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In chemical-looping, a fuel is oxidised by a solid metal oxide, MeO, in one reactor: (2$n+m$)MeO+C$_{n}$H$_{2m}\rightarrow$(2$n+m$)Me+$m$H$_{2}$O+$n$CO$_{2}$. The exit gas yields pure CO$_{2}$ after the steam has been condensed. The reduced metal oxide, Me, is transferred to an oxidation reactor and regenerated: Me+air$\rightarrow$MeO. Adding these reactions, the fuel has been combusted, but the CO$_{2}$ has been separated from the nitrogen in air. In fact, it is in a suitable form for sequestration in the Earth where prevention of greenhouse gas emissions to the atmosphere is desired. Generally, Me is a transition metal and, to withstand many such redox cycles, it has to be supported on a suitable refractory oxide, with particles of the resulting construct being termed the "oxygen carrier". This Dissertation is concerned with the release and uptake of gaseous oxygen when Me is copper. In particular, the interest is in the following reaction at temperatures exceeding ~900°C undertaken in a fluidised bed reactor:$\\$ 4CuO$_{(s)}\Leftrightarrow$2Cu$_{2}$O$_{(s)}$+O$_{2(g)}$. (1)$\\$ The value of this reaction is that the oxygen released as part of a chemical looping scheme is important in combusting unreactive solid fuels, e.g. coal chars, whilst the Cu$_{2}$O could, in principle, be further reduced to Cu by the more reactive components of the fuel. This Dissertation investigates the development and characterisation of suitable, Cu-based oxygen carriers, which must (i) be inexpensive and easy to produce at a large scale and (ii) remain stable in prolonged operation in terms of mechanical integrity and chemical reactivity when fluidised. Here, a suitable oxygen carrier was developed, satisfying the above criteria, using a wet-mixing method and containing nominally 60 wt% CuO, 23 wt% Al$_{2}$O$_{3}$ and 17 wt% CaO. In particular, it was found that this oxygen carrier could operate between CuO and Cu$_{2}$O without problem in a circulating fluidised bed but agglomeration and de-fluidisation was observed when the carrier was re-oxidised from the Cu form. For design, it is important to understand the thermodynamics and kinetics of the release of gaseous O$_{2}$ from the oxygen carrier, because the combustion of the solid fuel depends critically on this reaction. A novel method was developed to measure experimentally the thermodynamics of reaction (1) for the supported copper oxide. It was found that the thermodynamic equilibrium deviated slightly from that of the pure CuO/Cu$_{2}$O system reported in the literature and that the enthalpy of reaction was lower by ~ 15%; the reasons for this are discussed. The rate of release of O$_{2}$ from the oxygen carrier was investigated using a thermogravimetric analyser and the activation energy for the forward reaction of (1) was found to be 59.7$\pm$5.6 kJ/mol, obtained after appropriate modelling of the external mass transfer resistances present in the experimental apparatus. A critical analysis of the seemingly disparate activation energies reported in the literature revealed that the activation energy of the forward step in reaction (1) was, in fact, similar for many CuO-based oxygen carriers supported on different materials. The associated pre-exponential factor for the forward rate constant was also determined in the present research, and the kinetic parameters were used in a numerical model to predict the behaviour of the oxygen carriers in a fluidised bed reactor. Excellent agreement between theory and experiment was found, confirming that the kinetic parameters obtained in this work reflect the intrinsic chemical kinetics of the oxygen carrier, rather than being totally dominated by transport effects.

