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In part 1, we looked at the likely impact of the recent EPA rules for greenhouse gas emissions from new power plants. We also looked at some of the technology options for carbon capture from new coal-fired plants, and how they might affect the cost of compliance. Now we need to consider the other side of CCS: sequestration. What options do we have for disposition of captured CO2, and how do they affect the economics of the overall process?
There are three general options we'll examine here: (1) enhanced oil recovery (EOR); (2) other forms of geological sequestration; and (3) ocean sequestration. These are not the only sequestration options by any means, but they are the ones most widely considered.
Enhanced Oil Recovery
EOR, as it applies to sequestration of CO2, involves a form of geological storage. It is considered separately here because of its unique position: it has a negative cost to utilities. It costs them less than nothing to dispose of captured CO2 by supplying it to oil field operators for EOR; the operators will buy the CO2 from them and deal with pumping it into their oil reservoir. CO2 has positive value to the operators because it enables recovery of more oil from the reservoir. At the same time, the fact that it has held oil for millions of years virtually guarantees that the oil reservoir will be suited for semi-permanent sequestration of CO2.
EOR is a complex subject, and my treatment of it here will be greatly simplified. One point to be aware of is that injection of CO2 is by no means the only option for EOR. Nor is it presently the most widely used. Other options include water flooding, steam flooding, nitrogen injection, and injection of various types of chemical solutions. It's also common in fields that co-produce both oil and natural gas to re-inject the gas. In all cases, the objective is to maintain pressure in the formation and to "encourage" the oil to migrate through the porous rock of the reservoir toward the production wells tapping it.
Although there are other approaches for EOR, CO2 injection is often the most effective -- when it is an option at all. It requires an adequate nearby source of CO2, which may not exist. But where it is an option, CO2 injection works well. Supercritical CO2, at a pressure above 73 atmospheres, is an effective solvent for nearly all types of oil. The liquid oil and the supercritical gas are fully miscible, and the result is a thin fluid with little or no surface tension that seeps relatively easily through the porous source rock. When the oil and CO2 mix is brought up to the surface and depressurized, the CO2 "fizzes out", leaving the oil behind. The CO2 is then mixed with makeup gas and returned to the injection well for another round of oil recovery.
For every barrel of oil recovered by CO2 injection, the equivalent of one or more barrels of supercritical CO2 will typically be left behind in the reservoir. The ratio isn't precise; it will depend on the size and state of the reservoir rock and on how the operators choose to employ CO2 injection. If they're paying a high price for CO2, they will use it sparingly, in a manner that maximizes the ratio of oil recovered to CO2 left behind. If it's cheap, they will use it more freely, in a manner that leaves more CO2 in the reservoir but maximizes the amount of oil recovered.
Given those complications, it's impossible to assign a precise value to CO2 used for EOR. When the IPCC went through the exercise for its report on CCS in 2005, they cited a range of 10 -- 16 US$ per ton for what operators had actually paid1. However, that was from data taken when oil had been selling in the range of $25 -- $30 per barrel. The report's authors asserted that an oil price of $50 per barrel "could justify a credit" of $30 per ton to offset the cost of carbon capture. With oil at $100 per barrel, the figure would presumably be $60 per ton of CO2. I'm somewhat skeptical of that figure, but it amounts to a credit of roughly $60 per megawatt-hour of output from a coal-fired plant with CCS. If valid, that's ample to pay for the cost and operational overhead of the CCS system -- even using currently available technology. With the enhanced CCS technologies likely to be available within 5 -- 10 years, it would make CCS highly profitable.
Limitations of EOR
There is a significant "fly in the ointment" in relying on EOR to make CCS profitable without the political hurdle of a carbon tax. That's the limited size of the market. If CO2 could be supplied to all of the oil fields in the US that could profitably use it for EOR, a generous estimate of the amount of CO2 sequestered annually would be 250 megatons. In absolute terms that's a lot of CO2. However, it's less than 10% of what is currently produced by our coal-fired power plants.
As a result, supplying CO2 for EOR can never be much of a factor in reducing CO2 emissions to the degree needed to stabilize greenhouse gas levels in the atmosphere. It will, however, provide an economical means to fund prototype plants while CCS technology is being refined and tested.
