Article pubs.acs.org/EF
Diesel Particulate Filter Regeneration Strategies: Study of Hydrogen Addition on Biodiesel Fuelled Engines K. Theinnoi,⊥,† S. S. Gill,† A. Tsolakis,*,† A .P. E. York,‡ A. Megaritis,§ and R. M. Harrison∥ †
School of Mechanical Engineering, University of Birmingham, Birmingham B15 2TT, United Kingdom Johnson Matthey Technology Centre, Blount’s Court, Sonning Common, Reading RG4 9NH, United Kingdom § Mechanical Engineering, School of Engineering and Design, Brunel University, West London, Uxbridge UB8 3PH, United Kingdom ∥ School of Geography, Earth and Environmental Sciences, University of Birmingham, Birmingham B15 2TT, United Kingdom ‡
ABSTRACT: The diesel particulate filter (DPF) has attracted considerable attention for the reduction of particulate emissions to meet strict forthcoming emission regulations. The advantages associated with the current commercial diesel oxidation catalyst (DOC) have been implemented and combined with that of the diesel particulate filter for overall improvement in filtration and oxidation efficiencies. This study has been extended to include regeneration of the DPF, with the main focus being on the impact of hydrogen on the DOC performance by means of actual exhaust gas from a diesel engine operating on diesel, biodiesel (RME), and low temperature Fischer−Tropsch (LTFT) synthetic diesel (GTL). This technique is a key element for continuous DPF regeneration at low temperatures. A DOC has the ability to effectively oxidize hydrocarbon and carbon monoxide, as well as enhance the NO2 concentration with increased hydrogen concentration from 0 to 3000 ppm. However, hydrogen addition without the use of a DOC had no significant benefit for DPF regeneration. The combustion of different fuels illustrated in terms of NO2 formation (specifically NO2/NO ratio) over the DOC followed the order RME > GTL > diesel, which represented the ability of DPF regeneration activity in the presence of hydrogen (3000 ppm).
1. INTRODUCTION The challenge for compression ignition (CI) diesel engines lies primarily on the control of the three pollutant phases emitted from the exhaust. This is more complicated than the control of gas phase emissions from spark ignited (SI) gasoline engines. The emissions emitted from a diesel engine are composed of three phases (i.e., solids, liquids, and gases). The total particulate matter (PM) makes up the combination of solids, primarily of dry carbon (soot) and liquid phase hydrocarbons (HC), with a small amount of adsorbed sulphates (SO4) from the fuel. The liquid HC’s are a combination of unburned diesel fuel and lubricating oil, which together are called the soluble organic fractions (SOF).1 In respect to their difference in size distribution, the soot and SOF particles are sometimes called accumulation mode particles and nucleation mode particles, respectively. The gaseous phase constituents are made up of HC, carbon monoxide (CO), nitrogen oxides (NOX), and sulfur dioxide (SO2).1,2 Regulated emissions of NOX and PM emitted from diesel engines need to be controlled to keep within the current limits and meet forthcoming scheduled stringent emission standards (e.g., Euro 6, Tier 4, Low Emission Vehicle III (LEV III)). Aftertreatment technologies including selective catalyst reduction (HC-SCR), lean NOX trap, diesel oxidation catalyst (DOC), and diesel particulate filter (DPF) have been introduced to significantly reduce both emissions to meet new regulations. The DPF is currently the most efficient and commonly used method developed for the removal of PM from a diesel exhaust gas.3 Previous studies have shown a DPF to exhibit good performance for trapping soot with more than 90% filtration efficiency.4 However, continuous soot accumulation over the DPF can lead to an increase in back pressure and fuel penalty. © 2012 American Chemical Society
A continuously regenerating system is required under low load engine operation to improve the DPF performance.5 The soot can be oxidized in air producing CO2; however, this reaction takes place under high temperatures (>450 °C) that are higher than the normal diesel exhaust gas temperatures. There are many possible methods for DPF regeneration under a low temperature window (e.g., fuel additive system, electric heater, and microwave regeneration), but there are still causes for concern, primarily associated with compactness and simplicity. In earlier work, we have shown that hydrogen (H2) addition improves the NOX reduction performance of the hydrocarbon SCR (HC-SCR) system at low exhaust temperatures.6−8 H2 also allows DPF and NOX traps to regenerate at lower temperatures and within a reduced time frame, resulting in an improved fuel economy. However, one of the major obstacles at the present time is the storage of H2 on-board the vehicle for on-demand supply. Current methods include exhaust gas fuel reforming, which has been researched widely over the past decade. The process essentially involves the catalytic cracking of the source feed (i.e., diesel type fuels) into synthesis gas (H2 + CO) using the available oxygen (O2) and water vapor already present in the exhaust stream.9,10 Although this technique is promising, further work is still required to develop and optimize the process efficiency, especially with interest in renewable and alternative combustion fuels.10 The continuously regenerating trap (CRT) developed by Johnson Matthey is among the most effective technologies currently in practice for the control of diesel PM. This particular system Received: September 8, 2011 Revised: January 20, 2012 Published: January 20, 2012 1192
