Introduction     Most of the world energy derives from combustion of fossil fuel. One of the major environmental concerns is that fossil fuel combustion releases CO2, which is believed to be linked to global warming and climate change. Given that the usage of fossil fuel is not expected to decrease in the future, States are seeking for a major change to diversified energy supply and, possibly, to a more sustainable usage of fossil fuel with less environmental impact. We present a model for studying chemical looping combustion (CLC), the technology that is potentially best suited for capturing CO2 at low cost and efficiently providing a low energy option for the separation of CO2 from flue gases [1,2].The process consists in cyclic reduction and oxidation of a metal oxygen carrier (in the form of particulates) which is exchanged between two reactors, usually a circulating fluidised bed (the oxidation reactor) and a bubbling bed reactor (the combustor) �see Figure 1.  Figure 1  �The CCL process The oxygen carrier transfers oxygen from the air to the fuel, hence a direct contact between air and fuel is avoided. Consequently, the outlet gas from the combustor contains only CO2 and H2O: the latter is easily removed by condensation and the CO2 is readily captured for storage and/or utilisation. The main attraction of the process is that it does not involve any penalty for extra highly intensive separation of CO2 from the other combustion products: indeed separation always involves high energy consumption with production of additional CO2. Modelling      A number of challenges exists for the optimal operation of the CLC set-up and those involve the characterisation of the oxidation reactor, the optimal circulation rates, the thermal output, the reactors' residence time. The present paper presents the process integration analysis of the CLC set-up and the implementation of a novel method to simulate fluidised beds using Aspen Plus software. The fuel reactor is modelled as PFR and CSTR reactors in series: the PFR reactor is employed to describe the bubble phase whereas the CSTR describes the emulsion phase. The air reactor is modelled via four CSTRs in series which take into account the variation of the void inside the reactor (see Figures 2 and 3). Figure 2  �Aspen Plus fast fluidization bed scheme Figure 3  �Aspen Plus bubbling bed scheme Two different models are considered: the uniform conversion model (UCM) and the shrinking core model (SCM). In addition, various controlling steps of reaction are investigated and comparisons between the fluidised bed models proposed here and the Gibbs reactor model (usually implemented in Aspen Plus to simulate fluid beds) are carried out.Results and Discussion     Considering the comparison between the model proposed here and the largely used Gibbs reactor model, the Gibbs reactor assumes that the free energy of the reaction is minimum. Thus, the hydrodynamics is not considered and no information is available on the size of the reactors. The comparison between the two models shows that the results are close when the inlet superficial  gas velocity is close to Umf (equal to 0.01 m/sec): in this case, the volumetric fraction in the bubble phase si small and thus its influence on the overall gas conversion can be neglected. Most of the gas is consumed in the CSTR reactors: the assumption that the reaction kinetics is fast, allows to reach a nearly complete methane conversion. When Uo increases, the volumetric fraction of the bubble phase increases and thus the gas bypass; consequently, the difference between the present model and the Gibbs reactor becomes more pronounced. In this case, a Gibbs reactor gives higher conversions than the multistage system implemented here.A check of the net heat duty of the whole process is carried out. It is expected that the net heat duty of the process matches the net heat duty produced by a direct combustion of methane with air [3, 4]. The direct combustion of methane is implemented in Aspen Plus using a stoichiometric reactor at the same temperature of the two fluidised beds (750�C) with 92.5% of methane molar conversion as obtained through the fuel reactor. The simulation results show that the net heat duty of the process is -693.8 kWatts that is equal to a net heat duty produced by methane direct combustion.Subsequently, a computational fluid dynamics (CFD) code is used (Mfix) to simulate the precise hydrodynamics of the two reactors and the results are integrated with the model implemented in Aspen Plus. The CFD makes it possible to assess the kinetic models utilised for the oxidation and combustion reactions and more precise evaluation of the controlling resistances and the rate equations of the main reactions is obtained.  Additionally, the relative importance of the grain size, inter-particle forces and hydrodynamic forces can be studied via CFD �see Figure 4. The results are in good agreement with experimental data from the relevant literature.  Figure 4  �CFD results showing the relative importance of inter-particle forces and hydrodynamic forcesConclusions The model presented allows for including hydrodynamic and kinetic mechanisms into Aspen Plus, making it possible a more precise representation of the CLC system. Its complexity, however, is lower than CFD models implementing more rigorous description of the system at the particle level (e.g. two-fluid models). The model gives a good estimation of the main variables involved, namely diameter and height of the two beds, solid inventory, molar stoichiometric ratio between air and fuel. A comparison with a fluidised bed modelled using the Gibbs reactor is carried out shows differences in terms of gas/solid conversion since the gas by-pass through the bubble phase is neglected in a Gibbs reactor model; consequently, the model proposed here is more accurate (realistic) than the Gibbs one.The model makes it possible to estimate the thermal efficiency of the process and to undertake an economic analysis.References[1] E. A. Bryant, Climate Process and Change, Cambridge University Press, Cambridge UK (1997)[2] National Council for Science and the Environmental, Energy for a Sustainable and Secure Future: A Report of the Sixth National Conference on Science, Policy and the  Environment, edited by D. E. Blockstein and M. A. Shockley, Washington DC (2006)[3] Liang-Shih Fan, C. John Easton, Chemical Looping Systems for Fossil Energy Conversions, WILEY (2010)[4] T. Hatanaka, S. Matsuda, H. Hatano, A new-concept gas�solid combustion system �MERIT� for high combustion efficiency and low emissions, Proceedings of Intersociety energy conversion engineering conference, 30 (1997) 944�948

Chemical looping technology for clean energy production: Integrating Aspen Plus and CFD

247th National Meeting (2014)

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