Scope for Improvements in Design and Development of Diesel Particulate Filter

Transcription

1 Scope for Improvements in Design and Development of Diesel Particulate Filter Vishwajit Patil #1, Dr M.Talib *2, Dr.L.G.Navale #3 # Assist Prof., Mech Dept,SITS,Narhe,Pune, India; Dr.L.G.Navale,Prof.,Mech Dept.,DYP COE,Pimpri,Pune,India Abstract Global warming is the major concern nowadays and diesel engines are the major contributors for exhaust pollutions. These exhaust emissions are hazardous to human health and to the environment. To minimise the exhaust pollutants and to meet the stringent emission norms from the diesel engines some arrangement must have to done and the answer was filters called as Diesel Particulate Filter(DPF). From then the research is going on for different substitute materials to optimise its performance, cost. This paper describes research conducted by a group of researchers considering some of the important parameters that should be taken into account while designing the diesel particulate filter and the impact of particulate matter on engine performance in terms of back pressure for conventional particulate filter thereby improving the design parameter that can be applied to different materials also. Keywords Diesel Particulate filter, diesel filter materials, DPF parameters. I. INTRODUCTION A diesel particulate filter, sometimes called a DPF, is a device which is meant to remove diesel particulate matter or soot from the exhaust gas from a diesel engine. Diesel particulate filters have contributed to decreasing particulate matter (PM) in the exhaust gas of diesel cars, and they have become standard diesel exhaust gas after-treatment devices. Silicon carbide (SiC) is currently used as a material in these filters due to its high thermal stability. In order to halt the progress of global warming, attention has been increasingly given to technologies for reducing carbon dioxide (CO2) emissions, and the reduction of CO2 discharged from automobiles has garnered a lot of interest. It is considered that for the next two decades the internal combustion engine will be an appropriate selection as a power train for automobiles. Especially, diesel-powered automobiles have better fuel efficiency than gasoline-fueled automobiles, and thus they are recognized part of the eco-car category. However, it is crucial to implement environmental measures for dieselpowered automobiles also, including the reduction of particulate matter (PM) and nitrogen oxide (NOx) discharged from diesel engines. As part of these environmental measures, studies of the exhaust gas after-treatment systems for diesel engines have been actively conducted for quite some time, mainly in Europe and America. Particularly for the PM issue, the adoption of diesel particulate filters has enabled us to obtain certain prospective solutions. However, as is typically seen in Europe, regulations have been more and more stringent, and thus development of higher performance DPFs is the need of the situation which can perform better. A wall-flow type filter structure is mainly used in DPFs. This type of filter is mainly made from ceramics, and the inlet and outlet of the honeycomb structure are alternately plugged, and the honeycomb wall face is used as a filtration area. The honeycomb wall has a porous structure, and PM is removed from the exhaust gases as it passes through this porous layer as shown in Fig1. However, it is said that the fuel efficiency will be reduced by 1% to 3% once a DPF is installed because the exhaust gas pressure loss will be higher due to the PM accumulation on the DPF, hence PM accumulation on the DPF must be periodically burned and removed.[1] Fig1: Ceramic Wall flow filter Currently, silicon carbide (SiC) having outstanding heat resistance properties is the principal material used for DPFs. Because SiC has a high coefficient of thermal expansion. The DPFs are bind in segments. With this type, units with specific dimensions are bonded to each other, and the thermal expansion of the whole DPF is absorbed by the conjugation layers as shown in Fig 2. However, because the manufacturing cost for this type of DPFs is high and material loss is significant as compared to monolith type DPFs, it is one of the high cost factors for DPFs. Furthermore, it also has a disadvantage in terms of performance as the ISSN: Page 137

2 segmented layers do not perform its function in PM removal.[2] Fig 2: Segmented Filter II. PRESENT STATUS Direct oxidation catalysts have been of interest in the field for more than five years. These catalysts use oxygen conducting materials (such as ceria, zirconia, or manganate) to burn the soot at the soot-catalyst interface, rather than by oxygen in the gas phase. Complex ceria material can begin oxidizing soot with model gas phase oxygen at 160 C with completion at 220 C using no or very little precious metal.[3] Diesel oxidation catalysts (DOC) were once the premier choice for diesel exhaust after-treatment, however with ever tightening emissions standards, they are quickly becoming obsolete. DOCs are very effective in reducing CO, HC, aldehydes, and the soluble organic fraction (SOF), however they have little effect on NOX and limited effect on particulate emissions.