# Effect of Fuel Composition on Carbon Black Formation Pathways
## 3 Results and Discussion
### Carbon Black Formation--Dependence upon Fuel Compositional Differences  
CT composition is dependent upon both initial coal rank and pyrolysis conditions of temperature, rate, and quenching [18; 19; 20]. Yet general distinctions are remarkably consistent given the evolution of coals along the van Krevelen path and fit within the Seylar coal band [21]. Specifically, CT consists predominantly of small aromatics with ensuing pyrolysis reactions increasing their content facilitating their growth. Alkyl groups, ether linkages, etc. will have undergone cleavage during the destructive thermal pyrolysis [19; 22]. Light volatiles and saturates arise by cleavage of such linkages and dealkylation reactions [23]. The significance of the high concentration of small aromatics and high level of methylation [24] is that these species will readily form resonance stabilized radicals [25; 26]. Their low barrier to formation facilitates their participation in the resonance stabilized radical addition reactions considered central to soot nuclei formation and subsequent growth [6; 27]. For such reactions, feedstock aromatics are a direct precursor. High temperatures activating sufficient H-atom concentrations for hydrogen abstraction acetylene addition reactions (HACA mechanism) towards the buildup of the 1 st aromatic ring and subsequent growth are not required [28]. It should also be noted that fractionation studies on CT have found a continuum of cuts ranging from naphtha, kerosene, diesel, heavy gas oil, and even a small resid fraction (a few wt.%) [29]. Such a broad molecular size range is synergistic with the resonance stabilized radical formation path given aromatics in each cut.  
The FCC feedstock also contains considerable aromatic content, but with saturate content in the form of long-chain paraffins, i.e., waves and alkyl substituent groups on aromatics [24]. The paraffinic content lowers the overall aromatic concentration, slowing the rate of nuclei formation while requiring pyrolysis reactions to form aromatics thereby imposing a kinetic delay, \(\tau\). Perhaps more importantly the saturate decomposition (whether as paraffins or substituent groups) generates H-atoms and alkyl radicals that can cap aromatic radicals to restrict reactive radical addition reactions [30]. Preceding pyrolysis  
\begin{table}
\begin{tabular}{c c c} \hline \hline  & **CT** & **FCC** \\ \hline No. Measurements & 7 & 15 \\ Ratio: Edge/Center & 0.86 & 0.83 \\ Normalized Mean sp\({}^{2}\) (\%) & \(7.2\pm 1.8\) & \(24.4\pm 1.6\) \\ sp\({}^{2}\) referenced value & 46 & 78 \\ Rel. Ratio & 1 & 3.4 \\ \hline \hline \end{tabular}
\end{table}
Table 2: EELS analysis results for the three carbon blacks from the indicated feedstocks.  
saturates can serve as H-donors to terminate aromatic radicals--restricting their clustering via radical chain reactions [31; 32]. Though the reaction rates are fast and mechanistic steps few for these processes, any kinetic delay (\(\tau\)) allows turbulent mixing with flame gases, temperature elevation, and homogenization of the reactive environment under which nuclei form and particle growth occurs [33].  
Interpretation for the structural differences spanning nano- to micro-scales for the two carbon blacks is premised upon the feedstock compositional differences. Formation of carbon black using the two feeds is postulated to begin at different stages of the carbon black formation pathway for CT-CB and FCC-CB, illustrated by the schematic in Figure 5 --based on feedstock compositional differences as listed in Table 1, analogous to the well-known soot formation pathway [34].