Chemiluminescence based operating point control of domestic gas boilers with variable natural gas composition

Abstract

The current control strategy of domestic gas boilers optimizes the heating efficiency and pollutant gas emissions assuming a fixed fuel composition. However, larger and more frequent variations of gas composition are expected in the European natural gas network. New control systems capable of monitoring and regulating the flame equivalence ratio in real time are therefore necessary. The present work investigates one equivalence ratio sensing strategy by analyzing the flame chemiluminescence signal from a laminar premixed burner widely used in domestic gas boilers. The OH∗ chemiluminescence intensity is first identified as a reliable equivalence ratio indicator, valid for different natural gas composition scenarios with CH4 diluted by N2 or CO2 or enriched by H2, C2H6 or C3H8. These fuel mixtures are representative of biogas and hydrogen enriched natural gas. A demonstrative control loop is developed based on the OH∗ signal measured by a photomultiplier tube mounted with a bandpass optical filter. The system is tested by changing the equivalence ratio set value and the fuel composition. It is shown to be capable of regulating the equivalence ratio of these natural gas fuel blends with a good precision. The differences observed between the real and target values for the equivalence ratio ϕ are in most cases lower than |Δϕ|=0.01. Only fuel blends with C3H8 lead to slightly larger errors of |Δϕ|=0.03.