铜基氧载体吸释氧微观机理及组分间互相作用机制研究

发布时间：2018-01-20 19:43

本文关键词： 化学链氧解耦燃烧 Cu 基氧载体密度泛函理论抗烧结氧载体设计　出处：《华中科技大学》2015年硕士论文　论文类型：学位论文

【摘要】：化学链燃烧(CLC)是一种新型的燃烧方式,不仅可以提高燃料燃烧效率,而且可以实现CO2内分离,对解决当前面临的CO2排放问题具有重要意义。化学链氧解耦燃烧(CLOU)是一种特殊的化学链燃烧方式,氧载体颗粒可循环释放/吸收气态氧,可有效提高煤等固体燃料的燃烧速率及效率。其中高性能氧载体是化学链氧解耦燃烧技术的关键。但是目前对于氧载体的释氧吸氧微观机理和氧载体组分间相互作用机制的认识非常有限,对于氧载体设计优化的理论研究比较缺乏,制约了这一技术的发展。本文基于密度泛函理论(DFT)及热重分析实验,首先系统地研究了CLOU中应用最广泛的Cu基氧载体的释氧机理。恒温释氧实验发现,CuAl2O4释氧速率随反应进行逐渐减慢,而CuO保持匀速率释氧。DFT计算表明表面反应是CuO释氧的速控步骤,释氧快慢不受晶体内氧原子迁移路径的长短限制,因此能够保持匀速释氧;而CuAl2O4内部氧原子迁移势垒与表面反应势垒相当,是释氧的速控步骤,随着释氧过程的进行,氧原子迁移路径更长,造成释氧速率随着反应进行逐渐变慢。同时对CuFe2O4释氧进行了研究,发现CuFe2O4的O-O形成势垒比CuO和CuAl2O4略小,而O-O脱附势垒显著高于另两者;CuFe2O4晶体内部的氧原子迁移势垒同样比较高,这造成了CuFe2O4释氧困难。然后通过构建合理的负载型氧载体模型及开展相应的程序升温释氧实验(TPD),探索了负载的抗烧结机制和对CuO释氧活性的影响。DFT计算表明,CuAl2O4表面与CuO团簇间的相互作用可以有效抑制团簇的原子扩散,进而抑制烧结现象。通过比较CuO与不同负载的结合能,并据此判断相应负载的抗烧结能力,发现Cu-Zr和Cu-MgAl抗烧结性最好,而Cu-Ti抗烧结性较差。通过计算释氧过程的能量势垒及TPD实验研究,发现CuAl2O4添加并不能有效提高CuO释氧的速率。DFT计算表明对于不同负载的CuO团簇,抗烧结性能好的氧载体反应活性反而较低,与TPD实验结果相符合。最后,利用DFT计算方法研究了Cu2O、CuAlO2和CuFeO2吸氧初期的表面反应过程。计算结果表明自由O2分子的吸附解离最终在Cu2O表面形成了三重位的O3f和四重位的O4f,形成了类似CuO(111)表面的结构。三种氧载体的O2分子的表面吸附解离过程均是放热过程,说明吸氧过程中表面反应相对容易进行,同时进一步讨论了实际中限制吸氧过程进行的主要因素。
[Abstract]:Chemical chain combustion (CLC) is a new combustion method, which can not only improve the combustion efficiency of fuel, but also realize the separation of CO2. Chemical chain oxygen decoupling combustion (CLOU) is a special chemical chain combustion method. Oxygen carrier particles can recycle and absorb gaseous oxygen. The combustion rate and efficiency of solid fuels such as coal can be improved effectively. Among them, high performance oxygen carrier is the key of chemical chain oxygen decoupling combustion technology. But at present, the microscopic mechanism of oxygen release and oxygen absorption of oxygen carriers and the interaction between oxygen carrier components are very important. The understanding of the mechanism of action is very limited. The lack of theoretical research on oxygen carrier design optimization restricts the development of this technology. This paper is based on density functional theory (DFT) and thermogravimetric analysis (TGA) experiments. Firstly, the oxygen release mechanism of Cu-based oxygen carrier in CLOU was systematically studied. It was found that the oxygen release rate of CuAl2O4 decreased gradually with the reaction at constant temperature. The results show that the surface reaction is the rapid control step of CuO oxygen release, and the speed of oxygen release is not limited by the length of oxygen atom migration path in crystal, so the oxygen release rate can be maintained at a uniform rate. The oxygen atom migration barrier in CuAl2O4 is the same as the surface reaction barrier, which is the speed control step of oxygen release. With the oxygen release process, the oxygen atom migration path is longer. The oxygen release rate of CuFe2O4 decreases gradually with the reaction. It is found that the O-O formation barrier of CuFe2O4 is slightly smaller than that of CuO and CuAl2O4. The O-O desorption barrier was significantly higher than the other two. The oxygen atom transport barrier in CuFe2O4 crystal is also relatively high. This resulted in the difficulty of releasing oxygen, and then through the construction of a reasonable loaded oxygen carrier model and the corresponding temperature programmed oxygen release experiments. The anti-sintering mechanism and the effect on the oxygen release activity of CuO were investigated. The results show that the interaction between CuAl2O4 surface and CuO clusters can effectively inhibit the atomic diffusion of the clusters. By comparing the binding energy of CuO with different loads and judging the anti-sintering ability of the corresponding load, it is found that Cu-Zr and Cu-MgAl have the best sintering resistance. However, the sintering resistance of Cu-Ti is poor. The energy barrier of oxygen release process and the experimental study of TPD are calculated. It was found that the addition of CuAl2O4 could not effectively increase the oxygen release rate of CuO. The results showed that the oxygen carrier reaction activity with good sintering resistance was lower for CuO clusters loaded with different loads. Finally, Cu2O is studied by DFT method. The surface reaction process of CuAlO2 and CuFeO2 at the beginning of oxygen absorption. The results show that the adsorption and dissociation of free O _ 2 molecules finally form triple O _ 3f and quadruple O _ 4f on the surface of Cu2O. The surface adsorption and dissociation process of the O _ 2 molecules of the three oxygen carriers are exothermic, which indicates that the surface reaction is relatively easy to be carried out in the process of oxygen absorption. At the same time, the main factors restricting the process of oxygen absorption are discussed.
【学位授予单位】：华中科技大学
【学位级别】：硕士
【学位授予年份】：2015
【分类号】：TK16

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