Frequently asked questions on Carbon Capture and Storage (CCS)

Effectiveness of CCS for waste-to-energy plants

Aren't there cheaper ways to avoid or compensate CO2, e.g. renovation of buildings, electric mobility or reforestation?

In fact, CO2 storage is a rather expensive technology - and we should definitely use all other possibilities to avoid or compensate for CO2 emissions in parallel. However, according to the IPCC, achieving the climate targets requires CO2 storage: in practically all scenarios some of the CO2 will have to be filtered out of the air again (for example by direct air capture) [1] and then stored/bound in geological formations. Compared to this solution, it makes both ecological and economic sense not to let the CO2 escape into the atmosphere first, but to capture it immediately at the large point sources (capture from a waste-to-energy plant is about 2-3 times more energy-efficient than from the air).

[1] IPCC (8 October 2018), Press release on Special Report on Global Warming of 1.5 °C. Full report.

Shouldn't we rather work on reducing the quantities that are incinerated in waste-to-energy plants and recycle more plastic?

This should definitely be part of the solution and has the potential to keep waste volumes stable or even slightly reduce them. A Prognos study from the year 2017 ("Municipal Waste Generation in Switzerland 2050") has presented various scenarios. In a "business as usual" scenario, the quantities to be incinerated would increase by 36%, driven by economic and population growth - and even a very ambitious recycling strategy would only reduce the quantities by 10%.

It should also be noted that plastic cannot go through an unlimited number of recycling cycles. Recycling can significantly increase the useful life of plastic, but in the end plastic must be incinerated. Furthermore, the industry is not showing any signs of reducing the amount of plastic produced. On the contrary, the production of plastic will increase dramatically [1].

[1] The Future of Petrochemicals: Towards more sustainable plastics and fertilisers, Methodological annex, Figure A.1 S. 7, OECD/IEA 2018

Couldn't we reduce the amount incinerated by separating out more biological waste and, for example, processing it into methane in biogas plants?

Biogenic waste should be separated out, especially because it seriously affects the storability of municipal waste and because valuable nutrients and structural material for our soils are lost in incineration. But a further increase in the sorting of biogenic waste is hardly possible today. The main limitation here is the increase in unwanted materials, such as plastic, which goes hand in hand with each increase in the amount collected separately.

Couldn't CO2 be used as a raw material, for example for the synthesis of methane (power-to-gas)?

This is possible and is also being tested in various pilot plants. However, there are three fundamental limitations:

1. To carry out the chemical reaction CO2 + 4 H2 --> CH4 + 2 H2O, hydrogen (H2) must be produced first. The production of hydrogen by electrolysis is very energy-intensive. To convert one million tons of CO2 into methane (CH4), more than 180'000 tonnes of H2 or 9 TWh electricity is needed for the electrolysis [1]. This corresponds to 16% of the current electricity consumption in Switzerland [2]. In order for power-to-gas to be an acceptable part of climate policy, the electricity required would have to come from renewable sources.

2. From an efficiency perspective, it makes more sense to directly use the hydrogen produced as a fuel, for example in a fuel cell. A fuel cell produces electricity by converting the hydrogen together with oxygen from the air to water.

3. When the methane produced by power-to-gas is burned, CO2 escapes again. In this sense, power-to-gas is not a CO2 sink, but only short-term storage.

[1] Electrolysis on an industrial scale consumes about 50 kWh electricity per kg H2 produced.

[2] Switzerland's electricity consumption in 2017 was 57.6 TWh (BFE 2019).

From a life cycle perspective, does CO2 storage make any sense at all if we have to build new infrastructure such as pipelines?

Scientistis at the Paul Scherrer Institute conducted a full cycle analysis for CCS at a cement production plant. They found a reduction in CO2 emissions of up to 78%. The study assumed 200 km pipeline transport and 1000 m storage depth [1]. With biogenic materials, e.g. wood, even a negative emission balance is possible.

