Effects of low-temperature catalytic pretreatments on coal structure and reactivity in liquefaction

Abstract

Low-temperature catalytic pretreatment is a promising approach to the development of an improved liquefaction process. This work is a fundamental study on effects of pretreatments on coal structure and reactivity in liquefaction. The main objectives of this project are to study the coal structural changes induced by low-temperature catalytic and thermal pretreatments by using spectroscopic techniques; and to clarify the pretreatment-induced changes in reactivity or convertibility of coals in the subsequent liquefaction. This report describes the recent progress of our work. Substantial progress has been made in the spectroscopic characterization of structure and pretreatment-liquefaction reactions of a Montana subbituminous Coal (DECS-9), and thermochemical analysis of three raw and reacted bituminous coals. Temperature programmed liquefaction has been performed on three low-rank coals both in the presence and absence of dispersed molybdenum sulfide catalyst. We also performed a detailed study of the effects of mild thermal pretreatment-drying in air and in vacuum-on thermal and catalytic liquefaction of a Wyodak subbituminous coal. Important information on structure andstructural transformation during thermal pretreatment and liquefaction reactions of low-rank coals has been derived by applying solid-state CPMAS 13C NMR and flash pyrolysis-GC-MS (Py-GC-MS) for characterization of the macromolecular network of a Mon:ana subbituminous coal and its residues from temperature-programmed and nonprogrammed liquefaction (TPL and N-PL) at .al temperatures ranging from 300 to 425°C in Hdonor and non-donor solvents. The results revealed that this coal contains significant quantities of oxygen-bearing structures, corresponding to about 18 O-bound C per 100 C atoms and one Obound C per every 5 to 6 aromatic C. The oxygen-bearing components in the coal include catechollike structures, which seem to disappear from the liquefaction residues above 300°C; carboxyl groups, which almost disappear after 350'_C; and phenolic structures, which are most important in the original coal but diminish in concentration with increasing temperature. These results point to the progressive loss of oxygen functional groups and aliphatic-rich species from the macromolecular network of the coal during programmed heat-up under "/'PL conditions. The higher conversions in TPL runs in H-donor tetralin (relative to the conventional N-PL runs) suggest that the removal of carboxylic and catechol groups from the coal and the capping of the reactive sites by H-transfer from H-donors during low temperature (<_350°C) pretreatments have contributed to minimizing the retrogressive crosslinking at higher temperatures. Quantitative calculation of NMR data and mathematical correlation were also attempted in this work. For 24 liquefaction residues derived under significantly different conditions, linear correlations between C-distribution and reaction temperature (_>300°C)have been found, which can be expressed by a simple equation, Ci = 0t fi + 13T, where fi and C...