A full set of design theory was developed and some key elements of this

theory are presented in this paper.

First, a classification of state of the solid-gas two-phase flow in CFB boiler is given. TH’s

studies validated that a CFB boiler can be generally described as the superposition of a fast bed inthe upper part with a bubbling bed or turbulent bed in the bottom part. A concept model ofmaterial balance for the open system of CFB boiler was developed and later improved as a more

comprehensive 1-D model taking ash formation, particle attrition and segregation in bed into

account. Some results of the models are discussed.

Then the concept of State Specification of a CFB boiler is defined and discussed. The StateSpecification is regarded as the first step to design a CFB and a base to classify different style of

CFB boiler technologies for various CFB boiler manufacturers. The State Specification adopted

by major CFB boiler makers is summarized and associated importance issues are addressed.

The heat transfer model originally developed by Leckner and his coworkers is adopted andimproved. It is further calibrated with experimental data obtained on the commercial CFB boilermeasurements. The principle, improvements and application of the model are introduced. Some

special tools developed for heat transfer field test are also given.

Also, combustion behaviors of char and volatile content are studied, and the combustiondifference between a CFB boiler and a bubbling bed is analyzed. The influence of volatile contentand size distribution is discussed. The concept of vertical distribution of combustion and heat in

CFB boiler furnace is introduced and discussed as well.

In the last, the suggested design theory of CFB boiler is summarized.

Keywords: circulating fluidized bed boilers, design theory, state specification, fast bed

INTRODUCTION

Circulating fluidized bed (CFB) technology has gained a great progress in coal-firing boilers

since the successful operation of the world’s first demonstration of circulating fluidized bed (CFB)

boiler in Germany [1]. The largest CFB boiler, a supercritical unit with capacity of 460MWe made

by Foster Wheeler Corporation, is under construction in Lagisza, Poland [2]. In China, the number

of commercial CFB boilers that have been put into operation is over 800, among which the units

with capacity 100-150MWe are near 30 [3]. The first 300MWe CFB boiler (Alstrom licensed) is

in construction [3].

Studies on CFB boilers have being conducted at the Tsinghua University (TH), Beijing,

China since 1985, in both fundamental research and commercial development. A series of CFB


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boilers with capacities ranging from 20t/h to 460t/h have been put into commercial operation and

some other units with larger capacities and higher steam parameters are under design or feasibility

study [4,5,6], based TH’s research and development (R&D) achievements. In this paper, a

summary of the two-decade R&D works on CFB boilers by the TH research group, especially

those works linking the fundamentals with practical application is to be given.

TWO PHASE FLOW IN CFB BOILER

Typically, the main loop of a CFB boiler is composed of a riser, separators and loop seals.

For some small units, single separator and single loop seal might be applied. Nevertheless, the

main loop is a typical solid-gas two-phase flow system with chemical reaction. Appropriate

understanding of the fluid mechanics inside the furnace is of fundamental importance to design a

CFB boiler.

Theoretically, the regimes of fluidization can be classified into stationary bed (or say fixed bed), particulate fluidization, bubbling bed, slugging bed, turbulent bed, fast bed and pneumatic

transport, depending on the gas superficial velocity u f , bed voidage and physical properties (e.g.,

size and density) of the solid particles, as shown in Fig. 1[7]. Normally, the fluid mechanics inside

the furnace is separately described in two parts: a lower part and upper part. In the lower part, the

so-called dense bed, size distribution is rather wide with many coarse particles and bulk density is

rather high. Thus, the associated fluidization regime is not necessarily fast bed, it can be bubbling

bed or turbulent bed depending mainly on the uf .

Al2O3 Beads

d p=52 m

p=3580kg/m3

v p: particle velocity, m/s

Pneumatic Transport

u f (m/s)

Figure 1 Fluidization regimes for Al2O3 particles- bed voidage vs. superficial velocity [7]

However, in the upper part, the main portion and so-called free board of the bed, the

classification of its fluidization regime has been an argument in CFB boiler research community

for a long time. Since the bulk density of most coal-fired CFB boiler furnaces (tens of kg/m3

or

even less) [8] is much smaller than that of fast bed reactors in chemical engineering process (in the


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range of hundreds of kg/m3) [3], it was easily intended to classify the fluidization regime as

pneumatic transport. However, the authors suggest that the upper part of a CFB boiler still belongs

to fast bed rather than pneumatic transport.

