Gas Turbine Fuels and the Roles of Fuel Additives in Corrosion Protection

Transcription

1 Gas Turbine Fuels and the Roles of Fuel Additives in Corrosion Protection Presented By: Md. Moynul Islam Chemical Engineer, Special Interest and Expertise on Fuels & Lubricants Skype: moynulbd Special Article on Liquid Fuel Selection for Industrial Gas Turbine and the Role of Various Fuel Additives in Protecting Turbine Blades from High Temperature Corrosion 1 P a g e January 10, 2015

2 About GT Fuels and Fuels Specifications: Fuel related problem is a common problem in both diesel engine based and GT based power plant. The profitability of your power business is directly related to the quality of fuels. As long as you will feed quality fuel to your engine/turbine, you will get not only quality output but also extended service life of equipments. You will have to give less effort on preventive maintenance works; you can eliminate the risk of catastrophic failure or breakdown maintenance in your highly expensive turbine components. However, before proceeding discussion on GT fuel additives and their roles in corrosion protection in high temperature alloys used in blades and vanes gas turbines (aviation or industrial GT), let us know a bit about fuel oils and fuel oils specifications of gas turbines. The major difference between aviation GT and industrial GT is that the aviation generates thrust to propel the airplane by exhausting hot gases at high temperatures and velocities. The aviation turbine section is intended only to generate enough horsepower to drive the compressor. In industrial GT the hot gases are expanded across additional turbine stages to generate shaft horsepower that can be used to drive generators, pumps, gas compressors, etc. As we are concern about hot corrosions in industrial GT, we will concentrate more on requirements of industrial GT fuel and the effects of trace metals on GT blades and vanes. Many industrial GT use gaseous fuels but others use variety of liquid fuels ranging from naphtha to residual fuel oils. Aviation GT fuel requirements are quite narrow because of the varying operating conditions (altitude, temperature, etc.), which impose limitations on volatility, viscosity, distillation range, etc. Industrial GT are usually stationary; this means atmospheric conditions do not change as drastically as with aviation GT. The specification for industrial GT fuels is ASTM D 2880, Standard Specification for Gas Turbine Fuel Oils (Figure 01) which is slightly different from ASTM D 975, Standard Specification for automotive diesel fuel oils (Figure: 02). Figure 01: Standard Specification for Gas Turbine Fuel Oils (ASTM D 2880) If we compare two tables shown in Figure 01 and Figure 02, then we will see that the specification Requirements for different distillate fuels are varying based on applications. A comparison table for different distillate fuels has given in Figure 03. If we closely observe the parameters in Figure 01 and Figure 02 then we will see the similarities between No. 1 GT (in Figure 01) and No. 1-D (in Figure 02). Flash Point, Water and 2 P a g e

3 Sediment, Kinematic Viscosity Limit, Ash Content and Carbon Residue values are same in both specification tables. But automotive diesel fuel has Cetane number requirement whereas GT fuel does not. Figure 02: Standard Specification for automotive diesel fuel oils (ASTM D 975) Figure 03: Comparison of specification requirements for selected distillate fuels Parameter D 396 D 975 D 2069 D 2880 D 3699 Flash Point Water & Sediment Distillation Viscosity Carbon Residue Ash Copper Strip Corrosion Density Pour Point Sulfur Cetane # Cloud Point Freezing Point Burning Quality Saubolt Color indicates the property is included in the specification Major Components and Temperatures Ranges in a GT: There are three major components in a simple GT: compressor (intended to raise the pressure of operating fluid), combustor (intended to raise the temperature of operating fluid), turbine section (generates shaft horsepower). In some complecated cycle the heat in the exhaust gas is used to increase the combustor inlet temperature without consuming excess fuel. As a result the typical combustor outlet temperature ranges are in 1093 o C 1482 o C. 3 P a g e