[4]. Also, at high temperature it produces sulfates. Although DPFs have been in commercial production for more than 10 years, there is still much optimization activity in the field. The research is going on to find different materials for better performance and cost efficient.[5] Warner, et al., investigated present diesel filter regeneration dynamics and found that Passive regeneration is more efficient than Active regeneration.[6] DPF substrates are also improving. For low-soot applications, Heibel A shows that low-mass DPF prototype cordierite substrates can allow a downstream SCR catalyst to heat-up faster, dropping NOx emissions by 15% in cold start testing. Back pressure is also reduced 35% relative present US DPFs.[5][7] Cheng A S, studied Contribution of Lubrication Oil to Particulate Emissions from a Diesel Engine, when wear produces larger gaps that contribute to transport of lubrication oil into the combustion chamber and out the exhaust port. The consumption of oil is a concern for proper engine operation. The sensitivity of modern emission control catalysts, to sulphur poisoning has led to increased interest in determining the quantity of oil transported through the cylinder and out the exhaust port. Also, although the mass of lubrication oil consumption is quite low, it has been implicated as a source of nanoparticles which are harmful for human health.[8] According to study conducted by Timothy V., for review on diesel exhaust in which he had reviewed various developments in the field of exhaust emission after treatment systems where Cu-Zeolite filter used, reported that A DPF with Cu-zeolite behaves similarly to the uncoated filter, and has minimal impact on DPF regeneration when used with SCR system.[5] In selective catalytic reduction, ammonia is commonly injected into the waste gas stream and the combination is passed across a catalyst to reduce nitrogen oxides to nitrogen and water. Disadvantages of this approach include high cost, narrow temperature range of applicability, and ammonia emissions into the atmosphere. In selective noncatalytic reduction, ammonia or urea is injected into the engine or combustor itself or into the gas leaving the engine where temperatures are very high.[9] Disadvantages of this approach are low NOx removals and the problems associated with the handling of ammonia or urea. Reaction of the oxides with water or alkali has seen only limited success because of NO, the principal NOx species in most gases, does not readily dissolve in water or react with alkali in aqueous solutions. [10] Sidney C. et.al. conducted study on Activated carbon. Few materials sorb NOx well. An exception is activated carbon under certain conditions. The specially prepared activated carbons can sorb 10 percent or more of their weight in NOx under ideal conditions. After NOx is sorbed at a low temperature, heating the carbon to a higher temperature can release it. This process of sorbing NOx at a low temperature with activated carbon and releasing it at a high temperature has been used commercially in the past. This approach was pursued in treating exhaust gases from mobile sources and the study reported says of about 54 to 64 percent were achieved for mobile engine. Though, in the study it has not elaborated about the control of particulate matter and the regeneration of filter. Also the effect on back pressure is not mentioned.[11] Nett Technologies Inc. developed DPF based on zeolite also known as molecular sieves, which are most frequently used as the hydrocarbon trap. These zeolites traps and store diesel exhaust hydrocarbons during periods of low exhaust temperature, such as during engine idling. Then, when the exhaust temperature increases, the hydrocarbons are released from the washcoat and are oxidized on the catalyst. Zeolite is capable of 40-50% HC conversion and effective diesel odor control at very low exhaust temperatures[12]. ML Stewart et.al. conducted study on a silicate mineral which is processed to obtain the required filter material called Acicular Mullite, in his study he found ISSN: Page 138

3 the filter can be able to reduce the back pressure effect and exhaust emission of a diesel engine. The study also reported to have sufficient reduction in pressure drop across diesel filter.[13]. The research is going on various materials to find better possible alternatives which can reduce the exhaust emission (PM) by improving the engine performance. III. DPF DESIGN PARAMETERS DPF design items consist of the following factors: a) Material: DPF material will affect the thermal stability, thermal shock resistance, mechanical strength and reactivity with the base material of the exhaust gases. b) Cell structure: This will affect the pressure drop, thermal shock resistance and ash resistance characteristics which can sustain physical accumulation of ash. c) Pore size: This will affect the filter s basic properties, namely the pressure drop and filtration efficiency. As the substrate material s wall is used as a filtration area. a) Material The parts used for vehicle emission gas lines are exposed to various harsh conditions. For example, severe thermal conditions due to sudden changes in the engine operating status, vibrations from the engine and road