Energy & Fuels 2005, 19, 2335-2339
2335
Partially Sulfated Lime-Fly Ash Sorbents Activated by Water or Steam for SO2 Removal at a Medium Temperature Liming Shi* and Xuchang Xu Department of Thermal Engineering, Tsinghua University, Beijing, 100084, China Received April 6, 2005. Revised Manuscript Received July 15, 2005
Laboratory experiments were conducted to investigate the reactivity of partially sulfated limefly ash sorbents activated by water or steam for SO2 removal. Sulfation tests were performed at 550 °C using a fixed bed reactor under conditions simulating economizer zone injection flue gas desulfurization. Activation experiments were conducted with water or steam using a range of temperatures between 100 and 550 °C. The results showed that the reactivity of the sorbents was closely related to the content of Ca(OH)2 formed in the activation process, which varied with the water or steam temperature. The sulfur dioxide capture capacity of Ca(OH)2 in the sorbent is higher than that of CaO at a medium temperature. Water or steam temperatures in the range of 100-200 °C are favorable to the formation of Ca(OH)2 from CaO.
Introduction In comparison with wet processes for the removal of SO2 from flue gas, dry flue gas desulfurization (dry FGD) processes are much cheaper. There are three dry FGD approaches corresponding to three temperature windows: (1) in-furnace injection (900-1250 °C); (2) economizer injection (450-550 °C); (3) duct injection (typically below 200 °C). While previous work has shown that medium temperature (economizer injection) desulfurization technologies are practical, the calcium utilization in the sorbents (typically lime, quick lime, and lime-fly ash) is still not high. The low utilization of calcium-based sorbent is caused by the formation of calcium sulfite or sulfate, which have larger molar volumes than CaO or Ca(OH)2. Blockage of pores in the sorbents prevents SO2 diffusion into the surfaces of unreacted sorbent. Sorbent after the first sulfation reaction, called the spent (partially) sorbent, can be activated by water or steam in a process called hydration to further improve its calcium utilization. This method is simple and inexpensive. The hydration process is generally divided into two stages, slurrying and drying. Most of the hydration products are formed in the slurrying stage. The second stage is to dry the slurry. Previous work has shown that activity is improved due to the formation of microporous structure and active species in the sorbent.1,2 Tsuchiai et al. found that the activity of various sorbents was markedly increased up to 400 °C drying * To whom correspondence should be addressed. Present address: 407 Stocker Center, Ohio Coal Research Center, Ohio University, Athens, OH 45701. Tel: +01 (740) 597-1501. Fax: +01 (740) 593-0476. E-mail:
[email protected]. (1) Jozewicz, W.; Rochelle, G. T. Fly Ash Recycle in Dry Scrubbing. Environ. Prog. 1986, 5, 219-223. (2) Ueno, T. Process for Preparing Desulfurizing and Denitrating Agents. U.S. Patent 4629721, 1986.
temperature.3 The sorbents, including calcium oxide, calcium sulfate, and fly ash, were heated at 95 °C for 15 h before drying. The reaction temperature with SO2 was 130 °C to simulate low-temperature FGD. Other researchers have investigated the reactivation of dry solids from the atmospheric circulating fluidized bed combustors using water or steam.4,5,6 The sulfation temperature in these studies was around 850 °C. Their results indicate that water or steam is effective for reactivating the bed ash fractions tested and that the hydration temperature (150-250 °C) plays a very important role in the hydration of FBC ash with saturated steam. Although these studies provide promising results, it is very difficult to realize the slurrying stage on a large scale since it requires large amounts of water, heat, and space, especially when steam is the activation medium. The purpose of this research is to determine what calcium utilization levels are possible in mediumtemperature desulfurization if the sorbent does not go through the slurrying stage but is treated by small amounts of water or steam to enhance its reaction ability with SO2. Temperature for the sulfation experiments was set at 550 °C, and temperatures for water/ steam treatment were in the range of 100-550 °C. To help understand the mechanism of activation, the (3) Tsuchiai, H.; Ishizuka, T.; Ueno, T.; Hattori, H.; Kitat, H. Highly Active Absorbent for SO2 Removal Prepared from Coal Fly Ash. Ind. Eng. Chem. Res. 1995, 34, 1404-1411. (4) Couturier, M. F.; Marquis, D. L.; Steward, F. R.; Volmerange, Y. Reactivation of Partially-Sulphated Limestone Particles from a CFB Combustor by Hydration. Can. J. Chem. Eng. 1994, 72, 91-97. (5) Couturier, M. F.; Volmerange, Y.; Steward, F. R. Hydration of Partially Sulfated Lime with Water. In Proceedings of the 15th International Conference on Fluidized Bed Combustion, Savannah, GA, May 16-19, 1999; American Society of Mechanical Engineers (ASME): New York, 1999; Paper No. FBC99-0119. (6) Wu, Y. H.; Anthony, E. J.; Jia, L. Steam Hydration of CFBC Ash and the effect of Hydration Conditions on Reactivation. Fuel 2004, 83, 1357-1370.