Other Geological Sequestration
Formations of the type in which oil is found are well suited for sequestration of CO2, but only a tiny fraction of formations that are suitable for the latter also happen to hold oil. Any extensive sandstone formation that lies buried below an intact capstone layer is a candidate for storing CO2. Startup C12 Energy in Berkeley, CA, is conducting geological surveys to identify such locations . When they've identified a good formation, they negotiate contracts with the local land owners for CO2 storage rights. Storage rights can be secured cheaply, since there is as yet no price on carbon emissions and no certainty that the rights being secured will ever be exercised. But if and when a price is put on emissions, and the capacity for storage in EOR operations is exceeded, the rights held by the company will presumably rise in value.
It isn't actually necessary that a formation be covered by an intact capstone layer in order to sequester CO2. As long as it is far enough down and saturated with water (a deep saline aquifer, for example), then CO2 that has been compressed enough to inject into the formation will be denser than water. Part of it will dissolve into the water, but what doesn't dissolve will tend to sink deeper into the formation rather than rising. The pressure needed to achieve that is approximately 30 MPa. That corresponds to the pressure in the ocean at a depth of 3 km, or about 10,000 feet. That's also the minimum depth needed for a CO2 injection well, if the formation into which the CO2 is injected is not capped.
A third type of formation that's suited for CO2 sequestration is offshore sandstone. If it's in water deep enough that the formation lies below the ocean thermo cline, the water temperature and the temperature in the upper layers of the formation will be only a few degrees above freezing. In that case the formation can safely hold CO2 even if the injection point is much less than 10,000 feet down. Any CO2 that diffuses to the upper boundary of the formation will form a solid CO2 clathrate with the cold water, plugging the pores of the sandstone and allowing at most very slow diffusion of sequestered CO2 into the water above.
The capacity of formations suitable for geological sequestration of CO2 is large. Estimates vary and are all rough, but there's a consensus that availability of suitable formations won't be a limiting factor for at least the next century or so. However, the likely cost of drilling huge numbers of deep injection wells and compressing gigatons of CO2 to pressures for geological sequestration makes for ongoing interest in other options.
The most widely considered of those other options is ocean sequestration. It's not one option, however, but rather a varied set whose common feature is that the CO2 ends up being stored in the ocean.
As a carbon reservoir within the global carbon cycle, the oceans are immense. Especially the deep waters that comprise the bulk of the oceans' 1.37 billion cubic kilometers. Figure 1 is a diagram from a NASA web site that illustrates the major reservoirs and the annual fluxes of carbon that move between them. The atmosphere is estimated to hold 800 gigatons of carbon (GtC), plant biomass 550 GtC, soil carbon 2,300 GtC, and fossil carbon 10,000 GtC. Those reservoirs, however, are dwarfed by the oceans' 38,000 GtC. Of those 38000 GtC, 37000 reside in the deep ocean, while 1000 are in surface waters.
The division of ocean waters into "deep" and "surface" categories is not an arbitrary matter of labels. They are distinct subsystems with different characteristics. The surface waters are stirred by wind, waves, and shallow-water currents. They are "well mixed". But the effects of wind and waves fall off rapidly with depth. By 50 meters, only very major storm systems have much effect. Away from polar latitudes, the surface waters are also much warmer than the deep waters. With solar heat deposited at the top, they form a stratified system that floats on the cold and denser waters below. The transition between the two is the thermo cline, a narrow region in which temperature drops rapidly with depth.
The significance of this layering of ocean waters as it applies to the carbon cycle is that the CO2 content of surface waters is in rough equilibrium with CO2 in the atmosphere, while that of the deep waters is not. It takes on average from one to two thousand years for water that has been conveyed from the surface to the deep ocean to make its way back to the surface. That means that the bulk of deep ocean waters carry a dissolved CO2 content that was in equilibrium with the polar atmosphere of one to two thousand years ago -- before the industrial revolution when atmospheric CO2 levels were much lower.
Gases do not have fixed solubility limits in water. The amount of gas that will dissolve in water (or any liquid) is proportional to the partial pressure of that gas at the gas-liquid interface -- a relationship known as Henry's Law. As the partial pressure of CO2 in the atmosphere goes up, the amount of CO2 dissolved in the surface waters goes up with it. As a result, for every ton of CO2 released into the atmosphere in burning fossil fuels, only about half of it remains there for any length of time. A substantial fraction of it is quickly absorbed into the surface waters of the ocean.
