Dry Regenerable Metal Oxide Sorbents for SO2 Removal from Flue

Ind. Eng. Chem. Res. 2006, 45, 5859-5869

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Dry Regenerable Metal Oxide Sorbents for SO2 Removal from Flue Gases. 1. Development and Evaluation of Copper Oxide Sorbents Vasudeo S. Gavaskar and Javad Abbasian* Illinois Institute of Technology, 10 West 33rd Street, Chicago, Illinois 60616

The reactivities of a number of copper-based sorbent prepared by the sol-gel method toward SO2 were determined in a thermogravimetric analyzer as well as a fluid-bed reactor. The copper content of these sorbents ranged from 11.2 to 26.5%. The highest sorbent capacity was achieved with sorbent with copper content of 14.1%. All these sorbents have significantly higher mechanical strength than commercially available copperbased sorbents and commercial fluid catalytic cracking catalysts (i.e., 5-8 times higher), making them suitable for fluidized-bed applications. The reactivity of the sorbent increases with increasing temperature up to 450 °C, and starts to decrease significantly beyond 500 °C, possibly due to sintering of the sorbent. Although these sorbents can be readily and completely regenerated with natural gas, the overall sulfur capacity of the sorbents gradually decreases in the cyclic operation and appears to stabilize after about 20 cycles, making them suitable for regenerative flue gas desulfurization applications. Introduction Recent legislation concerning air pollution has forced many industries to review the quality of their stack gas emissions. Major sources of sulfur dioxide emissions are coal-based power plants and sulfuric acid plants. The petroleum industry also has its own sources of SO2 pollution, of which the most prevalent are Claus sulfur units and fluid cracking regenerators. A variety of methods have been proposed and developed for the removal of sulfur compounds from coal, either before combustion or from the flue gas after combustion. Processes for treating the flue gas include wet scrubbing, gas-phase reaction to make a removable solid or liquid product, sorption by a calcium-based once-through sorbent, and sorption by a regenerable sorbent. Of the above, the method receiving much attention in recent years has been the sorption or reaction of SO2 with dry regenerable solid sorbents, notably metal oxides.1 The dry regenerative processes for flue gas desulfurization (FGD) offer a number of advantages over the established once-through processes:2 (1) no solid/liquid waste is generated, (2) salable sulfur byproduct is produced, (3) reheating of the flue gas after SO2 removal process is not required, and (4) water requirements are minimal. The overall economics of dry regenerative FGD processes depends heavily on the sorbent properties. The desirable sorbent characteristics include high sulfation and regeneration reaction rates, high sulfur capacity, high physical strength (i.e., low attrition rate), and low sorbent deterioration rate (i.e., low fresh sorbent makeup rate). A considerable amount of research has been conducted on the suitability of various metal oxides for SO2 removal. A comprehensive theoretical examination of the thermodynamics of sulfite-sulfate formation and their thermal decomposition3 concluded that Al, Bi, Ce, Co, Cr, Cu, Fe, Ni, Sn, Ti, V, U, Zn, and Zr are potentially suitable as dry regenerable sorbents for flue gas desulfurization processes. The initial experimental studies performed on a wide range of metal oxides4 suggested that the bulk oxides of Mn, Co, and Cu were the most promising candidates as sorbents for SO2 capture. DeBerry and Sladek5 investigated the bulk metal oxides short-listed by Lowell et al.3 for the rate of SO2 sorption in a thermogravimetric analyzer (TGA) in the temperature range of 200-500 °C, concluding * To whom correspondence should be addressed. Tel.: (312) 5673047. Fax: (312) 567-8874. E-mail: [email protected].

that the oxides of Ce, Co, Cr, Cu, Fe, and Ni have measurable reaction rates with SO2. To improve the physical strength of the sorbents, research efforts also focused on the development of supported sorbents. Bienstock et al.6 experimentally demonstrated the suitability of an alkalized alumina sorbent (sodium oxide on alumina support) for SO2 removal in the temperature range of 300-350 °C and thermal regeneration of the sulfate in the temperature range of 650-700 °C. Vogel et al.7 evaluated various impregnated metal oxides supported on alumina for removal of SO2 at 343 °C in a fixed-bed reactor and concluded that the supported oxides of Na, K, Sr, Ca, Cr, and Cu reacted with sufficiently high reaction rates. Similar studies were performed by Koballa and Dudukovic,8 concluding that oxides of Ni, Mn, Fe, Co, and Zn were the most promising metal oxide sorbents for SO2 removal. Faltsi-Saravelou and Vasalos9 developed a model to simulate a dry fluidized-bed process and used the model to match the published data of the alkalized alumina process6 and the fluidized-bed copper oxide process.10 From the simulation they concluded that the oxides of Cu, Cr, Fe, Ni, Co, and Ce can be considered suitable candidates for industrial applications. Among these favorable metal oxides for flue gas desulfurization, sodium and copper oxides have been the most extensively studied. Since the introduction of the Shell flue gas desulfurization process (copper oxide process) in the late 1960s,11 a significant amount of work has been carried out to develop improved regenerative sorbents for flue gas desulfurization processes.12-18 Different porous materials including γ-alumina, R-alumina, silica, and titania were used as the sorbent supports. However, one of the persisting problems associated with the dry regenerable sorbent processes developed for FGD applications has been the high sorbent attrition rates.1 For instance, although over 90% SO2 removal efficiencies were reported in the fluidized-bed copper oxide process, more than 0.5% of the sorbent inventory was lost per cycle due to attrition, underlining the need for physically stronger/durable sorbents.10 Conventional sorbent preparation techniques typically consist of impregnating a porous support with a solution of the desired metal salt, which is then converted to the metal oxide upon heat treatment. However, to impart the necessary physical strength, it was necessary to subject the sorbents to high thermal treatment temperatures. This in turn leads to deterioration of one or more of the other desired sorbent characteristics such as high surface

