Provided is a method of controlling the operation of a combustion device (1) to provide safe and reliable operation while reducing NOx emission that includes providing a flow of fuel (14) and diluent (15) at a determined volume ratio to a flame (12) in the combustion device; providing a flame stability sensor (30, 40, 50) to generate a measurement of a characteristic of the flame, providing a flow measurement for each of the fuel and diluent, and controlling the determined volume ratio of fuel:diluent using the measurement from the flame stability sensor and/or flow measurements. A combustion system incorporating this method also is included.