A New Method to Minimize High-Temperature Corrosion Resulting

Energy & Fuels 2003, 17, 191-203

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A New Method to Minimize High-Temperature Corrosion Resulting from Alkali Sulfate and Chloride Deposition in Combustion Systems. I. Tungsten Salts† Keith Schofield* Materials Research Laboratory, University of California, Santa Barbara, California 93106-5121 Received July 24, 2002. Revised Manuscript Received October 22, 2002

Based on a recently ascertained understanding of the parameters that control flame generated deposition of Na2SO4, NaCl, or Na2CO3 onto cooled surfaces immersed in flames, a new process has been developed to inhibit their formation. It is seen that no alkali sulfate is formed if tungsten salts are added to a flame containing an alkali (sodium or potassium), sulfur, and chlorine, when the tungsten-to-alkali gas-phase ratio is larger than about 2-fold on an atomic basis. Instead, the alkali exhibits a greater affinity for the tungsten and benign alkali polytungstates are deposited. The presence of chlorine appears to have a negligible effect on these systems. This modified behavior is further confirmed in experiments in which an Na2SO4 deposit is initially formed on a probe and then is seen to be fully converted by the addition of sufficient tungsten to the flame and overlaid by a similar tungstate growth. For sodium, the preferred polytungstates formed are Na2W2O7 and Na2W4O13, explaining to some degree the required W/Na addition ratios. Deposition appears to reflect the relative condensed phase thermodynamic stabilities of these salts and follows the order Na2W4O13 > Na2W2O7 > Na2WO4 > Na2SO4 > NaCl > Na2CO3 . In the case of potassium, behavior is very similar and the dominant polytungstate produced is K2W6O19, with some contribution from K2W3O10 . Conversions can occur in the direction of greater stability but are irreversible. The method is insensitive to fuel type, equivalence ratio, or general flame parameters. Deposits have been acquired on stainless steel and platinum clad probes operating in the temperature range 600 to 900 K. Analysis has utilized Fourier transform Raman spectroscopy and inductively coupled plasma atomic emission spectroscopy. An examination of potential interferences for the tungsten addition also has been made. This concerns whether any other element might have a greater affinity for the tungsten over that of the alkali. Calcium, strontium, and barium appear to be the most likely to fall into this potential category having well-defined and stable tungstates. Initial experiments with and without calcium addition do, in fact, show an interference with evidence of CaWO4 formation and a need for enhanced additions of tungsten. The process, outlined in detail herein, appears to be the first viable active solution for preventing Na2SO4- and NaCl-induced corrosion in a variety of combustion systems. Coupled to present techniques for reducing burned gas alkali concentrations, the method appears to be economically feasible.

Introduction High-temperature corrosion of metallic components in a wide range of combustion systems has always been a severe practical engineering problem.1-5 As a result, there is a rich literature of operational observations. Quite early, the corrosion was identified as resulting primarily from the alkali sulfates, Na2SO4 and K2SO4. Corrosion is facilitated by the molten phase and, * Phone: (805) 681-0916. Fax: (805) 893-8797. E-mail: combust@ mrl.ucsb.edu. † Presented at the Western States Section of the Combustion Institute, held at the University of California at San Diego, March 25-26, 2002. (1) Stringer, J. High Temp. Technol. 1985, 3, 119-141. (2) Meadowcroft, D. B.; Stringer, J. Mater. Sci. Technol. 1987, 3, 562-570. (3) Gupta, A. K.; Immarigeon, J.-P.; Patnaik, P. C. High Temp. Technol. 1989, 7, 173-186. (4) Stringer, J. J. Phys. IV. Colloque 1993, 3 (C9), 43-61. (5) Bryers, R. W. Prog. Energy Combust. Sci. 1996, 22, 29-120.

although these two have melting points of 1157 and 1342 K, respectively, the formation of eutectic mixtures with, for example, CoSO4, NiSO4 and other depositing relatively inert molecules dramatically lowers these values. The action of the molten alkali sulfate appears to be its ability to dissolve off the scale and protective oxide layers, leaving the metal substrate exposed and vulnerable to attack. The action of chlorine still is not entirely clear.4,6-9 It appears to enhance the corrosion yet NaCl generally is not observed together with (6) Barkalow, R. H.; Pettit, F. S. Proceedings of the Symposium on High-Temperature Metal Halide Chemistry; Hildenbrand, D. L., Cubicciotti, D. D., Eds.; The Electrochemical Society, Inc.: Princeton, NJ, 1978; Vol. 78-1, pp 617-633. (7) Chlorine in Coal: Proceedings of an International Conference, Coal Science Technology 17; Stringer, J., Banerjee, D. D., Eds.; Elsevier: Amsterdam, 1991. (8) Salmenoja, K.; Makela, K.; Hupa, M.; Backman, R. J. Inst. Energy 1996, 69, 155-162. (9) Sorell, G. Mater. High Temp. 1997, 14, 207-220.

10.1021/ef0201681 CCC: $25.00 © 2003 American Chemical Society Published on Web 11/26/2002

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Na2SO4.1,4 One of its roles may stem from the actual mechanism of the alkali deposition. This is now known to be totally controlled by the influx of the alkali, that can be in either atomic or molecular forms. Once on the surface, depending on the specific molecules at hand, the alkali instantaneously forms, for example, either the sulfate, chloride, or carbonate. If, as is generally the case, sulfur is present in the burnt flame gases at a concentration level more than half that of the alkali, then the result will be dominant sulfate formation.10 In other words, any chloride or carbonate that may be momentarily formed in the first instant will be rapidly converted to sulfate and release chlorine or CO2. This release of chlorine in the vicinity of the corroding layer might partly explain its supporting role. The general feeling, however, is that chlorine has often received the blame for some problems that were really associated more with the alkalies. In fact, if the chlorine concentrations are less than 3000 ppm in coal on a dry weight basis, there appear to be no associated problems directly related to it.4,7 It is true, however, that many elements have quite volatile chlorides and a major role of chlorine in solid fuel combustion is the enhancement of volatility of many trace elements that would otherwise have been retained to a greater degree in bottom and fly ash components.11 No direct solution for this alkali sulfate corrosion problem exists, the difficulty being that the levels of sodium and potassium need to be NaCl > Na2CO3 > NaOH. Even in H2/air flames, the CO2 content of the air is more than sufficient to produce Na2CO3 in systems free of sulfur or chlorine.39 Attempts have been made to force the system into producing NaOH. This utilized ultrahigh purity H2, O2, and N2 gases and was partially successful, but it is difficult to remove all the dissolved CO2 from the small traces of water used in the nebulized salt solution. Such experiments did confirm, however, that NaOH, which has a greater alkalinity than Na2CO3, was in fact being produced.39 Rates of deposition are surprisingly similar for all the alkali metals (lithium through cesium) despite their 20fold difference in atomic weight. A normal boundary layer diffusional model appears inadequate in describing the deposition process.10,39 Results for potassium show that it closely mirrors the behavior of sodium. The controlling aspect of the thermodynamics is evident in experiments where the flame ingredients are modified. For example, if an Na2CO3 deposit is initially laid down on the probe and then chlorine is added to the flame, not only is an NaCl deposit formed but also the initial Na2CO3 layer converts fully to NaCl. Then

