(91b) Carbon Dioxide (CO2) Separation with Molten Carbonate Fuel Cell (MCFC) Systems

Molten carbonate fuel cell (MCFC) systems can be designed to separate carbon dioxide (CO₂) from a dilute stream into a highly-concentrated stream of CO₂ by mass, while also producing electricity. The use of MCFCs in this manner highlights their unique ability among various CO₂ separation techniques to achieve substantial CO₂ separation, while also producing a net amount of electric power. By contrast, other approaches to CO₂ separation consume a net amount of electricity. MCFC systems also are unique among fuel cell systems in that their particular electrochemistry allows them to separate out CO₂. Not all fuel cells can perform this function. MCFCs are special.

At the cathode of a MCFC, oxygen (O₂) and CO₂ react in the presence of electrons that have flowed through an external circuit to form carbonate ions (CO₃ ^2-). The cathode half-reaction is ½ O₂ + CO₂ + 2e^- à CO₃ ^2-. The molten electrolyte conducts CO₃ ^2- from cathode to anode. MCFC systems are typically designed to consume fuels such as natural gas fuel and renewable methane (CH₄), including anaerobic digester gas (ADG). Within the MCFC system, CH₄ is reformed into a hydrogen (H₂) and carbon monoxide (CO) rich gas either through external reforming (a chemical reactor upstream of the fuel cell stack that is typically a steam reformer), or internal reforming (steam reforming reactions that take place on the anode’s surface), or a combination of the two. At the anode, the H₂ electrochemically reacts with CO₃ ^2- that has traversed the electrolyte to form water (H₂O), CO₂, and electrons (e^-). The main anode half-reaction is H₂ + CO₃ ^2- à H₂O + CO₂ + 2e^-. The electrons then flow through an external circuit to the cathode to create an electric current. The CO can electrochemically react at the anode in a similar manner as the H₂. A secondary anode half-reaction is CO + CO₃ ^2- à 2CO₂ + 2e^-.

This process transports the carbonate ion across the electrolyte from cathode to anode, and, thereby, the CO₂ in the cathode inlet stream air mixture is separated and concentrated into a stream of just CO₂ and water at the anode exhaust. This anode exhaust stream can be cooled to condense the water.

Importantly, MCFC technology has been commercialized and is available for use in a variety of applications. The company FuelCell Energy Inc. has installed approximately 200 Megawatts-electric (MW) globally, with about 47 installation operating on natural gas fuel and 38 installations consuming renewable biogas fuel in the state of California (CA) alone.

These MCFC systems are also classified as ultra-low emission generators in CA. These generators achieve this classification in part because the upstream endothermal steam reforming process is not heated by combustion as in a traditional steam reformer, but rather by electrochemical ‘waste heat’ from the high temperature MCFC stack.

These MCFC systems can be sensitive to impurities in the inlet gas, so some upstream gas clean-up processes may be needed.