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MATERIALS AND INTERFACES Ca-Based Sorbents Modified with Humic Acid for Flue Gas Desulfurization Rongfang Zhao,*,†,‡ Haidi Liu,‡ Shufeng Ye,‡ Yusheng Xie,‡ and Yunfa Chen‡ Jiangxi Key Laboratory of Surface Engineering, Jiangxi Science & Technology Normal UniVersity, Nanchang 330013, P. R. China, and Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100080, P. R. China
With humic acid (HA) as a special additive, modified Ca(OH)2 sorbents referred to as M-Ca(OH)2-iHA, where i is the weight percentage of HA in the dry product and is in the range between 0 to 10, were prepared by hydrating CaO with water in which the expected amount of HA had been dissolved. The physicochemical properties of the sorbents were characterized using SEM, XRD, BET, etc., and the desulfurization activity of the sorbents, expressed as “Ca utilization”, was investigated in a differential fixed-bed quartz reactor under isothermal conditions at 65 °C and 70% relative humidity. The experimental results indicate that both hydration treatment and HA modification play important roles in improving the microstructure and desulfurization activity of M-Ca(OH)2-iHA. In particular, hydration treatment is the key contribution to M-Ca(OH)2-0HA, whereas both factors are significant for M-Ca(OH)2-iHA (i ) 4, 6, 8, 10). It was also found that the microstructure and desulfurization activity of M-Ca(OH)2-iHA improves considerably as i increases from 0 to 8 and then levels out when i increases further to 10. That is, M-Ca(OH)2-8HA has almost the optimal microstructure among the sorbents prepared in this study, i.e., volume mean diameter of 0.5 µm, BET surface area of 19.29 m2/g, and mesopore volume of 0.084 cm3/g, as well as the highest desulfurization activity, Ca utilization of 0.52 mol of SO2/mol of Ca. In addition, the mechanism of HA modification might be due to the acceleration effect of HA on the dissolution of lime, as well as the disturbance effect of Ca2+-HA chelates on the nucleation and growth processes of Ca(OH)2 crystals. 1. Introduction Flue gas desulfurization (FGD) has become a set of technologies used worldwide to abate SO2 from the combustion of fossil fuels, and Ca-based sorbents are widely studied for this purpose because of their economic advantages. For a Ca-SO2 system, the SO2 removal process involves two steps: SO2 diffusing into the sorbent particle and then reacting with the sorbent to form CaSO3/CaSO4. As the sulfation reaction proceeds, however, the sorbent particles gradually expand and the pores, especially micropores, in the sorbent are quickly blocked and plugged because the molar volume of CaSO3/CaSO4 is higher than that of CaO or Ca(OH)2. Then, a rate-limiting step for sulfur uptake is thought to be SO2 diffusing into the pores through the product layer on the particle surface. Therefore, modifying the microstructure of Ca-based sorbents, including particle size, specific surface area, porosity, nd so on, is generally considered to be an effective method for improving the sorbent reactivity toward SO2, to further enhance the SO2 removal capacity and the utilization efficiency of the sorbent.1 Commercial Ca(OH)2 is usually manufactured from the controlled slaking of quicklime. Slaking is a highly exothermic process releasing about 65 kJ/mol of hydration heat at 25 °C.2 In addition, the molar volume of the hydration product Ca(OH)2, 33 cm3/mol, is larger than that of CaO (16.9 cm3/mol), so the CaO particles immediately fragment to form smaller Ca(OH)2 particles after the addition of water as a consequence of the * To whom correspondence should be addressed. Tel.: +86-791380-1423. Fax: +86-791-380-1423. E-mail:
[email protected]. † Jiangxi Science & Technology Normal University. ‡ Chinese Academy of Sciences.