As we known, the most distinguished feature of a fast bed is the formation of cluster in the

riser, resulting in strong vertical mixing. According to our observation, temperature distribution is

rather uniform in the bed not only in the core region in radial direction but also along the furnace

height, even the combustion keeps going in the gas-solid flow and the furnace is surrounded by

water-cooled membrane. Such temperature uniformity can be only maintained by the existence of

strong vertical solids mixing and thus the existence of clusters.

During the CFB boiler evolution history in China, a CFB boiler was once regarded as nothing

else than the traditional bubbling bed boiler with an extended free board. However, the

fluidization regime inside a bubbling bed boiler is totally different from that inside a CFB boiler.

In a bubbling bed, only small amount of particles are entrained into the free board so that

combustion fraction in the dense bed is about 75-85%, and a rather amount of immersed tube has

to be arranged there. However, in a CFB boiler, much more particles are entrained into the free

board so that combustion fraction in the dense bed only occupies about 50-60%, and no

convective heat transfer surfaces are necessary to be arranged there. It was found that for a

bubbling bed boiler retrofitted with fly ash recirculation, if the recirculation flow rate is above a

critical amount, the hydrodynamic and thus combustion and heat transfer behaviors inside the bed

become CFB-alike and qualitatively different from bubbling bed. The temperature in the dense

bed can be even too low to keep stable combustion.

Given the upper part of a CFB boiler is a fast bed, shown in Fig. 1, for certain particles, flow

dynamics of the two-phase flow, or called hydrodynamic state can be defined by two parameters:

superficial velocity uf (m/s) and solid circulating rate Gs (kg/m2⋅s). For engineering simplicity, Gs

is also assumed to be the solid flux at the separator entrance.

Then the onset superficial velocity of fast bed for certain size particle is defined as uc [9]:

uc=(3.5-4)ut (1)

where, ut is the terminal velocity of particle, m/s.

The minimum solid circulating rate to enter the fast bed regime Rmin can be estimated by [7]:

2.25 1.627

c f min 0 627

p p f 0 164[g ( )].

u R

. d ρ ρ

ρ =

− (2)

where, ρ f is the gas density, kg/m3; ρ p is the particle density, kg/m

3; and g is the gravity, m/s

2.

It can be seen from (2), for a certain uf , CFB boiler can operate at various states in fast bed

regime because the bed inventory in CFB boiler is composed of different size particles.

MATERIAL BALANCE IN CFB BOILER

Based on the observation on coal-fired CFB boilers, the average size of bed inventory, which

is often called bed quality, is finer than that of bubbling bed boiler and even finer than that of

feeding raw coal. Thus, an accumulation process for size selection exists during the operation. In

order to study the material balance, a conceptual model was built up by TH’s CFB boiler research

group [10].


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CFB boiler is an open system for solid flow

Different from most chemical reactors, a CFB

boiler is an open system for both gas flow and solid flow.

The solid inputs are ashes formed from feeding fuel,

limestone and, sometimes, inert sands for making up.

There are two outlets for solids to exit: one is on the

furnace bottom for draining bed ashes and the other is on

the separator top for blowing fly ash, as shown in Fig.2

[11].

Conceptual model of material balance in CFB boiler

Solid particles of any size interval should be kept in

balance during the stable operation, so

Gin(i)=Gout(i)+ F (i) (3)where, Gin(i) is the flow rate of solids with size d i

entering the system, which is from the ash formation of

coal and limestone or make-up sands; F (i) is the flow

rate of fly ash with size d i; Gout(i) is the flow rate of

drained bed ash with size d i; X (i) is the fraction of particles with size d i in dense bed; and E (i) is

the entrainment rate of particles with size d i.

The entrained flow rate of particles with size di is accounted as E (i)× X (i).

