(476d) Three Dimensional CFD-Based Numerical Study of Chemically Reacting Carbon Particles Within a Randomly Packed Bed

This work is devoted to three-dimensional numerical study of the influence of gas flow (consisting of O2, H2O and N2) and its composition on the carbon consumption evolution within a randomly-packed 3D packing of spherical non-porous carbon particles. The monodisperse packing was numerically generated using gravity-forced sedimentation which was modeled utilizing a in-house developed Discrete Element Method code.

The primary interest of this work is the spatial distribution of the temperature and species concentrations in the so-called combustion zone of a fixed bed, where the oxygen reacts with carbon particles. It should be noted that due to the inaccessibility to the interior of a fixed bed reactor experimental studies are not always capable of characterizing basic features of flow and phenomena occurring inside the reactor. From this point of view numerical simulations can play the role of 'numerical-experiments' enabling to see the processes 'in situ'.

In this work the reacting particles are represented by moisture and ash free nonporous carbon. The model includes six gaseous chemical species (O2, CO2, CO, H2O, H2, N2). Three heterogeneous reactions (C+O2, C+CO2 and C+H2O) and two homogeneous semi-global reactions, namely carbon monoxide oxidation and water-gas shift reaction, are employed. Several semi-global reaction rate expressions taken from the literature were utilized. The Navier-Stokes equations coupled with the energy and species conservation equations are used to solve the problem in pseudo-steady state approach. At the surface of the particle, the balance of mass, energy and species concentration is applied including the effect of the Stefan flow. The range of parameters considered in this study are d = 2*10^-2 m, Tin=1000 K, Re =100, where d, Tin and Re are the diameter of the particle, the inlet temperature of the gas and the Reynolds number, respectively. The inflow gas composition corresponds to dry air gas. The unstructured grid used in simulations comprises of approximately 6 millions of control volumes.

The results of numerical simulations were compared with those for a simple cubic packing of chemically reacting carbon particles. The comparative simulations showed that the penetration length of the flame decreases from four particle diameters for regular packing to one particle diameter in the case of random packing. Additionally, we found out, that the temperature and species gradients between particle surface and gas phase predicted for the random packing decreases, too. Spatial distribution of species, temperature and gas velocity are illustrated and discussed.