固定源懸浮微粒的量測與管理

Particulate Matter Measurement and Management of Stationary Sources

黃玉玫(Yu-Mei Huang)

指導教授 ： 陳志傑

國立臺灣大學/公共衛生學院/環境與職業健康科學研究所/博士(2022年)

https://doi.org/10.6342/NTU202200570

摘要

固定污染源排放管道所產生之原生性粒狀物 (Particulate Matter, PM)可細分為可過濾性微粒 (FPM, Filterable Particulate Matter)，及可凝結性微粒 (CPM, Condensable Particulate Matter)，其中小於2.5 µm微粒為近年較受注目的污染物。固定污染源因排放量大、濃度高以及毒性高之特性，成為政府優先管控對象，以降低對環境及民眾的影響。然而在近幾年研究亦發現，現有粒狀物排放清單及管理政策並未完整納入固定污染源排放管道的CPM及微粒粒徑的影響。本研究方法共有三個部分探討，以建構完整的粒狀物管理架構。本研究第一部分探討冷凝法(US EPA Method 202)方法誤差，第二部分探討臺灣火力電廠粒狀物排放現況，第三部分探討粒狀物防制策略。
可靠的量測方法是管理的基礎，依本研究研究結果顯示，使用Method 202量測CPM時，除了常被討論的正向誤差外，還會受到氮氣迫淨、採樣時間、樣品分析方法以及系統設計造成結果的誤差。實驗中量測SO2於水中的吸附與脫附曲線，並改變衝擊瓶形式、凝結水體積、氧氣濃度以及等待時間，藉此評估SO2造成的正向誤差。負向誤差則是藉著評估靜電、CPM種類、溶劑體積、燒杯大小以及濾紙握持器的設計來達成。研究中也設計強迫換氣系統用來減少樣品乾燥時間。結果顯示氮氣迫淨無法完全移除水吸附的SO2，且改良式衝擊瓶無法增加SO2的回收效率，因為SO2與水在冷凝管中即已反應。而停留時間、凝結水體積與氧氣濃度的增加皆會增加SO2造成的正向誤差，因此應盡量減少採樣與等待時間。使用不良導電的容器在秤重前，應使用中和器，以避免靜電造成影響。在負向誤差方面，蒸氣壓較高且粒徑較小的CPM在迫淨時會因揮發而造成低估，而回收時的溶劑體積增加能夠增加回收效率。進行CPM樣品轉移時，燒杯越小則能夠減少殘留在燒杯內的CPM質量。約有4 %的CPM微粒可穿透過濾紙與握持器間的空隙，應將使用墊片避免洩漏。本研究設計之加速乾燥腔可來減少90%以上的乾燥時間，則僅需1.5~2.5小時即可完成乾燥且有98.5 %以上之有機樣品回收。CPM另一種量測方法 (稀釋法)則有設備過大及採樣參數如稀釋倍數等的問題待驗證。由研究結果顯示，冷凝法的正向誤差雖無法避免，但造成正向誤差的氣狀物如二氧化硫，排放標準已較以往嚴格，而且本研究也提供減少方法誤差的建議，因此，Method 202仍為目前量測CPM較佳的方法。
