Fuel evaporator

Details Fuel evaporator Fuel evaporator

2000-11-03

2003-09-09

Kalafut, Stephen (Department: 1745)

Chemistry: electrical current producing apparatus, product, and

With pressure equalizing means for liquid immersion operation

C429S006000, C429S010000, C261S157000

Reexamination Certificate

active

06617067

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to a fuel cell system that generates electricity by reforming raw fuel gas which is vaporized raw fuel liquid and feeding it to a fuel cell, as well as to a fuel evaporator that can be suitably applied to the fuel cell system.
BACKGROUND OF THE INVENTION
Conventional fuel cell systems are known that inject raw fuel liquid comprising a mixture of methanol and water or the like into a fuel evaporator (evaporation chamber) via a raw fuel injection apparatus and vaporize the raw fuel liquid into raw fuel gas, and then reform the raw fuel gas with a reformer while removing the carbon monoxide to yield hydrogen-rich raw fuel gas and feeding the raw fuel gas to a fuel cell to generate electricity. However, when a fuel cell system having such a construction is used under conditions with extreme load fluctuation, such as when it is mounted for use in a fuel cell-powered electric automobile, sudden injection of the raw fuel liquid into the fuel evaporator based on the requirement for increased output makes it impossible to vaporize all of the raw fuel liquid, and this often creates liquid pools of raw fuel liquid (hereunder referred to as “liquid pools”) in the fuel evaporator. When the fuel evaporator has not been adequately warmed up upon starting the fuel cell system, liquid pools tend to form due to a lack of heat for vaporization.
When liquid pools form in the fuel evaporator, the liquid pools vaporize due to heat retained inside even after terminating injection of the raw fuel liquid, and this produces raw fuel gas resulting in undesirable deterioration in the fuel evaporator response.
When the raw fuel liquid is a mixture, the highly volatile components of the formed liquid pools vaporize first thus creating an imbalance in the composition of the raw fuel gas, and the reformer often fails to exhibit adequate performance or the carbon monoxide cannot be sufficiently removed, thus lowering the performance of the fuel cell. In addition, it becomes impossible to satisfactorily control the humidity in the fuel cell, such that the fuel cell often fails to generate the prescribed output.
For these reasons, the present applicant has proposed in Japanese Patent Application No. Hei-11-125366 (unpublished) the fuel evaporator
100
shown in
FIG. 23
, with the object of effectively preventing creation of liquid pools for improved response of the fuel evaporator and to allow more rapid warming up of the fuel evaporator. The fuel evaporator
100
is provided with an evaporator body
110
, a superheater
130
downstream from the evaporator body
110
and a raw fuel gas injection apparatus
140
above the evaporator body
110
.
In the fuel evaporator
100
, heated gas HG serving as a high temperature heat medium obtained by catalytic combustion of off gas (hydrogen-containing gas) generated in a fuel cell (not shown) at a catalytic combustor (not shown) is supplied as a heat source. The heated gas HG passes from an inlet
112
in through a plurality of U-shaped heat medium tubes
112
arranged in the evaporation chamber
111
of the evaporator body
110
, and reaches an outlet
112
out. The heated gas HG then passes through a heated gas conduit
113
provided under the evaporation body
110
, and is conducted to a superheater
130
mounted downstream from the evaporator body
110
. Raw fuel liquid FL composed of a methanol and water mixture is injected as a mist from a fuel injection apparatus
140
and is heated to vaporization with the heat medium tubes
112
into raw fuel gas FG. The raw fuel gas FG is superheated as it passes through vapor tubes
131
of the superheater
130
and is conducted to a reformer (not shown) downstream from the superheater
130
. The heat medium tubes
112
are U-shaped with the top and bottom tubing as the horizontal tubing on either side of the curved sections R′, and as shown in
FIG. 24
, they are evenly arranged from top to bottom (vertically) and from side to side (horizontally).
FIG. 24
is a cross-sectional view of
FIG. 23
along line D—D′.
In this fuel evaporator
100
, the bottom
111
b
of the evaporation chamber
111
in the evaporator body
110
also serves as the top
113
t
of the heated gas conduit
113
. Thus, since heat is also supplied from the bottom
111
b
of the evaporation chamber
111
, creation of liquid pools is prevented, and even when liquid pools are created they are rapidly vaporized. The response of the fuel evaporator
100
is therefore improved.
The fuel evaporator must accomplish rapid and efficient vaporization of raw fuel liquid upon starting up or under extreme load fluctuations in order to yield the raw fuel gas.
In a conventional fuel evaporator
100
, however, the amount of heat on the bottom
111
b
or sides is not so great, and therefore liquid pools have formed on the bottom
111
b
and sides of the evaporation chamber
103
of the fuel evaporator
100
, due to pooling of raw fuel liquid that fails to vaporize.
Furthermore, since the heat medium tubes
112
are arranged horizontally, the raw fuel liquid FL injected from the raw fuel injection apparatus
140
has often pooled on the surface of the heat medium tubes
112
, thus reducing the heat transfer efficiency of the heat medium tubes
112
. The reduced heat transfer efficiency of the heat medium tubes
112
increases the amount of raw fuel liquid FL that falls under the evaporation chamber
111
without vaporizing above the evaporation chamber
111
, but since the heat medium tubes
112
are evenly arranged from top to bottom and from side to side, there have been heat medium tubes
112
that do not contact with the falling raw fuel liquid FL; i.e., that contribute little to vaporization of the raw fuel liquid FL.
In addition, since the temperature of the heated gas flowing through the heat medium tubes
112
is lower at the lower side of the heat medium tubes
112
(at the heat medium tube outlets
112
out), the amount of heat is insufficient at the bottom of the evaporation chamber
111
so that the raw fuel liquid FL often cannot vaporize at the lower side of the heat medium tubes
112
, tending to result in formation of liquid pools.
When pooled raw fuel liquid is present in the evaporation chamber
111
, the pooled raw fuel liquid can flow into the superheating chamber in cases where it runs along an upgrade. The unvaporized raw fuel liquid that has entered the superheating chamber can reach the reformer through the vapor tubes of the superheating chamber, and unreacted raw fuel liquid flowing into the reformer is a cause of reduced reforming efficiency and deterioration of the reformer.
SUMMARY OF THE INVENTION
Consequently, it is an object of the present invention to provide a fuel evaporator whereby warming up of the fuel evaporator can be rapidly accomplish liquid pools in the evaporator can be prevented with a simple structure, by effective utilization of the heat from the catalytic combustor.
It is another object to provide a fuel evaporator provided with a piping structure that allows rapid vaporization of raw fuel and effective utilization of the potential heat of the high temperature heat medium for vaporization of raw fuel liquid.
It is yet another object to provide a fuel evaporator that can effectively prevent flow of unvaporized raw fuel liquid in the fuel evaporator out of the evaporation chamber, and that can also prevent flow of unvaporized raw fuel liquid through the superheating chamber to the reformer.
It is yet another object to provide a fuel cell system with satisfactory response to loads by improved evaporation efficiency of raw fuel liquid in the fuel evaporator.
The present invention which overcomes the aforementioned problems is a fuel evaporator with an evaporation chamber that vaporizes raw fuel liquid with a high temperature heat medium, comprising a catalytic combustor installed adjacent to the evaporation chamber.
With this construction it is possible to apply more heat more rapidly to raw fuel liquid adhering as droplets onto the wall of the evaporation chamber or

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