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Ind. Eng. Chem. Res., Vol. 45, No. 17, 2006

Table 1. Typical Flue Gas Composition from a Coal-Fired Power Plant constituent

concentration range

thermodynamic calculation

carbon dioxide (CO2), % water vapor (H2O), % oxygen (O2), % sulfur dioxide (SO2), ppmv nitrogen oxides (NOx), ppmv HCl, ppmv nitrogen

12-14 2-10 0.5-5.0 1200-5000 200-500 10-50 balance

14 7 0.5 and 4.0 2500 500 50 balance

area and porosity, which adversely affects the reactivity of these sorbents.15,19,20 It has been reported that ceramic sorbents synthesized by the sol-gel method generally possess a rather high surface area and mechanical strength.17,21,22 Additionally, other physical characteristics such as pore structure of the solgel derived sorbents can be tailored by proper heat treatment to achieve higher reactivities and high sulfur capacities.23 However, a relatively complete set of experimental data of the physical and chemical characteristics of such sorbents (i.e., reactivity, physical strength, long-term durability, etc.) is needed to properly assess the economical viability of the regenerative FGD processes. This paper addresses the development and the evaluation of reactive and durable sol-gel derived copper oxide based sorbents for SO2 removal from flue gases, and provides the experimental results of their sulfation and regeneration characteristics, physical strength, long-term durability in the cyclic operation, and their suitability for FGD application. Process Development and Operating Conditions Flue gas desulfurization processes based on dry copper oxide based sorbents proceed via the following sulfation reaction at the sulfation temperature TS:

CuO + SO2 + (1/2)O2 f CuSO4

(A)

The sulfated sorbent can be regenerated, either thermally or through reaction with a reducing gas, such as methane at the regeneration temperature TR:

CuSO4 + CH4 f Cu + SO2 + (1/2)CO2 + H2O

(B)

This is followed by the immediate oxidation of Cu to CuO in the presence of oxygen in the flue gas to complete the sulfation and regeneration cycle:

Cu + (1/2)O2 f CuO

(C)

Reactive regeneration is generally preferred to thermal regeneration because the temperature range of operation is expected to be much lower, avoiding sorbent exposure to high temperature, which can adversely affect the sorbent performance in subsequent cycles. Furthermore, the rate of regeneration is likely to be higher than that in the case of thermal regeneration. Methane was selected as the regenerating gas because of the availability of natural gas in most industrial plants. For a coalburning power plant, the ranges of the flue gas compositions encountered are given in Table 1, signifying that the O2 content in the flue gas ranges from 0.5% to 5.0%, which is directly related to the amount of excess oxygen used to ensure complete combustion of the coal, which, in turn, varies widely with boiler type, type of coal, particle size of coal, and operator preference.24 To determine the optimum ranges of process conditions, thermodynamic calculations were performed to identify possible

