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DESIGN OF GAS DISTRIBUTORS FOR FLUID BED BOILERS
DESIGN OF GAS DISTRIBUTORS FOR FLUID BED BOILERS Prabir Basu Centre for Energy Studies, Technical University of Nova Scotia Halifax, Nova Scotia, Canada
ABSTRACT Principles in the design of air distributors for a fluidized bed boiler have been reviewed. Three major types of distributors are discussed. The criteria for uniform and stable fluidization are discussed. Finally, a step by step method has been suggested for the design of distributor plates. The design procedure is illustrated with an example on the design of a nozzle type distributor. INTRODUCTION The primary function of the distributor is to introduce gas uniformly into a fluidized bed. Except in the case of sparge pipe distributors, it supports the weight of the bed material too. The distributor has a strong influence on the size and frequency of the bubbles in a fluidized bed. This, in turn, affects the com bustion of coal in the bed. The distributor also affects the attrition of bed materials (Botterill, 1975). A poorly designed distributor may lead to low com bustion efficiency or even agglomeration. Bearing this in mind Botterill (1975) and Whitehead (1971) suggested the following desired properties of the distributor: 1. 2. 3. 4. 5. 6. 7.
It should promote uniform and stable fluidization. It should minimize the attrition of bed materials. It should reduce erosion of the bed-internals or in-bed heat exchanger tubes. It should prevent backflow of bed materials through holes. It should ensure complete fluidization over the entire range of operating velocities and temperature. It should minimize the dead zones on the distributor. Plugging of the orifices over extended period of operation should be minimum. TYPES OF DISTRIBUTORS
Following are the common types of distributor plates used in fluidized bed boilers or combustors. Some of the typical distributors are shown in Fig. 1. (a) Straight hole or perforated type; (b) Nozzle type; (c) Pipe grid type. Besides these, there are a few other special purpose distributors such as the ones described by Korenberg (1982), which are meant for fuels with tramp materials.
45
P. Basu
46 Straight Hole Distributors
Perforated plates, the simplest types of distributors, were widely used during the initial periods of fluidized bed boiler developments. The holes were drilled directly on the plate supporting the bed material. The holes could be either straight, divergent or convergent. Zenz (1968) suggests that upward diverging holes will help eliminate dead zones. There are few propietory distributor plates such as fConiduref, fProcedynef, fOrthos', etc. If the operating conditions match the design conditions of the plates, they perform satisfactorily. However, these have not been extensively used in fluidized bed boilers. Nozzle Distributors In nozzle type plates air enters the bed through nozzles at a certain height above the main distributor plate, which supports the bed materials. As a result of this, a layer of insulating inert bed material always remains on top of the load-bearing plate, which, therefore, does not distort under heat. The exit ports of the nozzle may be slightly inclined, but in general they are horizontal. Therefore, backflow of solids through the ports is greatly eliminated. The nozzles are arranged in different fashions on the plate. This determines the formation of dead zones. However, if the roofs of the nozzles are taperd, the dead zones are negligible compared to those in case of a perforated plate with holes at the same pitch as the nozzles (Wen, 1980). Pipe Grid Distributors Pipe Grid or sparge tube distributors are pipes with orifices drilled in them. They are being increasingly used in boilers. The major advantages of this distributor are as follows: (a) (b) (c) (d)
no no it it
elaborate air box - sometimes there is no air box at all; problem of sealing to prevent air by-passing the grid; is simple to install and build; does not support the bed material.
The major problem is that it cannot be used for a very wide bed as it would require a very long pipe, which, in turn, will be required to have a large diameter for a reasonable pressure drop. It is relatively difficult to achieve uniform fluidization in a long but low pressure drop pipe grid distributor because a large varia tion of flow along the length of the pipe is observed (Zenz, 1962). In order to reduce the dead zones, the orifices may be oriented in different radial directions. A rule of thumb for design of pipe grid distributor for turbulent flow is that the ratios of kinetic energy of the inlet stream to pressure drop across the outlet hole and friction loss in the pipe to pressure drop across the outlet hole should be equal to or less than one tenth (Perry, 1973). Other Distributors Water cooled distributors with orifices or nozzles on the fins joining two water carrying tubes have been successfully used in commercial fluidized bed boilers (Basu, 1981) (Fig. 1). It saves the cost of the distributor by making it a part of the boiler heating surface and at the same time it reduces thermal expansion problems. It is especially suitable for the beds which are ignited by hot air. Slits or grate bar distributors have fine gaps or slits formed by bars or fins of a tube. These slits prevent channelling (Gregory, 1967) and also help to reduce
FLUIDIZED BED BOILERS: DESIGN AND APPLICATION
47
back flow of solids into the air box, but are not yet very popular amongst the boiler designers.
DESIGN EQUATIONS Pressure Drop Through Distributors The pressure drop through the distributor can be written from simple orifice theory; AP
d ■ CD Uor2/2§
The orifice discharge coefficient, C is taken by Zenz (1968) to be equal to 0.8. Kunni and Levenspiel (1969) give values increasing from 0.1 to 0.7 while the Reynolds number (based on approach diameter) increases from 2 to 200. Thereafter, it remains steady at 0.6. Behie, et al. (1978) have demonstrated the effect of orifice Reynolds number and the presence of fluidized solids on C . For empty beds it varies between 0.84 to 0.76 whereas the presence of solids reduce it to the level 0.73 to 0.77. The presence of solids on the plate prevents sudden expansion of the gas issuing through the orifice. Thus it reduces the orifice discharge coefficient. Behie (1978) further observed that through extended operations, the square edges of the orifices are rounded by solids erosion. This could reduce the discharge coefficient. No data was provided. However, the designers may keep in mind the expected reduction in distributor pressure drop through this effect in the long run. The thickness of the plate will have an effect on the orifice discharge coefficient. Quereshi and Creasy (1979) presented an expression for C ; C = 0.82 (t/d ) ° · 1 3 D or
(2)
Non-Uniform Distribution of Air Non-uniform fluidization is most undesirable in a fluidized bed. This is often caused by inoperative orifices. Whitehead and Dent (1967) observed that in large (0.09 - 5.9 m ) and deep (1.2 m) beds with bubble cap type distributors, the minimum velocity for complete operation of all the orifices, u , was greater than 3 u f . For perforated plates Fakhimi and Harrison (1971) found this limit to be 1.53 u _ and it was observed to be lower for larger particles. Davidson, et al. (1977) mentioned that a small spout whose height is about twice the orifice spacing should be formed on each orifice for uniform fluidization. Based on this they stipulated that the minimum pressure drop for full operation of all the orifices would be ΔΡ
=
(1 - 2/ïï)hoPï(l 1
- <%fV
£lnf )s
(3)
2
Yue and Kolaczkoski (1982) presented a more complicated expression to recognize the effect of bed depth, H on the minimum velocity for full operation of all the orifices. For nozzle type distributors a correlation was presented by Whitehead (1971). After converting his correlation to SI units it appears as below.
