• Document: Carbon Dioxide Membrane Separation for Carbon Capture using Direct FuelCell Systems
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Carbon Dioxide Membrane Separation for Carbon Capture using Direct FuelCell Systems DFC Technology Used as Electrochemical Membrane for CO2 Purification and Capture during Power Generation FCE’s Direct FuelCell (DFC) is based on the carbonate fuel cell technology, where electrochemical reactions are supported by an electrolyte layer in which carbonate ions serve as the ion bridge that completes the electrical circuit. A side effect of this basic aspect of the technology is that carbon dioxide introduced at the air electrode is converted to carbonate ions and transferred through the electrolyte layer to the fuel electrode, where it is converted back to CO2. This means that a DFC stack can be used as a carbon purification membrane – transferring CO2 from a dilute oxidant stream to a more concentrated fuel exhaust stream. This has the potential to solve a problem that the US Department of Energy (and other agencies around the world) has been struggling with for years: is there a way to concentrate and capture the CO2 in the exhaust of large coal or natural gas powerplants to avoid the harmful greenhouse effects that these exhaust gasses cause? Conventional technologies that are being considered for carbon capture are expensive and have high power needs, consuming a significant fraction of the power output of the fossil plant they are trying to clean up. FCE is developing a carbon capture system which uses DFC stacks as CO2 concentration systems. Instead of consuming power, the DFC Carbon Capture system produces additional clean power – an added value stream which is key to reducing the cost of the carbon capture process. Direct FuelCell Electrochemical Reactions Carbonate ion transfer supports electrochemical reaction of hydrogen at anodes and oxygen at cathodes, creating cycle of CO2 production at anode and CO2 consumption at cathode The DFC Carbon Capture concept is an extension of the standard DFC system design, as illustrated below. In a standard DFC powerplant, CO2 produced at the anode is recycled back to the cathode by mechanical systems in the balance of plant. If the concentrated CO2 in the anode exhaust stream is extracted from the system and not recycled back to the cathode, an external source of CO2 can support the cathode reaction. This external source can be the exhaust from another powerplant or an industrial source. The dilute CO2 in the external flue gas will be reacted at the DFC cathodes and transferred to the anode stream, from which it can be easily separated for sequestration or utilization. In the standard system, a hydrocarbon fuel (e.g. natural gas or biogas) is sent to the anodes and reformed to hydrogen. Most of the hydrogen is consumed in the anode power production reaction. The anode exhaust contains residual hydrogen, any CO2 from the input fuel, and the CO2 produced as a result of the carbonate ion transfer. The anode exhaust is mixed with fresh air and sent to a catalytic oxidizer, where the residual hydrogen is used to heat the oxidant stream up to the stack temperature. The cathode consumes oxygen from the air and the CO2 from the carbonate ion transfer. Water vapor, residual oxygen, nitrogen and the CO2 from the input fuel pass through the cathode to the system exhaust. System Comparisons CO2 from powerplant or industrial source is sent to cathode, transferred and concentrated in anode, and removed from anode exhaust The modification for carbon capture involves cooling the anode exhaust and separating most of the CO2 from the exhaust stream. Since most of the CO2 is removed from the anode exhaust, the CO2 needed for the cathode reaction is provided by the exhaust of the external source. If this source is a conventional coal fired plant the CO2 concentration will be in the range of 12 to 15 percent. An advanced Integrated Gasification Combined Cycle coal plant will have 7 to 8% CO2 in its exhaust. A large scale combined cycle natural gas powerplant will have as little as 5% CO2 in its exhaust. Separating CO2 from these dilute streams is difficult, but once the CO2 is sent to the fuel cell cathodes it is transferred to the anode exhaust stream, which has a CO2 concentration of about 70%, and most of the balance is water, so it is very easy to remove CO2 from this stream. Using DFC powerplant stacks for this purpose has ancillary benefits beyond the CO2 capture. One benefit is that since the fuel cell product water is condensed and removed while separating CO2 from the anode exhaust, the DFC powerplant is a net water producer. This can reduce cost and environmental impact since many of the CO2 source systems are significant water consumers. Another benefit is that a large percentage of any NOX in the source powerplant will be destroyed as it flows through the DFC stacks. While NOX is usually not present in the cathode inlet stream (which is mostly fresh air), it has been shown that if flue gas containing NOX is used instead of fresh air, m

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