Molten Carbonate Fuel Cell

Prepared By:-

Nainesh M Patel

(13MCHN01)

Contents

• Introduction• Working MCFC.• Performance of Cell

• Effect of Pressure• Effect of Temperature• Effect of Reactant Gas Composition and Utilization• Effect of impurities

• Advantages & Disadvantages• Reference

INTRODUCTION

• The molten carbonate fuel cell operates at approximately 650 °C (1200 °F).low-cost metal cell components.

• A benefit associated with this high temperature is that noble metal catalysts are not required for the cell electrochemical oxidation and reduction processes.

• Molten carbonate fuel cells are being developed for natural gas and coal-based power plants for industrial, electrical utility, and military applications.

CELL REACTIONS

WORKING• A molten carbonate (MC)fuel cell consists of two flow field plates an anode a molten carbonate electrolyte and a cathode hydrogen is directly through channels in the flow field plate and feeds into the “anode” or negatively charged electrode• Oxygen and carbondioxide feed into the “cathode” or positively charged electrode.

WORKING MODEL

Anode :Ni-Cr/Ni-Al/Ni-Al-CrCathode : lithiated NiO-MgOElectrolyte : molten carbonate carbonate ion (Co3--)

• When the hydrogen reaches the anode, the catalyst encourage it to split into positively charged protons and negatively charged electrons.

• The negatively charged electrons are not allowed through the membrane.

• They are diverted so must go through an external circuit generating electricity.

• When the electrons enter the cathode they are combined with oxygen from the air and carbon dioxide recycled from the used fuel.

• These molecules from a carbonate ion (Co3--).

• The negatively charged carbonate ions then move through the electrolyte to the anode where they combined with the protons to maintain the charge balance.

• This is only possible if the electrolyte is very hot, above 600 degree Celsius.

• The byproducts at the anode are carbon dioxide and water.

• The carbon dioxide is separated and recycled to the cathode side.

• Some of the heat produced in the process is exhausted with the water in the from of vapor. A cooling system remove the rest of it.

• The obtain desired amount of electrical power individual fuel cells are combined in to fuel cell “stacks”.

• A typical stack may consist of hundreds of fuel cells.

• Increasing the number of cells in a stack increases the voltage.

Performance

Cell performance for any fuel cell is a function of

• Pressure• Temperature• Reactant gas composition & Fuel utilization• Impurities

Effect of Pressure

• Increasing the operating pressure of MCFCs results in enhanced cell voltages because of the…

• Increase in the partial pressure of the reactants.• Increase in gas solubility.• Increase in mass transport rates.• Opposing the benefits of increased pressure are the effects of

pressure on undesirable side reactions such as carbon deposition

2CO → C + CO2• And methane formation (methanation)

CO + 3H2→ CH4 + H2O

• The addition of H2O and CO2 to the fuel gas modifies the equilibrium gas composition so that the formation of CH4 is not favored.

• Increasing the partial pressure of H2O in the gas stream can reduce

• The change in voltage as a function of pressure change was expressed as

• was based on a load of 160 mA/cm2 at a temperature of 650 °C. • It was also found to be valid for a wide range of fuels and for a

pressure range of 1 atmosphere ≤ P ≤ 10 atmospheres.

Effect of Temperature

• Increases with temperature change in equilibrium composition

a - P is the partial pressure computed from the water gas shift equilibrium of inlet gas with composition 77.5 percent H2/19.4 percent CO2/3.1 percent H2O at 1 atmosphere.b - Cell potential calculated using Nernst equation and cathode gas composition of 30 percent O2/60 percent CO2/10 percent N2.c - Equilibrium constant for water gas shift reaction

• Changes with temperature and utilization to affect the cell voltage.

• Change in the equilibrium gas composition with temperature.

• The partial pressures of CO and H2O increase at higher T because of the dependence of K on T.

• Change in gas composition, and the decrease in E° with increasing T, is that E decreases with an increase in T.(E° =equilibrium electrode potential)

• Operating cell, the polarization is lower at higher temperatures

Effect of Reactant Gas Composition and Utilization

• The voltage of MCFCs varies with the composition of the reactant gases.

• The effect of reactant gas partial pressure, however, is somewhat difficult to analyze.

• As reactant gases are consumed in an operating cell, the cell voltage decreases in response to the polarization (i.e., activation, concentration) and to the changing gas composition.

• These effects are related to the partial pressures of the reactant gases.

Effect of Impurities

• Sulfur• The tolerance of MCFCs to sulfur compounds is strongly

dependent on temperature, pressure, gas composition, cell components, and system operation (i.e., recycle, venting, gas cleanup).

• <10 ppm H2S in the fuel can be tolerated at the anode.• The adverse effects of H2S occur because of:

• Chemisorption on Ni surfaces to block active electrochemical sites,

• Poisoning of catalytic reaction sites for the water gas shift reaction, and

• Oxidation to SO2 in a combustion reaction, and subsequent reaction with carbonate ions in the electrolyte.

Nitrogen Compounds

• NH3 and HCN do not appear to harm MCFCs in small amounts.

• NH3 tolerance of MCFCs was 0.1 ppm.

Effect

Advantages

• Support spontaneous internal reforming of light hydro-carbon fuels

• Generate high-grade waste heat • Have fast reaction kinetics (react quickly) • Have high efficiency • Do not need noble metal catalysts

Disadvantages

• Have a high intolerance to sulfur. The anode in particular cannot tolerate more than 1-5 ppm of sulfur compounds (primarily H2S and COS) in the fuel gas without suffering a significant performance loss.

• Have a liquid electrolyte, which introduces liquid handling problems.

• Require a considerable warm up period

Reference • http://www.fuelcellenergy.com/why-fuelcell-energy/types-of-fuel-cells/• http://www.ceret.us/HydrogenFuelcells/MC_FC.html (Animation)• http://en.wikipedia.org/wiki/Molten_carbonate_fuel_cell• Fuel Cell Handbook (Seventh Edition) By EG&G Technical Services, Inc.• Module 4 fuel cell technology :collage of the desert.

Thank you..