Talk:Selective catalytic reduction

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[edit] California and vanadium-containing catalyst

The decision to use vanadium, Cu-exchanged zeolites or Fe-exchanged zeolites for a urea-SCR application is impacted by much more than jurisdiction and whether or not "California" frowns on something. It fundamentally an engineering decision that is based on the expected operating temperature and the availability of fuels with very low sulfur. With respect to diesel fuel in the U.S., fuel that is essentially sulfur free (ULSD, <15 ppm S at the pump)is now widely available and mandated for road use.JoeM42 18:46, 8 February 2007 (UTC)

In fact the US EPA and California are not ready to approve vanadium containing SCR catalyst systems even though they might be technically superior to other technologies with respect to catalyst performance. The issue with vandadium is the potential (stress potential) public health issue with vanadium volatilization out of the vehicle's tailpipe. Vanadium hydroxy-oxides are known to be volatile and the EPA does not want to risk a public health issue with people breathing toxic vanadium emissions. So, the decision on what technology to use is subject to approval in the USA by the EPA. The interest in zeolites is driven to a large extent by a desire to avoid vanadium systems.Chem engineer 02:19, 14 February 2007 (UTC)

I disagree with the above statement. Vanadium SCR application on marine engines is directly funded by U.S. EPA in New York Harbor (the Alice Austen Ferry), and is directly referred to by EPA within Chapter 4 of their Regulatory Impact Analysis for the recently proposed Tier 4 Locomotive and Marine regulations. Vanadium-based SCR systems are also under development for EPA's Voluntary Retrofit Program. Vanadium SCR usage for NOx control for stationary sources has been approved via EPA permitting for approximately 20 years and is in fairly widespread usage in the United States for NOx control from stationary sources. It is certainly possible to limit vanadium emissions by limiting the temperature that the SCR catalyst is exposed to either passively or actively within the system design. Interest in zeolites is driven largely by the useful-life and NTE requirements for vehicles, trucks, buses and other applications in the U.S. For example, HD urban buses in the U.S. must meet the 2010 0.2 g/bhp-hr NOx standard at 435,000 miles (700,000 km) following the potential of repeated forced PM-trap regeneration events that can exceed 650 °C and must be under the 0.3 g/bhp-hr NOx NTE under virtually all loaded operating conditions, including rated conditions. Regarding the NTE, at rated conditions that can include exhaust temperatures of 550 °C in some high BMEP heavy-duty applications, there is not sufficient NOx reduction efficiency in a vanadium-based SCR system to meet the 0.3 g/bhp-hr NOx NTE, while current Fe- and Cu-exchanged gamma zeolite based systems under development for 2010 are above 95% NOx efficiency at this temperature. Similar European HD engine standards only apply to new engines for Euro IV(not at the end of useful life) or only up until 500,000 km in the case of Euro V, and only apply over the regulatory cycle (no NTE requirements). Also, the regulations of engine and vehicle emissions in Europe and most other parts of the world are based solely on type approval by the manufacturer, whereas the process in the U.S. requires engine emissions certification prior to sale that carries with it considerable financial risk for the engine manufacturer since the process includes selective enforcement audits, confirmatory testing by both U.S. EPA and California ARB, surveillance testing of emissions vehicles and/or engines sampled from the in-use fleet by EPA, as well as provisions for vehicle recall by both EPA and ARB that occur with some frequency and at great cost. The multi-billion dollar heavy-duty truck consent decree of 10 years ago for NOx exceedences is a notable example . As far as I know, there really is no comparable example of in-use enforcement of emissions regulations or manufacturer liability on the scale of the 1997 EPA HD Truck compliance action for in-use emissions that exists anywhere outside of the U.S., and manufacturers simply face far more risk of compliance liability in the U.S. market than they face anywhere else. Just ask the emissions engineers of any engine or vehicle manufacturer with sales of similar products both in the EU and the U.S. and particularly in California about how much liability risk impacts their choice of emission control systems for the U.S. market if you harbor any doubts regarding this. —Preceding unsigned comment added by JoeM42 (talk • contribs) 12 June 2007 (UTC)

[edit] Please read the above guidance on how to use a Discussion page

With all due respect, everyone is asked to read the guidance, at the top of this page, on how to use a Discussion page. Regards to all, - mbeychok 23:10, 16 February 2007 (UTC)

[edit] Uhh...

I thought the reaction 2NO − > N₂ + O₂ was thermodynamically favorable, at least at low temperatures. Why isn't this reaction catalyzed? Scythe33 (talk) 22:16, 14 February 2008 (UTC)