Don’t get too excited about turning CO2 into stone

Carbon capture and storage (CCS) has been trotted out year after year as the fabled solution to climate change, but so far it has produced little except a few pilot CCS plants that are political boondoggles. The costs of CCS in these pilot plants are so high that the companies which burn fossil fuels have decided that their money is better spent lobbying against government regulation and financing global warming denial and junk science. A 2014 report by the UK government estimates that capturing carbon dioxide (CO2) from steel production will cost around $75 per tonne and more than double that for oil refining. Sequestering it underground will add $15 more per tonne.

Even if the high costs of CCS can be solved, there is no guarantee that the CO2 dumped into old mine shafts and abandoned wells will stay locked underground. The CO2 could bubble up through seams in the earth and unexpected seismic activity could release large amounts of the stored gas at once. Nobody wants to live close to a CCS storage site, knowing that they might suddenly be suffocated by a CO2 leak and no fossil fuel or power company wants to be held financially liable for leaks that might occur decades or even centuries in the future.

Because CCS is fraught with so many problems, the press has eagerly covered the publication of a new technique to convert CO2 into stone, so it can be stably stored. A recent article (pdf and supplemental) in Science published the results the CarbFix pilot project at the Hellisheidi geothermal power plant in Iceland, which dissolved 230 tonnes of CO2 in water and pumped it through basaltic lava and hyaloclastites 400 to 800 meters below the ground. Within 550 days, over 95% of the CO2 had mineralized so it could be stored in a stable form underground. Although it was well known that the calcium, magnesium and iron in basaltic rock would react with CO2 to form a carbonate mineral, the experimenters at the CarbFix project thought that the process might take 8 to 12 years, but the mineralization of CO2 started within months.

One of the biggest problems of CCS is the high cost of separating out the CO2 from the other emissions. In phase I of CarbFix, 175 tonnes of pure CO2 were used in the test project. Phase II of the project, however, used 55 tonnes of CO2 mixed with 18 tonnes of hydrogen sulfide (H2S), which is a poisonous gas also released by volcanic activity in Iceland. The CO2 contaminated with H2S mineralized just like the pure CO2, which is important because it shows that CCS can be done without the costly step of first separating out the CO2. Edda Aradottir of Rejkjavik Energy calculates that CO2 can be turned to stone at the Hellisheidi plant for a cost of $30 per tonne, which is far below the cost of conventional CCS. The low cost has allowed the Hellisheidi plant to scale up its CarbFix project to bury 10,000 tonnes of CO2 per year, plus the noxious H2S which accompanies it.

The low cost of storing CO2 and H2S is important, because it means that most of the harmful emissions from geothermal energy in Iceland can be eliminated. The Hellisheidi plant, which is the largest geothermal facility in the world, has a capacity of 303 MWe and produces roughly 2,400 GWh of electricity per year. However, it also emits 40,000 tons of CO2 and 12,000 tons of H2S per year. For every kWh of electricity it generates, the plant is emitting roughly 17 grams of CO2. On top of that, the greenhouse gases from building and maintaining a geothermal plant need to be added. The IPCC estimates that the full life cycle emissions of the median geothermal plant is 38 grams of CO2-equivalent per kWh, but the Hellisheidi plant is probably emitting more than the median due to the large amounts of escaping volcanic gases. If we guesstimate that it is emitting 50 grams, than it emits four times as much greenhouse gases per kWh as wind energy and a quarter more than residential photovoltaic solar panels. Of course, this is far better than fossil fuels, which emit between 10 and 20 times as much greenhouse gases per kWh of electricity. According to the IPCC, the average coal power plant emits 1040 grams and the average natural gas plant emits 600 grams. The most efficient type of fossil fuel energy from an advanced combined-cycle natural gas plant emits a median of 490 grams.

