Combustion state monitoring of premixed heating appliances with flame ionization current and chemiluminescence

The development of a combustion operating point control system has received interests from gas boiler manufacturers in order to ensure optimized performances despite variations of natural gas composition. Fully premixed burners are widely used in these boilers and the equivalence ratio is a key parameter to control. Experiments are carried out in this study for CH4/air fuel blends diluted by N2 or CO2 or enriched by H2, C2H6, or C3H8. Two low cost solutions for monitoring the equivalence ratio based on the flame ionization current and the chemiluminescence signal are investigated on a laboratory laminar conical flame. The behavior of the ionization current with changes of gas velocity, fuel composition, and equiva- lence ratio is shown to be mainly related to the size of the dead space between the flame base and burner rim. In a second stage, changes of the chemiluminescence signal is studied and the use of CH*/OH* intensity ratio as an equivalence ratio indicator is verified. Numerical 1-D premixed flame simulations are performed to support the experimental results on the flame chemiluminescence. Finally, a third equivalence ratio sensing method is proposed, coupling both techniques and the three strategies are compared.