合成氣與烷類預混衝擊火焰之流場與燃燒特性研究

Characteristics of Reacting-Flow and Combustion of Syngas/alkane Premixed Impinging Flame

邱志斌(Chih-Pin Chiu)

指導教授 ： 楊鏡堂

國立臺灣大學/工學院/機械工程學研究所/博士(2018年)

https://doi.org/10.6342/NTU201801108

摘要

摘要
合成氣是再生且潔淨之替代能源，於燃燒工程應用中極具發展潛力；衝擊燃燒器可利用簡單的幾何結構，達到增強流場質量、動量和熱量傳遞的效果。本研究使用衝擊型流場進行合成氣與烷類燃料混燒之研究，透過融合合成氣與衝擊燃燒器兩者之優點，提出一潔淨、高效且低碳足跡的燃燒概念，也藉由燃燒流場診斷和火焰自由基螢光分布，探討合成氣混燒衝擊火焰之結構與流場。實驗分析包含火焰型態觀測、粒子影像測速法、化學螢光擷取、溫度與廢氣排放量測，並搭配火焰傳播速度計算與質傳效應分析，歸納不同氫氣/一氧化碳(H2/CO)比例與不同流速比(UC/UF)，對合成氣混燒衝擊火焰混合機制與燃燒性能之影響。
本文首先探討C3H8/H2/air和C3H8/CO/air於V型衝擊燃燒器之混燒火焰，建立後續合成氣混燒火焰研究的基礎。實驗結果顯示，氫氣與一氧化碳加入丙烷衝擊火焰皆能使火焰傳播速度提升，拓展貧油操作極限，其中一氧化碳比例超過82.9%時，火焰傳播速度轉變為下降趨勢。火焰面的反應強度隨著氫氣比例增加而提高，焰尖強度則呈相反趨勢，顯示氫氣比例增加可增強火焰面反應區的質傳效應，因此火焰面溫度隨之升高，但焰尖溫度降低。另一方面，添加過多的一氧化碳導致質傳效應減弱，造成火焰傳播速度下降並抑制火焰面反應強度；然而，透過V型衝擊燃燒器的迴流減速區，可有效增進燃氣預熱與延長滯留時間，有利於具三鍵結構之一氧化碳於衝擊區進行二次反應，因此衝擊區內的反應強度隨著一氧化碳比例增加而提升，且一氧化碳排放濃度下降。為了探討合成氣預混衝擊火焰特性，本研究進一步以甲烷/合成氣混合物為燃料於多向衝擊燃燒器中實施混燒。
研究結果顯示，H2/CO比例是影響火焰穩定性、燃氣混合與燃燒效能的關鍵因素。CH4/syngas/air預混衝擊火焰中氫氣含量增加具有提升火焰傳播速度的效果，火焰面沿火焰傳播速度方向的拉伸增強，有助於預混火焰面抵抗燃氣流速，因此穩定性提高。由於兩股燃氣噴流相互撞擊產生減速區，流場紊流強度增強，促進燃氣混合效率。一氧化碳比例提高時，不僅擴大減速區範圍且渦度場結構更完整，顯示燃氣混合效率隨一氧化碳含量增加而提升，反應強度因此增強。由Lewis number觀點而言，H2/CO富油火焰的Le > 1，且Le隨著氫氣比例增加而更大，質傳效應降低導致燃氣處於非均於擴散狀態。因此，反應強度隨著一氧化碳比例增加或氫氣比例降低而增強，且火焰中心軸溫度隨著一氧化碳含量增加而上升。此外，當一氧化碳比例增加4倍時，CH4/syngas/air預混衝擊火焰之一氧化碳排放濃度僅上升12%，顯示透過衝擊流場的設計有助於一氧化碳參與反應，燃料反應率大幅提升。證實具有噴流衝擊設置的燃燒器，可有效地促進燃料內的一氧化碳參與反應。
最後，透過注入中心空氣噴流探討主動輸入氧化劑對燃燒性能之增益效果。研究結果指出，在當量比超過1.6且UC/UF < 1時，燃料不僅在火焰上游處提早進行反應，且生成更多的OH推動一氧化碳反應式，燃燒反應強度大幅提升且火焰長度明顯縮短。相較於未添加中心空氣噴流之衝擊火焰，當UC/UF = 1.0時，高-一氧化碳比例之火焰高溫區域擴大，最高溫度上升約150 °C，一氧化碳排放濃度大幅降低。反之，當UC/UF > 1.5時，燃氣與中心空氣噴流交界面處的未然氣體被加速帶往下游逸散，造成燃料反應強度下降，一氧化碳排放濃度急遽上升。在當量比較小的情況下(1.4和1.6)，因空氣噴流的加入對交界面處的燃料可燃濃度產生稀釋效應，導致反應強度減弱，一氧化碳排放濃度亦呈現上升的趨勢。本研究提出結合生質合成氣與衝擊燃燒器二者之優點，利用衝擊型流場探討合成氣與烷類混燒火焰特性，說明合成氣混燒火焰於衝擊型燃燒器之燃氣混合機制，並證實本概念可有效促進合成氣中的一氧化碳進行反應，提升整體燃料的燃燒性能，研究結果可作為拓展合成氣應用範圍與其燃燒載具設計之參考，進而達到潔淨燃燒之目標。

