3 1 Corrosion&Scaling
3 1 Corrosion&Scaling
3 1 Corrosion&Scaling
Chapter 9
ing.
The objective of this chapter is to offer
a general description of the problems of
scaling and corrosion in geothermal plants
and to discuss the most common measures
to mitigate these problems.
9.2. SCALING IN GEOTHERMAL PLANTS
9.2.1. Definitions
Fouling is defined as the accumulation
of undesirable materials in the surfaces that
come in contact with certain fluid. Fouling
can be found in almost every industrial, domestic or physiological activity which involves fluid flow, with or without heat transfer
via the surface. The problems related to
fouling are not recent. The Ancient Greeks
and the Romans, more than twenty centuries ago, had encountered problems of calcium carbonate deposits in aqueducts
(Cowan and Weintritt, 1976).
Precipitation or crystallization fouling
occurs in a geothermal system whenever
the ionic product of a sparingly soluble salt
exceeds its equilibrium solubility product.
The terms scaling or scale formation
are commonly used when the precipitate formed is a hard deposit. Scaling often refers
to the formation of deposits of inverse-solubility
salts (e.g. CaCO3, CaSO4, Ca3(PO4)2), although this term in industry denotes the
hard and adherent deposits that form in
equipment from the inorganic constituents of
water.
181
' (M a + ) n (A b! ) m $
=
"G RT ln %
"
K sp
&%
#"
182
1/(n + m)
& IAP #
= RT ln $
!
%$ K sp "!
1/(n + m)
(2)
Various sites
Various sites
Nigrita (Greece)
Iron sulphide salts [in association with
Reykjanes (Iceland)
TDS: Total Dissolved Solids (mg/kg)
Component
Calcium carbonate
Figure 9.1. Pictures of scaled geothermal pipes. Left, CaCO3 scales in a pipe carrying lowenthalpy geothermal water at Nigrita, Greece. Right, mixed silica and sulphide scales in the
reinjection pipe of Milos geothermal plant.
In the above equation, R is the gas
constant, T the fluid temperature, Ksp the
thermodynamic solubility product of the phase forming compound and (IAP) the ion
activity product. Quantities in parentheses
183
( (M " + ) n (A ! ) m %
S =&
#
K sp
'&
$#
1/(n + m)
& IAP #
= $
!
%$ K sp "!
1/(n + m)
Often, in the literature, S is written without the exponent. Today, the solution speciation and the supersaturation ratios of the
various salts in geothermal waters are
readily computed by various computer codes taking into account all possible ion-pairs
and the most recent values for the solubility
products and the dissociation constants.
Of primary importance is the development of supersaturation which is the driving
force for nucleation and crystal growth. Provided that there is sufficient contact time
with a foreign substrate, scale formation
may take place. Supersaturation can be
achieved as a result of the change of the
operating conditions, most notably of temperature and of pH.
In Figure 9.2 a typical solubility diagram
for a sparingly soluble salt of inverse solubility (such as CaCO3, CaSO4) is shown.
The solid line corresponds to equilibrium. At
a point A the solute is in equilibrium with the
corresponding solid salt. Any deviation from
this equilibrium position may be effected
either isothermally (line AB), at constant
solute concentration, increasing the solution
temperature (AC), or by varying both con-
n
o
ti
a
rt
n
e
c
n
o
C
(3)
Labile
(precipitation)
C
D
A
(S=1)
Metastable
Stable region
Temperature
Figure 9.2. Solubility-supersaturation diagram of a sparingly soluble salt with inverse solubility.
184
Calcium Carbonate
Calcium carbonate forms a dense,
extremely adherent deposit. It is by far the
most common scale problem in low and
medium temperature geothermal systems.
Calcium carbonate deposits can be also
encountered in heat pump systems (Rafferty, 2000). The mechanism of CaCO3 scale formation can be described as follows:
almost all geothermal fluids contain significant quantities of dissolved CO2, in the
form of CO2(aq) and HCO3 . The flashing of
the vapour phase and the CO2 release
cause a pH increase. As a result supersaturation conditions are established and
CaCO3 is deposited:
2+
2Ca + CO3 # CaCO3 (solid)
Apart from the assessment of the
CaCO3 scaling tendency using the supersaturation ratio, this tendency can be predicted qualitatively by a plethora of indices
derived theoretically or empirically over the
past 70 years. The most common indices
are the Langelier Index and the Ryznar Index.
