Energy & Fuels 1988,2, 702-708
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straightforward matter to include the contribution of this factor to the chemical potentials and hence obtain expressions that can be used to interpret swelling and molecular-weight measurements with more precision. In conclusion, we have derived expressions for the contribution of hydrogen-bond formation to the free energy of mixing molecules. This has proved successful in predicting the phase behavior of synthetic polymers containing phenolic OH groups,3oand there is no reason to believe that the general approach should not apply to coal. The model predicts that low-molecular-weight, low-aromaticity coal macromolecules should be soluble in pyridine,
indicating that such coals must be cross-linked. Higher aromatic, high-molecular-weight molecules phase separate in solution into solvent-rich and solvent-poor swollen coal gels, however, indicating that many successive extractions would be required to remove the bulk of such non-crosslinked material from a particular sample. Acknowledgment. We gratefully acknowledge the support of the Office of Basic Energy Sciences, Division of Chemical Sciences, Department of Energy, under Grant NO. DE-FG02-86ER13537. Registry No. Pyridine, 110-86-1.
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Aluminosilicate Sorbents for Control of Alkali Vapors during Coal Combustion and Gasification W. A. Punjak and F. Shadman" Department of Chemical Engineering, University of Arizona, Tucson, Arizona 85721 Received February 1, 1988. Revised Manuscript Received May 10, 1988
Kaolinite is found to be a suitable sorbent for the removal of alkali from hot flue gases. The kinetics and mechanism of adsorption of NaCl vapor on kaolinite were studied a t 800 OC under both nitrogen and simulated flue gas (SFG) atmospheres. Under nitrogen, both chlorine and sodium were retained by the sorbent; however, under the simulated flue gas, only sodium was retained. In both cases the adsorption was irreversible. High-resolution scanning Auger analysis of kaolinite particles indicated the formation of a product layer during adsorption. Under the SFG atmosphere the product layer appears to be nephelite, a stable sodium aluminosilicate compound. The rate of adsorption dropped with the increase in alkali loading, and a maximum saturation limit was observed. In the SFG environment this saturation capacity was approximately 5 times greater than that under the nitrogen atmosphere. An analytical model is presented that facilitates the extraction of fundamental kinetic information from experimental results. The model allows for surface adsorption as well as diffusion through both the saturated product layer and pores of the active sorbent.
Introduction A significant fraction of the alkali-metal compounds present in coal are vaporized during combustion or gasifi~ation.'-~The released alkali vapors are the precursors of hot condensates that cause corrosion of various parts of the combustors, gasifiers, and the downstream systems for secondary energy r e c ~ v e r y . For ~ ~ ~example, the corrosion caused by alkali vapors is important in combined cycle processes where the hot flue gases come in contact with turbine materials. In such applications the alkali concentration should be reduced to less than 50 ppb to prevent significant hot corrosion. One of the promising techniques for removal of alkali from hot flue gases is by using solid sorbents. Various studies have considered the feasibility of passing the flue gases through a fixed-bed filter of appropriate sorbents.H This process seems to be quite feasible for application in pressurized fluidized-bed combustion before the turbine section. The use of additives for in situ capturing of alkali during pulverized coal combustion has also received some attention.1° Most of the previous studies have concentrated on the selection of sorbents and the overall feasibility of the
* To whom correspondence should be addressed 0887-0624 I88 12502-0702$01.50 I O I
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process. The mechanism of adsorption and, particularly, the fundamental kinetics of adsorption are not well understood. In recent years, there have been some studies of the fundamentals of alkali adsorption on various sub(1)Raask, E. Mineral Impurities in Coal Combustion; Hemisphere: New York, 1985; Chapter 7. (2) Huffman, G. P.; Huggins, F. E. In Mineral Matter and Ash i n Coal; Vorres, K. S., Ed.; ACS Symposium Series 301; American Chemical Society: Washington, DC, 1986; Chapter 8. (3) Berkowitz, N. The Chemistry of Coal; Elsevier: New York; Chapter 9. (4) Brio, R. W.; Levasseur, A. A. In Mineral Matter and Ash in Coal; Vorres, K. S., Ed.; ACS Symposium Series 301; American Chemical Society: Washington, DC, 1986; Chapter 21. ( 5 ) Raask, E. Mineral Impurities in Coal Combustion; Hemisphere: New York, 1985;Chapter 17. (6) Lee, S. H. D.; Johnson, I. J.Eng. Power 1980, 102, 397. (7) Lee, S. H. D.; Henry, R. F.; Smith, S. D.; Wilson, W. I.; Myles, K. M. Alkali Metal Vapor Removal from Pressurized Fluidized-Bed Combustor Flue Gas; ANL-FE-86-7;Argonne National Laboratory: Argonne, IL, 1986. (8) Bachovchin, D. M.; Alvin, M. A.; DeZubay, E. A.; Mulik, P. R. A
Study of High Temperature Removal of Alkali in a Pressurized Gasification System; DOE-MC-20050-2226;Westinghouse Research and Development Center: Pitssburgh, PA, 1986. (9) Jain, R. C.; Young, S. C. LaboratorylBench Scale Testing and Eualuation of A.P.T. Dry Plate Scrubber; DOE-ET-15492-2030;Air
Pollution Technology: San Diego, CA, 1985. (10) Klinzing, G. E.; Blanchere, M.; Weintraub, M.; Shannon, S.; Martello, D., University of Pittsburgh, personal communication.
