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Measurements of gas—particle flows and elutriation of an 18 inch i.d. cold vortexing fluidized-bed combustion model
Powder
Technology,
139
69 (1992) 139-146
Measurements of gas-particle flows and elutriation i.d. cold vortexing fluidized-bed combustion model G. Yang **, A. Q. Zhut and C. S. Zhaot
S. Nieh*, Combustion Washington
of an 18 inch
and Multiphase Flow Laboratory, DC 20044 (USA)
Department
of Mechanical
Engineering,
Catholic
Universiv
of America,
(Received November 2, 1990; in revised form June 3, 1991)
Abstract A novel vortexing fluidized-bed combustion (VFBC) technique was recently developed for commercial space/ water heating applications. The advantages of VFBC firing solid fuels include large combustion intensity and efficiency, wide turndown and fuel flexibility, and low pollutant emissions. An 18 inch i.d. cold VFBC test model was designed, built, and systematically tested. Measurements of gas flow field, and particle m&s fluxes and elutriation under different swirl numbers and secondary air fractions were conducted with mixed-size glass beads. The gas flow field in the VFBC model is characterized by a swirling, developing and recirculating flow. The particle flow field is characterized by alternate layers of dense and dilute circulating suspensions. The VFBC process can substantially reduce the elutriation of particles which was measured to be 3 to 5 orders of magnitude lower than the conventional bubbling FBC operation.
Introduction
The benefits of vortex flow in both reactive and nonreactive multiphase systems have been recognized for many decades [l, 21. Strong vortex flows occur in a wide variety of engineering applications: cyclone separators, jet pumps, vortex reactors, spraying machines, industrial furnaces, internal combustion engines, fluid heaters and utility boilers. In reactive systems, the design of strong vortexes for the injected air and fuel (or reactants) can have large-scale effects on entrainment and decay, heat and mass transfer, flame stability, fuel ignition and burnout, and pollutant abatement reactions [l, 31. By integrating the advantages of fluidized-bed combustion (FBC) and vortex flow, a new solid-fuel firing technique, known as vortexing FBC (VFBC), was developed for small- and medium-scale boiler/kiln applications for commercial and light industrial sectors [4-6]. Figure 1 shows a design example of a coal-fired VFBC unit. The combustion chamber is a gas-tight enclosure with a circular cross-section, where coal combustion and sulfur retention reactions take place. The combustor may be fully or partially water-cooled with vertical wall tubes or water jacket. Primary air is in*Author to whom correspondence should be addressed. **Current address: Department of Mechanical Engineering, Florida International University, Miami, FL 33199. wisiting scholars from Southeast University, Nanjing, China.
0032-5910/92/$5.00
traduced at the bottom of the lower combustion chamber (fluidized bed) to fluidize the bed materials-coal, ash and limestone-the same as those in conventional FBC boilers. A substantial amount of secondary air is tangentially injected at two or more elevations into the upper combustion chamber (freeboard), similar to the arrangement in cyclone burners and vortex combustors [7]. Coal particles are ignited, burned and entrained into the upper combustor. The strong vortexing flow promotes gas-gas and gas-particle mixing, creates internal gas-particle recirculation, and reduces particle elutriation from the center exhaust tube. Sulfur capture, No, reduction, and carbon burnout are time-controlled and completed in the combustor before the combustion products exit to a heat exchanger. Steam and/or hot water generated from the VFBC unit can be used for space heating, water heating, and industrial process heat. Since VFBC is a new combustion concept, only a few preliminary tests on the overall performance have been conducted [4-6]. These results measured on the design and operation of a VFBC unit were found to support the value of the concept. In VFBC design, for instance, the freeboard is not considered as a simple settling chamber; rather it is an active part of the combustor. Because of the advantageous effects ofvortex flow, the freeboard volume of a VFBC unit can be significantly reduced, giving a large combustion intensity (volumetric heat release rate). Our recent tests of a
0 1992 - Elsevier Sequoia. All rights reserved
140 TABLE 1. Major VFBC unit
design
Parameter thermal HEAT EXCHANGER
PRIMARY
AIR
Fig. 1. Vortexing
fluidized-bed
combustion
boiler.
0.15 MMBtu h-’ (0.15~ lo6 Btu h-’ or 440 kW) exploratory VFBC model firing Indiana bituminous coal [8] showed a high combustion intensity of 0.25 MMBtu tY3 h-’ (2.2~10~ kcal mm3 h-‘) which is an order of magnitude higher than a typical conventional FBC boiler. However, the technical data on combustion, thermal and gas-particle flows in thevortexing freeboard are far from adequate for use for a satisfactory boiler design and operation. In order to achieve a better understanding of gas-particle flow characteristics, an 18 inch i.d., 80 inch tall VFBC cold model was designed, built and tested. The measured results of the flow field in the freeboard and particle elutriation from the VFBC system are presented and discussed in this paper.
