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Catalytic reduction of NO and N2O on limestone during sulfur capture under fluidized bed combustion conditions
Science. Chemical Engineeting Printed in Great Britain.
Vol.
47,
No.
9-l
1, pp. 2419-2424.
1992. 0
ocw9-2509192 $5.00+0.00 1992 Pergamon Press Ltd
Catalytic R.eductionof NO and N,O on Limestoneduring Sulfur Capture under Fluidized Bed Combustion Conditions P.F.B.
Hansen, K. Dam-Johamen,
J.E. Jolumon and T. Hulgaard
Department of Chemical Engineering Technical University of Denmark 2800-DK, Lyngby, Denmark
AbatrZbCt sulfur retention by limestone (CaCOa) under fluidized bed combustion conditions is the net effect of a competition between sulfur capture and sulfur release during which the composition of the Ca surface changes continuously between CaO, CaS and C&04, the ultimate product being CaSO4. The presence of limestone in fluidized bed combustors interacts with the nitrogen chemistry; it acts as a catalyst for volatile-N oxidation to NO and as a catalyst results
for show
the that
reduction CaS
and
of
NO
CaO
and
N20,
are active
CM04 and CaCO3 are poor catalysts. conditions which leave NO unaffected. reduced simultaneously but apparently of limestone decreases during sulfation
the
latter
catalysts
for
being
the subject
reduction
of NO
of this and
paper.
N20,
The
whereas
N20 decomposes readily over CaO under oxidizing Under reducing conditions, both NO and N20 are not at the same active sites. The catalytic activity due to a 1~ in porosity and a lower activity of the
CM04 formed. The highest catalytic activity for NO a high sulfur capacity. The presence limestone with heterogeneous reactions involving NO and N20.
and N20 reduction was found of H20 appears not to affect
for the
Introduction One of the major advantages of fluidized bed combustion (FBC) is the low level of emissions of SO2 and NO that is a result of the low combustion temperatures of SOO-900’ C. The low temperatures allow in-situ retention of sulfur by injection of limestone and at the same time prevent formation of thermal NO. Moreover, high concentrations of volatile matter in the lower part of the bed reduce a large amount of the NO already formed to N2. On the other hand, the presence of limestone may result in increased NO emissions, and the low temperature is responsible for the relatively high level of NzO emissions. A comprehensive study of N20 emissions from different combustion systems showed that emissions from FBC are 5 to 50 times as high as emissions from conventional combustion systems operating at higher temperatures (Hulgaard and Dam-Johansen, 1992). Due to the high particle concentration heterogeneous reactions are believed to be as compared to other combustion systems. Laboratory-scale important in FBC investigations have shown that CaO is a stron catalyst in the conversion of fuel-N into NO and Ns, as well as for reduction of NO by 8 0 or Hz (Johnsson, 1989; Dam-Johansen et al., 1992). Johnsson (1991) reports that calcined limestone and analytical grade CaO are active catalyst in N20 decomposition; the activity of sulphated limestone, however, is unclear. CaS, which may be formed durin sulfur retention, was found to be a strong catalyst in the reduction of NO by CO ( # urusawa et al., 1985). No reports on the catalytic activity of CaS towards N20 reduction have been found in the literature. The net effect on the NOx chemistry of limestone addition into FBC depends on design and operation conditions (Leckner and Amand 1987; Lyngfelt and Leckner, 1989). NsO ecrease or remain unchanged when limestone is introduced into a FBC emissions either (Johnsson, 1991; % mand et al., 1992). 2419
P. F. B. HANSEN
2420
C6
et al.
Traditionally, bench-scale studies of sulfur capture by limestone under FBC conditions have been carried out under constant oxidizin conditions. However, limestone particles injected into stationary or circulating FB 8 will experience an atmosphere which alternates between oxidizing and reducing conditions. Recent studies on sulfur capture by limestone under alternating oxidizing and reducing conditions (Hansen et al.; 1990, 1991, 1992) have revealed that sulfur retention under FBC conditions is the net effect of a competition between sulfur capture and sulfur release. Hence the composition of the Ca-surface is constantly changing. The solid phase transformations during sulfur retention under alternating conditions are shown in Fig. 1 and a brief description is given below. More detailed information on this subject can be found in the literature (Hansen et al.; 1990, 1991, 1992). It follows from the molar volume of the solid phases that great structural changes will accomnanv the reactions: CaCOs, CaO, CaS04, CaS, 36.9, 16.9, 46.0, and 28.9 c&s/mole respectivkly. When limestone is heated to FBC temperatures at atmospheric pressure, the main constituent - CaCOa - calcines and forms CaO, which subsequently reacts with the gaseous sulfur compounds in the bed as shown in Fig. 1. Under oxidizin conditions SO2 is captured as CaSO4, un J er reducing conditions - and in the presence of CO - as CaS. Any transformation between CaS04 and CaS appears to proceed via CaO. The intermediate Ca compound is believed to be CaO and not CaSOs, since a release of SO2 can be seen when the intermediate is formed from either CaS or CaS04. Reactions catalyzed by CaO, CaS04, and/or CaS will be affected by the changing composition of the particle surface.
