502
Energy & Fuels 1992,6, 502-511
catalysts could have significant application when methylation is required on a large commercial scale. These reactions might also be promoted by basic oxide catalysts at the much-higher temperatures (700-900 "C) at which they exhibit methane coupling reactivity. Acknowledgment. We thank Mr. R. Quezada for
GC/MS analysis. The Australian Institute of Nuclear Science ACARP and the Australian Research Grant Scheme are acknowledged for financial assistance. Registry No. CHI, 74-82-8;Pb,7439-92-1; Ni, 7440-02-0;Si, 7440-21-3;Cu, 7440-50-8;naphthalene, 91-20-3;l-methylnaphthalene, 90-12-0;2-methylnaphthalene, 91-57-6.
Basic Hydrodynamic Characteristics of a Fluidized-Bed Incinerator S. C. Saxena,* V. N. Tanjore, and N. S. Rao Department of Chemical Engineering, The University of Illinois at Chicago, P.O.Box 4348, Chicago, Illinois 60680 Received September 30,1991. Revised Manuscript Received March 30, 1992
A pilot plant fluidized-bed incinerator facility is described, which consists of an air supply system, a fluidized-bed incinerator, a gas analysis unit, and an off-gas cleanup system. For preheating the combustion air a propane burner system was developed and successfully tested. The fluidization quality of the bed is investigated by computing and analyzing the pressure fluctuation history data of an inert dolomite bed over a period of 92 s using statistical functions such as standard deviation, probability density function, skewness, kurtosis, autocorrelation,and power spectral density function. Measurements are taken over a wide temperature range from ambient (298K)to 1287 K. Two different cofiiing fuels, propane gas and coal, used for the combustion of low calorific fuels are examined. Coal combustion and carbon utilization efficienciesare determined as a function of temperature and gas fluidization velocity. Conditions leading to the lowest carbon monoxide emission levels in the flue gas have been identified. All these data have revealed the optimum operating conditions for the efficient thermal destruction of waste materials in a fluidized-bed incinerator in relation to a specific inert bed and based on considerations of carbon monoxide emission.
Introduction Destruction and disposal of waste materials by incineration is a commonly adopted technology from the dawn of civilization. Numerous publications on the subject and considerable industrial manufacturing experience and design guides are availab1e.l4 Fluidized-bed incineration has been used with considerable success for a variety of wastes.613 These and other studies have indicated that fluidized-bed incineration can successfully handle, within stringent environmentalpollution emission limits,a variety of specialized wastes which cannot be thermally disposed of by other thermal techniques. This has become possible because of the well-known properties of fluidized beds. Of special relevance are the good gas-solids contacting, efficient solids mixing, high particle concentration, and uniform temperature distribution. Such systems can be operated even at higher temperatures, if necessary, particularly for hazardous and toxic wastes easily and this has encouraged the application of this technology to such materials which can be chemically decomposed to simple inert components only at high temperatures. Further, the successfulremoval of toxic pollutants by in situ absorption in the appropriate inert bed material has added another dimension in popularizing this technology. This work has explored some of these features in a specially designed pilot-plant facility and these results are reported here. *Towhom all correspondence should be addressed. 0887-0624/92/2506-0502$03.00/0
In particular, a dolomite bed was used, and its different hydrodynamic properties were investigated to establish those optimum conditions for incineration. In order to suitably combust to low calorific value wastes, it is essential to cofire the waste with an appropriate auxiliary fuel. (1) Corey, R. C., Ed. Principles and Practices of Incineration; John Wiley and Sons: New York, 1969. (2) Niessen, W. R. Combustion and Incineration Processes; Marcel1 Dekker Inc.: New York, 1984. (3) Brunner, C. R. Incineration Systems-Selection and Design; Van Nostrand Reinhold Co.: New York, 1984. (4) Martin, E. J.; Johnson, J. H. Hazardous Waste Management Engineering; Van Nostrand Reinhold Co.: New York, 1987. (5) Bartok. W.: Lvon. R. K.: McIntvre. A. D.: Ruth. L. A.: Sommerlund; R. E. Comb&tdrs: 'Appli&tiom &d bign'Considerations. Chem. Eng. Prog. 1988,84, 54. (6) Kotani, T.; Mikawa, K. Combustible Materials Recovery and MSW Fluidized-Bed Incineration in Japan. AIChE Symp. Ser. 1988,84, 44. (7) Furlong, D. A.; Wade, G. L. Combustion of Municipal Solid Waste in a Fluidized-Bed. AIChE Symp. Ser. 1972,68, 152. (8)Makanski, J. Agri-Waste-Fired Plants Settle on Bubbling Beds. Power 1989, 133,93. (9) Bulewicz, E. M.; Kandefer, S.; Jurys, C. Fluid Bed Combustion of Waste Materials and Difficult Fuels. R o c . Tenth Int. Conf. Fluidized Bed Combust., San Francisco, CA, 1989,1, 85. (10) Copeland, G. G. Industrial Waste Disposal by Fluidized Bed Oxidation. AIChE Symp. Ser. 1972, 68, 63. (11) Hickman, H. L.; Turner, W. D.; Hopper, R.; Hasselries, F.; Kuester, J. L.; Treznk, G. J. Therm1 Conuersion Systems for Municipal Solid Waste; Noyes Publication: Park Ridge, NJ, 1984. (12)McFee. J. N.: Rasmussen. G.P.: Yound. C. M. The Design and Demonstration of a 'Fluidized-Bed Incinerator' for the Destruciion of Hazardous Organic Materials in Soh.J. Hazardous Mater. 1986,12,1!29. (13) Dry, R. J.; LaNauze, R. D. Combustion in Fluidized Beds. Chem. Eng. Prog. 1990, 84, 31.
0 1992 American Chemical Society
Energy & Fuels, Vol. 6, No. 4, 1992 503
Hydrodynamics of a Fluidized- Bed Incinerator
1. Air Compressors 2. Silicagel Drier 3. Freon Cooler 4. Surge Tank 5. Primary Oil Filter 6. Secondary Oil Filter 7. Air Regulator 8. hssurc Gauges 9. Globe Valve IO. Air Rotameters
1 1. Propane Tanks
12. Ropane Regulators 13. Ressure Gauges 14. Propane Globe Valve 15. Ropane Rotameters 16. Ropane Sparger Bumer 17. Spark Plug 18. Distributor 19. Rimary Cyclone Separator 20. Secondary Cyclone Separator
21. Cock Valve 22. Container 23. Microfilter 24. Thermocouples (6) 25. Differential h s s u n Cells 26. Solids Feeder 27. Motor 28. Screw Feeder 29. Data Acquisition System 30.Memory Drive
31. Monitor with Key Board 32. Printer 33. Plotter 34. Gas Sample Robe 35. Cole-Pa" Combustion Analyzer a. Preheating Section b. Elbows c. Calming Section d. Test Section e. Freeboard Section
Figure 1. Schematic diagram of the UIC incinerator facility.
Dolomite was used to capture the sulfur present in the waste and coal if employed for cofiring needs. Propane gas and coal combustion were used here to maintain the inert dolomite bed at different desired temperatures. A special effort is made to characterize the fluidized-bed behavior by recording the bed pressure drop variation history as a function of fluidizing velocity and bed temperature. These data are employed to compute such statistical functions as standard deviation, probability distribution function (pdf), skewness, kurtosis, and power spectral density function (psdf) to quantify the nature and quality of bed fluidization. The adequacy of using propane and coal as auxiliary fuels for purposes of cofiring in incinerators is investigated. This work has also provided information on the dependence of coal combustion efficiency, carbon utilization efficiency, and carbon monoxide emission on operating parameters. Pilot-Plant Experimental Facility In connection with coal combustion studies a fluidized-bed The entire combustion system has been previously de~cribed.'~J~ experimental setup has been modified for incineration studied4 and its schematic is presented in Figure 1. It consists of an air supply system comprising of two two-stage 18.65-kW compressors (1) equipped with individual air driers (2, 3) and suitably manifolded through an air surge tank (41, two fiters 6 6 1 , pressure gauges (8),pressure regulator (7), and two air rotameters (10); an incinerator assembly (a-e); a propane sparger burner system (16); and an off-gas cleanup system (19-23). The fluidized-bed incinerator consists of a 1.52 m long air preheating section (a), and two elbows (b) which connect this section to a 0.305 m long calming section (c) above which are located the test bed (d) and freeboard (e) sections. This assembly has an overall height of 2.44 m and consists of four flanged sections, each 0.511 m long, bolted together with 0.038 m long spacer rings. The entire incinerator assembly ia fabricated from 0.254 m diameter, schedule 40,type 304 stainless-steel pipe sections and is provided with and (14) Saxena, S.C.; Rao, N. S.; Zhou, S.J. Fluidization Characteristics of Gaa Fluidized Beda at Elevated Temperatures. Energy 1990,15,1001. (15) Saxena, S. C.; Rao, N. S.;Rao, V. G.; Koganti, R. R. Coal Combustion Studies in a Fluidized-Bed Test Facility. Energy 1992,17,579.
