Active Regeneration of Diesel Particulate Filter Employing Microwave

Ind. Eng. Chem. Res. 2009, 48, 69–79

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Active Regeneration of Diesel Particulate Filter Employing Microwave Heating Sameer Pallavkar,† Tae-Hoon Kim,‡ Dan Rutman,§ Jerry Lin,§ and Thomas Ho*,† Department of Chemical Engineering, Department of Industrial Engineering, and Department of CiVil Engineering, Lamar UniVersity, Beaumont, Texas 77710

Wall-flow diesel particulate filters (DPFs) are considered the most effective devices for the control of diesel particulate emissions. A requirement for the reliable operation of the DPFs, however, is the periodic and/or continuous regeneration of the filters. While microwave heating has been considered a potential active regeneration method for the DPFs, past studies on the technology have identified several technical problems leading to filter failure. The problems are mainly associated with the use of inappropriate filter materials for the microwave system and the generation of local hotspots due to uneven microwave heating, resulting in the physical damage to the filters. The objective of this study was to develop and demonstrate the technology employing a microwave-absorbing filter material coupled with an effective waveguide design for the reliable regeneration of DPFs. In this study, a well-equipped diesel emission control laboratory was established to conduct the experiments. The experimental facilities included a 6-kWe diesel generator, an exhaust flow control system, a diesel particulate filter system, a microwave energy supply system, a soot sampling system, a differential-pressure measurement system, and a temperature measurement system. The DPF was a silicone carbide wall-flow monolith filter enclosed in a quartz filter holder. A commercial 1.4-kWe microwave oven was modified to accommodate the quartz holder and a waveguide was engineered to evenly supply the microwave energy to the enclosed filter to achieve filter regeneration. In the experiments, the diesel engine exhaust was lined up to flow through the filter with a fixed flow rate. The microwave regeneration was triggered after a specific amount of soot loading was reached based on the differential pressure drop reading. The results have indicated that the designed system has been able to achieve uniform temperature profiles both in the radial and the vertical DPF positions. The off-line regeneration of DPF by microwave energy has been observed to be highly efficient in terms of energy consumption and regeneration efficiency. The DPM filtration efficiency has remained comparably high after 150 cycles of filtration/regeneration with no apparent physical damage to the DPF being observed. The on-line microwave regeneration of the DPF, however, is not as efficient as the off-line regeneration due to the insufficient oxygen concentration in the engine exhaust stream. 1. Introduction Diesel engines are superior to conventional gasoline engines with regard to fuel consumption, which reduces CO2 emissions and helps suppress global warming.1 These engines, however, produce more NOx as well as particulate matter emissions as compared to the gasoline engines. In response to the environmental and health concern from the emission of diesel particulate matter (DPM),2,3 the United States Environmental Protection Agency (US EPA) has continued to impose more stringent regulations restricting its emissions. The new 2007 standard for on-highway heavy duty diesel engines sets an emission limit of 0.01 g/bhp-h, which is 10 times more stringent than the 2005 standard of 0.1 g/bhp-h.4 Control of DPM emissions from diesel engines has been a challenging issue. Wall-flow diesel particulate filters (DPFs) are currently considered the most efficient control devices for DPM.5 The filtration mechanism of the DPFs involves forcing the DPMcontaining exhaust gases to flow through the porous walls as they enter the DPF channels from the front end, trapping the DPM (or soot) in the porous walls or depositing it on the wall surfaces, and exiting out as cleaned exhaust gases through the * To whom correspondence should be addressed. E-mail: Thomas.ho@ lamar.edu. Tel.: 409-880-8790. Fax: 409-880-2397. Address: Department of Chemical Engineering, Lamar University, P.O. Box 10053, Beaumont, TX 77710. † Department of Chemical Engineering. ‡ Department of Industrial Engineering. § Department of Civil Engineering.

