Process Description

The process flow sheet of the calculated fuel cell system is shown in Figure 1. In the first step the natural gas used for the steam reforming is compressed to an absolute pressure of 3.8 bar (Kl). Catalysts are used for steam reforming and carbonmonoxide conversion. To avoid damaging of these catalysts a ZnO bed for the desulphurization has been integrated in the fuel cell system after the natural gas compressor. The steam reforming of the natural gas takes place in the reformer unit after adding the overheated process steam. The volumetric flow of the natural gas for the reforming is set to 1.3 m3NTp/h. The steam to carbon ratio is set to S/C = 3.1. The requirements described above are necessary to offer the fuel cell a hydrogen flow of 5.0 m3CTP/h. The reaction temperature in the reformer is set to 800 °C for the calculation. The principle chemical reactions for the steam reforming are:

CH4 + H20<=>C0 + 3H2 , ARH0=2O6kJ/mol, (1) CO +H20«-C02+ H2 , ARH0=-41kJ/mol. (2)

The posterior conversion of carbonmonoxide (2) is carried out in a high temperature step (shift 1, 320°C) and in a low temperature step (shift 2,220 °C) and is used to increase the ratio of hydrogen and to decrease the ratio of carbonmonoxide in the product gas. In the shift reactor also a slight methanation reaction takes place. The carbondioxide scrubbing in the absorber column is carried out at an absolute pressure of 3.1 bar with an activated 40 % methyldiethanolamine solution (MDEA). This purification step correspond to a patent of the BASF AG, Ludwigshafen Germany 151, using piperazin as activator. The overall reaction for the C02-scrubbing with an aqueous solution of MDEA is described by:

The final gas purification step in this fuel cell system is the methanation to convert the residues of carbonmonoxide and carbondioxide, with hydrogen as reactant.

In addition to the purified hydrogen, the FEM fuel cell needs to be supplied with reaction air (air ratio 2 - 4). The calculations of the process in this study are based on an absolute working pressure of the fuel cell of 2.9 bar. Apart from the production of electricity the PEM fuel cell produces a significant quantity of heat. Thus, a suitable cooling system is necessary. A separate flow of cooling water is used as a heat transport medium. The process water for the steam reforming is taken out of this water stream. The reaction heat for reforming offers a natural gas burner, additionally utilising the off gas from the fuel cell.

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