What is Selective Catalytic Reduction?

Selective Catalytic Reduction (SCR) is a method of converting harmful diesel oxides of nitrogen (NOx) emissions, by catalytic reaction, into benign nitrogen gas and water. SCR can deliver near-zero emissions of NOx, an acid rain and smog-causing pollutant and greenhouse gas, in modern highway clean diesel engines.
The SCR system does not alter the design of the modern Common Rail Diesel (CRD) engine, therefore it can continue to deliver excellent fuel economy and durability. Rather, SCR provides emissions after-treatment well into the exhaust stack, in a way similar to the soot containment achieved by the Diesel Particulate Filter (DPF).
SCR works by injecting Diesel Exhaust Fluid (DEF) , such as AdBlue, into the hot exhaust stack. DEF works in conjunction with the hot exhaust gases and catalyst to break NOx into two components of our normal atmosphere—water vapor and nitrogen.
The Process - How SCR Works

Engine: The NOx reduction process starts with an efficient CRD engine design that burns clean Ultra Low Sulfur Diesel (ULSD) and produces inherently lower exhaust emissions—exhaust that is already much cleaner due to leaner and more complete combustion.
Diesel Exhaust Fluid (DEF) tank and pump: Under the direction of the vehicle’s onboard computer, DEF is delivered in precisely metered spray patterns into the exhaust stream just ahead of the SCR converter.
SCR Catalytic Converter: This is where the conversion happens. Exhaust gases and an atomized mist of DEF enter the converter simultaneously. Together with the catalyst inside the converter, the mixture undergoes a chemical reaction that produces nitrogen gas and water vapor.
Control device: Exhaust gases are monitored via a sensor as they leave the SCR catalyst. Feedback is supplied to the main computer to alter the DEF flow if NOx levels fluctuate beyond acceptable parameters.

Honda acknowledges both approaches. The company also notes problems with each: that the adsorber can impose a fuel penalty due to the regeneration strategy, and the urea-injection approach requires the development of an infrastructure for another fluid.



In the proposed Honda system, the electrically-powered plasma reactor first converts oxides of nitrogen other than NO₂ to NO₂. In addition, and in conjunction with a reducing agent injected upstream of the reactor, it can also oxidize PM. Multiple reactors could be placed in series or in parallel, if needed.
The NO₂ exhaust stream then flows to the catalyst units where it is adsorbed or reduced by alkali metals and silver.
Other companies and laboratories are exploring the use of plasma-catalyst combinations for NO_x reduction.

• Research funded by the DOE and later by ArvinMeritor led to the deployment of a plasma reformer (Plasmatron) for use with heavy-duty diesel engines. In development since the 1990s, the system reduced NO_x emissions by up to 90% when used with an adsorber catalyst. It operated effectively at lower temperatures than other NO_x removal systems, and it reduced the amount of fuel required for adsorber regeneration in half.
• GM researchers have developed a plasma-assisted catalyst system (PAC) capable of reducing NO_x under highly lean conditions using E-diesel or ethanol as the reductant. The system consists of a compact, energy-efficient hyperplasma reactor followed by a dual-bed catalytic reactor. They also demonstrated good NO_x conversion (above 90% on average) over a wide temperature range of 200-400° C under steady-state optimum operating conditions.
• Pacific Northwest National Laboratory also developed a two-phase approach to a plasma catalysis system, also producing reduction of NO_x emissions by as much as 90%.
• Caterpillar has looked at reformer-assisted lean NO_x catalysis as well as plasma-facilitated catalysis.
• Researchers from Ford, GM, DaimlerChrysler and PNNL collaborated on a three-phase plasma-catalyst system. (They found that with hexene as a reductant, the system reduced NO_x by more than 90%; with diesel or Fischer-Tropsch reductant, however, the catalyst efficiency rapidly dropped off.)