Greenhouse gas emissions from electricity generation
(grams of CO2-equivalent per kWh)
Energy source Life Cycle Source
Median* (range)
European coal 1.070 (949 – 1280) Dones et al. 2004
World coal average‡ 1.040 (990 – 1090) IPCC 2014
Pulverized coal 820 (740 – 910) IPCC 2014
European petroleum (diesel or fuel oil) 880 (519 – 1200) Dones et al. 2004
European natural gas 640 (485 – 991) Dones et al. 2004
World natural gas average‡ 600 (520 – 790) IPCC 2014
Natural gas with combined cycle 490 (410 – 650) IPCC 2014
Biomass 230 (130 – 420) IPCC 2014
Geotérmica 38 (6.0 – 79) IPCC 2014
Hydroelectric 24 (1.0 – 2200) IPCC 2014
Tropical Amazonian hydroelectric† 2,556 (2524 – 26784) McCully 2006
Tropical Amazonian hydroelectric (only methane)† 2,044 (2006 – 2454) McCully 2006
Nuclear 66 (1.4 – 288) Sovacool 2008
Nuclear 12 (3.7 – 110) IPCC 2014
Concentrated solar 27 (8.8 – 63) IPCC 2014
Residential photovoltaic solar 41 (26 – 60) IPCC 2014
Utility photovoltaic solar 48 (18 – 180) IPCC 2014
Terrestrial wind 11 (7.0 – 56) IPCC 2014
Maritime wind 12 (8.0 – 35) IPCC 2014
Tidal 17 (5.6 – 28) IPCC 2014
* The median value in IPCC 2014, but the average in other sources.
** Dones et al. 2004 is the European average in the 1990s, when most plants didn’t have combined cycle or other new technologies.
† CO2-equivalent is recalculated using a GWP-100 of 34 for methane from IPPC AR5.
‡ Based on counting the pixels in the graphic (IPCC 2014, p. 539).
Sources: IPCC (Intergovernmental Panel on Climate Change) (2014) Working Group III: Fifth Assessment Report, p. 539, table A.III.2,; R. Dones, T. Heck, S. Hirschberg (2004) Greenhouse gas emissions from energy systems: Comparison and overview, Encyclopedia of Energy, vol 3, p. 27-40,; Patrick McCully (2006-11) Fizzy Science Loosening the Hydro Industry’s Grip on Reservoir Greenhouse Gas Emissions Research, International Rivers Network,; Benjamin K. Sovacool (2008) “Valuing the greenhouse gas emissions from nuclear power: A critical survey,” Energy Policy, 36: 2950–2963, doi:10.1016/j.enpol.2008.04.017,

It first glance, the CarbFix project appears to have solved the critical problems of carbon capture and storage, which is why so many stories about the project have recently appeared in the press. The project has been covered not only in science and technology sites like Scientific American,, Wired, Clean Technica and Climate Change News, but also in the mainstream press at Time, the Guardian, the Washington Post, BBC and Newsweek.

In an interview on Ecoshock Radio, Dr. Martin Stute, who coauthored the Science article about CarbFix project, is careful to note many of the potential problems in deploying his technique of turning CO2 into stone around the world. Although basalt rock forms most of the ocean floor and 10% of continental rock, the basaltic lavas in Iceland may react more readily with CO2 than other types of basalt, so experiments are needed in other parts of the world to see if the results can be replicated. Another limitation of this technique is the fact that it uses 25 tonnes of water for every tonne of CO2. Turning CO2 into stone may not be a viable option in the future when fresh water supplies will be limited, but Stute believes that his technique will also work with salt water. Power plants in coastal cities could pump water out of the ocean to turn their CO2 emissions into stone. If bacteria begin to feed on the carbonate mineral, they could create methane, which is a greenhouse gas 86 times more potent than CO2 over a period of 20 years. The CarbFix project hasn’t observed any bacterial activity, but it may be a problem in other parts of the world. Another critical question is whether sulfur dioxide (SO2) which is emitted by coal plants can be captured and buried like hydrogen sulfide using this technique.

While many of the articles in the press have mentioned these potential problems, they have failed to grasp the larger fact that this new technique of storing CO2 as stone probably won’t make CCS a viable option for fossil fuels. Even even if this new technique can be proven to work with all types of basalt rock and works with non-pure CO2 and salt water, it still won’t be widely deployed, because most fossil fuel thermoelectric plants won’t install it.

