Preparation of Formed Coke from Biomass by Sequence of Torrefaction, Binderless Hot Briquetting and Carbonization

抄録

This paper proposes a method of preparing high-strength formed coke from woody biomass without binder. Chipped and pre-dried Japanese cedar was heat-treated in an inert atmosphere (i.e., torrefied) at 225–325°C (T_t), pulverized to sizes in three different ranges, molded into briquettes (in the form of thick disk with diameter/thickness ≈ 2.5) at temperature up to 200°C by applying mechanical pressure of 128 MPa. The torrefied/briquetted cedar (TBC) was then converted into coke by heating to 1000°C in an inert atmosphere at normal pressure. This process sequence enabled to prepare coke having indirect tensile strength (S_t) of 8–32 MPa, which was much higher than that without torrefaction, below 5 MPa. The torrefaction greatly improved pulverizability of the cedar, which was further promoted by increasing T_t. S_t of TBC and that of coke both increased as the particle sizes of TBC decreased, but this explained only a minor part of significant effect of T_t on S_t of the coke. S_t was maximized at T_t = 275°C regardless of the degree of pulverization. The T_t effects on physicochemical properties of TBC and coke were investigated in detail. The difference in S_t of coke by T_t was mainly due to that in the increment of S_t along the carbonization at 500–1000°C. Fracture surfaces of the coke had particular morphologies that had been inherited from the original honeycomb structure of the cedar.