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Soot removal from diesel engine exhaust using a rotating fluidized bed filter
Advances in Environmental Research 8 (2004) 387–395
Soot removal from diesel engine exhaust using a rotating fluidized bed filter Gui-Hua Qiana, Ian W. Burdicka, Robert Pfeffera, Henry Shawa, John G. Stevensb,* a
Department of Chemical Engineering, New Jersey Institute of Technology, Newark, NJ 07102, USA b Department of Mathematical Sciences, Montclair State University, Montclair, NJ 07043, USA Received 5 September 2002; accepted 15 October 2002
Abstract A horizontal rotating fluidized bed filter was used to capture soot from diesel engine exhaust, achieving mass filtration efficiencies exceeding 80%. In the packed bed mode, the filtration efficiency increased with increasing gas flow rate at all rotating speeds. A mathematical model developed for this radial in-flow configuration was used to predict filtration efficiency as a function of superficial gas velocity. The resulting theoretical curve provided an upper bound for the data, exhibiting the same trend at higher rotating speeds. At low rotating speed, the efficiency after fluidization decreased with increasing gas velocity due to bubble bypassing. 䊚 2002 Elsevier Science Ltd. All rights reserved. Keywords: Diesel soot; Rotating fluidized bed filter; Filtration efficiency; Filtration modeling; Aerosol size distribution
1. Introduction Diesel engines are widely used in transportation and for small power applications. Due to their overall lean operation and higher compression ratio, they tend to emit less CO and unburned hydrocarbons with higher thermal efficiency than do alternate technologies. However, diesel engines tend to produce significant quantities of particulate matter (soot) and NOX (Heck and Farrauto, 1995). The soot consists of both solid and liquid components. There is evidence that particulates from diesel engines are biologically more active than those from spark ignition engines and may be carcinogenic (Russell-Jones, 1987). Therefore, diesel emission control is being addressed worldwide. More stringent standards have been proposed and promulgated to limit the emissions of particulates from diesel engines in *Corresponding author. Tel.: q1-973-655-7254; fax: q1973-655-7686. E-mail address: [email protected] (J.G. Stevens).
many parts of the world (Heck and Farrauto, 1995; Needham et al., 1991). Diesel engine manufacturers are trying to develop engines that will produce less soot and NOX. Nevertheless, it is likely that after-treatment technologies will be needed to meet the projected emissions standards. Several devices have been studied for use in diesel engine emission control, including the monolith or honeycomb (Charles, 1990; Farrauto et al., 1992), fibrous filter (Zhu et al., 2000), cyclone (Arcoumanis et al., 1994) and granular bed. In the mid-1980s, honeycomb filters were investigated extensively. The gases are forced to flow through the monolith wall and exit through an adjoining channel. The particulates, being larger than the pore size of the monolith wall, are consequently trapped. Since this device has limited capacity before pressure drop becomes excessive, it is necessary to regenerate it periodically by combustion. Thus, an elaborate control system is required which is not considered economical for many applications. Granular bed filters can be used for aerosol and dust capture with high filtration efficiency (Clift, 1983;
1093-0191/04/$ - see front matter 䊚 2002 Elsevier Science Ltd. All rights reserved. PII: S 1 0 9 3 - 0 1 9 1 Ž 0 2 . 0 0 1 1 7 - X
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G.-H. Qian et al. / Advances in Environmental Research 8 (2004) 387–395
tive advantage of using an RFBF is that catalyst granules can be used to filter soot and then catalyze its oxidation by NOX and O2 at high temperatures, with a concomitant reduction of NOX to N2 (Tsutsumi et al., 1994; Ma et al., 1998). This mode of operation can make the bed self-cleaning and avoid the need for regenerating the granules as they become saturated with soot. In this mode, we refer to the system as a rotating fluidized bed reactor (RFBR). This paper summarizes the results of an experimental and theoretical study of the filtration efficiency of soot removal from diesel engine exhaust as a function of exhaust flow rate using a horizontal RFBF. 2. Filtration theory and modeling the RFBF
Fig. 1. Schematic diagram of the horizontal rotating fluidized bed.
D’Ottavio and Goren, 1983). D’Ottavio and Goren (1983) reported single grain capture efficiencies for 0.6–4.5 mm solid and liquid aerosol particles at high speed gas flow through clean, packed granular beds. Their data show that when impaction is the dominant capture mechanism, the single grain capture efficiency depends on the Stokes number, Reynolds number and bed porosity. The overall collection efficiency can reach 90%. If the gas velocity is above the minimum fluidization velocity, an unconfined packed bed becomes fluidized, bubbles appear and aerosol bypassing decreases collection efficiency (Clift et al., 1981). Pfeffer and Hill (1978) proposed the use of a rotating fluidized bed as a filter (RFBF), as shown in Fig. 1. The drag force of the gas, which flows radially inward through the porous wall (distributor) supporting the bed particles, is balanced by the centrifugal force caused by the rotation of the bed. Therefore, the minimum fluidization velocity increases as the rotating speed is increased so that the formation of bubbles, which can cause bypass of dust particles, can be avoided even at high gas flow rate (Pfeffer et al., 1986). Kao et al. (1988) used an RFBF to separate different industrial pollutants containing micron and submicron sized particles from contaminated gas streams. They found that the filtration efficiency could reach 90% when the RFBF is operated at Stokes number higher than 0.04. The use of an RFBF has been shown to have many advantages, such as high collection efficiency, relatively low pressure drop, high flow rate of gas per unit area of distributor, ability to operate at high temperatures and a very small ‘footprint’ compared to other granular bed filters, such as conventional fluidized beds, packed beds or moving beds (Pfeffer et al., 1986). Another innova-
According to classical aerosol filtration theory, there are three main collection mechanisms occurring in a granular bed filter: inertial impaction, direct interception, and Brownian diffusion. Previous researchers have generated theoretical and semi-empirical models for the prediction of the collection efficiency of a single collector granule based on these mechanisms, expressed by the following correlations: Inertial impaction and direct interception (Pendse and Tien, 1982): w
B
y
D
hICs(1q0.04Re)xStq0.48C4y
4d d EB d1.0412 Ez FC F| y dU dU2 GD dU G~
(1)
where ´ is the bed porosity and d is the relative size parameter given by dp ydc, the ratio of the diameter of the aerosol particle to the bed granule diameter. This correlation is based on the representation of a granular bed by the constricted tube model. Here d* is the ratio of the diameter of the constricted flow tube to the bed granule diameter. In the case of a uniform unit cell size, Pendse and Tien (1982) indicate that it can be taken to be 0.35. The Reynolds number, Re, is given by dcUgrgy m, where Ug is the superficial gas velocity, rg is the gas density and m is the gas viscosity. The Stokes number, St, is dp2UgrgCy(9mdc), where rp is the aerosol particle density and C is the Cunningham slip factor. Diffusion (Paretsky et al., 1971): hDs5.04(f(´))y1y3Pey2y3
(2)
where f(´)sw2y3(1y´)1y3q3(1y´)5y3 y2(1y´)2xyw1y(1y´)5y3x
(3)
and the Peclet number, Pe, is given by dcUg yDB with DB being the Brownian diffusivity.
