FUEL CELLS – MOLTEN CARBONATE FUEL CELLS | Full-scale Prototypes

Abstract

The carbonate fuel cell power plant is an emerging high-efficiency, ultraclean electric power generation system utilizing a variety of gaseous, liquid, and gasified solid carbonaceous fuels such as coal for commercial and industrial applications. The power-producing component of this system is the carbonate fuel cell, which uses alkali metal carbonate mixtures as the electrolyte immobilized in a porous ceramic matrix. The fuel cell operates at 550–650 °C. The fundamental understanding and the state-of-the-art material and electrolyte choices are based on extensive research carried out in Japan, Europe, and the United States in the 1960s and the early 1970s. Present-day carbonate fuel cell construction employs commonly available stainless steels. The electrodes are based on nickel and fabricated using well-established manufacturing processes including sheet metal forming, tape casting, and low-temperature sintering. Commercial product development efforts began in the late 1990s, leading to prototype field tests and commercial customer applications beginning in the current decade. Cost reduction has been an integral part of the product development effort. Cost-competitive product designs have evolved as a result. Currently, seven teams around the world are pursuing carbonate fuel cell product development. The power plant development efforts to date have mainly focused on several hundred kilowatt- (submegawatt) up to megawatt-class plants, with more than 70 power plants placed at customer sites in the United States, Europe, and Asia. The worldwide carbonate fuel cell power generation capacity in early 2007 was >17 MW and growing. These units are mostly operating on pipeline natural gas, biogas, and liquefied natural gas. Power plants that operate on propane and methanol have also been demonstrated. The commercial units are operating at 45–49% efficiency in single cycle operation. Overall thermal efficiency approaching 90% has been achieved in combined heat and power applications where high-grade (process heat) and low-grade (hot water) heat recovery have been considered. Carbonate fuel cell products are also being developed to operate on coal-derived gases, diesel, and military logistics fuels. Innovative carbonate fuel cell/turbine hybrid power plant designs promising ultra-high energy conversion efficiencies approaching 70% have also emerged. A pilot-size 300 kW unit of this design has already demonstrated a record electricity conversion efficiency of 56% (lower heating value) at a hospital site in Montana, USA. The salient features of the carbonate technology, its development history, the key developers, and their commercialization status are discussed in this article.