What Is Selective Catalytic Reduction?

Selective catalytic reduction is a proven and advanced active emissions control technology system that injects a liquid-reductant agent through a special catalyst into the exhaust stream of a diesel engine. The reductant source is usually automotive-grade urea, otherwise known as Diesel Exhaust Fluid. The DEF sets off a chemical reaction that converts nitrogen oxides into nitrogen, water and tiny amounts of carbon dioxide (CO2), natural components of the air we breathe, which is then expelled through the vehicle tailpipe.

SCR technology is designed to permit nitrogen oxide (NOx) reduction reactions to take place in an oxidising atmosphere. It is called “selective” because it reduces levels of NOx using ammonia as reductant within a catalyst system. The chemical reaction is known as “reduction” where the DEF is the reducing agent that reacts with NOx to convert the pollutants into nitrogen, water and tiny amounts of CO2. The DEF can be rapidly broken down to produce the oxidising ammonia in the exhaust stream. SCR technology alone can achieve NOx reductions up to 90 per cent.

 Why is SCR important?

SCR technology is one of the most cost-effective and fuel-efficient technologies available to help reduce diesel engine emissions. Its effectiveness allows diesel engines to be tuned and optimised toward maximum fuel efficiency, while the SCR systems are highly efficient at treating the engine-out exhaust.

The largest sector for use of SCR technology in the US is heavy-duty commercial trucks.  All heavy-duty diesel truck engines produced after January 1, 2010 must meet the latest EPA emissions standards, among the most stringent in the world, reducing particulate matter and nitrogen oxides (NOx) to near zero levels. SCR can reduce NOx emissions up to 90 per cent while simultaneously reducing HC and CO emissions by 50-90 percent, and PM emissions by 30-50 per cent.

In the commercial trucking industry, some SCR-equipped truck operators are reporting fuel economy gains of 3-5 percent. Additionally, off-road equipment, including construction and agricultural equipment, must meet EPA’s Tier 4 emissions standards requiring similar reductions in NOx, PM and other pollutants.  SCR is also used in some of the many different applications of off-road engines and equipment.

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