Ongoing pressure to decarbonise the world’s economies has prompted process developers to take another look at using CO2. Vast quantities of CO2 produced from natural gas reforming are already used to make 150m t/year of urea, plus a few other lower-volume chemicals. The focus now, however, is to find ways of consuming CO2 found in the flue gas of a typical factory smokestack, or even in the ambient atmosphere.
So far, a handful of processes are profitable, and these only under special conditions.
Carbon Recycling International, for example, has been running its methanol manufacturing plant in Svartsengi, Iceland, on a commercial scale since 2011. Carbon dioxide, from the exhaust of an adjacent power plant, is reacted with hydrogen from an electrolyser that splits water into H2 and O2, to make methanol. The plant pumped out 5m litres of methanol in 2014, consuming 5.5kt of CO2. Its special conditions are Iceland’s dirt-cheap power prices (for the electrolyser) and, in Svartsengi, a very-low-cost source of CO2.
A project proposed near the Dutch city of Rotterdam, meanwhile, will feed CO2 from a waste incinerator into the atmosphere in greenhouses, where it will be harnessed to promote plant growth. The special factors here, explains technology supplier Carbon Oro, are two-fold. One, because the Netherlands suffers from an overcapacity of incineration, the incinerator’s operator is willing to invest, to offer an extra incentive for waste-generators to use his services. Two, a pipeline network for CO2 already exists among greenhouses in the region. All that need be added is an injector-station for the incinerator’s output.
Other process developers are trying to follow the path to profits. Most of them are trying three main steps: collecting CO2, creating hydrogen and then combining these to form a hydrocarbon that can be used either as a fuel, or as a feedstock for more complex chemicals.
Most developers prefer to start with a concentrated, industrial source of carbon. As Michael North, professor of green chemistry at the University of York points out, these are relatively plentiful (see table).
To remove the CO2, the typical approach is to wash the flue gas with a liquid amine absorbent which, when subjected to modest heating, desorbs the carbon back into its gaseous form. Although design vendors tinker continually with the precise solvent mixture and conditions, the process is well-known and proven.
As yet unproven is the CAIR process promoted by Dutch company Antecy, which extracts CO2 directly from air. Details on the actual workings are sketchy, but R&D manager Timo Roestenberg contends the process can make CO2 at about $80/t. If that can earn a tax credit of $40/t, he adds, the door is open to making cost-competitive methanol. The process-engineering arm of oil giant Shell Global Solutions has completed a design study with Antecy, which is now looking to find funding for a pilot plant.
Finding the hydrogen
The biggest economic hurdle, say CO2 recyclers, is finding a cost-competitive source of ‘clean’ hydrogen. The ‘dirty’ kind, largely made from steam reforming of natural gas, is not feasible because it defeats the purpose of CO2 recycling – it creates more greenhouse gas than it saves.
One possibility is to use conventional (alkaline) electrolysers, but hooked up to low-carbon energy, such as wind or solar powered electricity. In most parts of the world, this is also economically not feasible. Along with Iceland, another possible exception is South Africa. Because of the country’s sunniness – it receives twice the available radiation per m2 than in northern Europe – and windiness, a new solar photovoltaic can produce power at €0.059/kWh, while a new windfarm can produce at €0.046/kWh, reports Tobias Bischof-Niemz of South Africa’s Council for Scientific and Industrial Research (CSIR).
Clearly, Bischof-Niemz has found believers among his CSIR colleagues. His plan, currently under consideration, is to transfer CSIR’s main campus near Pretoria from coal-fired to wind/sun power. If that works out, then the aim would be to capture CO2 from an industrial source, electrolyse hydrogen and recombine the two into fuels, using nearby Fischer-Tropsch technology mastered by South African chemicals major Sasol.
A similar approach by groups in northern Europe instead focuses on excess electricity. This has always been available at night, but also now increasingly during sunny, windy days because of the spare wind and solar capacity. The power-to-gas or power-to-methane idea is to react the electrolysed hydrogen with captured CO2 to make methane, which can then be exported into the local gas grid. In Germany, a development group is being led by ZSW, the Centre for Sun Energy and Hydrogen Research. A similar group in Switzerland is led by Etogas, a process designer, and in Denmark, by a company called Electrochaea.
Other innovators are targeting improved electrolysers. One approach is to use polymer electrolyte membranes (PEMS), which so far have received little attention from CO2 recyclers. More popular is the use of solid oxide electrolysers, which use a ceramic as the electrolyte that selectively conducts negatively charged oxygen ions at elevated temperatures. Water at the cathode combines with electrons from the external circuit to form hydrogen gas and negatively charged oxygen ions. The oxygen ions pass through the solid ceramic membrane and react at the anode to form oxygen gas and generate electrons for the external circuit.
Solid oxides are being tried by:
- Israel-based NewCO2Fuels, which is aiming to tap industrial off-gases for its carbon source. The end-product, says ceo David Banitt, will likely be methanol.
- German-headquartered Bauhaus Luftfahrt, which hopes to make syngas and then jet fuel, dubbed ‘solar kerosene’.
- Sunfire, another German firm, which aims to use the solid oxide cell reversibly, ie as an electrolyser when cheap electricity is available, or as a fuel-cell when power is more expensive. Earlier in 2015, the company made a big media splash when Germany’s federal research minister, Johanna Wanka, filled the tank of her private car with methanol made at Sunfire’s Dresden pilot plant.
A research group at the California Institute of Technology, US, is trying another solid method. Nathan Lewis’s research group has developed a mat that can be rolled out like a fitted carpet and exposed to water and sunlight to generate hydrogen at the bottom and oxygen bubbles on top. The mat, made of silicon nanowires grown by chemical vapour deposition, can so far be produced with an area of only about 1cm2. But, if it can be scaled up, Lewis says, hydrogen might be generated at prices not far off those of existing commercial sources. ‘We followed the science where it led us,’ says Lewis, ‘and we think this could be a real breakthrough technology.’
Products and processes
Technologies for combining carbon and the hydrogen into a product are fairly conventional – usually a variation on the Fischer-Tropsch process. Products are mainly centred on methane, methanol and ethanol, although a few developers are aiming for longer-chain hydrocarbons. Fuels are the main target, because they have the best prospects of recognising carbon subsidies and credits.
Gaining those government hand-outs is still tricky, admits Andreas Pilzecker, who until recently dealt with the issue in the European Commission’s DG-Energy and now addresses it from DG-Climate. One hurdle is qualification: most subsidy-qualified fuels are biofuels, and those that use CO2 from the air and hydrogen from water are not ‘bio’ as such. Also, the Commission has yet to establish default carbon footprints for fuels made from recycled CO2. ‘This,’ adds the Commission official, ‘is something that companies should be lobbying the Commission about.’
As York’s North points out, many chemicals can be made from CO2. He thinks an attractive class would be ethylene and propylene carbonates, which are in big demand as electrolytes in popular lithium batteries.
So far the most-celebrated chemical application has been pioneered by Covestro, formerly Bayer MaterialScience. In 2016, the company will start commercial production of polyurethane foams that contains some recycled CO2 from a German power plant. Unclear at this point, however, is whether the product is actually profitable or just a green-marketing tool. In any case, project leader Christoph Guertler declares that for CO2 recycling, the foam ‘is just a start. There are lots of other products that could be made via similar process. What remains to be seen is if any of them will be commercially viable’.