internal 50 mm thick Purolite-30 insulating material lining. The incinerator test-bed section is also externally insulated with Fiberflax Lo-Con felt insulating wrapping of 25 mm thickness. The propane sparger burner (16) is installed in the calming section and consists of a 25.4 mm diameter cylindrical plug with a 12.7 mm internal diameter axial blind cylindrical cavity communicating with four stainlesa steel pipes of 9.5 and 3.18 mm outer and inner diameters and 63.5 mm long with blind ends. Each of these four arms and the central support housing have nine 0.5 mm diameter orifices symmetrically located15 to introduce propane in the calming section countercurrent to the incoming air stream and is thoroughly mixed. A spark plug located above the propane sparger ignites the ail-propane mixture. A bank of four propane tanks suitably manifolded with two rotamers and valves provides a regulated supply of gas to the sparger burner. The calming section gas distributor plate has 61 3.2 mm diameter holes as detailed by Saxena and Chattergee.16 The incinerator bed distributor (18)has 19 multiorifice nozzles mounted on a 6.3 mm thick stainless steel distributor plate with two arcsnap rings located on either side of the plate.15 Each nozzle is made out of a 19 mm diameter stainless steel rod with a conical taper on its top end. It has a 12.7 mm diameter blind axial hole and 27 strategically oriented holes designed to give adequate pressure drop. These nozzles are so arranged on the base plate that it is supplied by the same amount of gas per unit area. This will ensure a uniform flow of gas through the cylindrical incinerator-bed. The off-gas cleanup system" comprises two, primary (19) and secondary (20), stainless steel cyclones of 6 u P type manufactured by Universial Oil Products, and a Pall Trinity Microfilter (23). The filter will retain all particles greater than 2 pm. Six thermocouples (24), three in the test bed section, two in the freeboard section and one just below the distributor plate in the calming section, are used to monitor and measure the temperature distribution.'* Six pressure taps (25) are provided along the incinerator to measure the pressure profile.16 Two pressure probes (16) Saxena, S. C.; Chattergee, A. Heat Transfer Between a Fluidized Bed and an Immersed Vertical U-Tube. Energy 1979, 4, 349. (17) Saxena, S. C.; Mathur, A. Hydrodynamic and Heat Transfer Studies in Fine Paricle Gas-Fluidized Beds. Energy 1985,10, 57. (18) Saxena, S. C.; Rao, N. S.;Zhou, S. J. Fluidization Characteristics of Coal Combustors. Proc. First Znt. Conf. Energy Conversion and Energy Sources Eng., Wuhan, China, 1990, 577.
604 Energy & Fuels, Vol. 6, No. 4, 199.2 -
3 A P D = 0.532U1.74
T=298K
Sawena et al.
,/ 1
Table I. Size Distribution of Initial Dolomite Particlesn mass fraction of U S . sieve no. av size, pm solids retained 8-10 2180 0.3139 10-12 1850 0.3412 12-14 1550 0.2689 14-16 1290 0.0758 Average particle diameter = 1830 pm.
0
2
1
3
U, mls Figure 2. Variation of distributor pressure drop with superficial air velocity. located at 40 and 290 mm above the gas distributor plate in conjunction with a differential pressure transmitter is used to register the pressure drop variation with time. The response time is 1 s for 90% of a step change signal. For waste injection the incinerator is equipped with an Acrison Model 105B solids feeder (26), appropriate for continuous operation and a manually operated system for batch feeding.14J5 The down spout from the feeder introduces the coal in the bed at a distance of 10 cm from the base of gas distributor plate. An automated computerized data acquisition system (29) is used for on line recording and processing of temperature, pressure, and pressure-drop history. This Hewlett Packard system comprises a dedicated personal computer (30),monitor (31),printer (32),and a plotter (33).19 At the top end of the freeboard a 96-mm gas sample probe (34)is installed which communicates with the Cole-Parmer KM 9004 electronic combustion analyzer (35). I t monitors flue gas oxygen in percent, carbon monoxide in ppm, and temperature. The instrument also calculates the carbon dioxide concentration baaed on measured oxygen concentration for a given fuel of known carbon weight fraction. The unit has a display and a printer which will provide the print out on demand.
