Electron microscopy investigation of carbonaceous particulate matter generated by combustion of fossil fuels

The morphology, microstructure, and composition of individual carbonaceous particles generated during combustion of coal, residual oil, and diesel fuel were investigated by various electron microscopy (EM) techniques, including scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HRTEM), selected area electron diffraction (SAED)/dark-field imaging, electron energy loss spectroscopy (EELS), scanning transmission electron microscopy (STEM), and energy-dispersive X-ray spectroscopy (EDS). Carbonaceous components of all of these fossil-fuel-derived particulate matter (PM) samples contain ultrafine soot aggregates with fractal chain morphologies and spherical primary particles (sizes similar to 30-50 nm) that exhibit a concentric arrangement of stacked graphitic layers around the particle center. Some submicrometer soot aggregates with larger primary particles (similar to 100-400 nm) were also found in coal-derived carbonaceous PM. Larger spherical or irregular-shaped porous char particles dominate the micrometer-sized fraction of coal and residual oil PM samples. The relative fractions of sp(2) hybridized carbon atoms in carbonaceous particles were estimated from EELS measurements of the carbon K-edge taken at the magic angle. Although the carbonaceous particles generated by combustion of various fossil fuels may have quite different morphologies and microtextures, they possess similar internal structural parameters, such as the dimensions of basic structural units (BSUs) and the average hybridization state of the carbon atoms. Compositional differences between soot aggregates and char particles were observed, and such information could be used as fingerprints to identify the source of different types of carbonaceous particles.