The separator efficiency for size d i based on the entrained flow is:

s

( )

( )=1 ( ) ( )

F i

i E i X iη − ⋅ (4)

Then,

F (i)= E (i)⋅ X (i)⋅(1-η i) (5)

If we define the bed ash drain efficiency η o based on the entrained flow as:

η o(i)=1-Gout(i)/ E (i)× X (i) (6)

Then, the overall efficiency of the system η m to maintain particles with size d i is:

outm o

( )+ ( )( )=1 1

( ) ( )i i

G i F ii

E i X iη η η − = + −

⋅ (7)

Material balance equation can be expressed as:

Gin(i)=Gout(i)+ E (i)⋅ X (i)⋅(1-η i) (8)

Σ X (i)=1 (9)

Provided E (i) is properly given in literature and segregation in dense bed can be neglected,

then after solving the equation group, we have:

outout

( )=

( )

G iG

X i (10)

Some interesting and valuable results can be derived from the model. Figure 3 depicts the

variation of overall system efficiency η m with particle size d and the size distribution of bed

inventory for given separator efficiency η s and ash drain efficiency η o. It can be seen that η m first

Gin(i)

Gout(i)

F (i)

X (i)

( i ) × X

( i )

Figure 2 Concept of material

balance of CFB boiler


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increases with the increasing of d i and

after it reaches a peak value it

decreases with the further increasing

of d i. As d is smaller than the peak

value, η m is dominated by η s and as d

is larger than the peak value, η m is

dominated by η o. The particle size

distribution of bed inventory, also a

solution for given η s and η o, exhibits

a cap-like curve. Moreover, the

particle size corresponding to the

peak value on the size distribution

curve is consistent with that

corresponding to the highest η m.

The size distributions of bed

inventory for two separators with

different cut sizes d 50 and d 100 are

shown in Figure 4, for three different

uf s while the ash drain efficiency η o is

the same. It can be seen, as the

material balance is built up, the size

distributions of bed inventory are

remained. For a separator with better performance, namely smaller d 50 and

d 100, the particle size corresponding to

the peak value of the size distribution

curves is smaller. This result is

straight forward since more fine

particles are captured if separation

efficiency increases. For the same

separator, for different uf s, the particle

size corresponding to the peak values on the frequency distribution curves are nearly constant. As

uf increases, more fine particles are entrained into and stored in the free board. At the same time,Gs increases and the amount of returning particles increase, forcing more ash particles including

the particles less than d 100 are drained from the bottom. As a result, the mean particle size

decreases and fewer particles can be entrained and thus Gs decreases. When balance is reached,

more particles around the mean value are drained, and consequently the overall distribution of the

particles becomes wider though the mean particle value keeps nearly the same.

It is clear that although the size of feeding particles into system is widely distributed, the CFB

boiler system behaves like size selection machine. Coarse particles which can not be entrained are

drained out from bottom of bed, and very fine particles which are difficult to be capture by the

separator are carried out the system by flue gas. Only those particles that can be entrained by the

0

20

40

60

80

100

0 200 400 600 800 1000

Particle size d i µm

E f f i c i e n c y η i %

0.0

0.1

0.2

0.3

F r e q u e n c e d i s t r i b u t i o n P i

% / µ mOverall efficiency

Ash drain efficiency

Separator efficiency

Bed material size

distribution

Figure 3 Overall efficiency of the system

0

0.1

0.2

0.3

0.4

0.5

0.6

0 200 400 600 800


F r e q u e n

c e d i s t r i b u t i o n P i

% / µ m uf

m/sd 50µm

d 99µm

5.5 17 110

5.5 27 160

5.0 17 110

5.0 27 160

4.5 17 110

4.5 27 160

Figure 4 Size distribution of bed inventory for

different cyclone efficiencies


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flue gas and also be captured by the separators are retained in system for circulating. The results

indicate that the average size of bed inventory (bed quality) and the circulating rate of ash are

depending on the performance of separator and bed ash drain characteristics, besides the

superficial velocity and ash formation characteristics of coal and limestone. Thus the overall

system efficiency, especially the efficiency for circulating ash (near the d 99 of separator) is very

important and sensitive for the circulating rate. Our studies on the commercial CFB boilers

showed that G s is typically in the order of 103 larger than the feeding rate of such size particles, so

the efficiency near this size should be over 99.7%. This result is not only important for the design

of separator but also important for determination of bed ash drain characteristics. In engineering

practice, sometimes, ash drain facilities with specific size classification, combined with ash cooler,

are needed to keep fine circulating ash in bed.

