Induced Earthquakes A Potential Hazard for Geological Storage of Carbon Dioxide

Summary.  The world is burning fossil fuels at an ever-increasing rate, resulting in increased release of the greenhouse gas carbon dioxide into the atmosphere.  This results in an increase in the long-term globally averaged temperature.  Consequently there is great interest in developing carbon capture and storage in geological repositories to help abate the increase in atmospheric carbon dioxide.

Zoback and Gorelick have just published a paper that a) emphasizes the vast amounts of carbon dioxide that need to be captured and stored, and b) analyzes in detail the likelihood that small-scale earthquakes may be induced at the injection sites because of the increased fluids introduced into the storage sites.  Their concern is that even small to medium scale earthquakes may destroy the integrity of the sites, leading to significant leakage of carbon dioxide back into the atmosphere.  They conclude that extensive deployment of carbon storage involves considerable risk.

Background.  Zoback and Gorelick have analyzed the long-term geological storage of carbon dioxide (CO₂) as a means of permanently removing this greenhouse gas from the atmosphere by carbon capture and storage (CCS; see the next section).  First we present some introductory information on CCS. (Background on CCS may be found in this earlier post.)

The European Union (EU) has embarked on the only multinational program in the world, based on binding enacted policies, to reduce emissions below the emissions levels of 1990 by 20% by 2020, and by 80-95% by 2050 (the EU Roadmap; see this post).  Achieving such goals requires decarbonization of most energy sources.  The EU recognizes that a major portion of this reduction should come from use of CCS for large scale fixed sources involved in generating electricity. 

To begin research and development of CCS technology, the EU has selected six demonstration projects in six member countries, using differing capture and sequestration technologies.  The EU has committed EUR1 billion (US$1.25 billion) to them.  Variously, they range in size from one at 30 MW (to be scaled up to over 300 MW) to 900 MW, with most projects expected to capture about 90% of the emitted CO₂.  Storage will be in land-based or offshore saline aquifers, and depleted land-based or offshore gas fields.

In the U. S. the state of California is implementing a plan very similar to the EU’s Roadmap.  In a non-official report detailing how California might attain these goals, the California Science and Technology Council (CSTC) relies heavily on decarbonizing energy sources to the greatest extent possible (see this earlier post).  Electricity generation is to be decarbonized, to the extent that use of fossil fuels is maintained, by use of industrial-scale CCS, even though the report recognizes that this technology remains unproven.  Decarbonization of electricity generation is especially important because CSTC envisions use of electric vehicles to decarbonize transportation.

The U. S. Department of Energy (DOE) is sponsoring research on CCS, as reported in the Carbon SequestrationProgram: Technology Program Plan of the National Energy Technology Laboratory.  Its budget request for Fiscal Year 2011 was about US$140 million, with anticipated sharing by an equal amount from Regional Carbon Sequestration Partnerships with universities and corporations.  This budget has grown from about US$10 million in 2000.  Recent support from the American Recovery and Reinvestment Act of 2009 (the fiscal “stimulus”), included in the recent growth of this funding, is essentially exhausted at this time. All aspects of the various stages in capture, release and concentration, transportation and geological storage, as well as monitoring, verification and accounting, are being investigated at laboratory and small pilot scale.

Similar programs are also supported in the DOE Fossil Energy program.  Their requested budget for Fiscal Year 2013 is about US$276 million for CCS and Power Systems, which supports projects as large as industrial scale pilot projects.

