Abstract:
A method of operating a gas turbine that utilizes vanadium-containing fuels at a combustion stage of the turbine thereby producing a combustion product having vanadium gaseous species, the method comprising increasing the volatility of the vanadium gaseous species in the combustion product atmosphere generated in a combustor of the turbine by increasing concentration of water vapor in the combustion product.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    An embodiment of the invention relates to the operation of gas turbines. More specifically, embodiments of the invention relate to gas turbines that are operated using heavy fuels that contain vanadium and/or vanadium species. 
         [0002]    Vanadium impurity concentrations in petroleum fuels range from less than 0.5 ppm in distillate fuels to as much as 200 ppm in residual fuels. Corrosion problems associated with liquid vanadium-containing deposits on turbine surfaces limit the use of the cheaper fuels. Present practice is to treat such fuels with magnesium additives which convert the vanadium impurities to solid magnesium vanadates resulting in solid, non-corrosive ash deposits on the blades. 
         [0003]    Non-distillate fuels containing more than 2-3 ppm vanadium by weight can be burned in conventional gas turbines only if magnesium or calcium compounds are added to the fuel to form vanadates during combustion. While the liquid vanadium oxide (V 2 O 5 ) and sodium vanadate (NaVO 3 ) that would otherwise be formed are highly corrosive to gas turbine hot stage materials at moderate temperatures and above, the alkaline earth vanadates (magnesium/calcium vanadates) are solid and result only in deposits within the turbine that are comparatively innocuous from a corrosion standpoint. Solid vanadate-containing deposits can be detrimental to gas turbine operation in other ways, however. First, the deposits decrease aerodynamic efficiency of turbine airfoils, leading to the necessity of periodic removal procedures (“nutshelling” while the turbine is hot, water washing when the turbine is cold). This problem becomes particularly severe for machines with higher firing temperature and high turbine component temperatures where the deposits are hard and difficult to remove by nutshelling and water washing. 
         [0004]    Second, higher efficiency gas turbine with firing temperatures approaching 2300° F. depend upon nozzle and bucket cooling by injection of air across the surfaces of these parts (film cooling). Solid vanadate deposits can plug the air cooling ports and lead to overheating of the improperly cooled parts. Thus gas turbines capable of high efficiency operation on clean distillate fuels are generally de-rate for operation on vanadium-containing fuels. Despite these disadvantages, magnesium additives to the fuel are widely used as the only currently feasible approach to utilization of vanadium-containing fuels. No satisfactory method of removing vanadium from fuel prior to combustion has been found. It can be done by setting up or arranging refineries to create distillate fuel which may be complicated and expensive. 
         [0005]    Vanadium occurs in the form of soluble porphyrin complex molecules in fuels, but after combustion it would be present in form of gaseous oxides and hydroxides. In most publications, vanadium oxides have been assumed to be the major vanadium containing species in the gas phase. See, W. D. Halstead,  Deposition and Corrosion in Gas Turbines , J. Wiley, 1972, p. 22; W. D. Halstead, J.  