Energy Fuels 2010, 24, 302–308 Published on Web 10/07/2009
: DOI:10.1021/ef900796p
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Low-Load Dual-Fuel Compression Ignition (CI) Engine Operation with an On-Board Reformer and a Diesel Oxidation Catalyst: Effects on Engine Performance and Emissions A. Tsolakis,*,† R. Torbati,‡ A. Megaritis,§ and A. Abu-Jrai
School of Mechanical Engineering, The University of Birmingham, Birmingham B15 2TT, United Kingdom, ‡Department of Chemical Engineering, University of Naples “Federico II”, P. le Tecchio 80, 80125, Naples, Italy, §Mechanical Engineering, School of Engineering and Design, Brunel University, West London, Uxbridge UB8 3PH, United Kingdom, and Department of Environmental Engineering, Al-Hussein Bin Talal University, Ma’an, Post Office Box 20, Jordan )
†
Received July 28, 2009. Revised Manuscript Received September 17, 2009
In the exhaust gas fuel-reforming method, part of the engine exhaust gas reacts with small amounts of engine fuel in a mini-reactor fitted in the exhaust gas recirculation (EGR) loop to produce gaseous fuel named reformed EGR (REGR). In this study, hot REGR (gas containing H2, CO, CH4, and CO2) was fed back to the engine inlet, so that the diesel engine in effect operated in dual fuel with inlet heating and EGR (CO2 existence). The effects of the diesel engine dual fueling with different REGR percentages on the engine performance and emissions have been examined. The study focused at low-load engine operation, where the conditions are not favorable for efficient gaseous fuel oxidation and engine combustion stability. The addition of the premixed gaseous fuel resulted in a remarkable reduction of both NOx and smoke engine emissions. Carbon monoxide and hydrogen oxidation efficiency can be improved by careful selection of REGR addition (e.g., premixed air/fuel ratio, inlet temperature, and reduced in-cylinder O2 concentration) and in-cylinder diesel injection timing. The use of Pt supported on Al2O3 as an oxidation catalyst at the engine exhaust can eliminate both CO and H2 at temperatures lower than the engine exhaust gas temperature at part loads. Preferably, in an actual engine reformer system, the uncombusted CO and H2 will be used to enhance the aftertreatment system (i.e., diesel particulate filter, NOx traps, and hydrocarbon-selective catalytic reduction of NOx) performance.
which is related to the combustion noise.1-10 For example, engine misfires can occur at low load, and premature ignitions can occur at high loads. Therefore, under such conditions, the introduction of technologies such as variable valve timing, variable compression ratio, boost pressure, and inlet heating will be required.11-14 Considerable drawbacks associated with the dual-fueling method include the additional engine fueling system (e.g., natural gas, gasoline, and hydrogen) on board a vehicle, fueling infrastructure (e.g., hydrogen and bioethanol), system simplicity (e.g., safety), and compactness (additional fuel tank and fuel lines). A solution to the above problems can be the production of the high-octane fuel on board from the standard engine fuel [diesel, biodiesel, and FisherTropch gas to liquid (FT-GTL)] using a fuel reformer.
