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Suppression of SO2 Emission during Coal Oxidation with Calcium Loaded by Hydrothermal or Hydration Treatment Koichi Matsuoka,* Aki Abe, and Akira Tomita Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, 2-1-1 Katahira, Sendai, 980-8577 Japan Received September 14, 2000. Revised Manuscript Received February 8, 2001
Calcium was loaded on Illinois No. 6 coal by different methods; hydrothermal treatment, hydration treatment, and physical mixing, and then its effectiveness as a desulfurizing agent was compared under various experimental conditions. The amount of SO2 emitted from the coal hydrothermally treated with lime was quite small during the oxidation in comparison with the physical mixture of coal with lime. In the case of coal with hydrated lime, the amount of SO2 emission was as small as that of hydrothermally treated coal when compared at the same level of calcium loading. Calcium loaded by hydrothermal or hydration treatment can act as a good sulfur sorbent. Chemical species produced by these treatments were identified by using X-ray diffractometer and a scanning electron microscope attached with energy-dispersive X-ray analyzer, and we attempted to clarify the effectiveness of each species for SO2 sorption.
Introduction The emission of SOx from a coal-fired power plant is mainly suppressed by the wet process using a slurry of calcium compounds. The degree of calcium utilization is very high, but the process needs a large amount of water as well as facility for wastewater treatment. On the other hand, dry methods have some advantages such as low cost, simplicity in operation system, and no need of wastewater treatment. However, the degree of calcium utilization is generally low, because of the pore blocking of sorbent by reaction product like calcium sulfate. Many attempts have been made to explore the more efficient sorbents which can capture as much sulfur per calcium as possible. As one of such attempts, desulfurizing agents have been produced through the reaction of calcium compounds with fly ash.1-9 Most of these studies aim to remove SOx from flue gas at relatively low temperature. This approach is quite promising and some sorbents are already used commercially. Another dry process is the direct injection of calcium-containing species into the combustor. Some attempts to improve the efficiency of calcium utilization * Corresponding author. Fax: +81-22-217-5626. E-mail: matsuoka@ tagen.tohoku.ac.jp. (1) Jozewicz, W.; Rochelle, G. T. Environ. Prog. 1986, 5, 219. (2) Jozewicz, W.; Chang, J. C. S.; Brna, T. G.; Sedman, C. B. Environ. Sci. Technol. 1987, 21, 664. (3) Martı´nez, J. C.; Izquierdo, J. F.; Cunill, F.; Tejero, J.; Querol, J. Ind. Eng. Chem. Res. 1991, 30, 2143. (4) Ho, C.-S.; Shih, S.-M. Ind. Eng. Chem. Res. 1992, 31, 1130. (5) Sanders, J. F.; Keener, T. C.; Wang, J. Ind. Eng. Chem. Res. 1995, 34, 302. (6) Tsuchiai, H.; Ishizuka, T.; Nakamura, H.; Ueno, T.; Hattori, H. Ind. Eng. Chem. Res. 1996, 35, 851. (7) Tsuchiai, H.; Ishizuka, T.; Nakamura, H.; Ueno, T.; Hattori, H. Ind. Eng. Chem. Res. 1996, 35, 2322. (8) Davini, P. Fuel 1996, 75, 713. (9) Arthur, L. F.; Rochelle, G. T. Environ. Prog. 1998, 17, 86.
