Catalytic membrane system for converting biomass to hydrogen

Abstract

A two-reactor catalytic system including a catalytic membrane gasification reactor and a catalytic membrane water gas shift reactor. The catalytic system, for converting biomass to hydrogen gas, features a novel gasification reactor containing both hollow fiber membranes that selectively allow O.sub.2 to permeate therethrough and a catalyst that facilitates tar reformation. Also disclosed is a process of converting biomass to H2. The process includes the steps of, among others, introducing air into a hollow fiber membrane; mixing the O.sub.2 permeating through the hollow fiber membrane and steam to react with biomass to produce syngas and tar; and reforming the tar in the presence of a catalyst to produce more syngas.

Claims

1. A process of converting biomass to H.sub.2 and CO.sub.2, the process comprising: step (1): introducing air into hollow fiber membranes that selectively allow O.sub.2, not N.sub.2, to continuously permeate therethrough, the hollow fiber membranes installed around the perimeter of a circle inside a gasification reactor and controlling the amount of O.sub.2 flowing into the gasification reactor; step (2): mixing the O.sub.2 continuously permeating through the hollow fiber membranes and steam to react with biomass to produce syngas, tar, and ash, wherein the syngas contains H.sub.2 and CO; step (3): reforming the tar in the presence of a first catalyst to produce more syngas; step (4): mixing the syngas produced in step (3) and steam to react in the presence of a second catalyst to generate H.sub.2 and CO.sub.2; and step (5): allowing H.sub.2 to selectively permeate through a hollow metal-based membrane, thereby separating the H.sub.2 from the CO.sub.2.

2. The process of claim 1, wherein the permeation of H.sub.2 through the hollow metal-based membrane is conducted at 400° C. to 700° C.

3. The process of claim 1, wherein the permeation of O.sub.2 through the hollow fiber membranes is conducted at 650° C. to 900° C.

4. The process of claim 3, wherein the hollow fiber membranes are formed of BaBi.sub.0.05Co.sub.0.8Nb.sub.0.15O.sub.3-δ and has a thickness of 1 to 3 mm.

5. The process of claim 1, wherein the first catalyst is a Ni/phyllosilicate catalyst having a Ni content of 5-45 wt %, a Ni—La/SBA-15 catalyst having a Ni content of 5-10 wt % and a La content of 0.5-2 wt %, a Ni/Fe.sub.2O.sub.3—Al.sub.2O.sub.3 catalyst, or a Ni/perovskite catalyst.

6. The process of claim 1, wherein the second catalyst is a Ni—Cu/CeO.sub.2 catalyst, a Ni—Na/CeO.sub.2 catalyst, a Ni—Li/CeO.sub.2 catalyst, a Ni—K/CeO.sub.2 catalyst, or a Ni—Cu/SiO.sub.2 catalyst.

7. The process of claim 2, wherein the hollow metal-based membrane is a palladium alloy composite membrane.

Description

BRIEF DESCRIPTION OF THE DRAWING

(1) FIG. 1 is a schematic depiction of a two-reactor catalytic system of this invention.

DETAILED DESCRIPTION

(2) Within this invention is a two-reactor catalytic system for converting biomass to hydrogen gas, the system-including a catalytic membrane gasification reactor and a catalytic membrane water gas shift reactor. The biomass is a solid waste, e.g., empty fruit bunch, mesocarp fibre, or Palm Kernel Shell. FIG. 1 shows an embodiment of the two-reactor catalytic system. It includes (i) a gasification reactor 100 disposed in which are a number of hollow fiber membranes 101 for receiving air, a container 102, and a catalyst 103 confined in container 102; and (ii) a water gas shift reactor 200 disposed in which are a container 201, a catalyst 202 confined in container 201, and a hollow metal-based H.sub.2 membrane 203. The gasification reactor 100 is connected via a pipeline with container 201 in the water gas shift reactor 200.

