Carbon dioxide: turning the problem around

^{Webspecial Fraunhofer magazine 1.2024}

Rehabilitating the prime suspect responsible for climate change: More and more technological innovations aim to harness CO₂ as a sustainable source of carbon.

One part carbon, two parts oxygen: Carbon dioxide is a fairly simple chemical compound. It also makes up just a tiny fraction of the air we breathe, currently 0.04 percent. But that’s enough to cause big trouble for the entire world. The carbon dioxide in the atmosphere can absorb the heat emitted by the earth and reflect it back down to the planet’s surface. Among the various greenhouse gases, carbon dioxide is the prime suspect responsible for climate change. And that’s not even because it has an especially high greenhouse potential – nitrous oxide, hydrofluorocarbons, and methane rank much higher than CO₂ on that score – but because it is produced in the largest volumes and remains in the atmosphere for a relatively long time.

These days, reducing emissions of carbon dioxide is viewed as the single most effective way to combat climate change. But that alone won’t be enough: The amount of unavoidable CO₂emissions from Germany alone is estimated at some 60 million metric tons per year. And yet, the German federal government aims for negative emissions by 2050. How are we supposed to get there?
The trick will be to bind more carbon dioxide than is released. As for how to do that, three mechanisms have been proposed. Two of them focus on trapping the carbon emitted: carbon capture and storage (CCS) and carbon capture and usage (CCU). The third, carbon dioxide removal (CDR), takes a different approach. It calls for removing CO₂ from the atmosphere and permanently storing it in geological formations or oceanic carbon sinks, in biomass, or in durable products, thereby achieving true negative emissions. In terms of developing the technologies needed for this, time is pressing: Megatons of CO₂ will need to be separated to reach Germany’s climate targets alone between now and 2030.

Capturing carbon dioxide directly from the air

The principle of direct air carbon capture and storage (DACCS) is geared toward filtering carbon dioxide out of the atmosphere. To achieve this, a fan moves the air past a sorbent material that soaks up the CO₂. “With the natural concentration of CO₂ in the air being so low, capturing this greenhouse gas is associated with very high energy use,” says Dr. Barbara Breitschopf, a project manager at the Fraunhofer Institute for Systems and Innovation Research ISI, who explored the potential of DACCS with her team in a policy brief. So DACCS only makes sense in places where there is an adequate supply of renewable energy. “For energy efficiency reasons, though, we should prioritize capturing CO₂ at available point sources,” Breitschopf explains. It is more efficient to capture the gas right where it is emitted than to allow it to escape into the atmosphere first and then spend a lot of energy and money filtering it back out.
One of these point sources for CO₂ can be the production of hydrogen from or through biomass. This is because H₂ is formed not only through electrolysis, but also through conversion of organic residue and waste from fields such as food production and agriculture. In the H2Wood – Blackforest project, for example, researchers from the Fraunhofer Institute for Interfacial Engineering and Biotechnology IGB are working to use wood waste from Germany’s Black Forest to generate hydrogen. If the biogenic carbon dioxide produced as a byproduct of these kinds of processes is captured and used or stored on a long-term basis, the hydrogen production is said to be CO₂-negative – a win-win from an environmental standpoint.

“The overarching term for all kinds of approaches like this is hydrogen bioenergy with carbon capture and storage, or HyBECCS,” explains environmental scientist Sonja Ziehn. For her master’s thesis, Ziehn participated in the RhoTech project at the Fraunhofer Institute for Manufacturing Engineering and Automation IPA in Stuttgart, studying how a purple bacterium named Rhodospirillum rubrum can be used to produce hydrogen out of fruit and dairy waste through a process known as “dark photosynthesis.” In this method, the microorganisms use the sugar present in the residue as a source of energy instead of light. One big advantage of this “dark” production of hydrogen is that the process can be scaled up to almost any level using conventional stainless steel bioreactors. A follow-up project, RhoTech II, is now incorporating a bioreactor into the production workflow at a fruit juice company to set bacterial hydrogen production in motion using the residue generated there. “Fraunhofer’s focus is on economic and ecological process optimization,” Ziehn points out. “Under what conditions do we see mostly hydrogen, but also large amounts of CO₂ being produced?”