CaS04
CaS
11X JcII CaO
co
co2
Fire
1. Solid phase transformations during sulfur capture under FBC conditions
The aim of this study was to evaluate how NO and N20 are reduced over limestone under FBC conditions with and without simultaneous sulfur capture. Experimental
Study
The experimental study was carried oui in a laboratory scale fixed bed quartz reactor in the 800-950 C temperature range. An artificial flue gas was mixed from pure gases or gas mixtures. The reactor is shown schematically in Figure 2 (inner diameter: 24 mm, length: 630 mm). To simulate alternating oxidizing and reducing conditions, the 02 concentration was varied periodically between 0 and 4 vol. %, out of phase with the CO concentration which was also varied between 0 and 4 vol. %. The main gas was introduced through the gas inlet (1) and passed the preheating section (2) before entering the bed. The 02 and the CO were let into the reactor through the capillary tube (3). A typical gas composition was 1500 ppm SO2, 625 ppm NO, 400 ppm N20_, 10 % CO2 and 6 % H20. The limestone sample to be investigated was easily introduced onto the porous quartz plate 4) (14 mm in diameter) in the detachable bottom section of tI, e reactor without changing the reactor temperature significantly. The reaction temperature was measured by a thermocouple (6) placed just below the porous quartz plate and continuous monitors were used for measuring the SOs, CO2, CO, 02, NO, and NzO outlet concentrations. SO3 was not measured. Previous studies have shown that less than 1.5 % of the SO2 is oxidized to SOa.
Fire
2. The fixed bed quartz reactor
C6
Catalytic reduction of NO and N20 on limestone
242 1
More detailed information on the experimental setup and experimental procedure is available in Hansen et al. (1992). The dynamics of the N20 analyzer is slow: a 90 % response is obtained after about 300 sec. (Hulgaard et al., 1989). Experimental Results and Diion Catalytic reduction of NO over limestone under alternating conditions was tested at typical FBC temperatures - around 850° C - and under three different sets of conditions. Period A: over calcined limestone, period B: during simultaneous sulfur capture, period C: during reductive decomposition of sulphated limestone. Concentration rofiles for CO, N20, NO, and SO2 are shown in Fig. 3. Periods of 10 minutes were used P5 min. of oxidizin and 5 min. of reducing conditions); 02 is present when the CO level is zero, v$ ithout limestone present no reduction of NO and less than 10 % reduction of NT& zh observed. Period A: NO was reduced only in the presence of CO. A rapid increase in the outlet NO concentration was detected during the first period of NO reduction, indicating a deactivation of the CaO. Studies of NO reduction over CaO in the absence of CO2 do not show a similar decrease in activity. Sintering of CaO catalyzed by CO2 as described by Borgwardt (1989) cannot be the reason for these observations since the catalytic activity was immediately restored when CO2 was removed from the gas. Poisoning of the active sites is believed to be the reason for the gradual deactivation of CaO. During NO reduction a minor net formation of NzO may be detected. NzO is easily decomposed over CaO. Thus, it is not possible to estimate the amount of N20 actually formed. N20 formation decreased as the reduction of NO decreased. After the shift to oxidizing conditions, a sharp peak of N20 was seen. This is most likely due to an increase in the rate of formation of N20 (by oxidation of adsorbed N , since N20 decomposes readily over CaO even under constant oxidizing conditions (cf. r!rgure 4).
0 1500
SO2 bpmv>
-
500 -
1000
‘-
OO
I
50
Time
I
100
(minutes)
Figure 3. NO reduction over Stevns Chalk. Period 3.0 mmol CaO, 1.0-1.4 mm, 850-C, 600s (300s Ox/3OOs Red), 0.9 Nl/min 9.8 % [COZ]~, 680 ppmv [Nolo, 1480 ppmv [SOz]o, O-3.9 % [CO]o and [0210, 6 % [H2O]o.
150
Period B: When SO2 was introduced under reducing conditions, CaS was formed (Hansen et al.; 1991, 1992). The NO concentration dropped immediately indicating either that the catalytic activity of the solid was restored, that catalytically active CaS was formed or a combination of the two. After the shift to oxidizing conditions, CaS was rapidly oxidized to CaO and release of SO2 was reflected in a peak of SO2 and a distinct temperature peak, the latter of which is shown in Fig. 4. NO reduction was not observed under oxidizing conditions. After the return to reducing condiformed tions, some of the CaS04 decomposed to CaO and SO;z; CaS was subsequently formed and the reduction of NO resumed. Thus the SO2 profile for sulfur capture under alternating conditions contains two sets of SO2 peaks: one set caused by CaS oxidation and another set caused by reductive decomposition of CaS04.
The catalytic activity of the solid diminished as the degree of sulfation increased. This may be explained partly by a low catalytic activity of CaSO4 and partly _ _ . . . . . by a reduced porosity and thereby less sunace area. The reduced porosity of the particles after the sulfation is caused by the large differences in the molar volumes of the solids.
P. F. B. HANSEN
2422
et al.
C6
Contrary to the findings during period A, the catalytic activity of the solid for NO reduction was seen to increase durin a period of reducing conditions. This can be explained by the gradual formation of the catalytically active CaS, which took place primarily near the particle surface. There was some drift in the zero-point of the NzO analyzer during period B. However, a minor formation of NsO can be seen.
Period C: Addition of SO2 was stopped after 125 minutes. The SO2 peaks observed under reducing conditions originate from CaS04 being reductively decomposed. The SO2 maximum, seen after 2 minutes of reduction, reflects the point at which sufficient CaO has been formed for the rate of CaS formation to become higher than the rate of CaSOd decomposition. Moreover, under these reaction conditions, 100 ppm SO2 must be present for CaS to be thermodynamically stable (Hansen et al., 1992). The SO2 peak at the end of the period is caused by CaS oxidation to CaO and SO 2. It appears that the period of NO reduction is shorter than during periods A and B. This seems to be closely related to the period of CaS formation. No reduction of NO was observed during the early part of the CaS04 decomposition process. The decrease in NO concentration during period C refleets the gradual reduction in the degree of sulfation. As CaS04 decomposes the porosity of the particle increases. Formation of N20 then increases to the level seen during period A. Periods A and B shown in Figure 3 were repeated in the presence of NzO. The profiles for temperature and concentrations of NzO, NO, and SO2 are shown in Figure 4.
