Energy & Fuels 1992,6, 753-757
753
Effect of SO2 Removal by Limestone on NO, and N2O Emissions from a Circulating Fluidized Bed Combustor Tadaaki Shimizu,* Yutaka Tachiyama, Daisuke Fujita, Ken-ichi Kumazawa, Osamu Wakayama, Kazuya Ishizu, Sadamu Kobayashi, Shinobu Shikada, and Makoto Inagaki Department of Material and Chemical Engineering, Faculty of Engineering] Niigata University, Ikarashi, Niigata, 950-21, Japan Received March 30, 1992. Revised Manuscript Received August 14, 1992 ~~
~~
The effect of limestone addition for SO2 removal on emissions of NO, and N2O was evaluated with a laboratory-scale circulating fluidized bed combustor. It was found that NO, emission increased with increasing SO2 removal whereas N2O emission decreased. The rates of HCN oxidation, NH3 oxidation] NO reduction by COYN2O decomposition, and SO2 removal over limestone were evaluated with a fixed bed reactor. The major product of HCN oxidation was NO. N2O formation was observed slightly among the produce of HCN oxidation. The rates of HCN oxidation, NH3 oxidation, and N2O decomposition in the presence of CO were found to be higher than that of SO2 removal, while the rate of NO reduction by CO was far lower. The change in NO, and N2O emissions from CFBC is explained by the results of the kinetic study. The increase in NO, emission is attributed to the formation of NO through oxidation of HCN and NH3 over limestone. The decrease in N2O emission of CFBC is partly attributed to catalytic activity to decompose N2O. Also, consumption of HCN through oxidation over limestone is considered to reduce N2O emission since conversion of HCN to N2O for catalytic oxidation is far lower than reported value of the conversion for gas-phase oxidation. Introduction Recently, N2O has become a focus of attention not only as a greenhouse gas but also as an agent of ozone destruction in the stratosphere. Circulating fluidized bed combustors (CFBCs) are considered to be a source of N2O. A feature of CFBCs is in situ control of emission of SO2 by addition of sorbent (CaO). The sorbent feed rate is controlled so that SO2 emission is reduced to meet environmental regulations. Therefore, it is required to evaluate NzO and NO, (NO and NO2) emissions under conditions achieving desired SO2 removal. For CFBCs, the relation between N2O emission and SO2 removal has not yet been fully evaluated. Amand et al.,I Moritomi et al.,2 and Hiltunen et aL3 reported that N2O emission decreased with sorbent addition, while Brown et aL4 reported that limestone feed did not affect N2O emission. Mjiirnell et aL5reported that the change in NzO emission by limestone feed depended on the type and size of the limestone. Thus, a study to evaluate the effect of limestone feed on N2O and NO, emissions is still required. Although it is well-known that limestone feed in CFBCs (1)Amand,L-E.; Leckner, B.; Andersson, S.;Gustavsson, L. European Workshop on N20 Emissions LNETI/EPA/IFP (Lisbon, Portugal), 1990; p 171. (2) Moritomi,H.;Suzuki,Y.;Kido,N.;Ogisu,Y. CirculatingFluidized Bed Technology III; Basu, P., Horio, M., Haeatani, M., Eds.; Pergamon Press: Oxford, UK, 1991; p 399. (3) Hiltunen, M.; Kilpinen, P.; Hupa, M.; Lee, Y. Y. Proceedings of the 11 th International Conference on Fluidized Bed Combustion. (Montreal, Canada);ASME New York, 1991; p 687. (4) Brown, R. A.; Muzio, L. Proceedings of 11th International Conference on Fluidized Bed Combustion (Montreal, Canada);ASME: New York, 1991; p 719. (5) Mjbrnell, M.; Leckner, B.; Karlson, M.; Lyngfelt, A. Proceedings of the 11th International Conference on Fluidized Bed Combustion (Montreal, Canada);ASME New York, 1991; p 655.
