Capture of Potassium and Cadmium by Kaolin in Oxidizing and

Oct 7, 2004 - Figure 3 shows that, at 850 °C in air, the amount of potassium captured by kaolin ...... Hagström, M.; Engvall, K.; Pettersson, J. B. ...
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Capture of Potassium and Cadmium by Kaolin in Oxidizing and Reducing Atmospheres Quang K. Tran,* Britt-Marie Steenari, Kristiina Iisa, and Oliver Lindqvist Department of Environmental Inorganic Chemistry, Chalmers University of Technology, 412-96 Go¨ teborg, Sweden Received May 12, 2004. Revised Manuscript Received August 31, 2004

Nutrient elements including alkali metals in biomass ash need to be returned to the forest by ash recycling. It is, however, difficult to recycle the total biomass ash due to the high concentrations of heavy metals in some ash fractions. During fluidized bed combustion of biomass, alkali metals tend to attach on bed particles or deposit on heat transfer surfaces, causing bed agglomeration, slagging, and fouling problems, while heavy metals are found enriched in the fly ash. Therefore, it is important to control both alkali and heavy metals during combustion of biomass. In this study, the competition between potassium and cadmium adsorption on kaolin was investigated experimentally in a fixed bed reactor equipped with an alkali detector. The experiments were made at 850 °C under both oxidizing and reducing conditions, which are relevant to fluidized bed combustion of biomass. The oxidizing gas was air, while the reducing gas was composed of 80% N2, 10% CO, and 10% CO2. Fine aerosols of droplets containing the metals in water were produced from their aqueous solutions by means of an atomizer and subsequently vaporized in the reactor. Atomic absorption spectrometry (AAS) and X-ray powder diffraction (XRD) techniques were used for quantitative and qualitative analyses of the products. The results indicate that potassium is captured by kaolin in both oxidizing and reducing atmospheres. Kaolin captures cadmium in air, but not under reducing conditions. In addition, reducing conditions appear to promote potassium adsorption on kaolin. The copresence of potassium and cadmium compounds in the gas phase promotes the capture of both metals by kaolin. This promotion was more pronounced for potassium than for cadmium, indicating a preference of kaolin for potassium capture over cadmium capture. In air, the total amount of potassium captured by kaolin increased when cadmium was added, but the water-soluble fraction decreased slightly. Moreover, kaolin captures KCl slightly more effectively than KOH in air, but less effectively than KOH under reducing conditions.

Introduction In the course of efforts to abate the green house effect by reducing carbon dioxide emission, increased combustion of biomass for heat and power production has created a need to recycle mineral nutrients in biomass ash to forest and agricultural soils. Only when the ash is returned to the soils so that nutrient elements, including alkali metals, can re-enter the ecosystem will the consumption of biomass be fully sustainable. It is, however, difficult to recycle the total amount of biomass ash produced, due to the high concentrations of heavy metals in some ash fractions. During fluidized bed combustion of biomass, ashforming species may either attach on bed particles and leave the process as bottom ash or become released as fly ashes and flue dust. The fate of these species is dependent on their physical characteristics, chemistry, the boiler design, and the combustion conditions. The distribution of the inorganic ash-forming species, including alkali and heavy-metal compounds, in different ash fractions depends very much on their vapor pressure. The higher vapor pressure a species has, the more * Corresponding author. Tel: +46 31 772 2864. Fax: +46 31 772 2853. E-mail: [email protected].

likely it is bound in the finer ash fractions or even escapes the process as inorganic vapor. A study on combustion of different kinds of biomass showed that more than 80% of alkali metals was found in the bottom and cyclone fly ashes, while most of the Cd was found in the filter fly ashes and flue dust, which had Cd concentrations of about 27 times higher than the bottom ashes.1 Methods to overcome the problem of the presence of heavy metals in mobile forms in fly ash include capturing the heavy metals in the combustion chamber by suitable sorbents. Various materials have been tested for this purpose. Biswas and Wu2 have included a review of past studies of heavy-metal capture by hightemperature sorbents, including silica, alumina, aluminosilicates, bauxite, emathlite, kaolin, limestone, hydrated lime, and titania. Kaolin, mainly composed of kaolinitesAl2Si2O5(OH)4shas been identified among the most promising aluminosilicate sorbents for the capture of heavy metals at high temperatures.3-9 However, kaolin appears to capture also alkali metals very (1) Obernberger, I.; Biedermann, F.; Widmann, W.; Riedl, R. Biomass Bioenergy 1997, 12, 211-224. (2) Biswas, P.; Wu, C. Y. J. Air Waste Manage. Assoc. 1998, 48, 113127.

