Energy & Fuels 1993, 7,648-654
648
Decomposition of N20 over Limestone under Fluidized Bed Combustion Conditions Tadaaki Shimizu' and Makoto Inagaki Department of Material and Chemical Engineering, Faculty of Engineering, Niigata University, Ikarashi, Niigata, 950-21, Japan Received April 12, 1993. Revised Manuscript Received July 8, 1 9 9 P
A kinetic study was carried out to evaluate the rates of N2O decomposition and SO2 capture over calcined limestone by use of a fixed bed reactor. NzO decomposition was inhibited by HzO and COZ. On the other hand, SO2 capture was enhanced by HzO and it was not affected by COZ. A flue gas treatment study was also conducted by passing flue gas from a bench-scale circulating fluidized bed combustor through a fluidized bed of calcined limestone; thus the activity of calcined limestone to decompose NzO and the activity to capture SO2 were evaluated under an actual fluidized bed combustion condition. N2O decomposition and SO2 capture within the fluidized bed was numerically analyzed using the kinetic data obtained for the present fixed bed study. The results of the numerical simulation agreed well with the experimental results when the kinetic data obtained in the presence of HzO was used. On the other hand, the kinetic data obtained in the absence of HzO overestimated the N2O decomposition within the fluidized bed reactor. Introduction Recently, N2O emission from fossil fuel combustion has become a focus of attention since N2O is not only a greenhouse gas but also an agent of ozone destruction in the stratosphere. Atmospheric fluidized bed combustion has been developed as a promising coal combustion technology that can reduce SO2 emission by feeding limestone (CaC03) into the combustor. In atmospheric fluidized bed combustors (AFBCs), CaC03 is calcined to CaO then the CaO captures S02.132 However, the emission of N2O from AFBCs has been reported to be considerably higha3 Limestone feed to AFBCs has been reported to decrease N2O emission.&7 The calcined limestone (CaO) is known to be a catalyst of N2O decomposition.6*s10 The decrease Abstract published in Aduance ACS Abstracts, September 1,1993. (1)For pressurized FBCs, however, limestone is not calcined due to high COZ partial pressure. Only a trace of CaO was observed in bed material aa reported in: Ljungstrom, E.; Lindqvist, 0.Proceedings of the 7th Internationul Conference of Fluidized Bed Combustion, (PhilaDOE/METC/83-48,DOE (DepartmentofEnergy),1982, delphia, U.S.A.); p 465. (2) The activity of uncalcined limestone to decompose NzO is known to be far lower than that of calcined limestone as reported in: Shimizu, T.; Inagaki, M. Nihon-Energy-Gakkai-Shi (J.Jpn. Inst. Energy) 1993, 72, 199. (3) h a n d , L-E.; Andersson, S.Proceedings ofthe 1OthInternational Conference on Fluidized Bed Combustion (Sun Francisco, USA); ASME New York, 1989; p 49. (4) h a n d , L-E.; Lecher, B.; Andersson, S.;Gustavsson, L. European Workshop on N20 Emissions LNETI/EPA/IFP (Lisbon, Portugal), 1990; p 171. (5) Moritomi, H.; Suzuki, Y.; Kido, N.; Ogisu, Y. In Circulating Fluidized Bed Technology III; Basu, P., Horio, M., Hasatani, M., Eds.; Pergamon Press: Oxford, UK, 1991; p 399. (6) Shimizu, T.; Tachiyama,Y.; Fujita, D.; Kumazawa, K.; Wakayama, 0.; Ishizu, K.; Kobayashi, S.;Sikada, S.;Inagaki, M. Energy Fuekr 1992, 6, 753. (7) Shimizu, T.; Tachiyama, Y.; Kuroda, A.; Inagaki, M.Fuel1992,71, 841. (8) Moritomi, H.; Suzuki, Y.; Kido, N.; Ogisu, Y. Proceedings of the 11 thlnternational Conference on Fluidized Bed Combustion (Montreal, Canada);ASME New York, 1991; p 1005. (9) Iisa, K.; Salokoski, P.; Hupa, M. Proceedings of the 11th International Conference on Fluidized Bed Combustion (Montreal,Canada); ASME: New York, 1991; p 1027.
