Abstract: A regulation system for adjusting the influx of air to a combustion zone in which an off-gas and one or more auxiliary fuels are burned. Manifestations of the composition and mass flow rates of the various fuels flowing to the combustion zone are applied to the control system. The system accommodates the signals and determines the molar proportions of the fuel components. It further determines the molar flow rate of oxygen which is theoretically necessary for complete combustion. A representation of a desired amount of excess air is then factored in and a representation of the proportion of molecular oxygen which will appear in the combustion products for the desired excess air percentage. The latter factor is used to determine the setpoint in a control loop whereby the fuel/air ratio is maintained in accordance with an output from an oxygen-sensing transducer in the flue which receives the combustion products.