利用滑動電弧的電漿輔助觸媒進行自熱反應重組乙醇的研究:不同觸媒的比較

Abstract

本實驗探討利用滑動電弧的電漿輔助觸媒經過自熱反應的方式重組乙醇存並進行產生氫氣之研究。而滑動電弧的電漿可以由一個能夠產生20kHz的電源供應器來提供所需的功率。其中，在電漿輔助觸媒生產氫氣的過程搭配兩種不同種類的觸媒，分別是貴重金Rh的觸媒和非貴重金屬Ni0.35Mg2.65FeO4.5的觸媒，藉由這兩者去比較滑動電弧搭配觸媒各有怎樣的效果。首先在探討貴重金屬中，藉由成分為5 wt% Rh/CeO2/Al2O3的觸媒，並在空氣流量 0.5-2.0 SLM中，使乙醇水氣重組。在觸媒重組的實驗結果中，當氣體流量為1.0 SLM時，氣體轉換率達到100%，氫氣選擇比也達到最高的115%，但流量為較低的0.5slm和較高的1.5slm時，氫氣選擇比分別為95%和70%的選擇比。加入電漿輔助觸媒後，可以明顯地看到在1.0 SLM 及1.5 SLM時，氫氣選擇比分別為113%和111%。由此結果可以看出，PAC的系統在較高流量時，仍然可以藉由目前設備達到較好的氫氣選擇比。然而當流量在達到更高的2.0 SLM時，氫氣的選擇比急速的下降至70%上下並且和觸媒的氫氣選擇比幾乎一樣，可能的原因是，電漿處理氣體的停留時間下降，使得大部分的氣體僅藉由觸媒反應產生重組。 而在非貴重金屬的探討中，使用成分為10 wt% Ni0.35Mg2.65FeO4.5/Al2O3的觸媒使酒精重組並產生氫氣。而結果顯示出，當空氣流量在1 SLM，觸媒溫度為400 ℃時，觸媒可產生出最高的氫氣選擇比大約55%，還有的乙醇轉換效率接近100%，並且隨著溫度下降而下降。在經過電漿輔助後，氫氣選擇比可以來到約75%。但即使氫氣選擇比有提升，但是乙醇的轉換效率卻大幅下降至73 %並且氫氣選擇比仍然遠低於搭配貴重金屬的選擇比。而最有可能的原因是，Ni0.35Mg2.65FeO4.5本生身「水氣轉換氫氣」的這項反應式是幾乎不會發生的，對於氫氣的選擇比以及乙醇轉換效率會大幅下降有很好的解釋。而上述所解釋的現象和氣體成分詳細的分析皆可透過經由氣體層析儀得知。

In this study, the preheated ethanol steam flow is reformed using a gliding-arc plasma-assisted catalyst (PAC) system with a power frequency of 20 kHz. Among the PAC reforming, the noble catalyst Rh and non-noble catalyst Ni0.35Mg2.65FeO4.5 respectively is used to compare PAC with catalyst alone reforming by generating the hydrogen selectivity and conversion rate via different experimental parameters. In Rh catalyst alone reforming at the range of air flow rates of 0.5-2.0 SLM, the results show that a 100% conversion rate and a maximum of 115% hydrogen selectivity could be obtained at a C/O ratio of 0.7 with an air flow rate of 1.0 SLM. However, hydrogen selectivity decreases rapidly to 95% and 70% at lower (0.5 SLM) and higher (1.5 SLM) air flow rates, respectively. With the addition of a gliding arc prior to the catalyst, hydrogen selectivity reaches 113% and 111% at air flow rates of 1.0 and 1.5 SLM, respectively, with a plasma absorption power of approximately 200 W. This shows that very high hydrogen selectivity (>110%) can be obtained at air flow rates of both 1.0 and 1.5 SLM under the current experimental setup. However, at a 2.0 SLM air flow rate, the hydrogen selectivity of PAC drops down to 70% and is almost the same as that for reforming with the catalyst alone. The above observations correlate strongly with the residence time of the gas flow in plasma with the catalyst. In the Ni0.35Mg2.65FeO4.5 catalyst reforming at the range of catalyst temperatures of 200-400℃, the highest hydrogen selectivity (~55%) and conversion rate are close the 100% at temperature 400 ℃ and drop down rapidly with decreasing the temperature. However, the PAC with Ni0.35Mg2.65FeO4.5 catalyst can improve the selectivity of catalyst alone reforming to 75% at the temperature 400℃. Even though the selectivity could be raised, the conversion drop down to 73% and hydrogen selectivity is much lower than PAC with Rh catalyst. The reason maybe the plasma generates a lot of H2O but the catalyst is inefficient at water-gas-shift (WGS) chemical reaction equation, as estimated using measurements of the gas composition from gas chromatography.