水热环境下甲烷生成路径及催化剂稳定性综述

The reaction pathway of methane and catalyst stability under supercritical water

在超临界水条件下,生物质及固废气化转化为H_(2)或者CH_(4)等气体产物是其高值化利用的重要手段。鉴于高温高压的复杂水热环境以及生物质和固废的复杂组分,超临界水气化制气目前仍存在一些亟需解决的关键科学问题。在超临界水条件下,为了促进原料尽可能的定向转化为目标产物,如H_(2)和CH_(4)等,通常制备各种不同功能的催化剂并加入反应过程以实现目标产物的最大化,如Al_(2)O_(3)、SiO_(2)、TiO_(2)、ZrO_(2)、MgO、Y_(2)O_(3)、CeO_(2)、Si-Al、Zeolite、碳纳米管以及活性炭等不同载体负载不同活性组分(Cr、Ni、Zn、Ru、Rh)的催化剂。因而在水热环境下,尤其在超临界水条件下,催化剂的长期稳定运行十分重要。因此本论文针对目前超临界水环境下常用催化剂载体的稳定性以及活性金属组分失活等现象进行了探讨,以期对于超临界水环境下高稳定性催化剂的选取提供理论指导。在超临界水条件下,催化剂对于含碳原料的裂解、甲烷化以及水煤气变化反应程度的影响决定了系统整体气化效率;但甲烷的生成机制仍不明确,因而本论文重点探讨了甲烷的生成机制以及催化剂对甲烷的作用机制。

In supercritical water(SCW)condition,the gasification of biomass to H_(2) or CH_(4) has been studied extensively and proves to be critical in realising the upgrading of carbonaceous fuels.Given the extreme conditions of water at high pressure and temperature,along with the complex structure of biomass,the development of such a process still remains a challenge.In order to realize the complete decomposition of biomass and a high yield of desired products,such as CH_(4) and H_(2) at relatively milder conditions,various catalysts were synthesized and practiced.Different metals(such as Cr,Ni,Zn,Ru and Rh)were incorporated into various supports,such as mineral compounds of Al_(2)O_(3),SiO_(2),TiO_(2),ZrO_(2),MgO,Y_(2)O_(3),CeO_(2),silica-alumina,zeolites and carbon based supports of carbon nanotube,activated carbon.As a result,the long term stability of catalys is critical in the gasification of carbonaceous fuel in supercritical water condition.Therefore,this work focused on the stability of various support materials and deactivation of active metal components in supercritical water conditions with the purpose of choosing robust catalysts.In supercritical water condition,the effect of catalyst on carbonaceous fuels cracking,methanation reaction and water gas shift reaction determines the gasification efficienty as a whole.Unfortunately,the mechanism of methanation reaction is still unclear.Therefore,the CH_(4) formation mechanism and the effect of catalyst on CH_(4) conversion are emphasized in this work.