Test apparatus
and instrumentation
WBC cold test setup Based on Indiana bituminous coal, a thermal analysis and conceptual design of a 1.5 MMBtu h-l VFBC unit was conducted, and boiler configuration and operation conditions determined. The major design parameters are summarized in Table 1. Based on these results and our previous experience with 4 inch i.d. and 7 inch i.d. VFBC cold models [4-6], an 18 inch i.d. VFBC cold test system was designed and installed, as schematically shown in Fig. 2. Air from a forced draft fan (Buffalo
input
coal (Indiana bituminous) consumption rate combustion temperature combustion intensity combustor height inside diameter distributor diameter number of bubble cap diameter of bubble cap exhaust pipe diameter secondary air nozzles level x number tip diameter primary (fluidizing) air flow rate velocity secondary (tangential) air flow rate velocity flue gas (at exit) flow rate velocity in pipe
parameters
of the 1.5 MMBtu
h-’
Unit
Magnitude
MMBtu h-’ kW
1.5 440
lb h-’ “F MMBtu ft -‘h -’
148 - 1650 0.17
inches inches inches _ inches inches
63 17.5 12 49 1 10
inches
2x4 1
ft3 min-’ ft s-1
148 -4.25
ft3 mitt-’ ft s-1
124 -82
ft3 min-’ ft s-1
1187 32
Forge 45RD, 1200 ft3 min-‘, 47 inch water column) is introduced into the test chamber through three individually controlled pipelines. Two pipelines are connected to secondary air injection nozzles and one to the primary air wind box. Air flow rates are controlled and measured by flowmeters and three Kurz digital electronic manometers. Test particles are continuously fed into the test chamber by a screw type AccuRate model 302 dry material volumetric feeder with a nominal feed rate of 0.26-4.16 ft3 h-l. The particle-laden gas stream leaving the VFBC model is forced into a bag filter, where fines are collected and removed for microscopic examination, as needed. Eighteen inch i.d. VFBC test chamber Figure 3 shows the configuration of the 18 inch i.d., 80 inch tall VFBC cold test chamber which consists of four major parts: upper freeboard, lower freeboard, fluidized bed and wind box, all made of Plexiglas to facilitate visual observations. The 22 inch long upper freeboard has a 10 inch exhaust pipe at the top and four equally-spaced (90” on the circumferential wall) air nozzles at an elevation of 64 inches. The 28 inch long lower freeboard also has four air nozzles. These two freeboard sections were made exchangeable, which provided the flexibility for secondary air injection. In order to achieve a good quality of fluidization, the tapered chamber design was adopted for fluidized bed/
UPPER
CHAMBER
s'c"n:~ARy NOZZLES
LOWER
CHAMBER
FLUIDIZED
WIND
BED
BOX
PR I+$"' 6
Fig. 2. Cold VFBC model test system. Fig. 3. Configuration of 18 inch i.d. cold VFBC test model.
freeboard transition. A 3.5 inch long Plexiglas converter connects the 12 inch i.d. fluidized bed to the 18 inch i.d. freeboard. One of the major design considerations of the VFBC unit is the control of vortex intensity which is commonly characterized by the swirl number [l, 21. For the VFBC model, the swirl number can be expressed as (refer to Fig. 4): Swirl number, S= =-
inlet angular momentum outlet axial momentum x DJ2 ~2 D& 4
I
FLUE
I I’I t
I
PRIMARY
GAS
AIR
I t
AIR
A,
where D,=diameter of exit tube, D, = diameter of chamber (freeboard), Di = diameter of imaginary circle of air injection, Di = D, cos p, p = nozzle injection angle on a horizontal plane, A, = total tip area of secondary air nozzles, l=volumetric flow rate fraction of secondary air. For a VFBC model of tixed D,, D, and A,, the swirl number (or swirling intensity) can be varied by changing secondary air fraction E and injection angle /3. The flow rate fraction E is an operational parameter which can be easily adjusted in the tests. The adjustment of injection angle /3 needs to be accounted for in the air nozzle design. Figure 5 shows the configuration of the secondary air nozzles which are mounted at two levels
SEC~;ARY
Fig. 4. Definition of swirl number.
on the VFBC chamber. Each nozzle is sealed in a Plexiglas compartment and glued onto the test chamber. Secondary air is supplied from a 2 inch tube and horizontally injected into the test chamber via a 1 inch
142 INJECTION
When charged particles impact the metal sensor, a sequence of charge pulses are generated. These pulses are converted, amplified, counted and recorded for a preselected period of time. By multiplying the total number of impacts per unit time with the local averaged particle mass and dividing by the projected area of the metal sensor, the local particle mass flux can be determined. With independent calibrations against the isokinetic sampler system [ll], this probe represents a convenient secondary standard for measuring local particle mass flux of flowing glass beads. Corrections of probe readings due to electrical noises and fraction of impact were taken into account [lo, 121. A United Sensor DA&DAT 5-point directional probe with four manometers was used for measuring the local gas velocity [13]. Based on the pressure readings from the probe, the pitch angle and yaw angle of gas flow, and the axial, tangential and radial components of the gas velocity can be found. The data reduction process is accomplished by a microcomputer with a software program.
NUT
AIR INLET A)
8)
FRONTAL VIEW
TOP VIEW
Fig. 5. Configuration
of secondary
air injection nozzle.
circular tube controlled by an angle deflector. The injection angle of each nozzle can be independently adjusted and read from an angle indicator on the nozzle. Instiumentation The local particle mass flux was measured by an electrostatic impact probe and associated signal processing devices [9, lo], as shown in Fig. 6. The probe tip (sensor) is a 0.04 inch diameter metal ball, which is electrically isolated from the probe stem. Particles are naturally charged to a detectable level through contacts with solid surfaces in a multiphase system. ,--a------.e
’ O-TO-V I CCNVERER
Fig. 6. Electrostatic
impact probe system.
Results
and discussions
Gas pow field The VFBC process is distinguished by a strongly swirling gas flow which characterizes the flow and combustion performance in the freeboard. An understanding of the aerodynamic structure in the freeboard is crucial for the design and operation of VFBC units. A series of tests were therefore arranged. Since the gas flow field was essentially axisymmetric, measurements were made only on a vertical plane passing through the chamber axis. Effects of the following parameters were explored: (i) volumetric flow rate fraction of secondary air, E, (ii) injection angle of secondary air, p, and (iii) number of enabled air nozzles. ______