G.-H. Qian et al. / Advances in Environmental Research 8 (2004) 387–395
389
Fig. 2. Schematic diagram of the experimental set-up.
When these efficiencies are small, typically a valid assumption, the overall single granule collection efficiency can be calculated as the sum of the individual mechanisms with negligible error, i.e., hoverallshICqhD.
(5)
The relationship between the overall single granule collection efficiency and the local collection rate (kc), as given by Clift et al. (1981) is w 3h
kcsx y
v (1y´) z |r 2dc ~
overall r
(6)
where r is the soot particle number density, i.e., the number of soot particles per unit gas volume. 3. Model basis and assumptions The model basis is that particles (and thus mass) are conserved throughout the filtration process. Soot particles are either carried by the gas flow through the filter or are deposited and accumulate on the collector particle surface. We have developed a comprehensive filtration model for the RFBF that can be used to predict transient phenomena, such as bed saturation and breakthrough. However, the experimental efficiencies reported below were obtained with ‘clean’ beds. Thus, we present here a simplified version that does not consider the reentrainment of collected soot particles or changes in bed porosity. Other model assumptions are: a. Soot particles do not agglomerate or disintegrate in the aerosol phase. b. Aerosol soot and collector particles are monodisperse. c. Plug flow exists with negligible dispersion. d. In the coordinate frame that rotates at the rotational speed of the bed, the gas flow is radially inward (i.e.,
Fig. 3. Diesel engine exhaust temperature and flow rate as functions of engine load and rpm. (a) Exhaust Temperature; (b) Exhaust flow rate.
in a fixed frame, the circumferential component of its velocity matches that of the bed, as discussed below). Based on the conservation of particles and the continuity equation for the carrier gas, we have vr
≠r ≠r ykcs´ ≠r ≠t
(7)
where r is the radial coordinate and vr is the local superficial radial gas velocity. This equation was integrated in time until the steadystate particle density distribution was obtained. The value of r at the bed outlet divided by the inlet value of r gives the bed penetration, which when subtracted from unity gives the number filtration efficiency of the filter. Eq. (7) was solved in dimensionless form using
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Fig. 4. Soot size distribution (by volume) for ‘T’ connection and isokinetic sampling before and after the RFBF. (a) Before the RFBF; (b) After the RFBF.
a method-of-lines approach with spatial collocation by Chebyshev polynomials. The aerosol (soot) diameter was taken to be the volume-mean diameter obtained from measured distributions. We discuss modeling results below and compare predicted to experimental filtration efficiencies. 4. Experimental facility The experimental facility used in this study is shown in Fig. 2. The exhaust gas from the diesel engine (1981 Volkswagen Rabbit) is fed directly to the RFBF through a pipe with inner diameter of 35 mm diameter approximately 1 m in length. The flow rate of the exhaust to
the RFBF is controlled by two valves and a bypass. Downstream of the RFBF the gas flow rate is measured by two calibrated orifice meters. Pressure, temperature and gas sampling points were located at the inlet and outlet of the RFBF. Temperature is measured by thermocouples and the pressure drop between the inlet and outlet of the RFBF is measured by a manometer. A variable-speed motor provided rotating speeds between 0 and 5000 rpm. In these experiments we used a cylindrical perforated distributor (134 mm diameter, 152 mm long) made of stainless steel in which 460 holes, 10 mm in diameter were drilled, resulting in 52% open area. The distributor wall was covered on the inside with a 325 mesh (45 mm) stainless steel screen. The
G.-H. Qian et al. / Advances in Environmental Research 8 (2004) 387–395
391
Fig. 5. Mass average filtration efficiency as a function of superficial gas velocity using isokinetic and ‘T’ connection sampling methods.
filter granules (bed particles) consist of glass beads manufactured by MO-SCI Corporation which have a narrow size distribution, with no fines present. The mean size is 81 mm and the particle density is 2450 kgym3. We also used alumina particles as the filter media. However when the bed became fluidized, the filtration efficiency quickly dropped to zero and actually became negative. This phenomenon is due to the presence and generation of alumina fines in the bed. Therefore, only results obtained using glass beads are reported here. Fig. 3 shows the diesel engine exhaust temperature and flow rate as function of engine load and rpm. They show that the flow rate and temperature of exhaust increase with increasing of rotating speed and load of diesel engine. The exhaust gas sample is drawn by a vacuum pump through a 5 mm sampling tube to the Aerosizer system (TSI, Inc.) at a constant sampling flow rate of 2 lymin. The soot concentrations from sampling points before and after the RFBF were measured with the Aerosizer, which provides the particle size distributions by number and by volume. The soot particle diameter is based on a soot density of 1.25 gycm3, assuming the soot particles are spherical. Particles are counted over a 90-s period. The mass average filtration efficiency based on the particle mass concentrations before and after the RFBF is given by hs
CinyCout =100 Cin
(8)
where h is the mass average filtration efficiency (%), Cin is the particle mass concentration before the RFBF and Cout is the concentration after the RFBF. The sampling tube was attached to the exhaust pipe in a ‘T’ configuration. Due to the use of this connection, we did not initially sample the aerosol streams isokinetically. We have endeavored to estimate the error due to our sampling method to determine if our experimental results would be significantly affected by its nonisokinetic nature. We conducted experiments using both the ‘T’ sampling and isokinetic sampling methods at different gas velocities and compared the results. Fig. 4 shows the soot size distribution by volume before and after RFBF using these different sampling methods. The volume average soot sizes were 1.0 and 1.3 mm before the RFBF and 1.0 and 1.1 mm after the RFBF for the ‘T’ connection and isokinetic sampling methods, respectively. Before the RFBF, isokinetic sampling captured relatively more large soot particles than ‘T’ sampling. After the RFBF, the soot size distributions are very close. Normally, diesel soot particles when formed as products of diesel combustion are very small, below 0.1 mm. However, these small individual soot particles tend to agglomerate in the exhaust stream. The agglomerated soot size is typically between 0.1 and 1.0 mm (Baumgard and Johnson, 1996; Vuk et al., 1976; Johnson et al., 1994). Fig. 5 shows the mass average filtration efficiency as a function of gas velocity based on data obtained using the two different sampling methods. The calculated
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Fig. 6. Soot size number distribution before and after the perforated distributor with 45 mm screen at 700 rpm at engine low load conditions.