10.1021/ef0500991 CCC: $30.25 © 2005 American Chemical Society Published on Web 08/23/2005
2336
Energy & Fuels, Vol. 19, No. 6, 2005
Shi and Xu
Figure 1. Apparatus for the sulfation tests or steam activation tests. Table 1. Particle Size Distribution of Fly Ash size range (µm) 0-40 40-60
wt %
size range (µm)
wt %
27.32 27.68
60-74 74-100
25.54 19.45
Table 2. Chemical Composition of Lime, Fly Ash, and Lime-Fly Ash species
lime
fly ash
lime-fly ash
SiO2 Al2O3 Fe2O3 CaO MgO K2O Na2O MnO TiO2 P2O5 ignition loss
5.78 0.99 0.64 70.38 10.96 0.27 0.53 0.007 0.05 0.11 10.28
53.84 26.92 8.20 2.37 0.78 1.09 0.93 0.96 0.09 0.32 4.47
28.32 16.81 3.58 37.61 5.19 0.70 0.29 0.64 0.63 0.18 6.87
effective species for desulfurization were identified by XRD and SEM techniques. Experimental Section Materials. Lime used was obtained from the Building Materials Plant, Beijing, China. Fly ash used in this study was from the First Cogeneration Power Plant in Beijing, China. Lime and fly ash were sieved separately. The particle sizes of fly ash and CaO used in this study were less than 100 µm. The particle size distribution for fly ash (less than 100 µm) is given in Table 1. Note that particle sizes less than 90 or 75 µm have been used in similar studies by other researchers.7,8 Equal amounts of lime and fly ash were measured, loaded to a large plate, mixed evenly using a spoon by hand, and stored in the glass bottle. The chemical compositions of lime, fly ash, and lime-fly ash mixture are listed in Table 2. The content of CaO in the fly ash itself is low. Apparatus. A fixed bed reactor (shown in Figure 1) was used for the sulfation test to evaluate the influence of water or steam treatment on the utilization of lime-fly ash. About 3 g of sorbent was evenly loaded into a ceramic sample holder. The sample holder was centered horizontally in the fixed bed reactor. The simulated flue gas (a mixture of air and SO2) was preheated to the desired temperature and then passed through the sample holder in the reactor. The temperature for sulfation was kept at 550 °C, and the reaction lasted 1 h. Sorbent was collected after each test, stored in a glass tube, and sealed by a rubber stopper. The flow rate of gas was kept at 10 L/min to prevent particles from entraining into the gas stream. The concentration of SO2 at the inlet of the reactor was 1000 ppm. To activate the sorbent by steam, the inlet gas is switched from a mixture of air and SO2 to steam only, provided by the steam generator. (7) Bernardo, G.; Telesca, A.; Valenti, G. L.; Montagnaro, F. Role of Ettringite in the Reuse of Hydrated Fly Ash from Fluidized-Bed Combustion as a Sulfur Sorbent: A Hydration Study. Ind. Eng. Chem. Res. 2004, 43, 4054-4059. (8) Wu, Y. H.; Anthony, E. J.; Jia, L. An Experiemtnal Study on Hydration of Partially Sulfated FBC Ash. Combust. Sci. Technol. 2002, 174, 171-181.
