Environ. Sci. Technol. 1995, 29, 1660-1665
M t l c t i - m l Sorlrents for the Remwai of S dM M c Contaminants from High-Tern re Gases BAOCHUN W U , + K E I J O K . J A A N U , * A N D F A R H A N G SHADMAN*tt Department of Chemical Engineering, University of Arizona, Tucson, Arizona 85721, and Technical Research Centre of Finland, W, 401 01 Jyvaskyla, Finland
A multi-functional sorbent is developed for the simultaneous removal of alkali vapor, toxic metal vapors, and sulfur oxides from combustion gases. The sorbent is tested in a bench-scale reactor at the 800-1000 "C temperature range, using simulated flue gas containing controlled amounts of sodium, potassium, lead, and sulfur vapor compounds. The kinetics of sorption for these contaminants, both individually and in combination, are measured. In general, the sorption process consists of adsorption followed by the diffusion of the metal in the product layer and finally reaction with the sorbent. The product layer is a porous alkali aluminosilicate in the case of alkali and a molten lead aluminosilicate in the case of lead. SO2 reacts with the calcium sites distributed over the aluminosilicate matrix. The tailored sorbent is effective in simultaneous removal of the tested contaminants; it even shows synergistic removal in some cases.
Introduction The use of sorbents for the removal of harmful species during combustion and incineration is becoming increasingly important. Sorbents for SO2 capture have been the subject of various studies for many years. Other sorbents for capturing alkali and various trace metals have been investigated more recently. In general, sorbents can be used in two ways: in-furnace application (injection into the reaction zone) and postcombustion application (flue gas cleanup). In-furnace application has the advantage of higher temperature and higher reaction rates; the disadvantage is unfavorable equilibrium for some sorption reactions at high temperatures and the short residence time, which often results in low sorbent utilization. The post-combustion application for flue gas cleanup is at lower temperatures and gives lower reaction rates. However, when used as a flue gas treatment bed, the sorbent is stationary and has large enough residence time to be utilized efficiently. The use of sorbents has advantages over ceramic filter application, the technique used presently, for hot gas cleanup. Ceramic filters degrade due to reaction with alkali and other metallic compounds. Various sorbents have been suggested for SO2 removal ( I -4). The most widely studied sorbent is lime and various naturally occurringlimestones. While the intrinsic reaction of lime with sulfur dioxide is fast, the reaction is slow because of pore plugging. This is due to the fact that the reaction product, calcium sulfate,has a larger molar volume than calcium oxide. To reduce pore plugging, the application of hydrated lime and the use of structure modifiers have been suggested (5, 6). In recent years, there have been several studies of sorbents for capturing metal vapors (7-11). This includes alkali and trace toxic metals like lead, cadmium, and mercury. Alkali metals, although nontoxic, can cause a variety of problems including fouling and corrosion. The foulingand slagging are problems in dry ash processes while corrosion is a problem in all coal conversion processes, particularly at high temperatures. One of the most stringent alkali control requirements is for the combined cycle processes and pressurized fluidized bed combustion (PFBC) where downstream gas turbines are employed. The gas turbine blades are usually made of materials that are easily attacked by corrosive alkali compounds. Sulfur,as a major pollutant in the flue gas, needs to be also controlledtogether with the alkali metals. In general, gas turbines require alkali levels lower than 50 ppb. The flue gases from various coal conversion processes generally contain alkali levels that are several orders of magnitude higher. There are two principal ways that the alkali minerals accentuated the fouling and corrosion process. They contribute to the formation of eutectic mixtures with other compounds, resulting low melting point compounds which increase the stickiness and fouling propensity of particles already formed in the combustion environment. In the biomass combustion process, the ash melting point decreases as much as 150 "C due to alkali content of the ash * Fax: 520-621-6048;e-mail address:
[email protected]. University of Arizona. 4 Technical Research Centre of Finland. t
1660 1 ENVIRONMENTAL SCIENCE & TECHNOLOGY I VOL. 29. NO. 6.1995
0013-936)(/95/0929-1660509.00/0 0 1995 American Chemical Society
