| Abstract: | 焚化是台灣廢棄物主要的處理方式，該過程的最終產物為底渣和飛灰（FA）。因為富含高濃度的戴奧辛（PCDDs）及?喃（PCDFs），台灣 24 座大型都市垃圾焚化爐產生的飛灰無法再利用或回收，而進入掩埋場。因此，焚化過程產生的飛灰量不斷增加，加上垃圾掩埋場空間不足，引起了公眾的強烈關注。本研究的重點為透過催化熱裂解過程去除都市垃圾焚化爐產生之飛灰中的戴奧辛及?喃。由於氯化物含量可能會影響熱裂解過程之戴奧辛及?喃的減量效率，因此採用水洗和碳酸水洗滌兩種預處理方法來降低飛灰中的氯化物含量，並評估氯化物含量對戴奧辛及?喃減量的影響。在350oC溫度下熱裂解1小時，對未水洗飛灰（含23.4%氯）、水洗飛灰（含7.10%氯）及碳酸水(含2.70%氯)之戴奧辛及?喃去除效率分別為96.7%、98.5%和96.2%。此外，觸媒熱裂解顯示相當高的戴奧辛及?喃去除效率。具體而言，以Pd/γ-Al2O3及Pd/C為觸媒，在 350oC溫度下熱裂解15 分鐘，戴奧辛及?喃的去除效率分別為 64.1% 及 91.3%，明顯高於熱裂解（35.4%）。與γ-Al2O3作為熱裂解系統觸媒載體相比，活性碳被證明具有更高的活性。本研究製備各種過渡金屬觸媒並應用於觸媒熱裂解系統。結果說明以Fe/C、Co/C、Ni/C和Cu/C為觸媒，在350oC溫度下熱裂解15分鐘，戴奧辛及?喃的去除效率分別為91.1%、91.1%、93.7%和94.5%。此外，以尺寸為10~18 mesh、20~40 mesh、70~120 mesh和120~200 mesh之觸媒經過行熱裂解後，飛灰中戴奧辛及?喃濃度分別為235、231、101和77.5 pg I-TEQ/g。表明戴奧辛及?喃濃度隨著觸媒尺寸的減小而逐漸降低。 為瞭解觸媒熱裂解去除戴奧辛及?喃的機制，需要定義所有 210 種戴奧辛及?喃同源物的濃度和轉化。本研究成功開發具有成本效益的淨化方法，減少分析一至八氯戴奧辛及?喃所需的溶劑體積及時間。在未經處理的飛灰中，一至三氯戴奧辛及?喃同源物佔 63.8%，而四至八氯戴奧辛及?喃同源物佔總戴奧辛及?喃的 36.2%。經熱裂解後，一至三氯戴奧辛及?喃同源物占比上升至88.3%，而四至八氯戴奧辛及?喃同源物占比則減少至11.7%。在熱裂解過程中，一至三氯戴奧辛的濃度增加130 pmol/g，而四至八氯戴奧辛的濃度減少74.7 pmol/g。結果指出其他的熱裂解過程中也可以形成一至三氯戴奧辛，而不僅是從四至八氯戴奧辛的脫氯。另一方面，熱裂解後一氯?喃的濃度增加88.6 pmol/g，而二至八氯?喃的濃度減少478 pmol/g。結果指出熱裂解過程中大部分多氯?喃的減少是由於高度氯化到低氯化同源物的脫氯，且多氯?喃的最終產物將是非氯化二苯?喃。透過各種前驅物（氯苯酚、氯苯等）在飛灰表面縮合可形成戴奧辛及?喃。因此，在熱裂解系統中加入觸媒，這些前驅物可分解成其他化合物而非形成戴奧辛及?喃。與熱裂解相比，抑制前驅物冷凝可能是觸媒裂解去除效率更高的主要原因;Incineration is considered as the major method for waste treatment in Taiwan and the residues of this process are bottom ash and fly ash (FA). Due to the high concentration of polychlorinated dibenzo-p-dioxins (PCDDs) and polychlorinated dibenzo-furans (PCDFs), fly ash generated from 24 large-scale municipal waste incinerators (MWIs) in Taiwan can not be reused or recycled and is finally subjected to sanitary landfill. Therefore, the increasing amount of fly ash generated from incineration process combines with the lack of landfill space has caused intense public concerns. This study focuses on the removal of PCDD/Fs in fly ash generated from MWIs via catalytic pyrolysis process. Since the chloride content may affect PCDD/Fs reduction efficiency via pyrolysis, two pretreatment methods, i.e., water washing and carbonated water washing, are applied to reduce the chloride content in FA and the effect of chloride content on PCDD/Fs reduction achieved with pyrolysis is evaluated. Being pyrolyzed at temperature of 350oC for 1 hour, the TEQ removal efficiencies of 96.7%, 98.5% and 96.2% for PCDD/Fs are achieved for unwashed fly ash (23.4% Cl), water washed fly ash (7.10% Cl) and carbonated water washed fly ash (2.70% Cl), respectively. In addition, catalytic pyrolysis reveals significantly high PCDD/Fs TEQ removal efficiencies. Specifically, the removal efficiencies of PCDD/Fs achieved are 64.1% and 91.3% under