New Method To Minimize High-Temperature ... - ACS Publications

1898

Energy & Fuels 2005, 19, 1898-1905

New Method To Minimize High-Temperature Corrosion Resulting from Alkali Sulfate and Chloride Deposition in Combustion Systems. II. Molybdenum Salts Keith Schofield* Materials Research Laboratory, University of California, Santa Barbara, California 93106-5121 Received March 21, 2005. Revised Manuscript Received May 20, 2005

The use of fuels other than natural gas in gas turbine generators still is fraught with blade corrosion problems that result from the formation of sodium sulfate or potassium sulfate, in the presence of chlorine. The present work illustrates that the addition of molybdenum salts to synthetic fuels (synfuels) modifies this deposition process and benign protective coatings of alkali polymolybdates are produced instead. This study is a follow-up to research published earlier in this journal that showed closely similar behavior with tungsten salt additives. In the case of molybdenum, a new preferential ranking of product formation on the surface is established that is closely related to the thermodynamic stabilities, namely, Na2Mo2O7 > Na2SO4 > Na2MoO4 > NaCl and K2Mo4O13 > K2Mo3O10 > K2Mo2O7 > K2SO4 > K2MoO4 > KCl. This chemistry exists under fuel-lean conditions and is otherwise not sensitive to the fuel, combustion conditions, surface temperature, or material. The required additive trace levels of molybdenum salt are on the order of twice that of the gaseous alkali, on an atomic basis. Consequently, when used as a final pregas turbine polishing technique and following other fuel precleaning methods, the approach offers a very practical and inexpensive solution.

Introduction This is the author’s second paper in this journal that extends a method to minimize high-temperature corrosion in gas turbines that arises from the presence of gaseous alkalis in either the fuel or air.1,2 Previously (in Part I), the conceptual approach was described in detail.3 The research reported in Part I was intended to identify trace additives that might bind the alkali (primarily sodium and potassium), form a benign product, and thus prevent the formation of the alkali sulfate or chloride molecules that are responsible for the gas turbine blade corrosion. A survey of potential candidates indicated very few such possibilities, with tungsten and molybdenum salts being the most feasible. The successful bench testing with tungsten salt additions was presented. Herein, the corresponding evidence is presented that molybdenum salts behave rather similar to, and offer a valid alternative to, tungsten salts. This corresponding behavior of the two elements stems from their close relationship in the periodic table and their comparable chemistries. These potential approaches * Author to whom correspondence should be addressed. Telephone: (805)681-0916. Fax: (805)965-9953. E-mail: [email protected]. (1) Schofield, K. The Regents of the University of California, Oakland, CA, Method for the Prevention of High Temperature Corrosion due to Alkali Sulfates and Chlorides and Composition for Use in the Same, U.S. Patent No. 6,328,911, December 11, 2001. (International Application No. PCT/US01/03387.) (2) Schofield, K. The Regents of the University of California, Oakland, CA, Method for the Prevention of High Temperature Corrosion due to Alkali Sulfates and Chlorides Using Molybdenum, U.S. Patent No. 6,602,445, August 5, 2003. (International Application No. PCT/US01/03387.) (3) Schofield, K. Energy Fuels 2003, 17, 191-203.

now provide an additional means to fully polish alternate fuels down to negligible levels of alkali and, thus, permit the use of synthetic fuels (synfuels) in modern gas turbines. Alkali Corrosion in Gas Turbines Driven by environmental, economic, and political pressures, the energy supply sector is moving in a direction of utilizing cleaner fuels produced from biomass or coals.4,5 However, these raw materials can be rich in alkali content, with biomass generally having larger K/Na ratios than those observed in coals.6,7 Corrosion effects by alkali sulfates on differing blade alloys has been an ongoing study for more than 50 years and still continues.8,9 The role of chlorine in corrosion still is not fully understood. However, studies imply that an initial deposition of an alkali chloride will be converted to the sulfate in the presence of sulfur dioxide, with a resultant release of chlorine at the surface. This close proximity to the surface is considered to be the mechanism for the pronounced corrosion effects. In recent decades, the use of superalloys has been intro(4) Paisley, M. A.; Anson, D. J. Eng. Gas Turbines Power 1998, 120, 284-288. (5) Ruth, L. A. Mater. High Temp. 2003, 20, 7-14. (6) Westberg, H. M.; Bystrom, M.; Leckner, B. Energy Fuels 2003, 17, 18-28. (7) Knudsen, J. N.; Jensen, P. A.; Dam-Johansen, K. Energy Fuels 2004, 18, 1385-1399. (8) Meadowcroft, D. B.; Stringer, J. Mater. Sci. Technol. 1987, 3, 562-570. (9) Leyens, C.; Wright, I. G.; Pint, B. A. Mater. Sci. Forum 2001, 369/372, 571-578.

