Energy & Fuels 2004, 18, 405-409
405
Temperature-Time SievesA Case of Nitrogen in Coal Krzysztof Stan´czyk* Central Mining Institute, Pl. Gwarko´ w 1, 40-166 Katowice, Poland Received June 6, 2003. Revised Manuscript Received November 22, 2003
The purpose of this research was to advance the fundamental understanding of the chemistry of nitrogen in coals, in relation to nitrogen oxide formation in coal combustion. The structures of nitrogen in coal of different rank and model chars, and their transformation in pyrolysis, were investigated, as well as their influence on nitrogen oxide emissions in combustion. The method of transformation of different coal nitrogen structures during pyrolysis and combustion was explained, as well as the role of nitrogen primary structures in nitrogen oxide emission. The correlation of behavior of carbon and nitrogen structures was explained based on temperaturetime sieve phenomenon, which may help in better design of burners and boilers for low-emission technologies of coal combustion.
Introduction Coal combustion, which is still the main source of electricity in the world, causes the emission of harmful gases into the environment, and one of the main pollutants of that process are nitrogen oxides. Nitrogen oxides affect the environment through acid rain and ozone layer depletion, and they also contribute to the greenhouse effect. The limitation of that negative influence of coal combustion on the environment may be achieved by obtaining a better understanding of the relationship between the nitrogen structures present in a coal, their transformation at high temperature, and nitrogen oxide emissions in combustion. The results of research of nitrogen structures in coal that was performed using different techniques such as X-ray photoelectron spectroscopy (XPS),1-3 X-ray adsorption near edge structure (XANES),4 Fourier transform infrared spectroscopy (FT-IR),5,6 and nuclear magnetic resonance (NMR)7,8 revealed that, in coal, three main structures of nitrogen are distinguished: pyrrolic, pyridinic, and quaternary structures.9,10 However, in * Author to whom correspondence should be addressed. E-mail:
[email protected]. (1) Perry, D. L.; Grint, A. Application of XPS to Coal Characterisation. Fuel 1983, 62, 1024-1033. (2) Buckley, A. N. Nitrogen Functionality in Coals and Coal-Tar Pitch Determined by X-ray Photoelectron Spectroscopy. Fuel Process. Technol. 1994, 38, 165-179. (3) Jansen, R. J. K.; van Bekkum, H. XPS of Nitrogen-Containing Functional Groups on Activated Carbon. Carbon 1995, 33, 1021-1027. (4) Mitra Kirtley, S.; Mullins, O. C.; Branthaver, J. F.; Cramer, S. P. Nitrogen Chemistry of Kerogens and Bitumens from X-ray Absorption Near-Edge Structure Spectroscopy. Energy Fuels 1993, 7, 11281134. (5) Wallace, S.; Bartle, K. D.; Perry, D. L. Quantification of Nitrogen Functional Groups in Coal and Coal Derived Products. Fuel 1989, 68, 1450-1455. (6) Cagniant, D.; Gruber, R.; Boudou, J. P.; Bilem, C.; Bimer, J.; Salbut, P. D. Structural Characterization of Nitrogen-Enriched Coals. Energy Fuels 1998, 12, 672-681. (7) Knicker, H.; Hatcher, P. G.; Scaroni, A. W. Solid-State 15N NMR Spectroscopy of Coal. Energy Fuels 1995, 9, 999-1002. (8) Solum, M. S.; Pugmire, M. J.; Grant, D. M.; Kelemen, S. R.; Gorbaty, M. L.; Wind, R. A. 15N CPMAS NMR of the Argonne Premium Coals. Energy Fuels 1997, 11, 491.
Table 1. Structures of Nitrogen in Coal and Carbonaceous Materialsa structure of nitrogen
symbol
binding energy (eV)
pyridinic amino nitrile pyrrolic pyridonic pyrrolidonic quaternary N-oxide
N-6 N-H N-C N-5 N-6(O) N-5(O) N-Q N-O
398.5 ( 0.4 399.3 ( 0.4 399.1-400.1 400.5 ( 0.4 400.5 399.6 ( 0.2 401.1 ( 0.3 402.5-403.7
a
From refs 1-3 and 9-18.
