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Formation of HNCO from the Rapid Pyrolysis of Coals Peter F. Nelson,* Chun-Zhu Li, and Elmer Ledesma CSIRO Coal & Energy Technology, P.O. Box 136, North Ryde 2113, Australia Received September 18, 1995 Pyrolytic release of nitrogen from coal is the initial step in its conversion to NO during coal combustion in furnaces, and hence to its emission to the atmosphere. Pyrolysis experiments in inert gas atmospheres have shown that the major forms of gas phase N released are HCN and NH3.1-5 There is considerable debate about the source of these species, particularly the NH3, since the nitrogen-containing functional groups present in coal (pyrrolic and pyridinic heterocycles)5-6 are expected to thermally decompose to yield HCN rather than NH3.7-9 In this Communication, we report the first observation of the formation of isocyanic acid, HNCO, from the pyrolysis of coals and discuss possible implications of this observation for the release and subsequent fate of volatile N species during coal pyrolysis and combustion. The coals were pyrolyzed in nitrogen at atmospheric pressure in a fluidized bed reactor system, which has been described in detail elsewhere.10,11 Previous work10 has shown that this system achieves heating rates of at least 104 K s-1; temperature could be varied in the range 500-1050 °C and gas residence times were in the range 0.3-0.5 s, depending on the total gas flow rates. Particle residence times varied, and depended on the mixing patterns in the reactor and on the tendency for the coal to soften during pyrolysis and to adhere to the bed material (zircon sand). Coals chosen for the pyrolysis experiments were crushed and sized to 75-106 µm. For some experiments a two-stage reaction system was used in which the cracking of the volatiles could take place in a separately heated quartz tube; this system has also been previously described.12 Gaseous products from pyrolysis were routinely characterized by Fourier transform infrared (FTIR) spectroscopy, for determination of yields of HCN and NH3. The pyrolysis products from the reactor flowed through a trap containing a Soxhlet extraction thimble held at 80 °C; this trap removed the tars and char elutriated from the bed. The remaining gas phase products then * Corresponding author. Phone: 61-2- 887 8666/8660. Fax: 61-2887 8909. E-mail:
[email protected]. (1) Solomon, P. R.; Colket, M. B. Fuel 1978, 57, 749. (2) Freihaut, J. D.; Proscia, W. M.; Seery, D. J. Joint EPRI/EPA Symp. Stationary Combust. NOx Control 1987, 2, 36.1-36.37. (3) Freihaut, J. D.; Zabielski, M. F.; Seery, D. J. Symp. (Int.) Combust. [Proc.] 1982, 19, 1150. (4) Baumann, H.; Mo¨ller, P. Erdo¨ l Erdgas Kohle 1991, 44, 29. (5) Nelson, P. F.; Kelly, M. D.; Buckley, A. N. Symp. (Int.) Combust., [Proc.] 1992, 24, 1259. (6) Keleman, S. R.; Gorbaty, M. L.; Kwiatek, P. J. Energy Fuels 1994, 8, 896. (7) Axworthy, A. E. Jr.; Dayan, V. H.; Martin, G. B. Fuel 1978, 57, 29. (8) Mackie, J. C.; Colket, M. B.; Nelson, P. F. J. Phys. Chem. 1990, 94, 4099. (9) Mackie, J. C.; Colket, M. B.; Nelson, P. F.; Esler, M. Int. J. Chem. Kinet. 1991, 23, 733. (10) Tyler, R. J. Fuel 1979, 58, 680. (11) Nelson, P. F.; Smith, I. W.; Tyler, R. J.; Mackie, J. C. Energy Fuels 1988, 2, 391. (12) Nelson, P. F.; Tyler, R. J. Energy Fuels 1989, 3, 488.
0887-0624/96/2510-0264$12.00/0
Figure 1. FTIR Spectra in the 2400-2050 cm-1 region: (a) coal pyrolysis product gas with lines due to 13CO2 subtracted, collected at 0.25 cm-1 resolution; (b) isocyanic acid, HNCO, spectrum from QASoft Database,15 collected at 0.5 cm-1 resolution.
passed through a long path length IR gas cell (infrared analysis, 7.2 m total path length on a base length of approximately 20 cm). Coal feed rate during the collection of the gas sample was determined by continuously recording the weight of the coal feeder using an electronic balance and data logger. FTIR spectra were determined using a Digilab FTS15/ 80 spectrometer at 0.25 cm-1 resolution by the coaddition of 256 scans. Yields of HCN and NH3 were calculated based on peak absorbance at 712 and 1103 cm-1, respectively. Calibrations were performed with NH3 in He and HCN in nitrogen gas mixtures diluted using mass flow controllers with N2 to the range of concentrations observed in the present work. The HCN and NH3 calibrations were in excellent agreement with those reported by Freihaut et al.13 and also, in the case of HCN, with that calculated using the HITRAN IR Database.14 For a wide range of coal types and pyrolysis temperatures HNCO was observed in the gas phase products. Figure 1 shows spectra for a coal pyrolysis product gas from an Australian bituminous coal (coal analysis, dry ash-free basis: 83.2% C; 5.67% H; 1.75% N; 1.15% S; 8.2% O (by difference); air-dried basis: 2.3% moisture; (13) Freihaut, J. D.; Proscia, W.; Knight, B.; Vranos, A.; Hollick, H.; Wicks, K. United Technologies Research Center Report no. 957553F, 1989. (14) USF HITRAN-PC, Computer Programs to calculate transmission spectrum of individual gases and ambient atmosphere using 32gas HITRAN optical spectra database, version 2.3; Ontar Corp., North Andover, MA, 1994.
