Renewable fuels of non-biological origin (RFNBO)
In the EU, this refers to fuels that are produced using energy from other renewable energy sources. In practice this means the use of renewable power from geothermal, solar or wind power, where a local excess production can result during shorter or longer periods, thereby giving access to such energy at low costs. There are also developments using e.g. concentrated solar power or geothermal energy as a source for direct heat for use in fuel production.
Definition from RED II:
(36) ‘renewable liquid and gaseous transport fuels of non-biological origin’ means liquid or gaseous fuels which are used in the transport sector other than biofuels or biogas, the energy content of which is derived from renewable sources other than biomass;
Production of hydrogen from renewable power is produced by means electrolysis of water in electrochemical cells. Electrolyzers are composed of several cells arranged in “cell stack” modules that can then be multiplied to reach the desired output capacity. The hydrogen produced is then compressed or liquefied for storage.
Hydrogen production by means of alkaline electrolyzers has been around for more than a century and is a fully commercial technology. Another technology that has more recently been introduced is the so-called PEM (Proton Exchange Membrane) but which is now competing head-to-head with the alkaline electrolyzers. Other types (MCEC and SOEC, molten carbonate and solid oxide electrolyzer cells, respectively) are still in development. Electrolyzers installations have typically been up to a few MW in capacity, set by the hydrogen user’s requirements. However, with the increased use of wind and solar power generation and also demand for hydrogen, and RE hydrogen, installations are growing in capacity to 10 - 20 MW and installations of 100 MW are in planning and 1 000 MW installations are studied in the Netherlands
E.ON has a power-to-gas pilot unit in Falkenhagen, Germany with an electrolyzer capacity of 2 MW, the output mainly being injected to the gas grid, but also used by local users. A second PtG pilot unit is in construction outside Hamburg demonstrate a more compact and efficient electrolysis equipment.
In addition, hydrogen form renewable sources can also be added to other biofuel conversions pathways. One concept is a hybrid gasification-to-biofuel pathway. Hydrogen addition results in process savings and a more efficient use of the biomass resources as the yield of biogenic carbon to fuel increases up to twice the amount without hydrogen addition. Renewable hydrogen can also be used in other pathways where hydrogen is used, e.g. HVO, the upgrading of pyrolysis, HTL and lignin intermediate oils, thereby eliminating the internal production of hydrogen in stand-alone biofuel plants or replace fossil hydrogen in refineries. Another possibility is to add hydrogen to anaerobic digesters, and thereby assisting in the methanogenesis step to produce additional biogas from the CO2 present.
E-fuels (or PtG or PtL)
Electrofuels (E-fuels), Power-to-Gas (PtG) and Power-to-Liquid (PtL) refers to technologies, which convert renewable electric energy to another energy carrier, like for example methane, methanol or Fischer-Tropsch fuels.
In short, as a first stage, electricity is converted to hydrogen through electrolysis as described above. To produce a hydrocarbon or alcohol fuel, a carbon source is also required. This carbon source is typically CO2, which is readily available from many sources e.g. biogenic CO2 from bakeries, ethanol fermentation in breweries or ethanol fuel plants, as well as from biomethane upgrading or gasification plants, or CO2 from fossil sources, e.g. coal power plant flue gases or industrial waste gas streams in e.g. refineries and chemical plants. This use of CO2 is also often being referred to as an example of Carbon Capture and Utilization (CCU).
The production of fuels is essentially as described for gasification synthesis gas as described in PVC1: Transport fuels via gasification. In the case of production of biomethane, or methanol, or gas fermentation to ethanol, CO2 and H2 can be reacted directly. For other products such as FT hydrocarbons, a reverse water gas shift reaction is needed to convert CO2 to CO, prior to the catalytic synthesis process where the products are formed.
The largest Power-to-Methanol facility is the CRI’s ‘George Olah’ Renewable Methanol Plant in with a capacity of 4 000 tonnes per year.
In addition, there are a number of pilot initiatives to produce methane (Audi/Solar fuels (DE), BioCAT (DK), methanol (MefCO2 (DE), Thyssen Krupp (DE)) and FT liquids (Sunfire (DE)) based on hydrogen from electrolysis at a scale of 1-5 MW electrolyzer capacity.
Generally speaking, these technologies are at TRL 5-6.
 Hydrogen enhancement potential of synthetic biofuels manufacture in the European context: A techno-economic assessment. Illka Hannula. Energy, Volume 104, 1 June 2016, Pages 199-212