fuel cell system fuel processing

8/14/2019 Fuel Cell System Fuel Processing

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Fuel Cell SystemFuel Processing

Maree


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OverviewFuel cell fuel requirements - why is fuel processing required?Desulphurisation - removing organic sulphur from fuelSteam reforming- getting hydrogen from hydrocarbons and alcoholCarbon formation and pre-reforming - avoiding an accumulation ofcarbon deposits in a fuel cell system

Internal reforming - using the heat from the stack of high temperature fuelcells

Direct hydrocarbon oxidation - aka dry-reforming. Burning the methane

so most of the heat is converted directly into electrictyPartial oxidation (POX) and autothermal reforming - produce syngasor combination POX + steam forming

Hydrogen generation by pyrolysis or thermal cracking - heat in theabsense of air so hydrocarbon cracks into hydrogen and solid carbon

Carbon Monoxide removalcool the product gas and pass it throughshift reactors to reduce CO to reasonable level (to CO2)Stationary vs. Mobile applicationsElectrolysersBiological production


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Fuel cell fuel requirements

The lowerthe temperature, the more intolerantof other compounds...

PEMFC AFC PAFC MCFC SOFC

Temperature (C) 50-100 50-200 180-220 600-700 850-1000

* Note that diluent just means diluting agent.


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DesulphurisationNatural gas and petroleum liquids contain organic sulphurcompounds (Natural gas odour for leak detection)Sulphur levels deactivate some catalysts and reducelifetime of fuel processors and even poison the PEM anodecatalyst.Hydrodesulphurisation (HDS) reactor converts organic

sulphur containing compounds into hydrogen sulphide andthen absorbed onto a bed of zinc oxide to form zincsulphide; this is used for PEM and PAFC systems.Internal reforming MCFC or SOFC systems use a reformerreactor or absorbents (e.g. activated carbon), molecular

sieves or sulphur-tolerant catalysts.


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Internal reformingutilising the heat of the stack to sustain the reforming of low-weight hydrocarbons such as methane (MCFC or SOFC)

cooling requirements of the stack are reduced -> electricalefficiencyUses one of two approaches, or a combination:

IIR (Indirect internal reforming) - close thermal contactwith stack, but reforming and electrochemical separateDIR (Direct internal reforming) - reforming reactionscarried out within the anode compartment of the stack

Advantages cf external reforming: cost reduced, less steamrequired and methane conversion is high with DIR, system

efficiency is higher (due to cooling)can be applied to several hydrocarbon fuels


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Direct hydrocarbon oxidationaka Dry reforminghydrocarbon oxidized direction within the fuel cell, converting

it directly to electricity with maximum efficiencymain issue is carbon formation, but reduced with use ofcopper or special ceramic anodes for SOFC such as ceria-doped zirconia.probably has particular benefits for proton-conducting fuelcellsstill being being developed


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Partial oxidation and auto-thermal reforminghydrocarbons converted to hydrogen viapartial oxidation(POX) which can be carried out at high temperatures (1200-1500C) without a catalyst.attractive for heavier petroleum such as diesels.does not scale down well from large scale systemswith a catalyst and reduced temperature, process is known ascatalytic partial oxidation (CPO or CPOX)POX and CPOX are usually less efficient than steam reformingand oxidation reactions (heat goes to reforming reaction).does not require steam and therefore makes the system moresimple (e.g. for micro-cogen)

Autothermal reforming - both steam and air fed with fuel to acatalytic reactor (combination of POX and steam reforming);this also requires no complex heat management engineering(good for mobile applications)


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Carbon monoxide removalcool the product gas from the steam reformer and pass itthrough a reactor containing a catalyst which promotes theshift reaction (may need more than one shift reactor) ->converts CO to CO2. Often cool -> shift -> cool -> shiftsteps. [PAFC]For PEMFC further CO removal needed. One of three

ways:Selective oxidation reactor - air added, catalystabsorbed CO; catalyst $$$ and need to be carefullycontrolled.Methanation - the opposite of steam reformation, butchews hydrogenPalladium/platinum membranes - works but $$$

Shift reaction via bacteria is being researchedPressure Swing Absorption (PSA) - H first absorbed and

then on depressurisation of reactor, desorbed.


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Electrolysersuses electricity to divide water into hydrogen and oxygen (basically a fuelcell in reverse). Efficiency = 1.48 / cell voltage (1.6 - 2.0 V)

electrolytes include alkaline liquids, but usually solid PEM (in the pastpotassium hydroxide used)usually only makes sense to use electrolysis where FC is mobile applicationsuch as boats or cars converting surplus electricity into useful peak power(hydro, wind, solar, ... off peak coal-generated power).


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fuel cell system fuel processing

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