Selective Catalytical Reduction (SCR)


Selective Catalytic Reduction (SCR) is one of the most popular processes today to reduce emission of nitrogen oxides (NOx). These pollutants come from combustion sources in the industry, such as power plants, stationary power stations, ship engines or automobiles.

Nitrogen Oxides are a family of poisonous, highly reactive gases. These gases form when fuel is burned at high temperatures. Considering an over 100-year period, it has 310 times more impact 'per unit weight' (Global Warming Potential - GWP) than carbon dioxide (CO2) according to the United States Environmental Protection Agency (EPA). All major international environmental agencies, e.g. US-EPA, EEA, MEP-China, etc. strictly dictate NOx emission limits today.

Reduction of NOx Emissions as a Major Environmental Goal

One of the most popular techniques to reduce NOx emissions is reduction of NOx using urea (effectively ammonia (NH3)), whereby NOx is reduced to nitrogen (N2) and water (H2O). The reduction often taken place with the aid of a catalyst (Selective Catalytic Reduction – SCR), but at times a non-catalytic process (Selective Non-Catalytic Reduction – SNCR). Ammonia measurement is used both to manage urea injection and to control residual ammonia (known as "ammonia slip"). This offers many advantages such as more accurate process control, longer catalyst life, and reduced environmental impact from ammonia slip.

NOx Reduction in Industrial Processes

In power plants today, up to 99% of nitrogen oxides can be reduced. The prerequisite for this is precise control of urea and the entire process flow. Therefore, power plants as well as many other industrial sites such as cement and waste incineration plants or refineries have to be equipped with an SCR system more and more often. In Europe, new legal targets for reducing emissions are being imposed on operators of such plants under the IPPC Directive (Integrated Pollution Prevention and Control). These targets relate primarily to emissions of nitrogen oxides. In addition to this European directive, further targets must be met at state and regional level.

Automotive SCR Test Stations

Most major automotive manufacturers now offer SCR systems as standard equipment that effectively reduce nitrogen oxide emissions. After all, this is the only way for diesel engines to meet the Euro 6d emissions standard with a maximum of 80 mg NOx/km.


In the commercial vehicle sector, the addition of urea solution to reduce emissions has already been used for some time. As a result of the renewed tightening of emissions standards with real-world tests on the road, SCR technology is now essential for all larger diesel engines. The widespread introduction of SCR technology by the automotive industry has triggered numerous research activities to further improve existing systems or develop new solutions.

NOx Reduction in Shipping Sector

The shipping sector is a considerable contributor to global emissions of nitrogen oxides. Stricter regulations by the International Maritime Organisation (IMO) in the 2008 Amendments to MARPOL Annex VI apply globally from 2016.


All new ships sailing within an Emission Control Area (ECA) must reduce NOx emissions to meet the strict Tier III requirements. SCR is becoming a widely-accepted technology to meet the Tier III requirements for NOx emission limits, and can be used with marine engines using various standard fuel types.

Reliable Ammonia Measurement with the LGD F200

The LGD F200-NH3 is based on Tunable Diode Laser Spectroscopy (TDLS), and is designed for hot-wet (typically up to 220°C) gas measurements. The technology combines a precise, contact-less optical measurement with high target gas selectivity over a large measuring range.


 The OEM-module delivers a reliable and continuous measurement of NH3 with reaction times (T90) of less than five seconds. The LGD F200 is a standalone extractive solution, and can be integrated into various ammonia analyzer solutions. The measurement can take place close to the gas stream by drawing a sample (near in-situ), or through extraction of a heated sample to a common gas analyzer.