The evolution of gaseous oxygen-containing species (CO, CO2, and H2O) during carbonization of 10 types of caking coals has been investigated mainly using a fixed-bed quartz reactor to reveal the influence of inherent oxygen species on the Gieseler fluidity of the coal particles. The heating rate and temperature were 3 °C/min and 1000 °C, respectively. CO evolution apparently started after 350 °C, and the rate profile for CO evolved showed the main or shoulder peak at about 650 °C in many cases. On the other hand, CO2 started to evolve at low temperatures of 200–250 °C for almost all of the coals, and the profile for the rate of CO2 evolution exhibited a main peak at 400–450 °C and a shoulder or small peak at about 600 °C in all cases. H2O formation occurred significantly between 400 and 800 °C, irrespective of coal type. The Gieseler fluidity analyses also revealed that the initial softening, maximum fluidity (MF), and resolidification temperatures of the 10 coals were in the ranges of 375–435, 435–480, and 450–505 °C, respectively, and the MF values were 0.78–4.1 log­(ddpm). Interestingly, the MF values tended to decrease with increasing total amount of CO, CO2, or H2O up to the initial softening temperature mentioned above. Further, the addition of oxygen-containing compounds [phthalide (C8H6O2), 2-naphthoic acid (C11H8O2), and fluorescein (C20H12O5)] to an Australian caking coal decreased the MF value considerably; specifically, the value decreased from 2.2 log­(ddpm) originally to 0.28–1.5 log­(ddpm), and the degree of decrease was greatest with 2-naphthoic acid containing COOH. In contrast, benzofuran (C8H6O) mixed with the coal did not affect the MF value significantly. These observations indicate that some of the oxygen-containing functional groups naturally present in coal have a negative effect on coal fluidity and suggest that this effect is particularly strong for carboxyl and/or acid anhydride groups, which can be readily converted to gaseous oxygen-containing species during heating to the initial softening temperature.

In order to produce high-strength coke from low-quality coals, noncovalent bonds between O-functional groups in coal were cleaved by pyridine containing HPC pyridine soluble and HPC-derived thermoplastic components were introduced into the pores formed by swelling; thus, the synergistic effect during carbonization of the suppression of cross-linking reactions and the fluidity amplification due to close placement of coal and thermoplastic components was investigated. When HPC was extracted with pyridine, a decrease in O-functional groups was observed in the pyridine-soluble and pyridine-insoluble components. When HPC was extracted with MeOH, on the other hand, O-functional groups in HPC selectively moved into the soluble components. When non-or slightly-caking coal was chemically-modified with the prepared HPC pyridine-soluble components by utilizing the solvent-swelling effect of pyridine, the fluidity improved compared with the coals physically mixed with the soluble components or HPC. On the other hand, the fluidity of the chemically-modified sample with the MeOH-soluble components hardly changed from that of the original sample, and no effect of the modification with the thermoplastic component was observed. Furthermore, it was clarified that higher-strength coke can be produced from the chemically-modified sample with the HPC pyridine-soluble components than from the original coal or the physically mixed coal with the soluble components. The contraction behavior during carbonization of the chemically-modified sample with the soluble components and that of the original coal was investigated; as a result, a large difference was not observed between these two. Thus, it was found that high-strength coke can be produced from low-quality coals by the present method.

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The effect of various oxygen-containing compounds added and/or inherent O-species on coal fluidity and coke strength has been investigated in detail. When several O-containing compounds, which have different O-containing groups, are added independently to caking coal, the MF value almost decreases, and the extent of the decrease being ether < ketone < lactone < hydroxyl < acid anhydride < < ether/hydroxyl/lactone < carboxyl group. The COOH content in four coals used increases with decreasing C%, and the MF values decrease with increasing the content. The evolution of gaseous O-containing species (CO, CO2, and H2O) during carbonization at 3 °C/min of four coals up to 400 °C has been studied mainly with a flow-type quartz-made fixed-bed reactor to clarify the effect of the amount of O-containing gases evolved with the Gieseler fluidity of coal particles. A positive correlation is found between the amount of CO, CO2, or H2O evolved up to 400 °C and the COOH content in coal. However, a negative correlation between MF and O-containing gases evolved up to 400 °C is observed. It is suggested that the COOH amount and/or O-containing gases evolved have adverse effects on the thermoplasticity of coal. When the indirect tensile strength of coke prepared from pelletized samples is plotted against MF values, a positive correlation is found, whereas an inverse correlation is observed between the indirect tensile strength and COOH in coals used or the O-containing gases evolved up to 400 °C during carbonization. These observations indicate that some of the oxygen-functional groups naturally present in coal have a negative effect on coal fluidity and that this effect is particularly strong in carboxyl, which can readily be decomposed into gaseous oxygen-containing species during heating up to the initial softening temperature.

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