| Molecular models, which represent the constitution (the chemical and physical structure) of bituminous vitrinites from the Upper Freeport and Lewiston-Stockton coal seams, were generated computationally. Incorporation of physical data significantly reduced the number of possible structures and forced the inclusion of hydroaromatic structures. This limited the bonding types and the extent of bonding to the coal matrix. The use of vitrinites (telocollinite) rather than whole coals reduced the complexity of the molecular structures and precluded incorporation of other maceral functionalities (such as long chain aliphatics or highly condensed aromatic structures) into the models.; A narrow size fraction of each vitrinite was pyrolyzed in a drop-tube reactor under conditions representative of the devolatilization stage of pulverized coal combustion. The collected chars represented the transition from vitrinite to almost fully devolatilized char. Computational fluid dynamics were used to calculate the range of time-temperature histories of the particles in the reactor. Significant variation (up to 400{dollar}spcirc{dollar}C and 0.15 s residence time) was found within a cuts primarily due to different extents of swelling, different particle sizes and the associated differences in mass loss.; The vitrinite models were subjected to via various pyrolysis mechanisms and the resulting char structure compared to those of the experimentally-produced chars. The breaking of bonds in order of bond strengths did not produce a reasonable initial char structure; one which was similar to the parent vitrinite. The weaker bonds tend to be aliphatic in nature, hence breaking these bonds resulted in a structure that was too aromatic and hydrogen deficient.; 'Rules' of pyrolysis, which were reasonable for the transitions of this rank of vitrinite under rapid heating conditions were proposed. Five-membered rings, either formed via ring condensation or originally present in the vitrinite, introduce curvature into the structure and influence the stacking efficiency of the char lamellae. Internal hydrogen redistribution is primarily responsible for the fluidity, and hence the degree of ordering in the chars. It is speculated that the difference extent of incipient hydrogen transfer within the char structures is responsible for the inferred fluidity differences between the samples. |
