Manfred Koebel

Urea-SCR: a promising technique to reduce NOx emissions from automotive diesel engines

Abstract Urea-SCR, the selective catalytic reduction using urea as reducing agent, has been investigated for about 10 years in detail and today is a well established technique for DeNO x of stationary diesel engines. It is presently also considered as the most promising way to diminish NO x emissions originating from heavy duty vehicles, especially trucks. The paper discusses the fundamental problems and challenges if urea-SCR is extended to mobile applications. The major goal is the reduction of the required catalyst volume while still maintaining a high selectivity for the SCR reaction over a wide temperature range. The much shorter residence time of the exhaust gas in the catalyst will lead to higher secondary emissions of ammonia and isocyanic acid originating from the reducing agent. Additional problems include the control strategy for urea dosing, the high freezing point of urea, and the long term stability of the catalyst.

NOx-Reduction in Diesel Exhaust Gas with Urea and Selective Catalytic Reduction

Abstract High values of NOx reduction may be obtained with urea as a reducing agent and a standard SCR catalyst based on TiO2-WO3-V2O5. The process was carefully investigated for possible secondary emissions and it could be shown that urea-SCR does not lead to relevant emissions of nitrogen dioxide, nitrous oxide, hydrogen cyanide and isocyanic acid. Further investigations using a HPLC method have also proved that addition compounds of higher molecular mass than urea (and urea itself) are not emitted in appreciable amounts as long as the process is properly managed, i.e., as long as the emission of ammonia is kept low. The limiting secondary emission is ammonia slip, the major problem when ammonia is used directly as a reducing agent.

Adsorption and desorption of NO and NO2 on Cu-ZSM-5

Abstract The interaction of NOx from lean exhaust gases with copper-exchanged ZSM-5 has been studied by combined adsorption and temperature-programmed desorption. Experiments with NO at 200 °C show that this molecule cannot be stored on Cu-ZSM-5. On the other hand, NO2 may be stored over a wide temperature range from both dry and humid feeds. Therefore, in order to obtain storage of NO, NO must first be oxidized to NO2. However, the oxidation reaction is inhibited by water in the exhaust gas. The adsorption of nitrogen dioxide involves a disproportionation reaction yielding two stored nitrate species and one molecule of NO released to the gas phase: 3 NO 2 +[ CuO ]⇌ NO + Cu ( NO 3 ) 2 Water in the feed gas competes with the formation of nitrates, thus lowering significantly the amount of NO2 stored on Cu-ZSM-5. NO has also a negative effect on the storage of NO2. This effect is due to its reaction with the stored nitrates, causing a considerable shift in the equilibrium of the above reaction back to NO2.

Determination of urea and its thermal decomposition products by high-performance liquid chromatography

Abstract The thermal decomposition of urea can yield a wide variety of products; apart from ammonia and isocyanic acid, addition compounds of higher molecular mass may appear. In order to detect their presence in exhaust gases from a selective catalytic reduction (SCR) process using urea as a reducing agent, a chromatographic method was developed. The chromatographic separation is performed on an anion-exchange column using a phosphate buffer (pH 7) as eluent and UV detection at 190 nm. The method allows the simultaneous determination of neutral compounds (urea, biuret, melamine, ammeline) and of anions (cyanurate, isocyanate, acetate, formate, nitrite, nitrate, etc.). The value of the method for optimizing urea-SCR process design is illustrated.

Investigation of the ammonia adsorption on monolithic SCR catalysts by transient response analysis

Abstract The adsorption of ammonia on three different monolithic SCR catalysts was investigated experimentally. Two of the examined monoliths were coated cordierite honeycombs, containing either V2O5–WO3/TiO2 or V2O5/TiO2 as active components. The third sample was a commercially available extruded monolithic honeycomb consisting of V2O5–WO3/TiO2, and was tested for comparison. Different transient response methods and a modified temperature programmed desorption (TPD) method were used to investigate the ammonia adsorption in the presence of water (5%) and oxygen (10%). Under typical SCR conditions (stoichiometric ratio NH3:NO

Storage of NO2 on BaO/TiO2 and the influence of NO

Abstract The influence of NO on the adsorption and desorption of NO 2 on BaO/TiO 2 has been studied under lean conditions. The adsorption of NO 2 involves the disproportionation of NO 2 into an adsorbed nitrate species and NO released to the gas phase with a 3:1 ratio, BaO +3 NO 2 → NO + Ba ( NO 3 ) 2 . Three different nitrate species form on the catalyst: surface nitrates on the TiO 2 support, surface nitrates on BaO, and bulk barium nitrate. The stability of the three species in different gas feeds was investigated by temperature-programmed desorption (TPD). The reverse reaction of the NO 2 disproportionation has also been observed. If NO is added to the feed, nitrates previously formed on the sorbent will decompose into NO 2 . Therefore, the above chemical equation should be considered as an equilibrium reaction. Applying this finding to the NO x storage and reduction catalyst means that NO probably reacts with the previously formed nitrates yielding NO 2 as an intermediate product. This NO 2 is subsequently reduced by the reducing agents (hydrocarbons and CO) present during the regeneration period.

NOx-Verminderung in Dieselabgasen mit Harnstoff-SCR bei tiefen Temperaturen

Die selektive katalytische Reduktion (SCR) mit Harnstoff gilt zur Zeit als das aussichtsreichste Verfahren zur NOx-Verminderung von Nutzfahrzeug-Dieselmotoren. Um das erforderliche Katalysatorvolumen moglichst klein zu halten — und auch bei tiefen Temperaturen eine ausreichende NOx-Verminderung zu erreichen — ist vorgeschlagen worden, den NO2-Anteil des Abgases durch einen vorgeschalteten Oxidationskatalysator anzuheben und so das Potenzial der „schnellen SCR-Reaktion“ zu nutzen. In der vorliegenden Arbeit des Paul Scherrer Instituts wurde das Verhalten dieser Kombination bei tiefen Temperaturen untersucht. Ergebnis: Die Kombination aus vorgeschaltetem Oxidationskatalysator plus SCR-Katalysator fuhrt zu einer deutlich verbesserten Entstickungsleistung bei tiefen Temperaturen. Fur den sicheren Betrieb eines derartigen Systems gibt es aber weiterhin eine untere Grenztemperatur, die bei etwa 180 °C anzusetzen ist.

Aufbau und Vermessung eines DeNOx-Systems auf der Basis von Harnstoff-SCR

Bei Liebherr Machines Bulle SA wurde ein DeNOx-System fur die Entstickung von Dieselmotor-Abgasen mit einen einzigen hochaktiven SCR-Katalysator aufgebaut, dessen Volumen dem doppelten Hubvolumen des Dieselmotors entspricht. An diesem System wurden verschiedene Steuer- und Regelalgorithmen fur die Harnstoffdosierung erprobt. Erstmals stand fur diese Versuche ein serientauglicher NOx-Sensor zur Verfugung, der fur die Messung von NOx und Ammoniak nach Katalysator eingesetzt wurde und eine Regelung der Harnstoffdosierung ermoglichte. Der Beitrag stellt die dabei gewonnenen Ergebnisse vor.

Development and evaluation of a DeNOx system based on urea SCR

DeNOx system comprising a single, highly active SCR catalyst with a catalyst volume of about twice the swept volume of the engine was set up and evaluated at Liebherr Machines Bulle SA in Switzerland. Various control algorithms have been tested with this system. A novel NOx sensor became available for these experiments being used to measure NOx and NH3 after the catalyst. It allowed a NOx closed-loop control system to be used for the adjustment of the urea dosage. This article presents the results of the tests.