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Project Description and Objectives: |
Reforming of hydrogen from fossil fuels, which has previously been applied only on an industrial scale, could open up new applications for fuel cells in stationary and mobile systems for decentralised electricity generation.
Hydrogen can be generated by reforming from gaseous and liquid fossil fuels, e.g. natural gas or petrol - both being fuels which are already widely available today. Whereas hydrogen generation by reforming on an industrial scale is the state of the art, the development of small units for decentralised hydrogen production is a new challenge with considerable potential for application, not only in the energy sector.
Depending on the application, there are differing requirements on a reformer system for converting hydrogen to electricity in a fuel cell. For instance, a membrane fuel cell fundamentally requires a high degree of gas purity. In addition, a hydrogen generator which supplies a fuel cell for an electric motor must react immediately to the rapid load fluctuations which occur during driving. By contrast, continuous operation is possible in a thermally controlled, stationary fuel cell. The design of a reformer processing unit for the operating conditions encompasses the conception of the individual component reaction stages and their integration into the system.
The main components for reforming from natural gas are shown in fig. 1. In the first processing step, the reforming reactor creates a hydrogen-rich gas mixture by reacting natural gas and steam at temperatures between 700 and 900 °C. This mixture still contains a high proportion of carbon monoxide, which reacts with steam in two subsequent catalytic converters (shift reactors) to give carbon dioxide. Further hydrogen is released at this stage. Finally, the gas purification step removes the carbon monoxide which did not react in the shift reactor - about 0.5 vol. % - down to a low residue of 10 ppmv and thus guarantees the fuel gas quality required for the membrane fuel cell.
We are investigating two different principles for the reforming reactor: the steam reformer and the autothermal reformer (fig. 2). In the steam reformer, the endothermic reforming reaction and the combustion reaction required to maintain it are in spatially separated zones. The housing wall transfers much of the required heat from the combustion zone to the reforming zone. The steam reformer we developed generates 5 Nm3/h hydrogen at atmospheric pressure, but can also operate at pressures up to 6 bar. A low-emission radiative burner heats the metallic honeycomb catalyst. Gas flow control with integrated heat exchangers ensures effective internal use of the heat, so that the reactor efficiency value is 0.65 - 0.68 depending on the load. The efficiency value here is the ratio of the lower heating value of the generated hydrogen to that of the natural gas used.
In the autothermal reactor, both processes occur together in one reaction zone. Substoichiometric combustion runs parallel to and provides the heat for the reforming, allowing optimal heat transfer. Thus, an autothermal reactor can follow load fluctuations significantly faster than a steam reformer. However, the product gas contains a large proportion of nitrogen ballast from the combustion air. A test reactor with a thermal hydrogen generation power of 20 kW was constructed at Fraunhofer ISE and taken into operation. In it, the active element is also a metal honeycomb catalyst, through which a mixture of fuel, air and water flows. The direct thermal coupling of the combustion and reforming reactions in the catalyst honeycomb keeps the operating temperatures relatively low at 800 °C. Efficiency values exceeding 70 % are achievable because of the integrated heat exchanger.
A further option for autothermal reforming is the conversion of liquid fuels, e.g. petrol, which is particularly interesting for powering vehicles with fuel cells.
As part of a pilot project, we have developed an automated reformer system to continuously generate hydrogen for a 7.5 kW membrane fuel cell. It is based on steam reforming with a subsequent CO shift. Carbon monoxide is selectively oxidised with air to remove it from the reaction gas for the fuel cell. This system will be taken into operation at the beginning of 1998 in the Technology-Orientated Entrepreneurial Centre in Riesa-Großenhain, Saxony.
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