Characteristic Properties and Applications of Fine Particles in Biomass Gasification

Abstract: The gasification of biomass is a promising route to increase the share of renewable sources in the energy mix. Besides having an overall higher thermal efficiency than combustion, it also offers the possibility of producing gaseous and liquid biofuels that can be used in the transport sector. The use of biomass gasification for energy purposes can help lower the net emissions of greenhouse gases, and hence help counter the global warming. One of the problems impeding the exploitation of this technology is the lack of efficient high-temperature cleaning systems to limit the release of fine particle contaminants after gasification. These contaminants can penetrate through the filters presently in use, and be deposited on the surfaces of integrated thermal plants leading to corrosion and on catalysts in downstream upgrading processes. Condensable material may also pass through the high-temperature filters in the gas phase, and form significant amounts of particulate matter if the temperature is decreased for operational reasons. The overall aim of this work was to develop methodologies to aid the further development of post-gasification high-temperature cleaning systems. It included the high-temperature dilution particle sampling techniques, detection of agglomerated soot particles using a novel sensor concept and the investigation of catalysts deactivation due to particulates present in the producer gas. To accomplish this, a laboratory-based method of generating well-characterized model aerosol particles was developed. These particles were compact KCl particles generated by a nebulizer in order to represent the alkali particles, and soot generated by a flame soot generator to represent agglomerated particles in the gasifier. Di-octyl-sebacate (DOS) was used to model tar forming compounds present in gasifier producer gas. The characteristic properties of the particles, such as size, concentration, morphology and mass fraction of organic coating, were analyzed using sophisticated on-line aerosol characterization techniques, including a Scanning Mobility Particle Sizer (SMPS), and a Differential Mobility Analyzer – heater – Aerosol Particle Mass Analyzer (DMA-heater-APM). This provided useful information regarding the morphology and density of agglomerated particulate with a condensed phase, which is not possible with the SMPS technique. The soot and KCl particles mixed with DOS were sampled with a probe-denuder setup at 200 °C, and the effects of the concentration of condensable material, the dilution ratio in the probe, and the flow rate through the denuder were investigated. This setup demonstrated the capacity to collect >99% of organics when the denuder inlet concentration of DOS was below 6 mg/m3. The soot sampling was also performed by replacing the denuder with a packed bed, which exhibited an enhanced collection capacity for condensable material for inlet concentrations of up to 15 mg/m3. The sampling system developed was used to sample particles from a circulating fluidized bed gasifier downstream of an existing filter to assess the filtration performance and to allow characterization of the particulates released from the gasifier. This demonstrated the usefulness of the setup by revealing the presence of coarse particles of calcium and silica from bed material, and fine particles dominated by K and Cl released from biomass feedstock. In catalytic activity measurements on Ni and Pt/Rh catalysts, it was found that potassium and soot particles could reduce the activity by up to 50% when the catalyst was exposed to very small amounts of model particles (soot 0.5 wt %; potassium 0.0038 wt %). The physical blocking of the active sites, and hence reduced active metal surface area is thought to be the main cause of activity loss. The model soot particles were also used to develop an online detection method for soot particles detection at the high temperatures. The performance of the soot sensor was satisfactory in the soot concentration limits tested, with the possibility of enhancing the sensitivity by improving the design. Such soot sensors could be installed after downstream cleaning devices in thermochemical conversion processes. The research presented in this thesis contributes to the development of effective cleaning systems in the biomass gasification process by improving our understanding of particle formation, deposition and catalyst deactivation mechanisms. This will help us move towards renewable energy sources in an efficient way.