stable solid-state phases of sulfur compounds (i.e., sulfate, sulfite, etc.) and the corresponding equilibrium SO2 concentrations during sulfation over wide ranges of temperatures and oxygen concentrations. The flue gas composition used in these calculations is given in Table 1. Thermodynamic analysis for the regeneration reaction was based on the reaction of the stable sulfur compound (sulfate/sulfite) with methane. A typical predominance diagram (Figure 1a) was constructed to show the stable solid phases for the copper oxide (CuO) system in a flue gas containing 2500 ppmv SO2 with 0.5% and 5.0% oxygen over a wide range of sulfation temperatures. As shown in the figure, the system exhibits two stable compounds, namely CuSO4 and CuO*CuSO4, over the entire range of temperatures considered (350-850 °C). The results also suggest that varying the oxygen concentration from 0.5% to 5% does not appear to have any significant effect on the stable solid phases or the equilibrium SO2 concentration, other than up to 30 °C shift in the corresponding temperature (see Figure 1b). It is evident from Figure 1a that, above 450 °C, the oxysulfate (CuO*CuSO4) and sulfate (CuSO4) species coexist, while below 450 °C the equilibrium composition of the oxysulfate appears to be negligible. Additionally, the equilibrium SO2 composition for the sulfation reaction appears to be very low at temperatures below 450 °C. Based on the thermodynamic analysis, the range of sulfation temperatures in the experiments performed in this study ranged from 350 to 450 °C. Thermodynamic analyses performed for the regeneration of copper sulfate with methane indicated that the sulfate can be completely reduced to metallic copper over the entire temperature range of 350-850 °C. These results are consistent with the experimental data in the literature,1,25 which report that the copper sulfate is reduced to metallic copper (via reaction B), which in turn is instantaneously oxidized to its original oxide state (via reaction C). Experimental Section Sorbent Formulations. Five copper-based sorbent formulations were prepared for this study using a modified sol-gel technique. The synthesis process for preparation of 1500 mL of a 1 M boehmite (stable alumina) sol started with slow addition of a total of 380 mL of aluminum tri-sec-butoxide (ALTSB) in 1500 mL of distilled water, which was well stirred by a magnetic stirrer and controlled at 80 °C. Large irregularly shaped clusters appeared when the ALTSB made contact with water. The mixture was stirred continuously throughout the entire duration of the preparation of the sol. Continuous addition of the entire 380 mL of the ALTSB in water took about 30-35 min. This was followed by addition of 105 mL of 1 M HNO3 solution (resulting in H+/Al3+ ratio of 0.07) as the gelation agent to peptize and stabilize the sol, resulting in the rapid dispersion of the Al(OH)3 clusters. The temperature of the sol was increased and maintained at 90 °C. Water was intermittently added to the mixture at definite time intervals to maintain the volume of the sol. After 3 h, when the sol (milky white and translucent) was expected to stabilize with the evaporation of most of the alcohol formed due to the hydrolysis reaction, copper nitrate (Cu(NO3)2‚3H2O) solution was added as the precursor of the active copper oxide (CuO). The amount of the copper nitrate solution was determined based on the desired copper content in the sorbent. For instance, the amount of 1 M copper nitrate solution added to the sol for a copper content of 15% was approximately 220 mL. The nitrate solution was added steadily with a flow rate of approximately 10 mL/s, so that the entire copper nitrate solution was added to the sol in about 20 s. The entire matrix upon addition of the nitrate solution turned

Ind. Eng. Chem. Res., Vol. 45, No. 17, 2006 5861

Figure 1. (a) Stable solid phases of copper at FGD process conditions. (b) Effect of oxygen concentration on the equilibrium SO2 concentration.

to a sky-blue color. The gel formed almost instantaneously and became so viscous that the stirring was no longer possible by a magnetic stirrer. Further stirring/mixing was accomplished using a motor-driven impeller at a speed of 10 rpm, while maintaining the temperature of the gelled sol at 100 °C for 30 min. At this point further aging of the gel matrix was performed by the addition of 105 mL of 1 M ammonium hydroxide (NH4OH) solution to the gel mixture with constant stirring. The NH4OH was added extremely slowly using a buret at a rate of approximately 10 mL/min. After addition of the ammonium hydroxide the highly viscous gel matrix had a dark green color, the uniformity of which was an indication of the proper dispersion of the ammonium hydroxide throughout the mixture. The dark green gel matrix was further allowed to cool for about 30 min (with continuous stirring), after which it was transferred to drying pans and dried in a drying oven at 120 °C for approximately 2 days. The dried sorbent was then transferred to ceramic pans and calcined at 500 °C and 6 h in a calcining

oven. The heating ramp rate of the calcining oven was adjusted to 1 °C/min. The dried and calcined sorbents were then crushed, sized, and analyzed for their physical and chemical properties using standard characterization techniques, which included chemical compositions, BET surface area and pore-size distributions, and mercury porosimetry. The mechanical strengths of these sorbent formulations were determined by ASTM D5757, which is a standard test method for the determination of the relative attrition characteristics of solids granules by air jets that can be related to particle size reduction (i.e., attrition) in a fluidized environment.27 The size range of the sorbent particles used for testing the SO2 removal capacity of the sorbent was selected as 180300 µm. Thermogravimetric Analyzer Setup. The experiments pertaining to the kinetic studies for the sulfation and regeneration reactions of the copper-based sorbents were conducted in a thermogravimetric analyzer (TGA) unit (Cahn Instruments;

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Figure 2. Schematic diagram of the fluidized-bed reactor setup. Table 2. Ranges of the Experimental Operating Conditions fluidized-bed reactor operating variable temperature, °C gas flow rate, cm3/min weight of sample, mg gas composition, vol % SO2 O2 N2 CH4

sulfation 350-550 850 12

regeneration

sulfation

regeneration

450 850 12

450 667 (STP)

450 667 (STP)

