Preparation of Coke from Indonesian Lignites by a Sequence of Hydrothermal Treatment, Hot Briquetting, and Carbonization

Abstract

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.

Preparation of Coke from Hydrothermally Treated Biomass in Sequence of Hot Briquetting and Carbonization

“…The briquette undergoes carbonization with no or insignificant softening/fusion, but transforms into coke that has tensile strength of 5-40 MPa. 5,6) It was also found for every type of lignite that the tensile strength of coke was correlated linearly with its bulk density, and this trend was consistent with general knowledge of mechanical strength of coke.…”

“…The gross coke yields from the HT solids with HTT ≥ 250-260°C are well above 50 wt%, and are as high as those from hot-briquetted lignites. 5,6) As seen in Fig. 8(b), HT at 240°C and higher temperatures give net coke yield that is equivalent to or even higher than that without HT in spite of substantial mass release during HT.…”

“…Details of the procedure are reported elsewhere. 5,6) In brief, 0.9-1.0 g of solid was placed in a 14.1 mm diameter mold and heated to 130 or 200°C. A mechanical pressure (PB) of 12, 25, 55 or 114 MPa was then loaded for 8 min.…”

“…25) Details of the tests and the method of determining the tensile strength (TS) are reported elsewhere. 5,6) Compression strength was not employed because it depends on the shape (i.e., aspect ratio of cylindrical specimen). Fractured surface of the coke was observed by scanning electron microscopy (SEM) on a micrograph (Keyence, model VE-9800).…”

“…3,4) The present authors 5,6) proposed sequences of hot briquetting and carbonization 5) or hydrothermal treatment, hot briquetting and carbonization 6) for production of coke from lignite. The briquetting of pulverized lignite at 100-200°C causes its thermomechanical plasticization while macromolecular relaxation occurs due to breakage of non-covalent bonds such as hydrogen ones.…”