Process for separating synthesis gas into fuel cell quality hydrogen and sequestration ready carbon dioxide

[0002] This invention relates to a process for separating synthesis gas (“syngas”) containing hydrogen, steam, CO₂, and CO into fuel cell quality hydrogen and “sequestration ready” carbon dioxide.

[0003] Global warming due to CO₂and other Greenhouse gases is one of the major problems facing modern industrial society. One major source of CO₂emissions to the atmosphere is the combustion of fossil fuels in power plants producing electricity. The problem of CO₂emissions from fossil fuels could be significantly reduced, if not eliminated, by converting the fuel to a mixture containing H₂, CO₂, CO and water vapor, (syngas), separating the mixture into two streams, one containing hydrogen and the other containing CO₂, and then isolating the CO₂and using the hydrogen to produce electricity in PEM fuel cells.

[0004] 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 H₂S content must also be very small. Since gases like CO₂and CH₄are not catalyst poisons, larger amounts can be tolerated. Isolating CO₂normally requires compressing and cooling the gas into a liquid. Although some impurities such as SO₂will readily liquefy along with the CO₂and hence do not interfere with sequestration, others such as CO and CH₄are not readily liquefied and interfere with the process if present in excessive amounts.

[0005] The use of CaO to remove CO₂from 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 H₂, steam, CO and CO₂is passed through a bed of calcined limestone. CO₂is removed by the reaction with the calcined limestone, i.e., CaO+CO₂CaCO₃. Since calcined limestone serves as a catalyst for the water gas shift reaction, the CO and CO₂are in mutual equilibrium via the reaction CO+H₂O CO₂+H₂, and removal of the CO₂also removes the CO. Thus, hydrogen gas was purified in the Hann and Harrison

[0006] This 1994 reference is subject, however, to an important limitation. Although the process of purifying the hydrogen as described converts the CaO to CaCO₃, the article is silent with respect to the regeneration of the CaCO₃back into CaO. Even though it may be within the skill of the art to recalcine the CaCO₃back to CaO, the process has obvious disadvantages. Converting CaCO₃to 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 CaCO₃, reconverting it to CaO by heating the walls of bed involves a very awkward and inefficient heat transfer situation.

[0007] 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 CO₂and 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 CO₂and 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.

[0008] 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/nickel 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.

[0009] 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 CO₂which 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 CaCO₃. 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/CaCO₃is chosen so that the oxidation of the nickel catalyst liberates enough heat to decompose the CaCO₃back 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.

[0010] 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 SO₂in the presence of Fe₂O₃. The reaction between the coal and the SO₂reduces it to elemental sulfur and other reduced sulfur species that are oxidized back to SO₂by the Fe₂O₃. thus, the SO₂acts as a catalyst, facilitating the oxidation of the coal by the Fe₂O₃. In this process, the Fe₂O₃is reduced to FeO, which is then reoxidized back to Fe₂O₃in the presence of air.

[0011] For situations in which the fuel contains sulfur, the Lyon and Cole reference teaches the existence of a “threshold” amount of Fe₂O₂. If the conversion of the Fe₂O₃to 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 Fe₂O₃, the FeS is oxidized to Fe₂O₃and SO₂and emitted to the atmosphere. Keeping the Fe₂O₃conversion below the threshold prevents SO₂emissions.

[0012] 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 Fe₂O₃that can be reduced to FeO without exceeding the “threshold.”

[0013] Taken together, the above references reflect a number of limitations in the art. The Combustion Institute paper teaches a method of producing sequestration ready CO₂, 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 CO₂. The Hann and Harrison reference teaches the separation of hydrogen from gas mixtures, but not the production of sequestration ready CO₂.

[0014] Thus, a clear need exists in the art for a new and more efficient method of using syngas whereby gas mixtures containing hydrogen, steam, CO₂and CO can be separated into fuel cell quality hydrogen and sequestration ready CO₂.