efficiencies exhibit non-systematic error on the order of 10–20%, comparable to that observable in the ‘T’connection data reported below. The similarity of these results supports the use of our ‘T’ sampling data. 5. Experimental results and discussion 5.1. Filtration efficiency of the distributor alone The particle size number distributions at the inlet and outlet of the RFBF using a clean perforated distributor with screen (no bed particles) are shown in Fig. 6. The number size distributions are similar, indicating that almost all of the soot passes through the distributor. Fig. 7 exhibits the mass average filtration efficiency vs. superficial gas velocity at different rotating speeds. The maximum filtration efficiency is below 18%. The filtration efficiency due to the distributor alone is higher at low gas velocity and high rotating speed. It should be noted that after exposure to exhaust for 160 min at low engine load operation, the perforated distributor described above became plugged by soot. This plugging occurs because the soot is wet and contains a soluble organic fraction composed of fuel and lube components. Therefore, filtration efficiency experiments were run before any appreciable plugging of the screen occurred.
Fig. 7. Filtration efficiency of the perforated distributor alone (no bed granules) as a function of soot size at engine low load conditions.
of gas velocity. This charge resulted in a bed thickness of approximately 5 mm. During one set of filtration experiments at 900 rpm, glass beads from the RFBF were sampled every 1y2 h. The samples became darker because the glass beads became progressively covered with soot. Fig. 8 shows the color of glass bead samples, measured by a spectrophotometer, as a function of the total exhaust gas volume passed through the RFBF. The soot mass per unit surface area of the glass beads, also shown in Fig. 8, was measured by thermogravimetric analysis at 700 8C. These independent measurements
5.2. Soot accumulation in the RFBF In a set of experiments using 0.5 kg monodisperse glass beads as the filter medium, the soot concentration was measured at the inlet and the outlet as a function
Fig. 8. Color difference due to soot deposit on filter media and soot mass per collector surface area vs. total engine exhaust volume through the RFBF.
G.-H. Qian et al. / Advances in Environmental Research 8 (2004) 387–395
Fig. 9. Mass average filtration efficiency as a function of superficial gas velocity at different rotating speeds using the perforated distributor.
both show that the mass of soot collected increases with time. It should be noted that transient conditions during engine startup resulted in higher soot loading at the beginning of the run. After steady-state operating conditions are achieved (;30 m3), both curves become linear, as would be expected from theory. 5.3. Filtration efficiency of the RFBF Fig. 9 shows the calculated mass-average filtration efficiencies for three rotating speeds (525, 700 and 900
393
rpm). Using the experimental set-up described above, it is difficult to control flow rates, temperature, and soot generation. These factors produce the variability observed in the filtration efficiencies. Our goal is to understand the trends exhibited by these data. To that end, we have added quadratic regression curves for each of the data sets. Below the minimum fluidization velocity, the RFBF is in packed bed mode. When the velocity is further increased, the entire bed becomes fluidized at the critical minimum fluidization velocity (Umfc ). The values of Umfc for the three rotating speeds, calculated from the theoretical analysis given in Qian et al. (1999), are also given in the figure. Also shown in Fig. 9 is the theoretical curve obtained using the previously described model based on correlations developed for packed beds. Since no bypassing due to bubbling is considered, the model shows an increase in filtration efficiency with superficial gas velocity due to the inertial impaction, which increases with increasing Stokes number. However, at very low superficial gas velocity, inertial impaction becomes negligible and collection takes place mainly by interception and diffusion. The diffusion mechanism becomes more effective at very low gas velocities, resulting in the minimum at Ugs0.05 mys predicted by the model. To amplify these points, Fig. 10 shows the filtration efficiency as a function of soot size for a rotating speed of 900 rpm at Ugs0.28 mys. Under these conditions, the bed is fluidized with the gas velocity just exceeding Umfc. Above ;0.7 mm the filtration efficiency increases due to the greater effectiveness of inertial impaction for larger particles. Below ;0.3 mm the efficiency increases due to the enhanced contribution of the diffusion mechanism. In the interval between, a plateau of minimal
Fig. 10. Filtration efficiency as a function of soot particle size.
