Six ideas for CO2 reuse: a pollutant or a resource?

The world will have to start removing 12 billion tonnes of carbon dioxide from the Earth’s atmosphere every year by 2050 if it wants to limit global warming to the relatively safe rise of 1.5°C compared to pre-industrial times, according to the Intergovernmental Panel on Climate Change.

In 2017, CO2 emissions generated by the global energy sector reached 32.5 billion tonnes, according to the International Energy Agency. The amount of CO2 needing to be captured is almost one-third of current annual energy-related emissions globally.

Several companies have already demonstrated carbon capture either from industrial facilities and power plants or directly from the atmosphere. The technology, however, is costly and adopters are few – nowhere near the scale needed to meet the target.

Environmentally conscious innovators are therefore developing solutions that would turn the climate-warming pollutant into a marketable raw material, which would create a business case for costly climate-protecting technology. Production of CO2-based bioplastics, fuel and even rock and carbon fibre have been tested, with proponents of such technologies hoping to create novel carbon sinks from objects of daily use.

Here is our selection of six interesting things that engineers and scientists have done with carbon dioxide.

1. Turn it into rock

Terminal One of San Francisco International Airport might look like any other international airport terminal – but it actually is quite special: it has been partially built with concrete incorporating CO2 emissions. This concrete uses aggregate coated with artificial limestone developed by California-based Blue Planet. The company’s founder and CEO Brent Constantz spoke to E&T about the firm’s ambitious plan to turn the world’s construction projects into a massive carbon sink. Everything from roads and bridges to office buildings and regular homes could be made of rock that sequesters carbon emissions, according to Constantz.

“There is about 55 gigatonnes of rock mined, transported and used every year,” says Constantz. “The cost of rock depends mostly on the distance. If you get it at the quarry, it might be something like $10 per tonne. Here in California, we import most of our rock aggregate from British Columbia and then the cost goes up to $45 per tonne.”

Blue Planet wants to manufacture its carbon-sequestering rock where large CO2 emitters, such as coal- or gas-fired power plants, are located. And that is usually near major population centres, where most of the construction work takes place. Constantz believes that such a set-up would make the artificial green rock competitive.

Over the past four years, the company has been operating a pilot plant in California and hopes to benefit from the 2017 California Buy Clean Act, which requires state authorities to only award state infrastructure contracts to firms using low-carbon materials.

The process that Blue Planet uses takes flue gas from a natural-gas-fired power plant and turns it into calcium carbonate, essentially artificial limestone, a type of mineral with high-carbon content typical for natural sedimentary rock.

A piece of rock substrate is then coated with this artificial limestone, which permanently sequesters the CO2. Blue Planet can make rock aggregate sizes ranging from a grain of sand to gravel. Out of the 55 gigatonnes of rock aggregate used in concrete every year around the world, 70 per cent is limestone, says Constantz. Replacing this natural-mined limestone with one that isolates carbon dioxide emissions presents a considerable opportunity to help in the battle against climate change, Blue Planet believes.

“In one tonne of limestone that we make, we have 440kg of CO2 that is permanently converted into carbonate and mineralised,” says Constantz. “The International Energy Agency says that mineralisation of carbon is the most stable form of carbon sequestration available. Rocks persist for millions of years.”

The only way to get the CO2 out of the rock would be to heat it up to over 700°C or to dissolve it in a strong acid, Constantz adds.

Carbon sequestration in limestone is a process that spontaneously happens in nature. The UK’s famous White Cliffs of Dover are one example of nature’s ability to permanently store carbon. In the lab, however, the process can be greatly sped up.

Blue Planet’s process uses carbon dioxide exactly as it comes out of the power plant without having to purify it. That, according to Constantz, is a major advantage compared to other carbon-use technologies. “The purification step is an energy and capital-intensive process,” says Constantz. “We take the raw flue gas and convert it directly to carbonate. We don’t have the energy penalty.”

Constantz says the cost of Blue Planet’s limestone could be less than the cost of naturally mined limestone if it can be produced in local carbon dioxide-emitting industrial plants.

2. Make cement out of it  

Blue Planet is not Constantz’s first carbon sequestration venture. In 2007 he launched a company called Calera, which operates a cement-making factory next to a large power plant on the US west coast.

