Enhancing the performance of Vanadium based catalysts for selective catalytic reduction of nitrogen oxides with ammonia

Abstract

암모니아를 활용한 선택적 촉매 환원기술은 높은 효율성과 낮은 운영 비용으로 대기 오염물질 질소산화물 제거에 탁월하여 각광받아 오고 있으나 매년 강화되는 질소산화물 배출 규정을 준수하기 위해서는 종래의 기술들보다 더욱 효과적인 선택적 촉매 환원 기술이 도입되어야 하는 실정이다. 또한, 해당 기술은 발전소의 후처리 과정에서 많이 활용이 되는데 후처리 과정에 포함된 황산화물은 촉매 수명과 효율에 영향을 미치므로 이에 대한 영향도 고려되어야 한다. 공정의 운용 온도와 배기 가스 조성 등의 요인에 따라, 다양한 금속 산화물 기반 촉매와 제올라이트 기반 촉매들이 암모니아를 활용한 선택적 촉매 환원 기술에 활용되고 있다. 그 중에서도 바나듐 산화물 기반 촉매는 높은 열 안정성과 경제적인 가격 때문에 많이 활용된다. 하지만, 최근 배출 규제를 충족시키기 위해서는 몇 가지 문제점들에 직면해 있다. 첫째, 기존 바나듐 산화물 기반 촉매의 높은 질소산화물 전환율은 300℃ 이상의 높은 온도에서만 달성할 수 있으나 최근 산업에서 실제로 운전되는 온도는 대부분 300℃ 미만이다. 게다가, 후처리 과정에서 황산화물과 물이 존재하게 되는데 황산화물이 산화되어 암모니아와 물의 반응물로 인해, 비활성화 물질인 암모늄 바이설페이트(ABS)가 생성되게 된다. 생성된 암모늄 바이설페이트가 촉매에 침적되게 되어 촉매의 효율과 수명을 감소시킨다는 문제점이 있다. 따라서, 저온에서 높은 질소산화물 제거 효율과 함께 더 적은 에너지 소비로 우수한 내황성을 확보하는 혁신적인 선택적 촉매 환원 기술이 필요하다. 이 같은 목표를 가지고 저온에서 내황성을 향상시키기 위해 V2O5/WO3-TiO2 촉매와 다양한 온도에서 소성된 알루미나 촉매를 물리혼합방법으로 제조한 촉매들을 개발하였다. 혼합된 촉매 (V2O5/WO3-TiO2 + Al) 중에서 900℃에서 소성된 알루미나와 혼합된 바나듐 촉매가 강한 산점의 증가로 인해 가장 높은 내황성을 유지하였다. 물리 혼합 과정에서 생성된 물리적 접촉면을 통해 바나듐에 생성된 ABS가 혼합된 알루미나로 이동하여 바나듐 활성점이 황 피독으로부터 보호되어 저온에서도 우수한 내황성과 우수한 재사용성을 가지는 것으로 관찰되었다. 이 연구는 바나듐 산화물 기반 촉매(V2O5/WO3-TiO2)와 표면을 카본 처리한 제올라이트와 물리 혼합 방법으로 제조하여 물리적 및 화학적 비활성화를 동시에 해결할 수 있는 방법을 제시하였다. V2O5/WO3-TiO2 촉매 및 알루미늄 사이트가 많은 제올라이트 Y를 물리 혼합방법으로 제조하였을 때, 활성 저하가 관찰되었다. 활성 저하의 주요 원인은 물리 혼합 과정 중 바나듐 종과 제올라이트 표면에 존재하는 알루미나 종의 화학적 상호작용을 통한 활성화 에너지 증가로 밝혀졌다. 이에 따라 제올라이트 표면에 옥타데실트리클로로실란(OTS)을 코팅처리 하여 V2O5/WO3-TiO2 촉매와 물리 혼합하여 제조하였다. 개발된 촉매(V2O5/WO3-TiO2 + OTSY)는 물리적 및 화학적 중독을 동시에 억제하여 우수한 저온 활성과 내황성을 가지는 촉매 개발하였다. 더불어, 본 연구에서는 V2O5/WO3-TiO2 + 제올라이트 Y 촉매를 볼 밀링 머신을 통해 제조함으로서 볼 밀링 효과를 연구하였다. 볼 밀링을 이용한 촉매 제조를 통해 물리 혼합 방법에 따라 변화된 촉매 특성과 활성에 미치는 영향에 대해서 연구를 하였으며, 최적화 연구를 진행하였다.

