| 1. |
- Malik, Azhar, et al.
(författare)
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A Potential Soot Mass Determination Method from Resistivity Measurement of Thermophoretically Deposited Soot
- 2011
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Ingår i: AEROSOL SCIENCE AND TECHNOLOGY. - : Taylor and Francis. - 0278-6826 .- 1521-7388. ; 45:2, s. 284-294
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Tidskriftsartikel (refereegranskat)abstract
- Miniaturized detection systems for nanometer-sized airborne particles are in demand, for example in applications for onboard diagnostics downstream particulate filters in modern diesel engines. A soot sensor based on resistivity measurements was developed and characterized. This involved generation of soot particles using a quenched co-flow diffusion flame; depositing the particles onto a sensor substrate using thermophoresis and particle detection using a finger electrode structure, patterned on thermally oxidized silicon substrate. The generated soot particles were characterized using techniques including Scanning Mobility Particle Sizer for mobility size distributions, Differential Mobility Analyzer-Aerosol Particle Mass analyzer for the mass-mobility relationship, and Transmission Electron Microscopy for morphology. The generated particles were similar to particles from diesel engines in concentration, mobility size distribution, and mass fractal dimension. The primary particle size, effective density and organic mass fraction were slightly lower than values reported for diesel engines. The response measured with the sensors was largely dependent on particle mass concentration, but increased with increasing soot aggregate mobility size. Detection down to cumulative mass as small as 20-30 mu g has been demonstrated. The detection limit can be improved by using a more sensitive resistance meter, modified deposition cell, larger flow rates of soot aerosol and modifying the sensor surface.
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| 2. |
- Malik, Azhar
(författare)
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Characteristic Properties and Applications of Fine Particles in Biomass Gasification
- 2011
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)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.
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| 3. |
- Malik, Azhar, et al.
(författare)
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Methodology for Sampling and Characterizing Internally Mixed Soot-Tar Particles Suspended in the Product Gas from Biomass Gasification Processes
- 2011
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Ingår i: Energy & Fuels. - : American Chemical Society (ACS). - 0887-0624 .- 1520-5029. ; 25:4, s. 1751-1758
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Tidskriftsartikel (refereegranskat)abstract
- When biomass is used to produce fuels and green products by thermochemical conversion, the ability to handle or remove the fine particle phase in the product gas is crucial. The product gas from biomass gasification contains relatively volatile organic compounds (“tar”) condensed on nonvolatile cores of, for example, aggregated soot particles and char. The problems are, for example, that particles will poison catalysts used for upgrading of the gas and loss of thermal energy occurs when carbonaceous particles are being formed. The aim of the work is to design and use novel methodologies to characterize the particles in the product gas stream. A methodology has been developed to sample and characterize fine particles by a sampling probe connected to either a denuder or a packed bed device. The system was designed to avoid condensation of organic compounds when diluting the sample and decreasing the temperature. A flame soot generator connected to a condensation−evaporation unit was used to produce internally mixed model particles, i.e., particles consisting of a core of soot with an outer layer of condensed volatile compounds. A scanning mobility particle sizer (SMPS) and a differential mobility analyzer followed by an aerosol particle mass analyzer (APM) were used to characterize the particles. Because of the agglomerated structure of soot, the SMPS system was not adequate to fully characterize the mass of volatiles condensed onto the soot core, and therefore the DMA-heater-APM technique was used to determine the mass fraction of the condensed phase on the soot particles. The two different configurations were studied, and the sampling system was shown to work at a high load of organic mass. In both cases, the organic removal efficiency was >99.5%. Minor condensation of organics on the sampled soot was found for the denuder but not the packed bed. On the other hand, the particle losses were substantially higher for the packed bed compared to the denuder. The results showed that the tested sampling methodology can be used to get sufficient characterization of particles in the product gas and to evaluate the performance of biomass product gas cleaning systems at high temperature.
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| 4. |
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| 5. |
- Nilsson, Patrik, et al.
(författare)
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Laboratory evaluation of a gasifier particle sampling system using model compounds of different particle morphology
- 2011
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Ingår i: Biomass Conversion & Biorefinery. - : Springer Science and Business Media LLC. - 2190-6815 .- 2190-6823. ; 1:2
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Tidskriftsartikel (refereegranskat)abstract
- The objective of this work was to design and evaluate an experimental setup to be used for field studies of particle formation in biomass gasification processes. The setup includes a high-temperature dilution probe and a denuder to separate solid particles from condensable volatile material. The efficiency of the setup to remove volatile material from the sampled stream and the influence from condensation on particles with different morphologies is presented. In order to study the sampling setup model, aerosols were created with a nebulizer to produce compact and solid KCl particles and a diffusion flame burner to produce agglomerated and irregular soot particles. The nebulizer and soot generator was followed by an evaporation–condensation section where volatile material, dioctylsebacete (DOS), was added to the system as a tar model compound. The model aerosol particles were heated to 200°C to create a system containing both solid particles and volatile organic material in gas phase. The heated aerosol particles were sampled and diluted at the same temperature with the dilution probe. Downstream the probe, the DOS was adsorbed in the denuder. This was achieved by slowly decreasing the temperature of the diluted sample towards ambient level in the denuder. Thereby the supersaturation of organic vapors was reduced which decreased the probability for tar condensation and nucleation of new particles. Both the generation system and the sampling technique gave reproducible results. A DOS collection efficiency of >99% was achieved if the denuder inlet concentration was diluted to less than 1–6 mg/m3 depending on the denuder flow rate. Concentrations higher than that lead to significant impact on the resulting KCl size distribution. The choice of model compounds was done to study the effect from the particle morphology on the achieved particle characteristics after the sampling setup. When similar amounts of volatile material condensed on soot agglomerates and compact particles, a substantially smaller growth in mobility diameter was found for soot compared with compact KCl.
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