Environ. Sci. Technol. 2001, 35, 794-799
Capture of Gas-Phase Arsenic Oxide by Lime: Kinetic and Mechanistic Studies RAJA A. JADHAV AND LIANG-SHIH FAN* 121 Koffolt Laboratories, 140 West 19th Avenue, Department of Chemical Engineering, The Ohio State University, Columbus, Ohio 43210
Trace metal emission from coal combustion is a major concern for coal-burning utilities. Toxic compounds such as arsenic species are difficult to control because of their high volatility. Mineral sorbents such as lime and hydrated lime have been shown to be effective in capturing arsenic from the gas phase over a wide temperature range. In this study, the mechanism of interaction between arsenic oxide (As2O3) and lime (CaO) is studied over the range of 300-1000 °C. The interaction between these two components is found to depend on the temperature; tricalcium orthoarsenate (Ca3As2O8) is found to be the product of the reaction below 600 °C, whereas dicalcium pyroarsenate (Ca2As2O7) is found to be the reaction product in the range of 700-900 °C. Maximum capture of arsenic oxide is found to occur in the range of 500-600 °C. At 500 °C, a high reactivity calcium carbonate is found to capture arsenic oxide by a combination of physical and chemical adsorption. Intrinsic kinetics of the reaction between calcium oxide and arsenic oxide in the medium-temperature range of 300-500 °C is studied in a differential bed flowthrough reactor. Using the shrinking core model, the order of reaction with respect to arsenic oxide concentration is found to be about 1, and the activation energy is calculated to be 5.1 kcal/mol. The effect of initial surface area of CaO sorbent is studied over a range of 2.7-45 m2/g using the grain model. The effect of other major acidic flue gas species (SO2 and HCl) on arsenic capture is found to be minimal under the conditions of the experiment.
Introduction Arsenic and its compounds have been linked to harmful toxicological impacts despite the fact that arsenic is an essential trace element for humans and other animals. Coal combustion, waste incineration, and metallurgical operations such as smelting of ores generate hot flue gases containing arsenic. Coal-fired power plants have been identified as major sources responsible for trace metal emissions including arsenic and will most likely come under U.S. EPA regulations. The average arsenic concentration in U.S. coal is reported as 15 µg/g (1), with low rank lignite and subbituminous coals having less than 5 µg/g arsenic (2). The mode of occurrence of arsenic depends on the rank of coal; in many bituminous coals, arsenic is principally associated with pyrite in which it substitutes for sulfur in the pyrite structure, whereas in low-rank coals, arsenic is present as As3+ in oxygen coor* Corresponding author telephone: (614)292-7907; fax: (614)2923769; e-mail:
[email protected]. 794
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dination (2). Upon combustion, this arsenic leaves the coal matrix and gets partitioned between vapor and particulate phases. Concentration of arsenic species in the flue gas depends greatly on the temperature of the stream and location of the measurement. Chemical interactions with fly ash have been shown to be responsible for arsenic partitioning between vapor and particulate phases (3). Germani and Zoller (4) reported that, in coal-fired power plants, 0.7-52% of the in-stack arsenic remains in the vapor form with its average vapor-phase concentration amounting to 7 µg/m3. For the incineration of arsenic-laden hazardous soil matrixes, Thurnau and Fournier (5) found that between 5 and 20% of the arsenic remains in the flue gas. The elemental (As) and oxide forms (As2O3) have been postulated to be the most probable arsenic species in the oxidizing flue gas environment. However, arsenic is much more volatile as an oxide than as an element, and researchers have concluded that arsenic in the flue gas could only be in the oxide form (6, 7). The high volatility and existence in the vapor phase make arsenic control a very difficult task to accomplish. Furthermore, submicron arsenic fumes are difficult to control in conventional particle control devices such as an ESP or a baghouse. Capture of the arsenic species on sorbents by physical or chemical means is