increases NO, e m i s s i ~ n , ~ the - ~ lmechanism ~?~ of the increase in NO, emission is not yet clear. Catalytic oxidation of NH3, which is released from fuel during devolatilization, has been investigated intensively and calcined limestone (CaO) was found to be a catalyst to oxidize NH3 to mainly However, recent studies on devolatilization of coal under rapid heating conditions showed that conversion of coal bound nitrogen to HCN was higher than that toNH3.lO Catalytic activity of limestone on HCN oxidation to NO, has not been reported. The role of limestone in the formation and destruction of NzO in the CFBCs is not fully clarified. It has been reported that CaO is a catalyst for N2O decomp~sition.ll-~~ On the other hand, Iisa et reported that CaO was a catalyst for the production of N2O from NH3-02 and NH3NO-02 mixtures. For catalytic oxidation of HCN, no work (6) Hirama, T.; Takeuchi, H.; Horio, M. Proceedings of the 9th International Conference on Fluidized Bed Combustion (Bostbn, USA); ASME New York, 1987; p 898. (7) Hirama, T.; Kochiyama, Y.; Chiba, T.; Kobayaehi, H. NenryoKyokai-Shi (J.Fuel SOC.Jpn.) 1982,61, 268. (8)Furueawa, T.;Tsujimura,M.; Yasunaga, K.; Kojima, T.Proceedings of the 8th International Conference on Fluidized Bed Combustion (kouston, USA); DOE/METC-85/6021; U.S.Department of Energy, Office of Fossil Energy: Morgantown, WV., 1986; p 1095. (9) Lee, Y. Y.; Sekthira, A.; Wong, C. M. Proceedings of the 8th InternationaZ Conferenceon FluidizedBed Combustion (Houston, USA); DOE/METC-85/6021; US. Department of Energy, Office of Fossil Energy: Morgantown, WV, 1985; p 1208. (10) Kanbara, S.; Takarada, T.; Yamamoto, Y.; Nakagawa, N.; Kato, K. Preprinkof Autumn Meeting of Societvof ChemicalEnaineers.Jauan, 1991; Vol. 1, p 601. (11)Moritomi, H.; Suzuki, Y.; Kido, N.; Ogisu, Y. Proceedings of the 11 th International Conference onFluidizedBed Combustion (Montreal. Canada); ASME New York, 1991; p 1005. (12) Iisa, K.; Salokoski, P.; Hupa, M. Proceedings of the 11th International Conference on Fluidized Bed Combustion (Montreal, Canada);ASME New York, 1991; p 1027. (13) Miettinen, H.; Strbmberg, D.; Lindquist, 0. Proceedings of the 1 1thlnternational Conference onFZuidized Bed Combustion (Montreal, Canada);ASME New York, 1991, p 999.
0887-0624/92/2506-0753$03.00/00 1992 American Chemical Society
754 Energy & Fuels, Vol. 6,No. 6,1992
Shimizu et al. Table I. Analyses of Coals ultimate analysis (daf, w t %) H 0 ' N
coal C 90.2 4.2 3.6 semianthracite (SA) medium-volatile bituminous (MVB) 84.1 4.9 8.7 high-volatile bituminous (HVB) 80.1 6.5 11.1 0 By difference. b Combustible sulfur. Volatile matter. Fixed carbon.
has been conducted. HCN is considered to be an important precursor of NzO since conversion of HCN to N20 through gas-phase oxidation was reported to be far higher than J ~ J ~ a kinetic study, conversion of NH3 to N z ~ . ~ Therefore, including oxidation of HCN over limestone, is necessary to clarify the pathways of formation and destruction of NzO as well as NO, in CFBCs. There are two main objectives in this study. The first objective of this work is to evaluate the effect of SO2 removal by sorbent feed on emissions of NO, and NzO from CFBC. Limestone feed to a laboratory-scale circulating fluidized bed combustor was conducted. The second objective is to evaluate the rates of HCN oxidation, NH3 oxidation, NO reduction by CO, and NzO decomposition. A fixed bed study was conducted. The rate of SO2 was also measured and compared with the rates of reactions of nitrogen compounds. In general, limestone feed rate of CFBCs is controlled to achieve the desired $02removal. If the rate of a reaction is far lower than that of the SO2 removal, such reaction is considered to play no significant role in the combustor. The role of limestone in reactions of nitrogen compounds in CFBCs is discussed. Experimental Section Circulating Fluidized Bed Combustion Study. The combustor was made of SUS3lOS stainless steel. Ita inner diameter and height were 5.3 cm and 4.3 m, respectively. A secondary air nozzle was installed at 1.3 m above the primary air distributor. The reactor was heated by electric heaters, and the temperature in the bed was fixed at 1123 K. The primary air feed rate and the secondary air feed rate were fixed at 7.57 X and 7.22 X mol/s, respectively; i.e., the superficial gas velocity above secondary air inlet was 6.2 m/s at 1123 K. Quartz sand of mean size 147 wm was used as bed material. The inventory of the sand was 2 kg. Typical pressure drop of the riser was ca. 25 cmAq. Three