Chemical potential: turning carbon dioxide into fuel

A number of engineering companies are on a mission to make CO2 fuel production not just a physical reality but a commercial one too. Stephen Harris reports

Our entire way of life is built on a fundamental problem. Burning fossil fuels provides abundant cheap energy but produces a chemical widely believed to be dangerously altering the planet’s climate. But what if that chemical could be recycled to produce an endless supply of fuel without altering the delicate balance of the atmosphere? That’s the tantalising concept behind a slow but dramatic shift in thinking taking place among some scientists and engineers. Instead of viewing carbon dioxide (CO2) just as a cause of climate change, they see it as part of the solution: not something dangerous that needs to be hidden away underground but a useful material that could help reduce our dependence on fossil fuels.

Support for carbon capture and utilisation (CCU), rather than carbon capture storage (CCS), has been growing for several years, spurred on by research grants from, among others, the US departments of defense and energy and by companies who are successfully turning CO2 into plastics, building materials and now fuels on an industrial scale. None have yet proven they can profitably produce a sustainable, synthetic alternative to fossil fuels, but there are several persuasive reasons why they’re driving ever closer towards that goal. And thanks to some clever engineering, some may be on the verge of a breakthrough.

The most obvious reason for pursuing CCU is that turning CO2 into a valuable product could help make the process of capturing it more profitable and potentially even viable without government subsidies. It also reduces the need for locations and infrastructure to deposit the gas underground at the risk of it leaking out due to natural or induced seismic activity. But part of viewing CO2 as useful means understanding that the carbon that forms it is a vital part of our economy and, indeed, the natural world.

‘It is the molecule of life — very central to all the chemistry we do,’ said Prof Will Zimmerman of Sheffield University, whose work on improving algae production and anaerobic digestion has led him to a new CCU research project about to begin across several UK institutions (see below: Using carbon dioxide to grow algae for biofuels). ‘It doesn’t make sense to stop at carbon capture technology and sequester it. That may be a short-term win if you’re trying to fight global warming, but it’s a long-term loss when it comes to making economic use of that carbon.’

Another way to view CCU is not just as something that can directly reduce fossil-fuel emissions but also as a method of promoting renewable generation, namely as an energy storage medium. Turning CO2 into fuel requires energy, so hooking up production to wind or solar farms creates a way to save excess electricity from these intermittent sources in a form that can easily be used with our existing liquid fuel infrastructure.

This is the view of Rich Masel, a retired professor from Illinois University, whose company, Dioxide Materials, is using renewably driven electrolysers to convert CO2 into vehicle fuel. He argues that CO2-derived fuel also offers a more feasible alternative to producing biofuels from crops, pointing to a recent US Congress-commissioned report that found the current US biofuel programme may be ineffective at reducing global greenhouse-gas emissions. ‘The problem with biodiesel is you can never make enough of it,’ he said. ‘There’s not enough arable land to make enough biodiesel to make a really significant difference to CO2.’

Researchers at Dioxide Materials are using ionic liquids to break down CO2

The positive news about making fuel from CO2 is the variety of ways in which it can be done, although these fall into two broad categories. The first is by following nature’s own method for converting CO2, photosynthesis, either by processing biomass, algae or other micro-organisms or by artificially recreating the process using ultraviolet light to drive the reaction of CO2 with hydrogen from water. For example, Princeton University spin-out company Liquid Light says it can produce more than 20 different fuel products from CO2 using this technique.

But while it’s easy to think of photosynthesis as an elegant solution — and artificial leaves make for eye-catching headlines — using forms of energy other than light to drive the reaction can free scientists from some of the constraints of the natural process, not least its relatively low energy efficiency. The basic idea is to use heat or electricity and a catalyst to split the CO2 molecules, producing carbon monoxide that can be combined with hydrogen to create syngas. This can then be turned into liquid fuel via Fischer-Tropsch synthesis or another process.

Some groups have developed impressively novel ways of tackling this problem. For example, researchers at Sandia National Laboratories in New Mexico have built a giant solar concentrator that uses the sun’s heat to break down CO2 and water for syngas production, and a team at ETH Zurich university in Switzerland is pursuing a similar idea. But perhaps more striking is the number of companies that are using established techniques alongside cutting-edge research to make CO2-derived fuels not just possible but potentially commercially viable as well.

Air Fuel Synthesis (AFS) in the UK is one such company harnessing expertise from the petrochemical industry, using a technique developed by Mobil to turn gas into methanol. ‘It’s based on tried-and-tested catalytic processes that we know,’ said managing director Peter Harrison. ‘As it happens, that catalytic process actually converts carbon monoxide into CO2 before it converts that to methanol. So we said forget about the carbon monoxide.’ The company has set up a pilot plant in a 30ft (9m) container in Teeside that can produce more than 1,800 litres of renewable fuels a year and is now seeking funding to take production to a commercial level.

Dioxide Materials, meanwhile, is using the recent development of catalytic ionic liquids to help break down CO2 inside an electrolyser, a novel technique that allows the direct use of renewable energy but that was inspired by the chloralkali process used to make chlorine and sodium hydroxide from salt water. ‘The advantage of this is that the expertise and the infrastructure is in place for the large-scale electrolysers,’ said Dioxide Materials’ business development specialist Megan Atchley.

Dioxide Material’s three-electrode cell for lab-scale CO2 conversion

But possibly the most developed process so far has been created by Iceland’s Carbon Recycling International (CRI), which has moved from a pilot plant to a demonstration facility with a capacity of five million litres of methanol a year — which the company says could be blended into petrol to reduce Iceland’s volume of fuel imports. It uses around 5MW of power and about 4,500 tonnes of CO2 annually from a nearby geothermal power plant.