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efficiency exists due to the diminished effectiveness of both mechanisms. At superficial gas velocities below 0.10 mys, the rotating bed is in the packed bed regime for all three rotating speeds. Thus in this region, comparison to the theoretical curve is instructive. Many phenomena not considered in the model, such as leakage, bypassing, the failure of intercepted particles to ‘stick’, and reentrainment would all result in the measured efficiency being lower than that predicted. Consequently the theoretical curve should provide an upper bound for the measured efficiency, as is the case over the entire range of superficial gas velocity. It is interesting to note that the data in this regime (below 0.10 mys) appear to show a systematic decrease in filtration efficiency with increasing rotating speed. We outline one possible explanation for this effect. It is widely accepted that angular momentum is rapidly transported to the incoming gas as it enters the distributor and the outer bed layer. This notion is supported by the fact that the pressure drop through the bed is nearly independent of rotating speed once the contribution from the distributor is subtracted out (Qian et al., 1998). Thus, with respect to a coordinate frame that rotates at the same angular velocity as the bed, the gas flow is essentially radial through the bed. However, it has been suggested on theoretical grounds that the net pressure drop should exhibit a weak dependence on rotating speed—increasing slightly as the rotating speed is increased, although this effect has been difficult to observe experimentally (Chen, 1987; Kao et al., 1987; Qian et al., 1998). In any case it is likely that, especially in the region of the bed close to the distributor, the mean gas velocity has a small tangential component in the rotating frame that increases relative to the radial component as the bed is rotated more rapidly. The drag on the layer of collected particles due to this component would give rise to a shearing force exerted within this layer. This force would increase as the rotating speed increases and would result in increased re-entrainment and an overall decrease in filtration efficiency. The regression curve for the 525 rpm data exhibits the expected increase in filtration efficiency up to a superficial gas velocity of approximately 0.12 mys. This value is somewhat greater than the velocity at which the bed becomes fully fluidized (Umfcs0.10 mys). As the superficial gas velocity is increased further, the data show a clear decrease in filtration efficiency, departing from the trend of the theoretical curve for a packed bed. It is well accepted that in a fluidized bed that is bubbling, the aerosol in the bubble gas does not contact the collector granules, resulting in the observed filtration efficiency being lower than that predicted for a packed bed (Clift et al., 1981). In our earlier work (Qian et al., 1999), we indeed observed that at 525 rpm bubbling occurs for gas velocities just exceeding Umfc.
For the 700 and 900 rpm runs, we observe that both sets of data display the same trend, namely, a discernible increase, followed by a leveling off. The theoretical curve shows the same trend. For these two higher rotating speeds, we did not reach the point at which the filtration efficiency decreases significantly due to bypassing. It is likely in these runs the bed is bubbling after it becomes fluidized. However, we suspect that the bubbles are smaller due to the increased centrifugal force acting on the bed, which is proportional to the square of the rotating speed. 6. Concluding remarks The experimental results indicate that the RFBF can be used for removal of soot from diesel engine exhaust with a mass filtration efficiency that can exceed 80%. At low rotating speed (525 rpm), the filtration efficiency of the RFBF increases with superficial gas velocity as expected and then decreases after minimum fluidization due to bubble bypassing. At high rotating speeds (700 and 900 rpm), the bypassing appears to be less significant, suggesting that bubble size may be smaller at higher centrifugal force. The proposed model gives an upper bound for the filtration efficiency in a packed bed and exhibits the same trend as the data at the higher rpm. Thus, if the filtration granules are catalytically active for the reaction of NOX with soot, the RFBR has the potential to simultaneously remove soot and NOX from diesel exhaust and to operate as a self-cleaning filter. Acknowledgments This research was supported by the National Science Foundation under Grant No. CTS-9612483. References Arcoumanis, C., Barbaris, L.N., Crane, R.I., Wisby, P., 1994. Evaluation of a cyclone-based particulate filtration system for high-speed diesel engines. Proc. Inst. Mech. Eng. Part D: J. Automobile Eng. 208, 24–34. Baumgard, K.J., Johnson, J.H., 1996. The effect of fuel and engine design on diesel exhaust particle size distributions. SAE Paper No. 960131, pp. 37–50. Charles, J., 1990. Filtration system destroys diesel particulate emissions. Des. News 46, 170–171. Chen, Y., 1987. Fundamentals of a centrifugal fluidized bed. AIChE J. 33, 722–728. Clift, R., Ghadiri, M., Thambimuthu, K.V., 1981. Filtration of gases in fluidized beds. In: Wakeman, R.J. (Ed.), Progress in Filtration and Separation, vol. 2. Elsevier, Amsterdam. Clift, R., 1983. Fundamental processes in gas filtration. Mech. Eng. Trans. 181–190.
G.-H. Qian et al. / Advances in Environmental Research 8 (2004) 387–395 D’Ottavio, T., Goren, L.S., 1983. Aerosol capture in granular beds in the impaction dominated regime. Aerosol Sci. Technol. 2, 91–108. Farrauto, R.J., Heck, R.M., Speronello, B.K., 1992. Environmental catalysts. Chem. Eng. News 70, 34–44. Heck, R.M., Farrauto, R.J., 1995. Catalytic Air Pollution Control: Commercial Technology. Wiley, New York. Johnson, J.H., Bagley, S.T., Gratz, L.D., Leddy, D.G., 1994. A Review of Diesel Particulate Control Technology and Emissions Effects—1992 Horning Memorial Award Lecture, SAE 940233. Kao, J., Pfeffer, R., Tardos, G.I., 1987. On partial fluidization in rotating fluidized beds. AIChE J. 33, 858–861. Kao, J., Tardos, G.I., Pfeffer, R., 1988. Development of a rotating fluidized bed filter. FluidyParticle Separation J. 1, 44–52. Ma, K., Tang, X., Bagyi, I., Shaw, H., Pfeffer, R., Stevens, J.G., 1998. Catalytic Reduction of Nitric Oxide with Solid Reducing Materials. Presented at the Second World Congress on Environmental Catalysis, Miami, FL. Needham, J.R., Faulkner, S.A., Doyle, D.M., 1991. Developing the truck engine for ultra low emissions. Proc. Inst. Mech. Eng. Part D: J. Automobile Eng. 205, 149–159. Paretsky, L., Theodore, L., Pfeffer, R., Squires, A.M., 1971. Panel bed filters for simultaneous removal of fly ash and sulfur dioxide: II. Filtration of dilute aerosols by sand beds. APCA J. 21, 204–209. Pendse, H., Tien, C., 1982. General correlation of the initial collection efficiency of granular filter beds. AIChE J. 28, 677–686.
