Selective catalytic reduction

Selective catalytic reduction of NOx using ammonia as the reducing agent was patented in the United States by the Engelhard Corporation in 1957.

Development of SCR technology continued in Japan and the US in the early 1960s with research focusing on less expensive and more durable catalyst agents.

SCR systems are now the preferred method for meeting Tier 4 Final and EURO 6 diesel emissions standards for heavy trucks, cars and light commercial vehicles.

[3][4] Base metal catalysts, such as vanadium and tungsten, lack high thermal durability, but are less expensive and operate very well at the temperature ranges most commonly applied in industrial and utility boiler applications.

Thermal durability is particularly important for automotive SCR applications that incorporate the use of a diesel particulate filter with forced regeneration.

Plate-type catalysts have lower pressure drops and are less susceptible to plugging and fouling than the honeycomb types, but are much larger and more expensive.

Honeycomb configurations are smaller than plate types, but have higher pressure drops and plug much more easily.

In marine applications, this can increase fresh water requirements as the boiler must be continuously washed to remove the deposits.

Of more concern to SCR performance are poisons, which will chemically degrade the catalyst itself or block the catalyst's active sites and render it ineffective at NOx reduction, and in severe cases this can result in the ammonia or urea being oxidized and a subsequent increase in NOx emissions.

Without tuning, SCRs can exhibit inefficient NOx reduction along with excessive ammonia slip due to not utilizing the catalyst surface area effectively.

In applications using both SCR and an alkaline scrubber, the use of high-sulfur fuels also tend to significantly increase ammonia slip, since compounds such as NaOH and Ca(OH)2 will reduce ammonium sulfate and ammonium bisulfate back into ammonia: Temperature is SCR's largest limitation.

A significant operational difficulty in coal-fired boilers is the binding of the catalyst by fly ash from the fuel combustion.

This requires the usage of sootblowers, ultrasonic horns, and careful design of the ductwork and catalyst materials to avoid plugging by the fly ash.

[10] In 2007, the United States Environmental Protection Agency (EPA) enacted requirements to significantly lower harmful exhaust emissions.

These engines require the periodic addition of diesel exhaust fluid (DEF, a urea solution) to enable the process.