Among lean-NOx reduction technologies currently available for diesel engine emission control, selective catalytic reduction of NOx by urea (urea/SCR) and lean NOx trap technology (LNT) are the two most promising. Both technologies can provide good NOx reduction performance over a wide temperature range under well-controlled conditions, but each has its own serious drawbacks, hampering commercial application. As a promising alternative, plasma-assisted catalysis for lean-NOx reduction has been demonstrated in recent years [1-3]. In this approach, the exhaust gas is subjected to a strong electric field to generate non-thermal gaseous plasma which in turn produces highly reactive species such as ions, radicals and reaction intermediates. The major role of a plasma reactor in the engine exhaust stream is to produce NO2 from NO and partially oxidized hydrocarbons (POHC) from hydrocarbons (HC). The NO2 and POHC then react over suitable SCR catalysts located downstream to produce N2. Thus, it can be classified as a modified version of the HC/SCR technology.

On the other hand, it has been reported recently that oxygenated hydrocarbons such as ethanol and acetaldehyde are very effective reductants for NOx reduction over BaY (or NaY) catalysts [4,5] or over Ag/Al2O3 catalysts [6], while HCs such as propene, gasoline and diesel fuels tend to deactivate the BaY (or NaY) catalysts due to coking. One of recent developments in this area is a plasma-assisted catalyst system that includes a sidestream hyperplasma reactor, a dual-bed catalyst system, and ethanol or E-diesel as the reductant [4]. In this system, ethanol (or E-diesel) has been shown to provide superior NOx conversion performance to other HC reductants, such as propene, gasoline or diesel fuel, for NOx reduction without inducing catalyst deactivation [4]. Also, the addition of oxygenated hydrocarbon compounds to diesel fuels has been shown to improve engine performance while reducing exhaust emissions such as diesel particulate matter (PM) and CO emissions. Various oxgenates such as alcohols, ethers, esters and acetals have been tested in diesel engines to enhance engine performance and/or reduce exhaust emissions [7].

Encouraged by these recent developments in the use of oxygenated hydrocarbons to improve engine performance, reduce engine-out PM/CO emissions and/or remove NOx emissions through selective catalytic reduction, we have developed a plasma-assisted diesel fuel reformer to produce oxygenated hydrocarbons (OHC's) along with light hydrocarbons (HC's) by simultaneously reforming and fractionating raw diesel fuel. The system consists of an energy-efficient hyperplasma reactor and a continuous fuel reformer, which can produce OHC's such as alcohols and aldehydes that are known to be very effective for selective catalytic reduction (SCR) of NOx over suitable catalysts. It was also demonstrated that the OHC/HC ratio in the vapor reformate product can be significantly enhanced by doping the raw diesel fuel with a small amount of ethanol, as a result of the cooxidation effect. The residual liquid reformate in the reformer maintains essentially the same fuel properties as the raw diesel fuel after reforming treatment, except for its increased level of the cetane number and oxygen content. A reaction mechanism is proposed for the OHC production from raw diesel fuel treated by air plasma, which involves the liquid-phase oxidation of HC's by the free radical chain mechanism initiated by ozone from the hyperplasma reactor. Results indicate that the diesel fuel reformer developed in this work might be an enabler for SCR technology using diesel fuel as the source of the reductants, providing a way to overcome the inherent shortcomings of both the urea/SCR and the lean NOx trap (LNT) technology.

References

[1]. H. Miessner, K.-P. Francke, R. Rudolph, Appl. Catal. B 36 (2002) 53. [2]. K. G. Rappe, J. W. Hoard, C. L. Aardahl, P. W. Park, C. H. F. Peden, D. N. Tran, Catal. Today 89 (2004) 143. [3]. S. J. Schmieg, B. K. Cho, S. H. Oh, Appl. Catal. B 49 (2004) 113. [4]. B. K. Cho, J.-H. Lee, C. C. Crellin, J. G. Toner, “Plasma-Catalyst System for Diesel NOx Reduction Using Ethanol and E-Diesel: Laboratory Reactor and Engine Dynamometer Tests,” Presented at 2005 Annual Meeting of AIChE, Cincinnati, Ohio, Oct. 30-Nov. 4, 2005. [5]. B. Wen, Y. H. Yeom, E. Weitz, W. M. H. Sachtler, Appl. Catal. B 48 (2004) 125. [6]. Y. Yu, H. He, Q. Feng, H. Gao, X. Yang, Appl. Catal. B 49 (2004) 159. [7]. A. Mohammadi, T. Ishiyama, H. Kawanabe, N. Hiribe, SAE Technical Paper 2004-01-1866, 2004.