in NzO emission from AFBCs with limestone feed has been partly attributed to the catalytic activity of CaO for NzO decomp~sition.~JHowever, the contribution of N2O decomposition over limestone to overall N2O emission from AFBCs is not yet clear since the rate of NzO decomposition has not yet been fully evaluated, especially the effect of co-existing gases such as HzO and COZis not fully known. For the analysis of NzO decomposition over calcined limestone within AFBCs, not only NzO decomposition rate but also SO2 capture rate must be evaluated. Feed rate of limestone into an AFBC is controlled so that desired SO2 capture is attained. Thus, the concentration of limestone within the combustion chamber is determined by the rate of SO2 capture. If the rate of NzO decomposition is far lower than that of SO2 capture, NzO decomposition over calcined limestone plays only minor role in determining overall N2O emission from AFBCs since the concentration of the limestone is too low for N2O decomposition to occur. Therefore, it is necessary to evaluate the ratio of N2O decomposition rate to SO2 capture rate. In this work, a fixed bed study was conducted to evaluate the rates of N2O decomposition and SO2 capture. The effect of coexisting gases, COZand HzO, on the rates of N2O decomposition and SO2 capture was studied. Also, N20 decomposition and SO2 capture of flue gas from a bench-scale circulating fluidized bed combustor were conducted by use of a fluidized bed reactor. By making a comparison between the kinetic study and the flue gas treatment study, we discuss the validity of application of the kinetic data to the analysis of AFBC.
Experimental Section Kinetic Study. A flow sheet of the experimental apparatus for the kinetic s t u d y is shown in Figure 1. A quartz fixed bed reactor of 2 c m in inner diameter was employed. Quartz sintered (10) Miettinen, H.; Strcmberg, D.; Lincquist, 0. Proceedings of the 11 th International Conference on Fluidized Bed Combustion (Montreal, Canada); ASME New York, 1991; p 999.
Q~~7-Q624/93/25Q7-Q648$04.QQ/Q 0 1993 American Chemical Society
Decomposition of N20 over Limestone Flow
Energy & Fuels, Vol. 7, No. 5, 1993 649
Neede
Figure 1. Flow sheet of the fixed bed experimental apparatus for kinetic study. ~
CaCOa 96.9
Table I. Analysis of Limestone (wt % 1 MgCOs Si02 &os Fez03 1.4
0.6
0.8
0.3
plate was installed in the reaction zone to support the fixed bed. The temperature was fixed at 1123 K by heating the reactor with a electric furnace. Chichibu limestone was used as a sample. Its composition is shown in Table I. Its particle size was between 0.42 and 0.59 mm. Quartz sand of 0.3-0.5 mm was used to dilute limestone. The weight of quartz sand was 5 g. Calcination of limestone was conducted in a quartz sand bed which waa fluidized in 0 2 stream at a velocity of 0.2 m/s at 1123 K. Raw limestone of 120-200 mg was injected in the fluidized bed and calcined for 5 min. Thus, the limestone was heated up in the same way as in AFBCs. By using an oxidizing atmosphere, carbonaceous material which might exist in the limestone was removed; If the limestone is calcined in an inert atmosphere, it is possible that char produced from the carbonaceous material affects NzO decomposition.6 The physical properties of calcined limestone such as porosity and internal surface area could not be measured by the present experimental apparatus. After calcination, the reactant gas was introduced from the top of the reactor downward through the fixed bed. Catalytic effects of the reactor and quartz sand on reactions were found to be negligible. Total feed rate of reactant gas was fixed at 2 x 10-9mol/s. Helium was employed as a diluent. The reactant gaseous species except for H2O were fed from cylinders. H2O vapor was produced by feeding water with a pump into a heated vaporizer. Concentration of H2O vapor was controlled by regulating the feed rate of water. The concentration of 0 2 was fixed at 4 vol 5%. The produced gas was dried by cooling the gas in an ice bath, and then the gas waa intermittently stored in Tedlar gas bags. Concentrations of N2O and SO2 within the sample gas were meaaured by gas chromatography for N2O and by Kitagawa detector tubes for SO2, respectively. Concentration of NO was measured by an on-line analyzer using chemical luminescence. The concentration of NO in the produced gas was lower than the detection limit of the analyzer ( 5 ppm). For the present kinetic study, formation of N2O in the sample bag is considered to be negligible since the concentration of NO, was sufficiently low. Only under conditions that both SO2 concentration and NO, concentration are sufficiently high, N20 formation is known to occur in the presence of water." Flue Gas Treatment Study. Flue gas from a bench-scale circulating fluidized bed combustor (CFBC) was fed into a fluidizedbed of a mixture of calcined limestone and quartz sand;