10.1021/ef049881b CCC: $27.50 © 2004 American Chemical Society Published on Web 10/07/2004

Capture of Potassium and Cadmium by Kaolin

effectively at high temperatures.10-14 It has been used as an additive to fluidized bed biomass combustion to prevent problems of slagging and fouling of heattransfer surfaces and bed agglomeration, which are caused by the relatively high content of alkali metals in biomass.15-17 While most of the past studies have focused on the capture of either alkali or heavy metals separately, relevant systems involve both types of metals. Davis et al.8 and Gale and Wendt3,9 from the University of Arizona have reported results from their study on competitive capture of sodium and heavy metals by high-temperature sorbents, including kaolin, during coal combustion. For biomass, concentrations of heavy metals in biomass ash are normally of the same level as in ash from peat and coal,18 but potassium is of a greater concern, as it occurs naturally in plant material in a much larger amount than sodium.1,19-21 Therefore, the work reported in this present paper was carried out in order to investigate the competition between potassium and cadmium adsorption on kaolin. Cadmium was chosen for the study because its compounds are considered to be among the most toxic heavy-metal species emitted into the environment during combustion of biomass, and some plant species such as willow have very high uptake of cadmium. The experiments were done under both oxidizing and reducing conditions, which are relevant to fluidized bed combustion of biomass. Reducing conditions are especially relevant when air-staging combustion is applied for the reduction of NOx formation.22-24 (3) Gale, T. K.; Wendt, J. O. L. Combust. Flame 2002, 131, 299307. (4) Yang, H.-C.; Yun, J.-S.; Kang, M.-J.; Kim, J.-H.; Kang, Y. Korean J. Chem. Eng. 2001, 18, 499-505. (5) Yang, H.-C.; Yun, J.-S.; Kang, M.-J.; Kim, J.-H.; Kang, Y. Korean J. Chem. Eng. 1999, 16, 646-653. (6) Uberoi, M.; Shadman, F. ACS Symp. Ser. 1992, 515, 214-222. (7) Uberoi, M.; Shadman, F. Environ. Sci. Technol. 1991, 25, 12851289. (8) Davis, S. B.; Gale, T. K.; Wendt, J. O. L. Aerosol Sci. Technol. 2000, 32, 142-151. (9) Gale, T. K.; Wendt, J. O. L. Aerosol Sci. Technol. 2003, 37, 865876. (10) Punjak, W. A.; Shadman, F. Energy Fuels 1988, 2, 702-708. (11) Uberoi, M.; Punjak, W. A.; Shadman, F. Prog. Energy Combust. Sci. 1990, 16, 205-11. (12) Punjak, W. A.; Uberoi, M.; Shadman, F. AIChE J. 1989, 35, 1186-94. (13) Mwabe, P. O. Mechanisms Governing Alkali Metal Capture by Kaolinite in a Downflow Combustor. Ph.D. Thesis, University of Arizona, Tucson, AZ, 1993. (14) Tran, K.-Q.; Iisa, K.; Hagstrom, M.; Steenari, B.-M.; Lindqvist, O.; Pettersson, J. B. C. Fuel 2004, 83, 807-812. (15) O ¨ hman, M.; Nordin, A. Energy Fuels 2000, 14, 618-624. (16) Turn, S. Q.; Kinoshita, C. M.; Ishimura, D. M.; Zhou, J.; Hiraki, T. T.; Masutani, S. M. J. Inst. Energy 1998, 71, 163-177. (17) Steenari, B. M.; Lindqvist, O. Biomass Bioenergy 1998, 14, 6776. (18) Bioenergy: Sustainable Energy Solutions Project, Vattenfall AB, 2001. (Available via the Internet at http://www.vattenfall.se/ downloads/informationsmaterial/cadmium.pdf.) (19) Lundborg, A. Ecological and economical evaluation of biomass ash utilizationsThe Swedish approach. In Ashes and Particulate Emissions from Biomass Combustion: Formation, Characterisation, Evaluation Treatment, Vol. 3; Obernberger, I., Ed.; Institute of Chemical Engineering, Technical University of Graz, 1998. (20) Baxter, L. L.; Miles, T. R.; Miles, T. R., Jr.; Jenkins, B. M.; Milne, T. A.; Dayton, D.; Bryers, R. W.; Oden, L. L. Dev. Thermochem. Biomass Convers. 1997, 2, 1424-1444. (21) Baxter, L. L.; Miles, T. R.; Miles, T. R., Jr.; Jenkins, B. M.; Milne, T.; Dayton, D.; Bryers, R. W.; Oden, L. L. Fuel Process. Technol. 1998, 54, 47-78. (22) Spliethoff, H.; Hein, K. R. G. Proceedings-16th Annual International Pittsburgh Coal Conference; 1999; pp 1335-1348.