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utilizes the nitrogen dioxide (NO2) available in the exhaust stream as well as that produced over an oxidation catalyst in front of the filter during the HC oxidation for a low temperature (below 300 °C) DPF regeneration.11,12 In addition, the oxidation of CO, HC, and H2 over the DOC provides additional heat for soot oxidation, while in the presence of H2 the DOC can be used to further enhance the nitric oxide (NO) to NO2 oxidation.13 The amount of NO to NO2 conversion is dependent on the hydrocarbon fuels and the exhaust gas temperature window.14 However, as NO2 is more toxic than NO,15 concern for emitted NO2 emission will hold further difficulty. The work presented here is part of an ongoing research study with the overall aim of understanding the influence of H2 on the current CRT technology (i.e., a combined DOC and DPF system). The first aspect of the study looks at the influence of H2 over the DOC, particularly in terms of NO−NO2 oxidation under actual exhaust conditions. The second aspect follows on from the first, making use of the increased NO2/NO ratio for passive DPF regeneration under isothermal conditions. This has been chosen to prevent temperature (i.e., resulting from the exotherm that is generated during the addition of H2 over the DOC) from having an influence on the regeneration process/ filter loading. Thus, the direct effect of NO2−soot oxidation can be observed at low exhaust temperatures. The combustion of different fuels is also examined to extend the CRT work.
Table 1. Engine Specifications engine specification
data
number of cylinders bore/stroke connecting rod length displacement volume compression ratio rated power (kW) peak torque (Nm) injection system injection timing (°bTDC) engine piston
1 98.4 mm/101.6 mm 165 mm 733 cm3 15.5:1 8.6@2500 rpm 39.2@1800 rpm three holes pump-line-nozzle 22 bowl-in-piston
rpm with an engine load of 4 bar indicated mean effective pressure (IMEP), representing approximately 50% of the maximum load. The DOC used for the aftertreatment system was prepared by impregnating low loading supported platinum (Pt) based catalyst coated onto a cordierite honeycomb monolith substrate (diameter of 115 mm and length 75 mm) with a high cell density (600 cpsi), thus giving a space velocity (SV) of 60kh−1 based on the DOC volume and exhaust flow rate. H2 (99.95% purity supplied from a gas cylinder) was introduced upstream of the DOC, allowing it to mix with the exhaust gas before being directed through the DOC. The H2 concentrations (0, 1000, and 3000 ppm) were chosen on the basis of those that can be efficiently produced with on-board fuel reforming processes with diesel-type fuels. The uncatalyzed SiC-DPF of size 25 mm diameter × 76 mm length was positioned inside a minireactor, specifically designed to study the effect of regenerative activities under real diesel exhaust gas compositions. The minireactor was heated by a tubular furnace whose temperature was set to simulate that of the engine-out exhaust. The temperature of the DPF was measured using k-type thermocouples positioned at the inlet and midbed of the DPF, with the exhaust flow
2. EXPERIMENTAL SECTION The experimental apparatus is detailed and illustrated in Figure 1. The engine used for this research as a source of exhaust gas was an experimental single cylinder, naturally aspirated, direct injection diesel engine. The main engine specifications are given in Table 1. In this work, all tests were examined under a constant engine speed of 1500
Figure 1. Schematic diagram of DOC reactor system. 1193
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rate set to mimic a full size DPF (i.e., the exhaust flow rate was reduced to account for a reduced filter size). The influence of NO2 for passive regeneration was recorded in terms of its pressure drop across the DPF. The loading of the DPF was accelerated for each test condition by increasing the exhaust flow rate for a period of 30 min. A pressure transducer (Cole−Parmer high-accuracy compound transmitter model EW-68073-00) was used to monitor the pressure drop across the DPF providing an understanding of soot accumulation and/ or oxidation characteristics. The fuels used were ultra-low sulfur diesel (ULSD), synthetic diesel (GTL), and biodiesel (RME), provided by Shell Global Solutions, U.K. The fuel properties have been collated and illustrated in Table 2.
