Acoustic agglomeration as a pretreatment to enhance particle separation in syngas cleaning

MSc student: Joppe Buntsma

Final colloquium: 25 June 2012

This research investigates the necessary conditions for the application of ultrasonics as a pre-treatment, to enhance particle separation in biomass gasification hot gas clean-up for the 100 kWth CFB gasifier at Delft University of Technology. The current particle separation techniques suitable for hot gases (>800°C) are expensive and lack efficiency for the very fines (<1µm). A high power ultrasonic acoustic field (+150dB - >20kHz) could entrain the individual particles to induce a convergent motion, forcing the particles to collision and larger agglomerates which are assumed to be more easily separated.

This thesis focusses on the basic viscous forces and its induced hydrodynamic interaction with other particles, which causes collision. A model has been built and validated with experimental and computational results obtained from literature. The collision times for two particle pairs of interest have been simulated for varying parameters to draw a conclusion about the possibility of application in case of hot syngas clean-ups. Although ultrasonic acoustic agglomeration is expected to suffer from a lower performance, the modeling results still predict reasonable agglomeration rates as long as the gas velocity amplitude and the residence time are sufficient.

To get acquainted with the agglomeration process and high-power ultrasonics, also a practical experiment has been performed. In a 28 kHz – 150dB acoustic field, tobacco smoke aerosol (0.01-1µm) has been forced to agglomerate. Although the results are not indicating a shift in the PSD directly, an extended study shows several strong clues for the particle agglomeration taking place. Especially for extended treatment times, tobacco smoke has shown to be really affected by the ultrasonics.