Furthermore, a preliminary life cycle analysis was carried out for the transport of CO2 from Swiss waste incineration plants for deep geological storage off the Norwegian coast. The CO2 is transported by train to Rotterdam and then by ship to the storage site. This transport route causes CO2 emissions of below 6% of the amount of CO2 that is stored. The infrastructure and containers used for transport are designed in such a way that no transported CO2 can escape into the atmosphere.

[1] Volkart, K., Bauer, C., & Boulet, C. (2013). Life cycle assessment of carbon capture and storage in power generation and industry in Europe. International Journal of Greenhouse Gas Control, 16, 91–106.

How high are the CO2 emissions caused by transport to the storage site?

For the transport of CO2 from a Swiss waste-to-energy plant for deep geological storage off the Norwegian coast, a preliminary analysis of transport-related emissions was carried out by the Sustainability in Business Lab. The CO2 is transported by train to Rotterdam and then by ship to the storage site. This transport route causes CO2 emissions of just under 6% of the amount of CO2 that is stored. The infrastructure and containers used for transport are designed to prevent the transported CO2 from escaping into the atmosphere.


Do realistic possibilities for permament CO2 storage exist?

Yes, the most advanced options currently available in Europe are in the North Sea, where empty gas deposits can be used as storage facilities. In Norway, the industrial consortium "Northern Lights" consisting of Equinor (former Statoil), Shell and Total is working on a facility to store CO2 from an Oslo waste-to-energy plant and a cement factory.

Norway also plans to open these storage facilities to European emitters in around 2024 (provisional timetable). The Norwegian Parliament will make an investment decision for the project in 2020/2021, so that capacities of approx. 5 mio. tonnes of CO2 could be made available from 2023/2024. The timeline for larger capacities will then depend on the interest of other countries and industry.

Similar efforts are underway in the Netherlands: for example, the Porthos project is expected to make an investment decision in 2020 to provide a storage capacity of 2-5 mio. tonnes of CO2 per year in former gas deposits below the North Sea.

Do sufficiently large storage facilities exist?

Yes. An estimate for Norway, for example, has shown available offshore storage capacity of 70 gigatonnes of CO2, equivalent to about 20 years of EU28 emissions at current levels.

Is the technology proven?

There are >20 large-scale plants (existing or planned) worldwide with a total capture capacity of more than 400,000 tonnes of CO2 [1].

[1] Global CCS Institue (2020), Map of global carbon capture and storage facilities.

Do possibilities for CO2 storage in Switzerland exist?

Deep geological storage of CO2 will probably not be available in Switzerland within the next 10-20 years, as a systematic investigation of the underground is still pending and potential storage sites would have to be investigated in a long-term test [1].

In the longer term, however, there is potential: Switzerland also has "deep saline aquifers" at suitable depths of 800-2500 m and combined with impermeable rock layers. However, key information is currently still missing to enable the most promising sites to be selected, such as the exact location of suitable formations.

At present, public funds are being used in Switzerland to promote the characterisation of geology for geothermal energy production. It might be useful to combine this research with an examination of suitability for CO2 storage.

[1] Based on current estimates, theoretical (unproven) storage capacity in Swiss deep porous geological formations may be up to 2.6 Gigatonnes of CO2. This equivalent to storing approximately 70 years' worth of current Swiss CO2 emissions.


How much does the separation of one tonne of CO2 from the flue gas of a waste-to-energy plant cost?

The costs depend strongly on the price of the energy used. Without thermal energy, the lower limit is around CHF 20 per ton of CO2. If purification and liquefaction are required, the costs increase to about 56 CHF per ton of CO2.

In addition, there are opportunity costs for thermal energy, which could otherwise be used e.g. for electricity generation, of approx. 46 CHF per captured ton of CO2 [1] or 75 CHF if energy-intensive purification and liquefaction is required [2].

The total costs are therefore between 68 CHF and 131 CHF per tonne of CO2. These figures can be determined precisely when planning a specific plant.

The costs mentioned are for a separation of about 90% of the CO2 emitted. The International Energy Agency (IEA) has shown that the capture rate could be increased to 99%, with additional costs of ~8% per tonne of CO2 [3].

