Ind. Eng. Chem. Res. 1992,31, 1026-1030
1026
Sintering of Ash during Fluidized Bed Combustion Bengt-Johan Skrifvars* and Mikko Hupa Abo Akademi University, Turku, Finland
Matti Hiltunen A. Ahlstrom Corporation, Ahlstrom Pyropower, Karhula, Finland
Agglomeration of bed material and fuel ash may sometimes cause problems during fluidized bed combustion. In this paper a laboratory test method has been applied on different coal ashes to predict how they behave in temperatures typical for circulating fluidized bed boilers. The method is also useful when the influence of the different bed compounds on the sintering is studied or when the effect of the surrounding gas phase is investigated. The method is based on compression strength measurements of sintered cylindrical pellets and has been used earlier when slagging and fouling tendencies of different coal ashes in pulverized coal fired boilers have been studied. The results showed clear differences in sintering tendencies between the five different coal ashes studied. Temperatures where the sintering was initiated could vary between 500 and 900 "C, depending on the ash. The sintering tendency seemed also to correlate well with the experiences achieved from full-scale and pilot-scale operation. Addition of limestone decreased sintering of one coal ash when the amount exceeded the Ca/S ratio of 1.3. The decrease was even greater when an Al-Si-based clay mineral was used.
Introduction In circulating fluidized bed combustion, sintering of the bed material can sometimes cause severe problems (Smith, 1956; Goblirsch et al., 1980; Basu and Sarka, 1983; Manzoori, 1990; Moore et al., 1991). As a result of various sintering processes, deposits can be formed in different locations in the boiler. In the lower furnace, the deposits disturb the air distribution in the bed, disturbing the fluidization. In other parts of the furnace, the deposits affect the circulation of the gas/ bed material in a negative way. In most serious cases, sintering can lead to heavy agglomerate formation in the bed which finally may inhibit the fluidization completely. Recent results (Manzoori, 1990) show that ash material under suitable conditions is transferred from the char surface to bed particle surfaces, coating the bed particles with, in some cases, a sticky ash material, resulting in bed agglomeration. The sintering behavior of different fuel ashes varies widely. Predicting the ash behavior before a fuel is used would be desirable in many cases for avoiding problems in the operation. Reid (1984) gives a good review of different available techniques. Standard ash melting point measurements are often used when ash behavior predictions are made (DIN, 1984). These give, however, an inaccurate prediction of the ash sintering tendency, while significant sintering usually starts far below the temperatures of any detected melting of the ash. Other methods that have been used for ash agglomeration predictions are different kinds of shrinkage measurements (Manzoori, 1990; Smith, 1956; Raask, 1979) and electrical resistance tests (Raask, 1979; Frederikse and Hosler, 1973; Cumming, 1980, Cumming et al., 1985). There are also different types of in situ measurements (Sondreal et al., 1977). Ash pellet shrinkage tests can be done by dilatometric measurements (Manzoori, 1990; Raask, 1979). The method is a good way of detecting ash sintering when clear particle shrinkage occurs in the test sample. The electrical resistance measurement of an ash sample is based on ion mobility and detects first hand the existence of a liquid phase in the sample (Raask, 1979; Frederikse and Hosler, 1973; Cumming, 1980; Cumming et al., 1985). Partial 0888-5885/92/2631-1026$03.00/0
Table I. Analysis Data of the Coals Expressed as the Percent by Weight of the Coal on the Dry Solids Basis
brown 1 2 ash 20.0 22.0 volatiles 49.1 45.3 fixed carbon 30.9 32.7 C 55.3 53.5 4.3 4.4 H N 0.6 0.7 S 3.9 3.7 0 16.0 15.6 HHV" 22.4 21.8
3 5.2 50.8 44.0 66.4 4.5 0.9 0.4 22.7 24.7
bituminous coal anthracite coal 4 5 40.2 21.9 6.0 27.6 53.8 50.5 55.2 66.6 4.1 1.6 1.8 0.9 1.1 0.4 4.5 1.6 25.7 19.1
"Unit: higher heating value (HHV), MJ/(kg of fuel).
