Jasbir S Gill, Ph.D., Gregory jacobs, and Javier Florencio, Nalco Water. Iceland Geothermal Conference 2018 - Breaking the Barriers 24 - 27 April, 2018, Harpa, Reykjavík

1. MANAGING SILICA DEPOSITS IN
GEOTHERMAL
JASBIR S GILL, PH.D1., GREGORY JACOBS1, AND JAVIER FLORENCIO2
1. NALCO WATER, NAPERVILLE, USA
2. NALCO ESPANOLA S.L., SANT JOAN DESPI, SPAIN
ICELAND GEOTHERMAL CONGRESS 2018
Together, well ahead
PROS & CONS OF PH MOD VS. SILICA INHIBITOR

2. Mineral Scales in Geothermal Power Plants
2
 Most common: Silica, calcium carbonate, and calcium sulfate.
 Less common: Iron, iron silicate, Ba/Sr sulfate and calcium fluoride.
 In some specific geographies: Sulfides of antimony, arsenic, iron, lead and Zn.
 Mixed deposits that contain many of the above scales are often the case.
 Iron silicate and iron sulfide are common in Salton Sea and Philippines.
 Silica / silicate and Stibnite are found in many parts of the world.

3. Common Deposit Control Strategies
3
 Silica forms generally in Evaporators, Binary System, and Injection Wells
 pH mod – Acid feed to brine to pH<5  corrosion and other scales such as stibnite
 Keeping the temperature above silica saturation  loss of enthalpy and MWT
 Discharging to pond for silica removal before injection
 Scale inhibitor such as GEO981 / GEO982  allows maximum enthalpy capture.
 Antimony sulfide forms generally in Binary Systems and Injection Wells
 Discharging high enthalpy fluid  loss of enthalpy and MWT
 Dispersants like GEO905, GEO906, GEO907, GEO917
 Other most common scales: Calcite, Anhydrite, Barite, etc..
 Scale inhibitors
 Managing pH and temperature
Mostly it depends on the chemical composition of the scale and its location

4. MANAGING SILICA AND STIBNITE
SIMULTANEOUSLY
4

5. Stibnite Characteristics
5
 Highly insoluble mineral
 KSP = 1.6E-91 (ref. calcite KSP = 4.45E-09)
 Strong relationship with pH and temperature
 Supersaturated at <90 ºC at pH<9.7
 Supersaturated at,195 ºC and pH ~5.96

6. Stibnite Saturation Index – NZ Geothermal Plant
6
Ref: Nathaniel Wilson, et.al., Geothermics 36 (2007) 330–347

7. Managing Silica and Stibnite deposits simultaneously
7
 pH Mod for silica control is a standard strategy
 Slows down silica precipitation kinetics
 Increases corrosion
 Acid cost
 Safety concerns for handling acids
 Could enhance the precipitation of other scales such as Stibnite due to reduced pH
 Use of silica Inhibitor
 Stabilize silica species
 Does not increase corrosion
 No safety issues
 Mitigate Stibnite precipitation as the pH is not reduced

8. Simultaneous Management of Stibnite and Silica in a North American Power Plant
CASE STUDY
8

9. Plant Description
9
Case Study
 Located in a western state of USA.
 38 MW geothermal plant, two different generating units.
 26.1 MW Unit 1 uses “flash” technology and was commissioned in 1984.
 In 2007, the plant’s capacity expanded by 12 MW with the addition of the Unit 2
“bottoming” (Binary unit) cycle.
 The “bottoming” cycle employs binary heat-recovery process to extract more
energy from the hot geothermal brine left over from the steam separation cycle.

10. Plant Configuration
10
Case Study
Production
well
Injection
well

11. Geomizer® input to Predict Treatment
11
Case Study

12. Key parameters under pH Mod
12
Case Study
DP increase while Brine
Flow is decreasing
quickly  Clear indication
of flow restriction inside
equipment.

13. ORC Exchangers Inspection with pH Mod
13
Case Study
 Unit II heat exchangers were inspected after hydro blasting
 Hottest section  Silica scaling
 Coolest section  Heavy stibnite scaling
 Intermediate section  Mixture of stibnite and silica
 Very little scale is being removed with hydro blasting.
 Thickest scale is occurring in the coolest section impacting performance.
 Parasitic losses from reduced flow cross section may be the biggest effect on
performance.

14. Deposit Analysis Comparison
14
Case Study
Deposit Analysis during pH Mod
Upper level II preheater
Antimony (Sb) 37%
Sulfur 30%
Silica (SiO2) 22%
Scale is Stibnite mixed with amorphous
silica. It is a hard scale and found
throughout upper preheater and piping
after preheater with average thickness
of 0.085”. Hard to remove with hydro
blasting.
Deposit Analysis during inhibitor GEO982
Upper level II preheater
Silica (SiO2) 78%
Aluminum (Al2O3) 7%
Potassium (K2O) 3%
Scale is mostly amorphous silica and
with some silt. It is a soft scale and
found in cooler area of upper preheater
and piping after preheater with an
average thickness of 0.050”. Easy to
remove with hydro blasting.

15. Impact of fouling in Power Generation
15
Case Study
Net power is
clearly below
theoretical 15
days after start
up.

16. Power Generation with Inhibitor GEO982
16
Case Study
No Net Power
loss vs.
theoretical after
15 days of
operation.

17. BOOSTING POWER WITH SILICA INHIBITOR
17

18. Power Boost with Silica Inhibitor
18
Temperature Optimization
0
50000
100000
150000
200000
250000
300000
120 110 100 95 90 85 80 75 70
USD/year
ORCs Outlet Temperature, C
Silica Inhibitor Cost, $/y New binary plant project in a reservoir with
600ppm SiO2 in brine.
 Customer interested on boosting power
production by lowering Injection
Temperature at design stage.
 Geomizer™ software used to study
chemical treatment vs. Injection
Temperature.
 Lowering temperature from design 100C to
70C will deliver 1MWe more at a cost of
150 K$/y.
 200 K$/y net profit increase or 133% ROI.

19. Conclusions
19
 Better pH control through the acid program can alleviate silica scaling  but stibnite scaling
potential increases as pH decreases.
 Nalco offers stibnite scale inhibitors for pH mod program.
 Replacing the pH mod program with a silica inhibitor (GEO982) will also reduce both silica and
stibnite scaling, with no pH Mod required.
 Case study plant benefits by switching from pH mod to GEO982
 Differential Pressure decreased by 5PSI
 Brine Flow increased by 100 kph
 Heat Transfer Coefficient improved by minimum 10% across all unit heat exchangers
 Total Power output results improved by 0.5-1.0 MW
 Net savings of $168,000 to $336,000 annually ($40/ MWH)
 Additional savings are possible from optimizing the flow
 Silica inhibitor can also help optimizing injection temperatures at positive ROI.

20. MORE ENERGY
EVERY DAY
Increase capacity and reliability.
Maximize brine flow and equipment
efficiency. Minimize downtimes.
IN CONTROL
TODAY AND TOMORROW
Dynamic system management and
predictive approach through
GEOMIZER™ and 3DTRASAR™
digital solutions.
MORE SAVINGS
EVERY YEAR
Preserve asset integrity, avoid well
re-drilling and equipment
maintenance costs.
in geothermal
Together, well ahead