| 摘要: | 戴奧辛是焚化處理產生的污染物中，毒性最高且最引人注目者，世界各國均制定更嚴格的排放標準來加以限制戴奧辛的排放。就目前焚化系統中戴奧辛之控制技術而言，以活性碳吸附戴奧辛的方式最被工程界所接受，並可有效控制煙道排氣中戴奧辛之排放。然而，在高溫度(>110℃)下吸附有機污染物的特性卻甚多被提及。本研究將嘗試利用活性碳吸附床控制焚化過程中戴奧辛污染物的排放，主要的探討對象包括：(1)溫度及水氣含量對粒狀活性碳吸附之影響；(2)戴奧辛物種間的競爭吸附作用；(3)比較三種不同種類之活性碳對戴奧辛的吸附特性；(4)戴奧辛物種在吸附於活性碳過程中的轉化行為(5)戴奧辛前驅物質(四氯酚)在吸附於活性碳過程中的戴奧辛生成機制等。研究結果顯示，活性碳對戴奧辛的吸附效率主要是取決於活性碳吸附床之操作溫度及戴奧辛物種之特性，尤其當吸附床的操作溫度接近1,2,3,4-TCDD之熔點時，活性碳對1,2,3,4-TCDD的吸附容量有明顯下降之趨勢。在水氣條件方面，在有水氣條件下(>10%)，1,2,3,4-TCDD的吸附貫穿時間及飽和時間均有提前之趨勢，但當吸附環境中含有10%~20%的水氣含量時，活性碳對1,2,3,4-TCDD的吸附效能則無明顯的差異。在探討戴奧辛物種間的競爭吸附效應方面，OCDD相較於1,2,3,4-TCDD而言乃屬強吸附質，推測當活性碳接近飽和時，低氯數的戴奧辛物種會先脫附至煙道排氣中。在戴奧辛轉化的途徑中，溫度條件對戴奧辛在活性碳表面的脫氯反應影響最為深遠。此外，在添加四氯酚的實驗中可發現隨著三種活性碳之金屬催化能力的提升，戴奧辛生成量亦相對增加。 Ever since it was reported that toxic organohalogen compounds such as PCDD/Fs were emitted from the municipal solid waste (MSW) incineration facilities, most countries have been issuing strict limits to the permissible emission level of such pollutants. PCDD/Fs emissions levels less than 0.1 ng TEQ/dscm have been demonstrated on many medical waste combustor (MWC), municipal waste incinerator (MWI), and hazardous waste incinerator (HWI) using activated carbon based control techniques. However, the adsorption characteristics of gaseous organics at high temperature (typically larger than 110℃) are rarely investigated. This study looked into the adsorption characteristics and removal efficiency of 1,2,3,4-TCDD and OCDD in the simulated flue gas of MWI by the fixed-bed activated carbon adsorption system. In particular, we are interested in the effects of operating temperature and water vapor content on the dioxin adsorption efficiency. Additionally, the properties of activated carbon on dioxin adsorption capacity and transformation of dioxins on the activated carbon are experimentally investigated. Experimental results indicate that the operating temperature and homologues of dioxins have significant effects on the removal efficiency of dioxins with the activated carbon. Especially, the adsorption capacity of 1,2,3,4-TCDD on activated carbon starts to decrease at a temperature close to the melting point of 1,2,3,4-TCDD (i.e. 185℃). Experimental results also indicate that a higher water vapor content evidently decreases the adsorption capacity. Due to the competition effect, the adsorbed 1,2,3,4-TCDD on activated carbon will be replaced by OCDD which is a stronger adsorbate. In addition, the 1,2,3,4-TCDD and OCDD adsorbed on activated carbon may dechlorinate or decompose if no strong gaseous chlorinating agent is present. Furthermore, when the activated carbon with high heavy metals, contents is used as the adsorbent it can actually serve as the active catalyst for PCDD/F formation. |
