Reaction between Limestone and SO2 under Conditions Alternating


Sulfur capture by limestone under periodically changing oxidizing and reducing conditions was investigated in a quartz reactor. To simulate conditions...
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Energy & Fuels 2000, 14, 654-662

Reaction between Limestone and SO2 under Conditions Alternating between Oxidizing and Reducing. The Effect of Short Cycle Times Marı´a Jose´ Ferna´ndez,† Anders Lyngfelt,*,† and Britt-Marie Steenari‡ Department of Energy Conversion, Chalmers University of Technology, S-412 96, Go¨ teborg, Sweden, and Department of Environmental Inorganic Chemistry, Chalmers University of Technology, S-412 96, Go¨ teborg, Sweden Received October 25, 1999

Sulfur capture by limestone under periodically changing oxidizing and reducing conditions was investigated in a quartz reactor. To simulate conditions experienced by limestone particles in fluidized bed combustion, short periods of oxidizing and reducing conditions were used. Total cycle times (time under oxidizing conditions + time under reducing conditions) between 2 and 24 s were used. Furthermore, three different fractions of time under reducing conditions were tested: 50, 33, and 25%. The degree of conversion of CaO to CaSO4 under these conditions is very dependent on both total cycle time and fraction of time under reducing conditions. For a fraction of time under reducing conditions of 50%, a minimum in the conversion (1.9%) versus the total cycle time was found at 8 s. For shorter cycle times a dramatic improvement of the conversion was obtained, e.g., 14.2% for 6 s. These high conversion values were also 50% higher compared to those obtained for the same limestone sulfated under totally oxidizing conditions. When the fractions of time under reducing conditions were 33 and 25%, alternating conditions have a positive effect on the conversion compared to that obtained under oxidizing conditions for all cycle times.

Introduction Most experimental investigations reported in the literature related to capture of SO2 by dry limestone under fluidized bed combustion conditions have been carried out in the laboratory under constant oxidizing conditions. Such research does not consider the fact that the bed material in a stationary or circulating fluidized bed boiler (FBB), is exposed to reducing conditions part of the time.1-3 Reducing conditions may be caused by a bypass of the fluidization air in bubbles and jets through the dense bed. This results in incomplete mixing of gas and gives a low oxygen concentration and a high concentration of reducing gases (CO, H2, and hydrocarbons) in the particle phase. This effect is further accentuated by air staging, and therefore the fraction of time in which a location is under reducing conditions is affected by the degree of air staging. Lyngfelt et al.4 carried out in-situ measurements of the O2 partial pressure in a 12 MW CFBB using zirconia cell probes * Corresponding author. Tel: 00-46-31-772 1427. Fax: 00-46-31772 3592. E-mail address: [email protected] † Department of Energy Conversion. ‡ Department of Environmental Inorganic Chemistry. (1) Ljungstro¨m, E. B. Proc. Int. Conf. Fluid. Bed Combustion 1985, 8, 853-864. (2) Lyngfelt, A.; Leckner, B. Chem. Eng. Sci. 1999, 54, 5573-5584. (3) Hansen, P. F. B. Sulphur capture in fluidized bed combusters. Ph.D. Thesis. Department of Chemical Engineering, Technical University of Denmark, Lyngby, 1991. (4) Lyngfelt, A.; Bergqvist, K.; Johnsson, F.; A° mand, L.-E.; Leckner, B. Gas Cleaning at High Temperatures; Blackie Academic & Professional: Glasgow, 1993; pp 470-491.

and found that a position located at 0.65 m from the bottom was under reducing conditions 80% of the time under normal air-staging operation. As a result, limestone particles added to an FBB will experience an atmosphere changing between oxidizing and reducing conditions as they circulate in the boiler. Studies of the effect of reducing conditions on the sulfur capture including measurements in stationary and circulating fluidized bed boilers have been carried out by Lyngfelt and Leckner.5-7 These studies indicated that CaSO4, the desired product of the reaction between limestone and SO2, is reductively decomposed under these conditions. Therefore reducing conditions may have a negative effect on the sulfur capture. However, based on an experimental study in a bench scale FBB Jonke et al.8 suggested that the sulfur capacity of the limestone would increase when cyclically exposed to oxidizing and reducing conditions. In accordance with this theory, Mattisson and Lyngfelt9 found a considerable difference in conversion for a limestone with low reactivity when it was sulfated in two commercial Swedish fluidized bed boilers compared to sulfation in the laboratory under constant oxidizing conditions. The conversion in boilers was two or three times larger for all particle sizes. (5) Lyngfelt, A.; Leckner, B. Chem. Eng. Sci. 1989, 44, 207-213. (6) Lyngfelt, A.; Leckner, B. Fuel 1993, 72, 1553-1561. (7) Lyngfelt, A.; Leckner, B. J. Inst. Energy, 1998, 71, 27-32. (8) Jonke, A. A.; Vogel, G. J.; Carls, E. L.; Ramaswami, D.; Anastasia, L.; Jerry, R.; Haas, M. AIChE Symp. Ser. 1972, 68, 241245. (9) Mattisson, T.; Lyngfelt, A. Energy Fuels 1998, 12, 905-912.