coals, high-volatile bituminous coal (HVB), mediumvolatile bituminous coal (MVB), and semianthracite (SA), were used, the analyses of which are shown in Table I. The sample was crushed and the fraction smaller than 1.0 mm was used. The weight mean size of the coals was 0.35-0.42 mm. The coal was continuouslyfed through a rotary feeder, conveyedpneumatically in nitrogen, and injected into the bottom of the combustor 10 cm abovethedistributor. Thecoalfeedrate wascontrolledtoachieve desired 02 concentration in flue gas. Chichibu limestone was used as the sorbent. Ita composition (wt % ) was CaCOs 96.9; MgC03 1.4; Si02 0.6; A1203 0.8; and FezOa0.3. Its particle size was between 0.25 and 0.35 mm. The limestonewas impulsively injected into the combustion chamber. Continuous feed of limestone was also conducted for combustion of high-volatile bituminous coal and for medium-volatile bituminous coal. For continuous feed testa, limestone was previously mixed with coal at a Ca/S ratio of 6.5 before it was fed to the combustor. (14)Kramlich, J. C.; Cole, J. A.; McCarthy, J. M.; Lanier, W. M.; McSorley, J. A. Combust. Flame, 1989, 77, 375. (15) Hulgaard, T.; Glarborg, P.; Dam-Johansen,K. Proceedings of the
11th Internutionul ConferenceonFluidized Bedcombustion (Montreal, Canada); ASME: New York, 1991; p 991.
Sb
1.8
0.3
1.9
0.4 0.6
1.7
proximate analysis (dry, wt VMc FCd 11.8 73.2 25.2 58.6 42.5 42.5
%)
ash 15.0 16.2 15.0
to vent
Figure 1. Flow sheet of the experimental apparatus for fixed bed study. Concentrations of total NO, (NO and N02) and SO2 in the flue gas were continuously measured by on-line analyzers using chemical luminescence for NO, (Yanagimoto ECL-77A) and nondispersive infrared for SO2 (Shimadzu URA-B),respectively. The response of each gas analyzer was sufficiently fast to follow the change in concentration of each gaseous component. Concentration of NO2 was measured by Kitagawa detector tubes.16 However, NO2 was not detected in the flue gas under the present experimental conditions. A portion of sample gas was dried at 276 K by an electric gas cooler (Shimadzu CFP101) and stored in Tedlar bags. Concentrations of 0 2 , CO, Con, and N2O in the bags were measured by gas chromatography with thermal conductivity detection (TCD-GC). Muzio et al.17 have pointed out that N2O could be formed in such bags in the presence of water and high concentrations of SO2 (>600 ppm) and NO,. They also reported that drying the sample could reduce N2O formation and removing SO2 by scrubbing the sample through NaOH solution could eliminate NzO formation." For the present work, the change in N2O concentration in dry sample gas with time was measured for several hours, but formation of NzO in the gas bag was not observed. NO, concentration in the sample did not change with time. A test was also performed passing flue gas (SOZ: 390 ppm) through a gas washing bottle containing 0.56 N NaOH solution. Both sampling approaches gave the same results of the concentration of NzO in flue gas. Thus, under the present conditions (SO2 concentrations lower than 400 ppm), drying the samples at 276 K is considered to be sufficient to suppress NzO formation within the sampling system and gas bags. The concentrations of NO,, NzO, and SO2 [ppmls [PmoVmol of flue gas] reported here are the values corrected to dry flue gas at 6 vol 5% oxygen. Fixed Bed Study. Figure 1shows the flow sheet of the fixed bed experimental apparatus. The fixed bed reactor was made of quartz. The inner diameter and the length of reaction zone were 2 and 6 cm, respectively. Quartz sintered plate was installed in the reaction zone to support the fixed bed. The inner diameter of the inlet and the outlet was reduced to 0.8 cm to minimize the gas residence time thus to suppress gas-phase reactions. The temperature in the fixed bed was measured by chromel-alumel (16)Japanese Industrial Standard, K 0804-1985(Detector tube type gas measuring instruments), Japanese Standards Association, Tokyo, Jaaan. 1985. .(17) Muzio,L.J.;Teague,M.E.;Kramlich,J.C.;Cole,J.A.;McCarthy, J. M.; Lyon, R. K. JAPCA 1989,39,287. r--r
Effect of SO2 Removal on NO, and
t Liwdm inplsire i n 4
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Energy & Fuels, Vol. 6, No. 6, 1992 766
N20Emiseiom I
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Figure 2. Typical dynamic change in emissions of SOz, NO,, and N20 for medium-volatile bituminous coal combustion for (a) impulsive addition of 150 g of limestone and (b) continuous (0) feed of liestone. Total air ratio = 1.25 f 0.03;(A),902, NO,, ( 0 )NzO.