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on adding sulfur, this produces Na2SO4 and converts all the previous layers to sulfate. Done in reverse, however, a layered structure results with a first layer of Na2SO4 being covered by one of NaCl and in turn by Na2CO3 . Conversions occur up the chain of preference but are irreversible. It is with this level of understanding that it has now been possible to proceed further and assess potential candidate additives that might disrupt this chemistry. An appraisal has been completed of all the elements of the periodic table to find any sodium and potassium salts that may be more thermodynamically stable than Na2SO4 in a combustion environment. Numerous such potential candidates have been identified and tested experimentally to examine whether Na2SO4 formation can be inhibited by such an approach. Conceptual Approach As mentioned already, deposition of alkali sulfate has been shown to be zero-order with respect to sulfur concentration down to levels that are one-half the concentration of the alkali.10 In other words, attempts to minimize corrosion by reducing sulfur levels are fruitless and would need essentially a total removal. The observed deposition behavior indicates that thermodynamic stabilities appear to play a major role in the outcome. Consequently, if a molecule can be found that is more thermodynamically stable than the alkali sulfate, then it may be preferentially formed. An analysis of relative sulfate thermodynamic stabilities in the periodic table indicates that the five alkalies are among the most stable sulfates, with only those of calcium, strontium, and barium having comparatively similar magnitudes of stability.44 As a result, the concept of nullifying the sulfur with some other metal and making it unavailable to the alkali appears not to be possible. Moreover, it would be necessary to remove all the sulfur in this way, which would require an unfeasibly large addition of additive. Rather than trying to remove or tie up the sulfur, another concept is to bind the alkali such that it no longer reacts with the sulfur or chlorine. Table 1 is a representative list of sodium molecules that contain one other element of the periodic table besides oxygen. Some of these elements, in fact, form a series of sodium polyanionic salts. The question is whether any of these are more thermodynamically stable than Na2SO4. To help minimize the necessary number of experiments needed to answer this question, various approaches have been taken. An initial simple elimination could be made of the molecules that are known to be weakly bound and thermally unstable at the operational temperatures of about 700 to 1200 K. Also, past flame experiments have shown already that the alkali sulfates are preferentially formed over their carbonates, nitrates, hydroxides, oxides, or halides, so that these needed no further assessment. Some of the other potential additive elements could be tested by running thermodynamic equilibrium calculations for appropriate flames containing sodium/ (44) Stern, K. H.; Weise, E. L. High-Temperature Properties and Decomposition of Inorganic Salts. I. Sulfates; National Standard Reference Data Series - National Bureau of Standards, Report No. NSRDS-NBS 7, 1966.

Schofield Table 1. Potential Sodium Metal Salts Whose Thermodynamic Stabilities Have Been Assessed Relative to That of Na2SO4 either by Calculation or Experimentally for Flame Deposition Conditions NaAlO2b NaAsO2c,e NaAsO3c,e Na2HAsO4a Na3AsO4b NaBO2b NaB3O5b Na2B4O7b Na2B8O13b NaBiO2a NaBiO3a Na3BiO4c,e NaBrd Na2CO3d Na2CeO3c,e NaCld NaClO3a NaClO4a

NaCrO2a,d Na2CrO4d Na2Cr2O7a NaFb NaFeO2d Na2GeO3c,e Na2HfO3c,e NaId Na2IrO3c,e NaMnO2c,d NaMnO4a,d Na3MnO4c,d NaMoO2a,d Na2MoO4d Na2Mo2O7d Na2Mo4O13d NaNO3a NaNbO3c,d

NaO2d Na2Od Na2O2a NaPO3b NaH2PO4a Na3PO4b Na4P2O7b Na2PbO3c,e Na2PrO3c,e Na2PtO3c,e NaReO4a,d NaSbO3c,e Na3SbO4c,e Na2SeO3c,e Na2SeO4c,e Na2SiO3b Na2Si2O5b

Na4SiO4b Na6Si2O7b Na2SnO3c,e NaTaO3c,d Na2TbO3c,e Na2TeO3c,e Na2TeO4a,c Na2TiO3b Na2Ti2O5b Na2Ti3O7b NaVO3b Na3VO4b Na4V2O7b Na2WO4d Na2W2O7d Na2W4O13c,d Na2ZrO3d

a Not sufficiently stable thermally. b Eliminated by thermodynamic equilibrium calculations; thermochemical data are available for all except those that are so indicated. c No thermochemical data currently available. d Tested experimentally in this program. e Considered unlikely candidates from the general periodic table trends noted in the results obtained thermodynamically and experimentally.

sulfur and the additive element in various proportions and in this probe temperature range for fuel-rich or -lean conditions. This of course can only be done if a sufficient database of readily available thermochemical values is available for the element and all its appropriate stable molecules that might conceivably be involved. Such thermodynamic equilibrium calculations, but on a much larger scale, now are emerging for the partitioning of trace elements in coal combustion,45,46 and very extensive efforts that include up to 54 elements and 3200 species have recently appeared.47 However, the interpretation of such equilibrium calculations, particularly at lower temperatures, requires care and can be open to question. For burned flue gases that are uniformly cooling and adjusting to chemical equilibrium, the exercise can generally be of some value, except that in reality kinetic limitations will be controlling the molecular distributions. The situation of a flame or hot gases impinging onto a much cooler surface is far more complex, and equilibrium expectations can really lose all meaning. The flame can supply atoms and radicals that thermodynamically should not be present at such cooler temperatures. Nevertheless, in the present situation it is hoped that the thermodynamic stability differences are such that they will at least indicate trends, particularly at the highest probe temperatures. It is true, though, that uncertainties will remain in some cases. Consequently, for the molecules indicated in Table 1 for which thermodynamic data are available,48-50 (45) Yan, R.; Gauthier, D.; Flamant, G.; Badie, J. M. Fuel 1999, 78, 1817-1829. (46) Furimsky, E. Fuel Process. Technol. 2000, 63, 29-44. (47) Yan, R.; Gauthier, D.; Flamant, G. Combust. Flame 2001, 125, 942-954. (48) Chase, M. W., Jr.; Davies, C. A.; Downey, J. R., Jr.; Frurip, D. J.; McDonald, R. A.; Syverud, A. N. JANAF Thermochemical Tables, Third Edition; J. Phys. Chem. Ref. Data 1985, 14, Supplement No. 1. (49) Thermochemical Properties of Inorganic Substances, 2nd ed.; Knacke, O., Kubaschewski, O., Hesselmann, K., Eds.; SpringerVerlag: Berlin, 1991. (50) Barin, I.; Platzki, G. Thermochemical Data of Pure Substances, 3rd ed.; VCH Publishers: Weinheim, Germany, 1995.

