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Effective NOx Control in Fossil Fuel Combustion Systems

Learn about the formation of NOx in fossil fuels, strategies for NOx reduction, selective catalytic reduction, and the role of catalysts in controlling emissions. Discover efficient methods, such as staged combustion and three-way catalytic converters, for minimizing nitrogen impurities. Explore the importance of Lambda probes and zeolites in achieving optimal combustion efficiency.

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Effective NOx Control in Fossil Fuel Combustion Systems

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  1. .

  2. NOX CONTROL

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

  4. 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.

  5. Staged combustion Distributed mixing burner concept

  6. Selective Non-Catalytic Reduction (SNCR) NOx control: Ammonia is the reducing agent injected into exhaust 4 NO + 4 NH3 + O2 4 N2 + 6 H2O 2 NO2 + 4 NH3 + O2 3 N2 + 6 H2O 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]). 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).

  7. 3-way catalytic converter carbons

  8. reactions fostered by oxidative conditions reactions fostered by reductive conditions Thus we have to work at stoichiometric point Lambda probe

  9.  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.

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

  11. Adsorption

  12. Octahedral Layer (Alumina) Zeolites Tetrahedral Layer (Silica) 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.

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