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Figure 1. Schematic of the experimental setup.

Experimental Section Experiments were carried out in a fixed bed reactor assembly, as shown in Figure 1. The reactor assembly consists of two major components: a fixed bed for holding the sorbent and an alkali-metal detector. The fixed bed has an inner diameter of 12 mm and is situated in the center of the reactor. The detector is based on surface ionization of alkali metals on the hot surface of a Pt filament. The filament potential was kept at +400 V and the temperature of the filament at 1500 K. More details on the equipment are described in Tran et al.14 Crystalline KCl, KOH, and CdCl2 of pro analysis quality were used to produce gaseous metal species in a carrier gas. The compounds were first made into aerosol particles by means of an aerosol generator (TSI Constant Output Atomizer Model 3075). The aerosol generator includes a bottle containing an aqueous solution of the metal compounds, a nebulizer, a cyclone, and a dryer, as shown in Figure 1. The aerosol particles generated were then delivered to the reactor via a breakthrough located on the bottom cap of the reactor, and they were subsequently vaporized in the reactor. From known concentrations of the metal compounds in their aqueous solutions and a known gas flow rate of the carrier gas, concentrations of the metals in gas phase can be estimated. In this study, experiments were conducted at a constant gas flow of 3.6 L/min. The oxidizing gas used was air with approximately 20 ppm water. The reducing gas had a composition of 80% N2, 10% CO, and 10% CO2. CO2 was used in order to prevent formation of carbon black from CO according to reaction 1, which is thermodynamically favored:

2CO(g) ) CO2(g) + C

(1)

For experiments under reducing environments, inert nitrogen gas (3 L/min) was used for aerosol generation. CO (0.3 L/min) and CO2 (0.3 L/min) were added to the main flow before it entered the reactor. For experiments in oxidizing environments, only air was used (3 L/min) as carrier gas. However, a small side flow of air (0.6 L/min) was added to the main flow in the same way as for CO and CO2 additions in the experiments under reducing conditions, to maintain the conditions in the two sets of experiments identical. The alkali detector can directly monitor the concentration of potassium in the gas phase.14 However, it does not work for cadmium. Therefore, to be able to get a complete picture of multiple-metal adsorption on kaolin, the reaction products were subjected to a chemical analysis for the determination of potassium and cadmium by means of an atomic absorption spectrometer (AAS-800, Perkin-Elmer). The AA spectrometer (23) Lyngfelt, A.; Aamand, L.-E.; Karlsson, M.; Leckner, B. Institute of Energy’s 2nd International Conference on Combustion & Emissions Control, Proceedings of the Institute of Energy Conference; London, December 4-5, 1995; pp 89-100. (24) Jenkins, B. M.; Baxter, L. L.; Miles, T. R., Jr.; Miles, T. R. Fuel Process. Technol. 1998, 54, 17-45.

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Table 1. K and Cd Contents of Calcined Kaolin mg of metal/g of dehydrated kaolin fraction

