In the exhaust after-treatment business, Emitec GmbH (Lohmar, Germany; www.emitec.com) is a relatively small player when compared to the likes of ArvinMeritor and Faurecia, but it nevertheless punches above its weight by being innovative and pushing the technology boundaries. Recently, MAN, a German truck manufacturer, announced that it was going into series production with Emitec’s PM-Filter-Catalyst. It’s thought that the system may be used by an automobile manufacturer for a diesel car program in the near future. “The system is made up of an oxidation catalyst which eradicates the hydrocarbons and carbon monoxide whilst at the same time oxidizing up the NO to NO2,” says Wolfgang Maus, Emitec CEO. “This is essential for the conversion of the particles in the PM-Filter-Catalyst.”
The PM-Filter-Catalyst is a filtration system that directs an exhaust gas by-pass stream against the sintered metal fleece and through it into the neighboring channels using the MX-blade structure. The sintered metal fleece layers store a high quantity of the particles that accumulate in this way. These are then continuously converted at relatively low temperatures—from approximately 200ºC—under the influence of the NO2 gained from the upstream oxidization catalyst. Here the carbon and hydrocarbon react with the NO2, which gives off oxygen to become NO again. The final components are CO2 and nitrogen. Using this reaction, which is constantly repeating within the PM-Filer-Catalyst, the carbon that is stored along the entire length of the PM-Filter-Catalyst can be converted at a high level of efficiency. These advantages, says Maus, are especially noticeable when the PM-Filter-Catalyst is itself deployed as a coated oxidization catalyst. Not only are CO and HC oxidized to air components, but NO also becomes NO2.
“Particulate reduction using NO2 is therefore even more intensive. This is due to the fact that the NO2—in contrast to the ceramic-coated filter—is still available even after passing through the filter fleece in the next channel and in the bypass process. In addition, the mixer structure in conjunction with the porous fleece layer is particularly effective because the channels that have been opened due to the ‘blades’ effectively ‘communicate’ with each other. In this way they guarantee an internal balance of flow and concentration.” At a temperature of about just over 350°C, the particles stored in the metal fleece burn very effectively with the oxygen present in the exhaust gas—also a continuous process.
“The PM-Filter-Catalyst system always works in equilibrium between particle storage and particle conversion via oxidation,” says Maus. “When the metal fiber fleece becomes blocked by the particles, no more particles can be stored. As soon as they have been burnt off by NO2, new particles are collected again. The filter efficiency is therefore dependent on the fact that particles are stored in the ‘puffer’ and sufficient NO2 or oxygen is available at temperatures above 180°C to provide oxidation potential. Diameter and length of the PM-Filter-Catalyst elements are adapted to suit the engine size. Depending on the space available, it is possible to use one substrate with a large diameter or several substrates of a smaller diameter arranged in parallel may also be used. There is a proportional increase in the efficiency of the PM-Filter-Catalysts with length provided if there is sufficient oxidation potential. At a length of around 300 mm, an efficiency of over 80% can be achieved in a truck. Therefore, the efficiency of the PM-Filter-Catalyst is in the region of that seen in wall flow systems, without having their disadvantages such as clogging and increases in back-pressure and consumption.”
According to Maus, the PM-Filter-Catalyst can also be used with the conventional particle filter. “If a coated PM-Filter-Catalyst is inserted upstream of a conventional filter, it converts HC and CO and also reduces the particulate mass as a pre-filter using NO2 and the super-fine particles. The conventional particle filter is therefore relieved of load and can be kept more compact if required. This also leads to a lengthening of the operational period until regeneration of the particle filter is next required, which in turn reduces operational costs.”
In order to meet Euro 5—2.0 g/kWh and a 20-30% reduction in NOx—which comes into force in 2008, many engine manufacturers see Selective Catalytic Reduction (SCR) as the best solution, but there are no clear answers, as Maus explains. “Future requirements are posing enormous problems for engine developers and exhaust gas suppliers because of the conflicting demands that have to be fulfilled simultaneously. While internal engine measures can reduce particulate mass, they often simultaneously increase the levels of NOx emissions, but both must be reduced at the same time. Whoever optimizes engines for low NOx tends to have more particulate emission in the exhaust gas. The use of the only available ceramic particle filter brings with it the danger that unburned particles can clog the filter channels. There is also the danger that above the required lifetime performance of several hundred thousand kilometers, the filter surface can become blocked through the accumulation of oil incineration ash. This would not only increase exhaust gas back-pressure and therefore fuel consumption, but would also result in considerable costs caused by filter servicing or exchange.”
However, there are other issues that need to be addressed. One is the contradictory requirements of future levels of particulate reduction around the world. “In the U.S., future levels of particulate reduction are 68% and 60% in Europe. In the U.S., an exhaust gas after-treatment system must last for 1.5 million miles, but in Europe only one million kilometers. While the SCR system being developed in Europe in long distance transport for NOx reduction, the U.S. authorities favor the NOx adsorber. If there is an onboard diagnosis device or method to guarantee that urea has entered the system and works properly then there is a good chance SCR may be accepted in the U.S.”
In the SCR process, a 32.5% solution called “AdBlue” in Europe is turned into ammoniac in the exhaust gas flow using hydrolysis. This ammoniac then converts the nitrogen oxides in the exhaust gas into the molecular air components of nitrogen, CO2 and water in the downstream reduction catalyst. The downside is that the perfect infrastructure to the urea supply is essential for this system to work. Some commercial vehicle manufacturers rely only on the SCR system and maybe the ceramic diesel particle filter.
NOx adsorbers, also known as NOx storage catalysts, provide stable storage for the nitrogen oxides during lean running that is typical in diesel engines, in order to release them in the form of the air component nitrogen during short-term operation using additional fuel. However, in order to achieve NOx emission levels under the Euro 5 limit, more frequent catalyst regeneration will be necessary. These phases then lead to higher fuel consumption as well as an increase in particle emissions, therefore creating the need for the additional use of a PM-Filter-Catalyst or a conventional particle filter. A NOx adsorber and a PM-Filter-Catalyst used in series are able to reduce pollutants effectively. The NOx adsorber can also be used in the SCR system to reduce NOx.
In its development strategy for commercial vehicles, though, Emitec is therefore covering SCR. “By using the structured foils with its Metalit LS-Design and PE-Design, it has been possible to increase the efficiency of the catalysts significantly and to reduce the catalyst volume by more than 70% to today’s size of only one and a half times the engine displacement,” says Maus. The “LS-Design” refers to the Longitudinal Structure that is being currently introduced for the first time in a diesel car while the “PE-Design,” the perforated design structure, which is deployed in the new Audi RS6.
“Because of the structured foils, the straight-line, laminar flow pattern is broken up and turbulent, radial sections of flow come about. Within the catalyst this causes a more intensive mass transfer between the edge stream that has already been cleaned by the catalyst and the core stream which has a richer pollutant content. In addition, passages in the catalyst wall ensure that there is a cross-exchange and even distribution of the exhaust gas throughout the catalyst. This results in an additional improvement in the conversion performance. Both the SCR system and the PM-Filter-Catalyst now require the same amount of space in their respective designs. As far as the commercial vehicle manufacturers are concerned, this is a great advantage—each system fits into the conventional silencers thus avoiding additional development and costs,” says Maus.