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 NOX and SO2 compared to internal combustion engines. There are two types of fuel cells:

-solid oxid fuel cell

-Molten carbonate fuel cells (SOFC) 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 NOx, no SOx, 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]