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Pfeffer, R., Hill, F.B., 1978. A feasibility study on the use of a rotating fluidized bed as a high efficiency dust filter, BNL 50990. Pfeffer, R., Tardos, G., Gal, E., 1986. The use of a rotating fluidized bed as a high efficiency dust filter. Proceedings of the Fifth Engineering Foundation Conference on Fluidization, Denmark, pp. 667–674. ´ Qian, G.H., Bagyi, I., Pfeffer, R., Shaw, H., Stevens, J.G., 1998. A parametric study of a horizontal rotating fluidized bed using slotted and sintered metal cylindrical gas distributors. Powder Technol. 100, 190–199. ´ Qian, G.H., Bagyi, I., Pfeffer, R., Shaw, H., Stevens, J.G., 1999. Particle mixing in rotating fluidized beds: inferences about the fluidized state. AIChE J. 45, 1401–1410. Russell-Jones, R., 1987. The health effects of vehicle emissions. ImechE Conference on Vehicle emissions and their impact on European air quality, Paper C355y87, pp. 55–59. Tsutsumi, A., Mitsuru Demura, A., KunioYoshida, A., 1994. Reduction of nitrogen oxides with soot emitted from diesel engines by a centrifugal fluidized bed. AIChE Symp. Ser. 90, 152–156. Vuk, C.T., Jones, M.A., Johnson, J.H., 1976. The measurement and analysis of the physical character of diesel particulate emissions. SAE Trans. Section 1, SAE 760131, pp. 556– 597. Zhu, C., Lin, C.-H., Cheung, C.-S., 2000. Inertial impaction dominated fibrous filtration with rectangular or cylindrical fibers. Powder Technol. 112, 149–162.
Soot removal from diesel engine exhaust using a rotating fluidized bed filter Gui-Hua Qiana, Ian W. Burdicka, Robert Pfeffera, Henry Shawa, John G. Stevensb,* a
Department of Chemical Engineering, New Jersey Institute of Technology, Newark, NJ 07102, USA b Department of Mathematical Sciences, Montclair State University, Montclair, NJ 07043, USA Received 5 September 2002; accepted 15 October 2002
Abstract A horizontal rotating fluidized bed filter was used to capture soot from diesel engine exhaust, achieving mass filtration efficiencies exceeding 80%. In the packed bed mode, the filtration efficiency increased with increasing gas flow rate at all rotating speeds. A mathematical model developed for this radial in-flow configuration was used to predict filtration efficiency as a function of superficial gas velocity. The resulting theoretical curve provided an upper bound for the data, exhibiting the same trend at higher rotating speeds. At low rotating speed, the efficiency after fluidization decreased with increasing gas velocity due to bubble bypassing. 䊚 2002 Elsevier Science Ltd. All rights reserved. Keywords: Diesel soot; Rotating fluidized bed filter; Filtration efficiency; Filtration modeling; Aerosol size distribution
1. Introduction Diesel engines are widely used in transportation and for small power applications. Due to their overall lean operation and higher compression ratio, they tend to emit less CO and unburned hydrocarbons with higher thermal efficiency than do alternate technologies. However, diesel engines tend to produce significant quantities of particulate matter (soot) and NOX (Heck and Farrauto, 1995). The soot consists of both solid and liquid components. There is evidence that particulates from diesel engines are biologically more active than those from spark ignition engines and may be carcinogenic (Russell-Jones, 1987). Therefore, diesel emission control is being addressed worldwide. More stringent standards have been proposed and promulgated to limit the emissions of particulates from diesel engines in *Corresponding author. Tel.: q1-973-655-7254; fax: q1973-655-7686. E-mail address: [email protected] (J.G. Stevens).
many parts of the world (Heck and Farrauto, 1995; Needham et al., 1991). Diesel engine manufacturers are trying to develop engines that will produce less soot and NOX. Nevertheless, it is likely that after-treatment technologies will be needed to meet the projected emissions standards. Several devices have been studied for use in diesel engine emission control, including the monolith or honeycomb (Charles, 1990; Farrauto et al., 1992), fibrous filter (Zhu et al., 2000), cyclone (Arcoumanis et al., 1994) and granular bed. In the mid-1980s, honeycomb filters were investigated extensively. The gases are forced to flow through the monolith wall and exit through an adjoining channel. The particulates, being larger than the pore size of the monolith wall, are consequently trapped. Since this device has limited capacity before pressure drop becomes excessive, it is necessary to regenerate it periodically by combustion. Thus, an elaborate control system is required which is not considered economical for many applications. Granular bed filters can be used for aerosol and dust capture with high filtration efficiency (Clift, 1983;
1093-0191/04/$ - see front matter 䊚 2002 Elsevier Science Ltd. All rights reserved. PII: S 1 0 9 3 - 0 1 9 1 Ž 0 2 . 0 0 1 1 7 - X
388
G.-H. Qian et al. / Advances in Environmental Research 8 (2004) 387–395
tive advantage of using an RFBF is that catalyst granules can be used to filter soot and then catalyze its oxidation by NOX and O2 at high temperatures, with a concomitant reduction of NOX to N2 (Tsutsumi et al., 1994; Ma et al., 1998). This mode of operation can make the bed self-cleaning and avoid the need for regenerating the granules as they become saturated with soot. In this mode, we refer to the system as a rotating fluidized bed reactor (RFBR). This paper summarizes the results of an experimental and theoretical study of the filtration efficiency of soot removal from diesel engine exhaust as a function of exhaust flow rate using a horizontal RFBF. 2. Filtration theory and modeling the RFBF
Fig. 1. Schematic diagram of the horizontal rotating fluidized bed.
