(421b) Analysis of Chemical Looping Combustion for Power Plant Applications

Chemical Looping Combustion (CLC) is a technology able to convert energy whilst managing CO₂ emissions. CLC involves combustion of carbonaceous fuel such as coal-derived syngas or natural gas via the heterogeneous chemical reaction with a solid oxygen carrier exchanged between two fluidised beds. The two fluidised beds usually operate in different hydrodynamic regimes: the so called “air reactor” works in fast fluidisation regime whereas the so called “fuel reactor” works in bubbling bed regime. One of the issues for the feasibility of the process concerns with the solid inventory within the beds that must be minimised. In this work, a 10 MW CLC power plant, which uses methane as fuel gas and NiO supported by Al₂O₃ as oxygen carrier, was implemented in a software widely used in industry, Aspen Plus, to evaluate the minimum solid inventory required to achieve full gas conversion and competitive thermal and CO₂ capture efficiency. The modelling of the two fluidised beds takes into account both kinetic and hydrodynamic phenomena. Initially, actual mass transfer phenomena involved in the hydrodynamics of the fluidised beds are neglected; indeed, the CLC system is modelled as interconnected CSTR reactors. This simple approach allows for investigating how variables, such as the circulating solid flow-rates, the difference in solid conversion between the two reactors, the air/fuel molar ratio, the temperature and the pressure, affect the total solid loading within the beds. This methodology proved useful to minimise and thus optimise the amount of solid loading needed to achieve nearly full gas conversion which is believed to be counted as the major operational cost of the plant. However, being aware that the idealised CSTR reactor model does not consider the hydrodynamics, we included new features in modelling the beds into Aspen Plus to account for the real behaviour of the fluidised reactors. We then developed and simulated a different hydrodynamic model for each fluidised bed able to incorporate into the commercial software the mass transfer, the variation of solid void fraction and the limiting conditions affecting the conversion. The influence of the hydrodynamics in increasing the total solid loading required into the beds is analysed under different pressure conditions. The plant thermal efficiency, as well as the CO₂ capture efficiency, are evaluated for different plant configurations; the results are discussed in relation to the conditions needed to increase the thermal efficiency of the process and to reduce the utilities’ cost.