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Characteristics of a fluedized-bed incinerator for the combustion of solid wastes
Energy Convers. Mgmt Vol. 34, No. 2, pp. 81-97, 1993 Printed in Great Britain. All rights reserved
0196-8904/93 $5.00+ 0.00 Copyright © 1992 Pergamon Press Ltd
CHARACTERISTICS OF A FLUIDIZED-BED INCINERATOR FOR THE COMBUSTION OF SOLID WASTES S. C. SAXENA? and N. S. R A O Department of Chemical Engineering, The University of Illinois at Chicago, P.O. Box 4348, Chicago, IL 60680, U.S.A.
(Received 16 September 1991; receivedfor publication 17 April 1992) Ahstract--A laboratory-scale fluidized-bed incinerator has been designed for the efficient combustion and thermal destruction of solid waste materials. The experimental facility consists of a 153 mm dia cylindrical furnace, a propane gas burner, an air supply system, batch and continuous feeders, an off-gas cleanup system, flue-gas analyzer system, and an on-line automated data recording and analysis system. A continuous monitoring of temperature and pressure distributions in the incinerator, and composition of the flue gas is possible. Characteristics of the incinerator have been established. These include measurement of pressure drops across the calming and bed sections as a function of superficial air velocity and temperature, and axial temperature distribution in the test bed. Hydrodynamic characteristics of three inert sand beds of average diameters 641, 1312 and 2165/am have been investigated by recording the pressure-drop histories in the lower and upper regions of the bed as a function of fluidization number and bed temperature. Several statistical functions such as variance of the pressure-drop data, probability density function, skewness, kurtosis, autocorrelation function and probability density function have been computed, and the same have been related to the fluidization quality of the bed. These investigations suggest the optimum conditions for incineration of waste materials where the gas-solid contacting and solids mixing is most favorable, and hence, best results will be obtained for thermal destruction and pollution control. Fluidized bed
Incinerator
Combustion
Solid wastes
NOMENCLATURE a3 = a4 = do = dt = dx = f= fm= N= Nx = p(x) = p(w) = T= T~= t= U= Umf= U/Umf = x, = Z=
Relative skewness (dimensionless) Kurtosis (dimensionless) Mean particle diameter (m) Increment in time (s) Increment in pressure drop (kPa) Frequency (Hz) Major frequency (Hz) Number of data points (dimensionless) Number of data points in range dx around x (dimensionless) Probability density function (kPa- ~) Power spectrum (kPa 2) Average bed temperature (K) Bed temperature at given distance above distributor plate (K) Time instant (s) Superficial gas velocity computed at standard conditions of temperature and pressure (m/s) Minimum fluidization velocity (m/s) Fluidization number (dimensionless) ith pressure-drop value (kPa) Distance above the distributor plate (ram)
Greek letters AP = APd = APb = = ~L = ~u = /z =
Pressure drop (kPa) Pressure drop across distributor plate (kPa) Pressure drop across sand bed (kPa) Bed voidage (dimensionless) Voidage of lower bed section (dimensionless) Voidage of upper bed section (dimensionless) Mean of N pressure-drop values (kPa)
tTo whom all correspondence should be addressed. 81
82
SAXENA and RAO: FLUIDIZED-BED INCINERATOR CHARACTERISTICS a2a= Variance of residuals (kPa2) ap = Standard deviation (kPa) ~bl,~b2. . . . . Op= AR coefficientsin ARIMA model = Angular frequency (rad) Acronyms
AR = Auto regressive ARIMA = Auto regressive integrated moving averages pdf = Probability density function psdf = Power spectral density function SAS = Statistical analysis system SCA = Scientificcomputing associates UBJ = Univariate Box-Jenkins
INTRODUCTION Management of solid wastes produced from a variety of sources, such as domestic, industrial, medical, and agricultural, is becoming a serious and challenging problem. Many monographs have been written on this subject, and several conventional treatment technologies have been developed and reevaluated for this purpose with emphasis on environmental pollution control. Thermal degradation and combustion of waste materials is the subject matter of several recent books, such as those of Tillman et al. [1], Bartok and Sarofim [2], and Brunner [3]. Incineration of domestic waste has been a well known disposal procedure for centuries, and improved designs of incinerators have been continuously developed to cope with the changing composition of the domestic and industrial wastes. Innumerable articles dealing with the various design and operational features of incinerators have appeared in the technical literature which also emphasize its distinct advantages and disadvantages. Fluid-bed incinerators are one type of incinerator which has been preferred for this purpose in view of its temperature uniformity, good gas-solids contacting, good solids mixing and versatility in the nature of refuse material these can handle. Its potential for in situ retention of sulfur and chlorine has been well recognized in connection with coal combustion and is certainly of great relevance in cogeneration. Here, we present the design of a test fluidized-bed incinerator and preliminary data with three inert sand-beds heated to different temperatures with a specially designed propane burner. In addition to the characteristics describing the behavior of the incinerator and sand beds, pressuredrop history has been recorded and analyzed for several statistical functions, such as the standard deviation in pressure drop data (ap), probability density function (pdf), skewness, kurtosis, autocorrelation function, and power spectrum, to characterize incinerator hydrodynamic regimes and fluidization quality. It is hoped that such characterization of the fluidized-bed incinerator will help in its more controlled operation and thereby minimize the formation of pollutants and, hence, environmental pollution. FLUIDIZED-BED INCINERATOR The schematic of the fluidized-bed incinerator, consisting of several independent units, is shown in Fig. 1, and it is a modified version of our fluidized-bed coal combustion facility, Saxena and Mathur [4]. The facility consists of an air supply system (3, 4), propane gas supply and metering system (8, 9), incinerator (l), solids batch (7) and continuous (6) feeders, and a flue-gas cleanup system (13, 14). The combustion air is supplied by two 18.65 kW compressors and is dried by silica gel and refrigeration driers and then filtered by two filters to remove oil vapors before entering the incinerator preheater section (e). Propane gas is supplied to the sparger (10) by two 100 lb propane gas cylinders (8) and is metered on two rotameters (9). The incinerator is comprised of two preheating sections (e) with an overall height of 1.52 m and is equipped with a tube-fired propane burner which is not used in the current experiments. The two elbow sections (d) connect the preheater section to the calming section (a), 0.305 m long. The main test bed (b) and freeboard (c) sections are comprised of four flanged sections, each 0.571 m long and bolted together with 0.038 m long spacer rings. All these sections are made of 0.254 m diameter, schedule 40, 304 stainless-steel pipe. The entire preheater, elbows, calming and bed sections are provided with a 50 mm thick Purolite-30 insulating material lining. The incinerator wall is externally insulated with Fiberflax
SAXENA and RAO:
FLUIDIZED-BED INCINERATOR CHARACTERISTICS
C
1. Incinerator
q 1 ~ J 4I
I
~
~(
b ~ ~111~
• e -e i
'
1--'~ 1 5 J - - ~ ~ - ~
83
7
2 _ a ~ "~- 5 /10 ~_-----~ / /'// 9 i~-- -~ 8
2. Differentialpressuretransmitters 3. Air compressors 4. Air inlet 5. Gas distributerplates 6. Continuoussolidsfeeder 7. Batchsolidsfeeder 8. Propanetanks 9. Rotameters 10. Propanesparger 11 Solidsoverflow 12 Coolingcoils 13 Cyclones 14 Fabric-filter 15 Insulation a. Calmingsection b. Testsection c. Freeboardsection d Elbows e Preheater
Fig. 1. Schematic of the fluidized-bed incinerator and the support system.
Lo-Con felt insulating wrapping of 25 mm thickness. The solids feeders (6, 7) are not used in the present work dealing with sand beds operated in batch mode. The flue gas is cleaned by passing it through two stainless steel, 6 UP type, cyclones (13) and a Pall Trinity Micro-filter assembly (14), Saxena and Chatterjee [5]. The engineering design details of the propane sparger (10) are given by Saxena et al. [6], and it essentially consists of a central stainless steel housing equipped with four stainless pipe arms symmetrically placed. It has nine orifices, 0.5 mm in diameter, for propane injection into the calming section. The calming section gas distributor is a perforated plate with 61 holes, 3.2 mm in diameter, and its detailed design is given by Saxena and Chatterjee [5]. The test bed distributor plate has 19 multi-orifice nozzles, and its intricate design features are reported by Saxena e t al. [6]. Nine chromel-alumel thermocouples have been used to monitor the temperatures of the different regions of the incinerator. Three of these are located in the bed section at an elevation of 76.2, 152.0 and 304.8 mm above the top surface of the distributor plate. Six pressure taps are provided to establish the pressure distribution along the incinerator. In the bed section, these probes are located at heights of 40, 140 and 290 mm above the gas distribution surface. The bed height can be maintained and adjusted by an overflow pipe (11), with design details given by Saxena and Chatterjee [5]. The pressure drops across the distributors, bed sections and the entire bed are fed to a data acquisition system through Leeds and Northrup differential pressure transmitters (2). The electrical signals from the thermocouples and the pressure probes are measured over a HewlettPackard system consisting of a dedicated computer, monitor, printer and plotter. Its details are given by Saxena and Rao [7].
84
SAXENA and RAO: 30
v~
FLUIDIZED-BED INCINERATOR CHARACTERISTICS
-
/
2o-
~
% X
~ 0
1
I
I
I
I
2
3
4
5
U (m/s) Fig. 2. Variations of distributor plate pressure drop as a function of superficial air velocity at different temperatures.