open channels at the back end.6 However, with the accumulation of the DPM in the wall media, the pressure drop across the filter increases which in-turn exerts a back pressure on the engine exhaust, resulting in gradually poorer performance of the diesel engine in terms of increased CO emissions, poor fuel efficiency, and increased DPM production. Therefore, a requirement for reliable operation of DPFs is the effective regeneration of the filters by continuously or periodically burning off the trapped DPM.7-10 Two methods have been developed to achieve DPF regeneration.11 One is the passive regeneration which requires the use of oxidation catalysts upstream from the DPF to convert NO to NO2 and uses the converted NO2 to oxidize the trapped DPM in the DPF in the presence of catalysts.12,13 Since the DPM oxidation reaction can occur below 270 °C with the presence of the catalysts, no additional energy is needed to achieve the regeneration. However, the method is ineffective under high load conditions where the oxygen content in the exhaust is not sufficient enough for sustained DPM combustion.14 The other DPF regeneration method, termed active regeneration, involves the supply of additional energy to burn off the trapped DPM at a temperature higher than 450 °C.15-17 Since the diesel exhaust system cannot generate such a high temperature continuously, additional energy must be used to raise the DPF temperature to accomplish the burning. The energy may come from additional fuel combustion, electric furnaces, electric heating elements, or microwave irradiation. Although effective, two concerns are associated with active

10.1021/ie800780g CCC: $40.75  2009 American Chemical Society Published on Web 10/11/2008

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Table 1. Dielectric Properties of Various Materials material diesel soot23 quartz20 cordierite23 alumina ceramic Al2O320 silicon carbide SiC20

dielectric constant ′ dielectric loss factor ′′ 10.695 3.78 2.873 8.9 30

3.561 0.001 0.138 0.009 11

regeneration, one is the consumption of additional fuel and the other is the potential damage to the DPFs due to high thermal gradients induced by rapid DPM oxidation. Additional fuel combustion mainly involves the hydrocarbon source such as the diesel fuel itself which is mixed with the atomized air from a pressure reservoir of the vehicle, introduced in a retrofitted combustion chamber, and ignited by means of ignition electrodes.18 The key feature of this technology is the development of DPM-free flame irrespective of the engine operating conditions, which involves compressed air as the source of oxygen for clean combustion in the burner. However, this method essentially results in an added fuel penalty and increases the complexity of the system in terms of the engine control unit, retrofitting, and sensor management.19 The electric regeneration method involves the heating of the exhaust gases or the regeneration air by a resistive heater to reach DPM ignition temperatures and trigger the regeneration. The heater is placed upstream to the DPF where the energy from the heating element is deposited to the exhaust stream thus elevating its temperature to the DPM ignition temperatures. For automotive applications applying this strategy on-board means additional power consumption for heating the electric resistive element which in-turn has to be drawn from the battery of the automobile. This puts extra load on the engine and hence results in a relative higher fuel penalty.19 Microwave heating is very selective and can be an efficient energy source in many applications.20 With the choice of proper material as the filter substrate, it can easily provide high temperatures needed for DPM combustion and DPF regeneration.21-23 The extent of microwave energy absorption by the filter material depends on its dielectric properties (especially the dielectric loss factor). Table 1 illustrates the various dielectric properties of some of the common materials. With high dielectric constant and loss factor such as SiC, the material can readily absorb the microwave energy and convert it into heat energy. As indicated in Table 1, an additional advantage of using microwave-assisted DPF regeneration is its ability to deposit energy directly to the DPM (or soot) which is collected in the DPF. With the SiC being the filter material, the microwave energy can heat up both the DPM and the SiC DPF and trigger the regeneration process without the requirement of heating up the exhaust gases. Microwave-assisted DPF regeneration, however, has been experienced with uneven energy distribution and regeneration patterns, which results in hotspots during the exothermic DPM oxidation reactions.24 2. Objective The objective of this study was to carry out experiments to demonstrate the use of microwave heating for DPF regeneration involving SiC as the filter material for its excellent dielectric properties in converting microwave energy to heat energy. A metallic waveguide was designed to ensure uniform distribution of the microwave energy throughout the volume of the DPF. Specifically, the objectives of this study were to conduct experiments to (1) characterize the designed waveguide for the efficient use and uniform distribution of

