| 摘要: | 本論文針對貧油天然氣加氫預混紊流燃燒技術，提出一定量的實驗量測與分析研究。目前貧油預混紊流燃燒技術已應用至燃氣輪機和汽車引擎等領域，並證實可有效節省燃油、降低NOx和提高熱效率。但使用此燃燒技術有兩大問題須克服，因在接近天然氣貧油可燃極限時(當量比, equivalence ratio, 0.6)，會導致火焰燃燒速度降低及失效點火增加的問題，這會使引擎效能降低，並產生嚴重空氣汙染。本實驗採取在甲烷(天然氣主要成分)中，添加少量氫氣的方式，來克服貧油燃燒的缺點，因為氫氣有甚為寬廣的可燃極限和甚高的層流燃燒速度(SL)，可解決前述貧油預混燃燒問題。針對不同氫氣添加量(體積比10%、20%、30%)，於一十字型預混紊流燃燒爐，應用一對自製離子探針，定量量測貧油預混焰受不同紊流強度(u')變化時之紊流燃燒速度(turbulent burning velocities, ST)，並確認加氫後貧油天然氣預混紊流燃燒生成物之NOx和CO濃度的定量變化，進而深入評估天然氣加氫的效益，以瞭解並掌握貧油天然氣加氫燃燒的技術。 實驗結果證實，添加少量氫氣不僅可微幅擴展天然氣之貧油可燃極限，並使預混燃氣易於引燃，最重要的是，在固定值( =0.6或 =0.7)和 /SL下，加氫後之ST值大於未加氫之ST值，ST值隨加氫量之增加而增加。然而紊流不會無限制地線性增加紊流燃燒速度，當紊流強度(u')數倍於層流燃燒速度(laminar burning velocity, SL)，繼續增加 /SL則ST/SL增加幅度明顯趨緩，此即所謂的彎折效應(bending effect)，在 =0.6加氫10%的情況下，當u'/SL>20，ST/SL值不僅不再增加，甚至會降低。前述加氫後所有ST/SL結果可用一關係式來預測估算，此關係式由Shy et al. (2000c)首先提出，(ST/SL)/u'=C1Da^C2 ，其中Da為Damköhler數而C1和C2為實驗常數。在 =0.7和u'/SL=0~7之間，我們發現NOx濃度會隨著加氫量增加而增加，而CO之濃度則有相反趨勢，例如在 /SL=2時，加氫量0%、10%、20%到30%的[NOx]分別為98、136、158及188 ppm，這說明氫氣的加入會導致火焰溫度上升和thermal NOx的增加，顯示氫燃燒會有[NOx]太高的嚴重問題。在加氫量(10%~30%)固定時，[NOx]會先隨著u'/SL的增加而略為增加，當u'/SL>2時，[NOx]則隨u'/SL值之增加而遞減。在 =0.7和任何u'/SL值(0~7)的條件下，[CO]會隨加氫量之增加而減少(CO濃度介於40~55 ppm之間)。於 =0.7時，較佳的加氫量和u'/SL值應為不超過20%和介於4~10之間，則[NOx]會較低而ST/SL值可達6.5~12，本實驗結果可應用至天然氣火力電廠相關之燃燒研究。 This thesis investigates lean premixed turbulent combustion technology using natural gas with hydrogen addition. Lean premixed combustion has been applied to the areas of gas turbines and automobile engines and its advantages of saving fuel, reducing [NOx] emission, and increasing thermal efficiency were proved. However, when this technology is applied, there are two major problems: (1) a substantial decrease in flame speed and (2) the tremendous increase of the misfire probability, leading to poor engine outputs and heavy emissions. In this work, hydrogen as an additive is used to solve these aforementioned two problems because hydrogen has very low flammability limit and very high laminar burning velocity (SL). With three different hydrogen additions (volume fraction 10%, 20%, and 30%) in lean CH4/air mixtures, a series experiments are conducted in the cruciform burner where an intense near-isotropic turbulence can be generated via a pair of counter-rotating fans and perforated plates. Using a pair of self-designed ion-probe sensors, turbulent burning velocities (ST) are measured quantitatively. In addition, emission concentrations of NOx and CO are measured for the first time to understand combustion characteristics of lean natural gas with hydrogen addition. The results indicate that a small hydrogen addition not only can slightly expand the lean flammability of CH4/air mixtures, but also can increase largely values of ST. At fixed values of (=0.6 or 0.7) and u'/SL, values of ST increase with the amounts of hydrogen addition. However, turbulence cannot increase values of ST continuously. The slope of ST/SL vs. u'/SL is found to bend towards the horizontal when values of u'/SL are greater than about 4. This is the so-called bending effect, as also observed in burning CH4/air mixtures with hydrogen addition. All the present data can be correlated with an empirical relation with the form of (ST-SL)/u'=C1Da^C2 , where C1, and C2 are experimental constant and Da is the Damköhler number. This empirical correlation is first proposed by Shy et al. (2000c) using the same methodology without hydrogen addition. Emission concentrations of [NOx]/[CO] increase/decrease with the amounts of hydrogen addition, respectively. For examples, at u'/SL=2.0 and =0.7, values of [NOx] are respectively 98, 136, 158, and 188 ppm for 0%, 10%, 20%, and 30% hydrogen addition. This indicates that hydrogen combustion has a serious problem of NOx formation due to its very high flame temperature. Values of [CO] vary from 40 ppm to 55 ppm for any values of u'/SL ranging from 0~7 and at =0.7. These experiments suggest that the hydrogen should not exceed 20% for premixed CH4/air turbulent combustion and values of u'/SL should be controlled within the range of 4~10, such that lower [NOx] emissions and higher values of ST/SL range from 6.5 to 12 can be achieved. These results are relevant to the combustion performance of natural gas fire power plants. |
