化学循环燃烧中载氧体表界面反应与体相离子传递耦合过程的理论研究

Chemical looping combustion converts viable fossil fuels to clean energy with carbon dioxide ready for capturing. The key to commercialize the chemical looping combustion process is the development of an efficient and sustainable oxygen carrier. Understanding reactions between the oxygen carrier and the fuel molecules, ion transport in bulk oxygen carrier and through the interface as well as the possible phase transition during the looping process at mesoscale is key to design effective oxygen carriers and develop efficient technology for chemical looping combustion. There have been several successful examples of chemical looping combustion in laboratory and even at pilot plant scale but the underlying physicochemical processes is not well understood. Herein, we propose to use density functional theory calculations to investigate the surface/interfacial reactions between the oxygen carrier and fuel molecules. We selected CH4 and CO as model fuel molecules, and Fe- and Ni- based oxides as oxygen carriers. By varying the cluster sizes of oxygen carrier from a few molecular units to thin film, we will evaluate the corresponding changes in the geometric and electronic structures. We will also study the interfacial interaction and monitor the evolution of the carrier-support interface during the reduction and oxidation of the carrier. We will follow the elementary adsorption and reaction steps of the fuel molecules at the surface of the carrier, the ion transport through the carrier and interface, and the possible phase transition during the looping process. We anticipate that the proposed research projects provide mechanistic understanding into the interfacial chemical processes during chemical looping combustion and information to establish a kinetic models to describe the process. The understanding and model are critical to a rational design of oxygen carrier and will ultimately benefit the development of efficient chemical looping combustion processes for commercial application. The general understanding on heterogeneous process developed through the proposed research will have a broad impact beyond the present project.

化学循环燃烧可以将各种化石燃料转化为清洁能源，同时产生易于捕集的二氧化碳，而高效耐用载氧体是实现化学循环的主要障碍。载氧体的开发有赖与对其与燃料分子的表界面反应、离子在体相中的扩散及随反应进程可能发生的相变等过程及不同过程耦合的认识和理解。本项目拟采用密度泛函理论研究上述过程。选取CH4、CO作为模型燃料分子，Fe、Ni基氧化物作为载氧体。通过调变载氧体的颗粒大小及形态，考察载氧体几何电子结构变化及其与不同载体之间的界面作用，分析燃料在载氧体表界面上的吸附与氧化的基元反应步骤，考察过程中离子在载氧体体相与表界面的扩散及载氧体可能的相变行为，据此建立载氧体表界面的反应机理和相变机制及描述载氧体还原的动力学模型。本项目还将研究还原后载氧体的再生过程和机理。本研究结果将有益于载氧体的合理设计，对进一步开发化学循环燃烧及其它如固体氧化物燃料电池等涉及类似多相过程的应用具有重要意义。

随着现代社会的快速发展，我们对化石燃料（煤炭、石油和天然气）的需求迅速增加并向大气中排放过量的二氧化碳。CO2被认为是造成温室效应的主要原因。化学循环燃烧(CLC)技术,可以从排放的气体中分离二氧化碳而不消耗额外的能量。在化学循环燃烧过程中,燃烧反应在两个相连接的反应器中发生：在燃料反应器中，燃料被载氧体(OC)氧化,载氧体被还原；在空气反应器中,被还原的载氧体被氧气再次氧化。因此在化学循环燃烧中，载氧体是十分重要的。本项目借助第一性原理系统地研究了不同结构的载氧体的还原过程以及不同脱氧速率对于相变的影响。. 计算结果表明体相CuO和CuO(111)表面的平均反应能基本保持一致，分别为246.2和245.9 kJ/(mol O2)。与体相模型相比，使用平面模型并不能显著降低整体的反应能量。相比之下，团簇模型中的平均反应能量明显降低，仅仅为127.5 kJ/(mol O2)。同时，比较这几种模型的结构变化过程，可以看到在团簇结构中有大量Cu-Cu键的形成,这将会引起Cu金属相的形成。结果还表明,不同还原速率下达到相同还原度时将会产生不一样的结构。所有这些结果均表明了固相反应的复杂性。这项研究说明了粒子大小和反应条件在还原过程中的重要性。. 本研究表明了尺寸或者说边界效应对于还原过程的结构变化和相变过程具有重要的影响，同时强调了还原速率对于反应的调控作用。本研究补充了深度还原过程结构变化及相变过程的空白，将有助于合成和设计高效的载氧体。