One of the methods to reduce NOx in large industrial boilers is natural gas reburning, which is capable of providing a 50-70% reduction in NOx. In this method, natural gas is injected into the upper furnace region to convert the NOx formed in the primary fuel’s combustion gases to molecular nitrogen. The overall process occurs within three zones of the boiler as shown in Fig. 4.15.

Primary Combustion Zone: Burners fueled by coal, oil, or gas are turned down by 10-20%. Low excess air is used to minimize NOx.

Gas Reburning Zone: Natural gas between 10% and 20% of boiler heat input is injected above the primary combustion zone. This creates a fuel — rich region where hydrocarbon radicals react with NOx to form mole­cular nitrogen. Recirculated flue gases may be mixed in with the gas before it is injected into the boiler.

Burnout Zone: A separate overfire air system redirects air from the primary combustion zone to a location above the gas reburning reaction zone to ensure complete combustion of any unreacted fuel. All coal-, oil-, or gas- fired utility boilers are suitable for reburning. There must be enough room above the main firing zone for reburning and burnout. As the natural gas replaces the primary fuel (coal or oil), the emissions of SOx, CO2, and particulates are also reduced.