NOX REDUCTION AND PROBLEMS AT LOW TEMPERATURE

NOX is a result of the reaction of nitrogen and oxygen during engine combustion, especially during the lean-burn operation of heavy-duty diesel engines.

There are two main technologies for diesel NOX reduction: Implementing Exhaust Gas Recirculation (EGR) system and Selective Catalytic Reduction (SCR). With EGR, a controlled amount of the exhaust gas is cooled and fed back into the intake. This dilutes the air and reduces the peak combustion temperature, thereby limiting the chemical reaction responsible for creating NOX. This approach has been used for many years and is a relatively low-cost method to achieve moderate NOX reduction.

More recent legislation has required much higher levels of NOX reduction and has led to the widespread use of SCR technology, particularly on recent heavy-duty diesel engines. SCR involves the dosing into the exhaust stream of a precisely measured amount of reductant in the form of a urea solution, commonly known as diesel emission fluid (DEF) or AdBlue. This solution breaks down and releases ammonia, which then reacts over a catalyst to transform NOX into harmless nitrogen and water. At higher operating temperatures, these systems are capable of conversion efficiencies exceeding 90 percent. SCR systems usually incorporate an ammonia slip catalyst at the downstream side to mop up any small amounts of unreacted ammonia.

Low temperature operation, however, is a problem as the catalyst operates with limited efficiency below its lower operating or "light-off" temperature. The latest Euro 7 regulation will have a much wider range of on-road test conditions including low temperature urban operation and idling, combined with much lower heavy-duty NOX limits. This will create a major NOX compliance challenge. Leading commercial vehicle makers are responding by "close coupling," which involves placing an additional SCR catalyst very close to the engine to increase the temperature as much as possible (previous system layouts place the SCR catalyst further downstream, behind the soot particulate filter). It may also be necessary to build in an electrical catalyst heater to reduce NOX at start-up, although this will need a large amount of electrical energy which will increase fuel consumption and CO2 emissions.