Cautionary Analysis of CCS.  Zoback and Gorelick analyzed the dangers to maintenance of reservoir integrity in geological sequestration of CO₂, in a paper published in the Proceedings of the National Academy of Sciences, June 26, 2012, vol. 109, pp. 10164-10168 .  As background, the authors note:

·        CCS will be very costly;

·        in the U. S. use of coal for generating electricity produces about 2.1 billion metric tons of CO₂ a year, or about 36% of all U. S. emissions;

·        China’s emissions are about 3 times more than this from coal-fired generation, corresponding to about 80% of its emission rate;

·        annually, on a worldwide basis, CCS has to contend with 3.5 billion tons of CO₂, which requires injecting an amount of CO₂ underground roughly equal to the volume of all the oil extracted from oil wells worldwide;

·        this amount of injected CO₂ requires that worldwide about 3,500 functional industrial-scale injection facilities be operational by mid-century, averaged to about 85 facilities added per year; and

·        geological storage must remain faultlessly leak-tight in order to compare with freedom from emissions of renewable energy sources.

The authors include the following analyses:

o       The paper itemizes several instances of earthquakes apparently triggered by underground injection of liquids.  This can arise because many geological formations are already in states of unresolved stress, so that the relatively minor perturbation arising from fluid injection releases the stress in an earthquake.  The fluid in essence makes it easier for the stressed surfaces to slide over one another, which is the hallmark of an earthquake.  Zoback and Gorelick emphasize that it is not any land-based earthquake damage to human wellbeing that concerns them, but rather that even small earthquakes, likely not to produce damage to structures, are likely to damage the geological structures holding the pressurized CO₂.  CO₂ could then readily permeate to or near the surface, permitting release into the atmosphere and defeating the intent of the storage in the first place.  They present the results of calculations that even a small earthquake of Magnitude 4 could induce slippage of several cm. along a fault of about 1-4 km (0.6-2.4 mi).

o       In stressed geological formations, it is not only the pressure of injected CO₂ that is potentially hazardous, but also the rate of injection.  More rapid pressure buildup is more likely to trigger an earthquake event; the need to dispose of large volumes of CO₂ would be an incentive for high injection rates.

o       A widely known injection site is the Utsira formation of the Sleipner gas field in the North Sea.  About 1 million tons of CO₂ has been separated from natural gas and reinjected below ground every year, for the past 15 years.  There has been no earthquake activity to date.  The authors calculate that about 3,500 such sites would have to be identified and put into service to accommodate storage needs projected for 2050 (most of which would be needed right now, in fact).  The authors conclude “Clearly this is an extraordinarily difficult, if not impossible task” if only geologically suitable sites are to be used.

o       Depleted oil and gas wells, while seemingly attractive as potential injection sites, are not numerous enough to satisfy the need, and are not necessarily located conveniently for the need.

The authors conclude “multiple lines of evidence indicate that preexisting faults found in brittle rocks almost everywhere in the earth’s crust are subject to failure, often in response to very small increases in pore pressure. In light of the risk posed to a CO₂ repository by even small- to moderate-sized earthquakes, formations suitable for large-scale injection of CO₂ must be carefully chosen.”  Because of the extremely large volumes of CO₂ needing to be disposed of, the industrial-scale CCS needed will be “extremely expensive and risky for achieving significant reductions in greenhouse gas emissions”.

Certain CCS projects have been abandoned due to risk and lack of financing.  The very factors identified by Zoback and Gorelick are echoed in these two recent news reports. 

The Guardian on June 17, 2012 reported that Ian Marchant, chief executive of Scottish and Southern Energy, while still favoring CCS development, warned the British Parliament that a CCS project his company is undertaking is “the most risky project I’ll ever invest in….CCS is…at the demonstration stage….We do not know that this technology will work”.  He called for UK government support at this demonstration phase of the project.

The same article noted that another company, Scottish Power, abandoned CCS technology last year.  Together with Shell, the company evaluated it would need at least £1.5 billion (US$2.3 billion), and the UK government could not support such a funding level.

Similarly, theGuardian reported on June 26, 2012 that Ayrshire Power (Scotland) abandoned its planned new CCS-fitted 1852 MW power plant because it feared it could not obtain funding from the UK and the European Commission.  Nevertheless, the Scottish energy minister still strongly supports CCS development since it borders North Sea offshore CO₂ storage sites.