4 Trace Metal Limits in GT Fuels: We have seen the general requirements of designated parameters for GT fuels in Figure 01. Beyond those common parameters, an additional list of Trace Metals has been introduced for GT fuels. Vanadium, Sodium+Potassium, Calcium and Lead are considered as trace metals in fuels entering in turbine combustor. The limits of trace metals for different grades of GT fuels has given in Figure 04. The sulfur content is also important because of its interaction with sodium and potassium as well as the effects on exhaust emissions. Trace metals are metalic compounds dissolved or suspended particulates like rust either in fuel hydrocarbons or free water present in the fuel. Lower levels of trace metalas are beneficial for GT from corossion stand point but must consider the cost and availability of such a refined fuel. About Marine Diesel Oil, #2 Grade (Middle Distillate Fuels): Take a look on ISO 8217:2010, specification for Distillate Marine Fuels (Figure 05) and see the list of parameters, their limiting values and designated test methods. If we closely observe the parameters in ISO 8217 table then we will see that #2 MDO is a blend of DMX and DMA. It is obvious that the the parameters and test methods for MDO/#2 and GT fuels are different. Moreover there is no information about trace metals that is very important to know for GT operation. Perhaps the reason of excluding the requirements of trace metals from diesel oil specification is that the detrimental effects of trace metals on diesel engines are not significant as GT. Although valve burning is very common issue in diesel engines but the replacement cost of a burnt valve is negligible compared to that of GT s expensive blades. It is like that repairing a damaged brick in a wall and rebuilding a destroyed wall. Besides, the temperatures inside the diesel engine combustion chembers marely reaches above 600 o C. So, most of the time the temperatures of combustion chembers and subsequent hot gases paths are remaining below the vanadium containing ash melting point. Hence, the diesel engines operators are less concerned about trace metals in MDO. As a GT operator, you should have sound knowledge on selecting appropriate fuel, efficient fuel cleaning system and fuel testing methods to ensure the safety and uninturrupted productivity of your GT. Probably you are forwarding your MDO/#2 sample to a fuel testing laboratory for routine analysis. The laboratory will conduct analysis in accordance with ISO 8217 specification list and they have no obligation to look for trace metals in your fuel (as trace metals are not included in ISO 8217 specification). However, you will receive a fuel test report having all parameters withinh specification limits but remaining in dark about trace metal contamination that is very important to know for taking preventive measures on you fuel system before entering the fuel to your GT s combustor. ASTM D 3605 is the designated test method to determine trace metals in fuel, and you have to specify the method while forwarding fuel samples for testing. 4 P a g e Trace metal limits of fuel entering turbine combustor(s). a Trace Metals Limits, mg/kg Designation V Na + K Ca Pb No. 0-GT No. 1-GT No. 2-GT No.3-GT No.4-GT Consult turbine manufacturer a Test Method D 3605 may be used for determination of V, Na, Ca and Pb Figure 04: Trace Metals Limits in Gas Turbine Fuels

5 Figure 05: ISO 8217:2010 Specification for Distillate Marine Fuels Detrimental Effects of Trace Metals on Gas Turbine: Vanadium present in fuel can form low melting compounds V 2 O 5 which melts at 691 o C and which causes severe corrossive attack on all high temperature alloys used for gas turbine blades. However, if sufficient magnesium is present in fuel, it will combine with the vanadium and forms a self-spalling compounds with higher melting points and thus reduce the corrosion rate to an acceptable level. Sodium and Potassium can combine with vanadium to form eutectics compounds which melt at temperatures as low as 565 o C and with sulfur in the fuel to yield sulfates with melting points in the operating range of the gas turbine. See the V 2 O 5 -Na 2 O phase diagram in Figure 07. Calcium is not harmful from corrosion standpoint; in fact it serves to inhibit the corrosive actions of vanadium. However, calcium can lead to hard-bonded deposits that are not self-spalling when the gas turbine is shut down and not easily removed by water washing of the turbine. Lead can cause corrosion and in addition, it can destroy the beneficial inhibiting effect of magnesium additives on vanadium corrosion. But the significant quantities of lead present in fuel is very rare. Compounds Name Chemical Formula Melting Temperature o C ( o F) Venadium pentoxide V 2O (1250) Sodium metavanadate Na 2O.V 2O (1165) Sodium pyrovanadate 2Na 2O.V 2O (1185) Sodium orthovanadate 3Na 2O.V 2O (1560) Sodium vanadic vanadate Na 2O.V 2O 4.V 2O (1160) Sodium vanadic vanadate 5Na 2O.V 2O 4.HV 2O (995) Figure 07:Phase diagram of V 2 O 5 Na 2 O system showing a series of utectic compounds 5 P a g e Figure 06:Sodium Vanadium Compounds and Their Melting Point