FIGURE 1. Schematic diagram of the experimental apparatus: 1, mass flow controller; 2, saturator; 3, mixer; 4, reactor. (12). In addition, the alkali compounds vaporize and condense on other particulates or on the walls of combustors, gasifiers, and auxiliary equipment where alkali-rich deposits are known to be highly corrosive (13). The presence of moisture at high temperatures enhances the corrosion. There have been several recent studies on the use of aluminosilicatecompounds for capturing some toxic metals like lead and cadmium (14, 15). While the application of sorbents looks very promising and versatile, there are still major challenges left. The most important one is the control of interactions while several pollutants are present. Most studies have focused on sorbent development and evaluation with respect to one type of pollutant. Since it is not practical to introduce a variety of sorbents into the system for controlling various pollutants, an integrated approach in sorbent utilization is needed. This integrated approach requires understanding the interactions among various pollutants during a multicomponent sorption process. The purpose of this work was to study the simultaneous sorption/removal of sulfur oxides, alkali metal vapors, and lead compound vapors. Since various methods and sorbents are available for in-situ SO2 removal, this study is focused on the development of a sorbent optimized for metal vapor capture but capable of removing residual SOz, which is not removed by other methods. In addition to understanding the interactions of the sorption reactions, the objective was to develop, characterize, and evaluate effective multi-functional sorbents for simultaneous removal of these compounds.
Experimental Approach Apparatus. A schematic diagram of the experimental setup is shown in Figure 1. The setup consists of three sections. The first section is for preparing a simulated flue gas (SFG) with a controlled composition and flow rate. This section
consists of mass flow controllers, moisture introduction cell, and a gas mixer. The simulated flue gas then enters metal vapor sources. These are heated chambers containing the source metal (alkali or lead) where the evaporation rate can be controlled by temperature in each zone independently. These sources have been designed and tested to make sure that the metal vapor delivery rate to the inlet gas and the reactor remains constant during the course of an experiment. The second section is the reactor system equipped with a recording electronic balance (Cahn, Model 2000) for online monitoring of the sorbent mass during each experiment. The reactor contains the sorbent sample suspended from one arm of the electronic recording balance. The quartz reactor is inside the movable electric furnace which can be moved up and down for rapid startup and quenching of the reaction. The third section is the gas analysis and data acquisition. The gas analysis section consists of the gas chromatograph and a pulse fluorescence SO2 analyzer. Other analytical equipment used include a scanning electron microscope equipped with energy dispersive X-ray and scanning auger microprobe for detailed study of captured metal and sulfur distribution in the sorbent particles. Atomic absorption spectroscopy and X-ray diffraction spectroscopy were used for sorbent elemental and chemical characterization. Sorbent Materials. Four different sorbents were used in this study. All sorbents are mixtures of naturally occurring aluminosilicate compounds. The calcium was added by selective impregnation of the sorbent with lime. The compositions of the four sorbents are given in Table 1. For the reaction rate experiments, the sorbent powder was pressed into thin disks and cut into 5 x 3 x 1 mm flakes with an average mass of about 80 mg. VOL. 29, NO. 6,1995 I ENVIRONMENTAL SCIENCE &TECHNOLOGY
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TABLE 1
Composition of Multi-Functional Sorbents (wt %) oxide
constituents Si02 A1203
CaO
Fez03
Ti02 MgO K20 NazO
sorbent 1
sorbent 2
sorbent 3
sorbent 4
34.20 54.08 8.40 2.00 1.32
9.12 69.90 17.00 3.98
34.22 49.89 10.38 3.75 0.15 1.02 0.44 0.04 0.11
58.00 26.64 10.37 1.97 1.17 1.13 0.57 0.05 0.10
others
Experimental Procedure. In all experiments, the flue gas consisted of 80% Nz,15% COZ, 3% Oz, and 2% H20. Unless specified otherwise, the SO2 concentration in all SOz sorption experiments was 300 ppm. In a typical experiment, first the composition of the simulated flue gas and then the temperatures of the reactor and the metal source were stabilized. The metal source temperature was used to control the rate of evaporation and the metal vapor delivery to the reactor. Using controlled source temperature, the metal delivery rate and concentration remained very stable during each experiment. However, variations of 5-15% in metal vapor concentration (at the sorbent location) from run to run were inevitable. The methods for measurement and calibration of the metal vapor concentration in the reactor are given elsewhere (7-11). The average metal concentrations were 70 pprn for sodium, 55 ppm for potassium, and 60 ppm for lead. During the startup stage, metal source and sorbent reactor were hot; however, the gas flow was directed away from the sorbent to prevent any sorption reaction. To start the sorption process, the gas flow switched and directed to the sorbent section of the reactor. The sorption process was monitored by on-line recording of the sample mass change as well the SO2 concentration in the reactor effluent. In all experiments, the gas flow rate in the reactor and around the sample flakes was maintained at 440 mL/min. Systematic study of the effect of flow rate on the overall rate has shown that, under these conditions, the overall rate does not change with the flow rate; therefore, the mass transfer resistance at the flake-gas interphase is negligible.
