A climate killer is turned into feedstock: Using CO2 and hydrogen for sustainable chemicals
Artificial photosynthesis: Evonik and Siemens Energy start a demonstrator plant that uses carbon dioxide as a raw material, stores energy, and is capable of producing various specialty chemicals.
By Hubertus Breuer
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The plant in Marl, Germany, certainly doesn't look like a biological plant – there is no green, but plenty of metal. And yet photosynthesis takes place here – artificial photosynthesis. A large box with caps and knobs is located in a rigid frame structure in a container. It is an electrolyzer that uses green power, carbon dioxide (CO2), and water to produce a synthesis gas consisting of carbon monoxide (CO) and hydrogen. From there, a tube runs past instruments and control displays to an eight-meter high steel cylinder in an adjacent hall. In the cylinder, bacteria convert the gas into chemical substances such as hexanol and butanol. For sure, that’s not real photosynthesis which uses solar energy to produce sugar from water and CO2. But it certainly is a technical version of it.
Two companies are contributing core competencies to the research project called 'Rheticus'. Siemens Energy provides its completely automated low temperature electrolyzer, and the specialty chemicals company Evonik, which has its largest location in Marl, its expertise in biological gas fermentation. Together, they achieved a technical breakthrough for Power-to-X-solutions. After several years of upscaling, both technologies are being connected in a first test plant.
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Scalable for industry
The process thus can contribute to the transition from fossil-based feedstock to climate neutral ones – just as Germany intends to do with its ‘Energiewende’ by 2050. It can be used wherever industry produces sizeable amounts of CO2.
The conversion of CO2 into CO and water to H2 by itself allows for creating energy storage. But the primary goal of the Rheticus process is to use the synthesis gas in a bioreactor to produce specialty chemicals in a cost-effective and sustainable manner. Today, many chemical substances are often produced with the help of crude oil or natural gas. Now CO2 can also be used as a raw material for many of these processes. "It not only helps to reduce carbon dioxide in the atmosphere, it helps to reduce our dependence on fossil fuels," says Karl-Josef Kuhn, head of Technology Field Power-to-X and Storage at Siemens Energy. “And even better, the system is scalable for industry according to their needs.”
Sustainable in more ways than one
The Siemens Energy electrolyzer converts CO2 and water into carbon monoxide (CO) and hydrogen (H2). That’s achieved by means of 10 electrolysis cells with a total output of around 25 KW. The synthesis gas consisting of CO and H2 then flows into Evonik's bioreactor with a capacity of 2,000 liters. There it is transformed by two species of clostridia bacteria into butanol and hexanol. Both products are used, among other things, for special plastics or food supplements. And if there were a power outage, the process still could continue to run for several hours.
But the process is sustainable in more ways than one. The green electricity for the electrolyzer ensures that the energy source itself is CO2-free. Siemens Energy and Evonik, for example, assume that a plant that produces 10,000 metric tons of hexanol and butanol within a year will require about 25.5 megawatts of power. Depending on location, weather conditions, and type of photovoltaic modules installed, in Germany this is roughly equivalent to the annual energy yield of solar modules covering an area of 0.15 square kilometers. For comparison: According to estimates all by the end of 2019 photovoltaic modules had an output of 635 gigawatts, Germany 49 GW. And even if the electrolyzer is only used for energy storage, it can still help stabilizing power grids based on renewables when needed.
" It helps to keep carbon dioxide out of the atmosphere and to reduce our dependence on fossil fuels "Karl-Josef Kuhn, Head of Technology Field Power-to-X and Storage at Siemens Energy
A wide variety of chemicals
As the plant reduces CO2 pollution by using the greenhouse gas as a raw material, it prevents it from entering the atmosphere in the first place. In about five years’ time, an industrial plant based on this technology, could produce 10.000 tons and butanol per year using 25,000 tons of CO2 as feedstock. And there is certainly no shortage of CO2. German Federal Environment Agency estimated that industrial processes in Germany alone produce around 45 million tons of CO2 emissions annually.
The advantages do not end here. CO is needed in large quantities in the chemical industry - the global demand amounts to 80 million tons annually. The Siemens Energy CO electrolyzer can produce the needed CO directly from industry’s CO2 emissions. In addition, the Rheticus-platform potentially could use various fermentation processes, i.e. produce a wide variety of chemical substances such as components for synfuels.
As the platform is being tested in Marl, Siemens Energy and Evonik are asking various questions: What is the optimal design of the connection between the electrolyzer and fermenter? Will the Clostridia bacteria pass the test on the industrial scale? Will it be possible to design a fully continuous process? The answers will give valuable insights for a potential pilot plant, ready to serve the market.
Hubertus Breuer is an independent journalist specializing on technology reporting. He lives and works in Munich, Germany.
Combined picture credits: Siemens Energy, Evonik