Molten carbonate cells

A fuel cell cycle employing carbonate ions penetrating a solid matrix electrolyte at high temperatuures is schematically illustrated in Fig. 3.16. It is aimed at stationary applications and promises high efficiency. The electrode reactions for this electridty-produdng molten carbonate fuel cell (MCFC) are 

The carbon dioxide from (3.63) is in most designs recycled as input to (3.62) together with the hydrogen fuel. It has been suggested that CO2 emissions from fossil power plants could be used as input as a way of reducing greenhouse gas emissions from the current system. However, then the output CO2 

The electrolyte constitutes a separator for the gas flows shown in Fig. 3.16, in addition to providing ionic transport. Physically, it consists of a compressed powder behaving as a soft paste at the operating temperatures, thus substantiating the name molten carbonate for the liquid Li-Na or Li-K carbonates inhabiting the electrolyte substrate. The LiAlOj material used for the support structure can exist in three different forms (phase a, P and fj, and transitions have been observed (e.g., yto d) after extended fuel cell operation 

Simple models for performance simulation have been developed for stationary flow situations, with the options of verifying the voltage-current relations and calculating the overall efficiency imder consideration of total plant auxiliary power and heat inputs. The efficiency calculated with such models exhibits a few percent efficiency improvement from raising the pressure above ambient by the factors considered above and, as expected, a decline with increased gas flow rates (Simon et al, 2003). 

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Cathode materials of both high-temperature cells are composed of chemically and morphologically relatively stable oxide ceramics. For molten carbonate cells, lithiated NiO is used and the cathode of oxide ceramic cells usually is made of porous LaMnOj. 

In multiple-stack installations, it is important to control the performance of each stack separately to ensure that one stack cannot discharge into another. This is necessary, because the manufacturing of identical stacks is just about impossible with the current means of manufacturing in the industry. This is particularly a problem for active anode SOFCs and molten carbonate cell designs, because the 02 drawn through the cell electrolyte can oxidize and destroy the catalytic ability of the cell. 

There are four types of fuel cells in development. They differ in the electrolyte they use, but the mechanical and chemical fundamentals are similar. The electrolytes under investigation are Phosphoric Acid, Molten Carbonate, Solid Oxide and Solid Polymer. The Phosphoric acid cells operate at temperatures of 180 to 210 degrees Celsius. Molten carbonate cells operate at 600 to 700 degrees Celsius. Solid oxide Cells operate at 650 to 1000 degrees Celsius. These temperatures are uncomfortably high for home use and impractically high for automotive use. Only the Solid Polymer cells operate at a temperature range, 80 to 100 Celsius, a suitable for use in the home or automobile. 

The fuel cell has already proved its usefulness in space technology and there are excellent prospects for its commerical application. Application on a large scale is not expected during the 20th century. The alkaline cell and the phosphoric acid cell are technically well developed, but from a commerical point of view it is questionable whether or not they will be of interest when other types reach technical maturity. The molten carbonate cell and the solid oxide cell seem to have the best prospects. For mobile application the solid polymer cell is a strong candidate. 

To the present time, it has not been possible to obtain a reasonable performance from anodes with a primary fuel feed, and the candidate systems are based on H2 (as in phosphoric acid cells at 573K) or H2 + CO (molten carbonate cells at 973K). 

Molten carbonate cells have been operated on a variety of fuels carbon monoxide, hydrogen, kerosene and a variety of hydrocarbon gases mixed with steam. The reaction with carbon monoxide is... 

There are basically three types of construction of molten carbonate cells. In the most developed type, the electrolyte is contained in a porous diaphragm of magnesia. This type of construction reduces corrosion, but increases electrolyte resistance. The second type, that of free flowing electrolyte, has not been developed because of the serious corrosion problem at the temperatures of operation of these cells. The third type uses electrolyte which is mixed with magnesia powder to a stiff paste. This structure seems to have the merits of both the other structures in that it reduces corrosion without much affecting the resistance of the cell. 

The main factor that limits the life of molten carbonate cells is that of deterioration of the construction materials. Unless these cells can be more cheaply produced or of much longer life, it seems that they will be abandoned in favour of the solid oxide cells which will be described in the next section. Another disadvantage of the molten carbonate cells is the high temperature of operation. Experimental cells using concentrated aqueous acid solutions and operated at less than 200°C will be described presently. 

Molten Carbonate Fuel Cell. The electrolyte ia the MCFC is usually a combiaation of alkah (Li, Na, K) carbonates retaiaed ia a ceramic matrix of LiA102 particles. The fuel cell operates at 600 to 700°C where the alkah carbonates form a highly conductive molten salt and carbonate ions provide ionic conduction. At the operating temperatures ia MCFCs, Ni-based materials containing chromium (anode) and nickel oxide (cathode) can function as electrode materials, and noble metals are not required. 

Hydrogen use as a fuel in fuel cell appHcations is expected to increase. Fuel cells (qv) are devices which convert the chemical energy of a fuel and oxidant directiy into d-c electrical energy on a continuous basis, potentially approaching 100% efficiency. Large-scale (11 MW) phosphoric acid fuel cells have been commercially available since 1985 (276). Molten carbonate fuel cells (MCFCs) ate expected to be commercially available in the mid-1990s (277). 

Molten Carbonate Fuel Cell The electrolyte in the MCFC is a... 

Molten Carbonate Fuel Cell developed by Baur (1921)... 

The PAFC is, however, suitable for stationary power generation, but faces several direct fuel cell competitors. One is the molten carbonate fuel cell (MCFC), which operates at "650°C and uses an electrolyte made from molten potassium and lithium carbonate salts. Fligh-teinperature operation is ideal for stationary applications because the waste heat can enable co-generation it also allows fossil fuels to be reformed directly within the cells, and this reduces system size and complexity. Systems providing up to 2 MW have been demonstrated. 

As the name suggests these cells use an electrol5de of molten carbonates (generally of lithium and potassium) and in order to keep the carbonates molten and provide good conductivity the cells need to operate at around 650 °C. This type of cell is becoming increasingly favoured for commercial power production. The moderate operating temperature means that... 

The Surface Fractal Investigatioii of Anode Electrode of Molten Carbonate Fuel Cell... 

In order to describe the geometrical and structural properties of several anode electrodes of the molten carbonate fuel cell (MCFC), a fractal analysis has been applied. Four kinds of the anode electrodes, such as Ni, Ni-Cr (lOwt.%), Ni-NiaAl (7wt.%), Ni-Cr (5wt.%)-NijAl(5wt.%) were prepared [1,2] and their fractal dimensions were evaluated by nitrogen adsorption (fractal FHH equation) and mercury porosimetry. These methods of fractal analysis and the resulting values are discussed and compared with other characteristic methods and the performances as anode of MCFC. 

W. He and Kas Hemmes, Operating characteristics of a reformer for molten carbonate fuel-cell power-generation systems. Fuel Processing Technology, 67 (2000) 61. 

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