Readers of The Economist are a well-travelled lot. Many of them will be aware, perhaps slightly guiltily, that one of the biggest personal contributions to climate change is all that jet-setting. On average, each person on Earth going about their normal business produces the equivalent of five tonnes of CO2 a year. But a single transatlantic round trip produces the equivalent of about one tonne per passenger even in economy class.
For some, the problem with global warming is the idea that they may have to change their behaviour to fight it, not just by recycling or eating more seasonal food but by, heaven forbid, forgoing that holiday in Gstaad or the Maldives. Eventually zero-carbon technology may be able to avert some of those difficult sacrifices, by combining green-hydrogen production with a way of sucking CO2 out of the atmosphere to make synthetic fuels. This is currently very expensive, but it could help low-carbon aviation to take off.
On current trends, air-passenger numbers are expected to double within the next 20 years, mostly because of growth in Asia. That could push up today’s emissions of 1bn tonnes of CO2 a year to at least 1.7bn tonnes, mostly from long-haul flights. The International Air Transport Association, an industry group, has pledged to halve emissions by 2050. Airlines are buying more efficient aircraft to lower their emissions, some with more success than others (see chart). But it is not enough.
Batteries and hydrogen fuel cells are already finding their way into light aircraft for short trips, but they are too heavy or too bulky to propel a jetliner on a long flight. Instead, biofuels or synthetic fuels that combine clean hydrogen and CO2 could be used as “drop-in” fuels in existing engines.
In Finland a refiner called Neste has made biofuels from the waste products of slaughtered cows and pigs. It has dropped small quantities of them into the fuel systems of Boeing Dreamliners. But the availability of all biofuels, even those from energy crops, is constrained by the shortage of land to produce raw materials sustainably. At Finland’s Lappeenranta University of Technology researchers are looking at synthetic fuels as an alternative. Christian Breyer says that if electrolysis were used in places with abundant renewable resources, such as the Atacama Desert in Chile, hydrogen could be produced cleanly at low cost.
To turn it into aviation fuel, he suggests siting the electrolysers near plants extracting CO2 from the air—a process known as Direct Air Capture (dac). The gas would be converted into carbon monoxide and combined with hydrogen using the 100-year-old Fischer-Tropsch process that is used to make liquid fuels, all powered by renewable energy. The fuel could be refined into kerosene and other products, such as diesel for marine transportation and naphtha for use in the chemicals industry. When burned, there would be no net addition of carbon dioxide to the atmosphere. “It would work like nature,” says Dr Breyer.
Unfortunately dac is the most nascent of nascent technologies. Yet it is attracting the attention of influential promoters such as Bill Gates, founder of Microsoft. Initially it was conceived as a way to reduce the amount of CO2 in the atmosphere; if a captured CO2 molecule can be burned again to keep people flying, at least it does not add to the overall stock.
The firm backed by Mr Gates is Carbon Engineering, based in Canada, that has run a dac pilot project since 2015 that is capable of extracting one tonne of CO2 per day, and has produced synthetic fuels since 2017. Another firm is Climeworks of Switzerland. Estimates have suggested the dactechnology can cost up to $600 per ton of CO2 removed, but in a recent paper in Joule, an energy journal, Geoffrey Holmes of Carbon Engineering and others argue that costs can be below $100 per ton if done at scale.
Mr Holmes says the company has borrowed and modified tested technologies to ensure that it is not reinventing the wheel. The pilot plant sucks in lots of air using a modified version of cooling-tower technology, and draws it through corrugated sheets of plastic sprayed with a hydroxide solution. The CO2 absorbs into a liquid film to form a carbonate solution which goes through a pellet reactor, using chemistry common in water treatment, to form calcium carbonate pellets “like hailstones” that molecularly bind the CO{-2} for further processing. These pellets are heated to 900ºC in a high-temperature reactor to produce calcium oxide and CO2. The heating process can be fired by natural gas and both the atmospheric CO2 and that from combustion can be gathered and used.
The carbon thus captured does not have to be turned back into fuel; it could simply be buried. This is one of several ways of removing carbon dioxide out of the air for good. They include producing biomass such as forests, burning wood to generate electricity and capturing and sequestering the CO2. A report by America’s National Academy of Sciences says that even the cheapest negative-emissions technologies such as biomass with ccs are still too limited in scale to make a big dent in atmospheric CO2. A study by Britain’s Royal Society and Royal Academy of Engineering said a carbon price of $100 a tonne may be needed to make most negative-emissions projects feasible. The danger is that policymakers will delay curbing emissions now in the hope of being able to remove large amounts of greenhouse gases from the air in the future. In fact, both are needed on a massive scale.
Source: The Economist