plasma technology for diesel exhaust aftertreatment

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					1. INTRODUCTION
Today world is facing dangerous problem of „Air Pollution‟. The major cause is obviously emissions from automobiles. With the intentation to protect public health. Several nations are issuing stricter and stricter regulations for both solid and gaseous exhausts. In particular the trend is to restrict the limit of Particulates and NOX emissions. Diesel engines are reliable, fuel efficient, durable, easy to repair, inexpensive to operate, less CO & HC. However their future applications will greatly depend on the successful solutions of their NOX and Particulate Matter (PM) emissions. As the two pollutants are difficult to controlled simultaneously by only one of the control measures. To assure survival of diesel engines, research work is going on to find effective solutions to control diesel exhausts, especially Particulates & NOX. PLASMA-the Fourth State of Matter
[5]

has

found

increasingly wide applications in various industries. Some of them are especially for treatment of Particulates & NOX. However very few are concerned with the treatment of diesel Particulates. In this paper, Plasma technology is discussed for exhaust aftertreatment. Corona Discharge Device (CDD) The Corona Discharge Device (CDD) significantly reduces HC, CO and Nox emissions as well as NOX are discussed here.
[6]

and

non thermal Plasma (NTP) Reactor [3] which removes both Particulates

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2. THEORY BEHIND PLASMA TECHNOLOGY 2.1 WHAT IS PARTICULATE?
Particulates are results of incomplete combustion of diesel engines & lubricating oil. Anything collected on plastic coated fibre glass test filter at temperature below 125° F (51.7° c) and excluding condensed water droplets is known as diesel particulates. Nature of particulates: a) solid. b) Liquid compounds. Size: single particulate – about 0.01um Large chain – about 10-30 um [3] Composition: Diesel particulate = Black smoke soot + HC. Effects of Diesel Particulates 1) 2) 3) 4) 5) Aggregate Chronic Lung Problems of Bronchis and Emphysema, Effect Mortality and Morbidy, Affect visibility or opacity, Mutagenic, Most adverse effect is those particulates are carcinogenic.

Causes of Particulate Emissions 1) 2) 3) 4) 5) Interaction of Unburnt mixture with CC Wall. Poor Atomization. Local fuel rich or lean zone. Late Injection. Incomplete Combustion of fuel.

Control Methods: – 1) 2) 3) To improve CC design & fuel Injection. To Control Engine parameters. Exhaust Aftertreatment. 2

Due to optimization of Diesel Engines, first two methods are not suitable. The third way seems not only easiest to follow in future but especially the only one assuring survival of C.I. Engines as such. However; the problems to be solved in order to get effective filtering systems are considerable. In particular, it must work automatically, suitably without failure for long periods & have acceptable cost. Current aftertreatment methods: The systems for particulate removal can be grouped as A. Particulate Traps or Filters. E.g. Ceramic traps or Steel Wool Filters. etc. Limitations: 1. 2. 3. Increase in back pressure. Low filtration efficiencies. Limited capacity per unit volume.

B. Regenerators: The accumulations of particule on the trap cause a progressive increase of back pressure. The convenient method is to carry an effective oxidation process using regenerators it requires high temperature. Methods of Regeneration: A. Increasing trap temperature system. 1. External energy system. 2. Throttling regenerators. B. Reduction of Activation energy 3. Catalytic fuel additives. 4. Catalytic converters. Limitations: 1. Poor durability. 2. Thermal stresses cracking & melting of substrate. 3. Increase in back pressure decreases oxidation efficiency. 4. Lead is carcinogenic.

