4.4.4. Ash

4.4.4.a. Ash formation from heavy oils

Ash stems from the inorganic content of the fuel. The origin of these constituents is varied (Bellan and Elgobashi (1985)) :

1. The animal and vegetable sources from which the oil was formed
2. Contact of the oil with the underground rock structure
3. Production, storage, handling and transportation facilities.

Two aspects are particularly important when corrosion caused by ash is studied (Blanton et al. (1983)) :

1. The deposition of substances, resulting from the combination of three processes:
   1. Particle delivery to the surface, which depends on the particle size.
   2. Adherence and removal of particles (Bellan and Elgobashi (1985)) : Dry particles will adhere by metal attraction or roughness. Molten or semi-molten particles will obviously adhere readily. Because of their low thermal conductivity deposits will be at a higher temperature than the metal and in molten or semi-molten state, which enhances the adherence of more deposits. Ca and V compounds enhance the sticking tendency of deposits.
   3. Gross detachment of previously built-up deposits, sometimes caused by spallation (see section "4.4.4.c. Preventing or reducing ash deposition and corrosion").

2. The corrosion caused by the substances, which is increased exponentially by the metal temperature (Lee et al. (1973)) . In addition, higher temperatures make ash residues more difficult to remove as they are produced at a higher rate and are also harder.

4.4.4.b. Ash forming contaminants in fuel: Vanadium, sodium and lead

Vanadium appears in fuel in the form of oil-soluble porphyrins. These organic vanadium compounds decompose in the gas stream to give mainly V₂O₅. Vanadium pentoxide is most damaging since due to its low melting point (690 °C) it is in its liquid state at normal combustion temperatures.

The mechanism of formation of vanadium compounds can be explained as follows (Stevens and Tidy (1981)) : In high-temperature (approximately 1,730 °C), low-oxygen zones, the solid, non-volatile vanadium tetroxide (V₂O₄) is formed. Vanadium pentoxide is formed from the tetroxide in low-temperature (approximately 800 °C), high-oxygen zones. In fact, deposits are observed to comprise both oxides, although the tetroxide oxidises to the pentoxide in excess air (Bellan and Elgobashi (1985), Dooley and Wilson (1975)) .

The extent of the vanadium attack on metal is determined by two factors (Bellan and Elgobashi (1985), Dooley and Wilson (1975)) : The amount of corrosive vanadium compounds at the metal/oxide interface and the diffusion rate of oxygen to the metal oxide interface. In that respect, the nature of the oxide layer on the metal surface is very important as it may hinder oxygen diffusion towards the metal surface. For instance, Ni₃(VO₄)₂ is formed on nickel-based alloys which forms a stagnant phase on its surface which stops oxidation (Bellan and Elgobashi (1985)) .

Sodium is normally present in the fuel as NaCl collected by the fuel either from underground water or transportation facilities. It can be removed by combined water-washing and subsequent centrifugation (Blanton et al. (1983)) . Sulphidation attack (Patarini et al. (1979)) (also known as "hot corrosion") is corrosion caused by sulphates, mainly Na₂SO₄, on nickel and aluminium alloys, by dissolving the carbide network of the metal. These alloys (eg, Udimet 500) are commonly used in high metal temperature applications, in place of stainless steel and cobalt-based alloys (Lee et al. (1972)) . Experimental work by Stevens and Tidy (Stevens and Tidy (1981)) using Na-doped fuels shows that, regardless the initial form of sodium, its main post-combustion compound is Na₂SO₄, where sulphur arises from the fuel-S content. Whether Na₂SO₄ is formed in the flame or on the metal surface remains unclear (Steinberg and Schofield et al. (1990)) . Gaseous sodium sulphate is almost harmless, unlike its solid or liquid forms, which are particularly harmful when they exceed the theoretical Na "dew point", over 60 ppm (Patarini et al. (1979), Stevens and Tidy (1981)) . This threshold is lowered by the presence of Mg, which could be introduced as an anti-vanadium reagent, and also by other alkaline metals such as Ca and K, which form eutectic mixtures with sodium (eg, Na₂Mg(SO₄)₂·4H₂O).