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8 thermocouple covered with high-purity alumina. The temperature was fued at 1123 K. The same limestone as used for the CFBC study was used for the fied bed etudy. Ita particle size was between 0.42 and 0.59 mm. Quartz sand was used to dilute limestone. Ita average particle size was 0.61 mm. The larger solids were used for the Cued bed study rather than for the CFBC etudy since the pressure drop of the fued bed of fie particles was too high. Calcinationof liieetone was conducted by injecting limestone in fluidized quartz sand at 1123 K. Quartz sand of 3-5 g was fluidized in upward flow of oxygen at euperficial velocity of 0.2 m/s. Raw liiestone was weighed, injected in the fluidizedquartz sandbed,andcalcinedfor5min. Thus,thelimestonewascalcined in a similar way as in the CFBCs. After calcination, the reactant gas was introduced from the top of the reactor downward through the fixed bed. Total feed rate of reactant gases was 2.05 X moVs; i.e., superficial velocity was 60 cm/s at 1123 K. Catalytic effecta of the reactor and quartz sand on reactions were found to be negligible. Experiments of HCN oxidation, NHs oxidation, NzO decomposition, NO reduction by CO, and ,902removal were conducted. Helium was used as a diluent. The reactant gases except for HCN were fed from cylinders. HCN was produced by feeding KCN solution in HsSO, solution. To minimize the effect of hydrolysisof HCN to NHs catalyzed by liiestone,'8 water vapor in the HCN-He mixture was removed by CaC12. T h e typical H2O concentration in the feed gas was 60 ppm. T h e concentration of HCN was measured by gas chromatography with flnme ionization detection (FID-GC). The gas (18) W o k , 5.;Smaoka, E.; Ozaki, A. Nenryo-kyokai-ehi (J.Fuel Sox. Jpn.), 1982,62,1088.
1
8
O1
10
So2
COAC.
loo Cppml
1000
Figure 3. Effect of SO2 removal by liiwtone addition on emissions of NO, and N20, for semianthracite (SA),mediumvolatile bituminous coal (MVB), and high-volatile bituminous coal (HVB). Total air ratio = 1.25 f 0.03;Solid symbols, N10; open symbols, NO,; ( 0 , O )impulsive feed of limeatone,amount of limestone impulse, SA and MVB 150 g, HVB 100 g; (0, W) continuous feed to limestone at C 4 S = 6.6; (A,A) in F i e 3a: resulta for bituminous coal by Amand et al.' chromatograph was calibrated by injecting KCN dissolved in HCl solution.19 The concentration of NHs was measured by Kitagawa detector tubes.16 The concentration of NO was measured by an on-line analyzer using chemical luminescence (YanagimotoECL-30). Concentrations of 02,No, CO,COONoO, NOz, and SO2 were measured by the same methods as CFBC study.
Results Circulating Fluidized Bed Combustion Study. Figure 2 shows typical reeulta of the change in S02, NO,, and N2O concentratione with time for impulsive feed and (19! Japan- Industrial Standard, K 0109-1982 (Methodr for determination of hydrogen cyanide in exhaust gm),Japan- Strndardr hociation, Tokyo,Japan, 1982.
Shimizu et al.