48
P. Basu
[ where:
L , .7 + {.49 + 1
X
»
1.98 x 10 7 (N 0 # 2 2 K 2 H ) l· 1D ps s \
%2f
f
(A)
K D = 992.7 U/ΔΡ ^
Non-uniform fluidization is also caused by mal-distribution of incoming gas. A flow resistance across the orifice which is considerably larger than rearrangement resistance of incoming gas results in uniform distribution of gas to all the orifices. Kunii (1969) estimated this rearrangement resistance to be equal to the expansion loss. Richardson (1961) suggested that the distributor pressure drop should be about 100 times the rearrangement resistance for uniform distribution of gas P
a i r / 2 g (u o p x \ / Α ± ) 2 < ΔΡ^ΙΟΟ
(5)
A sparge tube distributor has rather erratic distribution of flow through uniform orifices (Zenz, 1962; Basu, 1978). For short and low resistance tubes, maximum flow occurs furthest from the air entry, but this trend changes with flow rate. Therefore, design of this type of distributor needs careful consideration. Channelling is also a major problem. It is particularly bad in case of a shallow fluidized bed. The kinetic energy of the orifice jets at times exceed the bed weight; and the jets punch holes right through the bed creating a low resistance path (Kunii, 1969). Federov (1955), however, concluded that if the grid resistance is equal to, or greater than the resistance of the bed, fluidization occurs uniformly without any channelling regardless of the bed-depth, bed area or particle size. Qureshi (1979) found the following stability criterion would ensure that a sudden channel formation will subside instead of aggravating with increasing velocity d(AP. + ΔΡ,) ^ Ìdu
> 0
(6)
3 Wen (1980) suggested that this value should be greater than 800 Ns/m . Based on these pictures, various workers have determined different ratios of pressure drop through the distributor and that across the bed for stable fluidization (Table 1). Qureshi and Creasy (1979) correlated these data to find the criterion for stable and uniform fluidization ΔΡα/ΔΡ
= 0.01 + 0.2 [1 - exp (-D/H
f )]
(7)
Dead Zones Formation of dead zones of particles between the orifices has been studied by Wen (1978), Kozin and Baskakov (1968), Pétrie and Black (1966). Such zones adversely affect the combustion and heat transfer. Kozin and Baskakov (1968) showed that in case of a nozzle or bubble cap type distributor, the gas jets issuing from the nozzles coalesce at the boundaries between the caps. These jets then form series of ascending bubbles with accompanying solids. Unless the top of the nozzles are tapered, the descending solids form a stagnant layer on top of the nozzle. There is some possibility of the bubble stream breaking the surface and forming spouts. This results in poor gas conversion. This can be avoided if bed height is 30-35 times the diameter of the cap exit opening (Kozin and Baskakov, 1968). Wen (1980) presented the following correlations for no dead space in nozzles type distribu tors.
49
FLUIDIZED BED BOILERS: DESIGN AND APPLICATION
P < Γκ (u. - u. f ) d . 2 l 0 ' 4 5 + d. - L
J
jmf i J
(8)
i
All units are in CGS. K = 0.62 for arrangements where the center line of the holes coincides with pitch line of the nozzle. K = 1.43 for arrangements where the center line of the orifices is turned 45 from the pitch line of the nozzles. Yia-Ching (1980) studied the effect of wide cut particles. Wen (1980) also proposed correlations to avoid dead zones in perforated plates, where the solid circulation is just the reverse of that of nozzles.
P
or
l - u - y +1 3on , 1NΓ
205 -|i/(o.7i6 d °· ) P I
(9)
L °- orJ
All variables are in CGS except d which is in micron. P Dead Zones Near the Walls Wen (1980) observed that higher flow rate of air is necessary to eliminate the dead zone near the walls more than those near the center for the bed. He presented different correlations for this minimum exit velocity for elimination of the dead zone, but proposed equations (8) and (9) for calculation of the orifice or nozzle spacing. It would appear from above that a smaller pitch will help eliminate the dead zone near the walls, but this may lead to short circuiting of air up the wall. So it is difficult to define a universal procedure to solve this problem. One has to rely on experiences by taking note of the above situations. Jet Heights Small vertical jets, formed on individual orifices in a perforated plate distribu tor, hit the heat exchanger tubes located within the jet region and seriously erode these. Furthermore, taller jets result in poor gas conversion (Basu and Grace, 1983). A large number of workers, including Wen (1977), Yang and Keairns (1979), Zenz (1968) proposed different correlations for determination of the jet height. Sit (1981) analyzed these correlations and found the one by Merry (1975) to give the best fit for vertical jets.
[ ι if] H * ·] P d -10.3 Γ
/u
2
\0.2
Ί
(10)
The designer should locate the tubes above the height L.. Bubble Coalescence Closer pitch avoids dead zones, but too close a pitch causes premature bubble coalescence which results in poor gas conversion. Zenz (1968) suggested that pitch should be greater than half bubble diameter. Wen et al. (1980) assumed this value 1.5 times the bubble diameter at the tip of the jets. It may be calculated from Davidson and Schüler (1960) theory;
50
P. Basu . - -ui rr)/ ,I2 -/ 5 , c_ I|6(u -V1.5 op mf 1.5 | op mf | πΝ 11/5 /5 g g L or J
^nj
The same equation can be used to determine the height of the nozzle holes on the nipples as it would prevent the loss of insulating layer on the nozzle plate through an updrift caused by a rising bubble.
(12)
Particle Backflow Even if the orifice velocity is well above the terminal velocity of the particles, particles fall into the wide boundary layer in the orifices where pressure fluctu ation sporadically force them down into the windbox. This is called 'weeping1. Besides this, at low flow rates particles fall through the whole area of the orifice (Serviant, 1970). This phenomenon is termed 'dumping'. In both cases particle backflow leads to particle attrition and erosion of the orifices. Kunii (1970) suggests that the kinetic energy of the jets should be adequate to counter this tendency. unr. = (.7 to .85)(2gAP /p ) or ß g uor <
2g(APB/pg)
1 / 2
(13)
1/2
Kassim (1972) observed dumping in beds where bubbles are formed on the orifices whereas weeping occurred where gas jets into the bed from the orifices. He further proposed a correlation for the rate of weeping as a function of operating and design conditions. Serviant et al. (1970) presented data on weeping of 60ym diameter solids through 12.5mm and 18mm orifices. Bubble cap distributors greatly reduce dumping, although at times local eddies near the orifice cause some backflow of particles. Gregory (1967) suggested that slots of high length to width ratio reduce backflow of solids. Wen (1980) suggests that in order to reduce weeping and dumping, the orifice diameter should not be more than 3 to 8 times the particle diameter. Cost of manufacturing prohibits orifice diameters less than 1.5mm. So a rough guide could be d
or
= 3 x d p
d = 4 x d or p
for rperforated rplates for nozzle plates
(14)
MECHANICAL CONSIDERATIONS Thickness of the Plate The thickness of the grid plate influences the pressure drop to some extent. Higher the thickness, lower the pressure drop through the plate (Razumov et al, 1972; Qureshi, 1979; Gvozdev et al, 1963). A thicker plate prevents distortion of perforated type distributor and it facilitates fitting nozzles on the plate in case of nozzle type distributors. On the other hand, thicker plate is expensive
51
FLUIDIZED BED BOILERS: DESIGN AND APPLICATION compared to the thinner ones.
It increases the cost of drilling holes considerably.
The thickness of the plate should be at least adequate to support the weight of the slumped bed under operating plate temperature. The operating plate temperature may be taken to be 50 C above the inlet air temperature in case of nozzle plate. It could be as high as 400 C in case of a perforated plate type distributor supporting a fluidized bed at 800-900°C. In case of hot air passing through the plate, either the material should stand the temperature (i.e., use of high temperature steel) or water cooled membrane wall type distributor (Fig. 1) should be used. Sealing of the Plate Leakage of air around the edge of the distributor causes non-uniform fluidization. In smaller beds (D< 1.0m) it is easy to control this leakage by use of suitable gasket and uniform tightening of the bolts. This type of sandwiching the plate between the two flanges proves difficult for larger and rectangular beds. The best way to prevent this leakage is to weld the plate with the containing walls (Fig. 2 ) . This practice is being increasingly adopted in boilers where the walls will have rooms for thermal expansion. Pipe grid distributors are relatively free from this problem. Thermal Expansion Expansion of the distributor plate could be substantial in a large fluidized bed. In a smaller bed one can allow for this expansion by providing larger holes for the holding bolts. Larger beds would require flexible supports such as grid pins (Comparato, 1982). This would allow horizontal movement of the plate and at the same time it will carry the plate's load. Percentage Opening Fractional open area, S of the distributor is an important practical index of distributor design. Vanecek (1966) gives the following empirical relation for fluidized bed dryer. S = 1.7 (u /u ,)°· 9 r op mf
(15)
This gives values in the range of 2-10% which is higher than the usual 1-2% used in fluidized bed boilers. Vanecek (1966) further suggests an auxiliary grid placed below the main grid to help rearrange the entrance flow. The effect of incoming flow is rather pronounced in case of low pressure drop distributor. DESIGN PROCEDURE A step by step design procedure has been developed based on the foregoing discus sion. This procedure is based on nozzle or bubble cap type distributors. The design procedure is identical for perforated plate distributors except in a few steps where separate expressions have been presented. In certain places one has to make some compromise, which may necessitate choosing a value of distributor
52
p. Basu
pressure drop higher than that required for stable fluidization. A typical worked out example of design of a nozzle plate for an FBC boiler is also presented with the procedure. Most of the criteria of the design method can be met by increasing the pressure drop through the distributor plate, but this value should be as low as possible without sacrificing the stability and the uniformity of fluidization. Since a boiler requires a large volume of air, a small increase in the distributor pressure drop substantially increases the power requirement of the forced draft fan. So, a designer should be?very cautious about the checks of step (16) to (22) if ΔΡ + ΔΡ exceeds 11,000 N/m . Beyond this range multistage fan or unusually large blower may be required. Design of distributor is more an art than a rigid science. Every step in the suggested design procedure may be violated by the experienced designers. However, the present example illustrates the application of the general principles involved in the design of distributor plates for fluidized bed boilers. In the present example (Table 2) the thermal design suggests a bed area equal to 8 sq. m. Air enters the plenum chamber from the side through a port whose area is 1 sq. m. We have chosen a nozzle type distributor with a taper head as this would eliminate the dead zones and a layer of inerts will insulate the main plate from the hot bed. This plate will carry the weight of the bed solids. First we deter mine the pressure drop through the distributor which would ensure uniform fluidization. This will dictate the size and number of nozzles. Thereafter, the pitch and the height of the nozzles are calculated. This essentially completes the hydrodynamic design of the distributor. Then this design has been checked for stable operation, channelling, full operation of the orifices, elimination of dead zones and premature coalescence. In the present example fortunately all the criteria have been satisfied. So it was not necessary to modify the original design to meet the criteria. When deciding the location of the in-bed tubes the heights of the vertical jets on the orifices will be calculated from equation (10) and the tubes will be placed above this level to avoid excessive erosion. For a rough check, the percentage opening based on the bed area has been calculated and it is found to be within acceptable limits. Then the total number of the orifices has been calculated and it is not too large to be prohibitively expensive.