10.1021/ef990218a CCC: $19.00 © 2000 American Chemical Society Published on Web 04/21/2000

Reaction Between Limestone and SO2

Investigations in the laboratory simulating the periodically changing oxidizing and reducing conditions experienced by limestone particles in a fluidized bed combustor were initiated by Hansen.3 In this study the reaction time was divided into a number of cycles. A complete cycle is composed by a period under reducing conditions plus a period under oxidizing conditions. The duration of the total cycle was varied from 30 to 240 s and the periods of oxidizing and reducing conditions were in most cases of equal length, corresponding to a fraction of time under reducing conditions of 50%. Limestones with different reactivities were tested. From Hansen’s study, no evidence of Jonke’s theory was found. However, Okamoto et al.10 conducted experiments similar to Hansen’s, but found much higher conversions when the limestone was sulfated during alternating conditions, compared to when the limestone was sulfated under oxidizing conditions, 34.4 and 23.9%, respectively. Mattisson and Lyngfelt9 studied in depth the reaction between limestone and SO2 under alternating conditions in the laboratory. They found that the conversion under alternating conditions is very sensitive to the fraction of time that the limestone particles are under reducing conditions. Their work showed that an increase in the conversion, compared to that obtained under oxidizing conditions, of 50-150% is obtained working under an optimal fraction of time under reducing conditions, i.e., 30-50%. Total cycle times from 30 to 900 s were used and the conversion increased with increasing cycle times. According to Mattisson and Lyngfelt11 alternating conditions promote diffusion of sulfur into the solid material. The previous laboratory work was made using cycle times higher than 30 s. However, by studying the behavior of a single particle in a cold CFBB, Weinell12 showed that the solid particles move between the dense bed and the riser section nearly every second in a small plant. In addition, measurements with zirconia cell oxygen probes inside the combustion chamber show rapid shifts between oxidizing and reducing conditions, typically the time period under oxidizing conditions is 0.1-0.2 s.7 These results show the conditions in a fixed position, however, and do not reflect the conditions experienced by freely moving particles. Apparently the total cycle times tested up till now are not representative for the conditions in a fluidized bed boiler where the sorbent is exposed to rapid shifts between oxidizing and reducing conditions. Consequently it is of great interest to find out how the reaction between limestone and SO2 is affected by rapid shifts between oxidizing and reducing conditions and this is also the goal of this work. Main Reactions Involving Sulfur Capture in FBB In the conditions present in an atmospheric FBB, limestone added undergoes an endothermal reaction 1. (10) Okamoto, T.; Sakaue, T.; Nakamichi, J.; Suzuki, Y.; Nishimura, M.; Moritomi, H. Proceedings of the 2nd SCEJ Symposium on Fluidization, Tokyo, 1996; pp 398-405. (11) Mattisson, T.; Lyngfelt, A. J. Inst. Energy 1999, 71, 190-196. (12) Weinell, C. E. Single particle behavior in circulating fluidized bed combustion. Ph.D. Thesis. Department of Chemical Engineering, Technical University of Denmark, Lyngby, 1994.

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During calcination the internal surface area and the porosity of the sorbent are developed due to structural rearrangements. The sulfur capacity of the limestone is closely related to the internal structure formed during this process.

CaCO3 f CaO + CO2

(1)

Sulfur retention by limestone involves formation of CaSO4, reaction 2, or CaS, reaction 3, depending on whether the conditions in the fluidized bed combustor are oxidizing or reducing.

CaO + SO2 + 1/2O2 f CaSO4

(2)

CaO + SO2 + 3CO f CaS + 3CO2

(3)

Reducing conditions at the temperatures present in a FBB may cause reduction of the CaSO4 formed.13,14 Depending on the temperature and gas composition, the solid reaction product may be either CaO or CaS, according to reaction 4 or 5.

CaSO4 + CO f CaO + SO2 + CO2

(4)

CaSO4 + 4CO f CaS + 4CO2

(5)

Large experiments carried out in a 16 MW stationary FBB and a 12 MW circulating FBB have verified that partly sulfated limestone can be reductively decomposed with release of SO2.5,6 Under oxidizing conditions CaS can be oxidized to CaO or CaSO4 depending on the temperature and oxygen concentration.

CaS + 3/2O2 f CaO + SO2

(6)

CaS + 2O2 f CaSO4

(7)

Finally, the solid-solid reaction between CaSO4 and CaS, reaction 8, has to be considered when studying sulfur capture in FBB.

