Module 1 CO2 capture and storage overview

Original text: W. Gunter, APEC Capacity Building in the APEC Region, Phase II Revised and updated by CO2CRC and ICF International

Overview

The introduction to this training manual outlines the role of CO2 capture and storage (CCS) in a range of measures to reduce greenhouse gas emissions and thereby avoid dangerous climate change. CO2 capture and storage represents an interim option to reducing CO2 emissions – the main greenhouse gas. This module introduces these technologies.

Learning objectives

By the end of this module you will:

  • Be familiar with general climate change science and the various technologies available to reduce greenhouse gases;
  • Understand how CO2 capture and storage can reduce greenhouse gas emissions; and
  • Understand the current status of CO2 capture and storage technology.

The science of climate change

The climate of planet earth is currently comfortable and hospitable to human civilization. Actual measurements averaged over the year and over all latitudes indicate that the mean temperature is about 15°C. Calculations show that this average temperature is about thirty five degrees warmer than if there was no atmosphere, in which case the mean temperature would be -20°C. This difference is due to the "greenhouse effect" of the Earth's atmosphere.

The greenhouse effect is a natural phenomenon. Most of the solar radiation hitting the earth is lost to space by reflection from the surface and the atmosphere. However, certain gases in the atmosphere, the so-called greenhouse gases (GHGs), absorb and then quickly re-emit the infrared or heat radiation. In effect, these gases trap heat in the atmosphere, resulting in a rise in the Earth's surface temperature. The most important greenhouse gases are water vapour (H2O), carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O), tropospheric ozone (O3) and chloroflorocarbons (CFCs such as CFCl3 and CF2Cl2) – the latter are strictly man-made or anthropogenic.

Figure 1.1 Carbon dioxide emissions are enhancing the greenhouse effect of the atmosphere (courtesy of CO2CRC)

An increase of greenhouse gases in the atmosphere due to human activities will increase the atmosphere's natural warming capacity. Global warming is the term commonly used for this process. Fossil fuel consumption, cement production, land-use changes and forest fires (natural and man-made) are the main causes of the excessive accumulation of carbon dioxide. The accumulated methane largely comes as a byproduct of digestion by livestock, agriculture production practices, garbage decomposition, and the production of fossil fuels. However, carbon dioxide is the most important GHG because of its abundance and longevity. It contributes more than half of the enhanced greenhouse effect – the rest being mainly due to increases in methane and CFCs.

Climate scientists predict that global warming will trigger changes in the average weather experienced in a region. At a global level, such changes in weather could impact the Earth's climate balance – a phenomena called climate change. A change in the Earth's climate does not require a very large increase in greenhouse gases in the atmosphere; and it is highly likely that the accumulated GHGs that are now in the atmosphere, and that will remain in the atmosphere for decades to come, could by itself trigger these reactions.

The Intergovernmental Panel on Climate Change (IPCC), which is made up of some of the world's leading climate scientists, have concluded that, despite uncertainties, the model simulations of a warming due to increased concentrations of greenhouse gases are broadly consistent with observations of global temperature changes (IPCC, 2007). The IPPC assessment is based on an ongoing scientific assessment of climate change science and its impacts.

There has been an increase in global mean temperature over the past century, due to increased introduction of carbon dioxide into the atmosphere from the burning of fossil fuels. The effects of climate change are such that they will likely be major deleterious social and economic consequences if the warming trend is not slowed, or even reversed. For example, low-lying areas and entire economies could be threatened by flooding, and crops would be affected by the change in climate. The IPCC predicts an average global temperature rise of about 0.4°C by 2020 and a rise of between 2°C and 6°C by the end of the 21st century. Sea level could rise by 20cm to 60cm by 2099 under the influence of these temperature rises (IPCC, 2007).

Sources of anthropogenic CO2 can be centralized, as in a power generating station, or diffuse, as in the use of motor vehicles. No single method of CO2 emissions reductions will be adequate to meet reduction objectives, since no single method can address the issues related to both large central and diffuse emission generators.

Technologies to reduce CO2 emissions to the atmosphere

CO2 emissions are a function of several factors. Population and the standard of living are two key variables that will not be discussed in this document as reductions in either of these variables are contrary to the public policies of most governments.