Period A; The NO concentration under reducing conditions was nearly constant throu hout period A, the initial deactivation of the catalytically active CaO (shown in perio f A in Fig. 3) is not included in Fig. 4. About 80 91 of the NzO decomposed over CaO under oxidizing conditions. Under reducing conditions about 90 % N20 is decomposed. Thus even though there is a higher degree of conversion under oxidizing conditions, it may be even higher under reducing conditions. The observed increase in NzO reduction under reducing condititions may be explained by either heterogeneous decomposition, homogeneous decomposition (Hulgaard et al., 1991), or both. However, further studies of the homogeneous decomposition of N20 are necessary. The presence of NO does not affect for the active sites is not the decomposition of NzO, indicating that competition important under these conditions. Unlike NzO, the NO molecule contains an unpaired electron which could indicate that different sites on the catalytic surface may be preferred by ;he two molecules. B I Period B: SO2 addition is started at reduTamp cc> cing conditions leading to formation of CaS. 875 From Fig. 4 it can be seen that CaS is at least as active a catalyst for N20 decomposition as CaO. CaS04, on the other hand, is 825 found to be a poor catalyst for NzO decomposition. This agrees with findings by Khan 200 et al. (1991) and Miettinen et al. (1991) whereas Iisa et al. (1991) found sulphated 0 limestone to be an effective catalyst. The 800 N20 reduction observed during sulfation may be explained by decomposition on the 300 free CaO surface situated deeper within the 0 limestone particles. As seen for NO in Fig. 1500 3, the catalytic activity of the solid decreases as the degree of sulfation pro res1000 ses. Also, the rate of decomposition for a 20 500 is seen to increase at reducing conditions 0 when CaS04 is reductively decomposed and 100 50 CaO and CaS are being formed. Time (minutes) Figure 4. Reduction of NO and N20 over Stevns I
Chalk. 2.98 PeriodSOOs,
mmol CaO, dp: 0.9 Nl/min,624
395 ppm PJ2010, 9.8 % [CO&, 04
1505 %
ppm
0.85-1.00 mm, ppm [NO],
W210, [OZ]~ and [CO]o.
850” C.
Drying the gases or addition of 6 % Hz0 to the gas does not effect the heterogeneous reactions involving NO and N20.
Catalytic reductionof NO and N20 on limestone
C6
2423
It appears from the two experiments described in Figures 3 and 4 that NzO decomposes readily on CaO under oxidizing conditions while NO remaim unaffected. Under reducing conditions both NO and N20 are reduced. When the temperatures were increased from 800 to 950° C, the rates of NO and N20 reduction increased significantly. During a slow calcination of Stevus Chalk at 800” C, the reduction of NO and N20 was negligible. After calcination the reduction of both NO and N20 became appreciable, indicating a low catalytic activity of CaC03. Different types of limestones act differently with respect to the nitrogen chemistry in FBC (MjBrnell et al., 1991). For this reason the simultaneous decomposition of NO and N20 was tested over six European limestones, ranked in Table 1, from the young porous Stevns Chalk with high sulfur capacity to the very old almost crystalline Kijping limestone with low sulfur capacity (Dam-Johansen and Bstergaard, 1991). The maximum reduction of NO and the average reduction of NO under reducing conditions (averaged over the first 45 minutes) versus sulfur capacity for the six limestones is shown in Figure 5. 1.0
- U&U
-Ah&b&
Max. reduction of
Ave.
NO
roductlon ot NO over
Final
degre?k
4s
Table 1. Six European limestones ranked according to sulfur capacity
mln.
sulphation
l
-
*
-
1.0
Bryozo (Fe) Gotland
Figure 5. Maximum reduction and 45 minutes average reduction of NO versus the sulfur capacity for the 6 limestones listed in table 1.
It appears that use of any of the six limestones results in a substantial reduction of NO. However, limestone with the highest sulfur capacity also appears to exhibit the highest catalytic activity towards NO reduction, an effect most likely caused by differences in porosity and surface area of the sorbent. In order to obtain the same degree of desulfurization during combustion, limestone with high sulfur capacity must be present in a lower concentration than limestone with low sulfur capacity. Thus the importance of the sorbent to the nitrogen chemistry may be hi her when limestone with a low sulfur capacity is used. The maximum reduction of N2 8 and the average reduction occurring under both oxidizing and reducing conditions shows a similar trend as for NO. Mjijrnell et al. (1991) reported that the addition of Ienaberga limestone to a CFBC resulted in a decrease in N20 emissions, whereas addition of Kiiping limestone had no effect on the level of NzO emissions. It was found that the catalytic activity of Ignaberga in N20 reduction is substantially higher than that of K5ping, indicating that CaO-catalyzed decomposition of N20 affects emissions in a CBFC to a great degree. Conclusions. -
CaO and CaS are strong catalysts in NO and N20 reduction by CO whereas CaS04 and CaCOs are poor catalysts. A high catalytic activity towards NO and N20 reduction has been found in limestones with high sulfur capacity. The catalytic activity of a limestone decreases as the degree of sulfation increase. NO does not decompose under oxidizing conditions; N20 decomposes readily on CaO under oxidizing conditions. Reduction of NO and N20 on CaO and CaS does not appear to take place on the same active sites.