K

Cd

water-soluble water-insoluble

0.32 9.40

0.01 0.27

is equipped with two atomization techniques: flame and a graphite furnace. For the present work the flame technique with zero-intercept and nonlinear calibration was employed. For this analysis, solid samples are pretreated in either deionized water or an acid solution for the determination of water- or acid-soluble fractions of the metals captured, respectively. For the determination of the acid soluble fraction, samples were dissolved in a mixture of HNO3, HCl, and HF with proportions of 3:1.1, respectively (ASTM D3683). Qualitative determination of crystalline compounds in the kaolin samples before and after experiments was carried out using a Siemens D5000 X-ray powder diffractometer with application of Cu characteristic radiation and a position sensitive detector. A software system, DIFFRAC AT, supplied by Siemens, and the PDF-1 database (Release 2000, International Center for Diffraction Data, 2000) were used for data processing and identification of crystalline components. This method allows for the identification of crystalline compounds present in amounts larger than 1%-2% (w/w). The kaolin used in this study was a French kaolin (MERCK, CAS No. 1332-58-7), supplied by Merck Eurolab. The original kaolin powder particles were too fine for the experiments due to a high pressure drop over the sorbent bed. Therefore, the powder was made into a slurry with deionized water, allowed to dry naturally at room temperature, crushed, and sieved to retain the particles with desired sizes. The size 1.0-1.4 mm in dynamic diameter (dp) was selected for the experiments. K and Cd contents of the kaolin, dehydrated by being baked at 850 °C for several hours, are given in Table 1.

Results Performance Test and Alkali Detector Measurements. In this study, experiments were made with relatively high concentrations of the metals in gas. The aim was to get high kaolin conversions and thus facilitate the qualitative and quantitative analyses. The concentrations of metals in gas can be increased by increasing their concentrations in the aqueous solutions, which will consequently increase the size of the aerosol particles generated. The larger the aerosol particles are, the more likely they deposit on the way to the reactor. Therefore, a performance test was made with the help of the alkali detector. It was found that the atomizer worked fine with concentrations of metals in their aqueous solutions of up to 3000 ppm (wt), which was estimated for potassium to correspond to 23 ppm (v) in the gas phase, and the diameter of the aerosol particles generated was 0.35 µm. Heat and mass transfer calculations were made for KCl aerosol particles of 0.35 µm in size. It was shown that the particles would be completely vaporized, when entering the reactor under the conditions of the experiments. These calculations are also valid for CdCl2, since the vapor pressure of CdCl2 at a given temperature is higher than that of KCl. When the KCl concentration in its aqueous solution was increased further, the detector signal became inconsistent. In addition, white metal deposits were observed in the bottom cap of the reactor when the reactor was opened and visually inspected. Even with

Figure 2. Alkali detector signal of experiments in oxidizing gas.

the solution of 3000 ppm KCl in water, similar problems started to occur at 8 h. These problems can be attributed to the deterioration in drying efficiency of the diffusion dryer. Clear changes in color of the silica gel as drying agent of the dryer were observed at the time. Due to these problems and for practical reasons, the experimental time of each run was limited to 5.5 h. It was then decided to carry out experiments with the chosen metal compounds, and their concentrations in liquid and gas phases as given in Table 2. Eight experiments were selected as the combinations of the four conditions represented in Table 2 and the two reaction atmospheres, oxidizing and reducing. Among them, experiments 1, 2, and 3 were the focus for obtaining information about the competition for potassium and cadmium capture on kaolin. Experiment 4 was brought about for a comparison study into the capture of the two different potassium compounds, KCl and KOH. Figure 2 represents the detector signals recorded from experiments 1 and 3 made in air, which are labeled 1 and 3, respectively. The signals were obtained after the bed (Figure 1) and thus are inversely proportional to the adsorption rate. For experiment 1 with a 0.01 M KCl solution (curve 1), the adsorption rate was constant for the first hour and then started decreasing at about 80 min from the start of the experiment. When cadmium was added, experiment 3 and curve 3, the absorption rate started to decrease already after about 20 min from the start, and the rate decreased somewhat faster than that in experiment 1, since curve 3 has a somewhat steeper slope than curve 1. At this stage, it is not possible to draw a firm conclusion about the effect of cadmium addition on the potassium removal efficiency of kaolin, because of the unavailability of the inlet signals. The alkali detector does not work properly under the conditions of relatively high concentrations of alkali metals, due to a decreasing ionization efficiency.25 In addition, the work function of the platinum filament surface is dependent on its surface chemistry.26-28 When (25) Ja¨glid, U.; Olsson, J. G.; Pettersson, J. B. C. J. Aerosol Sci. 1996, 27, 967-977. (26) Hagstro¨m, M.; Engvall, K.; Pettersson, J. B. C. J. Phys. Chem. B. 2000, 104, 4457-4462. (27) Davidsson, K. O.; Engvall, K.; Hagstro¨m, M.; Korsgren, J. G.; Lonn, B.; Pettersson, J. B. C. Energy Fuels 2002, 16, 1369-1377. (28) Zandberg, E. Y.; Ionov, N. I. Surface Ionization (translated from Russ.); Israel Program for Scientific Translations: Jerusalem, 1971.