1-D model for CFB material balance

A 1-D material balance model was developed by the co-research work between TU and EDF[12]. Standard bench-scale facility and test procedure were implemented to measure the coal ash

formation and attrition characteristics [13] that are used as input data for the model. The particle

segregation in dense bed was taken into account in the model to characterize the bed ash drain.

The prediction on resident time of different size particles and its impact on attrition is a novel

feature of the model. The model was calibrated by the field test data from three boilers in China

and successfully applied to predict the

material balance in the Gardanne’s

250MWe CFB boiler. Figure 5

compares the size distributions of fly

ash between the data measured in thefield of this boiler and those predicted

by the 1-D model. It can be seen that

there is an important impact of

attrition on ash size formation.

Without taking the attrition of solid

particles into account, remarkable

discrepancy would be induced.

Figure 6 is the comparison of the

bulk density along the height of

furnace by field test and model prediction.

STATE SPECIFICATION FOR CFB BOILER DESIGN

State Specification and its importance

The “State Specification” for a CFB boiler means to keep the CFB boiler in a specific state

such that it can operate stably and continuously. From previous discussions, the state of a CFB

boiler can be represented by the superficial gas velocity uf and solid recirculation rate Gs. The uf is

a design and operating parameter, while the Gs is a dependent variable on the uf , separation

¿ÅÁ£Ö±¾¶ (micron)

10 100 1000 10000

Ö Ê Á ¿ Æ µ ¶ È

( % / m i c r o n )

0.000

.002

.004

.006

.008

.010

.012

.014

.016

ProvinceÑ-»·²́²âÊÔÊý¾ ÝGardanneÃºÖÖ±¾Õ÷³É»Ò·Ö²¼Gardanne¼ÆËã³É»Ò·Ö²¼

Measurement

Intrinsic

Model prediction

F r e q u e n c e d i s t r i b u t i o n P i

% / µ m


Figure 5 Comparison of model prediction on the

ash formation w/o attrition with the data measured in

the field for a 250MWe CFB


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efficiency, ash drain efficiency and solid inputs in the open system. For an industrial combustion

process, the operating state has to be controlled to a stable state.

Figure 6 Comparison of model prediction on the pressure drop profiles along the

furnace height with the data measured in the field for a 250MWe CFB boiler

In case that the feedings of particles such as coal, limestone or make-up sands are varying,

the state of a CFB boiler might keep changing as well if Gs can not be controlled. Consequently,

the heat transfer coefficients between water-wall membrane and solid-gas flow in furnace, which

strongly depends on the bulk density [14], and the fractional fuel heat releasing along the furnace

height could not be kept stable during

operation. Fortunately, as we

discussed in the material balance

section, Gs can be manually

controlled by adjusting the bed

inventory.

Shown in Fig. 7, the increasing

of the bed inventory leads the

increasing of bulk density in furnace,

and thus the increasing of Gs at the

furnace outlet.

State Specification plays a

fundamental role in CFB boiler

design. In engineering practice,

before conducting the detailed design,

CFB boiler designers usually selected

a specific state in fast bed regime for

0.0

0.2

0.4

0.6

0.8

1.0

0.0 0.5 1 .0

D im e n s io n le s s P re s s u r e

D i m e n s i o n l e s s H e i g h t

0.0

0.2

0.4

0.6

0.8

1.0

0.0 0.5 1.0

Dimensionless Pressure

D i m e n s i o n l e s s H e i g h t

Dot: measurements

Line: Model Prediction

Case 2Case1

Dot: measurements

Line: Model Prediction

0

0.2

0.4

0.6

0.8

1

0 0.2 0.4 0.6 0.8 1

Demensionless bulk density

D i m e n s i o n l e s s h e i g h t

1- Case 12- Case 2

3- Case 3

1 2 3

Figure 7 Bed inventory vs. the bulk density in furnace

and circulating rate G s


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the CFB boiler, namely to perform State Specification for a CFB boiler. After State Specification