Calera produced two tonnes of CO2-containing calcium carbonate per day using raw flue gas from the plant. The flue gas first reacts with an alkaline solution, releasing the CO2; when combined with calcium, the CO2 is trapped in the form of calcium carbonate, which is subsequently dried to make cement.

Conventional manufacturing of cement is, on the other hand, a major source of carbon emissions. The global cement industry is responsible for about 6 per cent of global carbon emissions, according to the Carbon Disclosure Project, a UK-based charity that encourages companies and cities to disclose the environmental impact of major corporations. The industry has achieved only minor emission reductions.

Around 4.1 gigatonnes of cement were produced around the world in 2017, most of it deriving from China and India, according to the International Energy Agency.

Other companies have investigated green cement production with varying levels of success. London-based Novacem, a spin-off from Imperial College London, failed to find investors for its green cement technology that replaces conventional binding material in concrete with magnesium oxide, which subsequently captures carbon dioxide when mixed with water.

3. Feed it to algae to make carbon fibre  

Carbon fibre is a wonder material hailed for its superior strength and light weight. But it has a dirty little secret. The current carbon fibre production methods are extremely energy demanding and thus polluting. According to some estimates, production of carbon fibres is in an order of magnitude more energy-intensive than the production of steel. Moreover, the material is made from petroleum: a fossil resource the world is trying to wean itself off.

A team of scientists from the Technical University of Munich, Germany, unveiled their research into using algae to make precursors for carbon fibre manufacturing. “We have been researching algae-based biofuels for the past seven years and have made some really important strides in that respect,” says Professor Thomas Brück, the leader of the research group at the TUM AlgaeTec Cultivation Centre.

“We have always had glycerol as a side product that we would pitch to the cosmetics or other chemical industries. Now we have identified a process chain, which is already available at existing chemical industry parks, that enables glycerol conversion to carbon fibres via acrylonitrile.”

Acrylonitrile, Brück explains, is a precursor of polymer polyacrylonitrile, the basic component for the manufacture of all commercial carbon fibres.

“If you pyrolise (thermally decompose) polyacrylonitrile fibres, you end up with commercial carbon fibres,” says Brück, adding that the algae-based carbon fibres would be as strong and lightweight as the current commercially available carbon fibres but would be at least 10 times cheaper.

“Carbon fibres are conventionally produced at 3,000°C,” says Brück. “You need a lot of energy. We don’t have this problem.”

The researchers envisage they could grow the glycerol-producing algae in ponds near the Mediterranean coast. The warm climate and easy access to seawater would reduce the cost of algae cultivation. The algae would be fed carbon dioxide captured at a power plant or industrial facility nearby, incorporating the carbon dioxide into their cells.

Brück hopes the carbon sequestering material could catch the eye of a wider client base, not just traditional carbon-fibre buyers in the automotive and aerospace sector. “If we get the carbon fibres to the building market, the technology would become climate-relevant,” says Brück. “The automotive sector is too small. The building market is something that we want to address in the long term.”

Brück believes the carbon fibres could, for example, replace a portion of concrete, the production of which generates a lot of CO2. At the promised lower cost, the carbon fibre products might also replace steel or aluminium beams in the built environment.

The Munich team has already assembled a supportive industrial consortium that aims to have the technology in the market by the mid-2020s.

4. Turn it into insulation foam for housing  

Econic Technologies, a chemistry start-up based in Alderley Park, near Manchester, UK, has developed a process that incorporates carbon dioxide from emissions into polyurethane foams and other plastic materials.

Polyurethane foam is used to make various products including mattresses, furniture upholstery and car seats. It is also used to make thermal insulation for houses, a market Econic Technologies hopes to enter. Experts consider a large-scale retrofit of the UK’s notoriously energy-inefficient homes to be the number one step for the country to slash its CO2 emissions in line with the October 2018 recommendations. Econic Technologies hopes insulation used in this retrofit could not only slash the emissions from heating but also trap emissions produced by other sources.

“We are using carbon dioxide from emissions as feedstock for the production of polymers, especially polyurethane,” Leigh Taylor, Econic Technologies’ head of sales and licensing told E&T. “By doing this we are also preventing emissions that would be created in the production of the conventional feedstock.”