Figure 2. Flow diagram of the experimental process. Sample Preparation. The flow diagram of the experimental process is shown in Figure 2. First, fresh lime-fly ash reacted with SO2 at 550 °C for 1 h. Without any treatment, some of the spent sorbent reacted with SO2, again at 550 °C for 1 h. The remaining sorbent is named the resulfated sorbent. Some spent sorbent was treated by water or steam under different temperatures. Next, these resulting dry sorbents from the activation process were tested for SO2 removal at 550 °C for 1 h separately. To consider the effect of water temperature on sorbent activity, sorbents were prepared as follows: 2.6 g of spent sorbent was measured and placed in the ceramic crucible. A 5.2 g amount of deionized water (2:1 water:sorbent ratio) was added into the crucible. The crucible was shaken well and placed in the isothermal oven. The oven temperature was set at 100, 200, 300, 400, and 550 °C, respectively. The time for sorbent in the furnace was 15 min except for sorbents dried at 100 °C, which took 25 min. No visible water was in the crucible at the end of activation. To consider the effect of steam temperature on sorbent activity, sorbents were prepared as follows: 2.6 g of spent sorbent was measured and loaded in the sample holder. The holder was placed in the reactor shown in Figure 1. The inlet gas to the reactor was switched to steam. The rate of steam generation was 6.6 g/min. The reaction time of steam with sorbent was 15 min, and steam temperature was set at 120, 200, 300, 400, and 550 °C, respectively. The utilization rate is defined as the percentage of the amount of calcium reacted with SO2 to the initial amount of calcium contained in the sorbent. The amount of calcium reacted with SO2 was calculated on the basis of the sulfur content in the sorbent, which was measured by turbidity comparison.9 A mixture of the sorbent and a strong oxidizer like Na2O was melted at 700 °C. After the acid treatment and addition of BaCl2, the content of BaSO4 in the liquid was determined by comparing its turbidity with the standard sample using visible light (420-444 nm).
Results Calcium Utilization of Sulfated Sorbents. The results for sulfation ability of sorbents activated by water or steam are plotted in Figure 3. The error in these values is estimated to be about (10%.10 The dotted and dashed horizontal lines from the bottom of the chart represent the average calcium utilization of sorbent after the first sulfation and resulfation, respectively. The calcium utilization degree after the resulfation was slightly higher than that obtained after the first sulfation. When the spent sorbent was loaded in the ceramic plate for the resulfation test, some surfaces of the particles may have had a better exposure to the gas stream, which resulted in further reaction with SO2. The square symbols in the figure show the resulfation ability of sorbent treated by water versus the activation (9) Shi, L.; Xu X. Study of the effect of Fly Ash on Desulfurization by Lime. Fuel 2001, 80, 1969-1973. (10) Shi, L. Research on Mechanism of Medium-Temperature Dry Flue Gas Desulfurization and Steam Activation, Ph.D. Thesis, Tsinghua University, June 1999.
Partially Sulfated Lime-Fly Ash Sorbents
Figure 3. Calcium utilization of lime-fly ash sorbents sulfated at 550 °C vs activation temperature.
temperature. It can be seen that calcium utilization rate depended on the activation temperature. The sulfation ability was greatly enhanced after water treatment. The influence of water treatment was much more significant at the lower temperatures (100-300 °C) than that at the higher temperatures (300-550 °C). It should be noted that some particles agglomerated after water treatment. The amount of agglomerated particles decreased as the activation temperature increased. The agglomerated sorbent was crushed gently and sieved to ensure particle size less than 100 µm prior to sulfation. In this study, it is unknown how the crushing affected the resulfation ability of sorbents activated by water. Size of particles which contained calcium can be smaller or greater than their previous size after crushing. To determine such effect, it is recommended that the particle size distribution be measured before and after water activation and particles be separated into different size ranges to conduct the resulfation tests. The water activation process is actually equivalent to the drying stage of the regular hydration process. Because of the small amount of slurry and the exothermic reaction between CaO and water, the slurry temperature quickly increases to 100 °C in the oven, remains at this temperature during free water (not bonded to the sorbent) evaporation, and increases again until it reaches the target value. Therefore, the process is not isothermal but it can be used to guide future implementation of similar large-scale processes. The triangle symbols in Figure 3 represent the calcium utilization of sorbent treated at different steam temperatures. The calcium utilization also decreased with the increase of steam temperature. The improvement of sorbent activity treated by steam with temperature higher than 200 °C is negligible. The sorbent treated by water showed a higher utilization compared with that treated by steam under the same temperature. The sorbent prepared by water activation achieved up to 30% of utilization while those by steam achieved only 22%. These results indicate that it is possible to use small amounts of water or steam to improve sorbent activity without the slurrying stage and that the activity of sorbent for SO2 removal depends on the hydration medium.
