Energy & Fuels 1993, 7, 645-647
645
Hydrolysis and Oxidation of HCN over Limestone under Fluidized Bed Combustion Conditions Tadaaki Shimizu; Kazuya Ishizu, Sadamu Kobayashi, Satoru Kimura, Toshihiro Shimizu, and Makoto Inagaki Department of Material and Chemical Engineering, Faculty of Engineering, Niigata University, Ikarashi, Niigata, 950-21, Japan Received March 8, 1993. Revised Manuscript Received June 11, 1993
A kinetic study was conducted for hydrolysis and oxidation of HCN over limestone at 1123 K with a fixed bed reactor. In the absence of 02, HCN was hydrolyzed to mainly NH3. However, the reaction rate of hydrolysis was far lower than that of oxidation. In the presence of 0 2 a t concentrations higher than 0.5%, oxidation of HCN was dominant and most of HCN was oxidized to NO, while only a small amount of NH3 was produced. Conversion of HCN to N2O for oxidation over limestone was far lower than the conversions which had been reported for homogeneous oxidation of HCN. Introduction Circulating fluidized bed combustion has been developed
as a promising coal combustion technology that can reduce SO2 emission by feeding limestone (CaC03) into the combustion chamber. However, the emission of Nz0 from circulating fluidized bed combustors (CFBCs) has been reported to be considerably high.l Recently, NzO has become a focus of attention not only as a greenhouse gas but also as an agent of ozone destruction in the stratosphere. Limestone feed to CFBCs is known to decrease N20 emission." However, the mechanism of the decrease has not yet been fully clarified. It is considered that limestone affects not only N2O decomposition but also N2O formation from volatile matter. For the former, calcined limestone (CaO) is known to be a catalyst of Nz0 decompositionP7 while, for the latter, HCN in the volatile matter is considered to play a significant role in forming N20. It is widely accepted that the conversion of HCN to NzO for homogeneous oxidation is considerably high (>20 % ) -899 In a CFBC, for example, upto 40 ppm of HCN was observed.10 On the other hand, Shimizu et al. reported that conversion of HCN to NzO for catalytic oxidation (1) Amand,L.-D.;Andersson,5.Proceedingsofthe 1OthZnternational Conference on Fluidized Bed Combustion (SanFrancisco, USA);ASME New York, 1989; p 49. (2) Amand, L.-E.; Leckner, B.; Andereson, S.; Gustavsson, L. Eur. Workuhop NpO Emissiona LNETIIEPAIZFP (Lisbon, Portugal) 1990, 171. (3)Moritomi, H.; Suzuki, Y.; Kido, N.; Ogisu, Y. In Circulating Fluidized Bed Technology III; Basu, P., Horio, M., Hasatani, M., Eds.; Pergamon Preea: Oxford, UK, 1991; p 399. (4) Shimizu,T.;Tachiyema,Y.; Fujita, D.; Kumazawa,K.;Wakayama, 0.;Ishizu, K.; Kobayaahi, S.; Sikada, S.;InagaJsi, M. Energy Fuels 1992, 6,753. (5) Moritomi, H.; Suzuki, Y.; Kido, N.; Ogisu, Y. Proceedings of the IlthZnternational ConferenceonFluidized Bed Combustion (Montreal, Canada);ASME: New York, 1991; p 1005. (6) h a , K.; Salokoski, P.; Hupa, M. Proceedings of the 11th Znternational Conference on Fluidized Bed Combustion (Montreal,Canada); ASME: New York, 1991; p 1027. (7) Miettinen, H.; StrBmberg, D.; Lindquist, 0. Proceedings of the llthlnternational Conference on Fluidized Bed Combustion (Montreal, Canada); ASME: New York, 1991; p 999. (8)Kramlich, J. C.; Cole, J. A.; McCarthy, J. M.; Lanier, W. M.; McSorley, J. A. Combust. Flame 1989, 77, 375. (9)Hdgaard, T. Nitrous oxide from combustion. Ph.D. Thesis, Department of Chemical Engineering,TechnicalUniversityof Denmark, Lyngby, Denmark, 1991.
