E-mobility in cars is slowly gaining traction. In the coming years, numerous new models with fully electric or hybrid drives will be hitting the market. However, things aren’t looking quite so rosy when it comes to trucks, container ships and airplanes. It is as of yet unclear whether it will one day be possible to operate them with rechargeable batteries at all. However, it is paramount that reductions in carbon dioxide emission are also achieved in these areas. This is where research into power-to-liquid technology comes in, for synthetic fuels can in fact be produced with the help of green energy. In the third part of the “Power-to-X” series, we will be looking at the mode of action behind and the potential for this technology.
According to the German government’s climate protection plan, emissions from the transport sector are to fall by at least 40 percent by 2030, i.e. from 160 million metric tons of greenhouse gases in 2018 to less than 100 million metric tons. But even representatives of the Federal Ministry of Transport are sceptical as to whether these targets can be met, as reported by Süddeutsche Zeitung. Experts estimate that over the next ten years emissions will only fall to 142 million metric tons annually – and that only if vehicles consume significantly less fuel moving forward. The biggest driving factor is road-based freight transport, which has been steadily increasing for years.
The situation is even more dire when it comes to air transport: The European Commission forecasts that the number of flights within Europe will increase considerably and that CO2 emissions could increase by as much as a fifth by 2040. Any emissions savings made by more efficient motors are simply offset by increased traffic.
In light of these forecasts, there is an increased need for a technology that can significantly reduce emissions from the transport sector in the long term. That which is referred to as power-to-liquid technology uses green electricity to produce synthetic liquid fuels (e-fuels), which are considered to be climate-neutral. Plans are being made for larger plants, now that the first pilot projects have passed their practical tests. According to experts, e-fuels – also known as current-based fuels – and waste- and residue-based alternative fuels are particularly interesting to sectors where electric drives are currently not an alternative and will remain impractical for the foreseeable future.
The power-to-liquid technology uses green electricity to produce synthetic liquid fuels (e-fuels), which are considered to be climate-neutral. Click through the figure and learn more about the individual steps of the process.
Power-to-liquid technology uses electric energy to generate liquid energy sources. Or in other words: Power is turned into liquid fuel. In order for these synthetic fuels to help meet climate targets, the power used for this process needs to come from renewables.
Electrolysis produces hydrogen: Electricity is used to split water (H2O) into hydrogen (H2) and oxygen (O2). The oxygen is emitted into the air, whilst the hydrogen is used for the following steps.
In a two-stage process, carbon dioxide (CO2) and hydrogen are converted into hydrocarbon chains. First, carbon dioxide and hydrogen are reduced to carbon monoxide with water (H2O) as a by-product. The carbon monoxide is then combined with more hydrogen to form a synthesis gas. Various liquid hydrocarbons can then be produced from the synthesis gas using the Fischer-Tropsch process.
The liquid energy sources can, for example, be processed in a refinery to make synthetic petrol, diesel or kerosene.
Synthetic petrol, diesel or kerosene can be used to directly power cars, trucks, container ships or aircrafts. During combustion, the CO2 that was previously bound by the process is released once more. Furthermore, significantly fewer pollutants are emitted, compared to fossil fuels, as the products are purer.
“Aviation, maritime shipping and heavy goods traffic are areas that largely elude electrification,” explains Thomas Willner, Professor of Process Engineering at the Hamburg University of Applied Sciences (HAW). Willner chairs a working committee in which experts from process and chemical engineering examine the technology paths and potentials of alternative fuels. The fundamental problem of electric drives is that batteries have a much lower energy density than fuel and are much heavier per unit of energy. “This is a make-or-break criteria when it comes to aviation and heavy transport over long distances, but also in other areas such as agriculture,” says Willner.
Yet particularly aviation and freight transport are, from a global perspective, showing high growth rates, meaning greenhouse gas emissions are likely to continue to rise. “From our point of view, alternative fuels – both electricity-based and waste- and residue-based – are indispensable for hitting climate targets,” says Willner, summarising the position of the committee, which is made up of scientists from renowned institutes and representatives of relevant business enterprises and associations.