近年來，火力電廠排放的細微粒受到民眾的重視，多認為燃料是最主要的影響因素，而實際上，高效率的空污防制設備 (Air Pollution Control Device, APCD)能夠有效降低排放濃度，減少大氣污染，重要性更甚於燃料。而現行法規排放濃度與APCD僅能考慮FPM，未考量CPM，造成粒狀物排放量的低估。本研究探討電廠排放管道的FPM與CPM的排放特性，評估空污防制設備對PM質量濃度的影響，及評估CPM對PM排放量的影響，並納入發電成本考量，評估火力電廠的選擇。研究對象包含燃氣 (G)、燃煤 (C1~C4)及燃油 (O)電廠，結果發現CPM與FPM2.5、FPM10及FPMT比值4.5~93.2倍、3.3~77.7倍及2.2~7.9倍，表示CPM質量濃度排放量皆高於FPM。由成分來看，主要為硫酸根離子及氯離子是FPM2.5與CPM，SO2與CPM質量濃度有高度相關性 (R=0.77)，低排氣溫度有較低的CPM濃度，代表溫度與SO2是影響CPM質量濃度的主要因素。
從粒徑的角度來看，燃煤電廠廢氣中的細微粒以FPM2.5為主，FPM2.5/FPMT比值約介於0.4~0.7，燃氣電廠細懸浮微粒比例為0.4，燃油電廠細懸浮微粒比例最低為0.1。燃煤電廠大多具Electricstatic Precipitator (ESP) or Baghouse (BH)，顯示其去除大粒徑的效果較佳。經過測試，燃煤電廠BH防制設備最易穿透粒徑約 40 ~ 70 nm。
比較燃氣電廠(G) 與安裝較佳防制效率粒狀物防制設備的新式燃煤電廠(C1)，前者CPM平均排放濃度略高於後者，兩者FPM2.5平均排放濃度相近，顯示廢氣排放濃度與電廠的防制設備有較高的關係，安裝粒狀物收集效率較佳防制設備的燃煤電廠排放濃度與燃氣電廠相近，甚至更佳，由臺灣的發電成本來看，燃氣電廠成本約燃煤電廠1.5倍，若加入溫室氣體減量成本，燃氣電廠仍略高於燃煤電廠，顯示加入防制設備效率及溫室氣體排放等考量後，燃煤電廠仍為較佳的選項，即對於火力電廠評估，不應僅由燃料做為唯一考量。
相較於燃氣電廠，燃煤電廠被認為其管道排放的粒狀物對空氣品質細懸浮微粒的影響較劇。近年研究提出不同看法，以往僅考量FPM的排放量，未考量CPM的排放量，若同時考量FPM及CPM，燃氣電廠與具良好空污防制設備的燃煤電廠的粒狀物排放量差異不大。由於天然氣在運輸及保存上，仍有其限制，燃煤電廠仍為重要的發電設施。由於以往燃煤電廠的粒狀物防制設備，只能管制FPM質量濃度，未考量粒狀物在粒狀物防制設備前後粒徑分佈對收集效率的影響，但研究顯示最易穿透粒徑才能呈現粒狀物防制設備真實防制效率；也未考量非預期洩漏量(Unexpected Leakage)，如氣狀物防制設備操作過程中，可能產生的粒狀物，也未考量CPM的控制及廢氣特性的影響（如SO2及水份等）。溫度是控制CPM產生最重要的參數，而由於粒狀物的特性，氣狀物防制設備操作也可能是另一個產生源，粒狀物防制設備若未在防制設備配置最後面，將可能影響管末粒狀物排放濃度。為了減少CPM，降溫宜在粒狀物防制設備之前，而由於其他氣狀物防制設備在操作過程可能產生的粒狀物，粒狀物防制設備宜在最末端。由於污染源粒徑分佈改變，即會改變粒狀物防制設備收集效率，因此，未來宜增加相關研究，才能評估最佳的防制設備配置及操作。

