IVAN AMATO, NATURE, 2013
THE chemical means of capturing and storing CO2 are many and spectacularly varied. Direct air capture involves exposing ‘sorbents’ to the air, so that they absorb the CO2. The gas can then be compressed and stored in deep rock strata. Swiss company Climeworks has developed a mobile CO2 capture device that can remove one tonne per year of CO2 directly from the air around it using a new kind of recyclable chemical filter system. When the filter reaches capacity, the CO2 is driven off by heating the filter to 95°C. The CO2 stream released is very pure, making it a commodity with some value. The filter can be re-used many times, and the fact that about 90 per cent of the energy needed can be supplied by low-temperature heat makes the technology potentially relatively low cost.
The Canadian company Carbon Engineering has a broadly similar approach; it intends to use captured CO2 to make low-carbon fuels. Their experimental unit operated for about six months in 2012, and captured two tonnes of CO2. Carbon Engineering is yet to construct a complete pilot plant, but anticipates doing so by 2017.
Scale is a formidable problem for these types of technologies. About 10 million shipping-container sized units, using 4–5 exajoules (a quintillion joules) per year of electricity, would be required to sequester one gigatonne of CO2. By comparison, in 2008 humanity used 474 exajoules of energy, so this investment represents about one 150th of total human energy use.1 No commercial demonstration plant yet exists, and cost estimates are hugely varied, from US$20–$1000 per tonne of CO2 captured.
Another option is chemical capture of CO2 from the air by exposing silicate rocks to weathering. The proposal involves accelerating the weathering process that occurs in nature by breaking large rocks into smaller pieces to increase their surface area and thus rate of weathering, and exposing them in conditions where they will weather quickly. Olivine is a beautiful, glassy dark green mineral from deep in the Earth’s crust, which is transformed by naturally occurring chemical reactions into a variety of common rock types, including serpentinite. Grind olivine or serpentinite into a sand, and lay it on a beach, and it can continue to absorb CO2 for years. But between 3.6 and 5.1 gigatonnes of rock is required to sequester one gigatonne of atmospheric carbon. That’s a lot of rock to mine, crush and transport. The estimated cost is US$24–$123 per tonne of CO2 removed.
Derbigum, a roofing company with factories in Europe and the US, offers an interesting twist on this approach. It has developed a roofing product with a layer of olivine that reacts with rainwater to remove and permanently store atmospheric CO2. Dutch company Greensand offers yet another solution: an olivine-based, carbon-negative lime-replacement product (for soil remediation) for use in home gardens.
Many seemingly far-fetched applications have been suggested for olivine. They include constructing olivine hills as monuments or public recreation areas, using olivine to make artificial reefs for tourism and fishing purposes, as an additive in fire-fighting, to both improve fire-suppression and capture carbon, through to using olivine sand as a replacement for conventional sand in beach replenishment. Olivine grains might also be used to enhance the growth of diatoms in biofuel production or for constructing building ventilation systems that will also control CO2 levels during the day. Olivine-rich soils might be used to grow plants which hyper-accumulate nickel, or for producing magnesium carbonate spring waters similar to those that naturally occur in springs across Europe near olivine deposits. It’s even been proposed that olivine-based carbon-capture devices be installed on ships. Located in the exhaust of the ship’s engines, they would capture the CO2 emitted and turn it into a carbonate that, if released into the ocean, could lead to the sequestration of additional amounts of CO2 from seawater.