the supplied microwave energy, and (2) demonstrate the efficiency of the microwave DPF regeneration process involving SiC as the filter material. 3. Experimental Details A diesel emission test unit equipped with a diesel generator, an exhaust flow system, a SiC DPF, a DPM sampling system, a differential pressure measurement system, a temperature measurement system, and an on-line data acquisition system was established to conduct the experiments. A 1.4-kWe commercial microwave oven was modified and used as the microwave generator. A schematic diagram of the experimental facilities is shown in Figure 1. 3.1. Diesel Generator and Exhaust Flow System. A 6-kWe generator driven by a single-cycle diesel engine (Lombardi model 15LD 400) was used in the experiments. The engine exhaust pipe was connected to the DPF through a flow control valve (Badger Meter) and a flow meter (Asea Brown Boveri). A picture showing the generator and the flow control system is shown in Figure 2. The system allowed various resistive loads in terms of back pressure and various generator loads. The default generator load was 36.4 A, which was equivalent to 60% of generator capacity. During the regeneration experiments, the engine exhaust was routed through the DPF until the pressure drop across the filter reached a preset critical value for regeneration. The microwave was then turned on for on-line regeneration. For off-line regeneration, the engine exhaust to the DPF was switched to an air stream and the microwave was turned on for DPM oxidation. In such regeneration, the engine exhaust was routed to bypass the DPF and vent out through the exhaust hood. The diesel fuel (Citgo Clear No. 2) for the experiments was purchased in bulk from a local supplier to ensure the consistency of fuel quality. 3.2. Diesel Particulate Filter and Microwave. A silicon carbide (SiC, Ceramic Techniques et Industrielles) wall-flow monolith filter (50 mm diameter × 150 mm length, cell density ) 150 cpsi, pore size ) 20 µm) was enclosed in a custom-made quartz holder (Technical Glass Products), which was insulated and sealed by using Fiberfax alumina blanket and Interam mat (3M). A schematic diagram of the DPF filter/ holder assembly is shown in Figure 3. Two high temperature flange gaskets were also used as a seal between the filter element and the holder. Some of the important properties of the SiC filter used in our experiments are listed in Table 2. The microwave oven employed was a 1.4-kWe Sharp consumer microwave oven. It was modified with holes drilled in its top and bottom surfaces to accommodate the filter assembly. A waveguide was designed and installed in the oven to effectively direct microwave energy to the SiC filter. Figure 4 shows a picture of the microwave oven with the waveguide assembly in it. A schematic diagram of the waveguide assembly is shown in Figure 5. 3.3. DPF Temperature Measurement. The DPF filter was fitted with three thermocouples composed of three thin gauge thermocouple wires (J-type with 0.8 mm in diameter) with the wire leads placed inside the SiC channels both in the vertical and radial directions. For the vertical temperature profile, the three thermocouple leads were placed at depths of 50, 75, and 100 mm, respectively, at the center of the filter. In the case of radial temperature profile, the three leads were placed at three radial locations at a fixed depth of 75 mm. 3.4. DPM Sampling. The DPM sampling system consisted of sample selector valves, a sample holder (SKC model LS47), and a sampling pump (SKC model Hi Lite 30) with a

Ind. Eng. Chem. Res., Vol. 48, No. 1, 2009 71

Figure 1. Schematic diagram of the diesel test facilities.

Figure 2. Assembled microwave diesel emission test unit.

rotameter to maintain a sample flow rate of 30 cm3/min. The valves and the pump were integrated-controlled by the switches located on the control panel to ensure the right sampling sequence. The sample lines and the sample holder were wrapped with electric heating tapes to maintain tem-

perature at about 90 °C to avoid the condensation of water in the lines and the PM holder. After collecting PM for a preset time, the sample holder was allowed to dry in a desiccator for 24 h and the filter paper was weighed by a microbalance.

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Figure 4. Installed waveguide in the microwave oven.

Both single and multiple cycles of filtration and regeneration data were logged every second using a data acquisition system designed for the experiment (N.I. LabVIEW). Figure 3. Schematic diagram of the quartz filter holder.

4. Results and Discussion

Table 2. DPF Filter Specifications properties

value

material CPSI coding cell size, wall width (mm) wall thickness (mm/1000/inch) filtration efficiency (%) clean filter-PM10 filtration efficiency (%) 10% loaded filter specific SiC weight, massive material density (kg/dm3) specific DPF weight, porous wall density (kg/dm3) monolith weight, bulk density (kg/dm3)

