Tuesday, April 1, 2008 - 3:50 AM

Optimizing Hydrogen Production of a Steam-Methane Reformer by Analytical Modelling

dr. Angèle H. Reinders1, ing. Maarten de Jong1, dr. Jim B.W. Kok1, and Gerard Westendorp2. (1) University of Twente, (2) HyGear BV

An important process to generate hydrogen on basis of natural gas is steam reforming. In this endothermic process, steam is used to oxidize natural gas to carbon monoxide and hydrogen. The chemical reactions of natural gas and steam take place in a tubular reactor system. These tubes are catalyst filled at the inside, where the reactive mixture flows, and heated at the outside by combustion of natural gas with air.
In our paper the processes in such a reactor are modeled with the use of a lumped parameter thermodynamic model combined with chemistry and heat transfer models. With the use of the model, design changes have been investigated, and their effect on the reactor performance has been predicted. A fast and accurate design model has been created. The model enables the design of steam reformers for hydrogen generation of optimal efficiency and very high power density.
It has been observed that critical design parameters are the air fraction in the burner and the thickness of the ceramic shield. It was found that the design can be improved considerably by the following design modifications:
Up to 9 % more hydrogen can be produced by increasing the burner air fraction by 50 %
And up to 11 % more hydrogen can be produced by increasing the thickness of the ceramic shield plate by 50 %.

The model predicts the performance of a single reactor tube and is implemented in the mathematical computer program Matlab. The chemistry model calculates the chemical reaction rates on basis of two global reactions describing the reforming chemistry and taking into account diffusion and kinetic limitation. As a result profiles are calculated of temperature, concentrations of hydrogen, carbon monoxide and natural gas, and heat transfer.
Measurement results of the prototype developed at HyGear, producing 30 Nm­­3[H2]/hr at 60% power were used to validate the model. Next, the analytical model was used to analyse the results of six selected design modifications for which the model is applicable. The design modifications are quantitatively compared on performance and qualitatively on modification impact.


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