Fuel ash corrosion

John A. Bonar, Esso Research and Engineering Co., Florham Park, N.J. 

Sodium plus vanadium in the fuel in excess of 100 ppm can be expected to form fuel ashes corrosive to metals by fuel oil ashes. 

Vanadium-Sodium Compounds Most Corrosive. Physical property data for vanadates, phase diagrams, laboratory experiments, and numerous field investigations have shown that the sodium vanadates are the lowest melting compounds and are the most corrosive to metals and refractories. These compounds are thought to form by either the vapor phase reaction of NaCI and V2O5 or by the combination of fine droplets of these materials upon the cooler parts of combustion equipment. 

Excess sodium hydroxide present can also be troublesome as the alkali reacts with the SO3 present in the gas stream to form a range of alkali sulfates which in themselves are highly corrosive to metallic components. In addition, the combination of alkali sulfate -l- V2O5 can result in compounds having melting points as low as 600°E. This situation is only encountered when alkali is present in amounts in excess of that which can react stoichiometrically with V2O5, since the formation of alkali vanadates is favored over that of alkali sulfates. 

Corrosion Influenced by Variables. Corrosion oi metals by V2O5 or alkali vanadates is influenced by several variables, some of which are independent. Those found to affect the corrosion rate are  

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Corrosion of metals by fuel ashes only occurs where the fuel ash contains a liquid phase. Temperatures at which the first liquid will form are inversely proportional to the oxygen partial pressure. Thus, when firing fuels at high excess air ratios, fuel ash corrosion occurs at lower temperatures than when firing fuels with low excess air ratios. 

Fuel Asb Corrosion Control. A variety of methods have been used to control fuel ash corrosion of metallic members in process furnaces, utility boilers, and other combustion equipment. Among these are ... 

Use of excess air levels of 5 percent or less has been shown to reduce fuel ash corrosion in furnaces, most likely by stabilizing the vanadium as a refractory suboxide, VO2 or V2O3. Utility plants have had some success using this method to control vanadium ash corrosion. However, practical application of excess air control in refinery and chemical plant operations is difficult, and has not been particularly successful. Problems with particulates, smoke, pollution, and flame control are encountered unless the necessary expensive control systems and operator attention are constantly available. 

Monolithic refractory coatings have been applied to metallic components in furnaces for fuel ash corrosion control. Results have been less than satisfactory because of the large thermal expansion mismatch between the metal and refractory. Failure usually occurs upon thermal cycling which causes cracking, eventual spalling of the refractory, and direct exposure of the metal to the effects of the fuel ash. 

High Chromium Alloys. Field experience and laboratory data indicate that alloys high in chromium offer the best fuel ash corrosion resistance. The table below shows laboratory corrosion rates for engineering alloys which have been exposed to several types of vanadium-sodium fuel ash melts. 

The subject is also closely related to fuel-ash corrosion which in most cases is caused by a layer of fused salts such as sulphates and chlorides Attention has been focused on the electrochemistry of this type of corrosion and the relevant thermodynamic data summarised in the form of diagrams . Fluxing and descaling reactions also resemble in some respects reactions occurring during the corrosion of metals in fused salts. A review of some of the more basic concepts underlying corrosion by fused salts (such as acid-base concepts and corrosion diagrams) has appeared. ... 

The selection of materials must also consider oxidation/reduction processes that occur in the absence of an aqueous electrolyte. Examples include sulfidation, destructive oxidation of alloys in air or steam at high temperatures, carburization, nitriding, fuel ash corrosion, and high-temperature hydrogen attack. 

Thus, the failure of the tubes is most likely due to fuel ash corrosion. Presence of V and S in the tube surface deposits had accelerated the corrosion, leading to perforation of the tubes. Use of Cr-containing steel SA 213-T22 (K21590) was recommended to minimize corrosion. 

Srivastava SC, Godiwalla KM and Baneijee MK, Fuel ash corrosion of boiler and superheater tubes, J Mater Sci, 1997, 32(4) 835-849. 

Service testing to simulate ash/salt deposit corrosion is of importance to a number of industries. The fossil-fired power generation industry must deal with what is called "fuel ash corrosion fixrm sulfur- and vanadium-containing fuels and alkali, chlorine, and sulfur in coal. The gas turbine industry must deal with "hot corrosion" problems arising fixjm sulfur in fuel and sodium salts from ingested air. Waste incineration environments can become even more complex with refuse containing sulfur, chlorine, phosphorus, and numerous metallic elements. 

In several industrial environments, the deposition of ashes and salts on the surface of the components may occur at high temperatures. In this case, corrosion is determined by both the deposit and the gaseous environment. Underneath the deposits, the formation of a protective oxide scale may be impeded or, as is often observed, the formerly protective oxide scale is destroyed by a chemical reaction or by the formation of a low melting eutectic of the deposit and the oxide scale. This type of corrosion is often observed in fossil-fired boilers (fuel ash corrosion) and gas turbine components (hot corrosion). In recent years, it was also discovered that, in particular, low melting de-... 

This nonhardenable chromium steel exhibits good resistance to reducing sulfurous gases and fuel-ash corrosion. Alloy S44600 has good general corrosion resistance in mild atmospheric environments, fresh water, mild chemicals, and mild oxidizing conditions. 

Although all high-temperature corrosion is considered oxidation, there are other terms that are also encountered, such as oxidation-reduction, sulfidation, fuel ash corrosion, carburization, and nitridation, to name a few. 

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