Hydrothermal upgrading of Loy Yang Brown coal — Effect of upgrading conditions on the characteristics of the products

Hydrothermal treatment of the high-moisture containing (59%) Loy Yang brown coal was conducted in a 20 ml stainless steel autoclave at 200 to 350 °C for 30 and 180 min. Three treatment methods were employed: 1) conventional method (i.e., extra water was added before treatment); 2) as-received method (i.e., treatment without addition of extra water); and 3) separation method (i.e., treatment with physical separation of water and solid). Treatment using the as-received method revealed that hydrothermal conditions can be maintained in the autoclave without addition of extra water. Upgrading using the separation method yielded more effective drying especially during treatment at 350 °C, where the water content was reduced to 6%. Treatment above 250 °C led to progressive lower water content, increasing carbon and decreasing oxygen content. Calculations of the calorific values of the upgraded coals showed that hydrothermal treatment at 300 and 350 °C led to a net increase in the calorific value compared to the raw coal. Reductions in the volatile content and the changes in the physical properties of the upgraded coals significantly reduced self-ignition tendencies of the coal treated above 300 °C.