Environmental Technology ChimH409 (2-0-1) Michel Verbanck 2012 Universite Libre de Bruxelles Bruface Dept Water Pollution Control

Part 2. Air pollution control engineering Gaseous Particulates

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NO X CONTROL

Fuel NO x Organic, fuel bound nitrogen compounds in solid fuels C-N bond is much weaker than the N-N bond increasing the likelihood of NO x 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 + N 2  NO + N N + O 2  NO + O N + OH  NO + H Atomic oxygen O (resulting from the dissociation of O 2 ) 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 NH 3 + O 2  4 N H 2 O 2 NO NH 3 + O 2  3 N H 2 O Selective Non-Catalytic Reduction (SNCR) NO x 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 N 2 O (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 H 2 via reduction reactions on the surface of the catalyst. Remaining CO and HC are removed through an oxidation reaction forming CO 2 and H 2 O in the products. Light-off temperature: The temperature at which the catalytic converter becomes 50% efficient. It is approximately 270 o C for oxidation of HC and about 220 o C 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