0.1250-0.50 3.7 balance

0.25 3.7 balance 100

100

Model TG 131) that has a sensitivity of detecting weight changes on the order of 1 µg. The unit essentially consists of a threepiece quartz reactor that is externally heated by a single zone electrical tubular furnace positioned with respect to the sample holder to accomplish gas preheating and careful control of the sample temperature. The reactant gas mixture supplied from pressurized gas cylinders is introduced in the reactor tube through precalibrated flow meters. Helium is used as the purge gas to prevent corrosive gases from entering the balance chamber, to protect the balance against corrosion. Initial experiments were performed to determine the optimum sample size and gas flow rate to eliminate the effect of external factors on the sulfation kinetics, and blank runs were conducted to confirm the inertness of the reactor assembly. The results of the series of duplicate experiments performed on the sorbent formulations to demonstrate the reproducibility of the experimental data indicated that the experimental results obtained with the TGA setup used in this study can be reproduced with (2% error. The operating conditions for the TGA experiments are given in Table 2. The sulfation runs in this study were carried out at 450 °C and an inlet SO2 concentration of 2500 ppmv unless specifically stated otherwise. In a typical run, about 12 mg of sorbent particles is placed in the sample holder and spread uniformly over a wire mesh basket positioned inside the reactor tube in the furnace, which is suspended with a wire connected to the microbalance. Helium and nitrogen gases are initially passed through the reactor during heating at a predetermined ramp rate until the required reaction temperature is achieved.

At this point, a simulated gas mixture containing the desired level of SO2 is passed through the sorbent, resulting in an increase in the weight of the sample, which is directly proportional to the extent of copper oxide conversion to copper sulfate. The experiment is continued until the slope of the sample weight curve decreases and approaches nearly zero. The fractional CuO conversion is calculated from the weight vs time data using the following formula:

% conversion )

MW ∆W ∆MW WreactantyCuO

(1)

where ∆W is the weight gain by the solid during the reaction, mg; Wreactant is the initial sample weight, mg; yCuO is the copper content in the sorbent, w/w; MW is the molecular weight of solid reactant, g/mol; and ∆MW is the difference in the molecular weights of solid product and reactant, g/mol. ∆MW ) MWCuSO4 - MWCuO ) 80 g/mol. Fluidized-Bed-Reactor Setup. The effectiveness of the sorbents for removal of SO2 and their respective effective sulfur capacities were also determined in a fluidized-bed-reactor system (2.18 cm i.d. and 10 cm3 bed). The schematic diagram of the fluidized-bed unit is presented in Figure 2. The equipment consists of three sections: reactor assembly with the heating equipment, gas feeding system, and a gas analyzer. The reactor, consisting of a quartz tube with a quartz frit serving as the gas distributor, is heated using a single zone heating furnace and controlled by a PID temperature controller to maintain the reactor at the desired temperature. The temperature inside the bed is measured using a K-type thermocouple and is recorded using data acquisition software. In a typical test a 10 g batch of the sorbent is loaded in the reactor, and the reactor is brought to the desired temperature under an N2 atmosphere. The experiments, unless specified otherwise, were performed under a gas space velocity of 4000 h-1,with an inlet SO2 concentration of 2500 ppmv and a temperature of 450 °C. The gas flow rates are controlled using mass flow controllers. After the sorbent bed reaches the desired temperature, the sorbent is exposed to


Figure 3. Comparison of attrition indexes of various sorbent formulations. Table 3. Physical and Chemical Properties of the Sorbents BET Cu Al surface true sorbent content, content, area, particle density, expansion designation % % m2/g porosity g/cm3 factor Z Cu-1 Cu-2 Cu-3 Cu-4 Cu-5

11.2 14.1 17.4 21.3 26.5

44.3 43.2 41.2 37.4 34.2

153 150 141 137 134

0.4824 0.4814 0.4689 0.4671 0.4662

2.2062 2.2184 2.3446 2.4019 2.4564

1.5 1.6 1.7 1.9 2.1

a simulated flue gas mixture containing the desired levels of SO2 during sulfation runs, and to a pure stream of methane during regeneration experiments. The reactor exit gas mixture is analyzed by a gas chromatograph equipped with a flame photometric detector (FPD) with a detectability of about 0.2 ppmv SO2. The column used in the analysis is a 1/8-in. Teflon tube packed with Chromosil 330 (Supelco). The experiment is continued until the exit SO2 concentration reaches about 100 ppmv (breakthrough). The prebreakthrough SO2 content of the reactor effluent determines the effectiveness of the sorbents for removal of these species, while the SO2 breakthrough time represents the effective capacity of the sorbents. During regeneration steps the experiment was terminated when SO2 could not be detected at the reactor outlet. Duplicate runs on the various sorbent formulations were performed a priori, indicating that the experimental results obtained with the fluidized-bed reactor used in this study can be reproduced with approximately (3% error. Experimental Results and Discussion The BET surface area, particle porosity, sorbent density, and chemical composition of the sorbents are presented in Table 3. The pore-size distributions, obtained from the BET desorption isotherm, indicated that all the sorbent formulations prepared in this study essentially have a narrow pore-size distribution with an average pore diameter of around 40 Å. The BET surface areas reported in the literature range from as low as 85 m2/g 28 to as high as 244 m2/g.23 The low surface areas reported by Uysal et al.28 and Best and Yates29 can be attributed to the limitations of the conventional wet-impregnation preparation technique and the low surface area of the porous supports

