Characterization of particles generated by non-catalytic methane pyrolysis in a tubular flow reactor

This research investigates the influence of temperature on the physicochemical and morphological characteristics of particles produced during methane pyrolysis in a quartz tubular reactor. To achieve this, a range of analytical techniques, including Raman spectroscopy, thermogravimetric analysis (TGA), transmission electron microscopy (TEM), mobility and aerodynamic particle size spectroscopy, and tandem measurements with a centrifugal particle mass analyzer (CPMA) paired with a single particle soot photometer (SP2) and a scanning mobility particle sizer (SMPS), have been employed. The structural evolution of carbon particles, as depicted by Raman spectroscopy, shows a transition from polycyclic aromatic hydrocarbons (PAHs) to more graphitized forms of carbon with an increase in temperature, marked by the emergence of structural defects. This is further evidenced by TGA, where a significant increase in the mass fraction of coke was found, from roughly 23% to 94% at the pyrolysis temperatures of 950 °C and 1100 °C, respectively. Morphologically, this temperature-dependent transformation results in a shift in the particle size distribution, evolving from smaller, less dense particles with count median mobility diameters around 100 nm at 900 °C to larger, denser particles of approximately 400 nm median mobility diameter at 1100 °C. Additionally, the refractory carbon mass fraction is not strongly dependent on individual particle mass; instead, it increases from ∼0% to over 83% as the temperature rises from temperatures below 975 °C to 1100 °C, further highlighting the significant influence of thermal conditions on aerosol formation in methane pyrolysis. These results have implications for product purity and how to process undesirable PAHs, as well as the development of particle and gas phase separation techniques.