CONCLUSION The types and the desired properties of a fluidized bed gas distributor are discussed. A brief review of the design equations is also presented. A step by step method for the design of both nozzle and plate type distributors is explained with a worked out example. This method demonstrates the various considerations involved in the design of a distributor for a fluidized bed boiler. NOMENCLATURE Cross-section area of bed, m
\ A. 1 C
D
2
Cross-section area of the inlet to the wind box, m Drag Coefficient Equivalent diameter of bed, m
D d
Diameter of orifice, m or
2
FLUIDIZED BED BOILERS: DESIGN AND APPLICATION d
P
di
Ga
Hmf
I n t e r n a l d i a m e t e r of n o z z l e , m G a l i l i o number Height of i n c i p i e n t l y f l u i d i z e d bed, m Height o f t h e n o z z l e , m
h
hs KD
L
Mean p a r t i c l e d i a m e t e r , m
j
NT Nor NZ
n = f(Ga)
Height of s p o u t s above t h e o r i f i c e , m D i s t r i b u t o r flow f a c t o r of e q u a t i o n ( 3 ) Height of a i r j e t s above t h e o r i f i c e , m T o t a l number of n o z z l e s i n bed Numbers of h o l e s p e r n o z z l e Numbers of n o z z l e s p e r u n i t bed area, l / m Table I : G
=
2
1, n = 4.65
Ga = 100 n = 4.05
P e r i m e t e r of t h e bed, m
'e P
Pitch, m
QT
'r
Gas flow through n o z z l e s (scfm)
F r a c t i o n a l open area of d i s t r i b u t o r p l a t e Thickness of p e r f o r a t e d p l a t e o r n o z z l e wall, m
t uj U U
j mf OP
O r i f i c e e x i t v e l o c i t y , cm/s O r i f i c e e x i t v e l o c i t y of minimum f l u i d i z a t i o n , cm/s Operating v e l o c i t y , m / s
U
Minimum f l u i d i z a t i o n v e l o c i t y f o r f u l l o p e r a t i o n of a l l t h e o r i f i c e s , m/ s
U
Minimum f l u i d i z a t i o n v e l o c i t y , m / s
m
mf
Ut
Terminal v e l o c i t y , m / s
2
Height of expanded bed, m
B
Constant of Mori ti Moriyama
E
Bubble f r a c t i o n
B
E
mf
ps
T a b l e I ( . 3 8 f o r u/umf = 3)
Voidage a t minimum f l u i d i z a t i o n Density of bed m a t e r i a l , kg/m
3
53
54
p. Basu o
Density of air at operating temperature, kg/m Dynamic viscosity of air at operating temperature, Ns/m Pressure drop through the bed, N/m 2 Δρ
α
2
Pressure drop through distributor, N/m 2 BIBLIOGRAPHY
Basu, P., and K. L. Das (1978). Internal Report. Heat Power Laboratory, CMERI, Durgapur. Basu, P., and J. R. Grace (1983). Modelling of char Combustion in fluidized bed. Under preparation. Behie, L. A., B. E. Voegelin, and M. A. Bergougnou (1978). Design of fluid bed grids using the orifice equation. Can. J. of Chem. Eng., 56, 404-405. Botterrill, J. S. M. (1975). Academic Press, London. Davidson, J. F., D. Harrison, R. C. Darton, and R. D. LaNauze (1977). Chemical reactor theory. L. Lapidus, N. R. Amundson (Ed.). Davidson, J. R., and B. 0. G. Schüler. Transaction of Institution of Chemical Engineers, 38, 335. Fakhimi, S. and D. Harrison (1970). Institution of Chemical Engineers, Symp. Ser. 33. Chemica-70, Session 1, 29. Fakhimi, S., M. Sohrabi, and D. Harrison (1982). Proc. 32nd Canadian Chemical Engineering Conferences, _4, 160-172. Fedorov, I. M. (1966). In S. S. Zabrodskii (Ed.), Hydrodynamics and Heat Transfer in Fluidized Beds. MIT Press, . 40. Gregory, S. A. (1967). In Drinkenburg (Ed.), Int. Symp. on Fluidization. Netherlands Univ. Press, Amsterdam, . 751. Kassim, W. M. S. (1972). Ph.D. Thesis, University of Aston in Birmingham. Kelsey, J. R. (1968). Inst. Chem. Eng. Symp. Ser., 30, 86. Korenberg, J. (1982). Proc. 7th International Conference on Fluidized Bed Combustion, 339-349. Philadelphia. Kozin, V. E., and A. P. Baskakov (1968). Int. Chem. Eng., 8l, 257. Kunii, D., and 0. Levenspiel (1969). Fluidization Engineering, Wiley, 88. Merry, J. M. D. (1975). AIChE Journal, 2]_9 507-510. Mori, S. and A Moriyama (1977). Kogaku Kogaku, _3, 7-11. Pétrie, J. C. and D. E. Black (1966). Chemical Engineering Progress, 62 (3), 82. Qureshi, A. E. and D. E. Creasy (1979). Power Technology, _22, 113-119. Richardson, D. R. (1961). Chemical Engineering, 68, 83. Serviant, G. A., M. A. Bergongnou, C. A. Baker, and W. Bulani (1970). Canadian Journal of Chemical Engineers, 48, 496. Siegel, R. (1976). AIChE Journal, _22 (3), 590-592. Sit, S. P. (1983). Ph.D. Thesis. Department of ChemicalEngineering, McGill Univ. Vanecek, V., M. Markvant, and R. Drbohlav (1966). Fluidized Bed Drying, Leonard Hill, London, . 98. Wen, C. Y., D. F. King, and J. Shang (1980). Proc. DOE/WVU Conference on Fluidized Bed Combustion System Design and Operation, 165-216. Morgantown. Wen, C. Y., R. Krishnan, S. Dutta, and R. Khosravi (1978). Proc. Second Eng. Foundation Conference. Cambridge, England. 37-38. Whitehead, A. B. (1971). In Davidson and Harrison (Ed.), Fluidization. Academic Press, 781-814. Whitehead, A. B. and D. C. Dent (1967). In Drinkenburg (Ed.), Int. Symp. on Fluidization. Netherlands University Press, Amsterdam, . 802. Yang, W. C. and D. L. Keairns (1979). I & EC Fundamentals, 18, 317. Yia-Ching, R. (1980). M.S. Thesis, University of West Virginia. Yue, P. L. and J. A. Koloczkoszski (1982). Trans. I. Chem. Eng., 60, 164-170. Zenz, F. A. (1968). Symp. on Fluidization II, Int. Chem. Eng. Symp. Series. Montreal. Zenz, F. A. (1962). Hydrocarbon Processing and Petroleum Refiner, 41 (12), 125-130. Zuiduberg, F. J. (1967). Proc. Int. Symp. Fluidization, Eindhoven, Netherlands.