There is a direct relationship between dissolved CO2 and the pH of water. The anthropogenic CO2 that has been added to the atmosphere and to the surface waters of the ocean has lowered the water's pH (increased its acidity), but it has barely touched the vast body of deep ocean waters.
If the entire ocean were mixed over a period of just decades to the degree that the surface waters are, then we would have no problem with CO2 levels or ocean acidification. All the fossil carbon that has been burned since the start of the industrial revolution would have raised atmospheric and ocean CO2 levels slightly, but not to anything like the extent we have seen. From the pre-industrial 280 PPMv, we might have risen to around 295 PPMv. However we could continue burning coal, oil, and gas at the current rates for another two centuries before hitting the 350 PPMv regarded as the maximum safe level over the long term for avoiding a disruptive degree of climate change. (We're already beyond that now, at 395 PPMv and rising.)
Unfortunately for us, the time scale for mixing of the deep ocean waters is millennia, not decades. Every year, the ocean's thermohaline circulation carries a sizable volume of frigid surface water from polar regions down to the deep ocean basins. A matching volume of deep ocean water is pushed up and begins to mix with the surface waters around the globe. The volumes of water involved are huge -- many times larger than the combined flows of all the world's rivers -- but they amount to less than 0.1% of the total volume of the world's oceans. Thus it takes more than 1000 years to complete one turnover of the ocean waters.
Because the polar surface waters that sink each year have been exposed to higher atmospheric CO2 levels than the water pushed up elsewhere to replace them, they carry more CO2. In this manner, the natural thermohaline circulation carries about two GtC to the deep ocean annually -- about a quarter of what burning of fossil fuels and production of cement are currently dumping into the atmosphere. If we were somehow able to stop all such activity tomorrow, CO2 levels would stop rising and would begin a slow relaxation back toward 295 PPMv. It would take a few thousand years to get there, however.
Most of the proposed forms of ocean sequestration are methods of bypassing the thermohaline circulation to get CO2 quickly into the deep ocean waters. They shortcut the process by using direct injection.
Figure 2 illustrates options that have been considered. They differ in how the injection is accomplished, but are otherwise similar. The preferred method depends on local geographic factors. For instance, use of pipelines from the shore is generally more economical than use of ships, but not always an option. It requires the continental shelf to be close to shore, with a significant coastal current at the injection depth to disperse the CO2 plume. Similarly, the shallower injection options require less energy for compression of CO2 and would be more economical than deeper injection. However, they are only viable in areas where there is a sinking current around the injection point to carry dissolved CO2 down to deep ocean basins.
Boosting Ocean Alkalinity
An alternative to direct injection involves boosting the capacity of the ocean surface waters to pull CO2 from the atmosphere. That avoids the need for carbon capture at point sources, and can be done by increasing the water's alkalinity.
Absorption of CO2 in a water solution involves a complex balance of equilibrium reactions:
When the alkalinity of the solution is increased, as for example by adding caustic soda (NaOH), it reduces the concentration of H+ ions and shifts the above equilibrium in the direction that produces more H+ (to the right). That reduces the concentration of H2CO3, which also shifts the first reaction to the right, pulling in more gaseous CO2 from the atmosphere. It works out that for every mole of hydroxyl ions added, an additional 0.89 moles of CO2 are dissolved.
The most efficient means for boosting ocean alkalinity involves dissolving calcium carbonate (CaCO3). Carbonate-rich minerals, in the form of vast chalk or limestone beds, are the "final resting place" for most of the CO2 that volcanoes over the eons have spewed into the atmosphere. It is counter-intuitive that dissolving large quantities of them -- we're talking potentially gigatons per year -- could be a way to counter the rise in atmospheric CO2 levels. Yet it is -- or theoretically could be.
