Fuel Cell Reformers

Reforming of Biogas Feed for Fuel Cells

Introduction

In considering the use of fuel cells with biofuels as the primary fuel, a means of recovering hydrogen gas from this feed is required. The technique used with biogas feed is that of reforming. Here, either a feed gas is reacted with steam at around 800oC in the presence of a catalyst, which results in a gas mixture containing hydrogen and carbon dioxide, or internal reforming occurs.

In the case of high temperature fuel cells such as SOFC's, there is high grade waste heat which can be used to facilitate internal reforming. In lower temperature fuel cells such as the PEM and PAFC types, the waste heat is low grade and external reforming is required.

There are several types of external reformer available or being developed, these include-

  • Compact regenerative reformers
  • Plate reformers
  • Spiral steam reformers
  • Membrane reformers
  • Intensified combined reactors
  • Low temperature partial oxidation reactors
  • Auto reformers

    The different types offer different characteristics and may be chosen to suit the fuel cell type, fuel supply or load characteristics. A review of the principles of operation of these reformer types can be found in reference 1 below.


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    Feed Gas Pretreatment

    Prior to feeding the biogas to the reformer, sulphur species must be removed from the gas. All sulphur species are poisonous for catalytic processes employing reduced metals or metal oxides as the primary active phase, therefore a desulphuriser is employed. Here the sulphur compounds are reacted with hydrogen to produce hydrogen sulphide. The hydrogen sulphide is then reacted with zinc oxide to produce zinc sulphide and water. This process is known as chemisorption and may also be carried out using iron oxide. The treated gas is then passed to the reformer.

    In the case of the PEM fuel cell even trace amounts of CO are poisonous to the electrocatalysts in the anode. Therefore, for such a cell the hydrogen must be separated from the CO and cooled.



    The Reforming Reactions

    In the case of pretreated biogas, we are interested in the methane reforming reaction which takes place. Chemically the reaction is described as follows-

    CH4 + H2O = CO + 3H2

    CO + H2O = CO2 + H2 (this is the water gas shift reaction)

    This gives the overall methane reforming reaction

    CH4 + + 2H2O = CO2 + 4H2

    The overall process is endothermic and requires an external heat input. Excess steam and heat is required to shift the water-gas reaction equilibrium to the right and maximise the hydrogen to methane yield.


    External Reforming of Biogas to Fuel PEM and PAFC Fuel Cells

    The reforming reaction is achieved in external reformers by preheating the feed gas in a heat exchanger and reacting with steam at around 800oC in the presence of a nickel catalyst.


    Internal Reforming in MCFC and SOFC Fuel Cells

    With high grade heat generation in the MCFC and SOFC type it is possible to have internal reforming of methane gas at the anode. The fuel cell reaction products in the MCFC appear at 650oC and a nickel catalyst is required to promote reforming at this temperature. In the SOFC the reaction products appear at a temperature of 1000oC which is sufficient to perform the reforming process without the requirement for a precious metal catalyst. This reduces the sensitivity of this type of fuel cell to sulphur.

    Reference 2 demonstrates the case of internal reforming in a SOFC. FIGURE- Illustration of internal reforming in a solid oxide fuel cell.


    The above figure demonstrates the process of internal reforming in a solid oxide fuel cell. Note the work and heat output equations, where the remaining heat after reforming is shown.

    Internal reforming can be considered to occur in two ways:

    • Indirect internal reforming
    • Direct internal reforming
    In the former, the reforming reaction occurs at the anode, upstream and separate to the fuel cell reaction. Since the section of the anode where reforming occurs is adjacent to the section where the fuel cell reaction occurs, the heat of the fuel cell reaction supplies the reforming reaction by internal heat transfer with minimal losses.

    In the latter, the reforming reaction occurs in the anode fuel channels alongside the fuel cell reaction. In theory, the removal of hydrogen by the fuel cell reaction helps shift the reforming reaction to the right, but in practice indirect internal reforming predominates as all the methane is converted to hydrogen close to the inlet of the fuel cell.


    Demonstrations of the Reforming Processes

    There are default models for PEM, PAFC and SOFC type fuel cell systems in the assessment pages which detail the reforming reactions and their thermodynamics.

    The reformer efficiency is conventionally expressed as:

    n ref = heating value of products/heating value of reactants

    However, to model the electricity and heat output of a fuel cell system it is required to separate the electicity and heat effects.

    Electricty efficiency factor

    The electricity generated by a fuel cell is a function of the quantity of hydrogen input. Hence the reforming factor in electricity generation is given by:

    Eref = Actual yield of hydrogen from reforming/Theoretical yield of hydrogen from reforming if all fuel input was converted

    The difference between the actual yield and full conversion will be due to:

    • The equilibrium position of the reforming reaction (not being fully to the right)
    • Any fuel combusted at the inlet to the reformer to provide preheating for the reforming reaction
    The factor Eref has been included in the models, although the equilibrium position for the reforming reaction is not calculated as internal modelling of external reformers is beyond the scope of the project. For SOFC types with internal reforming, research papers suggest full conversion.

    Heat efficiency factors

    For external reformers the following considerations apply:

    • Heat and/or steam may be supplied from the fuel cell and/or from a preheater. As stated above, preheating will be required with low temperature fuel cells to achieve the required temperature
    • Excess heat may be supplied in addition to that required by the enthalpy of the reforming reaction, to shift the reaction equilibrium to the right
    • The fuel output from the reformer is cooled prior to entry to a low temperature fuel cell
    Modelling the reforming reaction equilibrium is beyond the scope of the project. Instead, the additional preheating supply and heat losses from the system are entered in the model. For external reforming the latter would include unrecovered heat from the exhaust steam and hydrogen cooling.

    With internal reforming for SOFC types, the excess heat supplied for the reforming reaction is contained within the fuel cell and should be recovered with the surplus heat from the fuel cell reaction, so the overall heat balance is valid.



    References

    1. Dicks, Andrew L, Hydrogen generation from natural gas for the fuel cell systems of tomorrow,Journal of Power Sources 61 (1996) 113- 124, Elsevier 1996,
    2. Gardener,F J,Thermodynamic processes in solid oxide and other fuel cells, Proc Instn Mech Engrs Vol211 Part A 1997,



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