surface, and coming into contact with chemical compositions such as ash having high reactivity and originating from fuel or oil. Above all, when using a DPF, the PM collected inside the filter must be burned and removed after accumulating for a certain period of time which is generally called as regeneration. The combustion heat generated during this process will cause the DPF to be exposed to more severe thermal conditions than any other part. Although DPF regeneration is normally conducted under relatively mild and controlled conditions, on rare occasions it is performed under an uncontrollable condition. Under such conditions, the greater the PM amount is, the more severe the combustion will be. Therefore, the regeneration start timing is set according to actual vehicles so as not to damage filters. The larger the SML (Smoke Mass Limit) value is, the greater is the PM amount that can be burned and removed at once, thus causing regeneration to occur less frequently, thereby improving the fuel efficiency. The thermal shock parameter (TSP) and heat capacity of a base material can be considered as factors that would affect the SML value. The TSP can be simply calculated as a correlation equation of material properties as shown below: Regarding MOR/eMod in this equation, since values usually fluctuate in sync with each other and the MOR/eMod ratio is nearly a constant value, it is assumed that reducing the coefficient of thermal expansion (CTE) is most effective to improve the TSP. TSP = MOR / (CTE emod) TSP ; Thermal shock parameter MOR ; Bending strength emod; Young modulus CTE ; Coefficient of thermal expansion Furthermore, the heat capacity of the DPF can be considered as a material property that directly contributes to the SML. The temperature of the DPF itself will increase due to the heat generated during the DPF regeneration. The larger the heat capacity is, the milder the heating behavior will be during the regeneration so the sudden temperature change is minimal. When comparing ceramic materials having identical volume, the material density is large can be rephrased as the heat capacity is large. The present available filter are SiC and Cordierite filter the following Table-I shows material properties which are also helpful to get favourable smoke mass limit properties. Table-I: Material Properties and its effect on SML Parameter SiC Cordierite SML Theoretical density [g/cc] High Heat Capacity [J/L K] High CTE/ 10 6 [1/K] 4 1 High Thermal Conductivity [W/m K] 50 2 Low b) Cell Structure Cell structure is one the important factor as it can affect contact surface area there by the pressure drop may vary causing the influence on engine performance. The cell structure also affects the ash resistant ability of the filter. Contribution of Cell Structure to Ash Capacity solid components (other than PM) contained in exhaust gas include non-organic components mixed in from the fuel and/or lubricating oil, as well as metal components mixed in due to the frictional wear of syringes. These solid components are collectively called ash components. Ash components gradually accumulate in the DPF, decreasing the effective filtration area, and can therefore cause an increase in pressure drop. According to the study conducted Hexagonal structure was found to be better in terms of performance for controlling back pressure. (a) Hexagonal Cell Structure ISSN: Page 139

4 (b) Squqre Cell Structure (c) Octa square Cell Structure Fig.3: Different Cell Structure of diesel particulate filter c) Pore Size DPF partition wall possesses a pore structure that consists of minute pores in the order of microns. This structure (i.e. pore size and distribution) greatly affects the filtration efficiency and pressure drop characteristics. This means that in order to efficiently collect PM, it is preferable that the porosity and pore diameter are as small as possible. Contrastingly, in order to reduce the pressure drop (= allow gas to flow efficiently), larger porosity and pore diameter are better. The optimal pore structure can be determined by taking both characteristics into account. We conducted pore structure optimization on DPF products already on the market, through a small-scale experiment using the filtration efficiency and pressure drop characteristics as parameters (Fig. 2). Fig. 2 shows the pore characteristics (porosity and mean pore diameter) of each product and the characteristics of the filtration efficiency. Fig. 4 also indicates the pressure drop characteristics using the value of Serial (A) as a standard. Additionally, Serial (A), Serial (B) and Serial (C) are DPF products obtained from the market. Of all those products, Serial (A) is made of the material that can be considered to be the most popular in the current global market. Fig 4: Effect of pore size IV. EFFECT OF PARTICULATE MATTER Fig. 5 depicts the measurement results of the pressure drop characteristics during PM accumulation. The PM accumulation mode after passing the initial PM accumulation region (where the amount of deposited PM is less than 1g/L) is considered as the cake filtration region. In this region a linear increase in pressure drop, which occurred in proportion to the amount of deposited PM, was observed. Additionally, the initial PM accumulation region was referred to as the transient region. It is considered that in this region the PM infiltrate into the pores of the partition wall and accumulates in them. Regarding SiC having relatively small pores which means low porosity, small mean pore diameter, it was surmised that the pressure drop increased sharply because the pore channels tend to be overlapped by PM.[14] Fig 5: Effect of particulate matter on back pressure ISSN: Page 140

