Multi-scale modelling, synthesis and characterization of materials for chemical looping reforming

Abstract

Chemical looping is an emerging technology for producing electricity, fuel or chemicals with low CO2 emissions. In chemical looping combustion (CLC) one often uses dual fluidized beds to circulate oxygen-carrier particles, which are used to provide oxygen for combustion to the fuel reactor. The reduced particles are cycled back to the air reactor where they are oxidized and circulated back to the fuel reactor. Apart from low CO2 emissions, other benefits are higher combustion efficiency and lower NOx emissions than in standard power plants. Chemical looping reforming (CLR) is a class of related techniques where less air is consumed and focus is on the catalytic production of CO and H2. Our work has focused on synthesis and testing of improved oxygen-carrier particles for use in CLR. In order to support the experiments, especially for the purpose of process intensification, an integrated multiscale modeling platform has been developed to provide virtual testing of the materials in a CLR process, from the nano-scale to process scale. A number of criteria need to be met by the synthesised particles, e.g., fast redox chemistry, chemical selectivity, chemical and physical stability, and being non-hazardous to health and environment. In order to eliminate health, safety and environmental issues connected to nickel-based materials, oxides based on, e.g., Fe or Mn are considered as the oxygen carrier material for CLR. Different approaches were applied to synthesize active oxide particles of different size (nm to m) supported on/embedded in a more mechanically strong structure. The oxygen capacity was evaluated by thermogravimetric analysis and lab-scale fixed bed reactor tests were performed in order to evaluate the potential of the prepared materials for use as oxygen carriers in CLR conditions, for the production of CO and H2 from CH4. Density Functional Theory (DFT) calculations and kinetic Monte Carlo (kMC) were used to calculate reaction rate constants, especially for evaluating the selectivity of the different oxides towards CO and H2 formation over the competing production of CO2 and H2O as a function of temperature. Particle scale models were then applied to simulate heat and mass transfer in mixtures of gas and reactive particles.To approach actual conditions on a laboratory and industrial scale, we used computational fluid dynamics (CFD) simulations (in 2D and 3D) of reactive flows and phenomenological 1D models of fluidized bed reactors.