EXPERIMENTAL
DATA
The pressure drop across the bed distributor plate is measured as a function of the superficial air velocity at three temperatures, with the results reported in Fig. 2. The air is heated to different temperatures in the calming section by the combustion of propane. A spark plug is provided above the propane sparger in the calming section to ignite the air-propane mixture. The U and Umrvalues in this paper always refer to standard conditions of temperature and pressure. It is found that the pressure drop can be represented as a power function of U at all temperatures and over the entire velocity range. The three sand beds examined in this work have the mean diameters of 641, 1312 and 2165/~m. Experiments have been conducted with these beds of about 32.5 cm height. From the knowledge of bed pressure drop as a function of decreasing gas velocity, the minimum fluidization velocities are determined following the procedures of Saxena and Vogel [8]. These results are reported in Table 1, and a typical data plot for 2165 #m is shown in Fig. 3. The pressure drops are also measured for two sections of the bed enclosed between 40 and 140mm, and 140 and 290mm. These are referred to as the lower and upper bed sections, respectively. The bed voidages computed from these data (qj and EL) are presented in Fig. 4 as a function of fluidization number (U/Umr)and temperature. From these data, it is clear that the bed voidage of a bed section is almost independent of temperature at temperatures above the ambient. It is also very weakly dependent upon the fluidization number and bed height. The pressure-drop history data for these three beds and, in each case, for the two bed regions at several temperatures and fluidization velocities have been taken at a sampling rate of 10.8 readings/s. In all, 1000 readings are taken in 92.4s. The standard deviation, trp, of these 1000 readings is computed from the following relation: ap
-
1
u
Y~ ( x , - ~)~
(1)
N--I~=I
Table 1. Values of minimum fluidization velocities for three different sand beds at different temperatures
d0
(#m) 641
T (K) Umf (m/s)
298 0.42
603 0.28
823 0.24
---
1312
T (K) Umf (m/s)
298 !.10
833 0.62
923 0.46
1033 0.52
2165
T (K) Umf (m/s)
298 1.44
823 0.45
1023 0.60
1143 0.52
SAXENA and RAO:
FLUIDIZED-BED INCINERATOR CHARACTERISTICS
(A)
(C)
8 --
8
-
-
e~oOoO
o
e,o~oe~eoeo
°[°~"
4--
T = a98K 06°
2 -ze/o
o
~
dp = 2165~.m
/2
2
l
I
I
1
2
3
U (m/s)
dp = 2165~,rn
I
I
I
1
2
3
U (m/s)
(D)
8 --
8
--
f~'cj (3 we
oa-o,o-oo-~o-O-~oQ-6 --
6
Q-
0-~o •
OOU
O@
13_ ~v 4
4 T =823K
0
T = 1023K
~t~
o
(B)
2
a-
/o
a~ 4
aY
<
85
t
~
dp = 2165p, m
2
l
I
1
1
2
3
U
T = 1143K
0
(m/s)
dp = 2165~rn
I
I
1
2
U (m/s)
Fig. 3. Variations of bed pressure drop as a function of increasing (C)) and decreasing (O) gas velocity at different temperatures.
where 1
N
= ~,=~, xi.
(2)
Here, N is the total number of AP readings, xi is the ith AP value, and/~ is the mean of N data points. Computed values of ap based on the above relations for the two bed regions and for the three sand beds at different temperatures and fluidization velocities are presented in Fig. 5. A detailed discussion of these figures is extremely interesting in establishing the hydrodynamic activity of the bed and is presented below. Figure 5(A), for the 641/~m sand beds, exhibits that the pressure fluctuations do not depend upon temperature (303-823 K) for the lower bed region. This suggests that the bubbling phenomenon, including bubble formation and coalescence, does not depend on T over the entire velocity range. However, as the fluidization number increases, the standard deviation also increases until about a value of 3, thereafter, further increases in U/Umrdo not produce appreciable changes in ap. This implies that, initially, bubble coalescence increases the bubble size with increasing U/Umf up to a value of 3 and this results in the continuously increasing value of ap. For U/Umfvalues greater than 3, the bubble dynamics change and there is an equilibrium between bubble growth and decay, resulting in a constant bubble size distribution and, hence, a constant value of a v. 'This is exhibited in a constant value of EL in Fig. 4(A). In Fig. 5(B), for the same bed, we find that, in the upper region, the ~p values are not dependent on T as long as it is above the ambient, in addition to U/Umf. For U/Umrvalues smaller than 3, the bubble induced AP and, hence, the ap values increase with U/Umrat all temperatures. This is due to the increasing bubble size as a result of coalescence. However, this coalescence is temperature dependent and the same decreases with an increase in temperature. The reduction in effective bubble size with increase in temperature decreases ap. Figure 5(B) also suggests that, for U/Umrgreater than 3 and at temperatures greater than ambient, temperature does not influence and, hence the effective bubble diameter and its induced pressure fluctuations remain almost constant. Figures 5(A) and 5(B) suggest that, along the bed height, there is a change in the fluidization quality in as much as the lower region is bubbling
86
and R A O :
SAXENA (A) 0.6
FLUIDIZED-BED
INCINERATOR
CHARACTERISTICS
(B)
dp = 641 p,m
--
0.6
dp = 641 p.m
-A
o
,.,~....+~.o .'.'+• "" 0.5
--
0.5
0.3
+
2
T(K) 0.4
•
T(K) 0.4
303
o
303
,',
603
"
603
•
823
•
823
I
I
I
3
5
7
--
o
0.3
]
I
3
5
I 7
U/Umf
U/Umf
(C)
(D) dp =13121~m
0.8
--
0.7
--
[]
dp = 1312p, m
[]
~-~+
z~ z~
0.5
z~ z~
--
0.7
--
^-~'~" +
0.6 --
0.8
[]
T(K)
+
--
T(K)
0
298
[]
833
+
923
~
0.60.5
--
,', 1033 0.4 0
I
I
I
I
2
3
4
0.4 0
A
I
I
i
2
3
4
0.7
--
0.6
;-- •
dp = 2 1 6 5 ~ m
A
[]
A
..~o~~ 0.6
923
I
(F)
-[]
+
U/Umf
dp =21651~m •
833
1
U/Umf
0.7
298
[]
,', 1033
1
(E)
o
[]"~-d-~.o
-
[]
[] ~ '
•m
zx ~
'
~
~
T(K) J
O
298
~
823 ,', 1023 • 1143
, - < "
[]
0.5
--
c,,,O O ~ "°
/: 0.4
0.5
I
I
I
I
1
2
3
4
U/Umf
0.4
•
~ /
T(K) o 298 [] 823 z~ 1023
I
I
2
3
•
1143
I 4
U/Umf
Fig. 4. Variations of lower and upper bed region voidages as a function of fluidization number at different temperatures for the three sand beds.
and its intensity depends upon the temperature, while the upper region will exhibit a more stable bubbling fluidization whose quality will not change as long as the temperature is above the ambient. For the 1312/~m sand bed, the ap for the lower bed region [Fig. 5(C)] increases monotonically with U/Umrup to 3 and with T. This implies bubbling fluidization and bubble growth increasing with T, which results in greater a n values with an increase in T. At greater U/Umfvalues in this region, the fluidization quality appears to stabilize, suggesting a stable constant bubble size distribution. In contrast, in Fig. 5(D), we find that the upper bed region exhibits a transition from bubbling to turbulent fluidization at a U/Umfvalue of about 2. The other features are the same as found for the lower bed region. The fluidization quality for such particle beds changes along
S A X E N A and RAO: (A)
FLUIDIZED-BED INCINERATOR CHARACTERISTICS
0.4 --
.o ~ ~ o
~
0.3-
(B)
dp = 6411~m Lower bed region
L-
~
0_
o ~ O ~ o. °
~
/
T(K) o
0.1
0
~
303
A
603
•
823
/
+
I
5
7
T(K) O
//.
o
(C)
0.4
0.2-
T(K)
~"
o
298
~
[]
833
o.
+
923
/ / A'(-/
/
+
/Ac,"OCY-
-v./ /
I
I
I
5
7
/
0.3-
-+
/~- A / A I /
T(K)
IA/
0.2-
[]
298 B33
+
923
O
+//~
'~'aa
o.,
fP
'/
1
2
I
I
3
4
0
I
I
I
I
1
2
3
4
U/Umf (E)
U/Umf
dp =21651xm
0.8 -
h
/ /
.~.
0.4
(F)
-~¢-
dp = 2165rtm
Lower bed region
q
0.6 --
/ I
~
i
a /~
I
Upper bed region
0.8 --
--x'... \ aiX-..eT.,
0.6 --
\x-\ \
/
A/
T(K)
~'ldl~'i
0
A
p~z
-
0
0.2
823
Upper bed region
0.4 --
/
603
•
dp = 13121,tm
0.5 --
--
v
0.1
(D)
dp =13121xm
+
a
U/Umf
Lower bed region
"~" 0.3 -z,¢
303
3
U/Umf
0.5 --
A
i
,/ 0.2
I
/
//
o.
3
_
//~ /
0.4 - -
o.
0.2 o_
dp = 641~m Upper bed region
0.6 - -
~
87
o a98
"O////
[]
'-~ 5/-
~
023 1023
•
1143
"~"
/
\~
~
.'/ •O 0.2--/L
~
•
/ //
i /
/
/a /
~--6---_ ~-a ...El ~
T(K)
."..- = /r~ '~/--
[] 823 A 1023 •
I
I
I
2
3
4
U/Umf
d o.o/ -/
0 /
1143
I
I
I
2
3
4
U/Umf
Fig. 5. Variations of standard deviation in pressure-fluctuation data for the lower and upper bed regions as a function of fluidization number and temperature for the three sand beds.
the bed height in as much as that, while the lower section is in the bubbling mode, the upper section is in turbulent fluidization. For the 2165 #m sand bed, the lower bed region [Fig. 5(E)] exhibits bubbling fluidization, ap increasing with U/Umr. However, unlike the other two cases for temperatures greater than ambient, the fluidization regime transition from bubbling to turbulent takes place at U/Umt value a little smaller than 3. For still larger values of U/Um¢, the quality of fluidization appears to improve with continuous reduction in the ap values. At room temperature, this bed region leads to increasing bubbling and the bubble growth results in steep increasing values of ap with U/Umr. The upper bed region [Fig. 5(F)] is qualitatively similar to the lower bed region, except the bubbling behavior seems to stabilize for U/Umf values greater than 3. Turbulent fluidization of such large particles
88
SAXENA and RAO:
FLUIDIZED-BED
INCINERATOR
CHARACTERISTICS
will be encountered only for much larger U values, as stabilized fluidization may prevail over a large range of U/Umfvalues greater than 3. This suggests that the lower bubbling bed region at U/Umf greater than 3 may become turbulent, while the upper bed region will remain at stable fluidization. It is also interesting to examine the temperature uniformity in the incinerator. For this purpose, the axial temperature profile is measured, with the results presented in Fig. 6, where the percentage deviation of the temperature at a particular height above the distributor plate (Z) from the mean temperature is presented as a function of Z for four temperatures and for the 2165 #m sand bed. In each case, several values are reported for the different U/Umfvalues. The deviations are negligible at 298 K [Fig. 6(A)] as expected. In all other cases, for U/Umfgreater than about !.5, the deviations are always smaller than +5%, and these are particularly smaller at higher velocities and temperatures. A more precise interpretation of the bed quality fluidization is probably possible by a more detailed analysis of this pressure fluctuation data in terms of its probabilistic distribution functions. 15 - - ( A )
298K U/Umf =
5 --
1.16-1.80 6. . . .
