Comparison of Reforming Technologies

So, when is each reforming technique best used? The first consideration is the ease with which the chosen fuel can be reformed using the respective method. Generally speaking, methanol is most readily reformed at low temperatures and can be treated well in any type of reformer. Methane and LPG require much higher temperatures but again can be processed by any of the methods discussed in Sections 5.4.1 to 5.4.3. With higher hydrocarbons, the current standard fuels used in the automotive sector, one usually resorts to POX reactors.

Table 5.2 shows typical gas compositions obtained as reformer outputs. Steam reforming gives the highest hydrogen concentration. At the same time, asystem relyingentirelyonsteam reforming operates best under steady-state conditions because it does notlenditself to rapid dynamic response. Thisalso applies to start-up.

Partial oxidation, in contrast, offers compactness, fast start-up, and rapid dynamic response while producing lower concentrations of hydrogen; compareEqs. (5.4)and(5.3). Inaddition todifferencesin product stoichiometries between SR and POX reformers, the output of a POX reformer is necessarily

TABLE 5.2 Typical Compositions of Reformate from Steam Reformers (SRs), Partial Oxidation Reformers (POXs), and Autothermal Reformers (ATRs), with Methanol as Fuel

Output Composition

SRs

POXs

ATRs

(dry gas, %)

(Pasel et al., 2000)

(Pasel et al., 2000)

(Golunski, 1998)

h2

67

45

55

CO2

22

20

22

N2

22

21

CO

2

further diluted by nitrogen. Nitrogen is introduced into the system from air, which is usually the only economical source of oxygen, and carried through as an inert gas (see Table 5.1). ATR offers a compromise, as was discussed in Section 5.4.3.

The fuel processor cannot be considered on its own, however. Steam reforming is highly endothermal. Heat is usually supplied to the reactor, for example by burning extra fuel. In a fuel cell system, (catalytic) oxidation of excess hydrogen exiting from the anode provides a convenient way of generating the required thermal energy. In stationary power generation, it is worth considering that the PAFC fuel cell stack operates at a high enough temperature to make it possible to generate steam and feed it to the fuel processor. Steam reforming may be appropriate here whereas autothermal reforming could be considered in a PEMFC system, which has only low-grade heat available.

Fuel efficiency also deserves careful attention. Though always important, the cost of fuel is the most important factor in stationary power generation (on a par with plant availability). Hence, the method offering the highest overall hydrogen output from the chosen fuel, usually natural gas, is selected. Steam reforming delivers the highest hydrogen concentrations. Therefore, the fuel cell stack efficiency at the higher hydrogen content may compensate for the higher fuel demand for steam generation. This is probably the reason why steam reforming is currently also the preferred method for reforming natural gas in stationary power plants based on PEM fuel cells (see Chapter 8).

In automotive applications, the dynamic behavior of the reformer system may control the whole drive train, depending on whether back-up batteries, supercapacitors, or other techniques are used for providing peak power. A POX reformer offers the required dynamic behavior and fast start-up and is likely to be the best choice with higher hydrocarbons. For other fuels, in particular, methanol, an ATR should work best. Nevertheless, the reformers used by DaimlerChrysler in their NeCar 3 and 5 vehicles are steam reformers. This perhaps surprising choice can be reconciled when one considers that during start-up, additional air is supplied to the reformer system to achieve a certain degree of partial oxidation (DaimlerChrysler, 2000). During steady-state operation, the reformer operates solely as a SR with heat supplied from excess hydrogen. Clearly, it is not always possible to draw clear borderlines between different types of reformers.

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