Olivine is not the only rock that can be used to capture CO2. Lime produced from carbonate rocks such as limestone can also do the job. Lime production, however, requires considerable heat, and between five and six gigatonnes of rock would be needed to sequester a gigatonne of atmospheric carbon, at an estimated cost of US$79–$159 per tonne. Because of the high costs involved, and the fact that no useful material is created in the process, the widespread use of olivine and other rocks for sequestering CO2 will require incentives, as well as much research and development aimed at lowering costs.2
Carbon negative cements represent another chemical pathway with huge potential. The production of Portland cement, which dominates the cement industry today, generates one tonne of CO2 for every tonne of cement produced. As a result, cement manufacturing contributes about 5 per cent of our current greenhouse gas emissions. Researchers are trying to understand the maddeningly complex chemistry of cement-making in order to find a way to reduce greenhouse gas emissions. One option being investigated is the use of a lower roasting temperature during production, which would involve burning less fuel. Another is to incorporate fly ash, a by-product from burning coal for electricity, in the cement-making process, which one company claims makes its cement carbon neutral.3
But it turns out that there are ways of making a cement that actually absorb and sequester carbon over long periods. These include a cement-curing process being developed by Solidia Technologies in the US that takes CO2 from industrial waste and incorporates it into the cement. Solidia claims that its cement can be used to produce concrete that is stronger, more durable and more flexible, and that it costs less than conventional cement.4 It uses the same raw materials and equipment, but less water, energy and time. Their product is now in the commercialisation stage.
About four billion tonnes of cement are produced worldwide each year5 at a market value of US$300 billion, and cement and concrete products are worth about $1.3 trillion annually. There is certainly the potential for such products to make a difference to the atmospheric carbon budget at a gigatonne or near-gigatonne scale. But to sequester a gigatonne of carbon using this technology, 80 per cent of the world’s cement production would need to incorporate carbon-negative processes. One huge impediment to the uptake of these new kinds of cement is the risk-averse nature of the industry. Nobody wants their building or bridge to fail, and until the novel products have stood the test of time, and a global price on carbon emissions is introduced, it’s hard to see them being adopted at scale.
Carbon-negative plastics offer a potential solution for storing CO2 captured from the air. One of the leaders in the field is Newlight Technologies in California. As a result of 10 years of research, Newlight has invented and commercialised a carbon-capture technology that combines air with methane-based greenhouse gas emissions to produce a material they have called AirCarbon. It is a carbon-negative material that can be used in place of oil to produce a range of plastics that can compete with oil-based plastics on both performance and price.6 From chairs to automotive parts to thin films, the material has already been trialled in a number of applications and proven to be equal to or superior to plastics sourced from fossil fuels. Newlight intends to lead its commercialisation efforts with the manufacture of office furniture. But the world’s annual plastic use would need to quintuple, with all of it coming from carbon-negative technologies, in order to sequester a gigatonne of carbon per year.
Newlight is one of a number of companies pioneering systems that allow us to do some of the things that some of the most primitive living things do naturally. The development pathway began in 1953, when American chemists Stanley Miller and Harold Urey stood at a laboratory bench before a flask filled with water vapour, methane, ammonia and hydrogen gas. The scientists hypothesised that the mix was similar to Earth’s atmosphere before the origin of life. Two electrodes sparked continuously through the flask, to stimulate the lightning storms that were thought to have occurred early in Earth’s history. Within a day, the gases had turned pink, and within a week 15 per cent of the carbon in the mixture had been incorporated into the organic compounds that are the building blocks of life.
The experiment sparked a global media sensation. Had the scientists, in a god-like act, created life in a flask—or at least its precursors? While providing a major breakthrough in our understandings of how life might have begun, the experiment also pointed towards another development that would gather pace more than half a century later. Using water enriched with CO2, instead of a mix of gases, through which to pass the current, researchers have created long-chain hydrocarbons (the basis of fossil fuels). We are on the very frontiers of science here. Could the creation of oil and other hydrocarbons from CO2, water and electricity one day become a major route on the third way?
A German company called Sunfire has recently announced that it has discovered a way of creating petrol and other fuels from water and CO2. The climate benefit of creating petrol this way lies in its potential to replace fossil fuels. The process involves producing steam, and then treating it to remove the oxygen from the H2O. Combining the remaining hydrogen with CO2 leads to the creation of long-chain hydrocarbons. While experimental, Sunfire’s work clearly has potential.
