Combustion control by means of a photodiode

The objective of this work was to assess the feasibility of a photodiode to control combustion, instead of a spectrometer (which is an expensive sensor to be used in commercial burners). For that purpose, the flame emission has been measured in a hot-water boiler fired with light fuel-oil, whose nominal output power goes up to 370 kW. The parameters investigated were: overall air excess, combustion air distribution, burner power and atmospheric conditions. High influence of air distribution on the flame emission was detected by the diode, since it modifies directly the mixing quality reducing rich-fuel regions where soot can be formed. Low influence of air excess and burner power was found. When these parameters increase, the flame emission decreases. This behavior is explained by the better mixing attained at high burner power and at high air excess. However the high emission level observed at low firing rate can be reduced with a good burner adjustment. With the photodiode, it was possible to find the optimum air excess value where NO, CO and soot are as low as possible. As the photodiode measures locally in the flame, results were compared with images obtained with a CCD camera. Finally, these results are in good agreement with those obtained previously with the spectrometer.