P. F. B. HANSEN et al.
2424 -
-
-
C6
Durin NO reduction by CO on CaO, a lower percentage of the NO may be reduced toN2 8 . With CO2 present in the gas, poisoning of the catalytic sites may be important during NO reduction by CO over CaO. The addition of SO2 under reducing conditions immediately restores the catalytic activity. Hz0 does not affect the heterogeneous reduction of NO and N20.
Acknowledgements: The authors gratefully acknowledge financial support from the Nordic Ministers Council, ELSAM (The Jutland-Funen Electricity Consortium), ELKRAFT (The Zealand Electricity Consortium) and the Danish Ministry of Energy. Cited literature: Bornwardt. R.H. (1989), Sintering of Nascent Calcium Oxide, Chem. Engng. Sci., 44, 1, 53-60 Dam-Johansen. K and Q)stergaard, K. (1991), High-Temperature Reaction Between sulfur Dioxide and Limestone-I. Comparison of Limestones in Two Laboratory Reactors and a Pilot Plant, Chem. Engng. Sci., 46 3, 827-837 and Leckner, B. (1992) In fl uence of SO2 on the NO/N20 Dam-Johansen. K., 8, mand, L.-E. Chemistry in Fluidized Bed Combustion. II: Interpretation of Full-scale Observations Based on Laboratory Experiments, Submitted. Furusawa. T., Koyama, M. and Tsujimura, M. (1985), N’It ric Oxide Reduction by Carbon Monoxide over Calcined Limestone Enhanced by Simultaneous Sulfur Retention, FUEL, 84, 413415. Hansen P.F.B., Dam-Johansen K., Bank, L.H. and astergaard, K. (1990), Sulfur Capture on Limestone under Periodically Changing Oxidizing and Reducing Conditions, 3rd Int.Conf. on Fluidized Bed, Japan. Edited by P. Basu. Dam-Johansen K., Bank, L.H. and IZlstergaard, K. (lQQl), sulfur Capture Hansen P.F.B., on Limestone under Fluidized Bed Combustion Conditions - An Experimental Study, 11th Int.Conf. on Fluidized Bed Combustion: Clean Energy for the World. Montreal. 73-82. Edited by E.J. Anthony. Hansen P.F.B., Dam-Johansen K. and (astergaard, K. (1992), High-Temperature Reaction Between sulfur Dioxide and Limestone-V. The Effect of Periodically Changing Oxidizing and Reducing Conditions, Submitted. Hulgaard. T., Dam-Johansen, K., Karlsson, M. and Leckner, B. (1989), Evaluation of an Infrared N20 Analyzer, CHEC-Article no. 8904, Presented at the first technical meeting, Amsterdam, Ott, 1989. Hulzaard. T. and Dam-Johansen, K. (1992), Submitted. Hulgaard, T., Glarborg, P. and Dam-Johansen, K. (1991), Homogeneous Formation and Destruction of N20 at Fluidized Bed Combustion Conditions, 11th Int.Conf. on Fluidized Bed Combustion: Clean Energy for the World. 991-998. Montreal. Edited by E.J. Anthony. Iisa. K., Salokoski, P. and Hupa, M. (1991), Heterogeneous Formation and Destruction of Nitrous Oxide under Fluidized Bed Combustion Conditions, 11th Int.Conf. on Fluidized Bed Combustion: Clean Energy for the World. 1027-1033. Montreal. Edited by E.J. Anthony. Johnsson. J.E. (1989), A kinetic Model for NO x Formation in Fluidized Bed Combustion, 10th Int.Conf. Fluid Bed Combustion: FBC-Technology for today. San Francisco, 1111-1118. Edited by A.M. Mar&car. Johnsson, J.E. (1991), Nitrous Oxide Formation and Destruction in Fluidized Bed Combustion - A Literature Review of Kinetics, Presented at the 23rd IEA meeting in Firenze, November 8, 1991. of Nitrious Oxide in the Khan. T., Lee, Y.Y and Young, L. (1991) Heterogeneous Decomposition Operating Temperature Range of Circulating Fluidized Bed Combustors, Proceedings from the 1991 EPRI/EPA Joint Symposium on Stationary Combustion NOx Control, Washington, D.C. Leckner, B. and amand, L.E. (1987) E missions from a Circulating and a Stationary Fluidized bed Boiler, A Comparison.Qth Int.Conf. on Fluidized Bed Combustion: FBC Comes of Age. Boston, 891-897. Edited by J.P. Mustonen. Lvnzfelt. A. and Leckner, B. (1989), SO2 Capture in Fluidized Bed Boilers: Re-emission of SO2 due to Reductive Decomposition of CaS04, Chem.Engng.J. 40, 207-213. Miettinen. H., Striimberg, D. and Lindquist 0. (1991) The Influence of Oxide and Sulphate Surfaces on N20 Decomposition, 11th Int.Conf. on Fluidized Bed Combustion: Clean Energy for the World. Montreal. 999-1004. Edited by E.J. Anthony. Miijrnell, M. Leckner,B,. Karisson, M and Lyngfelt, A. (1991), E mission Control with Additives in CFB Coal Combustion, 11th Int.Conf. on Fluidized Bed Combustion: Clean Energy for the World. ontreal. 655-676. Edited by E.J. Anthony. K. (1992) Influence of SO2 on the NO/N20 Leckner, B. and Dam-Johansen, x mand. L.-E., Chemistry in Fluidized Bed Combustion. I: Full-Scale Experiments, Submitted.
