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NOx reduction systems based on the selective catalytic reduction (SCR) technology are being developed for a number of mobile diesel engine applications in the EU, Japan, and the USA. The use of ammonia has been practically ruled out, due to safety concerns, and urea (in water solution) is the preferred reductant. SCR technology is approaching commercialization in Europe, where a number of heavy-duty vehicle manufacturers selected urea-SCR for meeting Euro IV (2005) and Euro V (2008) emission standards . In Japan, the first commercial urea-SCR system was introduced by Nissan Diesel on its Quon heavy-duty truck launched in November 2004 . The 2007 emission standards for heavy-duty engines in the USA, on the other hand, will be met using EGR and other in-cylinder approaches, rather than SCR. No consensus has been yet reached on 2010 emission strategies for heavy-duty engines in the USA, or for light-duty vehicles in the USA and EU; there is a possibility that SCR catalysts may be used in those applications as well.
Historically, the introduction of ammonia/urea-SCR systems for NOx control from mobile applications started with marine engines. Large sizes and steady-state operation of marine units, similar to stationary diesel engines, make the adaptation of stationary SCR technology straightforward. The first SCR units were installed in 1989 and 1990 on two Korean 30,000 metric ton carriers . The ship operator was seeking a permit from the Bay Area Air Quality Management District (BAAQMD) to allow the reduced-emission ships in its docks. Both ships were powered by MAN B&W 2-stroke 8 MW diesel engines. The ships were equipped with ammonia SCR system designed for 92% NOx reduction. The SCR reactor included a by-pass system. Exhaust gases were passed through the reactor only when the ships were sailing in waters subject to NOx emission regulations.
Commercial SCR systems have also been installed on ferries. In 1992 the ferry “Aurora of Helsingborg” that shuttled between Sweden and Denmark was equipped with a urea SCR system . The engine was 2.4 MW Wärtsilä, type 6R32E. The reactor included three layers of monolithic extruded SCR catalysts and one layer of an oxidation catalyst. The SCR technology has also been considered for NOx control from locomotive diesel engines .
Since the early 1990s, numerous development projects have been conducted to adapt the SCR technology for diesel truck and car engines. In a project sponsored by the Netherlands Ministry of Environment, a 12 liter heavy-duty diesel engine equipped with an urea SCR catalyst system was tested under both steady-state and transient conditions . In another study by FEV and Ford a urea SCR system was tested on a 2.5 liter light-duty diesel engine . Ongoing development of the Ford system has been targeting the US EPA Tier 2 Bin 5 emission limits . TNO Automotive and Engelhard have developed an SCR system for heavy-duty engines, which was tested on DAF and Renault trucks . Johnson Matthey has been developing the Compact SCR-Trap system—a device comprising a particulate filter (CRT) upstream of an SCR catalyst .
Not surprisingly, mobile systems have also been developed by companies with traditional expertise in stationary installations, such as Haldor Topsøe  or Argillon (formerly Siemens) with its automotive SCR system termed SINOx. Reported SINOx demonstration programs involved bench testing, chassis dynamometer tests, and vehicle road test with a number of heavy-duty trucks which were operated by trucking companies in regular duty . The SINOx system has also been tested on a heavy-duty truck on US highways . In more recent US tests, the SINOx system was coupled with a particulate filter positioned upstream of the SCR catalyst .