1. Field of the Invention
The present invention relates both to a hydrogen generator configured to generate a reformed gas by reacting a compound containing carbon and hydrogen with water and supply the reformed gas to a fuel cell configured to generate a power using hydrogen, and to a fuel cell system comprising the hydrogen generator.
2. Description of the Related Art
In general, a steam reforming reaction is employed as a method of generating hydrogen to be supplied to a fuel cell. The steam reforming reaction is to react a city gas and a steam as a feed material using a ruthenium catalyst at a high temperature of approximately 600° C. to 800° C. to generate a reformed gas containing hydrogen as a major component.
FIG. 12 is a cross-sectional view schematically showing a configuration of the conventional hydrogen generator disclosed in Japanese Laid-Open Patent Application Publication No. 2002-187705. The hydrogen generator in FIG. 12 is multiple-concentric cylindrical. As shown in FIG. 12, the hydrogen generator is comprised of a plurality of tubes provided concentrically with one another, a burner 73 provided at a center within an innermost tube 93 to heat a feed material to generate a combustion gas 74 as a heat source for causing the steam reforming reaction to occur, a reforming catalyst layer 88 provided within a space formed by the tubes, a shift catalyst layer 87, a selective oxidization catalyst layer 86, and the like.
In the conventional hydrogen generator, first of all, water 72 for a reforming process (reforming water 72) is supplied to a heater 75 and heated by the combustion gas 74 flowing under the heater 75. As a result, the reforming water 72 is partially evaporated. And, the reforming water 72 partially evaporated moves downwardly through a connecting tube 76 and is delivered to a supply passage 77 of a city gas 71.
The combustion gas 74 flowing under the heater 75 is discharged to outside of the hydrogen generator as an exhaust gas 92 having low calories. At a portion where the connecting tube 76 and the supply passage 77 are connected to each other, the reforming water 72 in a partially evaporated state that has passed through the connecting tube 76 is mixed with the city gas 71 that has passed through the supply passage 77. The resulting mixture of the city gas 71 and the reforming water 72 is supplied to an evaporator 78.
The evaporator 78 is comprised of an outer tube 79, an inner tube 80, an intermediate tube 81 disposed between the inner tube 80 and the outer tube 79, and a bottom plate 82. Between the intermediate tube 81 and the outer tube 79, there are provided round bars 83 spirally wound. The round bars 83 form a spiral downward flow passage 84. In addition, between the inner tube 80 and the intermediate tube 81, an upward flow passage 85 is formed.
The downward flow passage 84 is heated up to a temperature between 100° C. and 150° C. by a generated gas flowing within the selective oxidization catalyst layer 86 provided outside of and in contact with the downward flow passage 84. Meanwhile, the upward flow passage 85 is heated up to a temperature between 250° C. and 350° C. by a generated gas flowing within a shift catalyst layer 87 provided inside of and in contact with the upward flow passage 85. Most of the reforming water 72, of the mixture of the reforming water 72 in the partially evaporated state and the city gas 71, is evaporated while flowing downwardly within the downward flow passage 84, and the reforming water 72 remaining unevaporated is completely evaporated while flowing upwardly within the upward flow passage 85. And, the feed gas containing the mixture of the steam and the city gas 71 is heated up to a temperature between 200° C. and 300° C. within the upward flow passage 85 and supplied to the reforming catalyst layer 88.
While flowing within the reforming catalyst layer 88, the feed gas is heated up to a temperature between 600° C. and 800° C. by a high-temperature combustion gas 74 flowing as a counter flow in a space within the tube on the inner side, thereby causing the steam reforming reaction to occur. As a result, the feed gas is converted into the generated gas containing hydrogen, carbon monoxide, carbon dioxide, and steam.
While flowing upwardly within a return flow passage 89, the generated gas is cooled to be approximately 350° C. by heat exchange with the feed gas flowing downwardly within the reforming catalyst layer 88 and then flows into the shift catalyst layer 87. Within the shift catalyst layer 87, shift reaction is conducted in such a manner that carbon monoxide contained in the generated gas reacts with the steam to be converted into carbon dioxide and hydrogen, thereby resulting in reduced concentration of carbon monoxide contained in the generated gas. This reaction is an exothermic reaction. The resulting reaction heat and sensible heat owned by the generated gas are used to evaporate the reforming water 72 moving upwardly within the upward flow passage 85 and to heat the generated steam and the city gas 71. At an exit of the shift catalyst layer 87, the generated gas is cooled to approximately 150° C. The generated gas is mixed with an air 90 for oxidation of carbon monoxide which is supplied from outside of the hydrogen generator. Thereafter, selective oxidation reaction is conducted using carbon monoxide remaining within the selective oxidation catalyst layer 86, oxygen contained in the air which is supplied externally, and the generated gas. As a result, a generated gas 91 containing a carbon monoxide at a concentration of 10 ppm or lower is delivered to the fuel cell.
The above-mentioned selective oxidation reaction is also the exothermic reaction. The reaction heat and the sensible heat owned by the generated gas are used to evaporate the reforming water 72 flowing downwardly within the downward flow passage 84 of the evaporator 78. At an exit of the selective oxidation catalyst layer 86, the temperature of the generated gas 91 is cooled to approximately 100° C.