combination of thermal stress and swelling stress.3,4 Moreover, hydrating CaO produces powderlike Ca(OH)2 with some predetermined medium diameter at a cost much lower than crushing and grinding CaCO3 to a similar size.5 Therefore, hydrating lime with a modifying agent is one of the most important and viable ways to modify Ca-based sorbents. Previous literature reports have shown that “hydrationmodification” of Ca-based sorbents can be briefly summarized as follows: (1) Hydrated lime can be activated by making a slurry of it with siliceous materials, such as fly ash, diatomaceous earth, and silica fume.6-8 (2) Addition of certain inert compounds, e.g., NaCl, KCl, etc.,9,10 to Ca-based sorbents as alkali promoters can improve the sulfur retention process. (3) Hydrating lime with ethanol-water solution can produce sorbents with relatively optimized microstructures.10 (4) A strong increase in sulfur retention of Ca-based sorbents can be obtained by modification with surfactants, such as sodium and ammonium salts of a polycarboxylic acid, calcium lignosulfonate, lignosite, calgon, etc.,10-13 mainly because of the increasing density of pores with diameters of 5-20 nm, which provides a higher effective surface area for the sulfation reaction and is less susceptible to pore filling or pore mouth plugging.14-15 Humic acid (HA), extracted from any material containing well-decomposed organic matter, e.g., soil, coal, composts, etc., defies precise description in terms of molecular structure and formula except that it is a complex aromatic macromolecular polyelectrolyte with carboxylic, amino, peptide, phenolic hydroxyl, and aliphatic functional groups in linkages among the aromatic groups, which endows HA with surfactant activity. Moreover, a substantial mass fraction of HA contains carboxylic functional groups, which endow it with the ability to chelate
10.1021/ie051253+ CCC: $33.50 © 2006 American Chemical Society Published on Web 09/08/2006
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Figure 1. Schematic illustration of the experimental apparatus. Table 1. Characteristics of the Refined HA component Fe total acidity (daf,b mequiv of H+/mg) COOH phen-OH ash moisture content a
contentsa 0.1% 427 214 12.2% 8.5%
All percentages are by weight. b daf ) dry ash free basis.
positively charged multivalent ions.16 Just as mentioned above, lignosulfonate, another complex macromolecular polyelectrolyte that also exhibits surfactant activity, can be used to improve the sulfur retention of Ca-based sorbents.10 Therefore, it is interesting to determine whether it is feasible to use HA as a special additive to improve the reactivity of Ca-based sorbents toward SO2. In this article, a modified Ca(OH)2 sorbents were prepared by hydrating CaO with hydration water in which HA had been dissolved as a special additive. The sorbents were characterized using SEM, XRD, BET, etc., and the sulfur retention of the sorbents was tested in a differential fixed-bed reactor to gain some perspective on the effects of HA modification on the microstructure of the sorbents and their reactivity toward SO2. 2. Experimental Section 2.1. Materials. Powdered humic acid (HA) from brown coal was provided by Zhengfang Oil & Fat Chemical Co. Ltd, Jiangxi Province, China. Before being subjected to hydration, the powdered HA was refined as follows:17 The powdered HA was extracted by stirring in a 0.5 M NaOH solution under a N2 atmosphere. The alkaline supernatant was then separated from the residue by filtering. The filtrate was precipitated with 5 M HCl solution to pH 2.0, after which the precipitated HA was separated from the supernatant by centrifugation. The precipitate was flushed with deionized water to remove free acid and salt and finally dried under a N2 atmosphere. Characteristics of the refined HA are listed in Table 1. Commercial lumpy CaO was used directly for hydration, and commercial Ca(OH)2 with a volume mean diameter of 9.0 µm was used for reference and comparison and is denoted as C-Ca(OH)2. 2.2. Preparation of the Sorbents. It is well-known that CaO reacts readily with H2O to form Ca(OH)2, so hydrated lime is
