Energy & Fuels 2000, 14, 943-944
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Communications Detection of HNCO during the Low-Temperature Combustion of Coal Chars Patricia M. Nicholls and Peter F. Nelson* CRC for Black Coal Utilization & CSIRO Energy Technology, PO Box 136, North Ryde 1670, Australia Received May 3, 1999 Emissions of nitrogen oxides (NOx) from combustion sources are significant contributors to a number of detrimental atmospheric environmental effects, including acidic deposition processes and production of photochemical smog.1 Combustion of coal, both in pulverized-fuel fired furnaces and, increasingly, in fluidized-bed combustors, is an important anthropogenic source of NOx,1 and in the latter case of nitrous oxide (N2O), which has a large greenhouse warming potential. The predominant source of these emissions is the nitrogen bound in the organic structures present in all coals.2 A significant proportion of coal-bound nitrogen is released during the initial stages (devolatilisation) of combustion as simple gas-phase species (HCN, NH3) and in more complex organics, which are categorized as tar molecules.3-6 Oxidation of these nitrogen-containing species results in the conversion of some coal-bound N to NO and some to N2. Proportions of NO and N2 formed are a function of the overall stoichiometry, and the chemistry of this volatile-N is relatively well understood, based on extant models of fuel-N chemistry.7 However, not all coal-N is volatile, and, depending on the coal, varying amounts are retained in the char matrix, and only released during burnout of the char. The fate of this char-N is much more uncertain. This is although, under staged combustion conditions such as those found in low-NOx burners, it is probable that it is the major source of NOx. Molecular nitrogen (N2), NO, and N2O are generally agreed to be the most significant products of char combustion. In fact, for many reported experiments, only two of these products have been measured, and it is assumed that the third product (usually N2) completes the mass balance. * Corresponding author: CSIRO Energy Technology, PO Box 136, North Ryde 1670, Australia. Phone: 61-2-9887 8660. Fax: 61-2-9887 8909. E-mail:
[email protected]. (1) Seinfeld, J. H.; Pandis, S. N. Atmospheric Chemistry and Physics: From Air Pollution to Climate Change; Wiley-Interscience: New York, 1998, pp xxvii, 1326 pp. (2) Pershing, D. W.; Wendt, J. O. L. Proc. 16th Int. Symp. Combust. 1977, 389. (3) Freihaut, J. D.; Proscia, W. M.; Seery, D. J. In Proceedings 6th Annual Pittsburgh Coal Conference; 1989; vol. 2, pp 1184-1193. (4) Chen J. C.; Niksa, S. Energy Fuels 1992, 6, 254. (5) Nelson, P. F.; Buckley, A. N.; Kelly M. D. Proc. 24th Int. Symp. Combust. 1992, 1259. (6) Li, C.-Z.; Nelson, P. F.; Ledesma, E. B; Mackie, J. C. Proc. 26th Int. Symp. Combust. 1996, 3205. (7) Wendt, J. O. L. Combust. Sci. Technol. 1995 108, 323.
Figure 1. Concentrations of gas-phase nitrogen-containing products formed during the combustion of an Australian coal char at 600 °C, as a function of time after exposure to a 2% O2 in He gas mixture. (a) N2 and NO, (b) HNCO and HCN, and N2O.
Recently, in addition to these products, HCN has been reported as a significant product of char combustion,8-10 particularly at low temperatures10 or under conditions where product gases are directly sampled from a point close to the heated char sample.9 These results suggest that HCN is a primary product of char oxidation, since it persists throughout the burnout. Here we report the first observation of the formation of isocyanic acid (HNCO) from char combustion. In this work, the products of char combustion in a laboratory (8) Winter, F.; Wartha, C.; Lo¨ffler, G.; Hofbauer, H. Proc. 26th Int. Symp. Combust. 1996, 3325. (9) Jones, J. M.; Harding, A. W.; Brown, S. D.; Thomas, K. M. Carbon 1995, 33, 833. (10) Ashman, P. J.; Haynes, B. S.; Buckley, A. N.; Nelson, P. F. Proc. 24th Int. Symp. Combust. 1996, 3069.