Maximizing the production of carbon dioxide might sound like a paradox in light of climate change. But this biogenic CO₂ is easy to separate, plus it can be used as a raw material for chemicals and products that were previously based on fossil carbon dioxide, thus shrinking their carbon footprint. “Now that the financial incentives for biogas plants under the German Renewable Energy Sources Act are expiring, operators are looking for new business models,” Ziehn explains. “Switching to hydrogen production using a HyBECCS plant is one option, especially since hydrogen is practically crying out to be used as an alternative to diesel for tractors and other agricultural
machinery.” HyBECCS technologies could make marketing “green” carbon dioxide an interesting source of supplemental income for the agriculture and food production sectors.

The researchers at the Fraunhofer Institute for Microengineering and Microsystems IMM offer another potential path forward for agricultural operations with biogas plants. So far, the biogas produced has been used mainly to produce electricity and heat, or it is processed and then fed into the natural gas network in the form of methane. With the current methods, the carbon dioxide released in the process simply escapes unused into the atmosphere. Although the CO₂ involved is biogenic rather than being from fossil sources, Dr. Gunther Kolb, a chemical engineer and the head of the energy division at Fraunhofer IMM, says that is still a waste: “It would make more sense to convert the CO₂ part of the biogas – which accounts for 40 percent of the total anyway – to methane and then feed all of the biogas into the natural gas network,” he notes.
In the ICOCAD I project, the team of researchers developed reactors and catalysts that are able to use green hydrogen to methanize the carbon dioxide contained in the biogas – even with the existing methane being present. One of the challenges of doing this, Kolb explains, was building a pilot plant that also has good heat management: “Heat is generated inside the reactor, but it can be decoupled and used in a local district heating network, for example. This creates an overall process that is economically attractive for farmers, who are increasingly producing energy as well.”

In the follow-up project, ICOCAD II, work is now under way to install a demonstration plant along with a biogas feed-in unit to scale the process and optimize practical operation. Kolb says this technology offers huge environmental opportunities: “If all of the biogas from the 9,000 or so plants around Germany were fed into the natural gas network in full, it would be enough to cover about 13 percent of Germany’s demand for natural gas – all from purely biogenic sources and with better quality than natural gas derived from fossil sources.”

The Fraunhofer Institute for Environmental, Safety and Energy Technology UMSICHT is using plants as “middlemen” in its work on storing carbon dioxide. After all, plants are known to absorb CO₂ from the air and split it via photosynthesis. The oxygen released in the process is emitted into the environment, but the carbon remains inside the plant itself, including in the roots. The CO₂ is released again when biomass is burned. The KARBO-SELF project at Fraunhofer UMSICHT aims to use a carbonization technology developed at the institute to burn biogenic residue while excluding oxygen, with the result that the carbon present in the plants remains bound and stable in the form of plant carbon, acting as a carbon sink.

Fraunhofer itself is planning to prepare this technology at its own locations and, at the same time, develop a method of certifying the plant carbon products as carbon sinks. There are also plans to study potential roles for plant carbon as additives in construction materials or in agriculture.

Platform chemicals to store CO₂

However, carbon dioxide can also be stored chemically. After all, carbon is an essential element of many everyday products. In the e-CO₂Met project, coordinated by multi-energy company TotalEnergies, researchers from the Fraunhofer Center for Chemical-Biotechnological Processes CBP are working to convert green hydrogen and carbon dioxide into methanol. For this purpose, plans call to use a pilot plant at Fraunhofer CBP that has been specifically adapted to this process. True, this will not involve long-term storage of CO₂ − but as Dr. Ulrike Junghans, a chemist and head of the regenerative resources department at Fraunhofer CBP, points out: “Methanol is an excellent platform chemical, and it serves as a starting material for a whole host of products in the chemical industry and for the transportation sector.”