coal and coal chars, other nitrogen structures also may be present. Data regarding the structures of nitrogen in coal and carbonaceous materials, as distinguished by XPS,1-3,9-18 are presented in Table 1 . In the pyrrolic structure N-5, the sp3-hybridized N atom is connected to one H atom and two C sp2 atoms. (9) Kelemen, S. R.; Gorbaty, M. L.; Kwiatek, P. J.; Fletcher, T. H.; Watt, M.; Solum, M. S.; Pugmire, R. J. Nitrogen Transformation in Coal during Pyrolysis. Energy Fuels 1998, 12, 159-173. (10) Kelemen, S. R.; Freund, H.; Gorbaty, M. L.; Kwiatek, P. J. Thermal Chemistry of Nitrogen in Kerogen and Low-Rank Coals. Energy Fuels 1999, 13, 529-538. (11) Stan´czyk, K.; Dziembaj, R.; Piwowarska, Z.; Witkowski, S. Transformation of Nitrogen Structures in Carbonization of Model Compounds Determined by XPS. Carbon 1995, 33, 1383-1392. (12) Stan´czyk, K. Nitrogen Oxide Evolution from Nitrogen Containing Model Chars Combustion. Energy Fuels 1999, 13, 82-87. (13) Piwowarska, Z.; Stan´czyk, K.; Dziembaj, R. XPS Evidences for Changes in the Nitrogen Forms in Results of Hydropyrolysis of Model Chars. In Proceedings of International Conference on Coal Science (Oviedo, Spain, 1995); Pajares, J. A., Tascon, J. M. D., Eds.; Elsevier: Amsterdam, 1995; pp 1693-1696. (14) Kelemen, S. R.; Gorbaty, M. L.; Kwiatek, P. J. Quantification of Nitrogen Forms in Argonne Premium Coals. Energy Fuels 1994, 8, 896. (15) Pels, J. R.; Kapteijn, F.; Moulijn, J. A.; Zhu, Q.; Thomas, K. M. Evolution of Nitrogen Functionalities in Carbonaceous Materials during Pyrolysis. Carbon 1995, 33, 1641. (16) Kambara, S.; Takarada, T.; Toyoshima, M.; Kato, K. Relation between Functional Forms of Coal Nitrogen and NOx Emission from Pulverized Coal Combustion. Fuel 1995, 74, 1247. (17) Stohr, B.; Boehm, H. P.; Schlogl, R. Carbon 1991, 29, 707. (18) Wojtowicz, M. A.; Pels, J. R.; Moulijn, J. A. The Fate of Nitrogen Functionalities in Coal during Pyrolysis and Combustion. Fuel 1995, 74, 507.
10.1021/ef034018h CCC: $27.50 © 2004 American Chemical Society Published on Web 01/22/2004
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Energy & Fuels, Vol. 18, No. 2, 2004
Stan´ czyk
Table 2. Elemental and Proximate Analyses of Coals wt % sample number
name
C
N
wt % (daf) H
1 2 3 4 5 6
1 Maja Szczyglowice Debiensko Bolesław Smialy Komuna Paryska Turow
91.9 85.7 82.0 82.9 77.0 69.7
2.0 2.1 2.0 1.8 1.8 1.2
4.8 5.1 5.0 5.5 4.6 5.1
In the pyridinic structure N-6, the N atom is connected to two sp2-hybridized C atoms. In the quaternary structure N-Q, the N atom is situated in a graphen layer and connected to three sp2-hybridized C atoms. During pyrolysis, which is the first step of coal combustion, nitrogen structures transform to morestable structures. In combustion, they are oxidized and the oxidation rate, as well as oxidation degree, is dependent on the temperature-time history of the nitrogen structure and also on the stoichiometry in the burner and the boiler. Experimental Section Samples. The model chars were prepared from the following precursors: poly(vinylpyrrolidone), acridine, 9-aminoacridine, 9-cyanoanthracene, 2-aminoanthracene, and carbazole. The coal chars were prepared from six coals of different rank. Elemental and proximate analyses of the coals are presented in Table 2. Sample Preparation. Carbonization of the nitrogen compounds and coals was performed in a fixed-bed reactor in an argon flow. The apparatus is described elsewhere.19 The nitrogen compounds were heated to 300 °C at a rate of 15 K min-1 and then to 460 °C at a rate of 5 K min-1 under 5 MPa of argon and held at 460 °C for 3 h. The chars were subjected to further heating at a heating rate of 5 K min-1 under 5 MPa of argon at 600 °C for 15 min or heated to 800 °C and soaked for 15 min. Coals samples were heated to 500, 600, 700, or 800 °C, using a heating rate of 15 K min-1 under 5 MPa of argon, and were soaked for 15 min. Thermogravimetry-Mass Spectroscopy (TG-MS) Examination. The gasification experiments have been performed on a simultaneous thermogravimetry-mass spectrometry system (TG-MS), which consists of a model PL STA 500 thermobalance (Stanton-Redcraft) and a model VG Quadrupoles Micromass PC 300 D mass spectrometer (VG Instruments). The gasification experiments were conducted under isothermal conditions at a temperature of 550 °C in an oxygen/argon mixture (20% O2, 80% Ar). The gas flow rate was 50 mL min-1, and the heating rate to the final temperature was 50 °C min-1. The weight of the sample was 20 mg, and the particle diameter was 38-75 µm. The sample was heated in pure argon to the reaction temperature, and the weight was allowed to stabilize. The gas was then switched to 20% oxygen/argon. The reaction time ended when the chars were totally burned. The total pressure and the partial pressure of the evolved gases were monitored. The mass spectrometer was used as a specific detector working in multiple ion monitoring (MIM) mode to detect selected ions. The NO measurements were integrated over the 100% burnout. XPS Examination. XPS analyses of the chars were performed in a VSW model ESCA 100 spectrometer, using Mg KR radiation. The spectra were obtained in an analyzer at an (19) Wiatowski, M.; Fabis, G. Erdoel Kohle, Erdgas, Petrochem. 1993, 46, 74.
S
O
moisture
volatile matter
ash
0.5 1.8 1.9 1.0 1.2 0.6
0.8 5.3 9.1 8.8 15.4 23.4
1.0 2.0 2.1 5.8 15.1 7.5
24.0 33.0 40.0 38.3 38.3 67.5
5.8 7.4 20.3 9.2 7.3 9.4
energy of 44 eV in a fixed analyzer transmission mode. The operating pressure was