10.1021/ef990079x CCC: $19.00 © 2000 American Chemical Society Published on Web 05/16/2000
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scale quartz fixed bed reactor at temperatures of 600 °C and 900 °C were determined. An Australian bituminous coal char was used (carbon, 96.1%; nitrogen, 2.02%; hydrogen, 1.39% (daf basis)). Weighed samples of char (approximately 60 mg) were loaded into the reactor, and heated to the reaction temperature in an atmosphere of He. Once the sample had equilibrated, the flow was switched to a 2% O2 in He mixture. Concentrations of product species were determined during the heatup and combustion by a variety of techniques: N2 was measured on-line using an MTI micro-gas chromatograph (M200D, column:molecular sieve, 5 Å:10 m) and NO using a chemiluminescent analyzer (Monitor Labs ML 9841A), and other products (N2O, HCN, and HNCO) were determined by gas-phase Fourier Transform Infrared (FTIR) spectroscopy using a 2.4 m multipass gas cell (Infrared Analysis, G-1-2.4-PA-BA-AG). All measurements were performed on the gas exiting the reaction system, after it had cooled to room temperature. At 600 °C, concentrations of NO and N2 accounted for approximately 75% of the char-N. Concentrations of these species as a function of time after turning on the O2/He mixture are given in Figure 1a. Using FTIR spectroscopy resulted in the identification of the additional products HCN and N2O, as reported by previous workers. Concentrations of these species are also given in Figure 1b, as are the concentrations of HNCO which was identified by FTIR spectroscopy. Figure 2a shows a portion (1000-850 cm-1) of the FTIR spectrum of the product gas 8.5 min after starting the O2/He flow. A complex series of lines is observed, some of which (near 770 cm-1) are due to CO2. Figure 2b shows an authentic spectrum of HNCO. It is clear that char oxidation at these temperatures results in the formation of detectable concentrations of HNCO. As far as we are aware this species has not previously been detected explicitly in the products of char oxidation, although it has been observed in coal pyrolysis products.11 Concentrations of HNCO produced as a function of time after introduction of O2 are also shown in Figure 1b. For quantification of the HNCO the larger feature at 2300 to 2200 cm-1 was used after subtraction of the absorbance of 13CO2 in the region.5 Integration of the data in Figure 1 shows the following contributions to the total char-N released as gas-phase products: N2 (54 ( 5%), NO (22 ( 2%), HNCO (12 ( 4.5%), HCN (6 ( 1.5%), N2O (1 ( 1%). It is possible that HNCO is a product of HCN oxidation. Adding 11.5 ppm HCN to the oxidizing mixture during char combustion resulted in a significant increase in the amount of NO (from 22% to 39% of the total char-N) and N2O (from 1% -8.8% of the total char N). However, it was not possible to determine, from this experiment, whether adding HCN to the oxidizing mixture increased the amount of HNCO and N2 produced. Although this (11) Nelson, P. F.; Li, C.-Z.; Ledesma, E. Energy Fuels 1996, 10, 264. (12) Glarborg, P.; Miller, J. A. Combust. Flame 1994, 99, 475. (13) De Soete, G. G. Proc. 23rd Int. Symp. Combust. 1990, 1257. (14) Goel, S.; Zhang, B.; Sarofim, A. D. Combust. Flame 1996, 104, 213. (15) Atakan, B.; Wolfrum, J. Chem. Phys. Lett. 1991, 178, 157.
Communications
Figure 2. (a) FTIR spectrum (1000 to 850 cm-1) of the product gases produced during the combustion of an Australian coal char at 600 °C. (b) authentic FTIR spectrum of HNCO.
result suggests that HCN oxidation is important in char oxidation under these conditions, it is unclear whether HNCO is a significant product of this oxidation. The observation of HNCO may have important implications for understanding char-N chemistry, and the impact of char combustion on emissions of NOx and N2O. Isocyanic acid (HNCO) is recognized as a potentially important intermediate in the oxidation of HCN.12 A particular issue in the recent literature related to char combustion is the mechanism for the formation of N2O. Both homogeneous8 and heterogeneous13,14 pathways have had their supporters. Detection of HNCO provides additional support for a potential homogeneous route to N2O, since HNCO will be a precursor to NCO at higher temperatures than those used in this study, and the reaction of NCO and NO is known15 to produce N2O:
NCO + NO f N2O + CO Further experiments to determine effects of temperature and other variables on yields of HNCO, HCN and N2O from char combustion are currently in progress. Acknowledgment. The authors thank Tony Prokopiuk for production of the coal char and Dr Jeffrey Shi for elemental analysis of the char. The authors also wish to acknowledge the support of the CRC for Black Coal Utilization, which is funded in part by the CRC Program of the Commonwealth of Australia. EF990079X