thus the reduction of N2O and SO2 over calcined limestone was observed using actual flue gas instead of artificially prepared gas mixture. The detail of the CFBC is described elsewhere.6 Medium-volatile bituminous coal which had been employed for the previous study was used as a fuel. The temperature in the combustor was 1123 K. The diameter and the length of the quartz fluidizedbed reactor were 2 and 16 cm, respectively. The fluidized bed consisted of calcinedlimestone and quartz sand. The same limestone sample as used for the kinetic study was employed for this study. The limestone of 100-200 mg was mixed with quartz sand of 10 g, then packed in the fluidized bed reactor. The size of quartz sand was 0.3-0.5 mm. Calcination was conducted in the bed of quartz sand fluidized in air stream at 1123 K. Then N2O decomposition and SO2 capture of the flue gas were carried out at 1123 K. Transient experiments for NzO decompositionand SO2capture were conducted as follows: A portion of the flue gas from the bench-scale CFBC was continuously sampled from the stack at a rate of 0.9 X 10-9mol/s, the particles were removed by filtering the gas with a silica filter, and then the gas waa led to the quartz fluidizedbed reactor. The filter and the sample line were heated to prevent water condensation. Under the present conditions, residence time of gas in the reactor was ca. 0.9 a. The flue gas from the quartz fluidizedbed was dried by cooling the gas at 275 K and then the gas was led to gas analyzers. Concentration of SO2 was measured by an on-line analyzer using infrared. A portion of the dried gas was intermittently sampled in Tedlar gas bags, and then the concentrations of NzO, 0 2 , and COz were measured by gas chromatography. Drying the sample gas at 275 K was sufficient to suppress N2O formation within the bags containing the flue gas under the present experimental conditions.6
Results and Discussion Kinetic Study. 1. NzO Decomposition. N2O decomposition over unsulfated limestone was conducted. Table I1 shows the products and material balance of N20 decomposition. N20 was decomposed to N2. NO was not detected among the products. The error of the material balance of nitrogen was less than 10%. Figure 2 shows the change in the unreacted fraction of N2O (N~OOUT/N~OIN) after feed of C02 or H2O was started, where N2Oou~and N 2 0 1 ~are the concentrations of N2O at the outlet and at the inlet, respectively. Five minutes after the feed of C02 or H2O was started, the change in the outlet concentration with time was within the range of scattering of the data. Thus, stable catalytic activity was obtained after 5 min. Hereafter, the activity of the calcined limestone for N2O decomposition in the presence of CO2 or H2O was evaluated after 5 min or more. In the absence of C02 and H20, no change in the activity with time was observed for one hour. The reaction order of N2O decomposition over calcined limestone was determinedby two methods, (1)by changing limestone inventory and (2) by changing inlet N2O concentration. In Figure 3, the open symbols show the relation between the unreacted fraction of N2O and contact time (7)at inlet N2O concentrations of 225-269 ppm. The contact time is defined as follows: 7
= (volume of limestone)/(volume flow rate of gas)
where W ,p ~and , F are inventory of raw limestone, density of raw limestone, and gas volume flow rate at the reactor temperature, respectively. Decomposition of N2O without (11) Muzio,L.J.;Teague,M.E.;Kramlich,J.C.;Cole,J.A.;McCarthy, calcined limestone, (7 = 0) was negligible; thus, the N2O J. M.;Lyon, R. K.JAPCA 1989; 39,287. decomposition rate could be evaluated eliminating the (12) Borgwardt, R. H.Ind. Eng. Chem. Res. 1989,28,493.
Shimizu and Inagaki
650 Energy &Fuels, Vol. 7, No. 5, 1993 Table 11. Typical Material Balance for NzO Decomposition. inlet
coz,
NzO-02
HzO,
vol %
ms 0.251 0.393 0.224 T,
NzO-Coz-0~ 15.4 Nz0-Hz0-02 0 NO waa not detected among the products.
Y
,
I
02=4%
balance
02,
NzO,
NzO,
vol %
PPm
PPm
Nz, PPm
NOUJ
vol %
11.0
4.3 4.0 3.8
261 492 233
68 160 123
199 333 121
1.02 1.00 1.04
6000~ 1.0
outlet
I
Nill
I
I
I
I .*+*.....
z -20
-10
0
10
20
30
time [minute]
0
Key I Gas 0 N,O-CO,-OA
1
N,0,,=241
-
261 ppm
Figure 2. Change in the reacted fraction of NzO after starting feed of C02 or HzO (kinetic study, temperature 1123 K).
2
1
: ;1 1
OO
0.2
Contact time, lCO;/%l
0 .
