Simultaneous chlorination and sulphation of calcined limestone

Pergamon

Chemical Engineering Science, Vol. 5l. No. 1I, pp. 2529--2534.1996 Copyright © 1996Ebevie¢ScienceLtd

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S I M U L T A N E O U S CHLORINATION AND S U L P H A T I O N OF CALCINED LIMESTONE M. M A T S U K A T A , K. TAKEDA, T. MIYATANI AND K. UEYAMA Department of Chemical Engineering, Faculty of Engineering Science, Osaka University Machikaneyama, Toyonaka, Osaka 560, JAPAN A b s t r a c t - In order to analyze HC1 and SO2 retention in fluidized bed combustors of coal and wastes, chlorination and sulphation of calcined limestone were investigated at 1023 Kand atmospheric pressure using thermogravimetry. The rate of chlorination of calcined limestone slightly depended on its particle size and was kept almost constant against the progress of chlorination. In contrast, the rate of suiphation increased with decreasing particle size and steeply decreased with the progress of sulphation as commonly reported. It was found that the sulphation was markedly accelerated in the presence of HCI. Such acceleration of sulphation was remarkable for larger limestone. The level of conversion of CaO to (CaSO4 + CaC12) always approached 100% in the simultaneous absorption of HCI and SO2. It was observed by SEM that in the chlorination a number of spherical aggregates and large voids were formed on the surface of limestone and that large aggregates with very flat surface and large voids have been formed in the course of the simultaneous chlorination and sulphation. These chlorination behavior and the acceleration of SO2 absorption in the presence of HC1 can be due both to the formation of a mobile CI" ion-containing phase and to the formation of voids playing a role of the diffusion paths for HC1 and SO2 toward the interior of a limestone particle. Melting of a eutectic mixture of CaC12 and CaSO4 might largely contribute to the promotion of SO2 absorption in the case of simultaneous absorption of HC1 and SO2. INTRODUCTION Wastes like car shredder dust (CSD) and refuse derived fuel (RDF) should be utilized in an environmentally benign way instead of reclamation, the conventional way of disposal. Energy production is one of the selections to utilize these wastes. Fluidized Weds have been proposed as combustors of the wastes, because of the flexibility in choosing fuels and of low emission of toxic gases such as NOx and SOx. An important characteristic of these wastes is that they often contain chloride. Toxic hydrochloric acid (I-ICI) is considered to generate under the common fluidized Wed combustion conditions in the temperature range of 900 - 1200 K. Emission control of HC1 is thus a key issue for commercialization of fluidized Wed combustors (FBCs) of wastes. Co-combustion of coal and wastes has been proposed to increase the heat value of fuel. In this case, SO 2 is formed through coal combustion as well as HCI. Simultaneous removal of HCI and SO 2 must be accomplished. It is well-known that one of the advantages of FBCs is hot gas clean up by using limestone as sorbent, according to eq. 1. The same idea can be applied to HCI removal in FBCs. The retention of HCI in FBCs by calcined limestone is expected to occur according to eq. 2.

(1) (2)

CaO + SO2 + 1/202 -> CaSO4 CaO + 2HCI -> CaCI2 + H 2 0

While the sulphation of limestone with SO 2 in fluidized ~ coal combustors has been extensively studied over the last two decades, less attention has been paid to the chlorination of limestone with HC1. Chlorination studies have been carried out mostly in the temperature range, 473 - 923 K. It has been found that an attainable level of conversion of limestone deo'e.as~ with increasing particle size (Daoudi and Waiters, 1991a and b; Mura mad Lallal, 1992). Mura and Lalli (1994) reported that the chlorination of limestone in this temperature range can be explained by the grain model. There rarely existed reports on the kinetics of simultaneous absorption of HCI and SO2 under FBC conditions. Degnchi et at. (1982a and b) tested HCI retention (1982a) and simultaneous retention of HCI and SO 2 (1982b) in a bubbling fluidized Wed at 1123 K. They reported that HCI was effectively retained by limestone and the capacity of SO2 absorption was increased in the presence of HCI. In the present study, on the basis of thermodynamics, we discuss HC1 formation and chlorination of calcined limestone under FBC conditions. Kinetic studies on the chlorination and sulphation of limestone were preliminary performed at 1023 K.