Experimental Results In connection with incineration investigation^'^ in an inert 2165 mm average size sand particle bed, the distributor plate was damaged due to agglomeration of bed material and resulting local overheating. The mineral impurities in sand caused this malfunction. The incinerator was dismantled and the gas distributor plate was rebuilt after repairing and replacing five multiorifice nozzles. To check the performance of this grid plate, the pressure drop across it (A&) was measured at room temperature as a function of superficial air velocity (v). The results are presented in Figure 2. The correlated data for pressure loss are adequately represented by A P D = 0.532W4
(1)
The pressure drop has decreased considerably from earlier valud9 and this is attributed to open and clean nozzle orifices. This also suggests that periodic monitoring of A P D is essential because it is likely to increase with use. The increase in temperature on the other hand will increase the orifice size and hence decrease the pressure drop as observed by Saxena and Rao.le These measurements were conducted in the temperature range extending from ambient to 1143 K. In actual practice with prolonged use the orifices get partially plugged and the pressure drop may then even increase. At room temperature, the pressure drop is about 20% of the bed drop and this will ensure uniform bubbling behavior in the fluidized bed.20.21 (19) Saxena, S. C.; Rao, N. S. Characteristics of a Fluidized Bed Incinerator for the Combustion of Solid Wastes. Energy Conversion and Management-An International Journal. To be published. (20) Saxena, S. C.; Sathiyamoorthy, D.; Sundaram, C. V. Design Principles and Characteristics of Distributors in Gas-Fluidized Beds, Adv. Tramp. Processes; Wiley Eastem Ltd.:New Delhi, India, 1987; Vol. 8, p 241.
Table 11. Minimum Fluidization Velocities at Different Temperatures bed temp, K 298 523 695 1030 0.90 0.56 0.48 U,/ mls 1.32 1.57 1.30 1.65 U,f,bm/s 1.31 OFluidization gas at 291 K and at 191 kPa. bFluidizationgas at bed temperature and bed pressure (104kPa).
Figure 3. Variation of pressure drop across the major section of the bed with decreasing superficial air velocity.
A fresh charge of dolomite used in the present investigations has volume-surface average size of 1830 mm and ita size distribution is given in Table I. We determined the approximate value of minimum fluidization velocity by measuring the pressure drop across two probes located at heights of 40 and 973 mm above the distributor plate. The initial slumped bed height was about 30 cm. The procedure adopted is as detailed by Saxena and Voge1.22 Two typical plots are given in Figure 3. The U values in this figure refer to standard conditions of temperature (291 K)and pressure (101 P a ) . In presenting our subsequent results a dimensionless velocity ratio, fluidization number, given by U / Umf,is used. The measured values of Ud at temperatures of interest are given in Table 11. The first set of values is at ambient conditions of temperature and pressure while the bed is at stated temperature. The second set is at the actual conditions which prevail in the incinerator bed. The bed voidage is a very significant property of a fluidized-bed incinerator. It is computed by measuring the pressure drop across a section of the bed as a function of U. We have considered the pressure drop a c r w the bed section located between 40 and 290 mm above the distributor plate. The relationship between the different parameters is
AP =
Jw -
t)(P,
- P,)
(2)
AP is the pressure drop across a bed section of length L, E is the bed voidage, pa and pg are the densities of dolomite and gas, respectively, and g is the acceleration due to gravity. Computed values of at two temperatures as a function of fluidization number are given in Figure 4. It is to be noted that t increases with increase in U/Ud This
(21) Kunii, D.; Levenapiel, 0. Fluidization Engineering, 2nd ed.; Butterworth-Heinemann: Boston, 1991. (22) Saxena, S. C.; Vogel, G. J. The Measurement of Incipient Fluidization Velocities in a Bed of Coarse Dolomite at Temperature and Pressure. Tram. I m t . Chem. Eng. 1977, 55, 184.
