A sequence of briquetting of biomass solids (bamboo, larch and mallee) at temperature and mechanical pressure of respectively, and carbonization at 900°C produces coke with tensile strength (TS) of 5-19 MPa. Introduction of heat treatment in hot-compressed water (i.e., hydrothermal treatment; HT) of the biomass prior to the briquetting increases TS up to 44, 57 and 42 MPa for the bamboo, larch and mallee, respectively. TS of coke is correlated well and positively with the coke/briquette bulk density ratio, and HT increases the ratio if operated under appropriate conditions. The efficacy of HT is attributed primarily to increase in the coke yield on a basis of the briquette mass. HT hydrolytically removes highly volatile cellulosic material (i.e., cellulose and hemicellulose), transforms it into solid that contributes to coke as effectively as lignin, and thereby increases the mass yield of coke by a factor of 1.4 to 2.1. HT also enhances the plasticizability of the biomass during the briquetting by degradation of the lignin to reasonable extent, and then promotes particles' coalescence/fusion and densification of the briquettes. Applying mechanical pressure over a range of 12-114 MPa to the briquetting of a solid from HT of the bamboo at 240°C successfully results in production of coke with TS of 41-44 MPa.

In continuation of the present authors' studies on production of high strength coke from lignite by sequential binderless hot briquetting and carbonization, this study has been carried out aiming at proposing methods to produce high strength coke from non-/slightly caking coals of subbituminous to bituminous rank. This paper firstly demonstrates preparation of cokes with cold tensile strengths above 10 MPa from two single non-caking coals (particle size; < 106 μm) by applying briquetting at temperature and mechanical pressure of over 200°C and 100 MPa, respectively. Such strength of coke is obtained over a wide range of heating rate, 3-30°C/min, during carbonization with final temperature of 1 000°C. Then, a simple pretreatment, fine pulverization of coal to particle sizes smaller than 10 or 5 μm, is examined. This pretreatment enables to prepare coke with tensile strength even over 25 MPa, by decreasing porosity of resulting coke and more extensively the size of macropores simultaneously. The coke strength changes with carbonization temperature having a particular feature; significant development of strength at 600-1 000°C, i.e., after completion of tar evolution, in which macropores and non-porous (dense) part of coke shrink in volume, inducing bonding and coalescence of particles and thereby arising the strength.

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 1 000°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 500-1 000°C. Fracture surfaces of the coke had particular morphologies that had been inherited from the original honeycomb structure of the cedar.

A novel measurement method for coal thermoplasticity was developed, where permeation distance of thermally plastic coal into glass beads layer placed on the coal sample was measured. The characteristic of this method is simulating the condition in a coke oven, especially void structure around the plastic layer by using glass beads and coking pressure by applying a load. In a standard condition, the coal sample is heated to 550°C, and coal sample softens and permeates into the glass beads layer, then the permeation distance is measured after cooling the sample. The maximum permeation distance measured is roughly correlated with Gieseler fluidity, however large deviation is observed especially for high fluidity coals. Moreover, the deterioration of coke strength is observed in case that long permeation distance coal is used in a coal blend for cokemaking. This new measurement method clearly shows the difference in coking property of high fluidity coal as well as solving the problems in Gieseler plastometer method for evaluating high fluidity coals. By employing the permeation distance method, contribution to the production of high strength coke and effective usage of caking coal will be expected.

Sequential coal briquetting and carbonization was applied to preparation of cokes from 9 non-or slightly caking coals with carbon contents (f C) of 67-85 wt%-daf. Coal pulverization to sizes of < 106 μm and briquetting at 40°C enabled to prepare cokes with tensile strength (σ) over 10 MPa from 4 coals with f C of 82-85 or 67 wt%-daf. Then, by introducing fine pulverization to sizes of < 10 μm before the briquetting, 7 coals were converted successfully into cokes with σ = 11-25 MPa. Increasing the briquetting temperature to 240°C further increased σ to 19-35 MPa for all the 9 coals. It was thus demonstrated that the hot briquetting of finely pulverized coal was a method to prepare high strength coke regardless of the rank of parent coal. Cokes were also prepared from 14 binary coal blends. All the cokes prepared by applying the fine pulverization and hot briquetting had σ of 20-35 MPa, which agreed well with that calculated by weighted average of those from the component coals. On the other hand, positive and also negative synergistic effects of blending occurred when blends were briquetted at 40°C. Characteristics of bonding/ coalescence among particles of different types of coals were responsible for such synergies.