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-
TYPES OF FUEL CELLS
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.
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
Left: 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
TAB. I REFORMING METHOD
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.