© 1996 American Chemical Society
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7.6% ash; 36.8% volatile matter; 53.3% fixed carbon, and 1.04% total S) pyrolyzed in this case at 600 °C with the second stage temperature set at 800 °C and that for a standard sample of HNCO collected at 0.5 cm-1 resolution (obtained from infrared analysis QA Soft V3.1 IR Reference Spectral Database15). The spectrum of coal pyrolysis gas has had lines due to 13CO2 subtracted to reveal the underlying feature centered at 2269 cm-1. The sharp lines above about 2300 cm-1 are due to CO2 and those below about 2230 cm-1 to CO. Clearly, by comparison with the standard reference spectrum, the identification of this feature as HNCO is unequivocal. Investigation of the systematic dependence of HNCO yields on pyrolysis temperature and coal type will be the subject of future work. However, for the conditions of Figure 1, HNCO represents some 15% of the gas phase N species, with HCN and NH3 accounting for some 40 and 45%, respectively. This estimation is based on calibrations developed during recent previous FTIR observations of HNCO produced from interactions between propane and NO16 and from CO/H2/NO mixtures passed over car exhaust type catalysts.17 Hence HNCO is a not insignificant nitrogen-containing constituent of the products of coal pyrolysis. Examination of our previously collected spectra reveals that HNCO is present in the gas from many of the bituminous and subbituminous coals we have pyrolyzed over a range of temperatures. There is, however, little evidence for HNCO in the pyrolysis gas from lignites or brown coals, although in that case the amounts of CO2 formed are much greater and the consequent interference from 13CO2 is more problematic. Nitrogen contents of the brown coals and lignites we have studied also tend to be lower (less than 1% daf basis). There was also no HNCO produced from the pyrolysis of a US high-rank low-volatile coal (Pocahontas). Possible sources of HNCO formation from pyrolytic decomposition of nitrogen-containing functional groups in the coal are not difficult to identify. There is considerable evidence for hydroxyl groups associated with aromatic ring systems in coal, and some of these are likely to be in close proximity to ring N. Based on the observation that cyanuric acid (HOCN)3 readily sublimes and decomposes to form isocyanic acid18
(HOCN)3 f 3HNCO
(1)
decomposition of analogous groups in coals could lead to HNCO formation. The observation of HNCO in the pyrolysis products is also interesting in the context of NH3 formation from (15) Hanst, P. L.; Hanst, S. T.; Williams, G. M. Infrared Spectra for Quantitative Analysis of Gases; Infrared-Analysis Inc.: Anaheim, CA, 1995. (16) Nelson, P. F.; Haynes, B. S. Symp. (Int.) Combust., [Proc.] 1994, 25, 1003. (17) Du¨mpelmann, R.; Cant, N. W.; Trimm, D. L. Appl. Catal. B: Environ., in press. (18) Perry, R. A.; Siebers, D. L. Nature 1986, 324, 657. (19) Wo´jtowicz, M. A.; Zhao, Y.; Serio, M. A.; Bassilakis, R.; Solomon, P. R.; Nelson, P. F. 8th International Conference on Coal Science; Elsevier: Amsterdam, 1995; Vol. I, pp 771-774.
the pyrolysis of coals. The source of the NH3 is unclear since model compound studies of the thermal decomposition of compounds representative of the nitrogencontaining species in coals, such as pyridine and pyrrole, do not result in NH3 formation. Amine type functional groups would provide a possible source for NH3 formation, but N(1s) X-ray photoelectron spectra of coals do not reveal the presence of significant quantities of such groups, even for low-rank coals. Previous workers have postulated that NH3 arises from secondary reactions of HCN. Baumann and Mo¨ller4 invoked hydrogenation of HCN as a route to NH3. More recently, the functional group-depolymerization vaporization and cross-linking (FG-DVC) model has been modified19 to include HCN hydrogenation at the char surface to form NH3. The model accounts for some of the observations that have been made concerning yields of these volatile nitrogen-containing species during coal pyrolysis: evolution patterns are correctly modeled, and the relatively higher yields of NH3 observed in slow heating rate experiments are predicted. Formation and subsequent reactions of HNCO provide an alternative mechanism for the formation of NH3. Isocyanic acid readily hydrolyzes17 on alumina surfaces to yield NH3 and CO2
HNCO + H2O f NH3 + CO2
(2)
and such surface-catalyzed reactions are possible during coal pyrolysis. The failure to detect HNCO from brown coals is consistent with this mechanism since these coals have much higher moisture contents and will yield more water during pyrolysis. The observation of HNCO could also be important in N2O formation in fluidized bed combustion (FBC) of coals. Isocyanic acid will be rapidly broken down under combustion conditions to a variety of radical products, but most importantly, in the present context, to NCO.20 The reaction
NCO + NO f N2O + CO
(3)
has been shown to be the most important route to N2O in the RAPRENOx process20 and is also believed to be the most important route to N2O formation in FBC. The dependence of N2O formation on rank,21 with bituminous coals producing relatively higher quantities than brown coals, may be a result of the higher moisture contents in the lower rank coals rapidly converting HNCO to NH3 and thus changing the possible pathways for N chemistry. The mechanistic discussion reported in this brief communication is neccessarily speculative at this stage, but the observation of HNCO as a product of the thermal decomposition of coals does provide a plausible framework for some of the N species chemistry in pyrolysis and combustion of coals. EF950183O (20) Miller, J. A.; Bowman, C. T. Prog. Energy Combust. Sci. 1989, 15, 287. (21) Wo´jtowicz, M. A.; Pels, J. R.; Moulijn, J. A. Fuel Process. Technol. 1993, 34, 1.