5 10 15 20 H20, CO, conc. [%]
0.4 T
0.6 [ms]
H20~1x1~ 10.8-12.3
OAO : N20,=225-269ppm 0 A m : N20,=444 -499ppm Figure 3. Relation between unreacted fraction of NzO and contact time for N2O decomposition over unsulfated limestone (kinetic study, temperature 1123 K).
effects of N20 decomposition over quartz sand and homogeneous N2O decomposition. The relation between In ( N ~ O O U T / N ~and ~IN T was ) expressed as straight lines. In addition, as shown in Figure 3, the results obtained a t the inlet N2O concentrations of 444-499 ppm (closed symbols) are compared with those obtained a t inlet NzO concentrations of 225-269 ppm. The unreacted fraction of N2O was not affected by the inlet N2O concentration. Therefore, the decomposition of N2O was found to be first order with respect to NzO concentration. Figure 4 shows the effect of concentrations of C 0 2 and H2O on the first order rate constant of N20 decomposition (kNzO) over unsulfated limestone. The rate constant is given as
I
T
ms
0.39-0.40
Figure 4. Effect of concentration of COz and HzO on the firstorder rate constant of NzO decomposition over unsulfated limestone (kinetic study, temperature 1123 K).
The rate constant decreased with increasing C02 and H2O concentration. Both C02 and H2O were found to inhibit N2O decomposition over calcined limestone. The decrease in NzO decomposition rate in the presence of C02 or H2O can be explained by two mechanisms as follows: 1. Physical effect: Reduction of the internal surface area with sintering of CaO is accelerated by H2O and C02.l2 2. Chemical effect: Active sites of calcined limestone for N2O decomposition is covered with adsorbed HzO and C02; thus, N20 cannot reach the sites. To evaluate the contribution of each mechanism, the calcined limestone was exposed to a gas containing CO2 or H20, and the rate of N2O decomposition was measured for the N2O-02 system. Figure 5 shows the first-order rate constants (a) for the N20-02 system obtained before the limestone was exposed to CO2 or H20, (b) for the NzOCO2-02 and N20-H20-02 systems, and (c) for the NzO0 2 system obtained after the limestone was exposed to C02 or H20 for 27 min. The effect of physical property on the activity was evaluated by making a comparison between the cases a and c. A decrease in kNzoof ca. 20 % was observed after the limestone was exposed to C 0 2 or H2O (indicated as P in Figure 5). The chemical effect (adsorption C02 and H2O) was evaluated by making a comparison between the cases b and c (indicated as C in Figure 5). The decrease in kNzO was of ca. 40% for the N20-C02-02 system and it was ca. 50 5% for N20-H20-02 system. Thus, C02 and H20 were found to inhibit NzO decomposition over calcined limestone due to both the physical effect and the chemical effect. In contradiction to the present results, Iisa et aL9 reported that the presence of H2O increased the rate of N20 decomposition for Ignaberga limestone from Sweden. However, the chemical property of Ignaberga limestone is considered to be totally different from ordinary limestones; for catalytic oxidation of NH3 over calcined
Energy & Fuels, Vol. 7,No. 5, 1993 651
,
6000
-3; (4
(a)
7in -4000 Y
s2000
-
10000 c
I
I
0
5000
r-
2000
1
I
5
53
5000
1
1000-
0
0
0
0
so~-co,-o,
~
\t 2000 1000
ani
8
c 10000
in 4000
T
Y
2
3 2000
2000 Gas : N,O-0,
N,O-H,0(8%)
-02
N,O-0, after exposing limestone to H,O of 8%
Figure 5. Change in the first-order rate constant of NzO decomposition after the limestone was exposed to gas containing COz of HzO (kinetic study, temperature 1123 K).
.
xCaSO. r-1 lkeyl
T
~~
ms
I
I I
Figure 6. Effect of contact time on the relation between the reacted fraction of SO2 and the conversion of CaO to Cas04 (kinetic study; SOzm = 214-239 ppm; 0 2 = 4%; CO2 = 15% for SOz-C02-O~system; HzO = 5.6% for SOZ-HZO-OZsystem;lines: eq 19 for SO242 system; eq 20 for SO&OZ-OZ system; eq 21 for SOZ-H~O-OZsystem; temperature 1123 K).
