Low-rank coal-water slurries for gasification: Phase 2, analytic studies: Final report

A laboratory-scale study was conducted to compare alternate methods for thermally dewatering low-rank coal (LRC) as prospects for the preparation of LRC/water slurries for gasification. Conventional, hydrothermal, and high-temperature evaporative coal drying methods were investigated and modern analytical methods were used to characterize coal both before and after thermal treatment. Results showed that the moisture in low-rank coal may all be removed by conventional drying practices, however, it is rapidly reabsorbed when the dried coal is slurried in water. Several changes in the chemical and physical properties of low-rank coal were found important for preventing moisture reabsorption and for enhancing its slurring characteristics. These were: (1) decarboxylation of the coal to reduce the number of polar sites to which water is attracted; (2) removal of water-soluble inorganics; and (3) devolatilization of the coal to produce tars and waxes which can coat the surface of the coal and plug micropore entrances, all of which appear to inhibit moisture reabsorption and/or lower the viscosity of the coal/water mixture. The hydrothermal and high-temperature evaporative coal drying methods were found to be effective in causing some or all of these changes, and for increasing the propensity of low-rank coal for coal/water slurry applications, especiallymore » making it a viable feedstock for slurry-fed gasifiers. In effect, the equilibrium moisture content of low-rank coals was permanently reduced, by converting the coals from a hydrophilic to a hydrophobic substance. This makes it possible to form concentrated slurries using only slightly more water than that inherently found in the coal. 50 refs., 43 figs., 37 tabs.« less