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Table 2. Types of Experiments with Different Inlet Metal Compounds concentration metal compound KCl CdCl2 KOH total concentration in liquid total concentration in gas

exp 1

exp 2

exp 3

exp 4

0.01 M 0M 0M 746 ppm (w) 5 ppm (v)

0M 0.01 M 0M 1833 ppm (w) 5 ppm (v)

0.01 M 0.01 M 0M 2579 ppm (w) 10 ppm (v)

0M 0M 0.01 M 561 ppm (w) 5 ppm (v)

Figure 3. Potassium capture on kaolin in oxidizing gas.

Figure 4. Cadmium capture on kaolin in oxidizing gas.

cadmium was added, the work function may have changed, which consequently changed the ionization efficiency of the detector and made it impossible to compare the adsorption rates in the two experiments. For experiments carried out under reducing atmospheres, the detector signals were not consistent and therefore are not included in this report. The same problem was encountered during KOH experiments in air. However, the detector signals were still a good indicator for controlling the experimental performance, especially for detecting problems of bed channeling that may occur in the sorbent bed during experiments. Results from Experiments in Oxidizing Atmosphere. Figures 3 and 4 present results from the chemical analysis of potassium and cadmium captured on kaolin, respectively. The experiments were carried out in air for 5.5 h, and the results reported here were obtained after subtraction of the content of the metals in calcined kaolin, which was treated and analyzed in the same way as the reaction product samples. The amounts of metals captured are expressed in millimoles of the metals per gram of dehydrated kaolin.

Figure 5. Potassium capture on kaolin in reducing gas.

Figure 3 shows that, at 850 °C in air, the amount of potassium captured by kaolin increased when CdCl2 was added. While the water-insoluble fraction increased, the water-soluble fraction slightly decreased. Figure 4 indicates that the amount of cadmium captured on kaolin also increased when KCl was added but less significantly than for potassium, as shown in Figure 3. The water-insoluble fraction increased, but the watersoluble fraction remained almost unchanged. By comparing the data shown in Figures 3 and 4, one can see that kaolin captured cadmium less effectively than potassium from the gases containing a single metal, either CdCl2 or KCl, respectively. Similarly, kaolin captured more potassium than cadmium from the gas containing equal moles of KCl and CdCl2. In addition, it is shown in Figure 3 that kaolin captures KCl somewhat more effectively than KOH. More water-soluble products are formed from the interaction between kaolin and KOH than from the interaction between kaolin and KCl. In all cases the conversion of kaolin to products was relatively low. Assuming that the product from the reaction with KCl was KAlSiO4, the conversion of kaolin was 0.7% in 0.01 M KCl and 1.6% in the mixture of KCl and CdCl2. Results from Experiments in Reducing Atmosphere. The same types of experiments as described in Table 2 were carried out under reducing conditions. In contrast to the experiments in air, results from the experiments under reducing conditions show that no cadmium was captured by kaolin at 850 °C in the experiments with and without KCl addition. For potassium capture, results presented in Figure 5 indicate that the addition of cadmium had the same promoting effect on potassium capture by kaolin as in oxidizing atmospheres. The increase in the content of potassium captured on kaolin is, however, more significant than that in the same type of experiment in air. Both the water-soluble and water-insoluble fractions of potassium-containing products increased as CdCl2 was added.

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of the samples from the experiments in air clearly showed that the products were mainly noncrystalline. In addition, the diffraction patterns of products formed during experiments in the same atmosphere were very similar. Therefore, only two typical patterns were selected for illustration in this paper. Parts a and b of Figure 6 represent the powder XRD patterns obtained from the products of experiment 3 (0.01 M KCl + 0.01 M CdCl2) in oxidizing and reducing atmospheres, respectively. The patterns are also similar to those obtained from the samples of kaolin heated at 850 °C in the corresponding atmospheres without being exposed to metal-containing gas. In both patterns, apart from the main diffraction peaks originating from quartz and muscovite, some weak diffraction peaks indicating the presence of kalsilite (KAlSiO4) were also found. These diffractions were, however, too weak to draw any firm conclusions. In addition, a visual inspection (Figure 7) showed that there is a difference in the color of the reaction products from experiments in oxidizing and reducing atmospheres. The products of the experiments in reducing atmosphere are white, whereas those of the experiment in air have a beige color that can be seen as darker in Figure 7. This color difference is probably be due to the difference in crystallinity of the products as observed from the powder XRD analyses. Discussion Figure 6. Typical diffraction patterns of the reaction products of experiments in (a) oxidizing and (b) reducing atmospheres. Peaks from quartz are marked Q, those from muscovite are marked M, and those from kalsilite are marked K.