CaS + 3CaSO4 f 4CaO + 4SO2

(8)

According to Chen and Yang15 the rate of reaction 8 is negligible below 950 °C. However, it has recently been shown that CaS and CaSO4 react at a surprisingly rapid rate at temperatures common in a FBB.16 Reactions 3-5 can also take place with other reducing agents present in fluidized bed combustion, such as H2 or hydrocarbons. The above reactions are overall reactions and should not be interpreted as elementary processes. In these reactions at least four solid species are involved, i.e., CaCO3, CaO, CaSO4, and CaS. The molar volumes of CaCO3, CaO, and CaS are 36.9, 16.9, and (13) Wheelock, T. D.; Boylan, D. R. Ind. Eng. Chem. 1960, 52, 215218. (14) Ghardashkhani, S.; Ljugstro¨m, E.; Lindquist, O. Proceedings International Conference Fluid Bed Combustion 1989, 10, 611-615. (15) Chen, J. M.; Yang, R. T. Ind. Eng. Chem. Fundam. 1979, 18, 134-138. (16) Davies, N. H.; Hayhurst, A. N.; Laughlin, K. M. Twenty-fifth Symposium (International) on Combustion; The Combustion Institute, Pittsburgh, 1994; pp 211-218.

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Ferna´ ndez et al. Table 1. Reaction Conditions limestone particle size sample weight height of particle bed gas flow (0 °C, 1 atm) temperature reaction time total cycle time

Figure 1. Fixed-bed quartz reactor. (1) CO and O2 entrance, (2) capillary which transports CO and O2 to the sample; (3) sample holder; (4) N2, CO2, SO2 entrance; (5) gas exit; (6) thermocouple entrance.

28.9 cm3/mol.17 CaSO4 formed during sulfation, reaction 2, is in the form of anhydrite II with a molar volume of 46.0 cm3/mol.18 Because of the large molar volume of CaSO4, the sulfate plugs the pores of the oxide resulting in particles with an unreacted core of CaO. Therefore complete conversion of CaO to CaSO4 is normally not achieved. On the other hand, due to the smaller molar volume of CaS the reaction between the oxide and SO2 under reducing conditions can reach a higher conversion. Experimental Section Experimental Setup. All the experiments were carried out in a fixed-bed laboratory quartz reactor illustrated in Figure 1. The reactor was built as a reproduction of the one used by Mattisson and Lyngfelt.9 The inner tube of the reactor had an inner diameter of 19 mm. The main gas was introduced through the gas inlet (4), whereas O2 and CO were introduced alternatingly through a 4 mm capillary tube (1) and mixed with the main gas 10 cm from the top of the limestone bed. The limestone particles were placed on a sintered quartz plate (3). This system was designed to minimize the back-mixing and reaction of the O2 and CO before they reach the sample. Reactor temperature was measured by means of a thermocouple placed just below the sintered quartz plate (6). The gases SO2, CO2, and N2 were led from gas tubes through mass flow controllers (Brooks 5850E) where the mass flows were adjusted to obtain the right concentrations and flows. Simulation of a cyclic exposure to oxidizing and reducing conditions was performed by adding alternatingly O2 and CO to the flow. Cycles constituted by one oxidizing time period and one reducing time period were repeated throughout the (17) Weast, R. C.; Astle, M. J.; Beyer, W. H. Handbook of Chemistry and Physics: A Ready-reference Handbook of Chemistry and Physical Data, 67th ed.; CRC Press: Boca Raton, FL, 1986-1987. (18) Dam-Johansen, K.; Østergaard, K. Chem. Eng. Sci. 1991, 46, 839-845.

fraction of time under reducing conditions

Ko¨ping 0.5-0.7 mm 0.6 g 1.5 mm 1000 mL/min 850 °C 2h 2-24 s 240-720 s 25, 33, 50%

gas composition O2 concentration CO concentration SO2 concentration CO2 concentration N2 concentration