Inst. of Fuel,  42, (1969) 419; and N. S. Bornstein and M. A. DeCrescente, “Properties of High Temperature Alloys,”  The Electrochemical Society , Princeton, 1976, p. 626. However, combustion gases in gas turbines contain in excess of 3% H 2 O, and at a first stage inlet pressure of greater than 10 atm., the water vapor pressure would be quite significant (&gt;0.3 atm.) resulting in appreciable concentrations of hydroxides. Volatile hydroxides reported in the literature include V 2 0 7 H 4 , VO(OH) 3 , VO(OH) 2  and VO 2 (0H) 2 . Glemser and Miiller determined the vapor pressure of V 2 O 7 H 4  by transpiration experiments. See, O. Glemser and A. Muiller,  Z. Anorg. Allgem. Chem.,  325, (1963) 220. These workers, however, did not demonstrate that their results were in a flow-rate independent region, a necessary experimental condition for equilibrium measurements. Yannoupoulos (912-1172° K) and Taniguchi and Ooue (738-893° K) reported V0(OH) 3  to be the predominant vanadium hydroxide while Suito and Gaskell (1173-1373° K) reported VO(OH) 2  and V0 2 (OH) 2  to be the major species. See, L. N. Yannoupoulos,  J. Phys. Chem.,  72, (1968) 3293; M. Taniguchi and M. Ooue, 23 rd    Annual Conf. of the Chem. Soc. of Japan,  1970 Preprint 2, p. 1112; and H. Suito and D. R. Gaskell, “Metal-Slag-Gas Reactions and Processes,”  The Electrochemical Society , Princeton, 1975, p. 251. There is a good agreement between the vapor pressure data of Yannoupoulos and Taniguchi and Ooue; the results of Yannoupoulos have been used for the thermodynamic calculations in this work. 
       BRIEF DESCRIPTION OF THE INVENTION 
       [0006]    Disclosed herein are embodiments of the invention for a method of operating a gas turbine that utilizes vanadium-containing fuels. At a combustion stage of the turbine a combustion product having vanadium gaseous species is produced and may result in condensation of vanadium depositions on blades of the turbine. An embodiment of the invention comprises increasing the vapor pressure of the vanadium gaseous species in the combustion product atmosphere generated in the combustor of the turbine by increasing the water vapor pressure of the combustion product. Steam or water vapor may be introduced into the turbine component at location where deposition can occur and cause corrosion of the turbine components; or, the steam/water vapor may be introduced into the combustor. The steam/water vapor may be introduced at locations having moving items such as rotary blades and/or stationary components such as nozzles. Introduction of the steam/water vapor at predetermined temperatures and vapor pressures increases the volatility of the vanadium species. More specifically, vanadium oxide (V 2 O 5 ) and sodium vanadate (Na 2 V 2 O 5 ) are oxidized in the presence of the steam/water vapor to form VO(OH) 3  and sodium hydroxide (NaOH) gases, which may not as readily condense on the turbine blades forming the solid vanadium depositions. 
         [0007]    An embodiment of the invention may also be described as a gas turbine power generation system that comprises a water vapor source in fluid communication with the turbine element for the introduction of the water vapor into the turbine element and/or combustor to increase a vapor pressure of vanadium gaseous species in the combustion product. As noted above, increasing water vapor pressure results in an increase of the vanadium gas species vapor pressure, which may reduce the condensation of the vanadium gaseous species on to hot components of the turbine element. In an embodiment, the system may comprise a closed loop system that takes combustion product and steam (water vapor) exhausted from the turbine element, condenses the gases/vapors into liquid and cleans the liquid, which is then heated and introduced into the turbine element and/or combustor to increase the volatility of the vanadium gaseous species. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]    A more particular description of the invention briefly described above will be rendered by reference to specific embodiments thereof that are illustrated in the appended drawings. Understanding that these drawings depict only typical embodiments of the invention and are not therefore to be considered to be limiting of its scope, the invention will be described and explained with additional specificity and detail through the use of the accompanying drawings: 
           [0009]      FIG. 1  is a graph illustrating the effect of water vapor concentration on condensate (Na 2 V 2 O 6 —Na 2 SO 4 —V 2 O 5 ) composition. 
           [0010]      FIG. 2  is a graph illustrating effect of water vapor concentration on fuel vanadium tolerance. 