1. Introduction Dual-Fuel Systems. The principle of dual-fuel compression ignition (CI) engine operation is to introduce premixed highoctane fuel (e.g., natural gas, gasoline, hydrogen, and carbon monoxide) with air and directly injected (DI) diesel fuel into the combustion chamber. Ideally, homogeneous air fuel mixtures ignited spontaneously exhibit less pollutants and can improve engine efficiency compared to standard diesel combustion, which is based on diffusion combustion. Although optimization of the injection timing of the incylinder DI fuel (e.g., diesel) aims to ignite the mixture and control the start of combustion (SOC) for the different premixed fuel ratios, the fuel ignition timing is complicated and problematic for a dual-fueled engine under a number of engine-operating conditions (e.g., low loads and use of residual gas trapping). This can result in increased pollutants and deterioration of the vehicle drivability or engine knocking,
(6) Noguchi, N.; Terao, H.; Sakata, C. Bioresour. Technol. 1996, 56, 35–39. (7) Kusaka, J.; Okamoto, T.; Daisho, Y.; Kihara, R.; Saito, T. JSAE Rev. 2000, 21, 489–496. (8) Tsolakis, A.; Megaritis, A.; Yap, D.; Abu-Jrai, A. SAE Tech. Pap. 2005-01-2087, 2005. (9) Tsolakis, A.; Megaritis, A. Int. J. Hydrogen Energy 2005, 30, 731– 745. (10) Ishida, M.; Tagai, T.; Ueki, H.; Sakaguchi, D. Comodia04 2004, 51–58. (11) Christensen, M.; Hultqvist, A.; Johansson, B. SAE Tech. Pap. 1999-01-3679, 1999. (12) Christensen, M.; Johansson, B.; Einewall, P. SAE Tech. Pap. 972824, 1997. (13) Oaklay, A.; Zhao, H.; Ladommatos, N. SAE Tech. Pap. 2001-011030, 2001. (14) Law, D.; Allen, J.; Chen, R. SAE Tech. Pap. 2001-01-0421, 2001.
*To whom correspondence should be addressed. Telephone: þ44-(0)121-4144170. Fax: þ44-(0)121-4143958. E-mail: a.tsolakis@ bham.ac.uk. (1) Tsolakis, A.; Megaritis, A.; Wyszynski, M. L. Energy Fuels 2003, 17 (6), 1464–1473. (2) Daisho, Y.; Yaeo, T.; Koseki, T.; Kihara, R.; Saito, T.; Quiros, N. E. SAE Tech. Pap. 950465, 1995. (3) Daisho, Y.; Takahashi, K.; Iwashiro, Y.; Nakayama, S.; Kihara, R.; Saito, T. SAE Tech. Pap. 952436, 1995. (4) Tsolakis, A.; Hernandez, J. J.; Megaritis, A.; Crampton, M. Energy Fuels 2005, 19, 418–425. (5) Papagiannakis, R. G.; Hountalas, D. T. Energy Convers. Manage. 2004, 45, 2971–2987. r 2009 American Chemical Society
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: DOI:10.1021/ef900796p
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Figure 1. Proposed engine-reformer-diesel oxidation catalyst (DOC) system.
Fuel Reforming. For automotive applications [internal combustion (IC) engines and fuel cells], the partial oxidation, autothermal, and exhaust gas fuel reformers are suitable because of their compactness and rapid response under different operating conditions.15-19 All of these reforming methods aim to produce a combustible gaseous mixture, i.e., gaseous fuel containing gases such as H2, CO, and CH4, as well as CO2. Thus, in effect, the engine operates in a similar way as a dual-fueled engine with standard exhaust gas recirculation (EGR).8,9,19,20 The exhaust gas fuel-reforming technique involves the injection of hydrocarbon fuel (e.g., diesel) into a catalytic reformer fitted into the engine EGR system (Figure 1), so that the produced gas mixture (containing H2, CO, CH4, and CO2) is fed back to the engine as reformed EGR (REGR). This hydrogen production method is comparable to autothermal reforming (ATR) and has the advantages of both oxidation (compactness, fast start up and shut down, and rapid response) and endothermic (high calorific product fuel and increased hydrogen concentration) reactions.16,19 However, in the case where an autothermal reformer is coupled to an engine, additional water storage and supply units will be required on board the vehicle. Although the partial oxidation reformer is simpler and requires only air and engine fuel, the reduction of the fuel calorific value and, hence, system