If hydrocarbons and CO are present, NO2 formation will be inhibited and start to occur only at higher temperatures。 Moreover, as NO2 will react preferentially with hydrocarbons and CO, the NO2 fraction in the exhaust gas could even be decreased when the catalyst is not correctly dimensioned。

COMBINATION OF PREOXIDATION CATALYST WITH SCR CATALYST – A few tests have been run on the model gas setup with a combination of preoxidation catalyst (90 g/ft3 Pt) and a SCR catalyst。 The preoxidation catalyst was heated to a temperature 30 C higher than the SCR catalyst in a separate furnace upstream of the furnace with the SCR catalyst at a space velocity of 60000 hr-1 to simulate typical application conditions。 The NH3 was injected after this preoxidation catalyst。 The SCR catalyst was tested at a space velocity of 30000 hr-1 at different temperatures and alpha ratios。 The results in figure 9 show that now, even at temperatures below 300 C, the NOx conversion increases linearly with the NH3/NOx ratio。

Compared to the measurements were only NO was present (see figure 4) the maximum conversion at low temperatures was increased by typically 30-40%。

DISCUSSION OF MODEL GAS RESULTS – The model gas test results show that significant improvement in the low temperature activity of the SCR system can be obtained by increasing the NO2 fraction in the NOx。 The optimum NO2 fraction is 50%。 A Pt based oxidation catalyst can increase the NO2 fraction in NOx when the catalyst is correctly dimensioned to oxidize the hydrocarbons, CO and NO。 The oxidation catalyst must also not be too large as higher levels than the optimum level of 50% NO2 will be detrimental for the NH3 efficiency but will also yield secondary emissions of NH3。 At temperatures below 150° C no significant amounts of NO2 can be produced at the oxidation catalyst and the activity of the system, even with direct NH3 injection is low。 Also for other reasons injection of urea at these low temperatures is not advisable。

ENGINE BENCH EXPERIMENTS

SET-UP AND EXPERIMENTAL CONDITIONS – For the engine bench experiments reported here, a dynamic engine test bench (see figure 10) equipped with a state of the art 4。0 L heavy duty turbo-charged direct injection diesel engine was used。 The standard exhaust components were analyzed with the Horiba MEXA exhaust analyzer system。 Additionally a laser diode spectrometer (Altoptronics LDS3000) was used to detect NH3。

The catalysts used consisted of coated metallic substrates (400 cpsi)。 These could be combined in a modular fashion to change catalyst arrangement or space velocities。 The volume of the SCR catalyst was varied between 4。6 and 9。2 L。 The volume of the hydrolysis catalyst was in all experiments 4 L。 In the experiments with preoxidation catalyst 2 L of a diesel oxidation catalyst (90 g/ft3 Pt) was added upstream of the urea injection point。 The catalyst volumes have not been optimized and are probably oversized。 The potential of SCR as a NOx reduction technology on heavy duty engines can however be demonstrated by this nonoptimized system。 

Both gaseous NH3 and a urea solution (32。5%) could be injected into the system。 The urea solution injection system and the inlet cone of the hydrolysis catalyst were optimized to guarantee a homogeneous distribution of urea on the hydrolysis catalyst。 The urea injection system consists of a pump, a pulse controlled valve and an injection nozzle which enables injection rates between 1 and 30 g/min。 It was verified by comparing on one side experiments without hydrolysis catalyst and with direct gaseous NH3 injection to, on the other side, experiments with hydrolysis catalyst and with urea solution injection, that the distribution of urea on the hydrolysis catalyst and the activity of the hydrolysis catalyst were not influencing the results for the SCR system。 The results reported here have all been obtained with the urea solution injection system。