Rebuttals of Zoback and Gorelick’s warnings.  There has been response from the CCS community rebutting the serious concerns expressed by Zoback and Gorelick.  For example, two scientists were featured in the internet-based Carbon CaptureJournal (accessed June 27, 2012).

Dr. Malcolm Wilson, Chief Executive Officer, The Petroleum Technology Research Centre (PTRC), provided a detailed accounting of the experience gained at the Weyburn-Midale Project, an oil field storage development project in Saskatchewan, Canada, which it seems is an extended oil recovery project as well.  Storage has been under way there for 11 years, with a total of 21 million tonnes (metric tons) of CO₂ stored in that time.  Detailed research and characterization of the site has been undertaken throughout this time; indeed, seismic events with Magnitudes of -1 (extremely small) have been recorded.  Dr. Wilson considers this site now to be industrial scale, as 2.8 million tonnes of new CO₂, and more than 5 million tonnes when recycled CO₂ are included, have been injected; no earthquake activity or leakage has been identified.

PTRC is also conducting research on their Aquistore Project, for storage in saline aquifers.  Noting with approval that Zoback and Gorelick cite aquifers favorably because of their very large storage capacities, Dr. Wilson notes that the Aquistore Project will be the first industrial scale storage project, since it will receive CO₂ from a coal-fired power plant.

Dr. Bruce Hill, senior staff geologist at Clean Air Task Force (CATF) rebuts the concern over lack of integrity of storage sites due to earthquake activity by emphasizing the rate of CO₂ migration toward the surface, rather than the total amounts potentially released.  Dr. Hill emphasizes that there are many layers of rock structures, extending thousands of feet, overlaying injection sites, seeming to belittle the concerns of Zoback and Gorelick.  Dr. Hill feels that the examples cited by the authors are not representative.  He points out that “approximately 1 billion tons of CO₂ have been safely injected (and stored) in the process of enhanced oil recovery in the U.S. since the late 1970s, with no reported seismic incidents. In fact, there have been no earthquakes reported anywhere from saline CO₂ injections either”.

Dr. Hill concludes that CCS technology is “viable” and should play a significant role in potentially storing the very large amounts of CO₂ that need to be recovered to reduce atmospheric CO₂ accumulation.

George Peridas responded to the paper on the Natural Resources Defense Council Blog on June 22, 2012.  Mr. Peridas believes that Zoback and Gorelick raise valid issues, including whether CCS can cause earthquakes and whether such earthquakes could lead to leakage of the injected CO₂.  But in his opinion, the conclusions reached by the authors are more extensive than warranted by the evidence, for example with respect to the second issue, leakage.  He does not agree that an earthquake event would lead to migration of CO₂ all the way to the surface.  He believes that an experiment cited by the authors, performed on granite, a brittle mineral, is not representative of capstone layers anticipated in CCS, which would be more compliant, yet impermeable, shales.  In the case of existing fossil fuel geological reservoirs, large earthquakes have been known to occur without loss of the materials.  Mr. Peridas additionally cites Sally Benson (Stanford University and Lead Coordinating Author of the Underground Geological Storage Chapter in the Intergovernmental Panel on Climate Change Special Report on CCS) as stating that naturally care must be taken in choosing CCS injection sites, but that finding such sites should be feasible.

Discussion

Our earlier post, “Carbon Capture and Storage: A Needed yet Unproven Technology”, presented background information on the various technologies that may be employed in each phase of capturing CO₂, from the burning of fossil fuels for energy, to transporting the CO₂ to a storage site, and finally the actual storage process.   Many problems remain to make CCS industrially viable for utility-scale facilities.  Resolving these problems requires investment of large sums of money, worldwide, to arrive at practical CCS by about 2020.  Currently a relatively small number of demonstration and pilot projects are under way around the world.

