The present invention relates to a fuel cell, which supplies fuel gas obtained by reforming hydrocarbon based raw fuel has been reformed to a fuel cell to generate electric power.
In recent years, due to its cleanness and excellent energy efficiency, a fuel cell (proton exchange membrane type fuel cell) has found attracted as a power source for an electric vehicle or such. The fuel cell can be regarded to as one of a power generator which electrochemically generates power upon supplying fuel gas (hydrogen) for first reactant gas and oxidant gas (air) for second reactant gas.
In a fuel cell powered electric vehicle which carries a fuel cell, mileage of one refueling is required to be as long as possible. For this reason, the fuel cell powered electric vehicle has been developed, which possesses a container for raw fuel liquid such as methanol instead of a hydrogen tank, and the hydrocarbon raw fuel liquid is reformed to produce hydrogen-containing fuel gas to generate power.
Now referring to FIG. 6, a conventional fuel cell system FCS"" possessing a reformer will be described.
A water/methanol mixed liquid (hereinafter referred to as xe2x80x9craw fuel liquidxe2x80x9d), which is raw material stored in a storage tank for the raw fuel liquid is pumped to an evaporator 102 by means of a pump P. It is noted that why the raw fuel liquid comprises a mixed liquid of water and methanol is that hydrogen is produced from water and methanol by a steam reforming reaction, which will be described later on. Subsequently, in the evaporator 102, the raw fuel liquid is injected from a raw fuel injecting device 102b to a body 102a of the evaporator, and is evaporated due the heat generated by the catalytic combustion in a catalyst combustor 102c. Also, reforming air from an air compressor 104 is supplied to the body 102a of the evaporator. Within the body 102a of the evaporator, the vapor of the raw fuel liquid and the vapor are heated and mixed with each other to form raw fuel gas, which is supplied to a reactor 103.
The reactor 103 is composed of a reformer 103a and a CO remover 103b. A catalyst for an automatic thermal reaction (TAR) is incorporated into the reformer 103a, and the reformer 103a reforms the raw fuel gas supplied to the reactor 103 (reformer 103a) to thereby produce hydrogen-containing fuel gas. In the automatic thermal reaction, the following reactions (1) to (4) takes place to produce hydrogen, carbon dioxide, carbon monoxide (small amount), and water.
(1) Steam Reforming Reaction:
OH3OH+H2Oxe2x86x92H2Oxe2x86x923H2+CO2
(2) Oxidation
xe2x80x83OH3OH+3/2O2xe2x86x922H2O+CO2
(3) Thermal Decomposition
OH3OHxe2x86x922H2+CO
(4) Shift Reaction
CO+H2Oxe2x86x92H2+CO2
It is noted that the reactions (1) and (3) are endothermic reactions and the reactions (2) and (4) are exothermic reactions.
As a result of these reactions (1) to (4), the fuel gas produced in the reformer 103a contains a small amount (approximately 1%) of carbon monoxide. Supplying fuel gas containing carbon to the proton exchange membrane type fuel cell 101 is not preferable, because a platinum catalyst within the fuel cell 101 is poisoned. Consequently, the fuel cell produced in the reformer 103a is supplied to the CO remover 103b at which CO is converted into CO2 and is removed. Into the CO remover is supplied air for selective oxidation, and by the catalytic function of a catalyst disposed in the CO remover, CO is selectively oxidized. In front of the CO remover 103b, a heat exchanger (not shown) is provided for the purpose of controlling the temperature of the fuel gas within a given level to remove CO in an efficient manner.
In conventional, when the automatic thermal reaction is carried out, the amount of the reforming air to be mixed with the raw fuel gas (the amount of the air to be supplied in the body 102a of the evaporator) is set according to the following procedures (1) to (3):
(1) From the demand output from the fuel cell 101, the demand load from the reformer 103 is set. (2) Based on the demand load, the amount of raw fuel liquid (the injection amount of the raw fuel) to be injected to the body 102a of the evaporator is set. (3) Based on the injection amount of the raw fuel, the amount of reforming air required in the reformer 103a from a map for the amount of the reforming air.
