1 MW Direct FuelCell power plant operating on digester gas at King County, Washington.

Molten carbonate fuel cells are designed to operate at higher temperatures than phosphoric acid or proton exchange membrane (PEM) fuel cells and can achieve higher fuel-to-electricity and overall energy use efficiencies than these low temperature cells.

In a molten carbonate fuel cell, the electrolyte is made up of lithium-potassium carbonate salts heated to about 1,200 degrees F (650 degrees Celsius). At these temperatures, the salts melt into a molten state that can conduct charged particles, called ions, between two porous electrodes.

Molten carbonate fuel cells eliminate the external fuel processors that  lower temperature fuel cells need to extract hydrogen from the fuel, when natural gas is the fuel, methane (the main ingredient of natural gas) and steam are converted into a hydrogen-rich gas inside the fuel cell stack (a process called "internal reforming"). At the anode, hydrogen reacts with the carbonate ions to produce water, carbon dioxide, and electrons. The electrons travel through an external circuit creating electricity and return to the cathode. There, oxygen from the air and carbon dioxide recycled from the anode react with the electrons to form carbonate ions that replenish the electrolyte and provide ionic conduction through the electrolyte, completing the circuit.

Molten carbonate fuel cells can reach fuel-to-electricity efficiencies approaching 50%, considerably higher than the 37-42% efficiencies of a phosphoric acid fuel cell plant. When the waste heat is captured and used, overall thermal efficiencies can be as high as 85 percent.

Improved efficiencies are one reason why molten carbonate fuel cells offer significant cost reductions over phosphoric acid technology. Another is that the electrodes of a molten carbonate fuel cell can be made with nickel catalysts rather than the more costly platinum of phosphoric acid systems. The elimination of the external fuel reformer also contributes to lower costs.

Molten carbonate fuel cell development can be traced back to the late 1950s when Dutch scientists G.H.J. Broers and J.A.A. Ketelaar began experimenting with laboratory-scale fuel cells using electrolytes of fused (molten) carbonate salts. In the mid-1960s, the U.S. Army tested several small molten carbonate fuel cells - 100 watts to 1,000 watts - made by Texas Instruments, that were designed to run on "combat gasoline" using an external reformer.

The U.S. Department of Energy began its cooperative molten carbonate fuel cell research program with the private sector in the 1970s, focusing on scaling up the concept to utility-size stationary power units. By 2003, FuelCell Energy, a molten carbonate developer, had delivered its first commercial unit at the Kirin Brewery plant in Japan.

Today, commercial FuelCell Energy's demonstration and commercial units are operating at over 50 installations worldwide. Most of these are about 250 kW, although multiple units have been combined for larger installations.

Additionally, several other units have been successfully demonstrated, including:

• Renton, Washington, where a 1-megawatt power plant is operating at the King County wastewater treatment facility and fueled by wastewater digester gas.
• Cadiz, Ohio, where Northwest Fuel Development Inc., based on Lake Oswego, Oregon, operated a 250-kilowatt fuel cell on coal-mine methane gas from the Harrison Mining Corporation and supplied electricity to the mining operation.

Meeting Fuel Cell and Distributed Generation (DG) Standards

The molten carbonate fuel cell is one of the distributed generation technologies that received approval under California's Rule 21, which specifies standard interconnection, operating, and metering requirements for DG generators. It aims to significantly reduce the time, cost, and complexity of utility approval with prescriptive requirements jointly developed and accepted by the state's utilities. This Rule 21 standard could serve as a model for utilities and utility commissions in other states seeking to streamline their distributed generation application process.

In 2003, FuelCell Energy's Direct FuelCell (DFC) 300A power plant became state-certified meeting the California Air Resources Board's (CARB) stringent new distributed generation emissions standards for 2007. By meeting this standard, the Company's sub-megawatt DFC power plant is categorized as an 'ultra-clean' technology, exempting it from air pollution control or air quality district permitting requirements by CARB. In addition, this certification qualifies the Company's products for preferential rate treatment by the California Public Utilities Commission (CPUC) such as the elimination of exit fees and standby charges for customer electric generation.

Also in 2003, FuelCell Energy's Direct FuelCell (DFC) 300A power plant was certified to meet the American National Standards Institute (ANSI) products safety standard for stationary fuel cell systems, ANSIZ21.83. Collectively, these three certifications will reduce the time and cost for installation of FuelCell Energy's DFC power plants and enhance the products' acceptance throughout the United States. More recently, the CARB certification has been extended to FuelCell Energy's larger 1 MW model.

Fuel Cell/Turbine Hybrids

DOE and FuelCell Energy have developed and demonstrated an atmospheric molten carbonate DFC combined with an unfired gas turbine integrated in a hybrid system. One of the significant challenges for this technology was the development of high temperature heat exchangers that offer differential pressure operation. This hybrid system uses a network of heat exchangers to transfer waste heat from the DFC system to the turbine, which converts a portion of the waste heat to mechanical energy and then electricity. The system adds 10 to 15 percentage points to the efficiency of the fuel cell system.

The R&D efforts have resulted in significant progress in validating the DFC/Turbine cycle concept. FuelCell Energy has completed successful proof-of-concept testing of a DFC/Turbine power plant based on a 250 kWe DFC integrated initially with a Capstone 30-kWe and later with a 60-kWe modified micro turbine generator. The sub-megawatt system tests have achieved over 6,800 hours of successful operation with efficiency of 52 percent.

The FuelCell Energy hybrids work has provided valuable knowledge concerning complex hybrid fuel cells/turbine systems integration issues that need to be addressed for the development of SECA hybrids.