Simultaneous Removal of SO2 and Trace SeO2

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Environ. Sci. Technol. 2006, 40, 4306-4311

Simultaneous Removal of SO2 and Trace SeO2 from Flue Gas: Effect of Product Layer on Mass Transfer YUZHONG LI,* HUILING TONG, YUQUN ZHUO, CHANGHE CHEN, AND XUCHANG XU Key Laboratory for Thermal Science and Power Engineering of Ministry of Education, Department of Thermal Engineering, Tsinghua University, Beijing 100084, China

Sulfur dioxide (SO2) and trace elements are all pollutants derived from coal combustion. This study relates to the simultaneous removal of sulfur and trace selenium dioxide (SeO2) by calcium oxide (CaO) adsorption in the medium temperature range, especially the mass transfer effect of sulfate product layer on trace elements. Through experiments on CaO adsorbing different concentrations of SO2 gases, conclusions can be drawn that although the product layer introduces extra mass transfer resistance into the sorbent-gas reaction process, the extent of CaO adsorption ability loss due to this factor decreases with decreasing SO2 concentration. When the gas concentration is at trace level, the loss of CaO adsorption ability can be neglected. Subsequent experiments on CaO adsorbing trace SeO2 gas suggest that the sulfate product layer, whether it is thick or thin, has no obvious effect on the CaO ability to adsorb trace SeO2 gas.

Introduction Following SO2 and NOx, trace elements such as mercury, selenium, arsenic, and lead emitted from coal combustion have become a major concern for coal-burning utilities. Technologies to control SO2 and NOx have been maturely developed and widely used. Some research on trace element control has been conducted in recent years, but few practical technologies are brought into application. Until now, most technologies to control coal combustion pollutants are performed individually, that is, one technology controls only one pollutant. While more and more pollutants need to be eliminated, the divide-and-conquer approach will face challenges. Simultaneous removal technology for multiple pollutants will be more and more appealing. Low-cost calcium-based sorbent presents an attractive option for the technology because it has the ability to capture both sulfur species and trace elements such as selenium, arsenic, and lead species. Many studies have been performed on Ca-based sorbents adsorbing SO2 from hot flue gases (1-5). The previous study of our research team (6, 7) has revealed that the best desulfurization temperature is 700-800 °C so as to avoid CO2 competition and sorbent pores sintering, and 95% SO2 can be removed in a dry flue gas desulfurization (FGD)circulating fluidized bed (CFB) pilot device at 750 °C with the Ca/S ratio of 1.5. The following reaction scheme has been * Corresponding author e-mail: [email protected]. edu.cn. 4306

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proposed for SO2 capture under this condition:

CaO + SO2 + 1/2 O2 ) CaSO4

(1)

The dry FGD-CFB system mentioned above is in pilot scale. It can be fitted into the down stream of coal-fired boilers in the medium-temperature range. Although the retrofit of a boiler system is the most obvious obstacle for its application, the dry FGD technology is still attractive for the following two reasons: (1) It is an efficient desulfurization technology with low initial investment, high sulfur removal efficiency, and high operation reliability due to high gas-solid reaction rate under dry conditions; and (2) Trace elements such as selenium (8, 9) and arsenic (10, 11) can be absorbed by CaO in the medium-temperature window, therefore, the simultaneous removal of SO2 and trace elements may be performed through the dry FGD-CFB system. Matsushima et al (5) also support that the dry FGD technology has a good future. Previous literature (12, 13) reports that selenium exists as SeO2 for its entire course in the combustion environment. Its high volatility makes selenium emission control difficult. Ghosh-Dastidar et al.’s investigation involved study of the effectiveness of hydrated lime (Ca(OH)2) sorbent for selenium removal (8). The mechanism of capture is proven to be a chemical reaction between CaO and SeO2. It was found that the SeO2 removal efficiency increases with the temperature rise in the range of 400-600 °C. With further temperature increase, the thermodynamic equilibrium speeds up the decomposition of reaction product, therefore the amount of selenium capture is reduced. The chemical reaction pattern proposed by the authors is as follows:

CaO + SeO2 ) CaSeO3

(2)