           [0011]      FIG. 3  is a schematic illustration representation of the experimental arrangement used for volatilization studies from pure V 2 O 5 . 
           [0012]      FIG. 4  is a schematic illustration representation of the experimental arrangement used for volatilization studies from Na 2 V 2 O 6 —Na 2 SO 4 —V 2 O 5  melts. 
           [0013]      FIG. 5  is a graph illustrating effect of flow rate of an O 2 —H 2 O gas mixture on the apparent vapor density of V 2 O 5.    
           [0014]      FIG. 6  is a graph illustrating effect of P H     2     O  in O 2 —H 2 O gas mixture on the vapor density of V 2 O 5.    
           [0015]      FIG. 7  is a graph illustrating effect of P H     2     O  (O 2 —H 2 0-SO 2 —SO 3  gas mixture) on the volatilization of vanadium from Na 2 SO 4 —Na 2 V 2 O 6 —V 2 O 5  melts. 
           [0016]      FIG. 8  is a graph illustrating effect of P H     2     O  (O 2 —H 2 0-SO 2 —SO 3  gas mixture) on the volatilization of vanadium from Na 2 SO 4 —Na 2 V 2 O 6 —V 2 O 5  melts. 
           [0017]      FIG. 9  is a schematic illustration representation of vanadium volatilization using a steam film cooling process. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0018]    A more particular description of the invention briefly described above will be rendered by reference to specific embodiments thereof that are illustrated in the appended drawings. Understanding that these drawings depict only typical embodiments of the invention and are not therefore to be considered to be limiting of its scope, the invention will be described and explained. 
         [0019]    The major vanadium containing gaseous oxides under turbine conditions are V 2 O 5  and V 4 O 10 . Yannoupoulos observed that the vapor pressure of vanadium hydroxide was very much greater than that of the oxides in O 2 —H 2 O mixtures with P H     2     O  greater than about 0.1 atm. At the reduced activities of V 2 O 5  expected in condensates (due to association with Na 2 O), the vanadium hydroxide vapor pressure would predominate at even lower values of P H     2     O . This follows directly from the individual reactions since: 
         [0000]      P VO(OH)     3     i    
         [0000]    is proportional to (a V     2     O     5   ) 1/2 , the vapor pressure of vanadium hydroxide (VO(OH) 3 ) is proportional to the square root of vanadium oxide activity (a V     2     O     5   ) x , where x equals 1 for V 2 O 5  and 2 for V 4 O 10 . Thus, vanadium hydroxide is expected to be the major vanadium containing gaseous species in the first stage region of the turbine where P H     2     O  is greater than 0.1 atm. 
         [0020]    The formation of VO(OH) 3  can be described by the reaction: 
         [0000]        V 2 O 5   ( l )+3H 2 O( g )=2V0(OH) 3 ( g )   (1) 
         [0000]    The line below V 2 O 5  indicates that the vanadium oxide can exist in solution at less than unit activity. An increase in water vapor pressure (P H     2     O ) would increase the vapor pressure of vanadium compounds and thereby reduce the amount of vanadium in the condensate. Thus, it should be possible to safely use fuels containing higher vanadium impurities by increasing P H     2     O  in the combustion gases. 
         [0021]    Thermodynamic calculations have been conducted by the inventors to support this postulate. With respect to  FIG. 1 , the graph shows the effect of water vapor on the condensate composition at 900° C. For example, for a fuel containing 10 ppm vanadium, the combined sodium vanadate and V 2 O 5  content of the condensate can be reduced from about 36 mole % to about 0.2 mole % by increasing the steam content in the combustion gases from about 3% for normal operation to about 24%. 
         [0022]    In addition,  FIG. 2  provides a graphic illustration of the effect of water vapor on the allowable fuel level of vanadium (tolerance limit) to maintain the same total concentration of vanadium in the deposit. For example, if the concentration of H 2 O in the gas stream is increased from about 3% to 24%, the concentration of vanadium in the condensate obtained at 900° C. from a fuel containing about 23 ppm vanadium (V) would be about the same as that from a fuel containing 1 ppm V for no enrichment in H 2 O. Thus, the tolerance limit is increased from 1 ppm to 23 ppm under these conditions. 
