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Gas and liquid fuels may contain corrosive constituents at varying concentrations depending on the fuel source. Of greatest concern are gaseous fuels associated with oil recovery that contain significant levels of water, H2S, and CO2. Turbine operators must ensure that the fuel supply will meet the specified gas turbine fuel requirements. This process should also include a thorough evaluation of all possible fuel sources and the required fuel treatment whenever the fuel gas supply quality does not meet the specified limits.
This article contains excerpts from the paper, "Gas turbine durability in harsher environments" by Zaher Mutasim of Solar Turbines presented at the Middle East Turbomachinery Symposium in 2011.
Water in the presence of H2S or CO2 will form acids that can attack supply lines and components and then the turbine and turbine package components. Protection against water requires a careful comparison of the gas turbine manufacturer's fuel specification with the gas supplier's contractual limits, the proper coalescing filter selection, and a supply line layout with automatic or manual drains to remove accumulations of free water.
Fuel heating, or a reduction in the fuel gas pressure after the coalescing filter, may be required to provide enough superheat to prevent water dropout. Fuel line heat tracing may be required to prevent water dropout in static fuel lines during periods when the turbine is not operating.
Coalescing filters should include automatic or manual drains with level controls or alarms. The coalescing filters should include a “knock out” section to trap slugs of liquid and should be designed and instrumented to prevent filter breakthrough or collapse.
Multiple coalescing filters in series may be required to effectively remove water, since filters do not always operate as efficiently as advertised. On dual fuel systems, injector-to-injector crosstalk in the dormant liquid circuit can occur resulting in water condensation that will combine with H2S to form acid.
Acidic corrosion of brazed joints in the liquid circuit of the injector and fuel divider block is the result. Therefore, if the gas fuel contains significant concentration of H2S, a system using cooled PCD air to continually provide forward purge of the dormant passage preventing cross-talk should be part of the fuel system design.
Carbon Dioxide (CO2) is an acidic gas that forms acids when combined with water. CO2 is also very permeable and can cause explosive decompression problems in elastomers, including O-rings and diaphragms. High levels of CO2 raise the combustor lean blowout limits and increase the probability of flameout and/or reduce the offload transient performance of the turbine without flameout. Protection against CO2 in the gas requires removal of water from the gas, selection of appropriate seals, and adapting operating procedures for limitation.
Typically, the major sources of contaminants within liquid fuels that can cause corrosion are water and contaminants. Water in the fuel can cause problems if it contains contaminants such as sodium, potassium, calcium, and magnesium. Protection against water contamination requires installation of properly designed tanks with floors sloping to drains at the bottom and floating suction pipes using day tanks for delivery of fuel to the gas turbine, a properly sized centrifuge for the removal of water, allowing for adequate settling time in the day tank and the use of commercial additives. Water must also be drawn from the bottom of the tanks on a regular basis to prevent excessive build up.
Chemical contaminants in the fuel can by themselves, or through interaction with each other, adversely affect the engine system, particularly turbine hot section life. Sulfur in the fuel burns or oxidizes to form sulfur dioxide. In the presence of even minute quantities of sodium and potassium in the combustor environment (excess oxygen and high temperature), sodium and potassium sulfates are readily formed. These salts have melting points in the operating range of the gas turbine and condense onto turbine airfoil surfaces and react with the base metal, resulting in severe hot corrosion degradation.
Vanadium can form low melting point compounds such as vanadium pentoxide, which melts at 690°C, and alkali metal vanadates, which melt at temperatures as low as 570°C. These compounds can cause severe corrosive attack on all of the high temperature alloys in the gas turbine hot section.
Sodium and potassium can combine with vanadium to form eutectic compounds, which melt at temperatures as low as 570°C, and can combine with sulfur in the fuel to yield sulfates with melting points in the operating range of the gas turbine. Accordingly, the sodium plus potassium level must be limited. Blending fuel with lower sodium and potassium bearing fuel reduces the concentration of sodium and potassium. Contaminants such as mercury, cadmium, bismuth, arsenic, antimony, phosphorous, boron, gallium and indium are unlikely to be present except in unusual or accidental contamination of air, fuel or water supplies. If present, these contaminants can cause significant corrosion to the engine package and components.
Protective measures against chemical contaminants can be taken in various parts of the fuel delivery system. In the fuel supply to the site, the fuel should be inspected and tested to ensure that the chemical contaminant levels are below those detailed in the specification. Periodically, the fuel tank should be sampled and tested for chemical contaminants. If the levels are above the recommended values in the specification, then further action should be taken. This could include mixing the fuel in the fuel tank with low contaminant fuels, or cutting with other fuels in the supply, or introducing additives to react with the contaminants.