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The ash analysis receives special attention because of certain trace metalsin the ash that cause corrosion. Elements of prime concern are vanadium，sodium， potassium，lead， and calcium. The first four are restricted because oftheir contribution to corrosion at elevated temperatures; however， all these elements may leave deposits on the blading.
Sodium and potassium are restricted because they react with sulfur at elevated temperatures to corrode metals by hot corrosion or sulfurization.The hot-corrision mechanism is not fully understood; however， it can be discussed in general terms. It is believed that the deposition of alkali sulfates (Na2SO4) on the blade reduces the protective oxide layer. Corrosion resultsfrom the continual forming and removing of the oxide layer.Also， oxidation of the blades occurs when liquid vanadium is deposited on the blade.Fortunately， lead is not encountered very often. Its presence is primarily from contamination by leaded fuel or as a result of some refinery practice.Presently， there is no fuel treatment to counteract the presence of lead.
Fuel Properties
Natural gas has a Btu content of about 1000-1100 Btu/ft3(37，272-41，000 KI/m 3). Bydefinition， low-Btu gases can vary between 100-350 Btu/ft 3(3728-13，048 KI/m 3). Presently， little success has been achieved in burn-ing gases with a heating value lower than 200 Btu/ft3 (7456 KI/m3). Toprovide the same energy as naturalgas， a 150 Btu/ft 3 (5592 KI/m3) low-Btu gas must be utilized at the rate of seven times that of natural gas on avolumetric basis. Therefore， the mass flow rate to provide the same energy must be about 8-10 times that of natural gas. The flammability of low-Btu gases is very much dependent on the mixture of CH4 and other inert gases. Figure 12-2 shows this effect by illustrating that a mixture of CH4-CO2 of less than 240 Btu/ft3 (8947 KI/m3) is inflammable， and a CH 4-N2 mixture of less than about 150 Btu/ft3 is less inflammable. Low-Btu gases near these values have greatly restricted flammability limits when compared to CH4 in the air. Vaporized fuel oil gas is produced by mixing superheated steam with oil and then vaporizing the oil to provide a gas whose properties and heating value are close to natural gas.
Important liquid fuel properties for a gas turbine are shown in Table 12-5. The flash point is the temperature at which vapors begin combustion. The flash point is the maximum temperature at which a fuel can be handled safely.
Figure 12-2. ..ammab.efue. mixtureso. CH4-N2 and CH4-C02 atoneatmshowing .arious energy .e.e.s.
The pour point is an indication of the lowest temperature at which a fuel oil can be stored and still be capable of flowing under gravitational forces. Fuels with higher pour points are permissible where the piping has been heated. Water and sediment in the fuel lead to fouling of the fuel system and obstruction in fuel filters.
The carbon residue is a measure of the carbon compounds left in a fuel after the volatile components have vaporized. Two different carbon residue tests areused， one for light distillates， and one for heavier fuels. For the light fuels， 90%of the fuel is vaporized， and the carbon residue is found in the remaining 10%.For heavier fuels， since the carbon residue is large， 100% of the sample can be used. These tests give a rough approximation of the tendency to form carbon deposits in the combustion system. The metallic compounds present in the ash are related to the corrosion properties of the fuel.
Viscosity is a measure of the resistance to flow and is important in the design of fuel pumping systems.
Specific gravity is the weight of the fuel in relation to water. This property is important in the design of centrifugal fuel washing systems. Sulfur content is important in connection with emission concerns and in connection with the alkali metals present in the ash. Sulfur reacting with alkali metals forms compounds that corrode by a process labeled sulfidation.
Luminosity is the amount of chemical energy in the fuel that is released as thermal radiation.
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