Effect of processing conditions on organics in wastewater from hydrothermal dewatering of low-rank coal

A study was carried out into the effects of processing conditions on the nature and amounts of organics released into wastewater from hydrothermal dewatering (HTD) of low-rank coal. Processing conditions examined included temperature, residence time, water to coal ratio in the slurry, slurry loading, and coal lithotype. High temperature and pressure treatments of low-rank coal slurries were used to simulate the HTD process. It was found that the extent of extraction of organics during HTD processing depended on the coal lithotype and increased with increasing temperature, residence time, and the proportions of steam and water present during processing. For a constant residence time, the concentration of organics in the wastewater increased exponentially with temperature over the temperature range 250–350 °C. A maximum level of approximately 7 g/l (expressed as TOC) of organics was observed following processing at 350 °C. Hot filtration of the coal slurry produced a higher organics loading compared to that found in wastewater obtained at low temperature.