FLUIDIZED BED BOILERS: DESIGN AND APPLICATION
k& affitift;»tJ
foròUi^*Ki*y
Straight hole plate
Divergent hole plate
L.\\y tes\tf Kffli VkS\'\flfcv[ Slit type distributor
55
■^?<&kü%i Inclined hole plate (Conidure sheet)
TVTR iftSSA.
Nozzle with straight orifice
Bubble cap plate
Nipple plate
Nozzle with inclined orifice
'OSÒ'::
Y ss's ss /'s s's s, s ss-f:
\ Water cooled straight hole plate
Concave plate
Pipe g r i d distributor
MATERIAL AIR DISCHARGE
7T*F ^
^ T ^ f > f ^ ) w y y > f ^
S p e c i a l purpose d i s t r i b u t o r (Korenburg-1982)
Fig. 1. Different types of d i s t r i b u t o r s for Fluidized Bed Boilers
P. Basu
56
WSSSA
YS^SS/ASSSSZ
xfry//A
m. rw
v//A
&^A
f//i
F i g . 2 . S o m e m e t h o d s f o r s e a l i n g the d i s t r i b u t o r
v< r \ v \
plate
perforated
perforated perforated, but may be applicable to nozzles (Wen, 1980)
Fakhimi & Harrison (1982)
Kassim (1972)
Zenz (1968)
Whitehead & Dent (1971)
Siegel (1976)
Kelsley (1968)
nozzle
porous
perforated & porous
unspecified
0.185( H mf '
correlated for different types
Qureshi & Creasy (19 79)
Kunii (1969)
0.01 + .2 [l -
unspecified
d
p
1
T ^ " mf
Ο.ΟΙβί-^-^,υ
>
3uL mf
0.03(-|") % , u - 3u,l mf
n = f(Ga)
-(-ττ^)
1 ,«** ,
1 / n
0.046(-f-)
0.1
0.3
0.08(-|-)
%
exp(^-)] mf
0.012(1 - 1.4 u m£ /u op )
~ «W/V'
Zuiderberg (1967)
1
perforated & porous
Mori & Moriyama (1977)
Vapb
Type of Distributor
A
for Stable Fluidization
Author
TABLE I - Correlations for Minimum Values of ΔΡ,/ΔΡ
as
M O
f M Ω
5
co M O 2!
σ w
co
σ w ο M f w
σ
N M
P. Basii
58 TABLE 2.
DESIGN METHODS WITH EXAMPLE
Design parameters: Type - Nozzle plate with nozzles on square pitch u
= 1.52 4 m/s
u
= 0.9 34 m/s
mf
e
mf = 0.4 p
= 2 300 Kg/nT
p
= 1 . 2 2 Kg/m y = 1.77 X IO" 3 Ns/m 2
STEP
EXPRESSION
PARAMETER Bed pressure drop, ΔΡ, =
Orifice diameter, dor = but:
UNIT
27075
Ν/πΓ
p gH -(1 - ε -)
2300 X 9.81 X 2(1 - .4)
-
VALUE
3dp for perforated rplate 4d for nozzle plate P dor „ > 1.5 mm 4 X 1.1 X I O - 3 =
Equivalent diameter of bed, D
4
4.5 X 10
\
4 X 4 X 2 2 (4+2) Minimum distributor pressure drop for even distribution, Δ Ρ η
2.66 N/m z
ΔΡ [θ.ΟΙ + 0.2(1 - exp("2ïp—>"] b mf 2 6 27075 Γ .01 + .2 [l - exp( " y * )]]*:2897
lUop Aj J P air 2g
Check if rearrangement resistance, P R is less than ΔΡ β /100 ΔΡΡ <
N/m
100
1.22(1.52 X 8/1)' 2 X 9.81 9.78 <
(b)
(c)
Inlet diameter of nozzle, d. Nozzle wall thick ness, t
OK D
to satisfy this.
assume 4
1.5 dor
9.78
2897 100
If the check is not satisfied increase A P Nozzles: (a) Nos. of holes per nozzle, N
N/m z
4
Χλ/ΪΓ
1 . 5 X 4 . 5 X 1 0 " 3 X\fi assume 6 mm
13 X 10' 6 X 10'-3
N/m
FLUIDIZED BED BOILERS: DESIGN AND APPLICATION
STEP
59 VALUE
EXPRESSION
PARAMETER Thickness of the distribution plate, t
6
assume - 6 mm
ft*
.82
Orifice discharge coefficient, C n
u
0.853
Nos. of orifice per sqs. m of distributor, Ν Λ „
U U
*d
or
N
Nos. of nozzle per unit bed area M
m/s
58.79
4
op
1.52 58.79
11
V
2 X 2897 1.22
0.853
10
2A
c
or ■
Total nos. of nozzle, Nm
N
or
4 ττ(4.5
X !
1704
Λ -3 λ 2
426
14
Total number of holes on perforated distributor (a)
N
Pitch of nozzles, P
X A
or , N
3408
b (square)
1 (triangular) 7 N Z sin 60"
z
426 (b)
15
Pitch of orifices on perforated plate
Height of the nozzle h = ports
7 ^ — V or
0.05 (square)
ni
- i _ r 6(u°p - H 2 / 5 5
1/πΓ
b
426 X 8
13
1/m
or
X A
z
m/s
1/m2
2
1704 4
12
m
.13
•82(43->'13 Gas velocity through the orifice
X 10
UNIT
ft [ ""or
J
sin 60
(triangular)
m
60
P. Basu
STEP
PARAMETER
16
Check f o r operation
VALUE
EXPRESSION stable
-S-2EU. > 800 o r D orJ
L
N s /m
16 X 1 . 2 2 X 1 . 5 2 4
3482
[ 3 . 1 4 X 1704 X . 8 5 3 ( 4 . 5 X 1 0 " 3 ) 2 ] 2 3482
17
> 800
Check for channelling
=S£=
/2gAPb
.089 <
18
Vm
checked
< /pg
1
58.79 X 9.81 X 27075/1.22
y i
UNIT
.089
1
checked
Check for full operation of the orifices: (a) Height of spout, h
2 X
P
0.1
2 X .05 (b)
Perforated plate/ nozzles
(1
-
(1
-) X . 1 X 2 3 0 0 ( 1 1 -
.4)
(.934/1.52)'
789 < 2897 19
Check pitch for pre mature coalescence of bubbles in perforated plates
20
Check for elimination of dead zones Convert the values to CGS units
1.5 Γ 6 ( u o p * umf> 1 S/7 L IT NA J P'= d
d
u
P or
;f
=
% d N
β
i
6R
.
P dp d u
o r X 100 m £ x 100
V d N
X 100 X 10
i
X
100 x
Ν/πΓ
< ΔΡ,_
*- <%f/v2
X 9.81 789
Ν/πΤ
checked
2/5
cm micron cm cm cm
100
4 o r x IO'
Vcm 2
61
FLUIDIZED BED BOILERS: DESIGN AND APPLICATION
STEP
20
EXPRESSION
PARAMETER
(a)
Perforated plate
·205)>ρ·
716 d p
° (b)
VALUE
r
Nozzle
u
m/s
L20.1NÌRJ _2E_
m/s
-J-d. 2 N y
"3
4 1
z
1 . 5 2 4 1 X 100 . 7 8 5 ( 1 3 X 1 0 " 3 ) 2 X 426 (c)
UNIT
2696
mf
jmf ■i-
d
i2
cm/s
cm/s N
z
85.3 . 7 8 5 ( 1 3 X 1 0 " 3 ) 2 X 426
1509
cm/s
[.62 (u. - u j i n f ) d . 2 ] - 4 5 + d . 2 P
G 62(2696
- 1509)
1 . 3 2!Ί .45 +
]
1.3
checked
31.39 > 5
21
·5< N o r
Fractional opening
x
"T" dor
X 10
°<
3
1704 X -^- (4.4 X I O " 3 ) 2 X 100
Check for excessive number o f o r i f i c e s / 8 q . m ( for perforated plates )
23
2.58 checked
.5 < 2.58 < 3
22
31.39
Angle of inclination of the roof of the nozzle ,
N
or
<
100
°
greater than angle of repose <* bed material
45
degree
ABSTRACT Principles in the design of air distributors for a fluidized bed boiler have been reviewed. Three major types of distributors are discussed. The criteria for uniform and stable fluidization are discussed. Finally, a step by step method has been suggested for the design of distributor plates. The design procedure is illustrated with an example on the design of a nozzle type distributor. INTRODUCTION The primary function of the distributor is to introduce gas uniformly into a fluidized bed. Except in the case of sparge pipe distributors, it supports the weight of the bed material too. The distributor has a strong influence on the size and frequency of the bubbles in a fluidized bed. This, in turn, affects the com bustion of coal in the bed. The distributor also affects the attrition of bed materials (Botterill, 1975). A poorly designed distributor may lead to low com bustion efficiency or even agglomeration. Bearing this in mind Botterill (1975) and Whitehead (1971) suggested the following desired properties of the distributor: 1. 2. 3. 4. 5. 6. 7.