10.1021/ef050069y CCC: $30.25 © 2005 American Chemical Society Published on Web 07/08/2005

High-Temperature Corrosion Mitigation Method

duced as gas turbine operating temperatures have increased to improve efficiencies. Nevertheless, the requirement for low alkali concentrations has remained, because these materials still are subject to corrosion. It is estimated that levels on the order of tenths of parts per billion (ppb) are needed to ensure adequate operational lifetimes.5,8 As indicated in extensive laboratory studies of the deposition mechanisms, as long as the sulfur concentration is larger than that of the alkali, then sulfate formation will dominate.10 Moreover, because sulfur is invariably in excess of the alkalis, there are no advantages from the point of view of corrosion in attempting to reduce sulfur levels in the fuel. Although turbine blades with thermal boundary coatings are in an advanced stage of development, these will not resolve the dilemma. They are also extremely sensitive materials, in regard to fuel purity.11,12 Their porous nature will have a tendency to wick even slight traces of chemical deposits into their structure while operating at high temperatures. This will then lead to a catastrophic failure whenever the turbine is cooled. As a result, most practical gas turbines operating in a hot gas cleanup mode will need synfuel gettering with a final polishing to produce the tolerable levels of alkalis. Studies of various aluminosilicate-type absorbers indicate that high-temperature (800-1200 °C) gettering with materials such as kaolin or bentonite seems to be quite effective.13-15 The question of whether the quantities of absorber and the associated costs needed to remove all the alkali are feasible is yet to be determined. However, such an approach is certainly necessary as an initial step in removing most of the alkali. At low concentration levels of alkalis, the dominant mechanism producing the alkali sulfate is heterogeneous formation on a surface.10 Sodium and potassium are almost indistinguishable in their behavior.16 On combustion, the alkali impurity in a fuel becomes redistributed. Some can be retained in the bottom ash or in fly ash, but a substantial fraction remains in the gas phase as gaseous NaCl, NaOH, or atomic sodium. These gas phase components are all comparable precursors for this heterogeneous formation. After the sulfates are produced on a surface, they become effective fluxing agents, particularly when liquified,17 dissolving the surface protective oxide layers18 and opening the base metals to corrosive attack. Currently, this alkali sulfate formation is a major factor in preventing the use of biomass or coal gasification combined cycle methods that utilize the direct noncooldown firing of gas turbines. Presently, synfuels must be produced, cooled, and then extensively scrubbed (10) Steinberg, M.; Schofield, K. Proc. Combust. Inst. 1996, 26, 1835-1843. (11) Padture, N. P.; Gell, M.; Jordan, E. H. Science 2002, 296, 280284. (12) Schulz, U.; Leyens, C.; Fritscher, K.; Peters, M.; SaruhanBrings, B.; Lavigne, O.; Dorvaux, J.-M.; Poulain, M.; Mevrel, R.; Caliez, M. Aerosp. Sci. Technol. 2003, 7, 73-80. (13) Gale, T. K.; Wendt, J. O. L. Aerosol Sci. Technol. 2003, 37, 865876. (14) Wolf, K. J.; Muller, M.; Hilpert, K.; Singheiser, L. Energy Fuels 2004, 18, 1841-1850. (15) Tran, K.-Q.; Iisa, K.; Steenari, B.-M.; Lindqvist, O. Fuel 2005, 84, 169-175. (16) Schofield, K. Proc. Combust. Inst. 2005, 30, 1263-1271. (17) Rapp, R. A. Corros. Sci. 2002, 44, 209-221. (18) Ishitsuka, T.; Nose, K. Corros. Sci. 2002, 44, 247-263.

Energy & Fuels, Vol. 19, No. 5, 2005 1899

before they can be burned. As a result, most gas turbines remain restricted to using natural gas. With the gettering approaches being developed and the final polishing technique outlined below and in the first paper,3 it should now be possible to overcome this technological hurdle and return to developing the more-efficient hot gas cleanup combined cycle approaches. Experimental Section The same flame system as that fully described in Part I was similarly used here. Fuel-lean propane/oxygen/nitrogen (C3H8/ O2/N2) were mainly examined in the present study. Alkali flame concentrations were in the range of 10-40 ppmv (parts per million by volume) in the burned gases. Molybdenum salts were added such that atomic ratios of Na:Mo, for example, ranged from 1.0:0.25 up to 1.0:2.0 and were accurately controlled. Levels of sulfur dioxide (SO2) and molecular chlorine (Cl2) of ∼40-75 ppmv and 50 ppmv, respectively, were added when necessary to the unburned flame gases and were in excess of the metal additions. Mixed dilute aqueous solutions of alkali nitrates, ammonium paramolybdate ((NH4)6Mo7O24,4H2O, listed commercially as ammonium molybdate), and calcium nitrate have been used in an ultrasonic nebulizer and quantitatively added as aerosols to the unburned flame gases. No difference in performance was observed between deposits collected on either air-cooled Inconel-600 stainlesssteel probes or on an identical probe that was clad with platinum. Because of this, the latter mainly was used in this paper, to obtain clean deposits that were free from any probe corrosion contaminants for Raman spectral analysis. The probe was located generally ∼3-6 ms downstream from the reaction zone in a region where the burned gases are at temperatures of ∼2100-2400 K and are approaching equilibrium. Probe surface temperatures have been mainly in the 700-850 K range, which as seen later is marginally below or in the region of the melting points of the potential alkali polymolybdate product deposits. The analysis of deposits using a Nicolet Fourier transform Raman spectrometer with laser excitation with a wavelength of 1.06 µm generally has been preferred. The spectra are unique and not complex. In the majority of cases, the chemical nature of the deposit is readily identifiable by simple spectral inspection. However, it was noted in experiments that included the addition of calcium salts that these produced interfering luminescence from crystal defects in the deposit when excited at 1.06 µm. In such cases, additional analyses were run on an alternate in-house-built Raman spectrometer that is excited by a shorter-wavelength Ar+ laser line. These were observed not to be subject to the same interference. In addition, primarily because ionic halides are Raman inactive, it was necessary to monitor for their presence using a Bruker powder X-ray diffractometer. Because both of these analytical techniques are noninvasive, it was possible, when necessary, to obtain both the Raman and X-ray spectrum of the same sample. As indicated in the phase diagrams for mixtures of Na2MoO4 or K2MoO4 with MoO3, these can form a series of stable dimolybdates, trimolybdates, and tetramolybdates.19 The crystal structures of all eight such molybdates are known.20,21 However, other than the monomolybdates, the higher polymolybdates are not readily available commercially. As a result, samples of sodium and potassium dimolybdates, trimolybdates, and tetramolydates were synthesized. This involved the (19) Caillet, P. Bull. Soc. Chim. Fr. 1967, 4750-4755. (20) Fomichev, V. V.; Poloznikova, M. E.; Kondratov, O. I. Russ. Chem. Rev. 1992, 61, 877-888. (21) Iyer, V. S.; Keskar, M.; Jayadevan, N. C. In Proceedings of the 9th National Symposium on Thermal Analysis, Goa, India, 1993; pp 329-332.