In addition, kaolin captured KCl less effectively than KOH in reducing atmospheres, though more watersoluble products are formed from the interaction between kaolin with KCl than with KOH. XRD Pattern of the Reaction Products. X-ray powder diffraction (XRD) analysis was used to identify the final products formed by adsorption of potassium and cadmium in kaolin. The analyses were carried out in the 2θ range of 10°-70° at room temperature. The samples included kaolin as received, kaolin subjected to heat treatment (calcined kaolin), and the reaction product samples obtained from the experiments represented in Table 2 in different gas atmospheres (oxidizing versus reducing). Only kaolin as received and the samples from the experiments in reducing atmospheres gave distinct XRD patterns. The XRD analysis results

Kaolin as received contains mainly the mineral kaolinite, Al2Si2O5(OH)4. The heat treatment at 850 °C, which was carried out in order to remove moisture and traces of potassium present in the kaolin, resulted in the formation of meta-kaolinite containing some quartz, SiO2, and muscovite, KAl2Si3AlO10(OH)2. The fact that there are traces of muscovite present in kaolin after the heat treatment shows that some potassium ions are not easily released from the mineral matrix in the original kaolin. It is not surprising that the products of experiments in air are noncrystalline, because of the decomposition to amorphous meta-kaolinite. However, the rather good diffraction patterns shown in Figure 6b suggest that, in reducing atmosphere, kaolin does not lose its crystallinity or there might have been a recrystallization during heating at 850 °C. This observation is somewhat in conflict with an earlier study,17 which reported that, in reducing atmosphere (10% CO in He), the product of the reaction between kaolin and potassium was also mainly noncrystalline. However, in that study, solid

Figure 7. The products of experiments in different atmospheres have different colors.

Capture of Potassium and Cadmium by Kaolin

potassium compounds and kaolin were pressed together before being heated and kept at 900 °C for 2 h, a procedure that is very different from that used in this present work. Another possible cause of this discrepancy is the possible conversion of CO to CO2 and C according to reaction 1. Nonetheless, an important agreement between the two studies is that the reducing conditions appear to promote the chemical reaction between potassium species and kaolin. Due to the week diffractions obtained from the XRD analyses, it was not possible to identify all crystalline products formed. However, formation of cadmium and potassium aluminum silicates from meta-kaolinite and corresponding metal compounds has been discussed in the literature.4-7,10-12,17 For cadmium, the fact that most of the cadmium-containing products found in the present study (Figure 4) were water insoluble is in agreement with the literature. Uberoi and Shadman7 studied the removal of cadmium using high-temperature solid sorbents in a simulated flue gas and found that kaolinite reacted with gaseous CdCl2 to form CdAl2Si2O8, which is water insoluble. On the basis of XRD analysis results, they proposed the following reaction scheme for cadmium capture by kaolin:

Al2O3‚2SiO2 + CdCl2 + H2O f CdO‚Al2O3‚2SiO2 + 2HCl (2) When studying the high-temperature solid-state reactions between kaolin and potassium compounds, Steenari and Lindqvist17 found the formation of KAlSiO4 and KAlSi2O6, for which the following overall reactions were proposed:

