Gas Turbine Fuel Additives
FUEL ADDITIVES TO CONTROL HIGH TEMPERATURE CORROSION OF GAS TURBINE BLADES AND VANES
High Temperature Corrosion due to Trace Metal Impurities
Turbotect offers a range of fuel additives that are specially designed to control high temperature corrosion and ash fouling of gas turbine hot section components. Several different corrosion mechanisms can occur, but generally they are attributed to the formation of low melting point ash deposits originating from certain trace metal impurities in gas turbine fuels. For example, crude oils and residual-grade fuel oils typically contain small quantities of vanadium as a naturally-occurring component of the petroleum. During combustion, these type fuels create ash deposits composed mainly of vanadium pentoxide [V2O5], and with a low melting point of about 675°C (1247°F). Thus, at typical gas turbine operating temperatures the vanadic ash deposits are molten, and thereby accelerate the surface oxidation rate of blades and vanes. Other trace metal impurities such as lead and zinc will also initiate high temperature corrosion by similar mechanisms.
Crude oils and residual oils also contain sodium and potassium salts, originating from both oilfield and refinery sources. Distillate-grade fuels are often also contaminated with sodium due to marine transportation. The presence of these alkali metal impurities [Na and K] leads to another type of high temperature corrosion known as sulfidation attack. This mechanism involves the formation of sodium sulfates, through reaction with fuel sulfur, and results in serious intergranular pitting of hot section components. Note that in situations where both vanadium and sodium impurities are present, even lower melting point ash deposits can form, and the risk of high temperature corrosion is further increased.
Sodium and potassium salts are water soluble, and can be removed (or at least reduced to within acceptable specification limits) by on-site treatment processes known as "fuel washing". A quantity of fresh water is first mixed with the fuel to dilute and extract the water-soluble impurities, and is then separated using either centrifugal or electrostatic equipment. This type of treatment is normally applied to highly contaminated gas turbine fuels such as crude oils and residual oils. Distillate-grade fuels are usually not washed at the gas turbine power plant, although they are often delivered to the site containing some amount of sodium contamination. Vanadium and other oil-soluble trace metals can not be removed by fuel washing, and corrosion inhibition must therefore be achieved through the use of chemical additives as described below.
Liquid fuels are not the only source of ash-forming impurities. Sodium salts and other contaminants can also enter gas turbines via the compressor inlet air, and also from water and steam that may be injected for NOx control or power augmentation. Thus, the risk of contamination from non-fuel sources must also be considered in gas-fired applications.
Control of High Temperature Corrosion by Fuel Additives
The Turbotect product line includes more than 15 different additive formulations, each designed to address particular contaminant situations and operating conditions. Additive formulations based on the following active components are available, and are offered in various combinations and concentrations:
* Magnesium [Mg] * Chromium [Cr] * Silicon [Si]
Additives containing magnesium [Mg] are used primarily to control vanadic oxidation as described above, and function by modifying ash composition and increasing ash melting point. Through combination with V2O5 at an appropriate Mg/V treatment ratio, magnesium orthovanadate [3MgO.V2O5] with a high melting point of about 1243°C (2269°F) is formed as a new ash component. Corrosion is thus controlled by ensuring that the combustion ash does not melt, and that it remains in a solid state on gas turbine blades and vanes. Through reaction with fuel sulfur, the magnesium inhibition mechanism also generates magnesium sulfate [MgSO4] as an additional ash component. This compound is water-soluble and therefore facilitates the removal of combustion ash via periodic water washing of the hot gas path, thereby enabling the recovery of lost power.
Chromium [Cr] additives are specially designed to inhibit sulfidation corrosion promoted by alkali metal contaminants such as sodium and potassium. Chromium has also been shown to reduce ash fouling, and the mechanism is believed to involve the formation of volatile compounds which pass through the turbine without depositing. Turbotect offers products containing chromium alone or in combination with magnesium and other components. Additives containing silicon [Si] are also available to provide added corrosion protection and improved ash friability in specific applications.
Selection and Application of Turbotect Fuel Additives
Gas turbine fuels range from natural gas and high quality liquid distillates to crude oils and low-grade refinery residues. Additive requirements also vary widely, and are generally determined by the type of fuel and the nature and quantity of contaminants entering the gas turbine from all sources. Additional factors such as firing temperature and OEM specifications also need to be carefully considered. However, as a general guideline, additive selection is usually based on the following broad classification:
Washed crude oils and residual-grade fuel oils containing vanadium, lead, zinc and nickel trace metals. In these applications the fuel has been purified through a centrifugal or electrostatic treatment plant to reduce water-soluble sodium and potassium contaminants to an acceptably low level (typically 0.5 or 1.0 ppm Na+K max), according to gas turbine manufacturer’s specifications.
Unwashed crude oils and residual-grade fuel oils containing sodium and potassium contamination in addition to vanadium and other heavy metal impurities.
Contaminated distillate-grade fuel oils containing small quantities of sodium and/or other impurities that exceed gas turbine manufacturer’s specification limits – for example as a result of sea water contamination during transportation or accidental contact with low-grade fuels.
Gaseous or liquid fuel applications where contaminants (Na, etc.) may be introduced through the combustion air (via the compressor air inlet) or through water used for NOx control or for power augmentation.
Turbotect will assist in making the appropriate product selection and recommending optimum dosage requirements. Custom-designed formulations can also be provided for specific or unique applications.
Turbotect Additive Quality
All Turbotect fuel additives are oil-soluble and completely miscible in liquid hydrocarbon fuels. This simplifies site handling and dosing procedures, and enables the treated fuel to be reliably analysed to confirm and monitor treatment rates. The excellent solubility and low viscosity properties of these additives also ensure rapid mixing and uniform distribution in the fuel, which improves reaction efficiency in the combustion zone.
Turbotect uses only high quality raw materials to ensure that trace metal content is extremely low and in compliance with the strict quality specifications set by the gas turbine manufacturers. This is a very important factor in the selection and application of gas turbine-grade additives, because unacceptable quantities of sodium and other trace metal impurities can be introduced to the fuel if low-quality products or commodity materials are used. Turbotect has over 30 years experience in this specialized field, and additive quality is always considered to be the top priority.
Several other fuel additive properties are also extremely important, and can significantly influence the reliability and effectiveness of a treatment program. For example, the actual chemical composition of the organo-metallic component will affect critical properties such as stability, compatibility and water tolerance. Note that certain types of fuel additive – for example magnesium carboxylates – have a tendency to interact with small quantities of water that are commonly found in most fuels. This hydrolysis reaction leads to a decomposition of the additive and results in the formation of gels and sticky deposits that can cause numerous operational problems. Turbotect magnesium additives are of the sulfonate type, and experience has shown that this chemistry is far more resistant to hydrolysis. The tendency for gel formation as a result of water contact is extremely low, and problems such as the plugging of filters, flow dividers and fuel nozzles are alleviated.
Furthermore, Turbotect’s sulfonate type additives enable higher reactivity during combustion, which means that magnesium is consumed more efficiently in the vanadium inhibition process. This is mainly due to the extremely small particle size of this type additive, and is about 5 times smaller than measured in magnesium carboxylate products. This important feature means that Turbotect magnesium additives can be safely applied at minimum recommended dosage rates, thereby ensuring protection without the need for over-treatment.
Similar quality criteria also apply to all other additives in the Turbotect range. Products containing chromium, silicon and other active components are carefully formulated to ensure consistent high purity, low particle size, high water tolerance and complete compatibility with the fuel and other additives.