limestone, the selectivity of NH3 to NO has been reported to be considerably high (0.7-0.9) a t temperatures of 10731173 K as reported for Furano limestone (Hirama et al.'3), Iwato limestone (Furusawa et al.14),Reed limestone (Lee et al.15), and the present Chichibu limestone (Shimizu et al.eJ6), whereas the selectivity of NH3 to NO for Ignaberga limestone was only 0.3 a t 1123 K.9 In addition, for NH3of the rate of SO2 capture by calcined limestone under NO-02 reaction over calcined limestone, reduction of NO oxygen rich conditi~ns.l~-'~ Although Borgwardt and was observed for the Ignaberga limestone while increase Bruce2O reported a reaction order of 0.62 under conditions in NO was reported for other limestones.6J3-l6 Although that pore diffusion resistance was negligible, they obtained the reason for this discrepancy is not yet known, it is the data a t conversions of CaO to Cas04 higher than 10% possible that decrease in NzO decomposition rate occurs where the diffusion resistance in the product layer formed in the presence of H20 or COz for other limestones that over the grains was dominant. Simons et al.l9 reported have chemical property similar to Chichibu limestone. that the reaction was first order a t the early stage of 2. SO2 Capture. Transient experiments were conducted sulfation before the resistance of product layer diffusion for SO2 capture. Figure 6 shows the relation between controlled the overall rate. For the present study, the size conversion of CaO to Cas04 on -In (SOZ,OUT/SOZ,IN)/~,of limestone is relatively large (ca. 500 pm), since the where S O ~ J and N S02,o"~are concentrations of SO2 a t the practical FBCs use relatively large limestone particles inlet and the outlet, respectively. Conversion of CaO to (>lo0 pm) to suppress the loss of limestone due to Cas04 (Xcfio,) was calculated as follows: elutriation. Under such conditions, calcined limestone is known to lose its activity due to pore plugging a t the SO, removed from the gas [moll external surface; For the present study, the rates of SO2 (3) xcfi04 = CaO in the limestone [moll capture a t Xcfio,= 0.1 were nearly 1order of magnitude lower than the initial reaction rates as shown in Figure 6. The contact time did not affect -In (SOZ,OUT/S02,IN)/7. Under such conditions, the conversion of CaO to Cas04 Thus, SO2 capture rate of the present limestone was found is considered to be considerably low a t inner part of a to be first order with respect to SO2 concentration. calcined limestone particle; thus, the reaction rate is first The first-order dependency on SO2 concentration oborder with respect to SO2 concentration as reported by served for the present work agrees with previous findings Simons et al. a t the early stage of su1fati0n.l~ (13) Hirama, T.; Kochiyama, Y.; Chiba, T.; Kobayashi, H. NenryoFigure 7 shows the effect of concentrations of COZand Kyokai-Shi ( J . Fuel SOC.Jpn.) 1992, 61, 268. H2O on first-order rate constant of SO2 capture, where (14) Furusawa, T.; Tsujimura, M.; Yasunaga, K.; Kojima, T. Proceedings of the 8th International Conference on Fluidized Bed ComkSOa a t a conversion of CaO to Cas04 of 0.01 was obtained ~~
bustion (Houston, USA); DOE/METC-85/6021, U S . Department of Energy, Office of Fossil Energy: Morgantown, WV, 1985; p 1095. (15) Lee, Y. Y.; Sekthira, A.; Wong, C. M. Proceedings of the 8th Internutionul Conferenceon Fluidized Bed Combustion (Houston, USA); DOE/METC-85/6021;U. 5.Department o fEnergy,OfficeofFossil Energy, Morganbwn, WV, 1985; p 1208. (16) Shimizu, T.; Ishizu, K.; Kobayashi, S.; Shikada, S.; Inagaki, M. Nihon-Energy-Gakkai-Shi (J. Jpn. Inst. Energy) 1993, 72, 189.
(17) Kojima, T.;Take, K.; Kunii, D.; Furusawa,T. J . Chem. Eng. Jpn. 1985, 18, 432. (18) Hasatani, M.; Yuzawa, M.; Arai, N. Kagaku Kogaku Ronbunshu 1982,8, 45. (19) Simons, G. A.; Garman, A. R.; Boni, A. A. AZChE J . 1987,33,211. (20) Borgwardt, R. H.; Bruce, K. R. AIChE J. 1986. 32,239.
Shimizu and Inagaki
652 Energy &Fuels, Vol. 7,No. 5, 1993
m
Xcaso.=O.Ol
n0.8
10000
n Y
2000
'
0
n
5 10 15 20 H20, CO, conc. ["/I. Figure 7. Effect of concentrationsof CO2 and H2O on the firstorder rate constant of SO2 capture over calcined limestone at a conversion of CaO to Cas04 = 0.01 (kinetic study, temperature 1123 K).
0
"0
by interpolating XCfi04-kSOz curve. First-order rate constant of SO2 capture, kso,, is given as
kso, = -1n (S02,0UT/S02,1N)/7
XCaSO,
[-I
0.10
Key CO, H,O
A 15% 0.38ms 0 0% 5.6% 0.24ms Figure 8. Change in the ratio of rate constant for N20 decomposition to rate constant for SO2 capture with increasing conversion of CaO to CaSOI (kinetic study;lines: eq 22 for N2OSOZ-OZ system; eq 23 for Nz0-S02-C02-02 system; eq 24 for N20-S02-H20-02 system; temperature 1123 K).