766 Energy & Fuels, Vol. 6, No. 6,1992
Table 11. Typical Nitrogen Balance and Products of (1) HCN Oxidation, (2) NHs Oxidation, (3) NO Reduction by CO, and (4) NzO Decomposition concentrations in product gas concentrations in feed gas CO, HCN, NH3, N02, Nbalance W, NO, NzO, HCN, NH3, NO, N20, 02, Nz, P P ~ % % P P ~ P P ~ P P ~ P P ~ P P ~ P P ~ Nod% ppm ppm ppm mg 61 0 466 14 97 trace 1 27 750 2.5 1.00 2a 68 870 2.5 150 585 0 85 trace 1.04 2b 68 878 891 1.1 177 1306 0 135 trace 0.99 3 136 984 2.6 308 0 340 1.00 4a 136 498 0 58 450 1.02 4b 136 501 2.5 0 69 406 0.95 4c 136 510 1.0 0 14 508 1.02
continuous limestone feed experiments. For impulsive feed experiments SO2 and N2O emissions decreased to the minimum value within a few minutes after sorbent injection. Simultaneously, NO, emission increased to the maximum value. Then SO2 and N2O emissions increased while NO, emission decreased with time. For continuous feed experiments, the solid was not withdrawn from the bed and the limestone accumulated in the bed with time. SO2 and N20 emissions decreased gradually with time while NO, emission increased gradually. Steady state could not be achieved in the present study. In Figure 2b, the changes in emissions of NO,, N20, and SO2 after the limestone feed was stopped are also shown. SO2 and N20 emissions increased with time while NO, decreased. Figure 3 shows relation between NO, andN2O emissions and SO2 emission. For high-volatile bituminous coal (HVB) and medium-volatile bituminous coal (MVB), both impulsive feed and continuous feed of limestone were conducted. NO, and N2O emissions versus SO2 emission curves obtained for continuous limestone feed agreedwith the results obtained for impulsive feed of limestone. Therefore, only the impulsive feed experiment was conducted for semianthracite. For all coals, NO, emission increased with decreasing SO2emissionwhile N20 emission decreased. In Figure 3a the present results are compared with the result of transient test of limestone addition conducted by Amand et al.' The height and cross section of the combustor for the present study were 4.3 m and 2.2 X m2, respectively, while those employed by Amand et al. were 13 m and 2.9 m2, respectively. In spite of the difference in reactor sizes, both results agreed fairly well. Thus, the present experimental apparatus is considered to be able to estimate the effect of SO2 removal on NO, and N2O emissions of large-scale combustors. Fixed Bed Study. Table I1shows the typical material balance and products of (1) HCN oxidation, (2) NH3 oxidation, (3) NO reduction by CO, and (4) N2O decomposition. The major product of oxidation of HCN and NH3 was NO (Table 11, no. 1and 2). Conversion of HCN to N2O was only 4% (Table 11, no. 1). Neither the NH30 2 mixture nor the NH3-NO-02 mixture produced N20 (Table 11, no. 2a and 2b). N2O was not produced through NO reduction by CO (Table 11,no. 3). NO was not detected for N2O decomposition (Table 11, no. 4). Figure 4 shows the relation between solid inventory and conversion of gaseous component. X is the conversion defined as
x = 1- Cou,/Cin
(1)
where Cin and Gout are the concentrations of the reactant at the inlet and outlet, respectively. Wand F are limestone inventory as CaO and gas flow rate at reactor temperature, respectively. SO2 removal efficiency decreased with time,
ass transfer limit, Sh.3.1
-3 7- 2
4 1 & 0 0.5 4 1 1 J D
OO " W/F
t kg III-~S I
Figure 4. Effect of limestone inventory on the conversion for HCN oxidation, NHa oxidation NzO decomposition,NO reduction by CO and SO2 removal. Concentrations in feed gas: ( 0 )HCN oxidation, HCN 750 ppm, 02 2.5 % ;(A)NH3 oxidation, NH3 900 ppm, 02 1.1 % ;(0) N20 decomposition in the absence of CO, NzO 500 ppm; (0)NzO decomposition in the presence of CO, NZO 500 ppm, CO 2.5%; (B) NO reduction by CO, NO lo00 ppm, CO 2.6%; (0) SO2 removal, SO2 160 ppm, 02 2.5%.
i.e., with increasing conversion of CaO to CaSOr. So, for SO2 removal, the initial SO2 removal is shown in Figure 4. For other reactions, the change in conversion with time was not observed. For NH3 oxidation, N2O decomposition (both in the presence and absence of CO) and SO2 removal, -In (1- X) was proportional to W/F, Le., the order of those reactions was first order. The consumption rate of HCN was almost the same as the rate of mass transfer, where a Sherwood number of 3.1 was estimated by the Yagi and Wakao equation.20 Therefore, neither the rate nor the reaction order of HCN oxidation eliminating mass-transfer resistance could be evaluated. The rates of HCN oxidation and NH3 oxidation were higher than the SO2removal rate. The NzO decomposition rate in the absence of CO was as high as the SO2 removal rate. The N2O decomposition rate increased with increasing CO concentration. The rate of NO reduction by CO was only half of SO2 removal rate.