melting of ash is known to be an important contributor to deposit formation in some types of combustors (Hupa, 1977; Nowok et al., 1990). Ash pellet compression strength measurements for predictions of ash agglomeration during pulverized coal combustion has earlier been used by several authors (Nowok et al., 1990,Gibb, 1977; Babcock and Wilcox, 1975; Attig et al., 1963; Barnhart et al., 1956). We have used the compression strength measurements previously to predict flue gas dust sintering in black liquor recovery boilers (Skrifvars et al., 1991b; Hupa et al., 1989), in pulverized coal fired boilers with limestone injection for sulfur removal (Skrifvars et al., 1991a), and in peat gasification plants (Hupa et al., 1989; Moilanen et al., 1989). The purpose of the present work was to study the sintering tendency of ash from very different types of coals fired under circulating fluidized bed conditions. Laboratory test data obtained for five different coals, using the compression strength method, were compared with pilot- and full-scale experiments. Further, the laboratory test method was used to study the influence of two bed additives on the bed sintering tendency.
The Studied Coals Five different coals were studied. Three brown coals, one bituminous coal, and one anthracite were included. The three brown coals will be referred to as coals 1,2, and 3. The bituminous coal is 4, and the antracite, 5. 0 1992 American Chemical Society
Ind. Eng. Chem. Res., Vol. 31, No. 4, 1992 1027 Table 11. Analysis Data of the coal coal 1 coal 2 4.7 42.6 SiOz 0.5 9.7 A1203 2.4 1.3 FeZ03 8.9 CaO 8.9 35.7 6.2 Na20 0.5 0.9 KZO 23.5 15.6 so3 c1 29.9 1.7
Ash Tested in the Study" coal 3 coal 4 5 55.2 9.8 50.3 14.4 21.4 1.6 11.3 6.3 2.9 2.4 0.1 51.2 0.1 0.04 0.05 0.05 1.5 3.9 1.3 2.8 0.3 0.1 0.02 1470 flowing point 1460 1470 gas formation 720-1000 1230 a
HEAT TREATMENT
I
1 COMPRESSION STRENGTH TEST
Figure 1. Testing method. N/mm2
I T I
41
E
1
No melting point measurements available.
Brown coal 3 had a low ash content compared to the other coals. Brown coal 1was selected for the tests due to its high Na content. The more detailed analysis data of the coals are presented in Table I. Table I1 gives the ash analysis data of the coals. Also results from the DIN standard ash melting point measurement are presented in Table I11 (DIN, 1984). All of these coals have been tested in a pilot-scale furnace. Both coals 4 and 5 are used in full-scale CFB boilers at furnace temperatures up to 950 OC without any bed agglomeration problems. For fuel 3 some problems with deposit formation in the lower furnace have been experienced at temperatures below 900 "C. Coal 2 could be burned in a CFB boiler only with slagging inhibiting additives. Coal 1 is not used commercially. In pilot-scale tests its ash has shown to have a strong tendency to agglomerate and to form deposits on the furnace walls.
Experimental Section To be able to test the sintering tendency of the five coal ashes, each of the fuels were at first ashed in laboratory conditions according to a standard method (DIN, 1978). After this, the ash was passed through a 162-pm sieve. Ash remaining on the sieve was crushed and sieved again. This procedure was repeated until all ash material had passed the sieve. Cylindrical pellets were made out of the ash and heat treated in a tube furnace under controlled conditions. The force (pressure) by which the pellet was made influenced the final compression strength. Concequently, the pellet making pressure was standardized. A pressure of 10 bar was chosen as the lowest possible pressure, still giving the pellets enough strength so that they could be handled. Finally the compression strength of the heat treated cylindrical pellets were measured in a standard compression strength testing device, the strength being taken as the degree of sintering. For estimation of the scattering of the measured strength values, four pelleta were tested at each time. In the results the degree of sintering is given as the calculated average value divided by the cross-sectional area of the tested pellet perpindicular to the direction of the applied force. The scattering is given as the calculated standard deviation and is indicated in each figure as a vertical bar on top of each measured point. The testing method has been presented earlier (Hupa et al., 1989; Skrifvars, 1990), and further details of the test procedure are found in the references.
