The molten carbonate fuel cell (MCFC) gets its name because it uses molten carbonate salt as the electrolyte. MCFCs are high-temperature fuel cells in which the electrolyte is suspended in a porous, chemically inert ceramic matrix. They are currently being operated on natural gas and other fuels containing methane in stationary applications in the wastewater treatment, industrial, hotel and government markets.
They must operate at the extremely high temperature of 650°C (roughly 1,200°F) or above to obtain good conductivity of the electrolyte. This enables the use of non-precious metals as catalysts at the anode and cathode, reducing costs and enables reformation of gaseous fuels to hydrogen within the fuel cell eliminating the need to supply hydrogen to the fuel cell. MCFCs can reach efficiencies approaching 60 percent; when the waste heat is captured and used, overall fuel efficiencies can be as high as 85 percent.
The anode process involves a reaction between hydrogen and carbonate ions (CO3=) from the electrolyte which produces water and carbon dioxide (CO2) while releasing electrons to the anode. The cathode process combines oxygen and CO2 from the oxidant stream with electrons from the cathode to produce carbonate ions which enter the electrolyte. The need for CO2 in the oxidant stream requires a system for collecting CO2 from the anode exhaust and mixing it with the cathode feed stream.
The electrolyte in this fuel cell is usually a combination of molten alkali (Na, K, Li) carbonates, the composition of the electrolyte varies, but typically consists of lithium carbonate and potassium carbonate. The electrolyte is suspended in a porous, insulating and chemically inert lithium aluminum oxide (LiAlO2) ceramic matrix. The anode is a highly porous sintered nickel powder, alloyed with chromium to prevent agglomeration and creep at operating temperatures. The cathode is a porous nickel oxide material doped with lithium. At the operating temperature of about 1200°F (650°C), the salt mixture is liquid and a good ionic conductor.
The MCFC also produces excess heat at a temperature which is high enough to yield high pressure steam which may be fed to a turbine to generate additional electricity. In combined cycle operation, electrical efficiencies in excess of 70% (HHV) are possible with mature MCFC systems.
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. Early MCFCs suffered from lack of durability. The high temperatures at which these cells operate and the corrosive electrolyte used accelerated component breakdown and corrosion, decreasing cell life. These problems are being overcome as experience is gained in real world operating conditions, but remain a concern.
Fuel Cell Energy, Inc of Danbury, CT is the only company developing and manufacturing MCFCs, having started development over 30 years ago as Energy Research Corp. Their products are called Direct Fuel Cells (DFC) because they can use hydrocarbon fuels without the need to first create hydrogen in an external fuel processor. They claim that their fuel cells are the most efficient fossil fuel generators in the size range that they produce. They produce three module sizes; 250 Kw, 1 Mw and 2 Mw. The 250 Kw model is contained in a single shipping container while the large models are skid mounded and interconnected at the installation site.
The four largest repeatable markets for their products (megawatts installed or in backlog) are: wastewater treatment (4.25 MW), hotels/hospitality 2.75 MW), manufacturing (2.25 MW) and government/institutional 2.25 MW). Their geographical markets (megawatts installed or in backlog) are: Japan/Korea (8.25 MW), California (7.00 MW), Europe (4.25 MW) and other U.S. (4.5 MW).
Their wastewater treatment installations are especially successfully with their ability to run at high availabilities on the methane produced by the digestors.
The product is maturing with availability of about 80% in 2002, increasing to 93% in FY 2005 and expected to be fully mature in 2007 with an availability of 95%. Progress can be attributed to the Company's rigorous product testing program; resolution of earlier design issues with ceramic seals; a 24/7 call center and regional service teams in the U.S. and Japan to provide rapid response; and a robust data-gathering infrastructure to capture lessons learned, identify problems and drive root-cause corrective actions. The Company's key product development focus is to extend stack life from the current three years to five years and longer.
In addition to increasing the availability of their products they have an active cost reduction program which reduced costs of the 1 MW plant by 30% to approximately $4,300 per Kw in FY 2005 and 25% cost reductions on their 250 Kw model reduced its cost to $4,600 per Kw. They expect that costs will have to be reduced to $2,000 to $3,000 per kW, without subsidies, in order to be competitive with other generators.
They have a manufacturing facility capable of producing 50 MW of production. They believe (October 2004) that they can achieve operating break-even at a production volume of 100 MW. Fiscal 2004 production was approximately 6 MW. In FY 2005 sales of commercial products increased to $17.4 million from $12.6 million in 2004.
They are also developing a combined cycle system operating with an unfired gas turbine with an overall electrical efficiency of about 75%. They are currently testing the 2nd generation prototype of this system. A novel feature of their system is that the fuel cell does not need to operate at the turbine pressure, instead it operates at the preferred ambient pressure of the fuel cell and is independent of gas turbine cycle pressure ratio. The system works efficiently with a wide range of turbine compression ratios (3 to 15). This means that in principle the concept can be applied from the multi-MW scale (with industrial size turbines operating at 9 to 16 pressure ratios), to smaller systems using microturbines at a lower pressure ratio. The turbine does not add any fuel requirement to the system, but boosts the power output by up to 15%.