Liquid fuel, synthetically produced
02. November 2021
The conversion of electrical energy into liquid fuels goes by the name of power-to-liquid. The aim is to use green-generated electricity to produce CO2-neutral fuels for sectors that cannot do without liquid fuel, such as aviation. The process has great potential - but there are also some challenges.
Fraunhofer ISE's PS system enables investigations of the chemical state of almost all surfaces under realistic conditions. © Fraunhofer ISE
While the transformation of car traffic is predominantly contested by electromobility, the situation in aviation and logistics is different: Accumulators are either too weak or too heavy and expensive for use in aircraft. The solution is being sought in fuel cells on the one hand and in the use of synthetic fuels on the other, both based on hydrogen as a secondary energy carrier. One promising process that is being promoted by the German government and researched by renowned institutes is the "power-to-liquid" process: electrical energy becomes liquid fuel. Particularly in view of the prospect that wind and solar energy will continue to generate surpluses at times in the future, this creates a new customer field.
What sounds like a complicated process is actually a sequence of simple chemical reactions. Water is split into H2 and O by electrolysis, CO2 is added to the hydrogen in a kind of catalyst, creating synthesis gas. Under pressure and at high temperature, this is turned into hydrocarbons in the so-called Fischer-Tropsch synthesis (FTS), which are separated in the final step. Among these are petrol, diesel and paraffin: the fuels that once stood for a revolution in individual locomotion and today - produced from finite resources - are responsible for a considerable share of global CO2 and nitrogen emissions. For the time being, therefore, no conversion is necessary for the use of PtL fuels; synthetically produced paraffin can be mixed with its fossil predecessor without any problems. The practicality of the fuel has already been tested, but its large-scale applicability depends on many other factors that are still being investigated. These include whether FTS is the most productive production process. Alternatively, methanol synthesis is used, in which the synthesis gas is converted into methanol; the energy loss is somewhat lower here, and the efficiency is correspondingly higher.
Efficiency and applicability of power-to-liquid
The efficiency is an important parameter in the discussion of the applicability of PtL. It describes the calorific value of the products produced in relation to the amount of energy used and is in the order of 40 % for the PtL process chain, which already indicates that certain losses cannot be avoided. For this reason, the origin of the energy used is of crucial importance; after all, it makes little sense to use newly produced electricity if about half of it is lost. It therefore makes sense to use generated surpluses that would otherwise be forfeited. Equally important is the form in which the CO2 needed to produce synthesis gas is available. If it is obtained from biomass, for example, in which it already occurs in concentrated form, a completely greenhouse gas-neutral energy supply is possible. However, the use of CO2 from fossil combustion processes is also technically feasible and would at least ensure double use with the same emissions.
If the possibility exists, a purely battery-electric drive is more ecological than PtL, as all the energy fed in can be converted. In some applications, however, the use of stored electrical energy is not possible for weight or performance reasons. Here, liquid fuels are essential. Therefore, politics and industry have jointly agreed on a PtL roadmap for aviation, which was issued by the German government in April 2021. The goal is clearly formulated: The production of PtL paraffin is to be developed and expanded over the next few years. The project is in line with the National Hydrogen Strategy as well as the Paris Agreement and the EU's Green Deal. By 2030, it should be possible to use at least 200,000 tonnes in German air transport. CO2 emissions in the transport sector are to be reduced by 42% by 2030 compared to 1990, which is why every sector must be included: Thus, the propulsion of aircraft with synthetic paraffin is accompanied by efforts to reduce air traffic in general by improving the conditions for rail transport.
Power-to-liquid has the potential to make certain sectors climate-friendly that are still major drivers of emissions. However, a number of prerequisites must be met for its use to make ecological sense: first and foremost, the electricity used to produce the fuel must come from renewable energies. In addition, the CO2 that reacts with hydrogen during synthesis gas production must already be present in the atmosphere. Since PtL paraffin can be mixed with conventional fuel, a changeover is possible successively, depending on supply and demand. Under the current framework conditions, it is neither established on the market nor competitive, according to the German government. However, this is to change in the near future.