Geothermal energy is a renewable energy source
that gives off low emissions compared to conventional fossil fuel plants.
Even though emissions are low the steam used at the Hellisheidi Geothermal
Power Plant contains gases including carbon dioxide (CO2). CO2
is a green house gas and is believed to play a key role in climate
change. By capturing the CO2 and injecting it into selected
geological sites where it will react with basaltic rock and get stored as solid
calcium carbonate, the adverse effects of the power plant can be further
reduced.
Objectives
Inject water with high enough dissolved CO2 concentration so that
favorable reaction with the basaltic rock will occur. Mix tracers with
the fluids continuously so the CO2 sequestration can be monitored.
Methodology
Reykjavik Energy intends to move CO2 gas under pressure from its gas
purification plant at the Hellisheidi Power Plant to an injection well a few kilometers from the plant. There the CO2
will be dissolved in water and injected into the well at an approximate depth
of 500 meters. Prior to the mixing and injection, tracers will be added both
to the liquid and the gas stream. These tracers will be used to monitor
the sequestration process.
Mannvit Engineering proposed several designs for the process and in
collaboration with participants from the University of Iceland, the Earth
Institute of Columbia University, CNSR and Reykjavik Energy came up with the
final design. The final design combines cost efficiency, reliability and
low maintenance.
Design
The design that was developed for the project is as follows:
- The CO2 gas is lead in a plastic pipeline to the
wellhead.
- A gas tracer is dosed into the CO2 stream from a high pressure
storage tank.
- The water is pumped to the injection well.
- A tracer is dosed into the water stream with high accuracy dosing
pump.
- The water containing the tracer flows into the injection pipe.
- The gas pipe continues down the well inside the injection pipe down
to a water depth where the pressure in the gas pipe is only slightly
higher than the hydraulic pressure in the injection pipe. This
ensures that the driving force of the CO2 dissolution is high
and the gas volume is kept relatively low.
- The gas is injected into the outer pipe by maximizing the
interfacial area between the gas and liquid phase to ensure a rapid CO2
dissolution.
- A static mixer is placed lower in the injection pipe in order to
get a uniform mixing of the liquid and help dissolve any remaining gas
bubbles.
- The injection pipe leads the water containing the dissolved CO2
and tracers down to the desired depth.
This method is
preferred since no additional compression of the CO2 is required and
gravity provides the required water pressure. This method also makes it
possible to decrease costs by making most of the pipes out of plastic instead
of using steel.
Conclusion
Downhole mixing of CO2 in water was selected as the optimal solution
for the re-injection of CO2 captured from the geothermal steam used
at the Hellisheidi power plant. The solution combines cost efficiency,
reliability in operation and low maintenance.