The use of fossil fuels is projected to grow considerably in the coming decades around the globe, primarily in developing countries which will power their rapidly expanding economies with energy derived from burning fossil fuels.  This means that the annual rate of CO₂ emissions will continue expanding, and that the total accumulated concentration of atmospheric CO₂ likewise will continue increasing.  Even in developed countries having programs to abate CO₂ emissions at various stages of maturity, a major aspect of such abatement involves shifting transportation to electric power.  Thus the total demand for electricity is projected to grow in developed countries as well; to the extent that this demand is not met by renewable sources the need for contending with abatement of CO₂ emissions likewise will grow.  For this reason emission abatement programs will rely ever more heavily on technologies such as CCS.

The paper by Zoback and Gorelick serves at least three useful functions.  First, by arithmetic analysis, it underscores the vast, unprecedented need for functional and effective injection sites projected by 2050.  Some of this information has been summarized above.

Second, its geophysical modeling emphasizes the many unknown factors remaining in choosing and developing new CO₂ injection sites.  The seals installed surrounding well bores, and the many geological factors involved in retaining the injected CO₂ out of contact with the atmosphere for hundreds or thousands of years must be essentially fail-safe.  Yet this work emphasizes that the very act of injecting pressurized fluid facilitates potential small-scale earthquakes that, according to the modeling, have the potential of opening fissures in these seals that could lead CO₂ back to the surface.

Third, it has engendered fruitful debate in the CCS community about the integrity of proposed injection sites.  Although these issues were already known among workers in the community, they have now been aired among a wider public.  This has the effect of ensuring that research and data gathering, involved in characterizing new injection sites, will be carried out diligently and effectively so that wise siting choices may be made.

The critics of Zoback and Gorelick, such as those cited above, include examples in their rebuttals of injection sites taking advantage of pre-existing wells used in the extraction of oil and gas from their geological repositories.  These have kept the fuels underground for millions of years, and so are cited as justifying CO₂ injection for the same reasons.  These are likely not representative of the thousands of new storage injection projects that will be needed to accommodate the demand.  Overall the number of pilot injection sites worldwide is small, and many are new experimental projects.  The concerns raised by Zoback and Gorelick merit careful attention going forward as CCS technology is developed further and deployed in number.

© 2012 Henry Auer

Carbon Capture and Storage: A Needed yet Unproven Technology

Summary. Carbon capture and storage (or sequestration; CCS) is contemplated as a major group of technologies that would contribute to reducing the rate of emission of carbon dioxide (CO₂), a major greenhouse gas, in future decades.  Currently there are a handful of operating or demonstration CCS facilities worldwide.  CCS entails capturing CO₂ from a utility-scale source that burns a fossil fuel such as coal or natural gas; transporting the purified CO₂ to a storage site, and injecting or piping the CO₂ into the storage or sequestering formation.  Many problems remain to make CCS industrially viable for utility-scale facilities.  Resolving these problems requires investment of large sums of money, worldwide, to arrive at practical CCS by about 2020.  Successful development of CCS will make a major contribution to addressing the reduction of greenhouse gas emissions and reducing the extent of increase of the long-term global average temperature.

The development expenditures should be undertaken by nations around the world in recognition of the fact that, to date, mankind has not treated the disposal of CO₂ as a waste disposal obligation.  This contrasts with the way we deal with municipal and sanitary waste streams, where inhabitants are fully accustomed to pay for disposal either directly or as a tax burden. 

Introduction.  The nations of the world consume ever more amounts of fossil fuels each year to produce the energy they require.  Burning fossil fuels releases the greenhouse gas carbon dioxide into the atmosphere in correspondingly increasing amounts year by year.  Although emitted by myriad point sources, greenhouse gases are rapidly distributed in the atmosphere across the entire surface of the globe.  CO₂ is long-lived in the atmosphere, since there are insufficient natural mechanisms for removing the gas, once emitted.   Indeed, the atmospheric concentration of CO₂ has been increasing steadily, and more strongly, in recent decades, and is now higher than at any time since the beginning of the industrial revolution. 