Methanol produced via green methods is not yet competitive on price, but Junghans expects that to change soon, as processes grow more efficient and novel catalysts that are more efficient in handling CO₂ from industrial point sources are identified. “In the long run, industry will have no choice but to use non-fossil carbon sources,” she says with conviction. “And that means carbon capture, whether from the air or at point sources, will be increasingly important going forward.” Right now, there is plenty of CO₂ available, but that could change with advances in decarbonization across various industries. So will CO₂ be a sought-after commodity one day? Junghans: “I can certainly see it happening.” Which, she points out, makes it even more important to implement this source of raw materials as part of a circular economy.

Giving carbon dioxide a helping hand

The challenge with carbon dioxide is that the gas is “a really sluggish molecule,” as Dr. Thomas Schiestel, head of the membranes department at Fraunhofer IGB in Stuttgart, puts it. The two oxygen atoms and one carbon atom are slow to release their double bonds so they can form other compounds. “It takes a lot of energy to utilize CO₂ effectively,” Schiestel explains. And that very aspect is what makes using carbon dioxide expensive and not very sustainable, at least unless renewable energy is used. To address this, Schiestel is hunting for ways to make recycling CO₂ economically viable, either by transferring it into chemicals or products or by using it as an energy sink for volatile renewable energy.

With this in mind, the PiCK (Plasma-induced CO₂ Conversion) project is using excess electricity from renewables. Energy is used to split the stable carbon dioxide in a plasma – an ionized gas with highly reactive particles. To keep the products of this process, carbon monoxide and oxygen, from immediately re-bonding to form CO₂ in a reverse reaction, an innovative ceramic membrane is used to remove the oxygen particles from the system. “Since the membrane has to withstand the high temperatures in the plasma, which can be up to 1,000 degrees Celsius, along with the CO₂ concentration at the same time, we spun perovskites, a special kind of ceramic materials, together with polymers to form a thin-walled capillary,” Schiestel says, explaining the membrane’s special features. The NexPlas successor project plans to introduce hydrogen into the system as well to produce secondary reactions in the plasma – an additional challenge for the perovskite membrane.
The advantage of the plasma-membrane combination is its adaptability. It can be used wherever CO₂ arises: in combustion processes such as those that occur at power plants and in waste incineration, in the cement and glass industries, and at breweries, where carbon dioxide is a byproduct of alcoholic fermentation. “Industrial players have already signaled an interest in our plasma-membrane combination,” Schiestel says.

Building on sustainable material innovations

Dr. Michael Prokein, group manager for functional materials at Fraunhofer UMSICHT, is taking a different approach to long-term storage of carbon dioxide. In the NuKoS project, which aims to use the carbon dioxide in slags, he and his team have developed a method that uses carbon dioxide to produce ecofriendly masonry blocks from steel industry residue. These items could come to entirely replace construction materials with a large carbon footprint.

“We’re focusing on the part of steel slag that is too fine-grained to be reused for other purposes, which so far has meant it winds up in landfill, an expensive proposition,” Prokein explains. The carbon dioxide, in turn, is taken from sources such as process gases in the steel and iron industry or cement production. In this way, NuKoS is addressing two environmental challenges at once: First, the steel and cement industry is a top emitter of CO2, and second, Germany’s iron and steel industries produce about 14 million metric tons of slag from steel production each year. “The fine part of the steelmaking slag is ground up and mixed with sand and water,” Prokein says, describing the process. “The mixture is then pressed into the desired molded shape and then cured in a CO₂ atmosphere at 15 bars of pressure and 50 degrees Celsius.” In the process, the carbon dioxide forms a lasting chemical bond with the stone, creating a carbon sink in the form of a brick. “The moderate production conditions hold high potential for energy savings in comparison to other curing methods,” Prokein says. Another advantage is that for this process, the autoclaves currently used to make sand-lime brick can simply be retooled.