Reducing anthropogenic CO2 emissions into the atmosphere involves basically three approaches, as shown by the equation below, (Kaya, 1995; Bachu, 2003). These involve relationships between carbon emissions (C), energy (E) and economic growth as indicated by Gross Domestic Product (GDP):

In this equation, E/GDP is the "energy intensity" of the economy, C/E is the "carbon emission intensity" of the energy system, and S represents carbon removed from the atmosphere through carbon sinks. Carbon sinks are reviewed in more detail later in this module.

As a general trend, historical evidence shows that:

  • The emissions intensity (C/E) has decreased continuously since the beginning of the industrial revolution;
  • The carbon removed from the atmosphere (S) decreased slightly as a result of deforestation and agricultural practices; and
  • The net carbon emissions (C) increased at a faster rate than the decrease in emissions intensity, mainly as a result of the increase in economic growth (GDP).

Since the general trend in GDP is to increase, a reduction in net CO2 emissions into the atmosphere can only be achieved by:

Lowering the energy intensity of the economy E/GDP – by increasing the efficiency of primary energy conversion and end use. A very attractive and cost effective solution (which will reduce energy intensity) is energy conservation, although it will require tough policy measures. Solutions include improving energy and material efficiency or modifying industrial processes, which will lead to a lowering of the rate of CO2 generation. This could be a promising solution in the short- to medium-term.

Lowering the carbon intensity C/E of the energy system - by substituting lower-carbon or carbon-free energy sources for the current sources. An option to reduce carbon intensity is to increase the use of renewable resources. However, until such energy sources can be developed and applied on a large scale, fossil energy resources will continue to be the primary energy sources around the globe. During this period, reduction in carbon intensity could be achieved by switching to low carbon alternative fuels (for example switching to natural gas). Again, this is a long-term solution.

Artificially increasing the capacity and capture rate of carbon sinks The issue of emissions reduction is a complex one, and will only be solved by innovative responses that include both reducing the quantities of these gases emitted by anthropogenic activities, and enhancing and using greenhouse gas sinks by carbon sequestration in the biosphere, in materials and in the geosphere. These create short and medium term solutions to deal with the problem of increasing CO2 emissions.

However, short of revolutionary, large-scale new technological advances and major expenditures, the energy intensity of the economy will continue to decrease at a lower rate than the rate of GDP increase and mitigation strategies will have a limited impact (Turkenburg 1997). Similarly, fossil fuels, which currently provide more than 80% of the world's energy (88% according to the BP Statistical Review of World Energy, June 2009), will likely remain a major component of world's energy supply for at least this century because of their inherent advantages, such as availability, competitive cost, ease of transport and storage, and large resources. Thus, the carbon intensity of the energy system is not likely to decrease in any significant way in the medium term. This leaves the increase of carbon sinks and of their capture rate in a significant way as one of the major means of reducing net carbon emissions into the atmosphere in the short to medium term.

The IEA projects that more than 2.5 Gt of CO2 will need to be captured and stored annually by 2030 to meet the Blue Map Scenario of reducing CO2 emissions to 50% of 2005 levels (IEA, 2009). Several studies have been conducted over the past decade to estimate the amount of CO2 that can be stored in sedimentary basins. The IEA puts the estimates of global storage capacity at between 8,000 and 15,000 GtCO2 (IEA, 2009). This suggests that we have the capacity to store most, if not all, of the CO2 needed to prevent the build-up of harmful levels of CO2 in the atmosphere.

CO2 capture and storage

Capture

CO2 can be more easily captured from large stationary sources of CO2.

These sources include:

  • Fossil fuel power plants
  • Petroleum refineries
  • Oil and gas production
  • Iron and steel mills
  • Cement plants
  • Chemical plants

Figure 1.2: Options for capturing CO2 emissions: Natural gas separation, combustion of fossil fuels for power, chemical industries (courtesy of CO2CRC).

Early opportunities for CCS projects exist in the natural gas industry. CO2 naturally occurs with methane underground and it must be removed from the natural gas prior to sending to customers. Currently, the separated CO2 is mostly just released into the atmosphere. The Sleipner Project in Norway has been storing CO2 stripped from natural gas in a saline reservoir deep beneath the seabed since 1988. More recent projects at In Salah in Algeria and Snøhvit in Norway are also storing CO2 in this way, as will the recently approved Gorgon Project on the North West Shelf of Western Australia, which is expected to commence injection in 2015.