employed. The higher surface areas reported by Lin et al.20,23 for sol-gel derived sorbents are possibly due to the relatively lower calcination temperature and/or differences in the sorbent preparation techniques. Figure 3 shows the attrition indexes of the selected sorbent formulations determined by the ASTM D5757 method. To provide a basis for comparison, a commercially available alumina impregnated copper sorbent (∼7% copper prepared by wet impregnation of an alumina support) and a commercial fluid catalytic cracking (FCC) catalyst were also evaluated for their attrition indexes. The results indicate that the attrition indexes of the sol-gel derived sorbents formulated in this study are about 5-8 times lower than that of the commercially available alumina impregnated sorbent and approximately 3-4 times lower than that of the FCC catalyst, which is generally accepted as the baseline for fluid-bed application.30 These results are consistent with those reported by Wang and Lin.23 The higher mechanical strength of the sol-gel sorbent has been attributed to its close-knit structure that gives rise to a strong bonding between CuO and Al2O3.19,26 It should also be noted that the attrition indexes/resistances of the sol-gel derived sorbents were essentially independent of their copper content. The reactivities of the five sorbent formulations toward SO2 at 450 °C with a simulated flue gas mixture containing 2500 ppmv SO2 are shown in Figure 4, indicating that the overall reaction rate and the maximum CuO conversion decrease significantly with increasing copper content. Such incomplete conversions, also observed for the reaction of SO2 with limestone,31,32 have been related to the structural changes occurring within the solid particle due to the chemical reaction, mainly caused by the differences in the molar volumes of the reactant and the product, which results in the reduction of the porosity of the solid particle and leads to a drastic decrease in the effective diffusivity of the gaseous reactant through the porous solid matrix. A common parameter to check the extent of the swelling of the solid product is the expansion factor, Z, which is the ratio of the molar volume of the product to that of the reactant and is defined by

Z)

FreactantMWproduct FproductMWreactant(1 - s)

(2)

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Figure 4. Sulfation reactivities of different sorbent formulations.

Figure 5. Effectiveness of different sorbent formulations in fluid bed.

where Fj is the specific gravity of the species j, g/cm3; MWj is the molecular weight of species j, g/mol; and s is the porosity of the product layer owing to the cracks and fissures, dimensionless. The value of s has been estimated in the literature to be between 0.05 and 0.2.33 In this study, s was assumed to be 0.1. Values of Z < 1 indicate that the particles shrink during reaction, and with Z > 1, swelling occurs. The value of the expansion factor for sulfation of pure copper oxide is about 3.52. Since the solid sorbent particles consist of copper oxide on alumina support, the role of aluminum oxide (Al2O3) in the expansion was investigated by conducting experiments with pure sol-gel derived alumina. The results indicated a very low rate of reaction and negligible conversions of Al2O3 to Al2(SO4)3, compared to that observed during sulfation of alumina-supported copper oxide sorbents. Furthermore, the very small quantities of aluminum sulfate formed could not be regenerated when treated with methane under the regenerating conditions. There-

fore, it was concluded that alumina does not play any role in the subsequent sulfation reactions and can be regarded as an inert material in these sorbents. The expansion factors for the different sorbent formulations are given in Table 3, indicating that Z increases with increasing copper content and ranges from approximately 1.5 to 2.1. The results of an expanding grain model applied to describe and explain the reaction kinetics and the transport phenomena in the sorbent particle during the CuOSO2 reaction will be discussed in part 2 of this series of papers. The developed reaction rate model incorporates the physical and chemical properties of the sorbent and relates the behavior of the sorbent particle in the reacting system to the changes in the physical and chemical properties of the sorbents. The model takes into account the pore-closure phenomena, structural changes (i.e., changes in surface area and porosity) during the reaction, and parameters such as the changing effective diffusivity of the gaseous reactant through the sorbent particle and


Figure 6. Effect of copper content on maximum sulfur loading.

Figure 7. Effect of SO2 concentration on sulfation reactivity of the Cu-2 sorbent.

the diffusivity of SO2 through the product layer and accounts for the incomplete conversion of the solid reactant as seen in Figure 4. Although the initial reaction rate and the maximum CuO conversions show a decreasing trend with increasing copper content, the same is not true for the sulfur loading. Results of the experiments performed in the fluidized-bed reactor with selected sorbents are given in Figure 5, indicating that the optimum sorbent performance (highest sulfur loading) is achieved with Cu-2, which showed sulfur loading of about 2.4 g of S/100 g of sorbent, at approximately 96% SO2 removal (100 ppmv exit SO2 concentration). Figure 5 also indicates that all the copper-based sorbents prepared by the sol-gel technique were capable of reducing the SO2 concentration to negligible levels prior to breakthrough. These results are also consistent with those reported by Wang and Lin23 in a fixed-bed reactor using a similar sorbent with 9.0 wt % copper.

Figure 6 shows the sulfur loading (g of S/100 g of sorbent) as a function of the copper content obtained in the TGA and the fluid-bed experiments, confirming that the maximum sulfur loading was achieved with the sorbent Cu-2, which contained 14% copper. This suggests that the optimum combination of the physical and chemical characteristics may be achieved in the vicinity of about 14% copper content. Deng and Lin20 reported higher sulfur loading by impregnating high surface area alumina produced by the sol-gel technique and achieved the highest sulfur loading with a sorbent containing 16% copper, which is consistent with their theoretical monolayer calculations. It should be noted that CuO conversions greater than 100%, reported by Deng and Lin,20 were attributed to the possible simultaneous sulfation of the alumina. However, their data presented in Figure 6 are based on CuO conversion. In contrast, both McCrea et al.34 and Yeh et al.35 concluded that the optimum copper loading in their alumina-supported sorbents was ap-

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Figure 8. Determination of order of reaction with respect to SO2 concentration.