         [0023]    Laboratory experiments have been carried out to study the volatilization of vanadium from pure V 2 O 5  and Na 2 V 2 O 6 —V 2 O 5 —Na 2 SO 4  mixtures in the presence of steam. The vapor pressure of vanadium compounds was measured by a transpiration technique. When a carrier gas containing H 2 O vapor is passed over vanadium-containing melts at high temperature, the weight of the salt changes due to a loss of vanadium as result of the formation of vanadium hydroxide. Vanadium compounds condense in the cooler parts of the tube due to a decrease in their volatility at lower temperatures. The vapor density of vanadium in the gas stream can be determined by measuring the weight change of the melt as a function of time or by analyzing for the vanadium content of the condensed material. 
         [0024]    Volatilization of vanadium from pure V 2 O 5  was studied using oxygen as the carrier gas. However, for volatilization experiments over Na 2 V 2 O 6 —Na 2 SO 4  melts, O 2 —SO 2  mixtures providing SO 3  levels typical of gas turbine operating conditions were used. The SO 2 —O 2  mixture fixes the equilibrium activity of V 2 O 5  in the melt. 
         [0025]    Two kinds of experimental arrangements shown in  FIGS. 3 and 4  were used. In the arrangement shown in  FIG. 3 , P H     2     O  in the O 2 —H 2 O mixture was controlled by passing O 2  first through a water presaturator  10 , maintained at a few degrees above the desired temperature, and then through a saturator  12  filled with glass beads at the desired temperature. The glass beads provide a large surface area for equilibration of the gas. The entire section of glass tubing between the presaturator and the furnace tube  14  was heated to above 100° C. to avoid condensation of H 2 O. The O 2 —H 2 O mixture was passed over the salt mixture suspended in a crucible  16  from the arm of a Cahn thermogravimetric balance  18 . A thermocouple  20  was provided to monitor or estimate the temperature of the crucible  16 . Measurements of the steady state rate of weight loss from the V 2 0 5  sample provided the vanadium saturation limit of the gas stream. The experiments, however, had some uncertainties due to steep temperature gradients in the furnace tube and relatively poor (±4° C.) temperature control. 
         [0026]    The experimental arrangement shown in  FIG. 4  was used for volatilization studies on Na 2 V 2 O 6 —Na 2 SO 4 —V 2 O 5  melts. The volatilization experiments were carried out in O 2 —SO 2 —SO 3 —H 2 O gas mixtures over a range of water vapor pressures (0-0.83 atm.). The required amount of water vapor was obtained using a syringe pump  22  to inject a controlled flow rate of liquid water through a heated thin capillary  24  of about 0.5 mm internal diameter. The equilibrium value of P SO     3    was attained by passing the O 2 —SO2—H 2 O gas mixture over a platinum catalyst  26 . The specimen and catalyst were controlled at the same temperature to within ±2° C. After passing over the melt placed in a platinum crucible  28  in the wider section (12 mm i.d.) of the quartz tube  36  (disposed within a furnace  30 ), the gas mixture passed through the narrow section (6 mm i.d.) where vanadium oxide and vanadyl sulfate condensed (designated with number  32 ) on the cooler parts. The cooled gas mixture was finally led through a bubbler  34  to the exhaust. After the experiment was over, the condensate in the cooler parts of the tube was dissolved in dilute HF and analyzed for vanadium content. 
       Results and Discussion 
       [0027]    Pure V2O5: 
         [0028]    Volatilization of vanadium from pure V 2 O 5  was studied at 900° C. as a function of gas flow rate and P H     2     O    FIG. 5  shows the dependence of vapor density on gas flow rate. Vapor density (v) was obtained from the steady state weight loss rate by the following relation: 
         [0000]    
       
