1. Field of the Invention
The present invention relates to a reformation type fuel cell system, which reforms a fuel such as methanol into a hydrogen-enriched fuel gas and takes it, and particularly to a fuel cell system suitable as a power source for an electric vehicle.
2. Description of Related Arts
In recent years, various electric vehicles have been developed having a driving motor carried thereon instead of an engine. As one of such types of vehicles, development of a vehicle (hereinafter referred to as xe2x80x9cfuel cell electric vehiclexe2x80x9d) having a fuel cell system as a power source for the driving motor carried thereon has been sharply made. A so-called reformation type fuel cell system has been known as one of such fuel cell systems.
An example of the reformation type fuel cell system for use in the fuel cell vehicle will be described with reference to FIG. 7. A fuel cell system 50 depicted on FIG. 7 has a fuel cell 51, in which a hydrogen-enriched gas is supplied to an anode side thereof and air serving as an oxidant gas is supplied to a cathode side thereof to thereby generate electric power. The fuel cell system 50 also has an evaporator 52 which evaporates raw fuel liquid such as water/methanol mixed liquid to form raw fuel gas. To the evaporator 52 is connected a storage tank T for the water/methanol mixed liquid via a pump P, and the raw fuel liquid comprising the water/methanol mixed liquid is supplied to the evaporator 52 by the actuation of the pump P. The raw fuel gas obtained by the evaporation of the raw fuel liquid by means of the evaporator 52 is supplied to a reformer 53. In the reformer 53, the raw fuel gas undergoes a catalytic reformation reaction such as an automatic thermal reaction to produce hydrogen enriched fuel gas. The fuel gas produced in the reformer 53 is supplied to a CO remover 54 at which carbon monoxide by-product produced in the course of the reforming reaction, which is harmful for the fuel cell 51, is removed. The fuel gas from which carbon monoxide is removed by means of the CO remover 54 is then supplied to the anode side of the fuel cell 51. The fuel cell system 50 also has an air compressor 551 and by means of the air compressor 55, the air as the oxidant gas is supplied to the cathode side of the fuel cell 51. The air compressor 55 supplies the air as reforming air required for the reforming reaction (hereinafter referred to as xe2x80x9creforming airxe2x80x9d) to the reformer 53.
In the case where the fuel cell electric vehicle having the fuel cell system 50 carried thereon, which has been stopped, is started, the evaporator 52, the reformer 53, and the like are usually cooled. For this reason, in order to exhibit prescribed performances possessed by the evaporator 52 and the reformer 53, a prescribed degree of heat is required for heating them. For this reason, a combustion burner 56 for starting (hereinafter referred to as xe2x80x9cstarting combustion burner) which heats the evaporator 52 and a starting combustion burner 57 for heating the reformer 53 are provided on the conventional fuel cell system 50. After the catalyst layer of the evaporator 52 and the reforming catalyst of the reformer 53 are heated up to prescribed temperatures respectively by means of the combustion burners 56 and 57 for starting, the raw fuel liquid is supplied and the reforming air is supplied in the conventional fuel cell system 50.
Since the reforming air is directly introduced into the reformer 53 in the conventional fuel cell system 50, in some cases, the reforming air is not introduced into the reformer 53 in a uniform manner. In this case, differences in the density of the reforming air occurs in the reformer 53, changing the admixture of the raw fuel gas with the reforming air for the worse, which is apt to cause uneven temperatures on the surfaces of the reforming catalyst provided within the reformer 53. Typically, the temperature of the reforming catalyst becomes higher at the portion where the reforming air is concentrated, while the temperature of the reforming catalyst becomes lower at the portion where the reforming catalyst is diluted. Specifically, the oxidation represented by the formula (1), which is an exothermic reaction is accelerated on the portion where the reforming air is concentrated, and due to the heat generated at this time, the temperature of the reforming catalyst is increased.
CH3OH+3/2O2xe2x86x922H2O+CO2xe2x80x83xe2x80x83(1)
On the other hand, a steam reforming reaction represented by the following formula (2), which is an endothermic reaction, is promoted on the portion where the reforming air is diluted, and the temperature of the reforming catalyst is decreased due to the endothermic reaction.