, A400
-
~
&.
600 TEMPERATURE
4 800
1 l0OO0C
Figure 2. Compression strength values of the different heat treated ash pellets (N/mm2)expressed as a function of heat treatment temperature ("C): heat treatment time, 4 h; heat treatment atmosphere, dry air. The number of the tested fuel ash is expressed as the parameter. The mean strength values of untreated pellets were as follows: ash 1, 1.75 N/mm2; ash 2,0.67 N/mm2; ash 3, 1.10 N/mm2; ash 4, 0.27 N/mm2; ash 5, 0.12 N/mm2.
The test method is presented schematically in Figure 1.
Results and Discussion SinteringTendency of Different Coal Ashes. When the five coal ashes were treated in a temperature range of 200-1000 "C a broad variation in ash sintering behavior could be detected. The results are presented in Figure 2. Two main differences could be seen between the crushing strength vs temperature curves. The first was the initial strength which the pellets reached without any strengthening due to sintering having taken place. From comparison of the base-line strengths with the strengths of untreated pellets, it can be seen that the values are almost equal. The base-line strength can be seen in Figure 2 as the horizontal part on the left end of each curve. For ash 1,the base line has a strength of 1.9 N/mm2 and the strength of the untreated pellet is 1.75 N/mm2 on average. For ash 5, the base line has the strength of -0.1 and the untreated pellet strength is 0.12 N/mm2. This suggests that the base-line strength of the pellet is already achieved when the pellet is made. The key factor to the differences in base-line strength values between the ashes is probably found in the differences in the particle size distribution of the ashes. If the particle size distribution has been suitable, more contact area has been achieved between the particles in the pellet compared to a case where the size distribution has been less favorable. The second difference between the various ashes was the temperature where the strength values deviated from the base line. This temperature at which the deviation took place is taken as the initial sintering temperature and is indicated in Figure 3 as TsINT.For ash 1, this deviation
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1028 Ind. Eng. Chem. Res., Vol. 31, No. 4,1992 3 N/mmz
1
11,
,
,
j TSINT,
,
,
,
,
-
1
,I
Table IV. Chemical Analysis (I) of the Two Additives Clay Mineral Additive SiOz 48.7 K2O 2.12 11.9 A1203 36.0 LO1 Fez03 0.8 rest? 0.48 Limestone
E
CaO MgO COZ By balance.
31
53.9 0.7 43.9
" 1' '1
0 15 30
100
, '
" *
-
- 1
0 15 30 100 o/'
ADDITION IN FUEL
TEMPERATURE
Figure 3. Strength values of the heat treated ash pellets (N/mm2) as a function of the heat treatment temperature ("C). The sintering temperature for each tested ash is indicated as the Tsm: heat treatment time, 4 h; heat treatment atmosphere, dry air.