In order to stabilize the reaction in the reformer 103a, the temperature of the evaporator 102 or such is controlled so as to supply the raw fuel gas having a prescribed temperature.
However, the temperature of the raw fuel gas varies depending on the variation in the driving conditions of the fuel cell system FCS"" and external factors (such as atmospheric temperature). If the temperature of the raw fuel gas is changed, the temperature of the reforming catalyst is also changed accordingly. Specifically, when the temperature of the raw fuel gas to be supplied to the reformer 103a becomes higher, the temperature of the reforming catalyst becomes higher. Conversely, when the temperature of the raw fuel gas becomes lower, the temperature of the reforming catalyst becomes lower. Here, as shown in FIG. 8, if the temperature of the reforming catalyst is increased, the concentration of carbon monoxide (CO concentration) contained in the fuel gas produced is increased. If the CO concentration is too increased, CO is removed in the CO remover 103b only with an insufficient manner, which possibly has an influence upon the platinum catalyst of the fuel cell 101. On the other hand, if the temperature of the reforming catalyst becomes lower, the CO concentration in the fuel gas produced becomes smaller, but the total hydrocarbon (THC) concentration would be increased (in which case, the reforming reaction takes place slowly and unreacted raw fuel is passed as it is, together with the raw fuel gas not sufficiently reformed). For this reason, the amount of the hydrogen produced is decreased, changing the fuel ratio for the worse. If the amount of the hydrogen produced is too decreased, the fuel cell 101 cannot generate power any longer. Particularly, in the case where the fuel cell system FCS"" is carried on a vehicle like a fuel cell powered vehicle, the driving conditions and the external factors would be severely changed.
A main object of the present invention is, therefore, to provide a fuel cell system, which can be operated under appropriate conditions by keeping the temperature of the reforming catalyst at a prescribed level, even if the temperature of raw fuel gas is changed.
As a result of serious studies in light of the object described above, it has been found that as shown in FIG. 9 showing the relation between the temperature of the raw fuel gas to be supplied to the reformer and the amount of the reforming air in the case of keeping the temperature of the reforming catalyst at constant, if the amount (flow amount) of the reforming air to be supplied to the reformer is controlled even when the temperature of the raw fuel gas is changed, the temperature of the reforming catalyst can be kept at a prescribed level, reaching the present invention.
Specifically, the fuel cell system according to the present invention, which attains these and other objects, comprises:
a fuel cell in which first reactant gas and second reactant gas are supplied to generate electricity,
an evaporator for evaporating a raw fuel, which combustor exhaust gas discharged from said fuel cell and makes use of said exhaust gas to evaporate and volatile a raw material for said first reactant gas,
a reformer which reforms the evaporated and volatilized raw fuel vapor by means of a reforming catalyst to produce the first reactant gas, and
an air-introducing device, which introduces air used to be reformed into said evaporator or said reformed in a manner that the flow rate of the air is controlled depending upon the output from the fuel cell and upon the temperature of the raw fuel vapor.
In the embodiments of the present invention, which will be described later on, the first reactant gas is fuel gas, which is produced in the reformer, and then supplied to the fuel cell, while the second reactant gas is cathode air, which is compressed in the air compressor and supplied to the fuel cell. In the embodiments of the present invention, which will be described later on, the reformer reforms the raw fuel gas comprising vapor (raw fuel vapor) of the raw fuel (methanol+water) admixed with the reforming air in the presence of a reforming catalyst by the automatic thermal reaction to thereby produce fuel gas containing hydrogen. The hydrogen-introducing device in the embodiments of the present invention, which will be described later on, is composed of a control unit, a control valve for controlling the flow amount of the reforming air, and the like. It controls the amount of the air used to be reformed (the amount of reforming air) depending upon the injection amount of the raw fuel and upon the temperature of the fuel gas and supplies the reforming air to the body of the evaporator.
