實驗研究小型重組器產氫之性能

Abstract

本研究設計並組裝一套可將甲醇蒸汽轉化為氫的小型重組器，以提供可攜式燃料電池的原料，且並測試及分析該重組器在各種情形下的性能。該重組器可以分成甲醇蒸汽重組產氫與氫氣純化兩部分，其中甲醇蒸汽重組使用的觸媒為CuO-ZnO-Al2O3、CuO-ZnO-Al2O3-Pt-Rh及Pt-Rh。而由於甲醇水溶液經蒸汽重組會產生H2、CO及CO2與極少量的O2，其中CO會造成PEMFC白金電極的毒化，所以必須將其濃度降低到一定濃度以下，以提供PEMFC使用。因此在後端設計一小型氣體處理器系統，探討將不同含量的Pt與Ru/r-A12O3觸媒與加入促進物Co、Fe或水對CO轉化率與甲烷產量的影響。 甲醇蒸汽重組產氫的部分，CuO-ZnO-Al2O3、CuO-ZnO-Al2O3-Pt-Rh及Pt-Rh觸媒之氫氣產生率與甲醇轉化率均隨溫度的上升而增加，但是在溫度超過大約320℃時，則CuO-ZnO-Al2O3出現劣化現象。若在此觸媒中加入貴金屬Pt與Rh，則在測試溫度範圍內無高溫劣化情形，且經穩定性的測試結果，也呈現良好的穩定性。不過，在溫度低於300℃左右時，仍以CuO-ZnO-Al2O3之性能為最佳。另一方面，若觸媒中僅含Pt與Rh，則高溫劣化現象不明顯，但是催化性能最差。在甲醇進料方面，氫氣產生率隨進料率增加，但是甲醇轉化率會減少。而水對甲醇之莫耳比的增加會使甲醇轉化率與氫氣產生率都減少，但是會隨擔體孔徑密度及長度的增加而增加。另外在反應器裡擾流器之設計可以有效增加反應效果。 在CO去除的部分，當Pt含量從1wt%增加到5wt%時，CO轉化率在低溫區有明顯提升，且隨著溫度的增加，甲烷產量增加的幅度較不明顯。Ru含量越多，則CO轉化率在低溫80℃以前有較佳的效果，且甲烷產量隨著溫度的增加會變得更明顯。在觸媒蜂巢孔徑密度方面，Pt與Ru/r-A12O3觸媒的孔徑密度從300CPSI增加到400CPSI，則呈現出相似的高CO轉化率區間，但是甲烷產量沒有增加，反而下降。在添加過渡金屬Fe與Co方面，加Fe的效果可以使高CO轉化率的溫度區間加大，且可以稍微抑制甲烷化，但是添加Co後的CO轉化率在高溫時較為明顯，且甲烷產量比添加Fe的效果強。最後，5wt%Pt/r-A12O3與1wt%Ru-1%Fe/r-A12O3觸媒歷經12小時穩定性測試，呈現出極佳的穩定性。 另外在優先氧化反應中額外加入水參與反應，則可以發現，5wt%Pt/r-A12O3的觸媒可以提升CO轉化率，而且加水可降低各種觸媒於高溫時產生的甲烷化現象，尤其以Ru系列觸媒最為明顯，可在220℃時，將甲烷產量降至1414ppm左右，相較於無水參與優先氧化反應時，降低極多，因此加水的優點可以減少氫氣損失，進而提升系統效率。

This study details the design and fabrication of a small reformer for the generation of hydrogen gas from a solution of methanol and water. The catalysts used for the methanol steam reforming process were CuO-ZnO-Al2O3, CuO-ZnO-Al2O3-Pt-Rh and Pt-Rh. The solution of methanol and water, when passed through the reformer, produced H2, CO, CO2, and small amounts of O2. As the CO in the products can induce degradation of the electrode in a proton exchange membrane fuel cell, the concentration of CO must be reduced to an acceptable level by preferential oxidation. The purification process used different contents of catalysts Ru and Pt; moreover, Co, Fe or water were also added in order to study the effect on the CO conversion rate and methane yield. In general, the reformer system can be divided into two parts: one for the production of hydrogen and the other for the removal of carbon monoxide. The first experimental investigation produced hydrogen in the methanol reformer unit. The results show that both the methanol conversion and the hydrogen yield rates increase with temperature. Of the three catalysts tested, CuO-ZnO-Al2O3 provides the best performance at temperatures lower than 320℃; however, at higher temperatures, the performance of this catalyst deteriorates, while that of CuO-ZnO-Al2O3-Pt-Rh and Pt-Rh continues to improve. This suggests that the addition of Pt and Rh to the original CuO-ZnO-Al2O3 catalyst has a stabilizing effect upon the reforming process under higher temperature conditions. The results also show that a higher methanol feed rate reduces the methanol conversion rate but increases the hydrogen yield rate. It was found that both the methanol conversion and the hydrogen yield rates reduce as the steam-to-methanol ratio is increased. Finally, the performance can be significantly improved by introducing a turbulence inducer upstream of the catalyst carrier and by increasing both the length and the cell density of the honeycomb structure of the catalysts. The purification of hydrogen involves a preferential oxidation (PROX) reaction. When the content of Pt in the base catalyst r-A12O3 was increased from 1wt% to 5wt%, the CO conversion rate was increased significantly at lower temperatures; however, the methane yield was lower at higher temperatures. When the Ru content was increased from 0.5wt% to 5wt%, the CO conversion rate was increased at lower temperatures and the methane yield increased with increasing temperature. As the cell densities of catalysts Pt and Ru were increased from 300 to 400CPSI, the ranges of the high CO conversion rates were similar, while the methane yields decreased. The effect of addition of Fe can increase the range of the high CO conversion rate and can also slightly suppress methane production. The CO conversion rate for the addition of a Co catalyst was increased at higher temperatures, while the methane yield was higher than that found upon addition of Fe. Finally, catalysts 5wt%Pt/r-A12O3 and 1wt%Ru-1wt%Fe/r-A12O3 remained stable throughout the 12-hour stability test. The existence of water in preferential oxidation reactions can increase the CO conversion, especially with the 5%Pt catalyst; it can also reduce the methanation phenomenon of all catalysts used in the experiment at high temperatures, especially in the series of Ru catalysts. At a temperature of 220℃ and a water flow rate of 1 ml/min, the 1wt%Ru/r-A12O3 catalyst can reduce the methane concentration to 1414ppm. Therefore, it can decrease the loss of hydrogen and improved the efficiency of the reformer system.