It should promote uniform and stable fluidization. It should minimize the attrition of bed materials. It should reduce erosion of the bed-internals or in-bed heat exchanger tubes. It should prevent backflow of bed materials through holes. It should ensure complete fluidization over the entire range of operating velocities and temperature. It should minimize the dead zones on the distributor. Plugging of the orifices over extended period of operation should be minimum. TYPES OF DISTRIBUTORS
Following are the common types of distributor plates used in fluidized bed boilers or combustors. Some of the typical distributors are shown in Fig. 1. (a) Straight hole or perforated type; (b) Nozzle type; (c) Pipe grid type. Besides these, there are a few other special purpose distributors such as the ones described by Korenberg (1982), which are meant for fuels with tramp materials.
45
P. Basu
46 Straight Hole Distributors
Perforated plates, the simplest types of distributors, were widely used during the initial periods of fluidized bed boiler developments. The holes were drilled directly on the plate supporting the bed material. The holes could be either straight, divergent or convergent. Zenz (1968) suggests that upward diverging holes will help eliminate dead zones. There are few propietory distributor plates such as fConiduref, fProcedynef, fOrthos', etc. If the operating conditions match the design conditions of the plates, they perform satisfactorily. However, these have not been extensively used in fluidized bed boilers. Nozzle Distributors In nozzle type plates air enters the bed through nozzles at a certain height above the main distributor plate, which supports the bed materials. As a result of this, a layer of insulating inert bed material always remains on top of the load-bearing plate, which, therefore, does not distort under heat. The exit ports of the nozzle may be slightly inclined, but in general they are horizontal. Therefore, backflow of solids through the ports is greatly eliminated. The nozzles are arranged in different fashions on the plate. This determines the formation of dead zones. However, if the roofs of the nozzles are taperd, the dead zones are negligible compared to those in case of a perforated plate with holes at the same pitch as the nozzles (Wen, 1980). Pipe Grid Distributors Pipe Grid or sparge tube distributors are pipes with orifices drilled in them. They are being increasingly used in boilers. The major advantages of this distributor are as follows: (a) (b) (c) (d)
no no it it
elaborate air box - sometimes there is no air box at all; problem of sealing to prevent air by-passing the grid; is simple to install and build; does not support the bed material.
The major problem is that it cannot be used for a very wide bed as it would require a very long pipe, which, in turn, will be required to have a large diameter for a reasonable pressure drop. It is relatively difficult to achieve uniform fluidization in a long but low pressure drop pipe grid distributor because a large varia tion of flow along the length of the pipe is observed (Zenz, 1962). In order to reduce the dead zones, the orifices may be oriented in different radial directions. A rule of thumb for design of pipe grid distributor for turbulent flow is that the ratios of kinetic energy of the inlet stream to pressure drop across the outlet hole and friction loss in the pipe to pressure drop across the outlet hole should be equal to or less than one tenth (Perry, 1973). Other Distributors Water cooled distributors with orifices or nozzles on the fins joining two water carrying tubes have been successfully used in commercial fluidized bed boilers (Basu, 1981) (Fig. 1). It saves the cost of the distributor by making it a part of the boiler heating surface and at the same time it reduces thermal expansion problems. It is especially suitable for the beds which are ignited by hot air. Slits or grate bar distributors have fine gaps or slits formed by bars or fins of a tube. These slits prevent channelling (Gregory, 1967) and also help to reduce
FLUIDIZED BED BOILERS: DESIGN AND APPLICATION
47
back flow of solids into the air box, but are not yet very popular amongst the boiler designers.
DESIGN EQUATIONS Pressure Drop Through Distributors The pressure drop through the distributor can be written from simple orifice theory; AP
d ■ CD Uor2/2§
The orifice discharge coefficient, C is taken by Zenz (1968) to be equal to 0.8. Kunni and Levenspiel (1969) give values increasing from 0.1 to 0.7 while the Reynolds number (based on approach diameter) increases from 2 to 200. Thereafter, it remains steady at 0.6. Behie, et al. (1978) have demonstrated the effect of orifice Reynolds number and the presence of fluidized solids on C . For empty beds it varies between 0.84 to 0.76 whereas the presence of solids reduce it to the level 0.73 to 0.77. The presence of solids on the plate prevents sudden expansion of the gas issuing through the orifice. Thus it reduces the orifice discharge coefficient. Behie (1978) further observed that through extended operations, the square edges of the orifices are rounded by solids erosion. This could reduce the discharge coefficient. No data was provided. However, the designers may keep in mind the expected reduction in distributor pressure drop through this effect in the long run. The thickness of the plate will have an effect on the orifice discharge coefficient. Quereshi and Creasy (1979) presented an expression for C ; C = 0.82 (t/d ) ° · 1 3 D or
(2)
Non-Uniform Distribution of Air Non-uniform fluidization is most undesirable in a fluidized bed. This is often caused by inoperative orifices. Whitehead and Dent (1967) observed that in large (0.09 - 5.9 m ) and deep (1.2 m) beds with bubble cap type distributors, the minimum velocity for complete operation of all the orifices, u , was greater than 3 u f . For perforated plates Fakhimi and Harrison (1971) found this limit to be 1.53 u _ and it was observed to be lower for larger particles. Davidson, et al. (1977) mentioned that a small spout whose height is about twice the orifice spacing should be formed on each orifice for uniform fluidization. Based on this they stipulated that the minimum pressure drop for full operation of all the orifices would be ΔΡ
=
(1 - 2/ïï)hoPï(l 1
- <%fV
£lnf )s
(3)
2
Yue and Kolaczkoski (1982) presented a more complicated expression to recognize the effect of bed depth, H on the minimum velocity for full operation of all the orifices. For nozzle type distributors a correlation was presented by Whitehead (1971). After converting his correlation to SI units it appears as below.
48
P. Basu
[ where:
L , .7 + {.49 + 1
X
»
1.98 x 10 7 (N 0 # 2 2 K 2 H ) l· 1D ps s \
%2f
f
(A)
K D = 992.7 U/ΔΡ ^
Non-uniform fluidization is also caused by mal-distribution of incoming gas. A flow resistance across the orifice which is considerably larger than rearrangement resistance of incoming gas results in uniform distribution of gas to all the orifices. Kunii (1969) estimated this rearrangement resistance to be equal to the expansion loss. Richardson (1961) suggested that the distributor pressure drop should be about 100 times the rearrangement resistance for uniform distribution of gas P
a i r / 2 g (u o p x \ / Α ± ) 2 < ΔΡ^ΙΟΟ
(5)
A sparge tube distributor has rather erratic distribution of flow through uniform orifices (Zenz, 1962; Basu, 1978). For short and low resistance tubes, maximum flow occurs furthest from the air entry, but this trend changes with flow rate. Therefore, design of this type of distributor needs careful consideration. Channelling is also a major problem. It is particularly bad in case of a shallow fluidized bed. The kinetic energy of the orifice jets at times exceed the bed weight; and the jets punch holes right through the bed creating a low resistance path (Kunii, 1969). Federov (1955), however, concluded that if the grid resistance is equal to, or greater than the resistance of the bed, fluidization occurs uniformly without any channelling regardless of the bed-depth, bed area or particle size. Qureshi (1979) found the following stability criterion would ensure that a sudden channel formation will subside instead of aggravating with increasing velocity d(AP. + ΔΡ,) ^ Ìdu
> 0
(6)
3 Wen (1980) suggested that this value should be greater than 800 Ns/m . Based on these pictures, various workers have determined different ratios of pressure drop through the distributor and that across the bed for stable fluidization (Table 1). Qureshi and Creasy (1979) correlated these data to find the criterion for stable and uniform fluidization ΔΡα/ΔΡ
= 0.01 + 0.2 [1 - exp (-D/H
f )]
(7)
Dead Zones Formation of dead zones of particles between the orifices has been studied by Wen (1978), Kozin and Baskakov (1968), Pétrie and Black (1966). Such zones adversely affect the combustion and heat transfer. Kozin and Baskakov (1968) showed that in case of a nozzle or bubble cap type distributor, the gas jets issuing from the nozzles coalesce at the boundaries between the caps. These jets then form series of ascending bubbles with accompanying solids. Unless the top of the nozzles are tapered, the descending solids form a stagnant layer on top of the nozzle. There is some possibility of the bubble stream breaking the surface and forming spouts. This results in poor gas conversion. This can be avoided if bed height is 30-35 times the diameter of the cap exit opening (Kozin and Baskakov, 1968). Wen (1980) presented the following correlations for no dead space in nozzles type distribu tors.