0-4% 0-4% 1500 ppm 10% balance

tests. During the oxidizing time period oxygen was added to give 4% oxygen concentration, and during the reducing period carbon monoxide was added to give 4% CO concentration. This was achieved by means of two programmable three-way magnetic valves, which alternatingly, directed either O2 or CO through a quartz capillary at the top of the reactor, or to a fumehood. By having a constant flow of either O2 or CO, the total flow through the reactor was always constant. From the outlet of the reactor, the gas is led to a URAS 10E gas analyzer where the SO2 concentration was measured. SO2 concentration data were logged in a personal computer to a file with 1-s intervals. For the total flow used in this work the residence time in the SO2 analyzer has been calculated to be 2.3 s. In several trials the O2 concentration was also measured with a quadrupole mass spectrometer (Baltzers QMG 421) where data were logged every 2.5 s. Experiments were conducted with a geologically old and crystalline limestone called Ko¨ping. Details about its chemical composition, average pore radii, and surface are found in Mattisson and Lyngfelt.9 Normally, total cycle times used varied between 2 and 24 s, but some tests were also made in the range 240-720 s. The fractions of time under reducing conditions tested were 25, 33, and 50%. By fraction of time under reducing conditions is meant the ratio of the time period under reducing conditions to the complete cycle time. Table 1 shows the reaction conditions used in this work. Experimental Procedure. Initially, the reaction gas consisting of SO2, CO2, O2, and N2 was led to the analyzer bypassing the reactor and thus the inlet concentration was determined. Simultaneously, the reactor was heated to a temperature of 850 °C in an atmosphere of 100% CO2 in order to avoid any calcination of the sample before the experiment. Once the temperature reached steady state the reaction gas mixture was led through the reactor and alternation between O2 and CO started. Under constant oxidizing conditions, the experimental procedure was identical except that no CO was introduced into the reactor. To determine the final conversion the sample is carefully weighed before and after the experiment. Since the conditions in many experiments could lead to the formation of CaS, it was necessary to conduct a sulfide analysis on the sulfated samples. The method utilized is based on the conversion of CaS to H2S that was absorbed as ZnS and quantitatively determined by titration. This method is described by Mattisson and Lyngfelt,19 except that in their work H2S was absorbed as CdS. Once the mass fraction of sulfide sulfur, yS2-, present in the sample is obtained, the conversion to CaS (XCaS) and to CaSO4 (XCaSO4) can be determined by means the difference of mass before and after the reaction, eqs 9 and 10, if it is assumed that calcination is complete. (19) Mattisson, T.; Lyngfelt, A. Proceedings of the International Conference on Fluidized Bed Combustion 1995, 13, 819-829.

Reaction Between Limestone and SO2 XCaS )

Wf ys2- MCaCO3 Wi fMS

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XCaSO4 ) ((Wf - Wi(1 - f))MCaCO3 Wi f

- MCaO - XCaS(MCaS - MCaO)

MCaSO4 - MCaO (10) where Wi and Wf are the weights of the sample before and after the reaction, f is the fraction of CaCO3 in the limestone sample, and MS, MCaCO3, MCaSO4, and MCaS are the molecular masses of the respective compounds. The total conversion of the reaction, X, was defined as the sum of the conversions to CaS and CaSO4, XCaS + XCaSO4.

Effect of Non-Ideal Flow Rapid changes between oxidizing and reducing conditions add an experimental complexity. Deviations from the ideal plug flow, that is mixing or diffusion in the direction of the flow, can cause mixing of the oxidizing (O2) and the reducing gas (CO). This means a change in their concentrations compared to the theoretical input. The concentrations of O2 and CO are also lowered through reaction 11

CO + 1/2O2 f CO2

(11)

To minimize the effect of the possible mixing between the alternating gases their flow was maximized. Therefore, mixtures of 10% CO (v) in N2 and 10% O2 (v) in N2 were utilized instead of pure gases to prepare the appropriate mixture of reaction. This means that the flow of the alternating gases is 40% of the total flow (taking into account that their desired concentration was 4%). Also, the path between the motored valves that shift between these gases and the reactor inlet was minimized. The objective of the following section is to investigate the flow behavior in the present experimental setup in order to evaluate the extent of mixing of the alternating gases at the reactor, so that their real concentrations inside the reactor can be approximated. Dispersion Tests. To see whether the system behaves as an ideal plug flow reactor, five dispersion tests were done. For this purpose, an inert bed of silica sand particles was used as sample instead of limestone. The characteristics of the bed, particle diameter and bed height, were the same as for the limestone bed. As in the real experiments, cyclic exposure to oxidizing and reducing conditions was performed by adding alternatingly O2 and CO to the flow. During the oxidizing time period, oxygen was added to give 4% oxygen concentration; during the reducing period, carbon monoxide was added to give 4% CO concentration. Complete cycle times used were 2, 4, 6, 8, and 24 s, and the fraction of time under reducing conditions was 50% in all the cases. This means that the time in the presence of either O2 or CO for these five tests is 1 s/1 s, 2 s/2 s, 3 s/3 s, 4 s/4 s and 12 s/12 s, respectively. Assuming ideal plug flow behavior, corresponding to no axial mixing and uniform velocity in the direction of the flow, the O2 concentration in any of these experi-