Polyols, the building blocks of polyurethane, are conventionally made of propylene oxide, which is made from oil. The process generates carbon emissions.

Econic Technologies can replace up to 43 per cent of the propylene oxide required for the manufacture of polyols with carbon dioxide. Taylor says the final product is not only environmentally friendly but also cheaper since the price of carbon dioxide is two orders of magnitude lower than that of propylene oxide.

“Propylene oxide is roughly $2,000 per tonne of raw material as feedstock whereas carbon dioxide can be as low as $30 to $50 a tonne as feedstock,” says Taylor. “Even the most pessimistic views of the cost of carbon dioxide following capture are still substantially lower than the cost of the conventional propylene oxide raw material.”

When put through tests, Econic’s polyurethane performed as well as, and in some parameters even better than, conventional polyurethane.

“We are seeing distinct advantages in most application areas, such as increased resistance to chemicals, oil and water in things like coatings, as well as improved fire performance in things like insulating foams,” Taylor adds.

In March 2018, Econic opened its demonstration plant in Alderley Park with a 70-litre polymerisation reactor that produces samples of the company’s products.

Econic’s intention is not to manufacture the polymers themselves but to sell the innovative catalysts that allow the use of CO2 and license their technology and processes to large-scale chemical manufacturers.

Taylor believes that if 30 per cent of the industry adopted their technology, the green polymers would sequester or prevent about 3.5 million tonnes of CO2 emissions per year, which equates to taking about two million cars off the road.

5. Feed it to algae to revive oyster reefs

Scottish researchers are using CO2 produced during whisky fermentation to grow algae in special photobioreactors which could then be fed to baby oysters that marine scientists are nursing in an attempt to restore the population of native European oysters in the Dornoch Firth.

While the aim of this unique project, called DEEP (for The Dornoch Environmental Enhancement Project), is to revive the once rich ecosystem in the firth, it could, as a side-effect, isolate a bit of the climate-warming gas.

“Oysters sequester quite a bit of carbon in their shells in the form of calcium carbonate,” says Douglas McKenzie, founder and managing director at Xanthella, which is developing the technology for growing the algae. “We are talking about four million oysters. There is interest in doing it all up and down the North Sea coast and that could have a significant effect.”

The project is supported by the Glenmorangie distillery, which supplies not only the CO2 but also nutrients, essentially leftovers from the whisky-making that are processed through anaerobic digestion.

“We can feed it to the algae, which we can then feed to the oysters to get them to the point where they can then go to the firth,” says McKenzie.

Native oysters were plentiful in Scottish waters up until the 19th century, when the population became extinct as a result of overfishing and damge to the ecosystem. In 2017, 300 specimens from the UK’s last significant wild population in Loch Ryan were placed into the Dornoch Firth to try and revive the oyster reef.

Xantella, as part of the EU-funded Algal Solutions For Local Energy Economy project, tests whether algae could be grown in photobioreactors using surplus renewable electricity. In addition to oysters, they envision feeding the algae to locally farmed salmon.

6. Turn it into fuel   

The idea to turn carbon dioxide emissions into synthetic natural gas that could be used as a fossil-fuel replacement has been around for quite a while. However, most known processes that could do the job are rather energy-intensive.

In late 2017, researchers from the US Department of Energy’s Idaho National Laboratory published a paper in the journal Green Chemistry describing a process that offers a much more energy-efficient way.

“We demonstrated, as far as we know, for the first time that we can directly reduce captured CO2 into carbon monoxide and hydrogen using specialised liquid materials that make the CO2 more soluble and allow the carbon-capture medium to be directly introduced into a cell for electrochemical conversion to synthetic natural gas,” says Tedd Lister, the leader of the team behind the new process.

Previously developed methods require an intermediate step to dissolve the carbon dioxide in water – an energy-intensive reaction taking place at high temperatures.

“In our process, it’s all happening at the same temperature, so you have a potential saving in terms of energy,” says project lead Luis Diaz Aldana.

The team has recently obtained new funding to continue with the project and take the technology from concept to something that could be demonstrated to the industry.

“We want to reduce the energy consumption, the cost, make it competitive,” says Diaz.