0.2 1.3
resto
Pi 0'
so3
Figure 4. Effect of additive on the ash sintering tendency: ash from fuel 1; temperature, 500 "C; time, 4 h; atmosphere, dry air; left,
-
limestone-based additive; right, Al-Si-based additive. Key: 0% addition pure ash,100% addition * pure additive; 15% limestone added to dry fuel * Ca/S ration of 1.3;30% limestone added to dry fuel t* Ca/S ratio of 2.6; 15% clay mineral added to dry fuel Al/Na ratio of 0.5; 30% clay mineral added to dry fuel c)Al/Na ratio of 1.0. Q
took place at 500 OC, thus, giving a Tsm of approximately 500 "C. For ash 5, the temperature Tsm was found to be 900 OC. Sorting out the ashes in the order of increasing Tsm, the following order was achieved ash 1 < ash 2 < ash 4 < ash 3 < ash 5. According to these results, it would be expected that ash 1 would have the highest tendency for bed agglomeration while ash 5 would have the lowest. These results seem to correlate well with the experiences achieved by pilot- and full-scale tests. Also ash 2 followed our expectations. For ashes 3 and 4, a difference between laboratory tests and boiler experience could be detected. According to the laboratory tests, ash 4 would have a lower TSm than ash 3 (Figures 31, which is the oppsite trend that has been experienced in full-scale testa. The strength development of the pellets made from ash 3 starts however already at -600 "C, even if the strength increase is slow. Thie could be a sign of some initial sintering taking place already at these lower temperatures, thus maybe explaining the observed depoeiting of ash 3 in the full-scale boiler at 900 "C. Effect of Additives on the Sintering Tendency of the Ash. The effect of two additives on sintering was also studied. Both additives were tested with the laboratory test method together with the high sodium coal 1. One of the additives was a limestone, used for SO2 capture. Limestone was added to the fuel at two levels: 15 wt % limeatone added to the dry fuel corresponding to the Ca/S ratio of 1.3 and 30 wt % limestone added to dry fuel correaponding to the Ca/S ratio of 2.6. The other additive was a Al-Si-based clay mineral, used as a bed agglomeration inhibitor. Two levels of addition were used: 15 and 30 wt % additions to the dry fuel, the former corresponding to the Al/Na ratio of 0.5 and the latter to the Al/Na ratio of 1. The chemical analyses of the two additives are shown in Table IV. The fuel-additive mixtures were ashed and prepared according to the same procedure described earlier in the paper. After heat treatment of 4 h at three different
CLAY MINERAL
W
Ly
z 3
ADDITION IN FUEL
Figure 5. Effect of additive on the ash sintering tendency: ash from fuel 1; temperature, 600 OC; time, 4 h; atmosphere, dry air, left, limestone-based additive; right, Al-Si-based additive. Key 0% addition pure ash; 100% addition 0 pure additive; 15% limeetone added to dry fuel Ca/S ratio of 1.3;30% limestone added to dry fuel * Ca/S ratio of 2.6; 15% clay mineral added to dry fuel c, Al/Na ratio of 0.5;30% clay mineral added to dry fuel Al/Na ratio of 1.0.
-
-
temperatures in oxidizing conditione, the compression strength was measured. The general trend was that both additives decreased the compression strength in the conditione studied (Figurea 4 and 6). Obviously the additives diluted the ash in the pelleta, thus making them less sintered. When 15 wt % limestone was added to fuel 1and the mixture was prepared and heat treated in 700 "C, an increase in the compregeion strength could be detected when compared with the strength of the pure ash pellet (Figure 6). A further increase decreased the compression strength again. It has previously been found that the measured compression strength of the ash pellet at first increases as a function of heat treatment temperature when T s m is
Ind. Eng. Chem. Res., Vol. 31, No. 4,1992 1029 LIMESTONE
0 15 30 100
i
with the Al/Na ratio of 1 increased the compression strength. A further increase of the additive decreased the compression strength. 5. The decrease in the sintering tendency was slightly greater when the clay mineral was used as an additive compared to the case when limestone was used as an additive. This has also been experienced in commercially operating CFB boilers.
CLAY MINERAL
0 15 30 100 ADDITION IN FUEL
O/o
Figure 6. Effect of additive on the ash sintering tendency: ash from fuel 1; temperature, 700 OC; time, 4 h; atmosphere, dry air; left, limestone-based additive; right, Al-Si-based additive. Key: 0% addition * pure ash; 100% addition * pure additive; 15% limestone added to dry fuel 0 Ca/S ratio of 1.3; 30% limestone added to dry fuel cs Ca/S ratio of 2.6; 15% clay mineral added to dry fuel Al/Na ratio of 0.5; 30% clay mineral added to dry fuel AL/Na ratio of 1.0.