The test results for the slag-based building materials are encouraging: “We can achieve compressive strength equivalent to that of concrete,” Prokein confirms. Plus, production of one cubic meter of CO₂-cured stone results in an 80-kilogram carbon sink – while production of conventional sand-lime brick releases 250 kilograms of carbon dioxide per cubic meter. Or, as Prokein puts it: “Slag-based stone is amazing from a technological, economic, and environmental perspective!”

The construction industry has shown great interest, but bureaucracy presents a bit of a snag. “We’re not sure yet whether it will be possible to use steel slag as a construction material for this use case,” Prokein explains. Once questions like these are answered, industry could get down to work right away. From a technological standpoint, the production process for the CO₂-negative building material is ready to transfer.

The microalgae make the sneaker

The textile industry is also looking for ways to shift from petroleum to bio-based materials – possibly using tiny organisms. AlgaeTex, a subproject of the BioTexFuture innovation space funded by the German Federal Ministry of Education and Research (BMBF), brings together teams of researchers from Fraunhofer IGB and CBP, the University of Bayreuth, and the Institute of Textile Technology (ITA) at RWTH Aachen University, along with sporting goods manufacturer Adidas, to work on solutions for storing carbon dioxide in functional textiles. How is this supposed to work? “We grow a specific kind of microalgae that uses light to metabolize CO₂ and, under certain conditions, can produce fatty acids,” explains Dr. Grzegorz Kubik, head of the industrial biotechnology department at Fraunhofer IGB. These fatty acids, he says, can be chemically transformed into polymers, which Adidas then weaves into a kind of nylon fabric that can be used for things like the upper parts of sneakers – so the company is literally using shoes to shrink its environmental footprint.

Algen in Kellern kultivieren: Das Team am Fraunhofer IGB hat im Projekt AlgaeTex einen stapelbaren, indoor betreibbaren Photobioreaktor entwickelt. Der Vorteil: Wie bei allen Reaktoren wird keine fruchtbare Agrarfläche besetzt, außerdem entfällt die Abhängigkeit von Standorten mit guter Sonnenverfügbarkeit. Der Nachteil: Die Mikroalgen müssen mit künstlichem Licht statt Sonnenstrahlen versorgt werden – bis zu 100 Kilowattstunden pro Kilogramm Algenmasse. »Der Energieeinsatz ist deshalb eines der Kernthemen dieser Technologie«, erläutert Kubik. Je stärker sich aber der Anteil erneuerbarer Energien im Energiemix erhöht, desto umweltfreundlicher und auch günstiger wird es, Mikroalgen in Deutschland zu kultivieren. Kubik: »Die Verfügbarkeit von erneuerbarer Energie ist derzeit das Nadelöhr für die Speicherung von CO₂ durch Mikroalgen.« Auch deshalb arbeitet das Forschungsteam aktuell an einer Optimierung der benötigten Lichtmenge und damit des Stromverbrauchs. Ein anderes Augenmerk liege »auf der Weiterverwertung der Alge etwa als Düngemittel in der Landwirtschaft oder als Futtermittel in der Viehzucht. Wir wollen nicht nur die Effizienz der Algenproduktion erhöhen, sondern auch deren Weiterverwertung«.

Die Optimierung des mikrobiologischen Ansatzes in der CO₂-Nutzung könnte sich weit über die Textilindustrie hinaus lohnen. Denn Algen sind in der Lage, mit Lichtenergie und Kohlendioxid noch ganz andere Substanzen zu bilden und zu speichern, etwa Stärke: »Wir müssten Zucker dann nicht mehr aus pflanzlicher Biomasse herstellen, sondern könnten ihn aus Mikroalgen generieren und dadurch Agrarfläche einsparen«, erklärt Kubik. »Oder Algen nutzen, um CO₂ in Form von Kalk zu sedimentieren und dann zu lagern oder in der Bauindustrie zu nutzen.«

Im Projekt SmartBioH2 am Fraunhofer IGB produzieren Purpurbakterien in einem geschlossenen Bioreaktor aus Reststoffströmen Wasserstoff und Produkte wie Carotinoide. Das dabei entstehende Kohlendioxid wird von Mikroalgen in Biomasse gebunden – unter Freisetzung von weiterem Wasserstoff oder aber Produkten wie Proteinen.

