Combustion Control in a Fuel Oil Boiler from the Flame Spectrum

A new concept of combustion control system, based on an optical information from the flame spectrum, is applied to a domestic fuel oil boiler. The objective is to achieve the maximum thermal efficiency of the boiler, while keeping a stable flame and the pollutant emissions as low as possible. CH radicals and soot continuum emissions, measured with a spectrometer just downstream of the burner exit, provide the information needed. Characterisation tests of the systems dynamics in different operating conditions where made in order to optimise the controller performance and to verify the sensor response in face to external factors. For a given burner power, the control is performed by varying only the air damper position (the flame holder position was fixed). This solution led to minimal NO emission with an improved flame stability. On the other hand, the sensor follows well changes in fuel properties and ambient conditions (external disturbances). The control strategy was tested for two power conditions (high and low firing rates of the burner). Optimal values of air excess less than 6% have been achieved for both conditions. The main limitation arise from large soot production at low firing rate where the soot emission level was found around five times greater than at high firing rate.