over calcined limestone was far lower than that for homogeneous ~ x i d a t i o n . Thus, ~ consumption of HCN through oxidation over limestone is considered to contribute to N2O reduction. In addition, Gavin et al.11 pointed out a possibility that hydrolysis of HCN to NH3 catalyzed by limestone12 contributed to the decrease in NzO emission, since the conversion of NH3 to NzO for both heterogeneous and homogeneous oxidation is known to be far lower than the conversion of HCN to NzO for homogeneous o ~ i d a t i o n . ~ ~ ~ ~ ~ However, the contribution of hydrolysis of HCN within a combustion chamber is questionable since it has not yet been determined whether hydrolysis of HCN over limestone occurs in the presence of 0 2 . On a practical level, oxygen concentrations in combustion chambers of CFBCs are considerably high; Amand and LecknerlO reported that 02 concentration in the CFBC was higher than 1%. Ishizuka et al.13 also reported that 02 concentrations were higher than 2% even under strong air-staging conditions at a primary air ratio of 0.5. The objective of this work is to obtain kinetic data for hydrolysis and oxidation of HCN over calcined limestone. A fixed bed study was conducted to evaluate the rate of hydrolysis of HCN and the effect of 0 2 on this reaction. Experimental Section A quartz fixed bed reactor of i.d. 2 cm was employed. Quartz sintered plate was installed in the reaction zone to support the fixed bed. The temperature was fixed at 1123 K. Details of the experimental procedure are described in the previous study.' Chichibu limestone was used as a sample. Ita composition (wt %) waa CaC03 96.9,MgC03 1.4,Si02 0.6,A1203 0.8,and Fe2Os (10) Amand, L.-E.; Leckner, B.; Andersson, S.Energy Fuels 1991,5, 815. (11)Gavin, D. G.; Dorrington, M. A. Proceedings of the 1991 Znternationol Conferenceon CoalScience,(Newcastle,UK);Intemational Energy Agency Coal Research L a . ; Butterworth-Heinemann: Oxford, IJK. - - -, -1991: -. - n r 347. (12) Kasaoka, S.; Sasaoka, E.; Ozaki, A. Nenryo-kyokai-shi (J.Fuel Soc. Jpn.) 1982,62,1086. (13) Ishizuka, H.; Hyvarinen, K.; Morita, A,; Suzuki, T.; Yano, K.; Hirose, R. In Circulating Fluidized Bed Technology ZI; Basu, P., Large, J. F., Eds.; Pergamon Press: Oxford, UK, 1988, p 437. (14) Wakao, N.; Yagi, S.; Oshima, R. Kagaku Kogaku 1958,22,780. I
0887-0624/93/2507-0645$04.00/00 1993 American Chemical Society
Shimizu et al.
646 Energy & Fuels, Vol. 7,No. 5, 1993 c;
02
- "I
HCN
A 4
-
He0
HCN
0.1
"0
0.2
0.3
Contact time, T [ms] Figure 1. Effect of contact time on unreacted fraction of HCN. HCN inlet concentration= 694-738ppm. Solid line: unreacted fraction of HCN under mass transfer controlling condition. 0.3. Ita particle size was between 0.42 and 0.59 mm. Quartz sand of 0.3-0.8 mm was used to dilute the limestone. The total feed rate of reactant gases was 2.05 X 1W mob. With the exception of HCN and HzO, the reactant gases were fed
from cylinders. HCN, which was produced by feeding KCN solution into an HzSO, solution, was then swept out by He fed at a rate of 0.56 X 1W mol/s. Water vapor in the HCN-He mixture was removed by a CaClz packed column. Typical H2O concentrationin the HCN-He mixture at the outlet of the CaCl2 column was 220 ppm. Concentrationof HCN at the inlet of the reactor was fiied at 694-738 ppm. Water vapor was produced by passing He through a gas washing bottle containing ionexchangedwater; then the saturated H&He mixture was mixed with other gases. The temperature of the ion-exchanged water was kept constant at 288 K by cooling water with a temperature controller. The feed rate of water vapor was controlled by regulating the flow rate of He passing through the gas washing bottle. Concentrationsof HCN, N2, and N2O were measured by gas chromatography. Concentrations of NHg and NO2 were measured by Kitagawa detector tubes. Concentrationof NO was measured by an on-line analyzer using chemical luminescence.
Results and Discussion Figure 1 shows the relation between contact time and unreacted fraction HCN. Contact time, T , is given as 7
= W/(PF)
(1)
where W ,p, and F are raw limestone inventory, density of raw limestone, and gas flow rate at reactor temperature, respectively. In the absence of 0 2 , the relation between ln(unreacted fraction) and T was expressed as a straight line, and thus the rate of HCN hydrolysis was found to be first order with respect to HCN concentration. Concentration of H2O did not affect the rate of hydrolysis under the present conditions (HzO = 0.35-0.72%). The rate of HCN consumption in the presence of 0 2 was far higher than that in the absence of 0 2 as shown in Figure 1. The rate of HCN consumption for the HCNH20-02 system was as high as the rate for the HCN-02 system. For HCN oxidation in the presence of H20, there are two possible reaction pathways of HCN consumption. First, NH3 in formed through hydrolysis of HCN and then NH3 is oxidized over limestone (eq 2). HPO
HCN-
02-
NH,
NO, ...
(2)
Second, HCN oxidation and HCN hydrolysis are parallel processes (eqs 3 and 4).
NO, ... 02
NH, -NO,
(3)
...
