EXCESS AIR CONTROL FOR CRACKER FURNACE BURNERS

A method for control of the air/fuel ratio of the burner(s) (excess air) of a thermal cracker includes three steps. The thermal cracker has three consecutive zones or portions through which combustion gases pass, a firebox portion, a bridge wall portion and a convection portion The first step is to direct a wavelength modulated beam of near infrared light from two different tunable diode lasers located in the bridge wall portion through combustion gas from the burner to a pair of near infrared light detectors, each positioned to receive the wavelength modulated beam of near infrared light from a different one of the two tunable diode lasers to generate a detector signal. The second step is to analyze the detector signals for spectroscopic absorption at wavelengths characteristic of oxygen and carbon monoxide to determine their respective concentrations in the combustion gas. The third step is to adjust the air/fuel ratio of the burner(s) (excess air) in response to the concentrations of oxygen and carbon monoxide of the second step.