PMPLIFIER
1 ,
143 TABLE Run No.
1 2 3 4 5 6 7 8
2. Test conditions
Primary air (ft’ min-‘)
500 500 500 500 400 400 750 600
Secondary Upper
for gas flow measurements Swirl No. s
Air
nozzles
Lower nozzles
(ft’ min-‘)
p (“)
(ft’ min-‘)
p (“)
100 100 100 100 150 0 0 50
45 30 60 45 45 0 0 45
150 150 150 150 200 350 0 100
45 30 60 45 45 45 0 45
1.63 1.96 1.15 1.78 3.2 6.4 0 0.59
TANGENTIAL VELOCITY W
Fig. 7. Gas axial and tangential
along the axial direction except for the cross-section at the top exit. Both axial and tangential velocity profiles change rapidly near the top exhaust pipe. An intense, swirling gas-particle flow appears in the annular space at the top, which is advantageous for particle control and burnout. A recirculating flow is formed in the regions below the air nozzles, where a high particle concentration exists. This internal circulating toroidal flow promotes local particle recirculation, which is beneficial for solid combustion as it results in prolonged particle residence time and enhanced mixing. Basically, the recirculation/developing flow derives from the progressive injection of air together with the sudden change of chamber configuration. The size and location of recirculation can generally be controlled by the arrangement of secondary air injection. Three test runs, Nos. 5, 7 and 8 in Table 2, were conducted to explore the effect of swirl number on gas flow field. The total air flow rate and secondary air injection angles were kept unchanged at 750 ft3 mine1 and 45”, respectively. Figure 8 shows the gas axial velocity distributions at two bottom cross-sections: Section 2 at 6 inches below the lower level nozzles and Section 1 at 6 inches below Section 2 (see Fig. 8). It can be seen that at a high swirl number (Run No. 5, S = 3.2) the recirculating flow (reversal velocity) is strong at both cross-sections near the wall. It becomes weakened when the swirl number reduces to 0.59 (Run No. 8). For the non-swirling flow (Run No. 7, S= 0), the axial velocity is low. No gas recirculation can be found at these two sections.
velocity profiles.
Test conditions for the gas flow measurements and swirl numbers are given in Table 2. Figure 7 shows the typical results of tangential and axial velocity distributions in the VFBC freeboard (data of Run No. 1). The tangential velocity profiles show a strong vortex flow feature, while the axial velocity profiles show a recirculating/developing type of flow. The tangential velocities are high near the wall and decay rapidly toward the center, as expected. The swirling motion in the core region within half chamber radius is weak, although the velocity fluctuation is vigorous. The tangential velocity profiles are retained
0.5 r* - r/R
Fig. 8. Effect of swirl number on gas axial velocity distributions. (A) Run 7 S= 0, (0) Run 8 S = .59, (Cl) Run 5 S=3.2.
144
Figure 9 shows the effect of secondary air injection angle on recirculating gas flow. The axial velocity distributions at the same cross-sections explained earlier were measured. Test run Nos. 1, 2 and 3 (see Table 2) used a constant primary air flow rate of 500 ft3 min-’ and secondary air flow rate of 250 ft3 min-l, but different injection angles. The axial velocity distribution at Section 2 was clearly affected by /3. However, the axial velocity profile at Section 1 and tangential velocity profiles at both sections remained roughly unchanged at different ps. At Section 2 the axial velocities (or recirculating flow) exhibited distinct patterns at different /3s, although the swirl numbers were close. Run No. 3 (p=60”, S= 1.15) has a weak recirculating flow near the wall, as expected. Run No. 2 (p= 30”) also has a weak recirculating flow in spite of its larger swirl number (S= 1.96). At both sections, Run No. 1 with p = 45” appeared to be the most desirable arrangement of secondary air injection. It creates a strong recirculating flow in the wall region with the same feed rates of primary air and secondary air.
TABLE 3. Major test conditions
for particle
Parameter
Unit
Range
w
-
O-3000 Robin-Rammler 344 2.42 0.92
ft? min-’ ft s-1
66-477 1.9-13.7
ft3 min-r % ft s-1
O-221 O-O.47 O-146
test particles (glass beads) size range distribution mean diameter specific gravity spheric&y primary (fluidizing) air flow rate velocity secondary (tangential) air flow rate flow rate fraction velocity
-
pm
measurements
Particle mass flux distribution
In order to explore the flow behavior of particles in the VFBC freeboard, the mass flux distributions were measured. The major test conditions are listed in Table 3. Figure 10 shows a typical axial distribution of particle mass fluxes in the test chamber. The particle flow field is characterized by circulating dense suspension layers and dilute zones [6], which appear alternately along the chamber height. Particles generally follow spiral
0 ‘\.
1 I
2
3
I
5
MC/MM’S
Fig. 10. Typical distributions of particle mass fluxes in VFBC model. (0) R =5.5 inches, (0) R=8.5 inches.
t sn
0
-2
2
6
0
0.5
1.0
remr/R Fig. 9. Effect of secondary air injection angle on axial velocity distributions. 0 Run 1, /3 = 45”, S = 1.63; A Run 2, /3 = 30”, S = 1.96, •i Run 3, /3=60”, S=1.15.
ascending trajectories from the fluidized bed to the top exit [4, 51. In the wall region (rag.5 inches), two dense layers of particles and two dilute zones can be clearly seen, as shown in Figure 10. The average particle mass flux can differ by 5-fold among these layers. The particle mass flux distribution at r=5.5 inches (the opening of the exhaust pipe) is also shown in the figure. It generally decreases along the height, primarily due to the effect of gravity. In the vicinity of the exhaust pipe, the mass flux reduces to 0.014 mg rnrne2 s-‘, being only 5% of the averaged particle mass flux in the chamber. This indicates a low particle elutriation. Figure 11 shows the variations of particle mass fluxes along the freeboard axis and at r=5.5 inches. It is of interest to note that the mass fluxes along the axis are always higher than those at r=5.5 inches, primarily
145
r* =0.7-0.9,
which is believed to be caused by the reduced axial velocity and the air curtain formed by the strong swirling injection in this region. At a given r, the particle mass fluxes decrease along the height, as expected. It generally increases along the radial direction due to centrifugal action. The range of dilute region expands from r* = 0.7-0.8 at h= 27 inches to r*=0.6-0.9 at h=32 inches and to r*=0.34.9 at h= 37 inches. Particle elutriation
FREEBOARD
Fig. 11. Axial distribution (A) R =5.5 inches.
HEIGHT
(IN)
of particle mass fluxes. (0) R = 0 inches,
Elutriation of unburned char particles has been one of the major problems of conventional bubbling FBCs. In traditional FBC design, the size of the freeboard is often taken as the transport disengaging height 1141, at which the particle concentration reduces to a level which no longer changes with the height. Based on this concept, the freeboard serves essentially as a settling chamber for particles, which is unnecessarily tall from the combustion and heat transfer viewpoint. The VFBC design employing tangential air injection can effectively prevent particles from escaping the combustor. The freeboard of a VFBC unit can therefore be shortened and still gives low fines elutriation. The elutriation of particles is usually measured by the elutriation rate ki or the elutriation constant Ki defined as k.=1
dri
xi dt
Ki = ksi W/A,
0.5
0
.*‘.