have been pursued. For example, Fujiwara et al.10 showed that calcium loaded on lignite by ion-exchange method can effectively capture SO2 emitted in fluidized bed combustion, since a large amount of well-dispersed calcium can be loaded on low rank coal. However, this method cannot be applied for bituminous coal, because bituminous coal does not possess enough carboxyl or hydroxyl groups. Thus, there have been some trials to introduce ion-exchange sites on bituminous coal by mild oxidization.11-13 The attempt to utilize the hydrated lime for desulfurization was reported in a patent,14 where a bituminous coal was mixed with lime in aqueous solution. However, the details are unknown yet. Although sulfur emission could be suppressed to a large extent by these methods, the extent of suppression is still unsatisfactory. A more effective desulfurizing agent is desired. In our previous study,15 we proposed a new coal cleaning method to chemically remove mineral matter from coal. This method consists of the hydrothermal treating of coal with lime followed by acid washing. Preliminary experiment showed that the hydrothermal reaction products between lime and mineral matter, i.e., calcium-containing species such as calcium silicates and calcium aluminosilicates, are effective in desulfurization. In this paper, we carried out hydrothermal treatment of coal with lime and examined the ability of its (10) Fujiwara, N.; Takarada, T.; Kato, K. Proc. 7th Int. Conf. Coal Sci., 1993; Vol. 2, p 541. (11) Chang, K. K.; Flagan, R. C.; Gavalas, G. R.; Sharma, P. K. Fuel 1986, 65, 75. (12) Sharma, P. K.; Gavalas, G. R.; Flagan, R. C. Fuel 1987, 66, 207. (13) Freund, H.; Lyon, R. K. Combust. Flame 1982, 45 191. (14) Reuther, J.; Feldman, H. U.S. Patent 854223, 1986. (15) Wang, J.; Zhang, Z.-G.; Kobayashi, Y.; Tomita, A. Energy Fuels 1996, 10, 386.
10.1021/ef0001991 CCC: $20.00 © 2001 American Chemical Society Published on Web 04/11/2001
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Table 1. Analyses of Illinois No. 6 Coal S (wt %, dry)
ultimate analysis (wt %, daf) C H N O
Ash Pyritic Sulfate Organic (wt %, Total S S S dry)
77.7 5.0 1.4 10.3
4.8
2.8
0.0
2.0
15.5
Ash Analysis (wt %, ash) SiO2 Al2O3 Fe2O3 CaO K2O MgO TiO2 Na2O P2O5 SO3 43.7
18.3
18.0
7.9
2.9
1.2
1.0
0.0
0.2
6.8
reaction products in SOx capture during oxidation. We also added hydrated lime on coal as a reference, and determined its effectiveness. Furthermore, we attempted to identify the effective species for desulfurization by using XRD and SEM/EDX techniques. Experimental Section Sample Preparation. Illinois No. 6 coal, Argonne premium coal, was used as a coal sample. Table 1 presents its ultimate analysis and ash analysis. The pulverized coal particles in the size smaller than 75 µm were mixed with reagent grade lime (Wako Chemical Co. Ltd.). Fifteen grams of the mixture (CaO + coal) was loaded into an autoclave with 150 mL of distilled water. The atomic ratio of calcium to sulfur (Ca/S) in the slurry was 1.4 or 5.6. The slurry was heated at a rate of 10 °C/min up to 300 °C under autogenous pressure and held at this temperature for 3 h with stirring. The product was then filtered and dried at 90 °C in nitrogen atmosphere. As a reference, we prepared Ca-impregnated coal samples where the slurry was only stirred at ambient temperature for 3 or 24 h. Ca/S ratio in the mixture was varied from 0.7 to 5.1 by changing the initial load of lime. Oxidation. Ca-loaded coal particles (200 mg) were set in a vertical fixed bed reactor (i.d. ) 7.0 mm) and heated at 10 °C/ min up to 800 °C under a flow of 10% O2-N2 mixture at a flow rate of 100 cm3/min. The outlet gas was analyzed by a SOx meter (Shimadzu SOA-7000). Characterization. The amount of calcium in the sample was determined by XRF analysis (Shimadzu XRF 1700). SEM (Topcon ABT60) with an attached energy-dispersive X-ray analyzer (EDAX Phoenix) was used to examine the surface characteristics of Ca-loaded coals. XRD (Shimadzu XD-D1w) analysis was performed to determine the sample composition.