Air pollution has been a global environmental issue for decades and efforts to mitigate the global threat are a huge challenge to many environmental researchers. As the industries have been developing fast, desires for clean environment and better life have also increased for past years. The major air pollutions that have caught much attention are nitrogen gases (NOx), sulfurous gases(SOx), carbon dioxides(CO2), and other greenhouse gases(GHG). For recent years, the harmfulness of these emissions has been highlighted as cause of climate change. Hence, the global goals for clean environment have implemented to reduce the environmental impacts for public health. Among these air pollutants, NOx emission from stationary sources like coal power plants that uses fossil fuels as energy sources is a huge challenge because it can produce the secondary product when it emits to air. It also plays a critical role in increasing ozone and smog. Various technologies have been developed to suppress NOx emissions including selective catalytic reduction with NH3, lean burn engines and so on. Although selective catalytic reduction with NH3 as a reducing agent (NH3-SCR) has been applied to industries as an efficient technology, especially to after treatment process, to reduce NOx emission. The injected ammonia(NH3) into exhaust gas stream, passing through a catalyst bed where the NOx is converted to nitrogen (N2) and water (H2O), which are harmless compounds in air. The NH3-SCR technology are advantageous over other technologies because of its lower operating costs with high efficiency. However, new technology on NH3-SCR system must be introduced to fulfill the NOx emission regulations placed by governments and SOx are usually present in after treatment process of power plants. During the NH3-SCR process, the present of SOx must be considered as well because it can affect the lifetime of catalyst and efficiency. Various types of metal oxide-based catalysts and zeolite-based catalysts have been utilized in NH3-SCR systems depending on their properties and operating factors like temperature and composition of exhaust gas. Among various DeNOx catalysts, vanadium oxide-based catalysts are most used in after treatment process of stationary sources for its high thermal stability and affordable prices. Although the vanadium oxide-based catalysts (V2O5/TiO2 or V2O5/WO3-TiO2) are world widely used for its advantages, there are practical issues to meet the recent obligations. First, the high NOx conversion of the conventional vanadium oxide-based catalysts can only achieve only at high temperature above 300 oC while the recent operating temperature in actual industries are mostly below 300 oC. In addition, since SO2 and water exist during the after-treatment process. When SO2 are oxidized to SO3 and react with NH3 and H2O, ammonium bisulfate (ABS) is produced and it causes sulfur poisoning on catalysts, leading to reduce efficiency and lifetime of catalysts. Hence, innovative NH3-SCR technology that acquires high sulfur resistance with less energy consumptions and higher NOx removal efficiency at low temperature simultaneously are required as for environment-friendly industrial applications. Therefore, this research provides innovative insights to improve sulfur resistance of vanadium oxide-based catalysts via simple synthesis method that can be advantageous applying to practical industrial fields. In details, this study discussed that a series of V2O5/WO3-TiO2 and alumina calcined at different temperatures are prepared by physical mixing to enhance sulfur resistance and regenerability at low temperatures. Among the mechanically alumina mixed catalysts (V2O5/WO3-TiO2 + Al), the V2O5/WO3-TiO2 mixed with alumina calcined at 900 oC achieved the highest sulfur resistance due to increase of strongly adsorbed acid sites. This research also demonstrated that ABS formed on vanadia sites migrated to the mixed alumina sites and vanadia active sites were protected from sulfur poisoning, resulted in superior sulfur resistance at low temperature. The physical mixed V2O5/WO3-TiO2 catalyst with alumina can enhance sulfur resistance of V2O5/WO3-TiO2 catalyst and accomplish regenerability at low temperature. This study also discussed the physically mixed vanadia catalyst with surface modified zeolite that can resolve physical and chemical deactivation simultaneously. When V2O5/WO3-TiO2 catalysts and Al-rich zeolite Y (Si:Al2=5.1) were mechanically mixed are designed for sulfur-resistant deNOx catalysts, degradation of activity was observed with improved sulfur resistance. The main cause of degradation was the chemical interaction between VOx and mobile AlOx species, most likely extra-framework Al species, on zeolite surface during the mechanical mixing, which were confirmed by various characterizations including H2- temperature-programmed reduction (H2-TPR) and line energy dispersive X-ray spectroscopy (line-EDS). To resolve the problem, octadecyltrichlorosilnae (OTS) was coated on zeolite surface first and mechanically mixed with V2O5/WO3-TiO2 catalysts. In summary, the developed catalyst V2O5/WO3-TiO2 + OTSY catalyst obtained high NOx conversion and enhanced sulfur resistance by suppressing the physical and chemical poisoning simultaneously. The present study also investigated ball milling effect over hybrid catalyst composed of V2O5/WO3-TiO2 + zeolite Y catalysts. It included how the ball milling process affected on catalytic properties and activity depending on synthesis method.