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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
Particle size d i µm
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 ÝGardanneúÖÖ±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
Particle size d i µm
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