Once again, the advantage of this electro-thermo-chemical catalytic method over alternatives such as algae-based fuels was that it provided the fastest way to a commercial-scale operation and allowed the company to draw more on previous research and experience. ‘We needed to take into consideration the timeline and capital requirements for putting the project together,’ said chief executive KC Tran. ‘The design was about a way to put the system up rapidly but also cost effectively. Our plant is built by module, which means we built all the different components in different places around the world according the specification of the design and brought them together here in Iceland for rapid integration.’

The engineering challenge for all the groups that hope to make CO2 fuel production a commercial reality is scaling up the process while making it cost-effective. CRI’s modular solution has allowed the company to build each stage of the process individually: CO2 capture and purification, hydrogen production, syngas production and compression, methanol synthesis and finally distillation. But it also reflected the importance of a whole-system approach, where every stage is optimised for this specific production method.

Even for the most novel part of the system — the production and reaction of the syngas — the team was able to call on expertise from engineering partners. ‘We need uniform heat management and to design the reactor so there are no hotspots in the system,’ said Tran. ‘We need a catalyst that can tolerate some level of impurity and that can be deployed so that it can be exposed evenly throughout the reactor.

‘This was the design that we could not learn from the pilot scale. At the larger scale, we need to leverage what we’ve learned but also from people who know how to scale up systems. The choice we made on the design proved to be very good but we didn’t know that until we ran the longer cycles.’ By continuing to build on an established knowledge base and using this modular construction approach, the CRI team believes it can effectively upsize its plant to 10 times its current production levels — enough to turn a profit.

CRI’s demonstration plant can produce five million litres of CO2-derived fuel a year

There are other important considerations if CO2-derived fuels are to become commercially viable. Tran emphasised the need for flexible production so energy can be stored when electricity is cheapest. Another issue is what kind of fuel is produced and for what market: road vehicles, aviation fuel or something else.

‘We believe there is a market for a premium product for motorsport,’ said AFS’s Harrison, who hopes to capitalise on the sport’s move towards more sustainable fuel mixes. ‘A bioethanol blend is different to petrol; it has different characteristics and you can effect the energy performance. Having consistency of manufacture is key and when you’re using a variable product such as a bioethanol then that’s quite difficult but a synthetic fuel is pretty easy.’

Despite the appeal and rapid development of CCU, it’s very unlikely to remove the need for CCS in reducing carbon emissions in the short to medium term, given the scale of the challenge, the limited market for CO2-derived products and the capital costs involved (see box: more investment in CCU needed). But if a transition to a low-carbon economy can be managed, is there potential for CCU systems to one day pull CO2 directly from the atmosphere instead of relying on point sources such as power plants?

‘The process works almost down to atmospheric levels,’ said Dioxide Materials’ Masel. ‘The problem is that, if you take CO2 out of the air [surrounding the plant], how do we get the CO2 to come back again?’ Harrison of AFS suggested it might be possible using multiple capture units located over a vast area. ‘I would wonder what the dangers of sucking CO2 out of one area were,’ he said. Although many factors would have to be overcome, perhaps one day it might really be possible to make fuel out of thin air.

Using carbon dioxide to grow algae for biofuels

The challenge to make CO2-derived fuels economic is so great that every stage of the process needs to be optimised and perfected. The need for a holistic approach is especially apparent in algae biofuels, which appear to offer an alternative to biofuels that reduce arable land availability but which are also too expensive to be economic and can produce more CO2 through their production than they absorb.

Prof Will Zimmerman of Sheffield University points out that you can use the CO2 in biomass to grow algae for biofuels or for anaerobic digestion. ‘If you wanted to make that line more profitable, since the liquid biofuels aren’t profitable now perhaps the biomass to anaerobic digestion would be,’ he said. ‘If you can increase the profitability of the processing downstream then you could potentially run a biorefinery with a whole range of outputs, some of which are highly profitable and some of which may be at cost but are part of the processing.’

Zimmerman has developed a novel method of producing microbubbles with far less energy than conventional methods, which has the potential to create a much cheaper way of harvesting the algae for use in biofuels. But it could also enable the recycling of CO2 from anaerobic digesters to increase their methane production, which is the subject of a £5.7m CCU project led by Sheffield’s Prof Ray Allen and due to start later this year.

‘An engineer has to be an economist as well,’ said Zimmerman, who has launched spin-out company Perlemax to commercialise his technology. ‘If the overall process is not economic, then you haven’t solved the problem; you’ve just come up with nice technology that’s maybe bits of the solution.’

More investment in CCU needed

The UK should invest more money in CCU research in line with its competitors if it wants to benefit from the use and commercialisation of the technology, according to a policy document published last year by the Centre for Low Carbon Futures.

Carbon capture and utilisation in the green economy, authored by Peter Styring of Sheffield University and Daan Jansen of Dutch research institute ECN, found that while the UK government was investing £1bn in CCS demonstration, there were no plans for similar investment in CCU, unlike in Germany, the US and Australia.

However, CCU should be seen as a complementary technology to CCS and not as an alternative, the report said. It noted suggestions that the chemical industries could convert at most around 10 per cent of global CO2 emissions into synthetic fuels.

CCU was also limited by market demand for current products and high capital costs for plant construction, it said, but these issues could be addressed through research and development.