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This Dissertation describes the development of chemical looping for selective oxidations. Chemical looping is a reactor technology that achieves simultaneous reaction and separation. For a large subset of reactions (viz. abstraction or insertion of oxygen), this technology is based upon the use of oxygen carriers. These materials, typically metal oxides, reversibly store and release oxygen, and there is growing interest in using these materials for selective oxidations. This Dissertation describes work on the development of oxygen carriers for selective oxidations, including foundational work on a method for analysing periodic non-catalytic gas-solid reactions, of which chemical looping selective oxidations are a subset. The oxygen chemical potential of Ca2Fe2O5 was exploited to improve the efficiency of the steam-iron process to produce hydrogen. The ability of reduced Ca2Fe2O5 to convert a higher fraction of steam to hydrogen than chemically unmodified Fe was demonstrated in a packed bed. This demonstrates how the oxygen chemical potential might be manipulated and exploited for chemical looping reactions. The oxygen chemical potential determines the selectivity in thermodynamically-controlled selective oxidations, and, depending on the reaction mechanism, kinetically-controlled selective oxidations. A generic method for enhancing the oxygen-carrying capacity of oxygen carriers for use in selective oxidations is presented, where one material that is selective in the reaction is deposited on the surface of a second material acting as a reservoir of oxygen and as a support. The presence of ceria in the support was found to supply lattice oxygen additional to that provided by the bismuth oxide, without affecting the selectivity of bismuth oxide. The surface chemistry was decoupled from the bulk properties of the support, thus simplifying the design and formulation of composite oxygen carriers. Building upon the concepts of oxygen chemical potential and composite oxygen carriers, chemical looping epoxidation was demonstrated for the first time. The oxygen carrier was composed of Ag, for its unique catalytic properties, and SrFeO3 as the support, for its high oxygen chemical potential at low temperatures. A reaction mechanism was proposed based on the observations. Nonlinear frequency response theory was used to analysis a periodic non-catalytic gas-solid reaction. Generalised frequency response functions (which are higher order analogues to traditional, linear transfer functions) were derived to obtain the nonlinear frequency response of the archetypal reactor. Such a method lies between the traditional frequency response theorem and numerical methods in terms of accuracy and speed. A niche application was proposed for the analysis of experimental kinetics, avoiding convolution of measurements with the response time of measuring equipment. In summary, this Dissertation describes how materials might be formulated for selective oxidations in chemical looping mode. This was demonstrated for an industrially-significant reaction for the production of ethylene. A novel application of nonlinear frequency response theory was also demonstrated for chemical looping reactions.

Chemical looping combustion (CLC) is a promising fossil fuel combustion technology, which is able to separate CO2 from the flue gases without a large consumption of energy. In this thesis, the study was extended to look at the use of chemical looping materials within traditional fluidised bed combustion and investigation of the interaction between the fuel, the supplied air and the chemical looping agent. Three topics of chemical looping combustion are discussed, including 1) the Sherwood number in the fluidised bed; 2) properties of different oxygen carriers, Fe2O3 and CuO (with supporting materials), were tested in the fluidised bed reactor; 3) the simulation of a steady state and a dynamic model of a coal-fired CLC power plant using Fe2O3 as oxygen carriers. The Sherwood number, which represents the mass transfer rate, is important in the calculation of CLC process. With Sherwood number, the mass transfer rate kg around the acting particle can be calculated using correlation Sh=kg∙d/D, where d is the diameter of acting particle, and D is the diffusivity around the acting particle. Hayhurst and Parmar (Hayhurst and Parmar 2002) calculated the Sherwood number in the fluidised bed by using the CO/CO2 ratio, which was measured by the temperature difference between the carbon particle and the bulk phase (Hayhurst and Parmar 1998). However, the temperature of the particle could be overestimated, so the CO/CO2 ratio could be underestimated. In this thesis, a universal exhaust gas oxygen (UEGO) sensor was employed, which could measure the actual carbon consumption rate in the fluidised bed by oxidizing CO in the sample gas into CO2 and. Fe particles of the same size of the char particle is used to measure the O2 consumption rate, and thus eliminate uncertainty in the Sherwood number. The CO/CO2 ratio was calculated by using the carbon consumption rate and the O2 consumption rate. In contrast to Hayhurst and Parmar (Hayhurst and Parmar 2002) who assumed CO2 was the main product, for this char the actual ratio of CO/CO2 was almost zero. The measurement here is in agreement with Arthur. This more accurate determination of CO/CO2 allows a better estimate of the mass transfer coefficient and leads to a correction of the Hayhurst and Parmar’s (Hayhurst and Parmar 2002) correlation by a factor of 1⁄2. Interestingly, very small fluidised beds have mass transfer coefficients which are about twice that expected in a large bed (owing to the very different flow and indeterminate flow pattern). This means the correlation of Hayhurst and Parmar (Hayhurst and Parmar 2002), by fortuitous coincidence works wells for beds with diameters < 30 mm., without the