ments would shift between 0 and 4%. Furthermore, since the fraction of time under reducing conditions is 50%, the O2 concentration average should be 2%. The same can be said for the CO concentration. Deviation from the ideal plug flow produces mixing of the alternating gases and they react according to reaction 11. This will give a decrease of the average O2 concentration over a complete cycle to values lower than 2%. The O2 concentration measured for the dispersion tests is shown in Figure 2. It was measured by means of a quadrupole mass spectrometer (Baltzers QMG 421), with a residence time in the measuring cell lower than one second. As can be seen, when the total cycle time is 24 s O2 concentration oscillates between 0 and 4%, for shorter periods the range of the oscillation diminishes, thus for 2 s the O2 concentration fluctuates around a mean value of 1%. The average of the O2 concentration, Table 2, varied from 1.9 to 1.1% for total cycle times ranging from 24 to 2 s. It is obvious that mixing and reaction of the alternating gases take place, which means a deviation from ideal plug flow behavior. The observed decrease of the average O2 concentration from the theoretical value can be considered as a true indication of the total extent of the deviations occurring in the reactor if the following assumptions are made: (a) Cooling of gases is rapid after the reactor outlet, this means that no reaction between O2 and CO takes place after the gases leave the reactor; and (b) the flow in the capillary that injects the alternating gases 10 cm above the reaction zone behaves as an ideal plug flow. According to the latter assumptions, deviations from the ideal plug flow and chemical reaction in the tubular reactor (19 mm inside diameter and 45 cm from inlet of O2/CO to outlet) produce the decrease in the O2 average concentration shown in Table 2. However, the sample is placed 10 cm from the inlet O2/CO and the only deviations that are relevant for the reaction are those which take place before the reaction zone. Dispersion Model. Different models can be found in the literature to describe the deviations from the ideal plug flow in a tubular reactor. The dispersion model is one of the most commonly accepted and gives an analytical solution to the concentration profile in a tubular reactor. In this model the reactor is described as a tubular reactor where mixing in the axial direction takes place according to an effective diffusivity DL. This parameter can be theoretically calculated according to the operating conditions (gas species, flow, reactor dimensions). Frequently, the dimensionless number called dispersion number, DL/uL, is used in order to characterize the extent of the dispersion, u and L are the axial velocity and the reactor length, respectively. From correlations in the literature20 the effective diffusivity and the dispersion number can be calculated for the operating conditions used in this work, 5.21 × 10-4 m2 s-1 and 5.31 × 10-3, respectively. According to the dispersion model, eq 12 predicts the response of a reactor, i.e., the outlet concentration, Cstep, for a given change in input concentration, C0, in form of a step.21 The outlet concentration is a function of time (20) Levenspiel, O. Chemical reaction engineering, 2nd ed.; John Wiley & Sons: New York, 1972. (21) Smith, J. M. Chemical Engineering Kinetics, 2nd ed.; McGrawHill: New York, 1970.

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Figure 2. Concentration profile of O2 as a function of the time for dispersion experiments. Table 2. O2 Average Concentration and Fraction of Time under Reducing Conditions for the Dispersion Testsa Dispersion model experimental total cycle (s)

av. O2 concn (%)

2 4 6 8 24

1.08 1.47 1.72 1.81 1.94

a

DL/uL ) 0.02 whole reactor av. O2 (%) frac. red. (%) 1.18 1.55 1.70 1.77 1.92

23 42 45 47 49

DL/uL ) 0.1 whole reactor av. O2 (%) frac. red. (%) 1.00 1.18 1.38 1.50 1.82

0 23 36 40 47

DL/uL ) 0.45 reaction zone av. O2 (%) frac. red. (%) 1.24 1.50 1.64 1.72 1.9

28 40 44 45 49

Theoretical fraction of time under reducing conditions is 50%.

as well as the dispersion number and the mean residence time, θ.

[

(x

1 1 Cstep ) C0 1 - erf 2 2

)]

uL 1 - t/θ DL xt/θ

(12)

The input O2 concentration used in the dispersion experiments can be considered as two consecutive steps. In the first step, the O2 concentration reaches 4% instantaneously from an initial concentration of 0%. Then the O2 concentration is constant for a period of time. The second step starts from 4% and reaches 0% instantaneously. Thus, a theoretical prediction of the outlet O2 and CO concentration of the dispersion tests can be calculated with eq 12. Figure 3 represents the outlet O2 and CO concentrations predicted for a total cycle time of 2 s with three different flow behaviors. Figure 3a shows the concentration of O2 and CO with ideal plug flow behavior. Figure 3b represents the O2 and CO concentration calculated

by the dispersion model, eq 12, with the previously obtained dispersion number. Finally, Figure 3c shows the concentrations calculated according to the dispersion model also assuming that reaction between CO and O2 takes place. (The reason for the difference in peak width between O2 and CO is stoichiometric, reaction 11.) Similar calculations to those shown in Figure 3 were made for total cycle times of 4, 6, 8, and 24 s. Figure 4 shows the average O2 concentration as well as its maximum and minimum values predicted by the dispersion model for all the dispersion experiments. In addition, the values of the O2 concentration measured experimentally, Figure 2, are shown in this figure. The average O2 concentrations predicted by the dispersion model for the dispersion tests are higher than that obtained experimentally if the calculated dispersion number, DL/uL ) 0.00531, is used, see Figure 4. Thus, the calculated dispersion number underpredicts the extent of mixing that takes place in the reactor. This lack of agreement between predicted and measured

Reaction Between Limestone and SO2

Energy & Fuels, Vol. 14, No. 3, 2000 659 Table 3. Molar Conversion to CaS

Figure 3. Concentration profiles of O2 (solid lines) and CO (dashed lines) as a function of the time. (a) theoretical, (b) with back mixing, (c) back-mixing and reaction 11.