on the high reactivity of NO2 for PM oxidation reaction will occur at a temperature 200−300 °C lower than that with O2.11 This part of the study focuses on the effect of H2 addition directly over the DPF without the implementation of an upstream DOC. The engine-out concentrations in Figure 2a represent the emissions upstream of the DPF. The result of 0, 1000, and 3000 ppm of H2 shows the product emissions (downstream of DPF) after the mixture of H2 and exhaust gas has passed through the DPF. Figure 2a shows the effect of H2 on the emissions of HC, CO, and total NOX (mainly consisting of NO and NO2) over the DPF. It can be observed that as the H2 concentration was increased upstream of the DPF, there was a reduction in HC and an increase in the level of CO. Thus, further NO2 was utilized (with the total NOX being unaffected) in oxidizing the soot within the DPF, as shown in Figure 2b, resulting in a reduced pressure drop across the DPF (Figure 3).
Table 2. Fuel Properties diesel synthetic diesel biodiesel (ULSD) (GTL) (RME)
fuel analysis
method
cetane number density at 15 °C (kg m−3) viscosity at 40 °C (cSt) 50% distillation (°C) 90% distillation (°C) LCV (MJ kg−1) sulfur (mg kg−1) aromatics (wt %) O2 (wt %)
ASTM D613 ASTM D4052
53.9 827.1
80 784.6
54.7 883.7
ASTM D445 ASTM D86 ASTM D86
2.467 264 329 43.3 46 24.4
3.497 295.2 342.1 43.9 diesel, demonstrating the influence of different combustion fuels. However, with GTL, this available NO2 in the exhaust stream appeared to have no influence on the DPF regeneration, as observed by the pressure drop and soot loading across the DPF. Using RME as the combustion fuel proved to be the most effective, not only creating a greater NO2 concentration over the DOC but also showing a greater NO2−soot oxidation by possessing more reactive soot, thus enhancing the DPF regeneration process.
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SOF = soluble organic fraction SOM = soluble organic material SO2 = sulfur dioxide SO4 = sulfate ULSD = ultra-low sulfur diesel
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AUTHOR INFORMATION
Corresponding Author
*Tel.: +44 (0) 121 414 4170. E-mail:
[email protected]. Present Address ⊥
College of Industrial Technology, King Mongkut’s University of Technology North Bangkok, 1518 Pibulsongkram Road, Bangkok 10800, Thailand Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS Dr. K. Theinnoi thanks the Engineering and Physical Science Research CouncilEPSRC Project No. EP/F025483/1for supporting his postdoctoral research at Birmingham University. The School of Mechanical Engineering at the University of Birmingham (U.K.) is gratefully acknowledged for the PhD scholarship to Mr. S.S. Gill. Johnson Matthey Plc is also recognized for supporting this work and supplying the catalysts and DPFs to the University of Birmingham as part of Industrial CASE studentships.
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NOMENCLATURE CI = compression ignition CO = carbon monoxide CO2 = carbon dioxide CRT = continuously regenerating trap DOC = diesel oxidation catalyst DPF = diesel particulate filter GTL = gas-to-liquid HC = hydrocarbon HC-SCR = hydrocarbon-selective catalytic reduction IMEP = indicated mean effective pressure NOX = oxides of nitrogen (NO, NO2) NO = nitric oxide NO2 = nitrogen dioxide PM = particulate matter RME = rapeseed methyl ester SI = spark ignited 1200
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