-5
-0
o-
--
I
-15
]
I
15 - - ( S )
I 823K
q, , , / 1 . 4 6
5 --
\
\ \
~ ~.7
-5 --
0
~ -16 oO
1.67-3.96
I
I
I
15 - - ( C )
] 1023K
O./1"17 \ 1.61 1.96 --I:Y/
\
-- 2
i
-15 15 - -
(D)
~"-~.~
2
]
2.83-3.96
]
o~
I 1143K
\ f
1.00
\ \
5--
\% 1.25 ~e~
~L
~
~-.~ ~/15o-a 76
~_~
-5
\
-% \
-15 0
1
I
100
200
"" t300
I 400
Z (ram)
Fig. 6. A x i a l t e m p e r a t u r e d i s t r i b u t i o n s in 2165 p m bed at v a r i o u s t e m p e r a t u r e s a n d fluidization n u m b e r s as specified on i n d i v i d u a l curves.
SAXENA and RAO: FLUIDIZED-BED INCINERATOR CHARACTERISTICS
89
This may be done both in terms of its amplitude distribution and frequency distribution. The former is achieved by computing the probability density function (pdf) of the pressure fluctuation data, while the latter is investigated on the basis of the power spectral density function (psdf) or, more appropriately, by the power spectrum for the present case where the nature of pressure-drop data is in statistical equilibrium and is best approximated by a stationary series [9]. Such an analysis of data is presented in the next section. A N A L Y S I S OF E X P E R I M E N T A L
DATA
If N~ is the number of data points in the range dx around x, then the probability density function,
p(x), is given by [10] p(x) = N~/N dx.
(3)
Computed values of pdf for the lower and upper bed regions of the 641/~m sand particles are presented in Figs 7(A), (B) and (C) at temperatures of 303, 603 and 823 K. In each case, several values of U/Umf have been considered. At all the three temperatures, the distribution becomes sharper as the fluidization velocity is reduced. This implies that the bubble size distribution and, hence, the bubble coalescence increases with an increase in gas velocity. The nature of' the distribution functions is also different in the lower and upper bed regions. The sharpness of the distribution decreases, or the scatter in the AP values increases, in the upper section of the bed in relation to the lower section, and this difference decreases as the temperature increases. This implies a narrower size bubble distribution in the entire bed at higher temperatures, or the bed quality fluidization is more uniform. The apparent constancy in the spread of AP values for U/Umt. values greater than 3 in Fig. 7 corresponds to the earlier observation of constant ap in Figs 5(A) and (B). A better appreciation and quantification of these pdf plots in relation to fluidization quality is accomplished by computing the relative skewness (a3) defined as [11]: N
a3 = (1/Ua3p) ~ ( x i - ~,)~.
(4)
/-I
For a normal distribution, a3 = 0, and a3 is negative or positive depending upon whether the distribution is skewed to the left or to the right, respectively. A representative set of a3 computed values is given in Fig. 8. For the 641 # m bed [Fig. 8(A)], at lower U/Umf, a3 is positive and it decreases to negative values, attains a constant value, and thereafter, increases again to positive values with the increase in U/Umf. The same qualitative trend is observed for the larger particles at the two lower temperatures as seen in Fig. 8(B). This trend is not observed for the 2165 # m bed at higher temperatures (1023 and 1143 K) where a3 increases monotonically with the increase in U/Umf, and it is almost always positive. This information and trends directly translate to give us a quantitative nature of the pdfs and, hence, of AP which are related to bubble size distribution as discussed earlier. For our differential pressure probe arrangement, positive a~ values imply larger bubbles than for the normal distribution and negative values mean smaller bubbles than for the normal distribution. Further, positive a3 values will translate into more solids holdup than the negative a3 values for which gas holdup will be more. In the light of this interpretation, Fig. 8(A) suggests that, at lower U/Umf, the 641 # m bed has more gas holdup, i.e. smaller bubbles, and as the U/Umf is increased, the bubble size increases, obviously because of bubble coalescence. The same physical picture is valid for the larger particle bed at the two lower temperatures [Fig. 8(B)]. However, at the two higher temperatures, the bubble size is larger than for the normal distribution values of U/Umf, and at higher values, the bubble size distribution achieves a stable distribution. This information, while consistent with that obtained from the a m, tells us more about the structure and distribution of the AP values. These distributions are also compared with each other on the basis of the shape of their peaks. Flat topped and pronouncedly peaked curves are called platykurtic and leptokurtic, respectively.
90
SAXENA and RAO:
FLUIDIZED-BED INCINERATOR CHARACTERISTICS
(A)
4 ~- Lowerl--
IJ
Upper!
t
Sand " 641u.m
0
•
,,o
~
0
,.t,.
,a,
2
--J,L
J
l
i
..,,.s,'~J ~Ul~lt 2.50
I
'
1 ~']'"I hllfl, i ,
0
Iv
11JJtilll I
l~l
.,,,,,h,~.lI,1111]l,II Ih 1 Illilll h! hllllllh
J
it
l
• _ L t Ildl. !l*lll;lt,hJ,,.'.dil.,.,:,l., : ii!.,i;lilIJilfi h~h Jill .....
2
mrfllaI I[1[ Jr
.~ ,dl,.I,nllll,
I.,I
......... . .,,,hll . f, . .,.,,L~ , ......, ~,,!t,,, I,~l,,.~ 3.25
| h
g" :":~' 'th~It~,,.,..
o •__~_->"
J'J~
I llt
.~
I~
t
3.75 ,
o
_
t
0
'
I
'J:j ,,"T,,,I•/! ' ,. !! _ .,.,,,hll~i~i:l;il!!5:i, lli|::~:i:!Ir.i,];::.,**.,,
4.25
1
I
!
J
I ...
.
.... ~.,,,I.,~,lltl 'I
v., '
ll,,l:~Id,i
"
"0
~l,,I,~ t~,. ,Ih,,z,I.......... I ....'"'""~'" ,IJ,''*"''' 1 Pressure (kPa)
2
Fig.
Jl~lltIIII~l~lIdllllJ jlJ 2,Id,J,l,h,, I
L....,,,.,I~,~,II
1 Pressure (kPa)
Caption on p. 92.
7(A).
The peakedness of these distributions is measured in terms of kurtosis the following expression [11, 12]: a4=34
Ill, Iltl I Jllf Jlll,l,,l~.l !
N(N+I) (N--I)~-~-2~N
--
~, (x,-/~)4 3) i=1z... ~ O'p
(a4), and it is computed from
3(N- I)(N-1) (N -- 2)(N -- 3)
(5)
SAXENA and RAO: FLUIDIZED-BED INCINERATOR CHARACTERISTICS
9I
(B)
4
Lower
t
Upper
U/Umf = 1.1
1
1
l Sand 641~rn T = 603K
2
o 1[
I
.L
2.0
o
J~ll~lll
,
2"--'-3.5
+
o
°o ~-
I
J
l I I
i,it+l++tltlI++
.l .... ,.,.,H ~1
I, .L ,,Lm,,
t l ,Lift, .....
T
.,.
,
,F
tlJali[t+lit|hl tIll+tl~ ]
._ l,,,..,t.udlll I I~t I
It hllhtl,,, ....
J T
I
T
I
I
I'rlrI"l'
I lul~,Ililil~l 1 T'
67
....
t,,.h,l,l 0
i
"
5.0
1
,
J'F
1
4°J 2'
,,'
I.I
, j ~ ~ , . ,
a.o ~.
IJ
JLh,
,,~.,
"ili! l'ill,ItillhI!h,l.,., ........ I
Pressure(kPa)
2
I .....Lid II I
...
1I]llllilltgI,lmI,I',,!.,,.
Pressure(kPa)
2
Fig. 7(B). Capzion o~'erleall
For a normal distribution, d 4 = 3, and the coefficient of kurtosis, ( a 4 - 3 ) , is positive for a leptokurtic distribution and negative for a platykurtic distribution. Computed values of the coefficient of kurtosis are presented in Fig. 9 for the same beds and conditions as in Fig. 8. These ( a 4 - 3) values provide a quantitative measure of the flatness of the pdfs presented in Fig. 7. In Fig. 9, most of the values are negative, indicating that the distributions are platykurtic. This indeed is noted by the pronounced flatness of these distributions. This again implies that the bubble size distribution is wider than for the normal distribution. The spread of these peaks is related to the
92
SAXENA and RAO:
FLUIDIZED-BED INCINERATOR CHARACTERISTICS
(c)
1.1
0
i
,
.,,t,
L
2
I~ill, I
°o
.........
,
.... ~
,
"
~,
t
r
o--o ,-,
o
o..
2r~
1
L+
..
I
I
I
.
Jim/
3.5
,
,, .,..ilili.,
, ,,,l~llllll~J 1
__L_~ !
L
. mtl,i
Ill j ~'f
I.: .... i
0 ... ,.,,,,,411,,! I tfj>,,,J ........
_ 0
/
,.... ,,,,,=,,ItI t li 11i, ~h.,,,,,,,..,~
I ipl~ 1
1
P r e s s u r e (kPa)
II I t, 2
0
1
ti' 2
P r e s s u r e (kPa)
Fig. 7. Plots of pdf for the lower and upper bed regions at different UiU°, r values at (A) 303 K, (B) 603 K and (C) 823 K.
size range of bubbles in the bed. Obviously, for the 641 # m bed [Fig. 9(A)] at lower U/Umr values, the bubble size range is wide, and this becomes narrower as the U/Umrvalue increases. For the larger particle size bed [Fig. 9(B)], the bubble size range is always wider than the normal, and the spread is relatively smaller at greater temperatures and remains almost constant for the two highest temperatures. This last result is consistent with what is observed in
Fig. 8(B).
S A X E N A and RAO:
FLUIDIZED-BED INCINERATOR CHARACTERISTICS
(A)
93
(B)
0.6
•
[]
-
0.3
?
"0 /
o
.
0
,, ~ O "
y
+ "+
-0.3
o
-0.6
823K
--
/ 0~0
•
1143K
I
I
I
I
I
I
3
5
7
2
3
4
U/Umf U/Umrfor
Fig. 8. Variation of a 3 with
the upper bed section at different temperatures for (A) 641 # m and (B) 2165 # m beds.
The power spectrum of the pressure drop variation data, p(to), is defined by the following relation [13]: P(to) = 2~2a[1 - ~l e x p ( - j o g ) - ~b2 e x p ( - j 2 t o ) . . . . .