CH3OH+H2Oxe2x86x923H2+CO2xe2x80x83xe2x80x83(2)
For this reason, the temperature difference in the reforming catalyst occurs. FIG. 8 shows the relation between the concentration of carbon monoxide in the fuel gas and the temperature of the reforming catalyst. It can be proven from this figure that if the temperature of the reforming catalyst is low, an amount of the total hydrocarbons (THC) becomes unduly high, meaning that the raw fuel gas is passed through with no or insufficient reformation, and the CO concentration becomes low, while THC is decreased according to the increasing of the temperature of the reforming catalyst and the CO concentration has a tendency to be increased. Consequently, with such uneven temperatures of the surfaces of the reforming catalyst, there arises a problem that the raw fuel gas is passed through with no or insufficient reformation to be unreformed fuel gas on the portion where the temperature of the reforming catalyst is low, while the CO concentration becomes high at which the temperature of the reforming catalyst is high. If the amount of unreformed gas is increased, no sufficient amount of hydrogen can be obtained, considering that the power generation in the fuel cell system 51 sometimes has a trouble. On the other hand, if the CO concentration is high, there is a fear of poisoning the fuel cell system 51 with CO.
In order to solve such a problem as just mentioned, it could be considered that as shown in an ideal line of FIG. 7 a mixer 58 for mixing the raw fuel gas with the reforming air is separately disposed for the purpose of homogenizing the temperature distribution over the reforming catalyst. However, if such a mixer 58 is disposed, the fuel cell system 50 becomes large-scale, or the pressure loss of the total system becomes large, leading to poor system efficiency.
On the other hand, at the time of starting the conventional fuel cell system 50, two starting combustion burners, i.e., the starting combustion burner 56 for warming up the evaporator 52 and the starting combustion burner 57 for warming up the reformer 53, have been utilized. However, the use of many starting combustion burners as described above also leads to enlarge the size of the system, causing the problem of unsuitability for use in the fuel cell system for carrying a vehicle.
An object of the present invention is, therefore, to provide a fuel cell system which can appropriately mix the fuel gas in the reformer with the reforming air and which can rapidly operate the evaporator and the reformer at the time of starting the fuel cell system without enlarging the total size of the fuel cell system.
According to the present invention, which attains the object described above, there is provided a fuel cell system comprising:
a fuel cell in which fuel gas and oxidant gas are supplied to generate power;
an evaporator which evaporates raw fuel liquid by a combustion heat obtained by combusting exhaust gas exhausted from said fuel cell to provide raw fuel gas; and
a reformer which reforms the raw fuel gas supplied from said evaporator to provide said fuel gas;
said fuel cell system further comprising:
at least one air introduction member which introduces air for use in the reforming reaction (reforming air) in said reformer;
the air introduced from said air introduction member being supplied from said evaporator to said reformer.
In the fuel cell system according to the present invention, the reforming air in the reformer is introduced in the evaporator. For this reason, the reforming air is admixed with the fuel cell in a pipe which communicates the evaporator with the reformer; thus, the fuel gas and the reforming air are admixed in a uniform manner. As a result, there is no uneven temperature of surfaces of the reforming catalyst, making it possible to prevent the fuel gas within the reformer from remaining unreformed and to prevent the increasing of the CO concentration. Furthermore, since the reforming air is well admixed with the fuel gas and, thus, no additional device such as a mixer is required to be disposed, the fuel cell system is not enlarged as a whole.
Furthermore, according to the fuel cell system of the present invention, air can be previously introduced into the evaporator prior to the supply of the raw fuel liquid at the time of starting the fuel cell system. The use of the air as a thermal medium makes it possible to rapidly warm up the evaporator. The air making use of warming up the evaporator is supplied to the reformer in the state where it remains hot. As a result, since the temperature of the reforming catalyst can be increased through the hot air, there is no need for disposing any starting combustion burner, promoting miniaturization of the fuel cell system as a whole.