A bare carbonate ion in solution is not a happy camper. With its double negative charge, it really wants to latch onto a free hydrogen ion to become a bicarbonate ion. Consequently, adding carbonate ions to a solution has a similar effect to adding hydroxyl ions. It soaks up H+ and allows more CO2 to be dissolved. The net reaction can be written as:
CO3-2 (aq) + H2O (l) + CO2 (g) -- 2HCO3-
It seems like there ought to be a catch, and indeed there is. In fact there are two. The first is that the reaction is reversible, and if calcium carbonate should precipitate out of the solution, it will end up releasing as much CO2 back to the atmosphere as dissolving the limestone would absorb. The net reaction is:
Ca+2(aq) + 2HCO3-(aq) -- CaCO3(s) + CO2(g)
The second catch is that added calcium carbonate cannot be dissolved into the the oceans' surface waters to begin with. They are already super-saturated with dissolved CaCO3 . However, under the high pressures and low temperatures of the deep ocean waters -- below what's known as the carbonate compensation depth -- added CaCO3 will dissolve. So one way to boost alkalinity is simply to dump massive amounts of crushed chalk or limestone into the deep ocean and allow it to dissolve naturally. When the carbonate-enriched waters begin to reach the surface they will start pulling CO2 out of the atmosphere3. That would be around 1000 years later, in the normal course of events.
If the natural dissolution of CaCO3 in deep waters is to be useful within this century, it has to be coupled with a mechanism that brings the carbonate-enriched waters directly to the surface. One can envision a large OTEC system (Ocean Thermal Energy Conversion) with the inlet to its deep riser pipe buried in a heap of crushed carbonate minerals on the sea floor. The system would do triple service: (1) power generation from surface water as a heat source and cold deep water as a heat sink; (2) absorption of CO2 from the atmosphere when the high alkalinity water brought up is discharged at the surface; and (3) fertilization of surrounding surface waters for ocean farming or fishery enhancement.
Does that sound like a plan? Perhaps. But things are never simple. OTEC, and even the very idea of large scale sequestration of CO2, face opposition from multiple quarters. We'll step back and look at some of the larger issues around sequestration next week, in part 3.
For Chapter 8 (Costs and Economic Potential) from IPCC Special Report on CCS -- http://www.ipcc-wg3.de/publications/special-reports/.files-images/SRCCS-Chapter8.pdf
For home page of C12 Energy -- http://c12energy.com/
One may wonder, if the surface water are already super-saturated and we're bringing up water with yet higher levels of dissolved CaCO3, won't eh excess CaCO3 simply precipitate? That would neutralize the alkalinity added when the carbonate was dissolved, and defeat the purpose of the whole operation. That doesn't happen however. At moderate super-saturation levels even higher than found in warm surface waters, CaCO3 will not spontaneously precipitate. The potential energy barrier that prevents it is similar, conceptually, to the barrier that prevents free oxygen in the atmosphere from spontaneously oxidizing the carbon compounds in plant material.
For information on purchasing reprints of this article, contact sales. Copyright 2013 CyberTech, Inc.
I always wondered if injecting CO2 to release oil would actually be a net increase in CO2, as that oil would then be burned. It sounds like that is not the case, but in general, as you have stated, such "beneficial" injections are very limited and they distort the discussion into thinking that sequestering the CO2 isn't an added cost to how we do things now. It is.
Jim Hartung 8.1.12
Roger -- Thanks for the article. I believe that some form of sequestration will be necessary to stop global warming, because the only other solution to stop global warming is to stop using any fossil fuels and completely eliminate greenhouse gas emissions. This is nearly inconceivable. Have you done any thinking about the possibility of sequestering carbon dioxide through improved farming and forestry practices?
Roger Arnold 8.2.12
Have you done any thinking about the possibility of sequestering carbon dioxide through improved farming and forestry practices?
Oh yes. I'm a fan of "bio-char" as a way to increase soil carbon and provide buffering of plant nutrients. However I don't see it as offering much near term help for reducing net CO2 emissions. It's too expensive to be used on the scale that would be needed to offset a substantial fraction of the CO2 we're currently emitting. It would require 10 tons annually per hectare for every hectare of arable land in the world.
There's also some potential in moving agriculture from its current orientation on annual crops to deep-rooted perennials. The carbon retained in increased root mass would help, but again, not enough. (We burn a lot of fossil fuel annually.)
