Stability Limits of Biogas Jet Diffusion Flames

An investigation of the stability limits of biogas jet diffusion flames in a co-flowing air stream was conducted. Stability limits were determined experimentally for two different methane-carbon dioxide mixtures that are of typical biogas composition. Moreover, the effect of jet nozzle diameter was also investigated. It was found that with the presence of a significant amount of CO₂ in the fuel as in the case of biogas, the stability limits were very low and the flames can only be stabilized over a very small range of coflowing velocities. As expected, an increase in carbon dioxide concentration resulted in the narrowing of the region of the stable flames. However, it was shown that the flame stability of such mixtures can be enhanced very significantly and over a much wider range of co-flowing air velocities by introducing a small amount of hydrogen in the fuel. Results indicate an increase in the stability limits by approximately fourfold when 10% (by vol.) of hydrogen is added under the same operating conditions. The effect of addition of hydrogen on enhancement of biogas stability is most significant with the initial addition of 10%. The degree of enhancement diminishes with further increase in hydrogen addition from 10% to 30%.