THERMODYNAMIC CONSIDERATION Possible reactions to form HCI under FBC conditions a~e listed as follows: 2529

2530

M. MATSUKATAet aL

(3) (4) (5) (6)

2NaCI + SO 2 + 1/20 2 + H20 -> Na2SO4 + 2HCI R-CI + 02 -> HCI + CO2 + H20 2FeCI2 + 2H20 + 1/20 2 -> Fe203 + 4HCI 2HCI + 1/20 2 -> C12 + H20

Eq. 3 represents a reaction of alkali salts contained in garbage and coal, eq. 4, the decltlorination of Cl-containing plastics like polyvinyl chloride, and eq. 5, the oxidation of FeCI 2 that formed through corrosion of the surface of heat exchanger in a FBC. As a result of thermodynamic calculations, eq. 3 can be ruled out under FBC conditions because this reaction proceeds below 424 K. While eqs. 4 and 5 are thermodynamically favored under FBC conditions, it is considered that HCI can be formed by combustion of plastics via eq. 4. C12 can be formed mainly via oxidation of HC1 according to eq. 6. However, the equilibrium of eq. 6 lies far to the left: Kp = 0.417 at 973 K and 0.119 at 1173 IC Thus, we consider that main species containing chlorine is HCI under FBC conditions. We calculated equilibrium concentrations of HCI via eq. 2. Figures la and b show the results of the thermodynamic calculations for different concentrations of HCI and steam, respectively. The percentage of dechlorination is defined by eq. 7.

(7)

Dcchlorination(%) = (I-CHCI, e/CHCI, i)x 100

where CHCI, e represents the concentration of HCI remaining in a gas phase after reaching equilibrium, and CHCI, i represents the concentration of HC! formed in a FBC without sorbent. Figure la clearly indicates that the temperature of a fluidized bed significantly affects the efficiency of dechlorination in the presence of 10% steam. At CHCI, i = 1000 ppm, no absorption of HCI occurs at 1073 K and 500 ppm of HCI remains in the gas phase at 973 K. With increasing value of CHCI, i , dechlorination becomes effective: 89% of HCI can be absorbed by limestone at 973 K if CHCI, i = 5000 ppm. It should be, however, noted that the equilibrium concentration of HCI in gas phase is determined by eq. 2 irrespective of the values of CHCI, i. Namely, the minimum concentration of HCI that can be reached is restricted only by the combustion temperature and the H20 concentration, when a sufficient concentration of HCI forms in a FBC. Figure lb reveals that the efficiency of HCI retention in FBCs heavily depends on the concentration of steam in the temperature range commonly used for FBCs, 900 - 1200 K. As can be predicted by eq. 2, the efficiency of HCI retention, i. e., the percentage of dechlorination, decreases with increasing concentration of H20. When the concentration of steam is 1%, 580 ppm of HCI still remains at 1073 K, while the equilibrium HCI concentration can be depressed to 90 ppm at 923 K. Though we do not take the equilibrium of eq. 8,

(8)

CaCO3 -> CaO + CO2

into consideration here, this reaction might contribute to a decrease in the efficiency of HCI retention at lower temperatures, because the equilibrium of this reaction displaces to the left with decreasing temperature. These thermodynamic considerations indicate that the selection of bed temperature is the most important to control HCI emission fi'om FBCs. Drying of wastes prior to combustion in FBCs would markedly increase the efficiency of HCI retention with limestone. When HCI retention is difficult to reach an environmentally acceptable level in the presence of steam, the effect of HCI on SO 2 retention in FBCs would still be important.

100

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Figure 1. Results of thermodynamic calculations on dechlorination by calcined limestone. (a); Temperature dependency of the percentage of dechlorination for different HC1 concentrations in the presence of 10% of H20. The value of CHCI, i was shown in (a). (b); Temperature dependency of the percentage of dechlorination for different H20 concentrations. The value of CHC1, i in (b) was 1000 ppm.

