1. Solid Oxide Fuel Cells Thermo-Chemical Conversion HOME 8 8
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2. Global greenhouse gas emissions by sector from 1990 to 2010
Thermo-Chemical Conversion Introduction
Global greenhouse gas emissions by sector from 1990 to 2010
Reference :
Energy sector responsible for a large chunk of the GHG emissions
A renewable energy source and a highly efficient power generation technology is the need of the hour 8
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3. Thermo-Chemical Conversion Why fuel cells?8
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4. Efficiency potential of various power generation technologies
Thermo-Chemical Conversion Why fuel cells?
Efficiency potential of various power generation technologies
Redrawn from:
A. Boudghene Stambouli, Fuel cells: The expectations for an environmental-friendly and sustainable source of energy, Renewable and Sustainable Energy Reviews 15 (2011) 4507–4520 8
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5. Thermo-Chemical Conversion Why fuel cells?
Fuel Cell (FC) – An electrochemical device which converts chemical energy to electrical energy utilising a continuous flow of reactants stored outside the device
More efficient than the conventional power generation systems
High efficiency coupled with low environmental impact
Fuel cells - energy efficient – possibility of clean energy and pure water.
Pure hydrogen, natural gas are mainly used as fuel now
Here, we want to make use of a fuel derived from a renewable energy source in a FC
Main types: Polymer Electrolyte Membrane Fuel Cell and Solid Oxide Fuel Cell 8
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6. Thermo-Chemical Conversion Biomass – FC systems8
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7. Thermo-Chemical Conversion
Advantages of fuel cells
High efficiency
Good part load performance
Constant efficiency over a wide range of operating conditions compared to other energy conversion systems
Low levels of air, noise and thermal pollution
Visual aesthetics
Safety
Easy maintenance and low manpower requirements
Material requirements less than in nuclear and MHD systems
Multi-fuel ability
Siting flexibility – reduces T&D losses, helps in distributed power generation, large water bodies are not required for the powerplant
Modularity – makes maintenance and training easier
Economic benefits
Possible Challenges
Cost, technical and infrastructural development 8
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8. Thermo-Chemical Conversion Working of PEMFC and SOFC
SOFC can accommodate different fuels, PEMFC cannot
Operation of SOFC
Ref:
Fuel flexible, tolerant to impurities compared to the other types and internal reforming may be possible
Feasibility of operation at higher temperatures provides the option of using it for CHP
Simpler design (solid)
The high temperature operation reduces activation losses drastically – larger ohmic region 8
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Indian Institute of Science, Bangalore ;
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9. Thermo-Chemical Conversion Features of SOFC
FC can produce power with reduced emissions
FC utilizes electrochemical reactions to generate electric power
Cathode side: O2 + 4e-→ 2O2-
Anode side: 2H2 + 2O2-→ 2H2O + 4e-
CO + O2- → CO2+ 2e-
CH4 + 4O2- → 2H2O + CO2 + 8e-
Higher temperature & Ni catalyst helps in internal reforming
Typically used materials for SOFC:
Electrolyte: Yttria stabilized zirconia (YSZ): ZrO2/Y2O3 (Yttria replaces Zr4+ with Y3+ creates oxygen vacancies)
Anode: Ni/YSZ, Ni/GDC, Cu/YSZ (GDC: Gadolinium Doped Ceria)
Cathode: Strontium doped Lanthanum Manganate (La1-xSrx)MnO3, (LSM)
Interconnect: Metal / Glasses and glass ceramics
Sealants: metal or ceramic as required
Two major configurations – planar and tubular (here – planar button cell)
Classification based on support – anode, cathode or electrolyte
Based on temperature of operation – High, intermediate and low temperature 8
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10. Thermo-Chemical Conversion Planar and tubular arrangement of FC
Comparison of planar and tubular arrangement of FC
Planar
Tubular
Power per unit area
Higher
Lower
Power per unit volume
Fabrication method
Easier
Difficult
Ease of sealing
Long-term stability
Fair
Excellent
Thermo-cycling stability
Good
Efficiency
Ohmic losses
Smaller
Larger 8
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Indian Institute of Science, Bangalore ;
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11. Thermo-Chemical Conversion Major processes taking place in FC
Unlike in combustion, half reactions are separated,
permitting electron flow and work production. The major processes taking place in the fuel cell are:
Convective transport of gases in the gas flow channels
Diffusive transport of gases through the electrode, reforming reactions at anode
Ion formation and electron transport through the external circuit
Oxygen ion transport through the electrolyte
Combination of the oxygen ion with the ionised fuel molecules at the anode triple phase interface
Removal of the products by diffusive and convective transport 8
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12. Thermo-Chemical Conversion Operation of SOFC with Producer Gas
When the operation of an SOFC with producer gas is used, it has been reported that contaminants like tar present in the gas could potentially pose problems.
Tar is a mixture of condensable hydrocarbons present in the producer gas.
So, the quantitative and qualitative analysis of tar present in the producer gas becomes significant.
In this context, the open top downdraft twin air entry reburn gasification technology which has tar levels < 10 ppm becomes suitable for producer gas fuelled SOFC operation.
Ref: Monikankana Sharma, Rakesh N, S. Dasappa, Solid oxide fuel cell operating with biomass derived producer gas: Status and challenges, Renewable and Sustainable Energy Reviews 60 (2016) 450 – 463 8
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Indian Institute of Science, Bangalore ;
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13. Thermo-Chemical Conversion Operation of SOFC with Producer Gas
Some of the compounds found during GC-MS analysis of PG obtained from an open top downdraft twin air entry reburn gasification system, before cleaning and cooling
Ref: D. Prando, S. Shivananda, D. Chiaramonti, M. Baratieri, S. Dasappa, Tar analysis: quantitative and qualitative assessment, SAHYOG report 2014 8
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