Preliminary study has shown that the flue gas recirculation (FGR) is one of the effective ways to reduce the nitric oxides ($NOx$) emission in industrial furnaces. The sensitivity of the $NOx$ emission from a FGR industrial furnace to the change in three major furnace input variables—inlet combustion air mass flow rate, inlet combustion air temperature, and pressure head of the FGR fan—is investigated numerically in this study. The investigation is carried out in frequency domain by superimposing sinusoidal signals of different frequencies on to the furnace control inputs around the design operating condition, and observing the frequency responses. The results obtained in this study can be used in the design of active combustion control systems to reduce $NOx$ emission. The numerical simulation of the turbulent non-premixed combustion process in the furnace is conducted using a moment closure method with the assumed $β$ probability density function for the mixture fraction. The combustion model is derived based on the assumption of instantaneous full chemical equilibrium. The discrete transfer radiation model is chosen as the radiation heat transfer model, and the weighted-sum-of-gray-gases model is used to calculate the absorption coefficient.

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