5 V. PARTICULATE MATTER CHARACTERISTICS According to the study conducted by Akiyoshi NEMOTO et al., to find PM charactorystics.the PM collection properties called as filtration efficiency were measured using a PM generator and a particle counter. Also, in order to obtain a preparative gas in the measurement range that could detect the amount of PM, the gas was diluted first using a diluter and then introduced to the particle counter. The filtration efficiency was obtained using the equation shown below, denoting the start time needed to allow the gas containing PM to flow into the DPF as t = 0s: Filtration efficiency (%) = 100 (1 N600s/N0) N0 : The PM number concentration generated by the apparatus (number/cm3) N600s: The PM number concentration passing through the DPF after 600s (number/cm3) Fig.6 shows the chronological change in the PM number concentration in the calculation of the filtration efficiency. For each DPF, the PM number concentration decreased chronologically for particles of any diameter, and the PM was efficiently filtered by the DPFs. When the filtration efficiency of each DPF was calculated using equation (4), the results were 96% for SC-AT (HEX) and 94% for SiC (OS), thus indicating that SC-AT (HEX) possessed a PM collection capability equivalent to or greater than that of SiC (OS). Fig. 6 Changes in PM number distribution on downstream filter. VI. CONCLUSION To meet the ever changing demands of exhaust emission regulations the design of diesel particulate filters is undergoing a drastic change and requires a search for cost efficient substrate materials. DPF studies are showing improved understanding of regeneration. Passive and active regeneration parameters and influences are compared. New catalysts and substrates are described which are being used for forming the substrate of a filter. In short, regeneration temperatures are going down, catalysts are getting less expensive, and system back pressure and fuel efficiency are improving. As we can see from the above results there remains a vast scope in improving the diesel particulate filter by selecting different substitute materials so that overall improvement can be achieved. Thereby there remains a wide scope to search for other possible options which can reduce the cost of the filter can also be reduced to some extent. ACKNOWLEDGMENT I would like to acknowledge Prof P.D.Darade for his valuable guidance and timely help whenever and wherever required. REFERENCES 1. Ezio Alfieri, Emissions-Controlled Diesel Engine, Ph.D, Swiss Federal Institute of Technology Zurich, Tree D.R. and Svensson K.I., Soot processes in compression ignition engines, Progress in Energy and Combustion Science, vol. 33, no. 3, pp , Southward, B.W.L., Basso, S., and Pfeifer, M., On the Development of Low PGM Content Direct Soot Combustion Catalysts for Diesel Particulate Filters, SAE Technical Paper , 2010, doi: / Zelenka, P., Kriegler, W., Herzog, P., Cartellieri, W. (1990). Ways Toward theclean Heavy-Duty Diesel (SAE ). 5. Timothy V. Johnson Corning Inc., Diesel Emissions in Review, 2011 SAE International. 6. Warner, J.R., Dobson, D., and Cavataio, G., A Study of Active and Passive Regeneration Using Laboratory Generated Soot on a Variety of SiC Diesel Particulate Filter Formulations, SAE Int. J. Fuels Lubr. 3(1): , 2010, doi: / Heibel, A., Advancements in Substrate Technology, Presentation at SAE 2010 Heavy Duty Diesel Emissions Control Symposium, September, Cheng A.S., Rich D., Dibble R.W., Buchholz B.A., Quantifying the Contribution of Lubrication Oil to Particulate Emissions from a Diesel Engine, 3' Joint Meeting of the U.S. Sections of the Combustion Institute, Chicago, ILL, March 16-19, Kamasamudram, K., Currier, N., Szailer, T., and Yezerets, A., Why Cu- and Fe-Zeolite SCR Catalysts Behave Differently at Low Temperatures, SAE Int. J. Fuels Lubr. 3(1): , 2010, doi: / Nova, I., Colombo, M., Tronconi, E., Schmeisser, V., Weibel, M., Transient Kinetic Study and Modelling of the NO/NO2 NH3-SCR Reactions over Cu- and Fe-Zeolite Catalysts for Diesel Exhaust Aftertreatment, 3 rd International IAV MinNOx Conference, Berlin, June Sidney C. Nelson and Rick A. Babyak, Sorbent Technologies Corporation, Activated Carbon use in treating diesel engine exhausts, 1664 East Highland Road Twinsburg, OH Pg Diesel Exhaust Catalytic Converters Nett Technologies Inc Goreway Drive, Mississauga, Ontario Canada L4V 1P Stewart M.L., Maupin G D, Gallant T R, Zelenyuk A, Kim D H, Fuel Efficient Diesel Particulate Filter (DPF) Modeling and Development Prepared for the U.S. Department of Energy under Contract DE-AC05-76RL01830, August Akiyoshi NEMOTO et al., Development of Innovative Diesel Particulate Filters based on Aluminum Titanate: Design and Validation, SUMITOMO KAGAKU 2011-II. ISSN: Page 141