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2.2 WHAT IS PLASMA?
Plasma is often called as “Fourth State of Matter”, the other three being solid, liquid, and gas. Sir William Crookes, an English physicist, identified a fourth state of matter now called as Plasma, in 1879[5]. Plasma in stars and. Plasma in the stars and in the tenuous space between them makes up over 99% of the visible universe and perhaps most of that which is not visible. The Plasma state is gaseous collection of electrically charged particles with nearly equal numbers of negative particles and positive ions. Though they can exist at low temperature, they are typically come in existence when gases are heated at very high temperature. In ordinary gas each atom contains an equal number of positive and negative charges; the positive charges surrounded by an equal number of negatively charged electrons, and each atom is electrically neutral. A gas becomes Plasma when the addition of heat or other energy causes a significant number of atoms to release some or all of their electrons. The remaining part of those atoms is left with a positive charge and the detached negative electrons are free to move about. Those atoms and resulting electrically charged gas are said to be “ionized”. When enough atoms are ionized to significantly affect the electrical characteristics of the gas, it is “Plasma”. Plasma can be accelerated and steered by electric and magnetic fields, which allows it to be controlled and applied. Plasma research is yielding a greater understanding of the universe.

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3. WORKING OF PLASMA TECHNOLOGY 3.1 CORONA DICHARGE DEVICE
The Corona Discharge Device, called the CDD, was developed to increase the in-use efficiency of the exhaust gas aftertreatment system by mitigating the negative effects of undesirable material in the exhaust gas of the engine. The CDD permits the vehicle‟s catalyst to remain highly efficient during the full life of the vehicle. This allows the vehicle to meet in-use compliance regulations both during any emission test and also, during it‟s actual on-road use with a minimum of precious metals in the catalyst. 3.1.1 THE CDD SYSTEM. [6] 1. It converts low voltage direct current battery power to a high voltage high frequency supply. This is done in the power module. 2. The high voltage high frequency supply is delivered through the cable to an electrode to an electrode of a barrier discharge device, which has alternate electrode and dielectric barriers. as shown in figure 3.1. 3. The corona generated through the dielectric by means of this barrier discharge creates non-thermal Plasma in the exhaust gas. This takes place in the CDD itself as seen in Figure 3.2. The corona is found in the gap between the dielectric material and the ground. 4. The exhaust gas is directed, at a significant space velocity, through the corona so as to alter the exhaust gas constituent chemistry, creating radicals. 5. The radicals continue to be generated in a continuation reaction sequence that combine with and prevent material, such as sulfur, from contaminating the precious metal in the catalyst, keeping the catalyst more highly efficient. 5

3.2 The CDD PROCESS. [3] 1. Point Source Generation of Highly Active Species
A fraction of the exhaust flow (between 5% to 20% depending on flow conditions passes through the electric discharge region of the CDD. gas. The CDD produces radicals, such as Hydroxyl (OH), and other Oxidizing species, such as O3 and NO2 in the exhaust

2. Gas Phase Reactions Enable Radicals to Reach the Catalyst
Downstream of the CDD, various radicals, such as OH, generated by the CDD initiate gas phase reactions that generate oxidized hydrocarbon species (HCO), convert NO to NO2, and subsequently regenerate OH radicals. This reaction mechanism enables short-lived radicals, which are generated at the CDD, to reach the catalyst surface.

3. Radical Species Change the Catalyst Surface and Enhance Catalytic Activity
Fuel sulfur, even at levels as low as 30 ppm, is a significant poison to the catalytic activity of platinum group metals (PGM) such as Pt, Pd, or Rh. catalytic activity is a strong function of the surface coverage of the PGM sites. Small changes in the sulfur surface coverage of the catalyst, when the coverage is in the range of 15% 25%, significantly alters the overall catalytic activity. The OH and NO2 produced by the CDD, and propagated by subsequent gas phase reactions, act to reduce the sulfur coverage. It shows that both OH and NO2 react with the sulfur on the PGM surface and with nearby oxygen, to liberate the sulfur to the gas phase in the form of SO or SO2 . Such reaction mechanisms are known as a Rideal-Eley process.

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Figure 3.1 CDD Cross Section

Figure 3.2 Exhaust flow and process model.