High-Temperature Corrosion Mitigation Method

these with the help of the NASA Lewis PAC 97 Code,51 were added to the thermodynamic library used by the 1995 NASA CEA Thermodynamic Equilibrium Calculation Code.52 Their corresponding oxides, sulfides, sulfates, etc., also were added if these were not already in the database library. As indicated in Table 1, it was possible to consider numerous cases in this way. For the majority of those so tested, it was clearly apparent that the formation of such mixed metal salts would be very unlikely and the added element generally would prefer to produce some other molecule such as its own oxide, halide, or sulfate. The sodium would be ever present as Na2SO4. Only in the case of tungsten addition was there clear evidence from these preliminary calculations that Na2WO4 and Na2W2O7 might displace Na2SO4 as a result of an apparently greater thermodynamic stability. In an interesting paper, Mobin and Malik53 studied the interactions of TiO2, ZrO2, Nb2O5, Ta2O5, MoO3, and WO3 with Na2SO4 at 900-1200 K in flowing oxygen. These oxides were primarily chosen to represent the initial scaling products during the oxidation of industrial alloys and coatings. In each case, thermogravimetric measurements indicated the weight loss, assumed to result from SO2 emission. Although all showed some slight trace loss in weight, those observed for the Nb, Ta, and W systems were quite pronounced and it appeared that chemical modifications were occurring in these three cases. A previous study by them54 had similarly examined the interactions between the abovementioned oxides, as well as Al2O3, SiO2, Cr2O3, Fe2O3, Co3O4, and NiO, with NaCl at 800-1200 K in flowing oxygen. These experiments conveyed that many of the combinations react and form a variety of sodium metal oxides or metal halides in preference to NaCl. With tungsten trioxide, in particular, there was a pronounced loss of chlorine with Na2WO4 being produced. Nb and Ta also behaved similarly in this manner but also produced their halides NbCl2 and TaCl4. In numerous other examples, namely for Ge, Sn, Pb, Sb, Se, Te, Hf, Nb, and Ta, no appropriate thermochemical data are readily available. After considering the general trends that were apparent for all the other elements in the periodic table, it was decided that of these, only Nb and Ta had any probability of being more stable in their sodium/metal oxide forms than Na2SO4. As a result, it was decided to test experimentally in flames the combinations of Na/S/Cl with either Cr, Mn, Mo, Nb, Re, Ta, W, or Zr, in some cases to confirm the thermodynamic expectations and in others to examine if sodium salts other than Na2SO4 could be produced. Experimental Section The flame burner system has been described previously10,43 and provides for a well-defined one-dimensional atmospheric pressure flame. C3H8/O2/N2 and H2/O2/N2 flames have been (51) McBride, B. J.; Gordon, S. Computer Program for Calculating and Fitting Thermodynamic Functions; NASA Lewis Research Center, Cleveland, OH, Reference Publication NASA RP-1271, 1992. (52) Gordon, S.; McBride, B. J. Computer Program for Calculation of Complex Chemical Equilibrium Compositions and Applications. I. Analysis. II. Users Manual and Program Description; NASA Lewis Research Center, Reference Publication NASA RP-1311, Part I, 1994, NASA RP-1311, Part II, 1996. (53) Mobin, M.; Malik, A. U. J. Alloys Compd. 1996, 235, 97-103. (54) Mobin, M.; Malik, A. U.; Ahmad, S. J. Less-Common Metals 1990, 160, 1-14.

Energy & Fuels, Vol. 17, No. 1, 2003 195 used over a range of fuel-rich and -lean equivalence ratios. To produce measurable probe deposition rates in a reasonable length of time (generally 1-6 h), alkali flame concentrations of 10-30 ppm have been found most adequate. Metals are introduced as an aqueous salt solution aerosol from an ultrasonic nebulizer55 and solution strengths of about 0.05 N produce the required densities with minimal loss in the heated delivery line or heated burner during mixing with the unburned gas flows. Solutions, for example, of NaNO3, Na2S2O3 (as a source of both sodium and sulfur), KNO3 , ammonium metatungstate (3(NH4)2WO4‚9WO3), and Ca(NO3)2 have been used and found to be mutually compatible. In sodium salt experiments, a trace amount of potassium was added, K/Na ) 0.5%, to facilitate monitoring of the optically thin potassium resonance line flame emission as a measure of constant nebulizer delivery to the flame. When testing the behavior of potassium, a similar trace of sodium would facilitate this monitoring. In the present situation, when other metals have been added together with the alkali, suitable soluble salts were used such that a compatible mixed salt solution of the metal salt and the alkali salt could be aspirated. The ultrasonic nebulizer used has been reported as producing an aerosol of about 1-5 µm droplet size.55 In such a case, these droplets that also will have partially evaporated in the dry unburned gas flow en route to the burner are known to be even smaller and to be fully dissociated in passing through the flame’s reaction zone. It is often forgotten that although ionic, such salts dissociate into their gaseous neutral parts, forming initially the atomic metal and the unstable neutralized anion which then breaks apart further. This is the lowest energy dissociation channel. Even the addition of Na2SO4 nebulized salt solution has been shown to be a fully equivalent source both of elemental sodium and sulfur in the burnt flame gases. In cases where no water-soluble salts are available, such as with tantalum and niobium, experiments were formulated to use their sodium salts, NaTaO3 and NaNbO3, in a different manner. To alleviate such difficulties in the future, an alternate ultrasonic nebulizer now is being constructed. This will permit the use of organometallic salts dissolved in an organic solvent and allow a more extensive examination of, for example, the flame chemistries of Nb and Ta. Gas flows were controlled by calibrated electronic mass flowmeters. Small quantities of sulfur and chlorine were added using certified cylinder mixtures of 0.1% SO2 in N2, and 514 ppm Cl2 in N2. It is now well documented that whether SO2 is used or some other source of sulfur such as H2S, thiophene, or an inorganic salt such as Na2SO3 or Na2S2O3, at low levels of addition these all behave as equivalent sources in the burned gases. The same has been found for chlorine, and Cl2, HCl, or a chloroorganic produce equivalent distributions in the high-temperature burned gases of either fossil- or hydrogenfueled flames. Several cylindrical deposit collection probes have been used. These generally are about 12 mm in diameter and essentially interact with all of the seeded inner core burner flow at some point. They are of Inconel-600 stainless steel with a central channel for air or water cooling and have a built-in thermocouple to monitor the surface temperature. One was very tightly clad with a two micron thick foil of platinum to obtain cleaner deposits free of corroded probe materials. The probe was mounted horizontally in the vertical burned gas flows and the burner was raised or lowered by a computerized steppermotor. In this way, samples could be collected on the probe surface at various downstream times. In the propane flames mainly used in this program, these times would generally be of the order of 2-10 ms from the reaction zone. Probe surface temperatures have been in the 600-900 K range. Probe deposits have been characterized using a Nicolet Fourier transform Raman spectrometer that uses a 1.06 µm (55) Denton, M. B.; Swartz, D. B. Rev. Sci. Instrum. 1974, 45, 81-83.

