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Energy & Fuels 2005, 19, 744-752
Reaction Profiles during Exhaust-Assisted Reforming of Diesel Engine Fuels A. Tsolakis† and A. Megaritis* School of Engineering, Mechanical and Manufacturing Engineering, The University of Birmingham, Birmingham B15 2TT, United Kingdom
S. E. Golunski Johnson Matthey Technology Centre, Blount’s Court, Sonning Common, Reading RG4 9NH, United Kingdom Received October 28, 2004. Revised Manuscript Received January 29, 2005
On-board hydrogen production by exhaust-assisted fuel reforming offers a potential route to diesel emission control. In this study, different reformer configurations with the same catalyst composition were tested, as well as three types of diesel engine fuel, under conditions designed to simulate low- and high-load engine operating modes. The reforming efficiency was dependent on the fuel type and followed the general trend of bioethanol > rapeseed methyl ester > lowsulfur diesel fuel. In each case, a characteristic axial temperature profile was established in the catalyst. The profile can be deconvoluted into three phases, corresponding to the consecutive occurrence of combustion, steam reforming, and water-gas shift. The relative contribution of each process not only responds to changes in the inlet conditions but also to the aspect ratio and form of the catalyst bed. The heat produced in the first phase must be effectively transferred to the endothermic and much-slower steam-reforming reaction, whereas in the final phase, heat must be lost to ensure that the thermodynamically limited water-gas shift reaction is favored.
Introduction The use of exhaust gas recirculation (EGR) in diesel engines reduces nitrogen oxide (NOx) emissions but results in an increased release of smoke and particulate matter (PM), as well as higher fuel consumption.1,2 In an ongoing study, we have already shown the potential for reducing the adverse effects on smoke, particulates, and fuel economy, by incorporating an exhaust gas fuel reformer in the EGR loop.1,3-5 Some of the primary fuel is injected into the reformer, where it reacts with steam and oxygen on the surface of a catalyst to produce a hydrogen-rich gas, which is fed back to the engine. In this way, the reforming process utilizes several of the waste products (O2, H2O, heat) emitted during combustion. The system provides the engine with “reformed † Current address: Johnson Matthey Technology Centre, Blount’s Court, Sonning Common, Reading RG4 9NH, U.K. E-mail address:
[email protected]. * Author to whom correspondence should be addressed. Telephone: +44 (0)121 4144170. Fax: +44 (0)121 4143958. E-mail address:
[email protected]. (1) Tsolakis, A.; Megaritis, A.; Wyszynski, M. L. Energy Fuels 2003, 17 (6), 1464-1473. (2) Ladommatos, N.; Abdelhalim, S. M.; Zhao, H.; Hu, Z. SAE Tech. Pap. Ser. 1998, 980184. (3) Tsolakis, A.; Megaritis, A. SAE Tech. Pap. Ser. 2004, 2004-011844. (4) Tsolakis, A.; Megaritis, A.; Wyszynski, M. L. Fuel 2004, 83 (13), 1837-1845. (5) Teo, L. K. S.; Tsolakis, A.; Megaritis, A.; Wyszynski, M. L. Hydrogen and Biodiesel Mixtures as Fuels for the Compression Ignition Engine. In Proceedings of the 2nd International Conference on Thermodynamic Processes in Diesel Engines (THIESEL), Valencia, Spain, September 11-12, 2002, pp 389-395. (ISBN 84-9705-233-1.)
EGR” (REGR). Unlike standard EGR, where only the flow and temperature of the recycled gas are controlled, the REGR system additionally provides a means of chemically influencing the combustion process. Another possible function of the reformer can be to supply hydrogen to a catalytic converter (downstream of the EGR loop), where the H2 can be used to reduce NOx either by direct reaction6 or by promoting the reducing activity of hydrocarbons.7 In recent publications,1,4,8,9 we have presented results from exhaust gas fuel reforming of diesel fuel using a catalytic packed-bed mini reactor. These results suggested that, under the conditions that we used, the main reactions occurring in the reformer were combustion (reaction 1), steam reforming reaction (SRR, reaction 2), and water gas shift reaction (WGSR, reaction 3); however, there was no evidence of direct partial oxidation (POX, reaction 4) occurring.
CnH1.88n + 1.47nO2 f nCO2 + 0.94nH2O
(1)
CnH1.88n + nH2O f nCO + 1.94nH2
(2)
mCO + mH2O f mCO2 + mH2
(3)
n CnH1.88n + O2 f nCO + 0.94nH2 2
(4)
(6) Burch, R.; Coleman, M. D. J. Catal. 2002, 208 (2), 435-447. (7) Richter, M.; Bentrup, U.; Eckelt, R.; Schneider, M.; Pohl, M.M.; Fricke, R. Appl. Catal., B 2004, 51 (4), 261-274.
10.1021/ef049727p CCC: $30.25 © 2005 American Chemical Society Published on Web 03/10/2005
Exhaust-Assisted Reforming of Diesel Engine Fuels
Energy & Fuels, Vol. 19, No. 3, 2005 745
Table 1. Fuel Properties fuel analysis
method
cetane number density at 15 °C (kg/m3) viscosity at 40 °C (cSt) 50% distillation (°C) 90% distillation (°C) LCV (MJ/kg) sulfur content (mg/kg) aromatics (wt %) monoaromatics diaromatics triaromatics total molecular weight carbon (wt %) hydrogen (wt %) oxygen (wt %)
ASTM D613 ASTM D4052 ASTM D445 ASTM D86 ASTM D86
ultralow-sulfur diesel (ULSD)
rapeseed methyl ester (RME)
53.9 827.1 2.467 264 329 42.7 46
54.7 883.7 4.478 335 342 39 5
21.0 3.1 0.3 24.4 209 86.5 13.5
2 (i.e., 3 at test point 4) provides evidence for the occurrence of the WGSR, where some of the CO produced by the SRR reacted with water to generate more H2 and CO2. From these results and those in the previous section, it is clear that the GHSV cannot be standardized for the catalyst in different reforming modes (pure SRR with external heating; autothermal reforming; exhaust gas fuel reforming at high temperatures; exhaust gas fuel reforming at low temperatures). In the SRR and exhaust gas fuel reforming at high temperatures (with appropriately chosen flow rates of the reactants), the entire catalyst bed can be active in the production of H2 (and CO). On the other hand, in the case of lowtemperature exhaust gas fuel reforming, only part of the bed is effectively used. These are very important factors to consider in the design of an engine-reformer system, where the composition and temperature of the exhaust gas vary considerably during a driving cycle. For example, at engine part loads and low speeds, the exhaust gas contains high levels of O2 (>16%) and low levels of H2O (