6 Turbine Blades Composition: The gas turbine blades principally made of Nickel-based super alloy (Major ingredients base metal Ni ~ 68%, Cr ~ 20%, Mo ~ 6%, Tungsten (W) ~ 3%, Titanium (Ti) ~ 1.5% and Al ~ 1.7%). The excellent thermal stability, tensile and fatigue strengths, resistance to creep and hot corrosion, and micro-structural stability possessed by nickel-based super alloy. These Nickel based super alloy are the standard material for hot stages of gas turbines, where blades are subjected to high mechanical stresses, elevated temperatures and in aggressive environments. We have discussed about Sodium (NaCl) ingression in fuel system through salt water contamination. We know the presence of Sulfur in diesel fuels is very common that produces SO 2 in combustion process. Hot Corrosion (Type I corrosion) in turbine blades can be intensified in the presence of Vanadium, which produces V 2 O 5 that combines with sulfates of alkali metals (like Sodium Sulfate, Na 2 SO 4 ) and while in molten state, can aggressively dissolve protective metal oxide layers in turbine blades. The ratio of Sodium and Vanadium plays a vital role in dissolving oxide layers. Sodium (Na) to Vanadium (V) ratio of 1:3, as the lowest melting point corrosive sodium/vanadium/oxide salts are formed that we have seen in figure 07, Phase diagram of V 2 O 5 Na 2 O system showing a series of utectic compounds. Chemical Reaction involved in High Temperature Corrosion: 2S + 2O 2 = 2SO 2 4NaCl + 2SO 2 + 2H 2 O + O 2 = Na 2 SO 4 + 4HCl V 2 O 5 and Na 2 SO 4 + High Temperature >>> Molten deposit on blade surface >>> dissolving protective metal oxide layers from turbine blades. The removal of oxide layers exposing blade surfaces (having less strength and thermal stability) to facilitate erosion corrosion in turbine blades at elevated temperature, pressure and flow of hot combustion gas followed by changing shapes in blades. Progressive changes in turbine blades under hot corrosion environment: Step 1: Slight roughening of the surface caused by growth of localized breakdown in protective oxide layer is obvious. At this stage, neither Chromium depletion in the substrate layer nor loss of mechanical reliability are observed Step 2: At this stage, the roughness of the surface is more remarkable as oxide layer breakdown continues. Although chromium depletion begins at this phase but mechanical integrity is still unaffected Step 3: In this stage, oxidation of the base material has penetrated to significant depth, with obvious build-up of scale. At this level of corrosion, mechanical integrity should be considered as endangered and the blades require removal from service. Progression to step 4 will accelerate with or without the continued presence of sodium Step 4: At this stage, catastrophic attack occurs. The attack penetrates deeply into the blade while forming a large `blister' of scale. Failure is likely at this stage due to loss of structural material. 6 P a g e