Carbon dioxide is also captured as part of the manufacture of ammonia and other chemicals and/or fuels.. Ammonia is most commonly manufactured by converting methane gas into a syngas (largely carbon monoxide (CO), CO2 and hydrogen). The CO is converted to CO2 and removed. Ammonia is made from the hydrogen. Urea, an important fertilizer, is manufactured from ammonia and CO2. Several plants around the world capture CO2 for this purpose.

Iron and cement production creates large amounts of CO2 from the use of raw materials and from the energy to fuel the process. In the case of iron and steel, the CO2 is released from the use of coking coal and limestone to reduce iron ore and to remove excess carbon to form steel. In the case of cement, CO2 emissions come from the use of calcium carbonate to form clinker as well as from the large amounts of energy used in the process.

However, if significant reductions are to be made in the amount of CO2 released to the atmosphere, then the emissions from coal-fired power plants must be captured and stored too. This will mean adapting the technology to retrofit existing power stations to capture the CO2 after the coal is burnt (post-combustion capture) or build new higher efficiency power stations with CCS. Another technology is to apply new gasification technology to power production in which coal is "gasified" using oxygen or air to create a combustible gas then capture the CO2 before burning the gas for power (pre-combustion capture). There are operating gasification plants which capture carbon dioxide, but it is not the preferred method for power production due to higher costs and technology risks. Another new power production technology is to capture CO2 by first separating oxygen from air and burning coal in an oxygen-rich gas. This is called "oxyfuel", 'oxy-combustion' or "oxy-firing". The resulting gases contain largely CO2 and water which simplifies the capture process.

The major separations technologies for capturing CO2 currently are:

  • using a liquid solvent to absorb the CO2 (absorption);
  • using solid materials to attract the CO2 to the surface, where it becomes separated from other gases (adsorption); and
  • using membranes to separate the CO2 from the other gases.

Other technologies include chemical looping technology (a metal oxide reacts with the fuel, creating metal particles, carbon dioxide and water vapour), low temperature or cryogenic separation processes (which rely on different phase change temperatures for various gases to separate them) and dry regenerable solid processes.

Compression and transport

Figure 1.3: Once the CO2 has been captured, it is compressed to a supercritical state, a very dense gas with liquid-like properties (courtesy of CO2CRC).

The CO2 is dried and usually compressed before being transported to storage. The compression makes transporting the gas more efficient. CO2 is used commercially in a number of industries, notably the beverage industry, and it has been transported on a large scale in the US for use in recovering oil from reservoirs (enhanced oil recovery). While much of this CO2 is from natural sources (CO2 is formed as a result of geological processes and can remain trapped in sedimentary basins). CO2 captured from a variety of man-made sources will contain differing levels and types of impurities, which affects the compression and transport.

Injection and storage

Figure 1.4: Options for storing CO2 in geological formations (courtesy of CO2CRC).

There are a variety of types of geological formations that can be used to store CO2. These are found in sedimentary basins. Sedimentary basins are subsiding regions of the Earth's crust that, by their shape, permit the net accumulation of sediments that result from various processes, such as: a) erosion of preexisting rocks exposed on land (e.g., sands and muds); b) deposition of organic material; c) precipitation from water (e.g., salts); and d) volcanism (deposition of volcanic ash). As these sediments are piled and buried, they undergo a process of lithification and become sedimentary rocks, such as sandstones, carbonates, shales, coal, salt rock, tuffs and bentonites.

Deep saline aquifers-One of the more promising avenues for CCS has been the storage of carbon dioxide into saline aquifers deep in sedimentary basins. Of the various methods that have been suggested for the storage of carbon dioxide in sedimentary basins, saline aquifers have the largest capacity (Figure 1.4).

