NOx reduction technologies can be grouped into two broad categories: combustion modifications and post-combustion processes. Some of the more important NOx control technologies are briefly discussed below.

Combustion Modification

Low-NOx Burners — LNBs are designed to control the mixing of fuel and air to achieve what amounts to staged combustion. This staged combustion reduces both flame temperature and oxygen concentration during some phases of combustion, in turn, reduces both lower thermal NOx and fuel NOx production.

Overfire Air — OFA is air that is injected into the furnace above the normal combustion zone. Generally when OFA is employed, the burners are operated at a lower than normal air-to-fuel ratio, which reduces NOx formation. OFA, which is frequently used in conjunction with LNBs, completes the combustion process at a lower temperature.

Reburning — In the reburning process, part of the boiler fuel input (typically 10-25%) is added in a separate reburn zone. The fuel-rich reducing conditions in this zone lead to the reduction of NOx formed in the normal combustion zone. OFA is injected above the reburn zone to complete combustion. Thus, with reburn there are three zones in the furnace: (1) a combustion zone with an approximately normal air-to-fuel ratio; (2) a reburn zone, where added fuel results in a fuel-rich condition; and (3) a burnout zone, where OFA completes the combustion. Coal, oil, or gas can be used as the reburn fuel.

Flue Gas Recirculation — FGR, in which part of the flue gas is recirculated to the furnace, can be used to modify conditions in the combustion zone (lowering the temperature and reducing the oxygen concentration) to reduce NOx formation. FGR is also used as a carrier to inject fuel into a reburn zone to increase penetration and mixing.

Operational Modifications — Changing certain boiler operational parameters can create conditions in the furnace that will lower NOx production. Examples include burners-out-of-service (BOOS), low excess air (LEA), and biased firing (BF). In BOOS, selected burners are removed from service by stopping fuel flow, but airflow is maintained to create staged combustion in the furnace. LEA involves operating at the lowest possible excess air level without interfering with good combustion, and BF involves injecting more fuel to some burners (typically the lower burners) while reducing fuel to other burners (typically the upper burners) to create staged combustion conditions in the furnace.

Post-Combustion Treatment

Selective Catalytic Reduction — In SCR, a catalyst vessel is installed downstream of the furnace. Ammonia (NH₃) is injected into the flue gas before it passes over the fixed-bed catalyst. The catalyst promotes a reaction between NOx and NH₃ to form nitrogen and water vapor. NOx reductions as high as 90% are achievable, but careful design and operation, such as control of the reagent dosage and assuring good mixing, are necessary to keep NH₃ emissions (referred to as 
NH₃ slip) to a concentration of a few ppm.

Selective Noncatalytic Reduction — In SNCR, a reducing agent (typically NH₃ or urea) is injected into the furnace above the combustion zone, where it reacts with NOx as in the case of SCR. Critical factors in applying SNCR are sufficient residence time in the appropriate temperature range and uniform distribution and mixing of the reducing agent across the full furnace cross section.

Hybrid Processes — SNCR and SCR can be used together with some synergistic benefits. Also, either process can be used in conjunction with LNBs.