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The CRT filter is a two-stage passive diesel particulate filter system, where an uncatalyzed filter is regenerated using nitrogen dioxide (NO2) generated over an oxidation catalyst positioned upstream of the filter. By using NO2 to oxidize diesel soot —which was discussed in the paper on particulate filter regeneration—filters can be regenerated at relatively low exhaust temperatures. Indeed, on suitable applications and with the use of ultra low sulfur diesel fuel, the CRT filter is capable of regenerating at temperatures as low as 250-300°C.
“CRT”—an abbreviation for the “Continuously Regenerating Technology” trade name, originally introduced as “Continuously Regenerating Trap”—is a registered trademark of Johnson Matthey, whose researchers first described the use of NO2 for soot oxidation . This type of filter is also referred to as the CR-DPF, which stands for “continuously regenerating diesel particulate filter”. The CRT filter configuration was patented by Johnson Matthey in the USA  and in other countries, followed by market introduction in retrofit applications in the late 1990s, and as OEM system in the 2000s. As the patents expire in the late 2000s, this type of filter system will be available from a number of suppliers.
A schematic of the CRT configuration is shown in Figure 1. The filter system is composed of two devices—an oxidation catalyst (upstream) and a ceramic wall-flow diesel filter (downstream).
Figure 1. CRT Filter: Configuration and Principle of Operation
The NO2 necessary for filter regeneration is generated in the oxidation catalyst from nitric oxide (NO) present in diesel exhaust, according to the following reaction:
(1)NO + ½O2 ↔ NO2
Diesel particulate matter which is being trapped in the filter is continuously oxidized by NO2, as follows:
(2)NO2 + C → NO + CO
As it is the case with regeneration of all passive filter systems, the CRT operation depends on the vehicle’s duty cycle. A successful passive operation of the filter requires that the exhaust gas reaches a sufficient temperature and meets certain conditions, as discussed below. The regeneration of the system can be enhanced, within certain limits, by increasing the size of the catalyst and the filter and/or by increasing the noble metal loading in the catalyst. Nevertheless, if the application is unsuitable or the duty cycle is too cold, the filter may be plugged with soot or experience uncontrolled regenerations.