ALISTAIR DARLING, CHANCELLOR OF THE EXCHEQUER, 2008
A DECADE ago, geosequestration of carbon, also known as carbon capture and storage (CCS), was looking like an expensive, risky and potentially dead-end technology. That’s because most of the investments made have been relatively small and tied to trying to create a future for coal by capturing CO2 emissions from the smokestacks of power plants, or to injecting CO2 into oil wells to enhance oil recovery. In other words, CCS was being thought of as a way to prolong the life of fossil fuels. There are inherent limits to these approaches. But, worse, this narrow view of the technology has blinded us to other possibilities.
The role of CCS is particularly relevant to some third-way technologies and approaches. Like battery technology in relation to electric vehicles, large-scale, efficient forms of storage are required to unlock the full potential of many third-way technologies. Before going on to take a new and very different look at CCS, we need to examine progress made thus far.
In newly built power stations, two options exist. Integrated Gasified Combined Cycle (IGCC) methods involve transforming coal into a gas and mixing it with oxygen to fire a gas turbine. The second method involves burning the coal in oxygen-enriched air and recycling the exhaust gases back through the combustion chamber to create an exhaust gas that is mostly CO2. A large and expensive air separation plant is needed in both methods to provide the massive amounts of oxygen required.1
The CO2 must then be separated from the sulphur, ash, nitrogen and other materials mixed with it and then compressed for transport, usually via new gas pipelines. To store the gas in geological strata, the CO2 must be compressed even more, until it becomes a liquid. It is then injected into suitable underground rock formations, where it will potentially remain for geological time (thousands or millions of years).2
CCS can also be applied directly to natural gas, which sometimes contains large amounts of CO2. The CO2 must be separated out, and the purified gas compressed and injected in the earth, as occurs in gas from power plants. There are currently 12 commercial-scale CCS plants operating worldwide. Together they capture and store about 15–20 million tonnes of CO2 each year, which is only 0.4 per cent of a gigatonne. Eight of these CCS plants inject the captured CO2 into oil fields as part of the process of extracting the oil.
There is currently only one commercial-scale project capturing emissions from electricity generation (in Canada: it commenced operations in October 2014), though the Kemper County CCS power plant, another such project, is under construction in the United States. Both will use the captured CO2 for oil recovery.
The Global CCS Institute, headquartered in Melbourne, Australia, expects that by 2020 there will be 21 CCS plants worldwide capturing about 30 million tonnes of CO2 per year (about 3 per cent of a gigatonne of CO2).3
An inherent problem with using CCS in conjunction with power plants is that 20–25 per cent of the energy produced by the power station is needed to run the capture and storage processes, which makes the electricity that much more expensive to generate. And CCS power plants are also proving more difficult and expensive to build than anticipated. The 582-megawatt Kemper power plant in the US had its opening delayed until May 2015, and is now forecast to cost more than US$5 billion, a substantial increase on the original estimate.4 At best, the new power station is expected to capture only around 65 per cent of the CO2 produced (3.5 megatonnes per year). The machinery required to achieve even this is monumental. Photographs of the plant reveal a sprawling monster of pipes and tanks. It looks more like a gargantuan chemical factory than a power plant.
Many people are now concluding that CCS on the Kemper County model is a dead-end. But, recently, some geologists and climatologists have been revisiting CCS. Their ideas of how it might be done and what it might achieve are dramatically different from anything tried previously. And, although fraught with difficulty, some of their pioneering approaches might just achieve the volumes required to make a significant difference to the climate problem.5 The most promising new thinking is based on finding parts of Earth where conditions are right to store CO2 in either liquid or solid form. And, surprisingly, those conditions do exist in areas that are not entirely inaccessible.
One of these approaches involves the storage of CO2 in parts of the ocean crust. Hitherto, those seeking to sequester CO2 in the Earth’s crust on land have faced a daunting problem. As Bahman Tohidi, from the UK’s Institute of Petroleum Engineering, and his colleagues explain:
Because of the subsurface temperature profile of terrestrial storage sites, CO2 stored in these reservoirs is buoyant. As a result, a portion of the injected CO2 can escape if the reservoir is not appropriately sealed.6
This limits CO2 storage in continental rocks to geological structures, such as oil-bearing strata, that have impermeable cap-rock or saline aquifers, where natural chemical processes slowly solidify the CO2. But what if we could use the pressure of the waters of the ocean itself to help keep the gas in liquid form or, even better, to lock it into the rock?
In a series of laboratory experiments, Bahman Tohidi and his collaborators showed that, because of the enormous pressure of the water column, if CO2 is stored in marine sediments in waters 3000 metres or more deep, it remains in liquid form, although stored only a few hundred metres into the sediment. Over time, natural chemical processes in the water of the ocean sediments convert the liquid CO2 into a solid—in the form of stable hydrates. The great overlying pressure of the water prevents the CO2 rising towards the sediment surface, making the storage much more stable than when CO2 is stored in rocks on land.7 And when the CO2 becomes a hydrate, it is locked into the rock permanently.