         
           
             
               vapor 
                
               
                   
               
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                 density 
                 ( 
                 
                   μ 
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                     2 
                   
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                   litre 
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                   gas 
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                   STP 
                 
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                          
                         
                             
                         
                          
                         state 
                          
                         
                             
                         
                          
                         weight 
                       
                     
                   
                   
                     
                       
                         loss 
                          
                         
                             
                         
                          
                         
                           rate 
                            
                           
                             ( 
                             
                               ug 
                                
                               
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                                
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                   flow 
                    
                   
                       
                   
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                     rate 
                      
                     
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                         cc 
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                 273 
               
             
           
         
       
     
         [0000]    The curve for P H     2     O =0.045 illustrates typical effects of varying flow rates. At low flow rates, an erroneously high vapor density is obtained because vapor diffusion from the sample to the cooler sections of the tube contributes significantly to the volatilization rates. At high flow rates, evaporation is too slow to saturate the gas, and an apparently low vapor pressure or vapor density is obtained. The intermediate (plateau) region gives the true equilibrium vapor density. A plot of the results as a function of P H     2     O  is shown in  FIG. 5 . Yannoupoulos observed a vapor density of 65 ug V/litre (STP) for P H     2     O =0.6 at 900° C., which is in reasonably good agreement with the present value (from  FIG. 5 ) of 140 ug V/litre (STP). It can be seen that the vapor density (or the vapor pressure of vanadium containing species) increases almost linearly with P H     2     O . However, due to the narrow range over which the vapor density was independent of flow rate ( FIG. 5 ), poor control of specimen temperature, and small number of experiments, considerable uncertainty may be associated with the pressure dependence shown. In  FIG. 6 . Further volatilization experiments were carried out with Na 2 SO 4 —Na 2 V 2 O 6 —V 2 O 5  mixtures because of their greater relevance to turbine operation. 
         [0029]    Na 2 SO 4 —Na 2 V 2 O 6 V 2 O 5  Mixtures 
         [0030]    Equilibration Experiments: When a Na 2 V 2 O 6 —Na 2 SO 4  melt is exposed to O 2 —SO 2  environments, the salt gains weight as some of the sodium vanadate is converted to V 2 O 5 . Thermodynamic calculations show that the amount of higher order vanadates (Na 4 V 2 O 7 , Na 6 V 2 O 8 ) would he small. The equilibrium between the gas mixture and the melt can be expressed as: 
         [0000]      Na 2 V 2 O 6 (1)+SO 3 ( g )=Na 2 SO 4 (1)+V 2 O 5 (1)   (2) 
         [0000]    In volatilization experiments, Na 2 V 2 O 6 —Na 2 SO 4  salts were pre-equilibrated with the gas mixture containing the desired level of P SO     3    The equilibrium composition of the condensate was determined from the weight gain of the salt mixture during equilibration. Table I gives the results of experiments at 750° C. and 900° C. However, the combustion product temperature may range from about 600° C. to about 1100° C. The activity of V 2 O 5  was calculated from the final composition of the melt, assuming activity coefficient values of one for Na 2 SO 4  and Na 2 V 2 O 6  and using existing thermodynamic data for Na 2 V 2 O 6 , V 2 O 5  and Na 2 SO 4 . 
         [0031]    Volatilization Studies: A number of factors are important in volatilization studies from multi-component melts such as those containing Na 2 V 2 O 6 , V 2 O 5 , and Na 2 SO 4 : 
         [0032]    (1) More than one vapor species may have significant vapor pressures; (2) The loss of components at different rates, e.g., the predominant loss of V 2 O 5  from Na 2 SO 4 —Na 2 V 2 O 6 —V 2 O 5  mixtures, means that the overall composition of the melt changes with time. This difficulty can be circumvented experimentally by using a large quantity of melt and making measurements over a short time span where the overall composition of the melt stays essentially constant; (2) In the presence of a gas that reacts with the melt, e.g., SO 3  with Na 2 V 2 O 6 —Na 2 SO 4 —V 2 O 5  melt (as in reaction 2), additional weight changes will result even if the bulk composition of the melt stays nearly constant over the duration of experiment; and, (3) In addition to gas phase mass transport limitations that restrict experiments to a maximum flow rate for gas saturation, mass transfer inside the melt may also affect the vaporization kinetics. If mass transfer in the melt is slow relative to vaporization, composition gradients will arise both from the loss of a component and the ingress of compensating gaseous species. Thus, the activity of the evaporating species at the surface may be different from that corresponding to the bulk composition. This problem can be reduced by carrying out experiments at low flow rates so that the volatilization rates are small, but other limitations may arise; for example, the diffusion of vanadium species away from the sample may be faster than the flow of the bulk gas such that an apparently high vapor density is obtained. 
         [0033]    An attempt was made to measure by weight changes (for the set up shown in  FIG. 3 ) the rate of volatilization from a Na 2 SO 4 —Na 2 V 2 O 6 —V 2 O 5  melt in an O 2 —SO 2  environment. The experiments were started with Na 2 SO 4 -50 mole % Na 2 V 2 O 6  (Na 2 SO 4 -66.6% mole NaVO 3 ). After an initial period of equilibration with the gas (see reaction 2), the melt experienced a net weight loss which is the result of three major processes which are separate, but related: 
         [0000]    
       