Results and Discussion The experiments were designed to determine the removal characteristics of alkali metals, lead, and sulfur dioxide, first separately and individually and then in combination and simultaneously. Adsorption of Individual Compounds. The SO2 removal efficiency of the four sorbents is shown in Figure 2. The results show that the rate of adsorption decreases with time due to the increase in the diffusional resistance in the products layer. Sorbent 1 has the best performance both in having a high initial rate and in maintaining the sorption rate and capacity for a long time. Due to its superior performance, sorbent 1was used in most of the subsequent experiments. The time scale of the removal process is long in these experiments. This is due to the use of large flakes (disks) of sorbents instead of small particles. Sorbent in the particle form is not used in the rate measurements because using powder requires some form of support like a suspended pan: this adds complex and poorly defined mass transfer 1662 1 ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 29, NO. 6,1995
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70
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ua 40 e, 2 30 c,
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100 200 300 400 Reaction Time (min)
0
500
FIGURE 2. Sorbent utilization during SO, removal ( T = 800 O C ) .
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0
a
u
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75 100 125 150 175 200 Reaction Time (min)
FIGURE 3. Effect of SO, concentration on sorption temporal profile ( T = 800 OC).
as well as contact effects. The geometry of large disks is selected in these experiments because it allows the use of sorbent with a well-defined geometry as well as the suspension of the samples for negligible external masstransfer resistance. This geometry is also suitable in the modeling work aimed at determining the fundamental sorption kinetic properties from the raw data; this work is in progress. Using these models, the results can then be extended to determine the sorption characteristics of particles of various sizes. The time scale of the removal process depends on the process configuration. When sorbents are used in the semi-batch mode (a recirculating fluidized bed or a bed for after-treatment of the flue gas), the typical reaction time is large (minutes or hours). However, when sorbents are used as small particles for in-situ injection, the reaction time is in seconds, and the relevant kinetics are obtained from the initial slope of the curves similar to those given in Figure 2. The rate increases with the concentration of SO2 as shown in Figure 3.
0
2
4
6
e
10
Reaction Time (hr) FIGURE 4. Temporal profile of NaCl sorption.
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5
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0.00 R e a c t i o n Time (hr) FIGURE 5. Temporal profile of lead sorption.
