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Carbon capture and storage

Contents

The idea

Carbon capture and storage, also known as carbon capture and sequestration or CCS, describes a variety of ways to reduce the carbon dioxide emitted by burning fossil fuels. Most commonly it refers to methods based on capturing carbon dioxide from large point sources such as fossil fuel power plants and storing it so that it does not enter the atmosphere. It also refers to ways of removing CO2 directly from ambient air, for example by biological methods such as biochar.

In the following paper:

  • D’Alessandro et al, Carbon dioxide capture: prospects for new materials, Angew. Chemie Int. Ed. 49 (2010), 2–27.
the authors summarize two of the key issues as follows:

Two points must first be made with respect to capture materials and potential capture technologies, given the sheer magnitude of global CO2 emissions. First, any chemical employed to capture CO2 will rapidly exhaust its global supplies if it is used in a once-through manner; and second, any chemical produced from CO2 as a reactant will rapidly saturate global markets for that chemical. These considerations underscore the necessity that capture materials must be regenerable. In this case the energy input for regeneration is one of the key factors in determining the efficiency and cost.

List of methods

We need a listing of different methods of CCS. What follows is a start. Then: how they work, how well they can be scaled up, and some scientific and engineering challenges that need to be addressed for each one!

Injection into the ground

One widely studied approach to CCS is simply injecting CO2 into the ground. See:

In 1991, Norway became the first country in the world to impose a federal tax on atmospheric CO2 emissions from operations such as coal-fired power plants. Later this tax of $55 of per ton of CO2 was extended to cover offshore oil and gas production. This motivated Norwegians to set up an operation called Sleipner which strips CO2 from natural gas offshore in the North Sea and reinjects 0.3 megatons of carbon a year into a non–fossil-fuel–bearing formation. This is apparently the world’s largest carbon capture and storage project. However, a one-gigaton ‘wedge’ would require building 3500 Sleipner-sized projects (or fewer, larger projects).

For more details, see:

For safety issues, see these comments on the Sleipner project:

Among other things, they write:

The injected CO2 will potentially be trapped by geochemical processes. Solubility trapping has the effect of eliminating the buoyant forces that drive CO2 upwards, and through time it can lead to mineral trapping, which is the most permanent and secure form of geological storage.

Solubility trapping

Solubility trapping is where the CO2 dissolves in water. It’s harder for the CO2 to escape the reservoir when it’s dissolved, rather than just sitting on top of water in a light buoyant phase. The solution itself sinks deeper and more securely into the reservoir, because it’s heavier with CO2 in it. Mineral trapping is where the CO2 chemically reacts with minerals so that carbon gets sequestered in stable, solid precipitates.

On Azimuth, John Furey? writes:

I don’t have any confidence in high volume injection over the long term. Our species has been geologically injecting at all only about 50 years. The ground underneath our feet is not in homogeneous layers, contrary to the cartoons comprising most all modeling, and even if it were there are unpredictable side effects at any given site. If you’ve been following the news about fracking, you’re aware that the most experienced corporations with the most economic incentives to do things right environmentally fail miserably because they do not really understand even shallow injection:

Of course leaking, contamination, etc is to be expected (rational i.e. calculated risks). What is not being reported is volcanos of fracking fluids kilometers from injection sites. They scratch their heads, close that well, and move over horizontally a little bit.

