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.
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