| Abstract: | Lignite and sub-bituminous coals are by nature high in inherent moisture and oxygen content and low in calorific heating value. As-mined low-rank coals when mixed with water generally produce slurries with low solids content and with heating values generally less than 11.6 MJ/kg. These same slurries are usually unstable and form hard-pack sediments quickly, unless chemical additives or constant agitation are added. The low heating value and poor storage and flow characteristics of these coal-water mixtures discourage the use of raw lignite and subbituminous coals for preparation of slurries for fuel purposes.Hydrothermal conditioning, in a water slurry at temperatures above 230 °C and pressures above 552 MPa, is one method that can significantly aid in the preparation of low-rank coal-water fuels. High-pressure hot-water thermal conditioning of lignites and sub-bituminous coals has been found not only to change both the chemical and physical characteristics of the coal but also to alter the coal slurry's rheological properties. These changes are controlled by process variables (i.e. temperature, residence time, particle size, and mode of processing) and result because of decarboxylation, mild pyrolysis, extraction, dehydration, and surface modification; all of which occur during hydrothermal treatment. Using the hydrothermal process, concentrated low-rank coal-water slurries with heating values approaching or exceeding the heating value of the as-mined coal have been achieved with pseudoplastic flow behavior and stability towards settling, without the use of additives.Pilot-scale studies using a 90-kg/hr process development unit (PDU) are currently under way to produce hydrothermally treated low-rank coal fuel slurries for combustion tests in a pilot-scale, slurry-fed test furnace. |
