Preparation of coke from Indonesian lignites by a sequence of hydrothermal treatment, hot briquetting, and carbonization

Production of coke from lignites was studied in continuation of a previous study that demonstrated effectiveness of a sequence of hot briquetting and carbonization on preparation of high strength coke from a lignite. Cokes were prepared from four Indonesian lignites with or without pretreatments such as hydrothermal treatment (HT) at 200-300 °C, acid washing (AW), and a combination of them (HT-AW). The hot briquetting of the raw lignites at temperature and mechanical pressure of 200 °C and 128 MPa, respectively, enabled cokes to be produced with a tensile strength (TS) of 7-22 MPa. The pretreatments, AW and HT at 200 °C (HT200), increased TSs of resulting cokes to 18-24 and 13-36 MPa, respectively. A sequence of HT200 and AW further increased TSs of cokes to 27-40 MPa. AW and HT200 modified the macromolecular structure of the lignites by different mechanisms. AW removed alkali and alkaline earth metallic species that played roles of cross-links in the macromolecular network, while HT200 rearranged macromolecules physically. Both HT and AW enhanced plasticization and then deformation/coalescence of lignite particles during the briquetting, which formed high strength briquettes. There were strong correlations between TS of coke and that of briquette and also between TS and bulk density of coke from the individual lignites.