(with fixed uf and Gs), designers started to collect much referential data such as heat transfer

coefficients and fractional heat releasing along height of furnace, mainly from the field test on

demonstration boilers collaborated with laboratory researches. This accumulation is actually a

long-term R&D work. Based on State Specification and the following data accumulation, a

program, so-called Design Code, would be developed to design the layout and components of the

CFB boiler. Once CFB boilers designed by the Design Code are put into commercial operation,

more data are provided to improve and mature the Design Code. As a result, on one hand, each

CFB boiler manufacturer owns a specific Design Code as a commercial secret and makes CFB

boilers in different styles; on the other hand, it is also very difficult and challenging to change the

Design Code once it becomes a design standard because all design data based on a specific state of

a CFB boiler need to be re-accumulated. Consequently, special cautious should be paid in State

Specification.

Major Consideration in State Specification

The determination on superficial velocity uf

The uf in a CFB boiler should be higher than the onset velocity of fast bed corresponding to

particle size as mentioned before. Some designers favor higher uf s in order to obtain higher

specific cross section load. However, uf is limited by the erosion on the vertical water wall,

besides the resident time for fine coal particles burnout and de-NOX [15].

The determination of solid circulating rate Gs

The Gs should be more than the minimum solid circulating rate of fast bed regime - Rmin as

discussed before; otherwise the bed is a bubbling bed. The upper limitation for Gs depends on

several considerations. For example, since Gs is related to the total bed inventory (Fig. 7), it isrelated to the power consumption of draft fan. In addition, the total bed inventory can be divided

into circulating ash inventory that is important for keeping an enough amount of Gs, and the coarse

particle inventory that is important for keeping sufficient resident time for the burnout of coarse

coal particles. Recent research in China shows that the solid suspension in furnace influences gas

diffusion, thereby the burnout efficiency of coal char. Another factor limiting Gs is the erosion in

furnace.

State Specification Practices

The Gs-uf diagram shown in Fig. 8 summarizes the State Specification done by several major

CFB boiler manufacturers in the world. Because few data on circulating rate for commercial CFB

boilers were published, much of the data were by our estimation or by our field measurements

(most CFB boiler makers have demonstration boilers in China).

In above state diagram, the dot-dash curve close to the uf axis is the onset circulating rate of

fast bed which is based the calculation assuming the particle size is around 200µm (according our

observation, the cut size of circulating material for most CFB boilers is around 150-250µm

[4,16,17]. Below the line, fast bed state can not be realized. Above this curve, there are two curves

(one in dot-dash, and the other in dash) representing the maximum circulating rates for CFB boiler

with one stage cyclone and two stage cyclones in serial respectively, both predicted by TH- EDF

material balance model assuming no limestone or inert additives are added. Two dot curves


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approximately parallel to the Gs axis stand for the erosion limitation for lignite combustion and

hard ash content coal combustion respectively. These limitations are from our observation for a

group of CFB boilers with different design statuses and for burning different coals in China.

According to our observation, the hardness of ash and the superficial velocity have more

significant impact on erosion than the circulating rate. We have to point out here, for some CFB

boiler technologies of which uf is near or over 6m/s, serious erosion has been found on the vertical

water wall in furnace within limited operating period burning lean coal, bituminous or anthracite

coal. As shown in Fig. 8, those CFB boilers are operating at the states near to the erosion line.

Although, no erosion problem on vertical water wall has been reported for the boilers using same

technology while lignite is burning, it is safer for the designers to select uf to be lower than 5.5m/s

in case fuel quality can not be guaranteed.

Fluidizing velocity uf m/s

0 1 2 3 4 5 6 7 8 9

One stage cyclone

Soft coal

C

D

E

G

H

I

B

A

Commended

F

A s h c i r c u l a t i n

r a t e G

k

/ m

2 ⋅ s

10

30

5

10

15

20

25

Hard coal

Fast bed limit

Limits for erosion

protection

(Two stage cyclone)

Figure 8 State Specification by several major CFB boiler manufacturers

In fact, the selection of acceptable fast bed state is limited within a small area in the statediagram. TH also suggest its own state (marked in asteroid *), which is safe for most coal types

and the Gs is also far away from the material balance limit.

