Recently, Particulate Matter (PM) pollution received major attention based on scientific discoveries, health concerns, as well as climate change debates.  Especially, the area of organic PM (primary and secondary) has many uncertainties that need further exploration.  To help trace and determine the different source materials and points of origin of ultra small carbon particulate matter in the atmosphere, this study investigated fly ash-based ultra-fine PM carbon material (nanocarbons) derived from coal-fired power plants. The investigation only addressed the nature of the nanocarbons in fly ash from a high volatile bituminous coal although others are planned for future study.   The nanocarbons observed are associated with fly ash captured and collected within coal combustion pollution-control systems' electrostatic precipitators (ESP units). The study addresses (a) the spatial relationship of ultra-fine (< 10 nm) carbons with respect to the glassy ash spheres, (b) describes variations in the structural characteristics of the nanocarbons, and (c) analyzes the composition of nanosized metal inclusions in the ultra-small carbon source.  The most significant observation was the presence of carbon-based nanocoatings on the surface of individual fly ash particles.  The carbon nanocoatings are extremely porous and fragile. This delicate appearance of the nanocarbon deposits on the Al-Si glassy particles suggests that fractions of the nanocarbon coatings may become liberated during ash management (either in or before ESPs or after collection) which could affect the size of the nanocarbon fraction, cause separation of the nanocarbons from the bulk fly ash, and contribute nanocarbons in form of aerosol to atmosphere.  Given the seemingly delicate association of the nanocarbons with the glassy fly ash, it is suggested that this fraction could be a contributor to ultra-fine PM soot in the atmosphere and the associated trace elements released by power plants.   A secondary organic aerosol formation via detachment of nanocarbons from coal ash material may be an important contributor to the overall ambient organic aerosol mass.

The ultra fine PM carbon in fly ash was distinguished as either nanoscale sooty or graphitic fullerene-like carbons. They are well below the optical limit of resolution and, furthermore, their graphitic nature has not been described in earlier studies.  This ultra-fine PM carbon represents the carbon source in fly ash previously unaccounted for in optical microscopic investigations.   The nanocarbons (10's nm) are delicately deposited on and interlaced between larger aluminosilicate (Al-Si) glassy fly ash spheres. They form an ultra-thin layer on the coarser inorganic fly ash particles and emerge as nanocoatings covering the glassy spheres in a halo or shell-like fashion.  Nanocarbons can be traced as an ultra-thin halo around the majority of Al-Si glass fly ash spheres (carbon nanocoating). The carbon shells/nanocoatings are vastly porous and consist of agglomerated soot particles, each exhibiting concentric ring structures. High-resolution transmission electron microscopy (HRTEM), scanning transmission electron microscopy (STEM), and electron energy-loss spectroscopy (EELS) investigations of a high-carbon fly ash with relatively high concentrations of As (10²-10³ μg/g),  Se (10² μg/g), and Hg (1-3 μg/g) demonstrated that nanocarbons with  both soot-like structures and fullerene-like carbons contain finely dispersed nm-sized metal inclusions. The elemental associations of the metal inclusions follows the pattern Fe >>Ca>Ti>Pb>As>Ni>Co>Hg, with varying amounts of Se also present. The study showed a majority of Al-Si glassy spheres to have a carbon-based nanocoating or at least some fraction of the surface coated.  The presence of the carbon nanocoatings has not been taken into account during past routine petrographic studies of fly ash materials.  The delicate nature of the carbon deposited on the Al-Si glass was confirmed by the observed damage to the structure by the electron beam.  This suggests that processes, such as the transport of the fly ash in the turbulent flue gas, could potentially separate a fraction of the fullerene-like carbon, with the included trace elements, from the Al-Si glass.  If this happens in or before the electrostatic precipitators (ESP), the relatively coarser glass will be captured by the ESP (and ESP's routinely capture over 99% of the fly ash) and the finer carbons will escape into the atmosphere.  Since the thickness of the carbon nanocoating varies among different Al-Si spheres in fly ashes captured in ESPs and can be irregular on the surface of individual spheres, such nanocarbon deposits appear to obstruct the smoothness and, hence, fluidity of the ash particles.  HRTEM provides a superior resolution of the carbon nanostructure while STEM imaging emphasizes the difference between the low-atomic-number carbon and the inorganic entities captured or engulfed by the nanocarbons. The EELS spectra of selected metal inclusions in the nanocarbon agglomerates illustrate that a majority of the nanosized metals are iron-rich.  The results are based on the presence of the FeL3 (715 eV) and FeL2 (720 eV) peaks in the EELS spectra.  Elemental analysis indicated an association of Hg with the nanocarbon.  Arsenic, Se, Pb, Co, and traces of Ti and Ba are often associated with Fe-rich particles within the nanocarbon deposits.  A detailed high resolution STEM- X-ray map, with the corresponding STEM image, of a carbon-rich nanocluster indicates that As follows Fe and S signatures. 

Temperatures within the ESP array range from about 200 ^oC at the entry to the first row to <150 ^oC at the exit of the third row.  For both electrostatic precipitators (ESP) and fabric filters (FF), it is often observed that there is a relation between the collection point of the fly ash (ESP row) and the concentration of trace elements with the concentration of volatile elements, such as Pb, As, and Se, generally increasing with a decrease in temperature at the collection point and a decrease in ash particle size.  Coarser particles are preferentially collected in the first row of an ESP while finer, higher surface-area particles are trapped at the third row.  The combination of decreasing temperature and increasing surface area of fly ash particles leads to enhanced concentrations of volatile trace elements, but the concentration of trace elements in the fly ash is also strongly related to the concentration of elements in the feed coal.

The HRTEM investigation revealed abundant fullerene concentration over the nanocarbon agglomerate surfaces and the result is not surprising given the fact that the nanocarbons were formed in a combustion environment.  Individual fullerene molecules range in size which is typically observed in samples produced from a fullerene-forming flame. In addition, some nanocarbons in the current study exhibit a fullerene-like structure over larger areas, suggesting that the growth mechanisms of the nanocarbons involved fullerene formation.  The typical morphology of fullerene-like soot in the nanocarbons on Al-Si spheres shows chain-like assembly of primary fullerene particles, often exhibiting multiple shell structures that are interlinked and result in the appearance of strongly bent graphene sheets or ribbons and double shells. The key parameters required for fullerene synthesis on the nano fly ash carbons are suggested to depend in part on temperature and concentrations of H₂O vapor and CO₂ since those noticeably affect the amount of fullerenes, as well as their size formed under experimental conditions on carbon supports.  Future research should address the nature of the nanocarbons in fly ashes derived from these ranks and, very importantly, also the reactivity of such nanocarbons, as this may play important roles in pulmonary and other health effects.