低溫SCR觸媒處理NO之效率與SO2毒化影響研究 - 擔體效應

Abstract

選擇性觸媒還原法SCR (selective catalytic reduction)是多數工業(鋼鐵廠及發電廠等)用來處理NOx的最佳方法之一，傳統上以 V2O5/ WO3/ TiO2觸媒為主要使用觸媒，其操作反應溫度為250℃~400℃，而主要會遇到的問題在於釩鈦系觸媒之活性，易受工業製程中產生之大量粉塵及高濃度SO2之毒化，若經除塵或脫硫設備處理過後，則SCR入口端的煙道廢氣勢必降至200℃以下，且尚存有低濃度 SO2，因此若能發展低溫SCR觸媒並且抵抗硫之毒化，使SCR脫硝還原反應可在150℃以下順利發生，可顯著降低 SCR之耗能與操作成本。本研究利用共沉澱法合成單一擔體觸媒與多擔體觸媒，於低溫SCR系統下探討其受SO2毒化之影響，反應模擬溫度為150℃，空間速度為5000h-1。結果顯示以活性金屬Mn10wt%-Ce4wt%-Fe6wt%，搭配 TiO2、Al2O3與Lignite擔體比例8：1：1之多擔體觸媒可獲致最佳效率及最佳穩定性，在無SO2下30 hr之ηNO＝85%，而在30 ppm SO2下30 hr內仍可維持ηNO＝83%，同時透過BET、TPD (Temperature programmed desorption)、TGA (Thermogravimetry Analysis)等分析技術，找出低溫SCR反應中SO2毒化機制包括因為其促進觸媒NO之吸附量，使得NO與NH3吸附量不平均，阻斷SCR反應式4NO + 4NH3 + O2 □ 4N2 + 6H2O之發生；以及SO2與NH3結合形成硫銨化物附著觸媒表面，因而降低其活性。

Selective Catalytic Reduction (SCR) is one of the best techniques for nitrogen oxides removals. Many industrial plants such as power plants and steel plants commonly apply it for removing NOx. The WO3 promoted V2O5/ TiO2 catalyst is one of the most widely employed SCR catalysts, which is active within a temperature window of 250~400℃. If SCR unit can be installed at downstream of the Flue Gas De-sulfurization (FGD) and particulate control devices, the poisoning effects by high concentration SO2 and particles can be avoided. However, the temperature at this point is typically below 200℃, moreover, there is still low concentration of SO2 in the flue gas even after FGD system. Therefore, development of a superior low temperature SCR catalyst with high activity and resistant to SO2 would improve the cost-effectiveness of SCR. In this study, metal oxides were deposited on different supports using aqueous solution of metal precursor by co-precipitation method. A simulated SO2 poisoning system was designed for the reduction of NO with NH3 at low temperature. The reaction conditions were as follows：200 ppm NO, 30~150 ppm SO2, reaction temperature of 150℃ and GHSV of 5000h-1. The results showed that Mn10wt% Ce4wt% Fe6wt% supported on TiO2：Al2O3：Lignite＝8：1：1 catalyst exhibited the best resistance to SO2. Without SO2, the deNO efficiency was maintained at around 85% through the 30hr test. Under the presence of 30 ppm SO2, The efficiency was maintained at around 83% after 30 hr test. The inhibition effect of SO2 was characterized using BET, NH3-TPD and NO-TPD. As catalysts were poisioned by SO2, the NO adsorbed on the catalysts increased a lot, which resulted in the inhibition of NH3 and the subsequent reaction of NH3 and NO. Besides, the formation and deposition of ammonia sulfate at such low temperature also reduced the surface area for the active reaction.