Investigation on performance of SOFC in hydrocarbon fuel

Transcription

1 Investigation on performance of SOFC in hydrocarbon fuel S. Senthil Kumar, Sharad Chauhan, B. Shriprakash, S. T. Aruna CSIR National Aerospace Laboratories Bangalore Presentation at COMSOL 2016, Bangalore, Oct

2 Solid Oxide Fuel Cell (SOFC) At the anode: H 2 + 2O 2 2H 2 O + 4e. At the cathode: O 2 + 4e 2O 2 The overall cell reaction: O 2 + 2H 2 2H 2 O Roles of Electrolyte: Oxygen ion conduction Physically seperates the fuel from oxidant Roles of Electrode: Hosts triple phase boundary to support electrochemical reactions Provides path for O 2- ions/electrons Provides channels for gas diffusion Gives mechanical support to system

3 Hydrocarbon Compatible SOFC Most promising clean energy source Expansion of potential fuel range Eliminates fuel preprocessing Increases overall system efficiency Bottle Neck: Carburization of SOFC anodes

4 Genesis of Present work State of art anode cermet is prone to carburization in hydrocarbon fuel An optimum feed/ operating conditions have to be established to minimize carbon deposition. In the typical SOFC anode, carbon deposition is an unsteady state progress Analysis of unsteady variation of porosity and catalytic activity of SOFC anode would be helpful in establishing the durability of SOFC in given fuel feed conditions Objectives To simulate the performance of SOFC with the reformed feed (CH 4 +Steam) using COMSOL To predict the performance degradation over the long duration (3000 h)

5 Physics Involved Secondary current Distribution: Determines current profile. Accounts for the effect of the electrode kinetics and losses due to resistance Transport of Concentrated Species: Determines species flux across electrode. Involves flow of species across the porous electrodes via diffusion and transport of oxide ion Free and Porous Media Flow: Determines flow profile. Accounts for flow in channel and porous media

6 Reactions Involved Methane-steam reforming reaction rate: R r = a(k f,r p CH4 p H2 O k b,r p CO p 3 H2 ) CO water-gas shift reaction rate: R s = a(k f,s p CO p H2 O k b,s p CO2 p H2 ) Methane cracking reaction rate: Boudouard reaction rate: R C = a M C k f,c p CH4 p 2 H 2 K p,c 1 + k H p H2 2 R B = a 1 k f,bk CO p CO 3600M C 1 + K CO p CO + 1 K B p CO2 p CO 1 K CO2 K CO p CO2 p CO 2

7 Mathematical models for Parameter study Porosity variation rate: Where r C = (R C + R B ) Catalyst activity variation rate:. dε dt = εr CM C ρ C da dt = k ar C 2 c C a Permeability: κ = κ 0 ε ε Anode Exchange current densities : RT 11 i 0,H2 = F p H2 O p H exp RT RT 11 i 0,CO = F p CO p CO exp RT

8 Mole Fraction Efficiency in % Effect of Steam S/C=0.1 H K 900K 1000K CH4 C(s) CO2 CO K 1200K Temperature (K) S/C ratio Complete utilization of methane after >1023 K Efficiency decreases with increasing S/C Ƞ decreases steeply with increasing temperature

9 Fuel Utilization Hydrogen mole fraction in anode Carbon monoxide mole fraction in anode S/C =1, V cell =0.7 V and T=1073 K Good utilization of H 2 in fuel CO consumption was sluggish

10 Effect of fuel velocity Fuel utilization decreases with increasing velocity CO consumption was affected more than H 2 with increasing velocity Fuel velocity of 0.4 m/s had good depletion profile and reasonable current density

11 Carbon activity H 2 reduction reaction Boudouard reaction Carbon activity was <1 up to the fuel velocity of 0.9 m/s At higher velocity, the difference in carbon activity between above reactions was much pronounced

12 C in mol/m 3 Porosity Permeability (m 2 ) a Transient Studies Carbon concentration vs Time Catalytic activity vs Time Time in hours Time in hours Porosity vs Time Time in hours After 120 days of Operation Permeability vs Time 1.50E E E E Time in hours 15 % reduction in porosity 50 % reduced permeability

13 Carbon deposition vs. Time 720 h 1440 h 2160 h 2880 h Carbon builds-up quickly during the initial days Carbon deposit nears saturation in long duration

14 Power (W/cm 2 ) i (A/cm 2 ) V_cell Performance Degradation 1.8 Current vs Time 1.5 Polarization curve Time in hrs i (A/cm 2 ) Power curve i (A/cm 2 ) 0 h 1440 h 2880 h 29% drop in performance after 3000 h 0 h 1440 h 2880 h Concentration polarization become much pronounced with time

15 Conclusions S/C ratio of 1 was found to be suitable for SOFC operating temperature of 1073 K Fuel velocity of 0.4 m/s found to be suitable in the perspective of fuel utilization and carbon activity Transient studies showed 29 % drop in performances over the period of 3000 h due to carbon deposition

16 Acknowledgements The Director, NAL Head, SED Division CSIR-Network project (CERMESA )-Funding M/s. Carborundum Universal Ltd. - Funding THANK YOU