Energy & Fuels, Vol. 6, No. 4, 1992 505
Hydrodynamics of a Fluidized-Bed Incinerator 0.601
P
1
6
' ' ' '
1... 3 " '
2.1
1.3
2.4
0.7
2.1
0.1'
Ci
El
P.
e
E
2 0.50
1
2
3
4
Figure 4. Variation of bed voidage with fluidization number at two temperatures.
increase is more pronounced at higher temperatures. This result is important for incineration operation where higher temperatures are involved and a greater value of c indicates good bed aeration in the absence of excessive bubble coalescence. More light on this point will be shed later in this paper in connection with the discuasion of resulta given in Figure 7. In order to simulate the fluidized-bed incinerator environment, the inert dolomite bed is heated to different temperatures by the combustion of propane gas in the calming section. This hot air enters the fluid bed and after some time a good thermal equilibrium is reached between the fluidizing gas and the bed material. This is important if bed temperature uniformity is needed to ensure good quality controlled combustion. Further, to establish the quality of fluidization, bed pressure drop fluctuation history is recorded as a function of bed temperature and fluidization number. In each case a program which could sense lo00 data points was employed, to record over a period of 92 s at a sampling rate of 10.8 readings/s. A typical plot of this nature is presented in Figure 5 for TB of 523 K at different U/U,, Similar data have been generated at temperatures of 298,695, and 1030 K. The axial temperature distribution is measured at these temperatures in the bed and the results are displayed in Figure 6. The bed temperature at a particular height (TI) and ita deviation from the mean bed temperature (TB) are expressed in this figure as percentage deviation as a function of height of the point of measurement above the distributor plate 8.Parta A, B, and C of Figure 6 refer to average bed temperatures of 523,695, and 1030 K, respectively. The deviations are generally smaller than f2%. The bed is also heated by the combustion of Illinois Basin Coal IBC-103 coal and the coal feed rate (Fc)is adjusted to obtain a given bed temperature. This coal is a blend of 80% Springfield (No. 5) and 20% Herrin (No. 6) taken in 1983 from a Southern Illinois washing plant. The ratio of pyritic to organic sulfur approached 1. The size distribution of coal, ita proximate and ultimate analyses, rank, and heating value are reported in Table III, and ita average particle diameter is 1042 pm. The value of the feed rate depends upon the magnitudes of bed temperature and gas velocity. The start-up procedure involves heating the dolomite bed to about 1120 K by propane gas combustion and then adding coal. As the temperature starts to rise the supply of propane gas is gradually reduced and finally
-
'
I
t
'
'
I
0.
2.70
15
4s
7s
0.
Time, s
Time, s
Figure 5. Preeaure-drop history records at 523 K and at different fluidization numbers.
t
100
200
H,m m
300
400
Figure 6. Axial temperature distributionsat various temperatures and fluidization numbers (U/U,)for either propane (A, B, and C) or coal (D,E, and F) combustion. cut off completely. Three average temperatures (1085, 1177, and 1287 K) have been investigated and the bed temperature distributions are given in Figure 6, D, E, and F, respectively. The temperature uniformity is generally
Sarena et al.
506 Energy & Fuels, Vol. 6,No. 4, 1992 Table 111. Size Distribution of Coal and Dolomite Particles mass fraction of solids retained U.S.sieve no. av size, rrm coal dolomite 4-5 4331 0.289 5-6 3645 0.159 6-7 3061 0.186 7-8 2580 0.059 8-10 2180 0.026 0.129 10-12 1850 0.025 0.210 12-14 1550 0.026 0.202 14-16 1290 0.017 0.104 16-18 1090 0.014 0.079 18-20 925 0.014 0.046 20-30 723 0.033 0.059 30-35 543 0.021 0.027 35-40 463 0.013 0.021 40-45 390 0.014 45-50 328 0.018 40-50 363 0.025 50-70 256 0.020 0.018 70-80 196 0.023 0.022 80-100 165 0.020 0.026 100-120 138 0.022 0.032 1042 pmo 756 pma Analysis of IBC-103Coal (Moisture-Free Values) moisture 5.7 carbon 74.57 volatile matter 36.1 hydrogen 5.00 1.37 55.5 nitrogen fixed carbon ash 8.4 oxygen 7.85 13457 sulfur 2.30 But/lb rank HvBb chlorine 0.18 a
Average particle diameter.