1900

Energy & Fuels, Vol. 19, No. 5, 2005

Schofield

intimate mixing of powdered alkali molybdate or carbonate with MoO3 in the prescribed ratios. The two alternate approaches were useful for comparison and for enhancing the probability of success. However, as observed later, little difference was ever noted between either starting method. The mixtures were heated (750-1000 K) in covered platinum crucibles, which temperatures were either similar to or above the product melting point. A variety of times (5-66 h) were used, depending on the temperature and the nature of the product being synthesized. If the temperature used is too high, although the synthesis is more rapid, there can be potential evaporative losses that modify the stoichiometry. Also, melting with subsequent liquid creep out of the crucible was observed in some cases. Generally, it was determined better to operate at a lower temperature for longer times until the product reproduced the expected Raman or X-ray spectra. Such an approach for producing these six polymolybdates has been documented previously in the literature.19,21-28

Results Quantitative flame experiments have been designed to test the effectiveness of molybdenum, as compared to tungsten, in interfering with the deposition preferences of sodium and potassium and thereby preventing the normally observed sulfate formation. No previous similar efforts have ever been made in this connection. Numerous thermogravimetric experiments by Mobin and co-workers29,30 examined the interactions between several metal oxides including MoO3 with either NaCl or Na2SO4. Mixtures of these were heated at 900 K for up to 24 h in flowing oxygen, and the weight loss was monitored. Final remaining products then were identified qualitatively. With MoO3 and Na2SO4, a 10% evaporative loss was noted with indications of pronounced Na2MoO4 formation. Sulfides of both molybdenum and sodium also were observed. Obviously, reaction had occurred, indicating that Na2SO4 was not the most thermodynamically stable molecule under these conditions. Similar findings were reported with NaCl, with even more extensive conversion to Na2MoO4 being apparent, with possibly also some molybdenum chloride formation. In addition, in a study of K2Mo3O10, Leonyuk et al.31 successfully synthesized this material solely by heating a mixture of K2SO4 and MoO3 in the correct proportions. Consequently, there are strong indications in the literature that molybdenum does have an ability to compete with sulfur or chloride for binding alkali metals, and that the alkali molybdates can be thermodynamically more stable than the sulfates or chlorides. (22) Proshina, O. P.; Lazarev, V. M.; Suponitskii, Yu. L.; Balashov, V. A.; Maier, A. I.; Karapet’yants, M. Kh. Russ. J. Phys. Chem. 1976, 50, 945-946. (23) Suponitskii, Yu. L.; Proshina, O. P.; Karapet’yants, M. Kh. Russ. J. Phys. Chem. 1978, 52, 1699-1700. (24) Crouch-Baker, S.; Davies, P. K.; Dickens, P. G. J. Chem. Thermodyn. 1984, 16, 273-279. (25) Poloznikova, M. E.; Kondratov, O. I.; Fomichev, V. V. Russ. J. Inorg. Chem. 1988, 33, 1140-1143. (26) Iyer, V. S.; Agarwal, R.; Roy, K. N.; Venkateswaran, S.; Venugopal, V.; Sood, D. D. J. Chem. Thermodyn. 1990, 22, 439-448. (27) Iyer, V. S.; Venugopal, V.; Sood, D. D. J. Chem. Thermodyn. 1991, 23, 195-205. (28) Tangri, R. P.; Venugopal, V.; Bose, D. K. Thermochim. Acta 1992, 198, 259-265. (29) Mobin, M.; Malik, A. U.; Ahmad, S. J. Less-Common Met. 1990, 160, 1-14. (30) Mobin, M.; Malik, A. U. J. Alloys Compd. 1996, 235, 97-103. (31) Leonyuk, N. I.; Pashkova, A. V.; Gokhman, L. Z. Russ. J. Inorg. Chem. 1977, 22, 1175-1177.

Figure 1. Effect of molybdenum addition on Na2SO4 deposition. Raman spectra (relative intensities) are shown for deposits collected on a platinum clad probe immersed in flame gases with increased additions of molybdenum salt [C3H8/O2/ N2 (0.9/5/16) flame containing 75 ppmv SO2, 42 ppmv Na, and 10.5 ppmv Mo, probe at 750 K, and 4.1 ms downstream (spectrum b); C3H8/O2/N2 (0.9/5/14) flame with 44 ppmv SO2, 21 ppmv Na, and 10.5 ppmv Mo, probe at 850 K, and 3.1 ms downstream (spectrum c); C3H8/O2/N2 (0.9/5/16) flame with 75 ppmv SO2, 50 ppmv Cl2, 31 ppmv each of Na and Mo, probe at 750 K, 5.2 ms downstream (spectrum d)]; together with those for a purchased sample of pure Na2SO4 (spectrum a) and of a synthesized pure sample of Na2Mo2O7 (spectrum e).