Energy & Fuels, Vol. 19, No. 6, 2005 2337
Figure 4. XRD patterns of (a) spent sorbent, (b) sorbent activated at 100 °C, (c) sorbent activated at 200 °C, (d) sorbent activated at 300 °C, (e) sorbent activated at 400 °C, and (f) sorbent activated at 550 °C by water.
XRD and SEM Results of Sorbents Activated by Water. To determine the species produced during water treatment, XRD analysis was performed. The XRD patterns for the spent sorbent and water-activated sorbent are shown in Figure 4. CaO, CaSO4, and CaCO3 were found in the spent sorbent. When the activation temperature is in the range of 100-300 °C, the peaks for CaO disappeared and CaO was converted to Ca(OH)2. It should be noted that ettringite was also formed in the sorbent when dried at 100 °C. The peaks are corresponding to 2θ at 9.16, 15.94, 23.04, and 32.58°.3,7,11,12 Ettringite is represented by the molecular formula Ca6[Al(OH)6]2(SO4)3‚26H2O. Calcium sulfate in the spent sorbent provides SO4 ion for ettringite formation. Bernardo et al. also found that both ettringite and calcium hydroxide can be formed at the end of 30 min hydration process.7 Both CaO and Ca(OH)2 were detected when the activation temperature is 400 °C. Only CaO can be found when the temperature is around 550 °C. As the temperature increases, the dehydration of Ca(OH)2 to CaO cannot be ignored. In this study, Ca(OH)2 can dissociate completely at 550 °C. This is in agreement with the results given by Hartman and Coughlin.13 Therefore, the activation temperature greatly affects the conversion of CaO to Ca(OH)2 during water hydration process. No peak corresponding to ettringite is found in the sorbents dried at 200-550 °C. This indicates that it decomposes at a temperature between 100 and 200 °C, similar to the result of Bernardo et al.7 To examine the species formed in the water activation process, SEM pictures were also taken. Figure 5 gives the surface morphology of various sorbents activated by water. Figure 5a,b shows SEM results for surfaces of fly ash and lime in the sorbent activated at 100 °C, respectively. The needlelike product on the surfaces was (11) Anthony, E. J.; Iribarne, A. P.; Iribarne, J. V.; Jia, L. Reuse of Landfilled FBC Residues. Fuel 1997, 76, 603-606. (12) Taylor, H. F. W. Cement Chemistry; Academic Press: New York, 1990. (13) Hartman, M.; Coughlin, R. W. Reaction of Sulfur Dioxide with Limestone and the Grain Model. AIChE J. 1976, 22, 490-498.
2338
Energy & Fuels, Vol. 19, No. 6, 2005
Shi and Xu
Figure 5. SEM results of the (a) surface of fly ash in the sorbent activated at 100 °C by water, (b) surface of lime in the sorbent activated at 100 °C by water, and (c) surface of lime in the sorbent activated at 200 °C by water.
identified to be ettringite.3,14 Figure 5c shows the surface of lime in the sorbent activated at 200 °C. The spherical particles found on the surface were fly ash, and the hexagonal plate type particles which were partially embedded in the lime surface were Ca(OH)2. Lime (calcium oxide) has a cubic crystal structure. When calcium oxide reacts with water, the resultant hydrated lime (calcium hydroxide) has a hexagonal crystal structure. XRD Results of Sorbents Activated by Steam. XRD analysis of sorbent treated by steam is shown in Figure 6. CaO can be converted to Ca(OH)2 by steam at 120 °C. The peaks for Ca(OH)2 were rather weak in the sorbent treated by steam at 200 °C. The low conversion from CaO to Ca(OH)2 is probably because the reaction rate between gas and solid is slow. The amount of Ca(OH)2 was negligible as the steam temperature increased from 300 to 550 °C. No obvious physical change was observed for the particles after steam treatment. This is in agreement with the work of Couturier et al., whose experiments showed maximum hydration levels at 150 and 200 °C of 37% and 28%, respectively, after 50 min of steam treatment.4 It took only 15 min for CaO conversion to be close to the maximum level. Discussion The amount of Ca(OH)2 was calculated by the sum of intensity at 2θ ) 18.0 and 34.1° for sorbents activated (14) Moore, A. E.; Taylor, H. F. W. Crystal Structure of Ettringite. Acta Crystallogr., Sect. B 1970, 26, 386-393.