100% SiC 150 1.6 × 1.6 12-15 >98 >99 3.2 1.8 0.85

3.5. DPF Regeneration Experiments. The facilities described in Figure 1 were involved in the DPF regeneration experiments, including both off-line and on-line regeneration. In a regeneration experiment, the diesel engine was turned on and the DPF was loaded with DPM under constant exhaust flow rate conditions, e.g., 83.3 L/min. The DPF differential pressure drop was continuously recorded during the loading process. The regeneration process was triggered when the recorded pressure drop reached a preset critical value, for example, 50 in. of water (12.451 kPa). In the case of on-line regeneration, the microwave oven was turned on at this time and the regeneration was initialized while the DPM loading on the DPF was continued during the regeneration process. The microwave was then turned off when the differential pressure of the DPF returned to the clean status. In the case of off-line regeneration, when the differential pressure drop reached the preset regeneration pressure, the microwave oven was turned on and the engine exhaust gas was switched to a pure air stream at a smaller flow rate (10 L/min) for DPM oxidation. In this off-line regeneration, the engine exhaust stream was routed to bypass the DPF and vent to the hood without being treated. The microwave was turned off after the regeneration was completed, and the air stream was switched back to the engine exhaust for another cycle of DPM loading. The experiment may continue for many filtration-regeneration cycles depending on the design of the experiment. For a typical set of design, the experiment involved 10 filtration/regeneration cycles. An emissions analyzer (Testo 350-XL) was used to measure simultaneously the concentrations of O2, NOx, and CO during the regeneration process. In addition, on-line CO2 measurements were taken with a CEA Instruments GD444 portable analyzer.

The experimental results are reported in this section, which include DPF Temperature Profile, Characteristics of DPM Loading, Off-Line DPF Regeneration, and On-Line DPF Regeneration. 4.1. DPF Temperature Profile. The temperature profiles along the radial and vertical directions of the DPF were measured when the microwave system was turned on with and without the designed waveguide. Typical results are shown in Figures 6-8. The results shown in Figure 6 indicate that SiC is a good conducting material capable of absorbing microwave energy and distributing the absorbed energy efficiently across the entire DPF body as observed in its relatively uniform temperature profiles at three radial locations. With the installation of the designed waveguide, the results shown in Figures 7 and 8 strongly indicate that the recorded temperatures are substantially higher than those without the waveguide as shown in Figure 6. It is obvious that the designed waveguide has achieved its purpose by raising the energy efficiency by at least 30% while the temperatures across the entire DPF body remain relatively uniform. 4.2. DPF Particulate Loading. The loading of DPM in the DPF is a three-stage dynamic process as shown in Figure 9. The first stage (region 1) is the initial loading phase during which the DPM starts to fill the voids inside the DPF filtration walls, resulting in a fast increase of differential pressure drop across the DPF. Here, the fast-rising of the pressure drop is mainly caused by the filter wall permeability and porosity.6 Particulates disperse deep inside the filter-wall pores and form pore bridges thus causing a significant decrease in filter-wall porosity and increase in the pressure drop. During this stage, filter properties such as permeability and porosity change dynamically. With the wall filtration approaching its saturation state, the wall porosity and permeability in turn approach their saturated values and the filtration process starts to develop to the next stage where the DPM layer is forming on the filter-wall surfaces. This second stage is termed the transitional DPM layer formation stage where the DPM permeability is changing with time slowly and the DPM layer starts to form on the wall surfaces (see region 2 in Figure 9). 6,8 Once a critical mass of DPM has accumulated on the wall surfaces of the filter channels, a definite particulate layer of DPM starts to build up and this itself acts as a filter


Figure 5. Schematic diagram of the waveguide assembly.

Figure 6. DPF temperature profiles in different radial locations (without a waveguide). Table 3. Constants Used for Equations 1 and 2 Calculations constant

description

value

D Dp F H L U ww µ

DPF diameter wall pore diameter friction factor cell width channel length inlet cell entrance velocity dpf wall thickness exhaust gas viscosity

0.05 m 2.0 × 10-5 m 14.23 1.6 × 10-3 m 0.15 m 2.377 m/s 0.305 × 10-3 m 2.95 × 10-8 kPa-s

(constant DPM layer filtration). At this stage (see region 3 in Figure 9), the filter-wall permeability and porosity have reached their saturation values and the filter is in a steady-state filtration stage where the DPM layer continues to grow with a relatively constant density.

Figure 7. DPF temperature profiles in different radial locations (with a waveguide).