In one preferred aspect of the fuel cell system of the present invention, a second air introduction member which introduces the air into the evaporator, at the time of starting the fuel cell system is preferably disposed.
Comparing the introduction of the air into the evaporator at the time of staring the fuel cell system with that at the time of normal operation except for the starting, a much larger amount of the air is required at the starting, because a large amount of the air serving as the thermal medium is required for rapid warming-up.
In contrast, at the normal operation, only a small amount of the air is required (for the reformation), while fine adjustment of the amount of the air is required depending upon the operating situation of the fuel cell system. Consequently, the air introduction member for introducing the air at the normal operation is used to introduce a large amount of air required at the starting only with difficulty, taking into the consideration of the configuration of the air introduction member. For this reason, according the first preferred aspect of the present invention, the second air introduction member for the introduction of the air is separately disposed. (For the purpose of distinguishing from the second air introduction member, the air introduction member of the main configuration is sometimes referred to as xe2x80x9cfirst air introduction memberxe2x80x9d.) When the fuel cell system is started, the air is introduced both from the first air introduction member and the second air introduction member, whereby a large amount of the air required at the starting can be appropriately introduced.
In the first preferred aspect of the fuel cell system of the present invention, the second air introduction member is preferably configured so as to introduce the air into the evaporator in an amount larger than that of said first air introduction member.
In this preferred embodiment, the air can be introduced into the evaporator from the second air introduction member at starting the fuel cell system, and from the first air introduction member at the normal operation. Accordingly, the first and the second air introduction members may be simply configured, and may be controlled easily.
According to the second preferred aspect of the fuel cell system of the present invention, before the raw fuel gas is introduced into the evaporator and after the air introduction from the air introduction member is started, the raw fuel liquid is preferably supplied to the evaporator when at least one of a signal for the evaporator temperature based on the temperature of the evaporator and a signal for the temperature of the reforming catalyst based on the temperature of the reforming catalyst exceeds a prescribed level.
According to the third preferred aspect of the fuel evaporator of the present invention, in the first preferred aspect, it is preferred that before the raw fuel gas is introduced into the evaporator and after the air introduction from the second air introduction member is started, air introduction from the second air introduction member is stopped when at least one of a signal for the evaporator temperature based on the temperature of the evaporator and a signal for the temperature of the reforming catalyst based on the temperature of the reforming catalyst exceeds a prescribed level, and the raw fuel liquid is supplied to the evaporator.
In the second and third preferred aspects of the present invention, the raw fuel liquid is supplied to the evaporator when either or both of a signal for the evaporator temperature based on the temperature of the evaporator and a signal for the temperature of the reforming catalyst based on the temperature of the reforming catalyst exceeds a prescribed level. For this reason, after the situations for reforming the fuel in the fuel cell system have been ready, the raw fuel liquid is supplied to the evaporator to surely start the production of the fuel gas.
Also, according to the present invention, there is provided a fuel cell system comprising: a fuel cell in which fuel gas and oxidant gas are supplied to generate power; an evaporator which evaporates raw fuel liquid by a combustion heat obtained by combusting exhaust gas exhausted from said fuel cell to provide raw fuel gas; and a reformer which reforms the raw fuel gas supplied from said evaporator to provide said fuel gas; said fuel cell system having a configuration that at the time of starting said fuel cell system, air is introduced into said evaporator in an amount larger than that at the time of the normal operation, and the larger amount of the air and the raw fuel liquid are admixed with each other in said evaporator, after which the air having been utilized for warming up said evaporator is transferred to said reformer.
According to this configuration, the air can be previously introduced into the evaporator prior to the supply of the raw fuel liquid at starting the fuel cell system. The use of the air as a thermal medium makes it possible to rapidly warm up the evaporator. The air making use of warming up the evaporator is supplied to the reformer in the state where it remains warm. As a result, since the temperature of the reforming catalyst can be increased through the hot air, there is no need for disposing any starting combustion burner, promoting miniaturization of the fuel cell system as a whole.