         
           
             
               Net 
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         [0000]    Corrections for weight loss due to the evaporation of Na 2 SO 4  are available from existing vapor pressure data and known to those skilled in the art. If the diffusion rate inside the melt is much faster than the volatilization rate, there would be no concentration gradients in the melt and from equation (2) above: 
         [0000]    
       
         
           
             
               
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                 volatilization 
               
             
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         [0000]    and the melt should lose weight at a steady rate. 
         [0034]    Analysis of the experimental data showed that the melt did not lose weight steadily and that diffusion inside the melt was affecting the process. As a result, the rate of weight change varied with time, and it was not possible to correct accurately for the weight change due to SO 3  pickup by the melt. It was concluded that weight change measurements could not be used to obtain meaningful volatilization rates of vanadium from Na 2 SO 4 —Na 2 V 2 O 6 —V 2 O 5  melts. 
         [0035]    The experimental arrangement shown in  FIG. 4  was adopted as an alternative. Volatilization rates obtained by chemical analysis of condensates in the cooler section of the tube need no correction for the rate of Na 2 SO 4  loss or weight gain due to SO 3  reaction. However, since the rate of volatilization is affected both by flow rate and by the activities of V 2 O 5  and Na 2 SO 4  at the surface, which in turn are affected by gas/melt reactions and by interdiffusion in the melt, a flow-rate-independent region may not be obtained. 
         [0036]    Table I below gives the results of experiments at 750 and 900° C. 
         [0000]                                                                                        TABLE I                   Volatization Studies with Ns 2 SO 4 —V 2 O 5 —Na 2 V 2 O 6                                          Vapor                               Total   Density               Total flow rate           Reaction   Vanadium   (ug V/litre               at room temp.       Equilibrium   Time   Loss   of gas at       Gas Mixture   P H     2     O     (cc/min.)   Temp. ° C.   P SO     3       (Hours)   (ug)   STP                    O 2  +   0.0   50   750   8.7   24   &lt;6   &lt;0.09       0.15% SO 2         O 2  + 0.2%   0.21   50   750   8.7   57   145   0.93       SO 2         O 2  + 0.2%   0.17   55   750   9.3   66   185   0.92       SO 2         O 2  + 0.4%   0.55   50   750   8.7   41   675   6.00       SO 2         O 2  + 2% SO 2     0.87   50   750   8.7   24   602   9.13       O 2  + 2% SO 2     0.87   50   750   8.7   23.5   625   9.68       O 2  +   0.0   50   900   3.7   48   &lt;6   &lt;0.05       0.15% SO 2         O 2  + 0.2%   0.188   50   900   3.7   50   105   0.76       SO 2         O 2  + 0.2%   0.188   50   900   3.7   41.75   65   0.57       SO 2         O 2  + 0.2%   0.188   100   900   3.7   23.75   82   0.63       SO 2         O 2  + 0.2%   0.188   180   900   3.7   21.75   110   0.51       SO 2         O 2  + 0.4%   0.505   50   900   3.7   22   135   2.24       SO 2         O 2  + 2% SO 2     0.84   50   900   3.7   23.25   270   4.23                    
Most of the experiments were performed at a total gas flow rate of 50 cc/min. it can be seen from the results at 900° C. that under these experimental conditions the gas flow rate had very little effect on volatilization rates.
 