generally produced by adding water to quicklime, and the process is known as “hydration”. In addition, the highly exothermic process releases about 65 kJ/mol of heat at 25°C and generates some steam. Thus, in practice, about 50-65% water is added to counteract losses from steam, even though the theoretical amount of water needed is 24.3%. Additionally, empirical data indicate that the particle size and specific surface area of the hydrate particles are affected by many factors, especially slaking temperature. In detail, from a theoretical perspective, the closer the slaking temperature to the boiling point of water (100°C), the finer the particle size of hydrate particle and the greater the specific surface area. However, in practice, it is very difficult to slake successfully at such a high temperature without safety problems or adverse affects, such as agglomeration resulted from “hot-spot” formation. Therefore, slaking temperatures between 70 and 85 °C are more practical, and varying the water-to-lime (H2O/CaO) molar ratio and the slaking water temperature is the optimal way to control the slaking temperature.3 In this study, in an air-free hydrator equipped with a mechanically propelled stirrer, modified Ca(OH)2 sorbents, expressed as M-Ca(OH)2-iHA, where i is the weight percentage of HA in the dry product and is in the range between 0 to 10, were prepared by hydrating CaO with hydration water in which a predetermined amount of refined HA powder had been dissolved, and the hydration water temperature and H2O/CaO molar ratio were controlled at 70 °C and 2.6,13 respectively. The details of the preparation procedure based on 10 g of dry CaO were as follows: (1) The required amounts of refined HA powder and deionized water were calculated corresponding to the exact value of i and a H2O/CaO molar ratio of 2.6, respectively. (2) Aqueous HA solution was prepared by dissolving the calculated amount of refined HA powder in the calculated amount of deionized water. (3) Ten grams of dry CaO was loaded into the hydrator, the preprepared aqueous HA solution heated to 70 °C was added under stirring into the hydrator, and the stirring was continued until the hydration heat evaporated the excess water. (4) After hydration, the slightly moist sorbent was dried in a vacuum oven at 80 °C overnight, and then the dried powder sorbent was sealed in a bottle before being subjected to characterization and desulfurization activity
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Figure 2. Volume mean diameters of C-Ca(OH)2 and M-Ca(OH)2-iHA.
tests. The final concentration of HA in the sorbents ranged from 0 for M-Ca(OH)2-0HA to 100 mg/g for M-Ca(OH)2-10HA. 2.3. Desulfurization Activity of the Sorbents. In a differential tubular fixed-bed quartz reactor (10-mm i.d.), shown schematically in Figure 1, the desulfurization activity of the sorbents was studied at 65 °C and 70% relative humidity (RH). For each run, about 30 mg of sorbent was dispersed manually and supported on a porous quartz plate fitted into the reactor. The reactive gas flowing downward through the sample bed consisted of 2000 ppmv of SO2, 8 vol % of O2, some H2O vapor, and the balance N2. The SO2, O2, and N2 gases were supplied from cylinders, and the H2O vapor was provided by a humidification system wherein N2 contacts deionized water in flasks immersed in an isothermal water bath. The temperature of the water bath was adjusted to guarantee the expected RH at which the experiments were performed, and the RH was verified with a thermohydrometer without SO2 being admitted. The total flow rate of the reactive gas was controlled at 450 mL/min using mass flow controller. Furthermore, prior to each run, the sample bed was humidified for about 30 min using humid N2 with the expected RH to ensure water vapor equilibrium between the sorbent bed and the sweep gas. After humidification, the reactive gas was switched to conduct the 1-h desulfurization activity test. The desulfurization activities of the sorbents, expressed in terms of Ca utilization, i.e., moles of SO2 captured per mole of Ca(OH)2 (mol of SO2/mol of Ca), were determined from the change in the S/Ca molar ratio. A Dionex DX-120 ion chromatograph (IC) was used to analyze the concentrations of sulfite (SO32-), sulfate (SO42-), and calcium (Ca2+) ions of the sorbents after a nitric acid-hydrogen peroxide microwave digestion. 2.4. Characterization Methods. The particle size distribution and mean diameter of the sorbents were measured using a Coulter LS230 particle size analyzer. The morphology of the sorbents was observed using a JEOL JSM-6700F field-emission scanning electron microscope (SEM). X-ray diffraction patterns of the sorbents were analyzed using a PANalytical X′pert X-ray diffractometer (XRD). The specific surface area and pore size distribution of the sorbents were characterized by N2 sorption at 77 K using an Autosorb1 QuantaChrome apparatus. Then, the specific surface area was determined following the BET standard method, and the pore size distribution was calculated by applying the BJH method using the desorption branch. 3. Results and Discussion 3.1. Particle Size Distribution. Particle size distribution analyses of C-Ca(OH)2 and M-Ca(OH)2-iHA (see Figure 2) indicate that the volume mean diameter of C-Ca(OH)2 is about
Figure 3. SEM images of C-Ca(OH)2 and M-Ca(OH)2-iHA: (a) C-Ca(OH)2, (b) M-Ca(OH)2-0HA, (c) M-Ca(OH)2-8HA, (d) M-Ca(OH)2-8HA.