Since there is a common temperature window in which both SO2 and SeO2 can be captured by CaO, the simultaneous removal of sulfur and trace selenium by CaO is studied in the medium-temperature range of about 700 °C. Agnihotri et al. (9) reported that SO2 in the gas phase along with SeO2 can decrease the ability of CaO to capture SeO2. The possible explanation is that sorbent pore plugging/ blocking occurs due to the formation of a high molar volume CaSO4 product. However, Jadhav et al. (11) observed that there was no competition between SO2 and As2O3 when they were removed by CaO at 500 °C. Concerning more specialized studies on simultaneous removal, Wu et al. (14) developed a multi-function sorbent for the simultaneous removal of alkali vapor, lead vapor, and SO2. They claimed that the sorbent was effective in simultaneous removal of these contaminants, and synergistic removal even happened in some cases owing to a porous alkali aluminosilicate and a molten lead aluminosilicate product layer generated. Zhao et al. (15) also found the combined sorption of lead and sulfur and the enhancement of effectiveness as a result of a porous product layer. From the above studies, it can be concluded that the competitive effect between SO2 and trace elements is not determinate. The presence of CO2 may also bring competition via carbonate reaction. If the competition does exist, the trace element adsorption will be undesirably overwhelmed by SO2 or CO2 due to its very high concentration in flue gas. Although competition is one of the problems that we should focus on, the effect of CaSO4 product layer on trace elements removal is an even more basic problem. Above all, Agnihotri, Wu, and Zhao tried to explain the competitive mechanism through studies on product layer. It has been 10.1021/es052381s CCC: $33.50

 2006 American Chemical Society Published on Web 05/24/2006

TABLE 1. Different Sorbents for SeO2 Adsorption

FIGURE 1. Schematic of thermogravimetric analyzer (TGA) reactor system: (1) SeO2 vapor generator; (2) main TGA reactor; (3) scrubbers: (x) inlet of TGA protection gas (N2); (y) inlet of carrier gas (N2); (z) inlet of SO2 standard gas and diluent gases. proven that the product layer can prevent the internal CaO from further sulfation. As the concentration of trace elements is extremely low in the flue gas, to what extent the sulfation product layer influences their removal is not clear at present. To find out the effect of product layer on trace elements capture, relevant experiments in the thermogravimetric analyzer (TGA) are conducted in this research. The trace composition capture results by fresh CaO and used CaO after certain sulfation are compared. SO2 gas is used to replace SeO2 in our experiment for two reasons. One reason is that the concentration of SeO2 is more difficult to control than that of SO2. Normally, those trace species such as SeO2 are in solid state at room temperature. They must be heated to a certain temperature to be sublimed in special equipment to provide trace gases. Previous researchers (8-11, 14-17) all followed this method. Though a stable concentration trace gas can be obtained, 5-15% relative error still exists (17). Meanwhile commercial standard SO2 gas which is mixed with pure N2 has accurate concentration. It can substitute trace SeO2 gas in the experiment when diluted to trace level. The other reason is that a higher concentration of SeO2 gas cannot be provided during a long test period. Not only trace level but also much higher concentration gases (up to 2675 ppm) are involved in our experiments. To get higher concentration SeO2 gas, higher temperature is needed to increase the subliming rate. However, our experimental device is on a small scale and cannot sublime SeO2 in a steady high rate during one experiment period. Meanwhile it is easy to regulate the SO2 gas in any conditions. To verify the reliability of conclusions obtained from trace SO2 experiments, trace SeO2 gas experiments are also performed in the final part.

Experimental Section Assemblies. The reaction rate and capability of CaO adsorbing SO2 and SeO2 are obtained by measuring the mass change of a fixed amount of solid reactant in a gas-solid reactor system for a specific experimental time. The schematic of the experimental assembly is shown in Figure 1. It consists of three parts: SeO2 vapor generator, main TGA reactor, and gas scrubber. The SeO2 vapor generator, with reference to Sterling et al,’s method (17), consists of a 24 mm o.d. vaporization quartz tube housed in a horizontal furnace. A boat is used to hold the solid selenium (SeO2) inside the heated quartz tube. A

no.

name

pre-desulfurization (SO2, 2800 ppm) preprocess period of time (min)