A smart carbon cycle

From microorganisms to mega-scale: The Carbon2Chem® joint research project is also focusing on production of steel, cement, and lime as the biggest industrial source of CO₂ emissions. “We’re looking for methods and technologies of optimizing the circular carbon economy so that carbon isn’t released after it arises, but instead is locally reused and recycled sustainably,” says Prof. Görge Deerberg, a chemical engineer and the director of transfer at Fraunhofer UMSICHT. He is one of the coordinators of the huge and sprawling project, which was launched in 2016. With funding from the German Federal Ministry of Education and Research (BMBF), the project spans the fields of basic and applied research, along with various industrial sectors. “This cross-industry network is key to the success of the Carbon2Chem® project,” Deerberg says. His focus here is not solely on developing individual technologies, but also on integrating them into a cross-industry overall structure. Within that structure, the hope is that whole new forms of collaboration will take hold: “The opportunities and risks involved in CO₂ use need to be fairly distributed. That’s a prerequisite for long-term success.”

At the heart of Carbon2Chem® is the idea of substitution: The carbon required to produce many of the basic chemicals, plastics, and synthetic fuels used in industry will no longer come from fossil sources in the future, but rather from industrial process gases and waste incineration. The first phase of the project investigated topics such as gas purity. “In the steel industry, smelting gases are produced at blast furnaces and converters and also in coking plants, so the composition varies,” Deerberg explains. For this reason, the team first developed technologies to analyze the gases and then purify them to the point that they can undergo further processing without disrupting catalytic processes. Another challenge lay in the fluctuating concentrations of components in the process gases: “The technologies used at chemical plants have tight tolerances. They aren’t set up to deal with ranges,” Deerberg explains. “We needed to map out a systematic approach that was adaptable for overall conditions that change not just from one minute to the next, but also over years as a result of the industrial transformation.”
The second phase of the project, which is concerned with scaling the technology, launched in 2020. A pilot plant occupying 3,700 square meters was built right next to the plant grounds of thyssenkrupp Steel Europe AG in Duisburg, adding to the 500-square-meter lab on the Fraunhofer UMSICHT campus. The demonstration systems are hooked up to the wiring and plumbing for the steel mill. In 2018, the researchers succeeded in producing methanol from smelting gases for the first time. “It was only a small glass full,” Deerberg says. “But it was definitely a very special moment for all of us.” There are plans for a new plant to produce 12 metric tons a day.
The third and final phase of the project will deal with technology transfer to other high-emission, energy-intensive industrial installations like cement plants and waste incineration plants. “The goal of Carbon2Chem® is to support and advance the big industrial transformation,” Deerberg says. After all, even amid vigorous ongoing efforts to reduce CO₂ emissions, there will always be sectors that inevitably produce carbon dioxide. To Deerberg, this means that even as innovative CCU technologies are developed, we should also work on carbon’s image: “Right now, a lot of people are narrowly focused on strictly preventing carbon emissions. But we could flip that and think about continuing to use carbon, just not from fossil raw materials anymore.”

Will CO2 soon be a way to make money?

Dr. Jonathan Fabarius, senior scientist for microbial catalysis at Fraunhofer IGB, goes one step further at the biointelligence competence center: “Making money with CO₂?” is the title of a blog series. In one post, about turning the page on the fossil era, he discusses how carbon dioxide can be used to generate valuable and important chemical materials. Carbon dioxide as a rich source of funds? Fabarius’s colleague Dr. Arne Roth, head of the sustainable catalytic processes department at Fraunhofer IGB, also stresses the importance of CO₂, calling it a “key raw material that we would do better to recycle as part of a circular economy instead of continuing to pull new carbon-rich resources out of the ground.” However, creating the conditions for many of the proposed CO₂-based value chains to be commercially successful will require targeted and determined research and development to lay the technical groundwork.

