Gas: heating and illuminating – Processes – Manufacture from methane
Reexamination Certificate
2001-05-14
2004-12-28
Doroshenk, Alexa (Department: 1764)
Gas: heating and illuminating
Processes
Manufacture from methane
C422S186220, C422S198000, C422S198000, C422S211000, C422S222000, C048S061000, C048S127900, C048S198700
Reexamination Certificate
active
06835219
ABSTRACT:
TECHNICAL FIELD
This invention relates to a fuel processor for generating hydrogen gas by reforming hydrocarbon-based fuels, and more particularly, to a method and apparatus for heating fuel processor components, such as a water-gas-shift reactor, during startup of the fuel processor.
BACKGROUND OF THE INVENTION
A fuel cell is a device that converts chemical energy directly into electrical energy and heat. In perhaps its simplest form, a fuel cell comprises two electrodes—an anode and a cathode—separated by an electrolyte. During use, a fluid distribution system supplies the anode with fuel and supplies the cathode with an oxidizer, which is usually oxygen in ambient air. With the aid of a catalyst, the fuel undergoes oxidation at the anode, producing protons and electrons. The protons diffuse through the electrolyte to the cathode where, in the presence of a second catalyst, they combine with oxygen and electrons to produce water and heat. Because the electrolyte acts as a barrier to electron flow, the electrons travel from the anode to the cathode via an external circuit containing an electrical load that consumes power generated by the fuel cell. A fuel cell generates an electrical potential of about one volt or less, so individual fuel cells are “stacked” in series to achieve a requisite voltage.
Because fuel cells are more efficient than heat engines and can generate electricity with zero or near zero emission of pollutants, researchers have proposed replacing internal combustion engines in vehicles with fuel cells. Among the fuels that have been considered for vehicle applications, hydrogen (H
2
) appears to be the most attractive. Hydrogen has excellent electrochemical reactivity, provides sufficient power density levels in an air-oxidized system, and produces only water upon oxidation. Despite these advantages, however, its use in vehicles is hampered by on-board storage difficulties and by the lack of an established retail supply network of H
2
.
For these reasons, fuel cell engine designs often include a fuel processor, which employs steam reforming, autothermal reforming or partial oxidation to convert conventional hydrocarbon-based fuels, such as gasoline and methanol, to hydrogen. Most fuel processors include a primary reactor, a water-gas-shift (WGS) reactor, and a preferential oxidation (PrOx) reactor to generate “stack grade” H
2
. In steam reforming the fuel processor supplies the primary reactor with water (steam) and a hydrocarbon-based fuel (e.g., gasoline, methanol, etc.), which react to form a mixture of H
2
, carbon dioxide (CO
2
), carbon monoxide (CO), and excess steam. Since CO would poison the anode catalyst, the fuel processor channels the primary reactor effluent (reformate) to the water-gas-shift (WGS) reactor, which contacts the gas mixture with a catalyst and water to convert most of the CO to CO
2
and H
2
. Finally, the fuel processor converts residual CO to CO
2
in the PrOx reactor, which comprises a catalyst bed operated at temperatures (e.g., 150° C. to 250° C.) that promote preferential oxidation of CO by air with little attendant oxidation of H
2
. In steam reforming, fuel gas leaving the PrOx reactor typically contains (in mole %) about 70% H
2
, 24% CO
2
, 6% nitrogen (N
2
) and trace amounts (<20 ppm) of CO.
Autothermal reforming and partial oxidation share many features of steam reforming. For example, in one form of autothermal reforming, a portion of the hydrocarbon-based fuel may be burned or partially oxidized with oxygen or air within a reaction zone that is physically separate from the reforming reaction. Heat from the oxidation drives the endothermic conversion of water and the balance of the hydrocarbon-based fuel to H
2
, CO
2
, and CO in the reforming reaction zone. In another form of autothermal reforming, a portion of the hydrocarbon-based fuel is oxidized in the same reaction zone as the reforming reaction. Similarly, in partial oxidation, a fuel-rich mixture of the hydrocarbon-based fuel and air are reacted in the primary reactor, producing a gas mixture comprised mainly of H
2
, CO
2
, and CO. Autothermal reforming and partial oxidation also utilize WGS and PrOx reactors to reduce CO levels in the reformate stream leaving the primary reactor; the final reformate composition is about 42% N
2
, 38% H
2
, 18% CO
2
, less than 2% methane (CH
4
), and less than about 20 ppm CO. For further details of fuel processors for generating stack-grade H
2
, see U.S. Pat. No. 6,077,620 entitled “Fuel Cell System with Combustor-Heated Reformer,” which is herein incorporated by reference in its entirety and for all purposes.
One challenge facing developers of fuel cell engines is the ability to rapidly generate stack grade H
2
upon starting the fuel processor at ambient temperature (cold start conditions). Though many factors may affect fuel processor startup, it is particularly limited by the time required for the reactors to reach their operating temperatures. For example, a low temperature water-gas-shift reactor must reach about 200° C. before it can reduce CO in the reformate stream to levels low enough to be tolerated by the PrOx reactor and the fuel cell stack. A high temperature water-gas-shift catalyst must be even hotter (about 350° C.). Typically, the only heat available for raising the temperature of the water-gas-shift reactor is the sensible heat of the primary reactor effluent. This heat must be used to raise the temperature of the entire thermal mass downstream of the primary reactor, including the WGS reactor, the PrOx reactor and any heat exchangers.
Fuel processor startup is complicated by the presence of water vapor in the primary reactor effluent and the WGS reactor feed stream. Since water vapor may condense on the cold WGS catalyst, additional energy must be supplied during startup to vaporize any condensed water before the WGS catalyst is heated. Although the fuel processor may be run without water injection during startup to limit water vapor condensation, such practice may result in the primary reactor reaching excessive temperatures. As noted above, even if there is no water in the fuel processor feed at startup, the primary reactor generates water that may condense in the water-gas-shift reactor. Similarly, water may also condense on the cold PrOx catalyst during cold start, thus requiring additional energy to revaporize the condensed water.
Researchers have proposed several techniques for increasing heating rates of the fuel processor reactors, but each method has drawbacks. For example, the water-gas-shift reactor may be electrically heated at startup, but electric heating requires a secondary power supply that adds to the cost of the fuel processor. Alternatively, air or oxygen may be injected into the primary reactor effluent as it enters the water-gas-shift reactor, and an electrically heated catalyst (EHC) may be used to combust the H
2
and CO in the primary reactor effluent and subsequently heat the water-gas-shift catalyst. However, an EHC requires a secondary power supply, and air or oxygen injection may result in a loss of catalyst activity since many WGS catalysts are sensitive to oxygen. Non-pyrophoric water-gas-shift catalysts that “light off” or react in the presence of oxygen can generate sufficient heat to start the water-gas-shift reaction. However, such catalysts contain costly precious metals and still need to reach a light-off temperature of about 130° C. to become active.
The present invention overcomes, or at least mitigates, one or more of the problems discussed above.
SUMMARY OF THE INVENTION
The present invention provides an apparatus and method for supplying additional heat to fuel processor components—including the water-gas-shift reactor—during startup of the fuel processor at ambient temperatures. The additional heat is supplied without expending secondary power, and is accompanied by the removal of water from the fuel processor's primary reactor effluent. The added heat allows the water-gas-shift reactor to reach its operating temperature more rapidly, which reduces t
Brooks Cary W.
Doroshenk Alexa
General Motors Corporation
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