Off-stoichiometric, or staged, combustion is achieved by modifying the primary combustion zone stoichiometry – that is, the air/fuel ratio. This may be accomplished operationally or by equipment modifications. http://arator.fi/
An operational technique known us burners-out-of-service (BOOS) involves terminating the fuel flow to selected burners while leaving the air registers open.
The remaining burners operate fuel-rich, thereby limiting oxygen availability, lowering peak flame temperatures, and reducing NOx formation. The unreacted products combine with the air from the terminated-fuel burners to complete burnout before exiting the furnace. Figure 2 illustrates the effectiveness of this technique applied to electric utility boilers. Staged combustion can also be achieved by installing air-only ports, referred to as overfire air (OFA) ports, above the burner zone. redirecting a portion of the air from the burners to the OFA ports. A variation of this concept, lance air, consists of installing air tubes around the periphery of each burner to supply staged air.
BOOS, overfire air, and lance air achieve similar results. These techniques are generally applicable only to larger, multiple-burner, combustion devices.
Low-NOx burners are designed to achieve the staging effect internally. The air and fuel flow fields are partitioned and controlled to achieve the desired air/fuel ratio, which reduces NOx formation and results in complete burnout within the furnace. Low-NOx burners are applicable lo practically all combustion devices with circular burner designs.