X-Ray Diagnostics in Combustion - Study of Particle Formation in Flames Using Combined Small- and Wide-Angle X-Ray Scattering

Sammanfattning: In the thesis work, two X-ray scattering techniques, small- and wide-angle X-ray scattering (SAXS and WAXS) were combined to study particle formation in-situ and non-intrusively in ethylene laboratory flames. Combining these two techniques enabled information concerning the size and concentration as well as the subnanometer structure of the nanoparticles within the 1–100 nm range there to be obtained. Different stages in the particle formation process, and thus particles of different types were studied by varying the height above the burner surface. An increased understanding of these nanometer-sized particles and how they are formed in flames was sought. In a longer perspective, the work aims at contribute to a greater awareness of the emission of nanoparticles in combustion and ultimately to a reduction in the levels of emission. A large part of the work concerned the development and construction of a new detection system, one combining SAXS and WAXS and designed for flame measurements in particular. The detection system arrived at included a new SAXS detector, implemented together with an already existing WAXS detector, as well as specially designed software. The SAXS and WAXS detectors were characterized by several test measurements conducted at different beamlines at the synchrotron facility MAX-lab in Lund, Sweden. The SAXS/WAXS detection system was used initially at beamline D611 at MAX-lab for studying the particle formation process from the gas phase to the condensed phase in a diffusion ethylene flame just below a metal plate that was inserted into the flames there. The metal plate served as a flame stabilizer and as a cooling support for particles to condense on. A large number of particles smaller than 100 Å were found to be produced when the flame temperature was lowered. To study these nanoparticles at a higher level of resolution, use was made of another SAXS and WAXS detection system at beamline ID02 at the European Synchrotron Radiation Facility (ESRF), for conducting measurements close to the metal plate. This study enabled five main size groups of ≤ 8 Å, 15–30 Å, 30–100 Å, 100–200 Å, and fractal aggregates of 1000–2000 Å, to be identified. The concentrations for each of the size groups were determined quantitatively, the concentrations of particles below 50 Å in size being found to be very high at the end of the experiment. The new SAXS/WAXS detection system was also used at beamline ID12 at the ESRF for measurements of the gas-phase nanoparticles in different ethylene-rich, both sooting and non-sooting, flames at lower heights above the burner. Here vertically polarized X-ray radiation was employed, this extending the region involved in the case of WAXS and providing a more direct comparison of the chemical codes involved in evaluating the chemical composition of small species in different flame regions. The fitting of theoretical curves to the experimental data showed high concentrations of particles 1–2 nm in size.

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