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Characteristic Properties of Lignite to be Converted to High-Strength Coke by Hot-Briquetting and Carbonization Shinji Kudo, Aska Mori, Gentaro Hayashi, Takuya Yoshida, Noriyuki Okuyama, Koyo Norinaga, and Jun-ichiro Hayashi Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b03155 • Publication Date (Web): 15 Nov 2017 Downloaded from http://pubs.acs.org on November 20, 2017
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Energy & Fuels
Characteristic Properties of Lignite to be Converted to High-Strength Coke by Hot-Briquetting and Carbonization
Shinji Kudo†,*, Aska Mori†, Gentaro Hayashi†, Takuya Yoshida‡, Noriyuki Okuyama‡, Koyo Norinaga§, Jun-ichiro Hayashi†,ǁ
†
Institute for Materials Chemistry and Engineering, Kyushu University, 6-1 Kasuga Koen, Kasuga,
816-8580, Japan ‡
KOBE STEEL, Ltd. 2-3-1, Shinhama, Arima-cho, Takasago, Hyogo, 676-8670, Japan
§
Department of Chemical Systems Engineering, Graduate School of Engineering, Nagoya University,
Furo-cho, Chikusa-ku, Nagoya, 464-8603, Japan ǁ
Research and Education Center of Carbon Resources, Kyushu University, 6-1 Kasuga Koen,
Kasuga, 816-8580, Japan
* Corresponding author. E-mail:
[email protected]; Tel/Fax: +81 92 583 7793
ABSTRACT A sequence of hot-briquetting and carbonization (HBC) is a promising technology for the production of coke with a high mechanical strength from lignite, but factors affecting the coke strength have not yet been fully understood. The HBC cokes prepared from twelve lignites in this study showed diverse tensile strength (e.g., from 0.2 to 31.2 MPa in the preparation at 200 °C and 112 MPa for hot-briquetting and 1000 °C for carbonization), and the coke strengths could not be explained by differences in commonly used structural properties of the parent lignites, such as
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elemental composition and contents of volatile matter/fixed carbon and ash. In this study, two methods were proposed for correlating the coke strength with the lignite properties, which employed the chemical structure analyzed by solid-state
13
C NMR or the volumetric shrinkage during
carbonization. A stronger coke was obtained from lignite that contained more aliphatic carbons (less aromatic carbons) or shrank more considerably. These characteristics contributed to intensified compaction of lignite in the briquetting and suppression of the formation of large pores, which are a cause of coke fracture. Two empirical equations, predicting the coke strength from the parameters of lignite properties, were established to be criteria for selection of lignite as HBC coke feedstock, although further investigation with more experimental data would be necessary for the validation.
1. INTRODUCTION Lignite, a typical low-rank coal, is a potential feedstock for coke used in metallurgical industries and, especially, in the blast furnace. One of the most important properties required for the coke is mechanical strength. However, the absence of caking property in lignite renders it unsuitable as the coke feedstock, in spite of the huge reserves (proven global reserves are estimated at 280 gigatonnes1). Formed coke technology2 is an option for the use of lignite. Briquetting of coal with or without binders, followed by carbonization, can produce coke having a high mechanical strength, even from low-rank coals.3–11 Among the related studies, we have recently demonstrated binderless preparation of high-strength coke from low-rank carbonaceous materials, including lignite.5–8 A feature of the preparation method is briquetting at temperatures around 200 °C, where lignite hardly undergoes pyrolytic degradation, while its macromolecular structure is relaxed, resulting in the formation of a high-density briquette. The sequence of hot-briquetting and carbonization (HBC) produced a coke having a high tensile strength of over 30 MPa (obtained from a single Victorian lignite), which was far higher than that of conventional coke used in the iron-making industry (13 MPa), medium (13–5 MPa) or low (