Thus, within the evaporator 78, the reforming water 72 is evaporated by heating up to the temperature between 250° C. and 350° C. from the side of the inner tube 80 and by heating up to the temperature between 100° C. and 150° C. from the side of the outer tube 79, and the resulting steam is mixed with the city gas 71. And, the mixture of the steam and the city gas 71 are heated up to the temperature between 200° C. and 300° C.
The round bars 83 wound in a spiral shape within the downward flow passage 84 allows the reforming water 72 supplied to the downward flow passage 84 to move along the periphery of the inner face of the outer tube 79. Thus, in this configuration, a sufficient heat transmission area is ensured.
The reforming catalyst layer 88, the shift catalyst layer 87, the selective oxidation catalyst layer 86, and the evaporator 78 are integrated. This makes the hydrogen generator small-sized. In addition, since in the hydrogen generator, from a high-temperature portion to a low-temperature portion is arranged from a center portion to a peripheral portion thereof, loss of heat emission is thereby reduced. As a result, heat efficiency of the hydrogen generator is improved.
Meanwhile, as shown in FIG. 13, there has been proposed an evaporator comprising a water absorbing member having a capillary force, as a technology intended to improve a capability of the evaporator included in the hydrogen generator for the fuel cell (see Japanese Laid-Open Patent Application Publication No. 2001-64658). This evaporator is of a plate type and is comprised of single evaporators layered in multiple stages. The evaporator is configured to evaporate a liquid fuel such as methanol and supply the evaporated fuel to a reformer.
FIG. 13 is a schematic cross-sectional view showing a configuration of the evaporator included in the conventional hydrogen generator disclosed in Japanese Laid-Open Patent Application Publication No. 2001-64658. As shown in FIG. 13, this evaporator has a cylindrical evaporation chamber 115 within which liquid is evaporated. A cylindrical heating chamber 116 is provided on an outer peripheral side of the evaporation chamber 115 to be coaxially with the evaporation chamber 115. The evaporation chamber 115 and the heating chamber 116 are separated from each other by a separating wall 118.
A water absorbing member 119 is provided on a face of the separating wall 118 on the side of the evaporation chamber 115. The water absorbing members 119 is made of fabrics and porous-sintered material and has a capillary force. A liquid collecting member 111 is provided on an upper portion of the evaporation chamber 115, and a liquid distribution member 120 is provided on a lower portion thereof to supply a liquid fuel 113 supplied externally through a supply passage 114 to the water absorbing member 119.
The upper portion of the evaporation chamber 115 opens upwardly and a steam 10 generated within the evaporation chamber 115 is discharged from the upper portion to outside.
In the conventional evaporator configured as described above, the liquid fuel 113 that has passed through the supply passage 114 is supplied to the water absorbing member 119 through the liquid distribution member 120. As a result, the liquid fuel 113 spreads over the entire water absorbing member 119 by the capillary force of the water absorbing member 119. This increases an evaporation area of the liquid fuel 113, and therefore, the evaporation chamber 115 that has a relatively small volume is sufficient. Therefore, in the above configuration, the entire evaporator can be small-sized.
As described above, the conventional hydrogen generator in FIG. 12 is provided with the round bars 83 to form the spiral flow passage for sufficient heat transmission area of the downward flow passage 84. If there is a gap between the round bars 83 and the outer tube 79, and between the round bars 83 and the inner tube 81, then the reforming water 72 flows through the gap, and therefore, sufficient heat transmission area cannot be obtained. It is therefore necessary to arrange the round bars 83 without the gap between the round bars 83 and the outer tube 79, and between the round bars 83 and the inner tube 81. In some cases, it is necessary to fix the round bars 83 to a face of one of the tubes by welding or brazing without the gap and to press the other tube against the round bars 83. This makes manufacturing steps complex. In addition, manufacturing precision of parts requires strictness. Consequently, manufacturing cost becomes high.
In the case where the hydrogen generator is operated in a steady state, the evaporator 78 is heated to a temperature in the above-mentioned range from both of the inner and outer sides. During start, the hydrogen generator has an ambient temperature. So, in order to increase the temperature of the hydrogen generator, before the city gas 72 is supplied, the burner 73 is ignited to generate the combustion gas 74. The reforming water 72 is less likely to be evaporated because of its high evaporation latent heat. In addition, if the steam within the hydrogen generator is insufficient when the city gas 71 is supplied, then the city gas 71 is thermally decomposed to cause carbon to be deposited within the reforming catalyst layer 88, and to cause the catalyst to be thereby degraded. Therefore, it is necessary to supply the reforming water 72 before the city gas 71 to fill the steam within the hydrogen generator. Accordingly, in a short time after ignition of the burner 73, the reforming water 72 starts to be supplied. However, in the configuration in FIG. 12, since the evaporator 78 is not directly heated by the combustion gas 74, and is indirectly heated by conduction of heat from the center portion of the hydrogen generator and by the reforming water 72 heated by the heater 75, the evaporator 78 is incapable of generating the steam for some time after start. This follows that the reforming water 72 continues to remain within the evaporator 78. Long time is required to evaporate the reforming water 72 remaining within the evaporator 78. As should be appreciated from this, in the conventional hydrogen generator, start time becomes long.
On the other hand, since in the configuration in FIG. 13, the evaporator needs to be independently of the hydrogen generator, loss of heat emission occurs in a steam piping from the evaporator to the hydrogen generator and a piping through which a high-temperature fluid flows to the evaporator. For this reason, heat efficiency of the hydrogen generator is reduced.