[0015] The present invention provides a new energy efficient method of separating gas mixtures containing hydrogen, steam, CO₂and CO (syngas) into fuel cell quality hydrogen and sequestration ready CO₂. The method according to the invention also allows sulfur compounds present in small amounts in the syngas feed to leave as SO₂in the sequestration ready CO₂.

[0016] Specifically, the present invention concerns a new and improved method of separating gas mixtures containing synthesis gas into separate streams of wet hydrogen containing greatly reduced amounts of CO₂and less than 50 ppm CO, and of sequestration ready CO₂containing less than 1% fixed gases, i.e., less than 1% gases which at ambient temperature do not readily liquefy under pressure.

[0017] In one exemplary embodiment, a mixture of limestone and iron oxide circulates between two fluidized beds. One bed is fluidized with a gas containing H₂, steam, CO₂and CO, while the other bed is fluidized with a gas containing steam and oxygen. As the fluidizing gas containing hydrogen, steam, CO₂, and CO, or syngas, passes through the bed, the CO₂reacts with CaO, forming CaCO₃. Virtually all of the CO is removed by a combination of two reactions. Some CO is removed by the water gas shift reaction, forming hydrogen and CO₂with the CO₂being removed by reaction with the CaO. The rest of the CO is removed by reaction with the iron oxide, reducing Fe₂O₃to FeO, a process that also forms CO₂, which is removed by reaction with the CaO.

[0018] Some of the hydrogen is also removed by reaction with the iron oxide, reducing Fe₂O₃to FeO, while the remainder of the hydrogen passes through the fluid bed, leaving in a purified state for use in, for example, a PEM fuel cell. The circulation of solids between the two beds carries the CaCO₃and FeO, along with unreacted CaO and Fe₂O₃, to the second fluid bed where the FeO is oxidized to Fe₂O₃, liberating heat that decomposes the CaCO₃back to CaO.

[0019] Sulfur compounds in the syngas typically are present as H₂S. H₂S reacts with FeO and CaO to form FeS and CaS, respectively. When the solids circulation carries FeS and CaS to the second fluid bed, they are oxidized, liberating SO₂and forming Fe₂O₃and CaO, respectively.

[0020] Accordingly, in its broadest aspects, the present invention covers a process for separating syngas into fuel cell quality hydrogen and sequestration ready carbon dioxide, comprising the steps of (1) circulating a mixture of solids comprising calcium compounds present as CaO, Ca(OH)₂and CaCO₃and mixtures thereof, and iron compounds present as FeO, Fe₂O₃, Fe₃O₄and mixtures thereof, between first and second fluidized bed reactors; charging the first fluidized bed reactor with a first gas mixture containing high temperature steam, H₂, CO₂, H₂S and CO; (2) charging the second fluidized bed reactor with a second gas mixture containing high temperature steam and oxygen; (3) reacting CO₂present in the first gas mixture with Cao present in the first fluidized bed reactor to form CaCO₃; (4) reacting a portion of CO present in the first gas mixture with steam in a water gas shift reaction to form hydrogen and CO₂and reacting the remaining portion of CO present in the first gas mixture with iron oxide to form FeO and CO₂; (5) reacting any H₂S or other sulfur containing species in the first gas mixture with CaO to form CaS and H₂O; (6) circulating CaCO₃, CaS and FeO formed in the first fluidized bed reactor to the second fluidized bed reactor; (7) oxidizing FeO and CaS present in the second fluidized bed reactor to form Fe₂O₃, CaO and SO₂; (8) removing substantially pure hydrogen from the first fluid bed reactor; and finally (9) removing CO₂, high temperature steam and SO₂from the second fluid bed reactor.