I
r/R
Fig. 12. Radial distribution (0) 32, (A) 37 inches.
of particle
mass fluxes. H=(O)
27,
due to a weakened swirling flow field in the core region. At the elevation of 1 inch below the end plate of the exhaust pipe (or 38 inches), the mass flux in the center still amounts to 0.616 mg rnmm2 s-l, which is 45 times of that at r=5.5 inches. In the original VFBC design, a straight run exhaust pipe with an opening at the bottom was used, which resulted in a large elutriation of particles. The results in this figure show that the present design of exhaust pipe prevents direct escape of particles and effectively reduces the elutriation of fines. Figure 12 shows the radial distributions of particle mass fluxes at three elevations: 27 , 32 and 37 inches above the bed surface. For all three cases, a high mass flux was detected near the wall and a medium reading in the core region. A dilute region exists between
(2) (3)
where ki = elutriation rate of i th group of particles, s-‘, xi= weight fraction of ith range of particles in the fluidized bed, t = time, s, Ki = elutriation constant, W= total weight of particles in the fluidized mgcm-*s-l, bed, mg, A, =area of distributor plate, cm2. Four test runs were conducted to study the effect of secondary air injection on particle elutriation. The test conditions and major results are summarized in Table 4. In order to maintain the same average gas axial velocity for a fair comparison of elutriation, total air flow rate was kept at 650 ft3 min-1 and the secondary air injection angle kept at p= 45”. Run No. 1 using only primary air is designed to simulate the operation of conventional bubbling FBCs. Run Nos. 2 and 3 inject secondary air through both levels of nozzles but with different flow rates and velocities. Run No. 4 uses the lower nozzles for injecting secondary air, giving a large swirl number of 4.34. A large elutriation constant of 151 mg crne2 s-l was measured for Run No. 1, indicating a serious problem of particle elutriation for conventional bubbling FBC operation. When 23% of combustion air is injected through the secondary air nozzles in Run No. 2
146 TABLE 4. Test conditions
and major results of elutriation
tests
test particles (glass beads) size distribution specific gravity primary air flow rate velocity upper secondary air flow rate velocity lower secondary air flow rate velocity swirl number elutriation rate ki elutriation constant Ki mean diameter of elutriated particles
2
3
4
400 8.46
400 8.46
23.6
100 46.9
0 0
9570 151
100 46.9 0.78 59.5 0.93
150 70.5 2.17 8.92 0.14
250 117.8 4.34 0.138 0.0022
177
169
159
190
1
Run No.
w
105-3000 2.42
ft3 min-’ ft s-’
650 13.7
ft3 min-’ ft s-’
0 0
ft3 mitt-’ ft s-’
0 0 0
106s-1 mg cm-* s-r pm
(S= 0.78), the elutriation constant reduces by more than 2 orders of magnitude to 0.93 mg cm-’ s-‘, as listed in Table 4. As the fraction of secondary air further increases to 39% in Run No. 3 (S =2.17), the elutriation constant reduces by another order of magnitude to 0.14 mg cmp2 s-l. Run No. 4 of VFBC operation (S = 4.34) gives the lowest elutriation rate of 0.002 2 mg cm-’ s-‘, being about 5 orders of magnitude lower than that of bubbling operation.
500
10.6 50
0 to 4.34, the elutriation rate reduces by 5 orders of magnitude. Acknowledgement
The authors are grateful to the technical monitor, Dr. Tim T. Fu, of the Naval Civil Engineering Laboratory, U.S. Navy. References
Conclusion
A 18 inch i.d. cold VFBC model with associated auxiliary subsystems was designed and built based on a thermal analysis of a 1.5 MMBtu h-l VFBC boiler. Systematic measurements of isothermal gas-particle flows in the freeboard and particle elutriation from the test chamber were conducted. The following conclusions can be drawn: l The gas flow field in the VFBC model is characterized by a swirling, developing, and recirculating flow. The magnitude of gas recirculation increases with increasing swirl number. Secondary air injection at /3= 45” gives better performance of gas recirculation. 0 Particle mass flux measurements in the VFBC freeboard showed alternative circulating layers of dilute and dense suspensions. The particle mass flux generally decreases along the height and increases toward the wall. 0 The present design of VFBC models can substantially reduce particle elutriation. The change of swirl number via secondary air injection can effectively control elutriation. When the swirl number increases from
9 10 11 12 13
14
N. Syred and J. M. Beer, Combustion and Flames, 23 (1974) 143. A. K. Gupta, D. G. Lilley and N. Syred, Swirl Flow, Chs. 5 and 6, Abacus Press, Tunbridge Wells, 1984. .I. M. Beer and J. M. Chigier, Combustion Aerodynamics, Halsted-Wiley, New York, 1972. S. Nieh and G. Yang, Powder Technol., 50 (1987) 121. S. Nieh and G. Yang, Particulate Sci. Technol., 5 (1987) 323. G. Yang, S. Nieh and T. T. Fu, Powder Technol., 57 (1989) 171. S. Nieh and T. T. Fu, Proc. 7th International Coal ConjI, Pittsburgh, PA, 1990, pp. 223-232. S. Nieh, J. R. Chen and Y. C. Whang, Coal-Fired Vortexing Fluidized-Bed Combustion (WBC), final report to Naval Civil Engineering Laboratory, November 1989. S. Nieh, S. Y. Lin, S. W. Lee and T. T. Fu, Particulate Sci Technol., 6 (1988) 269. L. Cheng, S. L. Soo and S. K. Tung, J. Eng. Power, 92 (1970) 135. T. Nguyen, A. Nguyen and S. Nieh, Powder Technol., 59 (1989) 183. S. L. Soo, Multiphase Fluid Qvnamics, preliminary revised version, Chapt. 8, S. L. Soo Associates, Urbana, IL, 1983. G. Yang, Ph. D. Thesis, Department of Mechanical Engineering, The Catholic University of America, Washington DC, 1989. D. Kunii and 0. Levenspiel, Fluidization Engineering, Wiley, New York, 1969.
