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A stochastic model is presented of two coupled fluidised-bed reactors with a steady circulation of particles between them. The particles undergo reaction in each fluidised bed. The model uniquely accounts for the full conversion history of particles as they are circulated. Chemical looping combustion (CLC) is an example of such a process. We have previously used the model, in a general form, to understand the sensitivity of a CLC process to factors such as the nature of the gas-solid reactions or the residence time distribution of the particles in the reactors. To demonstrate that the stochastic model is also valuable for simulating and optimising specific configurations of CLC, it is applied in this paper to simulate CLC with methane as the fuel gas, conducted in a laboratory-scale circulating fluidised bed. Under the operating conditions of the circulating fluidised bed, it was found that the oxidation and reduction reactions were limited by the intrinsic chemical kinetics of the oxygen carrier particles. It was possible to conduct experiments in a packed bed reactor for reduction and a thermogravimetric analyser for oxidation where the reaction was also limited by the intrinsic chemical kinetics. This enabled a single particle model, for inclusion in the stochastic model, to be developed independently of the experiments in the circulating fluidised bed. The resulting stochastic model was able to simulate the performance of the circulating fluidised bed with reasonable accuracy. Unlabelled Image • Stochastic approach to model reactor-regenerator systems developed. • Model uniquely accounts for the full conversion history of particles. • Applied to simulate a specific configuration of chemical looping combustion. • Able to simulate process with reasonable accuracy. [ABSTRACT FROM AUTHOR]

Carbon capture and sequestration of CO 2 from the combustion of fossil fuels in thermal power plants is expected to be important in the mitigation of climate change. Deployment however falls far short of what is required. A key barrier is the perception by developers and investors that these technologies are too inefficient, expensive and risky. To address these issues, we have developed a novel retrosynthetic approach to evaluate technologies and their design based on the demands of the system in which they would operate. We have applied it to chemical looping combustion (CLC), a promising technology, which enables carbon dioxide emissions to be inherently captured from the combustion of fossil fuels. Our approach has provided unique insight into the potential role and value of different CLC variants in future electricity systems and the likely impact of their integration on the optimal capacity mix, the operational and system cost, and dispatch patterns. The three variants investigated could all provide significant value by reducing the total investment and operational cost of a future electricity system. The minimisation of capital cost appears to be key for the attractiveness of CLC, rather than other factors such as higher efficiency or lower oxygen carrier costs. [ABSTRACT FROM AUTHOR]

A model for chemical looping combustion has been developed to allow the effect of different residence time distributions of oxygen carrier particles in the air and fuel reactors to be investigated. The model envisages two, coupled fluidised bed reactors with steady circulation of particles between them. The results show that the process is sensitive to the residence time distributions, particularly when the mean residence time of particles in the reactors is similar to the time required for them to react completely. Under certain operating conditions, decreasing the variance of the residence time distribution, leads to a greater mean conversion of the particles by the time they leave the reactors and higher mean rates of reaction in the beds. In this way the required inventory and circulation rate of solids could be reduced, which would lower the capital and operating costs of a CLC process. Since the residence time distribution of solids is important, it should be taken into account when modelling or designing a chemical looping combustion process, e.g. by using a tanks-in-series model. This work indicates that if the number of tanks, N   ≤  5, knowing N to the nearest integer is generally sufficient, unless a high degree of accuracy is needed. As N increases, the sensitivity of the coupled system decreases, so for N   > 5, knowing the value to the nearest 5 or 10 tanks is sufficient. This is valid whether N is the same or different in the two reactors. Chemical looping combustion is one example of a reactor-regenerator system, so the results are also relevant for other processes of this type, such as fluidised catalytic cracking. [ABSTRACT FROM AUTHOR]