Simulation Methodology

The computer code Aspen PlusĀ® from Aspen Technology Inc. Cambridge, Mass. USA is used for the process simulation of the fuel cell system. The simulation employs definitions of the system components, data of incoming mass flows and their heat and mass linkage. The simulation methodology of the fuel cell system follows the flowsheet configuration shown in Figure 1.

The reformer furnace is modelled by an 'Equilibrium Reactor', where an isothermally reaction occurs, and a heater in which heat is transformed from the burner flue gas. The shift section is also modelled by an 'Equilibrium Reactor', where the water gas shift reaction occurs isothermally. Conversion rates and kinetic data for the parallel methanation in the shift reactors and the water gas reaction are estimated in own measurements. The estimated parameters were integrated into the computer simulation. The C02-scrubbing and the regeneration of the MDEA solution are modelled as a 'Rigorous Distillation' columns. The methanator is modelled as 'Equilibrium Reactor', where the methanation occurs isothermally. The compression of the natural gas and the fuel cell air is calculated as polytropic with an efficiency of 72 % (Kl, K2). The efficiency of the fuel cell is determined to be 60 %. This describes the conversion of the hydrogen energy flow to electricity. The fuel cell system is calculated based on fifteen heat exchangers to consider a nearly optimised heat integration of the system.

results and discussion

The main results of the energy and mass analyses are presented below. The computed compositions of the hydrogen gas with the volumetric flow are shown in Table I. The energy inputs of the calculated fuel cell system are the natural gas for reforming, the natural gas for the burner, the electrical power for the air compressor, the natural gas compressor and the MDEA pump. The calculation results show a volumetric flow of 0.71 mVrp/h for the burner fuel gas. The energy effort of the steam reforming conversion is calculated to 3.36 kJ/s, the CO shift step sets free 2.31 kJ/s and the methanator dismisses 0.07 kJ/s. The C02 absorption is calculated on the basis of a packed tower with five theoretical stages, operating with an absorbent circulation rate of 0.0374 m3Nip/h. 2.0 kJ/s thermal power is needed to achieve a regeneration of the aqueous solution of activated MDEA of 95 %. The necessary quantity of electrical power for the fuel cell system described as above is calculated to be 1.9 kJ/s.

Table 1: Calculated gas compositions after each process step

reaction step

product gas composition

total volumetric flow

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