Apart from hydrogen, carbon dioxide is the main ingredient for the synthesis of e-fuels in power-to-X processes. In order to be able to produce e-fuels in a climate-neutral way, CO2 must be supplied from renewable sources. In case of CO2 supply it makes sense to draw it from point sources in the first instance, such as biogas plants or bioethanol plants, or to generate it from residual biomass by burning it with pure oxygen in so-called oxy-fuel processes. It will be necessary to extract CO2 from the air in the medium to long term for any large-scale roll-out of e-fuel processes or produce CO2 and CO from residual biomass and waste by oxyfuel combustion or autothermal gasification.
Direct air capture (DAC) is a process in which carbon dioxide (CO2) is extracted directly from the atmosphere. In this process, the ambient air flows through a filter or an absorbent that removes part of the climate-damaging carbon dioxide. After desorption and concentration, pure CO2 is obtained, which can then be used for the above-mentioned synthesis processes, for example. which can then be used for various purposes.
Oxyfuel combustion or autothermal gasification of biogenic residues or plastic waste can provide CO2 or synthesis gases for power-to-X synthesis processes. For the thermochemical conversion of carbon-containing residues to carbon oxides (CO2 and CO), the oxygen (O2) produced in water electrolysis can also be used.
For the synthesis of CO2 and CO into eFuels, green hydrogen from water electrolysis can be used and thus sustainable carbon cycles can be realised. The ZSW can draw on many years of experience in synthesis processes in the technology fields of power-to-gas and power-to-liquids and acts as a contact partner and technology supplier due to its expertise. In addition to methanol synthesis, methane synthesis in particular has been fundamentally investigated and processes developed to ensure a high product yield.
Demand for carbon-based fuels is unlikely to evaporate any time soon with aircraft, ships and heavy-duty trucks unable to run on electricity alone. The chemical and primary industries also run on fossil fuels to some extent. Their production processes cannot do without carbon. Synthesizing e-fuels and base chemicals requires H₂, which can be sourced from water via electrolysis, and CO₂ which may be captured from the atmosphere.
It is to be expected that concentrated sources of CO₂ will be much harder to come by as climate action becomes an ever more pressing priority. This is sure to drive the development of solutions to actively capture CO₂ from the ambient air. Demand for this technology will also come from places where conditions for producing P2X fuels are favorable, say North Africa or Chile. These regions have no shortage of sun and wind, but may lack concentrated carbon resources. The fast-growing interest in these technologies is also down to their potential role in stabilizing global warming over the long term. These technologies’ CO₂ emissions are negative – they actually remove carbon from the atmosphere. Even if the forecast for rising CO₂ emissions by 2030 proves accurate, they could help hold carbon within the envisaged limits. The captured CO₂ would then have to be trapped permanently, for example, via mineralization. Today, there are but a few technologies that can extract CO₂ from the air with the help of a sorbent.
ZSW’s Renewable Fuels and Processes department has many years’ experience with technology that extract CO₂ from the atmosphere. Its researchers have developed new processes to this end in several projects and put them into practice in application-oriented prototypes. CORAL, a recently completed project sponsored by the German Federal Ministry of Education and Research (funding code 033RC005), is but one example among many. This project’s goal was to screen methods of extracting CO₂ from the air and benchmark their distinguishing features. Today, engineers generally use solid amines on various porous carrier materials such as SAB, MOF, zeolites, or cellulose to this end. CORAL focused on developing and testing a method of minimizing electricity consumption by integrating processes that use the waste heat from ancillary production steps such as electrolysis and synthesis.
Power-to-X (P2X) draws on surplus electricity from fluctuating sources to electrolyze water and produce hydrogen. This gas may be used straightaway or converted again with carbon dioxide to produce synthetic fuels such as methane or e-fuels. ZSW researches and develops the main components for this – electrolyzers, synthesis reactors and equipment to treat gas. These components are the building blocks for end-to-end application-oriented systems, designed and built to the given requirements.
End-to-end P2X systems draw on three resources – water, renewable electricity and sustainable carbon – to produce C-based fuels and base chemicals such as kerosene and methanol. This requires high power efficiency factors, the optimum integration of mass and energy balances (statements on the conservation of mass and energy) for the given location, and a source of renewable carbon. ZSW uses the commercially available simulation programs IPSEpro and HSC Chemistry to optimize the conceptual design, engineering and technical monitoring of P2X systems. These tools enable us to model P2X components with their chemical reactions and physical relationships while factoring the conservation of mass and energy into the equations. The resulting models map out the components and connections in P2X systems of varying complexity.
The chart below diagrams an IPSEpro process simulation environment for AUDI AG’s 6MWel P2G plant in Werlte, Lower Saxony, Germany. Up and running since 2013, this factory uses renewable electricity and CO2 sourced from a biogas plant to produce a natural gas substitute. It is piped into the natural gas grid for downstream use as a sustainable fuel for vehicles. This simulation environment was developed in a project called WOMBAT sponsored by the German Federal Ministry for Economic Affairs and Energy (funding code 0325428D). ZSW still uses it to monitor the plant’s equipment.
