Fuel Cells

Introduction

A fuel cell is an electrochemical cell that produces power through a reaction, triggered in the presence of an electrolyte, between the fuel (on the anode side) and an oxidant (on the cathode side). The reactants flow into the cell, and the reaction products flow out of it, while the electrolyte remains within it. Currently a couple of fuel cells are available, which can be allocated to different fuel cell families as shown in the table below.

Type	Efficiency	Operating temperature
Solid Oxide	45-65%	800°C
Molten Carbonate	50%	650°C
Phosphoric Acid	40%	200°C
Alkaline	50-60%	80°C
Direct Methanol	40%	80°C
Polymer (PEM)	40%	50°C

A modular design allows for custom power generation and generation close to the load, reducing transmission and distribution losses. Fuel cells are expected to generate power with higher efficiencies (between 40-60%) than turbines. The thermal output for heating can be used and the potential efficiency can rise over 80 percent (cogeneration). Some systems are air-cooled, so water is not needed. Methane driven fuel cells can work with gases in a concentration range between 30 – 100%. Generally CMM powered fuel cells could have some advantages in operation. They can use methane from mine pre-drainage and medium quality gob gas. Also methane diluted with air and/or carbon dioxide can be utilised. The exhaust gas should contain less NO_X and SO₂ compared to internal combustion engines. There are two types of fuel cells:

-solid oxid fuel cell(SOFC)

-Molten carbonate fuel cells (MCFC) These are described in the following

SOFC Fuel Cells

In a solid oxide fuel cell (SOFC) design, the anode and cathode are separated by an electrolyte that is conductive to oxygen ions but non-conductive to electrons. The electrolyte is typically made from zirconium doped with yttrium. In general, on the cathode side, oxygen catalytically reacts with a supply of electrons to become oxygen ions, which diffuse through the electrolyte to the anode side. On the anode side, the oxygen ions react with hydrogen to form water and free electrons. A load connected externally between the anode and cathode completes the electrical circuit. The real SOFC can be fed by other fuels than hydrogen (even without hydrogen at all). The general working principle of SOFC is passing oxygen ions through the electrolyte by the oxygen pressure difference between cathode and anode sides. Solid Oxide Fuel Cells (SOFC) offer the stability and reliability of all-solid-state ceramic constructions. High-temperature operation, up to 1.000°C, allows more flexibility in the choice of fuels and can perform very well in combined-cycle applications. SOFCs approach 60 percent electrical efficiency in the simple cycle system, and 85 percent total thermal efficiency in co-generation applications. According to EPA “at present, fuel cells are economically competitive with conventional forms of electricity generation only in certain cases. Fuel cells are, however, making steady progress toward the goal of widespread commercial use. Use of methane in fuel cells, recovered from gassy coal mines, may be an economical approach to on-site power generation or local use. Gob areas (collapsed rock over mined-out areas) release large volumes of gas and subsequently vent it to the atmosphere. Much of this gas is medium-quality and unsuitable for pipeline injection. However, fuel cells can operate on medium-quality gas, reducing methane emissions to the atmosphere while producing electrical power for on-site use. Because of their high efficiency, the use of fuel cells for power generation emits less carbon dioxide per kilowatt-hour of electricity produced than conventional turbine and internal combustion power generation methods. Solid oxide fuel cell (SOFC) power systems have already demonstrated extremely low emissions (less than 0.5 ppm NO_x, no SO_x, CO or unburned hydrocarbons), making permitting easier and less expensive. Solid Oxide Fuel Cells (SOFCs) are currently being demonstrated in sizes from 1kW up to 250-kW plants, with plans to reach the multi-MW range. A 200 kW SOFC sited at AEP Ohio Coal LLC’s Rose Valley Mine Site in Hopedale, Ohio can be given as an example of use. SOFCs utilize a non-porous metal oxide electrolyte material. SOFCs operate between 650 and 1000°C, where ionic conduction is accomplished by using oxygen ions.” [http://www.epa.gov/coalbed/docs/fuel_cells.pdf “COAL MINE METHANE USE IN FUEL CELLS”, EPA Coalbed Methane Outreach Program Technical Options Series, revised draft March 2004]

MCFC Fuel Cells

Molten carbonate fuel cells (MCFCs) are currently being developed for natural gas and coal-based power plants for electrical utility, industrial, and military applications. Because they are more resistant to impurities than other fuel cell types, scientists believe that they could even be capable of internal reforming of coal, assuming they can be made resistant to impurities such as sulphur and particulates that result from converting coal, a dirtier fossil fuel source than many others, into hydrogen. The primary disadvantage of current MCFC technology is durability. The high temperatures at which these cells operate and the corrosive electrolyte used accelerate component breakdown and corrosion. Thus are decreasing cells' life. Scientists are currently exploring corrosion-resistant materials for components as well as fuel cell designs that increase cell life without decreasing performance. MCFCs are medium high-temperature fuel cells that use an electrolyte composed of a molten carbonate salt mixture suspended in a porous, chemically inert ceramic lithium aluminium oxide (LiAlO2) matrix. Because they operate at extremely high temperatures of 500°C to 650°C (roughly 1,200°F) and above, non-precious metals can be used as catalysts at the anode and cathode, reducing costs. Molten carbonate fuel cells can reach efficiencies approaching 60%, considerably higher than the 37%–42% efficiencies of a phosphoric acid fuel cell plant. When the waste heat is captured and used, overall fuel efficiencies can be as high as 85%. Molten carbonate fuel cells are not prone to carbon monoxide or carbon dioxide "poisoning" —they can even use carbon dioxides as fuel— making them more attractive for fuelling with gases made from coal. In a MCFC methane is used as fuel. It has to be reformed, which can be done in the Indirect Internal Reformer (IIR). The IIR is a flat reactor located between the single cells of the fuel cell stack. Within this reactor the methane reforming as well as the water gas shift reaction take place. The reaction products CO and H₂ are used as fuel in the electrochemical reactions. Further on, the temperature distribution of the fuel cell stack is influenced by the endothermic reactions within the IIR. The temperature in turn is one of the important values to describe the state of the fuel cell stack. The reaction rates (efficiency) as well as the material degradation (life time) depend on the temperature. Therefore a model of the IIR is an important tool for the optimization of the temperature distribution within the fuel cell stack.

Examples of use

Moonlight Project:R&D in Japan is mainly conducted under the Moonlight project. The target of this project is the development of a 1kW external reforming MCFC pilot plant.

Further reading on fuel cells

Fuel cells 2000, "Worlwide Fuel Cell Installations" Update 10/05
Matthias Pfafferodt, Peter Heidebrecht, Kai Sundmacher, Uwe Würtenberger, Marc Bednarz, "Multiscale CFD simulation of a methane steam reformer for optimization of the spatial catalyst distribution" 17th European Symposium on Computer Aided Process Engineesing – ESCAPE17, 2007 Elsevier B.V.
U.S. Department of Energy "Types of Fuel Cells"
"COAL MINE METHANE USE IN FUEL CELLS", EPA Coalbed Methane Outreach Program Technical Options Series, revised draft March 2004
Fuel Cells 2000, "The online Fuel cell Information Resource"