The output from the fuel cell includes both the real output from the fuel cell and the fuel cell-demand output (target output). Furthermore, as used herein, the output from the fuel cell includes the output of the reformer (the real amount of the raw fuel gas produced, the demand of the reactant gas to be produced) and the output of the evaporator for the raw fuel (the real amount of the raw fuel gas evaporated, the demand of the raw fuel gas to be evaporated, the injection amount of the raw fuel liquid, . . . ), and it should not be interpreted to be narrower meanings. Incidentally, in the embodiments of the present invention, which will be described later on, the flow amount of the air used for the reformation (reforming air) is controlled on the basis of the injection amount of the raw fuel liquid (the injection amount of the raw fuel), because the injection amount of the raw fuel and the output from the fuel cell have a close relation (the fuel cell generates power utilizing reformed hydrogen), as the liquid fuel is injected in a large amount when the output from the fuel cell is increased, while the liquid fuel is injected in a small amount when the output from the fuel cell is decreased.
In a preferred aspect of the fuel cell system of the present invention, the air-introducing device carries out the correction based on the deviation between the measured temperature of the raw fuel and the set temperature of the raw fuel to control the flow amount of the air used to be reformed.
According to this configuration, the flow amount of the air used to be reformed is controlled based on the deviation between the measured temperature of the raw fuel and the set temperature of the raw fuel to control the flow amount of the air used to be reformed. In the preferred embodiments described later on, when the measured vapor temperature is enhanced to increase the deviation, the flow amount of the reforming air is decreased to prevent the increasing of the temperature of the catalyst. Conversely, if the measured vapor temperature is decreased to increase the deviation, the flow amount of the reforming air is increased to prevent the decreasing of the temperature of the catalyst. Also, in the embodiments, the measurement of the vapor temperature is carried out using a thermosensor disposed in front of the inlet of the reformer.
In the preferred aspect of the fuel cell system just mentioned, in addition to the correction based on the deviation, the air-introducing device carries out the correction based on the output from the fuel cell to control the flow amount of the air used to be reformed.
For example, when the output from the fuel cell is large, the raw fuel vapor to be supplied to the reformer becomes large accordingly. For this reason, the thermal mass of the raw material vapor also becomes large. In this case, the influences of the thermal mass and the thermal loss (heat loss due to the radiation) upon the reformer become small. Accordingly, in order to decrease the heating-up in the reformer, the flow amount of the air used to be reformed is corrected to be smaller than the usual case. On the other hand, if the output from the fuel cell is small, the raw fuel vapor to be supplied to the reformer becomes small accordingly. For this reason, the thermal mass of the raw material vapor also becomes small. In this case, the influences of the thermal mass and the thermal loss upon the reformer become great. Accordingly, in order to increase the heating-up in the reformer, the flow amount of the air used to be reformed is corrected to be larger than the usual case.
According to another preferred aspect of the fuel cell according to the present invention, the air-introducing device restricts the upper limit of the flow amount of the air used to be reformed based on the output from the fuel cell.
If the flow amount of the air used to be reformed is increased, the exothermic reaction can be accelerated to increase the temperature of the reforming catalyst. However, if the flow amount of the air is too increased, the concentration of the first reactant gas is decreased (i.e., the hydrogen concentration in the fuel gas is decreased), sometimes breaking down the fuel cell. Consequently, with regard to excessively increasing of the amount of the air used to be reformed, for example, when the temperature of the reforming catalyst should be largely increased, it is not preferable to increase the flow amount of the air relative to the output from the fuel cell over a prescribed level. Consequently, the upper limit of the flow amount of the air used to be reformed is restricted based on the output from the fuel cell.
Considering the removal of carbon monoxide, according to still another preferred aspect of the fuel cell of the present invention, the fuel cell system further comprises a CO remover which removes carbon monoxide utilizing the air from the first reactant gas having been reformed in the reformer, and the air-introducing device further introduces the air used for the removal of carbon monoxide into said CO remover in such a manner that the flow amount thereof is controlled based on the output from the fuel cell.
When the air for removing CO is introduced in a controlled flow amount, the performance for removing carbon monoxide to meet the flow amount of the reforming gas can be exhibited.