[1] Thermal energy of 0.8 MWh per tCO2, 66% electrical efficiency, electricity price of 8.7 Rp/kWh

[2] Thermal energy of 1.3 MWh per tCO2, 66% electrical efficiency, electricity price of 8.7 Rp/kWh.

[3] IEAGHG (March 2019), Towards zero emissions CCS in power plants using higher capture rates or biomass,

How much does the permanent storage of one ton of CO2 cost?

Current rough cost estimates for the entire process chain consisting of transport and geological storage in Norway are around 340 CHF per tonne of CO2 [1]. However, this very high price applies to pilot plants: it is expected that the costs will already be reduced to around 110 CHF per tonne of CO2 over the next 10 years [2]. This would correspond to a total amount of CHF 400 million in annual costs for the Swiss MSW incineration plants. This cost estimate does not take into account revenues from CO2 certificates, which could play a role in the future. A more precise cost estimate is currently being prepared and will be available by the end of 2020 at the latest.

[1] Assuming that the CO2 is transported from Switzerland to Norway by train and ship.

[2] Assuming that a pipeline is built to transport large quantities of CO2 to Norway.

Technical details

How high is the CO2 concentration in the flue gas of a waste-to-energy plant?

The concentration is about 10%. This also means that the separation is much more efficient than later separating CO2 from the air, where the concentration is only 0.04%. Separation from a MWIP is about 2-3 times more energy efficient than from air.

How much energy is required to separate one tonne of CO2 from the flue gas of a waste-to-energy plant?

CO2 separation from the flue gas is relatively energy-intensive: 1 MWh of thermal energy and 0.1 MWh of electrical energy is required per ton of CO2. To liquefy the captured CO2 before transport, additional electricity of 0.16 MWh per ton of CO2 is required. However, it is to be expected that intelligent integration of the plant with the heat and power generation of the MWIP will make it possible to cover a large part of the energy requirement from currently unused energy (depending on the plant).

Is a pipeline the only way to transport the CO2?

Transportation by rail or ship is also possible and useful for pilot projects where small amounts of CO2 are transported. In the long term, however, pipelines for the transport of the expected large quantities of CO2 are likely to be considerably cheaper. To this end, detailed studies are currently underway in international research projects in which researchers from ETH Zurich and the Paul Scherrer Institute are also involved. One such project is Elegancy.

How does CO2 capture work chemically?

The most widely used technology is capture with amines, where CO2 is bound first, separated from the flue gas and then released again to form a pure stream of CO2. The amine is therfore free again and the reaction can be repeated. Prof Greeves from the university of Liverpool modeled the reaction and you can watch it here

How CO2 storage works

Has the safety of geological CO2 storage already been tested?

The storage of CO2 in geological formations has been tested worldwide for several decades, partly in the exploitation of oil and gas, partly specifically in connection with carbon capture and storage. This has led to the development of regulatory frameworks and industry standards to make it safe, such as the EU directive on the geological storage of CO2 (so-called "CCS directive") or ISO/TC 265 - Carbon dioxide capture, transportation, and geological storage.

In Europe, for example, Equinor (formerly Statoil) has carried out a long-term test over 20 years since 1996 in the Sleipner project, in which 15.5 Mt CO2 (0.9 Mt per year) was injected into deep saline aquifers 800-1000 m below the seafloor. Regular monitoring and seismological measurements show that no CO2 escaped.

In 2018, a study published in Nature [1] examined the storage security of six large-scale carbon capture and storage projects* as well as other smaller projects. None of the plants has so far been able to detect any measurable CO2 leakage. Of course, further research on risk assessment and long-term storage is necessary.

*Sleipner and Snøhvit, Norway; Aquistore and Quest, Canada; In Salah, Algeria; and the Illinois Industrial Carbon Capture and Storage (IICCS) project, USA.

[1] Alcalde, Juan et al. "Estimating geological CO2 storage security to deliver on climate mitigation." Nature communications vol. 9.1 2201. 12 Jun. 2018, DOI: 10.1038/s41467-018-04423-1

In which state is the CO2 stored?