1 2 3 4 5

fresh CaO used CaO used CaO used CaO used CaO

0 5 10 20 30

CaO conversion (%) 0 15 30 43 50

9 mm o.d. quartz pipe wrapped by heat tapes connects the outlet of the quartz tube with the main TGA reactor. The temperature of heat tapes is controlled to be higher than that of the vaporization tube to avoid SeO2(g) condensation. The main TGA reactor is Dupont 951 type with a 24 mm o.d. quartz tube in its horizontal furnace. A platinum boat is used to hold the sorbent (about 10.3-10.5 mg). The weight signal can be recorded every 6 s. At the exit of the main TGA, gases pass through a latex pipe to a scrubbing assembly in which all residual toxic gases are removed by 7% HNO3 solution. The clean gas is then vented to the atmosphere. Materials. Analytic pure solid of SeO2 is used as vapor source. The desired SeO2 concentration in the gas can be attained by regulating the temperature of the vapor generator. As calibrated previously, the temperature and N2 carrier flow rate have been set as 230 °C and 200 mL/min, respectively, to give the desired SeO2 concentration. The CaO sample is obtained from the calcinations of analytic pure Ca(OH)2 at 600 °C for 30 min. Its BET specific area is 39 m2/g and mean particle size is 47 µm. Experiment Steps. The concentrations of 24, 48, 97, 194, 335, 669, 1338, and 2675 ppm SO2 are adopted in the experiments. To exclude the effects of CO2, the diluent gases contain 5% O2 and balanced N2. The total gas flow is 400 mL/min. Before the experiment is started, only diluent gases are introduced into the system and the temperature is maintained at 700 °C for 30 min. The experiment consists of four steps: in Step 1, low concentration SO2 is used for 2 h; in Step 2, 2675 ppm SO2 is used for 10-30 min to convert 43% CaO; in Step 3, SO2 is cut off and the system is purged by N2 for seconds. Then diluent gases are introduced again for 30 min to 3 h. This step is designed to reduce errors as explained in the Supporting Information. In Step 4, the same SO2 gas as that in the first step is absorbed for 2 h. Since it is easy to get 43% CaO conversion for the gas with 335, 669, 1338, and 2675 ppm SO2, in these situations, Steps 2 and 3 are omitted and Step 1 is directly extended to the end of Step 4. The SeO2 concentration in the gas is about 26 ppm balanced by air in the experiments of CaO absorbing SeO2. The total flow rate is 400 mL/min and the temperature is kept at 700 °C. Sorbents which have gone through different preprocesses are used in the SeO2 adsorption experiment for 2 h. The sorbents are described in Table 1.

Results and Discussion Effect of Product Layer. Experiments were performed at 700 °C to study the effect of CaSO4 product layer on the mass transfer of CaO-SO2 reaction. As an example, the complete TGA program curve of 48 ppm SO2 is shown in Figure 2. Part 1 of the curve shows the sorption course of fresh CaO; Parts 2 and 3 demonstrate Steps 2 and 3 to convert 43% CaO in a short time and to reduce error. Part 4 of the curve shows the 48 ppm SO2 sorption course by the used CaO which has gone through the above steps. If the starting points of Parts 1 and 4 are put together, and parts 2 and 3 of the curve are VOL. 40, NO. 13, 2006 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 2. Typical TGA program for the SO2 sorption by fresh CaO and used CaO with whole steps at 700 °C. Gases: (1) 48 ppm SO2; (2) 2675 ppm SO2; (3) 0 ppm SO2; (4) 48 ppm SO2. Diluent gases: 5% O2 and balanced N2.

FIGURE 3. Comparison of sorption curve by fresh CaO and used CaO for 48 ppm SO2: temperature 700 °C; diluent gases 5% O2 and balanced N2. omitted, then Figure 3 is obtained. Figure 3 indicates that the SO2 sorption rate of used CaO, r2(t), is lower than that of fresh CaO, r1(t), suggesting that the sulfate product layer brings resistance and decreases the CaO sorption ability. To analyze the extent of CaO’s adsorption ability reduced by product layer, a function of ξ(t) is defined as follows. ξ(t) has a value range of [0, 1]. The bigger the ξ value is, the smaller the extent of CaO sorption ability is decreased.

