Effect of different drying methods oil coal structure and reactivity toward liquefaction

Different methods of drying coal were investigated to determine if drying can be accomplished without altering the coal structure and reactivity toward liquefaction. Coal-drying methods included thermal and microwave drying at elevated temperatures and chemical drying at low temperature. Six coals from lignite to high volatile bituminous rank were studied. Laboratory-scale liquefaction experiments were carried out on premoisturized and dried coals to determine the effects of different drying methods on liquefaction yields. Solid-state nuclear magnetic resonance (NMR) and swelling measurements were made to assess any changes in coal structure brought about by the different methods of drying. The NMR measurements showed that, in general, there were no major structural changes in coals dried thermally or with microwaves other than partial decarboxylation. Chemically dried coals exhibited increased resolution in the aliphatic carbon region that was attributed to adsorbed methanol, which was a reaction product as well as solvent for the chemical drying method. The swelling ratios of thermally dried and microwave-dried coals were lower than those of premoisturized coals, indicating a greater degree of cross linking in coals dried using these methods. The swelling ratios of the chemically dried coals were greater than those of the premoisturized coals. Coals that were dried or partially dried thermally and with microwaves had lower liquefaction conversions than coals containing equilibrium moisture contents. However, chemically dried coals had conversions ranging from 11 to 60% greater than the premoisturized coals. The conversion behavior is consistent with changes in the physical structure and cross-linking reactions because of drying. Thermal and microwave drying appeared to cause a collapse in the pore structure, thus preventing donor solvents from contacting reactive sites inside the coals. Chemical dehydration did not appear to collapse the pore structure.