Oil and Gas Reservoirs-One of the most obvious, though not necessarily the best or largest, sinks for carbon dioxide in sedimentary basins are oil and gas reservoirs. There are four key reasons why the injection of CO2 into oil and gas reservoirs is relevant:

  • CO2 can be injected into depleted oil and gas reservoirs The CO2 may be injected into depleted oil and gas reservoirs, in which case the principle is that the conditions that allowed the hydrocarbons to accumulate in the first place, will also permit the storage of CO2 in the pore space vacated by the produced hydrocarbons. Intuitively, the carbon that is taken out as oil and gas can be returned as CO2.
  • Recoveries of oil can be increased. Under the right conditions, CO2 is a miscible solvent for oil, and oil recoveries can be increased substantially through miscible flooding of the reservoir (termed enhanced oil recovery, or EOR). In effect, the residual oil is washed from the reservoir rock by the CO2 solvent. Some of the CO2 is returned to the surface with the crude oil production, but this is recycled to recover further oil.
  • Recoveries of natural gas can be increased. CO2 may be used to displace residual natural gas in depleted gas reservoirs, in a process known as enhanced gas recovery (EGR).
  • Coalbeds-CO2 may be used to produce additional methane from coalbeds. This is a process known as enhanced coalbed methane recovery (ECBMR).

A number of conditions need to be satisfied to store CO2 geologically. In general, this includes the availability of:

  • Porosity – sufficient pore space to store the CO2 ;
  • Permeability – sufficient pathways for the CO2 to move through the rocks;
  • Sealing rock – sufficient impermeability to prevent upward movement of buoyant CO2; and
  • Depth – adequate enough to keep the CO2 as a supercritical fluid.

Figure 1.5: Tiny pore spaces in the storage rock shown by the blue spaces between the white grains of quartz in this photograph of a microscopic section of sandstone (courtesy of CO2CRC).

Sedimentary basins are suitable for CO2 storage because they possess the right type of porous and permeable rocks for storage and injection, such as sandstones and carbonates, and the low permeability-to-impermeable rocks needed for sealing, such as shales and evaporitic beds.

When CO2 is injected, it is not dissolved in formation water. It is free-phase and immiscible. At reservoir temperatures, it is less dense than the formation water and rises upwards. In order for the CO2 not migrate to the surface, there needs to be a trapping mechanism that keeps the CO2 in the subsurface for thousands of years or longer.

There are several ways in which dense carbon dioxide can be trapped at 800 m or deeper in saline formations or fossil fuel reservoirs in sedimentary basins.

  • Structural/stratigraphic trapping - traps CO2 as a buoyant fluid within geological structures and flow system (Also known as physical trapping or hydrogeological trapping).
  • Residual trapping- CO2 is trapped as small droplets by interfacial (or surface) tension.
  • Solubility trapping - the CO2 dissolves into the surrounding formation water making that water about 1% more dense.
  • Mineral trapping - dissolved CO2 reacts with the reservoir rock, forming solid carbonate minerals

In coal beds, the CO2 is adsorbed onto the coal bed (also known as adsorption trapping).

Choosing a site for storing CO2 is an extensive process. It begins with screening for sedimentary basins, then screening those basins for suitable formations. Site characterisation examines all aspects of a storage site, such as the geological characteristics together with engineering, economic, community and regulatory considerations. Annex I of the European CCS Directive provides a list of items that needs to be characterized for a good storage site.2 It is time consuming and expensive, but a well-designed project is essential to meet the objective of storing CO2 safely and securely.

Monitoring the stored CO2

Once the CO2 is injected, the storage site is monitored to show that the CO2 remains in the reservoir. The monitoring program begins before injection to establish baseline data. Monitoring during the operational phase of the project records the dynamic behaviour of the CO2 as it is injected and within the reservoir. After the injection ceases, the monitoring program is designed to ensure that the CO2 storage meets the environmental and safety conditions required.

A monitoring program covers three monitoring domains:

  • The sub-surface domain (the reservoir);
  • The near-surface domain (shallow zones and soil); and
  • The atmospheric domain, including wells, faults, and other geological features.

Many of the technologies for monitoring a storage project have been used by the oil and gas industries and are being adapted for CO2 storage and are used for site characterization. Geophysical and remote sensing uses seismic, electromagnetic, gravity, microseismic and displacement sensors and petrophysical logging measurements. Geochemical monitoring involves geochemical analysis of fluids, gases, rock/soil, groundwater, surface water and the atmosphere. Environmental sensing techniques include atmospheric gas detection and dispersion modelling, remote sensing techniques including multi spectral analysis. See related materials in Guidance Document 2 and Annex II of the European CCS Directive.3

Outlook

Sedimentary basins, fossil fuel resources, and deleterious greenhouse gas emissions are all closely associated. The main greenhouse gas produced by the burning of fossil fuels is carbon dioxide. With current technology, exploiting fossil fuels will produce greenhouse gases. However, this does not have to be so. Rather than discharge carbon dioxide to the atmosphere, it can be stored in deep aquifers in the same sedimentary basins from which the fuel was extracted - some of the strata can be hydrocarbon-bearing (reservoirs) with the carbon dioxide enhancing oil or gas production.