For the first two stages of DPF filtration, the following equation was proposed to describe the pressure drop across the DPF: 6 ∆Pw ) (µUHww ⁄ 4Lkw) + (2µFUL ⁄ 3H2)

(1)

In the above equation, the first term calculates the pressure drop due to the porous filter wall, which depends on the wall layer permeability (kw), filter wall thickness (ww), cell entrance velocity (U), and cell dimensions, and the second term calculates the pressure drop due to the frictional losses of the flow of the exhaust gases through the filter channels. This correlation implies that the filter pressure drop is a function of the physical properties of the filter as well as the properties of the engine

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Figure 8. DPF temperature profiles in different vertical locations (with a waveguide).

Figure 10. Wall-layer permeability during DPM loading at four filtration/ regeneration cycles corresponding to the differential pressure drop results shown in Figure 11 (exhaust flow: 83.3 L/min).

with the DPM layer thickness, which is a function of time. The following equation was proposed to describe the pressure drop as a function of the DPM layer thickness (wp) for this stage:6 ∆Ps ) (µUHwp ⁄ 4Lkp)

(3)

wp ) M(t) ⁄ (AfFs)

(4)

with

Figure 9. DPF pressure drop vs time in three consecutive stages (engine exhaust flow rate: 83.3 L/min).

exhaust gas such as its viscosity, temperature and flow rate. Given the measured pressure drop, the above equation can be used to calculate the wall layer permeability, that is, kw, during the two filtration stages. The corresponding filter-wall porosity can be calculated by using the following empirical correlation in the literature: 6,7 kw(t) ) [ε(t)5.5Dp2] ⁄ 5.6

(2)

where the wall layer permeability, kw, was empirically proposed to be a function of the wall layer porosity (ε) with respect to time and the pore diameter, Dp. Figure 10 shows the variation of filter-wall permeability for multiple cycles of DPF loading experiments corresponding to the results of the measured pressure drop shown in Figure 11. These permeability values were calculated based on equation 1 described above and the constants used in the calculations are summarized in Table 3. The calculated wall permeability values shown in Figure 10 indicate that the permeability reduces by 94% during the initial stage (region 1) of DPM loading and reaches its saturation value during the second stage (region 2) of loading. It remains constant during the third stage (region 3) of loading. The corresponding values of filter-wall porosity calculated on the basis of equation 2 are shown in Figure 12. The results indicate that the wall porosity drops from 0.33 to 0.22 during the initial stage of DPM loading and reaches a saturated value of about 0.18 during the filtration process. It is worth pointing out that, in the third stage of the DPM loading, the pressure drop across the filter increases linearly

It should be noted that, during this stage, the DPM layer permeability (kp) does not vary appreciably with time and the DPF pressure drop depends mainly on the DPM layer density (Fs), DPM layer thickness (wp), mass of DPM trapped in the filter (M), and the filtration area (Af). In this stage, the DPF is considered to have reached its steady filtration operation, and it is predominantly the stage associated with DPF regeneration. It should be noted that the total pressure drop across the DPF (∆PT) at any given time is the sum of the clean filter pressure drop (∆Pc), pressure drop due to wall flow filtration (∆Pw), and pressure drop due to DPM layer filtration (∆Ps) given below: ∆PT ) ∆Pw + ∆Ps + ∆Pc

(5)

where ∆Pc depends on the clean filter permeability, wall layer pore diameter, cell density, and channel length. It can be measured experimentally. 4.3. Off-Line DPF Regeneration. As described in the experimental section, the experiments for microwave DPF regeneration involved both off-line and on-line operations. In the offline regeneration, when the pressure drop across the DPF reached a predesignated value, the engine exhaust stream was switched to bypass the DPF and replaced with an air stream for DPM oxidation, while the microwave oven was simultaneously turned on to heat up the DPF and the trapped DPM. The two streams were switched back after the regeneration was completed. In the on-line regeneration, when the pressure drop across the DPF reached a predesignated value, the microwave oven was turned on and both the regeneration and the filtration were simultaneously occurring in the DPF. The microwave oven was turned off after the regeneration was completed. Typical experimental observations corresponding to the off-line regeneration are reported below. Figure 13 shows a typical set of results for an off-line microwave regeneration of a cold DPM-loaded DPF in an air stream. The figure shows profiles of five measured parameters, namely, the concentrations of O2, NOx, and CO, temperature, and pressure drop plotted against operation time after the microwave was turned on. The recorded temperature profile


Figure 11. Plot of CO concentration/temperature/pressure drop vs time for an off-line four-cycle filtration/regeneration operation (exhaust flow: 83.3 L/min).