Alan Belcher 8.2.12
This is a question for Roger, and any others who want to put in their two bits. CO2 is held up to be the culprit for our climate woes, yet it is the mildest of all the greenhouse gas (GHG) emissions. Why else would it have been assigned the value of 1 as a benchmark for all the other GHGs having greater global warming attributes? Referred to as “CO2 equivalent” it is written as CO2e but, often as not, the lower case ‘e’ is dropped. Consequently 1000 tons of CO2 is less than 1000 tons of CO2e.
Yet how many pundits, journalists, editors, and the general public at large understand the importance of this lower-case ‘e’? In strictly non-scientific terms, is CO2 getting a bum rap?
Roger Arnold 8.2.12
Molecule for molecule, CO2 is the mildest of the greenhouse gases. That's offset, however, by the fact that there's so much more of it. Atmospheric concentrations of other greenhouse gases are two to six orders of magnitude below that of CO2.
Since the radiative forcing caused by a greenhouse gas goes as the product of its molecular potency times the log of its concentration, CO2 still accounts for the lion's share of the radiative forcing that isn't due to water vapor. I don't recall its exact share, offhand, but it's something like 80%.
Water vapor is a special case. It's not only far more potent than CO2 molecule for molecule, but it's concentration in the lower atmosphere is normally far higher. So it does, in fact, contribute far more than CO2 to the greenhouse warming that keeps earth livable for us. As a condensable gas, however, its atmospheric concentration depends on temperatures. It functions as an amplifier for other radiative forcings. If there were insufficient CO2 or other greenhouse gases to provide a "signal", then global temperatures and water vapor content in the atmosphere would gradually spiral down in a positive feedback loop, until the earth was a cold, dry ball of ice and snow. That's apparently happened more than once in the distant past.
There's one wild card in this picture, and that's methane. There's currently much less of it in the atmosphere than CO2, and what there is only lasts on average about 20 years before being oxidized to CO2 and water vapor. But there are huge volumes of methane locked away in clathrate deposits on the cold sea floor and below arctic permafrost. There are credible fears that if arctic warming proceeds much further, then melting clathrates could release methane in sufficient quantities as to trigger catastrophic levels of global warming. Not just a matter of a degree or two, but enough to wipe out nearly all higher life forms on land.
There's no consensus among climate scientists as to how much warming would have to occur before that particular tripping point was hit, or even whether it's a realistic concern. Some researchers have found evidence that they interpret as indicating that such catastrophic warming pulses have occurred in the past, and were implicated in at least some of the mass extinctions found in the geological record. But others disagree. All that's certain is that the clathrate deposits exist, and that the volume of stored methane is easily sufficient to trigger catastrophic warming if a substantial fraction of the deposits were to melt.
I don't have any position on that issue, but given the magnitude of the threat, I think we should be taking it seriously.
Len Gould 8.2.12
/sarcon/ Sorry Roger, did you say something? I was busy on my cellphone complaining about the job my landscaping contractor does, and missed it...../sarcoff/ lol
Len Gould 8.2.12
/ I was busy on my cellphone complaining to my mechanic about having left a dirt smudge on the floormats of my Hummer /
/ I was on my cell complaining to the airline for making the plane wait on the runway for 30 minutes before takeoff /
/ I was on my cellphone complaining to the airport authority for allowing airplanes to take off over my house /
/ I was on my cellphone complaining to the governor for allowing wind turbines to be built offshore of my cottage /
etc. etc. etc.
Alan Belcher 8.3.12
Thanks, Roger. Appreciate the info. Guess I should have paid more attention during chemistry classes.
Roger Arnold 8.6.12
This is something of a "PS" to the article. In part 2, I deliberately talked only about the sequestration methods that have received the most attention in recent years. But there are a couple of more radical approaches that I think we may need to consider.
I've been studying the latest report from Jim Hansen's climate group at NASA. The report looks at climate change from a statistical point of view, comparing the distribution of extreme weather events for the last couple of decades versus that of the years up to around 1980. The data show that what would previously have been "100 year events" have now become routine. Although any one of the hot or cold spells, droughts or floods could have been chance events, the statistical probability that the overall pattern that we've seen recently could be a matter of chance is essentially zero.
Hansen's case looks sound. In talking about it, he summarized by saying "climate change is here, and it's worse than we expected". From someone how was derided by many as alarmist, the statement that the actual record has been worse than anything he predicted is saying something. Anyone who has doubts about the financial cost of our extended record of doing nothing might look at the disasterous corn and hay harvests this year, and what it has been doing to ranchers and hog farmers.
