垃圾焚化及燃煤程序之重金屬排放特性研究

Investigation of heavy metals emitted from MSWIs and coal-burning processes

王柏智(Po-Chin Wang)

指導教授 ： 張木彬

國立中央大學/工學院/環境工程研究所/碩士(2006年)

摘要

本研究探討國內三座大型都市垃圾焚化爐(A、B及C廠)之空氣污染防制設備對重金屬的去除效率進而推估其排放係數。此外並針對採用活性碳控制技術前後之金屬回收廠(D廠)排氣進行採樣分析以探討重金屬排放特性之異同，為了更加瞭解SCR對汞物種分佈及去除效率的影響，亦針對三座設有SCR燃煤鍋爐(E、F及G廠)採樣分析，最後則是針對僅以EP控制粒狀物之水泥窯(H廠)作採樣分析。
研究結果顯示A、B及C三廠汞排放係數 (72.5、257.1及12.1 mg /ton)，相較於世界其它國家，A、C兩廠汞排放係數低於其它各國，最主要的差異為採用活性碳噴入技術，而B廠汞排放係數相較於世界上先進國家之焚化廠大致相當，就未採用活性碳噴入技術控制之B廠及金屬回收廠(D廠)而言 ，由於原物料性質不同，D廠汞排放係數(622.9 mg /ton)高出B廠(257.1 mg /ton)甚多，而D廠在噴入活性碳後，汞排放係數(46.2 mg /ton)明顯下降許多，目前國內法規並未針對金屬回收廠之重金屬排放立法規範，未來是否需訂定合理之排放規範亦或建議加裝活性碳噴入控制設備，值得有關單位重視。現行空氣污染法規標準針對大型廢棄物焚化爐之汞排放標準值為0.3 mg/Nm3，A、B及C三廠分析結果皆符合法規規範。
燃煤電廠中SCR一般用以減少NOX的排放。然SCR卻具有將元素態汞轉換成氧化態汞的能力，E、F及G廠採樣分析結果皆顯示SCR具有轉化能力，就G廠而言，由於採樣地點限制，故所SCR後端採樣所得之數據僅通過一層觸媒，導致SCR轉化效率並不明顯；本研究亦探討F廠各空氣污染防制設備對重金屬之去除效率，結果顯示重金屬總去除效率皆可高達95%以上，而汞亦有接近90%的去除效率，在氣相汞中，元素態汞於各APCD去除效率普遍偏低，而SCR卻能將元素態汞轉化為氧化態汞，有利於提升汞於後續濕式洗滌塔系統中的去除效率。針對僅設置EP的H廠，其煙囪汞氣相比率高達98%，推論H廠汞排放量控制不佳。
過去之研究顯示影響汞物種比率最重要的兩個因子分別為HCl濃度及溫度，故本研究亦利用軟體(Chemistry HSC)模擬典型都市垃圾焚化廠及燃煤鍋爐鍋爐出口之汞物種組成，並改變HCl濃度及溫度探討氧化態汞及元素態汞之比率關係。

關鍵字

重金屬 ； 焚化爐 ； 燃煤程序 ； 活性碳 ； SCR ； 排放係數

並列摘要

On this study, stack samplings are conducted in three municipal solid waste incinerators (MSWI) with different air pollution controlling devices (APCDs) in Taiwan are conducted. To evaluate the removal efficiencies of heavy metals from the stationary sources, and the emission factors of heavy metals. Besides, this study also investigates Hg emission from a zinc recovery process (Zinc-D) which does not apply activated carbon injection (ACI) to control the emission of heavy metals in March, 2005. After applying ACI in November, 2005, stack samplings are conducted again at the end of the year. In order to understand how SCR affects the removal efficiency of Hg, this study selects three coal-burning plants equipped with SCR for NOX removal to conduct stack sampling.
The results indicate the emission factors of mercury at MSWI-A, MSWI-B and MSWI-C are quite different (72.5, 257.1 and 12.1 mg /ton, respectiviely). If we compared the results with other countries, MSWI-A and MSWI-C are relatively low, and the emission factor of mercury at MSW-B lies in between the range of other countries. The difference is caused by injection of activated carbon in domestic MSWIs to control dioxin emission. MSW-B is equipped with ACI, but it is not operated to avoid the increase of dioxin emission. As for the zinc recovery process with (DSC+CY+FF), the emission factor of mercury is calculated as 622.9 mg/ton waste, mercury emission from zinc recovery process is obviously higher than that of MSWI-A and MSWI-C, and it is also higher than MSW-B which does not apply ACI as APCD. When the zinc recovery process applies ACI, results indicate the emission factor of mercury is much lower than before (46.2 mg/ton). Although the feeding materials are different, we at least make sure factories which do not apply activated carbon injection technology might result in significant mercury emissions. The emission standard of Hg for large-scale MSWI in Taiwan is 0.3 mg/Nm3, all of the MSWIs investigated in this study are lower than the limit.
SCR is primarily applied to cut down NOX emissions from the power plants, however, SCR can transform elemental mercury into oxidized mercury. Results obtained in this study indicate that the removal efficiency of heavy metals (not including mercury) could reach 95%. As for mercury, the overall removal efficiency could also reach 90%, despite the removal efficiency of elemental mercury is relatively low. Based on this study (Plant-E, Plant-F and Plant-G), the SCR of all power plants can transform elemental mercury into oxidized mercury, but we only observe a little change of transformation from Plant-G, because the flue gas only passes through one layer of SCR reactor. When elemental mercury is transformed into oxidized mercury, it is easier to remove from gas streams through the APCD downstream such as wet scrubbers, and the overall mercury removal efficiency in power plant can be enhanced. Plant-H is equipped with EP only, so the percentage of vapor-phase mercury of the stack reaches 98%. We suspect the removal efficiency of mercury is not good at this plant.
Previous studies indicate that HCl concentration and temperature can affect mercury speciation. Our research changes the HCl concentration to investigate the percentage of mercury speciation. Besides, our study also simulates the mercury speciation of typical MSWI and power plant.

並列關鍵字

SCR ； activated carbon injection ； coal-burning processes ； MSWI ； heavy metals ； emission factor

參考文獻

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