| 摘要: | 本研究致力於以低溫電漿技術處理揮發性有機物─對二甲苯及乙醛，研究中自行設計實驗室規模的低溫電漿反應器，探討操作因子、氣體組成對低溫電漿系統的影響。研究結果顯示，低溫電漿技術對於對二甲苯及乙醛兩種不同分子結構之揮發性有機物，在特定操作條件下均可獲致相當高的去除效率(>99%)。許多操作因子會影響低溫電漿技術對揮發性有機物的去除率，其中，去除率與施加電壓、輸入功率、水氣濃度、氣流溫度及停留時間成正相關；與污染物負荷量成負相關；氧氣濃度的影響較特殊，氧氣為低溫電漿系統中最重要的活性物種─氧原子的提供者，換言之，氣流中存有愈多的氧分子應該愈有利於氧原子的生成，進而促進揮發性有機物的氧化；另一方面，由於氧氣本身電陰性會耗損電漿中的電子數，上述兩種現象在低溫電漿系統中彼此競爭，造成過高與偏低的氧氣濃度均不利於揮發性有機物在低溫電漿系統中的去除。研究中探討線管式與填充床式兩種介電質放電反應器，兩者對於有機物去除效率均可達到99%以上，上述操作參數對兩種反應器的影響趨勢一致，最大的差異在於能量消耗；填充床式反應器藉由填充料提高系統的蓄積能量，藉此增加系統內的表面放電，使得污染物可以在較短停留時間下達到較高的去除效率，然而介電質的蓄電卻導致反應器溫升而浪費能量，降低本反應器形式的能量效率。產物鑑定部份，乙醛上的碳經放電後95%轉化為CO2，少部份變成CO；對二甲苯上的碳經放電後70%轉化為CO2，另有醛類物質(15%)及CO (10%)之生成；除了有機物的轉換外，放電後會生成數十ppm的NO2及N2O副產物。本研究並發展一套電漿數學模式，以反應動力學的觀點進行揮發性有機物在低溫電漿中的電腦模擬，模擬結果與實驗結果的趨勢一致，顯示本模式具有良好的預測能力。整體而言，本研究證實低溫電漿對於揮發性有機物具有良好的去除效果，但操作費用偏高，未來電力供應系統若能提升，相信對低溫電漿技術的實廠化將有很大的助益。 The aim of this study is to treat volatile organic compounds (VOCs), e.g. para-xylene and acetaldehyde, via a nonthermal plasma technology (NTP). Laboratory-scale NTP reactors had been constructed and used to investigate the effects of operational parameters and gas compositions on VOCs' removal efficiencies. Experimental results indicated that removal efficiencies of these two VOCs with different chemical structures were relatively high (>99%). Many factors will affect the removal efficiency of VOCs for NTPs. For example, the removal efficiencies increase as the applied voltage, input power, water content, gas temperature and gas residence time are increased; but decrease with increasing inlet VOCs concentration and gas flow rate. The oxygen content plays a unique feature in the treatment of VOCs with NTPs. In NTPs, oxygen is the major source of atomic oxygen, being one of the most important active radicals in the processing of VOCs. From this viewpoint, a higher oxygen content should result in a higher VOCs removal efficiency. However, the electronegative characteristic of oxygen molecules cannot be overlooked. This property makes the existing electrons in NTP system can easily attach to oxygen molecules, resulting in a lower electron density in NTP system. Consequently, a competition between increasing active O atoms and reducing electron density is commonly observed in an NTP system. Therefore, an optimal oxygen content may exist when treating VOCs via NTP technologies. Two kinds of reactors, i.e. wire-tube dielectric barrier discharge reactor and packed-bed dielectric barrier discharge rector, had been investigated in this study. Both reactors showed good potential in VOCs removal. The trends affected by operational parameters for these two reactors were similar except for the energy consumption. A packed-bed reactor with packing dielectric materials can store more electric energy and increase the total discharge surface within the reactor; these two factors can enhance VOCs removal. Moreover, the dielectric loss might result in unexpected energy loss for this kind of reactor. Product analysis indicated that 95% of carbons in acetaldehyde and 70% of carbons in para-xylene were converted into carbon dioxide, respectively. The remaining products generated in the processing of para-xylene included 15% of aldehydes (on C1-basis) and 10% of carbon monoxide. Nitrogen-containing species, i.e. NO2 and N2O, with a typical concentration of tens of ppm were found during the VOCs processing. A computer program for plasma simulation based on chemical kinetics was successfully developed. The VOCs removal efficiency predicted by the model generally agreed well with the experimental data over a wide range of oxygen contents and gas flow rates. In brief, this study demonstrates the good potential of applying NTP technologies for VOCs removal. For the economic concerns, further works including improving the power supply are needed for better utilization of the energy. |
