Continuously regenerating trap CRT

The second is a neat idea coming from Johnson Ma tthey. They invented the so-called continuously regenerating trap (CRT) consisting of a monolithic preoxidizer and a particulate trap, see Figure 9.3 [24]. The first monolith (containing Pt) oxidizes hydrocarbons and CO to CO2 and NO into NO2, which is very reactive... 

In order to simulate decreasing emission levels a truck engine of Euro III emission level (6 cylinder Volvo, displacement 7 liters) has been equipped with a continuously regeneration trap (CRT) filter system and an additional bypass to this filter by the Swiss EMPA institute. By a variation of the flow ratio of filtered/unfiltered exhaust gas, an emission level of about 60% of Euro IV has been adjusted (Mohr and Lehmann, 2003). The temporal courses of EC mass concentration and EC-specific surface area are depicted in Figure 34. In this graph, the mass and surface emissions of the 13 operation points within this test yield a different behavior. Most pronounced is the small specific surface area for idle operation which is caused by relatively large particles and in agreement to the results of previous studies. 

Figure 7 Simplified schematic diagram of a continuously regenerating trap (CRT). The platinum catalyst oxidizes hydrocarbons and carbon monoxide, and also nitric oxide to nitrogen dioxide, which is used to oxidize soot retained in the filter. In the illustration this is a ceramic-wall flow filter, which has alternate channels blocked at the front inlet and rear outlet faces.

The emissions of PM can be reduced using filter technology, where the continuously regenerating trap (CRT) system appears promising [1]. The emission levels of NOx are however only affected by the CRT technology to some extent [1]. 

The continuously regenerating trap (CRT) system (by Johnson Matthey) con-skts of a DPF filter located downstream of a Pt-containing DOC catalyst. CO and HC are oxidized in the DOC, which ako oxidizes NO to NO2. NO2 k much more active in its oxidizing reactions than NO and O2 and reacts rapidly with the soot and initiates its ignition. 

Reaction (2.2) starts from as low as 250" C, so that the soot collected in the DPF is slowly and continuously oxidized under typical engine exhaust gas temperature conditions (180-300°C). As a result, Johnson Matthey refers to its developed DPF that uses this catal5dic process as a continuously regenerating trap (CRT). 

Another well-recognised approach for soot abatement is the use of a supported Pt oxidation pre-catalyst (upstream of the filter) aimed at producing NO2 (from NO oxidation), which decreases the non-catalysed oxidation temperature of soot by approximately 200 K (from ca. 773 to 573 K) relative to air oxidation.76,81,94 This is one of the basic concepts involved in the so called NOx-aided CRT (continuously regenerated trap), which is proposed as one of the most efficient technologies for soot abatement (Fig. 8.4).76,95 Under practical operating conditions, the oxidation precatalyst overall converts 90% of the CO and hydrocarbons and 20-50%... 

Continuous Regenerable Trap. Another approach is to generate NO2 upstream of an uncatalyzed DPF for combusting the particulate matter. This so-called continuous regenerable trap (or CRT) is based on phenomenon first published in 1989 by Cooper and Thoss (143), in which NO2 oxidized the dry carbon soot held within the trap at temperatures below that of similar oxidation with the O2 in the air. This technology is considered passive in contrast to this active system. 

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