Future projections for fossil fuel use and the resulting emission rate of CO₂ are even higher, as developing nations increase their demand for energy, and as the population of the world expands.  As the greenhouse effect from these accumulating greenhouse gases gets stronger, the long-term global average temperature likewise increases, leading to serious changes in regional climates and to intense extremes of distinct weather events that lead to damages and harms for the affected populations. 

For all these reasons climate scientists and policy makers around the world are grappling with ways to reduce the rate of greenhouse gas emissions, as one way to reduce the severity of the warming of the planet, and ultimately to try to stabilize the planet’s climate.

Carbon Capture and Storage (CCS), also called carbon capture and sequestration, has long been discussed as a way of potentially preventing the release of CO₂ once formed in energy production.  The term actually applies to the overall concept, since, as described below, there is an ensemble of CCS technologies that are being developed. 

CCS is widely mentioned as being an important means for abatement of CO₂ emissions as we strive to reduce greenhouse gas emissions in the coming decades.  For example, the European Union (see this earlier post) has set the objective of reducing emissions by at least 80% by 2050, and includes CCS as part of its strategy.  The state of California has established a similar objective.  A nonofficial report, “California’s

Energy Future: The View to 2050” by the California Council on Science and Technology likewise relies strongly on CCS to achieve this goal.  It is therefore important to understand the present status of development of CCS.

It is roughly estimated that overall, around the world, there exists sufficient geological (i.e., other than in oceans) capacity to store 10,000 gigatons (billions of tons) of CO₂, considered to be sufficient for 100 years of storage activity or more.  By far the highest capacity is found in deep brine formations (see below).

General features of CCS.  Since large facilities are required for CCS, it is appropriate only for use with fixed energy plants that rely on fossil fuels.  The objective of CCS is to capture CO₂ produced in energy production before it is released into the atmosphere, and to store it permanently out of contact with the atmosphere (sequestration).  Although some fixed energy plants may be sited appropriately for sequestration on site, in most cases the site where the CO₂ is captured is remote from the sequestration site, so that the CO₂ must be transported in order to reach a storage site.

There are important requirements for a CCS technology.  First, it must effectively and efficiently remove or capture CO₂ from a flue gas obtained from the fossil fuel.  Second, the CO₂ must in general be transported to a remote sequestration site with minimal risk of release into the atmosphere.  Third, the CO₂ has to be stored in a way that sequesters it from release back into the atmosphere for, say, one hundred or more years, and preferably for millennia.  Finally, CCS must be accomplished as economically as possible in order to minimize market resistance by the consumer.

The technologies deployed in CCS are explained below in the Details section following Conclusions.

Stages of development of CCS technologies.  Examples of the various CCS technologies are at various stages of development or implementation.  CO₂ has been used industrially for many years in enhanced oil recovery (EOR) with the objective not of storing CO₂, but rather extracting additional crude petroleum from wells for energy.  Generally, most of the methods summarized in Details use technologies that, individually, are already known.  The challenge for CCS is to combine them to successfully achieve the current objective, to scale them to the extent needed for the amounts of CO₂ product envisioned, and to achieve all this economically.  In addition, certain of the technologies have further research, development and deployment obstacles facing them, some of which are mentioned below.

Economics of CCS.  When considering economics, we have to realize that the more steps, or unit operations, that are involved in a technology, the more costly it becomes.  This is especially so because additional energy, compared to a non-CCS plant, has to be invested in some steps in order to proceed to the next step.  This is illustrated in the following graphic.

Conceptual diagram of additional costs, and additional CO₂ burden, arising in a CCS technology.  The upper panel schematically shows the additional costs arising from equipment and loss of efficiency.  The lower panel illustrates a conceptual accounting of the additional CO2 required for operation of a CCS plant.  The third line for “CO₂ captured” accounts for the fractional efficiency of the capture step (ca 80%).

Source: Ref. 1.