ξ(t) )

r2(t) r1(t)

(3)

r1(t) )

dx1(t) dt

(4)

r2(t) )

dx2(t) dt

(5)

Figure 4 shows ξ values in the sorption experiments with different low concentrations of SO2. It is obvious that the ξ value increases with the decrease of SO2 concentration, which suggests that the adsorption ability of CaO covered by CaSO4 product layer is decreased less with the lower SO2 concentration. In practical application, the initial reaction rate draws more attention since the time of contact between sorbent and flue gas would not be long enough. Even in a CFB reactor, 4308

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FIGURE 4. Comparison of ξ values among different low SO2 concentrations.

FIGURE 5. Comparison of ξ(10) values among different SO2 concentrations. ξ(10) value when t ) 10 min. the contact time is only 10-20 min. Therefore, ξ value should be studied in detail in the initial reaction period. The TGA data in the first few minutes are not accurate because of the time lag before reaching the well-proportioned gas diffusion in the sorption tube. Therefore, ξ(10) calculated at the reaction time of 10 min is adopted as the initial reaction ξ value. Figure 5 shows the relationship between the ξ(10) value and the SO2 concentration in the range of 24-2675 ppm. It indicates that the ξ(10) value increases sharply with the decrease of SO2 concentration, suggesting that the effect of sulfate product layer on CaO adsorption ability decreases with decreasing SO2 concentration. Diffusion Analysis. To explain the results, the relationship of reaction rate with CaO conversion is presented in Figure 6. The derivation of Figure 6 is provided in the Supporting Information. The dashed lines denote the stages of Steps 2 and 3 of the low concentration experiments. When the CaO conversion is small, the reaction rate is relatively high and is greatly influenced by SO2 concentration in the gas: the higher the SO2 concentration, the bigger the reaction rate. With the increase of CaO conversion, the reaction rate, as well as the effect of SO2 concentration on it, decrease simultaneously. The curves of 2675 and 1338 ppm overlap each other after point A. The CaO conversion is about 41% at point A. The curve’s superposition suggests that the reaction rate is virtually not influenced by SO2 concentration in the range of 1338-2675 ppm when the conversion is higher than 41%. Besides, the curve of 669 ppm merges into these overlapped curves at point B when CaO conversion exceeds 45%. The curve of 335 ppm also merges into these curves at

FIGURE 6. Dependence of reaction rate on CaO conversion. Temperature 700 °C. point C when the conversion is about 50%. It can be deduced that the curves of lower concentration will merge together at higher conversions. All these phenomena indicate that the higher the CaO conversion, the smaller the effect of gas concentration on the reaction rate. As a matter of fact, a certain conversion corresponds to a certain product layer status with certain diffusion resistance. This is proved in the following paragraphs. The high CaO conversion range, in which the reaction rate tends to be controlled by product layer diffusion, has been paid much more attention. The diffusion includes both external and internal diffusion. The external diffusion means the mass transfer between the mainstream and the sorbent surface. It relates to many factors such as hydrodynamics condition, gas-solid contact pattern (fixed bed or fluid bed), temperature, and physical and chemical properties of gas and sorbent. All these factors remain the same in our experiments, so the external diffusion resistances are considered as the same here. Furthermore, external diffusion can hardly become the control step in gassolid reaction process especially in the high CaO conversion range, so the effect of external diffusion is neglected here. In general, the internal diffusion is regarded as the control step. In the initial reaction stage, there is no product layer on the sorbent surface and the gas can contact with the sorbent easily, thus the diffusion of SO2 and O2 through the pores of sorbent becomes the control step. After a period of reaction, the conversion of sorbent reaches a certain degree and a continuous product layer has covered the sorbent surface, then the reaction rate tends to be controlled by diffusion through the product layer. There are two standpoints on the mechanism of the product layer diffusion (18, 19): One is that SO2 and O2 penetrate inward through the CaSO4 product layer meeting CaO at the interface between CaO and CaSO4, where sulfation takes place. This is called “inward growth mode”. The other is that the solid reactant migrates outward through the product layer by Ca2+ and O2ionic diffusion to react with SO2 and O2 on the surface. This is called “outward growth mode”. We first presume that the inward growth mode is acceptable to explain the product layer diffusion, then we will prove whether the presumption is correct. The following discussions are carried out on the basis of this presumption in which the whole stage of sulfation is controlled by gas diffusion. The gas diffusion will obey the Fick Law (20) described as