Technology,
139
69 (1992) 139-146
Measurements of gas-particle flows and elutriation i.d. cold vortexing fluidized-bed combustion model G. Yang **, A. Q. Zhut and C. S. Zhaot
S. Nieh*, Combustion Washington
of an 18 inch
and Multiphase Flow Laboratory, DC 20044 (USA)
Department
of Mechanical
Engineering,
Catholic
Universiv
of America,
(Received November 2, 1990; in revised form June 3, 1991)
Abstract A novel vortexing fluidized-bed combustion (VFBC) technique was recently developed for commercial space/ water heating applications. The advantages of VFBC firing solid fuels include large combustion intensity and efficiency, wide turndown and fuel flexibility, and low pollutant emissions. An 18 inch i.d. cold VFBC test model was designed, built, and systematically tested. Measurements of gas flow field, and particle m&s fluxes and elutriation under different swirl numbers and secondary air fractions were conducted with mixed-size glass beads. The gas flow field in the VFBC model is characterized by a swirling, developing and recirculating flow. The particle flow field is characterized by alternate layers of dense and dilute circulating suspensions. The VFBC process can substantially reduce the elutriation of particles which was measured to be 3 to 5 orders of magnitude lower than the conventional bubbling FBC operation.
Introduction
The benefits of vortex flow in both reactive and nonreactive multiphase systems have been recognized for many decades [l, 21. Strong vortex flows occur in a wide variety of engineering applications: cyclone separators, jet pumps, vortex reactors, spraying machines, industrial furnaces, internal combustion engines, fluid heaters and utility boilers. In reactive systems, the design of strong vortexes for the injected air and fuel (or reactants) can have large-scale effects on entrainment and decay, heat and mass transfer, flame stability, fuel ignition and burnout, and pollutant abatement reactions [l, 31. By integrating the advantages of fluidized-bed combustion (FBC) and vortex flow, a new solid-fuel firing technique, known as vortexing FBC (VFBC), was developed for small- and medium-scale boiler/kiln applications for commercial and light industrial sectors [4-6]. Figure 1 shows a design example of a coal-fired VFBC unit. The combustion chamber is a gas-tight enclosure with a circular cross-section, where coal combustion and sulfur retention reactions take place. The combustor may be fully or partially water-cooled with vertical wall tubes or water jacket. Primary air is in*Author to whom correspondence should be addressed. **Current address: Department of Mechanical Engineering, Florida International University, Miami, FL 33199. wisiting scholars from Southeast University, Nanjing, China.
0032-5910/92/$5.00
traduced at the bottom of the lower combustion chamber (fluidized bed) to fluidize the bed materials-coal, ash and limestone-the same as those in conventional FBC boilers. A substantial amount of secondary air is tangentially injected at two or more elevations into the upper combustion chamber (freeboard), similar to the arrangement in cyclone burners and vortex combustors [7]. Coal particles are ignited, burned and entrained into the upper combustor. The strong vortexing flow promotes gas-gas and gas-particle mixing, creates internal gas-particle recirculation, and reduces particle elutriation from the center exhaust tube. Sulfur capture, No, reduction, and carbon burnout are time-controlled and completed in the combustor before the combustion products exit to a heat exchanger. Steam and/or hot water generated from the VFBC unit can be used for space heating, water heating, and industrial process heat. Since VFBC is a new combustion concept, only a few preliminary tests on the overall performance have been conducted [4-6]. These results measured on the design and operation of a VFBC unit were found to support the value of the concept. In VFBC design, for instance, the freeboard is not considered as a simple settling chamber; rather it is an active part of the combustor. Because of the advantageous effects ofvortex flow, the freeboard volume of a VFBC unit can be significantly reduced, giving a large combustion intensity (volumetric heat release rate). Our recent tests of a
0 1992 - Elsevier Sequoia. All rights reserved
140 TABLE 1. Major VFBC unit
design
Parameter thermal HEAT EXCHANGER
PRIMARY
AIR
Fig. 1. Vortexing
fluidized-bed
combustion
boiler.
0.15 MMBtu h-’ (0.15~ lo6 Btu h-’ or 440 kW) exploratory VFBC model firing Indiana bituminous coal [8] showed a high combustion intensity of 0.25 MMBtu tY3 h-’ (2.2~10~ kcal mm3 h-‘) which is an order of magnitude higher than a typical conventional FBC boiler. However, the technical data on combustion, thermal and gas-particle flows in thevortexing freeboard are far from adequate for use for a satisfactory boiler design and operation. In order to achieve a better understanding of gas-particle flow characteristics, an 18 inch i.d., 80 inch tall VFBC cold model was designed, built and tested. The measured results of the flow field in the freeboard and particle elutriation from the VFBC system are presented and discussed in this paper.