Results Sulfur Emission from Ca-Loaded Coal. Figure 1a shows SO2 concentration in the outlet gas during oxidation of raw coal, physical mixture of coal with lime, and hydrothermally treated coals. Figure 1b represents the cumulative amount of sulfur emission per gram of coal. Ca/S in the parentheses shows the atomic ratio of Ca to S in the sample as determined by XRF analysis. The ratio of Ca/S in the hydrothermally treated coal was somewhat lower than the initial mixing ratio. For example, the Ca/S ratio was 5.2 when the initial Ca/S ratio was 5.6. This is because some amount of calcium was dissolved into water. Four SO2 peaks are seen at around 240, 280, 340 and 570 °C during the oxidation of raw coal. In the physical mixture, only two emission peaks are observed and the peak areas are small in comparison with those for raw coal. The peaks of SO2 emission from the hydrothermally treated coal at a Ca/S ratio of 1.4 is smaller than that of physical mixture. Almost no peak is observed for the sample with a Ca/S ratio of 5.2. The hydrother-
Figure 1. SO2 emission from raw coal, physical mixture of coal with lime, and hydrothermally treated coals during oxidation. Heating rate: 10 °C/min; gas; 10% O2/N2.
mal products between mineral matter and lime are likely to be active sorbent in a wide temperature range. The cumulative amount of sulfur emitted from raw coal is 4.3 wt %-dry which is a little bit lower than the total amount of sulfur in coal (4.8 wt %-dry). This is because some sulfur was captured by inherent calcium in coal. On the basis of the amount of sulfur emitted from raw coal, the desulfurizing yields of the physical mixture and hydrothermally treated coals for Ca/S of 1.4 and 5.2, are calculated as 41, 61 and 95%, respectively. The desulfurization yield for the hydrothermally treated coal with a Ca/S ratio of 1.4 is higher than the physical mixture by 20%, in spite of the fact that its Ca content is only a quarter of that of the physical mixture. Some effective species for SO2 removal might be formed during the hydrothermal reaction. To determine the species produced during the hydrothermal treatment, XRD analysis was carried out. The XRD patterns for the raw coal and hydrothermal coals before the oxidation are shown in Figure 2. Main components in the raw coal are quartz (SiO2), kaolinite (Al2Si2O5(OH)4), pyrite (FeS2) and calcite (CaCO3). The peak height of quartz in the hydrothermal coal is relatively lower than in the raw coal and negligibly small for Ca/S ratios of 1.4 and 5.2, respectively. Kaolinite peaks disappeared in both of the hydrothermal coals. It is evident that quartz and kaolinite were converted to other species during the hydrothermal reaction. Since kaolinite is more reactive than quartz,16,17 no kaolinite peak remained even in the hydrothermal
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Figure 3. SEM image and mapping analysis of hydrothermally treated coal at a Ca/S ratio of 5.2.
Figure 2. XRD patterns of raw coal and hydrothermally treated coals before and after the oxidation. A: anhydrite (CaSO4), C: calcite (CaCO3), H: hematite (Fe2O3), K: kaolinite (Al2Si2O5(OH)4), L: lime (CaO), P: pyrite (FeS2), Po: portlandite (Ca(OH)2), Q: quartz (SiO2).
coal with a Ca/S ratio of 1.4. The peaks for pyrite, which emits SO2 upon oxidation, still remained even after the hydrothermal reaction. As for calcium compounds, calcite is present in both of the hydrothermal coals, while portlandite (Ca(OH)2) is present only in the case of Ca/S ) 5.2. No other peaks of calcium-containing species were observed. However, in our previous studies,16,17 calcium silicates (R-dicalcium silicate hydrate: Ca2SiO4‚H2O, tobermorite-like compounds: (CaO)x.SiO2. nH2O (x ) 0.8-1.2), jaffeite: Ca6(Si2O7)(OH)6) were formed by the hydrothermal reaction of lime with quartz16 under the same condition as the present condition. Also, jaffeite and calcium aluminosilicates (hibschite: Ca3Al2(SiO4)1.25(OH)7, Ca3Al2(SiO4)2(OH)4) were formed by the hydrothermal reaction of lime with kaolinite.17 These species are likely to be formed during the hydrothermal reaction. They could not be detected by XRD, perhaps because of poor crystallinity. The XRD patterns for the samples after the oxidation are also shown in Figure 2. In the ash from raw coal, hematite (Fe2O3) converted from pyrite is seen in addition to quartz and calcite. In the ashes from hydrothermal coals, strong peaks of hematite are observed. Pyrite which was stable upon the hydrothermal treatment was converted into hematite during the oxidation. Anhydrite (CaSO4) is the only sulfur-contain(16) Wang, J.; Tomita, A. Ind. Eng. Chem. Res. 1997, 36, 1464. (17) Wang, J.; Tomita, A. Ind. Eng. Chem. Res. 1997, 36, 5258.