correction factor, should be ignored. In the fluidised bed in a typical CLC process, different fluidising material could have different influence on the reactions. Thus, it is worth discussing different kinds of fluidising materials. The char combustion in the fluidised bed was simulated by using inert (sand) and active (Fe2O3 or CuO) fluidising materials, and air as fluidising gas. The results indicated that 1) CO combustion in the boundary layer leads to smaller carbon consumption rate and larger oxygen consumption rate; 2) Using Fe2O3 particles as fluidising materials slows down the carbon consumption rate, since the diffusivity of CO2 is smaller than CO; 3) CuO particles slow down the carbon consumption rate at large Sherwood number (Sh=2 or 2.5). The influence of using CuO as fluidising material is further discussed experimentally by using low O2 fluidising gas. The results indicated that since the amount of CuO used in the experiment is small, when the O2 concentration in the bulk phase is lower than the equilibrium concentration, the O2 concentration in the bulk phase gradually decreases, and the O2 concentration in the bulk phase has large influence on the char particle combustion. A steady state model of a coal-fired CLC power plant was simulated. The aim of the model was to test the suitable operating conditions of the power plant, such as recycle rate of oxygen carriers, for the power plant design. In the steady state model, the power plant consists of a combustor and a steam cycle. Hambach lignite coal, Polish bituminous coal and natural gas were tested as fuels. The results indicated that: (1) The effect of the fuel is largely due to the amount of oxygen required per GJ released; (2) Preheating is important, but seems to have a minor effect since the most of the heat is released at temperatures well above the pinch point; (3) since the temperatures of heat source in this research is well above the pinch point, all heat are usable for the steam cycle. In this case, the steam cycle and the chemical looping plant could be optimised separately; (4) As long as the preheat temperature of the air flow into the air reactor is higher than the temperature of turbines, in most of cases the power output is unaffected by the choice of variables, leaving the designer free to choose the most convenient. With the conclusions above, a dynamic model of a coal-fired CLC power plant using Fe2O3 as oxygen carrier is then simulated. The aims of this simulation include: 1) explaining the kinetics of Fe2O3 oxygen carriers at high temperature (1223K) in a fluidised bed reactor using Brown’s data (Brown 2010); 2) a 1GWth dynamic power plant was simulated to test different cases including changing power supply and power storage. In the dynamic model, a chemical looping power plant using Hambach lignite char is tested, and the parameters of the system are adjusted so as to simulate the operations of a real chemical looping power plant. The two-phase model is employed for the fluidised bed reactors. Experimental data from Brown (Brown 2010) was simulated using this model first to test its validity. Then the model is scaled up to simulate a 1GWth dynamic power plant. The ideal operation conditions are found, and a char stripper is found helpful for carbon capture.

To prevent the global surface temperature from increasing past the 2 oC target, it is necessary to address CO2 emissions from small point sources. Within Canada’s heavy oil industry, SAGD facilities use natural gas combustion to produce the large amounts of steam required for the process, which produces approximately 0.5-2 Mtonnes of CO2 per annum. A suitable technology for CO2 mitigation from a SAGD facility is Pressurized Chemical Looping Combustion. PCLC is an oxy-combustion, carbon capture technology with a relatively low predicted energy penalty of 3-4%. The process requires a dual, interconnected fluidized bed reactor system with circulating solids. Natural gas is converted in the fuel reactor via a solid metal oxide, which is then circulated to the air reactor for reoxidation with air. As the cost of air compression is significant, the economical feasibility of the process is reliant on air reactor performance. The objective of this study is to investigate the oxidation reaction and derive a kinetic model for reactor design and performance assessment purposes. Ilmenite ore was chosen as the metal oxide, as it is low cost and has desirable oxygen transport properties for PCLC. Pressurized TGA tests were conducted to study the effects of oxygen concentration, temperature and pressure on the rate of the oxidation reaction. The total pressure was varied from 1-16 bara at 900 oC with air. The oxygen concentration was varied from 2.5-21 vol%, and the temperature from 800-1000 oC at 8 bar. Temperatures below 850 oC resulted in segregation of the Fe and Ti phase in the ilmenite ore, leading to a reduction in the overall oxygen carrying capacity. Crack formation was observed at higher oxygen partial pressures, resulting in increased surface area for reaction and a fast reaction rate. At lower oxygen partial pressures, a solid-state diffusion controlled regime was observed due to the absence of fissures. A dual mechanistic oxidation kinetic model was derived at 8 bar, with 2nd order random nucleation dominating at lower conversions, and Jander’s solid state diffusion model dominating at higher conversions. The transition from the nucleation and growth to the diffusion-controlled portion occurred at higher conversions with higher oxygen partial pressure. The activation energy was 16.6 kJ/mol and 48.7 kJ/mol while the order of reaction with respect to oxygen was 0.3 and 1.3 for respectively the nucleation and growth, and diffusion-controlled regimes. A preliminary air reactor model is constructed as a turbulent bed. The turbulent bed is modelled as an axial dispersion reactor for a basic performance assessment.

