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NOX CONTROL

Fuel NOx

Organic, fuel bound nitrogen compounds in solid fuels

C-N bond is much weaker than the N-N bond increasing the likelihood of NOx formation

Nitrogen impurities in fossile fuel are amines and amides, together with molecules such as:

Formation of thermal NO:

 • The NO formation kinetic model is based on the extended Zeldovich mechanism:

  O + N2 NO + N

 N + O2 NO + O

 N + OH NO + H

Atomic oxygen O (resulting from the dissociation of O2 ) will only exist

at temperatures over 1250 °C

We can therefore consider a staged combustion strategy.

Staged combustion

Distributed mixing burner concept

4 NO + 4 NH3 + O2 4 N2 + 6 H2O

2 NO2 + 4 NH3 + O2 3 N2 + 6 H2O

Selective Non-Catalytic Reduction (SNCR)

NOx control: Ammonia is the reducing agent injected into exhaust

Urea is often preferred to ammonia because it is safer and easier to handle. The risk, however, is the neo-formation of N2O (which is a greenhouse gas).

Efficiency 30-40%; can be enhanced to 70% by inserting a catalytic bed (NOx control by the SCR process [see technical visit at the Brussels-Energy combustion plant]).

3-way catalytic converter

carbons

reactions fostered by oxidative conditions

reactions fostered by reductive conditions

Thus we have to work at stoichiometric point Lambda probe

 Three-way catalyst consists of: 

• Rhodium – the principal metal used to remove NO 

• Platinum – the principal metal used to remove HC and CO NO reacts with CO, HC and H2 via reduction reactions on the surface of the catalyst.

 Remaining CO and HC are removed through an oxidation reaction forming CO2 and H2O in the products.

Light-off temperature: The temperature at which the catalytic converter becomes 50% efficient. It is approximately 270oC for oxidation of HC and about 220oC for oxidation of CO. Conversion efficiency at fully warmed up condition is 98-99% for CO and 95% for HC, depending on the HC components. 

Lambda probe: The closed-loop fuel metering system maintains the Air/Fuel ratio to the stoichiometric mixture

Adsorption

Zeolites

Tetrahedral Layer (Silica)

Octahedral Layer (Alumina)

Zeolites are crystalline alumino-silicates, which can work as molecular sieve adsorbents. They have a 3D interconnecting network of silica tetrahedra and alumina octahedra. Natural water of hydration is removed from this network by heating. This produces very small cavities (e.g. 4 Angstrom, for a given zeolite) which, due to their uniform pore size, will selectively adsorb molecules of a specific size.