Figure 9. Effect of sulfation temperatures on sulfation reaction of the Cu-2 sorbent.

proximately 5-6 wt %. However, the sorbents used in those studies were synthesized by wet impregnation of low surface area supports causing pore-plugging problems at higher copper loadings. The effect of SO2 concentration on CuO conversion is shown in Figure 7, indicating that the initial rate of the reaction increases with increasing SO2 concentration, and as shown in Figure 8, a plot of ln[dXA/dt] vs ln[CSO2] indicates that the order of the overall reaction with respect to SO2 concentration is approximately 1. This compares favorably with the values reported in the literature7,29,35 for ambient pressure sulfation of CuO/γ-Al2O3 sorbents. Figure 9 shows the effect of temperature on the reaction rate of the Cu-2 sorbent using a gas mixture containing 2500 ppmv SO2, indicating that the sulfation rate increases with increasing temperature up to 450 °C and decreases at 500 °C and above. This can be attributed to the sintering of the sorbent particle at temperatures around 500 °C, because all the sorbent formulations were calcined at 500 °C. Since the pore structures of the sol-gel derived sorbents are very sensitive to their thermal treatment history, care should

be taken in subjecting these sorbents to high temperatures, as this may have an adverse effect on their physical characteristics. It should, however, be noted here that the thermodynamic calculations predict the formation of the oxysulfate (CuO*CuSO4) in the vicinity of 550 °C. However, since this temperature was beyond the desired range of temperature, further analysis on the solid sample was not performed. The Arrhenius plot of the surface-dependent reaction rate constant (ks) for the sulfation reaction is shown in Figure 10. Because of the detrimental structural changes expected to occur beyond 450 °C, the apparent activation energy for the sulfation reaction was considered in the temperature range of 350-450 °C; it was determined to be approximately 8.2 kcal/mol. This value of the activation energy compares reasonably well with 11.33 kcal/mol reported by Best36 (temperature range 300-450 °C) and 7 kcal/mol reported by Uysal et al.28 (temperature range 111-256 °C). However, higher values of activation energies have also been reported by Deberry and Sladek5 (26.9 kcal/ mol, 325-482 °C, for pure CuO) and Wang and Lin23 (23 kcal/


Figure 10. Arrhenius plot of the sulfation reaction rate constant.

Figure 11. Effect of cycling on sulfur loadings of the Cu-2 sorbent. Table 4. Effect of Cycling on the Physical Properties of Cu-2 Sorbent oxidized form BET surface area, porosity

m2/g

fresh

1st cycle

25th cycle

150 0.4814

125 0.4640

111 0.4535

mol, 300-500 °C). Yeh et al.35 have also reported an activation energy as low as 4.8 kcal/mol (350-450 °C). The results of the 25 sulfation-regeneration experiments performed in the TGA and the fluid-bed reactor with the sorbent Cu-2 are shown in Figure 11, indicating that the sulfur loading initially increases during the first few cycles, followed by a gradual decrease during the next 10-15 cycles before reaching a constant value around the 20th cycle. The gradual decrease in the reactivity of the sorbent is indicative of physical deterioration of the sorbent in the cyclic process. Table 4 shows the effect of cycling on the BET surface area and the particle porosity of the sorbent, confirming that the surface area and the porosity decrease with the increasing cycles and appear to

be stabilizing after 20 cycles. The increase in the sulfur loading during the initial cycles is a common characteristic of regenerable sorbents, as reported by Abbasian and Slimane37 for a regenerable copper-based sorbent for H2S removal. They suggested that regenerable sorbents often experience a so-called “activation/conditioning” phase during which the sorbent performance improves during the initial cycles. Similar observations with regenerable zinc titanate sorbents for sulfur removal from hot coal gas have also been reported by Swisher et al.38 and Ryu et al.39 This “activation” phase may be related to the favorable changes in the pore structure, such as opening to initial “blind” pores and reconfiguration of pore-size distribution, occurring during the initial sulfation-regeneration cycles. The structural changes associated with these transformations appear to have a beneficial effect on the diffusion of the gaseous reactant through the sorbent particle, resulting in higher reaction rates and sulfur loadings in the first few cycles. As the sorbent is further “accustomed” to the sulfation-regeneration cycles, this conditioning effect is expected to diminish resulting in a