(4)
The first-order rate constant at Xcdo, = 0.01 was not affected by CO2 concentration. On the other hand, kS02 increased with increasing H2O concentration. Although the reduction of the internal surface area with sintering of CaO occurred in the presence of H2O or C02 as mentioned previously, the rate of SO2 capture increased with increasing H20 concentration and it did not decrease in the presence of CO2. This can be explained by the increased mobility of Ca2+and 0%ions in the presence of H2O or COz. The acceleration of sintering of CaO is attributable to the increased mobility of the ions along the surface.l2I2l On the other hand, Hsia et a1.22 have observed the direction of the growth of the product layer for SO2 capture by CaO and they have concluded that Ca2+ and 0% ions diffuse through Cas04 product layer outward to the CaSOdgas interface. Under such conditions, the rate of SO2 capture is dependent on the mobility of Ca2+and 02-ions. Therefore, the enhanced SO2 capture rate in the presence of H2O can be explained by the increased mobility of these ions. In the presence of C02, where no change in the SO2 capture rate was observed for the present kinetic study, it is considered that the acceleration of the intrinsic SO2 capture rate with accelerated mobility of ions is canceled out by the decrease in the internal surface area. 3. SimultaneousNzO Decomposition and SO2 Capture. N2O and SO2 were simultaneously fed in the fixed bed, thus the rates of N2O decomposition and SO2 capture over partly sulfated limestone were evaluated. Figure 8 shows the change in the ratio of first-order rate constants, kNzO/ kSO, with increasing conversion of CaO to CaSOr. kNzo/ ksoz increased with increasing Xcdo,. There are two factors which may cause the change in kNfl/kSO, with increasing Xcdo,: 1. Cas04 catalyzes N2O decomposition. 2. The change in effective diffusion coefficient with pore closure affects the ratio of N2O decomposition rate to SO2 capture rate. Under the present conditions, however, N20 decomposition by Cas04 is considered to play only a minor role in overall N2O decomposition. Miettinen et a1.'0 examined (21)Anderson. P. J.: Morean. P. L. Trans. Faraday SOC. 1964.60.930. (22) Heia, C.; St. Pierre, 6.R:; Raghunathan, K.; Fan, L.-S.h C h E J.
1993, 39, 698.
0.05
the activity of Cas04 of analytical grade and reported that the activity was poor. They reported that the activity of a fixed bed which consisted of CaSOr:SiO2 = 0.52:7.5 by weight for N2O decomposition was as low as that of Si02 bed. For the present study, CaSO4:SiOz is ca. 0.035 by weight (at Xcdo, = 0.1 for a limestone inventory of 200 mg). Under such conditions, the contribution of Cas04 to N2O decomposition is considered to be as low as quartz sand, whose activity is negligible as shown in Figure 3. Therefore, the increase in kNfllkS0, with increasing X c a , cannot be explained by the catalytic activity of CaSO4. Although Iisa e t aL9reported that N2O decomposition was catalyzed by Cas04 which had been produced by sulfating limestone, it is possible that the unreacted CaO in the limestone catalyzed the reaction since the conversion of CaO to Cas04 was ca. 0.6-0.7 for their experiments. On the other hand, the decrease in effective diffusion coefficient with pore closure is considered to change kN#l kso,. With increasing Xcdo4, pores close a t the external surface of the particle. Finally, the overall rates of NzO decomposition and SO2 capture are controlled by the diffusion through partly closed pores a t the surface, Le., by the effective diffusion coefficient a t the external surface, De,NzOand De,SOZ, r e ~ p e c t i v e l y .The ~ ~ effective diffusion coefficient is given as De = (porosity1tortuosity)D (5) where D is the diffusion coefficient. Thus, kNfllkso, is given as the ratio of diffusion coefficient = De,NzdDe,SO1 D N ~ o ~ D s o ~ (6) Under molecular diffusion conditions, D N ~ and O Dso, a t 1123 K are 5.6 X 10-4 and 4.8 X 10-4 m2 s-l, respectively. Thus, kN,O/kSO, is 1.2. Under Knudsen diffusion conditions, diffusion coefficient is given by molecular weight (M), thus, we obtain kN,dkSO,
Under the conditions that overall rate is controlled by the diffusion resistance in the partly closed pores a t the (23) For the SO2 capture by calcined limestone, the resistance of pore diffusionis the largest at the external surface. Thus, the apparent activity can not be given as a product of rate constant and effectivenew factor which is based on the assumption of uniform diffwion resistance in a particle. In this explanation, the diffusion resistanced is assumed to be lumped at the external surface.