Clearly, after stated of a CFB boiler is specified, a reliable material balance model is needed

for designers to validate the material balance for design coal. The bench-scale tests on ash

formation and attrition characteristics for coal and limestone are strongly suggested to be done

first. With those experimental data, designers can use the model to check if the maximum ash

circulating rate has enough margins for the specified state. If it does not, the model can estimate

the quantity and quality of make-up sands.


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HEAT TRANSFER IN CFB FURNACE

There are enormous literatures

on heat transfer research in CFB boiler s [18~21]. They are valuable

for understanding the mechanisms

of heat transfer in bed, but difficult

to be directly used into application.

For engineering purpose, TU has

conducted a series of experimental

studies on the commercial CFB

boilers. A Heat Flux Probe and a

Solid Suspension Density Probe

were developed to measure the heattransfer coefficients and solid

density respectively and

successfully applied in the field

tests. The schematics of heat flux

probe and local bulk density probe

are shown in Fig. 10 and Fig. 11

[22]. At the same, a semi-empirical

model was developed based on the suggestions from Bo Andersson and Leckner [23] and further

correlated with the field data.

The overall heat transfer coefficient between two phase flow and the water wall, α b, is mainly

composed of two components – particle suspension convective heat transfer coefficient α c and

particle suspension radiative heat transfer coefficient α r .

α b=α c+α r (11)

The α c is expressed as the function of local bulk density of solid suspension ρ as:

α b=a ρ b (12)

where, a and b are correlation parameters with data from the field test. The α r is calculated by

following equation:

)(*)()111

/(1 w b2

w

2

b

w b

T T T T r ++⋅−+= σ ε ε

α (13)

where, T and ε denote for temperature emissivity respectively, and the subscripts of b and w

denote respectively the suspension and water wall.

Later on, the heat transfer model was improved by taking the geometric factor of water

membrane into account. TH’s heat transfer mode has been proved to be simple and with satisfied

accuracy for engineering purpose, and it has been practiced in the design of more than one

hundred units of CFB boilers with different capacities.

More detailed information about the model can be found in other publications [24].

COMBUSTION IN CFB

BottomCover

Upper

Cover

SlideGuide

SlideControl Bar

Figure 10 Sampling probe for bulk density

Thermal insulation layer water outlet Protecting shell

Thermocouples in probe surface

water inlet

Thermocouples in center

Probe

Figure 11 Schematic of Heat Flux Probe


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Coal combustion modeling and bench-scale experiments also have been extensively

conducted at TH. It was found that the coal particles, as soon as fed into CFB boiler furnace,

experience a primary fragmentation by devolatilization or by thermal stress, and then a secondary

fragmentation by combustion of char [25]. The volatile combustion occurs mainly in bubbles in

the dense bed and in dilute phase in the freeboard. The char combustion occurs in emulsion phase

in the dense bed and also in dilute phase in the freeboard. The combustion rate of char is

controlled by both reaction kinetics and gas diffusion.

Our studies also found that the combustion occurring in the dense bed of a CFB boiler is in

fuel lean condition, which is on opposite of a bubbling bed boiler [26]. The result matched the

experimental observation by Leckner [27], who reported the vigorous fluctuation of oxygen in bed.

Our later research proved such phenomena is contributed to the average particle size in CFB

boilers (around 200µm) is much smaller than that in bubbling bed (around 1mm) [28]. Compared

with bubbling bed boilers, in CFB boilers, the fraction of fluidization air into emulsion phase is

smaller and the resistance of gas exchange between bubble phase and emulsion phase bed is

stronger. Char combustion mostly occurs in emulsion phase in the dense bed, consuming most of

oxygen over there. Since oxygen can not be compensated from bubbling phase, the CO

concentration on the boundary of dense bed of CFB boiler is very high [29]

Again, the combustion theory was applied to the commercial CFB boiler design. The

concept so-called “vertical distribution of combustion and heat in furnace” was introduced by TH

[28]. This concept is useful for boiler designers to arrange heating surfaces in furnace and it was

also validated by gas sampling along the furnace height of some commercial CFB boilers. The

field test data of vertical distribution of combustion and heat were also used to correlate the 1-D

combustion model developed by TH. Figure 12 shows the experimental results of accumulative

heat released along the

height of a bench scale CFB

apparatus.

