1. Field of the Invention
The present invention relates to a hydrogen generator configured to generate hydrogen to be supplied to fuel cells, as well as a fuel cell system using the hydrogen generator. More specifically, the invention relates to a hydrogen generator configured to heat a carbon monoxide reducing portion during a stop operation period so as to inhibit steam from condensing within the carbon monoxide reducing portion, as well as a fuel cell system using the hydrogen generator.
2. Description of the Related Art
Attention is being focused on the fuel cell cogeneration system which offers high power generating efficiency and high overall efficiency as a distributed electric power generator capable of effective energy utilization.
Many of fuel cells, including for example the phosphoric acid fuel cell having been put to practice and the polymer electrolyte fuel cell (hereinafter will be abbreviated as “PEFC”) under development, generate electric power using hydrogen as fuel. However, the infrastructure for hydrogen supply has not been established yet and, hence, it is required that hydrogen be generated at the site where a fuel cell system is installed.
Seam reforming is a kind of hydrogen generation methods. The steam reforming method is adapted to generate hydrogen by a process including: mixing water vapor with a hydrocarbon material such as natural gas, LPG, gasoline, naphtha, or kerosene, or an alcohol material such as methanol; and allowing a steam reforming reaction of the resulting mixture to occur in a reformer provided with a reforming catalyst.
The steam reforming reaction produces carbon monoxide (hereinafter will be referred to as CO) as a by-product and the resulting reformed gas contains about 10% to about 15% of CO. Because CO contained in the reformed gas poisons an electrocatalyst of a PEFC thereby to lower the power generating performance of the PEFC, a CO reducing portion need be provided to lower the CO concentration of the reformed gas to 100 ppm or less, preferably 10 ppm or less at the exit of the hydrogen generator.
Usually, the CO reducing portion of the hydrogen generator reduces the CO concentration of the reformed gas to 10 ppm or less by a shifter and a CO removing portion coupled to each other, the shifter having a shift reaction catalyst configured to cause a water gas shift reaction to proceed in which CO and steam react with each other to produce hydrogen and carbon dioxide, the CO removing portion having at least one of a selective oxidization catalyst configured to cause a selective oxidization reaction between oxygen contained in supplied air and CO, or a methanation catalyst configured to methanize CO for CO reduction.
Meanwhile, the PEFC is required to start and stop according to electric power requirement for its energy utilization efficiency to be improved. The hydrogen generator is also required to start and stop accordingly.
In view of the safety of operation and the durability of the reforming catalyst, a method has been proposed of purging combustible gases remaining within the hydrogen generator using the steam in the stop operation period of the hydrogen generator (see Japanese Patent Laid-Open Publication No. 2002-93447 for example.)
Since the temperatures of respective portions of the hydrogen generator are relatively high when stopping the hydrogen generator to stop the PEFC in operation, condensation of steam into liquid will not occur within the hydrogen generator if purging with steam is followed by purging and discharging of steam out of the hydrogen generator with air or material gas.
When starting the hydrogen generator, on the other hand, the temperatures of respective of the shifter and the CO removing portion are raised by a process including: supplying the reformer with a source material and water from a material supply portion and a water supply portion, respectively; heating the reformer with a reformer heater to allow the steam reforming reaction to proceed; and passing reformed gas resulting from the steam reforming reaction through the shifter and the CO removing portion thereby to transfer heat from the reformed gas to the shifter and the CO removing portion.
For this reason, it takes a relatively long time for the temperatures of the shifter and CO removing portion to rise sufficiently. In one example it took 30 to 40 minutes for the temperatures of the shifter and CO removing portion to rise to higher than 100° C. according to actual temperature measurement, though depending on the size and structure of the hydrogen generator and like factors.
Thus, for example, in cases where the hydrogen generator has to be stopped during the start operation period of the hydrogen generator, particularly the CO reducing portion, which is located on the downstream side in the hydrogen generator, is often at a temperature close to a room temperature. If purging with steam is conducted at that time, steam condenses to liquid water within the CO reducing portion and, in some cases, such condensation occurs on the CO reducing catalyst placed within the CO reducing portion undesirably. Such condensation of steam to water on the CO reducing catalyst causes the characteristics of the CO reducing catalyst to deteriorate problematically.
There exists a hydrogen generator using heating means such as a heater. However, the heating means is used to heat the catalyst only in the start operation period, not in the stop operation period. Therefore, in cases where the operation of the hydrogen generator is stopped immediately after having been started, the temperature of the CO reducing portion is not sufficiently raised and, hence, it is possible that the water contained in the purge gas condenses and deteriorates the catalyst.
In view of the problems essential to the prior art described above, the present invention intends to provide a hydrogen generator which is configured to inhibit condensation of water within the CO reducing portion during the stop operation period, as well as a fuel cell system using the same.