摘要
全球暖化造成人類對於二氧化碳排放的現象採取嚴密監控的政策。富氧燃燒搭配引流煙道氣中的二氧化碳迴流至燃燒室為當前有效降低二氧化碳排放的有效方向之一。本論文之主要研究方向為探討甲烷擴散火焰在併流氣體為不同比例二氧化碳混合氧氣狀況下，穩駐及跳脫的操作特性。首先，運用對沖流燃燒器來針對火焰的結構及熄滅特性進行基本實驗觀測，並與數值模擬結果進行比對，初步結果相當吻合。數值模擬是以CHEMKIN套裝軟體之OPPDIF程式進行對火焰結構及相關化學機制的探討。實驗結果與數值模擬在火焰溫度及OH, H 及 O產物得比較之下，發現對沖流火焰在受到二氧化碳的影響之下，火焰顏色變淡(OH值變弱)且火焰偏向於燃料端。在添加二氧化碳的對沖火焰中H 的最高值竟低於O的最高值，其中顯示H因添加二氧化碳消耗過多且有可能二氧化碳直接參與火焰的化學反應。
在富氧燃燒技術中，引流煙道氣中的二氧化碳回到燃燒室是要以取代氮氣為目的，以降低氮氧化合物(NOX)的產生。本論文有針對併流氣體中不同比例之二氧化碳與氧氣的組成對甲烷擴散火焰特性的影響進行實驗研究及數值模擬，並將此結果與併流氣體為空氣(21% O2 +79% N2)的狀態相互比較。針對不同比例二氧化碳的添加，實驗之火焰溫度及火焰結構螢光觀察結果與數值模擬均相當接近。更進一步數值模擬針對二氧化碳對富氧狀態下甲烷擴散火焰的燃燒特性研究顯示火焰有開口現象，此一相當特殊現象，目前實驗上仍需克服操作技巧來加以驗證。併流氣體由空氣改為二氧化碳及氧氣的組成，促成許多反應步驟之速率以倍速增加。這意味著併流氣體中氧氣比例增加，反應步驟之速率亦增加;當併流氣體中二氧化碳比例增加，部分化學反應會有正向及逆向反應加速現象產生。
一般而言，甲烷擴散火焰在空氣中燃燒，當燃料速度增加到跳脫極限時，會直接跳脫吹熄不會有穩駐現象。富氧狀態下甲烷擴散火焰在併流氣體中氮氣被二氧化碳取代狀況時，實驗觀測到火焰產生跳脫穩駐現象。這與一般認知中二氧化碳氣體會因比熱較高，添加在併流氣體中使火焰溫度降低的想法有所差異。本論文以實驗及數值模擬來探討富氧狀態下甲烷擴散火焰的此一特殊現象。實驗結果顯示，在併流氣體中氧氣比例高於15%時，甲烷擴散火焰會有穩駐跳脫現象。數值模擬在氧氣/二氧化碳比例為20%/80%時，其結果與併流氣體為空氣之狀況來比較討論。結果顯示，甲烷火焰會跳脫穩駐在甲烷當量火焰外型的貧油側。與一般三岐火焰的穩駐現象截然不同的是，流場速度圖顯示，進入火焰底部的流速降低至甲烷燃燒速度的30%左右。推斷此富氧甲烷擴散火焰穩駐現象與二氧化碳接近火焰底部所產生分解後進入火焰，再與甲烷跳脫火焰下流處之連續熱源反應，產生提供穩定的反應核心及化學反應所致。本論文對詳細之OH 、火焰溫度、CO2 、CO 及熱釋放率均有圖解說明，以推論驗證二氧化碳氣體在富氧甲烷擴散火焰跳脫過程中扮演重要的角色。
關鍵詞: 富氧燃燒, 併流氣體, 二氧化碳, 跳脫,甲烷擴散火焰

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

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