Gas: heating and illuminating – Processes – Fuel mixtures
Reexamination Certificate
1999-09-09
2003-05-13
Johnson, Jerry D. (Department: 1764)
Gas: heating and illuminating
Processes
Fuel mixtures
C048S127900, C048S198300, C422S198000, C422S198000, C422S211000
Reexamination Certificate
active
06562088
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a hydrogen generating apparatus for producing hydrogen, which is supplied to a fuel cell or the like.
A fuel cell for electricity generation used in a residential cogeneration system or mounted in an electric vehicle generates electricity by causing hydrogen gas to react with air. Hydrogen supplied to such a fuel cell is generated by a steam reforming method or a partial oxidation method, using as feedstock hydrocarbons such as LPG, naphtha, gasoline, kerosene, alcohol, coal or the like, or natural gas composed principally of methane.
Of these methods, the steam reforming method consists mainly of a reforming process and a shifting process. The steam reforming reaction results in the production of carbon monoxide as well as hydrogen and carbon dioxide. In fuel cells such as molten carbonate fuel cells operating at high temperatures, carbon monoxide generated as a byproduct by steam reforming can also be used as a fuel. However, in the case of phosphoric acid fuel cells and solid polymer fuel cells, which operate at low temperatures, the platinum-based catalyst used as an electrode is poisoned by the carbon monoxide and sufficient electricity performance cannot be obtained. In view of this fact, in Japanese Laid-Open Patent Publications Sho 62-27489 or Hei 3-276577, it is proposed to provide a hydrogen generating apparatus used for a fuel cell operating at low temperatures with a shift catalyst reactor or a purifying catalyst reactor. This shift catalyst reactor causes the carbon monoxide contained in the reformed gas to react with water. And, the purifying catalyst reactor selectively oxidizes the carbon monoxide.
Here, a brief description will be given of the steam reforming method by taking an example in which methane is used as the feedstock. The reaction equations for steam reforming reaction are given as (Equation 1) and (Equation 2), which represent the reforming reactions as the primary reactions, and as (Equation 3), which represents the shift reaction as the secondary reaction.
CH
4
+H
2
O
CO+3H
2
(Equation 1)
CH
4
+2H
2
O
CO
2
+4H
2
(Equation 2)
CO+H
2
O
CO
2
+H
2
(Equation 3)
These reactions are reversible reactions, exhibit large variations in equilibrium composition depending on temperatures, and require high temperatures to achieve sufficiently high reaction rates. First, in the reformer, the reactions of (Equation 1) and (Equation 2) proceed in parallel.
As for the reforming catalyst used here, a nickel-based metal or ruthenium-based metal supported on an oxide, for example, is known. Since the reforming reaction using steam is an endothermic reaction, the reaction is performed while maintaining the temperature of the catalyst at 600° C. or higher. For heating, it is known to combust part of the feedstock methane and to utilize the resulting combustion heat, for example. To reduce the amount of heat consumption in consideration of the generation efficiency of hydrogen, the reforming reactor and gas flow passage are designed so as to reduce heat dissipation as much as possible. Japanese Laid-Open Patent Publication Nos. Hei 5-301701 and Hei 7-291602, for example, propose a method for reducing heat dissipation by providing an apparatus having a concentric multi-turn tube configuration with a heating section located at the center.
Next, in the shift reactor, carbon monoxide in the reformed gas is shifted to carbon dioxide. The shift reaction proceeds in accordance with the reaction represented by (Equation 3).
The reformed gas contains more than few percents carbon monoxide as a byproduct, and by the reaction of (Equation 3), hydrogen is generated and the hydrogen concentration is increased to reduce the carbon monoxide concentration. However, since this carbon monoxide is poisonous to the electrode catalyst of the fuel cell, the concentration must be further reduced.
Known examples of the shift catalyst used here include an iron-chromium based high-temperature shift catalyst which exhibits high activity at around 350° C., and a copper-zinc based low-temperature shift catalyst which exhibits high activity at around 200° C.
The reaction of (Equation 3) is an exothermic reaction, and lower catalyst temperatures are advantageous since equilibrium moves toward the right-hand side at lower temperatures. That is, the carbon monoxide concentration in the shifted gas can be reduced down to several thousands ppm.
In particular, when hydrogen is supplied to a solid polymer fuel cell, the process of removing carbon monoxide by selective oxidation or methanation using a catalyst becomes necessary in order to further reduce the carbon monoxide concentration. However, if the reactivity of the shift reactor can be increased sufficiently, the carbon monoxide concentration in the shifted gas can be held within a specified value, making it easier to remove the carbon monoxide by the subsequent selective oxidation or methanation reaction.
In the case of phosphoric acid fuel cells and solid polymer fuel cells operating at low temperatures, the fuel reforming reaction and the carbon monoxide shift reaction and selective oxidation reaction (purifying reaction) are required, as earlier noted. However, since the reaction temperature greatly differs from one reaction to another, it is important to perform temperature control so that each reactor is held at the appropriate temperature for their operations. In this case, the reaction temperature for the reforming reaction must be the highest, and the reaction temperature must be lower for the shift reaction and the oxidation reaction in this order. Furthermore, to increase the operating efficiency of the apparatus, it is desirable that excess heat from each reactor be recovered to control the temperature.
In the presently available solid polymer fuel cells, a fluorocarbon resin with a terminal substituted by a sulfonic group is used for the proton conducting membrane, which is a constituent element of the cell. At this time, the proton conducting membrane must be swelled with water. Considering this, it is desirable to supply the hydrogen gas with as high humidity as possible. However, adding steam to the fuel gas requires much energy. It thus becomes necessary to utilize the excess heat as effectively as possible.
Development has been proceeding vigorously for practical implementation and commercialization of fuel cell systems, which is integrally constituted by combining a hydrogen generating apparatus based on the steam reforming method as described above, with a fuel cell, a DC-AC converter and other auxiliaries.
In particular, in residential or vehicular fuel cell systems, compared with traditional large-scale fuel cell systems, electricity output must be varied quickly to meet changing load. Accordingly, to operate fuel cells efficiently, it is desirable that the hydrogen generating apparatus be capable of adjusting the amount of hydrogen gas production in accordance with changing load, without entailing a decrease in hydrogen concentration or an increase in carbon monoxide concentration.
In practice, however, it is difficult to vary the hydrogen gas production amount while maintaining the fuel cell efficiency at a high level. In particular, in the shift reactor in the hydrogen generating apparatus, it is possible to hold the carbon monoxide concentration in the hydrogen gas within a specified value and yet bring the hydrogen concentration close to the theoretical value, while maintaining the amount of hydrogen gas production constant. However, the problem is that, if the amount of hydrogen gas production is varied even slightly, the amount of non-reacted carbon monoxide tends to increase and the hydrogen concentration tends to decrease. The reality is that presently no means is available that can easily control the hydrogen production amount by alleviating such a phenomenon, and this has been a major problem yet to be resolved.
It is accordingly an object of the present invention to provide a hydrogen gene
Honda Kimiyasu
Kitagawa Kouichiro
Shono Toshiyuki
Taguchi Kiyoshi
Tomizawa Takeshi
Akin Gump Strauss Hauer & Feld L.L.P.
Johnson Jerry D.
Ridley Basia
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