Wind energy is already cheaper than fossil fuels in many parts of the world. According to the US Department of Energy, the average cost of installing a wind turbine in the US dropped from $2290 per kW in 2009/10 to $1710 per kW in 2014, decreasing 6.3% per year. During the same time period, the average price of a purchasing power agreement for wind energy dropped from $70 to $23.5 per MWh, decreasing 21.5% per year.

Likewise, the price per kWh of solar energy is dropping so fast that it is starting to challenge fossil fuels. In recent power purchase agreements, the price of a MWh of solar energy is $58.4 in Dubai and $40 (or $57.1 when excluding subsidies) in Texas. Between 1977 and 2015, the price of a crystalline solar module has dropped from $76.67 to $0.54 per watt, decreasing 12.2% per year. For every doubling of global solar panel production, the price of solar modules drops 22%.

Because of the falling cost of renewables, wind energy accounted for 40.9% of new US electricity generating capacity in the year 2015 and solar energy accounted for another 25.9%, but fossil fuels only accounted for 30.6%.

New electricity generating capacity in the US in 2015
Energy type GW Percent
Wind 7,977 40.9%
Natural gas 5,942 30.5%
Other solar 2,890 14.8%
Utility-scale solar 2,157 11.1%
Biomass 308 1.6%
Water 153 0.8%
Geothermal 48 0.2%
Oil 15 0.1%
Coal 3 0.02%
Total 19,493 100%

Once the cost of CCS is added to fossil fuels, they simply can’t compete with wind and solar energy, even at the low price of $30 to store a tonne of CO2. At any rate, it is unlikely that most fossil fuel plants will be able to match the low cost of CCS at the Hellisheidi plant. First of all, at the average US price of $1.50 per 1000 gallons of water, the 25,000 liters of water used per tonne of CO2 will cost $10. Even if the plant can get its water for free out of the sea, the pumps and pipes to pull water from the sea will probably cost more than the water used in Iceland. Second, the Hellisheidi plant probably was able to use existing wells to carry out its pilot project, but fossil fuel plants will have to bear the expense of drilling new injection wells. If a fossil fuel plant isn’t conveniently located over a basalt rock formation with a cheap water supply, then it will have the added cost of pumping the water from far away or transporting the CO2 to another location. Another big question is whether the basalt rock in an area can get oversaturated by too much CO2, so that it looses the capacity to mineralize the CO2. If this occurs, then it will be necessary to keep drilling new injection wells, which will add to the costs.

It is hard to see how fossil fuels can compete when the price of wind and solar energy keeps falling. At this point, natural gas is still competitive with renewables in the US and Russia and coal is still competitive in China, India and Australia, but they probably won’t be in a couple years. If government regulation forces fossil fuel plants to add CCS, most fossil fuels plants will shut down, because it will be cheaper to build new wind or solar farms than convert an existing coal or natural gas plant to use CCS.

Shutting down fossil fuel plants is the best solution for the environment, because CCS only captures 80% to 90% of CO2 emissions, so it is still dirtier than wind, solar, geothermal and most hydroelectric energy (with the exception of tropical dams). A combined cycle natural gas plant with CCS would emit between 50 and 100 grams of CO2 per kWh and coal with CCS would emit double that amount. It is also critical that the transition to low carbon energy happen as fast as possible. A new photovoltaic solar plant can be constructed in a year and a new wind farm in 2 years, whereas converting an existing coal or gas plant to use CCS will take far longer, even for new plants that are advertised as “CCS ready”. By all measures, using CCS with fossil fuels is worse for the environment than simply switching to alternative energy.

The new technique of turning CO2 to stone is unlikely to save fossil fuel plants even if it is substantially cheaper than other forms of CCS, because fossil fuels simply can’t compete with the falling prices of wind and solar in the long term. It is also unlikely that geothermal plants outside of Iceland will use the CO2 to stone technique, because most geothermal plants don’t have significant emissions, especially poisonous hydrogen sulfide which needs to be captured.