D’Ottavio and Goren, 1983). D’Ottavio and Goren (1983) reported single grain capture efficiencies for 0.6–4.5 mm solid and liquid aerosol particles at high speed gas flow through clean, packed granular beds. Their data show that when impaction is the dominant capture mechanism, the single grain capture efficiency depends on the Stokes number, Reynolds number and bed porosity. The overall collection efficiency can reach 90%. If the gas velocity is above the minimum fluidization velocity, an unconfined packed bed becomes fluidized, bubbles appear and aerosol bypassing decreases collection efficiency (Clift et al., 1981). Pfeffer and Hill (1978) proposed the use of a rotating fluidized bed as a filter (RFBF), as shown in Fig. 1. The drag force of the gas, which flows radially inward through the porous wall (distributor) supporting the bed particles, is balanced by the centrifugal force caused by the rotation of the bed. Therefore, the minimum fluidization velocity increases as the rotating speed is increased so that the formation of bubbles, which can cause bypass of dust particles, can be avoided even at high gas flow rate (Pfeffer et al., 1986). Kao et al. (1988) used an RFBF to separate different industrial pollutants containing micron and submicron sized particles from contaminated gas streams. They found that the filtration efficiency could reach 90% when the RFBF is operated at Stokes number higher than 0.04. The use of an RFBF has been shown to have many advantages, such as high collection efficiency, relatively low pressure drop, high flow rate of gas per unit area of distributor, ability to operate at high temperatures and a very small ‘footprint’ compared to other granular bed filters, such as conventional fluidized beds, packed beds or moving beds (Pfeffer et al., 1986). Another innova-
According to classical aerosol filtration theory, there are three main collection mechanisms occurring in a granular bed filter: inertial impaction, direct interception, and Brownian diffusion. Previous researchers have generated theoretical and semi-empirical models for the prediction of the collection efficiency of a single collector granule based on these mechanisms, expressed by the following correlations: Inertial impaction and direct interception (Pendse and Tien, 1982): w
B
y
D
hICs(1q0.04Re)xStq0.48C4y
4d d EB d1.0412 Ez FC F| y dU dU2 GD dU G~
(1)
where ´ is the bed porosity and d is the relative size parameter given by dp ydc, the ratio of the diameter of the aerosol particle to the bed granule diameter. This correlation is based on the representation of a granular bed by the constricted tube model. Here d* is the ratio of the diameter of the constricted flow tube to the bed granule diameter. In the case of a uniform unit cell size, Pendse and Tien (1982) indicate that it can be taken to be 0.35. The Reynolds number, Re, is given by dcUgrgy m, where Ug is the superficial gas velocity, rg is the gas density and m is the gas viscosity. The Stokes number, St, is dp2UgrgCy(9mdc), where rp is the aerosol particle density and C is the Cunningham slip factor. Diffusion (Paretsky et al., 1971): hDs5.04(f(´))y1y3Pey2y3
(2)
where f(´)sw2y3(1y´)1y3q3(1y´)5y3 y2(1y´)2xyw1y(1y´)5y3x
(3)
and the Peclet number, Pe, is given by dcUg yDB with DB being the Brownian diffusivity.
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Fig. 2. Schematic diagram of the experimental set-up.
When these efficiencies are small, typically a valid assumption, the overall single granule collection efficiency can be calculated as the sum of the individual mechanisms with negligible error, i.e., hoverallshICqhD.
(5)
The relationship between the overall single granule collection efficiency and the local collection rate (kc), as given by Clift et al. (1981) is w 3h
kcsx y
v (1y´) z |r 2dc ~
overall r
(6)
where r is the soot particle number density, i.e., the number of soot particles per unit gas volume. 3. Model basis and assumptions The model basis is that particles (and thus mass) are conserved throughout the filtration process. Soot particles are either carried by the gas flow through the filter or are deposited and accumulate on the collector particle surface. We have developed a comprehensive filtration model for the RFBF that can be used to predict transient phenomena, such as bed saturation and breakthrough. However, the experimental efficiencies reported below were obtained with ‘clean’ beds. Thus, we present here a simplified version that does not consider the reentrainment of collected soot particles or changes in bed porosity. Other model assumptions are: a. Soot particles do not agglomerate or disintegrate in the aerosol phase. b. Aerosol soot and collector particles are monodisperse. c. Plug flow exists with negligible dispersion. d. In the coordinate frame that rotates at the rotational speed of the bed, the gas flow is radially inward (i.e.,
Fig. 3. Diesel engine exhaust temperature and flow rate as functions of engine load and rpm. (a) Exhaust Temperature; (b) Exhaust flow rate.
in a fixed frame, the circumferential component of its velocity matches that of the bed, as discussed below). Based on the conservation of particles and the continuity equation for the carrier gas, we have vr
≠r ≠r ykcs´ ≠r ≠t
(7)
where r is the radial coordinate and vr is the local superficial radial gas velocity. This equation was integrated in time until the steadystate particle density distribution was obtained. The value of r at the bed outlet divided by the inlet value of r gives the bed penetration, which when subtracted from unity gives the number filtration efficiency of the filter. Eq. (7) was solved in dimensionless form using
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Fig. 4. Soot size distribution (by volume) for ‘T’ connection and isokinetic sampling before and after the RFBF. (a) Before the RFBF; (b) After the RFBF.
a method-of-lines approach with spatial collocation by Chebyshev polynomials. The aerosol (soot) diameter was taken to be the volume-mean diameter obtained from measured distributions. We discuss modeling results below and compare predicted to experimental filtration efficiencies. 4. Experimental facility The experimental facility used in this study is shown in Fig. 2. The exhaust gas from the diesel engine (1981 Volkswagen Rabbit) is fed directly to the RFBF through a pipe with inner diameter of 35 mm diameter approximately 1 m in length. The flow rate of the exhaust to
the RFBF is controlled by two valves and a bypass. Downstream of the RFBF the gas flow rate is measured by two calibrated orifice meters. Pressure, temperature and gas sampling points were located at the inlet and outlet of the RFBF. Temperature is measured by thermocouples and the pressure drop between the inlet and outlet of the RFBF is measured by a manometer. A variable-speed motor provided rotating speeds between 0 and 5000 rpm. In these experiments we used a cylindrical perforated distributor (134 mm diameter, 152 mm long) made of stainless steel in which 460 holes, 10 mm in diameter were drilled, resulting in 52% open area. The distributor wall was covered on the inside with a 325 mesh (45 mm) stainless steel screen. The
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Fig. 5. Mass average filtration efficiency as a function of superficial gas velocity using isokinetic and ‘T’ connection sampling methods.