49
FLUIDIZED BED BOILERS: DESIGN AND APPLICATION
P < Γκ (u. - u. f ) d . 2 l 0 ' 4 5 + d. - L
J
jmf i J
(8)
i
All units are in CGS. K = 0.62 for arrangements where the center line of the holes coincides with pitch line of the nozzle. K = 1.43 for arrangements where the center line of the orifices is turned 45 from the pitch line of the nozzles. Yia-Ching (1980) studied the effect of wide cut particles. Wen (1980) also proposed correlations to avoid dead zones in perforated plates, where the solid circulation is just the reverse of that of nozzles.
P
or
l - u - y +1 3on , 1NΓ
205 -|i/(o.7i6 d °· ) P I
(9)
L °- orJ
All variables are in CGS except d which is in micron. P Dead Zones Near the Walls Wen (1980) observed that higher flow rate of air is necessary to eliminate the dead zone near the walls more than those near the center for the bed. He presented different correlations for this minimum exit velocity for elimination of the dead zone, but proposed equations (8) and (9) for calculation of the orifice or nozzle spacing. It would appear from above that a smaller pitch will help eliminate the dead zone near the walls, but this may lead to short circuiting of air up the wall. So it is difficult to define a universal procedure to solve this problem. One has to rely on experiences by taking note of the above situations. Jet Heights Small vertical jets, formed on individual orifices in a perforated plate distribu tor, hit the heat exchanger tubes located within the jet region and seriously erode these. Furthermore, taller jets result in poor gas conversion (Basu and Grace, 1983). A large number of workers, including Wen (1977), Yang and Keairns (1979), Zenz (1968) proposed different correlations for determination of the jet height. Sit (1981) analyzed these correlations and found the one by Merry (1975) to give the best fit for vertical jets.
[ ι if] H * ·] P d -10.3 Γ
/u
2
\0.2
Ί
(10)
The designer should locate the tubes above the height L.. Bubble Coalescence Closer pitch avoids dead zones, but too close a pitch causes premature bubble coalescence which results in poor gas conversion. Zenz (1968) suggested that pitch should be greater than half bubble diameter. Wen et al. (1980) assumed this value 1.5 times the bubble diameter at the tip of the jets. It may be calculated from Davidson and Schüler (1960) theory;
50
P. Basu . - -ui rr)/ ,I2 -/ 5 , c_ I|6(u -V1.5 op mf 1.5 | op mf | πΝ 11/5 /5 g g L or J
^nj
The same equation can be used to determine the height of the nozzle holes on the nipples as it would prevent the loss of insulating layer on the nozzle plate through an updrift caused by a rising bubble.
(12)
Particle Backflow Even if the orifice velocity is well above the terminal velocity of the particles, particles fall into the wide boundary layer in the orifices where pressure fluctu ation sporadically force them down into the windbox. This is called 'weeping1. Besides this, at low flow rates particles fall through the whole area of the orifice (Serviant, 1970). This phenomenon is termed 'dumping'. In both cases particle backflow leads to particle attrition and erosion of the orifices. Kunii (1970) suggests that the kinetic energy of the jets should be adequate to counter this tendency. unr. = (.7 to .85)(2gAP /p ) or ß g uor <
2g(APB/pg)
1 / 2
(13)
1/2
Kassim (1972) observed dumping in beds where bubbles are formed on the orifices whereas weeping occurred where gas jets into the bed from the orifices. He further proposed a correlation for the rate of weeping as a function of operating and design conditions. Serviant et al. (1970) presented data on weeping of 60ym diameter solids through 12.5mm and 18mm orifices. Bubble cap distributors greatly reduce dumping, although at times local eddies near the orifice cause some backflow of particles. Gregory (1967) suggested that slots of high length to width ratio reduce backflow of solids. Wen (1980) suggests that in order to reduce weeping and dumping, the orifice diameter should not be more than 3 to 8 times the particle diameter. Cost of manufacturing prohibits orifice diameters less than 1.5mm. So a rough guide could be d
or
= 3 x d p
d = 4 x d or p
for rperforated rplates for nozzle plates
(14)
MECHANICAL CONSIDERATIONS Thickness of the Plate The thickness of the grid plate influences the pressure drop to some extent. Higher the thickness, lower the pressure drop through the plate (Razumov et al, 1972; Qureshi, 1979; Gvozdev et al, 1963). A thicker plate prevents distortion of perforated type distributor and it facilitates fitting nozzles on the plate in case of nozzle type distributors. On the other hand, thicker plate is expensive
51
FLUIDIZED BED BOILERS: DESIGN AND APPLICATION compared to the thinner ones.
It increases the cost of drilling holes considerably.
The thickness of the plate should be at least adequate to support the weight of the slumped bed under operating plate temperature. The operating plate temperature may be taken to be 50 C above the inlet air temperature in case of nozzle plate. It could be as high as 400 C in case of a perforated plate type distributor supporting a fluidized bed at 800-900°C. In case of hot air passing through the plate, either the material should stand the temperature (i.e., use of high temperature steel) or water cooled membrane wall type distributor (Fig. 1) should be used. Sealing of the Plate Leakage of air around the edge of the distributor causes non-uniform fluidization. In smaller beds (D< 1.0m) it is easy to control this leakage by use of suitable gasket and uniform tightening of the bolts. This type of sandwiching the plate between the two flanges proves difficult for larger and rectangular beds. The best way to prevent this leakage is to weld the plate with the containing walls (Fig. 2 ) . This practice is being increasingly adopted in boilers where the walls will have rooms for thermal expansion. Pipe grid distributors are relatively free from this problem. Thermal Expansion Expansion of the distributor plate could be substantial in a large fluidized bed. In a smaller bed one can allow for this expansion by providing larger holes for the holding bolts. Larger beds would require flexible supports such as grid pins (Comparato, 1982). This would allow horizontal movement of the plate and at the same time it will carry the plate's load. Percentage Opening Fractional open area, S of the distributor is an important practical index of distributor design. Vanecek (1966) gives the following empirical relation for fluidized bed dryer. S = 1.7 (u /u ,)°· 9 r op mf
(15)
This gives values in the range of 2-10% which is higher than the usual 1-2% used in fluidized bed boilers. Vanecek (1966) further suggests an auxiliary grid placed below the main grid to help rearrange the entrance flow. The effect of incoming flow is rather pronounced in case of low pressure drop distributor. DESIGN PROCEDURE A step by step design procedure has been developed based on the foregoing discus sion. This procedure is based on nozzle or bubble cap type distributors. The design procedure is identical for perforated plate distributors except in a few steps where separate expressions have been presented. In certain places one has to make some compromise, which may necessitate choosing a value of distributor
52
p. Basu
pressure drop higher than that required for stable fluidization. A typical worked out example of design of a nozzle plate for an FBC boiler is also presented with the procedure. Most of the criteria of the design method can be met by increasing the pressure drop through the distributor plate, but this value should be as low as possible without sacrificing the stability and the uniformity of fluidization. Since a boiler requires a large volume of air, a small increase in the distributor pressure drop substantially increases the power requirement of the forced draft fan. So, a designer should be?very cautious about the checks of step (16) to (22) if ΔΡ + ΔΡ exceeds 11,000 N/m . Beyond this range multistage fan or unusually large blower may be required. Design of distributor is more an art than a rigid science. Every step in the suggested design procedure may be violated by the experienced designers. However, the present example illustrates the application of the general principles involved in the design of distributor plates for fluidized bed boilers. In the present example (Table 2) the thermal design suggests a bed area equal to 8 sq. m. Air enters the plenum chamber from the side through a port whose area is 1 sq. m. We have chosen a nozzle type distributor with a taper head as this would eliminate the dead zones and a layer of inerts will insulate the main plate from the hot bed. This plate will carry the weight of the bed solids. First we deter mine the pressure drop through the distributor which would ensure uniform fluidization. This will dictate the size and number of nozzles. Thereafter, the pitch and the height of the nozzles are calculated. This essentially completes the hydrodynamic design of the distributor. Then this design has been checked for stable operation, channelling, full operation of the orifices, elimination of dead zones and premature coalescence. In the present example fortunately all the criteria have been satisfied. So it was not necessary to modify the original design to meet the criteria. When deciding the location of the in-bed tubes the heights of the vertical jets on the orifices will be calculated from equation (10) and the tubes will be placed above this level to avoid excessive erosion. For a rough check, the percentage opening based on the bed area has been calculated and it is found to be within acceptable limits. Then the total number of the orifices has been calculated and it is not too large to be prohibitively expensive.