NSO2 ) -De

∂CSO2 ∂R

(6)

FIGURE 7. Dependence of reaction rate on CaO conversion in different processes. Temperature 700 °C. The diffusion coefficient, De, is determined by many factors such as (a) experimental temperature; (b) physical properties such as gas density, viscosity, mole mass, particle density; (c) sorbent structure characteristics such as specific surface area, pore volume, and zigzag factor. For the curves in Figure 6, nearly all experimental conditions are identical except SO2 concentrations in the experiments, thus the factors (a) and (b) are the same. Nevertheless, the factor (c) is not known well whether it is the same under the same conversion condition because the product layers are formed by SO2 with different concentrations. We designed the following experiments to identify this problem. At first, about 34% CaO conversion is achieved quickly by the gas with SO2 as high as 5350 ppm, then the SO2 concentrations are shifted to 2675, 1338, 669, and 335 ppm, respectively, and the CaO conversions are extended to higher than 50%. These curves are shown in Figure 7. The derivation of Figure 7 is provided in the Supporting Information. The conversion value between 34% and 37% relates to the gas shift process, and these parts of the curves in this range are denoted by dashed lines. Some curves in Figure 6 are also shown in Figure 7 for comparison. Taking the two curves of 2675 ppm SO2 as an example, one curve shows that CaO reacts with 5350 ppm SO2 to 34% conversion and then switches to 2675 ppm SO2, the other curve shows that CaO reacts with 2675 ppm SO2 all along. It can be found that the two curves overlap each other. This overlap suggests that a given CaO conversion corresponds to the certain reaction rate under the same SO2 concentration condition and it is independent of its history. Also, we can conclude from eq 6 that the diffusion coefficient, De, is also the same when CaO conversion is certain. The similar conclusions can also be drawn when the other three SO2 concentrations (1338, 669, and 335 ppm) are discussed. To sum up, one CaO conversion corresponding to the certain diffusion coefficient resulted from the product layer does not matter with the path the SO2 sorption reaction takes in our experiments. This conclusion can help to analyze the mechanism behind the test results shown in Figure 6. If the inward growth mode were correct in this situation, the reaction rate should satisfy Fick’s law (eq 6). It can be seen that the front parts of the curves comply with Fick’s law well, but the latter parts are contrary. From the above analysis, the diffusion coefficients De should be the same at the same CaO conversion. The curves’ overlapping violates Fick’s Law, which means that the reaction rate is not related to the SO2 concentration in the latter curve part. We can assume that, instead of the inward growth mode mechanism, the product layer diffusion may fit into the mechanism of outward growth mode, namely, VOL. 40, NO. 13, 2006 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 8. Typical desulfurization curve in TGA: temperature 700 °C; 2800 ppm SO2, balance air. the mechanism of ionic outward diffusion. This mechanism has been experimentally determined by Hsia et al. using inert marker method (18) and isotopic labeling technique (19). The ionic diffusion rate is possibly affected by the thickness of the product layer and temperature rather than the external SO2 concentration. If the quantity of ions diffusing to the gas/CaSO4 interface is limited comparing with the quantity of SO2 molecules there, the increase of SO2 concentration will not contribute much to the reaction rate and the curves superposition will occur naturally. Hence, the phenomena of curves superposition can be well explained in this way. Explanation to the Rule of ξ. In the following section, the conclusion drawn from the discussion of Figures 6 and 7 is used to explain the rule of ξ which is focused on in the previous part. ξ(10) means the ratio of the reaction rate at 43% conversion to the reaction rate at initial stage. These two reaction rates are represented by r2(10) and r1(10), respectively, according to the experiment steps. At the high conversion of 43%, the overlapping of curves of 1338 and 2675 ppm means the effect of SO2 concentration on r2(10) is little. Although other r2(10) values corresponding to lower concentrations are smaller, they tend to be equal. In contrast, r1(10), which represents initial reaction rate, is affected by SO2 concentration obviously: it increases with the rise of SO2 concentration. Considering the two factors, it can be concluded that ξ(10) will increase with the decrease of SO2 concentration. ξ(10) represents the extent of CaO adsorption ability reduced by sulfate product layer: the higher the value, the less the ability loss. The conclusion drawn from Figure 4 can also be explained in this way. Significance. The conclusion drawn above, i.e., the sulfate product layer has no obvious effect on preventing CaO from removing trace SO2, is very important in commercial practices for simultaneous removal of SO2 and trace elements. Assuming that SO2 and trace elements such as SeO2, As2O3, are coexist in the gas and will be removed by CaO at 700 °C in a dry FGD-CFB reactor, the relatively high concentration SO2 can react rapidly with CaO and the product layer can soon be formed. It is difficult to maintain the initial high reactivity of CaO when removing high-concentration SO2 owing to the product layer resistance. However, for trace elements, we can infer from the above conclusion that CaO’s absorption ability for trace elements may be reduced less by the product layer, which means that the used CaO may remove the trace elements as efficiently as the fresh CaO. Validation. To validate the above assumption, a series of specific experiments was conducted. Figure 8 shows a typical TGA desulfurization curve using CaO at 700 °C. The CaO samples after desulfurization process in TGA for 0, 5, 10, 20, and 30 min are respectively used for the 2 h SeO2 (26 ppm) 4310

