Structure and stability analysis of the ultra-rich H2CO2O2 premixed flames at various CO2 concentrations

A novel high-temperature hydrogen reduction method based on the hydrogen-oxygen ultra-rich premixed combustion technology is proposed to achieve nearly zero CO 2 emission when reducing the limestone or metallic mineral in our previous work. As the first step of the aforementioned study, the dynamics of the self-sustaining ultra-rich H 2 CO 2 O 2 premixed flame needs to be revealed in detail. In this study, the structure and stability of ultra-rich H 2 CO 2 O 2 premixed flame in a micro tube-burner with a backward step at three volume percentages of CO 2 (73%H 2 –10%O 2 –15%CO 2 , 68%H 2 –10%O 2 –20%CO 2 , and 63%H 2 –10%O 2 –25%CO 2 ) are investigated for the first time by employing experiment and simulation. It is found that the flame tip is brighter than its root primarily owing to a larger combustion intensity at the former. The CO sensitivity is also analyzed based on identifying ten sensitive reactions, including five inhibitory and five facilitatory reactions in the fast production region of CO. The CO is primarily generated by the reaction of CO 2 with H and OH radicals. Hydrogen is partially consumed by the combustion reaction first just before the flame front, and then it reacts with CO 2 to form CO via the RWGS (Reverse Water-gas Shift) reaction in the downstream high-temperature zone. Unexpectedly, the heat release rate and the corresponding reaction rate at the flame tip in the case of the volume percentage of CO 2 X CO2 =20% are larger than those in the case of X CO2 =15% and X CO2 =25%. This is primarily because the equivalent equivalence ratio of H 2 O 2 in the fresh mixture is closer to the stoichiometric ratio and the negative CO 2 dilution effect is smaller in the case of X CO2 =20%. The flame displacement speed along the flame front is smaller at a larger X CO2 value primarily due to a more remarkable negative CO 2 dilution effect. It is found that the thermal interaction of flame-wall effect contributes to anchoring the flame root rather than the flame tip. The relative fuel-rich region at the flame root reduces its stability. To the best of our knowledge, the present ultra-rich H 2 CO 2 O 2 premixed flame structure and stability analysis have not been reported yet. This study expands our understanding of the ultra-rich premixed flame dynamics.