Methods for NOx control can be classified into two broad categories:

1. Postcombustion methods: methods that are deployed after flue gases are generated.

2. Combustion control methods: methods that are deployed during the combustion process.

Postcombustion Methods

As the name implies, postcombustion methods deal with the flue gases obtained after combustion. They are more expensive than combustion control methods, because they handle large quantities of flue gases generated in the process of combustion. The ratio of flue gas to fuel on a weight basis is about 21 for natural gas and 18 for fuel oils in steam generators. In gas turbines, the exhaust gas quantity generated is very large because on the order of 200-300% excess air is used. The two commonly used methods of control are

1. Selective noncatalytic reduction (SNCR) methods

2. Selective catalytic reduction (SCR) methods