No abstract

Flame spray pyrolysis (FSP) is a versatile process for the production of inorganic nanoparticles featuring the advantage that the reagents are directly dissolved in the liquid fuel that is atomized to form the burning flame. A majority of previous studies on flame spray pyrolysis is focused on the formation and growth processes of the nanoparticles but neglect the preceding step of precursor atomization and spray formation. In this work an atomization concept for large‐scale nanoparticle production by flame spray pyrolysis is presented. A pressure swirl nozzle is applied for creating a liquid hollow cone, and in a second step, different dispersion gas nozzles are utilized to enhance the atomization of the liquid phase and to influence the spray cone formation and geometry. The relevant parameters influencing the atomization process (dispersion gas feed rate, liquid feed rate) are investigated (for air, water) in non‐burning (cold) spray conditions in order to access the utilization of the different atomizer concepts for the flame spray pyrolysis‐process. Measurements are conducted by applying high speed camera imaging (HSC), particle image velocimetry (PIV) and laser diffraction spectroscopy (LDS). Computational fluid dynamics (CFD) revealed further insight into the gas entrainment and the trajectory of droplets within the spray. Results show that the liquid volume flow rate (and thus the productivity of the process) may be increased significantly while still maintaining an appropriate droplet size compared to the conventional atomization process conditions in flame spray pyrolysis reactors.

Nanoparticle production in flames was modeled in an Eulerian-Lagrangean framework, considering droplet evaporation and fuel combustion to predict the flame chemical species concentration and the flame temperature fields by means of Computational Fluid Dynamics (CFD). A mathematical model was carried out considering two-way coupling between the gas phase and the droplets. For the combustion model, the eddy dissipation concept model was applied, taking into account the droplets vaporization, the chemical reaction mechanisms, and the chemistry-turbulence interaction. 2D axisymmetric and 3D approaches were investigated in standard operations conditions. The initial conditions for the droplet sizes and droplet velocities were taken in experiment test facility by means of Laser-Diffraction. The grid independence study was made according to the Grid Convergence Index (GCI) methodology for both approaches. The droplets mass evaporated, temperature and velocities profiles were used to compare the 2D and 3D results. The results show similar behavior for both approaches, however, with some quantitative difference. The 2D approach showed lower temperature resulted by a larger mass fuel not evaporated and unburned.

The hydrodynamics of circulating fluidized beds (CFBs) is a complex phenomenon that can drastically vary depending on operational setup and geometrical configuration. A research of the literature shows that studies for the prediction of key variables in CFB systems operating at high temperature still need to be implemented aiming at applications in energy conversion, such as combustion, gasification, or fast pyrolysis of solid fuels. In this work the computational fluid dynamics (CFD) technique was used for modeling and simulation of the hydrodynamics of a preheating gas-solid flow in a cylindrical bed section. For the CFD simulations, the two-fluid approach was used to represent the gas-solid flow with the k-epsilon turbulence model being applied for the gas phase and the kinetic theory of granular flow (KTGF) for the properties of the dispersed phase. The information obtained from a semiempirical model was used to implement the initial condition of the simulation. The CFD results were in accordance with experimental data obtained from a bench-scale CFB system and from predictions of the semiempirical model. The initial condition applied in this work was shown to be a viable alternative to a more common constant solid mass flux boundary condition.