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