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decrease in the sorbent reactivity, which is indicative of the physical/chemical deterioration of the sorbent in the cyclic process. However, the effective sorbent capacity of the sorbent appears to stabilize after the 20th cycle, which indicates that this sorbent can be used in cyclic fluidized application over a large number of cycles, resulting in a very low fresh sorbent makeup rate. Conclusions On the basis of the results obtained in this study, it can be concluded that highly reactive, attrition-resistant and durable sorbents can be prepared using the sol-gel technique with copper contents up to 30%. These sol-gel derived sorbents are generally 5-8 times more attrition resistant than a typical commercially available sorbent prepared by conventional techniques, and about 3-4 times stronger than a commercial FCC catalyst, making them suitable for fluidized-bed applications. Although the overall sulfation reaction rate decreases with increasing copper content, the optimum copper content (with respect to sulfur capacity) appears to be in the vicinity of 14%. The reactivity of the sorbents gradually decreases with increasing cycles in the cyclic operation and appears to stabilize after about 20 cycles. The gradual decrease in the sulfur loading can be attributed to the changes in the physical structure of the sorbent in the cyclic operation. The reactivity of the sorbent increases with increasing temperature up to 450 °C, beyond which it decreases significantly, possibly due to sintering of the sorbent. In summary, the experimental results presented in this paper indicate that the sorbents prepared by the sol-gel technique are highly reactive, durable, and mechanically strong, making them suitable for regenerative FGD applications. However, a suitable gas-solid reaction model is needed to relate the sulfation behaviors of the sorbents to their physical and chemical characteristics, which will be presented in part 2 of this series of papers. Literature Cited (1) Hoffman, J. S.; Smith, D. N.; Pennline, H. W.; Yeh, J. T. Removal of Pollutants from Flue Gas Via Dry, Regenerable Sorbent Processes. Presented at the 1992 AIChE Spring National Meeting, March 29-April 2, 1992. (2) Livengood, C. D.; Markussen, J. M. FG Technologies for Combined Control of SO2 and NOx. Power Eng. 1994, Jan, 38. (3) Lowell, P. S.; Schwitzgebel, K.; Parsons, T. B.; Sladek, K. J. Selection of Metal Oxides for Removing SO2 from Flue Gas. Ind. Eng. Chem. Process Des. DeV. 1971, 10 (3), 384. (4) Bienstock, D.; Field, J.; Myers, J. Process Development in Removing Sulfur Dioxide from Hot Flue Gases 1. Bench-Scale Experimentation. Bur. Mines Rep. InVest. 1961, 5735. (5) DeBerry, D. W.; Sladek, K. J. Rates of Reaction of SO2 with Metal Oxide. Can. J. Chem. Eng. 1971, 49, 781. (6) Bienstock, D.; Field, J. H.; Myers, J. G. Process DeVelopment in RemoVing Sulfur Dioxide from Hot Flue Gases. Report Investigation 7021; U.S. Bureau of Mines: Washington, DC, 1964. (7) Vogel, R. F.; Mitchell, B. R.; Massoth, F. E. Reactivity of SO2 with Supported Metal-Oxide-Alumina Sorbents. EnViron. Sci. Technol. 1974, 8 (5), 432. (8) Koballa, T. E.; Dudukovic, M. P. Sulfur Dioxode Adsorption on Metal Oxides Supported on Alumina. AIChE Symp. Ser. 1978, 73, 199. (9) Faltsi-Saravelou, O.; Vasalos, I. A. Simulation of a Dry Fluidized Bed Process for SO2 Removal from Flue Gases. Ind. Eng. Chem. 1990, 29, 251. (10) Yeh, J. T.; Demski, R. J.; Strakey, J. P.; Joubert, J. I. Combined SO2/NOX Removal from Flue Gas. EnViron. Prog. 1985, 4, 223. (11) Dautzenburg, F. M.; Nader, J. E.; van Ginneken, A. J. I. Shell’s Flue Gas Desulfurization Process. Chem. Eng. Prog. 1971, 67, 86.