The CO2 is injected into the reservoir rock under pressure. The injection pressure must be higher than the pressure in the reservoir to displace the water in the storage rock. At the same time, it is ensured that the injection pressure is well below the fracture limit of the storage and barrier rock. Geomechanical analyses and models are used for this purpose.

In the Sleipner project, for example, which has been storing one million tons of CO2 annually at a depth of 800 meters below the bottom of the North Sea since 1996, the injection pressure is 60 bar. The Northern Lights project will store CO2 in the Johansen Formation, which is located at a depth of around 2000 to 3000 meters. The pressure in this reservoir is correspondingly higher, at 200 to 300 bar.

The figure below shows the relationship between depth below the earth's surface and the density of CO2. The rock layers underground are saturated with water, so the pressure increase per km depth is about 100 bar. At the same time, the temperature increases by 25 to 40 °C per km depth (geothermal gradient), depending on the geographical region.

With increasing depth, the gaseous CO2 is compressed further and further until it enters the so-called "supercritical phase" at a depth of about 800 metres. In addition to "gaseous", "solid" and "liquid", this is another state of matter in which gas and liquid no longer differ: supercritical CO2 has a density similar to that of liquid CO2, but has the same viscosity as a gas. Viscosity is a measure of the viscosity of a liquid: a gas is a thin liquid and therefore has a lower viscosity than a liquid. The high density of supercritical CO2 means that a volume of 1000 m3 of CO2 at the earth's surface at a depth of 2 km only occupies a space of 2.7 m3. Accordingly, the supercritical phase is optimal for storing the CO2.
When the CO2 is injected into the underground, it becomes a supercritical fluid at a depth of about 800 meters. This reduces the volume of the stored CO2 to a fraction of the volume at the surface (source: [1]). This video clearly explains how the density of CO2 changes during the transition to the supercritical phase:
[1] CO2GeoNet (2009), Geologische CO2-Speicherung - was ist das eigentlich?,

How is a CO2 storage reservoir structured?

A geological storage reservoir for CO2 consists of three layers:

  1. Impermeable cap rock
  2. Permeable reservoir rock
  3. Impermeable bottom rock
The storage layer is a deep aquifer that carries salt water (saline aquifer). It is located at a depth of at least 800 metres, i.e. well below the groundwater level used for drinking water production (see figure below). The reservoir layer consists mostly of sandstone or limestone, as this has many pores and is permeable to CO2. In contrast, the cap and bottom rocks consist of clay or salt rock, which is impermeable to CO2 and water.

The figures below illustrate the structure of the geological reservoir and the mechanisms for CO2 storage in the reservoir rock. Structure of the underground rock. The saline aquifer in which the CO2 is stored is located hundreds of metres below the aquifers used for drinking water. The Eiffel Tower (330 metres high) is shown as a size reference. The cap and bottom rocks (above) are impermeable to water and CO2, whereas the reservoir rock (below) is permeable.

What happens to the CO2 in the reservoir during the storage process and over time?

The CO2 is injected in the lower part of the storage layer. During and after injection into the reservoir rock, four different processes take place to ensure reliable storage ("trapping") of the CO2:

1. Accumulation below the barrier rock (structural trapping)
The injected CO2 displaces the salt water that is in the pores of the reservoir rock. Because the CO2 has a lower density than water, it rises until it meets the barrier rock. Since the barrier rock is impermeable to CO2, it collects underneath. This process takes place on a scale of years.

2. Binding in small pores (residual trapping)
If the rock pores of the reservoir layer are sufficiently small, part of the rising CO2 is captured by capillary forces. This process takes place on a scale of decades.

3. Solution in the salt water of the reservoir rock (solubility trapping)
Over a period of hundreds of years, part of the injected CO2 dissolves in the salt water of the reservoir rock. This increases the density of the salt water so that it slowly sinks to the bottom. In the Sleipner project, it is estimated that 10 years after injection, about 15% of the injected CO2 has dissolved.

