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Energy & Fuels 1998, 12, 1356-1362
Char-Nitrogen Functionality and Interactions between the Nitrogen and Iron in the Iron-Catalyzed Conversion Process of Coal Nitrogen to N2 Yasuo Ohtsuka,* Takashi Watanabe, Kenji Asami, and Hiroshi Mori Research Center for Organic Resources and Materials Chemistry, Institute for Chemical Reaction Science, Tohoku University, Sendai 980-8577, Japan Received May 6, 1998
Iron catalysts precipitated on brown and bituminous coals promote conversion reactions of coal nitrogen to N2 during pyrolysis at 900 °C, and the N2 evolved from the brown coal arises mainly from char nitrogen (char-N). The present study, thus, focuses on clarifying the effect of the iron on char-N functionality and elucidating interactions between the two mainly with X-ray photoelectron spectroscopy (XPS). The N 1s spectra of brown coal chars revealed that the catalyst lowered the ratio of pyrrolic-N/pyridinic-N irrespective of the depth of the char, indicating the preferential formation of N2 from pyrrolic-N. In the Fe 2p and N 1s XPS spectra of the Febearing chars, Ar sputtering removed surface iron oxides to expose the metallic phase and concurrently increased the proportion of pyridinic-N with the corresponding decrease in quaternary-N. It is suggested that there are strong interactions between the iron and char-N even in the process of N2 formation. Iron particles were more finely dispersed on the brown coal char and in closer contact with it, which accounts for higher conversion of char-N to N2. A schematic mechanism for this conversion process is discussed in terms of solid-solid reactions of metallic iron and char-N.
Introduction In previous publications, we have shown that fine iron particles, which are derived from not only externallyadded iron catalysts1-3 but inherently-present Fecontaining minerals,4-6 can remarkably promote N2 formation from the nitrogen present in coal (coal-N) during atmospheric-pressure pyrolysis and that conversion of coal-N to N2 at 900-1000 °C reaches 50-60%. Efficient conversion to N2 during coal pyrolysis can be expected to reduce the emissions of fuel NOx and N2O during subsequent combustion, since such pollutants originate mostly and exclusively from coal-N in pulverized coal-fired plants and fluidized bed combustion, respectively.7-10 It has been suggested that N2 formation proceeds predominantly through solid-phase reactions in which iron nanoparticles catalyze the conversion * Author to whom correspondence should be addressed (e-mail:
[email protected]). (1) Ohtsuka, Y.; Mori, H.; Nonaka, K.; Watanabe, T.; Asami, K. Energy Fuels 1993, 7, 1095-1096. (2) Ohtsuka, Y.; Mori, H.; Watanabe, T.; Asami, K. Fuel 1994, 73, 1093-1097. (3) Mori, H.; Asami, K.; Ohtsuka, Y. Energy Fuels 1996, 10, 10221027. (4) Wu, Z.; Ohtsuka, Y. Energy Fuels 1996, 10, 1280-1281. (5) Wu, Z.; Ohtsuka, Y. Energy Fuels 1997, 11, 477-482. (6) Wu, Z.; Ohtsuka, Y. Energy Fuels 1997, 11, 902-908. (7) Hjalmarsson, A.-K. In NOx Control Technologies for Coal Combustion; IEACR/24; IEA Coal Research: London, 1990. (8) Boardman, R.; Smoot, L. D. In Fundamentals of Coal Combustion for Clean and Efficient Use; Smoot, L. D., Ed.; Coal Science and Technology 20; Elsevier: Amsterdam, 1993; pp 433-509. (9) Wo˘ jtowicz, M. A.; Pels, J. R.; Moulijn, J. A. Fuel Process. Technol. 1993, 34, 1-71. (10) Takashita, M.; Sloss, L. L.; Smith, I. M. In N2O Emissions from Coal Use; IEAPER/06; IEA Coal Research: London, 1993.
