Catalyst: what is? how does it work?
Generally speaking, a catalyst is a material whose function is to increase the rate of a chemical reaction.
Due to the fact that its participation to the process is only external, unlike other reagents in the chemical
reaction, a catalyst is not consumed and it may participate in multiple chemical transformations.
The effect of a catalyst may vary due to the presence of other substances known as inhibitors or poisons
(which reduce the catalytic activity) or promoters (which increase the activity).
The reason why users have to replace its elements from time to time, varying on the usage conditions (fuel
type, lube oil consumption, etc.) is that all the poisoning elements contained in the exhausts are reducing
the catalyst activity. The phenomenon occurs when the poisons react with catalyst’s solid surface, chemically bonding to its active surface and creating a coating over the working surfaces which encapsulates the
catalyst so that it cannot contact and treat the exhausts.
Different existing exhaust gas treatment systems.
Every kind of engine, depending on its working cycle and on the used fuel has its own typical emission
limits on different pollutant gases.
On diesel and miller cycle engines using diesel oil/heavy fuel/biofuel the most important pollutants are Nitrogen oxides (NO and NO2), HC and CO:
• NOx can be successfully converted to harmless components as Nitrogen (N2) and water (H2O) by
means of the Selective Catalytic Reduction (SCR) unit. Usually divided in multiple layers, it lets the mentioned gases react with the ammonia(normally urea is used) sprayed on the catalytic elements, usually made
by a ceramic carrier enriched by precious metals (usually Vanadium pentoxide) which actually are the
catalytic active components. With this process up to 98% NOx reduction is achievable.
• HC and CO can be converted to CO2 and water by means of oxidation catalysts which are physically
located downstream the other layers and which are responsible for the oxidation of the slight ammonia
slip coming from the SCR in order to harmless components as Nitrogen (N2) and water (H2O). The
catalytic elements are usually made by a ceramic carrier enriched by platinum or palladium (catalytic active
components).
On gas engines we have two kind of possible combustions: lean burn and stoichiometric combustion engines:
• On lean burn engines, the most harmful gas which is possible to threat is CO; therefore the goal is to
oxidize it to CO2, using the massive O2 presence inside the exhausts. During the process also nonmethane hydrocarbons (NMHC) and volatile organic compounds (VOC) are oxidized to CO2 and water
(H2O). For this kind of application metallic foils are usually used as support for the precious metals
(platinum and palladium) acting as catalyst. This layout grants very low backpressure levels thanks to high
void fraction metal substrate. The conversion efficiency is variable and can arrive up to 99% for CO.
• On stoichiometric combustion engines it is possible to use the three-way catalyst for simultaneous conversion of oxides of nitrogen (NOx), carbon monoxide (CO), hydrocarbons (HC), formaldehyde (CH2O)
and EPA classified Hazardous Air Pollutants (HAPs). Support is usually metallic foils in this application as
well, in order to reach very low backpressure (high void fraction metal substrate). The conversion
efficiency is variable and can arrive up to 99% both for CO and NOx.
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1. Control Cabinet with Touch.Screen PLC
2. Integrated NOx Analyzer (optional) for maximum
NOx reduction
3. SCR reactor with High-Efficiency Catalyst Technology
• up to 98 % reduction in NOx
• up to 95 % reduction in CO
• up to 90 % reduction in HC
• minimal ammonia slip
4. Mixing Pipe with Two-Phase Injection Nozzle
5. IPrecision Dosing System for fine metering of urea or
ammonia solution
6. Pump Station with accurate flow control
7. Storage tank for Urea or Ammonia Solution (optional)
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