Hank Roberts writes:

Worry seems to be that old oil and gas fields will have had lots of old wells drilled over time – often not mapped or recorded – plugged with concrete and abandoned, and increasing CO2 pressure will accelerate failure:

Here are some further references discussing trapping mechanisms and sometimes failure modes:

Here is the executive summary:

The success of geologic carbon dioxide (CO2) sequestration as a large-scale carbon management strategy is critically dependent on the ability of the geologic sinks and trapping mechanisms to confine the injected CO2 for hundreds to thousands of years. Leakage of CO2 from geologic sinks could result in significant release of the CO2 back to the atmosphere, potentially reducing, if not negating altogether, the benefits of geologic CO2 sequestration. For example, a leakage rate of 1% per year from 1 billion tons of geologically stored CO2 (10 million tons) would exceed the current annual CO2 emissions from all the power plants in North Dakota (4 million tons). Further, leakage could have negative ecological effects and present the potential for health problems other than global warming. Clearly, the more CO2 that is stored, the greater the potential that leakage from geologic sinks could result in adverse environmental and atmospheric impacts. It has recently been proposed that leakage rates of 0.01% per year be established as a performance requirement for geologically sequestered CO2 (White et al., 2003).

This report provides an analysis of how the physicochemical properties of CO2 would affect its trapping potential and mobility in various types of geologic environments. Analog studies of geologic environments containing large, concentrated amounts of CO2 or hydrocarbons gas were also used to derive insight regarding the leakage processes that would be inherent in geologic CO2 sequestration. The analogs included a) naturally occurring deposits of high-purity CO2, b) a mature CO2 flood enhanced oil recovery (EOR) project, c) an aquifer natural gas storage reservoir, and d) coalbed natural gas deposits.

Injected CO2 can be trapped in geologic sinks by four types of mechanisms. Different types of geologic sinks in combination with their site-specific properties would trap CO2 by different mechanisms. More than one type of trapping mechanism would typically be present in a single geologic sink. Most trapping mechanisms do not permanently immobilize CO2. Thus leakage of CO2 to the surface can potentially occur from all types of geologic sinks.

In the right types of geologic settings, a large, concentrated amount of CO2 could be stored for a geologically long time period without the risk of significant CO2 leakage to the surface. The dominant, but by no means sole, barrier to CO2 leakage to the surface from geologic sinks is not the trapping mechanism(s) but rather the permeability characteristics of the rock layers overlying or adjacent to the geologic sinks. The hydrologic properties of the formations containing the geologic sinks would also affect the potential for CO2 leakage. Geologic settings with relatively static hydrology, i.e., low formation.

Injecting CO2 into deep geological strata is proposed as a safe and economically favourable means of storing CO2 captured from industrial point sources. It is difficult, however, to assess the long-term consequences of CO2 flooding in the subsurface from decadal observations of existing disposal sites. Both the site design and long-term safety modelling critically depend on how and where CO2 will be stored in the site over its lifetime. Within a geological storage site, the injected CO2 can dissolve in solution or precipitate as carbonate minerals. Here we identify and quantify the principal mechanism of CO2 fluid phase removal in nine natural gas fields in North America, China and Europe, using noble gas and carbon isotope tracers. The natural gas fields investigated in our study are dominated by a CO2 phase and provide a natural analogue for assessing the geological storage of anthropogenic CO2 over millennial timescales. We find that in seven gas fields with siliciclastic or carbonate-dominated reservoir lithologies, dissolution in formation water at a pH of 5–5.8 is the sole major sink for CO2. In two fields with siliciclastic reservoir lithologies, some CO2 loss through precipitation as carbonate minerals cannot be ruled out, but can account for a maximum of 18 per cent of the loss of emplaced CO2. In view of our findings that geological mineral fixation is a minor CO2 trapping mechanism in natural gas fields, we suggest that long-term anthropogenic CO2 storage models in similar geological systems should focus on the potential mobility of CO2 dissolved in water.

Direct capture from the air

In capturing CO2 from point sources, the dominant cost is supposedly the cost of capture rather than storage. However, David Keith has argued that direct capture from the air could be viable:

Enhanced weathering

One of the main long-term mechanisms that removes carbon dioxide from the ocean and atmosphere is the natural weathering of rocks. We can vastly accelerate this process by digging up suitable rocks, crushing them into powder and spreading them around. The rock dust then ‘weathers’ by reacting with carbon dioxide. This method of carbon capture and storage is called enhanced weathering. In principle it can also serve as a source of carbon negative energy.

