A group of Spanish scientists at the University of Seville, Spain, has reinvented the recipe to turn rubbish into fuel for aeroplanes, lorries and ships. The team has filed a patent for catalysts that improve the process of making kerosene or diesel from urban waste, textile or agricultural residues, and sewage sludge.

The innovation will be particularly helpful for the aviation, maritime, and land freight transport sectors, which currently cannot be electrified (such as electric cars). This is due to the energy density of batteries, which is about 50 times lower per kilogram than, for example, aircraft kerosene, and for this type of transport, it is inefficient because a fuel tank would have to be replaced by tonnes of batteries.

According to the researchers, the solution lies in biofuels produced from the waste and pollution we generate to reduce to zero or even eliminate part of the emissions of our way of life. Currently, the recipe and the technology exist, but the result is expensive and difficult to scale up sufficiently to supply this enormous fleet. However, the research group Surfcat (Surface Chemistry and Catalysis) has succeeded in improving the formula to obtain biofuels from urban rubbish, waste and CO2.

Synthesising biofuels

The technology for the synthesis of advanced biofuels through carbon capture and utilisation allows the use of CO2 emissions as raw material and has been the subject of intense laboratory study for a decade, according to the scientists. However, it has not taken off due to limitations in energy efficiency and technical barriers, such as the lack of suitable catalysts.

To begin with, the raw materials do not come from fossil resources but from landfills. The group uses urban waste, carbon dioxide, biomass remains (pruning products or agricultural waste), clothing and even sewage sludge. Interestingly, some projects are experimenting with algae.

Very briefly, these compounds undergo a thermal treatment called gasification. Once the gas or liquid is obtained, it undergoes a catalytic process in a reactor, a pressure cooker where advanced catalysts are added. These allow the selective extraction of the desired chemical compound and accelerate the process, thus reducing the energy required and, therefore, the costs.

The basis of this advanced catalysis, which is to achieve a chemical reaction more quickly and with lower temperature and pressure ranges, which has been presented as a European patent, is based on metallic oxides and could also be used to improve current refining processes. The result is analogous to the fuel used by aeroplanes, ships and lorries.

Hydrogen, a second line of work

The scientists highlight the importance of this flexibility because it is the basis of the second line of research, still in its early stages, which would not only allow the biofuel to be obtained from the liquid generated in the pyrolysis but also make use of hydrogen, which would not depend on electrolysis or fossil sources. Until the hydrogen method is viable, the researchers plan to build a pilot plant that uses olive pruning remains. The research group explains that the cost of the final product is still higher than that obtained from fossil sources; however, it is a greener alternative that contributes to the circular economy.