~bpexp( - j p t o ) ]
:
(6)
for 0 ~< to ~< z. Here, tra is the variance of the residuals and ~b~,tk2. . . . . ~bp are model based coefficients. In view of the nature of our data, which is comprised of both a periodic as well as a random component, we have adopted a Univariate Box-Jenkins (UBJ) model [14], also known as the Auto-Regressive Integrated Moving Average (ARIMA) model, developed for forecasting time series data. On the basis of computed autocorrelation and partial autocorrelation functions, the model is identified as auto-regressive of order varying from two to six. Next, the coefficients (ap) are calculated using the software of Scientific Computing Associates (SCA) [15]. The adequacy of the model is checked by calculating the autocorrelation function of the residuals for the existence of any probable structure in the set. For the correct order of the model, the residuals will exhibit no structure. Computed values of the power spectrum for the 641 # m sand bed at various fluidization numbers and at three temperatures 303, 603 and 823 K are shown plotted in Figs 10(A), (B) and (C), respectively. In each case, we have identified a major frequency, fro, which is listed in these plots. These major frequencies are graphed in Figs I I(A) and (B) for the different temperatures as a function of fluidization number for the two sand beds of average diameters 641 and 2165 #m, respectively. It is clear that, for both the beds, at lower ftuidization numbers (smaller than about 2.5)
(A)
(B)
1.0
+ t3 0
303K 603K 823K
o o " •
0.5 o~
0
o
o
298K 823K 1023K 1143K
\ .
-0.5 -1.0
-1.5
~k --
or+
\
+" +-+'+
"~'c~
"°c~
i ~
n
_
..D
I
I
I
I
I
I
3
5
7
2
3
4
U/Umf Fig. 9. Variation of the coefficientof kurtosis with U/Umrfor the same beds as in Fig. 8.
94
SAXENAandRAO:FLUIDIZED-BED INCINERATOR CHARACTERISTICS (A)
A
0.08 ~ 0.06 - -
~ /
E
U/Umf = 1.10 ~
2 --
frn=1.63Hz
fm = 2 " 2 8 H z
~_/ y _ 0.02
1- ~
0.04
0
U/Umf = 2.75
I
I-"~
2
4
I 0
2
f (Hz)
t~
1.2
0.3
0.9
0.2
0.6
0.1
~--
I 0
2
2
, I 6
F
U/Umf = 1.60
•-~
E
4 f (Hz)
B 0.4 --
--
I
6
4
U/Umf = 3.25
0.3 --
a.
6
E
0
2
=
f (Hz)
~
4
6
f (Hz)
13..
00040 o Z00 ¢n
C
10-
r=*
~
U/Urn,=210
G
=~ 1 2 - - 1 ~
o
U/Umf = 3.75
o
I 0
2
4
6
0
I
C"----~
I
2
4
6
f (Hz)
f (Hz) D
1.2 --
H
U/Umf = 2.50
1.2
0.9
0.9
0.6
0.6
0.3 - -
0.3
U/Umf = 4.25
I 0
2
4
I
6
0
f (Hz)
2
4
6
f (Hz)
Fig.10(A).
Caption on p. 96.
and/or temperatures above the ambient, the temperature has a pronounced influence on fro. This dependence is a reflection of the bubble coalescence in the bed, as it is influenced by temperature. At higher fluidization numbers, the bed is in the turbulent regime and has an approximately constant fm which is not much influenced by either temperature or particle size. This is consistent with the inferences derived above suggesting a stable bubble size distribution at large U/Umrvalues. It is a very practical result from the incineration view point, as incinerators are generally operated in this regime. At ambient conditions, the bed is in the bubbling regime and fluidization velocity has a significant influence on fro, as expected.
SAXENA and RAO: (B) 0.04
FLUIDIZED-BED INCINERATOR CHARACTERISTICS A
E
U/Umf = 1.1 fm =2"17Hz
--
0.4
--
U/Umf = 4 . 0
,/~
0.03 --
0.3 --
0.02 -
-
~
,/"
fm=L63Hzfm = 1.63 Hz ~
0.2
0.01 --
0.1
I 0
1
2
3
4
5
I
6
0
1
2
f (Hz)
3
--
U/Umf = 2 . 0
~
0.3 --
0.2 -
~
0.2
0.1
~
0.1 --
2
3
4
U/Umf = 5 . 0
5
I 6
no 0
1
2
f (Hz)
u~ 0.5 ~
O 13-
0.4
~
3
4
5
I 6
5
6
f (Hz)
C
~:
6
fm= 1.52 Hz
v E
n E 2
0,4 --
~_.
0.3 -
1
5
F
fm = 2.61 Hz
0
4
f (Hz)
B 0.4
95
G
U/Umf = 3 . 0
0.4 ~-
U/Umf = 6 . 7
fm = 1.19 Hz
0.3 ! i
]
i.,~... *
0.3 0.2
0.2 0.1
0.1
I 0
2
4
I
6
0
f (Hz)
2
3
4
f (Hz)
D
0.5 0.4 !
1
y
U/Umf H z
~
= 3.5
0.3 -0.2 0.1 0
I
]
I
I'~'1
1
2
3
4
5
6
f (Hz) Fig. 10(B).
Caption overleaf.
CONCLUSIONS
On the basis of test runs conducted on the incinerator, several conclusions can be drawn regarding its design and operating characteristics. (a) The design of the propane sparger is certainly found to be adequate and the operation reliable for either initial heating of the inert bed or for supplying the supplemental thermal energy to sustain combustion of wastes of small calorific values. ECM 34/2-- B
96
S A X E N A and RAO:
(C) 0.02 - -
FLUIDIZED-BED INCINERATOR CHARACTERISTICS
A
D
U/Umf = 1.1 fm= 2.72 Hz
0.2 i'D /
U/Umf = 2.5 fm = 1.58 Hz
/
O0 0
OQ I
I
I
2
4
6
I 0
2
f (Hz)
E
fm = 3.04 Hz
~" n
0.4
E 2
E 2
0.3--
¢,~
Q.
0 n
I1.
cn
-
0
2
--
4
¢n
0.2
0
0.1
I
6
0
I
J
2
4
f (Hz)
""--"
I
6
f (Hz) C
0.2 --
6
f (Hz)
B ~" 0..
4
F
U/Umf = 2.0 =
0.5 --
U/Umf = 5.8 Hz
"
fm = 1.19
0.4 _-0.3 0.1 0.2 0.1
0
I
I
I
2
4
6
f (Hz)
I 0
2
4
6
f (Hz)
Fig. 10. Plots of power spectrum for the upper bed region (dp = 641 g m ) at different (A) 303 K, (B) 603 K, (C) 823 K.
U/Umfand at
(b) The bed voidage of inert sand beds is found to be almost independent of bed height, fluidization velocity and temperatures which are greater than ambient. This result is of significant value for incineration studies. (c) The standard deviation values suggest that, in general, the bubble coalescence increases with gas velocity and temperature, and the transition from bubbling to turbulent fluidization occurs at some characteristic value of U/Umfwhich depends primarily on particle size. The small particle beds (641 Izm) behave somewhat differently, and these may remain in the bubbling fluidization regime even at higher velocities where the bubble size distribution stabilizes, as exhibited by constant trp values. Above ambient, with increase in temperature, a general trend of bed quality deterioration (higher ~p values) is observed. The inert bed
SAXENA and RAO:
FLUIDIZED-BED INCINERATOR CHARACTERISTICS
(A)
(B)
43I3~40 dp= 641p.m ~"
~ /+/+~ ++: ,,.E 2 ~ ~/ / \
+-
~-- /° []dP= 21651J.m •
+ 303K m 603K O 823K
• O 298K e~o 823K ~"~,..,.=e.,.,,~.. A• 1023K 1143K
~- +- + - o A
1
97
1
I
3
5
o
°1
7
A
~o ~oo I
"8" i o
2
3
AO
[]
4
U/Umf
Fig. 11. Variations of fro in power spectrum with U / U s at different temperatures for the upper regions of beds of (A) 641 #m and (B) 2165 tam.
(d) (e)
(f)
(g)
particle diameter appears to play an important role for beds larger than about 1 mm as far as the influence of temperature on bed quality fluidization is concerned. The temperature uniformity in the bed along the vertical direction is found to be within + 5%, and it improves with increase in gas velocity and temperature. It is shown that the behavior of the 641/~m bed, as revealed by the pdf plots of Figs 7(A), (B) and (C), is consistent with the conclusions drawn on the basis of the t~pvalues in Figs 5(A) and (B). This reinforces the conclusions drawn in relation to bubble coalescence and bubble size distribution on the basis of ap. The two approaches based on % and pdf for investigating the bed quality fluidization are thus consistent and support each other. The computed values of the relative skewness and coefficient of kurtosis are found to provide valuable information about the size and size range of the bubbles in relation to a normal distribution, respectively. The inferences so derived are found to be consistent with those obtained on the basis of ap and pdfs. The computation of the power spectrum and the values of major frequencies so obtained as a function of fluidization number exhit a very practical result of value in the operation of incinerations. Specifically, the incinerator hydrodynamics will not be appreciably influenced over a range of fluidization numbers as long as it is greater than a specific value which depends upon particle size.
Acknowledgement--This work is supported by the Office of Solid Waste Research of the Univeristy of Illinois at Urbana-Champaign under grant number OSWR-04-002. REFERENCES 1. D. A. Tillman, A. J. Rossi and K. M. Vick, Incineration of Municipal and Hazardous Solid Wastes. Academic Press, New York (1989). 2. W. Bartok and A. F. Sarofim (Eds), Fossil Fuel Combustion, A Source Book. Wiley-Interscience, New York (1991). 3. C. R. Brunner, Handbook oflneineration Systems. McGraw-Hill, New York (1991). 4. S. C. Saxena and A. Mathur, Energy--The International Journal 10, 57 (1985). 5. S. C. Saxena and A. Chanerjee, Energy--The International Journal 4, 349 (1979). 6. S. C. Saxena, N. S. Rao, V. G. Rao and R. R. Koganti, Energy--The International Journal 17, 579 (1992). 7. S. C. Saxena and N. S. Rao, Energy--The International Journal 6, 489 (1990). 8. S. C. Saxena and G. J. Vogel, Trans. lnstn Chem. Engrs 55, 184 (1977). 9. G. M. Jenkins and D. G. Watts, Spectral Analysis and its Application. Holden-Day, San Francisco, Calif. (1968). 10. J. S. Bendat and A. G. Piersal, Random Data: Analysis and Measurement Procedures. Wiley, New York (1986). 11. G. Simpson and K. Kafka, Basic Statistics. Norton, New York (1952). 12. SAS Institute Inc., SAS ® User's Guide: Statistics, Version 5 Edition. Cary, N.C. (1985). 13. G. E. P. Box and G. M. Jenkins, Time Series Analysis, Forecasting and Control. Holden-Day, San Francisco, Calif. (1971). 14. A. Pankratz, Forecasting with Univariate Box-Jenkins Models, Concepts and Cases. Wiley, New York (1983). 15. L. M. Liu, G. B. Hudak, G. E. P. Box, M. E. Miller and G. C. Tiao, The SCA Statistical System, Reference Manual for Forecasting and Time Series Analysis, Version III. Scientific Computing Associates, Dekalb, Ill. (1986).