Decomposition of NzO over Limestone
Energy & Fuels, Vol. 7, No. 5, 1993 653
Table 111. Typical Composition of Flue Gas from the Bench-Scale Circulating Fluidized Bed Combustor composition of dry flue gas H2O 02, COZ, N2, NzO, SO2, concentration,' vol% vol % vol% ppm ppm vol % 3.8-3.9 14.6-14.9 80.4-81.0 209-215 193-208 5.3
~~~
reactor model. The fluidized bed consists of two regions, bubble and emulsion. Both NzO decomposition and SO2 capture are first-order reactions; thus, the change in concentration in the bubble (Cb,i) and that in the emulsion (C,i) with increasing height (2) are expressed as follows:
a Calculated from the moisture content and hydrogen content of the fuel.
( a)W,=100[mgl
(b)WL=200[mg]
(1- 6)(1- €&)kav,jCe,i(9)
E P
where subscript i denotes gaseous component (i = NzO and SOz). The boundary condition of eqs 8 and 9 is given
F -01
i 2
)002
000 0-
time
[SI
as
K -01 0
0
0
0
time ~
y
[SI
1
0
0
0
Figure 9. Dynamic change in the concentrations of N2O and SO2 at the reactor exit of fluidized bed reactor after starting flue gas feed for the flue gas treatment study (symbols: experimental results; lines: results of the numeric analysis; temperature 1123
cb,i = Ce,i= Cm,i at z = 0 (10) CIN,~ is the concentration of the component i a t the inlet of the fluidized bed reactor, i.e., concentration in the flue gas as shown in Table 111. CINis 210 ppm for Nz0 and 200 ppm for SOZ, respectively. The parameters in eqs 8 and 9 are described below: K b is the gas interchange coefficient between bubble and emulsion given as follows:
where umf and db are superficial velocity a t minimum fluidizing condition and bubble diameter, respectively. Bubble diameter is assumed to be db = 5 mm. The rise velocity of bubble gas (Ub*)is given by bubble rise velocity (Ub), superficial gas velocity (UO)and u d as follows: ub* = ub 3umf (12)
+
K). extemalsurface, k ~ f i / k sbecomes o~ 1.2. On the other hand, a t the early stage of sulfation, when mass-transfer resistance through pores is not so large, kNzO/kSOz was 0.160.47 as shown in Figure 8. Therefore, kNzO/kSoZincreases from 0.16-0.5 to 1.2 with pore plugging, i.e., with increasing
xcaso,.
Flue Gas Treatment Study. Flue gas from a benchscale CFBC was fed to a fluidized bed consisted of limestone and quartz sand; thus, the activity of calcined limestone for N20 decomposition and SO2 capture was evaluated by use of actual flue gas. Typical analysis of the flue gas is shown in Table 111. Batch experiments were conducted. Figure 9 shows the dynamic change in the concentrations of NzO and SO2 after the flue gas feed was started, where concentrations are corrected to dry gas which containing 02 of 6%. Both NzO and SO2 concentrations increased with time, i.e., with increasing conversion of CaO to CaS04. For the quartz sand bed without limestone, neither N2O decomposition nor SO2 capture took place. Numerical Analysis of Fluidized Bed for Flue Gas Treatment Study. A numerical analysis was conducted for the flue gas treatment within the fluidized bed reactor, using kinetic data obtained for the present fixed bed study for SO2 capture and NzO decomposition. By comparison of the results of simulation and the experimental results, the validity of application of kinetic data to the analysis of NzO decomposition under AFBC conditions is discussed. The bubbling bed model for intermediate size particle proposed by Kunii and L e ~ e n s p i e lwas ~ ~ employed as a ~~
~
(24) Kunii, D.; Levenspiel, 0. Fluidization Engineering, 2nd ed.;
Butterworth-Heinemann: Stoneham, MA, 1991;Chapter 12.
ub = uo - umf + 0.711(gdb)1'2
(13)
where g is the acceleration of gravity. 6 is the volume of bubbles per unit volume of bed:
6 = (UO - umf)/(ub+ 2 U d ) (14) and Yb are void fraction in the bed a t minimum fluidizing condition and volume of solids per unit volume of bubble, respectively. Yb and emf are assumed to be Yb = 0 and emf = 0.5, respectively. The height of the bed (20) is given by the inventory of the solids, cross-sectional area of the reactor (A), emf and 6. tmf
where WL, WQ,p ~ and , PQ are inventory of limestone, inventory of quartz sand, density of limestone and density of quartz sand, respectively. kav,iis the average first-order rate constant. In the present work, the limestone was diluted with quartz sand; thus, average rate constant is given as follows: (16) where ki is the rate constant obtained for the kinetic study. The concentration of each component a t the outlet of the reactor (&UT) are given by the concentrations in the bubble phase and the emulsion phase a t the upper surface of the bed ( z = 2 0 ) as
The change in the conversion of CaO to Cas04 with time is calculated from the difference in the concentration
654 Energy &Fuels, Vol. 7, No. 5, 1993
Shimizu and Znagaki
of SO2 between the inlet and the outlet.
