Evaluating new Oxygen Transfer Materials for Air Separation, Oxy-Fuel Combustion and other Processes
Creators: name: Goerke, Richard Hinrich
Publisher: name: Apollo - University of Cambridge Repository
Year: 2020
Subjects: Chemical looping
Air separation
Oxygen Production
Carbon Capture
CCS
Clean Coal
Perovskites
Oxygen Carrier

description: Chemical looping air separation was the pre-dominant commercial process to produce oxygen until it was superseded by the Hampson-Linde cycle in the early $20^{th}$ century. A chemical air separation loop is characterised by the cyclic reduction and oxidation of solid material serving as an oxygen carrier, which has the capacity to release gas phase oxygen. At the time, the so-called Brin process suffered from the deactivation of the oxygen carrier material due to carbonation if the air was not pre-treated. In the past decade, however, the process has gained considerable attention due to advances in material science. The objective of this Dissertation is to suggest novel materials suitable for this air separation process. In this Dissertation, the entire process from the identification of, in theory, suitable oxygen transfer materials $via$ the synthesis and evaluation (regarding thermochemical properties) of these materials through to the investigation of the kinetic parameters and stability over many redox cycles is carried out. Although oxygen is an important industrial gas used for many applications, here, chemical looping air separation is investigated as a method of avoiding the emission of CO$_2$ from a power plant $via$ oxy-fuel combustion and subsequent CO$_2$ capture. First, a fully heat integrated model of a power station, linked to a model of chemical looping air separation system is developed to back calculate the thermochemical properties, $i.e.$ enthalpy ($\Delta H_{r,298K}^{o}$) entropy ($\Delta S_{r,298K}^{o}$) reaction, of the oxygen carrier required for efficient operation. It is demonstrated that an oxy-fuel steam power plant fed with oxygen from chemical looping air separation can be operated with an energy penalty of as low as 1.5 percentage points. However, this is only possible for materials possessing a narrow range of $\Delta H_{r,298K}^{o}$ and $\Delta S_{r,298K}^{o}$. From a list of oxygen carriers possessing favourable thermodynamic properties, the perovskite SrFeO$_{3-\delta}$ is selected for further investigation. In a tubular micro-reactor, developed and optimised for minimum noise and response time, oxygen transfer capacities and reaction kinetics of the pure, Mn-doped and Co-doped SrFeO$_{3-\delta}$ are investigated. The rate constants of the reduction reaction have been successfully extracted, however, this was not possible for the oxidation which proceeded too rapidly. The apparent activation energy, $E_{a,app}$, the pre-exponential factor, $A'$, for SrFeO$_{3-\delta}$, SrMn$_{0.1}$Fe$_{0.9}$O$_{3-\delta}$, SrCo$_{0.4}$Fe$_{0.6}$O$_{3-\delta}$ are 128.9, 45.3 and 271.3 kJ/mol, and 3.9$\cdot$10$^{5}$, 6.29 and 35.5$\cdot$10$^{12}$ mol/s/Pa/m$^3$, respectively. SrMn$_{0.1}$Fe$_{0.9}$O$_{3-\delta}$ showed both the highest oxygen transfer capacity and rate of reduction and SrCo$_{0.4}$Fe$_{0.6}$O$_{3-\delta}$ the lowest; around 325 and 117 $\mu$mol of O$_2$ per gram of oxygen carrier material at 898 K, respectively. When subjecting SrFeO$_{3-\delta}$ and SrMn$_{0.1}$Fe$_{0.9}$O$_{3-\delta}$ to 1000 redox cycles, both showed an increase in the maximum observed rate of reduction and the oxygen transfer capacity.