Hydrothermal Pretreatment Approaches To Enhance Slurryability And Rheological Behaviors Of Low-rank Coals

To figure out a way to enhance the slurryability of the varieties of lignite and coals that are hard to make slurry, a lignite and a subbituminous coal were first selected as target research sample coals through a series of experiments. Thermal and hydrothermal treatments were then used to improve the slurryability and rheological behaviors of two coals. The main research contents are included as follows.First, four coals were used to prepare coal-water slurry, including a lignite (Yuxi lignite), a subbituminous coal (Zhundong coal), and two bituminous coals (Xinwen coal and Shenfu coal), and. Xinwen coal and Shenfu coal show good slurryability, Yuxi worse and Zhundong worst. It was evident that the coal rank can not be a simple index to judge the slurryability of a coal. The higher-rank coal is not always easier to make slurry. The mineral components present in coal had great influence on the coal slurryability. Calcium in coal showed a negative impact, whereas silicon and aluminum were favorable for stability. In this section, the relationship between solid concentration and rheological behavior, as well as the influence of dispersant and stabilizer on apparent viscosity and stability were also inspected.Second, thermal treatment was performed to upgrade Zhundong coal and Yuxi lignite. Thermal treatment was conducted in an air-isolated atmosphere at temperatures from145℃to700℃. While the two raw coals had the poor slurryability, the upgraded coals or char exhibited significant improvements in the slurryability depending on the treatment conditions. A coal or char slurry (CWS) with the solid loading of59wt%was prepared from the YX lignite by the thermal treatment at400℃, with the apparent viscosity of1208mPa s and the good stability. The apparent viscosity of CWS was found to be related to, but not only to the elimination of hydrophilic groups by pretreatment. The CWSs prepared from the thermally treated YX coals or chars were less pseudoplastic and less easily subjected to a pseudoplastic change with increasing solid loading than the CWSs prepared from the parent coal.Third, various upgraded coals or chars were derived from Zhundong coal and Yuxi lignite by hydrothermal treatments. Hydrothermal treatment of coal was carried out in an autoclave at temperatures from150℃to300℃with autogenous pressure from0.8MPa to9.2MPa. Both the thermal and hydrothermal treatments were proved to be effective for improving the slurryability of the two coals, but the effectiveness varied depending on the methods and the coals. And the chars resulted from hydrothermal treatment at300℃could be made into the char-water slurry with solid loading of60%and apparent viscosity of1285mPa s. The hydrothermal treatment was more effective to destruct the hydrophilic groups than the thermal treatment for Yuxi lignite, while this was not the case for Zhundong coal. The enrichment of carboxyl-bound calcium in Zhundong coal enabled the hydrophilic groups to be more thermally refractory, and this calcium-combining structure appeared to have a negative effect on the slurryability and the dispersion stability. In this section, we also tried to use the mixtures of Zhundong coal or Yuxi lignite with Shenfu coal to prepare slurries. The mixtures demonstrated higher solid loading with good dispersion stability as a result of an incorporated effect exerted by the characteristic properties of two coals.