filter granules (bed particles) consist of glass beads manufactured by MO-SCI Corporation which have a narrow size distribution, with no fines present. The mean size is 81 mm and the particle density is 2450 kgym3. We also used alumina particles as the filter media. However when the bed became fluidized, the filtration efficiency quickly dropped to zero and actually became negative. This phenomenon is due to the presence and generation of alumina fines in the bed. Therefore, only results obtained using glass beads are reported here. Fig. 3 shows the diesel engine exhaust temperature and flow rate as function of engine load and rpm. They show that the flow rate and temperature of exhaust increase with increasing of rotating speed and load of diesel engine. The exhaust gas sample is drawn by a vacuum pump through a 5 mm sampling tube to the Aerosizer system (TSI, Inc.) at a constant sampling flow rate of 2 lymin. The soot concentrations from sampling points before and after the RFBF were measured with the Aerosizer, which provides the particle size distributions by number and by volume. The soot particle diameter is based on a soot density of 1.25 gycm3, assuming the soot particles are spherical. Particles are counted over a 90-s period. The mass average filtration efficiency based on the particle mass concentrations before and after the RFBF is given by hs
CinyCout =100 Cin
(8)
where h is the mass average filtration efficiency (%), Cin is the particle mass concentration before the RFBF and Cout is the concentration after the RFBF. The sampling tube was attached to the exhaust pipe in a ‘T’ configuration. Due to the use of this connection, we did not initially sample the aerosol streams isokinetically. We have endeavored to estimate the error due to our sampling method to determine if our experimental results would be significantly affected by its nonisokinetic nature. We conducted experiments using both the ‘T’ sampling and isokinetic sampling methods at different gas velocities and compared the results. Fig. 4 shows the soot size distribution by volume before and after RFBF using these different sampling methods. The volume average soot sizes were 1.0 and 1.3 mm before the RFBF and 1.0 and 1.1 mm after the RFBF for the ‘T’ connection and isokinetic sampling methods, respectively. Before the RFBF, isokinetic sampling captured relatively more large soot particles than ‘T’ sampling. After the RFBF, the soot size distributions are very close. Normally, diesel soot particles when formed as products of diesel combustion are very small, below 0.1 mm. However, these small individual soot particles tend to agglomerate in the exhaust stream. The agglomerated soot size is typically between 0.1 and 1.0 mm (Baumgard and Johnson, 1996; Vuk et al., 1976; Johnson et al., 1994). Fig. 5 shows the mass average filtration efficiency as a function of gas velocity based on data obtained using the two different sampling methods. The calculated
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Fig. 6. Soot size number distribution before and after the perforated distributor with 45 mm screen at 700 rpm at engine low load conditions.
efficiencies exhibit non-systematic error on the order of 10–20%, comparable to that observable in the ‘T’connection data reported below. The similarity of these results supports the use of our ‘T’ sampling data. 5. Experimental results and discussion 5.1. Filtration efficiency of the distributor alone The particle size number distributions at the inlet and outlet of the RFBF using a clean perforated distributor with screen (no bed particles) are shown in Fig. 6. The number size distributions are similar, indicating that almost all of the soot passes through the distributor. Fig. 7 exhibits the mass average filtration efficiency vs. superficial gas velocity at different rotating speeds. The maximum filtration efficiency is below 18%. The filtration efficiency due to the distributor alone is higher at low gas velocity and high rotating speed. It should be noted that after exposure to exhaust for 160 min at low engine load operation, the perforated distributor described above became plugged by soot. This plugging occurs because the soot is wet and contains a soluble organic fraction composed of fuel and lube components. Therefore, filtration efficiency experiments were run before any appreciable plugging of the screen occurred.
Fig. 7. Filtration efficiency of the perforated distributor alone (no bed granules) as a function of soot size at engine low load conditions.
of gas velocity. This charge resulted in a bed thickness of approximately 5 mm. During one set of filtration experiments at 900 rpm, glass beads from the RFBF were sampled every 1y2 h. The samples became darker because the glass beads became progressively covered with soot. Fig. 8 shows the color of glass bead samples, measured by a spectrophotometer, as a function of the total exhaust gas volume passed through the RFBF. The soot mass per unit surface area of the glass beads, also shown in Fig. 8, was measured by thermogravimetric analysis at 700 8C. These independent measurements
5.2. Soot accumulation in the RFBF In a set of experiments using 0.5 kg monodisperse glass beads as the filter medium, the soot concentration was measured at the inlet and the outlet as a function
Fig. 8. Color difference due to soot deposit on filter media and soot mass per collector surface area vs. total engine exhaust volume through the RFBF.
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Fig. 9. Mass average filtration efficiency as a function of superficial gas velocity at different rotating speeds using the perforated distributor.
both show that the mass of soot collected increases with time. It should be noted that transient conditions during engine startup resulted in higher soot loading at the beginning of the run. After steady-state operating conditions are achieved (;30 m3), both curves become linear, as would be expected from theory. 5.3. Filtration efficiency of the RFBF Fig. 9 shows the calculated mass-average filtration efficiencies for three rotating speeds (525, 700 and 900
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rpm). Using the experimental set-up described above, it is difficult to control flow rates, temperature, and soot generation. These factors produce the variability observed in the filtration efficiencies. Our goal is to understand the trends exhibited by these data. To that end, we have added quadratic regression curves for each of the data sets. Below the minimum fluidization velocity, the RFBF is in packed bed mode. When the velocity is further increased, the entire bed becomes fluidized at the critical minimum fluidization velocity (Umfc ). The values of Umfc for the three rotating speeds, calculated from the theoretical analysis given in Qian et al. (1999), are also given in the figure. Also shown in Fig. 9 is the theoretical curve obtained using the previously described model based on correlations developed for packed beds. Since no bypassing due to bubbling is considered, the model shows an increase in filtration efficiency with superficial gas velocity due to the inertial impaction, which increases with increasing Stokes number. However, at very low superficial gas velocity, inertial impaction becomes negligible and collection takes place mainly by interception and diffusion. The diffusion mechanism becomes more effective at very low gas velocities, resulting in the minimum at Ugs0.05 mys predicted by the model. To amplify these points, Fig. 10 shows the filtration efficiency as a function of soot size for a rotating speed of 900 rpm at Ugs0.28 mys. Under these conditions, the bed is fluidized with the gas velocity just exceeding Umfc. Above ;0.7 mm the filtration efficiency increases due to the greater effectiveness of inertial impaction for larger particles. Below ;0.3 mm the efficiency increases due to the enhanced contribution of the diffusion mechanism. In the interval between, a plateau of minimal
Fig. 10. Filtration efficiency as a function of soot particle size.