-
*r)
exceeded. The compression strength reaches however a mnximumat some point and may decrease back if the heat treatment temperature is further increased (Hupa et al., 1989;Skrifvars, 1990). In the case illustrated in Figure 6, the heat treatment conditions were such that the pure ash ,T by some 200 "C. The compellet had exceeded its, pression strength of the pure ash pellet had already passed its maximum. When heatone was added to the ash pellet, and the heat treatment took place at 700 "C, the ash material in the pellet probably formed a sticky, flowing mas, "gluing" the solid lime particles together to a hard and dense pellet. This same effect could be seen when the clay mineral was added to ash 1and the heat treatment was performed at 700 "C. However a slight difference between the effect of the two additives on the ash sintering could be detected. The clay mineral seemed to decrease the sintering more effectively than the limestone. This is an effect which also was experienced in operating boilers. Conclusions 1. A laboratory sintering testing method, based on compression strength measurements of heat treated ash pellets, has been found suitable for predicting bed agglomeration problems in CFB boilers. 2. Clear differences in ash sintering tendencies could be detected between the five coals tested in this study. The ash sintering tendencies of the different coals correlated fairly well with the sintering problems experienced from full-scale and pilot-scale CFB-boilers. The order of increasing sintering tendency of the various coals was based on the laboratory testa coal 5 < coal 3 < coal 4 < coal 2 < coal 1 based on operating experiences coal 5 < coal 4 < coal 3 < coal 2 < coal 1 3. The addition of limestone decreased the sintering tendency of ash 1 at 500 and 600 "C. At 700 "C an addition of limestone with the Ca/S ratio of 2 increased the compression strength. A further increase of limestone addition decreased the strength of heat treated pellets. 4. The addition of the Al-Si-based clay mineral showed a trend similar to that with the addition of limestone. A decrease in the sintering tendency could be detected at 500 and 600 OC. At 700 "C the addition of the clay mineral
Acknowledgment The collaboration with the Technical Research Center of Finland (I" Mr.) Antero Moilanen is acknowledged. This work is a part of the Combustion Research Program LIEKKI in Finland. The laboratory work has been done by Mr.Klaus Tyrkkii, which is acknowledged. We also acknowledge the Engineering Foundation, New York, for their permission to publish this paper. The paper is based on the presentation given at the Engineering Foundation Conference on Inorganic Transformations and Ash Deposition during Combustion, held in March 1991,Palm Coast, FL. Literature Cited Attig, R. C.; Barnhart, D. H. A laboratory method of evaluating factors affecting tube bank fouling in coal boilers, Proceedings. The mechanism of corrosion by fuel impurities; Marchwood Engineering Laboratories: Hampshire, England, 1963. Babcock & Wilcox. Steam, its generation and use, 39th ed., 3rd printing; Babcock & Wilcox: New York, 1975; p 15-14. Barnhart, D. H.; Williams, P. C. The sintering test, an index to ash fouling tendency. Trans. ASME 1956, 78,1229-1236. Basu, P.; Sarka, A. Agglomeration of coal ash in fluidized beds. Fuel 1983,62 (Aug), 924-926. Cumming, J. W. The electrical resistance