31.19: Fuelling Requirements
- Page ID
- 33001
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Cell Type |
Fuelling requirements |
Solution |
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AFC |
Alkaline fuel cells, using KOH as the electrolyte, require pure H2 and O2 as their fuel. This is because even the small amounts of CO2 in the air (about 300 ppm) are enough to prevent the cell from functioning properly. This is because CO2 reacts with the KOH as follows: 2KOH + CO2 → K2CO3 + H2O The presence of potassium carbonate in the electrolyte may reduce cell activity by:
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Use of Pure Oxygen at the cathodes is necessary. This can be achieved by:
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Species: |
H2 |
CO |
CH4 |
CO2 and H2O |
S (e.g. H2S and COS) |
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Effect: |
Fuel |
Poison |
Diluent |
Poison |
Unknown |
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PAFC |
The phosphoric acid (H3PO4) systems are almost always fuelled by a hydrocarbon, which must undergo some sort of fuel processing (add link) to release the H2 gas that reacts at the anodes. The PAFC systems can tolerate CO2 and un-reacted hydrocarbons (e.g. methane). These act as diluents to the fuel and their concentrations should be minimised. CO gas however will poison the Pt catalysts at concentrations from 0.5% |
|
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Species: |
H2 |
CO |
CH4 |
CO2/H2O |
S etc |
|||||||
Effect: |
Fuel |
Poison (>0.5%) |
Diluent |
Diluent |
Poison (>50 ppm) |
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MCFC |
CH4 can either be a diluent or it may be internally reformed (see SOFCs). Very low sulphur tolerance in the MCFC due to poisoning, especially in the catalysts that aid fuel reforming. The reactions of the MCFC requires CO2 to be present in the fuel: Recall that CO32– ions are transported through the electrolyte from cathode to anode (see section on high temperature fuel cells). This requirement for CO2 contrasts with the AFC where it must be excluded. The CO2 is usually recycled externally by passing exhaust gases through a combustor to convert unused fuel into water and CO2, which can be fed back to the cathode inlet. This also serves to preheat the reactant air. |
|
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Species: |
H2 |
CO |
CH4 |
CO2/H2O |
S etc |
|||||||
Effect: |
Fuel |
Fuel (via shift reaction) |
Can be internally reformed |
Diluent |
Poison (>0.5 ppm) |
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SOFC |
SOFCs (and MCFCs) run at high enough temperatures to internally reform CO and hydrocarbons (e.g. petrol or methane) via reaction with H2O, producing CO2 and H2 via “shift” (or oxygenolysis) reactions: |
|
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Species: |
H2 |
CO |
CH4 |
CO2/H2O |
S etc |
|||||||
Effect: |
Fuel |
Fuel (via shift reaction) |
Can be internally reformed |
Diluent |
Poison (>1.0 ppm) |
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PEMFC |
PEM Cells generally use pure Hydrogen as the fuel, especially in portable applications where complicated reforming apparatus would be impractical. CO poisons PEM systems very easily because they rely on platinum catalysts. CO has a high affinity for Pt and occupies catalytic sites, preventing hydrogen fuel from preaching them. The processing equipment needed to reduce CO partial pressures to less than 10 ppm adds considerably to the cost of the system.
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|
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Species: |
H2 |
CO |
CH4 |
CO2/H2O |
S etc |
|||||||
Effect: |
Fuel |
Poison (>10 ppm) |
Diluent |
Diluent |
Unknown |