Power plant generation efficiency can span a range of about 42% to about 55%, depending on the technology used.  When comparable technologies include CCS, It is estimated that losses of roughly 14% in efficiency from these numbers can occur.  Or, considered from the point of view of the graphic above, depending on the technology and the particular features of a given installation, the cost of the “CO₂ captured” line (third line in the lower panel) can range from US$44 to US$90 per ton of CO₂ captured.  In a separate analysis, it is estimated that the cost per megawatt-hour (unit of energy provided) increases by 75-78% using CCS for pulverized coal combustion, or by 39% for IGCC (Ref. 1).

These figures may appear alarming to the consuming public.  However, it must be realized that the cost of treating CO₂ as a waste product of the lifestyle that we humans lead has never been included in the price of the fossil fuels used in its creation (see this previous post).  This contrasts with the ready willingness in developed countries of the world, to pay, for example, for waste water treatment facilities or for disposal of solid residential and commercial waste.  The need to reduce emissions of greenhouse gases, principally but not exclusively CO₂, in order to address the adverse effects of global warming necessarily involves expenses that society must bear. 

Implementation of CCS.  Presently CCS technologies remain experimental.  The following graphic shows a world-wide map of sites where CCS is being implemented on a research, a development, or a deployment basis.

Active or planned large-scale integrated projects by capture facility, storage type and region.  The numbers refer to labels in the text of the original.

(Original Source: Global CCS Institute 2010)

Source: Ref. 1.

An important aspect of the graphic above is that the enumerated label numbers do not even reach 100.  (Ref. 1 also includes an accompanying map restricted to the U. S. and Canada.)  In other words, these are the principal “active or planned” large scale CCS projects identified worldwide.  This shows clearly that at this time CCS is not anywhere near having reached maturity as an ensemble of technologies that can be implemented to sequester CO₂ to a meaningful extent.  Four large scale CCS facilities are currently in operation, successfully sequestering CO₂ at this time.

Unfulfilled needs required for research, development and deployment of CCS.

Ref. 1 points out that, in comparison with earlier assessments by the Carbon Sequestration Leadership Forum, progress is being made.  For example, it states that a group of developed countries has pledged US$26 billion that can fund between 19 and 43 large scale demonstration projects by 2020.

Ref. 1 includes a long section detailing gaps in our present state of understanding or capability in CCS.  A short selection of these needs is listed here.

There is a strong need for several large-scale CCS projects in order to demonstrate technical and commercial capability to achieve sequestration,  helping attain the objective of being a viable technology by 2020.  The scale of the projects needs to be large enough to accommodate the output of today’s power generating facilities.

Pipeline networks need to be planned and constructed to transport CO₂ between a power facility and its storage location. 

Research and development is needed to adequately and properly identify suitable storage sites that fulfill the requirements of the industry.

There is a need to expand CO₂ capture from power plants to other large industrial facilities, such as cement factories and steel mills.

New research modules need to be developed to consider the technologies and their associated costs for the entire CCS process train, and for the entire perceived life cycle of installed facilities.

The costs of the CO₂ capture process are the highest in the CCS technology.  New methods for capture need to be identified to help make it more efficient and economical.  These must be tested at scale for implementation.

Conclusions

CCS has the potential to play a major role in lowering the rate of emission of greenhouse gases in coming decades.  As such, CCS is a technology that attracts interest and development efforts worldwide.  A factor of paramount importance is that, to the extent it becomes a viable set of technologies, CCS would permit mankind to continue burning fossil fuels.  With present capabilities up to 80% of the resulting CO₂ will be removed from the effluent gas stream and stored out of contact with the atmosphere for long periods of time.  Thus even with CCS, greenhouse gases would continue to accumulate in the atmosphere, albeit it a considerably lower rate.

Unfortunately, the present status of CCS has not reached a level at which it can be implemented on a scale sufficient to fulfill the need.  Considerable efforts in research, development and full-scale deployment are needed to achieve this objective.