Vol.
47,
No.
9-l
1, pp. 2419-2424.
1992. 0
ocw9-2509192 $5.00+0.00 1992 Pergamon Press Ltd
Catalytic R.eductionof NO and N,O on Limestoneduring Sulfur Capture under Fluidized Bed Combustion Conditions P.F.B.
Hansen, K. Dam-Johamen,
J.E. Jolumon and T. Hulgaard
Department of Chemical Engineering Technical University of Denmark 2800-DK, Lyngby, Denmark
AbatrZbCt sulfur retention by limestone (CaCOa) under fluidized bed combustion conditions is the net effect of a competition between sulfur capture and sulfur release during which the composition of the Ca surface changes continuously between CaO, CaS and C&04, the ultimate product being CaSO4. The presence of limestone in fluidized bed combustors interacts with the nitrogen chemistry; it acts as a catalyst for volatile-N oxidation to NO and as a catalyst results
for show
the that
reduction CaS
and
of
NO
CaO
and
N20,
are active
CM04 and CaCO3 are poor catalysts. conditions which leave NO unaffected. reduced simultaneously but apparently of limestone decreases during sulfation
the
latter
catalysts
for
being
the subject
reduction
of NO
of this and
paper.
N20,
The
whereas
N20 decomposes readily over CaO under oxidizing Under reducing conditions, both NO and N20 are not at the same active sites. The catalytic activity due to a 1~ in porosity and a lower activity of the
CM04 formed. The highest catalytic activity for NO a high sulfur capacity. The presence limestone with heterogeneous reactions involving NO and N20.
and N20 reduction was found of H20 appears not to affect
for the
Introduction One of the major advantages of fluidized bed combustion (FBC) is the low level of emissions of SO2 and NO that is a result of the low combustion temperatures of SOO-900’ C. The low temperatures allow in-situ retention of sulfur by injection of limestone and at the same time prevent formation of thermal NO. Moreover, high concentrations of volatile matter in the lower part of the bed reduce a large amount of the NO already formed to N2. On the other hand, the presence of limestone may result in increased NO emissions, and the low temperature is responsible for the relatively high level of NzO emissions. A comprehensive study of N20 emissions from different combustion systems showed that emissions from FBC are 5 to 50 times as high as emissions from conventional combustion systems operating at higher temperatures (Hulgaard and Dam-Johansen, 1992). Due to the high particle concentration heterogeneous reactions are believed to be as compared to other combustion systems. Laboratory-scale important in FBC investigations have shown that CaO is a stron catalyst in the conversion of fuel-N into NO and Ns, as well as for reduction of NO by 8 0 or Hz (Johnsson, 1989; Dam-Johansen et al., 1992). Johnsson (1991) reports that calcined limestone and analytical grade CaO are active catalyst in N20 decomposition; the activity of sulphated limestone, however, is unclear. CaS, which may be formed durin sulfur retention, was found to be a strong catalyst in the reduction of NO by CO ( # urusawa et al., 1985). No reports on the catalytic activity of CaS towards N20 reduction have been found in the literature. The net effect on the NOx chemistry of limestone addition into FBC depends on design and operation conditions (Leckner and Amand 1987; Lyngfelt and Leckner, 1989). NsO ecrease or remain unchanged when limestone is introduced into a FBC emissions either (Johnsson, 1991; % mand et al., 1992). 2419
P. F. B. HANSEN
2420
C6
et al.
Traditionally, bench-scale studies of sulfur capture by limestone under FBC conditions have been carried out under constant oxidizin conditions. However, limestone particles injected into stationary or circulating FB 8 will experience an atmosphere which alternates between oxidizing and reducing conditions. Recent studies on sulfur capture by limestone under alternating oxidizing and reducing conditions (Hansen et al.; 1990, 1991, 1992) have revealed that sulfur retention under FBC conditions is the net effect of a competition between sulfur capture and sulfur release. Hence the composition of the Ca-surface is constantly changing. The solid phase transformations during sulfur retention under alternating conditions are shown in Fig. 1 and a brief description is given below. More detailed information on this subject can be found in the literature (Hansen et al.; 1990, 1991, 1992). It follows from the molar volume of the solid phases that great structural changes will accomnanv the reactions: CaCOs, CaO, CaS04, CaS, 36.9, 16.9, 46.0, and 28.9 c&s/mole respectivkly. When limestone is heated to FBC temperatures at atmospheric pressure, the main constituent - CaCOa - calcines and forms CaO, which subsequently reacts with the gaseous sulfur compounds in the bed as shown in Fig. 1. Under oxidizin conditions SO2 is captured as CaSO4, un J er reducing conditions - and in the presence of CO - as CaS. Any transformation between CaS04 and CaS appears to proceed via CaO. The intermediate Ca compound is believed to be CaO and not CaSOs, since a release of SO2 can be seen when the intermediate is formed from either CaS or CaS04. Reactions catalyzed by CaO, CaS04, and/or CaS will be affected by the changing composition of the particle surface.
