Coal-Fired Power Plants (CFPPs)
NOx – What is it and where does it come from?
NOx is actually a generic term for a group of gases called nitrogen oxides. Nitrogen
oxides contain one molecule of nitrogen and a varying number of oxygen molecules. Some of these gases are
colorless and odorless, but one of them, nitrogen dioxide (NO2) combined with particulate, can appear as
a reddish-brown layer of smog over urban areas.
How NOx Forms
Air is mostly nitrogen molecules (green in the diagram) and oxygen molecules (purple in the diagram).
When heated to around 3000 degrees Fahrenheit, the molecules break apart and oxygen atoms link with
the nitrogen atoms to form NOx, an air pollutant.
Although NOx can occur naturally in the environment, most NOx is formed from manmade sources when
fuel is burned at high temperatures. More that half of all NOx emissions come from motor vehicles, but
other contributing sources include industrial processes, power generation (utilities), and
residential fuel combustion. (See the following diagram for the most recent annual information available on NOx
emissions - 2003).