Both modeling and

measurement showed that

the vertical distribution of

combustion and heat in CFB

boilers are strongly impacted

by the volatile content and

size distribution of fuel. Theresults shown in Fig.13

indicate that volatile matter

prefers to be burnt in the

upper part of the furnace and

so does fine char particles.

Therefore, proper size

distribution of specific

feeding fuel is required to satisfy a uniform temperature distribution in CFB boiler furnace.

An interesting result should be mentioned is that the accumulation of heat releasing in dense

0.0

0.2

0.4

0.6

0.8

1.0

0.0 0.2 0.4 0.6 0.8 1.0

Dimensionless height

A c c u m u

l a t i v e c o m b u s t i o n h e a t f r a c t i o n

Figure 12 Distribution of the accumulative heat released along

the height of a bench-scale CFB boiler


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bed of CFB boiler is much less than

that of in bubbling bed. This was

explained before, and also tells us

why we have to put certain amount of

immersed heating surface in bubbling

bed to keep heat balance, but it is not

needed for CFB dense bed.

CONCLUSIONS

A set of design theory for CFB

boiler has been developed by the

researchers at Tsinghua University,

based on twenty-year research anddevelopment experience on CFB

boiler. The theory couples the

fundamental studies in the laboratory

with the experiments on the

commercial CFB boilers, and has

been applied in designing more than

100 commercial CFB boilers. Followings are a few main points of the design theory.

1. The flow pattern inside CFB boiler furnace is classified as the superposition of a fine

particle fast bed in the upper part and a bubbling bed or turbulent bed in the bottom part with bed

coarse particle segregation.2. CFB boiler is an open system for solid-gas flow. Modeling studies shows the bed quality

strongly depends on the overall system efficiency and ash size formation and attrition of coal on.

3. The state of a CFB boiler is defined by superficial velocity u f and circulating rate G s. A

CFB boiler can operate at different states in fast bed regime with a given u f and dependent G ss by

adjusting the bed inventory during operation.

4. As first step of process design, CFB boiler designers specified a firm state of fast bed for

the CFB boiler burning design coal. This step is called State Specification and is the base of CFB

boiler design. The State Specification is mainly performed on engineering experience.

5. After State Specification, double check the material balance for design fuel by material

balance model and corresponding ash formation and attrition experiments is suggested. If the

material balance does not satisfy, certain amount make-up inert sands instead of selecting a new

state is recommended, because almost all design data are based on the specified state, including

local heat transfer coefficients and combustion heat releasing profiles.

6. A simple model on heat transfer suggested by Bo Leckner and his coworkers can be

adopted and improved, and integrated in the Design Code for commercial CFB boiler design. The

model has satisfied accuracy in engineering practices.

7. Combustion of char and volatile content shows different behaviors in CFB boilers. The

coal combustion also is different between a CFB boiler and a bubbling bed. The concept of

vertical distribution of combustion and heat in CFB boiler furnace was introduced. Modeling and

0

0.2

0.4

0.6

0.8

1

0 1 2 3 4 5

Height h m

A c c u m u l a t i v e c o m b u s t i o n

h e a t f r a c t i o n

V daf 34.4%

Char

0.5~0.6mm

1.0~1.6mm

Figure 13 Vertical distributions of combustion and

heat inside CFB furnace burning coals with different

volatile content and coal size

0

0.2

0.4

0.6

0.8

1

0 1 2 3 4 5

Height h m

A c c u m u l a t i v e c o m b u s t i o n

h e a t f r a c t i o n


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experimental studies were conducted on the coal combustion in CFB boilers indicated that volatile

matter prefers to be burnt in the upper part of the furnace and so does fine char particles. Therefore,

proper size distribution of specific feeding fuel is required to satisfy a uniform temperature

distribution in CFB boiler furnace.

ACKNOWLEDGMENTS

Financial supports of the present investigation by EDF and Chinese National Key Projects of

Tenth-Five Plan are gratefully acknowledged.

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Note: Will be published in the 18th International Conference on Fluidized Bed Combustion,Toronto Canada, 2005

DESIGN THEORY OF CIRCULATING FLUIDIZED BED.pdf

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