Figure 4,which is the temporalprofile of sodium chloride capture, represents the typical sorption of alkali by the sorbent. The fractional weight change represents grams of alkali adsorbed per gram of sorbent. The reaction rate decreases slowly as time goes on and increases with temperature. The primary effect of temperature appears to be in changing the diffusional resistance. Two factors contribute to the observed temperature effect: (1)increase in the alkali mobility as temperature increases; this results in an increase in the observed rate with temperature; (2) pore closure and loss of intraphase area and porosity; this results in a decrease in the rate. The combination of these two opposing factors determines the overall observed temperature effect. The adsorption of lead is shown in Figure 5. The detailed micrographsof partiallyreacted sorbent particles show that the products layer in the case of lead sorption (lead aluminosilicate) is in a molten state at the reaction temperature. This is in contrast with the sorption of alkali, which generates a porous solid product layer. Lead
diffusion toward the sorbent surface is an ionic diffusion through a nonporous molten silicate layer. This diffusion is more activated than the diffusion of alkali in the porous aluminosilicate layer. This explains the reason for the significant increase in the sorption rate of lead from 800 to 1000 "C. The sorption of metals on,the sorbent is a combination of three processes: adsorption or condensation, transport to the reaction site, and reaction with the sorbent. The resistance to the transport of the metals increases as the sorption reaction progresses. However, there is a difference between lead and alkali regardingthe nature of the product layer and the primary resistance against the transport of the metal to the unreacted sorbent. To analyze this difference, elemental maps of the cross sections of partially reacted sorbent disks were obtained typical micrographs are shown in Figure 6. The results show that in the case of lead, the formation of the molten product layer (lead aluminosilicate) causes pore plugging of the void between the grain particles. This generates diffusional resistance in the sorbent disk and a distinct drop in the lead concentration from the edge to the center of the sorbent disks. Lead in the vapol; phase diffuses through this molten layer to reach the unreacted sorbent. In the case of alkali, there is no melt formation; therefore, disks remain porous, and the primary diffusional resistance is that of the product layer in the grains. This results in an almost uniform distribution of alkali across the sample disk. The comparison of product structures and the sorption processes for lead and alkali is illustrated in Figure 7. Adsorption of Multiple Compounds. In most practical applications of sorbents, a number of contaminants (adsorbates) are present and influence the sorption process. In most cases, the interaction ofvarious compounds during simultaneous sorption is complex and not understood. In fact, the simultaneous sorption has received very little attention in sorbent studies. The present study is the first phase of a comprehensive multi-pollutant sorbent development that is underway in our laboratory. The comparison between the rates of sodium chloride, potassium chloride, and the combined chlorides is shown in Figure 8. The rate of sorption of KC1is significantlyhigher than that of NaCl, even though the concentration of KCl (55 ppm) is less than the concentration of NaCl(70 ppm). The rate of combined sorption of the mixed chlorides (70 ppm NaCl 55 ppm KCI) is less than additive, indicating some inhibition effect on potassium sorption. Comparing the sorption profiles of NaCl with that of NaClIKCl, it appears that in the mixed chlorides the rate of KC1 sorption is reduced to almost that ofNaCl and that the mixed sorption rate is almost proportional to the total alkali concentration. Results on the combined adsorption of sodium and lead are shown in Figure 9. This figure shows the adsorption profile for NaCl alone, PbC4 alone, the expected (calculated) profile if the simultaneous adsorption were simply additive (no interactions between the sorbates), and finally, the observed profile for simultaneous sorption while the interactions are present. The comparison of profiles shows a positive interaction (synergistic) effect for the first 4 h of the reaction. It appears that the formation of a molten lead product layer enhances the alkali diffusion in the product layer, therefore increasing the rate of sorption. The results on the combined adsorption of sodium, lead, and SO2 are shown in Figure 10. The combined adsorption is due to the additive effects of adsorbing SO2 on calcium
+
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AGUREi 6. Distribution of lead, alkali, and sulfur inside the panially reacted sorbent samples yeacted co~e
Lead SorpUon AMI Somion FIGURE 7. Comparison of lead and alkali sorption processes.
Reaction Time (hr) FIGURE 8. Combined sorption ol NaCl and KCI I T = 8Dl "C).
sites while sodium and lead react with the aluminosilicate sites. The results indicate apositive (synergistic)interaction between the SOz and the metal sorption processes. The synergismisduetotwoeffects: Oneisthe panidconversion of metal chlorides to sulfates. Since sulfates have a lower vaporpressuresas compared to chlorides, they may partially condense after formation. The condensation is particularly 1 O M m ENVIRONMENTAL SCIENCE k TECHNOLOGY I VOL. 29. NO. 6.1995
favored in the micropores of the sorbent particles. Condensation of sulfate removes metal and SOz hom the gas phase simultaneously; it is also the first step toward the metal sorbent reaction. The other possible cause of synergism is the catalytic effect of metals on the S02-lime reaction.
