煙流之不透光度模擬

Abstract

煙囪不透光度為工廠煙道可見排放物之判斷標準，而白煙產生一直是空氣污染待解決的課題。在本研究中，發展了Plume opacity model模擬光電廠與燃煤電廠煙囪排放之常溫煙流與高溫煙流分佈狀況，模式中考慮煙流擴散、質能平衡、微粒成長、潮解作用與米氏散射理論，模擬煙流各項特性、微粒消光特性與不透光度分佈狀況。 本模式模擬不同條件之煙流狀況，其中煙流的邊界、溫度、水蒸氣濃度、微粒粒徑與不透光度分佈皆可被預測出來。結果顯示，某光電廠排放管道多為次微米級微粒，成份主要為SiO2與銨鹽，數目中間粒徑(NMAD)介於88.6~332.5nm、幾何標準偏差1.6~2.0，在冬季因煙流內濕度過高，導致煙囪出口處微粒即開始成長在此處可明顯看到白煙，不透光度最高可達91%、不透光度大於20%之煙流長度約10公尺(風速0.5 m/s)，並隨風速增加而遞減，影響不透光度的主因為煙流內溼度過高導致水蒸氣附著於微粒上使得微粒成長。若降低煙囪出口相對溼度可使煙流無法到達過飽和狀態微粒無法成長，煙流不透光度約可降至3%符合法規。某燃煤電廠排放管道多為微米級微粒，成份主要為硫酸鹽類，數目中間粒徑(NMAD)為180nm、幾何標準偏差2.51，微粒由煙囪排出後由於降溫速率快，在距離煙囪直徑2.0~5.7倍之處開始微粒成長，不透光度可達100%，煙流長度約13(風速10m/s)-35(風速1 m/s)公尺，影響不透光度的主因為煙流達過飽和狀態使得微粒成長與微粒濃度過高所致，若將煙囪排氣溫度降至100℃，可使煙流無法到達過飽和狀態微粒無法成長。此外，減少煙囪之微粒濃度排放亦可降低白煙不透光度，而粒狀物消光係數最高的粒徑發生在0.5-1μm範圍內，若能有效處理此粒徑範圍的微粒，預期將可大幅改善白煙的發生。

Plume opacity is a criterion of visible discharge of a factory. The solutions of white smoke are important topics in air pollution. In this thesis, plume opacity model has been developed to simulate the dispersion of normal temperature and high temperature plumes emitted by a stack of an electro-optical factory or a coal-fired power plant. The plume dispersion, mass balance, energy balance, particle growth and deliquescence were considered in the model. Mie scattering theory was used to calculate the light extinction of white plumes. The properties of the plume and plume opacity will be simulated. The plume boundaries, temperature, water vapor concentration, particle diameter and plume opacity can be predicted by the model under different conditions. The results indicate that the flue gases of an electro-optical factory are submicron particles, mainly composed of SiO2 and (NH4)2SO4. The number median diameter of the particles is found between 88.6-332.5nm, and geometric standard deviation is between 1.6-2.0. In winter, particulate matter (PM) emitted by the stack mixed with the ambient air and induced particle growth due to supersaturated water vapor. White smoke is observed clearly near the exit of a stack. The plume opacity is about 91% and the length of 10 m(wind speed 0.5 m/s), decreasing while wind speed is faster. If the relative humidity of flue gases is lower, the opacity is about 3% without particle growth which fits in with the regulations. The flue gases of a coal-fired power plant are micron particles, mainly composed of sulfate. The number median diameter of the particles is found between 180 nm, and geometric standard deviation is between 2.51. The plume temperature decrease fast, and the particle growth occurs at 2-5.7 times length of source diameter. The plume opacity reaches 100% and the length of 13(wind speed 10 m/s)-35 m(wind speed 1 m/s). The influences on plume opacity are due to particle growth and high number concentration of particles. If the emission temperature is about 100℃, particle growth will not occur in the plume. Furthermore, the decrease of discharging particles is also to reduce opacity of white smokes. The highest extinction coefficient of the particle was found to range between 0.5-1μm. As a result, removal of these particles should be able to reduce the plume opacity.