These efforts will require the expenditure of large sums of money, originating from greenhouse gas-emitting countries the world over.  Once emitted from a source, greenhouse gases are distributed across the entire planet, becoming a matter of global concern.  Correspondingly, technologies contributing to resolving global warming should be addressed by nations around the world.  Progress by one becomes shared progress for all.

Funding support for deploying CCS should come from governments, as well as possibly from public-private joint efforts.  Adequate support should be given to encourage private enterprise to undertake investments in presumably risky ventures entailed in deploying CCS. 

Installing facilities that include CCS capabilities necessarily increases the cost of the energy obtained from them.  As consumers of the energy generated by burning fossil fuels, we have become accustomed not to think about our energy sources.  After all, we can’t see or smell CO₂; fossil fuels provide “invisible energy”.    We should instead consider the costs of disposing of CO₂ as a waste product just as we do for municipal and sanitary waste streams. 

The harms to humanity arising from use of fossil fuels are significant, considering, for example, recent extremes of weather and climate scattered across the globe.  These harms create the need to expend large sums of money for remediation and relief on an emergency, and unscheduled, basis.  As an alternative, we should consider accepting the expenses of restricting the emission of greenhouse gases as a preventive measure.  

Details

There are three principal technologies under study for capturing CO₂, post-combustion capture, pre-combustion capture, and metal oxide capture.  In addition, there are several technologies for storing the captured CO₂ (see the following graphic), including on-land or offshore  injection into deep brine aquifers, on-land or offshore injection into existing oil wells frequently as part of a method for enhancing oil recovery, and injection into coal seams that cannot be exploited commercially as sources for coal. 

Schematic diagram of options for storing CO₂ in deep underground geological formations [(Courtesy Cooperative Research Centre for Greenhouse Gas Technologies, Australia)]

Source: © IPCC, 2005, Ref. 2.

In addition, direct release into ocean waters is also under consideration (see graphic below), involving pipeline release onto the deep ocean floor (CO₂ lakes, sinking CO₂ plumes), pipeline release at shallow depths (rising CO₂ plumes), and release from ships or ocean platforms by pipe injection (CO₂ lakes).

                               Some strategies for storing CO₂ in ocean waters.

                               Source: © IPCC, 2005, Ref. 2.

Post-combustion capture.  In this technology, a fossil fuel such as coal or natural gas is first burned to release its energy, typically in an electric generation plant.  When burned in air, the resulting CO₂ is relatively dilute in the flue gas, making its recovery slightly more challenging.  The flue gas is passed through an amine composition that combines chemically with the CO₂ while letting other components pass without capture.  The amine capture proceeds spontaneously and includes the release of heat during the reaction.  In the next step the compounded mixture now has to be heated sufficiently to reverse this combination and release pure CO₂, simultaneously regenerating the free amine for re-use.  The CO₂ finally obtained at this stage is essentially pure, ready for transport and sequestration.  As with all the CCS technologies, here the post-combustion capture step reduces the overall energy yield by significant amounts (the “Efficiency penalty” in the upper panel in the first graphic above).

A variation of this method burns the fuel in enriched or pure oxygen (oxyfuel combustion).  But oxygen can only be provided by ultra-low temperature fractionation of air into oxygen and nitrogen at cryogenic temperatures.  This step obviously requires additional energy for its operation, detracting from the overall energy yield.  The advantage of this method is that the flue gas itself has a high concentration of CO₂, making the capture step more effective and efficient, thus lowering the cost of this step.  Oxyfuel combustion has to date only been implemented on a small demonstration scale.

Pre-combustion capture is primarily used with coal, and is based on the earlier “syn-gas” (synthetic gas) method developed some decades ago.  This technology is also called integrated gasification combined cycle (IGCC) power generation.  The details may be too complicated to explain fully here.  In several steps pulverized coal is converted to a gas stream containing CO₂ and hydrogen gas (gasification).  The hydrogen is burned in air to produce hot gas containing water vapor that drives a first generating turbine.  The excess heat in the gas stream is used to generate steam which then drives a second generating turbine (combined cycle).  The CO₂ is captured and isolated for storage.

