Structural compositions of tars from hydropyrolysis of coal. Effect of reaction severity

Abstract

amounts of oxygen in the saturate and aromatic fractions. They proposed that oxidation of bitumen at low temperatures involves conversion of original resins to asphaltenes with simultaneous conversion of oils to resins. These authors suggested that in these processes oxygen is for the most part a catalyst for condensation reactions, being initially incorporated as labile functional groups that promote these reactions and subsequently eliminated as water. Our chemical kinetic model is consistent with this interpretation, indicating a weak dependence of the rate of depositional processes on oxygen pressure and a stronger effect of pressure of oxygen on the rate of combustion processes. The increase in saturates and decrease in polyaromatic and polar fractions that we observed in the produced oil relative to the original bitumen, in combination with the increasing aromaticity (decreasing molar H/C ratio) of cokes produced with increase in temperature, also support this interpretation. Acknowledgment. We are grateful to D. D. McIntyre for performing chromatographic separations and for many helpful discussions. We also thank Y .-M. Xu for preparation of the calorimetric sample, and Z.-L. Zhang and J.F. Smith for much good advice. We thank the Alberta Oil Sands Technology and Research Authority for support of this and related research.The tars from five hydropyrolysis testa of Highvale (subbituminous, Alberta) coal were studied to determine the effects of reaction severity on concentrations of structural groups. The yields of gases and vapors (carbon oxides + light hydrocarbons + benzene, toluene, and xylene) were used as an internal measure of reaction severity for tars produced from an entrained flow reactor operated at 6 0 0 0 "C, 7 MPa hydrogen pressure, and 0.5-5 s residence times. The structural groups in the tars were estimated by combining data from ' H NMR, ' % NMR, and infrared spectroscopy, nitrogen titration, and elemental analysis. The samples processed at higher severity showed overall concentration increases in aromatic groups and decreases in aromatic side chains, naphthenic rings, and oxygen-containing structures relative to low-severity tars.

Trends in Aromatic Ring Number Distributions of Coal Tars during Secondary Pyrolysis

“…8 coals suggests that oxygen heteroatoms are mostly released from within the rings . This is expected to be caused by the opening of ring structures, because releasing additional CO through the disruption of furan-type rings would not occur until higher temperatures are imposed, ,, as during the later stages of secondary pyrolysis in our experiments.…”

“…Since the nitrogen content of Dietz coal is less than 2%, the concentration of N-containing polycyclic PAC is small relative to the concentration of PAC with oxygenated substituents in the Dietz coal. In addition, other researchers have observed substantial amounts of PAC with hydroxyl substituents during the very early stages of secondary pyrolysis. , Therefore, it is likely that most of the transferred compounds are PAC with oxygen-containing substituents.…”

“…It is known that cleavage of methylated PAC occurs early in secondary pyrolysis, − ,− resulting in methyl groups and parent PAC which appear to be more stable than PAC with substituents. Since the elution order and absorptivity of I-PAC via HPLC are little affected by alkyl substituents, − the absence of alkylated groups should not significantly contribute to the observed reduction in I-PAC.…”

“…Since the elution order and absorptivity of I-PAC via HPLC are little affected by alkyl substituents, − the absence of alkylated groups should not significantly contribute to the observed reduction in I-PAC. Furthermore, since PAC with furan-type (oxygen-containing) rings appear to be insensitive to thermal conditions at the early stages of pyrolysis, ,, they should not be responsible for the decreasing concentrations seen for I-PAC. Therefore, the reduction in I-PAC concentrations during early secondary pyrolysis, as shown in Figure , appears to be mainly caused by direct conversion of ring compounds to soot.…”

“…The mechanisms of polymerization, neutralization, and ring rupture occurring among PAC at high temperatures have been mentioned by many researchers, − and the relative prominence among isomerization, biaryl formation, and condensation has been reported by Wornat et al for a standard compound at the late stages of secondary pyrolysis. However, the relative importance of these mechanisms as a function of different coal types throughout secondary pyrolysis has not been fully characterized.…”