選擇性催化還原除硝技術之觸媒孔隙內擴散及最佳孔隙結構研究

Abstract

在NOx控制技術中，以選擇性催化還原法（Selective Catalysis Reduction，簡稱SCR）對NOx有最佳的去除效率。由於SCR觸媒本身是多孔性物質，其孔隙結構對觸媒活性是有很大的影響，因此本研究即利用孔隙內擴散理論，藉由效率因子分析來探討孔隙內擴散對SCR反應的影響，另一方面，亦尋求最佳的孔隙結構以求達到最佳的NO去除效率。 本研究結果顯示，經由效率因子分析發現，SCR反應之孔隙擴散限制會隨著溫度增加而增加，而孔徑較小、孔隙度較小、活性較大之觸媒受到孔隙擴散影響較大。另外就反應器外擴散對整體反應之影響程度來看，本研究結果顯示，蜂巢狀觸媒之開口較大者，受外擴散影響較大，而填充床式觸媒受外擴散影響就比較不明顯。至於最佳孔隙結構方面，對於單孔徑分布之觸媒，最佳之孔隙度應設為0.7，而對於尚未完全受到孔隙擴散影響之觸媒，最佳孔徑（半徑）應該設為80Å（表面積最大）。對於雙孔徑分布之觸媒，若設定巨孔徑為1μm，最佳微孔徑應為80Å，而最佳巨孔隙度是0.3，最佳微孔隙度是0.4，因此在最佳孔隙度方面，應該要在表面積增加以及孔隙擴散限制之間找出一平衡點。另外，經模式計算結果，孔隙結構最佳化之觸媒可節省約40∼48%體積，另一方面，最佳化之觸媒亦可降低操作溫度，這對SCR操作成本有很大的幫助。

Selective Catalysis Reduction (SCR) is one of the NOx control techniques which has the best NOx removal efficiency. The pore structure of the SCR catalyst has great influence on SCR reaction performance. In this study, pore diffusion effects are discussed by analysis of the effectiveness factors and optimum pore structures for the best NO removal efficiency are presented. According to the analysis of the effectiveness factor, the pore diffusion limitation is increased as the temperature increases. The pore diffusion limitation has more influence on the catalyst with small pore size, less porosity and high activity. For the monolith type catalyst with bigger channel pitch, it is influenced by external diffusion rather than the one with smaller channel pitch. The optimum pore structure for the catalyst with the mono-dispersed pore size distribution is porosity of 0.7 and pore average radius of 80Å because of the highest surface active area. However, for the catalyst with great pore diffusion limitation, the optimum pore average radius doesn’t exist. For the catalyst with bimodal pore size distribution, if the macro pore average radius is 1μm, the optimum micro pore average radius is 80Å, and the optimum macro pore porosity is 0.3 and optimum micro pore porosity is 0.4. Therefore, searching for the balance point of pore diffusion limitation and higher active area is important for the catalyst with bimodal pore size distribution. Besides, the model results show that the reductions of the catalyst volume with the optimum pore structure could be up to 40~48% and operation temperature could be decreased.