燃煤發電廠中排煙脫硫之不透光率研究

Abstract

燃煤發電廠之排放煙囪內不透光率通常是用來檢測微粒濃度排放量的方法，它是利用光穿透儀，作為一個連續排放監測系統的一部分。然而，安裝排煙脫硫裝置，採用濕式洗滌方法後，濕氣會影響不透光率的測量，水氣產生干擾甚至可能在極低微粒濃度下，不透光率仍超過20%法定上限。本文研究以燃煤電廠(發電量14.3 MW，煙囪直徑為2.4 M，高度為70 M)之排煙溼式脫硫洗滌器對不透光率影響進行實驗與分析。透過實驗調整鍋爐負荷、靜電集塵器和排煙溼式脫硫，以確定在不同狀況下不透光率影響因素的變化量之多寡。研究結果顯示，影響不透光率的兩個重要參數為微粒和水氣消光因子。本文研究不透光率影響因子乃先定義其獨立的質量消光係數kp (微粒)與kw (水氣)，而利用比爾定律且以非線性迴歸方法計算kp和kw值的變化。結果顯示，kp從0.199到0.316 m2/g和kw從0.000345到0.000426 m2/g，其整體平均估計值kp為0.229 m2/g、kw為0.000397 m2/g。雖然kw小於kp 3個數量級，但實驗結果顯示，水氣的消光效應可與微粒的消光效應相比擬，此乃受溼式脫硫洗滌器大量的水氣所影響。理論消光係數也使用米氏理論加以計算(光學折射率為1.5－ni)，米氏理論所求出的消光係數範圍從0.282到0.286 m2/g，稍大於平均值0.229 m2/g的實驗消光係數值，其差異原因為粉煤飛灰可能已經形成了以球連著小顆粒或是中空的圓體球形，而非米氏理論的球狀假設。而過去的文獻研究在測定kp值則不考慮水氣的影響以致於高於本文的研究，此乃排煙濕式脫硫之水氣反射所致。除此之外，本文研究結果顯示微粒吸收水氣的影響可以忽略不計。 本研究亦使用敏感度分析不透光率經驗式中各項參數改變時最適切的解及預測某數值有變動時，研究該數值如何變動，以確定不透光率與獨立變量的定量影響。敏感度分析結果表示，不論是微粒質量消光係數或是水氣質量消光係數，若質量消光係數值較大，則對不透明度的影響將隨之變大。最後，根據環保署的微粒物排放限值，我們建立微粒排放濃度與不透光率相關係式，在考慮水氣與二氧化氮的影響下，可得到不透光率為33.8%時仍能符合微粒排放濃度的限制。本研究中更進一步將水氣、二氧化氮物及氧氣的影響加以校正，在95％信賴度區間求得不透光度和微粒濃度的經驗關係式，從而提供不透光率監測與實際微粒排放濃度的相互關係。

In-stack opacity, which is used as a surrogate for particle concentrations, can typically be measured using in-situ light transmission meters as part of a continuous emission monitoring system (CEMS) for coal-fired power plants. However after installing flue gas desulfurization (FGD) which utilizes a wet scrubbing method, water moisture can affect the measured opacity which may exceed the limitation of 20% even with lower particulate emissions. In this study, numerous experiments are investigated on factors influencing opacity at a 14.3-MW coal-fired power plant with FGD wet scrubbers. The inside diameter of the stack is 2.4 m and the height is 70 m. The factors of in-stack opacity are set with adjusting the boiler load, Electrostatic Precipitator (ESP) and FGD. Experiments are performed to determine variations in opacity for different values of the variables. The results show that two important factors that affect in-stack opacity-light extinction by emitted particles and that by water moisture after a FGD unit-are investigated. The mass light extinction coefficients for particles and water moisture, kp and kw, respectively, were determined using the Lambert-Beer law of opacity with a nonlinear least-squares regression method. The estimated kp and kw values vary from 0.199 to 0.316 m2/g and 0.000345 to 0.000426 m2/g, respectively, and the overall mean estimated values are 0.229 and 0.000397 m2/g, respectively. Although kw is 3 orders of magnitude smaller than kp, experimental results show that the effect on light extinction by water moisture was comparable to that by particles because of the existence of a considerable mass of water moisture after a FGD unit. The mass light extinction coefficient was also estimated using Mie theory with measured particle size distributions and a complex refractive index of 1.5-ni for fly ash particles. The kp obtained using Mie theory ranges from 0.282 to 0.286 m2/g and is slightly greater than the averaged estimated kp of 0.229 m2/g from measured opacity. The discrepancy may be partly due to a difference in the microstructure of the fly ash from the assumption of solid spheres because the fly ash may have been formed as spheres attached with smaller particles or as hollow spheres that contained solid spheres. Previously reported values of measured kp obtained without considering the effects of water moisture are greater than that obtained in this study, which is reasonable because it reflects the effect of extinction by water moisture in the flue gas. Additionally, the moisture absorbed by particulate matter, corresponding to the effect of water moisture on the particulates, was clarified and found to be negligible. Sensitivity analyses using a correlation equation are also conducted to determine the quantitative effect of the independent variables on plume opacity. Results on sensitivity analyses illustrate that at larger value of the mass light extinction coefficients of either particles or water moisture, the influence of the exhaust emission on the opacity becomes larger. Finally, we also discuss the opacity according to the particulate emission limit of Environmental Protection Agency (EPA) in Taiwan. Results indicate that the in-stack opacity could increase to 33.8% but still meet the requirement of EPA limit when water moisture is taken into consideration. Further, in consideration of water moisture, NOx and oxygen calibration, an empirical correlation between opacity and particulate concentration is given with 95% confidence intervals. The results provide useful information concerning the influence of various factors on in-stack opacity and may be utilized for possible modifications in measurements for monitoring particulate emissions by opacity.