Fuel Processing for Fuel Cells - Institut für Technische Chemie und ...

Author's personal copy
Fuel Processing for Fuel Cells 15
3.4 Fuels from natural gas
Even though the methane content in natural gas may vary between 70%
and 96%, the remaining hydrocarbons, mainly ethane, propane, and
butane, have only minor effects on the fuel processor performance.
In both crude oil and natural gas-derived fuels, the sulfur content is
crucial for the fuel processor, in particular for the activity of the catalysts
used. For most applications in fuel cell operation, the natural gas has to be
converted into syngas in a fuel processor, providing a hydrogen-rich fuel
gas for the fuel cell stack. Furthermore, the produced syngas can be
processed into other hydrocarbon compounds, such as alcohols or
Fischer–Tropsch fuels.
3.4.1 Propane
There are two technologies for recovering propane from natural gas,
either cryogenic separation or adsorption plants. The former includes
simple refrigeration which typically operates at 23 C for dew point
control operations or at 40 C for propane recovery. Turbo-expander
systems are well established for propane recovery. When operated at
temperatures down to 73 C, most of the ethane and all C 3 þ hydrocarbons
are liquefied and afterward fractionally distilled for components
separation. Figure 2 shows a typical natural gas processing diagram.
3.4.2 Methanol
Methanol can be produced by a variety of different processes. The most
frequently used industrial process is the formation of methanol from
syngas of Cu/ZnO- or CuO/ZnO-based catalysts, mainly supported on
Al 2 O 3 (Liu et al., 2003; Phan et al., 2011). The synthesis is carried out at
moderate temperatures of about 250–300 C at pressures of 50–100 bars
and is a moderately exothermic reaction.
CO þ 2H 2 ! CH 3 OH; DH 0 298 ¼ 91kJmol 1 (2)
CO 2 þ 3H 2 ! CH 3 OH þ H 2 O; DH 0 298 ¼ 49:5kJmol 1 (3)
CO þ H 2 O ! CO 2 þ H 2 ; DH 0 298 ¼ 41:2kJmol 1 (4)
In the first two reactions, (2) and (3), methanol is produced under
thermodynamic control at low temperatures to avoid hot spots in the
reactor. In parallel, water–gas shift (WGS) reaction consumes the produced
water in reaction (3), resulting in a strong overall driving force
of the reaction to the product side. Methanol is particularly used as a
transportation fuel or as a convenient means of energy storage for fuel cell