EGEE 101
Energy and the Environment

NOx Reduction Approaches (Utilities)

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 Graphic of air showing about half nitrogen and half oxygen.
"Air is mostly nitrogen molecules (green in the above diagram) and oxygen molecules (purple). When heated hot enough (around 3000 °F), the molecules break apart and oxygen atoms link with the nitrogen atoms to form NOx, an air pollutant. "
Credit: DOE

Nitrogen is 78% of the volume of air. Normally, nitrogen atoms float around joined to each other like chemical couples. But when air is heated - in a high-temperature boiler's flame, for example - these nitrogen atoms break apart and join with oxygen. This forms "nitrogen oxides" - or, as it is sometimes called, "NOx" (rhymes with "socks"). NOx can also be formed from the atoms of nitrogen that are trapped inside coal (fuel NOx).

Part of the Clean Air Act is a reduction in NOx emissions from power plants using Low NOx burners. Three others are worth mentioning here:

  • Fluidized Bed Combustion: This technique utilizes low combustion temperatures, which produce much less NOx.
  • Addition of a Catalyst Bed: This approach is also used to remove NOx concentrations - however, as these catalytic reactors are very expensive to operate, this technology is often run only during those summer months when smog pollution is most significant (called a selective catalytic reduction (SCR) reactor in the figure below).
  • Urea or ammonia injection: Another approach is to add nitrogen-containing compounds that will be reactive and bind with the nitrogen atoms (single) to form nitrogen molecules (N2)
Diagram of the Selective catalytic reduction system
Here the selective catalytic reduction system is shown along with an ESP particulate collector. Urea or ammonia is also injected into the flue gas to react with the NOx in the presence of the catalyst beds.
Credit: LANL

Related Topics for NOx Reduction

Many of the approaches for NOx reduction are similar to those we discussed in the previous and upcoming lessons (Acid Deposition), so for the details on these techniques, I'd suggest you keep this in mind when we research those materials.