[0021] Another aspect of the present invention concerns a new apparatus for separating syngas into fuel cell quality hydrogen and sequestration ready carbon dioxide, comprising first and second fluidized bed reactors containing solids containing calcium compounds present as CaO, Ca(OH)₂and CaCO₃and iron compounds present as FeO, Fe₂O₃and Fe₃O₄; first and second solids transfer lines for circulating said solids mixtures between said first and second fluidized bed reactors; a first inlet feed for charging said first fluidized bed reactor with high temperature steam and syngas; a second inlet feed for charging said second fluidized bed reactor with high temperature steam and oxygen; means for removing substantially pure hydrogen from said first fluidized bed reactor.; means for removing high temperature steam, CO₂and SO₂from said second fluidized bed reactor.

[0022] FIG. 1 is a schematic diagram of a syngas separation method and apparatus in accordance with the invention.

[0023] An exemplary embodiment of the present invention is illustrated in FIG. 1. A mixture of limestone and iron oxide circulates between two fluid beds 10, 12. The solids transfer lines (or conduits) 14, 16 between the beds are purged with steam via respective steam inlet purge lines 18, 20. One of the fluid beds 10 is fluidized with a gas containing H₂, steam (normally superheated), CO₂and CO (syngas) via a first inlet feed 22, and preferably operates at a temperature in the range of 400° C. to 450° C. and a pressure in the range of 100 psig to 500 psig. The other fluid bed 12 is fluidized with a gas containing steam (typically superheated) and oxygen via second inlet feed 24 and preferably operates at a temperature in the range of 800° C. to 1200° C. and a pressure in the range of 100 psig to 500 psig.

[0024] The fluidizing gas containing hydrogen, steam, CO and CO₂passes through bed 10 in a dense phase 11 and a bubble phase 13 as it entrains the mixture of limestone and iron oxide. As the fluidizing gas containing hydrogen, steam, CO, and CO₂passes through the bed 10, the CO₂reacts with CaO, forming CaCO₃. Virtually all of the CO is removed by a combination of two reactions. Specifically, some CO is removed by the water gas shift reaction, forming hydrogen and CO₂, and the latter is removed by reaction with the CaO. The remainder of the CO is removed by reaction with the iron oxide, reducing Fe₂O₃to FeO, a process which also forms CO₂that is removed by reaction with the CaO. Some of the hydrogen is also removed by reaction with the iron oxide, reducing Fe₂O₃to FeO and the remainder passes through the fluid bed 10, exiting in a purified state via outlet 26, fluid solid separation device 15 and outlet 28. As noted above, the purified hydrogen can then be used to produce electricity in, for example, PEM fuel cells.

[0025] The circulation of solids carries the CaCO₃and FeO formed in bed 10 to the second fluid bed 12 via fluid solid separation device 15 and conduit 16 where the FeO is oxidized to Fe₂O₃. This reaction liberates heat that decomposes the CaCO₃back to CaO. The ratio of atoms of calcium (present as CaO/CaCO₃) to atoms of iron (present as FeO/Fe₂O₃) in the solids that circulate between the beds preferably ranges between 1.5 and 2. The rate of solids circulation between the beds is such that the ratio of FeO entering the second bed 12 to gaseous O₂entering the bed with the fluidizing gas preferably is greater than 4 to 1, but less than 5 to 1.

[0026] Sulfur compounds in the syngas typically are present in minor amounts and predominantly as H₂S. In the embodiment of FIG. 1, H₂S reacts in the first fluid bed 10 with FeO and CaO to form FeS and CaS, respectively. When the solids circulation carries FeS and CaS to the second fluid bed 12, they become oxidized, liberating SO₂and forming Fe₂O₃and CaO, respectively.

[0027] The SO₂and CO₂liberated from the FeS, CaS and CaCO₃in reactor 12 are discharged along with steam at outlet 30. The steam contained in the gas exiting at outlet 30 can be readily condensed, leaving a stream consisting primarily of CO₂and some SO₂for recovery and possible sequestration.

[0028] While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, it is to be understood that the invention is not to be limited to the disclosed embodiment, but on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.