Simultaneous chlorination and sulphation of calcined limestone

2531

EXPERIMENTAL

Kinetic studies on chlorination and sulphation were performed using a thermobalance as a reactor made of quartz. The inner diameter of the reactor was 22 ram. All experiments were carried out at atmospheric pressure =KI 1023 K that is lower than the melting point of CaCl2, 1045 K. Both concentrations of HCI and SO2 were 1000 ppm and that of 0 2 was 5%. Particle size of limestone (Chichibu, CaCO3 98%) used was in the three ranges of 32 - 75, 250 - 355 and 710 - 1000 I~m. About 30 mg of limestone was placed on a basket, heated to 1023 K in a stream of CO2 and decomposed to CaO by switching the atmosphere from CO2 to N 2. After the completion of decomposition was confirmed, the reactant containing HCI and/or SO2 was fed. The total flow rate of the feed was 1.5 I-STP rain "1. We confirmed that the flow rate hardly influenced on chlorination or sulphation rates. When the absorption experiments of HCI and SO2 were performed separately, the level of conversion of calcined limestone was determined directly from the increase in the sample weight. When the simultaneous absorption of HC1 and SO2 was carried out, for determining the levels of sulphation and chlorination, it is necessary to separate the contribution of these two reactions to the increase in the sample weight. The amount of chlorine absorbed in limestone was determined by measuring the concentration of Cl- ion using an ion meter (Horiba) after the sample was dissolved in 0.1M KNO 3 aqueous solution. The amount of sulfur absorbed in the limestone was evaluated by subtracting the increase of the sample weight by chlorination from the total weight increase. Morphological changes of the surface of limestone in the courses of sulphation and chlorination were investigated using scanning electron microscopy (SEM). RESULTS AND DISCUSSION C H L O R I N A T I O N OF L I M E S T O N E First, the chlorination of calcined limestone was carried out in the absence of SO 2 at 1073 K by using different ranges of particle size. We found that the chlorination behavior of calcined limestone at 1023 K was different from that observed at lower temperatures by Daoudi and Waiters (1991a and b) and Mum and Lallai (1992) who reported that chlorination rate decreased with increasing particle size. Figure 2 shows the progress of chlorination with time at 1023 K. The ordinate expressed the level of conversion of calcined limestone in the form of 2CI/Ca [mol tool-l]. It should be noted that at a given reaction time the 2CI/Ca ratio slightly depended on its particle size in the initial stage but those with different sizes almost overlapped after 90 rain on stream. The linear relation of the 2CI/Ca ratio with reaction time suggests that the chlorination rate was almost constant till at least 120 rain on stream. Comparison of the chlorination behavior of calcined limestone with its sulphation behavior, which has extensively been investigated, is useful to discuss the mechanisms of the chlorination. The sulphation behavior is in general understood as follows: (i) First, numerous pores are generated with CO2 evolution during the calcination of limestone according to eq. 8 and then these pores serve as the paths for the diffusion of SO2 into the interior of a limestone particle, (ii) pore closure occurs with the progress of sulphation with SO 2, resulting in the deterioration of the activity for sulphation, because the molar volume of the product, CaSO4, is much greater than that of CaO, and (iii) sulphation finally stops, after the pores are fully closed. As a result, the outer surface area of calcined limestone is a dominant factor governing the sulphation rate. That is, the rate of sulphation of calcined limestone heavily depends on its particle size. The sulphation rate of limestone steeply decreased with the pore closure and the final conversion of calcined limestone is commonly in the range of 0.2 - 0.5. It is believed that the diffusion of SO42" ions or couples of Ca 2+ and SO42" ions occurs in the product layer of CaSO4 from the surface to the interior of limestone. With the progress of sulphation, the CaSO4 layer becomes thicker and as a result, the barrier against the diffusion in the CaSO 4 layer becomes greater. Hence, the diffusion in the solid phase becomes difficult to contribute to the progress of sulphation of calcined limestone. The results shown in Figure 2 clearly suggest that in the case of the chlorination, deactivation of limestone by pore closure did not occur significantly. Taking it into account that particle size dependency of the chlorination rate was 0.8 a

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0.6

32-75/~m

b 250-355/z m c 710-1000/~m

0.4

0.2

0.0

30

60

90

120

Time [mln]

Figure 2. Progress of chlorination of calcined limestone with different particle sizes at 1023 K.

2532

M. MATSUKATAet al.

obscure, these chlorination results cannot be explained by the mechanisms Proposed for the sulphation. We, thus,

strongly believe that the chlorination of calcined limestone follows mechanisms different fxom that for su]phation. The mechanisms of chlorination will be discussedlater. SIMUL TANEO US CHL ORINA TION AND SULPHA TION

