Conversion of a Wide Range of Low-Rank Coals into Upgraded Coals and Thermoplastic Extracts Having Similar Chemical and Physical Properties Using Degradative Hydrothermal Extraction

We have recently presented a method that converts low-rank coal into upgraded coal and thermoplastic extract through the hydrothermal degradative extraction in a flow of hot water at less than 350 °C. To examine the validity of the presented method and to characterize the upgraded coals and the thermoplastic extracts obtained in more detail, seven kinds of low-rank coals, a biomass, an O-methylated brown coal, an oil rock, and a bituminous coal were subjected to the hydrothermal degradative extraction at either 300 or 350 °C. The elemental compositions of the upgraded coals were found to be rather close to each other, and their carbon and oxygen contents were larger than 71% and less than 23%, respectively, meaning that the heating values are well over 30 MJ/kg on a dry ash-free (daf) sample basis. The solid extracts (deposits) obtained at 350 °C from the variety of samples were very close to each other in elemental composition, molecular-weight distributions, pyrolysis behavior, and softening/melting behavior. All of the deposits interestingly completely melted without decomposition below 200 °C. Thus, the proposed method was found to be effective to convert low-grade carbonaceous resources into solid fuel having a higher heating value and compounds having thermoplastic property under rather mild conditions.