The Effects of Manganese on Inhibition of Vanadium/Sodium Deposits and Corrosion in Gas Turbine and Diesel Engine Fuels (1)

Walter R. May

SFA International, Inc.

3355 West Alabama, Suite 640

Houston, Texas 77098-1718

ABSTRACT

The corrosion and slag characteristics of sodium sulfate, vanadium pentoxide and lead sulfate melts containing manganese were evaluated with electrochemical measurements of the corrosion rates on non-ferrous alloys. Manganese promotes corrosion via the formation of low melting salts. It interferes with magnesium and silicon corrosion inhibitors at higher temperatures.

INTRODUCTION

There are several metals which are known to catalyze combustion and reduce particulate matter in exhausts from combustion turbines, steam boilers and Diesel engines. These metals are manganese, iron, copper, barium and cerium. The use of manganese is well known in boiler applications. In those cases, it is used to reduce excess air which leads to production of higher melting ash by limiting oxygen and preventing vanadium from reaching the +5 valence state. (2)

Treatment of ash from the combustion of petroleum fuels with magnesium, silicon, chromium and aluminum is also a recognized approach for controlling deposits and corrosion. In this case, the additive reacts with the sodium and vanadium species to form higher melting species which do not adhere to hot metal surfaces.

Some combustion turbine fuels containing vanadium and produce carbonaceous particulate matter exceeding environmental regulations. A combustion catalyst is required to reduce carbonaceous particulate matter to required levels. This paper presents data which indicate that the use of manganese for the control of particulate emissions interferes with magnesium and/or silicon to inhibit deposits and corrosion caused by vanadium and lead.

RESULTS

The corrosion rate measurements were made in an electrochemical cell consisting of three electrodes immersed in a slag of the desired composition. The corrosion current was determined from polarization curves or by the polarization admittance technique. The cell, corrosion rate measuring apparatus, source of materials and sample preparation are described elsewhere (3).

Corrosion rate data for Na₂SO₄ - V₂O₅ - MnO₂ systems were measured over the range of Na/V = 0.01 to 1.0, Mn/V = 1 to 5 and temperatures from 700^o to 950^o C. The corrosion rate data are available from the author. The correlation equations presented in Table I were computed by a technique described earlier (5).

Correlation equations for systems containing MgSO₄ (4) and MgSO₄ - SiO₂ (6) are included for comparison with data presented in this paper. Unpublished data for systems containing MgSO₄ -SiO₂ with MnO and PbO are also included.

DISCUSSION

The reliability of predictions made by the equations presented here correlated with field observations was discussed in earlier papers (4, 5). The results of these laboratory evaluations compare well with field observations. In all cases, the Na/V coefficients are positive indicating the strong corrosion promoting effect of sodium. The coefficient for Mn/V is positive indicating that manganese promotes corrosion.

The Mn/V ratio was held constant in most of this work which reflects the situation in actual practice. Mn is held constant to control particulate matter whereas the Mg and Si levels are varied to control vanadium.

In Table II, corrosion rates predicted by the correlation equations are presented for several situations. For Mg, Mn and Mn + Mg, the temperatures at which corrosion is totally inhibited with no sodium present were calculated. These temperatures are 873^o for Mg, 613^o for Mn and 751^o for Mg + Mn. For the Mn system, the zero corrosion temperature is 661^o with no Mn present, that is, only vanadium pentoxide was present.

Another approach was to calculate the Na/V ratio for no corrosion at temperatures below the temperature at which corrosion is inhibited with Na/V = 0. For Mg, corrosion is inhibited at 800^o with Na/V = 0.136 and at 850^o with Na/V = 0.0431. These results compare well with field data. At 800^o metal temperature, corrosion is inhibited with a fuel containing 1.0 ppm Na, 7.3 ppm V and 22 ppm Mg.

With manganese present at 22 ppm in the fuel described above, corrosion cannot be inhibited above 724^o C. Normal combustion turbine operating temperatures would be contraindicated.

Other data (available from the author) for systems including silica indicate that some corrosion control can be obtained in the presence of large quantities of silicon. However, this is not good practice in that sodium and silicon form sodium silicates at high temperatures which coat blades leading to deposits and corrosion.

Lead is sometimes found in combustion turbine fuels although it does not naturally occur in crude oil. It generally comes from lead treated gasoline storage tanks. Lead is recognized as being equivalent to vanadium in deposit and corrosive characteristics. Manganese promotes corrosion in the presence of lead just as it does with vanadium.

CONCLUSIONS

Manganese contributes to corrosion in sodium sulfate - vanadium pentoxide deposit and will not inhibit corrosion by forming higher melting compounds. The use of manganese as a smoke inhibitor in combustion fuels is contraindicated.

REFERENCES

1) The data and results discussed in this paper were extracted from a paper presented by the author at the American Chemical Society Division of Fuel Chemistry, Inc., Symposium on Heavy Fuel Oil Additives, New York, N. Y., April 5, 1976, entitled "High Temperature Corrosion in Gas Turbines and Steam Boilers by Fuel Impurities. Part VIII. Evaluation of the Effects of Manganese, Calcium, and Several Heavy Metals on Corrosion and Slag Formation", by W. R. May, M. J. Zetlmeisl, R. R. Annand and D. F. Laurence.

2) Boiler Fuel Additives for Pollution Reduction and Energy Saving, ed. R. C. Eliot, Noyes Data Corporation, Park Ridge, NJ, 1978.

3) "High Temperature Corrosion in Gas Turbines and Steam Boilers by Fuel Impurities. I. Measurement of Nickel Alloy Corrosion Rates in Molten Salts by Linear Polarization Techniques", W. R. May, M. J. Zetlmeisl, L. Bsharah and R. R. Annand, I&EC Product Research and Development, 11, 438 (1972).

4) "High Temperature Corrosion in Gas Turbines and Steam Boilers by Fuel Impurities. III. Evaluation of Magnesium as a Corrosion Inhibitor", W. R. May, M. J. Zetlmeisl, L. Bsharah and R. R. Annand, I&EC Product Research and Development, 12, 145 (1973).

5)"High Temperature Corrosion in Gas Turbines and Steam Boilers by Fuel Impurities. VI. A Statistical Study of the Effects of Sodium, Temperature and Additive on Corrosion Rate and Slag Friability", M. J. Zetlmeisl, W. R. May and R. R. Annand, J. Eng. Power, Trans. ASME, Series A, 98, 506 (1976).

6)"High Temperature Corrosion in Gas Turbines and Steam Boilers by Fuel Impurities. Part IV. Evaluation of Silicon and Magnesium-Silicon as Corrosion Inhibitors", W. R. May, M. J. Zetlmeisl and R. R. Annand, J. Eng. Power, Trans. ASME, Series A, 96, 124 (1974).

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