Page 48 - Energize July 2021

TECHNICAL

Selective catalytic reduction (SCR) abatement method under lean burn conditions, especially when the
As mentioned, NOx is a general term referring to NO and NO₂ gases. diesel exhaust is used as a reducing agent.
These gases are generated from nitrogen and oxygen under the high In this process the system injects a small amount of diesel fuel or
pressures and temperature combustion conditions. other hydrocarbon reductant into the exhaust upstream of the catalyst.
NOx gases can however be successfully converted to N2 and water The fuel or hydrocarbon reductant serves as a reducing agent for the
using SCR – one of the most effective technologies available today. SCR catalytic conversion of NOx to N₂ .
systems are classified into two groups, Urea-SCR and Hydrocarbon- A lean NOx catalyst often includes a highly ordered porous channel
SCR - the latter is most commonly known as a lean NOx catalyst (LNC). structure made of zeolite, along with either a precious metal or base
metal catalyst. The zeolites provide microscopic sites that are fuel/
Urea-SCR hydrocarbon rich where reduction reactions can take place.
Urea-SCR uses a reductant known as a diesel exhaust fluid (DEF), which
is injected into the exhaust gas to help reduce NOx emissions, over a NOx adsorber catalysts (NAC)
catalyst. Aqueous urea has been the reductant of choice in SCR systems NOx adsorber catalysts (NAC), also referred to as lean NOx traps (LNT),
for mobile diesel engines, but this fluid will be covered in more detail provide another catalytic pathway for reducing NOx in an oxygen-rich
later in this article. exhaust stream. They are known as adsorbers or traps because part
The urea-SCR system uses a metallic (such as vanadium-based) or of their function also includes trapping the NOx in the form of a metal
ceramic (like zeolite-based) wash-coated catalysed substrate and the nitrate during lean operation of the engine.
chemical reductant – usually aqueous urea – to convert nitrogen oxides Typically, NACs consist of precious metals (like platinum or
to molecular nitrogen and oxygen in oxygen-rich exhaust streams like palladium), a storage element (such as barium hydroxide or barium
those encountered with diesel engines. Upon thermal decomposition carbonate) and a high surface area support material.
in the exhaust, urea decomposes to ammonia (NH₃) which serves as Under lean air to fuel operation, NOx reacts to form NO₂ over the
the reductant. As exhaust and reductant pass over the SCR catalyst, precious metal catalyst, followed by reaction with the barium compound
chemical reactions occur that reduce NOx emissions to nitrogen and to form barium nitrate. Following a defined amount of lean operation, the
water. Urea-SCR catalysts are often combined with a particulate filter for trapping function will become saturated and must be regenerated. This
combined PM and NOx reduction. is commonly done by operating the engine in a fuel-rich mode for a brief
period of time to facilitate the conversion of the barium compound back
Ammonia slip catalyst (ASC) to its original state and giving up NOx in the form of N₂ or NH₃ gas – the
The reaction between NOx and NH₃ is never perfect and, even though latter being an unwanted emission from the process. However, NACs
SCR systems can achieve efficiency rates often higher than 95%, there can be combined with SCR catalyst to trap NH₃ and further reduce NOx
is sometimes a waste stream of un-reacted NH₃ that goes into the via a selective catalytic reduction reaction to N₂.
atmosphere. This excess NH₃ is known as NH₃ slip. So, let’s put this symphony of technologies together into something
For this reason, SCR systems may also include an oxidation catalyst we can visualise. Figure 6 is an example of a typical Euro VI / Tier 4
downstream of the SCR catalyst to control ammonia slip. This catalyst is configuration.
aptly referred to as the ammonia slip catalyst (ASC). Be sure to look out for the next instalment in this two-part article,
The task of the ASC is the selective oxidation of the ammonia where we will explore how all these emission control strategies will affect
slip to harmless N₂ and water – usually over a platinum or aluminium the selection of fuels and lubricants that will have to feed the changing
oxide base. The ACS becomes increasingly important in SCR systems appetite of the diesel engine. n
designed for high NOx conversion efficiency, especially in the higher-
rated Euro engines. Contact WearCheck, Tel 031 700-5460, support@wearcheck.co.za,
www.wearcheck.co.za
Lean NOx catalyst (LNC)
Although the urea-SCR catalyst is widely recognised as a promising
de-NOx technology, it has some drawbacks such as NH₃ slip, the
additional process of urea injection, storage space limitations and the
low freezing point of the aqueous urea-based reductant. In an attempt
to overcome the inherent shortcomings of existing urea-SCR catalysts,
hydrocarbons have been considered as alternative reducing agents for
the SCR process.
Catalytic reduction of NOx with hydrocarbons is an attractive NOx

Figure 5: Selective cayalytic reduction Figure 6: Tier 4 technology

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