0196-8904/93 $5.00+ 0.00 Copyright © 1992 Pergamon Press Ltd
CHARACTERISTICS OF A FLUIDIZED-BED INCINERATOR FOR THE COMBUSTION OF SOLID WASTES S. C. SAXENA? and N. S. R A O Department of Chemical Engineering, The University of Illinois at Chicago, P.O. Box 4348, Chicago, IL 60680, U.S.A.
(Received 16 September 1991; receivedfor publication 17 April 1992) Ahstract--A laboratory-scale fluidized-bed incinerator has been designed for the efficient combustion and thermal destruction of solid waste materials. The experimental facility consists of a 153 mm dia cylindrical furnace, a propane gas burner, an air supply system, batch and continuous feeders, an off-gas cleanup system, flue-gas analyzer system, and an on-line automated data recording and analysis system. A continuous monitoring of temperature and pressure distributions in the incinerator, and composition of the flue gas is possible. Characteristics of the incinerator have been established. These include measurement of pressure drops across the calming and bed sections as a function of superficial air velocity and temperature, and axial temperature distribution in the test bed. Hydrodynamic characteristics of three inert sand beds of average diameters 641, 1312 and 2165/am have been investigated by recording the pressure-drop histories in the lower and upper regions of the bed as a function of fluidization number and bed temperature. Several statistical functions such as variance of the pressure-drop data, probability density function, skewness, kurtosis, autocorrelation function and probability density function have been computed, and the same have been related to the fluidization quality of the bed. These investigations suggest the optimum conditions for incineration of waste materials where the gas-solid contacting and solids mixing is most favorable, and hence, best results will be obtained for thermal destruction and pollution control. Fluidized bed
Incinerator
Combustion
Solid wastes
NOMENCLATURE a3 = a4 = do = dt = dx = f= fm= N= Nx = p(x) = p(w) = T= T~= t= U= Umf= U/Umf = x, = Z=
Relative skewness (dimensionless) Kurtosis (dimensionless) Mean particle diameter (m) Increment in time (s) Increment in pressure drop (kPa) Frequency (Hz) Major frequency (Hz) Number of data points (dimensionless) Number of data points in range dx around x (dimensionless) Probability density function (kPa- ~) Power spectrum (kPa 2) Average bed temperature (K) Bed temperature at given distance above distributor plate (K) Time instant (s) Superficial gas velocity computed at standard conditions of temperature and pressure (m/s) Minimum fluidization velocity (m/s) Fluidization number (dimensionless) ith pressure-drop value (kPa) Distance above the distributor plate (ram)
Greek letters AP = APd = APb = = ~L = ~u = /z =
Pressure drop (kPa) Pressure drop across distributor plate (kPa) Pressure drop across sand bed (kPa) Bed voidage (dimensionless) Voidage of lower bed section (dimensionless) Voidage of upper bed section (dimensionless) Mean of N pressure-drop values (kPa)
tTo whom all correspondence should be addressed. 81
82
SAXENA and RAO: FLUIDIZED-BED INCINERATOR CHARACTERISTICS a2a= Variance of residuals (kPa2) ap = Standard deviation (kPa) ~bl,~b2. . . . . Op= AR coefficientsin ARIMA model = Angular frequency (rad) Acronyms
AR = Auto regressive ARIMA = Auto regressive integrated moving averages pdf = Probability density function psdf = Power spectral density function SAS = Statistical analysis system SCA = Scientificcomputing associates UBJ = Univariate Box-Jenkins
INTRODUCTION Management of solid wastes produced from a variety of sources, such as domestic, industrial, medical, and agricultural, is becoming a serious and challenging problem. Many monographs have been written on this subject, and several conventional treatment technologies have been developed and reevaluated for this purpose with emphasis on environmental pollution control. Thermal degradation and combustion of waste materials is the subject matter of several recent books, such as those of Tillman et al. [1], Bartok and Sarofim [2], and Brunner [3]. Incineration of domestic waste has been a well known disposal procedure for centuries, and improved designs of incinerators have been continuously developed to cope with the changing composition of the domestic and industrial wastes. Innumerable articles dealing with the various design and operational features of incinerators have appeared in the technical literature which also emphasize its distinct advantages and disadvantages. Fluid-bed incinerators are one type of incinerator which has been preferred for this purpose in view of its temperature uniformity, good gas-solids contacting, good solids mixing and versatility in the nature of refuse material these can handle. Its potential for in situ retention of sulfur and chlorine has been well recognized in connection with coal combustion and is certainly of great relevance in cogeneration. Here, we present the design of a test fluidized-bed incinerator and preliminary data with three inert sand-beds heated to different temperatures with a specially designed propane burner. In addition to the characteristics describing the behavior of the incinerator and sand beds, pressuredrop history has been recorded and analyzed for several statistical functions, such as the standard deviation in pressure drop data (ap), probability density function (pdf), skewness, kurtosis, autocorrelation function, and power spectrum, to characterize incinerator hydrodynamic regimes and fluidization quality. It is hoped that such characterization of the fluidized-bed incinerator will help in its more controlled operation and thereby minimize the formation of pollutants and, hence, environmental pollution. FLUIDIZED-BED INCINERATOR The schematic of the fluidized-bed incinerator, consisting of several independent units, is shown in Fig. 1, and it is a modified version of our fluidized-bed coal combustion facility, Saxena and Mathur [4]. The facility consists of an air supply system (3, 4), propane gas supply and metering system (8, 9), incinerator (l), solids batch (7) and continuous (6) feeders, and a flue-gas cleanup system (13, 14). The combustion air is supplied by two 18.65 kW compressors and is dried by silica gel and refrigeration driers and then filtered by two filters to remove oil vapors before entering the incinerator preheater section (e). Propane gas is supplied to the sparger (10) by two 100 lb propane gas cylinders (8) and is metered on two rotameters (9). The incinerator is comprised of two preheating sections (e) with an overall height of 1.52 m and is equipped with a tube-fired propane burner which is not used in the current experiments. The two elbow sections (d) connect the preheater section to the calming section (a), 0.305 m long. The main test bed (b) and freeboard (c) sections are comprised of four flanged sections, each 0.571 m long and bolted together with 0.038 m long spacer rings. All these sections are made of 0.254 m diameter, schedule 40, 304 stainless-steel pipe. The entire preheater, elbows, calming and bed sections are provided with a 50 mm thick Purolite-30 insulating material lining. The incinerator wall is externally insulated with Fiberflax
SAXENA and RAO:
FLUIDIZED-BED INCINERATOR CHARACTERISTICS
C
1. Incinerator
q 1 ~ J 4I
I
~
~(
b ~ ~111~
• e -e i
'
1--'~ 1 5 J - - ~ ~ - ~
83
7
2 _ a ~ "~- 5 /10 ~_-----~ / /'// 9 i~-- -~ 8
2. Differentialpressuretransmitters 3. Air compressors 4. Air inlet 5. Gas distributerplates 6. Continuoussolidsfeeder 7. Batchsolidsfeeder 8. Propanetanks 9. Rotameters 10. Propanesparger 11 Solidsoverflow 12 Coolingcoils 13 Cyclones 14 Fabric-filter 15 Insulation a. Calmingsection b. Testsection c. Freeboardsection d Elbows e Preheater
Fig. 1. Schematic of the fluidized-bed incinerator and the support system.
Lo-Con felt insulating wrapping of 25 mm thickness. The solids feeders (6, 7) are not used in the present work dealing with sand beds operated in batch mode. The flue gas is cleaned by passing it through two stainless steel, 6 UP type, cyclones (13) and a Pall Trinity Micro-filter assembly (14), Saxena and Chatterjee [5]. The engineering design details of the propane sparger (10) are given by Saxena et al. [6], and it essentially consists of a central stainless steel housing equipped with four stainless pipe arms symmetrically placed. It has nine orifices, 0.5 mm in diameter, for propane injection into the calming section. The calming section gas distributor is a perforated plate with 61 holes, 3.2 mm in diameter, and its detailed design is given by Saxena and Chatterjee [5]. The test bed distributor plate has 19 multi-orifice nozzles, and its intricate design features are reported by Saxena e t al. [6]. Nine chromel-alumel thermocouples have been used to monitor the temperatures of the different regions of the incinerator. Three of these are located in the bed section at an elevation of 76.2, 152.0 and 304.8 mm above the top surface of the distributor plate. Six pressure taps are provided to establish the pressure distribution along the incinerator. In the bed section, these probes are located at heights of 40, 140 and 290 mm above the gas distribution surface. The bed height can be maintained and adjusted by an overflow pipe (11), with design details given by Saxena and Chatterjee [5]. The pressure drops across the distributors, bed sections and the entire bed are fed to a data acquisition system through Leeds and Northrup differential pressure transmitters (2). The electrical signals from the thermocouples and the pressure probes are measured over a HewlettPackard system consisting of a dedicated computer, monitor, printer and plotter. Its details are given by Saxena and Rao [7].
84
SAXENA and RAO: 30
v~
FLUIDIZED-BED INCINERATOR CHARACTERISTICS
-
/
2o-
~
% X
~ 0
1
I
I
I
I
2
3
4
5
U (m/s) Fig. 2. Variations of distributor plate pressure drop as a function of superficial air velocity at different temperatures.