50% reducing conditions

33% reducing conditions

cycle time, (s)

CaS/Catot, mol %

CaS/Stot, mol %

CaS/Catot, mol %

CaS/Stot, mol %

2 3 4 6 8 12 18 24

0.31

2.15 0.03

0.25

0.06 0.31 0.41 0.35 0.18 0.21

0.42 2.18 21.81 6.80 2.74 3.50

0.02

0.16

0.05 0.06 0.05

0.37 0.35 0.39

240 480 720

0.40 0.70 1.25

2.84 3.77 6.59

0.21 0.45 0.85

1.37 2.52 4.03

Consequently other values of the dispersion number were tried in order to find one that gives a better agreement. As can be seen in Figure 4, the dispersion numbers tried do not give an exact fit of both maximum/ minimum and average O2 concentration at the same time. This can be due to some uncertainties in the measurement of the maximum and minimum O2 concentrations produced by the rapid alternation of these gases. Thus, for a dispersion number of 0.02, the average O2 concentration calculated fits well with the experimental values but the maximum and minimum O2 concentrations are overpredicted. The best fit to the maximum and minimum O2 concentration, on the other hand, was obtained for a dispersion number DL/uL ) 0.1. Therefore the value of the dispersion number that agrees best with the experimental results is in the range 0.02 to 0.1. The highest value of the dispersion number was selected for further calculations. From this value of the dispersion number, which characterizes the dispersion in the whole reactor, a new dispersion number was derived which characterizes the part of the reactor from the inlet of O2/CO to the reaction zone. To do this it was assumed that the effective dispersion (DL) is the same before and after the position of the sample. With this new dispersion number, DL/uL ) 0.45, the concentration of alternating gases and the fraction of time under reducing conditions in the reaction zone was calculated. Table 2 shows the values obtained. According to these values, it can be concluded that the limestone sample experiences changes between oxidizing and reducing conditions even for the shortest cycle time used (2 s). The latter calculations were, as previously mentioned, made based on the highest fitted value of the dispersion number, and should therefore be conservative, i.e., overestimate the mixing. However, it should be noted that there is obviously a decrease in the fraction of time under reducing conditions for short cycle times. Results

Figure 4. Average (O), maximum (+), and minimum (×) O2 concentration as measured in the dispersion tests as a function of the total cycle time. For each of the three measured concentrations are also shown three dashed lines with results of the dispersion model, i.e., for three values of the dispersion number.

values is probably explained by pressure fluctuations that might occur when the flows of O2 and CO are shifted.

CaS formation. In Table 3 values of the conversion to sulfide sulfur for the majority of the experiments are shown. As can be expected the conversion to sulfide was higher for the higher fraction of time under reducing conditions, 50%. The maximum conversion to sulfide determined using short cycle times, 2 to 24 s, was only 0.4%. For longer cycle times the maximum conversion to sulfide increased, with a maximum value of 1.2%. Also, the ratio of sulfide sulfur to total sulfur is low for

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Figure 6. Data from Figure 5 shown with a higher resolution.

Figure 5. SO2 profile as a function of time for experiments with a constant fraction of time under reducing conditions, 50%, but different total cycle times, (a)-(e) 24, 12, 8, 6, and 2 s. For comparison results under constant oxidizing conditions are also shown (f). The horizontal dashed lines show the inlet SO2 concentration, 1500 ppm.

most of the samples. It can be concluded that under the present experimental conditions the major product of the reaction between limestone and SO2 is CaSO4, and only a minor conversion to CaS is seen. 50% Reducing Conditions. Figure 5a-e shows the SO2 concentration as a function of the reaction time for experiments carried out using cycle times ranging from 24 to 2 s and with 50% of the time under reducing conditions. Furthermore, the SO2 profile for an experiment performed under totally oxidizing conditions is also shown, Figure 5f. In each graph, a dashed horizontal line indicating the SO2 inlet concentration is shown, and the total conversion, X, obtained for each trial is specified. The large black areas seen in Figure 5a-c correspond to rapid variations of the SO2 concentration that in this long time scale are not possible to distinguish. Figure 6 shows a short scale time, 100 s, from Figure 5 and here the changes of the SO2 concentration can be observed.