Test apparatus
and instrumentation
WBC cold test setup Based on Indiana bituminous coal, a thermal analysis and conceptual design of a 1.5 MMBtu h-l VFBC unit was conducted, and boiler configuration and operation conditions determined. The major design parameters are summarized in Table 1. Based on these results and our previous experience with 4 inch i.d. and 7 inch i.d. VFBC cold models [4-6], an 18 inch i.d. VFBC cold test system was designed and installed, as schematically shown in Fig. 2. Air from a forced draft fan (Buffalo
input
coal (Indiana bituminous) consumption rate combustion temperature combustion intensity combustor height inside diameter distributor diameter number of bubble cap diameter of bubble cap exhaust pipe diameter secondary air nozzles level x number tip diameter primary (fluidizing) air flow rate velocity secondary (tangential) air flow rate velocity flue gas (at exit) flow rate velocity in pipe
parameters
of the 1.5 MMBtu
h-’
Unit
Magnitude
MMBtu h-’ kW
1.5 440
lb h-’ “F MMBtu ft -‘h -’
148 - 1650 0.17
inches inches inches _ inches inches
63 17.5 12 49 1 10
inches
2x4 1
ft3 min-’ ft s-1
148 -4.25
ft3 mitt-’ ft s-1
124 -82
ft3 min-’ ft s-1
1187 32
Forge 45RD, 1200 ft3 min-‘, 47 inch water column) is introduced into the test chamber through three individually controlled pipelines. Two pipelines are connected to secondary air injection nozzles and one to the primary air wind box. Air flow rates are controlled and measured by flowmeters and three Kurz digital electronic manometers. Test particles are continuously fed into the test chamber by a screw type AccuRate model 302 dry material volumetric feeder with a nominal feed rate of 0.26-4.16 ft3 h-l. The particle-laden gas stream leaving the VFBC model is forced into a bag filter, where fines are collected and removed for microscopic examination, as needed. Eighteen inch i.d. VFBC test chamber Figure 3 shows the configuration of the 18 inch i.d., 80 inch tall VFBC cold test chamber which consists of four major parts: upper freeboard, lower freeboard, fluidized bed and wind box, all made of Plexiglas to facilitate visual observations. The 22 inch long upper freeboard has a 10 inch exhaust pipe at the top and four equally-spaced (90” on the circumferential wall) air nozzles at an elevation of 64 inches. The 28 inch long lower freeboard also has four air nozzles. These two freeboard sections were made exchangeable, which provided the flexibility for secondary air injection. In order to achieve a good quality of fluidization, the tapered chamber design was adopted for fluidized bed/
UPPER
CHAMBER
s'c"n:~ARy NOZZLES
LOWER
CHAMBER
FLUIDIZED
WIND
BED
BOX
PR I+$"' 6
Fig. 2. Cold VFBC model test system. Fig. 3. Configuration of 18 inch i.d. cold VFBC test model.
freeboard transition. A 3.5 inch long Plexiglas converter connects the 12 inch i.d. fluidized bed to the 18 inch i.d. freeboard. One of the major design considerations of the VFBC unit is the control of vortex intensity which is commonly characterized by the swirl number [l, 21. For the VFBC model, the swirl number can be expressed as (refer to Fig. 4): Swirl number, S= =-
inlet angular momentum outlet axial momentum x DJ2 ~2 D& 4
I
FLUE
I I’I t
I
PRIMARY
GAS
AIR
I t
AIR
A,
where D,=diameter of exit tube, D, = diameter of chamber (freeboard), Di = diameter of imaginary circle of air injection, Di = D, cos p, p = nozzle injection angle on a horizontal plane, A, = total tip area of secondary air nozzles, l=volumetric flow rate fraction of secondary air. For a VFBC model of tixed D,, D, and A,, the swirl number (or swirling intensity) can be varied by changing secondary air fraction E and injection angle /3. The flow rate fraction E is an operational parameter which can be easily adjusted in the tests. The adjustment of injection angle /3 needs to be accounted for in the air nozzle design. Figure 5 shows the configuration of the secondary air nozzles which are mounted at two levels
SEC~;ARY
Fig. 4. Definition of swirl number.
on the VFBC chamber. Each nozzle is sealed in a Plexiglas compartment and glued onto the test chamber. Secondary air is supplied from a 2 inch tube and horizontally injected into the test chamber via a 1 inch
142 INJECTION
When charged particles impact the metal sensor, a sequence of charge pulses are generated. These pulses are converted, amplified, counted and recorded for a preselected period of time. By multiplying the total number of impacts per unit time with the local averaged particle mass and dividing by the projected area of the metal sensor, the local particle mass flux can be determined. With independent calibrations against the isokinetic sampler system [ll], this probe represents a convenient secondary standard for measuring local particle mass flux of flowing glass beads. Corrections of probe readings due to electrical noises and fraction of impact were taken into account [lo, 121. A United Sensor DA&DAT 5-point directional probe with four manometers was used for measuring the local gas velocity [13]. Based on the pressure readings from the probe, the pitch angle and yaw angle of gas flow, and the axial, tangential and radial components of the gas velocity can be found. The data reduction process is accomplished by a microcomputer with a software program.
NUT
AIR INLET A)
8)
FRONTAL VIEW
TOP VIEW
Fig. 5. Configuration
of secondary
air injection nozzle.
circular tube controlled by an angle deflector. The injection angle of each nozzle can be independently adjusted and read from an angle indicator on the nozzle. Instiumentation The local particle mass flux was measured by an electrostatic impact probe and associated signal processing devices [9, lo], as shown in Fig. 6. The probe tip (sensor) is a 0.04 inch diameter metal ball, which is electrically isolated from the probe stem. Particles are naturally charged to a detectable level through contacts with solid surfaces in a multiphase system. ,--a------.e
’ O-TO-V I CCNVERER
Fig. 6. Electrostatic
impact probe system.
Results
and discussions
Gas pow field The VFBC process is distinguished by a strongly swirling gas flow which characterizes the flow and combustion performance in the freeboard. An understanding of the aerodynamic structure in the freeboard is crucial for the design and operation of VFBC units. A series of tests were therefore arranged. Since the gas flow field was essentially axisymmetric, measurements were made only on a vertical plane passing through the chamber axis. Effects of the following parameters were explored: (i) volumetric flow rate fraction of secondary air, E, (ii) injection angle of secondary air, p, and (iii) number of enabled air nozzles. ______