1988 American Chemical Society
Aluminosilicate Sorbents
Energy & Fuels, Vol. 2, No. 5, 1988 703
Table I. Composition of As-Received Sorbents (Volatile-Free Basis) composition, w t % bauxite' kaoliniteb Si02 7.0 52.1 A1203 88.3 44.9 Fez03 1.2 0.8 Ti02 3.5 2.2
'Engelhard Corp. Burgess Pigment Co. s t r a t e ~ . ~ , &However, l~ there is no agreement in the literature on the nature and mechanism of adsorption. Lee and Johnsons studied the adsorption of NaC1, KC1, and K2S04on a number of substrates including bauxite, silica, and diatomaceous earth. They proposed chemical fixation on silica and physical adsorption on bauxite. They also found activated bauxite to be a very efficient sorbent. In another study, Lee et al.13 reported that the adsorption of alkali on activated bauxite was by physical adsorption and chemical fixation, the latter being dominant in the presence of sufficient water vapor. Luthra and LeBlancll measured the extent of adsorption of KCl and NaCl on bauxite under different temperatures and oxygen concentrations. Their results indicated nonpreferential and reversible physical adsorption of both compounds on alumina and activated bauxite. Bachovchin et al.* compared a number of additives including emathlite, which was found to be a very suitable adsorbent a t temperatures below 900 "C. The overall process was explained as a reaction with the adsorbent controlled by diffusion of alkali through a layer of reaction product around the emathlite particles. The choice of a suitable sorbent depends on many factors dealing with the coal properties and the process operating conditions. In general, however, the important characteristics desired in a potential sorbent are as follows: rapid rate of adsorption; high loading capacity; transformation of alkali into a less corrosive form; irreversible adsorption to prevent the release of adsorbed alkali during process fluctuations. Previous studies by us and other investigators are still inadequate to compare various potential sorbents systematically on the basis of the above characteristics. In particular, there is very little data on the fundamental kinetics and mechanism of adsorption that can be used for this kind of systematic evaluation. The purpose of the present work was to explore some of these fundamentals related to aluminosilicate type adsorbents. Kaolinite has not been studied as extensively as bauxite or emathlite, but appears to be a very good sorbent for the capture of alkali.14J5
Experimental Approach Kaolinite was selected RB a model compound to represent a class of aluminosilicate type sorbents. In this study, two types of tests were conducted. First, a series of tests were performed to evaluate kaolinite as a sorbent and, specifically, compare it with bauxite, which is known to be a good s ~ r b e n t . ~ JThe ~ J ~second series of experiments were designed for a detailed study of the kinetics (11)Luthra, K. L.; LeBlanc, 0. H. J. Phys. Chem. 1984, 88, 1896. (12) Shadman, F.; Punjak, W. A. Presented at the Fall Meeting of the Western States Section, The Combustion Institute, Tucson, AZ, 1986. (13) Lee, S. H. D.; Henry, R. F.; Myles, K. M. Remoual of Alkali Vapors by a Fixed Granular-Bed Sorber Using Activated Bauxite as a Sorbent; CON-8503513; Argonne National Laboratory: Argonne, IL,
1985.
(14) Shadman, F.;Peterson, T. W.; Wendt, J. 0. L.; Punjak, W. A.; Rizeq, G. Mechanism of Surface Enrichment and Ahesion of Coal Combustion Particulates; second quaterly report; DE-FG22-86PC90505;US. Department of Energy (PETC): Pittsburgh, PA, 1987. (15) Wall, C. J.; Graves, J. T.; Rober, E. J. Chem. Eng. (N.Y.) 1975, (April 14), 77.
F i g u r e 1. Schematic diagram of the apparatus: GC, gas chromatograph; IR, nondispersive infrared analyzer.
E
Reactor
Hangdow Wire
Pt Pan
Alkali
(a)
:,;> 1, one can determine the initial rate of mass increase as follows:
kCAb
A schematic of the slab during the second stage is shown in Figure 10. During this stage, eq 2 is still valid for the inner core of the slab; however, in the outer layer, where there is no adsorption, the following equation would be applicable:
where DLA is the effective diffusivity in the outer layer. Equations 2 and 9 can be solved by using the boundary conditions given in eq 3 and 4 as well as the matching of concentrations and fluxes a t the boundary between the inner and the outer zones (y = y m ) . After some mathe-
Equation 12 suggests that a plot of the initial adsorption rate vs CAb/L should be linear. This plot for experiments performed in both carrier gases is given in Figure 12 and is observed to follow a straight line rather closely. From the slope of this line and the values of C*s/k determined from Figure 11, the values of 4 can be calculated for each experiment. The resulting Thiele moduli range from 5.8 to 8.7 for the nitrogen carrier gas experiments and from 20 to 26 for the SFG carrier gas experiments. Values of 4 much larger than unity indicate a large influence of diffusion on the overall kinetics. This is in agreement with the steep gradients of alkali concentration in the core zone as observed by the Auger analysis.
