Building on the simulation models presented in previous posts (Multiphase Flow Model, and FlashReactor and Refiner Simulation), which described sewage sludge combustion in the FlashReactor, the modelling efforts at TU Graz have recently shifted toward understanding the behaviour of molten slag inside the furnace.
When sewage sludge is injected into the hot reaction chamber, most of the organic material combusts, leaving behind an inorganic residue known as slag. These droplets fly around inside the reactor and eventually deposit on the reactor walls. From there, a thin film of slag is expected to flow downward toward the phase separator vessel, where it is collected and extracted for further processing – because this slag is what still contains the phosphorus!
The goal of the current simulation models is to better understand how the slag film at the walls behaves – specifically its thickness, flow speed, and how these variables vary across the reactor walls. This influences the dynamics of the system, e.g., the time it takes until the conditions at the bottom reactor react to changes to the input. It also affects the wear on the furnace’s internal refractory lining. The figure below shows simulation results for slag film thickness along the walls of the main reaction chamber. Since slag viscosity depends heavily on the composition of the input material, three different viscosities were tested, increasing from left to right in the figure. For context, a viscosity of 0.1 Pa·s is similar to light motor oil, while 1.0 Pa·s is about 1,000 times thicker than water, behaving more like honey.
Figure: CFD simulation results showing the slag film thickness (top) and the surface velocity (bottom) in the phase separator. The slag viscosity is increasing from left to right.
The simulation results show six distinct streams forming along the reactor walls. These streams flow downward, gradually merging before dripping into the phase separator. When the slag is more viscous, the film thickness increases – reaching up to 1 mm as shown in the upper colour scale – but the surface velocity of the slag in the lower part of the phase separator, as shown in the lower scale bar, decreases significantly.
In our scientific publication “CFD modeling of sewage sludge combustion in an experimental entrained flow reactor with integrated slag flow dynamics”, you can find more information about how this model works, as well as its connections to our previous results.
One thing remains clear: Simulation models like these allow researchers and engineers to gain insight into processes that are otherwise challenging or even impossible to examine in reality. They are therefore valuable tools in research and development.
For further information contact
Benjamin Ortner
Institute of Thermal Engineering, Graz University of Technology
Christoph Hochenauer
Institute of Thermal Engineering, Graz University of Technology