of coal ash at elevated temperatures. J. Znst. Energy, September 1980, 53 (Sept), 153-154. Cumming, J. W.; Joyce, W. I.; Kyle, J. H. Advanced techniques for the assessment of slagging and fouling propensity in pulverized coal fired boiler plant. J. Znst. Energy 1985,58 (Dec), 169-175. DIN. Testing of solid fuels, determination of ash content (in German). Priifung No. 51719-A, June 1978. DIN. Testing of solid fuels, determination of ash melting (in German). Prtifung No. 51730,1984. Frederikse, H. P. R.;Hosler, W. R. Electrical Conductivity of coal 1973,56,41&419. slags. J. Am. Ceram. SOC. Gibb, W. H. The elagging characteristics of coal ashes by viscosity and sintering measurementa. VGB-Konferenze "Rauchgasseitige Korrosionen und Verschmutzungen in Wdrmekraftwerken",June 1977; VGB Technieche Vereinung der Groaekraftwerkebetrieber e.V.: Eeeen, Germany, 1977. Goblirsch, G.; van der Molen, K. H.; Wileon, K.; Hajicek, D. Atmospheric fluidized bed testing of North Dakota lignite. 6th International Conference on Fluidized Bed Combustion, 1980. Hupa, M. Deposit formation in boilers f i e d with bark alone or in combination with other fuels. Paperi ja Puu-Paper and Timber No. 8; The Finnish Paper and Timber Journal Publishing Co.: Helsinki, Finland, 1977. Hupa, M.; Skrifvars, B.-J.; Moilanen, A. Measuring the sintering tendency of ash by a laboratory method. J. Znst. Energy -. 1989,62 (Sept), i3i-137. Manzoori, A. R. Role of the inorganic matter in agglomeration and defluidktion during the circ&ting fluid bed &&bustion. Ph.D. thesis, The University of Adelaide, Adelaide S. A., Australia, 1990. Moilanen, A.; Skrifvare,B.J.;Hupa, M. The effect of temperature, chemical composition and gas atmosphere on sintering of peat fly ash. Proceedings of the 1989 International Conference on Coal Science, Tokyo, Japan, Oct 1989; International Energy Agency (IEA): 1989. Moore, R. E.; Zahradnik, R. L.; Vawter, €2. G. Simultaneous combustion of oil shale, low-BTU gas and coal in a circulating fluidized-bed combustor. Proceedings of the 1991 International Conference on Fludized Bed Combustion, April 21-24, 1991, Montreal, Canada; 1991; Vol. 1,pp 553-558. Nowok, J. W.; Benson, S. A.; Jones, M. L.; Kalmanovitch, D. P. Sintering behavior and strength development in various coal ashes. Fuel 1990, 69 (Aug), 1020-1028.
I n d . Eng. Chem. Res. 1992,31, 1030-1035
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Raask, E. Sintering characteristics of coal ashes by simultaneous dilatometry-electrical conductance measurements. J . Therm. Anal. 1979,16,91-102. Reid, W . T. The relation of mineral composition to slagging fouling and erosion during and after combustion. Prog. Energy Combust. Sci. 1984, 10, 159-175. Skrifvars, B.-J. Sintering of fuel aph, a laboratory testing method (in Swedish). Licentiate thesis, Abo, Akademi University, Turku, Finland, 1990. Skrifvars, B.-J.; Hupa, M.; Hyoty, P. Superheater fouling due to limestone injection in coal-fired boilers. J. Znst. Energy 1991a, 64 (Dec), 196-201.
Skrifvars, B.-J.;Hupa, M.; Hyoty, P. Composition of recovery-boiler dust and its effect on sintering. Tappi J. 1991b, 74 (No. 6, June), 185-189. Smith, E. J. D. The Sintering of Flv-ash. J. Znst. Fuel 1956, XXZX (185, June), 253-260. Sondreal. E. A.: Tufte. P. H.: Beckerinn. W. Ash fouline in the combustion of low rank wes&rn U.S. ccks. Combust. Sci. Technol. 1977, 16, 95-110.