a very attractive alternative. Many studies exist in the literature dealing with the interaction between trace metals and sorbents. In the lowtemperature range, activated carbon has been shown to be an effective sorbent for trace metal capture. Wouterlood and Bowling (3) found that activated carbons were quite effective in reversibly trapping arsenic oxide at 200 °C. Jadhav et al. (8) have shown that the mechanism of selenium dioxide capture by activated carbon involves a combination of physisorption and dissociative chemisorption. Fly ash has also been shown to remove trace elements such as As, Cu, Mo, Pb, and Zn from power station flue gas (9). In the medium- and high-temperature range, mineral sorbents have shown great promise in capturing these trace metal species. Uberoi and Shadman (10) have studied the use of mineral sorbents such as silica, alumina, kaolinite, emathlite, and lime for the removal of cadmium compounds from high-temperature (800 °C) flue gases and found the overall sorption process to be a complex combination of physical adsorption and chemical reaction. Mineral sorbents were also shown to be effective for arsenic, cadmium, and lead capture in the medium- and high-temperature range (11, 12). Ghosh-Dastidar et al. (13) have shown that the optimum capture of selenium dioxide by calcium hydroxide takes place at 600 °C by the formation of calcium selenite. Hydrated lime was found to be the most effective of the mineral sorbents Mahuli et al. (14) studied for arsenic capture, and the amount of arsenic captured was shown to be maximum around 600 °C. Hydrated lime was shown to capture arsenic oxide by forming an irreversible calcium arsenate (Ca3As2O8) product at 600 and 1000 °C. In recent years, research has been focused on finding multifunctional sorbents that can capture more than one trace metal or air toxin. Low-cost calcium-based sorbents offer an attractive option to be used as multifunctional sorbents because of their ability to capture sulfur species as well as trace elements such as selenium and arsenic species. This study investigates the mechanism of interaction between calcium oxide and arsenic oxide over a wide range of temperature (300-1000 °C). Intrinsic kinetics of this interaction is studied over the range of 300-500 °C. The competitive effect of acidic species (SO2 and HCl) on arsenic capture at 10.1021/es001405m CCC: $20.00
2001 American Chemical Society Published on Web 01/18/2001
TABLE 1. Chemical Composition and Structural Properties of CaO Powders Investigated CaOa
in-house CaO composition (wt %) CaO SiO2 Al2O3 MgO Fe2O3 mass median particle size, d50 (µm) BET surface area (m2/g) pore volume (cm3/g) a
(S1) 97.0 0.9 0.6 1.0 0.5 1.1
(S2) 97.0 0.9 0.6 1.0 0.5 1.2
(S3) 97.0 0.9 0.6 1.0 0.5 1.2
(S4) 97.0 0.9 0.6 1.0 0.5 1.1
(S5) 98.5 0.3 0.3 0.9 0 1.8
2.7 11 28 45 3.2 0.0062 0.022 0.074 0.143 ∼0
Johnson Matthey Co.
500 °C is also studied for different concentrations of these species.
Experimental Procedures The details of the experimental procedure and differential reactor assembly used are given elsewhere (8). Arsenic oxide (As2O3) is used as the source of arsenic in all the experiments. Arsenic oxide vapors are generated in the vapor generation assembly by heating the oxide source to suitable temperature, and the vapors are carried with the carrier gas (dry air) to the sorbent bed (10-15 mg of calcium oxide dispersed on quartz wool) housed in the furnace. Before the carrier gas enters the furnace, it is diluted with dry air to adjust the concentration of the arsenic species in the reactor to the desired levels. The flow rates of the carrier and diluent gas are maintained at 0.2 and 1.8 L/min (STP), respectively. The interaction of CaO with As2O3 is studied over a wide temperature range of 300-1000 °C. The kinetics of this gassolid interaction is studied over the range of 300-500 °C. At temperatures below 300 °C, physical adsorption may dominate, making the kinetic parameter determination complicated. The effect of As2O3 concentration is studied over a range, from 7 to 32 ppm (mg/m3). The concentration of As2O3 in flue gas is in the parts per billion range (µg/m3); however, much higher concentrations are chosen because of the experimental difficulties with maintaining lower trace metal