Al2O3 + 2SiO2 + 2KCl + H2O f 2KAlSiO4 + 2HCl (3) Al2O3 + 4SiO2 + 2KCl + H2O f 2KAlSi2O6 + 2HCl (4) In the present study, the presence of KAlSiO4 was verified. However, it is not certain to what extent KAlSiO4 was a product of the reaction between metakaolin and KCl and to what extent it was present as an impurity in the calcined kaolin. It was found in the kalsilite form, but also possibly in the kaliophilite form, a polymorph of kalsilite. Both kalsilite and kaliophilite reported in the literature have a hexagonal structure but different unit cell dimensions.17 The X-ray powder diffraction patterns of the two forms are rather similar but differ sufficiently to permit identification if significant amounts of the phases are present. It has been suggested by Mwabe13 that potassium capture by kaolin is likely to form both crystalline insoluble and amorphous water-soluble products. The water-insoluble products are formed from reactions similar to reaction 3. The amorphous water-soluble products are potassium silicates, which are formed from the reaction between potassium vapor and quartz (SiO2).13,29 The fact that both quartz and water-soluble products were found in the present study is favored by this suggestion. It also explains why only weak diffrac(29) Wilhelm, E. The Physical Chemistry of Silicates. Ph.D. Thesis, University of Chicago Press: Chicago, 1954.

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tions were obtained from the XRD analyses of the reaction products, considering that the water-soluble products are amorphous and found in considerable fractions. Similar results were found by another research group,11 and it has been concluded that the adsorption of alkali in the gas phase on kaolin is a combination of physisorption, chemisorption, as well as chemical reaction. Results from the chemical analyses of the present study indicate that the copresence of the two potassium and cadmium compounds promotes the capture of both metals on kaolin under oxidizing conditions. This is somewhat in contrast with an observation of Davis et al.8 from a study of the competitive capture of sodium and cadmium by kaolin. They reported that the presence of sodium inhibited the capture of cadmium by kaolinite at around 1200 °C. However, the promotion effect observed in the present study was more pronounced for potassium than for cadmium. This indicates a preference of kaolin for potassium capture over cadmium capture and is in agreement with a finding from the same study8 as mentioned earlier. In another work reported latter by Gale and Wendt3 from the same research group as Davis et al.,8 cadmium capture by kaolin at 1160 °C was found to be enhanced by the presence of sodium. The enhancement was attributed to the melt-associated restructuring of the meta-kaolinite crystal, caused by eutectics formed with sodium reaction products. Furthermore, cadmium itself was found to enhance the adsorption of sodium on kaolin at temperatures in the range 1000-1300 °C by slowing down the deactivating, excessive melt driven by the sodium-sorbent eutectics at these temperatures. Overall, the total metal capture enhancement in the range of temperature studied for the Cd/Na system was attributed to the formation of an optimum eutectic melt, whereby significant melt enhancement was induced without sorbent deactivation. The findings from this work are in great agreement with the results from the experiments under oxidizing conditions of the present study, though there is a difference in the ranges of temperature studied between the two works. However, since the effect of slowing down the deactivating, excessive-melt driven by sodium-sorbent eutectics was attributed to the cadmium-sorbent product, the explanation of the enhancement effect caused by the presence of cadmium does not work for the observations from the experiments under reducing conditions of the present study, which showed no capture of cadmium by kaolin under reducing conditions, but kaolin still captured more effectively potassium when cadmium was added. It seems that cadmium itself in the gas-phase had the effect of slowing down the deactivating excessive melt caused by sodium-sorbent eutectics by altering the surface activity of kaolin. Conclusions Multimetal captures by kaolin have been investigated in oxidizing and reducing atmosphere at 850 °C. The following conclusions can be drawn from the present study: (1) Potassium is captured by kaolin in both oxidizing and reducing atmospheres. Kaolin captures cadmium

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in air but not under reducing conditions. These results imply that kaolin can be used for in situ, selective capture of metals under FBC conditions with varying environments. (2) The reducing atmosphere appears to promote the reaction between gaseous potassium and kaolin, compared to the oxidizing atmosphere. (3) The copresence of the two metal compounds appears to promote the capture of both the metals on kaolin. Under oxidizing conditions, this promotion was more pronounced for potassium than for cadmium, indicating a preference of kaolin for potassium capture over cadmium capture. (4) In air, the total amount of potassium captured on kaolin increased when cadmium was added, but its water-soluble fraction decreased.

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(5) Kaolin captured KCl slightly more effectively than KOH in air but less effectively than KOH under reducing conditions. At the moment, it is not possible to draw firm conclusions about the mechanism of the competition between potassium and cadmium adsorption on kaolin. Further investigations in the direction of increasing kaolin conversion with applying analytical techniques such as XRD and scanning electron microscopy (SEM) for phase identification and morphological analyses are needed. Acknowledgment. This research has been financially supported by the Swedish Energy Administration Board, which is greatly acknowledged. EF049881B