Urea SCR

ASPECTS OF CATALYST DEVELOPMENT FOR MOBILE UREA-SCR SYSTEMS - FROM VANADIA-TITANIA CATALYSTS TO METAL-EXCHANGED ZEOLITES... 

Below 200°C, reliable urea thermohydrolysis is very hard to achieve, therefore urea dosage is usually stopped in real-world urea-SCR systems in this temperature regime. Another serious problem connected with the urea injection at low temperatures is the formation of white to yellowish deposits, which are observed when urea solution is injected at very low exhaust gas temperatures or if the urea spray forms a thick film at the walls of the SCR system. The analysis of these deposits [26] showed that they mainly consist of urea and some biuret at low temperatures and of cyanuric acid and some biuret at higher exhaust gas temperatures around 350°C. From laboratory investigations of the urea decomposition, it is known that biuret is easily formed from 150 to 190°C [27], whereas the formation of cyanuric acid is predominant from 200 to 300°C, according to the following reactions [12] ... 

Hug, H.T., Mayer, A. and Hartenstein, A. (1993) Off-Highway Exhaust Gas After-Treatment Combining Urea-SCR, Oxidation Catalysis and Traps, SAE Technical Paper Series 1993-0363. 

Koebel, M., Elsener, M. and Madia, G. (2001) Recent Advances in the Development of Urea-SCR for Automotive Applications, SAE Technical Paper Series 2001-01-3625. 

Koebel, M., Elsener, M., Krocher, O., et al. (2004) NOx Reduction in the Exhaust of Mobile Heavy-Duty Diesel Engines by Urea-SCR, Topics Catal., 30/31, 43. 

Tennison, P., Lambert, C. and Levin, M. (2004) NOx Control Development with Urea SCR on a Diesel Passenger Car, SAE Technical Paper Series 2004-01-1291. 

Piazzesi, G. (2006) The Catalytic Hydrolysis of Isocyanic Acid (HNCO) in the Urea-SCR Process, PhD. Thesis No. 16693, ETH Zurich. 

Schar, C.M., Onder, C.H., Geering, H.P., et al. (2003) Control of a Urea SCR Catalytic Converter System for a Mobile Heavy Duty Diesel Engine, SAE Technical Paper Series 2003-01-0776. 

Concerning the application of the urea SCR technology to the abatement of NO emissions from vehicles, although the first commercialization for heavy-duty diesel... 

They focus on the ID simulation of an urea SCR system. The system includes a model for N02 production on a DOC, a model for urea injection, urea decomposition and hydrolysis catalyst, a model for a vanadium-type SCR catalyst and a model for NH3 decomposition on a clean-up catalyst. The catalyst models consist of a ID monolith model with global kinetic reactions on the washcoat surface, kinetic parameters have been taken from literature or adjusted to experimental data from literature. The complete model was implemented in AVL BOOST (2006). AVL BOOST is an engine cycle and gas exchange simulation software tool, which allows for the building of a model of the entire engine. 

In the first stage of the investigation the catalyst can be considered in the form of powder in order to derive intrinsic transient kinetics of all the relevant reactive processes. To this purpose, dynamic reactive experiments can be performed in a simple tubular fixed-bed microreactor over small quantities (50-200 mg) of finely powdered catalyst in principle, this guarantees negligible transport limitations and more controlled conditions (e.g. isothermal catalyst bed), hence enabling a direct estimation of intrinsic rate parameters by kinetic fit. Internal diffusion limitations are particularly relevant to the case of bulk (extruded) monolith catalysts, such as vanadium-based systems for NH3/urea SCR however, they... 

Thus, special converters and advanced engine control techniques are necessary to meet the upcoming stringent NOx emission limits. Two types of catalytic deNOx systems for mobile applications—NSRC using periodic lean/rich operation and the SCR of NOx by NH3 (urea-SCR)—are discussed in Sections VI and VII, respectively. 

Finally, an application example for a combined aftertreatment system simulation has been given and discussed, investigating a combination of a DOC and a SCR catalyst. It shows how NOx conversions in the urea-SCR converter over the ESC and ETC driving cycles can be greatly improved by N02 formation in the DOC placed upstream. The system model is used to evaluate DOC size under steady state, but also under transient cycle operation. Further examples for such combined systems which can be investigated are DOC and NSRC or combinations of different catalyst technologies with DPF. 

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