concentration during the experiments. The CaO used for obtaining the kinetic parameters is procured from Johnson Matthey Company. Because of its low surface area and pore volume, these CaO particles can be assumed to be nonporous. To see the effect of the sorbent structural properties on the utilization of the sorbent, CaO powder with a range of surface areas is prepared by calcination of Ca(OH)2. For example, CaO with the highest surface area of 45 m2/g is prepared by calcining calcium hydroxide at 800 °C for 10 min in a horizontal tubular furnace, whereas the sorbent with the least surface area of 2.7 m2/g is prepared by calcining at 1100 °C for 30 min. The surface areas and pore volumes of these sorbents are determined using low-temperature nitrogen adsorption in Quantachrome’s NOVA-2200 instrument and are given in Table 1. Sorbents S1-S4 represent in-house CaO prepared by the above procedure, while sorbent S5 is a commercial sorbent. A high reactivity carbonate (HRC) having a surface area of 40 m2/g and pore volume of 0.18 cm3/g is prepared by carbonation of Ca(OH)2 slurry in water (15). The post-reaction solid product is analyzed using a PerkinElmer model 3110 graphite-furnace atomic absorption spectrometer (AAS). A measured amount of the solid sample is dissolved in dilute acid, and the solution is analyzed for total arsenic content in the AAS. X-ray diffraction (XRD) analysis on the product is conducted on a Scintag PAD-V powder diffractometer. The sulfation and chlorination extents
FIGURE 1. Arsenic oxide capture by CaO in the medium- and hightemperature range. As2O3 concentration: 14 ppm in air. Reaction time: 10 min. of the reaction are determined using Suppressor-based singlecolumn ion chromatography (IC). The experiments are performed in triplicates, and the average values of arsenic capture are reported. The deviation around the average value is found to be within (10%.
Results and Discussion Mechanism of Interaction between As2O3 and CaO. Arsenic oxide exists in the crystalline form as As4O6 (also known as arsenolite), and the crystal structure is retained even in the vapor phase. This structure is maintained till 800 °C, above which it dissociates into As2O3 (16). Arsenic oxide in the presence of oxygen can interact with calcium oxide in various stoichiometric ratios to give different calcium arsenates: CaAs2O6, Ca2As2O7 (dicalcium pyroarsenate), and Ca3As2O8 (tricalcium orthoarsenate). The thermal stability of these arsenates increases in the order CaAs2O6 (750 °C) < Ca2As2O7 (950 °C) < Ca3As2O8 (1400 °C) (17, 18). Figure 1 gives the amount of arsenic captured at temperatures in the range of 300-900 °C when CaO is exposed for 10 min to 14 ppm of arsenic oxide in air. As can be seen from the figure, the amount of arsenic captured increases in the temperature range of 300-600 °C and goes on decreasing with temperature beyond 600 °C. Mahuli et al. (14) also observed a maximum in the arsenic capture at 600 °C. This behavior is analyzed in detail below. Since the arsenic capture increases with temperature in the medium-temperature range (300-600 °C), it can be concluded that physical adsorption is not dominating. This is further confirmed by desorbing post-reaction CaO at 500 °C for 16 h in arsenic-free air, which did not indicate any loss of arsenic. The sudden drop in arsenic capture after 600 °C (Figure 1) could be due to the decomposition of the reaction product at higher temperatures. However, there was no apparent loss of arsenic when the reaction product at 500 °C was exposed to arsenic-free air stream at 1000 °C for 16 h. The decrease in the arsenic capture at higher temperature due to sintering-induced loss in the surface area of calcium oxide is also ruled out as the CaO sorbent is already sintered at high temperature and has a very low surface area (3.2 m2/g) and pore volume (≈0 cm3/g). Since the capture of arsenic decreases beyond 600 °C, it is suspected that two different reactions are occurring in these two temperature ranges. VOL. 35, NO. 4, 2001 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 2. XRD pictograms of the CaO/As2O3 reaction products. (a) CaO exposed to 60 ppm As2O3 at 500 °C for 30 h (Ca3As2O8, File No. 01-0933), (b) CaO exposed to 60 ppm As2O3 at 700 °C for 20 h (Ca2As2O7, File