CaS04
CaS
11X JcII CaO
co
co2
Fire
1. Solid phase transformations during sulfur capture under FBC conditions
The aim of this study was to evaluate how NO and N20 are reduced over limestone under FBC conditions with and without simultaneous sulfur capture. Experimental
Study
The experimental study was carried oui in a laboratory scale fixed bed quartz reactor in the 800-950 C temperature range. An artificial flue gas was mixed from pure gases or gas mixtures. The reactor is shown schematically in Figure 2 (inner diameter: 24 mm, length: 630 mm). To simulate alternating oxidizing and reducing conditions, the 02 concentration was varied periodically between 0 and 4 vol. %, out of phase with the CO concentration which was also varied between 0 and 4 vol. %. The main gas was introduced through the gas inlet (1) and passed the preheating section (2) before entering the bed. The 02 and the CO were let into the reactor through the capillary tube (3). A typical gas composition was 1500 ppm SO2, 625 ppm NO, 400 ppm N20_, 10 % CO2 and 6 % H20. The limestone sample to be investigated was easily introduced onto the porous quartz plate 4) (14 mm in diameter) in the detachable bottom section of tI, e reactor without changing the reactor temperature significantly. The reaction temperature was measured by a thermocouple (6) placed just below the porous quartz plate and continuous monitors were used for measuring the SOs, CO2, CO, 02, NO, and NzO outlet concentrations. SO3 was not measured. Previous studies have shown that less than 1.5 % of the SO2 is oxidized to SOa.
Fire
2. The fixed bed quartz reactor
C6
Catalytic reduction of NO and N20 on limestone
242 1
More detailed information on the experimental setup and experimental procedure is available in Hansen et al. (1992). The dynamics of the N20 analyzer is slow: a 90 % response is obtained after about 300 sec. (Hulgaard et al., 1989). Experimental Results and Diion Catalytic reduction of NO over limestone under alternating conditions was tested at typical FBC temperatures - around 850° C - and under three different sets of conditions. Period A: over calcined limestone, period B: during simultaneous sulfur capture, period C: during reductive decomposition of sulphated limestone. Concentration rofiles for CO, N20, NO, and SO2 are shown in Fig. 3. Periods of 10 minutes were used P5 min. of oxidizin and 5 min. of reducing conditions); 02 is present when the CO level is zero, v$ ithout limestone present no reduction of NO and less than 10 % reduction of NT& zh observed. Period A: NO was reduced only in the presence of CO. A rapid increase in the outlet NO concentration was detected during the first period of NO reduction, indicating a deactivation of the CaO. Studies of NO reduction over CaO in the absence of CO2 do not show a similar decrease in activity. Sintering of CaO catalyzed by CO2 as described by Borgwardt (1989) cannot be the reason for these observations since the catalytic activity was immediately restored when CO2 was removed from the gas. Poisoning of the active sites is believed to be the reason for the gradual deactivation of CaO. During NO reduction a minor net formation of NzO may be detected. NzO is easily decomposed over CaO. Thus, it is not possible to estimate the amount of N20 actually formed. N20 formation decreased as the reduction of NO decreased. After the shift to oxidizing conditions, a sharp peak of N20 was seen. This is most likely due to an increase in the rate of formation of N20 (by oxidation of adsorbed N , since N20 decomposes readily over CaO even under constant oxidizing conditions (cf. r!rgure 4).
0 1500
SO2 bpmv>
-
500 -
1000
‘-
OO
I
50
Time
I
100
(minutes)
Figure 3. NO reduction over Stevns Chalk. Period 3.0 mmol CaO, 1.0-1.4 mm, 850-C, 600s (300s Ox/3OOs Red), 0.9 Nl/min 9.8 % [COZ]~, 680 ppmv [Nolo, 1480 ppmv [SOz]o, O-3.9 % [CO]o and [0210, 6 % [H2O]o.
150
Period B: When SO2 was introduced under reducing conditions, CaS was formed (Hansen et al.; 1991, 1992). The NO concentration dropped immediately indicating either that the catalytic activity of the solid was restored, that catalytically active CaS was formed or a combination of the two. After the shift to oxidizing conditions, CaS was rapidly oxidized to CaO and release of SO2 was reflected in a peak of SO2 and a distinct temperature peak, the latter of which is shown in Fig. 4. NO reduction was not observed under oxidizing conditions. After the return to reducing condiformed tions, some of the CaS04 decomposed to CaO and SO;z; CaS was subsequently formed and the reduction of NO resumed. Thus the SO2 profile for sulfur capture under alternating conditions contains two sets of SO2 peaks: one set caused by CaS oxidation and another set caused by reductive decomposition of CaS04.
The catalytic activity of the solid diminished as the degree of sulfation increased. This may be explained partly by a low catalytic activity of CaSO4 and partly _ _ . . . . . by a reduced porosity and thereby less sunace area. The reduced porosity of the particles after the sulfation is caused by the large differences in the molar volumes of the solids.
P. F. B. HANSEN
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et al.
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Contrary to the findings during period A, the catalytic activity of the solid for NO reduction was seen to increase durin a period of reducing conditions. This can be explained by the gradual formation of the catalytically active CaS, which took place primarily near the particle surface. There was some drift in the zero-point of the NzO analyzer during period B. However, a minor formation of NsO can be seen.
Period C: Addition of SO2 was stopped after 125 minutes. The SO2 peaks observed under reducing conditions originate from CaS04 being reductively decomposed. The SO2 maximum, seen after 2 minutes of reduction, reflects the point at which sufficient CaO has been formed for the rate of CaS formation to become higher than the rate of CaSOd decomposition. Moreover, under these reaction conditions, 100 ppm SO2 must be present for CaS to be thermodynamically stable (Hansen et al., 1992). The SO2 peak at the end of the period is caused by CaS oxidation to CaO and SO 2. It appears that the period of NO reduction is shorter than during periods A and B. This seems to be closely related to the period of CaS formation. No reduction of NO was observed during the early part of the CaS04 decomposition process. The decrease in NO concentration during period C refleets the gradual reduction in the degree of sulfation. As CaS04 decomposes the porosity of the particle increases. Formation of N20 then increases to the level seen during period A. Periods A and B shown in Figure 3 were repeated in the presence of NzO. The profiles for temperature and concentrations of NzO, NO, and SO2 are shown in Figure 4.