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excitation source. Occasionally an alternate in-house built Raman system also has been used with a shorter wavelength Ar+ laser line excitation. This is necessary at times if crystal luminescence is evident that interferes with the Raman spectrum. This was the case particularly with CaSO4 deposits and several other metal systems when using the longer wavelength laser. After collecting a deposit on the probe, the flame is extinguished with ultrapure N2 and the probe is cooled in such a flow. The sample then is removed from the probe while remaining in this flow and sealed in a 1.5 mm diameter capillary glass tube for analysis. Contact with moisture or air is minimal. Rates of deposition in this system are about 2 mg/h of sodium and generally 5 mg is more than sufficient for an accurate spectral analysis. Rates of deposition were measured by dissolving the deposit off the probe with deionized water and analyzing the solution for Na, K, other metals and S utilizing a Thermo Jarrell Ash High-Resolution ICP emission spectrometer using carefully matched calibration solutions. Analysis for certain metals such as Na, K, and Ca also were obtained with a flame photometric Beckman analytical burner and values compared favorably with those acquired with the ICP. In such cases, monitoring sensitivity was very high, accurate, and reproducible results could be obtained for deposits collected in 1 h. Chlorine content was analyzed using an Orion Chloride Electrode calibrated against standard solutions.

Results Preliminary Screening Experiments. The major thrust of this program has been to identify an element that might combine with the alkali and nullify its preference for sulfur or chlorine. The preliminary screening of equilibrium calculations indicated that the most likely candidates would arise in the first few transition element groups (IVB-VIIB) of the periodic table with tungsten being a prime candidate. A compatible aqueous mixture of a sodium salt and the metal salt were nebulized as a fine aerosol spray, mixed with the fuel-rich or -lean unburned gases, and flowed into the burner together with trace additions of SO2 and chlorine. Such experiments are typified by that for zirconium illustrated in Figure 1. A mixed solution of NaNO3 and zirconium oxynitrate together with 40 ppm SO2 were used to test whether Na2SO4, Zr(SO4)2, or Na2ZrO3 might be predominant. The resulting flame deposit is readily identified as being composed solely of Na2SO4 and monoclinic ZrO2. There is no hint of the zirconate or any other sulfate. It is obvious that zirconium is incapable of modifying the predominance of Na2SO4 formation. The slight differences seen in the weak Raman lines in the 1080-1200 cm-1 region of Figures 1a and 1b are inconsequential but of some interest. Na2SO4 has two low-temperature solid phases. The purchased compound Na2SO4(V) is the stable room-temperature phase below 458 K. This is indicated in Figure 1 and is seen to compare with the flame deposit of Figure 1b. Above 517 K, Na2SO4(III) becomes the stable phase and if cooled in the absence of moisture can remain in this metastable state at room temperature. This apparently occurred in the Figure 1a flame sample which reflects this Na2SO4(III) Raman spectral scattering pattern.58 It appears that the flame sample of Figure 1b managed to largely relax to the Phase(V) form during handling. (56) Nyquist, R. A.; Putzig, C. L.; Leugers, M. A. Infrared and Raman Spectral Atlas of Inorganic Compounds and Organic Salts. Vol. 2. Raman Spectra; Academic Press: San Diego, 1997.

Schofield

Figure 1. Raman spectra of deposited probe material (relative intensities): (a) fuel rich flame, φ ) 1.2 (C3H8/O2/N2; 1.2:5: 20), containing 40 ppm SO2 and an atomic ratio Na/Zr ) 2:1 (Na = 30 ppm). NaNO3 and zirconium oxynitrate solutions used. Platinum probe at 600 K, located 11 ms downstream. (b) φ ) 0.9 (0.9:5:16) flame containing 75 ppm SO2, Na/Zr ) 2:1 (Na = 20 ppm) and a probe temperature of 850 K, 4 ms downstream. The deposits are Na2SO4 and monoclinic ZrO2, as indicated by comparison with their known spectra.56,57

Similar negative results to that of Zr addition were obtained for corresponding additions of Cr, Mn, and Re salts. This was expected for Mn and Re in that NaMnO4 and NaReO4 are not thermally stable at these probe temperatures. The case of Mo proved to be unexpectedly interesting and will be discussed at length in Part II of this paper series. In experiments where Na/Mo flame additions were made in the ratio of 2:1 with excess SO2, it was seen that exactly the same result could be reproduced by first coating the probe with Na2MoO4 and inserting it in a flame seeded only with the SO2 addition. The observed conversions were significant and also illustrated another potential testing approach. This proved to be very useful in the cases of niobium and tantalum where it was difficult to follow the normal procedure. Their salts are all insoluble in water. Instead, in these two cases, thin layers of their NaNbO3 or NaTaO3 salts were coated onto the probe surface and then immersed in a flame containing sulfur. Evidence of Na2SO4 formed by conversion was not apparent even after long exposure times. It appears that these two elements remain in their preferred states and cannot be converted by sulfur. Of all the elements, however, tungsten and molybdenum appeared to be the most appealing in mitigating the formation of the alkali sulfate, and are readily available in a wide variety of chemical sources that can be easily handled. Niobium and tantalum salts also appear to be viable candidates but are commercially less attractive. Experiments with Tungsten Salt Addition. As a result of these preliminary experiments, a much more detailed study was carried out for tungsten. Ammonium metatungstate, 3(NH4)2WO4‚9WO3, is very soluble in (57) Kim, B.-K.; Hahn, J.-W.; Han, K. R. J. Mater. Sci. Lett. 1997, 16, 669-671. (58) Hapanowicz, R. P.; Condrate, R. A., Sr. Spectrosc. Lett. 1996, 29, 133-141.


