Effect of Sodium on Turbine Blades

EFFECT OF SODIUM ON TURBINE BLADES D EFFECT OF SODIUM ON TURBINE BLADES The heavy duty gas turbines operating in power plants can burn various fuels ranging from natural gas to heavy oils. Ash-forming fuels can have harmful effects on the turbine hardware such as: combustion troubles, erosion, corrosion and fouling by ashes. For decades, progress has been made by the gas turbine industry, especially in the fields of super alloy metallurgy, coating and cooling technology. Furthermore, fuel treatments inspired by the petroleum and marine-engine industries (electrostatic and centrifuge desalting systems) and a vanadic corrosion inhibition philosophy based on magnesium additives have been developed to fully control corrosion. The consequences of fuel flexibility: The fuel is likely to produce three main adverse effects on the hot gas path hardware: Erosion due to impinging particles Hot corrosion by fusible slags Fouling by solid ash deposits The formation of ashes in gas turbine combustors: Due to the high pressures (10 to 14 bars) and temperatures (300 to 350 "C) of the combustion air delivered by the compressor, the gas turbine develops very hot flames (2100 to 2300 "C in the "reaction zone". Another important feature is the short residence time of the fuel molecules inside the combustors that is offset by the high excess of air (14 to 15% 02 in the exhaust gases), that creates strongly oxidative conditions in the combustors. The liquid fuel that is swirl-sprayed by the atomization air and projected into the flame as thin droplets begin to volatilize and the resultant vapors to burn. Some heavy hydrocarbon molecules cannot burn in the oxygen depleted atmosphere around the droplets, within the short residence times available and are emitted as gaseous unburnt hydrocarbons (HC) or further combine to soot particles. Effect of Alkaline Metals: The alkaline metals (Na, K) are tolerated only as traces (less than 1 ppm). They form trace amounts of alkali oxides and hydroxides in the flames, which entirely react with SO3 (present in relatively large excess) to form alkali sulfates. These sulphates are stable up to 2500 K but have low melting points. Vanadium when burnt in the presence of air forms V205. This oxide has a very low melting point. It reacts in the post-flame zone with the alkaline and earth-alkaline sulfates to form three types of vanadates, with evolution of the volatile SO3: V205+Na2S04 + 2 NaV03+S03:  metavanadate; MP = 630 "C V205 + 2 Na2S04 -+ 112 Na4V207 + 2S03: pyrovanadate; MP = 635 "C V205 + 3 Na2S04 -+ 2 Na3V04f3S03: orthovanadate; MP = 850 "C The sodium sulfate that travels as thin liquid particulates in the combustion gas impacts the nozzles and blades walls and can "film" the hottest areas. (Na2S04 has a relatively high melting point: 884 "C, but traces of metals like V, Ca, Pb, and W decrease it.) Na2S04 further fluxes the natural protective layers of blade coatings, exposing the underlying active metal to an anodic attack by the oxidizing sulfates. HOT CORROSION/ sulfidation: The metals discussed above cause hot Corrosion: sodium, vanadium, lead and potassium. It is important and critical to be aware that the corrosion mechanism discussed only occurs while these metals are in a liquid or molten state.  The corrosion mechanism does not take place while these metal oxides are solids. The problem of high temperature corrosion of metallic parts arises when the hot combustion gases contain materials that can deposit as a liquid on these parts. The corrosive materials cause destruction of the previously described protective oxide layer. The elements known to form these corrosive agents in combustion systems are vanadium (V), sodium (Na), potassium (K), and lead (Pb). The presence of these alkali metal impurities [Na and K] leads to another type of high temperature corrosion known as “Hot Corrosion or Sulfidation Attack”. These metals can react with each other and with oxygen and sulfur in combustion gases to form volatile compounds such as oxides, alkali sulfates, and vanadates when the fuel is burned. High temperature corrosion is due to vanadium and sodium contained in the fuel. When fuel containing vanadium is combusted in fired equipment, vanadium combines with oxygen to typically form vanadium pentaoxide (V2O5).  Vanadium pentaoxide normally melts about 675° C. When sodium in the fuel is combusted, sodium sulfate (Na2SO4, M.P. 880° C) can be formed from the sodium and sulfur also from the fuel.  A lower melting compound can be formed when amounts of sodium sulfate are present in the vanadium pentaoxide (as low as 300° C for 65% sodium sulfate, 35% vanadium pentaoxide).  While in the liquid state vanadium oxides and sodium sulfate can dissolve the natural metal oxides that form on the alloy surfaces.  When the metal oxide coating is dissolved by the vanadium oxides, the metal forms a new oxide coating which again dissolves, the metal forms a new oxide coating and the cycle continues. Each cycle removes a thin layer of the alloy metal.  The overall effect is corrosion. It is also possible for vanadium oxide to penetrate into the grain boundaries of an alloy causing small particles of metal to be removed.  This leads to more rapid metal losses. When sodium is present the corrosion rate is accelerated. The higher the sodium to vanadium ratio, the greater the corrosion rate. The corrosion rate is also accelerated the higher the temperature of the gas path. REMOVAL OF WATER SOLUBLE IMPURITIES: 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. 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.