The only place where the new technique of converting CO2 to stone might be widely deployed is in biomass thermoelectric plants. In the long term, biomass energy will face the same problem competing with wind and solar energy as fossil fuels, since its costs aren’t falling and the cost of transporting the biomass to the plant will probably rise in the future. Biomass energy only makes sense in cases such as sugarcane bagasse, nut shells, rice husks, corn husks, waste from lumber operations, etc., where the biomass is already collected in one place for processing. However, the world will face a shortage of artificial fertilizers in the future, due to limited supplies of phosphorus and potassium, so that biomass may be more valuable as compost than as energy. Most of the plant matter will be left in the field to decay and fertilize the next year’s crop.

Edit: Larmion on the Clean Technica forum points out that the ash from biomass burning will still contain P, K and other nutrients, so it can still be used as fertilizer. However, the value of this ash as fertilizer is unlikely to pay for the cost of first transporting the biomass to the thermoelectric plant and then transporting it back to the field, so biomass energy will continue to only make sense for material which is already being collected in a central point for processing.

It will cost just as much to bury the CO2 from burning biomass as it costs for fossil fuels, but the world might be willing to pay that additional cost. The CO2 in the atmosphere is already at 404 parts per million (ppm), but that amount needs to reduced down to 350 ppm or possibly even 300 ppm in order to ensure a stable climate. CCS is designed to capture CO2 when it is concentrated at the smokestack, but there are very few technologies in the offing that can suck unconcentrated CO2 out of the atmosphere. Growing plants to concentrate the carbon and then capturing that carbon while burning those plants is one of the few viable ways of removing carbon from the atmosphere.

However, biochar will probably be a cheaper method of CCS than turning CO2 to stone, since it doesn’t require capturing the CO2, drilling injection wells, pumping large amounts of water, dissolving CO2 in water and injecting it underground. Like turning CO2 into stone, biochar also locks the carbon in a stable state that can be stored for thousands of years, but it has the added benefit of fertilizing the soil and it is compatible with the localized, organic agriculture of the future. Biochar can be created in the agricultural field, so it doesn’t require that the biomass be transported to a distant thermoelectric plant with CCS.

Turning CO2 into stone might make sense in industrial plants which produce large amounts of CO2, but those types of plants will become fewer and fewer as the world switches to a low carbon economy. For example, cement manufacturing emits roughly 900 kg of CO2 per tonne of cement, and the industry as a whole emits between 5% and 7% of total anthropogenic greenhouse gases. Roughly 50% of those emissions come from the calcination of limestone, when calcium carbonate (CaCO3) is turned to calcium oxide (CaO) which in turn releases CO2. Another 40% comes from burning fuel to heat the limestone up to 1450 C in order to create cement. The final 10% comes from transportation and the electricity used in the plant. Before cement plants decide to invest in CCS, it is far more likely that they will invest in greater energy efficiency and in using a higher percentage of pozzolans (fly ash, metal slag, ash from rice, nut and palm oil husks, calcined clays, etc.) in the cement mixtures, which saves them money. A study of 3 cement plants in West Germany found that they had been able to reduce their emissions down to 385 kg of CO2-equivalent per tonne of cement, and anticipated that they would be able to reduce them further to 227 kg per tonne. It is far more likely that cement plants will fall the path of these West German plants, rather than invest in CCS, which just adds to their costs with little benefit to them. In the distant future, when the climate crisis comes full bore, I suspect that new processes will be adopted by the cement and steel industries which emit zero CO2, rather than turn to CCS which can only capture 80% to 90% of the CO2.

The press is hailing the idea of turning CO2 to stone, but it is unlikely to do much to solve our climate crisis. This new technique might find use in some limited applications, but it is unlikely to be widely deployed. Proponents of fossil fuels are likely to use it as one more talking point to delay the needed transition to cleaner alternative energies. CCS with fossil fuels is still dirtier than wind and solar energy, and making CCS cheaper and less risky still won’t make fossil fuels viable on either economic or environmental grounds in the long run. Like many of the technological fixes which have been offered up as solutions to global warming, turning CO2 into stone is a way to provide us with the false hope that present-day practices can continue with just a few minor changes around the edges. The sooner we come to accept the uncomfortable truth that 80% of fossil fuel reserves need to be left in the ground, the sooner we can start the real work of tackling climate change.


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