热气化生物质制氢催化剂及热力学分析研究

Hydrogen Production by Hydrothermal Gasification of Biomass: Catalysts and Thermodynamic Analysis

在反应过程中加入催化剂可在更温和的水热条件下实现生物质高效气化制氢。在众多催化剂中,Ni系催化剂因其廉价并在反应中表现出较高的活性等优点而被认为是很有发展前景的制氢催化剂。该课题组针对Ni/Al_2O_3酸性位易于积碳引起催化剂失活的弱点,通过金属助剂的辅助作用对Al_2O_3为载体的负载Ni催化剂进行改性。选用Cu、Co、Sn、Ce和碱性Mg,通过共浸渍、分步浸渍和共沉淀等方法制备双金属Ni-M或者复合氧化物载体催化剂。结果表明,金属助剂Ce的引入有效提高了Ni系催化剂的催化产氢活性,催化剂的抗积碳性能亦得到有效改进,表明金属Ce是非常合适的金属助剂。MgAl_2O_4使催化剂水热稳定性得到改善,碱性Mg助剂可以有效抑制Ni-Al催化剂的表面结晶碳的形成。较低的热处理温度,或者分步浸渍制备Ni-Mg-Al催化剂能获取更多有效的活性位。碱性Mg助剂可以改善Ni-Al催化剂的催化活性及水热稳定性。对于溶胶-凝胶法制备的Rutile TiO_2负载Ni催化剂,降低催化剂热处理温度可以获取分散性更高的Ni晶粒,从而提供较多的活性位,以促进生物质在水热条件下C-C键的断裂,水汽转化反应和甲烷化反应,从而提高生物质转化率。通过利用Aspen Plus软件根据Gibbs自由能最小化原理采用PR状态方程,以MHV2混合规则建立热力学模型来模拟水热条件下气化生物质及其模型化合物产氢的过程,对水热气化生物质及模型化合物进行了理论分析,计算出在一定温度和压力条件下达到平衡时系统的产气量,提供了催化气化生物质的方向和限度的数据。

Highly efficient hydrogen production from biomass can be realized by using catalysts in the reaction process under more mild hydrothermal conditions. Among various catalysts, Ni based catalysts are considered as the most promising catalyst for hydrogen production due to its low cost and good catalytic activity. In order to solve the problem of catalyst deactivation caused by carbon deposion at the acid site of Ni/Al2O3,the additives were studied to modify the Ni based catalysts loaded on AI203 in our research group. Cu, Co, Ce and Mg were chosed as additive and bimetal Ni--M or complex oxide catalysts were prepared by co--impregnation, impregnation by step, and co--precipitation, etc. The results show that the introduction of metal Ce can effectively improve the catalytic activity of hydrogenproduction, and the resistance of carbon deposition can also be improved, indicating that metal Ce is a suitable additive. Mg additive can restrain the formation of crystalline carbon on the surface of Ni--AI catalyst. Much lower heat--treating temperature or impregnation by step in preparation can obtain more effective active sites for Ni--Mg--A1 catalyst. Moreover, Mg additive can improve the catalytic activity and hydrothermal stability of Ni--A1 catalyst. As to the Ni catalyst loaded on rutile TiO2 prepared by sol-gel process, Ni crystals with much higher dispersity can be achieved by lowering the heat-treating temperature, which provide more active sites and promote the C--C bond breaking of biomass under hydrothermal conditions as well as water gas shift reaction and methanation reaction, thus improving the conversion efficiency of biomass. The thermodynamic analysis was performed with Aspen Plus V7.3.2. The thermodynamic models was built using the P--It equation of state and the MHV2 mixing rule to predict the gas yields in the hydrothermal gasification process. Hydrothermal gasification of biomass and its model compounds was theoretically analyzed. The equilibrium state of hydrothcrmal gasification reaction was obtained by calculating the gas production under certain temperature and pressure, which points out the direction and limitation of hydrothermal gasification reaction.