Figure 2. Raman spectra of purchased samples of Na2SO4, Na2WO4, WO3, and synthesized samples of Na2W2O7, Na2W4O13 (relative intensities).

water and was primarily used as the nebulized source of tungsten. It was also found to be miscible with other salt solutions. Figure 2 illustrates the types of Raman spectra displayed by some of the various possible molecules that might be formed in Na/W/S flame systems. This figure also shows the powerful analytical ability of Raman spectral scattering for solids. Such spectra of inorganic solids generally are relatively simple and the resulting sharp vibrational lines or broader bands clearly stand out as a specific fingerprint for a particular molecule. In the present work, these spectra generally have been recorded with a 4 cm-1 resolution and frequencies of the strongest spectral features such as the 993 cm-1 line of Na2SO4 can readily be distinguished not only from that of K2SO4 at 984 cm-1 and other sulfates, but more importantly from the lower frequencies observed with the various tungsten molecules. Anhydrous sodium tungstate has a relatively simple Raman spectrum characterized by three sharp frequencies at 927, 811, and 311 cm-1. However, the molecule has the ability to incorporate additional units of WO3 into its formulation and form polytungstates. As indicated in the well-characterized phase diagram of Na2O/WO3 mixtures at various temperatures, sodium can form such stable structures as Na2WO4, Na2W2O7, Na2W4O13, etc.59,60 Potassium behaves similarly but can also produce the 3WO3 molecule K2W3O10.59,60 The crystal structures of these seven tungstates are characterized by different space groups and so have differing Raman spectral features.61,62 Although not commercially available, these polytungstates are easily synthesized. For example, heating mixtures of Na2WO4/WO3, Na2WO4/ H2WO4, or Na2CO3/H2WO4 in the correct proportions60,63,64 each produced an identical Raman spectrum of Na2W2O7 (melting point 1019 K). Its Raman spec(59) Caillet, P. Bull Soc. Chim. Fr. 1967, 4750-4755. (60) Chang, L. L. Y.; Sachdev, S. J. Am. Ceram. Soc. 1975, 58, 267-270. (61) Fomichev, V. V.; Poloznikova, M. E.; Kondratov, O. I. Russ. Chem. Rev. 1992, 61, 877-888. (62) Range, K.-J.; Hegenbart, W.; Heyns, A. M.; Rau, F.; Klement, U. Z. Naturforsch. B 1990, 45, 107-110. (63) Okada, K.; Morikawa, H.; Marumo, F.; Iwai, S. Acta Crystallogr. 1975, B31, 1200-1201. (64) Knee, F.; Condrate, R. A., Sr. J. Phys. Chem. Solids 1979, 40, 1145-1146.

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trum, reproduced in Figure 2, is in full agreement with that previously published.64,65 As a result, all of the simple polytungstates of sodium and potassium were synthesized and their Raman spectra recorded to facilitate flame deposit compositional analysis. Tungsten has an interesting chemistry in that when solutions of sodium tungstate are acidified with mineral acids, sodium paratungstate crystals are produced. This can have the structure Na6W7O24, xH2O (3Na2WO4‚ 4WO3) (Paratungstate A) or when aged in solution transforms to Paratungstate B, Na10W12O41, 28H2O (5Na2WO4‚7WO3). After longer aging, the dominant crystal becomes the metatungstate, Na6W12O39, xH2O (3Na2WO4‚9WO3).66,67 These salts are not thermally stable and the paratungstate dissociates at 733 K into Na2W2O7 and Na2W4O13.68 Samples of the tetratungstate, Na2W4O13, melting point 1108 K, have been synthesized by heating mixtures of either Na2WO4/WO3 or Na2CO3/WO3 in the correct proportions60 at 1000 K for 180 h followed by a slow cool. Identical spectra were obtained also by heating sodium metatungstate at 925 or 1025 K for 20 h, followed by a slow cool. The thermal dissociation of sodium metatungstate appears not to have been reported, but its formulation of Na6W12O39 is proportionally correct to convert directly to Na2W4O13. This reflects the pyrolysis behavior of the paratungstate mentioned above but produces solely the one product. As a result of the exact agreement between the four spectra so obtained, the Raman spectrum in Figure 2 is labeled as Na2W4O13 with a high degree of confidence. The Raman shift frequencies also closely agree with those tabulated by Fomichev et al.61 It displays two equally strong transitions at 948 and 776 cm-1, the latter being quite broad and an easily recognized characteristic of this molecule. Because of their thermal instability, salts such as the alkali para- and metatungstates are not expected to play any role in combustion chemistry or deposition. Sodium paratungstate currently is not readily available commercially but its Raman spectrum has been published.56,69 I have recorded that for sodium metatungstate and it displays a very similar spectrum with its strongest band at 980 cm-1 (as compared to 965-970 cm-1) and showing the same pattern of unresolved structures to shorter frequencies. Unfortunately, halides such as NaCl and KCl are Raman-inactive and so have to be analyzed for separately. In the collection of flame-generated deposits of Na2SO4, it is normal, particularly on a platinum-surfaced probe, to obtain a thick, smooth layer of a white, low-density, powdery nature. However, on adding tungsten salts to a sodium/sulfur flame system it was apparent immediately that there was a very different behavior. The nature of the deposit altered and was no longer white and fluffy but more compact with a glittering metallic appearance of a grayish white color. Figure 3 illustrates a series of experiments in which varying concentration (65) Becher, H. J. Z. Anorg. Allg. Chem. 1981, 474, 63-73. (66) Liu, S.; Chen, Q.; Zhang, P.; Li, S. Trans. Nonferrous Met. Soc. China 1998, 8 (4), 688-692; Chem. Abstr. 1999, 130, 204321t. (67) Smith, B. J.; Patrick, V. A. Aust. J. Chem. 2000, 53, 965-970. (68) Liu, S.; Chen, Q.; Zhang, P.; Liu, M. Wuli Huaxue Xuebao 1998, 14 (9), 821-825; Chem. Abstr. 1998, 129, 339073p. (69) Hartl, H.; Palm, R.; Fuchs, J. Angew. Chem., Int. Ed. Engl. 1993, 32, 1492-1494.