efficiency is unavoidable because of the promotion of exothermic reactions. In earlier work, we have shown that the engine operation on dual fuel using gaseous fuel produced in a reformer leads to simultaneous reduction of NOx and smoke emissions, but it also results in incomplete combustion of the added H2, CO, and CH4. The incomplete oxidation of the gaseous fuel is more noticeable when the engine operates at part loads. The low in-cylinder pressure and temperature with the very lean air-fuel mixtures make it difficult to achieve efficient gaseous fuel oxidation. The gaseous fuel oxidation efficiency improves with the increase of the engine load. Thus, the fuel economy of a CI dual-fueled engine is improving mainly
at medium- and high-load engine operation compared to operation on diesel fuel only.8 The elimination of CO can be achieved in the reformer by promoting the exothermic water-gas shift reaction (WGSR, eq 1) with the use of an additional catalyst. However, a penalty in fuel consumption and system compactness will be unavoidable.19 CO þ H2 O f H2 þ CO2 ð1Þ The use of a diesel oxidation catalyst can reduce effectively the CO and hydrocarbon emissions in the engine exhaust gas, thus allowing operation of the fuel reformer under efficient conditions. Platinum (Pt) catalysts are the most active for the oxidation of CO and hydrocarbons in diesel exhaust, and highly dispersed Pt catalysts supported on γ alumina are widely used as automotive catalysts.21 However, the inclusion of aftertreatment systems, such as NOx traps, hydrocarbon-selective catalytic reduction (HCSCR) of NOx systems, and diesel particulate filters (DPFs) in the engine exhaust system may make use of the exhaled H2 and CO to improve the NOx conversion in the catalyst (mainly at low temperatures) or reduce the soot oxidation temperatures in the DPF while at the same time also reducing CO and HC emissions.22,23 The objectives of this study were to examine the ignition characteristics, combustion process, and exhaust gas emissions from the engine operation on duel fuel (diesel and REGR) at low-load engine operation. The incorporation of a diesel oxidation catalyst Pt/Al2O3 in the engine exhaust for the oxidation of CO and H2 has also been investigated. 2. Experimental Section Engine Test Rig and Experimental Apparatus. The experiments were carried out on a Lister-Petter TR1 engine. The engine is a 773 cm3, naturally aspirated, air-cooled, singlecylinder DI diesel engine with a compression ratio of 15.45:1. The engine has been modified to operate under dual-fuel conditions with a flexible selection of diesel/gaseous fuel ratios. An electric dynamometer with a motor and a load cell was coupled to the engine and used to load and motor the engine. The TR1 engine was used in this study to prove the concept that the REGR technology can provide significant emissions and fuel
(15) Trimm, D. L.; Adesina, A. A.; Praharso; Cant, N. W. Catal. Today 2004, 93-95, 17–22. (16) Edwards, N.; Ellis, S. R.; Frost, J. C.; Golunski, S. E.; Van Keulen, A. R. J.; Lindewald, N. G.; Reinkingh, J. G. J. Power Sources 1998, 71, 123– 128. (17) Tsolakis, A.; Megaritis, A. Biomass Bioenergy 2004, 27, 493–505. (18) Ahmed, S.; Krumpelt, M. Int. J. Hydrogen Energy 2001, 26, 291– 301. (19) Tsolakis, A.; Megaritis, A.; Golunski, S. E. Energy Fuels 2005, 19, 744–752. (20) Abu-Jrai, A.; Tsolakis, A.; Theinnoi, K.; Cracknell, R.; Megaritis, A.; Wyszynski, M. L.; Golunski, S. E. Energy Fuels 2006, 20, 2377–2384.
(21) Yao, H. C.; Sieg, M.; Plummer, H. K. J. Catal. 1979, 59, 365–374. (22) Rodrı´ guez-Fernandez, J.; Tsolakis, A.; Cracknell, R. F.; Clark, R. H. Int. J. Hydrogen Energy 2009, 34 (6), 2789–2799. (23) Abu-Jrai, A.; Rodrı´ guez-Fernandez, J.; Tsolakis, A.; Megaritis, A.; Theinnoi, K.; Cracknell, R. F.; Clark, R. H. Fuel 2009, 88, 1031– 1041. (24) Theinnoi, K.; Sitshebo, S.; Houel, V.; Rajaram, R.; Tsolakis, A. Energy Fuels 2008, 22 (6), 4109–4114.
