High-Strength Formed Coke from Torrefied Biomass and Its Blend with Noncaking Coal

Abstract

In continuation of our previous study on production of high-strength metallurgical coke from torrefied softwood (cedar), we studied coke production from a mixture of torrefied cedar (TC) and noncaking coal by pulverization to sizes <100 μm, mixing, binderless hot briquetting, and carbonization. These sequential processes produced coke with a tensile strength of 5–17 MPa, which was equivalent to or greater than that of conventional coke (5–6 MPa), from TC-coal mixtures over the entire ranges of TC mass fraction in briquette of 0–100%, torrefaction temperature of 250–300 °C, and choice of coal (sub-bituminous or medium-volatile bituminous coal). The mixing of TC and coal hindered densification of coke due to hindrance of shrinkage of more-shrinkable TC-derived particles during the carbonization under many of the conditions. Nevertheless, positive synergy occurred in the coke strength at TC mass fractions of over 50%, where coal-derived particles were dispersed in the matrix of TC-derived particles, bonded to them during the carbonization, and behaved as a reinforcement of the matrix. The bonding between TC-derived and coal-derived primary particles was revealed by scanning electron microscopy. Copulverization of mixed TC and coal to sizes <40 μm before the briquetting gave cokes having strengths as high as 23–28 MPa. The fine pulverization increased the frequencies of mutual bonding of TC-derived particles and coal-derived particles and bonding between TC-derived and coal-derived particles per coke volume. The strength of coke from the TC-coal mixture generally followed volume-based additivity of strengths of cokes from TC and coal. This was realized by mixing primary particles of TC and coal within ≈10 μm scale or even smaller.