Although not all regions of the ocean deeper than 3000 metres are suitable for the storage of CO2, the potential scale of this approach is large. At least a few hundred metres thickness of permeable sediments is required in an area where the topography is not too steep. Steep topography must be avoided because injection of CO2 there could trigger submarine landslides, which can generate tsunamis. Even with such limitations, researchers note that ‘the total CO2 storage capacity within the 200-mile economic zone of the US coastline is enormous, capable of storing thousands of years of current US CO2 emissions’.8
While the research is at an early stage, it does suggest that the option should be examined further, and even prioritised. Together with a steep reduction in emissions and seaweed farming, CO2 storage in deep water marine sediments might just be planet-saving.
Another intriguing proposal for the geosequestration of CO2 was recently published by Professor Ernie Agee and his colleagues at Purdue University in Indiana. It concerns the potential capture and storage of the gas in the Antarctic ice cap. The research was triggered by the observation that Mars has polar ice caps composed of frozen CO2, and the scientists wondered if Earth’s ice caps might be capable of storing frozen CO2 as well.
It turns out that conditions over the Antarctic ice cap are so severe that the storage of solid CO2 (dry ice) might be possible. At sea level, CO2 freezes at −78.5°C. The Antarctic ice cap has an average elevation of about 2500 metres, and temperatures of −89.2°C have been recorded at Vostok Station on the ice cap. This is close to the temperature required for CO2 to freeze and begin to accumulate as snow. At higher temperatures, CO2 freezes over the Arctic ice cap, but sublimates (changes back to a gas) as quickly as it freezes out. The average temperature over the interior of the Antarctic ice cap is −57°C, so in most weather conditions only about 30°C of cooling would be required to cause CO2 to fall out of the air.
Professor Agee and his colleagues propose building a series of 100 × 100 × 100 metre refrigeration chambers high on the Antarctic ice cap. Air cooled with liquid nitrogen to below CO2’s feezing point, would cause the precipitation of about 40 centimetres of CO2 snow per day, leaving all other components of the air in gaseous form. The CO2 snow could be stored in pits in the Antarctic ice, and covered with ice and snow to prevent its loss through sublimation on exposure to the slightly warmer air. The researchers estimate that only sixteen 1200-megawatt wind plants (less wind power than currently exists in Germany) could provide all of the energy required to drive 446 such cooling chambers. And that would be enough to capture and store one gigatonne of CO2 per year.9
Antarctica is a windy place, and wind power is already in use at research stations on the continent. Moreover, an existing global treaty—the Antarctic Treaty—provides a framework for scientific co-operation and international governance. Because the proposed refrigeration chambers are modular, it’s possible to build a trial plant in order to investigate this proposal further.
One important objection to the proposal is that Antarctica is Earth’s last continental-scale wilderness. Many people would be reluctant to see large refrigeration cubes and wind farms scattered over its surface. But it seems to me that these objections share much with those of people who object to wind farms in the countryside on the basis they don’t like looking at them. As the climate problem grows, such objections will surely need to be weighed against the growing climate impacts.
What other possible downsides might there be to storing CO2 ice in Antarctica? One potential problem is that the concentration of CO2 in the air over Antarctica might become greatly reduced, and this could affect the surrounding ocean, or indeed the southern hemisphere as a whole. And it’s possible that the very cool air might affect global atmospheric circulation. Both possibilities are easily investigated with climate modelling, and this should be done. Indeed even simple observations of what happens when air temperatures drop below –78.5°C naturally would be highly useful in understanding the potential effects. In any case, CO2 levels could be kept within the historical range experienced over Antarctica by regulating the speed of the drawdown, and since the atmosphere mixes readily, the local CO2 depletion would not last long after the plant throttled back.
There is also the risk of CO2 escape, if the ice cap were to warm. The conditions under which such a release might occur require investigation, though they are extremely unlikely even on the thousand-year timescale. If they did, Earth would be facing a full-blown climate crisis in any case. Conversely, if we take a long view and examine a future Earth threatened with an ice age, the trapped CO2 could be exposed and allowed to warm the atmosphere.
The cost of building the proposed infrastructure in Antarctica is very difficult to estimate, but it is likely to be huge. Back of the envelope cost estimates of up to a trillion dollars might be conservative.10 But technologies and cost structures change, and with the project unlikely to be seriously considered before 2050, both costs and funding models may then be very different. If the technology, the functionality and the economics of the proposal became increasingly robust, it’s not difficult to imagine such a project as the first globally crowd-funded project to save humanity’s future.
While these possible applications of geological storage of CO2 may seem distant and even dangerous to some, we must calibrate that against the dangers of living with 450 ppm or more of CO2 in the atmosphere for a century or longer. Whether that danger becomes a reality will be determined by the success of our efforts in reducing carbon pollution. However, we are on a worst-case emissions trajectory at present, so if nothing changes, in coming decades the idea of storing CO2 as snow at the South Pole, or deep in the ocean’s sediments, may not look so risky after all.