關鍵字

可過濾性微粒 ； 可凝結性微粒 ； Method 202 ； 最易穿透粒徑 ； 粒徑分布

並列摘要

Primary particulate matter (PM) emitted from stationary source includes Filterable Particulate Matter (FPM) and Condensable Particulate Matter (CPM). PM2.5 emitted from stationary sources draws a lot of attention recently. Primary PM is the priority to be controlled by the authority due to its characteristics of high level of emission, high concentration and toxicity. However, the CPM emissions and the impacts of particle size in the flue gas are not thoughtfully considered in primary PM emission inventory, making these issues a future challenge for PM management. To further elaborate these research issues, this study focus on three parts, including evaluation the bias of condensation method (US EPA Method 202), investigation of particulate matter emitted from fossil fuel power plants in Taiwan and assessment of control strategies of PM emission from coal-fired power plants.
A stable and precise measuring method is essential for policy making. First, positive, and negative biases when measuring condensable particulate matter (CPM) was evaluated according to US EPA Method 202. Four factors were considered to improve CPM overestimation: SO2 absorption, the volume of condensate, oxygen content, and residence time. Negative biases were evaluated using weight loss caused by static electricity, vapor pressure of the particles, rinsing volume, beaker size, and filter paper holder. Finally, a forced ventilation chamber was designed to reduce the drying time of the organic sample. The results revealed that artifacts produced by SO2 could not be eliminated. The homemade condensate-divert impinger did not reduce the amount of artifacts because the oxidation of SO2 already occurred when it came into contact with water while passing through the Graham condenser. The residence time, volume of condensate, and oxygen content can all increase CPM overestimation. Therefore, field sampling should be performed as quickly as possible. Furthermore, electrostatic neutralization of the beaker with poor conductivity should be performed before weighing it. Because of the high volatility of the material with high vapor pressure, nitrogen purging may reduce the particle mass with high vapor pressure. Furthermore, when the CPM sample was transferred from the beaker to the tin, a smaller beaker size led to lower residual mass. A 4% particle loss was measured according to the leakage of particles between the filter paper and the filter paper holder provided by the original manufacturer. A gasket is therefore recommended to minimize leakage. Finally, it took only 1.5–2.5 h for the organic sample to dry when it was placed in the forced ventilation chamber, and a sample recovery rate of >98.5% was achieved. Although the artifact produced by SO2 is unavoidable, estimation of CPM can be more precise by considering our proposed improvements. The results show that when using condensation measurement method (US EPA Method 202), CPM was not only affected by SO2 artifact, biases may also come from sampling time, nitrogen purging, analysis method and the filter holder design. However, the other common CPM method, dilution method, has issues, such as dilution ratio and retension time, needed to be verified. And its huge sampling devices is hard to applied in the field.
In recent years, the public has become increasingly aware of PM emitted by fossil fuel power plants, with many believing that fuel type plays a key role in PM emissions. Fossil fuel power plant with high-efficiency PM control device also can effectively reduce PM emissions. Furthermore, current PM emissions standards only consider FPM emission, not CPM, thus, PM emissions are underestimated. This study investigated the characteristics of FPM and CPM emissions from fossil fuel power plants. In addition, the cost of power generation was incorporated into considerations of the optimal choice of fossil fuel power plant. This research focused on gas, coal, and oil power plants. The results revealed that the ratios of CPM to FPM with an aerodynamic diameter of or less than 2.5 μm (FPM2.5), FPM with an aerodynamic diameter of or less than 10 μm (FPM10), and total FPM (FPMT) were 4.5 to 93.2, 3.3 to 77.7, and 2.2 to 7.9, respectively, indicating that the mass concentration of CPM in emissions is higher than that of FPM with different particle size. Sulfate and chloride ions were the main components of FPM2.5 and CPM. Low flue gas temperatures wase associated with lower CPM concentration, and SO2 and the CPM mass concentration were highly correlated (R = 0.77), indicating that temperature and SO2 were the main factors affecting CPM mass concentration.
Regarding particle size, the majority of PM in the flue gas of coal-fired power plants were FPM2.5, and the FPM2.5 to FPMT ratio was ranged from 0.4 to 0.7. For gas-fired power plants, the FPM2.5/FPMT ratio was 0.4, and for oil-fired power plants, it was 0.1. Most coal-fired power plants have an Electricstatic Precipitator (ESP) or Baghouse (BH), and the results of this study demonstrated that PM control devices have higher removal efficiency on larger particles size. Collection efficiency of BH was also tested in the field. The results reveals that the most penetrating particle size (MPPS) of BH in coal-fired power plants was range from 40 to 70 nm.
The average emission concentrations of CPM were compared, that of G was slightly higher than that of C1 with renovated PM control device. The average emission concentrations of FPM2.5 for these two were similar. From the perspective of power generation costs in Taiwan, the cost of gas-fired power plants is approximately 1.5 times that of coal-fired power plants. And the cost of gas-fired power plants is still higher when greenhouse gases were considered. This demonstrates that coal-fired power plants remain an option after considering the collection efficiency of PM control devices and greenhouse gas emissions. Thus, when evaluating fossil fuel power plants, the type of fuel should not be the only consideration.
Coal-fired power plants are considered to have a greater impact on fine particles in the atmosphere. However, recent studies have also pointed out different views. The results of this study shows that PM emission was underestimated when CPM was ignored. The PM emissions of gas-fired power plants and coal-fired power plants with high-efficiency PM control devices were about the same if CPM had been taken into consideration.
To reduce CPM, temperature drop is the key factor and it is recommended to be processed at the inlet of the PM control device. And PM control device should be at the end of APCD layout due to the possible generation of fine particles during the operation of non-PM air pollution control devices. And collection efficiency based on MPPS needed be developed for a comparable index among different types of PM control devices. Sound PM control strategies should include more studies on particle size distribution in the flue gas, considering CPM emissions reduction on PM control and incorporated them into a performance standard, to further regulate new and existing sources.

並列關鍵字

Filterable Particulate Matter ； Condensable Particulate Matter ； Method 202 ； Most penetrating particle size ； Particle size distribution

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