As can be seen in Figure 5, low conversions were obtained for experiments made with total cycle times between 24 and 8 s. In these experiments, the conversion was smaller than that determined under oxidizing conditions. However, when the total cycle time was decreased to 6 s, the conversion increased dramatically reaching values more than 50% higher than the conversion under oxidizing conditions. The abrupt change detected in conversions when the total cycle time decreases from 8 s to a period of 6 ss the conversion increases from 1.9% to 14.2% while the cycle time decreases only 2 ssis accompanied with a change in the SO2 profile of the experiments. In the experiments carried out using total cycle times between 24 and 8 s, a periodical release and capture of SO2 is observed. Peaks of SO2 which exceed the inlet SO2 concentration are followed by drops of the SO2 concentration below the SO2 inlet level. The variation in SO2 concentration below and above the SO2 inlet level is approximately symmetrical, indicating that the net capture of SO2 is very small. This is confirmed by the low conversion obtained. In contrast, the SO2 profile versus time for experiments conducted with periods lower than 6 s is different. In these cases, a smooth curve similar to that obtained under oxidizing conditions was observed and higher conversions were determined. Even though the smooth SO2 profile found for the

Reaction Between Limestone and SO2

Energy & Fuels, Vol. 14, No. 3, 2000 661

Figure 8. The degree of conversion, i.e., molar S/Ca percent, after 2 h as a function of the total cycle time. The fraction of time under reducing conditions is 50% (O), 33% (0) and 25% (∆). Filled symbols are data from Mattisson and Lyngfelt.9 The horizontal dashed line shows the conversion under constant oxidizing conditions.

Figure 7. SO2 profile as a function of time for experiments with a constant fraction of time under reducing conditions, 33%, but different total cycle times, (a)-(d) 24, 18, 12, and 6 s.

experiments made with cycle times shorter than 6 s could also be an effect of the analyzer response time, unpublished experiments made with a cycle time of 6 s and other reaction conditions indicate the opposite. Thus, when H2 is used as reducing agent instead of CO or when the SO2 concentration is lowered, the SO2 profile is not smooth but shows peaks of release and capture of SO2. Therefore, the change in the profile and the abrupt change in conversion indicate a change in the reaction behavior when the total time of the cycle is lowered from 8 to 6 s. Another notable fact of the experiments carried out with cycle times shorter than 6 s, is the high conversion found using a total cycle time of 2 s, 14.4%, compared to the conversion obtained under oxidizing conditions, 9.2%. This means that alternating, 1 s oxidizing/1 s reducing, conditions gives a high improvement of the conversion in relation to totally oxidizing conditions. To verify that this improvement is not simply due to the presence of CO but is really caused by alternating conditions, an experiment using 2% CO and 2% O2 simultaneously was made. In this case the conversion obtained (10.8%) was more similar to that obtained under oxidizing conditions. Although it is interesting to note that the presence of CO gave some improvement of the conversion, it is clear that the effect of alternating conditions improves the SO2 capture for short cycle times. 33% Reducing Conditions. Profiles of the SO2 concentration as a function of time are shown in Figure 7 for experiments with a fraction of time under reducing

conditions of 33%. Contrary to what was observed from the corresponding profiles of the experiments made under 50% reducing conditions, peaks of SO2 exceeding the inlet concentration are scarcely observed. For all the experiments the conversion is higher than that found under oxidizing conditions. Conversion vs Total Cycle Time. The conversion obtained in experiments carried out under alternating conditions is shown in Figure 8 as a function of the total cycle time. Data obtained for the following three fractions of time under reducing conditionss50%, 33%, and 25%sare included. For total cycle times ranging from 30 to 900 s, some of the conversions were determined by Mattisson and Lyngfelt9 (solid symbols). A good agreement between these values and those obtained in the present work is observed. In Figure 8 the first fact that calls our attention is the existence of a minimum in the conversion when the fraction of time under reducing conditions is 50%. The minimum was found for a total cycle time of 8 s and at a conversion as low as 1.9%. Thus, the conversion decreases when the total cycle time decreases from 720 to 8 s, but for a further decrease of the total cycle time a sharp increase of the conversion was observed. For total cycle times lower than 8 s, the conversion is 50% higher than that obtained under oxidizing conditions, horizontal dashed line. With a fraction of time under reducing conditions of 33%, the decrease in the conversion when the cycle time was lowered was less marked and no clear minimum is seen. Independent of cycle time the conversions determined were higher than that obtained under oxidizing conditions. The same can be said for a time fraction of 25% under reducing conditions, and the improvement compared to oxidizing conditions is approximately similar.

