An industrial reheating furnace with flue gas recirculation modeled by linear transfer functions

To design an effective active combustion control system, it is important to have detailed knowledge about the dynamic relationships of the furnace from its inputs to its outputs. A novel approach has been proposed to construct a set of dynamic models in the form of transfer functions for an industrial furnace with flue gas recirculation (FGR) based on the computational fluid dynamics (CFD) simulation results. The inputs to the furnace are the flow rate of the combustion air, the temperature of the combustion air, and the pressure head of the FGR fan. The outputs are nitric oxide (NO) and oxygen (O-2) concentrations. Therefore, the furnace has been represented by a three-input and two-output system. The model construction relies on the CFD simulations of the combustion process and NO formation in the furnace as well as frequency domain system identification techniques. Low-amplitude sinusoidal signals of different frequencies have been administered at the furnace inputs around a designed furnace operating condition. The dynamic relationships among the furnace inputs and outputs at these frequencies are first established in terms of frequency responses. These frequency responses are further processed by a least-squares-based system identification technique to convert them to a set of parametric models. These models are validated against the results obtained from the CFD simulations.