within fl%. It is important to note that in-bed coal combustion gives a more uniform bed temperature distribution than the hot gas heating obtained by propane combustion. In-bed propane combustion was also tried in our earlier work%%but sustained combustion was found difficult to achieve over long periods. At these three bed temperatures obtained by coal combustion, the flue gas is also monitored for concentrations of CO, C02,and 02.These data are presented in Table IV with operating variables U,Fc,and TB.These data are employed to compute carbon utilization and coal combustion efficiencies and are discussed later in this article. The dolomite particle size distribution was reduced considerably during these high-temperature and combustion runs. The sieve analysis results are reported in Table I11 and it gave a mean particle size of 756 pm. Fluidization Characteristics of the Incinerator In a fluidized-bed incinerator it is very important to know the operating hydrodynamic regime26which may vary from discrete bubbling to bubble coalescing to slugging to turbulent. The variation of bed pressure drop with time has been employed2’ to establish the fluidization regime. This may be accomplished by computing a set of statistical functions which indicate in varying degrees the detailed fluidization or bubbling behavior in the bed as (23) Saxena, S. C.; Mathur, A.; Wang, 2.F. Incipient Fluidization at Different Temperaturesand Powder Characterization. AIChE J. 1987. 33, 500. (24)Mathur, A.; Saxena, S. C.; Zhang, 2.F. Hydrodynamic Characteristica of Gae-Fluidized Beds Over a Broad Temperature Range. Powder Technol. 1986,47, 241. (25) Mathur, A.; Saxena, S. C. Total and Radiative Heat Transfer to an Immersed Surface in a Gas-Fluidized Bed. AIChE J. 1987,33,1124. (26) Saxena, S. C.; Ganzha, V. L. Heat Transfer to Immersed Surfam in Gas-Fluidized Beds of large Particles and Powder Characterization. Powder Technol. 1984,39,199. (27) Saxena, S. C.; b o , N. S. Pressure Fluctuation in a Gas Fluidized Bed and Fluidization Quality. Energy 1990, 15, 489.
0.20
rB:
o 298: a 1085:
523; + 695: A 1030 1177:
0.15
B 6
0.10 0.05
0.00
1
2
3
4
U”f, Figure 7. Standard deviation of AP with fluidization number at temperatures obtained either by propane (open symbols) or coal (closed symbols) combustion.
explained by Saxena, Rao, and Tanjore.= This approach has been adopted here. Details may be found in the work of Saxena et al.28 Computed values of the standard deviation of pressure fluctuation data are plotted in Figure 7 as a function of U/U, at several temperatures, ranging from ambient to 1030 K, obtained by propane gas combustion. The different curves in Figure 7 indicate a distinct trend for the variation of up with TB. At ambient temperature up monotonically increases with U/U,, signifying that the amplitude of pressure drop fluctuation increases consistently with increase in gas velocity. This in turn implies a rapid increase in bubble size with increase in U. Hence the bed is in bubbling or bubble coalescence regime. At temperatures greater than ambient, it is noticed that till a value of about 1.5 for U/U,, up increases with U. This is the bubbling regime and is also encountered around room temperature. However, at higher temperatures and for larger values of U/U, greater than 1.5,up undergoes a characteristic change which depends both on T and U. At temperatures around 600 K, the up values tend to assume a constant value for U/U, values greater than about 2.5. This means that, for V/U, values between 1.5 and 2.5, the bubble coalescence continues and up values monotonically increase with increase in U/U ,. But for still larger values of U/U, between 2.5 and 3.5,up is almost constant implying that further bubble growth hae ceased. At this stage a stable bubble size distribution exists in the bed and there is an equilibrium between the bubble growth and breakup. This is conventionally referred to as the turbulent regime. At temperatures around lo00 K, the up values mume an approximately constant value at a much lower value of U/U, of (about 1.5). This suggests that the bubble size distribution becomes stable at a much lower value of U at higher temperatures. This is particularly good for incineration where bed temperatures are around 1100 K and this dolomite bed will be in the turbulent fluidization range for gas velocities above 2 m/s (referenced to ambient conditions). up variation for the two temperatures (1085 and 1177 K) obtained by coal combustion indicates a much smoother turbulent fluidization over the entire gas velocity range than that obtained by propane gas combustion. Bubble size distribution is apparently more uniform and stable for coal combustion than for propane combustion under these operating conditions. The bed material apparently plays an important role and as evident from Table III dolomite particles have undergone appreciable reduction. The size distribution of dolomite and coal particles together provides a uniform and continuous wide range for particle size. Further, as seen in Figure 6, the bed temperature is more uniform for (28) Saxena, S. C.; Rao, N. S.; Tanjore, V. N. Diagnostic Procedures for Establishing the Quality of Fluidization of Gas-Solid Systems. Exp. Thermal Fluid Sci., to be published.