Table 1. Ultimate and Proximate Analyses of Coal Samples Used ultimate analyses, wt % (daf) coal
code
Loy Yang LY Blair Athol BA
C
H
N
S
proximate analysis, wt % (db) O
65.9 4.7 0.60 0.3 28.5 78.6 4.4 2.0 0.3 14.7
ash
VM
FC
0.5 7.5
51.0 28.7
48.5 63.8
of the nitrogen in char (char-N) and/or its precursors to N2.3,6 The main objective of the present work is, thus, to determine char-N functionality and elucidate interactions between char-N and iron catalyst with X-ray photoelectron spectroscopy (XPS) and transmission electron microscope (TEM). It has been accepted that XPS is one of the most powerful techniques to examine nitrogen functionalities of coal and char.11-14 Experimental Section Coal Sample and Iron Addition. Australian Loy Yang brown coal and Blair Athol bituminous coal, denoted as LY and BA, respectively, were used in this study. After drying in laboratory air at room temperature, these samples were ground and sieved to 150-250 µm. The ultimate and proximate analyses are shown in Table 1. Unless otherwise stated, (11) Nelson, P. F.; Buckley, A. N.; Kelly, M. D. 24th Symposium (International) on Combustion; The Combustion Institute: Pittsburgh, PA, 1992; pp 1259-1267. (12) Kelemen, S. R.; Gorbaty, M. L.; Kwiatek, P. J. Energy Fuels 1994, 8, 896-906. (13) Davidson, R. M. In Nitrogen in Coal; IEAPER/08; IEA Coal Research: London, 1994. (14) Wo˘ jtowicz, M. A.; Pels, J. R.; Moulijn, J. A. Fuel 1995, 74, 507516.
10.1021/ef980100e CCC: $15.00 © 1998 American Chemical Society Published on Web 10/09/1998
Char Nitrogen Functionality and Interactions
Energy & Fuels, Vol. 12, No. 6, 1998 1357
Table 2. Pyrolysis Results at 900 °C conversion of coal-N (%)
atomic ratio in char
coal
iron loading (wt %)
weight loss (wt %, daf)
CO2 yield (wt %, daf)
N2
NH3
HCN
oil-N
tar-N
char-N
total
H/C × 10
N/C × 10
LY LY LY BA BA
0 0.73 2.8 0 1.2
44.1 44.2 45.8 67.9 68.2
11.1 11.9 13.8 2.3 2.6
3.9 49 53 6.9 21
16 10 13 18 12
5.8 2.1 1.3 5.3 3.2
22 10 14 12 8.2
7.6 5.4 3.6 3.5 3.2
52 25 17 63 54
107 102 102 109 101
0.87 0.16 0.13 1.0 0.96
0.074 0.036 0.022 0.17 0.15
LY coal was used because iron catalyst showed a larger effect on N2 formation from LY coal than BA coal.3 An aqueous solution of FeCl3 was used as a catalyst precursor, since it is readily available as the main component of acid wastes from iron and steel pickling plants. A predetermined amount of Ca(OH)2 powder was added to a stirred mixture of coal particles and FeCl3 solution so that fine particles of Cl-free iron could be precipitated onto coal.15,16 The procedure has been described in detail elsewhere.16 After the Fe-loaded coal was separated by filtration and then washed with deionized water, it was dried in an inert gas at 110 °C. Iron loading in the dry sample, expressed in weight percent of iron metal, was determined by atomic absorption spectroscopy after extraction of the metal from the sample with hot HCl. The Fe-loaded samples, LY coal with 0.73 and 2.8 wt% Fe and BA coal with 1.2 wt % Fe, are denoted as 0.7% Fe/LY, 2.8% Fe/LY, and 1.2% Fe/BA, respectively. Pyrolysis and Nitrogen Analysis. Pyrolysis runs were carried out with a fluidized-bed quartz reactor, which was heated electrically with a transparent furnace. About 5 g of the sample was heated in flowing high-purity He at 600-700 K/min to 900 °C, soaked for 10 min, and quenched to room temperature. No fluidizing agents were used to avoid their possible influence on the fate of coal-N upon pyrolysis. The details of the apparatus and procedure have been described elsewhere.2,3 Pyrolysis products were separated into gas, an oil comprising liquid hydrocarbons and water, tar in solid-like forms, and char according to the method described previously.1,2 N2 in the gas was determined with a gas chromatograph. For analysis of HCN and NH3, another run was performed separately in the same manner as above and the compounds evolved were dissolved into deionized water, and the CN- and NH4+ ions were determined with a specific ion electrode.3 The total nitrogen in oil, tar, or char, denoted as oil-N, tar-N or char-N, respectively, was determined with a conventional elemental analyzer.3 XPS Spectra. The spectra were obtained with a Shimadzu (Kyoto, Japan) ESCA-750 using a Mg KR X-ray source operating at 240 W. Char samples after pyrolysis at 900 °C were made into fine powders just before each XPS analysis and mounted onto a sample holder with Ag paste. The chars derived from LY coal without and with 2.8 wt % Fe were used unless otherwise stated. Since nitrogen contents in the Febearing chars were as low as