ing species, suggesting that sulfur emitted was partly trapped as sulfate. During the oxidation, portlandite was dehydrated and converted into lime, and then lime was sulfated. At a Ca/S ratio of 5.2, lime is seen in addition to anhydrite. If the excess amount of calcium was loaded, some portion of lime remained without being sulfurized even after the oxidation. From the above results, calcite and portlandite are considered to act as desulfurizing agents. There is a possibility that some amorphous Ca-containing species were formed as described above and they also worked as desulfurizing agents. To examine the detailed composition of mineral species in the hydrothermal coal, they were characterized with SEM/EDX analysis. Morphology and Composition. Figure 3 shows typical SEM image and mapping analysis for the hydrothermal coal at a Ca/S ratio of 5.2. Many small particles were found on coal particles. Ca, Si, and Al were detected in these particles. Most particles contained Ca, Si, and Al, which can be ascribed to calcium aluminosilicate. Some particles contained only Ca (portlandite or calcite) or Ca and Si (calcium silicate). As shown in Figure 2, portlandite and calcite were detected by XRD while calcium aluminosilicate and calcium silicate were not detected. Thus, these silicates are amorphous. The SEM image and mapping analysis of the hydrothermal coal at a Ca/S of 1.4 is not presented here, but the results were almost the same as above, except that the size of silicates were smaller than that shown in Figure 2. Hydration. As mentioned above, Ca-containing species formed by the hydrothermal treatment effectively captured SO2. To check whether the hydrothermal treatment is essential or not, we examined the SO2 capture ability of calcium loaded on coal by a simpler method, that is the addition of hydrated lime. Figure 4 shows cumulative amounts of sulfur emission from the coals with hydrated lime upon heating in O2 up to 800 °C. The amount of sulfur captured increases with increasing Ca/S ratio but it is almost the same as above Ca/S of 4.7. Hydrated calcium is also a good desulfurizing agent. XRD and SEM/EDX analysis before oxidation were made on the five samples listed in Figure 4. Quartz, kaolinite, pyrite, calcite, and portlandite were
Suppression of SO2 Emission during Coal Oxidation
Figure 4. Cumulative amount of sulfur emitted from raw coal and the coals with hydrated lime during oxidation. Heating rate: 10 °C/min; gas: 10% O2/N2.
Figure 5. SEM image and mapping analysis of the coal with hydrated lime at a Ca/S ratio of 5.1.
observed in XRD patterns for all the samples and the ratio of these species were independent of Ca/S ratio. Figure 5 shows SEM image and mapping analysis for the coal with hydrated lime (Ca/S ) 5.1). Many particles are seen on the coal surface. XRD pattern suggests that these are quartz, kaolinite, calcite, and portlandite. Since these samples were prepared at ambient temperature, no calcium aluminosilicate and calcium silicate were observed. Discussion SO2 Emission Behavior. In Figure 1a, four SO2 emission peaks were observed. LaCount et al.18,19 carried out temperature-programmed oxidation to determine structural types of sulfur in coal. They also observed four peaks for Illinois No. 6 coal and identified each peak by comparison with the results for model compounds or demineralized coal. By referring to their (18) LaCount, R. B.; Anderson, R. R.; Friedman, S.; Blaustein, B. D. Fuel 1987, 66, 909. (19) LaCount, R. B.; Kern, D. G.; King, W. P.; LaCount, R. B., Jr.; Miltz, D. J., Jr.; Stewart, A. L.; Trulli, T. K.; Walker, D. K.; Wicker, R. K. Fuel 1993, 72, 1203.