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Discussion The effect of alternating oxidizing and reducing conditions on the reaction between limestone and SO2 has previously been studied in the laboratory using cycle times longer than 30 s. Different conclusions were made from these studies. Thus, according to Hansen et al.22 the conversion obtained by the reaction between limestone and SO2 is not improved when the limestone is alternately exposed to oxidizing and reducing conditions. On the contrary, Mattisson and Lyngfelt9 found that an improvement in the conversion of 50 to 150% compared to that obtained under oxidizing conditions can be found under certain alternating conditions. The present work addressed the effect of rapid changes between oxidizing and reducing conditions on the reaction between limestone and SO2. Understanding of the complex processes taking place is limited. The interpretation of the results involves uncertainties owing to a number of difficulties: The elementary reactions are not really known; the reactions considered, reactions 2-8, are overall reactions and may not accurately describe what happens under these transient conditions. Possible intermediates are not known and under rapidly alternating conditions the presence and role of intermediates may be accentuated. Examples of possible intermediates were given by Low et al.23 who studied the reaction between SO2 and CaO in absence of O2 with infrared spectra. The infrared bands detected were attributed to the formation of species of the general formula SyOx-n resulting from the polymerization of SO42-, S2-, and undecomposed SO32-. The rapid shifts in gas composition experienced on the particle surface may not penetrate so rapidly to the particle interior. This is true especially for the solidphase diffusion, which is slow. The complexity of the reactions is further illustrated by the finding of a long induction period for the reduction of CaSO4 with CO.14 This induction period varied depending on limestone, temperature, and gas concentrations but was typically in the range 10 to 60 min. In view of these uncertainties, it must be concluded that a comprehensive understanding of these processes is lacking and any attempts to explain the results in detail would be more or less speculative. Nevertheless, the general effects of cycle time and fraction of time under reducing conditions on the conversion can be observed and provide a better understanding of how alternating conditions affect sulfur capture under FBB conditions. From previous laboratory work made under alternating conditions but with longer cycle times it was concluded that an increase in the conversion compared to oxidizing conditions of 50 to 150% is obtained under optimal conditions.9 However, as can be seen in Figure 8, Mattisson and Lyngfelt found that a decrease in the conversion occurs when the total cycle time is decreased. An extrapolation of this tendency to short cycle times (2-24 s) would lead to low values of the conversion, even (22) Hansen, F. B.; Dam-Johansen, K.; Østergaard, K. Chem. Eng. Sci. 1993, 48, 1325-1341. (23) Low, M.; Goodsel, A.; Takezawa, N. Environ. Sci. Technol. 1971, 5, 1191-1195.

Ferna´ ndez et al.

lower than those obtained under oxidizing conditions. This suggested that the improvement for long cycle times compared to constant oxidizing conditions might not be relevant under boiler conditions where particles are more likely to experience rapid shifts between oxidizing and reducing conditions. However, from the present results, it is possible to conclude that a higher conversion compared to that determined under oxidizing conditions can also be obtained with short cycle times. The conversion was very sensitive to the fraction of time under reducing conditions, and with 50% of the time under reducing conditions, the improvement took place for total cycle times shorter than 6 s. But with a fraction of time under reducing conditions of 33%, an improvement in the conversion related to oxidizing conditions was determined for the whole range of total cycle times tested (3-720 s). The same also goes for a fraction of time under reducing conditions of 25%. It has to be pointed out that the changing conditions to which sorbent particles are exposed in a fluidized bed boiler are not known in detail and cannot be exactly reproduced in the laboratory. Nevertheless, the results clearly indicate that rapidly alternating conditions may have a positive effect on the conversion. Conclusions The reaction between limestone and SO2 under periodically changing oxidizing and reducing conditions was studied using short periods, 2 to 24 s. Three different fractions of time under reducing conditions were tested. The following conclusions can be made: • Conditions similar to those present in boilers, i.e., conditions changing between oxidizing and reducing rapidly, may have a positive effect on the conversion to CaSO4 compared to the degree of conversion reached under totally oxidizing conditions. Conversion is closely dependent on the total cycle time and the fraction of time under reducing conditions. • While previous studies indicated a decrease in conversion with cycle time, the present results show a much more complex picture. When the reaction is carried out with 50% of the time under reducing conditions, a minimum in the conversion as a function of the total cycle time was detected at 8 s. Decreasing the total cycle time to values lower than 8 s results in a dramatic improvement of the conversion from a 1.9 to 14.2%. For cycle times in the range 2 to 6 s the conversion was increased by 50% compared to that determined under oxidizing conditions. • Alternating oxidizing and reducing conditions have a positive effect on the reaction of limestone and SO2 when the fraction of time under reducing conditions is 33%, independent of the total cycle time in the range 3-720 s. The same can be said for a fraction of the time under reducing conditions of 25%. Acknowledgment. The authors gratefully acknowledge financial support from the European Community with the award of a Marie Curie grant. We would also thank Tobias Mattisson for his help and advice on the experimental work. EF990218A