I can't know for sure, but it appears to me that we're pretty much out of time for any slow, gradual approach to reducing CO2 emissions. I think we may be forced to consider approaches that fall into the rhelm of geoengineering. I touch on that just a little in part 3, but not in detail. Another article?
Len Gould 8.6.12
Yes please, another article.
Ferdinand E. Banks 8.7.12
By all means another article. Then the environmental people here in Sweden - where the environment is among the best in the world - could tell each other about the disaster that will take place unless the nuclear sector is closed as soon as possible, and some nutty CCS takes place.
Roger Arnold 8.7.12
Your last comment, Fred, suggests that you view CCS and nuclear power as antagonistic options -- that if someone is in favor of one, they must be opposed to the other. That would explain much.
In fact, however, they are not antagonistic at all. I'm a strong proponent of next generation nuclear power, and against the premature shutdown of existing nuclear plants. But I don't believe in making "the best" be the enemy of "the good". I care about what will work. My interest in CCS stems from the question "how can we move quickly with the resources we have to achieve cost-effective results in reducing CO2 emissions".
I like the earth as it has been; I'd rather not see it cooked. Especially not because idiots refuse to see past the blinders of their ideologies.
bill payne 8.7.12
CO2 is apparently produced in New Mexico for sale for different applications..
Cost comparison of CCS producted CO2 might be valuable?
Len Gould 8.7.12
Always known that Bill. CO2 is often a co-product of natural gas as well. And as long as it is 1 penny per ton cheaper to get it from those sources than from coal-fired generating plants, the market system won't allow any CCS, so we're all simply dreaming if we hope for any such.
bill payne 8.7.12
Perhaps we should beware of msm energy writers?
Do not fear the enemy, for your enemy can only take your life. It is far better that you fear the media, for they will steal your HONOR. That awful power, the public opinion of a nation, is created in America by a horde of ignorant, self-complacent simpletons who failed at ditching and shoemaking and fetched up in journalism on their way to the poorhouse.
Look, these schemes to sweep CO2 under the rug (hide it) ignore the problem. We were assured that the CO2 rates of 1990 were the maximum. Abandon all hope to venture beyond those rates. We are double the 1990 rates. More coal was burned in 2011 than in any previous year. But we don't hear it.
If the US stopped all CO2 emissions overnight that would not solve the problem. With 7 billion people and still growing at about 0.1 billion per year our civilization is doomed.
Ferdinand E. Banks 8.8.12
Well Roger, if you really and truly feel the way that you say you feel, then we are on the same team. I am NOT against CCS - iI am not against it because, I never think about CCS. I am also NOT against renewables. I am against the people who want to close down the nuclear sector. A certain amount of nuclear probably fits in everywhere because of its reliability.
Reliability said the man - what the ____ is he talking about? He is talking about optimal reactors burning the huge amount of nuclear fuel that is actually available. Of course, that by itself will not result in the paradise we deserve, but assuming that US voters will not elect another president who starts a war on the basis of a lie, we at least have a shot.
Mark McClurkin 8.8.12
Roger, I believe there is some use for CO2 in the production of fertilizers. Probably a small amount of the total CO2 produced. Would you have any estimates of that need or potential need? And as seems to be true with any solution there must be some negative impacts for such use?
Len Gould 8.8.12
Mark, the only use of CO2 as plant food I've heard of is for some greenhouse crops, as a greenhouse is the only way to maintain artificially supplied CO2 near enough to the plants to be worth anything. Most if not all that (very tiny amount) is, I believe, supplied by re-directing part of the heater exhausts into the houses. Couldn't amount to a tiny fraction of the amount released by human activities, like perhaps a millionth or less.
Roger Arnold 8.9.12
CO2 is used in making urea, and also in ammonium carbonate. But the amounts are tiny compared to the amount of CO2 produced in burning fossil fuels. Also, both urea and ammonium carbonate are really supplying nitrogen; CO2 is released by the plant (or possibly used in place of CO2 from the atmosphere) when the nitrogen is taken up by the plants.
Bottom line: use of CO2 in fertilizers is not a candidate for sequestration.
bill payne 8.11.12
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