For more details, see Enhanced weathering.

Using natural materials for products

Plants are more or less carbon neutral over their lifetime. In particular during growth they on average capture more CO2, while upon decay they on average release more CO2. If one thus prevents natural materials from decaying, like by using them for products (like wood for housing) then one postpones CO2 release and thus may be able to capture some CO2 for a while. It should however be remarked that decomposition is important for maintaining soil nutrients.

Biochar

The idea here is to partially burn crop residues, forming charcoal, and then bury this charcoal, where it helps fertilize the soil. That way, we’d be letting plants suck CO2 out of the atmosphere for us, while simultaneously improving soils for farming.

For more details, see Biochar.

CROPS (Crop residue oceanic permanent sequestration)

CROPS stands for Crop Residue Oceanic Permanent Sequestration. The idea is to dump a lot of crop residues—stalks, leaves and stuff—on the deep ocean floor. As with biochar, this lets plants suck CO2 out of the atmosphere for us, taking advantage of the fact that agriculture is the world’s biggest industry. The US produces about 0.5 gigatonnes of crop residue per year, and worldwide the total is about times that:

Benford and Strand are testing the chemistry of how farm waste interacts with deep ocean sites offshore Monterey Bay right now. Here’s a picture of a bale 3.2 km down:

Algae photosynthesis

The general idea here is to remove carbon dioxide from the atmosphere by growing algae, and then somehow fix the algae in a form where the carbon that it contains will not be returned to the carbon cycle (at least for a long time).

There is a known technique called “iron fertilization,” which can be used to seed the growth of ocean algae blooms. Experiments have been conducted with the aim of carbon sequestration in mind. Although the idea has not reached practical fruition, there is a lot of potential carbon capture that can be produced by these abundant organisms.

For more details, see Iron fertilization.

Cement manufacture

The manufacture of ordinary Portland cement uses a large proportion of limestone. This is heated in a kiln, driving off the CO2. By replacing the calcium from limestone with magnesium from minerals such as serpentine it is possible to make the manufacture low- or even negative-carbon, producing a cement which has similar properties. Annual world production of cement, 2010, is around 2 billion tonnes.

See:

Making methanol

This is not exactly a form of carbon capture and storage, but related. See Methanol economy.

Making plastics

Carbon dioxide can be used to make plastics. Unfortunately, the amount of CO2 to be dealt with exceeds worldwide plastic demand by about 2 orders of magnitude. In 2007, worldwide production of plastic was 260 megatonnes, while 29.3 gigatonnes of CO2 was emitted by production of energy and cement manufacture. Plastic production is expected to reach 365 megatonnes by 2015.

Coal bed methane extraction

Disused and uneconomic coal mines continue to emit methane. They can also be used to sequester carbon dioxide. Coal has a stronger affinity for CO2 than methane. So, putting CO2 in mine shafts displaces the methane, which can be captured and used as it rises to the surface. This idea is called coal bed methane extraction. It may constitute a source of carbon negative energy.

See:

Methane hydrate reactions

Another idea, due perhaps to ‘Uncle Al’ of internet fame, is to pump liquid CO2 into methane hydrate formations. He claims that the recovered natural gas plus sequestered CO2 has net zero carbon footprint. The reactions involved are

CH4 + nnH2O(s) \to CH4(g) + nnH2O(l) + 54.44 kJ/mol gas

CO2 + nnH2O(s) \to CO2(g) + nnH2O(l) + 63.6 kJ/mol gas

For more on the chemistry of methane hydrates (also known as methane clathrates), see

Carbon capture and storage for coal-fired power plants

See the page Carbon capture and storage for coal-fired power plants.

References

For starters, try:

Other references:

  • Interagency Task Force on Carbon Capture and Storage, Final report, 2010, USA — a series of recommendations on overcoming the barriers to the widespread, cost-effective deployment of CCS within ten years.