As a result, sodium can work to decrease the efficiency of Mg additives on vanadium corrosion (Lee et al. (1971)) , by forming eutectic mixtures of low melting point (below 590 °C) with liquid vanadium pentoxide (Bellan and Elgobashi (1985)) . Sodium vanadyl vanadate (Na₂O·V₂O₄·V₂O₅), very corrosive above 647 °C, is of particular importance (Lee and Young (1976)) . However, there seems to exist a threshold of Na₂SO₄ concentration beyond which the addition of more sodium sulphate decreases the corrosion activity of vanadium pentoxide (Bellan and Elgobashi (1985)) .

Solid carbon deposits increase hot corrosion by reducing Na₂SO₄ to the very corrosive Na₂S (Stevens and Tidy (1983)) .

The harmful effects of Na are enhanced when Pb is present in the fuel (May et al. (1976)) . However, lead corrosion can also be tackled by Mg by means of the formation of compounds such as PbO and PbSO₄ (May et al. (1975)) of high melting point. However, in the more realistic case of Pb, V and Na mixtures acting at high temperatures, Mg alone does not suffice against corrosion (May et al. (1975)) , and combinations of magnesium and silica are efficiently used if the concentration of sodium is reduced below 0.5 ppm by water-washing (Lee et al. (1973), Spengler and Lee (1983)) . Silica does not inhibit corrosion on its own (Lee et al. (1973)) , but enhances the efficiency of Mg additives (Lee and Young (1976)) and increases the friability (capacity of deposits to break into small pieces) of the ash deposits (Lee et al. (1973), May et al. (1976)) . Finally, greater amounts of Mg are to be used if the metal surface is exposed to higher temperatures (May et al. (1976)) .

4.4.4.c. Preventing or reducing ash deposition and corrosion

1. Coatings (Patarini et al. (1979)) : Oxide scales build up a protective layer on the metal surface, separating the substrate from the corrosion environment. Coatings provide active elements for building-up this protective oxide scale (Dooley and Wilson (1975), Lee et al. (1972)) .

2. Cooling of metal surfaces (Lee et al. (1972)) , intended to solidify deposits before their attack is initiated. Corrosion is increased exponentially by the metal surface temperature.

3. Additives (Blanton et al. (1983), Lee and Young (1976), Lee et al. (1973), Stevens and Tidy (1983)) : Additives combine with fuel constituents and combustion products to form solid, innocuous products that pass harmlessly through the combustion equipment (Lee et al. (1972)) . Additives may contain metals such as Mg, Al, Si, Mn or Ba (Lee et al. (1973)) , or combinations like Mg-Si, Mg-Al-Si, Al-Si (May et al. (1975), May et al. (1976), May et al. (1976)) . Data obtained by May et al. (May et al. (1976)) indicate that at certain temperatures and with sufficient additive the corrosion rates can be reduced to zero. Additives also help to decrease corrosion by preventing the catalytic formation of SO₃ (Bellan and Elgobashi (1985)) . However, large concentrations of additives may increase particulate output.

4. Surface cleaning procedures (Blanton et al. (1983)) in gas turbines: These can be split in three categories:
   1. On-line techniques: Injection of an abrasive material such as crushed nutshells or coke. These techniques are effective on low temperature (under 970 °C) deposits.

   2. Off-line techniques: Usually a sequence of successive water-wash, soak and re-start is used to soften the MgSO₄ deposits, then followed by the breakage of other deposits. Off-line techniques are efficient on deposits formed at any temperature, although at the cost of efficiency loss as the engine must be switched off.

   3. Thermal excursions: Deposits suffer spallation if a sudden momentary increase of the turbine inlet gas temperature is caused.

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