Discussion The present kinetic study proposes an explanation of the increase in NO, emission with decreasing SO2emission by limestone feed; HCN oxidation catalyzed by limestone is considered to play an important role in NO formation since the rate of this reaction is very high and most of HCN converts to NO. Oxidation of NH3 catalyzed by limestone is also considered to contribute the increase in NO, since the rate is higher than that of sulfur removal. Reduction of NO by CO occurs over limestone but this reaction is considered to play only a minor role in determining overall NO emission, since the rate of NO reduction by CO is far lower than the rate of SO2 removal. (20) Wakao, N.; Yagi, S.; Oshima, R. Kagaku Kogaku 1958,22,780.
Effect of SO2 Removal on NO, and NzO Emissions
The decrease in NzO emission of a circulating fluidized bed combustor by limestone feed (Figure 3) is explained by the results of the present kinetic study. The rate of N2O decomposition over limestone is as high as that of SO2 removal (Figure 4). Thus, the reduction of NzO is considered to take place when limestone is fed to the combustor. HCN oxidation over limestone is also considered to contribute the decrease in N2O emission; the conversion of HCN to N2O through catalytic oxidation over limestone was only 4 % whereas a conversion of HCN to N2O of up to 50 % has been reported for homogeneous 0xidation.~J~J5 The rate of HCN oxidation is far higher than that of SO2 removal. Thus consumption of HCN over limestone is considered to occur in the CFBC and this reaction contributes the reduction of NzO emission from CFBC. A possibility was discussed by Gavin et alS2l that hydrolysis of HCN to NH3 catalyzed by limestone18 contributes the decrease in N2O emission. Conversion of NH3 to NzO for gas-phase oxidation is reported to be far lower than that of HCN.3J4J5 However, the contribution of this pathway is not clear since the rate of HCN hydrolysis has not yet been reported. The present kinetic study showed that NO was produced but N2O was not produced from NH3-02 and NH3-NO0 2 mixtures. Thus NzO formation through NH3 reactions over limestone is not considered to take place in the present CFBC study. Iisa et a1.12 reported that ca. 15% of total N (NO + NH3) converted to NzO while NO reduction took (21) Gavin, D. G.; Dorrington, M. A. Proceedings of the I991 International Conferenceon Coal Science, (Newcastle,UK);International Energy Agency Coal Research Ltd.; Butterworth-Heinemann: Oxford, UK, 1991; p 347.
Energy & Fuels, Vol. 6, No. 6,1992 7S7
place through NH3-NO-02 reaction in CaO bed. For the NH3-02 reaction, they reported conversion of NH3 to Nz0 of ca. 6 % . This difference could be attributable to the difference in the limestone. Conclusion Limestone feed to the circulating fluidized bed combustor increased NO, emission while it decreased NzO emission. The kinetic study showed that the rates of HCN oxidation and NH3 oxidation were far higher than that of SO2 removal and the major product of these reactions was NO. Increase in NO, by limestone feed to CFBC is explained by oxidation of HCN and NH3. The kinetic study showed that the rate of N2O decomposition over limestone is as high as SO2 removal rate. Decrease of Nz0 by limestone feed to CFBC is partly explained by NzO decomposition over limestone. HCN oxidation is considered to contribute NzO reduction of CFBC since coversion of HCN to N2O over limestone is far lower than that through gas-phase HCN oxidation. Acknowledgment. The authors thank the Steel Industry Foundation For The Advancement Of Environmental Protection Technology for financial aid and Idemitsu Kosan Co. Ltd. for cooperation. T.S. expresses his thanks to Prof. Sadakata, Tokyo University, and toDr. Adschiri, Tohoku University, for valuable discussions. Registry No. SO*, 7446-09-5; NzO,10024-97-2;nitrogen oxide, 11104-93-1; HCN, 74-90-8;NH3, 7664-41-7.
