Production of high-strength cokes from non-/slightly caking coals. Part I: Effects of coal pretreatment and variables for briquetting and carbonization on coke properties

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.