Adaptive pore network modelling of thermochemical processes in single porous particles
A single particle model with high accuracy is central to DEM/CFD simulations of a bed packed with a population of thermally-thick solid particles and exposed to a thermal process (such as drying) or a thermochemical process (such as calcination, pyrolysis, or combustion). A model as such must essentially account for heat and mass transfer within a single porous particle, morphological changes of its pore structure, chemical reactions and the connection to the particle’s fluid-solid surroundings. Project B4 aims at performing a major breakthrough in the modelling and simulation of these porescale phenomena at the level of a single particle and under realistic process conditions. In the first funding period (FP1), B4 will concentrate on microscopic discrete and macroscopic continuum modelling as well as on experimental characterisation of the drying and calcination processes. However, extensions for more complex thermochemical conversion processes are foreseen (these are tightly related, but not limited, to CRC/TRR applications): In FP2 pyrolysis of single particles with variable size and complex shape, and in FP3 combustion of single particles will be investigated.
A single porous particle is represented by an idealized network of cylindrical throats interconnected to spherical pores. The drying kinetics and the size evolution of the particle’s pores and throats during the drying process are visualized.
Discrete models will be developed based on first principles. Since the pore size will change over time due to thermal stress (shrinkage during drying) or chemical reactions (consumption of solid phase), the pore structure must be traced over time and updated accordingly. Full consideration of structural changes is one of the major advances that will be made with the help of adaptive discrete pore network models – a new family of discrete models. Model extensions shall be made to account for internal temperature gradients and unstructured networks with physically realistic pore structures. The interior pore structure and volumetric change of a particle will be characterised by techniques such as µ-CT imaging. Pore-scale phenomena are directly accessible by discrete models. This fact will be used to revisit the classical continuum models, taking inputs from representative discrete pore network simulations and feeding effective parameters to a macro-scale continuum model. To endow the continuum model with predictive capabilities, high-quality and trustworthy gravimetric measurements will be conducted for single particles in thermo-balance reactors under controlled conditions. On this basis, the classical continuum models will be upgraded and thus implemented in the DEM/CFD libraries (project C7) after their model-order reduction (project C4).
The pore network modeling and simulation of the calcination process of a single limestone particle. The movie provides detail insights into the transformation of the limestone into an active lime during the conversion process.