Let us discuss the results of the simultaneous chlorination and sulphation of calcined limestone at 1073 K in comparison with the results when SO2 and HCI absorption experiments with calcined limestone were performed separately. Figure 3a compares the progress of chlorination in the presence and absence of SO2. As to HC] absorption, the simultaneous presence of SO2 hardly influenced the chlorination behavior, while the chlorination rate of calcined limestone seems to be slightly depressed in the presence of SO2. On the other hand, significant effecl of coexisting HCI was observed on the sulphation of calcined limestone, as shown in Figure 3b. The sulphation was markedly acceleraled in the presence of HCI. The acceleration of sulphation became remarkable for larger limestone. The result using limestone with the particle sizes in the range of 32 - 75 ~m was the exception: The effect of coexisting HCI was hardly observed. When the limestone with particle sizes of 710 - 1000 ~m was used, the S/Ca ratio after 120 rain on stream attained to 0.35 in the presence of HCI, while the absorption of SO 2 ceased at around 0.25 of the S/Ca ratio in the absence of HCI. It is worth noting that for the particle sizes of 710 - 1000 p.m, the level of total conversion of calcined limestone, (2CI+S)/Ca, attained 0.88 after 120 rain on stream since the 2CI/Ca ratio was 0.35 after 120 min. Judging from Figure 3, further absorption of both HCI and SO 2 ocamed even at a level of the total conversion as high as about 0.9. The value of (2CI+S)/Ca increas~ with decreasing particle size at a given time on stream and limestone with the particle sizes of 32 - 75 rtm was fully used after 120 min. This is another important finding on the simultaneous chlorination and sulphation. As described above, the sulphation of calcined limestone in the absence of HCI becomes slower with the prowess of sulphation and ceased before limestone is fully used, in general. This tendency holds in the present study as can be seen in Figure 3b. It has been repoded (Deguchi et at. 1982a) that the absorption capacity of limestone for SO 2 in the presence of HCI was similarly improved. They, however, carded out the absorption experiments at 1073 K that is higher than the melting point of CaCI2, 1045 K. The results obtained in this study indicate that the absorption capacity of limestone for SO 2 was enhanced even at temperatures lower than the melting point of CaCI2. Improvement of limestone utilization has been a key issue in the development of fluidized bed coal combustors. We can anticipate that full utilization of limestone is easily accomplished in the case of co-combustion of coal and wastes in FBCs.

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Figure 3. (a) Progress of chlorination of calcined limestone in the presence and absence of SO2. (b) Progress of sulphation of calcined limestone in the presence and absence of HCI. Calcined limestones with different three ranges of particle size were used. Both concentrations of HC1 and SO2 were 1000 ppm.

Simultaneous chlorination and sulphation of calcined limestone

2533

The acceleration of sulphation in the presence of HC1 was not apparent in the early stage as shown in Figure 3b. The difference in the S/Ca ratios between the results obtained in the presence and absence of HCI became greater with the elapse of time. Since the deterioration of absorption activity for SO2 with time on stream is mainly due to pore closure of limestone, this tendency implies that pore closure was difficult to occur in the presence of HCI. MORPHOLOGICAL CHANGES The results of the kinetic study described above strongly stimulated us to observe morphological changes of limestone panicles in the courses of chlorination and sulphation by SEM. Figure 4 typically shows the SEM images for the surfaces of limestone particles. After the calcination at 1023 K, limestone has relatively fiat surface on which tiny particles have been attached as shown in Figure 4a. It is observed that numerous cracks of which the widths are in the order of submicrons have been formed. Figure 4b shows that when only SO 2 was absorbed for 20 min, the surface of calcined limestone has changed to ragged morphology. The cracks are difficult to find, indicating that those cracks generated by the calcination have been plugged owing to the formation of a layer comprising bulky CaSO4 on the limestone surface. As shown in Figure 3c, a different morphological feature is observed when HCI was absorbed in the absence of

(a)

(b)

(c)

(d)

i¸¸¸¸~:~i

Figure 4 Morphological change in the surface of Limestone (a) after calcination, (b) after 20 min of SO2 absorption, (c) after 20 rain of HC1 absorption and (d) after 20 min of simultaneous absorption of SO2 and HC1. Absorption experiments were carried out after calcination at 1073 K. (i) crack, (ii) large voids and (iii) particles in the course of occlusion.

2534

M. M^TSUrAT^et

al.