Energy & Fuels, Vol. 6, No. 4, 1992 607
Hydrodynamics of a Fluidized-Bed Incinerator
Table IV. Data Relatine to Coal Combustion in a Dolomite Bed flue gas comDosition I
U,m / s
1+Z 1.03 1.24 1.15 1.12 1.06 1.01 0.96 0.97
TB,K
CO,ppm
COP,I
496 592 716 842 938 1138 1058 1034
13.8 13.2 14.2 14.5 15.3 16.1 16.9 16.7
02,% 4.6 4.0 4.3 4.3 4.5 4.7 4.8 4.8
A 0.080 0.068 0.072 0.076 0.079 0.078 0.083 0.083
B 0.665 0.677 0.673 0.669 0.666 0.667 0.662 0.662
BCUS %
1095 1060 1070 1085 1095 1108 1081 1095
89.3 90.9 90.4 89.8 89.4 89.5 88.9 88.8
91.5 92.6 92.2 91.7 91.4 91.4 91.0 90.9
1.17 1.58 1.92 1.94 2.61 3.00 4.00
1.18 1.05 1.01 1.14 1.14 0.92 0.79
1166 1174 1167 1181 1186 1191 1176
421 444 455 468 489 523 404
13.8 15.5 16.1 14.3 14.3 17.6 17.8
4.2 4.6 4.7 4.3 4.3 4.6 4.8
0.074 0.079 0.082 0.075 0.074 0.088 0.087
0.671 0.666 0.663 0.670 0.671 0.657 0.658
90.1 89.4 89.1 90.0 90.0 88.2 88.3
92.1 91.5 91.3 92.0 92.0 90.6 90.8
1.50 1.92 2.08 2.44 3.08 3.18 3.94
0.92 0.87 0.93 0.91 0.81 0.92 0.93
1289 1288 1283 1279 1285 1300 1270
182 248 376 389 504 480 450
17.6 18.1 17.4 17.8 17.4 17.3 17.2
5.0 5.2 5.0 5.0 5.4 5.3 5.1
0.090 0.125 0.089 0.089 0.098 0.111 0.121
0.655 0.620 0.656 0.656 0.647 0.634 0.624
88.0 83.3 88.1 88.1 87.9 87.8 87.9
90.5 86.9 90.6 90.5 85.3 86.7 86.5
0.707 0.883 1.010 1.136 1.260 1.390 1.591 1.844
U/Ud 1.36 1.70 1.94 2.18 2.42 2.67 3.06 3.54
F,, g/s 1.50 1.56 1.92 2.22 2.61 3.00 3.64 4.14
0.631 0.758 0.883 1.010 1.136 1.263 1.440
1.00 1.20 1.40 1.60 1.80 2.01 2.28
0.631 0.758 0.883 1.010 1.136 1.263 1.440
1.03 1.24 1.45 1.66 1.86 2.07 2.36
~
$CEI
%
Table V. Variations of Statistical Parameters with Fluidization Number at Different Temperatures U.m/s U/U,, normalitv test-D skewness kurtosis TB = 298 K ~
0.963 1.137 1.389 1.516 1.642 1.768 2.020 2.147 2.399
0.73 0.86 1.05 1.15 1.24 1.34 1.53 1.63 1.82
0.758 1.010 1.263 1.516 1.768 1.895 2.021 2.274 2.526
0.84 1.12 1.40 1.68 1.96 2.10 2.24 2.53 2.81
0.823 0.969 1.090 1.453 1.695 1.937
1.47 1.73 1.95 2.59 3.03 3.46
0.437 0.606 0.849 1.333 1.575 1.817
0.910 1.26 1.77 2.77 3.27 3.79
0.031 0.036 0.117 0.029 0.035 0.048 0.059 0.092 0.052
+0.108 -0.832 -1.390 -0.355 -0.325 -0.567 -0.789 -0.965 -0.800
+0.670 +3.538 +2.763 -0.172 -0.161 +0.128 +0.765 +0.984 +0.908
523 K 0.032 0.037 0.036 0.031 0.026 0.029 0.030 0.045 0.036
-0.254 -0.407 -0.349 -0.442 -0.318 -0.409 -0.435 -0.634 -0.591
+0.030 +0.003 -0.065 +0.421 +0.151 +0.632 +0.523 +1.221 +0.786
695 K 0.051 0.035 0.028 0.021 0.033 0.052
-0.379 -0.634 -0.413 -0.395 -0.400 -0.477
-0.451 +0.839 +0.049 +0.551 +0.155 +1.087
-0.047 -0.081 +0.095 -0.102 -0.109 -0,346
-0.564 -0,384 -0.171 +0.256 +0.344 +0.304
TB
Pressure, kPa
TB
Pressure, kPa Figure 8. Plots of pdf at different fluidization numbers at 523
K.
