Improved SCR Systems for HeavyDuty ApplicationsABSTRACT This paper describes the function and application of the preoxidation, hydrolysis and SCR catalysts inpidually and as a combined system for urea SCR both in model gas and engine bench tests。 Using the basic system and a non-optimized urea injection strategy 45% NOx conversion was achieved in the ESC engine test。 Adding a preoxidation catalyst significantly improved the NOx conversion in the low temperatur e region of the engine mapping。 NOx conversions over 75% can be achieved in the ESC test using this improved system。 With a 50% reduced SCR catalyst volume still a NOx conversion of over 65% could be achieved。 Tests after 200 hours engine aging show that the activity of the system is stable。 83181

INTRODUCTION 

In view of the US 2002/2004 and EU IV/V legislation, the major concern of exhaust gas aftertreatment on the future heavy duty diesel vehicles is reducing the NOx and/or the particulate emissions (see figure 1)。 Several solutions for this problem seem to be technical feasible, e。g。 various diesel particulate traps, cooled EGR。 Most of these solutions, however, make a clear trade off between emissions and fuel consumption。

Some of these are even associated with a distinct fuel penalty。 Especially in the heavy duty market fuel efficiency is a major engineering target as the end users will see a clear effect on their operation costs。 A fuel efficient mode typically gives high NOx raw emissions and low particulate emissions as is visible from the NOx-particulate trade-off curve。 A SCR (Selective Catalytic Reduction) exhaust gas aftertreatment system using urea as reductant has high NOx reduction potential。 

Therefor the engine can be operated fuel efficient, and the SCR system can reduce the emitted NOx in most cases enough to meet legislation。 Depending on the engine design, however, it can be expected that additionally also some form of particulate aftertreatment is needed。 In this paper the technical feasibility of the SCR catalyst system will be shown and discussed。 Other problems which have to addressed before the SCR system could be successfully applied, like e。g。 injection strategy, injection system, urea availability and OBD (On Board Diagnosis) will not be discussed in this paper。

DESCRIPTION OF THE SCR SYSTEM 

The basic SCR system (see figure 2), the HSO system, consists of three different catalysts in series after the urea injection point; a hydrolysis catalyst (H), a SCR catalyst (S) and a guard oxidation catalyst (O)。 The urea is sprayed evenly onto the hydrolysis catalysts which converts the urea with water selectively to ammonia (NH3) and carbon dioxide (CO2)。 Then the ammonia reacts on the SCR catalyst with the NOx present in the exhaust gas to form nitrogen (N2)。 Finally an oxidation catalyst is added as guard catalyst to avoid any secondary emissions of ammonia during dynamic operation。

MODEL GAS EXPERIMENTS 

SET-UP AND EXPERIMENTAL CONDITIONS – The equipment which was used to test the catalysts in the laboratory is schematically shown in figure 3。 The set-up consists of a set of mass flow controllers for the various gasses used, a HPLC pump to control the feed of water, an evaporator in which a nitrogen stream can be saturated with a hydrocarbon, a quartz reactor with inset in which the catalyst core  is mounted, a furnace to heat the reactor and a FTIR spectrometer to analyze both the inlet and outlet gas。 The FTIR spectrometer is connected to the rest of the set-up by lines heated to 180 C。 The pressure in the FTIR cell is kept constant。 This set-up was used to determine the steady state conversion at a standard space velocity of 30000 hr-1 at different temperatures between 150 and 520 C using, unless referred otherwise, the standard gas composition given in table 1。

In the model gas experiments ammonia was used to determine directly the activity of the SCR catalyst。 The NH3/NOx molar ratio, alpha, was for the standard conditions 0。9, which essentially limits the expected maximum NOx conversion to 90%。 It was checked that the absence of CO2 and CO did not influence the NOx conversion over the SCR catalyst used in this study。 The water concentration was kept relatively low (typically diesel exhaust gas contains ca。 6 vol。-% water) to minimize the problems in the quantitative FTIR gas analysis。 In the absence of water the activity of the SCR catalyst is completely different from the activity in the presence of water。 Above water concentrations of typically 0。5 % no effect of the water concentration is observed other than a dilution effect。