Chemical Characteristics of Tars Produced in a Novel Low-Severity, Entrained-Flow Reactor

Understanding tar formation, desorption, and gas-phase secondary reactions during rapid coal devolatilization is critical to the formulation of a comprehensive understanding of rapid coal devolatilization. To overcome some of the limitations of traditional entrained-flow systems, a novel flow reactor and separation system was established to investigate the tar devolatilization properties of a range of coals. Results indicate a wide range of coal ranks devolatilize in a phenomenologically similar sequence, with tar formation and evolution dominating the initial phases of particle hydrocarbon mass loss under the heat-transfer conditions characteristic of this system. The average structural characteristics of the tars change significantly during the temperature-resolved evolution process, with hydrogen-rich, lower molecular weight species evolving before the relatively hydrogen-poor, high-molecular-weight species in the latter stages. The lower the rank index of the parent coal, the more dissimilar the primary tars are relative to the parent coal at any given extent of tar evolution. The integrated mass of tars evolved from medium- and low-volatile bituminous coals appears to approach the parent coal elemental and functional group infrared absorbance characteristics as an asymptotic limit. But for a given coal, the earlier in the tar evolution process, the more dissimilar, relative to the parent coal, are the evolved tars with respect to these characteristics. Generalizations concerning the “similarity” of primary tars to the parent coal structure have limited applicability in the formulation of general models of tar formation and evolution. Such generalizations apply to particular coal ranks and describe the integrated sum of a process that, upon deconvolution, reveals systematic differences between chemical characteristics of tars and the parent coal.