Figure 12. Wall porosity during DPM loading at four filtration/regeneration cycles corresponding to the differential pressure drop results shown in Figure 9 (exhaust flow: 83.3 L/min).

indicates that, because of the microwave energy, the DPF temperature rises from the initial temperature of 25 °C to about 350 °C where a CO peak appears indicating the onset of the DPM oxidation process. Although it is not shown, the CO2 peak follows the CO peak when the temperature is higher. Also shown in the figure is the formation of NOx after the CO peak due to an increase in temperature. The temperature is seen to continue to climb up to about 660 °C after 10 min of the regeneration process mainly because of the heat of combustion due to the DPM oxidation as well as the continuous addition of microwave energy. It then cools down after the combustion is completed and the microwave turned off. In the mean time, the pressure curve indicates that it increases slightly at the beginning of the microwave heating process due to the increase in temperature. The pressure continues to rise with the temperature until after it passes the onset of DPM oxidation, where it reaches

its peak and then starts to decrease. The decrease is mainly because of the combustion of solid diesel particulates into gaseous CO and CO2, while in the process, opens up the blocked DPM pores and results in the lower pressure drop. The pressure curve is seen to drop down to the initial pressure reading after the regeneration is completed. The off-line regeneration experiments associated with multicycle filtration/regeneration operations were also performed. A typical set of such results are shown in Figures 11 (referred to previously) and 14. The results shown in Figure 11 indicate that the differential pressure across the DPF increases during the filtration process where the temperature is around 260-280 °C and no CO is observed. When the differential pressure reaches the designated readings (64 in. of water or 15.937 kPa for the first regeneration and 50 in. or 12.451 kPa thereafter as indicated in the figure), the exhaust gas stream was switched off and the air stream switched on with the observed pressure immediately dropped down to about 4 in. of water (0.996 kPa) mainly because of the much slower air flow rate, 10 L/min for the air stream vs 83.3 L/min for the exhaust gas stream. At these moments, for example, at 33, 75, 128, and 130 min, the microwave oven was simultaneously turned on and the regeneration operation was initialized as shown in Figure 11. The temperature and CO peaks are observed in the figure similar to those shown in Figure 13 and the pressure reading is seen to reduce to about 2 in. of water (or 0.498 kPa) after the regeneration is completed within about 5 min. The engine exhaust stream was then switched back on, and the filtration process for the next cycle was started. It has been observed that the DPM filtration efficiency has remained to be comparably high (>90%) after 150 multicycles of filtration/regeneration operations and no apparent physical damages to the DPF have been observed. It should be noted that the corresponding experiment results shown in Figure 14 indicate that the NOx

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Figure 13. Plot of concentration of O2/NOx/CO/temperature/pressure drop vs time for an off-line single-cycle filtration/regeneration operation (exhaust flow: 83.3 L/min).

Figure 14. Plot of concentration of O2/CO/NOx vs time for an off-line four-cycle filtration/regeneration operation (exhaust flow: 83.3 L/min).

peaks are also observed, similar to that shown in Figure 13 during the single-cycle experiment. It is worth pointing out that, during the multicycle off-line regeneration, the time required for microwave heating for completing a regeneration cycle is only 5 min because of the DPF being at a relatively high temperature already heated by the engine exhaust gas. An attempt was made to compare the microwave regeneration processes with 5 and 10 min of

microwave heating and the results indicated that they are almost identical as shown in Figures 15 and 16. The observation appears to suggest that the microwave energy is only needed to heat up the DPM to the ignition temperature, and after that, the released heat from the DPM combustion will support the continuous burning of the remaining DPM without the need for additional microwave energy. This observation is significant in the efficient use of microwave energy for DPF regeneration.


Figure 15. Plot of concentration of O2/NOx/CO/temperature/pressure drop vs time for one regeneration cycle of an off-line multicycle filtration/regeneration operation with 5-min microwave heating (exhaust flow: 83.3 L/min).

Figure 16. Plot of concentration of O2/NOx/CO/temperature/pressure drop vs time for one regeneration cycle of an off-line multicycle filtration/regeneration operation with 10-min microwave heating (exhaust flow: 83.3 L/min).

