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Articles & Resources From Combustion Controls Solutions & Environmental Services, Inc.

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Phone: 419.841.9984    Fax: 419.841.9535

Scrubbing Diesel: Selective Catalytic Reduction Systems and transportation

The future of clean diesel may reside in the industrial cleansing power of SCR


The use of selective catalytic reduction – also known as SCR systems – has been in use for decades to remove nitrogen oxides (NOx) from industrial exhaust. Any of these NOx chemicals released into the atmosphere will combine with other elements as a primary component of smog and acid rain.

The use of SCR systems creates a chemical reaction to industrial exhaust, combining it with ammonia before passing it over a catalyst and heat to form harmless water vapor and nitrogen. This selective catalytic reduction process can destroy up to 99 percent of NOx chemicals in industrial exhaust.

But industrial output is only one of several points of NOx pollution. In fact, the EPA reports in California that personal transportation and shipping lead the emissions factors for NOx in the state. Thankfully, the older science of SCR systems is currently being adapted to account for this pollution source and provide abatement.

The process as adapted to diesel engines is virtually the same as the industrial selective catalytic reduction. But instead of the ammonia stream entering a giant combustion chamber as it would with an exhaust flue – it enters the emission stack or tailpipe. The combined ammonia and exhaust mixture then passes over a catalyst insert in the vein of a catalytic converter, which is already mandatory in many states for highway vehicles. The result, emissions free of NOx and filled with harmless water and nitrogen.

There are already applications of SCR systems in large-scale diesel engines for both rail and commercial ocean-going ships. Both of these transportation networks are among the worst offenders in NOx emissions, so taking selective catalytic reduction out of the fixed-point world of industry and to the road makes a lot of sense.

However, there are some hitches in the technology that are holding SCR systems from becoming more widely used in applications such as diesel trucking. First, the variation of NOx from the stack depending on the amount of horsepower used by the truck means a very sensitive computer monitoring the ammonia concentration in the SCR chamber is required. If the mixture fails to become rich enough, the reaction doesn’t take place and the system is worthless. If it’s too rich, it could mean toxic ammonia exposure.

In short, Selective Catalytic Reduction systems are promising if not ready for primetime players making the move from the staid industrial setting to overcoming one of the largest sources of NOx air pollutants in commercial transportation.

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