7 Gas Turbine Fuel Treatment Program: There are many possible sources of ingressing contaminants in fuels such as inefficient processing at refinery level, during transportation, inside fuel storage tanks. Whatever the sources of contaminants ingression, the fuel must be clean before entering to the gas turbine combustor. It is true that finding a fuel supplier capable to supply MDO/#2 having contaminants and trace metal impurities always within specified limits for gas turbine fuel. Sometimes we have to operate our gas turbines accepting fuels having one or more parameters (especially trace metals) residing beyond the specified limits. But selecting appropriate fuel cleaning system can remove (or reduce to an acceptable level) the contaminants and thus damaging effecs of contaminants on turbine components can be controlled to a certain extend. Contaminants may present in fuel oils in the form of suspended solids, dissolved in free water present in fuels and dissolved in hydrocarbon (oils). Two possible remedies which can be applied to deal with impurities in GT fuels. Remedy A: removal of contaminants by onsite fuel treatment and Remedy B: inhibiting the damaging effects of trace metals by using suitable additives in fuels. Suspended contaminants like rust, CatFines, Dirts etc can efficiently be removed (Remedy A) by filtration or centrifugal separators.sodium and Potassium salts are highly soluble in water, and can be removed (or at least reduce to a an acceptable specification limits) by on-site treatment process known as Fuel Washing. A high quality fresh water is first mixed with the fuel to dilute and extract the water soluble impurities and then separated using either centrifugal separator or electrostatic desalter. Fuel washing is generally applied to treat highly contaminated GT fuels such as crude oils and residual oils to remove water soluble contaminants. Distillate-grade fuels like MDO #2 are relatively clean and usually not washed at the gas turbine power plant, although they are often delivered to the site containing trace amount water soluble Sodium and Potassium contamination, oil-soluble vanadium and other trace metal contamination. Vanadium and other oil-soluble trace metals can not be removed by fuel washing or by centrifuge method, and corrosion inhibition must therefore be achieved through the use of chemical additives (Remedy B) based on the level of contamination. Selecting Appropriate Fuel Additives for GT Fuels: A fuel additive composition is an admixture made up of heavy aromatic petroleum naphtha and a halogenated hydrocarbon such as 1,1,1 - trichloro-ethane solvent where Polysilicone (Polymers of SiO 2 ), Magnesium Sulphonate (or Magnesium Carboxylate) and Chromium Naphthanate remain dissolved or stably dispersed. The formulation process and ratios of different ingredients depends on many factors such as types of fuels, types of impurities and contamination levels, operating temperatures, composition of alloy materials and safety related issues, etc. Magnesium based additives (such as Magnesium Sulphonates or Magnesium Carboxylate) are used mainly to control vanadic oxidation by modifying ash composition and increasesing the ash melting point temperature. Through combination with V 2 O 2 at an appropriate Mg / V ratio (generally 3:1) treatment ratio, Magnesium Orthovanadate [3MgO.V 2 O 5 ] having high melting point (about 1243 o C) is formed. Corrosion is thus controlled by ensuring that the combustion ash does not melt, and that remain in solid state in gas turbine blades and vanes. Through combination with fuel sulfur, magnesium also generates an water soluble magnesium sulfate as an additional ash component which can be removed from hot gas path by periodic water wash. 7 P a g e

8 Chromium based additives (Chromium Naphthanate, Chromium 2 Ethylhexanoate) are especially designed to inhibit sulfidation corrosion promoted by alkali metals Sodium and Potassium. Chromium reacts with the oxides and chlorides of alkali metals and form volatile compounds which pass through turbine without depositing. Silicon based additives (Polysilicone) are also available to manage fuels having high sodium contamination. It provides added corrosion protection and improved ash friability. Fuel Additives Dosing Methods and Appropriate Dosing Ratios: Fuel additives can be mixed with fuel by injecting directly in to the bunker receiving line by a on-line metering pump while receiving fuel. Thus a balancedmixture of additives and fuel can be achieved in the storage tank. But the main drawback of this method is that the fuel additives may be settled down inside the storage tank while storing fuel for a longer period. Although by agitation or recirculation, the additive can be remixed with the fuel. Again in this method, the dosing ratios of additives can not be optimized. Hence, this method is no more a recommended method for mixing additives with fuel. Another widely used and recommended method of injecting fuel additives just before entering the fuel in to the gas turbine combustor using on-line inhibitor injection system. In this method the the dosing ratios can finely be tuned based on contamination level in the fuel. The dosing ratios of additives are not fixed and it will varies based on nature of impurities and contamination levels. So it will change based on your receiving fuel quality. Summary: Fuel quality plays an important role in the profitability and performance of a GT based power plant. A large proportion of the total capital in a turbine based power plant is required to invest behind the installation of GT. The components of a GT s are much more expensive compared to the parts of diesel engines. Hence, intense care must be taken while selecting fuels and their designated test methods, selecting appropriate fuel additives etc. A small unintentional mistake on fuels or additive selection can lead a catastrophic/breakdown maintenance followed by collapsing your profitable power business. So before feeding fuels to the GT, make sure you have selected correct grades of fuel for your GT and all vital parameters required for GT fuels are within specification limits. And also make sure you have selected the correct fuel additives in your fuel treatment. 8 P a g e