4.4. On-Line DPF Regeneration. Although the off-line microwave regeneration appeared to be effective as reported above, the on-line microwave regeneration has not shown to be as promising. A typical set of such results is shown in Figure 17, where O2, NOx, CO, temperature, and pressure drop are plotted

against the operation time. As indicated, during the operation from 80 to 124 min, the pressure increases steadily from 40 to 50 in. of water (or 9.961 to 12.451 kPa) with temperature remaining at about 128 °C and CO at a low value. When the pressure reaches 50 in. of water (12.451 kPa) after 124 min,

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Figure 17. Plot of concentration of O2/NOx/CO/temperature/pressure drop vs time for an on-line regeneration experiment (exhaust flow: 16.7 L/min).

the microwave was turned on, and the temperature started to rise while the pressure also increased because of the increase in temperature. The first CO peak then appeared indicating the onset of DPM oxidation at a relatively lower temperature followed by another CO peak at a higher temperature due to additional DPM oxidation at a higher temperature but without sufficient supply of oxygen. The temperature was seen to peak at about 780 °C and then it dropped because of less intensity in DPM combustion, although the microwave was still on. The pressure appeared to have a step drop corresponding to the DPM combustion; however, the drop was not significant indicating the incomplete regeneration due to insufficient oxygen supply in the engine exhaust gas, that is, 5% in on-line regeneration (see Figure 17) as compared to 18% in off-line regeneration (see Figure 15). When the microwave was turned off after 149 min, the temperature started to drop substantially but the pressure only decreased slightly confirming that the regeneration was not as complete as desired. The lack of oxygen in the engine exhaust stream is considered the main reason for this incomplete regeneration. Further research is needed to investigate this problem associated with the on-line microwave regeneration of DPF. 5. Conclusions An experimental study has been carried out to demonstrate the effectiveness of DPF regeneration employing microwave energy. A well-equipped diesel emission control laboratory was established to conduct the experiments. The experimental facilities included a 6-kWe diesel generator, an exhaust flow control system, a diesel particulate filter system, a microwave energy supply system, a soot sampling system, a differentialpressure measurement system, and a temperature measurement system. The DPF tested was a silicone carbide wall-flow monolith filter (50 mm diameter × 150 mm length, cell density ) 150 cpsi, pore size ) 20 µm) enclosed in a quartz filter holder. A commercial 1.4-kWe microwave oven was modified to accommodate the quartz holder, and a waveguide was engi-

neered to evenly supply the microwave energy to the enclosed filter to achieve filter regeneration. In the experiments, the diesel engine exhaust was lined up to flow through the filter with a fixed flow rate. The microwave regeneration was triggered after a specific soot loading was reached based on the differential pressure drop reading. The results have indicated that the designed system has been able to achieve uniform temperature profiles both in the radial and the vertical DPF positions. The off-line regeneration of DPF by microwave energy has been observed to be highly efficient. The DPM filtration efficiency has remained to be comparably high after 150 cycles of filtration/regeneration with no apparent physical damage to the DPF being observed. The on-line microwave regeneration of the DPF, however, is not as efficient as the off-line regeneration due to the insufficient oxygen concentration in the engine exhaust stream. Further research to address the low oxygen concentration problem is required to develop the on-line microwave regeneration process. Acknowledgment The funding support for this project from the Texas Commission on Environmental Quality through the New Technology Research and Development (NTRD) Program (Grant No. 5825-70807-0007) is gratefully acknowledged. Nomenclature Af ) Filtration area, m2 Dp ) pore diameter, m F ) friction factor, dimensionless H ) cell width, m kp ) soot layer permeability, m2 kw ) wall layer permeability, m2 L ) channel length, m M ) mass of soot trapped, kg ∆Pc ) clean filter pressure drop, kPa ∆Ps ) filter pressure drop due to soot layer filtration, kPa

Ind. Eng. Chem. Res., Vol. 48, No. 1, 2009 79 ∆PT ) total filter pressure drop, kPa ∆Pw ) filter pressure drop due to wall filtration, kPa U ) cell entrance velocity, m/s ′ ) dielectric constant, dimensionless ′′ ) dielectric loss factor, dimensionless ε ) filter wall porosity, dimensionless µ ) exhaust gas viscosity, kpa-s ww ) cell wall thickness, m wp ) DPM (soot) layer thickness, m Fs ) DPM (soot) layer density, kg/m3

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ReceiVed for reView May 15, 2008 ReVised manuscript receiVed August 24, 2008 Accepted August 27, 2008 IE800780G

Active Regeneration of Diesel Particulate Filter Employing Microwave

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