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Scheme of a molten-carbonate fuel cell
Molten-carbonate fuel cells (MCFCs) are high-temperature fuel cells, that operate at temperatures of 600ºC and above. They have the highest efficiencies of any type fuel cell, including solid oxide fuel cells, proton exchange membrane fuel cells and phosphoric acid fuel cell and are not subject to the high-temperature material issues that affect solid-oxide technology.
Molten carbonate fuel cells (MCFCs) are currently being developed for natural gas and coal-based power plants for electrical utility, industrial, and military applications. MCFCs are high-temperature fuel cells that use an electrolyte composed of a molten carbonate salt mixture suspended in a porous, chemically inert ceramic matrix of beta-alumina solid electrolyte (BASE). Since they operate at extremely high temperatures of 650°C (roughly 1,200°F) and above, non-precious metals can be used as catalysts at the anode and cathode, reducing costs.
Improved efficiency is another reason MCFCs offer significant cost reductions over phosphoric acid fuel cells (PAFCs). Molten carbonate fuel cells can reach efficiencies approaching 60 percent, considerably higher than the 37-42 percent efficiencies of a phosphoric acid fuel cell plant. When the waste heat is captured and used, overall fuel efficiencies can be as high as 85 percent.
Unlike alkaline, phosphoric acid, and polymer electrolyte membrane fuel cells, MCFCs don't require an external reformer to convert more energy-dense fuels to hydrogen. Due to the high temperatures at which MCFCs operate, these fuels are converted to hydrogen within the fuel cell itself by a process called internal reforming, which also reduces cost.
Molten carbonate fuel cells are not prone to carbon monoxide or carbon dioxide "poisoning" —they can even use carbon oxides as fuel—making them more attractive for fueling with gases made from coal. Because they are more resistant to impurities than other fuel cell types, scientists believe that they could even be capable of internal reforming of coal, assuming they can be made resistant to impurities such as sulfur and particulates that result from converting coal, a dirtier fossil fuel source than many others, into hydrogen.
The leading manufacturer of MCFCs is FuelCell Energy.
The primary disadvantage of current MCFC technology is durability. The high temperatures at which these cells operate and the corrosive electrolyte used accelerate component breakdown and corrosion, decreasing cell life. Scientists are currently exploring corrosion-resistant materials for components as well as fuel cell designs that increase cell life without decreasing performance.
MTU fuel cell
The German company MTU Friedrichshafen presented on the Hannover Fair 2006 a MCFC. The unit weighs 20t and can produce 240kW electric power out of different gaseous fuels, biogas included. The waste air is so clean, that it is not classified as a polluting exhaust fume. The fume temperature is 400 degrees Celsius, enough to be used for many industrial processes. Another possibility is to make more electric power via a Steam turbine. Depending on feed gas type, the electric efficiency is between 42 and 49%. A steam turbine can increase this by up to 64%. The unit can be used for cogeneration.
- MTU 240kW fuel cell presented on the Hannover Fair 2006
Categories: Fuel cells | Environment | Sustainability | Sustainable technologies | Climate change