Size, effective density, morphology, and nano-structure of soot particles generated from buoyant turbulent diffusion flames

With a global gas flaring volume of ∼140 billion cubic meters, flares are an important source of particulate emissions; however, very little is known about the physical and morphological properties of these particle emissions. To study these properties, a laboratory pipe flare producing a buoyant turbulent diffusion flame was used which allowed controlled experiments on flames up to ∼3 m tall. Three flare diameters (38.1, 50.8, and 76.2 mm) were used in this study with fuel exit velocities of 0.5, 0.9, and 1.5 m/s. ‘Light’, ‘medium’, and ‘heavy’ fuel compositions (consisting of C1 to C4 alkanes, carbon dioxide, and nitrogen in concentration representative of flares in the Alberta, Canada upstream oil and gas sector) were used, where heavier compositions refer to a greater concentration of higher order alkanes. Size distributions of soot particles were measured using a scanning mobility particle sizer. Mass-mobility relationship and effective density of particles were determined using a tandem arrangement of a differential mobility analyzer, a centrifugal particle mass analyzer and a condensation particle counter. Morphology and nano-structure of the particles were studied using transmission electron microscopy and Raman spectroscopy, respectively. Results showed that the particle median diameter and concentration increased as the fuel composition was changed from light to medium to heavy. On the other hand, particle morphology, measured by the relationships between particle mass vs. mobility (or effective density) and primary particle size vs. particle aggregate size, was independent of fuel composition, flow rate, or flare size and was in good agreement with previously reported values for that of soot particles from different internal combustion engines. Previously developed relations between effective density and primary particle size work well for the soot particles of this study. Raman spectroscopy indicated slightly lower D1/G ratios (more graphitic content) for the heavier fuels.