Yet another approach is being trialled in Germany. Some years ago the giant engineering company Siemens began working with German universities in an effort to replicate photosynthesis. Were this to succeed, it would bring enormous benefits through the production of many usable materials created from atmospheric CO2 and water. But photosynthesis is such a complex process that replicating it in the lab was found to be impossible. So, as a first stage, instead of using light energy to split water molecules (as plants do in photosynthesis), the team set about transforming CO2 into complex hydrocarbons using electricity. The chemistry is complex and as yet poorly understood, but the basic approach involves providing CO2 with energy. Just what will be produced in commercial quantities using this technology is currently uncertain, but it may be ethylene or various alcohols. A large-scale demonstration facility is due to open in 2015.
Professor Maximillian Fleischer of Siemens says of the project:
On windy and sunny days, Germany already has more electricity generated from renewable sources than it needs. What it lacks is sufficient energy storage capacity . . . However, if the electricity were fed into photosynthesis modules, it could be used to produce valuable chemicals. This would help to reduce demand for petroleum and thus cut greenhouse gas emissions. What’s more, human beings will have incidentally managed to imitate the most productive chemical process on Earth. The dream of operating biochemical factories efficiently with sunlight could become a reality.7
Currently, all of the chemical technologies outlined require the use of energy—in most cases electricity—and while energy is generated by burning fossil fuels, unless the processes are highly efficient, it’s hardly worthwhile using ‘dirty electricity’ to capture atmospheric carbon. But as Fleischer notes, the wavy baseload provided by wind and solar, with its clean, zero fuel-cost electricity generation, will at some point change all that. The waves in the baseload are caused by the varying generation from wind and solar. But the point is that electricity is always available, and at times is excess to demand. Once wavy baseload comes to dominate Germany’s electricity network, using the excess electricity often available to capture and sequester CO2 will make complete sense.
Who would pay for the costs of capturing carbon? For almost all approaches that deal with CO2 as waste, the costs will be in the many billions, if not trillions, per year. Under current thinking a carbon price could be used to fund an industry in CO2 capture and storage, or governments could pay for the service directly out of general tax revenue. The costs are prohibitively high at present. But as the technologies mature, the cost per tonne of CO2 captured using third-way technologies provides a guideline for the capital that must be raised via taxation and the price of carbon in trading schemes. Additionally, the cost per tonne of CO2 removed should act as a guide to the price polluters should pay to emit in the first place. Just as the size of a fine given, for example, to the polluter of a lake, should reflect the clean-up cost.
Funding opportunities for environmental remediation (and we can think of third-way technologies as such) are rapidly diversifying. Perhaps, in years to come some measures will be crowd-funded. Other technologies, such as turning CO2 into plastics and fuels, may well become the foundation stones of profitable industries, albeit industries that are decades away from profitability at the scale required to draw down gigatonnes of CO2.
Given their present state, how should we be thinking about third-way technologies in the short term? A recent paper in the journal Science argues that we should regard third-way technologies as a valuable series of additional measures that complement our efforts to cut emissions. They should not, however, be used as an excuse for failing to cut emissions from burning fossil fuels. Many challenges remain: the uncertainty regarding costs, side effects and the effectiveness of carbon storage measures, not to mention the need for meaningful accounting. But delaying adoption of these approaches into the political process means that early opportunities may be missed, and research and development may slow, risking the possibility that the technologies will not reach their full potential.8
Where might the third way take us by 2050? If we put aside seaweed farming, with its stupendous potential but great difficulties in realisation, the following optimistic scenario is within the bounds of possibility. Forestry and soil carbon might together sequester a gigatonne of carbon per year, and biochar a similar amount. Direct air capture and silicate rocks might capture another gigatonne between them, and carbon-negative cement and carbon-negative plastics another gigatonne. That’s four gigatonnes of carbon per year, or around 15 gigatonnes of CO2—just one quarter of current global emissions and still below the 18 gigatonnes that the combined US academies found we’d need to draw down to reduce atmospheric CO2 by one part per million per year.9
The third way is unlikely to be a cure-all for runaway greenhouse gas emissions. Nor can it ever be considered an excuse for failing to reduce emissions as fast as possible. But, long term, it has the potential to hold back a warming climate to within a range in which human civilisation can thrive. And as such it’s a valuable asset that we should be seeking to develop and make the most of right now.