CONCLUSION The types and the desired properties of a fluidized bed gas distributor are discussed. A brief review of the design equations is also presented. A step by step method for the design of both nozzle and plate type distributors is explained with a worked out example. This method demonstrates the various considerations involved in the design of a distributor for a fluidized bed boiler. NOMENCLATURE Cross-section area of bed, m
\ A. 1 C
D
2
Cross-section area of the inlet to the wind box, m Drag Coefficient Equivalent diameter of bed, m
D d
Diameter of orifice, m or
2
FLUIDIZED BED BOILERS: DESIGN AND APPLICATION d
P
di
Ga
Hmf
I n t e r n a l d i a m e t e r of n o z z l e , m G a l i l i o number Height of i n c i p i e n t l y f l u i d i z e d bed, m Height o f t h e n o z z l e , m
h
hs KD
L
Mean p a r t i c l e d i a m e t e r , m
j
NT Nor NZ
n = f(Ga)
Height of s p o u t s above t h e o r i f i c e , m D i s t r i b u t o r flow f a c t o r of e q u a t i o n ( 3 ) Height of a i r j e t s above t h e o r i f i c e , m T o t a l number of n o z z l e s i n bed Numbers of h o l e s p e r n o z z l e Numbers of n o z z l e s p e r u n i t bed area, l / m Table I : G
=
2
1, n = 4.65
Ga = 100 n = 4.05
P e r i m e t e r of t h e bed, m
'e P
Pitch, m
QT
'r
Gas flow through n o z z l e s (scfm)
F r a c t i o n a l open area of d i s t r i b u t o r p l a t e Thickness of p e r f o r a t e d p l a t e o r n o z z l e wall, m
t uj U U
j mf OP
O r i f i c e e x i t v e l o c i t y , cm/s O r i f i c e e x i t v e l o c i t y of minimum f l u i d i z a t i o n , cm/s Operating v e l o c i t y , m / s
U
Minimum f l u i d i z a t i o n v e l o c i t y f o r f u l l o p e r a t i o n of a l l t h e o r i f i c e s , m/ s
U
Minimum f l u i d i z a t i o n v e l o c i t y , m / s
m
mf
Ut
Terminal v e l o c i t y , m / s
2
Height of expanded bed, m
B
Constant of Mori ti Moriyama
E
Bubble f r a c t i o n
B
E
mf
ps
T a b l e I ( . 3 8 f o r u/umf = 3)
Voidage a t minimum f l u i d i z a t i o n Density of bed m a t e r i a l , kg/m
3
53
54
p. Basu o
Density of air at operating temperature, kg/m Dynamic viscosity of air at operating temperature, Ns/m Pressure drop through the bed, N/m 2 Δρ
α
2
Pressure drop through distributor, N/m 2 BIBLIOGRAPHY
Basu, P., and K. L. Das (1978). Internal Report. Heat Power Laboratory, CMERI, Durgapur. Basu, P., and J. R. Grace (1983). Modelling of char Combustion in fluidized bed. Under preparation. Behie, L. A., B. E. Voegelin, and M. A. Bergougnou (1978). Design of fluid bed grids using the orifice equation. Can. J. of Chem. Eng., 56, 404-405. Botterrill, J. S. M. (1975). Academic Press, London. Davidson, J. F., D. Harrison, R. C. Darton, and R. D. LaNauze (1977). Chemical reactor theory. L. Lapidus, N. R. Amundson (Ed.). Davidson, J. R., and B. 0. G. Schüler. Transaction of Institution of Chemical Engineers, 38, 335. Fakhimi, S. and D. Harrison (1970). Institution of Chemical Engineers, Symp. Ser. 33. Chemica-70, Session 1, 29. Fakhimi, S., M. Sohrabi, and D. Harrison (1982). Proc. 32nd Canadian Chemical Engineering Conferences, _4, 160-172. Fedorov, I. M. (1966). In S. S. Zabrodskii (Ed.), Hydrodynamics and Heat Transfer in Fluidized Beds. MIT Press, . 40. Gregory, S. A. (1967). In Drinkenburg (Ed.), Int. Symp. on Fluidization. Netherlands Univ. Press, Amsterdam, . 751. Kassim, W. M. S. (1972). Ph.D. Thesis, University of Aston in Birmingham. Kelsey, J. R. (1968). Inst. Chem. Eng. Symp. Ser., 30, 86. Korenberg, J. (1982). Proc. 7th International Conference on Fluidized Bed Combustion, 339-349. Philadelphia. Kozin, V. E., and A. P. Baskakov (1968). Int. Chem. Eng., 8l, 257. Kunii, D., and 0. Levenspiel (1969). Fluidization Engineering, Wiley, 88. Merry, J. M. D. (1975). AIChE Journal, 2]_9 507-510. Mori, S. and A Moriyama (1977). Kogaku Kogaku, _3, 7-11. Pétrie, J. C. and D. E. Black (1966). Chemical Engineering Progress, 62 (3), 82. Qureshi, A. E. and D. E. Creasy (1979). Power Technology, _22, 113-119. Richardson, D. R. (1961). Chemical Engineering, 68, 83. Serviant, G. A., M. A. Bergongnou, C. A. Baker, and W. Bulani (1970). Canadian Journal of Chemical Engineers, 48, 496. Siegel, R. (1976). AIChE Journal, _22 (3), 590-592. Sit, S. P. (1983). Ph.D. Thesis. Department of ChemicalEngineering, McGill Univ. Vanecek, V., M. Markvant, and R. Drbohlav (1966). Fluidized Bed Drying, Leonard Hill, London, . 98. Wen, C. Y., D. F. King, and J. Shang (1980). Proc. DOE/WVU Conference on Fluidized Bed Combustion System Design and Operation, 165-216. Morgantown. Wen, C. Y., R. Krishnan, S. Dutta, and R. Khosravi (1978). Proc. Second Eng. Foundation Conference. Cambridge, England. 37-38. Whitehead, A. B. (1971). In Davidson and Harrison (Ed.), Fluidization. Academic Press, 781-814. Whitehead, A. B. and D. C. Dent (1967). In Drinkenburg (Ed.), Int. Symp. on Fluidization. Netherlands University Press, Amsterdam, . 802. Yang, W. C. and D. L. Keairns (1979). I & EC Fundamentals, 18, 317. Yia-Ching, R. (1980). M.S. Thesis, University of West Virginia. Yue, P. L. and J. A. Koloczkoszski (1982). Trans. I. Chem. Eng., 60, 164-170. Zenz, F. A. (1968). Symp. on Fluidization II, Int. Chem. Eng. Symp. Series. Montreal. Zenz, F. A. (1962). Hydrocarbon Processing and Petroleum Refiner, 41 (12), 125-130. Zuiduberg, F. J. (1967). Proc. Int. Symp. Fluidization, Eindhoven, Netherlands.
