熱裂解系統對戴奧辛之去除特性研究;Characteristics of PCDD/F removal from fly ash and soil via pyrolysis

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Title:	熱裂解系統對戴奧辛之去除特性研究;Characteristics of PCDD/F removal from fly ash and soil via pyrolysis
Authors:	洪保鎮;Hung,Pao-Chen
Contributors:	環境工程研究所
Keywords:	戴奧辛;熱裂解;脫氯反應;中間處理;土壤整治;空氣污染防制流程;PCDD/Fs;Pyrolysis;Dechlorination;Intermediate treatment;Soil remediation;Air pollution control process
Date:	2014-08-27
Issue Date:	2014-10-15 17:26:21 (UTC+8)
Publisher:	國立中央大學
Abstract:	本研究探討熱裂解技術於高溫缺氧環境下處理都市廢棄物焚化集塵灰及中石化安順廠污染土壤中PCDD/Fs之去除效率與降解特性。集塵灰與污染土壤之PCDD/Fs濃度及物種分佈有顯著的差異。集塵灰濃度較低（3.54 ng-TEQ/g），2,3,4,7,8-PeCDF為主要的毒性貢獻者；污染土壤濃度達114 ng-TEQ/g以上，質量或毒性濃度皆以OCDD/F為主。實驗室批次試驗結果指出溫度對PCDD/Fs去除之影響較反應時間顯著。集塵灰經溫度300oC、反應時間2小時以上之處理後，PCDD/Fs殘餘濃度可低於法規標準；污染土壤因濃度高，物種多為化學性質穩定的高氯數物種，因此溫度需達600oC以上，才可在30分鐘的反應時間內使PCDD/Fs毒性符合法規標準。實驗結果顯示PCDD/Fs有顯著的脫氯現象，其中又以污染土壤PCDD/Fs脫氯途徑對毒性去除的影響最為顯著，因質量及毒性濃度皆由高氯數物種所貢獻，故脫氯是否完全將顯著影響PCDD/Fs的毒性去除。集塵灰及土壤PCDD/Fs在相對較低的操作溫度可發現2,3,7,8-TCDD的生成，並發現HxCDD/Fs有顯著的脫氯門檻。在連續熱裂解系統方面，集塵灰於操作溫度350oC時可快速降解PCDD/Fs，停留時間20 min.以上皆可確保殘留濃度符合法規標準；在連續處理污染土壤方面，因土壤於有效高溫區之停留時間較短（<33 min.），造成PCDD/Fs去除效率在相同溫度下略低於實驗室批次試驗之結果，操作溫度達600oC以上才可降低殘留濃度至法規標準。在連續熱處理系統中仍可觀察到2,3,7,8-TCDD的顯著生成，導致PCDD/Fs之毒性去除效率低於質量去除效率。在氣相脫附方面，以連續熱裂解系統處理集塵灰時，溫度相對較低，脫附率僅佔總輸入量的0.005%以下，氣相PCDD/Fs濃度已低於0.1 ng-TEQ/Nm3之排氣標準；當連續處理污染土壤時，脫附量雖僅佔總輸入量的0.05%以下，但因操作溫度相對較高且污染土壤PCDD/Fs濃度亦高，故排氣PCDD/Fs濃度顯著高於我國嚴格之排放標準。爰此，本研究針對連續處理污染土壤後之系統排氣特性設計一空氣污染防制流程，設備依序為袋濾式集塵器、驟冷塔及多層活性碳吸附系統，以有效控制處理污染土壤過程中產生之含高濃度PCDD/Fs、五氯酚及汞之氣流。測試結果指出袋式集塵器可有效去除粒狀污染物，並確保出口粒狀物濃度低於3 mg/Nm3，但因氣流溫度高，污染物多以氣相存在，總去除效率皆在4.5%以下。於驟冷塔中，過飽和之元素汞蒸氣及水氣快速冷凝聚集，並排入冷凝液收集槽，各污染物的去除效率皆高於75%，但驟冷塔出口PCDD/Fs及汞濃度仍高於法規標準，故以我國專利技術多層活性碳吸附床作最後之把關，測試結果證實各污染物之排氣濃度皆可符合我國固定污染源最嚴格之排放標準。本研究最終成功建置並驗證一套可有效去除集塵灰及污染土壤中PCDD/Fs污染物，並有效避免二次污染之熱裂解處理系統及空氣污染防制程序。;Pyrolysis with oxygen-lack condition is applied to remove PCDD/Fs from the fly ash of a municipal waste incinerator (MWI) and contaminated soil of the pentachlorophenol factory in this study. PCDD/F concentration of the raw fly ash (3.54 ng-TEQ/g) is significantly lower than that of the contaminated soil (>114 ng-TEQ/g). Regarding TEQ distribution of PCDD/F congeners, 2,3,4,7,8-PeCDF and OCDD/F are the main toxic contributors of the fly ash and the contaminated soil, respectively. The results obtained with the laboratory-scale batch-type system indicate that influence of temperature is more important than reaction time. For the fly ash, residual concentration of PCDD/Fs in treated ash is efficiently reduced to lower than 1.0 ng-TEQ/g with pyrolysis temperature of 300oC and reaction time of 2 hours. For the contaminated soil, the operating temperature higher than 600oC is needed to ensure that residual PCDD/F concentration in remediated soil is lower than the limit with reaction time of 30 min. because PCDD/F concentration in the contaminated soil is significantly higher than that in the fly ash. Significant dechlorination of PCDD/Fs is significantly found in pyrolysis system. TEQ removal efficiency of PCDD/Fs may be significantly decreased due to formation of low chlorinated congeners. Formation of 2,3,7,8-TCDD, the most toxic PCDD/F congener, is found with specific operating parameters and accumulation of HxCDD/Fs due to dechlorination of highly chlorinated congeners is also found. Regarding the continuous pyrolysis system (CPS), the operating parameters to ensure that residual PCDD/F concentrations can meet the standard are significantly different between the fly ash and the contaminated soil. For fly ash, operating the CPS at a temperature of 350oC and retention time of ≥20 min. is feasible, but temperature ≥600oC and retention time ≥33 min. are needed for effective remediation of contaminated soil. Formation of 2,3,7,8-TCDD is still found, resulting in relatively lower TEQ removal efficiency compared with mass removal efficiency. Nevertheles, PCDD/F concentration in the exhaust of CPS is significantly higher than the emission limit of Taiwan even though less than 0.05% of PCDD/Fs input rate is desorbed. For better control pollutant emission, an air pollution control process consisting of baghouse, quench tower and multi-layer activated carbon (AC) bed is installed for the removal of particulate matter, PCDD/Fs, pentachlorophenol (PCP) and mercury. Firstly, particulate matter (PM) is efficiently filtrated by baghouse and the PM concentration at the outlet of baghouse is lower than 3 mg/Nm3. However, removal efficiencies of PCDD/Fs, PCP and mercury achieved with baghouse are relative low (≤4.5%) due to high temperature of exhaust in the baghouse. As flue gas passes through the quench tower, temperature is rapidly decreased. Water vapor and elementary mercury are effectively condensed and collected. Removal efficiencies of three pollutants achieved with quench tower are higher than 75%, but pollutant concentrations are still higher than emission limits. Therefore, multi-layer AC bed is essential to adsorb gaseous pollutants to make sure the emission of PCDD/Fs, PCP and mercury achieved with three-layer AC bed are lower than the regulatory limits. Overall, a continuous pyrolysis system with effective air pollution control process is developed in this study and experimental results indicate that PCDD/Fs can be efficiently removed from fly ash, contaminated soil and exhaust of CPS.
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