Calcium carbonate can exist in three
different polymorphs, namely calcite, aragonite and vaterite, in order of increasing solubility. All three polymorphs have been
identified in scales, although vaterite is rather rare. Thermodynamics predicts that calcite, the least soluble polymorph, should be
the phase favoured in the precipitation process. Aragonite is also encountered in geothermal systems, forming scales sometimes
as tenacious as those of calcite. The water
temperature and chemistry (e.g. pH and
ionic strength) have been shown to play a
determining role for the nature of the precipitating calcium carbonate phases. It is
also well known that the presence of magnesium ions, in solutions supersaturated
with respect to CaCO3, favours the precipitation of aragonite and appears to hinder the
formation of vaterite.
b)
185
t
i
l
i
b
u
l
o
S
0
-1
ZnS
FeS
-2
PbS
(a)
-3
-4
-5
-6
-7
1
y
t
i
l
i
b
u
l
o
S
pH
-2
-3
FeS
-4
-5
-6
ZnS
-7
-8
(b)
PbS
0
50
100
150
200
250
300
350
Temperature ( C)
186
d[Si(OH)4 ]
(7)
= k [Si(OH)4 ]- [Si(OH)4 ]e2 [OH! ]!0.7
dt
In the above relation k is the reaction
easier and more economically than the welconstant, which depends upon the surface
lbore. The use of submersible pumps to
area of the deposits and e denotes the silica
keep the pressure of the whole system at a
concentration at equilibrium with amorphous
pressure higher that needed for flashing is
SiO2. In practice for pH less than 5 this
recommended for the low enthalpy situareaction is very slow and the silica depotions.
sition is practically zero.
!
Careful adjustment of primary flashing
In certain geothermal fields, such as
pressure in high-enthalpy plants, at a sufSalton Sea in California and Kyushu in Jaficiently high level, can drastically reduce
pan, iron and aluminium are incorporated in
scale formation by keeping solids saturation
the amorphous silica deposits by forming
relatively low. This pressure adjustment is
bonds of the type Fe-O-Si and Al-O-Si (Galusually effective on silica deposition, but
lup, 1993), to form the so-called metalapparently cannot influence sulphide scasilicates. It is believed that the rate of deling.
position of silica is enhanced in the pre!
Large pipe diameters may offer some
2+
3+
sence of aluminium an iron (Fe and Fe )
advantages in reducing the impact of carions. Although the aluminium concentration
bonate and sulphide scaling, especially in
in geothermal fluids rarely exceeds 5 mg/kg,
areas where deposition is expected (after
its contribution to scale can reach 10% w/w
flashing point). This suggestion is a direct
(as Al2O3).
consequence of the fact that for these scaAnother characteristic of the silica deling systems the deposition mechanism appears to be controlled by the rate of tranposits is that they are present in every part
sport of scale-forming ions towards the pipe
of the geothermal installation and they are
walls.
not confined to a relatively short part im!
The prevention of shut-downs and of
mediately after the flashing point. A sigoperating condition changes may be of help
nificant problem is encountered in brine
in certain cases, by avoiding the formation
reinjection systems, where the precipitated
of bands of deposits of reduced adhesion
silica colloids can block the pores of the
strength. Such deposits can be sometimes
reinjection formation.
shattered and dislodged from the pipe walls,
transferred by the flow, and finally accu9.2.6. Scale Control and Prevention
mulated and cemented at certain places.
There are numerous methods in use to
Such problems have been observed for both
control scale formation in geothermal syslow and high enthalpy geothermal fluids in
tems. Some of the most common measures
Greece. In these cases, special ports at the
are the proper design of the geothermal
lower parts of the geothermal system must
plant and selection of operating conditions,
be installed to collect these deposit fragpH adjustment, use of chemical additives
ments.
and the removal of deposits by chemical or
Carbonate deposits can be prevented
mechanical means. Some typical measures,
by the use of scale inhibitors, as will be
to be taken in the selection of design and
discussed below, but the use of inhibitors in
operating conditions, are outlined below:
preventing sulphide and silica scaling is met
!
Flashing in the wellbore should be avoby limited success. It seems that these scale
ided by maintaining higher pressure in the
types can be prevented (at least partially) by
well. Reaming of the well is the most efpH decrease with the addition of a mineral
fective way to remove the deposits in the
acid. The least desirable method is the
casing, but it is rather difficult to remove
removal of deposits. However, under some
scale from the slotted liner. Surface equipcircumstances this method is the only apment plugged by scale can be cleaned
plicable technique. CaCO3 can be dissolved
!
