Prediction of Reactive Multiphase Flows in Chemical Looping Combustion
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The fundamental motivation behind this work is the large mismatch between the current trajectory of the global economy and the recommendations of climate science. Given the economic pressures created by the seemingly permanent quadrupling of the oil price and the large burdens of debt and unfunded liabilities carried by most developed nations, this mismatch is likely to persist for many years into the future. The result is a paradoxical situation where the need for rapid development and deployment of low-carbon energy technologies is greater than ever, but the amount of capital available for such developments remains far from adequate. In order to most effectively address this unique problem, this thesis is focussed on accelerating the development of one very promising low-carbon technology: Chemical Looping Combustion (CLC). It is postulated that reactive multiphase modelling can contribute greatly towards advancing CLC technology to the stage of commercial readiness within the very strict funding limitations faced by CO2 Capture and Storage (CCS) technologies. The primary objective is therefore to advance the current state of the art in reactive multiphase flow modelling to the point where it can make significant positive contributions to the CLC development process. As a first step, the limits of current state of the art models were determined to find that even the mature Two Fluid Model (TFM) can already provide industrially interesting simulation results in certain cases featuring dense fluidized beds, larger particle sizes and slower reaction rates. Substantial effort was invested in order to form a more fundamental understanding of the grid independence behaviour of TFM simulations, revealing the particle relaxation time as a surprisingly reliable predictor of the cell size required for grid independence. However, as the particle size was decreased, the fluidization velocity was increased, the bed width was decreased and the reaction rate was increased, the simulation problem rapidly increased in complexity, both due to the fine meshes required and uncertainties related to various closure model coefficients. For the wide range of cases that will remain out of the reach of the TFM approach for the foreseeable future, four alternative modelling methodologies were investigated: 2D simulations, a Lagrangian parcel-based approach, a filtered Eulerian approach and a phenomenological 1D approach. The bulk of work was dedicated to the relatively new Lagrangian parcel-based approach which was thoroughly tested and improved by adding the transport of granular temperature and the influence of the full stress tensor on particle motion. Practical experience was gained with the other approaches as well, ultimately allowing for the tabulation of the pros and cons of these approaches and the formulation of clear recommendations for future work. Experience suggests that no single approach will be generically applicable over all fluidization cases within the foreseeable future. Modellers should therefore respect the different strengths and weaknesses of each approach in order to select the most efficient modelling approach for any given application. A large amount of effort was also dedicated to model validation, both against published experiments and experiments carried out within the project. Although a number of unexplained discrepancies remain, comparisons to experiments focussing on hydrodynamics, species transfer and heterogeneous reactions were generally encouraging. Furthermore, experience gained with the operation of the novel reactive unit constructed in this project will be very valuable in directing future reactive validation studies. It was also found that building dedicated experiments was a significantly better investment than the inefficient practice of trying to validate against published experimental data that was not collected for the primary purpose of model validation. Finally, practical experience was gained with two possible applications of reactive multiphase flow modelling to accelerate the development of CLC: virtual prototyping of new process concepts and process optimization. In both cases, the fundamental advantages of such a simulation-based process design strategy were found to be highly attractive. Virtual prototyping granted complete creative freedom when it comes to the design of new reactor concepts and statistical optimization methods that previously were practically impossible became highly practical. In summary, it was found that the simulation-based process design of fluidized bed reactors such as those employed in the CLC process is already feasible over a range of flow conditions. A number of alternative modelling approaches which have been further developed and tested in this project will gradually extend the range of model applicability over coming years. These results together with encouraging comparisons to dedicated validation experiments clearly indicate that reactive multiphase flow modelling can now start the transition from development to application. It is therefore recommended that industry is gradually engaged through intelligently selected applications where current state of the art models can reliably predict the performance of industrially relevant fluidized bed reactors. Such a conscious shift to model application is vital to accelerate the development of CLC technology so that a commercially viable process can be made available for deployment as soon as the policy environment finally becomes favourable for CCS. The thesis is presented three parts: 1. Setting the stage: This introductory section gives background information on the necessity for second generation CO2 capture technology such as CLC and also the role that reactive multiphase flow modelling can play in accelerating the development process. 2. Review of technical work: This is the main section of the thesis and will bind together the conclusions drawn from all the papers completed in this project in a coherent manner. 3. Collection of papers: Finally, all the technical papers referenced in the review section (published and unpublished) will be included as an appendix for ease of reference.