Geothermal Energy

Enhancing the Recovery of Geothermal

Energy by Preventing Problematic Silica

Deposition Through the Precipitation of a

Useful Calcium Silicate Product

Professor Jim Johnston

School of Chemical and Physical Sciences

Victoria University of Wellington

New Zealand

Geothermal Systems

Geothermal Fields

Geothermal Systems

Naturally occurring hot water / steam system where water comes in

contact with subsurface hot rock.

Heat and chemical constituents are exchanged between the hot rock

and the water.

Hot water dominated: Water maintained in liquid phase by

hydrostatic pressure of overlying rock.

Pressure released at the surface.

About 30% of superheated water is flashed into steam to

drive a turbine for electricity generation.

Only about 15% of the available heat energy is converted into

electrical energy

New Zealand and Pacific rim countries, Iceland.

Geothermal Systems

Boiling point vs.

depth relationship

Dilute salt brine saturated in silica

Geothermal Heat Energy Recovery

for Electricity Generation

Geothermal Silica Precipitation

H3SiO4- + H+ SiO2 + 2H2O

−SiOH + HOSi− −SiOSi− + H2O

Complex precipitation process

Separated water supersaturated in

dissolved silica

Geothermal Silica Precipitation - The Nice Side

Geothermal Silica Precipitation - The Problem Side

A substantial problem in geothermal resource utilisation worldwide

Blocks drains, pipes, process equipment and re-injection wells


Currently addressed by:

Keeping the steam-water separation

temperature above 100oC typically 130oC

Lowers driving force for silica precipitation

Less heat energy recovered in steam phase

Acid dosing – retards silica precipitation rate

Small pH window or will dissolve pipes


Compounded with the use of Binary Cycle

technology to recover heat energy from

separated water stream

Limits temperature drop over heat

exchanger

Limits amount electricity produced

Input temperatures typically 120-130oC

Output temperatures typically 100 – 80oC

(risky)

Further exacerbates silica deposition

Geothermal Silica Precipitation - The Solution

Remove the dissolved supersaturated silica by rapid precipitation as a

nanostructured calcium silicate

Ca2+ + H3SiO4- + OH- CaSiO3-x(OH)2x + H2O

Addition of Ca(OH)2 (commodity slaked

lime) during induction period

Forms discrete particles

Do not adhere to metal surfaces

Do not form a scale

NCS particles flow freely through the

pipes and heat exchangers

Provide a saleable product

High surface area – up to 600 m2 g-1

High liquid absorption – up to 6x



Initial field trials


Comparison of laboratory and field results


Proving nanostructured calcium silicate does not

adhere to pipes and heat exchangers

Nanostructured Calcium Silicate NCS

Properties

Proprietary material protected by Patent

• Open Framework structure of nano-size platelets - “Desert

Rose”.

• High accessible pore volume - Oil Absorption up to 600g oil.

100g-1 .

• High accessible surface area - up to 600 m2.g-1

• Functionalised surface for specific applications using particular

cations, anions, conducting polymers.

• Excellent sorption properties – environmental remediation

Applications of Nanostructured Calcium Silicate

Utilises the unique nanostructure, high surface area and high

absorption properties of NCS

• Paper - Improving ink jet print quality of paper

• Food packaging and transport – composite NCS-phase change

thermal buffering package for transporting temperature sensitive

food

• Antimicrobial, Antifouling – composite NCS-silver antimicrobial

material as an additive to paint.

• Environmental – absorption of phosphate from lake and stream

waters

Calcium phosphate formed that can be re-used as a fertilizer

• Environmental – absorption of copper, zinc and other heavy

metals from industry and mine wastes.

Recovery and re-use


NCS - AgCl/Ag

NCS - Ag

Control 1 wt % Ag 1 wt % AgCl/Ag

Marine Antifouling Paint


Improving print quality of paper

Enhances quality of

ink-jet printing

New opportunity

Enhances quality of

ink-jet printing

New opportunity

Reduces print

through Solves

current problem

uncoated paper paper coated with NCS

Print Through vs. Filler Loading for 55gsm TMP

Newsprint

0

0.02

0.04

0.06

0.08

0.1

0.12

0.14

0.16

0.18

0 2 4 6

Filler Loading (wt%)

Pri

nt

Th

rou

gh Calcium-Silica

Calcined Clay

GCC

Sipernat 820A


NCS absorbs Cu2+ and SO42-

Forms Brochantite Cu4(OH)6SO4

NCS dissolves

P uptake, c0(P)=0.3 mg/L from added KH2PO4 with 250ppm nCaSil (from

100g as made slurry) in 10L batch, pumped for mixing by circulation

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

time / hr

c(P

) /

mg*

L-1

c (P) [mg/L]


• Nanostructured calcium silicate particles form immediately

• Dissolved silica level substantially reduced to below saturation levels

• No more silica precipitation

• Nanostructured calcium silicate particles travel as a dilute suspension

through pipes and heat exchangers

• Lower steam-water separation temperatures – down to 100oC

• Lower binary cycle heat exchanger – down to

• Nanostructured calcium silicate recovered as a useful product - $ return

• Reinject silica-free water at a lower temperature

• No blocking of reinjection wells

What Does it Offer

• Enhanced recovery of energy as electricity – approx 15-20%

• Obviates silica deposition in pipework – reduced maintenance costs

• Obviate silica deposition in reinjection wells – reduced maintenance and

need for new wells

• Better utilisation and greater $ return from the geothermal resource

Summary

Dr Thomas Borrmann Dr Mathew Cairns

Dr Andy Mcfarlane Dr Giancarlo Barassi

James Grindrod Tobias Asam

Matthias Krapf Christian Krauss

David Flynn

School of Chemical and Physical Sciences, VUW

The Foundation for Research Science and Technology

Acknowledgements

Documents

Geothermal Energy