(4)
Accordingto the results shown in Figure 1,HCN oxidation and HCN hydrolysis are considered to be parallel processes since the rate of HCN hydrolysis is far lower than the rate of oxidation. If the consumption of HCN is a series reaction (oxidation after hydrolysis), hydrolysis of HCN must be the rate-controlling step. The consumption rates of HCN for both the HCNH2O-02 and the HCN-02 systems were almost the same as the rate of mass transfer across the boundary layer. Under mass transfer controlling conditions, the change in concentration of HCN, CHCN,with contact time is given as dCHcN/dT = -Gk&c,/d, (5) where kf and d, are mass-transfer coefficient and particle diameter (d, = 0.5 mm), respectively. The Sherwood number (Sh) of 3.1 was estimated by the Yagi and Wakao equationl4 as follows Sh = k P d D = 2 + 1 . 4 5 S ~ ~ ~ ~ R e ' ' ~ (6) where D, Sc, and Re are diffusivity, Schmidt number, and Reynolds number, respectively. Diffusivity of HCN in He of D = 4.7 X 10-4 m2/s was estimated by use of Hirschfelder's equation. Figure 2 shows the effect of 0 2 concentration on the reacted fraction of HCN, XHCN, conversions of HCN to NH3, VNH*, total NO,, V N O ~and , N20, VND. In the absence of 0 2 , HCN was hydrolyzed to NH3 while neither NO, nor N2O was produced. The reacted fraction of HCN, XHCN, increased with increasing 0 2 concentration. At 0 2 concentrations higher than 0.5%, oxidation was dominant. HCN was oxidized to mainly NO, whereas only a small amount of NH3 was produced. The conversion of HCN to NO, for the HCN-H20-02 system was as high as that for the HCN-02 system. The conversion of HCN to N2O increased with increasing H2O concentration and with increasing 0 2 concentration. However, the conversion to N20 for heterogeneous HCN oxidation over limestone was at most 10% and this value is considerably lower than the reported value (>20% ) for homogeneous HCN oxidation.ag For the increase in NzO formation with increasing H2O concentration for the HCN-H20-02 system (Figure 2), there are two possible mechanisms: (1) homogeneous oxidation of HCN is enhanced by H2O and (2) intrinsic conversion of HCN to N2O for heterogeneous oxidation changes with H20 concentration. NH3 produced through hydrolysis is not considered to be involved in the increase in N2O since NHs is known to produce only a small amount of N2O for both heterogeneous and homogeneous oxidati0n.~18*~ For homogeneous HCN oxidation (the first pathway), Hulgaardg reported that the presence of H2O increased the rate of homogeneous HCN oxidation. As shown in Figure 1, a slight consumption of HCN was observed in the absence of limestone ( T = 0) and the consumed fraction of HCN in the absence of limestone for the HCN-H20-02 system was more than that for the HCN-02 system. Thus the increase in N20 formation with H2O feed is partly attributable to the increase in the rate of homogeneous oxidation which produces more N2O than heterogeneous oxidation.
Hydrolysis and Oxidation of HCN
1 n Y
P
X
“““
0.5
F
0, c m . [“I/. 0,c m . [“La] Figure 2. Effect of 02concentration on reacted fraction of HCN and conversionsto NHa, NO,, and NzO. 7 = 0.096 ms;HCN inlet concentration = 698-738 ppm.
For the change in intrinsic conversion of HCN to N2O for heterogeneous oxidation with H2O concentration (the second pathway), however, a conclusioncannot be obtained since the present experimental results could not be completely free from homogeneous oxidation. It was difficult to suppress homogeneous oxidation by decreasing gas residence time by increasing gas feed rate since the pressure drop of the fixed bed became too high. The intrinsic conversion of HCN to N2O for heterogeneous oxidation has to be evaluated in future studies. According to the present results, hydrolysis of HCN to NH3 over limestone is considered to play only a minor role
Energy & Fuels, Vol. 7, No. 5, 1993 647
in the behavior of HCN within CFBCs. On a practical level, 0 2 concentrations in combustion chambers of CFBCs are higher than 1% .10J3 Under such conditions, the rate of HCN oxidation is far higher than the rate of HCN hydrolysis. Thus, most of the HCN reacted over limestone is consumed through oxidation and only a small amount of HCN is consumed through hydrolysis. Consequently, the decrease in N20 emission with limestone feed to CFBCsU is mainly attributable to (1)less N20 formation through limestone-catalyzed HCN oxidation compared with homogeneous HCN oxidation4 and (2) N2O decomposition catalyzed by limestone.”’ Hydrolysis of HCN over limestone is considered to play only a minor role in the decrease in N2O emission with limestone feed into CFBCs.
Conclusion In the absence of oxygen, hydrolysis of HCN over limestone occurred and the major product was NH3. The rate of HCN hydrolysis was found to be first order with respect to HCN concentration. However, in the presence of 02,even under the lean 0 2 condition of 0.5%, oxidation of HCN was dominant and most of the HCN wa8 oxidized to mainly NO,. It is concluded that the hydrolysisof HCN over limestone plays only a minor role in determining the behavior of HCN within CFBCs since the concentrations of 0 2 in the combustion chambers are higher than 1%
.
Acknowledgment. The authors thank the Steel Industry Foundation for the Advancementof Environmental Protection Technology for financial aid and to Idemitsu Kosan Co. Ltd. for cooperation.