Received for review May 23, 1991 Reuised manuscript received November 18, 1991 Accepted December 4, 1991
Mechanistic Effects of Arsenic Oxide on the Catalytic Components of DeNO, Catalysts Erich Hums Power Generation Group (KWU),Siemens AG, P.O. Box 3220,D-8520 Erlangen, Germany
The interaction of arsenic oxide as a DeNO, catalyst poison was studied with a TiOz-Mo-V composite oxide catalyst (11). It showed differences in catalytic and other properties from a TiO2-MoO3-VzO5 catalyst (I) obtained by monomolecular dispersion of Moo3 and Vz05 on TiOz. Since the arsenic poisoning mechanism on Moo3 is obviously identical in both catalyst systems, the vanadium-molybdenum composite oxide phase used here is responsible for the differences in behavior from TiO2-MoO3-VzO5 catalyst (I). As a poisoned intermediate, an As4Mo3OI5phase was identified by X-ray diffraction. This phase is formed initially and subsequently transformed to the more stable MoAsz07phase. The latter phase was analyzed by structural analysis and possesses mixed oxidation numbers, namely, As3+and As5+. Mo6+,however, retains its valence. Replacing Moo3 with Mooz suppresses formation of a composite oxide phase containing arsenic.
Introduction It is a well-known fact that DeNO, catalysts installed on the high-dust side of wet-bottom furnaces are deactivated far more rapidly than those in dry-bottom furnaces. This rapid deactivation is, on the basis of current knowledge, essentially caused by the arsenic oxide content of the flue gas. By comparison with dry-bottom furnaces in which the catalysts are merely exposed to the catalyst poison burden entrained in the coal, a concentration of catalyst poisons takes place in wet-bottom furnace plants, which varies with the amount of ash returned to the furnace (Kautz et al., 1975; Gerhard et al., 1985; Cramer, 1986). Since arsenic oxide has been under discussion as a catalyst poison for DeNO, catalysts containing TiO,, numerous attempts have been made to find plausible models and explanations of the poisoning mechanism (Hums, 1991). A property specific to catalysts containing tungsten oxide which are exposed to flue gas containing arsenic oxide is that they deactivate at much faster rates than catalysts containing molybdenum oxide; reference is also made to this fact in the patent literature on this subject. Comparison of the performance of three catalysta after 1800 h of exposure in the flue gas of a wet-bottom furnace shows this very impreasively, but focusea attention in particular on the TiO,-Mo-V composite oxide catalyst (11)and on the Ti02-Mo03-V,0s catalyst (1)-a catalyst obtained by formation of a monomolecular dispersion of Moo3and V20s(monolayer) on TiOP Discrepancies in the catalysts occur in the distribution of pore radii (Hums, 1991) and the deactivation rates (Figure 1). The material compositions also do not lead to SO, SO3 oxidation rates that correspond to their vanadium content (Figure 2; Table I). Due to the fact that the two catalyst systems are made up of the same elements, this behavior can only be at-
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Table I. Catalyst Composition component
TiW
88.50
VZO, additives
catalysts, wt % I 80.58
79.93
10.42 0.41 8.59
10.41 1.16 8.50
0.60 10.90
I1
tributed to differences in the structure of active sites. These differences in the structure would in particular be evident in the reactivity with arsenic oxide and affect the respective catalytic components of the catalysts. In those chemical reactions in which solids are involved, surface layers are by nature the starting point of reactions which may propagate into the solid. In this case we have investigated the influence of As203on the phase composition of V205-M003mixtures free of TiOz by X-ray diffraction (XRD). We established that As203triggers the same phase transformation (V9Mo6OU V6M0402s)in V205-Mo03systems both with and without TiOz (Hums and Gabel, 1991). A crystalline composite oxide phase containing arsenic is not involved. It cannot be denied that this phase transformation occurs on the Ti02-Mo03-V20s catalyst (I) which we endeavor to prevent in the case of the TiOpMo-V composite oxide catalyst (11). But it has not yet been possible to find direct proof of this with low surface dispersion as with monomolecular layer. In the following, this topic is expanded to include the Moo3 component and a check made to establish whether A s 2 0 3 reacts with molybdenum oxide to form composite oxides at temperatures approaching those encountered in power plants. In this way it was thus possible to synthesize the A L ~ ~ M phase, O ~ Owhich ~ ~ is as yet unknown in the literature,
0888-588519212631-1030$03.00/00 1992 American Chemical Society
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