抽取式粒狀物連續排放監測系統採樣管道優化設計

Abstract

Part 1 連續排放自動監測系統CEMS (Continuous Emission Monitoring Systems)是用來監測與計算排放管道即時排放量的最佳方式，抽取式PM CEMS具有敏感度與準確度高之優點，但其採樣管道的設計卻會導致量測結果的誤差。採樣管道中的採樣入口、管道彎曲、稀釋管設計與管徑變化會造成微粒沉積而低估採樣濃度。本篇將結合計算模式和實驗探討抽取式PM CEMS整體採樣管道中的傳輸效率，並對採樣管道的後續改進優化提供方向。 本研究根據文獻中之經驗公式，結合採樣管道實際尺寸與採樣流量等特性進行不同粒徑微粒在採樣管道中沉積率的預估，再進行實驗進行驗證。實驗中使用超音波霧化器產生1-10µm的氯化鈉微粒，於測試腔內對採樣入口、採樣直管和90°彎管分別進行穿透率測試。以測試腔內濃度為作為上游，通過採樣管道後濃度為下游，將下游濃度除以上游濃度即可獲得採樣管道的穿透率。 根據某型號PM CEMS預設條件在計算模式中進行穿透率預估，而後進行實驗驗證，結果顯示10µm的微粒在傳輸管道中的傳輸效率小於5%。實驗發現採樣入口內部的未知結構將導致更高的微粒沉積，且隨著微粒粒徑增大，其在採樣管路中的傳輸效率不斷降低，而在較小粒徑的部分沉積損失較小，在採樣探頭前端加裝分徑器或許是提高監測精准度的一個方法。 Part 2 旋風式分徑器是利用離心力達到分離固-氣兩相的靜態設備，因其設計簡單、結構堅固且對壓力、溫度的耐受性較高以及優於衝擊式分徑器的負載能力而被廣泛應用於大氣環境、作業環境及室內空氣品質的採樣。火力發電廠、工廠等固定污染源的排放管道內的溫度達到100-200℃。由於外界環境的原因導致空氣特性的改變 ，此時的分徑器的分徑效率曲線也有所不同。目前的研究多以模式計算和計算流體力學為主，鮮少有實驗進行實際的驗證。本研究將設計高溫測試系統以探討25℃-200℃溫度對旋風分徑器性能的影響。 本研究以Apex公司的PM2.5旋風分徑器為研究對象，採用固定的採樣流量在25℃-200℃范围内，探討旋风分径器压降、截取粒徑的變化，同時研究還將探討高溫下的分徑器負載特性。本實驗採用氯化鈉水溶液經由超聲波霧化器產生微粒，經Am-241靜電中和後通入潔淨空氣進行乾燥以產生穩定氣膠環境，經加熱後傳輸到保溫腔中的測試分徑器及管路，高溫氣體隨後經冰浴冷卻系統降溫後進入量測設備，並繪製分徑效率曲線，本研究中主要的量測設備為氣動粒徑分析儀。 測試結果顯示，在抽氣流率不變的情況下，提高溫度會增加通過旋風分徑器的壓降。分徑效率則產生比較複雜的變化，因温度升高后气体的密度、粘度改变导致截取粒徑並未隨溫度的增加產生明顯變化，較小粒徑的微粒(1-2 µm)補集效率上升，而相對較大的微粒(2.7-5 µm )的補集下降。實驗同時發現在高溫環境中的負載效應由於大微粒撞擊的原因並不明顯。美國環保署Method 201A中提供的校正方法顯然需要再進行評估。

Part 1 The USEPA has promulgated major Maximum Achievable Control Technology standards, requiring many plants to continuously measure the emissions concentrations of particulate matter in the stack gas, using a continuous emissions monitoring system (CEMS). There are several extractive PM CEMS currently commercially available. Performance Specification 11 (PS11) had been promulgated by the USEPA to establish the initial installation and performance procedures that are required for evaluating the acceptability of a PM CEMS. However, based on the viewpoint of the aerosol sampling, the monitoring accuracy of PM CEMS might be influenced by the particle deposition of the sampling train. This potential and significant defect of PM CEMS has never been addressed before. Adding a size-selected separator to the front of the sampling probe can effectively reduce the monitoring deviation caused by the deposition of large particles. An ultrasonic atomizer was used to generate micro-meter-sized challenging NaCl or PST particles. The concentration and the size distribution of the test aerosol can be controlled by the aerosol produce system. An Aerodynamic Particle Sizer was employed to measure the aerosol size distributions and number concentrations upstream and downstream of the sampling tube. The sampling train could be divided into three parts: goose-neck probe, connecting horizontal straight tube, and elbow adaptor connecting to a dynamic aerosol mass monitoring sensor. The experimental data were then compared with the empirical models of particle deposition in bends of circular cross section, gravitational deposition of particles from laminar flows and turbulence in channels. The loss of aerosol deposition of sampling train was found to be significant. The overall loss of 10 μm particles was up to 95%. That means the effect of curvature-diameter ratio of the elbow on the deposition loss needs to be further studied. when the size-selected separator had been used to remove the particles that large than 2.5 μm, the deviation had been reduced to 11.58%, and for the PM1.0 separator, the result was 1.38%. The deviation of mass concentration monitoring for PM2.5 and PM1.0 is significantly reduced. Part 2 Cyclone Separator is a static equipment which uses centrifugal force to separate solid-gas two-phase. Because of simple design, strong structure, high tolerance to pressure and temperature and better load capacity than impactor, cyclone separators widely used in sampling air environment, working environment and indoor air quality. The temperature in the discharge pipeline of stationary pollution sources such as thermal power plants and factories reaches 100-200℃. Separation efficiency of cyclone separator is affected by the change of air characteristics in the environment. At present, most of the related studies are based on model calculation and computational fluid dynamics, and few experiments have been carried out to verify them. In this study, a high temperature test system will be designed to investigate the effect of 25-200℃ temperature on the performance of cyclone separator. This research took PM2.5 cyclone separator produced by Apex Company as the research object. The variation of pressure drop and cut-size of cyclone separator was discussed by using a constant sampling flow rate in the range of 25- 200℃. The loading effect of cyclone separators at high temperature were also discussed. The test results show that the pressure drop through the cyclone separators with the increase of temperature under the condition of constant pumping flow rate. The diameter separation efficiency has a complex change. The cut-size of separator didn’t change significantly with the increase of temperature due to the change of gas density and viscosity after the increase of temperature. The separator efficiency of smaller particles (1-2 µm) increases, while that of larger particles (2.7-5 µm) decreases. The calibration method provided in the US EPA Method 201A clearly needs to be further evaluated.