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Figure 3. Raman spectra (relative intensities) showing the flame inhibition of Na2SO4 formation (evident by its 993 cm-1 line). Sodium, tungsten salts, and sulfur are added to a φ ) 0.9 (C3H8/O2/N2; 0.9:5:16) flame with sodium flame concentrations of about 25 ppm. Samples collected 3.5 ms downstream on a platinum clad probe with one exception. (a) Na/W ) 2:1 atomic ratios, 75 ppm SO2, stainless steel probe, 850 K, (b) Na/W ) 1:1, 25 ppm SO2, 900 K, (c) Na/W/S ) 1:1.5:1, 900 K, (d) Na/W ) 1:2, 25 ppm SO2, 850 K.

ratios of Na/W were added to sulfur-bearing flames. In Figure 3a with an atomic ratio Na/W ) 2:1, Na2SO4 formation (the first line from the left in the spectrum at 993 cm-1) is still apparent but is reduced in intensity from that observed with no tungsten salt addition. Tungstate lines along with the three characteristic frequencies of Na2WO4 (927, 811, and 311 cm-1) can clearly be identified as being due to Na2W2O7. It is immediately clear that a competition for the alkali is occurring between the sulfur and the tungsten. By taking an equimolar mixture of purchased Na2WO4 and Na2SO4 salts it is apparent that the Raman scattering efficiency is approximately 1.4 times more efficient for the tungstate (927 versus 993 cm-1 lines) at 1.06 µm. Taking the relative intensities for these two lines in Figure 3a (the peak of the Na2WO4 is clipped in that shown), that for the tungstate is 1.7 times stronger, implying that the Na2WO4/Na2SO4 molecular ratio in this deposit is about 1.2:1. This is approximate as it assumes that the relative densities of the powdered salts in purchased samples is the same as in the flame deposits. Even so, the spectrum indicates formation of solely Na2SO4, Na2WO4, and Na2W2O7, clearly showing the presence of the latter with its characteristic Raman pattern at lower frequencies. From the input ratio of Na/W ) 2:1 and this determination of the ratio of Na2WO4 to Na2SO4, the mass balance equations can be solved. Consequently, Figure 3a tends to suggest that at 850 K the deposit composition is approximately 31% Na2SO4, 38% Na2WO4, and 31% Na2W2O7. As seen in Figure 3b, as the amount of tungsten is increased to Na/W ) 1:1, the spectrum changes and the presence of Na2SO4 is noticeably reduced. Na2WO4 is no longer present and there are indications now solely of Na2SO4, Na2W2O7, and Na2W4O13. In Figure 3a the concentrations of Na2SO4 and Na2W2O7 appeared to be approximately equal, as were the intensities of their respective 993 cm-1 (Na2SO4) and 832 cm-1 (Na2W2O7) lines. This suggests that the scattering efficiency of these two lines is essentially equal. Extending this fact into Figure 3b and assuming the mass balance for

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Figure 4. Raman spectra of deposits obtained from flames utilizing a platinum clad probe, each containing Na/W ) 1:1 at about 30 ppm each. (a) and (b) refer to a φ ) 0.9 (C3H8/O2/ N2; 0.9:5:16) flame, 4 ms downstream with the probe at 900 K. Both contain 25 ppm SO2 but (b) also has the addition of 74 ppm Cl2. (c) 0.9:5:20 flame with 30 ppm SO2 and 50 ppm Cl2. The probe, 6 ms downstream, is at 600 K. Each shows a similar slight presence of Na2SO4 as indicated by its 993 cm-1 line, the first peak from the left in the spectra.

sodium and tungsten is constituted by Na2SO4, Na2W2O7, and Na2W4O13, the implied composition is 8% Na2SO4, 84% Na2W2O7, and 8% Na2W4O13. The formation of Na2SO4 ties up some of the sodium, enabling some of the tungsten to form the tetratungstate. Increasing the tungsten addition further so that Na/W ) 1:1.5 essentially prevents Na2SO4 formation as seen in Figure 3c. There is possibly a slight hint of its existence (e2%) and all spectral features reflect the frequencies of Na2W2O7 and Na2W4O13 as illustrated in Figure 2. Using Figure 3b to get an approximate relative scattering efficiency of these two molecules, the observed intensities suggest a composition of NaCl > Na2CO3. Adding sulfur to a flame that has been depositing NaCl or Na2CO3 results solely in a final Na2SO4 deposit. Similarly, chlorine converts Na2CO3 deposits to NaCl. In this present effort, a similar behavior could be induced by the presence of tungsten salts, further confirming its dominant control. This is shown in Figure 7 that illustrates the results of two such experiments. Figure 7a indicates the Raman spectrum of the deposit resulting from an experiment where initially a deposit of Na2SO4 was collected with Na/S in the flame in the ratio of 1:1. After this period, tungsten was additionally added to the flame for a period of time such that overall the additions were equivalent to Na/ S/W ) 1:1:1. As seen, analysis of the final deposit indicated no evidence of Na2SO4. That initially laid down was converted to an essentially pure deposit of Na2W2O7. Figure 7b reflects the result of a similar experiment. In this case, the flame initially contained Na/W ) 2:1 with 75 ppm SO2. Then, for the same length of time, collection continued without tungsten but with sodium and sulfur in the flame. Na2SO4, the first peak from the left in the spectrum (993 cm-1) is clearly evident. However, it appears that the situation is complex. The initial deposit should reflect that of Figure 3a which was for similar conditions and indicated almost equal amounts of Na2SO4, Na2WO4, and Na2W2O7. That of Figure 7b shows enhanced Na2W4O13 formation together with some Na2W2O7 and the expected NaSO4. The expected simple layering of sulfate over the initial tungstate layer does not occur, but a readjustment takes place in the full deposit and the apparently more stable (73) Jensen, D. E.; Miller, W. J. J. Chem. Phys. 1970, 53, 3287-3292. (74) Farber, M.; Srivastava, R. D. Combust. Flame 1973, 20, 33-42.

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Figure 7. Raman spectra of flame deposits collected about 3.5 ms downstream. (a) On either stainless steel or platinumclad probes at 900 K, collected first with an atomic ratio Na/S ) 1:1 in the flame and then with Na/S/W ) 0.67:0.67:1 for a length of time such that overall the ratio of Na/S/W ) 1:1:1. φ ) 0.9, C3H8/O2/N2 (0.9:5:14). (b) On a platinum-clad probe at 825 K. Collected initially with Na/W ) 2:1 and 75 ppm SO2 and then similarly continued but without the tungsten for the same length of time, φ ) 0.9 (0.9:5:16). Sodium flame concentrations were about 30 ppm throughout.