187
188
O
||
-RCH2 - P - O |
OThis structure of phosphonates with CP-O bonding is more stable to hydrolysis
than the polyphosphates. The crystal distortion effect of the inhibitors can be seen in
the Scanning Electron Micrographs presented in Figure 9.4.
Figure 9.4. SEM micrographs of calcite scales in the laboratory in the absence (a) and in the
presence (b) of 5 mg/L of a phosphonate inhibitor. Evident is the reduced scale mass and the
crystal distortion in the presence of the inhibitor. In both cases XRD analysis has shown that the
deposits are composed of calcite. Micrographs (c) and (d) represent CaCO3 bulk precipitates in
the absence and in the presence of the inhibitor, respectively (Andritsos et al, 1996).
The most suitable method of the use of
additives in geothermal systems is the continuous downhole injection at a point upstream of the vapour flashing. Rarely the injection of the inhibitors is done batchwise. A
189
able, but for the highly corrosive environments of the high-temperature fluids corrosion resistant materials are needed (e.g. Incolloy, Hastelloy). Downhole inhibitor injection has the advantage of allowing the parallel use of corrosion inhibitors, but it is also
associated with the following operating
problems:
(1) Ineffective scale control due to low
inhibitor concentration.
(2) Probable formation of pseudo-scales
(e.g. calcium phosphates). In this case
a reduction of the inhibitor concentration or
change of the inhibitor is required.
(3) Corrosion of the injection tube due to
the corrosive nature of both the geothermal fluid and the inhibitor itself. The
phosphonates are strongly corrosive at high
concentrations. Pieri et al. (1989) suggest
the use of alloy Hastelloy C-4 with inner
Teflon coating.
(4) Blocking by scales of the tube exit. A
continuous additive injection is required in
dealing with this problem.
(5) Most inhibitors are unstable (or ineffective) at high temperature (>200C).
Finally, it is pointed out that the use of
inhibitors does reduce the thermodynamic
tendency of the fluid to precipitate due to its
supersaturation. The fluid remains supersaturated and it is possible that at long re-
Injection
tank
PI
low-pressure
fi lter
metering
pump
hi gh-press ure
fi lter
injection
tube
PDI
Production wel l
Figure 9.5. Simplified schematic diagram of the inhibitor injection system (Pieri et al, 1989)
190
pH Reduction
The method of sulphide scale control
by reducing the pH of geothermal brine is
based on the well known fact that the solubility of sulphides increases markedly in
acidic solutions. In the range of pH values
which are typical of high salinity and high
enthalpy brines (pH<6), a ten-fold increaseof the sulphide solubility is obtained by
reducing the pH by one unit. Even a small
reduction of pH may sometimes reduce
drastically the amount of deposits. A reduction of pH will also have a beneficial
effect on the formation of silica and metalsilicate deposits. However, two negative factors must be also considered; i.e. the possible corrosion of pipes at low pH values
and the cost of acid.
The method of brine acidification has
been successfully applied in several cases.
Gallup (1996) reports that inhibition of ironsilicate scale is achieved by lowering the pH
by only 0.1 to 0.3 units. Also, Harrar (1981)
notices that the pH reduction in laboratory
experiments inhibits the deposition of both
silica and sulphides. However, for complete
scale prevention the brine pH should be
reduced to about 3, which tends to increase
significantly the corrosion rate of steel.
Brine handling
The process aims at precipitating and
subsequenty settling of the soluble species
in specially designed equipment, so that no
scaling can occur in the downstream installlation. It is often used when there is the risk
of clogging the reinjection well and aquifer.
9.3. CORROSION IN GEOTHERMAL
PLANTS
9.3.1. Definitions
Corrosion of equipment employed for
handling geothermal fluids as well as in the
production and injection well casings is a
serious problem limiting the exploitation of
geothermal energy. In this section the corrosion types most commonly encountered in
geothermal systems are presented and the
main approaches for mitigating this problem
are outlined.
191
Figure 9.6. Underdeposit corrosion in carbon steel pipes from the villeneuve La Garenne
192
Figure 9.7. Picture of a corroded geothermal iron pipe section in contact a bronze valve.
193
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IN EUROPE, Orleans, Febr. 8-9, pp.
91-97, 1994
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th
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preventive measures. Geothermics 15,
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194