         [0037]    With respect to  FIG. 7  there is graphically shown the inferred values of vapor densities as a function of P H     2     O . It is clear that the vapor density of vanadium increases with P H     2     O . Since the vapor density of vanadium should increase with increasing activity of V 2 O 5  (reaction 1), the higher vapor density of vanadium at 750° C. than at 900° C. (the combustion product temperature may be between about 600° C. to about 1100° C.) is expected as a result of the higher activity of V 2 O 5  in the melt, as referenced in Table II below. 
         [0000]                                                      TABLE II                   Equilibration of Na 2 SO 4 —50% Na 2 V 2 O 6  mixtures in O 2 —0.15% SO 2                      Weight                                   of               starting   Weight               mixture   gain   Final X   Composition X   X   Calculated a       Temp.° C.   P SO     3       (mg)   (mg)   Na 2 SO 4     V 2 O 5     Na 2 V 2 O 6     V 2 O 5                 750   8.74 × 10 −4     199.7   36.7   0.654   0.307   0.04   0.0565       900   3.65 × 10 −4     462.5   41.0   0.583   0.252   0.252   0.0136                    
The agreement between the experimental vapor density data and the calculated vapor densities using Yannoupoulos&#39; data is extremely good. The experimental vapor density at 750° C. and P H2O =0.5 atm. is 4 ug V/litre (STP) compared to a value of 3 ug V/litre (STP) obtained by using Yannoupoulos&#39; data and the activity of V 2 O 5  in table I. Furthermore, for the calculations in  FIGS. 1 and 2 , VO(OH) 3  was assumed to be the predominant vanadium containing gaseous species in the presence of steam; a log-log plot of the experimental vanadium vapor density ( FIG. 8 ) as a function of P H     2     O  shows that the vapor density increases linearly with (P H     2     O ) 1.5 , which is consistent with VO(OH) 3  being the predominant vanadium containing gaseous species.
 
         [0038]    The theoretical calculations and laboratory experiments discussed above show that by injecting steam into the combustion product atmosphere it should be possible to increase the volatility of vanadium to allow the use of fuels containing higher vanadium levels. To increase the overall P H     2     O  of the combustion gas to effective levels would, however, require unacceptably large quantities of steam. A more practical solution may be to introduce steam locally, where the condensation (and corrosion) problems are encountered. Presently the hot stage components, such as shrouds, blades and vanes of many turbines have air-film cooling. If air cooling is replaced by steam cooling (or air containing steam) a high P H     2     O  would be attained locally at the surface of the blade, and the condensate would be expected to have a reduced vanadium content. 
         [0039]    An embodiment of the invention is shown schematically in  FIG. 9 . V 2 O 5  and Na 2 V 2 O 6  reaching the blade  38  surface will tend to volatilize in the form of vanadium hydroxide and NaOH by reactions (1) and (3): 
         [0000]      Na 2 V 2 O 6 (1)+4H 2 O( g )=2VO(OH) 3 ( g )+2NaOH( g )   (3) 
         [0000]    In addition to the vanadium volatilization, steam should give better cooling of the blades in comparison to air because of its higher specific heat and thermal conductivity. This approach appears to be directly applicable to present turbines where the blades or vanes have air-film cooling with an add-on system for steam generation. The laboratory experiments with P H     2     O  up to 0.83 atmosphere demonstrate the feasibility of the above approach. 
         [0040]    While the preferred embodiments of the present invention have been shown and described herein, it will be obvious that such embodiments are provided by way of example only and not of limitation. Numerous variations, changes and substitutions will occur to those skilled in the art without departing from the teaching of the present invention. For example, if the combustion of the fuel is performed by oxygen or oxygen-enriched air, the concentration and vapor pressure of the water vapor will be higher, and the vanadium tolerance limit will be higher. Accordingly, it is intended that the invention be interpreted within the full spirit and scope of the appended claims.