Experiments with Flames Containing Na/Mo/S/ Cl. Normally, a probe immersed in burned flame gases containing traces of sodium, sulfur, and chlorine will collect a deposit of white Na2SO4 over a wide range of probe temperatures (325-1100 K).10 Various such fuellean propane flames have been burned but, in addition, quantitative traces of an aerosol of an aqueous solution of ammonium paramolybdate also have been added to the unburned gases. Visually, it is immediately apparent that the nature of the deposit is different in appearance and morphology. Spectra b, c, and d in Figure 1 show the Raman spectral analyses of the resulting deposits for three such cases. As shown in spectrum a in Figure 1, sodium sulfate (Na2SO4) has an easily recognized simple Raman spectrum that has one strong characteristic sharp line, corresponding to a vibrational frequency of 994 cm-1. Spectra b, c, and d in Figure 1 all indicate that the deposit is modified chemically in a dramatic manner with gradual increases in the addition of molybdenum to the flame. Traces of molybdenum salt that are only one-quarter of that of the sodium, on an atomic basis, already show decreases in the amount of sulfate and new Raman peaks appear in the spectrum. If the addition ratio is increased so that the Na:Mo ratio is 1:1, then the Na2SO4 Raman peak at 994 cm-1 disappears altogether. By comparison with spectrum e in Figure 1, it is observed that a deposit of pure sodium dimolybdate (Na2Mo2O7) is produced preferentially. This behavior is not modified by the presence of chlorine, nor is any NaCl formation evident. The flame in spectrum d in Figure 1 did, in fact, also contain 50 ppmv of Cl2. Figure 2 illustrates the characteristic Raman spectra of the first four disodium molybdates. Their spectra are sufficiently different to facilitate ready identification. The spectrum for Na2MoO4 is for a purchased pure

High-Temperature Corrosion Mitigation Method

Figure 2. Raman spectrum of a purchased sample of Na2MoO4, compared with those of synthesized Na2Mo2O7, Na2Mo3O10, and Na2Mo4O13. X-ray analysis indicates, as implied from its Raman spectrum, that Na2Mo3O10 is difficult to make and the spectrum shown is actually that of a mix of Na2Mo2O7 and Na2Mo4O13; its true Raman spectrum remains unknown.

sample and that of the synthesized sample of Na2Mo2O7 is as expected.20,32,33 The Raman spectrum of sodium tetramolydate does not seem to have been published and is presented here for the first time. It has been validated against its known X-ray spectrum.34 The case of the trimolydate is quite interesting. The corresponding sodium tritungstate does not exist, and there similarly seemed to be no evidence for the trimolybdate in previous phase diagram studies.19 It is only in the past decade or so that confirmation has been obtained that it can exist. An X-ray spectrum has been published that differs from either of the dimolybdate or the tetramolybdate.21,27,28 However, its synthesis seems to be more difficult than the other polymolybdates and was reported as taking 400 h at 830 K for mixtures of either Na2MoO4 or Na2CO3 with MoO3.27 In the present work, only two conditions were used: 20 h at 1000 K and 40 h at 825 K. Consequently, although labeled as Na2Mo3O10 in Figure 2, this is not correct. As clearly evident from the adjacent Raman spectra and confirmed by X-ray analysis, the product obtained here was a 50/50 mixture of the dimolybdate and tetramolybdate; its true Raman spectrum remains unknown. Nevertheless, in this work, all indications were that only the dimolybdate and tetramolybdate are deposited and it seems that the trimolybdate not only is difficult to produce but also is prone to dissociative redistribution to these. Goel et al.,35 in thermogravimetric experiments, also noted that the sodium tetramolybdate dissociates on melting at 795 K to Na2Mo2O7 and MoO3. As a result, in the sodium/ molybdenum system, deposition at higher temperatures seems to be dominated by Na2Mo2O7 formation. In none of these experiments with sulfur present is there any indication of a tendency to deposit Na2MoO4. The sodium seems to have a preferred ranking for deposition product in these systems:

Na2Mo2O7 > Na2SO4 > Na2MoO4 > NaCl (32) Becher, H. J. Z. Anorg. Allg. Chem. 1981, 474, 63-73. (33) Poloznikova, M. E.; Kondratov, O. I.; Fomichev, V. V. Russ. J. Inorg. Chem. 1988, 33, 1447-1449. (34) Balashov, V. A.; Maier, A. A. Inorg. Mater. 1970, 6, 1276-1279. (35) Goel, S. P.; Verma, G. R.; Kumar, S.; Mehrotra, P. N. Thermochim. Acta 1989, 146, 101-106.

Energy & Fuels, Vol. 19, No. 5, 2005 1901

Figure 3. Spectrum a is a Raman spectrum of the flamegenerated deposit previously displayed as spectrum c in Figure 1 with flame concentrations of Na:Mo ) 2:1 and 44 ppmv SO2. Spectrum b is a Raman spectrum from the probe, which is initially coated directly with Na2MoO4 crystals and then immersed at 850 K and 4.8 ms downstream for 3 h in a C3H8/ O2/N2 (0.9/5/16) flame with no additives other than 75 ppmv of SO2.

