The Molten Carbonate Fuel Cell (MCFC)

The MCFC takes its name from its electrolyte of alkali car­bonates, which is used in molten form at the operating tem­perature of 650° C. The electric current is carried in the electrolyte by carbonate (CO32-) ions. Maintaining the charge transport within the electrolyte thus requires CO2 circulation between the fuel gas used and the air, which is usually achieved by using the CO2-containing anode ex­haust gas. The peripheral system components such as con­necting tubes and heat exchangers require only a limited high-temperature serviceability; however, the extremely ag-


Fuel cells can be classified as low – temperature and high-temperature cells. Low-temperature fuel cells oper­ate in a range from room temperature up to about 120° C, while high-temper­ature fuel cells require an operating temperature between 600 and 1000° C. The phosphoric-acid fuel cell, with an operating temperature around 200° C, lies between these two rough groups.

Подпись: characteristic properties of the corre-sponding fuel cells, such as their operat-ing temperatures and conductivity mechanisms. Directly related to this is the choice of usable catalysts, require-ments for the process gas, etc. Low-temperature fuel cells require noble-metal catalysts in their electrodes (platinum or noble-metal alloys), in order to activate the electrochemical reaction. These catalysts are in general sensitive to CO poisoning at low tem-peratures. The higher operating temperature of the PAFC, in contrast, allows it to tolerate CO concen-trations of 1-2 %, so that instead of pure hydrogen, so-called reformer gas can be directly input to the cells. The high operating temperatures of the MCFC and the SOFC make these cells insensitive towards CO and even allow the direct reaction of methane (natural gas). Подпись:Подпись:Fuel cells are named for the type of electrolyte they use. There are alkaline fuel cells (AFC), the PEMFC (polymer – electrolyte membrane fuel cell), the molten carbonate fuel cell (MCFC), the phosphoric acid fuel cell (PAFC), and the solid oxide fuel cell (SOFC). Only the direct methanol fuel cell (DMFC) does not mention the electrolyte in its name, but instead indicates its ability to react methanol directly as fuel. Typically, it is based on a PEMFC. The different elec­trolytes determine to a large extent the


The Molten Carbonate Fuel Cell (MCFC)

Подпись:Подпись: SOFC TUBE CELLSimage197"image198Left: This 100 kW SOFC plant (Siemens) has achieved a world – record operating life of nearly 40,000 hours (i. e. almost five years). For comparison: In the automotive sector, operating lifetimes of 5,000-10,000 hours are planned. Right: Cross section through a tube­shaped single cell.


Currently, hydrogen is produced on an industrial scale by steam reform­ing. In most cases, a fossil energy carrier such as methane is used as reactant for this endothermal, i. e. heat consuming reaction; it is chemi­cally decomposed. Reforming can also be used as a preliminary step within a fuel-cell system. In the high – temperature cell types SOFC and MCFC, the operating temperature is already sufficient to supply the necessary process heat; this at the same time provides the required cooling of the cells. For low-tempera­ture cells, in contrast, the necessary heat must be supplied externally, which reduces the system efficiency in the overall energy balance.

The resulting gas mixture is called reformer gas or reformate. Its main components in the industrial process are hydrogen, carbon dioxide, water vapor and about one percent of carbon monoxide. The technical alternatives to steam reforming are autothermal reforming and partial oxidation of fossil fuels. In industrial- scale processes, the fuel used is burned sub-stoichio-metrically, i. e. with reduced oxygen input, and provides the required process heat using suitable catalysts. The exother­mal variant is called partial oxidation, while the energy-balanced sum of combustion and reforming is termed autothermal reforming. The latter has gained a certain importance in space heating applications. Operational differences, besides the time for preheating, are the different yields of H2.

As byproduct of the reaction, CO is formed. The CO content at the output of the reformer is lowered by subse­


gressive electrolyte causes corrosion of the cell and stack components.

MCFC installations with output power up to 2.8 MW have already been built; their electrical efficiencies were 47 %, and overall efficiencies (electrical and thermal) of over 85 % were measured. However, in over 20 years of tri­als of these installations, the goals for their operating life­times and costs have not yet been met.

Updated: October 27, 2015 — 12:10 pm