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3.2 NON THERMAL PLASMA REACTOR
Full scale packed NTP prototype systems (active volume 2600 cm6) were tested on a vehicle and PM removal was determined by gravimetric, opacity, ultrafine particulate and PAH measurement. A schematic of the NTP reactor design can be seen in Figure 3.3 The exhaust gas from diesel engine is passed through perforated inner electrode. The gas then moves in region of Plasma between inner amounts outer electrode over the packing material such as ceramic bed, soot etc. and then goes out through rear portion of reactor. It is seen by introducing a packing material in NTP reactor it is possible to increase the residence time, which allows complete oxidation to be achieved more economically than previously predicted. It happens because certain species from the exhaust can adsorb or trap and be removed from the gas phase. Experiments were performed to confirm the NTP oxidation of diesel soot in packed reactors and to explore the mechanisms involved in the process. NTP reactors had the capability of being operated empty, or being filled with packing material such as ceramic beads, fibre filters, foams or monoliths as desired. The vehicle used in the tests was a 1994 model LTI Fairway London Taxi equipped with a Nissan 2.7l IDI diesel engine In the vehicle experiments the Plasma system was fitted in place of the vehicle's original muffler and was designed to have a similar backpressure characteristic over the operating load and speed range of the engine. The pressure drop across the packed NTP system was

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20 mbar at 1500 rpm full throttle increasing to 90 mbar at 3500 rpm full throttle.

Figure 3.3 Photography of plasma reactor

Figure 3.4 Cut-away of the NTP reactor

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4.EXPERIMENTAL RESULTS AND DISCUSSIONS
4.1 EXPERIMENTAL RESULTS FOR CORONA DICHARGE DEVICE
[6]

(A) Table 4.1 illustrates percentage reductions in hydrocarbons (HC), carbon monoxide (CO) and oxides of nitrogen (NOx) when a Litex CDD is used with fuel having sulfur content from 0 to 300 parts per million. (B) Figure 4.1 illustrates additional reductions in pollutants when a catalytic converter, poisoned by driving 50,000 miles exhaust (C) The following figures illustrate automobile exhaust emission levels across varying fuel-sulfur levels during engine testing of the Corona Discharge Device. In every case, the top line represents exhaust emissions with the CDD turned off, while the bottom line represents emissions with the CDD turned on. Figure 4.2 indicates that the CDD can reduce hydrocarbons by 33 percent even in fuel that contains zero sulfur. Figure 4.3 indicates that the CDD can reduce oxides of nitrogen by more than 50 percent. Figure 4.4 indicates that a CDD-equipped automobile running on fuel with a sulfur level of 100 ppm emits the same amount of carbon monoxide as an unequipped car running on 30 ppm. Fuel sulphur level Parts Per Million 0 30 100 300 10 (80,650 km.)With high-sulfur fuel, is run for 50 hours with a Litex CDD in the

HC 33% 31% 26% 26%

CO 58% 61% 50% 40%

NOX 54% 55% 46% 22%

Table 4.1 Harmful emission using CDD. reduction

Figure 4.1 Emissions v/s hours of operation. Figure 4.2 THC v/s fuel sulphur.

Figure 4.3 CO v/s fuel sulphur.

Figure 4.4 NOX v/s fuel sulphur.

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4.2

EXPERIMENTAL REACTOR. [3]

RESULTS

FOR

NON

THERMAL

PLASMA

4.2.1 LABORATORY BASED EXPERIMENTAL SYSTEM. Laboratory scale experiments were conducted on a small scale NTP system (active volume 200 cm6) in order to confirm oxidation of carbonaceous PM and to explore the mechanisms involved. In the laboratory experiments a bank of gas analysers and an FTIR spectrometer were used to study the composition of the gaseous exhaust emissions. PM measurements were performed by filter paper sampling (Whatman GFA). Exhaust was provided by a Lombardini diesel generator set (5 kVA rating), working at 3.6kW loading and running on RF73 diesel fuel containing between 350 ppm and 500 ppm Sulphur The exhaust gas composition was typically NO- 500ppm, NO2 - 10 ppm, CO2 - 4.2.4%, O2 - 15.25%, H2O - 3.0 %, Total hydrocarbons (THC) in C1 equivalent - 300ppm, N2 - balance, PM 0.06 g/h (equivalent to a challenge of 0.78 g/h in the full scale prototype system). 4.2.2 VEHICLE BASED EXPERIMENTAL SYSTEM. Two configurations were employed to take measurements on the full scale NTP system, a light duty transient engine dynamometer and a vehicle chassis dynamometer. The engine dynamometer was connected to emissions monitoring equipment comprising, Horiba MEXA 7200TDR and 71000DEGR gaseous analysers and a Horiba 110 DPAS particulate measuring system, which incorporates a TEOM. The chassis dynamometer was a 65 kW Consine dyno with full dynamic simulation housed in a thermostatically controlled test enclosure.

