Article pubs.acs.org/EF
Influence of Inherent Oxygen Species on the Fluidity of Coal during Carbonization Naoto Tsubouchi,*,† Yuuki Mochizuki,† Ryo Naganuma,† Kyohei Kamiya,† Megumi Nishio,† Yohei Ono,‡ and Kazuya Uebo§ †
Center for Advanced Research of Energy and Materials, Hokkaido University, Sapporo 060-8628, Japan Sakaide Plant, Mitsubishi Chemical Corporation, Kagawa 762-8510, Japan § Technical Research & Development Bureau, Nippon Steel & Sumitomo Metal Corporation, Chiba 293-8511, Japan ‡
ABSTRACT: 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.
1. INTRODUCTION Coal contains not only carbon and hydrogen but also heteroelements, such as oxygen, sulfur, nitrogen, chlorine, and fluorine. As is well-known, oxygen is the most abundant heteroelement in coal, and its concentration usually ranges from 5 to 30 mass % on a dry, ash-free (daf) basis. In addition, it has been widely accepted that this oxygen affects the efficiency of coal utilization during combustion, gasification, and liquefaction. The physicochemical states of oxygencontaining functional groups present in coal have thus been examined extensively with various methods, such as quantitative chemical analysis,1−4 X-ray photoelectron spectroscopy (XPS),5 and 13C nuclear magnetic resonance (NMR) spectroscopy.6 Moreover, the fates of the functional groups during coal conversion processes have also been studied with various techniques, for example, thermogravimetry/Fourier transform infrared (TG/FT-IR) spectroscopy7,8 and pyrolysis/triplequadrupole mass spectrometry.9 According to an important review on the chemical structures and thermoplastic properties of coals,10 it is widely accepted that the extent of thermoplasticity of coal particles during carbonization is one of the key factors determining the strength and reactivity of the coke formed. When an American caking coal (83 mass % daf) was oxidized with O2 gas at 100 ± 3 °C for 5−40 h, the Gieseler fluidity decreased almost linearly with increasing oxidation time, and the maximum fluidity value © XXXX American Chemical Society
decreased from 2600 to