Energy & Fuels, Vol. 15, No. 3, 2001 651
Figure 6. Relationship between sulfur removal and Ca/S ratio.
results, it can be deduced that the four peaks in Figure 1a were due to nonaromatic sulfur, aromatic sulfur, pyrite, and sulfate in the order of increasing temperature, respectively. As shown in Figure 1, hydrothermal reaction products effectively captured SO2 emitted from all of these sulfur species. Effective Species. Figures 2 and 3 show that the Ca-containing species in the hydrothermal coal are portlandite, calcite, calcium aluminosilicate and calcium silicate. According to the equilibrium data for the decomposition of calcite,20 the decomposition temperature of calcite into lime and CO2 is about 650 °C at a CO2 partial pressure of 1 kPa. Therefore, calcite would not decompose to lime in the SO2 emission temperature range. The calcium compounds in the sample with a Ca/S ratio of 1.4 before oxidation were calcite, calcium aluminosilicate and calcium silicate. In spite that the formation of lime from calcite is implausible, desulfurization took place to a considerable extent, i.e., 61%. It is thus considered that amorphous calcium aluminosilicate and/or calcium silicate act as desulfurizing agents. In the case of Ca/S ratio of 5.2, portlandite in addition to calcium aluminosilicate and calcium silicate could be active species. If calcium aluminosilicate and calcium silicate can capture SO2, sulfated calcium aluminosilicate and calcium silicate would be formed. However, no obvious XRD peaks of such sulfated calcium compounds were seen in Figure 2. Yang and Shen21 examined SO2 sorption capacities of calcium silicate (CaSiO3 and β-Ca2SiO4) at 900 °C and characterized the sulfated calcium silicate by XRD and IR. They reported that CaSiO3 was converted into calcium silicate sulfate, while β-Ca2SiO4 was converted to SiO2 and CaSO4 upon sulfation. The reason why no sulfated calcium aluminosilicate and calcium silicate was seen in Figure 2 is considered as follows: (1) calcium aluminosilicate and calcium silicate were converted to CaSO4, and (2) sulfated calcium aluminosilicate and calcium silicate kept being amorphous even after the sulfation because of rather low temperature employed in the present study. In the case of the coal with hydrated lime, only calcite and portlandite were present as calcium compounds. (20) Baker, E. H. J. Chem. Soc. 1962, 464. (21) Yang, R.; Shen, M.-S. AIChE J. 1979, 25, 811.
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Since calcite cannot be a desulfurizing agent under the present condition, it is considered that portlandite is responsible for desulfurization. Calcium Utilization. Sulfur removal ratios for the hydrothermal coals, coals with hydrated lime, and physical mixture of coal with lime are summarized in Figure 6. The plots for sulfur removal ratios for the hydrothermal coal and the coal with hydrated lime are on the same curve. As discussed before, effective species in the hydrothermal coal at a Ca/S ratio of 1.4 may be calcium aluminosilicate and calcium silicate, and that of the coal with hydrated lime was only portlandite. Therefore, the capacity of amorphous calcium compounds seems to be almost the same as that of portlandite. It can be said that the hydrated lime is very easy to impregnate on coal and it acts as a good desulfurizing agent, but still Ca utilization is insufficient. Further investigation is needed to disperse calcium more effectively on coal to increase the Ca utilization ratio.
Matsuoka et al.
Conclusions SO2 emission upon the oxidation was determined with Ca-loaded coals prepared by the hydrothermal or hydration treatment. Quite high level of the suppression of SO2 emission was attained in the case of the hydrothermally treated coal. The effective species were either portlandite, amorphous calcium silicate and calcium aluminosilicate. The desulfurization ability of the hydrated lime was almost the same as that of the hydrothermal coals, where only portlandite played a role as desulfurizing agent. SO2 emission can be almost completely suppressed when the sufficient amount of Ca was loaded by either of these methods. Acknowledgment. This work was partially supported by a Grant-in-Aid for the Scientific Research on Priority Areas from the Ministry of Education, Science, Sports and Culture, Japan. EF0001991