Punjak and Shadman
708 Energy & Fuels, Vol. 2, No. 5, 1988
1
-- O - -
3.0
0
Lo ’
1
Np
SFG
//
3.0 cAb/L
6.0 (gmo1/cm3 g a s - c m )
9.0
x
Figure 12. Dependence of the initial rate of adsorption on alkali concentration and flake size.
Conclusions Kaolinite was found to be very effective in removing vapors of both NaCl and KC1 from the gaseous products of coal combustion and gasification. In addition, the adsorption on kaolinite is irreversible. This could be an advantage over bauxite for applications where sorbent stability is important. Comparison of data for adsorption experiments under SFG and Nzshows a significant effect of gas composition on the adsorption. This seems to be primarily due to water and not oxygen. For example, the alkali-loading capacity of kaolinite under SFG is higher than that under N2. Moreover, in the Nzatmosphere both chlorine and sodium are retained upon adsorption, while in the SFG atmosphere only sodium is retained. It appears that the adsorbed NaCl reacts with kaolinite in the presence of water to form nephelite and volatile HC1. The rate of adsorption decreases with loading and approaches zero when a final saturation limit is achieved. The initial adsorption rate is proportional to the alkali concentration in the gaseous bulk and is nearly the same under Nzand SFG atmospheres. The kinetics of adsorption is largely influenced by two types of diffusion: diffusion through the adsorbent pores where adsorption is simultaneously taking place and diffusion through a saturated layer of sorbent formed on the outside of the sorbent particles. This layer expands toward the center as adsorption progresses. An analytical model that incorporates these findings is presented. The model gives a good fit to the data for conversions up to about 75%. Acknowledgment. Financial support from the Office of Fossil Energy, Pittsburgh Energy Technology Center, U.S. Department of Energy, under Grant No. DE-FG2286PC90505, is gratefully acknowledged.
/
A 0
15
30
45
Time (hr)
Figure 13. Comparison of experimental data with model predictions: (-) data; (- -) model. The parameter D was then evaluated by fitting the model to two complete temporal profiles (Figure 6). The resulting values were 0.38 for the nitrogen carrier gag runs and 0.23 for the SFG carrier gas runs. Finally, these calculated values of 4 and D were used to predict adsorption profiles for other experiments. The data and model prediction for several experiments performed with the SFG carrier gas are shown in Figure 13. In general, the model predictions agree well with the data up to about 75% sorbent saturation; beyond this point, the predictions are too high. This is expected because the local adsorption rate given by the model (eq 5) does not include the dependence on the available surface area of the kaolinite. This is particularly important as kaolinite gets depleted. A study of the adsorption kinetics for determining a more accurate rate expression is currently under way. Another conclusion suggested by the modeling results is that the alkali diffusivity in the product layer is about 3-4 times greater than the diffusivity in the flake interior. This is different from most noncatalytic gassolid reactions in which pore closure with conversion results in a lower diffusivity. A possible explanation is that saturation of the pores in the product layer results in the formation of a thin liquid film that facilitates the alkali transport in this zone.
CA CAb
CS
C*S DeA
D ’eA D k
L m mQ m€ RA
t
t*
X XT Y Ym
Nomenclature external surface area of the sorbent flakes, cm2 concentration of alkali, g mol/cm3 of gas concentration of alkali in bulk gas, g mol/cm3 of gas alkali loading, g mol of adsorbed alkali/cm3of solid sorbent the maximum value of Cs at saturation effective diffusivity of alkali in sorbent, cm2/h effective diffusivity of alkali in saturated outer layer, cm2/h ratio D,A/D ‘ e ~ rate coefficient, cm3 of gas/(cm3 of solid sorbent h) half-thickness of the slabs (sorbent flakes), cm mass of the sorbent at time t, g initial mass of the sorbent, g final mass of the sorbent at saturation, g local rate of adsorption,g mol/(cm3of bulk sorbent h) time, h duration of the first stage or onset of the saturated layer formation, h local fractional loading of sorbent, C s / C * s overall fractional loading of sorbent, (1IL)StXdy distance from the center of the slab, cm distance from slab center to the saturated layer interface, cm Greek Letters
( m f- mo)/mo € porosity of the sorbent Thiele modulus, L[k(l 4J Registry No. NaC1, 7647-14-5; KCl, 7447-40-7; kaolinite, 1318-74-7; nephelite, 12251-27-3. CY