Na2W4O13 is formed to counteract the presence of the increased Na2SO4. Similar effects have been seen with potassium ,and the laying down of a K2SO4 deposit on top of a preformed polytungstate layer forces the lower layer to modify slightly. Consequently, it is apparent that there can be a constant readjustment of the deposit on the surface if conditions change and a modified distribution is preferred by the chemistry. In this obvious competition between sulfur and tungsten for the alkali, it is seen that it depends on having sufficient tungsten for a dominance of the alkali polytungstates. The same conversion effects have also been recorded by burning flames with Na/S/Cl2 initially and then with only tungsten in the flame. Again, there is no trace of the initial Na2SO4 formation. Consequently, the previously established preference of, for example, Na2SO4 > NaCl > Na2CO3 now must be preceded with Na2WO4, or more appropriately by the di- and tetratungstates. This applies equally to the deposition chemistry of potassium. Conversions occur in the direction of the most stable alkali molecule and are not reversible. Thermodynamic Considerations. Some uncertainties appear to remain in the value for the heat of formation of Na2WO4(s). The NBS Tables75 recommend a value of -1548.9((8.4) kJ mol-1 at 298 K, that is supported by an empirical correlation analysis76 and an emf cell measurement.77 More recent measurements based on the reaction of liquid sodium with WO3 derive values of the free energies of formation of Na2WO4 that imply a greater stability and a heat of formation that is 40-90 kJ mol-1 more negative.78,79 Very recently, (75) Wagman, D. D.; Evans, W. H.; Parker, V. B.; Schumm, R. H.; Halow, I.; Bailey, S. M.; Churney, K. L.; Nuttall, R. L. The NBS Tables of Chemical Thermodynamic Properties: Selected Values for Inorganic C1 and C2 Organic Substances in SI Units, J. Phys. Chem. Ref. Data 1982, 11, Supplement No. 2. (76) Hisham, M. W. M.; Benson, S. W. J. Phys. Chem. 1988, 92, 6107-6112. (77) Lin, R. Y.; Elliott, J. F. Metall. Trans. 1983, 14A, 1713-1720. (78) Krishnamurthy, D.; Bhat, N. P.; Mathews, C. K. J. Chem. Thermodyn. 1991, 23, 581-591.


the thermodynamic properties of Na2W2O7(s) have been measured80 and are anchored by its heat of formation. The latter is the accepted value75 and is quoted with an accuracy of 3.3 kJ mol-1.81 Both ∆Hf values for Na2WO4(s) and Na2W2O7(s)75 were determined by reactive solution calorimetery82 in the same study but are otherwise independent. The reaction

Na2WO4(s) + WO3(s) ) Na2W2O7(s) is spontaneous and samples of Na2W2O7(s) have in fact been made in this study by heating such ingredients together. Assessments have been made of the free energy change for this reaction up to 1000 K using the accepted thermochemical values for WO3 and Na2W2O7. This indicates that the value for ∆Hf(Na2WO4) can at the most be no more than 25 kJ mol-1 more negative than the NBS Tables value. As a result, the occurrence of this reaction appears to be incompatible with the more recently determined smaller values.78,79 Equilibrium calculations for the flame deposits confirm the greater stability of Na2W2O7 over Na2WO4 when the ratio of W/Na increases above 0.5. This is reflected in the Raman spectra of many of the flame deposits when it is somewhat difficult to observe the lines of Na2WO4 and the spectra of Na2W2O7 and Na2W4O13 are most commonly encountered. Observations in fact suggest that Na2W4O13 is the most stable of the three. One consequence of this is the everincreasing melting point of the tungstate deposit, from 970 K (Na2WO4) to 1019 K (Na2W2O7) and 1108 K (Na2W4O13),60 but still less than that of Na2SO4 (1157 K). Whether these deposits are controlled by thermodynamic considerations or not, thermochemical predictions do mirror the data and illustrate a greater stability of the polytungstates over Na2SO4 in these situations. Thermodynamic values for the case of potassium are not as available. Knacke et al.49 have listed values for K2WO4(s) but no data yet exist for the corresponding K2W2O7(s), or the polytungstates K2W3O10, K2W4O13, etc. The thermodynamic data considering K2WO4(s) alone in fact suggests that tungsten might fail in competing with sulfur for potassium. This is reflected in the data in that Raman frequencies of K2WO4 have not been observed in any of these deposits when sulfur is present. However, stability appears to increase in the alkali polytungstates and, as shown in Figure 6, K2W4O13 and more complex stoichiometries become increasingly dominant with sufficient tungsten. The consequence of this also will be a pronounced increase in polytungstate deposit melting point, K2W2O7 (957 K), K2W3O10 (1115 K), K2W4O13 (1185 K), and K2W6O19 (1237 K),60 but again less than K2SO4 (1342 K). However, in practical systems, the dissolution of other metals into these structures becomes a noted modifier, particularly of such melting points. Interferences. It has been clearly illustrated that tungsten has a greater affinity for alkalies than does (79) Bhat, N. P.; Borgstedt, H. U. J. Nucl. Mater. 1997, 244, 59-65. (80) Liu, S.; Chen, Q.; Zhang, P. Thermochim. Acta 2001, 371, 7-11. (81) Naumov, G. B.; Ryzhenko, B. N.; Khodakovsky, I. L. Handbook of Thermodynamic Data; Atomizda: Moscow, 1971; U.S. Geological Survey Engl. Transl., 1974, NTIS Report PB-226722. (82) Koehler, M. F.; Pankratz, L. B.; Barany, R. U.S. Bureau of Mines, Report of Investigation, RI 5973, 1962.


sulfur. It also appears to be a potentially viable additive for preventing alkali sulfate formation. It certainly succeeds in this role in the environment of a relatively clean experimental laboratory flame system. An important question, however, is whether there may conceivably be other trace elements in practical combustors that have a greater affinity for tungsten than do the alkalies. This could minimize its effectiveness or require too large a compensatory additive quantity. As a result, an analysis of thermodynamic stabilities of tungstates of the periodic table has been assessed. Available data, however, are limited and many tungstates either tend not to exist or have not been thermodynamically characterized. Also, with respect to numerous transition elements such as Cr, Hf, Mo, Nb, Ta, and Zr, for example, these can be absorbed by Na2WO4 and the polytungstates to form alkali tungstate bronzes. Such chemistries can become extremely complex.83,84 Nevertheless, the only metals identified at present that appear to have specific tungstates of significant stability are those of calcium, strontium, and barium. Calcium, for example, can form CaWO4 and Ca3WO6, which are thermodynamically well characterized.49,50 Using the available thermodynamic data, free energy values can be assessed for reactions such as CaSO4 + Na2WO4 ) CaWO4 + Na2SO4 ∆G°700K ) -66 kJ mo1-1 ∆G°1000K ) -71 2CaSO4 + Na2W2O7 ) 2CaWO4 + Na2SO4 + SO2 + 0.5O2 ∆G°700K ) +94 kJ mo1-1 ∆G°1000K ) +4

Consequently, although it seems that Na2WO4 is insufficiently stable relative to CaWO4, Na2W2O7 and undoubtedly Na2W4O13 appear thermodynamically to be able to compensate for this potential interference. As a result, numerous flame experiments have examined deposits from flames containing Na or K with Ca/W/S and Cl2 mixtures. Two such results are indicated in Figure 8 together with the measured Raman spectra of anhydrous CaSO4 and CaWO4 that agree with previously published values.56,85,86 These are readily recognized by their sharp and intense lines at 1018 and 911 cm-1, respectively. In the case of sodium, Figure 8a, with Na/W/Ca ratios of 1:2:2, a pronounced interference is seen. CaWO4 formation is dominant and small amounts of Na2SO4, Na2W2O7, and Na2W4O13 are evident. The corresponding run for potassium, Figure 8b, with similar ratios again shows CaWO4, but K2W6O19 also is present with a little K2SO4. These spectra have yet to be quantified to assess the exact reapportionment of the alkali/Ca/W. Although the relative Raman scattering cross sections have not yet been measured, there appear to be too little Na2SO4 and K2SO4 formed raising the question of where is the alkali. Clearly no CaSO4 or CaCO3 are present. (83) Gmelin Handbook of Inorganic Chemistry, 8th ed.; SpringerVerlag: Berlin, 1990. (84) Lassner, E.; Schubert, W.-D. Tungsten: Properties, Chemistry, Technology of the Element, Alloys and Chemical Compounds; Kluwer Academic: New York, 1999. (85) Griffith, W. P. J. Chem. Soc. A 1970, 286-291. (86) Berenblut, B. J.; Dawson, P.; Wilkinson, G. R. Spectrochim. Acta A 1973, 29, 29-36.