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4 .2.3 VEHICLE BASED EXPERIMENTS (A) OPACITY STUDIES. A Celesco Research smoke meter was used to measure the opacity of the exhaust under several different sets of conditions. Tests were conducted at 120kph steady state and under free acceleration. The results are shown in Figure 4.5. Emissions at steady state were low even with the standard exhaust system it can be seen that the Plasma device significantly reduced the visible smoke emissions under free acceleration. (B) GRAVIMETRIC STUDIES. The reduction in total mass of PM can be seen by reference to Figure 4.6. Measurements were taken over the standard European drive cycles (ECE/EUDC), at ambient temperature of 25°C. An additional test was performed at 0°C to simulate cold-start conditions. (C) PM SIZE DISTRIBUTION STUDIES. The effect on the concentration of ultrafine particulate matter with mean diameters of less than 300 nm was tested using a scanning mobility particle sizer (SMPS). Figure 4.7 effect of the reactor on the ultrafine particulate distribution measured under steady state conditions. The packed Plasma system removed 99.9% of PM of average diameters around 60nm. (D) PAH REMOVAL STUDIES. The concentrations of sixteen PAHs were monitored using gas chromatography and mass spectrometry (GC/MS). The results of this work are shown in Figure 4.8. Amounts of these compounds in the exhaust were reduced significantly with the use of the packed NTP aftertreatment system.

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Figure 4.5 Opacity testing

Figure 4.6 Total particulate Mass Testing

Figure 4.7 Ultrafine particle testing with the NTP reactor

Figure 4.8 PAH testing with the NTP reactor

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4.2.4 LABORATORY-SCALE EXPERIMENTS
(A) IMPACT OF SOOT AND PACKING ON NO TO NO2 CONVERSION. All of the results presented are for systems running on unfiltered exhaust unless stated otherwise in the text. Figure 4.9 shows the conversion of NO to NO2 in an unpacked reactor. The conversion increases with increasing energy density and NO2 becomes the majority NOx species at energy densities above 25 joules per litre. (B) PACT OF SOOT AND PACKING ON HYDROCARBON OXIDATION. Figures 4.10 show the changes in concentrations of CO and CO2 relative to the engine out emissions as functions of energy density in each of the reactor configurations. Figure 4.11 shows the changes in the absolute concentration of THC, measured as C1, as a function of energy density in each of the reactor configurations. It can be seen that when using a packed reactor with soot-filtered exhaust more of the hydrocarbon is removed and converted to CO at a given energy density than for the experiment when soot was present. (C) SOOT OXIDATION IN A SYNTHETIC EXHAUST STREAM. In order to further study the PM oxidation mechanism, the packing material in the laboratory reactor was pre-coated in either diesel exhaust PM or carbon black. The reactor was then operated at 70 J/l and 200_C in a CO2 free simulated exhaust comprising 90% N2 and 10% O2. The CO2 evolution was then recorded as a function of time. The Plasma was initiated at time t=0 minutes and extinguished at time t=240 minutes. The data from this experiment, using Acetylene black (Denka Carbon), can be seen in Figure 4.12. A steady state concentration of approximately 200ppm of CO2 was liberated from the soot oxidation. After approximately 200 minutes, the CO2 returned to zero and visual observation of the packing confirmed removal of the acetylene black. 15

Figure 4.9 NO and NO2 concentrations in an UN packed reactor

Figure 4.10 The concentrations of CO.

Figure 4.11 The THC concentration.

Figure 4.12 Evolution of CO2 as a function of time.

