Reverion is a German company that is developing carbon-neutral power plants. The company’s technology doubles the electricity production of existing biogas plants and stabilizes the power grid in electrolysis mode by producing renewable gasses with zero carbon emissions.
Challenges: biogas carbon emissions
Biogas is a renewable and eco-friendly fuel produced by the decomposition of various forms of organic waste. It is produced through the anaerobic digestion of organic matter, such as animal manure, food waste, and sewage in the absence of oxygen. Biogas consists primarily of methane (50–75%) and carbon dioxide (25–50%), along with other minor amounts of gasses. Methane (CH₄) and carbon dioxide (CO₂) are greenhouse gas that, if released into the atmosphere, can contribute to climate change.
Biogas can be utilized as a fuel for internal combustion engines, which convert it to mechanical energy to power an electric generator or other machinery. However, this process is inefficient and can release pollutants into the atmosphere. As a result, more sustainable and cleaner methods of conversion have been developed, such as solid-oxide fuel cells (SOFC).
The biogas-fed solid-oxide fuel cell generates direct current electricity through an electrochemical process as opposed to combustion, which drives a mechanical process. This process is much more efficient than biogas combustion.
By selecting suitable electrolyte and electrode materials, solid-oxide fuel cells can operate at temperatures ranging from low (400–600 ºC) to high (over 1000 ºC) temperatures. The internal reforming reactions of methane in a fuel cell can benefit from operation at intermediate or high temperatures. Moreover, direct heat exchange between the endothermic internal reforming reactions and the exothermic electrochemical reactions in the fuel cells increases the overall system efficiency. Additionally, biogas fuel cells produce extremely low atmospheric emissions, making them an environmentally friendly option.
There are some challenges associated with using solid-oxide fuel cells to convert biogas into electricity. Due to the use of biogas rather than high-purity refined natural gas, the electrical conversion efficiency of these devices is typically 30-40%. The power density of biogas-fueled SOFC is lower than these fueled by hydrogen. The presence of impurities in biogas can also affect the performance of SOFCs.
Reverion has developed a biogas-fueled solid-oxide fuel cell system that doubles the electricity production of existing biogas plants and stabilizes the power grid in the reversible electrolysis mode by producing renewable gasses of methane or hydrogen with zero carbon emissions. The efficiency of the fuel cell system can achieve about 80% by a high percentage of the utilization of fuel supplied to the anode of the fuel cell and heat utilization.
Reversion biogas fuel cell system
The diagram below depicts the Reverion biogas fuel cell system. The (reversible) fuel cell system mainly comprises a fuel cell stack, an anode inlet conduit, and an anode exhaust conduit.
- Anode inlet conduit
Before entering the anode inlet conduit, the mixture of biogas and anode exhaust is passed through a CO₂ separator to remove CO₂ from the gas. After CO₂ removal, biogas consists primarily of methane (CH₄), and the anode exhaust, which has undergone a methanation reaction, a water-gas shift reaction, a water removal unit, consists primarily of CH₄ and hydrogen (H₂). The removal of CO₂ from the feed gas mixture can improve the efficiency of the fuel cell and prevent the formation of solid carbon in the fuel cell.
After passing through a compressor and two heat exchangers, the hot feeding gas mixture enters the anode chamber of the fuel cell stack, where chemical reactions produce electricity.
- Fuel cell stack
The stack’s fuel cell consists of an anode, a cathode, and a solid-oxide electrolyte layer between the anode and cathode. CH₄ and H₂ are fed to the anode. The cathode is fed with air.
The diagram below depicts the chemical reactions that occur in the solid-oxide fuel cell that uses CH₄ and H₂ as anode-feeding fuel. Since there is initially only few water steam and CO₂ present (less than 3 volume%) at the anode inlet, only water and CO₂ produced during the fuel cell reaction are provided for the internal reforming. The hot anode exhaust mainly contains H₂O, CO, and CO₂.
The fuel cell with only CH₄ and H₂ as fuel can reduce thermal stress, resulting in a longer fuel cell lifetime. In addition, the efficiency is greatly improved because the reforming reactions consume a significant portion of the product molecules that would otherwise dilute the fuel stream.
- Anode exhaust conduit
The anode exhaust conduit is equipped with a water-gas shift reactor, a methanation reactor, a water vapor condenser, and a carbon dioxide separator in order to effectively utilize the fuel.