By comparing the results of fixed bed study and those of the flue gas treatment study, the authors conclude that the presence of H2O is necessary to obtain a rate of N2O decomposition over calcined limestone which is applicable to the analysis of AFBCs. On the other hand, it is likely that the kinetic data obtained in the absence of H2O overestimate the N2O decomposition within the combustion chamber of AFBCs.
dXCaSO, - ~oA(~rN,so, - couT,so,) -dt
~caco,WL/Mcaco,
(18)
where Olcacos and Mcacos are CaCOs content of the limestone and molecular weight of CaC03, respectively. The rate expressions obtained for the NzO-SO2-02 system, the N20-SOd02-02 system, and the N2O-SOzH2O-02 system were applied in the model; thus, the validity of application of kinetic data to the analysis of N2O decomposition under AFBC conditions is discussed. The first-order rate constant for SO2 capture &so2 [s-ll) is approximated as a function of the conversion of CaO to Cas04 (XcasoJ a t X C ~ O 0.1, however, these expressions could not be applied for fitting the experimental results since the difference in the concentration of SO2 between the inlet and outlet was so slight due to very low activity that the accuracy of the value of kSOa was questionable. Also kNzol kSOz can be approximated as functions of X c m 4a t Xcaso4 < 0.1 as shown in Figure 8. for the N,O-SO2-0,
Conclusion A kinetic study was carried out to evaluate the rates of N 2 0 decomposition and SO2 capture over calcined limestone by use of a fixed bed reactor. The decomposition rate of N2O decreased in the presence of COz whereas SO2 capture rate was not affected by COz. The decomposition rate of N2O decreased with increasing H2O concentration while SO2 capture rate increased. Flue gas from a benchscale CFBC was fed in a fluidized bed of calcined limestone and the reduction of N2O and SO2 was observed under practical AFBC conditions. The results of the numeric simulation of the fluidized bed based on the kinetic data obtained in the presence of H2O agreed with the experimental results. On the other hand, the kinetic data obtained in the absence of H20 resulted in the overestimation of N2O reduction.
Acknowledgment. T.S. expresses his thanks to the Steel Industry Foundation for the Advancement of Environmental Protection Technology for financial aid and to Idemitsu Kosan Co. Ltd. for cooperation. The authors thank the students of Niigata University, Mr. Daisuke Fujita, Mr. Satoshi Kusakai, Mr. Masashi Miura, and Mr. TakeshiTogashi, for their assistance in CFBC experiments. Glossary
system:
for the N2O-CO2(15%)-SO2-O2 system:
for the N20-H20(5%)-S02-02 system: (24) The results of the calculation are shown in Figure 9 compared with the experimental results. The calculated outlet concentration of SO2 agreed well with the experimentalresulta for all rate expressions. Since the difference in kSO, among the rate expressions eqs 19-21 was less than 20 5% ,the difference in the calculated outlet concentration was slight. On the other hand, considerable difference in the outlet concentration of N2O was observed among the rate expressions. By employing kinetic data obtained for NzOSOrH20-02 system, the results of the calculation agreed fairly well with the experimental data. The kinetic data obtained in the absence of HzO, Le., for the N20-SO2COz-O2 system and for the N20-S02-02 system, were found to overestimate N20 decomposition.
Yb
6 PL PQ 7
concentration in bubble phase concentration in emulsion phase concentration in feed gas concentration at the outlet of the reactor diffusion coefficient effective diffusion coefficient bubble diameter gas volume flow rate at the reactor temperature acceleration of gravity rate constant obtained for the kinetic study average rate constant of bed solids gas interchange coefficient between bubble and emulsion molecular weight of CaCOa time bubble rise velocity rise velocity of bubble gas superficial velocity at minimum fluidizing condition superficial gas velocity inventory of raw limestone inventory of quartz sand height dense bed height content of CaCOa of limestone by weight volume of solids per unit volume of bubble volume of bubbles per unit volume of bed void fraction in the bed at minimum fluidizing condition density of raw limestone density of quartz sand contact time = W d p S
Subscripts
i
gaseous component (NzO and SOz)