PMPLIFIER
1 ,
143 TABLE Run No.
1 2 3 4 5 6 7 8
2. Test conditions
Primary air (ft’ min-‘)
500 500 500 500 400 400 750 600
Secondary Upper
for gas flow measurements Swirl No. s
Air
nozzles
Lower nozzles
(ft’ min-‘)
p (“)
(ft’ min-‘)
p (“)
100 100 100 100 150 0 0 50
45 30 60 45 45 0 0 45
150 150 150 150 200 350 0 100
45 30 60 45 45 45 0 45
1.63 1.96 1.15 1.78 3.2 6.4 0 0.59
TANGENTIAL VELOCITY W
Fig. 7. Gas axial and tangential
along the axial direction except for the cross-section at the top exit. Both axial and tangential velocity profiles change rapidly near the top exhaust pipe. An intense, swirling gas-particle flow appears in the annular space at the top, which is advantageous for particle control and burnout. A recirculating flow is formed in the regions below the air nozzles, where a high particle concentration exists. This internal circulating toroidal flow promotes local particle recirculation, which is beneficial for solid combustion as it results in prolonged particle residence time and enhanced mixing. Basically, the recirculation/developing flow derives from the progressive injection of air together with the sudden change of chamber configuration. The size and location of recirculation can generally be controlled by the arrangement of secondary air injection. Three test runs, Nos. 5, 7 and 8 in Table 2, were conducted to explore the effect of swirl number on gas flow field. The total air flow rate and secondary air injection angles were kept unchanged at 750 ft3 mine1 and 45”, respectively. Figure 8 shows the gas axial velocity distributions at two bottom cross-sections: Section 2 at 6 inches below the lower level nozzles and Section 1 at 6 inches below Section 2 (see Fig. 8). It can be seen that at a high swirl number (Run No. 5, S = 3.2) the recirculating flow (reversal velocity) is strong at both cross-sections near the wall. It becomes weakened when the swirl number reduces to 0.59 (Run No. 8). For the non-swirling flow (Run No. 7, S= 0), the axial velocity is low. No gas recirculation can be found at these two sections.
velocity profiles.
Test conditions for the gas flow measurements and swirl numbers are given in Table 2. Figure 7 shows the typical results of tangential and axial velocity distributions in the VFBC freeboard (data of Run No. 1). The tangential velocity profiles show a strong vortex flow feature, while the axial velocity profiles show a recirculating/developing type of flow. The tangential velocities are high near the wall and decay rapidly toward the center, as expected. The swirling motion in the core region within half chamber radius is weak, although the velocity fluctuation is vigorous. The tangential velocity profiles are retained
0.5 r* - r/R
Fig. 8. Effect of swirl number on gas axial velocity distributions. (A) Run 7 S= 0, (0) Run 8 S = .59, (Cl) Run 5 S=3.2.
144
Figure 9 shows the effect of secondary air injection angle on recirculating gas flow. The axial velocity distributions at the same cross-sections explained earlier were measured. Test run Nos. 1, 2 and 3 (see Table 2) used a constant primary air flow rate of 500 ft3 min-’ and secondary air flow rate of 250 ft3 min-l, but different injection angles. The axial velocity distribution at Section 2 was clearly affected by /3. However, the axial velocity profile at Section 1 and tangential velocity profiles at both sections remained roughly unchanged at different ps. At Section 2 the axial velocities (or recirculating flow) exhibited distinct patterns at different /3s, although the swirl numbers were close. Run No. 3 (p=60”, S= 1.15) has a weak recirculating flow near the wall, as expected. Run No. 2 (p= 30”) also has a weak recirculating flow in spite of its larger swirl number (S= 1.96). At both sections, Run No. 1 with p = 45” appeared to be the most desirable arrangement of secondary air injection. It creates a strong recirculating flow in the wall region with the same feed rates of primary air and secondary air.
TABLE 3. Major test conditions
for particle
Parameter
Unit
Range
w
-
O-3000 Robin-Rammler 344 2.42 0.92
ft? min-’ ft s-1
66-477 1.9-13.7
ft3 min-r % ft s-1
O-221 O-O.47 O-146
test particles (glass beads) size range distribution mean diameter specific gravity spheric&y primary (fluidizing) air flow rate velocity secondary (tangential) air flow rate flow rate fraction velocity
-
pm
measurements
Particle mass flux distribution
In order to explore the flow behavior of particles in the VFBC freeboard, the mass flux distributions were measured. The major test conditions are listed in Table 3. Figure 10 shows a typical axial distribution of particle mass fluxes in the test chamber. The particle flow field is characterized by circulating dense suspension layers and dilute zones [6], which appear alternately along the chamber height. Particles generally follow spiral
0 ‘\.
1 I
2
3
I
5
MC/MM’S
Fig. 10. Typical distributions of particle mass fluxes in VFBC model. (0) R =5.5 inches, (0) R=8.5 inches.
t sn
0
-2
2
6
0
0.5
1.0
remr/R Fig. 9. Effect of secondary air injection angle on axial velocity distributions. 0 Run 1, /3 = 45”, S = 1.63; A Run 2, /3 = 30”, S = 1.96, •i Run 3, /3=60”, S=1.15.
ascending trajectories from the fluidized bed to the top exit [4, 51. In the wall region (rag.5 inches), two dense layers of particles and two dilute zones can be clearly seen, as shown in Figure 10. The average particle mass flux can differ by 5-fold among these layers. The particle mass flux distribution at r=5.5 inches (the opening of the exhaust pipe) is also shown in the figure. It generally decreases along the height, primarily due to the effect of gravity. In the vicinity of the exhaust pipe, the mass flux reduces to 0.014 mg rnrne2 s-‘, being only 5% of the averaged particle mass flux in the chamber. This indicates a low particle elutriation. Figure 11 shows the variations of particle mass fluxes along the freeboard axis and at r=5.5 inches. It is of interest to note that the mass fluxes along the axis are always higher than those at r=5.5 inches, primarily
145
r* =0.7-0.9,
which is believed to be caused by the reduced axial velocity and the air curtain formed by the strong swirling injection in this region. At a given r, the particle mass fluxes decrease along the height, as expected. It generally increases along the radial direction due to centrifugal action. The range of dilute region expands from r* = 0.7-0.8 at h= 27 inches to r*=0.6-0.9 at h=32 inches and to r*=0.34.9 at h= 37 inches. Particle elutriation
FREEBOARD
Fig. 11. Axial distribution (A) R =5.5 inches.
HEIGHT
(IN)
of particle mass fluxes. (0) R = 0 inches,
Elutriation of unburned char particles has been one of the major problems of conventional bubbling FBCs. In traditional FBC design, the size of the freeboard is often taken as the transport disengaging height 1141, at which the particle concentration reduces to a level which no longer changes with the height. Based on this concept, the freeboard serves essentially as a settling chamber for particles, which is unnecessarily tall from the combustion and heat transfer viewpoint. The VFBC design employing tangential air injection can effectively prevent particles from escaping the combustor. The freeboard of a VFBC unit can therefore be shortened and still gives low fines elutriation. The elutriation of particles is usually measured by the elutriation rate ki or the elutriation constant Ki defined as k.=1
dri
xi dt
Ki = ksi W/A,
0.5
0
.*‘.
