Global warming and air pollution are problems that all the members of the global society must take joint actions for, and ships are not an exception in this case. Ships have begun to be really regulated to curb emission of air pollutants such as Nox and Sox caused by the engine, and various countermeasures from simple mechanical manipulation of the engine to a system change of attaching new devices have been prepared.
The solutions for environmental problems such as global warming and air pollution often conflict with the economic value of pursuing high efficiency.
Thus, much interest on new engines and systems has risen that can pursue both environmental and economic values together, and one of them is the hydrogen fuel cell system. Though the hydrogen fuel cell system is an old technology, active development of the technology is under progress to commercialize it for such uses in mobile power source, household energy supply, automobile engines, ship engines, and dispersion-type power generation.
One of the important challenges a ship faces is the eco-friendly use of energy. Therefore, a multilateral effort to raise the efficiency of ship diesel engines and reduce the discharge of air pollutants is under way. As one of its improvement measures, the application of a fuel cell system is being carefully reviewed.
One of the biggest problems when applying a fuel cell system as a power system of medium and large ships is a method of storing a lot of hydrogen fuel. The method of directly storing hydrogen fuel in a liquid or gas state should handle an ultra low temperature and a supercritical pressure, so it has many problems in terms of ship safety, handling convenience and large capacity storage. Accordingly, it seems that a method of storing the fuel in a liquid state at a normal temperature that can be easily hydrogenated depending on loads is desirable. Because LNG must be treated at a very low temperature of -162oC at the atmospheric pressure though it is a liquid fuel(hydrogen carrier) that can easily be reformed to generate hydrogen, its general use is thought to be limited except for such special ships as LNG carriers. Thus, such fuels as methanol, ethanol, DME, gasoline and diesel oil have been reviewed as liquid materials that can be stored and provide hydrogen at the atmospheric temperature and pressure. Thermal decomposition, partial oxidation and steam reforming methods are available to make hydrogen from the hydrogen carriers, but the steam reforming method is desirable for the case where efficiency is considered important due to consumption of a large amount of fuel as shown in medium/large ships.
In this thesis, the steam reforming characteristics of various hydrogen carriers adopted as fuels for ships were evaluated on the basis of thermodynamic simulation, and on the basis of the evaluation results, the effects of the operating temperature and current density of the stack, S/C, and hydrogen fuel efficiency on the characteristics of the system were reviewed and compared through simulation and modeling on the constitution and performance evaluation of the solid oxide type fuel system (SOFC) that uses hydrocarbon and alcohol fuels.
This paper concluded the performance of a system through a simulation, obtaining the following conclusions within the calculation condition and scope. The results show that the steam reforming characteristics of both hydrocarbon fuels and ethanol appear similar and that the reaction temperature for maximal hydrogen production is near 1000K. By contrast, the reaction temperature for the maximal hydrogen production of methanol and DME appears to be as low as 500K. The results show that the cell voltage and O2 utilization ratio are major factors on the performance of system. The efficiency of a system is largely affected by induced voltage and oxygen availability: the more these values, the more the efficiency increases. The efficiency of a system increases largely as the operating temperature of a stack increases and the current density decreases, but is not largely affected by S/C. The performance characteristics for the fuels used in characteristic comparison show a similar trend, but the induced voltage and efficiency were always small in the order of methane, methanol and gasoline although they were not a big difference.