EXPERIMENTAL
DATA
The pressure drop across the bed distributor plate is measured as a function of the superficial air velocity at three temperatures, with the results reported in Fig. 2. The air is heated to different temperatures in the calming section by the combustion of propane. A spark plug is provided above the propane sparger in the calming section to ignite the air-propane mixture. The U and Umrvalues in this paper always refer to standard conditions of temperature and pressure. It is found that the pressure drop can be represented as a power function of U at all temperatures and over the entire velocity range. The three sand beds examined in this work have the mean diameters of 641, 1312 and 2165/~m. Experiments have been conducted with these beds of about 32.5 cm height. From the knowledge of bed pressure drop as a function of decreasing gas velocity, the minimum fluidization velocities are determined following the procedures of Saxena and Vogel [8]. These results are reported in Table 1, and a typical data plot for 2165 #m is shown in Fig. 3. The pressure drops are also measured for two sections of the bed enclosed between 40 and 140mm, and 140 and 290mm. These are referred to as the lower and upper bed sections, respectively. The bed voidages computed from these data (qj and EL) are presented in Fig. 4 as a function of fluidization number (U/Umr)and temperature. From these data, it is clear that the bed voidage of a bed section is almost independent of temperature at temperatures above the ambient. It is also very weakly dependent upon the fluidization number and bed height. The pressure-drop history data for these three beds and, in each case, for the two bed regions at several temperatures and fluidization velocities have been taken at a sampling rate of 10.8 readings/s. In all, 1000 readings are taken in 92.4s. The standard deviation, trp, of these 1000 readings is computed from the following relation: ap
-
1
u
Y~ ( x , - ~)~
(1)
N--I~=I
Table 1. Values of minimum fluidization velocities for three different sand beds at different temperatures
d0
(#m) 641
T (K) Umf (m/s)
298 0.42
603 0.28
823 0.24
---
1312
T (K) Umf (m/s)
298 !.10
833 0.62
923 0.46
1033 0.52
2165
T (K) Umf (m/s)
298 1.44
823 0.45
1023 0.60
1143 0.52
SAXENA and RAO:
FLUIDIZED-BED INCINERATOR CHARACTERISTICS
(A)
(C)
8 --
8
-
-
e~oOoO
o
e,o~oe~eoeo
°[°~"
4--
T = a98K 06°
2 -ze/o
o
~
dp = 2165~.m
/2
2
l
I
I
1
2
3
U (m/s)
dp = 2165~,rn
I
I
I
1
2
3
U (m/s)
(D)
8 --
8
--
f~'cj (3 we
oa-o,o-oo-~o-O-~oQ-6 --
6
Q-
0-~o •
OOU
O@
13_ ~v 4
4 T =823K
0
T = 1023K
~t~
o
(B)
2
a-
/o
a~ 4
aY
<
85
t
~
dp = 2165p, m
2
l
I
1
1
2
3
U
T = 1143K
0
(m/s)
dp = 2165~rn
I
I
1
2
U (m/s)
Fig. 3. Variations of bed pressure drop as a function of increasing (C)) and decreasing (O) gas velocity at different temperatures.
where 1
N
= ~,=~, xi.
(2)
Here, N is the total number of AP readings, xi is the ith AP value, and/~ is the mean of N data points. Computed values of ap based on the above relations for the two bed regions and for the three sand beds at different temperatures and fluidization velocities are presented in Fig. 5. A detailed discussion of these figures is extremely interesting in establishing the hydrodynamic activity of the bed and is presented below. Figure 5(A), for the 641/~m sand beds, exhibits that the pressure fluctuations do not depend upon temperature (303-823 K) for the lower bed region. This suggests that the bubbling phenomenon, including bubble formation and coalescence, does not depend on T over the entire velocity range. However, as the fluidization number increases, the standard deviation also increases until about a value of 3, thereafter, further increases in U/Umrdo not produce appreciable changes in ap. This implies that, initially, bubble coalescence increases the bubble size with increasing U/Umf up to a value of 3 and this results in the continuously increasing value of ap. For U/Umfvalues greater than 3, the bubble dynamics change and there is an equilibrium between bubble growth and decay, resulting in a constant bubble size distribution and, hence, a constant value of a v. 'This is exhibited in a constant value of EL in Fig. 4(A). In Fig. 5(B), for the same bed, we find that, in the upper region, the ~p values are not dependent on T as long as it is above the ambient, in addition to U/Umf. For U/Umrvalues smaller than 3, the bubble induced AP and, hence, the ap values increase with U/Umrat all temperatures. This is due to the increasing bubble size as a result of coalescence. However, this coalescence is temperature dependent and the same decreases with an increase in temperature. The reduction in effective bubble size with increase in temperature decreases ap. Figure 5(B) also suggests that, for U/Umrgreater than 3 and at temperatures greater than ambient, temperature does not influence and, hence the effective bubble diameter and its induced pressure fluctuations remain almost constant. Figures 5(A) and 5(B) suggest that, along the bed height, there is a change in the fluidization quality in as much as the lower region is bubbling
86
and R A O :
SAXENA (A) 0.6
FLUIDIZED-BED
INCINERATOR
CHARACTERISTICS
(B)
dp = 641 p,m
--
0.6
dp = 641 p.m
-A
o
,.,~....+~.o .'.'+• "" 0.5
--
0.5
0.3
+
2
T(K) 0.4
•
T(K) 0.4
303
o
303
,',
603
"
603
•
823
•
823
I
I
I
3
5
7
--
o
0.3
]
I
3
5
I 7
U/Umf
U/Umf
(C)
(D) dp =13121~m
0.8
--
0.7
--
[]
dp = 1312p, m
[]
~-~+
z~ z~
0.5
z~ z~
--
0.7
--
^-~'~" +
0.6 --
0.8
[]
T(K)
+
--
T(K)
0
298
[]
833
+
923
~
0.60.5
--
,', 1033 0.4 0
I
I
I
I
2
3
4
0.4 0
A
I
I
i
2
3
4
0.7
--
0.6
;-- •
dp = 2 1 6 5 ~ m
A
[]
A
..~o~~ 0.6
923
I
(F)
-[]
+
U/Umf
dp =21651~m •
833
1
U/Umf
0.7
298
[]
,', 1033
1
(E)
o
[]"~-d-~.o
-
[]
[] ~ '
•m
zx ~
'
~
~
T(K) J
O
298
~
823 ,', 1023 • 1143
, - < "
[]
0.5
--
c,,,O O ~ "°
/: 0.4
0.5
I
I
I
I
1
2
3
4
U/Umf
0.4
•
~ /
T(K) o 298 [] 823 z~ 1023
I
I
2
3
•
1143
I 4
U/Umf
Fig. 4. Variations of lower and upper bed region voidages as a function of fluidization number at different temperatures for the three sand beds.
and its intensity depends upon the temperature, while the upper region will exhibit a more stable bubbling fluidization whose quality will not change as long as the temperature is above the ambient. For the 1312/~m sand bed, the ap for the lower bed region [Fig. 5(C)] increases monotonically with U/Umrup to 3 and with T. This implies bubbling fluidization and bubble growth increasing with T, which results in greater a n values with an increase in T. At greater U/Umfvalues in this region, the fluidization quality appears to stabilize, suggesting a stable constant bubble size distribution. In contrast, in Fig. 5(D), we find that the upper bed region exhibits a transition from bubbling to turbulent fluidization at a U/Umfvalue of about 2. The other features are the same as found for the lower bed region. The fluidization quality for such particle beds changes along
S A X E N A and RAO: (A)
FLUIDIZED-BED INCINERATOR CHARACTERISTICS
0.4 --
.o ~ ~ o
~
0.3-
(B)
dp = 6411~m Lower bed region
L-
~
0_
o ~ O ~ o. °
~
/
T(K) o
0.1
0
~
303
A
603
•
823
/
+
I
5
7
T(K) O
//.
o
(C)
0.4
0.2-
T(K)
~"
o
298
~
[]
833
o.
+
923
/ / A'(-/
/
+
/Ac,"OCY-
-v./ /
I
I
I
5
7
/
0.3-
-+
/~- A / A I /
T(K)
IA/
0.2-
[]
298 B33
+
923
O
+//~
'~'aa
o.,
fP
'/
1
2
I
I
3
4
0
I
I
I
I
1
2
3
4
U/Umf (E)
U/Umf
dp =21651xm
0.8 -
h
/ /
.~.
0.4
(F)
-~¢-
dp = 2165rtm
Lower bed region
q
0.6 --
/ I
~
i
a /~
I
Upper bed region
0.8 --
--x'... \ aiX-..eT.,
0.6 --
\x-\ \
/
A/
T(K)
~'ldl~'i
0
A
p~z
-
0
0.2
823
Upper bed region
0.4 --
/
603
•
dp = 13121,tm
0.5 --
--
v
0.1
(D)
dp =13121xm
+
a
U/Umf
Lower bed region
"~" 0.3 -z,¢
303
3
U/Umf
0.5 --
A
i
,/ 0.2
I
/
//
o.
3
_
//~ /
0.4 - -
o.
0.2 o_
dp = 641~m Upper bed region
0.6 - -
~
87
o a98
"O////
[]
'-~ 5/-
~
023 1023
•
1143
"~"
/
\~
~
.'/ •O 0.2--/L
~
•
/ //
i /
/
/a /
~--6---_ ~-a ...El ~
T(K)
."..- = /r~ '~/--
[] 823 A 1023 •
I
I
I
2
3
4
U/Umf
d o.o/ -/
0 /
1143
I
I
I
2
3
4
U/Umf
Fig. 5. Variations of standard deviation in pressure-fluctuation data for the lower and upper bed regions as a function of fluidization number and temperature for the three sand beds.
the bed height in as much as that, while the lower section is in the bubbling mode, the upper section is in turbulent fluidization. For the 2165 #m sand bed, the lower bed region [Fig. 5(E)] exhibits bubbling fluidization, ap increasing with U/Umr. However, unlike the other two cases for temperatures greater than ambient, the fluidization regime transition from bubbling to turbulent takes place at U/Umt value a little smaller than 3. For still larger values of U/Um¢, the quality of fluidization appears to improve with continuous reduction in the ap values. At room temperature, this bed region leads to increasing bubbling and the bubble growth results in steep increasing values of ap with U/Umr. The upper bed region [Fig. 5(F)] is qualitatively similar to the lower bed region, except the bubbling behavior seems to stabilize for U/Umf values greater than 3. Turbulent fluidization of such large particles
88
SAXENA and RAO:
FLUIDIZED-BED
INCINERATOR
CHARACTERISTICS
will be encountered only for much larger U values, as stabilized fluidization may prevail over a large range of U/Umfvalues greater than 3. This suggests that the lower bubbling bed region at U/Umf greater than 3 may become turbulent, while the upper bed region will remain at stable fluidization. It is also interesting to examine the temperature uniformity in the incinerator. For this purpose, the axial temperature profile is measured, with the results presented in Fig. 6, where the percentage deviation of the temperature at a particular height above the distributor plate (Z) from the mean temperature is presented as a function of Z for four temperatures and for the 2165 #m sand bed. In each case, several values are reported for the different U/Umfvalues. The deviations are negligible at 298 K [Fig. 6(A)] as expected. In all other cases, for U/Umfgreater than about !.5, the deviations are always smaller than +5%, and these are particularly smaller at higher velocities and temperatures. A more precise interpretation of the bed quality fluidization is probably possible by a more detailed analysis of this pressure fluctuation data in terms of its probabilistic distribution functions. 15 - - ( A )
298K U/Umf =
5 --
1.16-1.80 6. . . .
