Flue gas recirculation (subsequently called "FGR") is an effective measure for temperature control and hence for reducing thermal nitrogen oxide emissions.
The theoretical principle is illustrated as an example in the sequence diagram linked below.
In plants with FGR, purging takes place in two phases depending on the parameterization:
First phase
In the first phase, the furnace and flue gas channels are purged with the FGR channel closed. After RECI: Delay time purge position has expired, the FGR channel is opened. The purge time is paused until the control damper for the FGR rate is in the purge position.
Second phase
In the second phase, the FGR channel is purged for the remaining Purge time minus RECI: Delay time purge position.
If a RECI fan is available, it is switched on and off together with the combustion air fan. The flue gas pressure of the RECI fan must be monitored in the safety interlock circuit after fan start. If the safety interlock circuit is not included in the FSC input configuration after fan start-up, then monitoring must take place in the fuel 1 and/or fuel 2 safety interlock circuit(s).
The FSC input configuration FSC configuration for digital inputs must include the "Flue gas recirculation release" input signal.