Chemistry of inorganic compounds – Carbon or compound thereof – Oxygen containing
Reexamination Certificate
2001-08-14
2003-12-23
Langel, Wayne A. (Department: 1754)
Chemistry of inorganic compounds
Carbon or compound thereof
Oxygen containing
C422S142000, C422S198000, C423S655000, C423S656000
Reexamination Certificate
active
06667022
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to a process for separating synthesis gas (“syngas”) containing hydrogen, steam, CO
2
, and CO into fuel cell quality hydrogen and “sequestration ready” carbon dioxide.
Global warming due to CO
2
and other Greenhouse gases is one of the major problems facing modern industrial society. One major source of CO
2
emissions to the atmosphere is the combustion of fossil fuels in power plants producing electricity. The problem of CO
2
emissions from fossil fuels could be significantly reduced, if not eliminated, by converting the fuel to a mixture containing H
2
, CO
2
, CO and water vapor, (syngas), separating the mixture into two streams, one containing hydrogen and the other containing CO
2
, and then isolating the CO
2
and using the hydrogen to produce electricity in PEM fuel cells.
Each of the separated streams of gas must meet the purity requirements for which it is intended. Thus, hydrogen to be used in a PEM fuel cell must be free of substances that tend to “poison” the fuel cell, i.e., the hydrogen must have a CO content of less than a few ppm. The H
2
S content must also be very small. Since gases like CO
2
and CH
4
are not catalyst poisons, larger amounts can be tolerated. Isolating CO
2
normally requires compressing and cooling the gas into a liquid. Although some impurities such as SO
2
will readily liquefy along with the CO
2
and hence do not interfere with sequestration, others such as CO and CH
4
are not readily liquefied and interfere with the process if present in excessive amounts.
The use of CaO to remove CO
2
from gas streams in general, and specifically from syngas, has been described in the literature. A reference by Han and Harrison (Chemical Engineering Science, 49, 5875-5883, 1994) is typical of such prior art and describes a process in which a mixture containing H
2
, steam, CO and CO
2
is passed through a bed of calcined limestone. CO
2
is removed by the reaction with the calcined limestone, i.e., CaO+CO
2
CaCO
3
. Since calcined limestone serves as a catalyst for the water gas shift reaction, the CO and CO
2
are in mutual equilibrium via the reaction CO+H
2
O CO
2
+H
2
, and removal of the CO
2
also removes the CO. Thus, hydrogen gas was purified in the Hann and Harrison
This 1994 reference is subject, however, to an important limitation. Although the process of purifying the hydrogen as described converts the CaO to CaCO
3
, the article is silent with respect to the regeneration of the CaCO
3
back into CaO. Even though it may be within the skill of the art to recalcine the CaCO
3
back to CaO, the process has obvious disadvantages. Converting CaCO
3
to CaO is a strongly endothermic process, requiring a large input of heat energy. Typically, the purification of hydrogen is done using a packed bed of CaO. When heat is put into a packed bed by heating the walls of the bed, the sections of the bed closest to the walls tend to insulate the interior portions of the bed. Thus, once the bed is converted to CaCO
3
, reconverting it to CaO by heating the walls of bed involves a very awkward and inefficient heat transfer situation.
In U.S. Pat. No. 5,339,754 and related U.S. Pat. Nos. 5,509,362 and 5,827,496 (incorporated herein by reference), a new method of burning fuels is disclosed using catalyst materials. The '362 teaches the use of a metal oxide catalyst that can be readily reduced. Similarly, in the '496 patent, the catalyst consists of a material that can be readily reduced when in an oxidized state and readily oxidized when in a reduced state. The fuel and air are alternately contacted with the metal oxide. The fuel reduces the metal oxide and is oxidized to CO
2
and water vapor. The air re-oxidizes the catalyst and becomes depleted of oxygen. Thus, combustion can be effected without the need for mixing the fuel and air prior to or during the combustion process. If means are provided whereby the CO
2
and water vapor and the oxygen depleted air can be directed in different directions as they leave the combustion process, then mixing can be completely avoided.