This finding was confirmed by a subsequent and different type of flame experiment. In this case, the probe was first coated with Na2MoO4 crushed crystals and then immersed at 850 K into the burned gases of a fuel-lean C3H8/O2/N2 (0.9/5/16) flame that contained 75 ppmv SO2. After 3 h, the Raman spectrum of the coating is as shown in spectrum b in Figure 3. This is essentially the same as spectrum a in Figure 3, which is the same trace as that previously shown as spectrum c in Figure 1, resulting from a flame containing sodium and molybdenum in a 2:1 ratio together with sulfur. Obviously, the initial coating has been converted extensively to the dimolybdate and Na2SO4, with just a trace of the original Na2MoO4 still evident, as indicated by the extra small line at 891 cm-1. This propensity for forming a polymolybdate is further confirmed by additional conversion-type experiments. For example, in a flame containing 22 ppmv of sodium, 75 ppmv SO2, and 50 ppmv Cl2, a deposit of Na2SO4 was initially collected on a probe at 750 K for a specific time. Collection then was continued, but now with an additional 44 ppmv of molybdenum added for the same length of time. Overall, the probe experienced an average Na:Mo ratio of 1:1 and analysis of the final deposit reflects this by showing a totally pure dimolydate that is no different from the more-direct experiment whose results are shown in spectrum d in Figure 1. The initial layer of sulfate was fully converted, such that the final deposit was homogeneous. Obviously, as witnessed previously in these studies of deposition processes, a prior deposit can change if circumstances become modified and the opportunity arises for a preferred product. The final deposit, in fact, reflects an integration of the surface environment with time through the collection process. Experiments with Flames Containing K/Mo/S/ Cl. A similar series of experiments were performed but with potassium additions to the flame, rather than sodium. One set is reproduced in Figure 4 for four experiments in which the molybdenum additions ranged

1902

Energy & Fuels, Vol. 19, No. 5, 2005

Schofield

seems to be yet unpublished and, here, is validated not only by X-ray analysis but also in essence by the flame deposit itself in spectrum e in Figure 4. Consequently, potassium behaves slightly differently than sodium, in regard to showing a greater dominance of the trimolybdate and tetramolybdate. The order of the preferential ranking for potassium is as follows:

K2Mo4O13 > K2Mo3O10 > K2Mo2O7 > K2SO4 > K2MoO4 > KCl

Figure 4. Raman spectra showing the effect of molybdenum addition on K2SO4 deposition. Deposits have been collected on a platinum clad probe at 700 K (750 K in spectrum b), 5 ms downstream under fuel-lean conditions (φ ) 0.9), C3H8/O2/N2 (0.9/5/16) flames containing 75 ppmv SO2 and with K:Mo ratios as indicated. Flame potassium concentrations were 36 ppmv for spectra b and c, 25 ppmv for spectrum d, and 13 ppmv for spectrum e). These spectra are compared to that of a pure sample of K2SO4 (spectrum a).

Thermodynamic Considerations. Quite accurate values now are available for the heats of formation at room temperature for the molybdates through tetramolybdates of sodium23,28 and potassium.22,24 At 298 K, the step change in -∆H°f for the sequences of Na2MonO3n+1 (n ) 1-4) and K2MonO3n+1 is larger than ∆H°f (MoO3).37 This implies that the formation of a higher molybdate via the addition of MoO3 to the previous one in the series can be thermodynamically driven. This explains the ease with which most of these polymolybdates can be synthesized from the molybdate and MoO3. The only exception found was Na2Mo3O10, which, as discussed earlier, requires a long period of heating, in comparison to the other five polymolybdates. To some degree, this can be understood from its thermochemical values:26,27,37

Na2Mo2O7 + MoO3 ) Na2Mo3O10 (∆H°298K ) +0.9 (( 3) kJ/mol; ∆G°800K ) -44 (( 30) kJ/mol) 2Na2Mo3O10 ) Na2Mo2O7 + Na2Mo4O13 (∆H°298K) -31 (( 5) kJ/mol)

Figure 5. Raman spectrum of a purchased sample of K2MoO4, together with those of synthesized K2Mo2O7, K2Mo3O10, and K2Mo4O13. The latter of these is published here for the first time.

from one-half to twice that of the potassium. All flames had an excess of SO2, and the presence of chlorine did not seem to be a factor. As seen by the K2SO4 Raman reference spectrum (spectrum a in Figure 4), as molybdenum addition to a flame increases, the deposit’s intensities of the sulfate strong peak at 984 cm-1 and its smaller peaks at 620 and 456 cm-1 gradually weaken and disappear. In this case, a level of molybdenum that is a factor of 1.5-2 greater than that of the potassium is necessary to prevent sulfate formation. As realized by comparing these spectra to those compiled in Figure 5, it is apparent that, as the molybdenum additions increase, conditions initially favor the formation of the dimolybdate, but then the trimolybdate and, finally, the tetramolybdate becomes dominant. In Figure 5, the Raman spectrum for K2MoO4 is from a purchased pure sample. That for the synthesized K2Mo2O7 20,32,36 and K2Mo3O10 20,25 are as expected and agree also with X-ray databases. That for K2Mo4O13 (36) Poloznikova, M. E.; Kondratov, O. I.; Fomichev, V. V. Russ. J. Inorg. Chem. 1988, 33, 348-351.

