Fate of aromatic ring systems during thermal cracking of tars in a fluidized-bed reactor

A suite of brown and bituminous coals have been pyrolyzed in a fluidized-bed reactor between 400 and 1040 °C. The reactor featured a relatively long heated freeboard where the nascent volatiles were thermally cracked. Tars were characterized with size exclusion chromatography (SEC), and gases were analyzed with gas chromatography. The yield of dichloromethane-soluble tar was observed to reach a maximum at around 500 °C for the brown coals and at around 600 °C for the bituminous coals. SEC experiments, however, suggested that the release of aromatic tar components from the brown coals studied was not complete at temperatures lower than about 700 °C under the present experimental conditions. Tars from the pyrolysis of the bituminous coals showed higher thermal stability than those from the brown coals under the same conditions. The difference in the concentrations of larger (with at least three or more fused benzene rings) aromatic ring systems in coal substrates and tars is thought to be the main structural reason for the observed difference in pyrolytic behavior between the brown and bituminous coals. Larger aromatic ring systems in the coals and in the tars serve as cross-linking sites of higher coordination number than the smaller aromatic ring systems. During tar thermal cracking, larger aromatic ring systems in the bituminous coal tars (containing higher concentrations of larger aromatic ring systems) are more likely to disappear from the tars between 800 and 1040 °C than those in brown coal tars (containing lower concentrations of larger aromatic ring systems). One of the main fates of the aromatic ring systems lost from the tars during tar thermal cracking processes is the formation of soot. Concurrent with the release of simple (mainly C₁ and C₂) hydrocarbon gases, significant amounts of single-ring aromatic systems were also released from the more complex tar molecules as a consequence of the thermal cracking of the tar. In agreement with our previous studies, significant amounts of polymethylene groups are thought to be chemically connected to aromatic ring systems in low-rank coal and their low-temperature tars.