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FIGURE 9. Comparison of SeO2 adsorption with different CaO desulfurization utilizing different starts. Temperature 700 °C; SeO2 26 ppm, balance air. Horizontal axis explanation: (1) 2 h SeO2 sorption; (2) 2 h SeO2 sorption following 5 min 2800 ppm SO2 sorption; (3) 2 h SeO2 sorption following 10 min 2800 ppm SO2 sorption; (4) 2 h SeO2 sorption following 20 min 2800 ppm SO2 sorption; (5) 2 h SeO2 sorption following 30 min 2800 ppm SO2 sorption. removal in TGA. The details of the samples are listed in Table 1. Sample 1 (Point 1 in Figure 8) represents the fresh CaO, and other samples (Points 2-5 in Figure 8) represent the used CaO with conversions from 15% to 50%, which have the corresponding product layers. The SeO2 absorption results by these samples are shown in Figure 9. It can be seen that there is no obvious difference in absorption of trace SeO2 between fresh CaO and used CaO samples, which suggests that the CaSO4 product layer could not result in noticeable resistance to the reaction between CaO and trace SeO2. Therefore our assumption above has been proved by the experiment results. Considering only the effect of product layer except that of the reaction competition, we can say that the scheme of removing sulfur and trace elements in a dry FGD-CFB system will be a good choice to control the multi-pollutants. Under the presumption when the sulfur is removed by CaO in the FGD-CFB system, the CaO sorbent must be covered with a thin or thick CaSO4 product layer after many circulations. The used sorbent can hardly contribute to SO2 removal any more, but its absorption ability to trace elements has almost not been reduced. Therefore, an exhausted sorbent for high concentration SO2 can be used as the trace elements sorbent again. Agnihotri et al. (9) performed experiments to remove sulfur and trace selenium simultaneously. They found that the ability of CaO to adsorb selenium would decrease in the presence of SO2 and gave the explanation that extensive pore plugging/blocking due to the CaSO4 product renders the sorbent inaccessible for trace metal species. However, our observation differs from Agnihotri’s explanation. The main reason may not be the mass transfer resistance but another possible competition mechanism between SO2 and SeO2 which will be studied further in our research project.

Acknowledgments This work is supported by the State Key Development Program for Basic Research of China (2006CB200301).

Supporting Information Available Experimental error reduction methods; Figures S1-S3 related to Figures 6 and 7. This material is available free of charge via the Internet at http://pubs.acs.org.

Nomenclature CSO2

concentration of SO2

De

effective diffusion coefficient

NSO2

diffusion flux of SO2

R

radius of the sorbent grain

r

reaction rate

t

reaction time

x

conversion of CaO

ξ

function to scale the weakening extent of CaO adsorption ability

Subscripts 1

fresh CaO

2

used CaO

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Received for review November 27, 2005. Revised manuscript received March 13, 2006. Accepted April 10, 2006. ES052381S

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