pyrolysis at 350oC in 15 minutes with Pd/γ-Al2O3 and Pd/C as catalyst, respectively, which are significantly higher than that of thermal pyrolysis (35.4%). Activated carbon is proven to obtain higher activity compared with γ-Al2O3 as catalyst support in pyrolysis system. Various transition metals catalyst are prepared and applied in the catalytic pyrolysis system developed. The results indicate that PCDD/Fs removal efficiencies via pyrolysis at 350oC in 15 minutes with Fe/C, Co/C, Ni/C and Cu/C as catalysts are 91.1%, 91.1%, 93.7% and 94.5%, respectively. It indicates that PCDD/Fs concentrations decrease gradually with the reduction of catalyst size. Moreover, PCDD/Fs concentrations in FA are measured as 235, 231, 101 and 77.5 pg I-TEQ/g after being pyrolyzed with Ni/C catalyst sizes of 10-18 mesh, 20-40 mesh, 70-120 mesh and 120-200 mesh, respectively. In order to understand the mechanism of PCDD/Fs removal with catalytic pyrolysis, concentration and transformation of all 210 PCDD/Fs congeners need to be defined. A cost-effective clean-up method is successfully developed to reduce the solvent volume and working time required to analyze mono- to octa-CDD/Fs. In untreated FA, mono- to tri-CDD/Fs homologues account for 63.8% while tetra- to octa-CDD/Fs homologues account for 36.2% of total PCDD/Fs. After thermal pyrolysis, the contribution of mono- to tri-CDD/Fs homologues is increased to 88.3% while that of tetra- to octa-CDD/Fs homologues is reduced to 11.7%. During thermal pyrolysis, the concentrations of mono- to tri-CDDs increase by 130 pmol/g while those of tetra- to octa-CDDs decrease by 74.7 pmol/g. The results also indicate that 42.5% mono- to tri-CDDs measured in treated FA can be formed during thermal pyrolysis from other pathway rather than merely via dechlorination from tetra- to octa-CDDs. On the other hand, the concentration of mono-CDFs increases by 88.6 pmol/g while those of di- to octa-CDFs decrease by 478 pmol/g. This result indicates that most of PCDFs reduction during thermal pyrolysis is due to dechlorination of highly chlorinated congeners to low chlorinated congeners and the final product of PCDFs would be non-chlorinated dibenzofuran if the reaction time is sufficient. It is suggested that PCDD/Fs can be formed via the condensation of various precursors such as chlorophenol and chlorobenzene on FA surface. Thus, with the presence of appropriate catalyst in pyrolysis system, these precursors can be decomposed into other chemical compounds instead of forming PCDD/Fs. Inhibition of precursor condensation could be the main reason for the higher PCDD/F removal efficiency achieved with catalytic pyrolysis compared with thermal pyrolysis. |