The process engineers are not alone in their assessment. The German Energy Agency (dena) sees much potential in the European transport sector for synthetic fuels and combustibles produced using renewables: “The study shows that, even in a scenario that relies heavily on battery-powered drives, more than 70 percent of the necessary energy would be covered by powerfuels, in particular when it comes to shipping, air and freight traffic.”
Technically speaking, the production of electricity-based fuels is already possible today. “Hydrogen and carbon dioxide are necessary resources for the power-to-liquid process,” explains Professor Roland Dittmeyer, Head of the Institute of Microprocess Engineering at the Karlsruhe Institute of Technology (KIT). The fuels are referred to as current-based because the hydrogen is produced through electrolysis. This means that electricity is used to split water into hydrogen and oxygen. If the electricity comes from renewable sources then the gas is considered to be CO2-neutral. The hydrogen produced in this way can be used directly as an energy carrier, for example in a fuel cell, or converted to methane together with carbon dioxide. These processes are part of the power-to-gas technology.
On the other hand, power-to-liquid technology, as the name suggests, converts hydrogen and carbon dioxide to liquid fuel. This requires additional processes. “The most widespread processes are methanol synthesis and Fischer-Tropsch synthesis,” says Professor Dittmeyer, whose institute is looking into power-to-x technologies through several research projects.
During methanol synthesis, for example, hydrogen and carbon dioxide are converted to methanol, which is classed as an alcohol. Up to 3 volume percent of methanol can be blended with gasoline. However, methanol is not an established fuel. If methanol is to be added in higher proportions, gasoline engines would have to be specially adapted for the purpose – so far pure methanol has only been used to power HP racing cars in motor sports. In order to circumvent the problem of upper admixture limitations, methanol could in fact be converted to gasoline in an additional process step. However, this would increase production costs.
The areas of application of e-fuels generated by Fischer-Tropsch synthesis (FTS), on the other hand, are much broader. However, FTS, which has been around since the 1920s and was once used to liquefy coal, calls for carbon monoxide. Thus, carbon dioxide, together with hydrogen, is reduced to make carbon monoxide which is then combined with additional hydrogen to form a synthesis gas. Various hydrocarbons can then be produced from the synthesis gas using the Fischer-Tropsch process.
These can then be further processed to produce synthetic petrol, diesel or kerosene. “In principle, the entire fuel market can be supplied in this way,” says Hamburg native Professor Willner, pointing out two advantages that are not to be scoffed at: on the one hand, the entire existing fleet can be safely and securely involved in climate protection and, on the other hand, the existing infrastructure can be maintained in order to reduce costs.
More research is being carried out in relation to other processes in addition to the two existing methods: on the one hand, new fuel products such as oxymethylene ethers (OME) and octanol, which reduce pollutant emissions; on the other hand, increasing the number of suitable raw materials by using waste and residual materials as renewable carbon sources. “There is an infinite number of hybrid forms between electricity-based and waste- and residue-based alternative fuels,” explains Willner. However all synthetic fuels have one thing in common: “Combustion can be improved depending on the purity or molecular structure of the products, meaning significantly fewer pollutants are emitted compared to fossil fuels,” adds Willner.
But where does the necessary carbon dioxide come from? “On the one hand, large emitters such as chemical production plants or steel and cement plants could serve as CO2 sources. As these plants generate carbon dioxide during production anyway and sometimes in high concentrations,” explains Roland Dittmeyer from KIT. On the other hand, biogas plants could serve as decentralised sources. “The carbon dioxide was once absorbed by plants. It is therefore a closed cycle,” says the expert. A third option is to filter the carbon dioxide directly out of the air. However, this process requires more energy, partly as the CO2 level in the atmosphere is low, but has the added advantage that the carbon dioxide is not bound to any particular location.
According to experts, power-to-liquid technology will play an important role in the energy transition and it is hard to deny the advantages associated with synthetic fuels. In the next part of the power-to-x series we will be looking at instances in which power-to-liquid is already in use and what hurdles the technology faces.
Photo credits: Hanna Kuprevich, shutterstock.com