The formation process of Na2Mo3O10 is not particularly exothermic, and more importantly, the molecule is essentially metastable. It is prone to proportionate branching to its neighboring dimolybdate and tetramolybdate structures. Thermal properties, as a function of temperature, are still not well-defined for these molybdates and are available only for Na2MoO426,37 and Na2Mo2O7,26,38 with additional values of ∆G°f (840-915 K) for Na2Mo3O10.27 As a result, it is difficult to perform equilibrium calculations for these systems other than with Na2MoO4 and Na2Mo2O7. These were added to the thermochemical database of NASA’s Chemical Equilibrium Analysis code39,40 and numerous conditions were tested. Thermodynamic predictions at ∼850 K indicate a formation (37) Barin, I.; Platzki, G. Thermochemical Data of Pure Substances, 3rd Edition; VCH Publishers: Weinheim, Germany, 1995. (38) Knacke, O., Kubaschewski, O., Hesselmann, K., Eds. Thermochemical Properties of Inorganic Substances, 2nd Edition; SpringerVerlag: Berlin, 1991. (39) McBride, B. J.; Gordon, S. Computer Program for Calculating and Fitting Thermodynamic Functions; Reference Publication NASA RP-1271; NASA Lewis Research Center: Cleveland, OH, 1992. (40) (a) Gordon, S.; McBride, B. J. Computer Program for Calculation of Complex Chemical Equilibrium Compositions and Applications. I. Analysis; Reference Publication NASA RP-1311; NASA Lewis Research Center: Cleveland, OH, 1994. (b) McBride, B. J.; Gordon, S. Computer Program for Calculation of Complex Chemical Equilibrium Compositions and Applications. II. Users Manual and Program Description; Reference Publication NASA RP-1311; NASA Lewis Research Center: Cleveland, OH, 1996.

High-Temperature Corrosion Mitigation Method

preference that reflects the observed deposition data:

Na2Mo2O7 > Na2SO4 > Na2MoO4 Potassium shows a continuous transition with increasing MoO3 addition to K2MoO4 in forming the dimolybdate, trimolybdate, and tetramolybdate. As mentioned already earlier, in their study of the volatility and thermal stability of K2Mo3O10, Leonyuk et al.31 synthesized their sample by fusing K2SO4 and MoO3. They also noted, at high temperatures, that when the trimolybdate dissociates, it produces the tetramolybdate. Over the decade 1983-1992, Goel et al. published eleven papers examining the thermal dissociation behavior of numerous complex sodium and potassium oxalated molybdates, ammonium oxalated molybdates, and one sodium formate molybdate.35,41-50 They monitored the thermal breakdown of these compounds, using thermogravimetric methods, and analyzed the final products using infrared and X-ray techniques. They synthesized compounds such as K4(NH4)2[Mo6O18(C2O4)3] and heated them in air. What can be concluded from their studies parallels the present observations and similarly follows thermochemical expectations in all cases. Consequently, in the case of sodium,35,41-43 depending on the initial Na:Mo ratio in the complex salt, the expected sodium polymolybdate was formed with this stoichiometry in a range of 550-720 K. Unfortunately, the Na:Mo ratio of 2:3 was not tested. However, it was noted that, at 793 K, the tetramolybdate dissociates to the more stable dimolybdate.35 For potassium, all ratios appropriate to this paper were tested leading to the expected polymolybdate.44-49 When a K:Mo ratio of 2:7 was synthesized into the salt, this dissociated on heating and produced K2Mo4O13 and MoO3.50 This confirmed the accepted fact that the tetramolydates are the highest di-alkali product for sodium and potassium.20 In deposition studies from the gas phase, a constant consideration is whether the process is solely a condensation of a gas-phase species or whether a heterogeneous surface mechanism is occurring. Few experiments have examined the nature of molybdenum in flame gases. Thermodynamic calculations indicate the potential importance of MoO3 and MoO2 as dominant gaseous molybdenum species. Also, it is known that Na2MoO4 and K2MoO4 evaporate unchanged and their vapor pressures have been measured in Knudsen cell/mass (41) Goel, S. P.; Mehrotra, P. N. Thermochim. Acta 1983, 68, 137143. (42) Goel, S. P.; Mehrotra, P. N. Thermochim. Acta 1984, 76, 127132. (43) Goel, S. P.; Mehrotra, P. N. Thermochim. Acta 1985, 95, 295299. (44) Goel, S. P.; Mehrotra, P. N. Thermochim. Acta 1983, 70, 201209. (45) Goel, S. P.; Verma, G. R.; Sharma, M. P.; Kumar, S. Thermochim. Acta 1989, 141, 87-92. (46) Goel, S. P.; Verma, G. R.; Kumar, S.; Mehrotra, P. N. Thermochim. Acta 1989, 154, 219-224. (47) Goel, S. P.; Verma, G. R.; Kumar, S.; Gulati, A. J. Anal. Appl. Pyrolysis 1990, 18, 91-96. (48) Goel, S. P.; Verma, G. R.; Kumar, S.; Sharma, M. P. J. Therm. Anal. 1990, 36, 2349-2356. (49) Goel, S. P.; Kumar, S.; Sharma, M. P. Thermochim. Acta 1991, 188, 201-205. (50) Goel, S. P.; Verma, G. R.; Kumar, S.; Sharma, M. P. J. Therm. Anal. 1992, 38, 1399-1404.