SO2. Aggregates consisting of many spherical particles with a diameter of about 0.5 ~tm have been formed. In addition, large voids have been generated among the spherical particles on the surface of limestone. It is presumed that the phase of CaCI2 formed on the limestone surface is readily sintered to generate the spherical particles, resulting in the formation of numerous voids. These voids generated in the course of chlorination seemingly play as a role of paths through which the interior of a limestone particle is easily accessible to HCI. Namely, while the formation of CaSO4 leads to pore closure in the absorption of SO2, the paths for HCI diffusion from the outer surface to the interior of a limestone particle are oeated in the course of the HCI absorption by the formation of the aggregates and the resultant voids. Thus, the formation of the voids can be a reason why the chlorination rate was kept almost constant against the progress of chlorination as seen in Figure 2. This discussion, however, seems difficult to fully explain thal the effect of the particle size of limestone was obscure on the rate of chlorination. Judging from the SEM images (Figure 3c), the constituent of the limestone surface, mainly CaCI2, seems mobile. In contrast to the case of the sulphation, it is supposed that chlorine trapped on the surface of limestone in the form of CI- ion is readily lransported from the surface to the bulk by the diffusion of CI- ions in the solid phase and/or the migration of the CaCI2 phase. This mechanism seemingly played a significant role on the progress of chlorination as web as the diffusion of HCI through the voids. We found that the morphological feature was dramatically changed when HCI and SO 2 were simultaneously absorbed, as shown in Figure 3d. Large aggregates with very flat surface and large voids have been formed in the course of the simultaneous chlorination and sulphation. In Figure 3d, one can see a number of small particles that have been buried in the large aggregates. This possibly implies formation of a molten phase composed of a entectic mixture of CaCI 2 and CaSO 4. We observed that by means of DSC an equimolar mixture of CaCI2 and CaSO4 melted at 997 K which is much lower than the melting point of CaCI2, 1045 K. It has been reported (Delmon and van Houte, 1978; van Houte el al., 1981) that the addition of a slight amount of CaCI2 (2wt%) on CaCO3 promoted SO2 absorption in the temperature range of 573 - 908 K. Their TEM observations showed that by the addition of CaCI2, the morphology of CaCO3 crystals was dramatically altered to form large aggregates and voids. They proposed that such voids allowed SO 2 to enter into the bulk of CaCO3 particles readily. Similarly to their proposal, we suppose thai the voids among aggregates possibly play a role of paths for the diffusion of HCI and SO2 toward the interior of limestone particles. However, the melting of the eutectic mixture possibly contributed to the promotion of SO2 absorption more largely at higher temperatures as high as 1023 K used in the present study, leading to full utilization of limestone. In the present study, we observed that the rate of chlorination was independent of the simultaneous presence of SO2, although SO 2 absorption was accelerated by the simultaneous presence of HCI, as shown in Figure 3. Further study is requhed to fully understand the absorption mechanisms of HCI and SO 2. CONCLUSIONS Thermodynamic calculation showed that HCI can be funned from plastics under the FBC conditions. The HCI retention in fluidized beds using limestone is possible under FBC conditions. It is, however, predicted that the efficiency of HCI retention in FBCs heavily depends on the bed temperature and the H20 concentration. [t should be noted that at higher concentrations of H20 and higher temperatures, the emission control is thermodynamically restricted. Being different from the results on the absorption of SO 2 by calcined limestone, the absorption of HCI by calcined limestone showed that the dependency on the particle size was obscure and that the chlorination rote was kept constant against the progress of chlorination. Furthermore, the capacity of limestone for SO 2 absorption was found to be improved in the presence of HCI. This improvement of the absorption capacity is anticipated to make possible 100% utilization of limestone in FBCs. Two reasons have been emerged to explain this chlorination behavior and the acceleration of SO2 absorption in the presence of HCI: (1) the formation of a CI" ions-containing phase that makes CI" and SO42" species very mobile and (2) the formation of voids playing a role of the diffusion paths for HCI and SO2 toward the interior of a limestone particle. The former may be caused by the melting of a eutectic phase of CaCI2 and CaSO4. ACKNOWLEDGMENT The authors thank Idemitsu Kosan Co. Ltd. for kindly supplying the limestone sample. REFERENCES

Delmon, B., G. van Houte: USP 4115518 Deguchi, A., Y. Kochiyama, H. Hosoda, M. Miura, T. Himma, H. Nishizaki and M. Horio: J. Jpn Inst. Energy, 61 (1982a) 1105-1108 Deguchi, A., Y. Kochiyama, H. Hosoda, M. Miura, T. Hirama, H. Nishizaki and M. Horio: J. Jpn Inst. Energy, 61 (1982h) 1109-1112 Daoudi, M., J.K. Walters: Chem. Eng. J., 47, (1991a), 1-9 Daoudi, M., J.K. Waiters: Chem. Eng. J., 47, (1991b), 11-16 Mura, G., A. Lallai: Chem. Eng. Sci., 47, (1992) 2407-2411 Mura, G., A~ Lallai: Chem. Eng. Sci., 49, (1994) 4491-4500 Van Houte, G, L. Roddque, M. Genet, B. Delmon, Env. Sci. Technol., 15, 0981) 327-332