Selective Catalytic Reduction (SCR)

Selective Catalytic Reduction (SCR) is a means of converting nitrogen oxides (NO_X) with the aid of a catalyst into nitrogen (N₂) and water (H₂O). Commercial selective catalytic reduction systems are typically found on large utility boilers, industrial boilers, and municipal solid waste boilers and have been shown to reduce NO_Xby 70-95%. More recent applications include diesel engines, such as those found on large ships, diesel locomotives, gas turbines, and even cars.

SCR is the dominant technology used to meet NO_X emissions standards for heavy-duty diesel vehicles in the United States, Europe, and Japan. In developing markets such as China and India, which are planning to implement Euro IV standards, SCR is also the manufacturers preferred technology option. It allows for NO_X control with little or no fuel-economy penalty, and can permit manufacturers to continue marketing the same engine/after-treatment combination even as emission standards become tighter over time.

But SCR poses unique implementation challenges stemming from its reliance on a reducing agent (typically aqueous urea, though others can be used), which is injected into the exhaust gas upstream of the catalyst, and which must periodically be replenished. SCR requires an extensive urea delivery infrastructure for geographically dispersed mobile sources and robust fail-safes to ensure that drivers properly fill onboard urea tanks. Under certain circumstances SCR-equipped vehicles in use may also pose problems with off-cycle and unregulated emissions due to the temperature dependence of catalytic activity, improper urea dosing, catalyst poisoning, and the formation of catalytic by-products.

In terms of SCR amongst road transport the technology may be encouraged or implemented by directive.

For SCR technology amongst power plants this measure falls under the remit of the Large Combustion Plant Directive (2001/80/EC), the purpose of which is to limit the amount of sulphur dioxide, nitrogen oxides and dust emitted from large combustion plants each year.

Although in theory the use of SCR can potentially reduce NO_X emissions to zero, this is only under certain driving conditions. Furthermore, recent research in Europe found that under low-load, low-exhaust-temperature conditions during urban driving, SCR-equipped Euro V-compliant vehicles had NO_X emissions roughly three times what is allowed under the standard. A study by the Tokyo Metropolitan Government produced comparable data (see figure).

NO_X emissions by mean speed and test cycle for MY 2008 SCR-equipped Japanese truck

Another issue attracting scrutiny is the potential generation of nitrous oxide (N₂O), a powerful greenhouse gas, from SCR systems as a by-product of controlling ammonia slip. On a carbon dioxide (CO₂) equivalent basis, SCR-equipped vehicles in Japan have been estimated to emit, on average, N₂O emissions equal to 20% of their exhaust CO₂. From a climate perspective, this more than offsets the climate benefits those vehicles enjoy by virtue of using SCR instead of rival NO_X control strategies.

Generic categories of costs and benefits associated with this measure are listed below. However, this is not an exhaustive list and there may be additional features to each.

Costs:

Cost of fitting technology to the vehicles in production, Costs of technology development

Benefits:

Reduced emission levels with all associated benefits

Modelling SCR as a technical measures is achieved by examining the removal efficiency of the technology for a given fuel type, and applying the revised (abated) emission factor to the activity going through that standard.

Examples of models which are currently used to model this measure include COPERT (link) and GAINS (link). Whilst adopting somewhat varied methodologies, in both cases the broad approach is to define disaggregated vehicle types, associated activity levels and to estimate emissions based upon the specific euro standard technology in place. The euro standard performance as an abatement technology is estimated in applied research and reformatted for use in the specific modelling framework.