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. 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.
Direct air capture is a process in which carbon dioxide (CO2) is extracted directly from the atmosphere. The ambient air flows through a filter which removes some of the climate-damaging carbon dioxide from the air, leaving pure CO2 which can then be used for various purposes.
The oxy-fuel process involves the combustion of residual biomass using the oxygen produced in the electrolysis process, thereby producing a stream of pure CO2 which can be used in the synthesis processes.
In view of the climate policy challenges ahead, using regenerative carbon sources to produce renewable energy carriers will play an important role in efforts to reduce greenhouse gas emissions. P2X technology is a key component in this context – it provides the means of converting and, if necessary, storing renewable (surplus) electricity into C-based raw materials for industry, renewable fuels and combustibles. Replacing fossil energy sources with renewables will leave countries less exposed to geopolitical dependencies.
There are several categories of P2X processes distinguished by the physical state of the product: power-to-gas, power-to-liquid and power-to-chemicals
The technology that links all three conversion paths is electricity-to-hydrogen electrolysis. Hydrogen can be converted into a C-based material or energy carrier by adding a carbon carrier (e.g. carbon dioxide). This takes place in a process downstream from hydrogen production known as synthesis and involves various catalysts, operating conditions and process controls.
ZSW draws on many years’ experience with synthesis in power-to-gas and power-to-liquid technologies. This expertise enables us to serve as a contact, partner and technology supplier. We have thoroughly investigated methanol and methane synthesis, especially, and have developed processes to ensure high product yields.
Methane synthesis, the process by which carbon oxides are converted into methane using hydrogen, has been a research priority at ZSW for close to 20 years. We have amassed a trove of knowledge over the years encompassing everything from the first step of selectively methanating carbon monoxide to produce a fuel gas suitable for fuel cells to synthesizing the P2X technology’s stoichiometrically adjusted gas mixtures to produce a natural gas substitute. This expertise is needed to make the adjustments required by varying applications and gas composition. To this end, we adapt the operating conditions and process control to suit methane synthesis, a highly exothermic process, and select suitable catalysts. Cooled fixed-bed reactors are our preferred choice of medium for methane synthesis processes.
Various test facilities are available to investigate heterogeneous catalysts and develop reactors and even entire process chains. These assets enable us to conduct both fundamental research and engage in application-oriented development.
Extracting renewable energy from regenerative carbon sources is part of a larger process chain that usually involves treating raw gases or conditioning product streams. Impurities can trigger deactivation mechanisms, which is why minor components such as sulfur compounds are removed from raw gases to prevent problems in downstream catalytic processes. ZSW uses mainly sorbents and catalytic processes to eliminate minor components.
Membrane gas separation technology conditions gas flows to modify gas quality to the given specifications. It uses the different mass transfer rates (permeation rates) of individual gas components traveling through a membrane as a filtering mechanism. Membrane-based technology requires a lot less process engineering effort than other gas conditioning processes, and its modular design quickly adapts to changing operating conditions and a wide load range. Equipped with a suitable membrane, this technology can serve other separation purposes and also lends itself to smaller plants.
ZSW has various experimental facilities at its disposal to investigate individual gas generation, conditioning and purification steps, and to characterize materials such as sorbents and catalysts. We use temperature-controlled micro and macro reactors with downstream gas analysis systems as well as thermogravimetric analysis (TGA) to determine the relevant parameters. TGA involves measuring a sample’s change in mass as a function of temperature and time to track conversion in gas-solid reactions. The influence of individual parameters such as temperature, pressure and gas composition can be determined by varying the conditions in which the reaction takes place.
ZSW has two test facilities for characterizing materials via thermogravimetric analysis:
Oxyfuel combustion uses a mixture of O2, CO2 and water vapor rather than air as the oxidant, thereby producing an N₂-free oxyfuel flue gas consisting mainly of CO2 and water vapor. ZSW is developing a near-stoichiometric method of oxyfuel combustion using solid (biogenic) residues to provide process heat and oxyfuel flue gases with nearly 99% CO2 content by volume and residual O2 content greater than 0.5% by volume in dry conditions. This efficient integrated process serves to generate CO2 independently of CO2 sources for the production of C-based P2X products. The O2 required for oxyfuel combustion can be obtained directly from electrolysis when this process is combined with a P2X process, which again increases efficiency. ZSW has a comprehensive application-oriented test environment consisting of a fluidized bed reactor and a FLOX® reactor (flameless oxidation, FLOX®) at its disposal for development projects. The fluidized bed is designed for combustion temperatures up to 950°C. The FLOX® reactor can handle temperatures up to 1,100°C. The peak fuel thermal heat input is 15 kWth in each case.
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.