No. 17-0444), (c) CaO exposed to 70 ppm As2O3 at 900 °C for 10 h (Ca2As2O7, File No. 17-0444), (d) product of the reaction at 900 °C heated at 1000 °C for 24 h in arsenic-free air (Ca3As2O8, File No. 26-0295), and (e-h) pictograms published in the JCPDS files. These reaction products (arsenates) give the same arsenate ion upon dissociation in water; therefore, they cannot be distinguished using IC. However, these arsenates have distinctive crystal structures; the pyroarsenate (Ca2As2O7) possesses an octahedral structure (19), whereas the tricalcium orthoarsenate (Ca3As2O8) has a rhombohedral crystal structure (20). Therefore, XRD can be effectively used to identify the chemical and crystalline forms of these reaction products. The reaction of CaO with As2O3 in the presence of air is carried out for longer duration at a particular temperature to get sufficient conversion of CaO. A high surface area (SA) CaO (SA ) 14 m2/g) made from calcining the HRC is used in this 796
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case to get higher conversion of CaO. Diffractograms obtained at two different temperatures, 500 and 700 °C, are shown in Figure 2a,b, respectively. These figures also contain the comparative diffractograms published in the JCPDS files. The diffractogram in Figure 2a indicates that the reaction between CaO and As2O3 in air at 500 °C yields a product Ca3As2O8 (JCPDS File No. 01-0933), whereas at 700 and 900 °C, the reaction product is Ca2As2O7 (Figure 2b,c; JCPDS File No. 17-0444). At 600 °C, the XRD analysis of the reaction product indicated the presence of Ca3As2O8, Ca2As2O7, and unreacted CaO (diffractogram not shown). All these reactions did not go to completion, and the presence of unreacted
CaO is evident from the diffractograms. The reaction product consists of a nonporous high molar volume arsenate product layer surrounding the unreacted CaO core. The product of the reaction in the temperature range of 300-500 °C, Ca3As2O8, is reported in the literature to be stable till 1400 °C (17, 18). This explains the earlier finding that the arsenic content of the product at 500 °C did not change even after heating at higher temperature of 1000 °C for 16 h. The decrease in the arsenic capture in the high-temperature range (Figure 1) indicates that the reaction product (Ca2As2O7) is unstable at these temperatures. However, when the product of the reaction at 900 °C (Ca2As2O7, Figure 2c) was heated at 1000 °C in arsenic-free air for 24 h, no apparent loss of arsenic was observed. To further probe into this, XRD analysis of the heat-treated product is carried out and compared with that of Ca2As2O7 (Figure 2c,d). These diffractograms indicate a change in the structure of Ca2As2O7; the pyroarsenate (Figure 2c) is changed into orthoarsenate (Figure 2d, JCPDS File No. 26-0295). Note that even if the product in Figure 2d is identified as Ca3As2O8, its crystal structure is different than that in Figure 2a. As described next, two possible mechanisms explain this formation of Ca3As2O8 from Ca2As2O7 and CaO. One possibility is the decomposition of Ca2As2O7 into Ca3As2O8 above 950 °C via the following reaction (17, 21):
3Ca2As2O7(s) f 2Ca3As2O8(s) + 1/2As4O6(g) + O2(g) (1) Another possibility is the formation of calcium orthoarsenate (Ca3As2O8) from the reaction of Ca2As2O7 with unreacted CaO. This reaction can be given as
Ca2+ + O2- + Ca2As2O7(s) f Ca3As2O8(s)
(2)
Calcium and oxygen ions diffuse out of the CaO core, through the formed Ca3As2O8 product layer, and react with Ca2As2O7 to form Ca3As2O8. The formation of Ca3As2O8 in Figure 2d is difficult to definitively attribute to either reaction 1 or reaction 2, because reaction 2 can also occur above the decomposition temperature of 950 °C. One of the differences in these two reactions is the amount of arsenic lost; according to reaction 1, the amount of arsenic per mass of arsenate decreases by about 14% when it changes from pyroarsenate to orthoarsenate, whereas it stays the same in reaction 2. This decrease in arsenic content can be detected in the case of pure Ca2As2O7 using the AAS analysis. However, since these samples contained only partially formed Ca2As2O7 (the rest was unreacted calcium oxide), the small decrease in the arsenic content (per gram of