-5
-0
o-
--
I
-15
]
I
15 - - ( S )
I 823K
q, , , / 1 . 4 6
5 --
\
\ \
~ ~.7
-5 --
0
~ -16 oO
1.67-3.96
I
I
I
15 - - ( C )
] 1023K
O./1"17 \ 1.61 1.96 --I:Y/
\
-- 2
i
-15 15 - -
(D)
~"-~.~
2
]
2.83-3.96
]
o~
I 1143K
\ f
1.00
\ \
5--
\% 1.25 ~e~
~L
~
~-.~ ~/15o-a 76
~_~
-5
\
-% \
-15 0
1
I
100
200
"" t300
I 400
Z (ram)
Fig. 6. A x i a l t e m p e r a t u r e d i s t r i b u t i o n s in 2165 p m bed at v a r i o u s t e m p e r a t u r e s a n d fluidization n u m b e r s as specified on i n d i v i d u a l curves.
SAXENA and RAO: FLUIDIZED-BED INCINERATOR CHARACTERISTICS
89
This may be done both in terms of its amplitude distribution and frequency distribution. The former is achieved by computing the probability density function (pdf) of the pressure fluctuation data, while the latter is investigated on the basis of the power spectral density function (psdf) or, more appropriately, by the power spectrum for the present case where the nature of pressure-drop data is in statistical equilibrium and is best approximated by a stationary series [9]. Such an analysis of data is presented in the next section. A N A L Y S I S OF E X P E R I M E N T A L
DATA
If N~ is the number of data points in the range dx around x, then the probability density function,
p(x), is given by [10] p(x) = N~/N dx.
(3)
Computed values of pdf for the lower and upper bed regions of the 641/~m sand particles are presented in Figs 7(A), (B) and (C) at temperatures of 303, 603 and 823 K. In each case, several values of U/Umf have been considered. At all the three temperatures, the distribution becomes sharper as the fluidization velocity is reduced. This implies that the bubble size distribution and, hence, the bubble coalescence increases with an increase in gas velocity. The nature of' the distribution functions is also different in the lower and upper bed regions. The sharpness of the distribution decreases, or the scatter in the AP values increases, in the upper section of the bed in relation to the lower section, and this difference decreases as the temperature increases. This implies a narrower size bubble distribution in the entire bed at higher temperatures, or the bed quality fluidization is more uniform. The apparent constancy in the spread of AP values for U/Umt. values greater than 3 in Fig. 7 corresponds to the earlier observation of constant ap in Figs 5(A) and (B). A better appreciation and quantification of these pdf plots in relation to fluidization quality is accomplished by computing the relative skewness (a3) defined as [11]: N
a3 = (1/Ua3p) ~ ( x i - ~,)~.
(4)
/-I
For a normal distribution, a3 = 0, and a3 is negative or positive depending upon whether the distribution is skewed to the left or to the right, respectively. A representative set of a3 computed values is given in Fig. 8. For the 641 # m bed [Fig. 8(A)], at lower U/Umf, a3 is positive and it decreases to negative values, attains a constant value, and thereafter, increases again to positive values with the increase in U/Umf. The same qualitative trend is observed for the larger particles at the two lower temperatures as seen in Fig. 8(B). This trend is not observed for the 2165 # m bed at higher temperatures (1023 and 1143 K) where a3 increases monotonically with the increase in U/Umf, and it is almost always positive. This information and trends directly translate to give us a quantitative nature of the pdfs and, hence, of AP which are related to bubble size distribution as discussed earlier. For our differential pressure probe arrangement, positive a~ values imply larger bubbles than for the normal distribution and negative values mean smaller bubbles than for the normal distribution. Further, positive a3 values will translate into more solids holdup than the negative a3 values for which gas holdup will be more. In the light of this interpretation, Fig. 8(A) suggests that, at lower U/Umf, the 641 # m bed has more gas holdup, i.e. smaller bubbles, and as the U/Umf is increased, the bubble size increases, obviously because of bubble coalescence. The same physical picture is valid for the larger particle bed at the two lower temperatures [Fig. 8(B)]. However, at the two higher temperatures, the bubble size is larger than for the normal distribution values of U/Umf, and at higher values, the bubble size distribution achieves a stable distribution. This information, while consistent with that obtained from the a m, tells us more about the structure and distribution of the AP values. These distributions are also compared with each other on the basis of the shape of their peaks. Flat topped and pronouncedly peaked curves are called platykurtic and leptokurtic, respectively.
90
SAXENA and RAO:
FLUIDIZED-BED INCINERATOR CHARACTERISTICS
(A)
4 ~- Lowerl--
IJ
Upper!
t
Sand " 641u.m
0
•
,,o
~
0
,.t,.
,a,
2
--J,L
J
l
i
..,,.s,'~J ~Ul~lt 2.50
I
'
1 ~']'"I hllfl, i ,
0
Iv
11JJtilll I
l~l
.,,,,,h,~.lI,1111]l,II Ih 1 Illilll h! hllllllh
J
it
l
• _ L t Ildl. !l*lll;lt,hJ,,.'.dil.,.,:,l., : ii!.,i;lilIJilfi h~h Jill .....
2
mrfllaI I[1[ Jr
.~ ,dl,.I,nllll,
I.,I
......... . .,,,hll . f, . .,.,,L~ , ......, ~,,!t,,, I,~l,,.~ 3.25
| h
g" :":~' 'th~It~,,.,..
o •__~_->"
J'J~
I llt
.~
I~
t
3.75 ,
o
_
t
0
'
I
'J:j ,,"T,,,I•/! ' ,. !! _ .,.,,,hll~i~i:l;il!!5:i, lli|::~:i:!Ir.i,];::.,**.,,
4.25
1
I
!
J
I ...
.
.... ~.,,,I.,~,lltl 'I
v., '
ll,,l:~Id,i
"
"0
~l,,I,~ t~,. ,Ih,,z,I.......... I ....'"'""~'" ,IJ,''*"''' 1 Pressure (kPa)
2
Fig.
Jl~lltIIII~l~lIdllllJ jlJ 2,Id,J,l,h,, I
L....,,,.,I~,~,II
1 Pressure (kPa)
Caption on p. 92.
7(A).
The peakedness of these distributions is measured in terms of kurtosis the following expression [11, 12]: a4=34
Ill, Iltl I Jllf Jlll,l,,l~.l !
N(N+I) (N--I)~-~-2~N
--
~, (x,-/~)4 3) i=1z... ~ O'p
(a4), and it is computed from
3(N- I)(N-1) (N -- 2)(N -- 3)
(5)
SAXENA and RAO: FLUIDIZED-BED INCINERATOR CHARACTERISTICS
9I
(B)
4
Lower
t
Upper
U/Umf = 1.1
1
1
l Sand 641~rn T = 603K
2
o 1[
I
.L
2.0
o
J~ll~lll
,
2"--'-3.5
+
o
°o ~-
I
J
l I I
i,it+l++tltlI++
.l .... ,.,.,H ~1
I, .L ,,Lm,,
t l ,Lift, .....
T
.,.
,
,F
tlJali[t+lit|hl tIll+tl~ ]
._ l,,,..,t.udlll I I~t I
It hllhtl,,, ....
J T
I
T
I
I
I'rlrI"l'
I lul~,Ililil~l 1 T'
67
....
t,,.h,l,l 0
i
"
5.0
1
,
J'F
1
4°J 2'
,,'
I.I
, j ~ ~ , . ,
a.o ~.
IJ
JLh,
,,~.,
"ili! l'ill,ItillhI!h,l.,., ........ I
Pressure(kPa)
2
I .....Lid II I
...
1I]llllilltgI,lmI,I',,!.,,.
Pressure(kPa)
2
Fig. 7(B). Capzion o~'erleall
For a normal distribution, d 4 = 3, and the coefficient of kurtosis, ( a 4 - 3 ) , is positive for a leptokurtic distribution and negative for a platykurtic distribution. Computed values of the coefficient of kurtosis are presented in Fig. 9 for the same beds and conditions as in Fig. 8. These ( a 4 - 3) values provide a quantitative measure of the flatness of the pdfs presented in Fig. 7. In Fig. 9, most of the values are negative, indicating that the distributions are platykurtic. This indeed is noted by the pronounced flatness of these distributions. This again implies that the bubble size distribution is wider than for the normal distribution. The spread of these peaks is related to the
92
SAXENA and RAO:
FLUIDIZED-BED INCINERATOR CHARACTERISTICS
(c)
1.1
0
i
,
.,,t,
L
2
I~ill, I
°o
.........
,
.... ~
,
"
~,
t
r
o--o ,-,
o
o..
2r~
1
L+
..
I
I
I
.
Jim/
3.5
,
,, .,..ilili.,
, ,,,l~llllll~J 1
__L_~ !
L
. mtl,i
Ill j ~'f
I.: .... i
0 ... ,.,,,,,411,,! I tfj>,,,J ........
_ 0
/
,.... ,,,,,=,,ItI t li 11i, ~h.,,,,,,,..,~
I ipl~ 1
1
P r e s s u r e (kPa)
II I t, 2
0
1
ti' 2
P r e s s u r e (kPa)
Fig. 7. Plots of pdf for the lower and upper bed regions at different UiU°, r values at (A) 303 K, (B) 603 K and (C) 823 K.
size range of bubbles in the bed. Obviously, for the 641 # m bed [Fig. 9(A)] at lower U/Umr values, the bubble size range is wide, and this becomes narrower as the U/Umrvalue increases. For the larger particle size bed [Fig. 9(B)], the bubble size range is always wider than the normal, and the spread is relatively smaller at greater temperatures and remains almost constant for the two highest temperatures. This last result is consistent with what is observed in
Fig. 8(B).
S A X E N A and RAO:
FLUIDIZED-BED INCINERATOR CHARACTERISTICS
(A)
93
(B)
0.6
•
[]
-
0.3
?
"0 /
o
.
0
,, ~ O "
y
+ "+
-0.3
o
-0.6
823K
--
/ 0~0
•
1143K
I
I
I
I
I
I
3
5
7
2
3
4
U/Umf U/Umrfor
Fig. 8. Variation of a 3 with
the upper bed section at different temperatures for (A) 641 # m and (B) 2165 # m beds.
The power spectrum of the pressure drop variation data, p(to), is defined by the following relation [13]: P(to) = 2~2a[1 - ~l e x p ( - j o g ) - ~b2 e x p ( - j 2 t o ) . . . . .