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5.PROCESS LIMITATIONS AND IMPLICATIONS FOR PLASMA-CATALYST REACTOR DESIGNS [3]
5.1 PROCESS EFFICIENCY. A version of the full-scale system was run in steady state mode with the engine running at 155 Nm torque and 2500 rpm. 2.34 gm/Hr of soot were continuously oxidized from a flow of 2700 l/min. By varying the operating frequency of the system the Plasma power was varied from 1400 W (31J/L) to 800 W (18 J/L). At the lower power range, determined by the limit of resonant operation of the power supply and reactor system, continuous oxidation was still observed. From this we can determine an upper bound to the power required for soot oxidation to be 0.34 kW-hr/gm. 5.2 PLASMA-CATALYST REACTOR DESIGN CONCEPTS. In a two-stage system relying upon the Plasma oxidation of hydrocarbons to promote NO to NO2 conversion as a precursor to NO2 reduction over a catalyst, the presence of PM may lower the number of O atoms and hydroxyl radicals available for reaction. This will reduce the capacity for enhanced NO to NO2 conversion. The implication is that in circumstances where a filter may act to retain soot in the Plasma reactor, the NOx reduction capacity of the overall system may be reduced, unless hydrocarbon levels are high enough to allow hydrocarbon oxidation to compete with the faster soot oxidation process. This would suggest that the packed Plasma reactor may not be suitable as the first stage in a Plasma-catalyst system where the catalyst is sensitive to NO2. Even in situations where packing materials are not used, the effects of electrostatic trapping of PM in the Plasma region together with time-varying PM loads may have to be considered in order to fully optimise the control strategy for a twostage Plasma catalyst system of this type.

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The

use

of

packed

NTP

reactors

may

however,

is

advantageous for some two-stage Plasma-catalyst aftertreatment systems. It may be possible to select packing materials that optimise the chemical composition of the exhaust particularly with respect to modifying outlet hydrocarbon compositions. In these situations a tailored mixture of NOx and activated hydrocarbon could be supplied to a selected NOx reduction catalyst. In addition, by utilizing catalytic materials, which are selective to NO rather than NO2, it may be possible to use a packed NTP system to demonstrate combined NOx and PM removal.

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6. CONCLUSION
The Corona Discharge Device (CDD) significantly reduces HC, CO and Nox emissions by increasing the efficiency of existing catalytic converters. The CDD essentially creates a “self-cleaning” environment in which existing sulfur poisoning is reduced and new sulfur poisoning is prevented, thereby increasing the effectiveness and prolonging the life span of the catalyst. The CDD is simple to install. It is easily added to an automobile‟s exhaust system with minor interface modifications. It has been shown that a suitably designed NTP reactor, containing a packing material designed to filter and retain particulates, can effect the oxidation of PM in diesel exhausts at low temperatures characteristic of passenger car operation. It has also been shown that such a reactor is effective for oxidation of the ultrafine fraction and PAHs present in the exhaust. In Plasma exhaust aftertreatment systems, the carboneous particules will effectively compete with hydrocarbon for O. for removal of NOX, we can employ an NO catalyst; as oxidation of particulates may deplete the key radicals necessary for NO to NO2 conversion. For future work we need to do a long term test on Plasma reactor for regeneration and optimize the cost.

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7.REFERENCES
1. C. Bertoli et al “Performance Evaluation of Particulate Traps for Passenger Car Diesel Engines” SAE-SP-1247. 2. F. XiaO et al “Plasma Treatment of Diesel Particulates Minibus” SAE-2000-01-1926. 3. J. Christopher Whitehead et al “ Non Thermal Plasma Aftertreatment of Particulates –Theoretical Limits and Impact on Reactor Design” SAE-2000-01-1926. 4. http://www.aeat.com/electrocat/sae/saepaper.htm 5. http://www.Plasmas.org/rot-Plasmas.htm 6. http://www.osti.gov/fcvt/deer2000/nowakpap.pdf 7. http://www.litexcorp.com/docs/cdd_process.pdf for a

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