關鍵字

合成氣 ； 衝擊燃燒器 ； 粒子影像測速法 ； 化學螢光 ； 混合機制 ； 反應強度

並列摘要

Abstract
Syngas, a clean and alternative fuel, has a great potential to replace hydrocarbon fuels in combustion applications. An impinging flow field has attracted interest in the investigation of its mixing characteristics of fuel and oxidant in many fuel-injection systems, but up to now the research on jet-impinging flames has been focused mainly on diffusion flames with hydrocarbon fuels. For a practical application, we therefore propose a concept of clean combustion through combining the advantages of syngas and an impinging burner. Furthermore, the varied proportions of H2 and CO are the crucial causing a variation in the fuel mixing and combustion reaction when using syngas as a principal fuel. We performed experimental measurements of particle image velocimetry (PIV), chemiluminescence of free radicals, flame temperature, and CO emission to examined how and why the varied proportions of H2 and CO affected the fuel mixing and combustion reaction of a syngas premixed impinging flame.
For a C3H8 premixed impinging flame on the V-shaped burner, its flame propagation speed increased with the addition of H2 and CO into the fuel mixture, which expanded its lean flammability. The addition of H2 in the fuel mixture enhanced the reaction intensity of flame sheet, but, decreased the reaction intensity of flame tip, which shows that the reaction zone was dominated by strong mass diffusivity. The temperature of flame sheet hence increased, and the temperature of flame tip decreased with increasing H2 proportion. Although the mass diffusivity of reaction zone on the flame sheet became weaker when CO presented a large proportion of fuel, the fuel mixture conducted the second reaction within the impinging zone through the well preheating and deceleration. The reaction intensity of impinging zone hence increased, and the emission of CO decreased.
We further examined the characteristics of fuel mixing and reaction of CH4/syngas/air impinging flame with H2/CO in varied proportions using a multi-way impinging burner. The results showed that a deceleration area in the main flow formed through the mutual impingement of two jet flows, which enhanced the mixing of fuel and air because of an increased momentum transfer. The deceleration area expanded with an increased CO proportion, which indicated that the mixing of fuel and air also increased with the increased CO proportion. CO provided in the syngas hence participated readily in the reaction of the CH4/syngas/air premixed impinging flames when the syngas contained CO in a large proportion. Our examination of the OH* chemiluminescence demonstrated that its intensity increased with increased CO proportion, which showed that the reaction between fuel and air accordingly increased.
Finally, to enhance the reaction intensity, we introduce a central air jet injecting into a CH4/syngas/air impinging flame. For a fuel-rich CH4/syngas/air impinging flame, the added central air jet caused no acceleration of the fuel mixture flowing toward downstream when ratio UC/UF was less than 1.0. The fuel mixture obtained additional oxidant from the central air jet, which increased its reaction intensity; the CO emission hence decreased and the flame temperature increased when the UC/UF ratio was less than 1.0. When UC/UF exceeded 1.5, however, the central air jet caused the fuel mixture to accelerate in its escape downstream because of the increased upward momentum; the reaction intensity thus exhibited a decreasing trend and the CO emission greatly increased. The results shown in our work provide a significant reference and a prospective concept for the utilization of syngas, which improves the feasibility of fuel-injection systems using syngas as an alternative fuel.

並列關鍵字

syngas ； impinging flame ； particle image velocimetry ； chemiluminescence ； fuel mixing ； reaction intensity

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