This new method of combustion is now generally referred to in the art as “unmixed combustion.” The '754 patent discloses various metal oxides that can be readily reduced, including oxides of silver, copper, iron, cobalt, nickel, tungsten, manganese, molybdenum and mixtures thereof, supported on alumina. The '496 patent discloses that the readily reduced metal oxides are selected from a group consisting of nickel
ickel oxide, silver/silver oxide, copper/copper oxide, cobalt/cobalt oxide, tungsten/tungsten oxide, manganese/manganese oxide, molybdenum/molybdenum oxide, strontium sulfide/strontium sulfate, and barium sulfide/barium sulfate.
One embodiment of the '362 patent also teaches a process for steam reforming of hydrocarbons. In this process, the reaction between the hydrocarbon and steam is carried out over a nickel catalyst in the presence of CaO. While this steam reforming reaction is endothermic, it produces CO
2
which reacts exothermically with the CaO, making the overall reaction weakly exothermic. Thus, the need to supply the heat consumed by the steam reforming reaction by putting heat in through the reactor walls can be avoided. Eventually, the CaO is largely converted to CaCO
3
. When this occurs, the production of hydrogen by steam reforming is halted. Air is passed through the reactor, oxidizing the nickel catalyst to nickel oxide. The ratio of nickel catalyst to CaO/CaCO
3
is chosen so that the oxidation of the nickel catalyst liberates enough heat to decompose the CaCO
3
back to CaO. When hydrocarbon and steam are again fed through the reactor, the NiO is reduced to Ni and the production of hydrogen begins again.
Another embodiment of the '362 patent, relating specifically to coal combustion, is discussed in Paper 98F-36 by R. K. Lyon and J. A. Cole at the 26 & 27 October 1998 meeting of the Western States Section of the Combustion Institute. This paper discloses a process in which coal is oxidized in a fluid bed by SO
2
in the presence of Fe
2
O
3
. The reaction between the coal and the SO
2
reduces it to elemental sulfur and other reduced sulfur species that are oxidized back to SO
2
by the Fe
2
O
3
. thus, the SO
2
acts as a catalyst, facilitating the oxidation of the coal by the Fe
2
O
3
. In this process, the Fe
2
O
3
is reduced to FeO, which is then reoxidized back to Fe
2
O
3
in the presence of air.
For situations in which the fuel contains sulfur, the Lyon and Cole reference teaches the existence of a “threshold” amount of Fe
2
O
2
. If the conversion of the Fe
2
O
3
to FeO is allowed to exceed this threshold, the sulfur in the fuel forms FeS during the coal oxidation step. During the subsequent reoxidation with air of the FeO back to Fe
2
O
3
, the FeS is oxidized to Fe
2
O
3
and SO
2
and emitted to the atmosphere. Keeping the Fe
2
O
3
conversion below the threshold prevents SO
2
emissions.
While the Western States paper generally discloses the existence of a “threshold,” the reference does not state what the threshold is, i.e., it does not teach or otherwise quantify the amount of Fe
2
O
3
that can be reduced to FeO without exceeding the “threshold.”
Taken together, the above references reflect a number of limitations in the art. The Combustion Institute paper teaches a method of producing sequestration ready CO
2
, but does not disclose any method for producing fuel cell quality hydrogen. Nor does the paper teach any method of gas separation. The embodiment of U.S. Pat. No. 5,509,362 (which relates to steam reforming) teaches the production of hydrogen, but not the separation of mixtures containing hydrogen or the production of sequestration ready CO
2
. The Hann and Harrison reference teaches the separation of hydrogen from gas mixtures, but not the production of sequestration ready CO
2
.
Thus, a clear need exists in the art for a new and more efficient method of using syngas whereby gas mixtures containing hydrogen, steam, CO
2
and CO can
General Electric Co.
Langel Wayne A.
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