TB 1030 K
coal combustion than for propane combustion, implying better mixing for the former than for the latter. To get a greater understanding of the nature of AP variation and ita structure, the knowledge of up is not enough and one must consider the probability density functions (pdfs). In Figure 8, a typical set of such plots at 523 K are displayed at different U/Ud values. With increasing UlU,, the shape and width of the peak changes. Customarily, these changes are quantified by comparing the dietxibutions with the corresponding normal distributions and computing such quantities as skewness and kurtosis. Skewness is a measure of the actual magnitudea of AP values in relation to the normal distribution and kurtosis is related to the peakedness of the distribution.
0.026 0.034 0.021 0.019 0.018 0.032
To estimate the departure of the pdfs from normal distributions, the Kolmogrov-Smirnov test was conducte d . % ~The ~ ~ computed statistical D values are listed in Table V at each temperature. For normal distributions (29) Bandat, J. S.; Piersal, A. G. Random Data: Analysis and Measurement Procedures; John Wiley and Sons: New York, 1986. (30) SAS Users'Guide: Statistics, Version 5 Edition; SAS Institute, Inc.: C a y , NC (1985).
Saxena et al,
508 Energy h Fuels, Vol. 6,No. 4, 1992 l r .
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-
i
*I
t 0.5
t
0.0
0.0
-
0.0
0.0
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0
Tm: 0 268: a 523;
.I' 0
0
695; A 1030;
I 1
0.5
2
u/vnlf, -
3
4
6
8
IO -O+J
1
4
k
8
l'0
Lag, s Lag, s Figure 10. Plots of autocorrelation function at different fluidization numbers at 523 K.
4
Figure 9. Variation of relative skewness (A) and coefficient of kurtosis (B)with fluidization number at different temperatures. these numbers should be smaller than 0.01 and hence it may be concluded that the distributions of AP for all velocities and temperatures are not normal. To obtain more understanding of these departures, skewness31and kurt0SiS31have been calculated and these are also recorded in Table V. For a normal distribution the values of relative skewness reported in Table V and graphed in Figure 9A should be zero. Almost always these values are negative and hence the distributions are skewed to the left. Figure 9A shows that the skewness becomes increasingly negative with decreasing temperature at a given value of U/Ump On the other hand, at a given temperature, skewness is almost constant when U/Umfis greater than 1.5. Large negative values at ambient conditions and the continuous decrease in skewness with gas velocity suggest that bubble coalescence increases with gas velocity so that increasingly smaller AP values are encountered. This tendency decreases with increasing temperature and further increase in gas velocity does not alter the relative difference between the mean and the mode of the AP values. This implies that the bubble size distribution stabilizes and doea not change with increasing gas velocity. This inference indeed is consistent with that obtained from up measurements. Further, this inference is characteristic of the system dimensions as detailed by Kunii and Levenspiel.21 The coefficientsof kurtosis listed in Table V and plotted in Figure 9B are mostly positive at all temperatures for U /U, values greater than about 1.5. Such distributions are referred to as lepokurtic and have pronounced peaks in relation to normal distributions which are called mesokurtic. This peakedness of the distribution is also dependent on temperature and fluidization number, as shown in Figure 9B. At a given temperature above the ambient, the kurtosis coefficient increases with fluidization number and approaches a constant value a t higher U/U, values for all temperatures