I
r/R
Fig. 12. Radial distribution (0) 32, (A) 37 inches.
of particle
mass fluxes. H=(O)
27,
due to a weakened swirling flow field in the core region. At the elevation of 1 inch below the end plate of the exhaust pipe (or 38 inches), the mass flux in the center still amounts to 0.616 mg rnmm2 s-l, which is 45 times of that at r=5.5 inches. In the original VFBC design, a straight run exhaust pipe with an opening at the bottom was used, which resulted in a large elutriation of particles. The results in this figure show that the present design of exhaust pipe prevents direct escape of particles and effectively reduces the elutriation of fines. Figure 12 shows the radial distributions of particle mass fluxes at three elevations: 27 , 32 and 37 inches above the bed surface. For all three cases, a high mass flux was detected near the wall and a medium reading in the core region. A dilute region exists between
(2) (3)
where ki = elutriation rate of i th group of particles, s-‘, xi= weight fraction of ith range of particles in the fluidized bed, t = time, s, Ki = elutriation constant, W= total weight of particles in the fluidized mgcm-*s-l, bed, mg, A, =area of distributor plate, cm2. Four test runs were conducted to study the effect of secondary air injection on particle elutriation. The test conditions and major results are summarized in Table 4. In order to maintain the same average gas axial velocity for a fair comparison of elutriation, total air flow rate was kept at 650 ft3 min-1 and the secondary air injection angle kept at p= 45”. Run No. 1 using only primary air is designed to simulate the operation of conventional bubbling FBCs. Run Nos. 2 and 3 inject secondary air through both levels of nozzles but with different flow rates and velocities. Run No. 4 uses the lower nozzles for injecting secondary air, giving a large swirl number of 4.34. A large elutriation constant of 151 mg crne2 s-l was measured for Run No. 1, indicating a serious problem of particle elutriation for conventional bubbling FBC operation. When 23% of combustion air is injected through the secondary air nozzles in Run No. 2
146 TABLE 4. Test conditions
and major results of elutriation
tests
test particles (glass beads) size distribution specific gravity primary air flow rate velocity upper secondary air flow rate velocity lower secondary air flow rate velocity swirl number elutriation rate ki elutriation constant Ki mean diameter of elutriated particles
2
3
4
400 8.46
400 8.46
23.6
100 46.9
0 0
9570 151
100 46.9 0.78 59.5 0.93
150 70.5 2.17 8.92 0.14
250 117.8 4.34 0.138 0.0022
177
169
159
190
1
Run No.
w
105-3000 2.42
ft3 min-’ ft s-’
650 13.7
ft3 min-’ ft s-’
0 0
ft3 mitt-’ ft s-’
0 0 0
106s-1 mg cm-* s-r pm
(S= 0.78), the elutriation constant reduces by more than 2 orders of magnitude to 0.93 mg cm-’ s-‘, as listed in Table 4. As the fraction of secondary air further increases to 39% in Run No. 3 (S =2.17), the elutriation constant reduces by another order of magnitude to 0.14 mg cmp2 s-l. Run No. 4 of VFBC operation (S = 4.34) gives the lowest elutriation rate of 0.002 2 mg cm-’ s-‘, being about 5 orders of magnitude lower than that of bubbling operation.
500
10.6 50
0 to 4.34, the elutriation rate reduces by 5 orders of magnitude. Acknowledgement
The authors are grateful to the technical monitor, Dr. Tim T. Fu, of the Naval Civil Engineering Laboratory, U.S. Navy. References
Conclusion
A 18 inch i.d. cold VFBC model with associated auxiliary subsystems was designed and built based on a thermal analysis of a 1.5 MMBtu h-l VFBC boiler. Systematic measurements of isothermal gas-particle flows in the freeboard and particle elutriation from the test chamber were conducted. The following conclusions can be drawn: l The gas flow field in the VFBC model is characterized by a swirling, developing, and recirculating flow. The magnitude of gas recirculation increases with increasing swirl number. Secondary air injection at /3= 45” gives better performance of gas recirculation. 0 Particle mass flux measurements in the VFBC freeboard showed alternative circulating layers of dilute and dense suspensions. The particle mass flux generally decreases along the height and increases toward the wall. 0 The present design of VFBC models can substantially reduce particle elutriation. The change of swirl number via secondary air injection can effectively control elutriation. When the swirl number increases from
9 10 11 12 13
14
N. Syred and J. M. Beer, Combustion and Flames, 23 (1974) 143. A. K. Gupta, D. G. Lilley and N. Syred, Swirl Flow, Chs. 5 and 6, Abacus Press, Tunbridge Wells, 1984. .I. M. Beer and J. M. Chigier, Combustion Aerodynamics, Halsted-Wiley, New York, 1972. S. Nieh and G. Yang, Powder Technol., 50 (1987) 121. S. Nieh and G. Yang, Particulate Sci. Technol., 5 (1987) 323. G. Yang, S. Nieh and T. T. Fu, Powder Technol., 57 (1989) 171. S. Nieh and T. T. Fu, Proc. 7th International Coal ConjI, Pittsburgh, PA, 1990, pp. 223-232. S. Nieh, J. R. Chen and Y. C. Whang, Coal-Fired Vortexing Fluidized-Bed Combustion (WBC), final report to Naval Civil Engineering Laboratory, November 1989. S. Nieh, S. Y. Lin, S. W. Lee and T. T. Fu, Particulate Sci Technol., 6 (1988) 269. L. Cheng, S. L. Soo and S. K. Tung, J. Eng. Power, 92 (1970) 135. T. Nguyen, A. Nguyen and S. Nieh, Powder Technol., 59 (1989) 183. S. L. Soo, Multiphase Fluid Qvnamics, preliminary revised version, Chapt. 8, S. L. Soo Associates, Urbana, IL, 1983. G. Yang, Ph. D. Thesis, Department of Mechanical Engineering, The Catholic University of America, Washington DC, 1989. D. Kunii and 0. Levenspiel, Fluidization Engineering, Wiley, New York, 1969.