Injection and storage technologies, developed by the oil and gas industry, are fairly mature. The volume of storage depends on the current and ultimate pressures of the reservoir or aquifer. Experience in injection of CO2 has been gained from repressurizing oil reservoirs using CO2 in enhanced oil recovery and from acid gas re-injection. Similar technology is being developed for production of methane from coal beds (i.e. coalbed methane or CBM). The techno-economic capacity of geological storage of carbon dioxide is likely sufficient for large scale storage, contingent upon identifying secure traps in sedimentary basins. However, there are still significant challenges ahead in making the technology commercial.

Major challenges for the deployment of CO2 capture and storage

Some of the challenges ahead for CCS include:

  • Reducing the cost of capture and scaling up the capture processes;
  • Identifying the environmental impact of capture;
  • Determining the implications of pressure build-up in a storage formation;
  • Determining where the displaced water goes in a large scale injection and what the risk is to ground water;
  • How to reliably predict the size of the CO2 plume and where it migrates;
  • How to gain confidence in site selection;
  • Developing cost effective monitoring strategies and detection limits;
  • Engaging finance and insurance industries;
  • Greater regulatory and political certainty at all levels of government;
  • Training a workforce for large scale deployment;
  • Improve public awareness and acceptance (GHGT-9 conference summary).

Summary

The greenhouse effect is a natural phenomenon. Greenhouse gases trap heat in the earth's atmosphere maintaining a climate hospitable for life. However, when too much greenhouse gases are present in the atmosphere (mostly due to human activities), then the warming potential of the atmosphere increases. Increased average temperatures could trigger climate change.

At a global level, such changes could impact the Earth's climate balance, and alter rainfall and temperature patterns. Climate change has been labelled by some as the most significant threat facing our ecosystems and economy. Reducing anthropogenic greenhouse gases – gases that help to trap heat in the earth's atmosphere – could help to reduce potential climate change effects. However, changes in complex systems such as the global climate are very difficult to predict with a high level of accuracy.

Possible climate responses to increased CO2 concentration in the atmosphere are: increased desertification of semi-arid regions; higher levels of precipitation and flooding in other regions; the possibility of more intense storms such as hurricanes; and sea level rise.

As a general trend, emissions intensity has been decreasing, carbon sinks have been decreasing and net carbon emissions have been rapidly increasing. All of these have contributed to global warming and climate change.

There are three main approaches that can be used to reduce GHGs:

  • energy conservation;
  • reduction of carbon intensity; and
  • carbon sequestration.

In order to arrest climate change, very large reductions in the amount of GHGs emitted to the atmosphere would need to be made. Geological storage is currently one of few greenhouse gas reduction technologies available that can reduce large enough amounts of GHGs.

A reduction in net CO2 emissions into the atmosphere can only be achieved by:

  • Lowering the energy intensity of the economy E/GDP – by increasing the efficiency of primary energy conversion and end use;
  • Lowering the carbon intensity C/E of the energy system - by substituting lower-carbon or carbon-free energy sources for the current sources; or
  • Artificially increasing the capacity and capture rate of carbon sinks – through measures such as CO2 capture and storage.

CO2 can be captured from natural gas processing facilities, fossil fuel-fired power plants, cement plants, chemical plants and iron and steel manufacturing facilities. It can be captured before or after the combustion of fossil fuel depending on the process for combustion. The most common separation method is using a liquid solvent to absorb the CO2.

The captured CO2 is compressed and transported to a geological storage site.

The geological storage of CO2 requires access to large subsurface volumes in the rock pore space which can act as sealed pressurized containers. Aquifers have the largest capacity for all feasible sedimentary basins for CO2 storage. The volume of pore space in aquifers far exceeds that of oil, gas and coal bed reservoirs.

The storage site is monitored before, during and after injection to ensure that the CO2 can remain stored for a significant long time.

While the technology need for CCS can be adapted from other applications, there are still challenges ahead to making the technology commercial.

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