Energy & Fuels, Vol. 19, No. 5, 2005 1903

spectrometric systems.51,52 The only flame study using mass spectrometric sampling did monitor gaseous K2MoO4, but the flame concentrations of potassium and molybdenum were quite large.53 However, they did derive a value for its heat of formation that agrees very closely with that of Choudary et al.51 In the present work, during Na/Mo deposition studies, a trace of potassium (0.5% that of sodium) also was added to the nebulizer solutions. This was to produce an optically thin spectral emission of the 766.5 nm atomic potassium line. This is useful as a general spectral monitor of the flame system and provides an easy check on stability over a period of several hours. During these studies, it was consistently noted that the addition of molybdenum to the flame reduced the normally expected potassium radiation levels by 15%-40%. Previously, this effect also was noted in flames by Jensen and Miller.54 As a result, there is evidence that, particularly, when molybdenum is in excess of the second trace alkali, in the present case by ∼2 orders of magnitude, the formation of gaseous molybdate seems to be possible within several ms in the burned gases. Nevertheless, whether it be gaseous MoO3, MoO2, sodium, NaOH, NaCl, or Na2MoO4, the surface processes these ingredients to its own specific preference, making the actual gas-phase speciation a somewhat irrelevant consideration. Potential Interferences. In combustion systems, there is now little doubt that molybdenum displays a greater affinity for sodium or potassium than does sulfur or chlorine. As a result, it successfully inhibits sulfate deposition in experimental bench-type flame gases. However, a remaining important question is whether potential fuels may contain other species that have an even greater affinity for the molybdenum. This would neutralize the additive, leave the alkali free again to form the sulfate, and require the addition of enhanced levels of additive to remain effective. As with tungsten additions, an assessment has been made of the thermodynamic stabilities of ∼28 known molybdates, relative to the alkali polymolybdates.55-57 Similar trends to tungsten are apparent, and the most likely elements to interfere are calcium, strontium, and barium, for example:37

CaSO4 + Na2MoO4 ) CaMoO4 + Na2SO4 (∆G°800K ) -40 (( 30) kJ/mol) The question of whether the greater thermodynamic stability of the alkali polymolybdates can compensate for this Ca/Mo affinity needs to be addressed. (51) Choudary, U. V.; Gingerich, K. A.; Kingcade, J. E. J. LessCommon Met. 1975, 42, 111-126. (52) Ermilova, I. O.; Kazenas, E. K.; Zviadadze, G. N. Russ. J. Phys. Chem. 1976, 50, 1309-1310. (53) Farber, M.; Srivastava, R. D. Combust. Flame 1973, 20, 3342. (54) Jensen, D. E.; Miller, W. J. Proc. Combust. Inst. 1970, 13, 363370. (55) Naumov, G. B.; Ryzhenko, B. N.; Khodakovsky, I. L. Handbook of Thermodynamic Data; Atomizdat: Moscow, 1971. (English translation by U.S. Geol. Survey 1974, NTIS Report PB-226722.) (56) Suponitskii, Yu. L.; Lazarev, V. M.; Karapet’yants, M. Kh. Russ. J. Phys. Chem. 1979, 53, 1755-1756. (57) 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 (Suppl. No. 2).

1904

Energy & Fuels, Vol. 19, No. 5, 2005

Figure 6. Raman spectra of two deposits collected on a platinum clad probe, 4 ms downstream under fuel-lean conditions (φ ) 0.9), C3H8/O2/N2 (0.9/5/16) flames containing 75 ppmv SO2 and 50 ppmv Cl2: Na:Mo:Ca additive ratios of 1:1: 1, 17 ppmv each, probe at 750 K (spectrum b), and K:Mo:Ca ) 1:2:2, potassium concentration of 11 ppmv, probe at 700 K (spectrum c). These are compared to spectra of purchased samples of CaMoO4 (spectrum a), CaSO4 (spectrum d), and MoO3 (spectrum e).

Several preliminary experiments have been completed to examine this possibility. Two are presented in Figure 6 as the Raman spectra of acquired deposits in alkali/ Mo/Ca/S/Cl flame additive systems. The reference spectra of CaMoO4, CaSO4, and MoO3 are also shown. These flames contained 75 ppmv SO2 and 50 ppmv Cl2. Spectrum b in Figure 6 had equal amounts of sodium, molybdenum, and calcium (16 ppmv). As is quite frequently observed with deposited calcium salts, the Raman spectrum shows a very broad luminescent interference. This results from crystal defects in the deposits that happen to be excited by 1.06 µm radiation. Nevertheless, the regions of the spectrum that are less affected clearly illustrate that CaMoO4 is being produced predominantly over the expected Na2Mo2O7. This is confirmed by X-ray analysis and also shows that no other calcium product is present. The calcium fully attaches the molybdenum, leaving the sodium to revert to producing Na2SO4, which it does. Using a known mixture of anhydrous CaMoO4 and Na2SO4, approximate scattering cross sections have been obtained for both the Raman and X-ray analyses to enable these spectra to be quantified. The scattering cross sections of CaMoO4 are observed to be larger than the alkali sulfates, both in their Raman and X-ray spectra. In this way, it has been confirmed that, in the case of sodium (see spectrum b in Figure 6), the deposit is, in fact, CaMoO4/Na2SO4 in a molecular ratio of 2:1, as expected for such an interference. This is not obviously apparent in spectrum b in Figure 6, because the satellite Raman lines for the Na2SO4 are too weak to be observed and its strong peak is lost in the luminescence. It is apparent in the X-ray spectrum. As seen in spectrum c in Figure 6, a similar experiment with K/Mo/Ca in a ratio of 1:2:2 shows extremely similar behavior to that of sodium. This deposit similarly turns out to be CaMoO4/K2SO4 in a molecular ratio of 4:1. The calcium again attaches all the molybdenum. For such a case, the potassium Raman lines at 620 and

