Stability of Propane-air and oxyfuel diffusion flames in a swirl-stabilized combustor; an experimental study

Abstract

Emission of greenhouse gases like CO2 is associated with global warming and ultimately climate change. Oxy-fuel combustion as a carbon capture technique is among the most widely recommended means of tackling CO2 emission. The use of oxygen/carbon dioxide (O2/CO2) mixture as oxidizer in place of air will result in distinct combustion characteristics. Flame blowout is one of such characteristics and is one of the critical operational issues in real combustion systems. In this study, blowout characteristics of air and oxyfuel combustion of propane in a non-premixed, swirl-stabilized combustor were studied. The effect of the equivalence ratio on flame blow off for three oxidizer mixtures (air, oxyfuel I, and oxyfuel II) was first studied. Furthermore, the effect of CO2 addition and swirl number on oxyfuel flame blowout was also studied. Results show that the propane–air flame transits from: attached flame - lifted flame - no flame regimes with the lifted flame regime occurring at a critical velocity ratio (ratio of fuel velocity to oxidizer velocity). The oxyfuel I and oxyfuel II flames, however, transit directly from attached flame to no flame regime at all firing rates below 4.5MW/m3 bar. This is due to the fact that oxyfuel I and oxyfuel II could not reach the critical velocity ratio for oxy-flames (Critical velocity ratio observed = 3) before flame blowout. We observed that the critical velocity ratio for propane oxyfuel combustion is six times more than that of propane–air combustion (about 0.5). At 5 MW firing rate, the amount of CO2 required in the oxidizer at the attached flame - lifted flame transition point decreases from 76% to 74% as the equivalence ratio decreases from 1 to 0.9. The velocity ratios at this operating condition are 3.1 and 3.2, respectively and are more than critical velocity ratio for propane oxyfuel combustion. This indicated that attached flame - lifted flame transition is strongly dependent on the critical velocity ratio irrespective of the equivalence ratio. Further studies showed that oxyfuel flames are more stable at stoichiometry conditions and swirl number of unity.