In the EU’s EcoFuel project, this idea is to be incorporated into the electrochemical production of synthetic fuels made from carbon dioxide and water. “Working with various European partners, we’ve devised an innovative process chain that starts with CO₂ from direct air capture,” Roth explains. Then, as the next step, the gas is electrocatalytically converted to ethylene, a C₂ gas, which is then converted in turn into liquid fuels. “Power-to-X” is the term used for this cascade approach. At the Center for Sustainable Fuels (ZENK), in Bavaria, researchers from Fraunhofer IGB and UMSICHT are working to identify new production methods for fuels based on CO₂, biomass, and renewable electricity and scale them to pilot plant levels.

Roth is convinced: “If we develop suitable process technologies and combine them in smart ways, we can use carbon dioxide to produce a wide range of chemical products.” He is especially interested in combining CO₂ conversion with biotechnology: C1 chemicals like formic acid or methanol generated electrocatalytically from carbon dioxide can be used as fodder for microorganisms that produce higher-value chemicals out of them. Fabarius’s team has already demonstrated these kinds of promising hybrid approaches: In the CELBICON project, for example, the researchers at Fraunhofer IGB managed to harness the synthesizing prowess of bacteria to convert CO₂ from the atmosphere into a terpenoid dye, a type of natural pigment also found in plants and algae. “Microorganisms are sensational chemists,” Roth points out. “Even without high temperatures or pressures, they can metabolize carbon dioxide into products, some of which are even capable of long-term greenhouse gas storage. We should spend a lot more time looking to nature for examples.”

That same idea also fascinates Ulf-Peter Apfel, a chemist who serves as the head of the electrosynthesis department at Fraunhofer UMSICHT and a professor at Ruhr University Bochum: “In nature, CO₂ isn’t a problem chemical. As a C1 source, it’s a hugely important base material. When it comes to using CO₂, we have a lot to learn from nature.” That is why Apfel does not talk much about “decarbonizing” industry, preferring the idea of “defossilization” — reducing the amount of CO₂ derived from fossil raw materials. He is working on this in the CO2-Syn project, which was launched in 2022, for example.

The project focuses on the cement industry, which accounts for as much as eight percent of global carbon dioxide emissions. And that isn’t likely to change anytime soon, since when calcium carbonate – one of the main ingredients used to make cement – is burned to produce calcium oxide, carbon dioxide is inevitably produced as well. “That is a huge point source we can use,” Apfel says. Much like in the Carbon2Chem® project, the team here is also working on the use of CO₂ from waste gas streams – in this case, to produce synthetic gases (a mixture of carbon monoxide and hydrogen) that are then used as basic chemicals for olefins and higher alcohols.
The CO₂ generated during cement production is contaminated with dust and other potential pollutants, however. How can it be purified at reasonable cost? Or might there even be ways to avoid cleaning it altogether? “To do that, we need catalysts that are highly resistant to contamination,” Apfel explains. Right now, he is focusing on sulfide-based catalysts. “They’re really resilient. You can’t contaminate them all that easily.” The first tests of possible process routes have already been concluded, and now a pilot plant is to be commissioned. “We’ll be able to convert 100 kilograms of CO₂ a day with the new plant,” Apfel says. “It’s the world’s first plant on this scale.”

Apfel is surprised German companies are often still skeptical about the idea of using CO₂. “It won’t be long now before industrial processes are producing less and less carbon dioxide. There will be a lot of money in sustainable C1 point sources,” he predicts. “But for that to occur, we need to invest in the right processes and systems now.” Looking at the technological advances of the past five years, especially in the area of CCU, he offers another prediction: “I think we’ll have really big plants everywhere by ten years from now.” Germany cannot become carbon-neutral by 2050 solely by preventing emissions of the greenhouse gas, he notes. A much more diverse approach will be needed: “We have to stick with it and keep looking for new and promising routes to get there.”