Figure 6. XRD patterns of (a) spent sorbent, (b) sorbent activated at 120 °C, (c) sorbent activated at 200 °C, (d) sorbent activated at 300 °C, (e) sorbent activated at 400 °C, and (f) sorbent activated at 550 °C by steam.
by water or steam. The results are presented in Figure 6. For the same activation temperature, the amount of Ca(OH)2 formed with water treatment is higher than with steam treatment. In comparison of Figures 3 and 7, it can be seen that the change of the amount of Ca(OH)2 in the sorbent versus temperature is similar to that of calcium utilization versus temperature for both water activation and steam activation. The amount of Ca(OH)2 in the sorbent
Partially Sulfated Lime-Fly Ash Sorbents
Figure 7. XRD peak intensity of Ca(OH)2 under various activation temperatures.
can be estimated from its intensity in the XRD results. However, using a near-quantitative way to measure Ca(OH)2 will be more convincing. Since the amount of Ca(OH)2 in the spent sorbent is negligible, it is apparent that the amount of Ca(OH)2 produced from the activation affects the effectiveness of sorbents. The amount of Ca(OH)2 has a significant effect on the reaction between the sorbent and SO2. The desulfurization ability increases as more CaO converted to Ca(OH)2. The effect of ettringite on desulfurization cannot be determined since the calcium utilization rate is too close from 100 to 300 °C while ettringite was found in the sorbent only at 100 °C. Two possible explanations are that the amount of ettringite is little or that its decomposition temperature is low (below 200 °C) in contrast to the reaction temperature with SO2 set at 550 °C. It also should be noted that it took an additional 10 min for sorbent activated at 100 °C by water than sorbents activated at higher temperatures. Previous work of Wu et al. indicated that 5 min of hydration can result in CaO conversions of about 80-90% for fly ash less than 75 µm.15 Since the amount of CaO conversion during the additional 10 min was negligible, its effect on the resulfation ability of sorbent can be ignored. As can be seen from Figure 8, CaSO4 was identified as the product from the sulfation reaction. Peaks corresponding to CaSO3 were not found. This indicates that it may dissociate at 550 °C. In contrast to the spent and resulfated sorbent, the peak height of CaSO4 was (15) Wu, Y. H.; Anthony, E. J.; Jia, L. Experimental Studies on Hydration of Partially Sulphated CFBC Ash. Can. J. Chem. Eng. 2003, 81, 1200-1214.
Energy & Fuels, Vol. 19, No. 6, 2005 2339
Figure 8. XRD results of (a) spent sorbent, (b) resulfated sorbent, (c) resulfated sorbent, activated by water at 100 °C, and (d) resulfated sorbent, activated by steam at 120 °C.
much higher in the resulfated sorbent which was activated by water at 100 °C or steam at 120 °C. Therefore, the newly formed CaO from Ca(OH)2 dehydration is more reactive toward SO2 than the original CaO in the sorbent. Conclusions Sorbents activated by water or steam for SO2 removal from flue gas at a medium temperature have been investigated. The results show that sorbents can be activated using small amount of water without the slurrying stage, which is important to the large-scale application. Sorbents treated by water have higher reaction activity than those treated by steam under the same temperature. The amount of Ca(OH)2 formed during the treatment contributes to the effectiveness of sorbents. More CaO can be converted to Ca(OH)2 at water or steam temperatures in the range of 100-200 °C. Acknowledgment. This research was carried out in the State Key Laboratory of Clean Coal Combustion at Tsinghua University. The work was financially supported by the Ministry of Science and Technology of China and the New Energy and Industrial Technology Development Organization (NEDO) of Japan. EF0500991
