Scientist explores deep ocean storage of carbon - another option for carbon negative bioenergy

Imagine a gigantic, inflatable, sausage-like bag capable of storing 160 million tonnes of CO2 - the equivalent of 2.2 days of current global emissions. Now try to picture that container, measuring up to 100 metres in radius and several kilometres long, resting benignly on the seabed more than 3 kilometres below the ocean's surface and staying there forever. That would offer a way of storing carbon dioxide permanently and thus help in the fight against climate change.

At first blush, this might appear like science fiction, but it's an idea that gets serious attention from Dr. David Keith, one of Canada's foremost experts on carbon capture and sequestration. Keith will talk on the subject at the 2008 Annual Conference of the American Association for the Advancement of Science in Boston at a session entitled "Ocean Iron Fertilization and Carbon Sequestration: Can the Oceans Save the Planet?"

Shooting rockets full of sulphur into the high atmosphere to emulate the cooling effect of volcanic eruptions, launching space mirrors, making artificial reflective clouds, or building costly synthetic trees - there are a lot of gee-whiz (and risky) "geo-engineering" ideas for dealing with global warming that are really silly, remarks Keith, an NSERC grantee and director of the Energy and Environmental Systems Group at University of Calgary-based Institute for Sustainable Energy, Environment and Economy. At first glance his own idea looks nutty, but as one looks closer it seems that it might technically feasible with current-day technology. But, adds Keith, who holds the Canada Research Chair in Energy and the Environment, it's early days and there is not yet any serious design study for the concept.

Carbon storage is receiving more and more attention as climate change needs radical interventions. Capturing and storing CO2 from (biomass) power plants and other point sources comes in a variety of forms:

1. the gas can be captured and stored in geological formations such as depleted oil and gas fields, saline aquifers, unmineable coal seams or special rock formations; this storage technique is called "geosequestration"
2. alternatively, carbon can be captured and stored either in gaseous or in a solid form at the bottom of the ocean, where it would remain contained for millennia - "ocean storage"
3. carbon dioxide can also be captured and transformed into stable, inert products, via mineralisation processes
   
4. last but not least, carbon can be stored in an inert form in soils; this technique is based on biochar, obtained from biomass; soil sequestration has many advantages: it is both relatively simple and cost-effective, and improves soil qualities considerably; biochar systems yield negative emissions, because the biomass delivers both energy as well as a carbon sink
   

All these carbon sequestration techniques can be applied to CO2 from biomass power plants and decarbonised biofuel production. In this case, a "negative emissions" energy system can be designed that removes CO2 emissions from the past. Actively taking CO2 out of the atmosphere with such carbon-negative bioenergy systems, is the most radical tool in the climate fight. No other energy system can become carbon negative. Such bio-energy with carbon storage (BECS) systems are seen as one of the few cost-effective, feasible and safe geo-engineering option.

But these systems require efficient carbon capture, transportation and storage strategies. Biopact has been focusing both on carbon capture and storage (CCS) based on geosequestration and biochar, but ocean storage could be an alternative.

The original idea of ocean storage was conceived several years ago by Dr. Michael Pilson, a chemical oceanographer at the University of Rhode Island, but it really took off last year when Keith confirmed its feasibility with Dr. Andrew Palmer, a world-renowned ocean engineer at Cambridge University. Keith, Palmer and another scientist at Argonne National Laboratory later advanced the concept through a technical paper prepared for the 26th International Conference on Offshore Mechanics and Arctic Engineering in June 2007.

The Abyss
Keith sees this solution as a potentially useful complement to CO2 storage in geological formations, particularly for CO2 emanating from sources near deep oceans. He believes it may offer a viable solution because vast flat plains cover huge areas of the deep oceans. These abyssal plains have little life and are benign environments. Abyssal plains are flat or very gently sloping areas of the deep ocean basin floor, covering approximately 40% of the ocean floor and reaching depths between 2,200 and 5,500 m (7,200 and 18,000 ft). They generally lay between the foot of a continental rise and a mid-oceanic ridge (image, click to enlarge):
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If we stay away from the steep slopes from the continental shelves, the abyssal plains are a very quiet environment, says Keith.

For CO2 to be stored there, the gas must be captured from power and industrial point sources, compressed to liquid, and transported via pipelines that extend well beyond the ocean's continental shelves. When the liquid CO2 is pumped into the deep ocean, the intense pressure and cold temperatures make it negatively buoyant.

This negative buoyancy is the key, explains Keith. It means the CO2 wants to leak downwards rather than moving up to the biosphere.

The use of containment is necessary because CO2 will tend to dissolve in the ocean, which could adversely impact marine ecosystems. Fortunately, says Keith, the cost of containment is quite minimal with this solution. He and his colleagues calculate that the bags can be constructed of existing polymers for less than four cents per tonne of carbon.

The real costs lie in the capture of CO2 and its transport to the deep ocean. If we can drive those down, he notes, then ocean storage might be an important option for reducing CO2 emissions.

Image: the Abyssal Plains. Credit: Encyclopedia Britannica.

References:
David Keith, "Engineered Storage on the Abyssal Plain: prospects to a new approach to ocean carbon storage and some thoughts about geoengineering", Department of Chemical and Petroleum Engineering, University of Calgary, AAAS Annual Meeting 2008, Ocean Iron Fertilization and Carbon Sequestration: Can the Oceans Save the Planet.

Natural Sciences and Engineering Research Council of Canada: Into the Abyss: Deep-sixing Carbon - February 18, 2008.

Biopact: The end of a utopian idea: iron-seeding the oceans to capture carbon won't work - April 26, 2007

Biopact: WWF condemns Planktos Inc. iron-seeding plan in the Galapagos - June 27, 2007

Biopact: Simulation shows geoengineering is very risky - June 05, 2007

Biopact: Climate change and geoengineering: emulating volcanic eruption too risky - August 15, 2007

Biopact: Capturing carbon with "synthetic trees" or with the real thing? - February 20, 2007