How the SCRT® Works
The SCRT® system combines two major components, the CRT® and the SCR, integrated into a single compact unit.
The CRT® precedes the SCR section. It uses a process that functions on the basis that soot will begin to oxidise in the presence of NO2 at around 250°C, compared to 600°C with oxygen. This lower temperature is compatible with the typical exhaust temperature of diesel engines.
The CRT® system is made up of two chambers, a highly active platinum oxidation catalyst followed by an extremely fine ceramic wall-flow filter.
As the dirty exhaust gas enters the first chamber, it hits a diffuser plate which distributes the gas evenly through the catalyst. The platinum oxidation catalyst oxidises the CO and HC into CO2 and H2O, removing them from the exhaust gas. It also oxidises some of the NO to NO2. This is the key to the removal of particulate matter collected by the CRT® filter.
CO + ½O2 -> CO2
[HC] + O2 -> CO2 + H2O
NO + ½O2 -> NO2
In addition to the removal of particulate matter the CRT® is designed to optimise the exhaust gas composition so that excellent reduction of nitrogen oxides is achieved in the SCR element of the system.
The wall-flow filter in the second chamber traps the particulate matter in the exhaust gas. This trapped particulate matter is continually oxidised by the NO2, removing it from the exhaust gas.
[C]+ 2NO2 -> CO2 + 2NO
This process removes over 90% of all particulate matter, hydrocarbons and carbon monoxide.
The exhaust gas now passes in to the SCR section, where the nitrogen oxides are removed. The industry standard AdBlue®, an aqueous solution of urea, is used as the SCR reducing agent. Like the CRT®, the SCR also consists of two chambers, one to mix the urea with the exhaust gas and the second, a reducing catalyst specifically developed to optimise the chemical reaction.
The mixing chamber creates a homogeneous mix of urea and the exhaust gas. A high pressure injector system and specially designed pipe-work are key to the mixing process. During this process, the urea begins to decompose to form ammonia. It is the ammonia which neutralises the nitrogen oxides.
CO2 + 2NH3
The reduction catalyst in the second chamber speeds up the reaction between the ammonia and nitrogen oxides, producing water and nitrogen, both harmless.
4NO + 4NH3 + O2 -> 4N2 + 6H2O
This process demands an exact dosing of urea, otherwise all the nitrogen oxides will not be consumed and ammonia could be emitted from the tail pipe. Nitrogen oxide and temperature sensors provide the necessary information to be able to dose the urea accurately.