Period A; The NO concentration under reducing conditions was nearly constant throu hout period A, the initial deactivation of the catalytically active CaO (shown in perio f A in Fig. 3) is not included in Fig. 4. About 80 91 of the NzO decomposed over CaO under oxidizing conditions. Under reducing conditions about 90 % N20 is decomposed. Thus even though there is a higher degree of conversion under oxidizing conditions, it may be even higher under reducing conditions. The observed increase in NzO reduction under reducing condititions may be explained by either heterogeneous decomposition, homogeneous decomposition (Hulgaard et al., 1991), or both. However, further studies of the homogeneous decomposition of N20 are necessary. The presence of NO does not affect for the active sites is not the decomposition of NzO, indicating that competition important under these conditions. Unlike NzO, the NO molecule contains an unpaired electron which could indicate that different sites on the catalytic surface may be preferred by ;he two molecules. B I Period B: SO2 addition is started at reduTamp cc> cing conditions leading to formation of CaS. 875 From Fig. 4 it can be seen that CaS is at least as active a catalyst for N20 decomposition as CaO. CaS04, on the other hand, is 825 found to be a poor catalyst for NzO decomposition. This agrees with findings by Khan 200 et al. (1991) and Miettinen et al. (1991) whereas Iisa et al. (1991) found sulphated 0 limestone to be an effective catalyst. The 800 N20 reduction observed during sulfation may be explained by decomposition on the 300 free CaO surface situated deeper within the 0 limestone particles. As seen for NO in Fig. 1500 3, the catalytic activity of the solid decreases as the degree of sulfation pro res1000 ses. Also, the rate of decomposition for a 20 500 is seen to increase at reducing conditions 0 when CaS04 is reductively decomposed and 100 50 CaO and CaS are being formed. Time (minutes) Figure 4. Reduction of NO and N20 over Stevns I
Chalk. 2.98 PeriodSOOs,
mmol CaO, dp: 0.9 Nl/min,624
395 ppm PJ2010, 9.8 % [CO&, 04
1505 %
ppm
0.85-1.00 mm, ppm [NO],
W210, [OZ]~ and [CO]o.
850” C.
Drying the gases or addition of 6 % Hz0 to the gas does not effect the heterogeneous reactions involving NO and N20.
Catalytic reductionof NO and N20 on limestone
C6
2423
It appears from the two experiments described in Figures 3 and 4 that NzO decomposes readily on CaO under oxidizing conditions while NO remaim unaffected. Under reducing conditions both NO and N20 are reduced. When the temperatures were increased from 800 to 950° C, the rates of NO and N20 reduction increased significantly. During a slow calcination of Stevus Chalk at 800” C, the reduction of NO and N20 was negligible. After calcination the reduction of both NO and N20 became appreciable, indicating a low catalytic activity of CaC03. Different types of limestones act differently with respect to the nitrogen chemistry in FBC (MjBrnell et al., 1991). For this reason the simultaneous decomposition of NO and N20 was tested over six European limestones, ranked in Table 1, from the young porous Stevns Chalk with high sulfur capacity to the very old almost crystalline Kijping limestone with low sulfur capacity (Dam-Johansen and Bstergaard, 1991). The maximum reduction of NO and the average reduction of NO under reducing conditions (averaged over the first 45 minutes) versus sulfur capacity for the six limestones is shown in Figure 5. 1.0
- U&U
-Ah&b&
Max. reduction of
Ave.
NO
roductlon ot NO over
Final
degre?k
4s
Table 1. Six European limestones ranked according to sulfur capacity
mln.
sulphation
l
-
*
-
1.0
Bryozo (Fe) Gotland
Figure 5. Maximum reduction and 45 minutes average reduction of NO versus the sulfur capacity for the 6 limestones listed in table 1.
It appears that use of any of the six limestones results in a substantial reduction of NO. However, limestone with the highest sulfur capacity also appears to exhibit the highest catalytic activity towards NO reduction, an effect most likely caused by differences in porosity and surface area of the sorbent. In order to obtain the same degree of desulfurization during combustion, limestone with high sulfur capacity must be present in a lower concentration than limestone with low sulfur capacity. Thus the importance of the sorbent to the nitrogen chemistry may be hi her when limestone with a low sulfur capacity is used. The maximum reduction of N2 8 and the average reduction occurring under both oxidizing and reducing conditions shows a similar trend as for NO. Mjijrnell et al. (1991) reported that the addition of Ienaberga limestone to a CFBC resulted in a decrease in N20 emissions, whereas addition of Kiiping limestone had no effect on the level of NzO emissions. It was found that the catalytic activity of Ignaberga in N20 reduction is substantially higher than that of K5ping, indicating that CaO-catalyzed decomposition of N20 affects emissions in a CBFC to a great degree. Conclusions. -
CaO and CaS are strong catalysts in NO and N20 reduction by CO whereas CaS04 and CaCOs are poor catalysts. A high catalytic activity towards NO and N20 reduction has been found in limestones with high sulfur capacity. The catalytic activity of a limestone decreases as the degree of sulfation increase. NO does not decompose under oxidizing conditions; N20 decomposes readily on CaO under oxidizing conditions. Reduction of NO and N20 on CaO and CaS does not appear to take place on the same active sites.