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tions with other elements. The experiments of Mobin et al.53,54 have been discussed already wherein mixtures of Ta2O5 or Nb2O5 were heated with either Na2SO4 or NaCl, suggesting the greater stabilities of NaTaO3 and NaNbO3 . Similarly, Belyaev et al.89 have heated mixtures of NaTaO3, NaNbO3, KTaO3, and KNbO3 with Ag2SO4 and Tl2SO4 at 1070 K. They did observe extensive exchange with the formation of Na2SO4 and K2SO4. Species such as silver and thallium appear to be potential interferents in these cases, but these are rarely encountered in practical combustion systems.87,88 However, when so little data are available some other more relevant interference might exist. One such case could be with calcium. Room-temperature free energy values are available75,81 for the reaction

2NaNbO3 + CaSO4 ) Ca(NbO3)2 + Na2SO4 Figure 8. Raman spectra of purchased samples of anhydrous CaSO4 and CaWO4, together with those of flame deposits collected 3.5 ms downstream on a Pt-clad probe at 750 K. Atomic ratios in the burned gases are Na/W/Ca ) 1:2:2 in case (a) that utilized NaNO3, Ca(NO3)2, and ammonium metatungstate aqueous solutions. (b) The corresponding experiment but with KNO3 replacing NaNO3. Both for φ ) 0.9 (C3H8/O2/N2; 0.9:5:16) flames containing 75 ppm SO2 and 50 ppm Cl2. The alkali flame concentration is about 10 ppm.

These results appeared to be at odds with earlier measurements that were testing whether the presence of calcium in the flame modified preexisting deposits or whether deposits were affected by underlying calcium deposits. These experiments involved burning a flame first with Na/W/S and then continuing with Ca/S, or doing the same but in the reverse order. Overall, the experiments in essence involved Na/W/Ca additions in the ratios of 2:1:1 in the presence of excess sulfur and chlorine. In such cases, Na2W4O13 was dominant in both modes and the presence of calcium did not appear to affect the process. However, it does now appear that there can be additional competitive interactions by calcium vying for the tungsten. In practical combustors, the alkaline earths can be present at concentration levels comparable to or larger than the alkalies. However, most of their concentration is removed in bottom and fly ashes.87,88 Consequently, in such cases, it will be more beneficial or necessary to inject tungsten salts after initial fly ash separation. Applications to fluidized bed combustors based on a limestone sorbent are probably unlikely. The full nature of these interactions with calcium clearly need further study. As to whether Sr or Ba may also constitute an interference has yet to be studied, but these are of lesser significance in practical combustion systems. Niobium and Tantalum Salt Addition Potential. Little has been said of the corresponding behavior of additions of Nb and Ta which also appear to have a greater affinity for the alkalies than does sulfur. Only a few experiments have been attempted at present due to the insolubility of their salts in water. The tantalates and niobates are much less characterized than the tungstates, and little thermodynamic data are available. Various studies, however, have examined their interac(87) Ondov, J. M.; Ragaini, R. C.; Biermann, A. H. Environ. Sci. Technol. 1979, 13, 946-953. (88) Querol, X.; Fernandez-Turiel, J. L.; Lopez-Soler, A. Fuel 1995, 74, 331-343.

∆G°298 K ) -19 kJ mol-1 suggesting a potential interference. However, the situation at higher temperatures and the possibility of significantly more stable polyniobates or tantalates remains unknown.83,90 Even so, the economical and supply aspects in the cases of niobium and tantalum tend to exclude them from further consideration as potential practical additives. This is not the case with tungsten. Its global supply now extends to over 40 000 tons/year. Moreover, because additions of tungsten are molecularly on a par with those of the alkali, and on a ppm scale, requirements particularly in those cases utilizing alkali reduction techniques are quite minimal. Incurred requirements and costs consequently appear to be quite economically feasible. Concluding Remarks It is quite remarkable that, although alkalies and sulfur may be present in flames at concentrations of only several parts per million, they end up together chemically bound on a cooled surface immersed in the burned gases. High-temperature corrosion, induced by such alkali sulfate formation, is in fact the consequence of such very small trace impurities. The present work has shown that this process can be disrupted by the addition of tungsten salts to the flame in atomic quantities that are preferably about two times or more the concentration of the alkali in the flame. This not only prevents normal sulfate formation but should also alleviate the additional problems that result from chlorine in many cases. Sodium and potassium behave very similarly, both forming polytungstates. The process appears to be a viable solution with regard to the mitigation of hightemperature corrosion problems that relate to alkalies and has been patented as such.40 Particularly in cases where burned gas cleanup can be invoked to partially reduce the levels of gaseous alkalies, only low-level additions of tungsten salts will be required to neutralize the otherwise deleterious corrosive actions. Such trace additions are a significant aspect in making this an economically very acceptable approach. More practical full scale testing of this process now is required. (89) Belyaev, I. N.; Lupeiko, T. G.; Nalbandyan, V. B.; Abanina, E. V. Russ. J. Inorg. Chem. 1978, 23, 18-22. (90) Bridgeman, A. J.; Cavigliasso, G. J. Phys. Chem. A 2001, 105, 7111-7117.


Another very noteworthy aspect of this work has been to illustrate the dominance of the heterogeneous surface chemistry in these flame deposition processes. In the present case, the surface fully controls the chemistry and uses the flame solely as a source of ingredients. The outcome on the surface can be totally independent of the gas phase and its composition, although occasionally it is true that flame equivalence ratio can play some role. Nevertheless, the general indications are, as is the case here, that detailed studies of the gas-phase flame


chemistry may be of little, if any, value, in understanding and predicting deposition behavior. Acknowledgment. This work made use of the Material Research Laboratories Central Facilities which are supported by the Materials Research Section Program of the National Science Foundation under Award No. DMR00-80034. EF0201681

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