Metal oxide capture involves mixing natural gas with a metal oxide to yield CO₂ and pure metal; the latter is reoxidized in air for re-use. 

Other capture modalities are also under investigation.

Transport is envisioned primarily to occur by pipeline.  CO₂ is readily compressed to a liquid at ordinary temperatures.  Liquid CO₂ has been sent by pipeline already in the U. S. in EOR applications; there are a few thousand miles of such pipelines already in use.  Liquefied CO₂ can also be transported by truck or by sea.  Any leaks that may occur are potentially dangerous or lethal because of the hazard of asphyxiation.

Deep brine injection.  Concentrated brines exist in deep aquifers (see the schematic for geologic storage above) throughout the world.  The important feature of usable brine aquifers is that they be overlain with a layer of impermeable “caprock”.  This caprock is a mineral that does not allow escape of liquid or gaseous CO₂ into overlying geological layers.  Depending on the brine and mineral characteristics of the adjacent geology, injected CO₂ could react to form insoluble carbonate minerals that would have essentially infinite lifetimes in situ.

Oil well injection, depleted natural gas reservoir injection.  Injection into existing oil wells is used already in EOR.  While this would serve to sequester CO₂ underground, it may be considered counterproductive from the point of view of minimizing global warming, since additional fossil fuel is forced up the well in the process and would result in additional CO₂ being emitted upon combustion.  Depleted natural gas reservoirs pose less concern in this regard because much less residual gas remains.  Both oil and gas reservoirs most likely are overlain with suitable caprock formations, since the reservoirs remained intact for millions of years.  They thus are highly unlikely to leak CO₂ back into the atmosphere or into potable water aquifers.

“Un-mineable” coal seams are considered to occur too deep or be otherwise unsatisfactory for commercial mining.  Coal seams can absorb injected CO₂ and store the gas for long times.  Coal seams frequently contain methane (natural gas), a much stronger greenhouse gas than CO₂.  Injected CO₂ can expel methane to the surface, which would exacerbate greenhouse gas concerns, or expelled methane can be harvested as a fuel in which case the natural gas would carry the same concerns as mentioned above for recovered crude oil.

Ocean injection directly into sea water is also mentioned as a storage modality.  Deep ocean injection could occur from ships, stationary platforms, or pipelines originating on land (see the schematic for geologic storage above).  If deep enough, the ocean pressure and temperature keep the CO₂ liquefied, and it sinks to the bottom to form CO₂ lakes.  Escape from these lakes would occur slowly only by diffusion.  Shallow injection on the other hand would not keep the CO₂ liquefied, and it would dissolve or rise.

The principal hazard with ocean injection is that, CO₂ being a weak acid, any CO₂ that enters the general ocean body would increase its acidity.  The oceans are already being subjected to acidification by absorption of atmospheric CO₂ at the surface, leading, it is thought, to loss of coral reefs and shellfish, which disrupts the entire oceanic ecosystem.  It may not be advisable to risk further acidification by ocean injection of liquid CO₂.

References

1. Carbon Sequestration Leadership Forum 2011 Technology Roadmap; http://www.cslforum.org/aboutus/index.html?cid=nav_about.

2. Intergovernmental Panel on Climate Change (IPCC), 2005: IPCC Special Report on Carbon Dioxide Capture and Storage. Prepared by Working Group III of the Intergovernmental Panel on Climate Change [Metz, B., O. Davidson, H. C. de Coninck, M. Loos, and L. A. Meyer (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA, 442 pp. http://www.ipcc.ch/pdf/special-reports/srccs/srccs_wholereport.pdf. 

3. Carbon Sequestration Program: Technology Program Plan, National Energy Technology Laboratory, U.S. Department of Energy, 2011 http://www.netl.doe.gov/technologies/carbon_seq/refshelf/2011_Sequestration_Program_Plan.pdf .

© 2011 Henry Auer