P. F. B. HANSEN et al.
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Durin NO reduction by CO on CaO, a lower percentage of the NO may be reduced toN2 8 . With CO2 present in the gas, poisoning of the catalytic sites may be important during NO reduction by CO over CaO. The addition of SO2 under reducing conditions immediately restores the catalytic activity. Hz0 does not affect the heterogeneous reduction of NO and N20.
Acknowledgements: The authors gratefully acknowledge financial support from the Nordic Ministers Council, ELSAM (The Jutland-Funen Electricity Consortium), ELKRAFT (The Zealand Electricity Consortium) and the Danish Ministry of Energy. Cited literature: Bornwardt. R.H. (1989), Sintering of Nascent Calcium Oxide, Chem. Engng. Sci., 44, 1, 53-60 Dam-Johansen. K and Q)stergaard, K. (1991), High-Temperature Reaction Between sulfur Dioxide and Limestone-I. Comparison of Limestones in Two Laboratory Reactors and a Pilot Plant, Chem. Engng. Sci., 46 3, 827-837 and Leckner, B. (1992) In fl uence of SO2 on the NO/N20 Dam-Johansen. K., 8, mand, L.-E. Chemistry in Fluidized Bed Combustion. II: Interpretation of Full-scale Observations Based on Laboratory Experiments, Submitted. Furusawa. T., Koyama, M. and Tsujimura, M. (1985), N’It ric Oxide Reduction by Carbon Monoxide over Calcined Limestone Enhanced by Simultaneous Sulfur Retention, FUEL, 84, 413415. Hansen P.F.B., Dam-Johansen K., Bank, L.H. and astergaard, K. (1990), Sulfur Capture on Limestone under Periodically Changing Oxidizing and Reducing Conditions, 3rd Int.Conf. on Fluidized Bed, Japan. Edited by P. Basu. Dam-Johansen K., Bank, L.H. and IZlstergaard, K. (lQQl), sulfur Capture Hansen P.F.B., on Limestone under Fluidized Bed Combustion Conditions - An Experimental Study, 11th Int.Conf. on Fluidized Bed Combustion: Clean Energy for the World. Montreal. 73-82. Edited by E.J. Anthony. Hansen P.F.B., Dam-Johansen K. and (astergaard, K. (1992), High-Temperature Reaction Between sulfur Dioxide and Limestone-V. The Effect of Periodically Changing Oxidizing and Reducing Conditions, Submitted. Hulgaard. T., Dam-Johansen, K., Karlsson, M. and Leckner, B. (1989), Evaluation of an Infrared N20 Analyzer, CHEC-Article no. 8904, Presented at the first technical meeting, Amsterdam, Ott, 1989. Hulzaard. T. and Dam-Johansen, K. (1992), Submitted. Hulgaard, T., Glarborg, P. and Dam-Johansen, K. (1991), Homogeneous Formation and Destruction of N20 at Fluidized Bed Combustion Conditions, 11th Int.Conf. on Fluidized Bed Combustion: Clean Energy for the World. 991-998. Montreal. Edited by E.J. Anthony. Iisa. K., Salokoski, P. and Hupa, M. (1991), Heterogeneous Formation and Destruction of Nitrous Oxide under Fluidized Bed Combustion Conditions, 11th Int.Conf. on Fluidized Bed Combustion: Clean Energy for the World. 1027-1033. Montreal. Edited by E.J. Anthony. Johnsson. J.E. (1989), A kinetic Model for NO x Formation in Fluidized Bed Combustion, 10th Int.Conf. Fluid Bed Combustion: FBC-Technology for today. San Francisco, 1111-1118. Edited by A.M. Mar&car. Johnsson, J.E. (1991), Nitrous Oxide Formation and Destruction in Fluidized Bed Combustion - A Literature Review of Kinetics, Presented at the 23rd IEA meeting in Firenze, November 8, 1991. of Nitrious Oxide in the Khan. T., Lee, Y.Y and Young, L. (1991) Heterogeneous Decomposition Operating Temperature Range of Circulating Fluidized Bed Combustors, Proceedings from the 1991 EPRI/EPA Joint Symposium on Stationary Combustion NOx Control, Washington, D.C. Leckner, B. and amand, L.E. (1987) E missions from a Circulating and a Stationary Fluidized bed Boiler, A Comparison.Qth Int.Conf. on Fluidized Bed Combustion: FBC Comes of Age. Boston, 891-897. Edited by J.P. Mustonen. Lvnzfelt. A. and Leckner, B. (1989), SO2 Capture in Fluidized Bed Boilers: Re-emission of SO2 due to Reductive Decomposition of CaS04, Chem.Engng.J. 40, 207-213. Miettinen. H., Striimberg, D. and Lindquist 0. (1991) The Influence of Oxide and Sulphate Surfaces on N20 Decomposition, 11th Int.Conf. on Fluidized Bed Combustion: Clean Energy for the World. Montreal. 999-1004. Edited by E.J. Anthony. Miijrnell, M. Leckner,B,. Karisson, M and Lyngfelt, A. (1991), E mission Control with Additives in CFB Coal Combustion, 11th Int.Conf. on Fluidized Bed Combustion: Clean Energy for the World. ontreal. 655-676. Edited by E.J. Anthony. K. (1992) Influence of SO2 on the NO/N20 Leckner, B. and Dam-Johansen, x mand. L.-E., Chemistry in Fluidized Bed Combustion. I: Full-Scale Experiments, Submitted.