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efficiency exists due to the diminished effectiveness of both mechanisms. At superficial gas velocities below 0.10 mys, the rotating bed is in the packed bed regime for all three rotating speeds. Thus in this region, comparison to the theoretical curve is instructive. Many phenomena not considered in the model, such as leakage, bypassing, the failure of intercepted particles to ‘stick’, and reentrainment would all result in the measured efficiency being lower than that predicted. Consequently the theoretical curve should provide an upper bound for the measured efficiency, as is the case over the entire range of superficial gas velocity. It is interesting to note that the data in this regime (below 0.10 mys) appear to show a systematic decrease in filtration efficiency with increasing rotating speed. We outline one possible explanation for this effect. It is widely accepted that angular momentum is rapidly transported to the incoming gas as it enters the distributor and the outer bed layer. This notion is supported by the fact that the pressure drop through the bed is nearly independent of rotating speed once the contribution from the distributor is subtracted out (Qian et al., 1998). Thus, with respect to a coordinate frame that rotates at the same angular velocity as the bed, the gas flow is essentially radial through the bed. However, it has been suggested on theoretical grounds that the net pressure drop should exhibit a weak dependence on rotating speed—increasing slightly as the rotating speed is increased, although this effect has been difficult to observe experimentally (Chen, 1987; Kao et al., 1987; Qian et al., 1998). In any case it is likely that, especially in the region of the bed close to the distributor, the mean gas velocity has a small tangential component in the rotating frame that increases relative to the radial component as the bed is rotated more rapidly. The drag on the layer of collected particles due to this component would give rise to a shearing force exerted within this layer. This force would increase as the rotating speed increases and would result in increased re-entrainment and an overall decrease in filtration efficiency. The regression curve for the 525 rpm data exhibits the expected increase in filtration efficiency up to a superficial gas velocity of approximately 0.12 mys. This value is somewhat greater than the velocity at which the bed becomes fully fluidized (Umfcs0.10 mys). As the superficial gas velocity is increased further, the data show a clear decrease in filtration efficiency, departing from the trend of the theoretical curve for a packed bed. It is well accepted that in a fluidized bed that is bubbling, the aerosol in the bubble gas does not contact the collector granules, resulting in the observed filtration efficiency being lower than that predicted for a packed bed (Clift et al., 1981). In our earlier work (Qian et al., 1999), we indeed observed that at 525 rpm bubbling occurs for gas velocities just exceeding Umfc.
For the 700 and 900 rpm runs, we observe that both sets of data display the same trend, namely, a discernible increase, followed by a leveling off. The theoretical curve shows the same trend. For these two higher rotating speeds, we did not reach the point at which the filtration efficiency decreases significantly due to bypassing. It is likely in these runs the bed is bubbling after it becomes fluidized. However, we suspect that the bubbles are smaller due to the increased centrifugal force acting on the bed, which is proportional to the square of the rotating speed. 6. Concluding remarks The experimental results indicate that the RFBF can be used for removal of soot from diesel engine exhaust with a mass filtration efficiency that can exceed 80%. At low rotating speed (525 rpm), the filtration efficiency of the RFBF increases with superficial gas velocity as expected and then decreases after minimum fluidization due to bubble bypassing. At high rotating speeds (700 and 900 rpm), the bypassing appears to be less significant, suggesting that bubble size may be smaller at higher centrifugal force. The proposed model gives an upper bound for the filtration efficiency in a packed bed and exhibits the same trend as the data at the higher rpm. Thus, if the filtration granules are catalytically active for the reaction of NOX with soot, the RFBR has the potential to simultaneously remove soot and NOX from diesel exhaust and to operate as a self-cleaning filter. Acknowledgments This research was supported by the National Science Foundation under Grant No. CTS-9612483. References Arcoumanis, C., Barbaris, L.N., Crane, R.I., Wisby, P., 1994. Evaluation of a cyclone-based particulate filtration system for high-speed diesel engines. Proc. Inst. Mech. Eng. Part D: J. Automobile Eng. 208, 24–34. Baumgard, K.J., Johnson, J.H., 1996. The effect of fuel and engine design on diesel exhaust particle size distributions. SAE Paper No. 960131, pp. 37–50. Charles, J., 1990. Filtration system destroys diesel particulate emissions. Des. News 46, 170–171. Chen, Y., 1987. Fundamentals of a centrifugal fluidized bed. AIChE J. 33, 722–728. Clift, R., Ghadiri, M., Thambimuthu, K.V., 1981. Filtration of gases in fluidized beds. In: Wakeman, R.J. (Ed.), Progress in Filtration and Separation, vol. 2. Elsevier, Amsterdam. Clift, R., 1983. Fundamental processes in gas filtration. Mech. Eng. Trans. 181–190.
G.-H. Qian et al. / Advances in Environmental Research 8 (2004) 387–395 D’Ottavio, T., Goren, L.S., 1983. Aerosol capture in granular beds in the impaction dominated regime. Aerosol Sci. Technol. 2, 91–108. Farrauto, R.J., Heck, R.M., Speronello, B.K., 1992. Environmental catalysts. Chem. Eng. News 70, 34–44. Heck, R.M., Farrauto, R.J., 1995. Catalytic Air Pollution Control: Commercial Technology. Wiley, New York. Johnson, J.H., Bagley, S.T., Gratz, L.D., Leddy, D.G., 1994. A Review of Diesel Particulate Control Technology and Emissions Effects—1992 Horning Memorial Award Lecture, SAE 940233. Kao, J., Pfeffer, R., Tardos, G.I., 1987. On partial fluidization in rotating fluidized beds. AIChE J. 33, 858–861. Kao, J., Tardos, G.I., Pfeffer, R., 1988. Development of a rotating fluidized bed filter. FluidyParticle Separation J. 1, 44–52. Ma, K., Tang, X., Bagyi, I., Shaw, H., Pfeffer, R., Stevens, J.G., 1998. Catalytic Reduction of Nitric Oxide with Solid Reducing Materials. Presented at the Second World Congress on Environmental Catalysis, Miami, FL. Needham, J.R., Faulkner, S.A., Doyle, D.M., 1991. Developing the truck engine for ultra low emissions. Proc. Inst. Mech. Eng. Part D: J. Automobile Eng. 205, 149–159. Paretsky, L., Theodore, L., Pfeffer, R., Squires, A.M., 1971. Panel bed filters for simultaneous removal of fly ash and sulfur dioxide: II. Filtration of dilute aerosols by sand beds. APCA J. 21, 204–209. Pendse, H., Tien, C., 1982. General correlation of the initial collection efficiency of granular filter beds. AIChE J. 28, 677–686.
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