The water-gas shift reactor produces H₂ from anode exhaust via the chemical reaction:
CO + H₂O → CO₂ + H₂
The methanation reactor produces methane from anode exhaust via the chemical reaction:
CO₂ + 4H₂ → CH₄ + 2H₂O
The water-gas shift reactor is arranged upstream from the methanation reactor. There is a controlled share of gas flow between the two reactors, so that the hydrogen to carbon ratio of the anode exhaust can be controlled appropriately to prevent the formation of solid carbon in the fuel cell. The formation of solid carbon would cause side reactions and lead to carbon corrosion of the fuel cell, thereby decreasing the fuel cell efficiency.
Downstream of the methanation reactor, there is a water vapor condenser followed by a carbon dioxide separator to separate water and CO₂ from the anode exhaust. The removal of water and CO₂ can greatly improve the efficiency of the fuel cell system and prevent the formation of solid carbon. When pure CO₂ is captured, the overall process becomes drastically carbon-negative and enables cost-competitive carbon removal from the atmosphere.
The feed gas mixture is compressed in the compressor and reaches the first heat exchanger. This heat exchanger transfers heat from the anode exhaust gas that has undergone the methanation reaction and the water-gas shift reaction to the feed gas mixture. Thus, the feed gas mixture is initially preheated from about room temperature (20 ºC) to 300 ºC.
The heated feed gas mixture is then introduced into the second heat exchanger. This heat exchanger transfers heat from the 630 ºC anode exhaust that exits directly from the fuel cell after internal reformation reaction to the feed gas mixture. Thus, the feed gas mixture is further preheated to about 580 ºC.
The hot feed gas mixture then enters the anode chamber of the fuel cell, where it undergoes the internal reformation reaction. The high temperature can prompt the reaction. The fuel cell reaction also releases heat that is effectively consumed by the reformation reaction.
Anode exhaust exits at about 630 ºC. This heat is used to preheat the feed gas mixture that is supplied to the anode inlet via the heat exchanger. The cooled anode exhaust then enters the water-gas shift reactor and methanation reactor. Since both the water-gas shift reaction and methanation reaction are exothermic, the low temperature of the anode exhaust can promote these reactions.
The methanation reaction and the water-gas shift reaction both produce heat as a side-effect of these exothermic reactions. This heat can be used to produce water vapor from water, which can then be used to drive the turbine and generate extra electrical power.
The anode exhaust gas after the methanation reactor and the water-gas shift reactor further transfers heat to the feed gas mixture via the heat exchanger, so that the feed gas mixture is preheated for the first time and the temperature is increased from about room temperature (20 °C) to about 300 °C, as described above.
The temperature of the anode exhaust further drops to about 80 ºC, and water vapor is condensed and separated in the water vapor condenser. This further reduces the temperature of the anode exhaust to about 30 ºC. Carbon dioxide is separated and the resultant treated anode exhaust is then again mixed with biogas entering the fuel inlet conduit.
Air supplied to the cathode is preheated by a heat exchanger, which transfers heat from the cathode exhaust to the air, so that thermal energy at the cathode side of the fuel cell is effectively used.
Performance of biogas fuel cell system
Reverion’s biogas fuel cell system is highly efficient. The fuel cell electrical efficiency can achieve about 80%, meaning that 80% of the heating value of the feeding fuel is converted into electric energy in the fuel cell.
Additionally, around 7% electric power can be produced by the steam cycle. On the other hand, the auxiliary consumption, mainly originating from the carbon dioxide separation, may require 5-7% electric power, such that overall the system will typically have an electrical efficiency of around 80% of the hydrocarbon feed gas lower heating value.
Reversion’s fuel cell system can operate reversibly. When there is surplus of renewable electricity, the fuel cell can perform electrolysis reactions to produce green hydrogen or methane.
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Reversion has developed a 10kW prototype operating at a biogas plant and successfully validated the technology over more than 1,500 hours under real operating conditions. The company aims to build, commission, and pilot the first full-scale 100 kW unit by 2023 and to ramp up production in order to put many 100 kW units into operation.
Reverion power plant
Reverion’s power plants are designed to be modular and housed in 20-foot shipping containers, making them easy to transport and install. Reversion power plants can double the output of existing biogas plants, by achieving 80% electrical efficiency.
Reverion has raised a total of €9.5M in funding over 3 rounds, including a Seed round, a Non-equity Assistance round, and a Grant round. Their latest funding was raised on Dec 19, 2022 from a Grant round.
Reverion is funded by 8 investors, including European Innovation Council, SpinLab – The HHL Accelerator, Possible Ventures, Federal Ministry for Economic Affairs and Energy (BMWi), Doral Energy-Tech Ventures, Extantia Capital, European Social Fund (ESF), and LANDWÄRME. European Innovation Council and SpinLab – The HHL Accelerator are the most recent investors.
Stephan Herrmann is CEO.