(12) Centi, G.; Riva, A.; Passarini, N.; Brambilla, G.; Hodnett, B. K.; Delmon, B.; Ruwet, G. Simultaneous Removal of SO2/NO2 from Flue Gases, Sorbent/Catalyst Design and Performance. Chem. Eng. Sci. 1990, 45, 2679. (13) Duisterwinkel, A. E.; Doesburg, E. B. M.; Hakvoort, G. Comparing Regenerative SO2 Sorbents Using TG: The SRO Test. Thermochim. Acta 1989, 141, 51. (14) Hakvoort, G. C. M.; van der Bleek, J. C.; Schouten, J. C.; Valkenburg, P. J. M. The Study of Sorbent Material for Desulfurization of Combustion Gases at High Temperature. Thermochim. Acta 1987, 114, 103. (15) Hedges, W. W.; Yeh, J. T. Kinetics of Sulfur Dioxide Uptakes on Supported Cerium Oxide Sorbents. EnViron. Prog. 1992, 11, 98. (16) Kiel, J. H. A.; Prins, W.; van Swaaij, W. P. M. Performance of Silica-Supported Copper-Oxide Sorbents for SOx/NOx-Removal from Flue Gas. I. Sulfur Dioxide Absorption and Regeneration Kinetics. Appl. Catal. B 1992, 1, 13. (17) Wolff, E. H. P.; Gerritsen, A. W.; Verheijen, P. J. T. Attrition of an Alumina-Based Sorbent for Regenerative Sulfur Capture from Flue Gas in a Fixed Bed. Powder Technol. 1993, 76, 47. (18) Yang, R. T.; Shen, M. S. Calcium Silicates: A New Class of Highly Regenerative Sorbents for Hot Gas Desulfurization. AIChE J. 1979, 25, 811. (19) Abbasian, J.; Ho, K. K. DeVelopment of Sorbents for a Fluid-Bed Process to Control SOX and NOX; Final Technical Report to the Illinois Clean Coal Institute, Nov 1, 2000, through Oct 31, 2001. (20) Deng, S. G.; Lin, Y. S. Synthesis, Stability, and Sulfation Properties of Sol-Gel-Derived Regenerative Sorbents for Flue Gas Desulfurization. Ind. Eng. Chem. Res. 1996, 35, 1429. (21) Deng, S. G.; Lin, Y. S. Sol-gel Preparation and Properties of Alumina Adsorbents for Gas Separation. AIChE J. 1995, 41, 559. (22) Wang, Z. M.; Lin, Y. S. Sol-Gel Synthesis of Pure and Copper Oxide Coated Mesoporous Alumina Granular Particles. J. Catal. 1998, 174, 43. (23) Wang, Z. M.; Lin, Y. S. Sol-Gel-Derived Alumina-Supported Copper Oxide Sorbent for Flue Gas Desulfurization Ind. Eng. Chem. Res. 1998, 37, 4675. (24) Slack, A. V.; Hollinden, G. A. Sulfur Dioxide RemoVal from Waste Gases, 2nd ed.; Noyes Data Corporation: Park Ridge, NJ, 1975. (25) Harriott, P.; Markussen, J. M. Kinetics of Sorbent Regeneration in the Copper Oxide Process for Flue Gas Cleanup. Ind. Eng. Chem. Res. 1992, 31, 373. (26) Abbasian, J.; Carty, R. H. A Fluid Bed Process to Control SOx and NOx; Final Technical Report to the Illinois Clean Coal Institute, Nov 1, 2001, through Oct 31, 2002. (27) Determination of Attrition and Abrasion of Powdered Catalysts by Air Jets. Petroleum Products and Lubricants (III); ASTM Annual Book of Standards; ASTM: West Conshohocken, PA, 1985; D-2981, Vol. 05.03. (28) Uysal, B. Z.; Aksahin, I.; Yucel, H. Sorption of SO2 on Metal Oxides in a Fluidized Bed. Ind. Eng. Chem. Res. 1998, 27, 434. (29) Best, R. J.; Yates, J. G. Removal of Sulfur Dioxide from a Gas Stream in a Fluidized Bed. Ind. Eng. Chem. Process Des. DeV. 1977, 16, 347. (30) Slimane, R. S.; Abbasian, J. Copper-Based Sorbents for Coal Gas Desulfurization at Moderate Temperatures. Ind. Eng. Chem. Res. 2000, 39, 1338. (31) Hartman, M.; Coughlin, R. W. Reaction of Sulfur dioxide with Limestone and the Influence of Pore Structure. Ind. Eng. Chem. Process Des. DeV. 1974, 13, 248. (32) Hartman, M.; Coughlin, R. W. Reaction of Sulfurdioxide with Limestone and the Grain Model. AIChE J. 1976, 22, 490. (33) Hartman, M.; Trnka, O. Influence of Temperature on the Reactivity of Limestone Particles with Sulfur Dioxide. Chem. Eng. Sci. 1980, 35, 1189. (34) McCrea, D. H.; Forney, A. J.; Myers, J. G. Recovery of Sulfur from Flue Gases Using a Copper Oxide Absorbent. J. Air Pollut. Control Assoc. 1970, 20 (12), 819. (35) Yeh, J. T.; Drummond, C. J.; Joubert, J. I. Process Simulation of the Fluidized-Bed Copper Oxide Process Sulfation Reaction. EnViron. Prog. 1987, 92, 29. (36) Best, R. J. A Study of a Non-Catalytic Gas-Solid Reaction in Fixed and Fluidized Bed Reactors. Ph.D. Dissertation, Department of Chemical Engineering, University College London, 1974.

Ind. Eng. Chem. Res., Vol. 45, No. 17, 2006 5869 (37) Abbasian, J.; Slimane, R. B. A Regenerable Copper-Based Sorbent for H2S Removal from Coal Gases. Ind. Eng. Chem. Res. 1998, 37, 2775. (38) Ryu, S. O.; Park, K. N.; Chang, C. H.; Kim, C. J.; Lee, J. T. Multicyclic Study on Improved Zn/Ti-Based Desulfurization Sorbents in Mid-Temperature Conditions. Ind. Eng. Chem. Res. 2004, 43, 1466.

(39) Swisher, J. H.; Yang, J.; Gupta, R. P. Attrition Resistant Zinc Titanate Sorbent for Sulfur. Ind. Eng. Chem. Res. 1995, 34, 4463.

ReceiVed for reView January 30, 2006 ReVised manuscript receiVed June 8, 2006 Accepted June 16, 2006 IE060123D

Dry Regenerable Metal Oxide Sorbents for SO2 Removal from Flue

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