Gas reformer for recovery of hydrogen

Details Gas reformer for recovery of hydrogen Gas reformer for recovery of hydrogen

2001-06-25

2004-08-10

Heinrich, Samuel M. (Department: 1725)

Gas: heating and illuminating

Generators

Reexamination Certificate

active

06773472

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates to a gas reformer for recovering hydrogen gas generated by thermal decomposition of hydrocarbon gas.
Hydrogen has been used in broad industrial fields, as basic raw material in a chemical industry, a fuel for a fuel cell or atmospheric gas for heat treatment. A representative process to cope with a small demand is reformation of hydrocarbon with steam. Since a product obtained by the reforming process contains CO, CO
2
and residual H
2
O other than H
2
, it cannot be used as such for a fuel cell due to the inclusions; otherwise performance of the fuel cell would be worsened. In this regard, removal of subspecies such as CO, CO
2
and residual H
2
O from H
2
is necessitated, before the reformed product is supplied to a fuel cell
A conventional method of removing subspecies uses a hydrogen-permeating membrane made of a catalytic element such as Pd—Ag or Ta, which enables selective permeation of hydrogen The hydrogen-permeating membrane has been formed so far as a thin layer on a heal-resistant porous body, as disclosed in JP 63-294925 A1 and JP 1-164419 A1. Recently, feasibility of a metal body, which is perforated with holes for passage of hydrogen has been studied instead of a conventional heat-resistant porous body.
In a conventional method using a hydrogen-permeating membrane, a double-pipe
2
is located in a jacket
1
, a plurality of hydrogen-separating pipes
3
each composed of a perforated body
3
a
and a hydrogen-permeating membrane
3
b
are inserted between inner and outer walls of the double-pipe
2
, and a cavity of the double-pipe
2
is filled with a catalyst
4
. A box-shape hydrogen-separator, which has an external surface coated with a hydrogen-permeating membrane
3
b
, may be used instead of the hydrogen-separating pipe
3
. The catalyst may be Ni or the like supported by alumina or the like.
A fuel F is fed together with air A through a burner
5
and a burner tile
6
into an inner space of the double-pipe
2
, and burnt therein. Hydrocarbon gas G to be reformed is blown together with steam through a nozzle
7
into a cavity between inner and outer walls of the double-pipe
2
, and decomposed to H
2
and CO
2
according to a reforming reaction of CH
4
+2H
2
O=4H
2
+CO
2
for instance.
A reaction product H
2
selectively permeates through the membrane
3
b
into the hydrogen-separator
3
, and flows out through a takeout pipe
8
. Selective permeation of hydrogen H
2
from a reacting zone through the hydrogen-permeating membrane
3
b
accelerates the reforming reaction of CH
4
+2H
2
O=4H
2
+CO
2
. A by-product CO
2
is discharged as waste gas W together with excessive H
2
O and combustion gas through an exhaust pipe
9
to the outside.
The reforming reaction of CH
4
+2H
2
O=4H
2
+CO
2
is accelerated at a temperature above 690° C., and the reaction rate quickens as increase of the temperature. Another reaction of CO+H
2
O=CO
2
+H
2
is exothermic on the contrary, and the reaction does not advance over 707° C. In order to efficiently promote these reactions, the double-pipe
2
is conventionally heated with combustion heat of a fuel F in the manner such that an inner space of the double-pipe
2
is held at a temperature in a range of about 600-900° C. with a proper temperature gradient.
Heat-resistant stainless steel is representative material for high-temperature use, but an atmosphere in the gas reformer contains steam for reformation of hydrocarbon. Such the wet atmosphere causes oxidation and intergranular corrosion of a perforated body made of a conventional heat-resistant stainless steel such as SUS410L, SUS430 or SUS304. As a result, the hydrogen-permeating membrane
3
b
is peeled off or cracked, and H
2
gas flowing through the takeout pipe
8
reduces its purity due to inclusion of C
2
H
2n+2
, H
2
O and CO
2
.
Due to selective separation of H
2
from the reacting zone through the hydrogen-permeating membrane
3
b
, equilibrium in the reaction of CH
4
+2H
2
O=4H
2
+CO
2
collapses, and the reaction progresses to the rightward. Consequently a temperature necessary for the reforming reaction can be lowered to 450-600° C. However, the reacting atmosphere is still at a high temperature. When the reformer is operated at such a high-temperature atmosphere over a long term, the hydrogen-separator
3
is significantly damaged due to peel-off of the hydrogen-permeating membrane
3
b
as well as occurrence of cracks. Damage of the hydrogen-separating pipe
3
means inclusion of C
2
H
2n+2
, H
2
O and CO
2
in H
2
flowing through the takeout opening
8
, resulting in degradation of an objective gas H
2
.
SUMMARY OF THE INVENTION
The present invention aims at provision of a gas reformer which can be driven with higher performance even in case of long-term driving at a high-temperature atmosphere by use of a perforated body made of a ferritic stainless steel containing Cr at a proper ratio in response to a driving temperature.
A new gas reformer proposed by the present invention involves a plurality of hydrogen-separators each having a substrate, which is made of a ferritic stainless steel perforated with holes for passage of H
2
gas and coated with a hydrogen-permeating membrane at its external surface. The hydrogen-separators are inserted into inner and outer walls of a double-pipe filled with a catalyst. Hydrocarbon gas is decomposed with combustion heat of a fuel fed into an inner space of the double-pipe, and a decomposition product H
2
permeates through the hydrogen-permeating membrane and then flows to the outside.
The ferritic stainless steel, which is used as the substrate for formation of the hydrogen-permeating membrane of the hydrogen-separator to be exposed to an atmosphere of 600-900° C., contains 16-25 mass % Cr and Ti and/or Nb at a ratio of (C+N)×8 or more. Ti and/or Nb concentrations are preferably controlled in ranges of 0.1-0.7 mass % Ti and 0.2-0.8 mass % Nb, respectively, under the condition of (Ti, Nb)≧(C+N)×8. The ferritic stainless steel may contain at least one or more of Y and lanthanoids at a ratio up to 0.1 mass % for improvement of oxide resistance, and further contain one or more of Si Mn, AL Mo, Cu, V, W and Ta at a proper ratio for improvement of heat resistance.
The ferritic stainless steel, which is used as the substrate for formation of the hydrogen-permeating membrane of the hydrogen-separator to be exposed to an atmosphere of 450-600° C., contains Cr up to 15 mass % and Ti and/or Nb at a ratio of (C+N)×8 or more. Ti and/or Nb concentrations are preferably controlled in ranges of 0.1-0.7 mass % Ti and 0.2-0.8 mass % Nb, respectively, under the condition of (Ti, Nb)≧(C+N)×8.


REFERENCES:
patent: 5458857 (1995-10-01), Collins et al.
patent: 6527832 (2003-03-01), Oku et al.
patent: 63294925 (1988-12-01), None
patent: 01164419 (1989-06-01), None

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