9.0 µm, whereas that of M-Ca(OH)2-iHA decreases distinctly with increasing i from about 2.5 µm for M-Ca(OH)2-0HA to 1.0 µm for M-Ca(OH)2-4HA and even to 0.5 µm for M-Ca(OH)2-10HA. Thus, it can be deduced that both the hydration treatment and the HA modification contribute to the decrease in particle size of M-Ca(OH)2-iHA. Moreover, it must be pointed out that the particle size measured might be that of agglomerates of very fine primary particles. Additionally, commercial Ca(OH)2 is usually manufactured from the controlled slaking of quicklime, and the highly exothermic process is affected by many factors such as stirring intensity, slaking temperature, water-to-lime ratio, and so on. Therefore, the difference in volume mean diameter between C-Ca(OH)2 and M-Ca(OH)2-0HA can be ascribed to the
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Figure 4. XRD patterns of M-Ca(OH)2-iHA.
difference between the bulk commodity stored for a long time and the fresh product manufactured under special conditions. 3.2. SEM and XRD Analyses. Figure 3 shows typical SEM micrographs of C-Ca(OH)2 and M-Ca(OH)2-iHA. It can be seen that C-Ca(OH)2 particles agglomerate too extensively to easily discern the primary particles, although the scattered particles seem to be smooth-surfaced and spherical. For M-Ca(OH)2-0HA, there are some large cubic particles with many smaller granular particles attached to their surfaces, whereas for M-Ca(OH)2-8HA, almost all particles are primary spherical grains with a mean diameter of about 100 nm. Therefore, it can be further deduced that the rough surface and smaller particle size of M-Ca(OH)2-iHA depend on the hydration effect and HA modification. XRD patterns of M-Ca(OH)2-iHA are shown in Figure 4. According to the Joint Committee on Powder Diffraction Standards (JCPDS) card, only the characteristic peaks of Ca(OH)2 are present at 2θ values of about 34.10°, 18.01°, 47.12°, 50.81°, and 28.67° for all M-Ca(OH)2-iHA samples with different weight fractions of HA. Furthermore, no significant peak intensity difference can be observed, verifying that HA modification does not lead to any difference in the crystalline form of the sorbent, but exhibits mainly dispersant properties. 3.3. BET Surface Area and Pore Size Distribution. The N2 sorption experimental data better fit a type II sorptiondesorption isotherm that is common for absorbents with pore diameters larger than 20 nm.18 The pore size distributions of the sorbents according to the BJH method indicate that the pore volume is mainly attributable to macropores or, more specifically, that the attribution rate of macropores to the total pore volume is higher than 70%. Figure 5 and Table 2 show the multipoint BET surface area and pore volume results for M-Ca(OH)2-iHA samples based on the N2 adsorption isotherms. The BET surface area of M-Ca(OH)2-iHA increases from 14.18 to 19.29 m2/g as i increases from 0 to 8 and then decreases to 17.12 m2/g as i increases further to 10, suggesting that the optimum HA weight fraction might be about 8%. In addition, the influences of the hydration treatment and the HA modification on the specific surface area of the sorbent can be explained by referring to the corresponding SEM images and pore size distribution analyses. The pore volume distributions of M-Ca(OH)2-iHA in Table 1 indicate that the total pore volume appears to decrease, whereas the density of mesopores, especially those with diameters of 5-20 nm increases, with increasing i, which is conducive to the avoidance of rapid pore blockage during the desulfurization process and, further, to the enhancement of the reactivity of the sorbent.
Figure 5. BET surface areas of M-Ca(OH)2-iHA. The dotted line is the BET surface area of C-Ca(OH)2.
Figure 6. Ca utilization of C-Ca(OH)2 and M-Ca(OH)2-iHA after a 1-h desulfurization activity test. Table 2. Pore Volumes (cm3/g) of M-Ca(OH)2-iHA Samples i
total (
