Article pubs.acs.org/EF
Fate of Coal-Bound Nitrogen during Carbonization of Caking Coals Naoto Tsubouchi,*,† Yuuki Mochizuki,† Yohei Ono,‡ Kazuya Uebo,§ Naoto Sakimoto,∥ and Toshimasa Takanohashi∥ †
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 and Development Bureau, Nippon Steel and Sumitomo Metal Corporation, Chiba 293-8511, Japan ∥ Energy Technology Research Institute, National Institute of Advanced Industrial Science and Technology, Tsukuba 305-8569, Japan ‡
S Supporting Information *
ABSTRACT: Seven kinds of caking coals with carbon contents of 80−88 wt % dry and ash-free basis (daf) have been carbonized in high-purity He at 3 °C/min up to 1000 °C with a fixed-bed quartz reactor, and the fate of coal-bound nitrogen (coal N) has been investigated in detail. The nitrogen mass balances fall within 97−104%. Most coal N is retained as quaternary N in the cokes, and the rest is released as volatile N (tar N, HCN, NH3, and N2). NH3 is the main N species evolved below 650 °C, irrespective of the kind of coal, and the profile for the rate of NH3 formation shows the main peak at about 450 °C, followed by a small peak at around 670 °C in every case. Significant amounts of HCN are also observed below 650 °C, and the rate profiles for HCN as well as NH3 exhibit two peaks at around 450 and 670 °C, whereas most N2 is formed at the temperature range of 650−1000 °C. The distribution of volatile N at 1000 °C is in the order of tar N < HCN < NH3 ≈ N2 for almost all coals. Each coal used gives a Gieseler maximum fluidity (MF) of 1.1−4.0 log(ddpm) at around 450 °C (440−480 °C), and it seems that the MF value tends to increase when the total amount of either HCN or NH3 evolved up to 450 °C increases. on coal fluidity. Therefore, it is of interest to examine the fate of coal-bound nitrogen (denoted as coal N) during carbonization. In this paper, we first examine the chemical forms of nitrogen present in several caking coals by use of the X-ray photoelectron spectroscopy (XPS) method, then clarify the behavior of the release and retention of coal N in the carbonization process, and finally evaluate the relationship between the nitrogen distribution and the Gieseler MF value.
1. INTRODUCTION As is well-known, caking coal for metallurgical coke production softens around 400 °C during carbonization to give a coherent porous plastic mass, which subsequently resolidifies at about 500 °C to be transformed into coke.1,2 Because the quantity and quality of the plastic mass are recognized to be very important factors that determine the physical and chemical properties of the coke that is subsequently formed, the thermoplastic behavior of coal particles upon carbonization has been studied extensively with various analysis techniques,3−14 such as the Gieseler fluidity measurement, the Ruhr and Audibert-Amu dilatation analyses, and the dynamic viscoelastic measurement. Thus far, two major theories for the appearance and development of the plastic mass have been proposed:15−19 (1) the γ-component model and (2) the metaplast model. On the basis of these theories, it is believed that chloroform/pyridine solubles (γ components) and volatile low-molecular-weight constituents (metaplasts) naturally present in coal may control some thermoplastic properties (e.g., fluidity and dilation) of coal particles upon carbonization. According to previous studies, the importance of hydrogentransfer reactions from low-molecular-mass species, including the γ components and metaplasts, to the plastic mass has also been discussed in view of coal thermoplasticity,5,13,20−22 and it has been reported that the amount of transferable hydrogen can correlate well with the Gieseler maximum fluidity (MF).5 Furthermore, it has recently been reported that caking coal with a higher content of nitrogen or sulfur tends to give a larger Gieseler MF value23 and that the addition of denzo-[c]-acridine (C17H11N) to a coal blend, even at a low amount of 3 wt %, increases the tensile strength of the coke formed.24 These observations may suggest that nitrogen has an important effect © 2013 American Chemical Society
2. EXPERIMENTAL SECTION 2.1. Coal Sample. Seven kinds of caking coals (LL, GY, NW, GA, TR, SI, and GC) for metallurgical coke production were supplied from the Iron and Steel Institute of Japan (ISIJ) and used in the present work. All of them were first ground with a mortar, then sieved to obtain coal particles of the size fraction of 99.99995%) was flowed at 200 cm3 [standard temperature and pressure (STP)]/min into the whole system, including the reactor and tar traps, and the effluent was Received: August 29, 2013 Revised: November 11, 2013 Published: November 18, 2013 7330
dx.doi.org/10.1021/ef401735w | Energy Fuels 2013, 27, 7330−7335
Energy & Fuels
Article
Table 1. Ultimate and Proximate Analyses of Coals Used ultimate analysis (wt % daf)
a
proximate analysis (wt % dry)
coal
countrya
C
H
N
S
Ob
ash
VMc
FCb,c
LL GY NW GA TR SI GC
AUS AUS NZL AUS USA AUS AUS
79.9 82.8 83.5 85.6 86.4 87.3 87.5
5.7 5.6 5.3 5.3 5.2 4.9 4.9
2.1 2.3 1.2 2.0 1.6 2.0 2.1
0.57 0.70 1.8 0.62 1.4 0.75 0.55
11.7 8.6 8.2 6.5 5.4 5.0 4.9
9.2 7.6 3.1 8.5 8.5 10.2 9.4
35.2 31.8 30.8 25.4 23.8 18.4 19.0
55.6 60.6 66.1 66.1 67.7 71.4 71.6
AUS, Australia; NZL, New Zealand; USA, United States of America. bEstimated by difference. cVM, volatile matter; FC, fixed carbon.
analyzed with a high-speed micro-gas chromatograph (GC) (Agilent Technologies) to ensure that all of the air was replaced completely with He. Such prudent precautions against leakage are needed for precise determination of N2 formed from coal N. Finally, the reactor with He was heated electrically to a predetermined temperature (350− 1000 °C) and then quenched to room temperature. The heating rate was 3 °C/min, unless otherwise stated. The tarry materials and water condensed in the tar traps during carbonization were recovered by solvent extraction using n-butyl alcohol, and this fraction is denoted as tar throughout the present paper. Carbonaceous materials remaining in the cell were recovered as solid products (e.g., cokes and semicokes). 2.3. Nitrogen Analysis. N2 evolved during carbonization was analyzed in situ at intervals of 4 min by the GC, whereas HCN and NH3 were measured at 3 min intervals with a multi-gas monitor (INNOVA) employing the photoacoustic infrared detection method. The N in the tar (tar N) was determined with a total nitrogen analyzer (Mitsubishi Kasei) equipped with a chemical luminescence cell. The N in the solid products (residual N) was also measured with a conventional, combustion-type elemental analyzer (LECO). The yields of N2, HCN, NH3, tar N, and residual N are expressed as percentage of the total nitrogen in the feed coal. When the carbonization run was repeated for a given sample, the reproducibility was within ±2% for N2, ±4% for HCN or NH3, ±5% for tar N, and ±2% for residual N. 2.4. Fluidity Measurement. The fluidity of each coal was measured with a constant-torque Gieseler plastometer (Yoshida Seisakusho) employing the Japanese Industrial Standard (JIS) M 8801 method. In the experiments, about 4.5 g of coal particles with the size fraction of