Schofield

456 cm-1 will only be 1% of the intensity of the CaMoO4 peak at 322 cm-1 and, therefore, are lost in the background. The analysis in this case is also better using X-ray and confirms the presence of the sulfate. Even so, these experiments were still quantitatively marginal and further work needs to be done with higher alkali/Mo ratios, particularly in the presence of calcium. Nevertheless, these preliminary experiments were sufficient to show that the alkaline-earth elements will interfere if they are present. Fortunately, the alkaline-earth elements are extensively removed in the bottom or fly ash in fuel processing as their oxides or sulfates. Whether sufficient remains in synfuels is unlikely, especially after an additional gettering, but needs to be kept in mind. If present, the consequence to overcome this interference would be a need for an enhanced addition of molybdenum proportional to the atomic level of the alkaline-earth element. Concluding Remarks The fact that such minute traces of an alkali impurity in a fuel can gradually destroy a superalloy turbine blade is one of those unexpected aspects in technology development. Its importance remains significant, because it continues to impose limits to further technological advancement. However, as indicated in this paper, there are now approaches that may resolve this dilemma. Molybdenum salt addition is seen to be a viable option for polishing synthetic fuels (synfuels) of alkalis for gas turbine use and has a performance similar to that of tungsten additives reported in a previous complementary paper. These methods disrupt the normal formation mechanism of alkali sulfate and chloride depositions in combustors. The preferred product ranking now displayed by a surface in such systems is modified:

Na2Mo2O7 > Na2SO4 > Na2MoO4 > NaCl K2Mo4O13 > K2Mo3O10 > K2Mo2O7 > K2SO4 > K2MoO4 > KCl These rankings are to be compared to those previously observed with tungsten additions, namely,

Na2W4O13 > Na2W2O7 > Na2WO4 > Na2SO4 > NaCl and

K2W6O19 > K2W4O13 > K2W3O10 > K2SO4 > K2W2O7 > K2WO4 > KCl In all cases, if the additive-to-alkali gas-phase ratio is at least 2-fold, on an atomic basis, then corrosion will be inhibited. Because these additions relate to neutralizing the effects of alkali concentrations on the order of parts per million or less, the economic costs offer this as a practical solution to this enduring problem. Using the data for a 500-MWel coal-burning plant,14 coupled with a 90%-efficient alkali gettering system, the chemical cost of either of these molybdenum or tungsten additives can be estimated for these polishing methods. Currently, it would be slightly less than $1.00/ton of coal for either approach and one-half of this figure if a 95%

High-Temperature Corrosion Mitigation Method Table 1. Melting Points of Sulfates, Molybdates, and Tungstates of Sodium and Potassium Melting Point compound

(°C)

(K)

reference(s)

884 681 610 540 530 695 738

1157 954 883 813 803 968 1011

796

1069

58 26, 58 21, 26, 59 21 19, 21, 35 58 59-61 a 59, 61

Potassium 1069 1342 919 1192 495 768 568 841 546 816 921 1194 638 911 718 991 873 1146

58 58 19 19, 59 19, 59 58 19, 62, 63 19, 62, 63 19, 62, 63

Sodium Na2SO4 Na2MoO4 Na2Mo2O7 Na2Mo3O10 Na2Mo4O13 Na2WO4 Na2W2O7 Na2W3O10 Na2W4O13 K2SO4 K2MoO4 K2Mo2O7 K2Mo3O10 K2Mo4O13 K2WO4 K2W2O7 K2W3O10 K2W4O13 a

Unstable structure.20,61

getter was available. The additive in the form of a dilute aqueous solution of commercial ammonium molybdate or ammonium metatungstate can be mixed with the

Energy & Fuels, Vol. 19, No. 5, 2005 1905

precleaned fuel just prior to combustion or sprayed into the hot combustion gases just ahead of the gas turbine. One interesting difference between the tungsten or molybdenum additive approaches is the slightly lower melting points of polymolybdates. These are all listed in Table 1, together with the polytungstates. Depending on the operational turbine blade temperatures, the deposition of such a protective molybdate coating as a liquid, rather than possibly a solid, might be advantageous. This would ensure retention of only a thin protective film on the blades with no buildup over time. Acknowledgment. This work made use of the Material Research Laboratories Central Facilities, which are supported by the National Science Foundation (NSF), under Award No. DMR00-80034. EF050069Y (58) Lide, D.R., Ed. CRC Handbook of Chemistry and Physics, 82nd Edition; CRC Press: Boca Raton, FL, 2001. (59) Reau, J.-M.; Fouassier, C. Bull. Soc. Chim. Fr. 1971, 398-402. (60) Nolte, G.; Kordes, E. Z. Anorg. Allg. Chem. 1969, 371, 149155. (61) Chang, L. L. Y.; Sachdev, S. J. Am. Ceram. Soc. 1975, 58, 267270. (62) Gelsing, R. J. H.; Stein, H. N.; Stevels, J. M. Rec. Trav. Chim. 1965, 84, 1452-1458. (63) Chen, Q.; Liu, S.; Zhang, P. J. Chem. Thermodyn. 1999, 31, 513-519.