4. Mineral precipitation (mineral trapping)
In the long term, typically on a scale of about a thousand years, part of the CO2 is converted by chemical reactions into minerals called carbonates. Through this process of mineralization, the CO2 is permanently bound as a solid deposit in the reservoir rock. However, in certain reservoirs, such as the basalt rock in Iceland, this process can take place much faster - over 1-2 years.

The relative importance of the four retention mechanisms depends on the location of CO2 storage. On this website you can find some videos (in English) to better understand the retention mechanisms: The next question deals with how and at what speed the injected CO2 is distributed in the storage rock.

How and at what speed does the injected CO2 propagate through the storage layer?

This video, which illustrates the processes after injection of CO2, was produced as part of a project at ETH Zurich:

  1. The CO2 is injected in the lower part of the saline aquifer. The aquifer is shown in blue, the cap and bottom rock layers are black.
  2. The CO2 rises upwards within the reservoir rock, as it has a lower density than the salt water in the rock. After about 40 years it reaches the impermeable cap rock. In the simulation, the CO2 injection is terminated after 50 years.
  3. After about 100 years most of the CO2 will have diffused to the top of the reservoir rock. Due to the structural trapping by the cap rock, it spreads further to the left and right. The CO2 diffuses increasingly slowly through the rock and is converted more and more into solid minerals over hundreds of years. In this way it is safely stored.

How safe is geological CO2 storage? Is there a risk of leaks?

The CO2 is stored in the same layers of rock that have safely trapped natural gas reserves for millions of years. Research into the properties of these natural storage systems and the careful selection of CO2 reservoirs ensure that the risk of leaks is very low. In fact, especially in volcanically active regions, there are even natural geological deposits of large amounts of CO2: the Bravo Dome field in New Mexico (USA) contains 1.6 gigatons of CO2, which have been stored there for about 1.3 million years [1].

In the figure below, possible mechanisms that could cause leaks and thus lead to a slow release of the stored CO2 are illustrated.
Possible leakage paths for geologically stored CO2 (source: CO2CRC/ETH Zurich).
In order to ensure that no CO2 escapes, the selection of the storage site is based on an investigation to ensure that no leakage paths such as a leaking cap or geological faults exist. Furthermore, the CO2 reservoir is continuously monitored during and after CO2 injection. Three different techniques are used for this purpose:

  1. Geophysical methods, for example seismic measurements. These can be used to map the properties of the rock layers and geological faults in order to assess the suitability of a potential reservoir for CO2 storage. During and after CO2 injection, seismic data shows where the CO2 is located (see the figures below for the Sleipner project).
  2. Computer models of the CO2 storage facility, which model the dispersion of the CO2 in the rock. These models serve to understand the behaviour and movement of the CO2 over hundreds of years.
  3. Continuous measurements at the surface and in the wells to directly detect escaping CO2.
If, contrary to expectations, a leak should occur, the CO2 does not suddenly escape in large quantities, but escapes slowly. Furthermore, methods have been established to seal leaks, including the injection of cement or smart polymer gels. Lastly, there is always the possibility of pumping the CO2 out again via the injection wells and storing it in another reservoir. Seismic studies of the Sleipner project, in which one million tonnes of CO2 have been stored annually at a depth of 800 metres in the Utsira formation under the floor of the North Sea since 1996 (source: Philipp Ringrose, NTNU). Above: Cross-section through the rock layers. Before the CO2 injection, seismic measurements allow to map the different layers of the Utsira Formation (1994). In 2008, the reflection amplitude is higher where CO2 was injected. This is illustrated by a more intense coloration.
Below: Top view of the Utsira Formation. The seismic measurements show how the CO2 plume spreads in the plane after injection.

[1] K.J. Sat[haye, M.A. Hesse, M. Cassidy, D.F. Stockli, Constraints on the magnitude and rate of CO2 dissolution at Bravo Dome natural gas field, Proc. Natl. Acad. Sci. U. S. A. 111 (2014) 15332-15337.


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