| Summary: | 博士 === 國立成功大學 === 航空太空工程學系 === 102 === ABSTRACT
Global warming calls for stringent regulation on carbon dioxide emissions. Oxyfuel combustion diluted with recirculated carbon dioxide can generate high concentration CO2 in the flue gas to facilitate effective carbon sequestration. The purpose of this study is to investigate the characteristics of methane diffusion flame at attached, liftoff situations affected by coflow which is composed of various carbon dioxide and oxygen. The opposed jet flame burner is a convenient flat flame burner especially suitable for detailed theoretical/numerical and experimental studies of the flame structures and flame extinction behavior. For a preliminary study of the CO2-oxy-methane flame characteristics, the opposed jet burner is first used for experimental phenomenological observation of flame behavior in conjunction with numerical simulation using OPPDIF code of the CHEMKIN package for detailed delineation of the flame structure and flame chemical reactions. The experimental and numerical results of temperature and major reaction radicals of OH, H and O profiles of the opposed methane jet diffusion flame show that the flame location and thickness indicated by OH profile implies that with CO2 dilution methane flame becomes weaker (lower leak OH value) and the flame shifts further into the fuel side. The peak H concentration in the profile is significantly reduced and becomes lower than the peak O concentration, indicating excessive consumption of H radicals with CO2 in the flame, and further implying the involvement of the CO2 in the flame chemical reactions.
Recirculated carbon dioxide is often used to replace air nitrogen for dilution in oxy-fuel combustion. Experimental and numerical studies are performed to investigate the effects of different ratios of CO2/O2 in the coflow on laminar CH4 jet diffusion flame characteristics. The various ratios of CO2/O2 in the coflow are used to compare with air-coflow (21% O2 and 79% N2) condition. Experimental measurements of the temperature and flame chemiluminescence profiles are used to validate numerical simulation results for various CO2 dilution conditions. The numerical simulation is employed to further investigate the flame and reaction characteristics of the CO2 diluted oxy-methane combustion. One of the interesting findings is the open flame tip which is difficult to identify from experiment. The O2 and CO2 concentrations within the coflow promote most of the reaction rates up to many times higher than the situation of air-coflow. It means that reaction rate is enhanced when oxygen concentration is high but high concentration of CO2 may significantly promote some forward and reverse reactions involving CO2
In general, methane-air diffusion flame blows off directly without experiencing stable liftoff process when the jet velocity is increased. When nitrogen is replaced by CO2 in Oxy-methane diffusion flames, liftoff process is observed, in contradiction to general concept of weaker CO2/oxy-methane flames as CO2 has a higher specific heat than nitrogen leading to a lower flame temperature. Experimental and numerical studies are performed to investigate this peculiar liftoff process of the oxy-methane diffusion flame. Experimental results show that obvious liftoff process can be identified for cases of the coflow oxidizer stream composing of O2 and CO2 with the oxygen concentration 〉15%. Numerical results for the case of O2/CO2 ratio of 20%/80% are compared with the experiments and with the CH4-air case to further delineate the stabilization mechanism. The results show that the liftoff oxy-methane diffusion flame is stabilized on the lean side of CH4 stoichiometric contour in the CO2/O2 stream. Discordance with the triple flame model, the velocity plot along the streamline that goes into the flame base shows that the velocity decreases continuously to the flame base at a peculiar low velocity of about 30% of the CH4 laminar burning velocity. It is believed that the stabilization of the oxy-methane liftoff flame is strongly related to the reaction kernel of the flame base that facilitate a convenient reaction path of CO for flame stabilization with the heat coming from the adjacent CH4 reaction downstream. The detailed resultant profiles of OH, CO2, CO and heat release rate further verify and delineate the role of CO2 in the stabilization process of the liftoff oxy-methane diffusion flames.
Keywords: oxy-fuel, coflow, carbon dioxide, liftoff, methane diffusion flame
|
|---|