arsenate) was not detectable with the AAS. The formation of Ca3As2O8 by either of these mechanisms is believed to be the reason that the amount of arsenic did not decrease when Ca2As2O7 was heated at 1000 °C for 24 h. The mechanism given in reaction 2 is in accordance with the ionic mechanism proposed by Yanagase et al. (22). On the basis of their findings of the Pt-marker experiments, they concluded that, for the reaction between As2O5 and CaO at around 800 °C, Ca2+ and O2- are the major diffusing species. Calcium and oxygen ions diffuse out of the CaO core, through the nonporous calcium arsenate product layer, and react with arsenic oxide and oxygen on the surface of the solid. Since As4O6 is much larger in size than the calcium and oxygen ions, diffusion of latter is favored over arsenic oxide. Thus, on the basis of these observations, it is concluded that calcium oxide reacts with arsenic oxide in the presence of oxygen to give Ca3As2O8 below 600 °C, and that in the range of 700-900 °C, they react to give unstable Ca2As2O7. This formation of unstable arsenate is responsible for the decrease in arsenic capture in the higher temperature range. These reactions can be represented by
below 600 °C: As2O3(s) T 1/2As4O6(g)
(3)
3CaO(s) + 1/2As4O6(g) + O2(g) f Ca3As2O8(s)
(4)
between 700 and 900 °C: As2O3(s) T 1/2As4O6(g)
(5)
2CaO(s) + 1/2As4O6(g) + O2(g) f Ca2As2O7(s)
(6)
Note that in reaction 5, as mentioned before, arsenic oxide exists in the crystal form as As2O3 (and not as As4O6) at temperatures higher than 800 °C. At higher temperatures (>900 °C), the reaction between arsenic oxide and calcium oxide in the presence of oxygen can be expected to give Ca3As2O8 because of the decomposition of unstable Ca2As2O7 into Ca3As2O8 at temperatures higher than 950 °C (via reaction 1). This may be the reason Mahuli et al. (14) found the product of the reaction to be Ca3As2O8 at 1000 °C. Even though oxygen is present in the flue gas, arsenic trioxide does not get oxidized into arsenic pentoxide (As2O5), as the reaction is known to happen only under pressure (23). However, if arsenic pentoxide is used as the source of arsenic, it readily dissociates into arsenic trioxide (As4O6) and oxygen near its melting point of 300 °C (23). Therefore, the reaction between calcium oxide and arsenic pentoxide even in the absence of oxygen can be expected to give Ca3As2O8 at 500 o C. Using XRD, the product of this reaction at 500 °C is confirmed to be Ca3As2O8. Interaction of arsenic oxide is also studied with high reactivity calcium carbonate (CaCO3) at 500 °C. XRD analysis of the CaCO3 exposed to arsenic oxide for 24 h in air did not indicate the presence of any reaction product. The capture of arsenic by this carbonate is suspected to be due to physisorption and/or chemisorption. Desorption of the reaction product at 500 °C for 24 h in arsenic-free air is found to result in about 40% arsenic loss. The remaining arsenic on the HRC surface is believed to be chemisorbed. Kinetics of the As2O3-CaO Reaction. The mechanistic studies indicate that arsenic oxide reacts with calcium oxide to give a stable product Ca3As2O8 below 600 °C, whereas in the high-temperature range (700-900 °C) it gives an unstable product, Ca2As2O7. Therefore, it was decided to find the interaction kinetics of the reaction in the medium-temperature range (300-500 °C). Since the CaO particles (S5 in Table 1) have very low surface area and pore volume, they can be assumed to be nonporous and to react with arsenic oxide by the shrinking core mechanism. The extent of conversion of CaO by reaction 4 as a function of time can be given by (24)
(3kCAn/Fm)(1/rp)t ) 1 - (1 - X)1/3 ) f(X)
(7)
where X is the overall conversion of the particle and rp is the initial radius of the particle. Oxygen is maintained in excess as compared to arsenic oxide; therefore pseudo-zero order reaction with respect to oxygen is assumed. As the reaction is carried out for short contact times, it is also assumed that the structure of the particle remains unchanged during the course of the reaction. To study the kinetics of the reaction, the reaction is carried out under differential conditions using 10-15 mg of the sorbent particles. High velocity of the gas through the sorbent bed (≈0.4 m/s) and smaller particles (