Detailed sensitivity numerical studies have shown that the mesh cell-size may have a drastic effect on the modelling of circulating fluidized bed with small particles. Typically, the cell-size must be of the order of few particle diameters to predict accurately the dynamical behaviour of a fluidized bed. Hence, the Euler-Euler numerical simulations of industrial processes are generally performed with grids too coarse to allow the prediction of the local segregation effects. Appropriate modelling, which takes into account the influence of unresolved structures, have been already proposed for monodisperse simulations. In this work, the influence of unresolved structures on a binary mixture of particles is investigated and models are proposed to account for those effect on bidisperse simulations of bidisperse gas-solid fluidized bed. To achieve this goal, Euler-Euler reference simulations are performed with grid refinement up to reach a mesh independent solution. Such kind of numerical simulation is very expensive and is restricted to very simple configurations. In this work, the configuration consists of a 3D periodical circulating fluidized bed, that could represent the established zone of an industrial circulating fluidized bed. In parallel, a filtered approach is developed where the unknown terms, called sub-grid contributions, appear. They correspond to the difference between filtered terms, which are calculated with the reference results then filtered, and resolved contributions, calculated with the filtered fields. Then spatial filters can be applied to reference simulation results to measure each sub-grid contribution appearing in the theoretical filtered approach. A budget analysis is carried out to understand and model the sub-grid term. The analysis of the filtered momentum equation shows that the resolved fluid-particle drag and inter-particle collision are overestimating the momentum transfer effects. The analysis of the budget of the filtered random kinetic energy shows that the resolved production by the mean shear and by the mean particle relative motion are underestimating the filtered ones. Functional models are proposed for the subgrid contributions of the drag and the inter-particle collision.

Production of hydrogen from biomass steam gasification can be enhanced by using calcium oxide sorbents for CO2 capture in the gasifier. Calcium looping suffers from two main drawbacks: the need for high-purity oxygen in order to regenerate the sorbent under oxy-fuel combustion conditions and the loss of sorbent reactivity over several cycles due to sintering of pores upon calcination at high temperatures. One method of addressing the issue of oxygen supply for calcination in calcium looping is to combine the calcium looping and chemical looping processes, where the heat produced by the reduction of an oxygen-carrier by a fuel such as natural gas or gasification syngas, drives the calcination reaction. The technologies can be integrated by combining an oxygen carrier such as CuO with limestone within a composite pellet, or by cycling CuO and limestone within distinct particles. The goal of this project is thus to investigate the different sequences of solids circulation and the cyclic performance of composite limestone-CuO sorbents under varied operating conditions for this novel process configuration. Using a thermogravimetric analyzer (TGA), it was found that using composite CaO/CuO/alumina-containing cement pellets for gasification purposes required oxidation of Cu to be preceded by carbonation (Sequence 2) as opposed to the post-combustion case where the pellets are oxidized prior to carbonation (Sequence 1). Composite pellets were tested using Sequence 2 using varying carbonation conditions over multiple cycles. While the pellets exhibited relatively high carbonation conversion, the oxidation conversion underwent a decrease for all tested conditions, with the reduction in oxygen uptake particularly drastic when the pellets were pre-carbonated in the presence of steam. It appears that the production of a layer of CaCO3 fills up the pellets pores, obstructing the passage of O2 molecules to the more remote Cu sites. Limestone-based pellets and Cu-based pellets were subsequently tested in separate CaL and CLC loops respectively to assess their performance in a dual-loop process (Sequence 3). A maximum Cu content of 50% could be accommodated in a pellet with calcium aluminate cement as support with no loss in oxidation conversion and no observable agglomeration.

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