Effect of Different Drying Methods on Coal Structure and Reactivity Toward Liquefaction

Abstract

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. Laboratoryscale 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.

Supercritical gas sorption on moist coals

“…These effects, however, diminish with increasing rank and it has been suggested that drying does not significantly alter the structure of bituminous coals (Miknis et al, 1996). To avoid the effects of structural changes induced by drying, which may affect the sorption results, the samples selected for this study were all bituminous coals.…”

Recent Advances in Direct Coal Liquefaction

“…By swelling pretreatment, coal molecules can be dissociated, rearranged and re-associated in lower free energy conformations, or probably in a different molecular structure. The swelling solvents may break weaker bonds, create macro-pores or enhance the coal porosity, and hence facilitate the dispersion of the catalyst onto the coal structure, and more importantly assist the dispersion of the hydrogen donor solvent into the coal [37][38][39]. The yields of liquefaction products were strongly dependent on the type of swelling solvent used.…”

“…Although it is commonly believed that the swelling solvents can enhance the coal porosity, and improve the dispersion of the catalyst onto the coal structure as discussed above [37][38][39], negative impacts of the swelling solvents were also observed [41,43,44]. Shui et al [44] demonstrated that the presence of swelling solvents in the swollen coal was detrimental for the subsequent liquefaction of the swollen coal.…”

“…Direct preheating treatment, i.e., drying a coal, was found to be detrimental for DCL. Miknis et al [39] demonstrated that liquefaction of thermally dried coals led to a decrease in the amount of liquid products. For most coals tested, there is a general trend of decreased liquefaction conversion with increasing the percentage of water removed.…”

Influence of Coal Particle Size on Coal Adsorption and Desorption Characteristics

“…The preliminary information about this type of coal was shown in Table 1 and Table 2, tested by Saghafi and Roberts (2008) from CSIRO. It is widely recognised that thermal drying of low rank coals can cause irreversible coal structure changes and these effects can be diminish with increasing rank and it has been reported that drying does not significantly change the structure of bituminous coals (Miknis et al, 1996). The bituminous coal from Bulli Seam of the Sydney Basin is suitable to carry the test to avoid the effects of structural change induced by drying, which may affect the isotherm results of this study.…”