0.10 I
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I
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0
NaCl Adrorption
0 PbCIL Adsorption
-
vV NaCl, PbCl, Combined Sum of 0 and 0 0
5 10 R e a c t i o n Time (hr)
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FIGURE 9. Comparison of NaCI, PbClh and the combined Na/PbClz sorption profiles ( T = 800 OC).
OS3 M
2
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FIGURE 10. Effect of SO2 on the combined removal of NaCl and PbCI2
( r= 800
Acknowledgments This work was partially supported by VTT, Technical Research Centre of Finland, and LIEKKI 2 Combustion and GasificationResearch Programme. Financial support from the U.S. Department of Energy and the Arizona Mining and Minerals Resources Research Institute is also acknowledged.
’
e,
.c,
particular, it is suitable for the simultaneous removal of alkali, lead, and SOz. When used under multi-pollutant sorption conditions, no negative interaction or loss of activity due to competitive interactions was observed. In fact, the interactions in some cases are positive and synergistic. The presence of the lead molten phase assists the diffusion and reaction of alkali. Sulfur oxides assist the sorption process by increasing the rate of metal condensation that precedes the reaction. Metals also may catalyze the rate of S02/calciumreactions. In the developed sorbent, the aluminosilicate matrix acts as structure modifiers increasing the rate and the extent of calcium utilization.
oc).
Conclusions The new aluminosilicate sorbent with calcium additive formulated in this study is effective in simultaneous removal of several impurities from typical combustion gases. In
literature Cited
(1) Borgwardt, R. H. NChEJ. 1985, 31 (l),103. (2) Gavalas, G. R.; Edelstein, S.; Flytzani-Stephanopolous,M.; Weston, T. A. NChEJ. 1987, 33 (2), 258. (3) Cho, M. H.; Lee, W. K. J. Chern. Eng. Jpn. 1983, 16 (2), 127. (4) Simons, G. A.; Garrman, A. R.; Boni, A. A. NChEJ. 1987,33 (2), 211. (5) Kramlich, J. C.; Pershing, D. W.; Silcox, G. D.; Payne, R.; Chen, S. L. Presented at Combustion Institute MtglWestem States Section, Tucson, AZ,Oct 1986. (6) Shadman, F.; Dombek, P. E. Can./.Chem.Eng. 1988,66 (6),930. (7) Punjak, W. A.; Uberoi, M.; Shadman, F. NChEJ. 1989, 35 (7), 1186. (8) Rizeq, G.; Shadman, F. Chem. Eng. Commun. 1989, 81, 83. (9) Uberoi, M. High Temperature Removal of Metal Vapors by Solid Sorbents, Ph.D. Dissertation, University of Arizona, 1989. (10) Punjak, W. A.; Shadman, F. Thermochim. Acta 1988, 131, 141. (11) Ubeori, M.; Shadman, F. Ind. Eng. Chem. Res. 1991,30 (41,624. (12) Jaanu, K.; Orjala, M. Presented at Second International Conference on Combustion Technologies for Clean Environment, Lisbon, Portugal, July 1993. (13) Ross, J. S.; Anderson, R. J.; Nagarajan, R. Energy Fuels 1988, 2 (31, 282. (14) Uberoi, M.; Shadman, F. AIChEJ. 1990, 36 (9), 1433. (15) Uberoi, M.; Shadman, F. Enuiron. Sci. Technol. 1991, 25 (3, 1285.
Received for review October 31, 1994. Revised manuscript received February 2, 1995. Accepted March 3, 1995.@ ES940673F @Abstractpublished in Advance ACS Abstracts, April 15, 1995.
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