~bpexp( - j p t o ) ]
:
(6)
for 0 ~< to ~< z. Here, tra is the variance of the residuals and ~b~,tk2. . . . . ~bp are model based coefficients. In view of the nature of our data, which is comprised of both a periodic as well as a random component, we have adopted a Univariate Box-Jenkins (UBJ) model [14], also known as the Auto-Regressive Integrated Moving Average (ARIMA) model, developed for forecasting time series data. On the basis of computed autocorrelation and partial autocorrelation functions, the model is identified as auto-regressive of order varying from two to six. Next, the coefficients (ap) are calculated using the software of Scientific Computing Associates (SCA) [15]. The adequacy of the model is checked by calculating the autocorrelation function of the residuals for the existence of any probable structure in the set. For the correct order of the model, the residuals will exhibit no structure. Computed values of the power spectrum for the 641 # m sand bed at various fluidization numbers and at three temperatures 303, 603 and 823 K are shown plotted in Figs 10(A), (B) and (C), respectively. In each case, we have identified a major frequency, fro, which is listed in these plots. These major frequencies are graphed in Figs I I(A) and (B) for the different temperatures as a function of fluidization number for the two sand beds of average diameters 641 and 2165 #m, respectively. It is clear that, for both the beds, at lower ftuidization numbers (smaller than about 2.5)
(A)
(B)
1.0
+ t3 0
303K 603K 823K
o o " •
0.5 o~
0
o
o
298K 823K 1023K 1143K
\ .
-0.5 -1.0
-1.5
~k --
or+
\
+" +-+'+
"~'c~
"°c~
i ~
n
_
..D
I
I
I
I
I
I
3
5
7
2
3
4
U/Umf Fig. 9. Variation of the coefficientof kurtosis with U/Umrfor the same beds as in Fig. 8.
94
SAXENAandRAO:FLUIDIZED-BED INCINERATOR CHARACTERISTICS (A)
A
0.08 ~ 0.06 - -
~ /
E
U/Umf = 1.10 ~
2 --
frn=1.63Hz
fm = 2 " 2 8 H z
~_/ y _ 0.02
1- ~
0.04
0
U/Umf = 2.75
I
I-"~
2
4
I 0
2
f (Hz)
t~
1.2
0.3
0.9
0.2
0.6
0.1
~--
I 0
2
2
, I 6
F
U/Umf = 1.60
•-~
E
4 f (Hz)
B 0.4 --
--
I
6
4
U/Umf = 3.25
0.3 --
a.
6
E
0
2
=
f (Hz)
~
4
6
f (Hz)
13..
00040 o Z00 ¢n
C
10-
r=*
~
U/Urn,=210
G
=~ 1 2 - - 1 ~
o
U/Umf = 3.75
o
I 0
2
4
6
0
I
C"----~
I
2
4
6
f (Hz)
f (Hz) D
1.2 --
H
U/Umf = 2.50
1.2
0.9
0.9
0.6
0.6
0.3 - -
0.3
U/Umf = 4.25
I 0
2
4
I
6
0
f (Hz)
2
4
6
f (Hz)
Fig.10(A).
Caption on p. 96.
and/or temperatures above the ambient, the temperature has a pronounced influence on fro. This dependence is a reflection of the bubble coalescence in the bed, as it is influenced by temperature. At higher fluidization numbers, the bed is in the turbulent regime and has an approximately constant fm which is not much influenced by either temperature or particle size. This is consistent with the inferences derived above suggesting a stable bubble size distribution at large U/Umrvalues. It is a very practical result from the incineration view point, as incinerators are generally operated in this regime. At ambient conditions, the bed is in the bubbling regime and fluidization velocity has a significant influence on fro, as expected.
SAXENA and RAO: (B) 0.04
FLUIDIZED-BED INCINERATOR CHARACTERISTICS A
E
U/Umf = 1.1 fm =2"17Hz
--
0.4
--
U/Umf = 4 . 0
,/~
0.03 --
0.3 --
0.02 -
-
~
,/"
fm=L63Hzfm = 1.63 Hz ~
0.2
0.01 --
0.1
I 0
1
2
3
4
5
I
6
0
1
2
f (Hz)
3
--
U/Umf = 2 . 0
~
0.3 --
0.2 -
~
0.2
0.1
~
0.1 --
2
3
4
U/Umf = 5 . 0
5
I 6
no 0
1
2
f (Hz)
u~ 0.5 ~
O 13-
0.4
~
3
4
5
I 6
5
6
f (Hz)
C
~:
6
fm= 1.52 Hz
v E
n E 2
0,4 --
~_.
0.3 -
1
5
F
fm = 2.61 Hz
0
4
f (Hz)
B 0.4
95
G
U/Umf = 3 . 0
0.4 ~-
U/Umf = 6 . 7
fm = 1.19 Hz
0.3 ! i
]
i.,~... *
0.3 0.2
0.2 0.1
0.1
I 0
2
4
I
6
0
f (Hz)
2
3
4
f (Hz)
D
0.5 0.4 !
1
y
U/Umf H z
~
= 3.5
0.3 -0.2 0.1 0
I
]
I
I'~'1
1
2
3
4
5
6
f (Hz) Fig. 10(B).
Caption overleaf.
CONCLUSIONS
On the basis of test runs conducted on the incinerator, several conclusions can be drawn regarding its design and operating characteristics. (a) The design of the propane sparger is certainly found to be adequate and the operation reliable for either initial heating of the inert bed or for supplying the supplemental thermal energy to sustain combustion of wastes of small calorific values. ECM 34/2-- B
96
S A X E N A and RAO:
(C) 0.02 - -
FLUIDIZED-BED INCINERATOR CHARACTERISTICS
A
D
U/Umf = 1.1 fm= 2.72 Hz
0.2 i'D /
U/Umf = 2.5 fm = 1.58 Hz
/
O0 0
OQ I
I
I
2
4
6
I 0
2
f (Hz)
E
fm = 3.04 Hz
~" n
0.4
E 2
E 2
0.3--
¢,~
Q.
0 n
I1.
cn
-
0
2
--
4
¢n
0.2
0
0.1
I
6
0
I
J
2
4
f (Hz)
""--"
I
6
f (Hz) C
0.2 --
6
f (Hz)
B ~" 0..
4
F
U/Umf = 2.0 =
0.5 --
U/Umf = 5.8 Hz
"
fm = 1.19
0.4 _-0.3 0.1 0.2 0.1
0
I
I
I
2
4
6
f (Hz)
I 0
2
4
6
f (Hz)
Fig. 10. Plots of power spectrum for the upper bed region (dp = 641 g m ) at different (A) 303 K, (B) 603 K, (C) 823 K.
U/Umfand at
(b) The bed voidage of inert sand beds is found to be almost independent of bed height, fluidization velocity and temperatures which are greater than ambient. This result is of significant value for incineration studies. (c) The standard deviation values suggest that, in general, the bubble coalescence increases with gas velocity and temperature, and the transition from bubbling to turbulent fluidization occurs at some characteristic value of U/Umfwhich depends primarily on particle size. The small particle beds (641 Izm) behave somewhat differently, and these may remain in the bubbling fluidization regime even at higher velocities where the bubble size distribution stabilizes, as exhibited by constant trp values. Above ambient, with increase in temperature, a general trend of bed quality deterioration (higher ~p values) is observed. The inert bed
SAXENA and RAO:
FLUIDIZED-BED INCINERATOR CHARACTERISTICS
(A)
(B)
43I3~40 dp= 641p.m ~"
~ /+/+~ ++: ,,.E 2 ~ ~/ / \
+-
~-- /° []dP= 21651J.m •
+ 303K m 603K O 823K
• O 298K e~o 823K ~"~,..,.=e.,.,,~.. A• 1023K 1143K
~- +- + - o A
1
97
1
I
3
5
o
°1
7
A
~o ~oo I
"8" i o
2
3
AO
[]
4
U/Umf
Fig. 11. Variations of fro in power spectrum with U / U s at different temperatures for the upper regions of beds of (A) 641 #m and (B) 2165 tam.
(d) (e)
(f)
(g)
particle diameter appears to play an important role for beds larger than about 1 mm as far as the influence of temperature on bed quality fluidization is concerned. The temperature uniformity in the bed along the vertical direction is found to be within + 5%, and it improves with increase in gas velocity and temperature. It is shown that the behavior of the 641/~m bed, as revealed by the pdf plots of Figs 7(A), (B) and (C), is consistent with the conclusions drawn on the basis of the t~pvalues in Figs 5(A) and (B). This reinforces the conclusions drawn in relation to bubble coalescence and bubble size distribution on the basis of ap. The two approaches based on % and pdf for investigating the bed quality fluidization are thus consistent and support each other. The computed values of the relative skewness and coefficient of kurtosis are found to provide valuable information about the size and size range of the bubbles in relation to a normal distribution, respectively. The inferences so derived are found to be consistent with those obtained on the basis of ap and pdfs. The computation of the power spectrum and the values of major frequencies so obtained as a function of fluidization number exhit a very practical result of value in the operation of incinerations. Specifically, the incinerator hydrodynamics will not be appreciably influenced over a range of fluidization numbers as long as it is greater than a specific value which depends upon particle size.
Acknowledgement--This work is supported by the Office of Solid Waste Research of the Univeristy of Illinois at Urbana-Champaign under grant number OSWR-04-002. REFERENCES 1. D. A. Tillman, A. J. Rossi and K. M. Vick, Incineration of Municipal and Hazardous Solid Wastes. Academic Press, New York (1989). 2. W. Bartok and A. F. Sarofim (Eds), Fossil Fuel Combustion, A Source Book. Wiley-Interscience, New York (1991). 3. C. R. Brunner, Handbook oflneineration Systems. McGraw-Hill, New York (1991). 4. S. C. Saxena and A. Mathur, Energy--The International Journal 10, 57 (1985). 5. S. C. Saxena and A. Chanerjee, Energy--The International Journal 4, 349 (1979). 6. S. C. Saxena, N. S. Rao, V. G. Rao and R. R. Koganti, Energy--The International Journal 17, 579 (1992). 7. S. C. Saxena and N. S. Rao, Energy--The International Journal 6, 489 (1990). 8. S. C. Saxena and G. J. Vogel, Trans. lnstn Chem. Engrs 55, 184 (1977). 9. G. M. Jenkins and D. G. Watts, Spectral Analysis and its Application. Holden-Day, San Francisco, Calif. (1968). 10. J. S. Bendat and A. G. Piersal, Random Data: Analysis and Measurement Procedures. Wiley, New York (1986). 11. G. Simpson and K. Kafka, Basic Statistics. Norton, New York (1952). 12. SAS Institute Inc., SAS ® User's Guide: Statistics, Version 5 Edition. Cary, N.C. (1985). 13. G. E. P. Box and G. M. Jenkins, Time Series Analysis, Forecasting and Control. Holden-Day, San Francisco, Calif. (1971). 14. A. Pankratz, Forecasting with Univariate Box-Jenkins Models, Concepts and Cases. Wiley, New York (1983). 15. L. M. Liu, G. B. Hudak, G. E. P. Box, M. E. Miller and G. C. Tiao, The SCA Statistical System, Reference Manual for Forecasting and Time Series Analysis, Version III. Scientific Computing Associates, Dekalb, Ill. (1986).