FLUIDIZED BED BOILERS: DESIGN AND APPLICATION
k& affitift;»tJ
foròUi^*Ki*y
Straight hole plate
Divergent hole plate
L.\\y tes\tf Kffli VkS\'\flfcv[ Slit type distributor
55
■^?<&kü%i Inclined hole plate (Conidure sheet)
TVTR iftSSA.
Nozzle with straight orifice
Bubble cap plate
Nipple plate
Nozzle with inclined orifice
'OSÒ'::
Y ss's ss /'s s's s, s ss-f:
\ Water cooled straight hole plate
Concave plate
Pipe g r i d distributor
MATERIAL AIR DISCHARGE
7T*F ^
^ T ^ f > f ^ ) w y y > f ^
S p e c i a l purpose d i s t r i b u t o r (Korenburg-1982)
Fig. 1. Different types of d i s t r i b u t o r s for Fluidized Bed Boilers
P. Basu
56
WSSSA
YS^SS/ASSSSZ
xfry//A
m. rw
v//A
&^A
f//i
F i g . 2 . S o m e m e t h o d s f o r s e a l i n g the d i s t r i b u t o r
v< r \ v \
plate
perforated
perforated perforated, but may be applicable to nozzles (Wen, 1980)
Fakhimi & Harrison (1982)
Kassim (1972)
Zenz (1968)
Whitehead & Dent (1971)
Siegel (1976)
Kelsley (1968)
nozzle
porous
perforated & porous
unspecified
0.185( H mf '
correlated for different types
Qureshi & Creasy (19 79)
Kunii (1969)
0.01 + .2 [l -
unspecified
d
p
1
T ^ " mf
Ο.ΟΙβί-^-^,υ
>
3uL mf
0.03(-|") % , u - 3u,l mf
n = f(Ga)
-(-ττ^)
1 ,«** ,
1 / n
0.046(-f-)
0.1
0.3
0.08(-|-)
%
exp(^-)] mf
0.012(1 - 1.4 u m£ /u op )
~ «W/V'
Zuiderberg (1967)
1
perforated & porous
Mori & Moriyama (1977)
Vapb
Type of Distributor
A
for Stable Fluidization
Author
TABLE I - Correlations for Minimum Values of ΔΡ,/ΔΡ
as
M O
f M Ω
5
co M O 2!
σ w
co
σ w ο M f w
σ
N M
P. Basii
58 TABLE 2.
DESIGN METHODS WITH EXAMPLE
Design parameters: Type - Nozzle plate with nozzles on square pitch u
= 1.52 4 m/s
u
= 0.9 34 m/s
mf
e
mf = 0.4 p
= 2 300 Kg/nT
p
= 1 . 2 2 Kg/m y = 1.77 X IO" 3 Ns/m 2
STEP
EXPRESSION
PARAMETER Bed pressure drop, ΔΡ, =
Orifice diameter, dor = but:
UNIT
27075
Ν/πΓ
p gH -(1 - ε -)
2300 X 9.81 X 2(1 - .4)
-
VALUE
3dp for perforated rplate 4d for nozzle plate P dor „ > 1.5 mm 4 X 1.1 X I O - 3 =
Equivalent diameter of bed, D
4
4.5 X 10
\
4 X 4 X 2 2 (4+2) Minimum distributor pressure drop for even distribution, Δ Ρ η
2.66 N/m z
ΔΡ [θ.ΟΙ + 0.2(1 - exp("2ïp—>"] b mf 2 6 27075 Γ .01 + .2 [l - exp( " y * )]]*:2897
lUop Aj J P air 2g
Check if rearrangement resistance, P R is less than ΔΡ β /100 ΔΡΡ <
N/m
100
1.22(1.52 X 8/1)' 2 X 9.81 9.78 <
(b)
(c)
Inlet diameter of nozzle, d. Nozzle wall thick ness, t
OK D
to satisfy this.
assume 4
1.5 dor
9.78
2897 100
If the check is not satisfied increase A P Nozzles: (a) Nos. of holes per nozzle, N
N/m z
4
Χλ/ΪΓ
1 . 5 X 4 . 5 X 1 0 " 3 X\fi assume 6 mm
13 X 10' 6 X 10'-3
N/m
FLUIDIZED BED BOILERS: DESIGN AND APPLICATION
STEP
59 VALUE
EXPRESSION
PARAMETER Thickness of the distribution plate, t
6
assume - 6 mm
ft*
.82
Orifice discharge coefficient, C n
u
0.853
Nos. of orifice per sqs. m of distributor, Ν Λ „
U U
*d
or
N
Nos. of nozzle per unit bed area M
m/s
58.79
4
op
1.52 58.79
11
V
2 X 2897 1.22
0.853
10
2A
c
or ■
Total nos. of nozzle, Nm
N
or
4 ττ(4.5
X !
1704
Λ -3 λ 2
426
14
Total number of holes on perforated distributor (a)
N
Pitch of nozzles, P
X A
or , N
3408
b (square)
1 (triangular) 7 N Z sin 60"
z
426 (b)
15
Pitch of orifices on perforated plate
Height of the nozzle h = ports
7 ^ — V or
0.05 (square)
ni
- i _ r 6(u°p - H 2 / 5 5
1/πΓ
b
426 X 8
13
1/m
or
X A
z
m/s
1/m2
2
1704 4
12
m
.13
•82(43->'13 Gas velocity through the orifice
X 10
UNIT
ft [ ""or
J
sin 60
(triangular)
m
60
P. Basu
STEP
PARAMETER
16
Check f o r operation
VALUE
EXPRESSION stable
-S-2EU. > 800 o r D orJ
L
N s /m
16 X 1 . 2 2 X 1 . 5 2 4
3482
[ 3 . 1 4 X 1704 X . 8 5 3 ( 4 . 5 X 1 0 " 3 ) 2 ] 2 3482
17
> 800
Check for channelling
=S£=
/2gAPb
.089 <
18
Vm
checked
< /pg
1
58.79 X 9.81 X 27075/1.22
y i
UNIT
.089
1
checked
Check for full operation of the orifices: (a) Height of spout, h
2 X
P
0.1
2 X .05 (b)
Perforated plate/ nozzles
(1
-
(1
-) X . 1 X 2 3 0 0 ( 1 1 -
.4)
(.934/1.52)'
789 < 2897 19
Check pitch for pre mature coalescence of bubbles in perforated plates
20
Check for elimination of dead zones Convert the values to CGS units
1.5 Γ 6 ( u o p * umf> 1 S/7 L IT NA J P'= d
d
u
P or
;f
=
% d N
β
i
6R
.
P dp d u
o r X 100 m £ x 100
V d N
X 100 X 10
i
X
100 x
Ν/πΓ
< ΔΡ,_
*- <%f/v2
X 9.81 789
Ν/πΤ
checked
2/5
cm micron cm cm cm
100
4 o r x IO'
Vcm 2
61
FLUIDIZED BED BOILERS: DESIGN AND APPLICATION
STEP
20
EXPRESSION
PARAMETER
(a)
Perforated plate
·205)>ρ·
716 d p
° (b)
VALUE
r
Nozzle
u
m/s
L20.1NÌRJ _2E_
m/s
-J-d. 2 N y
"3
4 1
z
1 . 5 2 4 1 X 100 . 7 8 5 ( 1 3 X 1 0 " 3 ) 2 X 426 (c)
UNIT
2696
mf
jmf ■i-
d
i2
cm/s
cm/s N
z
85.3 . 7 8 5 ( 1 3 X 1 0 " 3 ) 2 X 426
1509
cm/s
[.62 (u. - u j i n f ) d . 2 ] - 4 5 + d . 2 P
G 62(2696
- 1509)
1 . 3 2!Ί .45 +
]
1.3
checked
31.39 > 5
21
·5< N o r
Fractional opening
x
"T" dor
X 10
°<
3
1704 X -^- (4.4 X I O " 3 ) 2 X 100
Check for excessive number o f o r i f i c e s / 8 q . m ( for perforated plates )
23
2.58 checked
.5 < 2.58 < 3
22
31.39
Angle of inclination of the roof of the nozzle ,
N
or
<
100
°
greater than angle of repose <* bed material
45
degree