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was examined in a synthetic gas rig operating under simulated diesel exhaust gas conditions. Both reforming and diesel oxidation catalysts were prepared as a powder, which was then either granulated (for packed-bed testing) or made into a suspension that was precision-coated onto ceramic monolith substrates with a high cell density (900 cpsi). The diesel oxidation catalyst was tested fresh and after thermal aging in air at 750 °C for 48 h.
economy benefits. The application of REGR is expected to have similar effects independent of engine technology (e.g., when integrated within a modern engine system). The engine test rig has been described in detail in previous publications.1,9 Emission analysis included measurement of carbon dioxide, carbon monoxide (both by nondispersive IR), unburned hydrocarbons (FID), oxygen (electrochemical method), and NOx (chemiluminescence) emissions. Smoke was measured using a Bosch smoke meter, giving smoke emissions in terms of Bosch smoke numbers (BSNs). Testing Procedure. The engine-reformer system was closedloop but not fully close-coupled (i.e., reactant flows in the reformer were externally controlled). Experimental results were obtained at the engine operating condition of 1500 rpm speed and 25% load. The reformer was externally heated to a temperature similar to the engine exhaust gas in this engine condition (T = 250 °C). The hydrogen content in the reactor outlet and in the engine exhaust gas was measured by gas chromatography. For all of the engine tests, cylinder pressure data was acquired for 200 consecutive engine cycles, and the average values are presented here. The reactor product gas composition (REGR) is shown in Table 1. There were some small changes in the engine exhaust gas composition when REGR was added, but this was corrected by controlling the reactants in the reformer. The ratios O/C, O2/C, and H2O/C in the reformer were calculated by taking into account engine out O2, H2O, CO2, and CO. The brake-specific fuel consumption (BSFC in g kW-1 h-1) was calculated using the mass flow rate of the fuel (diesel) added to the engine and reformer. When the mass flow of the diesel fuel in the reformer was increased to produce more premixed fuel to supply the engine, the mass flow of the engine diesel fuel had to be reduced to keep the engine at the same operating point. Details of Catalysts. The reforming catalyst was a nickel-free material, containing a low loading of precious metal promoted by metal oxides. It was a Johnson Matthey proprietary formulation designed to promote all of the desired reactions (oxidation, steam reforming, dry reforming, and WGSR) while at the same time inhibiting coke formation. The diesel oxidation catalyst used in this study was a Pt supported on γ alumina. The catalyst was prepared by the standard impregnation method using platinum nitrate solution. The catalyst was dried at 105 °C overnight and calcined at 500 °C for 2 h. The CO and H2 oxidation activity of the catalyst
3. Results and Discussion Engine Performance. Figure 2a shows the reduction (mass %) of the in-cylinder injected diesel fuel for the different REGR percentages used. For example, for a 30% REGR addition, the in-cylinder injected diesel fuel was reduced by over 80% so that the engine operating condition could be maintained constant. The inlet charge temperature shown in Figure 2b was increased for the different hot REGR percentages used. For example, for the engine operation on diesel fuel only, the inlet temperature was 22 °C and was then increased to approximately 40-45 °C when 30% REGR was added. The in-cylinder pressure and heat release patterns for the different REGR percentages (0-30%) used with diesel fuel injection timing set at 23 crank angle degree (CAD) before top dead center (BTDC) are shown in Figure 3. The lower cylinder pressure observed when the engine operated with REGR is due to the later gas mixture ignition and not significantly faster combustion. In earlier studies, we have reported that, at higher engine loads and speeds with the increased in-cylinder temperature, the ignition delay was
Table 1. Reformer Out Gas Compositiona reformer out H2 (vol %) CO (vol %) CO2 (vol %) CH4 (vol %) H2O (vol %) N2 (vol %) a
∼16.7 ∼18.5 ∼2.1 ∼0.03 ∼1.8 ∼60.88
Figure 3. In-cylinder pressure and rate of heat release (ROHR) for different REGR percentages, 0-30%. Injection timing at 23 CAD BTDC.
O/C, 1.3; O2/C, 0.5; H2O/C, 0.3.
Figure 2. (a) Diesel fuel replaced (mass %) and (b) inlet temperature with the addition of different REGR percentages.
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Energy Fuels 2010, 24, 302–308
: DOI:10.1021/ef900796p
Tsolakis et al.
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