(3) As shown in FIG. 1, the process of converting biomass to hydrogen gas starts with O.sub.2 separation from air through the hollow fiber membranes 101, which are installed in a circle inside the gasification reactor 100 for receiving air. Air is introduced into the gasification reactor 100 and distributed to the hollow fiber membranes 101. The hollow fiber membranes 101 selectively allow O.sub.2, not N.sub.2, to permeate therethrough. A sweep gas, e.g., Ar or He gas, is also introduced into the gasification reactor 100 to create a pressure gradient across the hollow fiber membranes 101 to facilitate continuous permeation of O.sub.2 into container 102. The N.sub.2 remaining in the hollow fiber membranes 101 is removed from the gasification reactor 100. Biomass and steam are introduced into container 102 simultaneously to react with the O.sub.2 therein.

(4) FIG. 1 also shows that, upon introduction of biomass and steam into the gasification reactor 100, the O.sub.2 permeating through the hollow fiber membranes 101 reacts with the biomass and steam to convert the biomass to ash 104, tar (not shown), and syngas containing H.sub.2 and CO. The ash 104 collected at the bottom of container 102 can be further utilized in production of cement, brick, and asphalt. Catalyst 103 confined in container 102 facilitates reformation of the tar with steam to produce more syngas. The syngas produced in the gasification reactor 100 is then transported to container 201 disposed in the WGS reactor 200.

(5) Upon entering into container 201 together with steam, the CO in the syngas reacts with the steam in the presence of catalyst 202 confined in container 201 to produce H.sub.2 and CO.sub.2. A carrier gas, e.g., N.sub.2, Ar, or He gas, is introduced into the hollow metal-based H.sub.2 membrane 203 to create a pressure gradient inside the membrane 203 to facilitate continuous permeation of the H.sub.2 thus produced therethrough and its exit therefrom, resulting in separation of the H.sub.2 from the CO.sub.2 thus produced. The CO.sub.2 remaining in container 201 is subsequently removed from the WGS reactor 200 for collection.

(6) A hollow fiber membrane plays two key roles in biomass gasification: (1) separating O.sub.2 from air to supply pure O.sub.2 required for optimal gasification; and (2) controlling the amount of O.sub.2 flowing into a gasification reactor. As the gasification reactor receives pure O.sub.2, its required size is smaller than those not including or connected to a hollow fiber membrane. The amount of oxygen present in the gasification reactor must be well controlled to achieve high reaction efficiency and minimize formation of by-products. An excess amount of oxygen can lead not only to more water and CO.sub.2 production but also to formation of undesired nitrogen oxides, e.g., NO and NO.sub.2, due to the presence of nitrogen compounds in the biomass. Optimization of the oxygen amount is effected based on both the air flow rate and the O.sub.2 permeation efficiency.

(7) Regarding the catalyst for tar reformation, it may contain one or more of metals Ni, Fe, Co, Cu, La, Ca, Mg, Sr, Al, and Si or oxides thereof, including a combination of a metal(s) and a metal oxide(s). Particularly, a nickel-based catalyst containing Fe, Co, Cu, La, Ca, Mg, Sr, Al, or Si can catalyzes the tar reformation. A Li, Na, K, Mg, Ca, or Sr, independently or in combination, greatly improves the catalyst performance. Examples of a nickel-based catalyst include a Ni/phyllosilicate catalyst, a Ni—La/SBA-15 catalyst, a Ni/Fe.sub.2O.sub.3—Al.sub.2O.sub.3 catalyst, or a Ni/perovskite catalyst. A preferred nickel-based catalyst is a Ni—La/SBA-15 catalyst having a Ni content of 5-10 wt % and a La content of 0.5-2 wt %.

(8) A Ni—La/SBA-15 catalyst combined with hollow fiber membranes unexpectedly improves conversion of toluene (a major component of tar) by >20%, compared with the nickel catalyst only. This nickel-based catalyst also efficiently promotes cellulose (biomass) gasification at 700° C. to increase the contents of H.sub.2 and CO in the syngas thus generated. Additionally, use of this nickel catalyst efficiently converts various types of biomass to gaseous products containing H.sub.2, CO, CH.sub.4, and CO.sub.2. The formation rates of these gaseous products increase substantially, compared with gasification without any catalyst.

(9) Referring to the catalyst and the hollow metal-based membrane in a WGS reactor, they in combination effectively promote the WGS reaction resulting in greater production of more hydrogen gas. Co-based catalysts have been traditionally used in the industry for facilitating this reaction. However, the use of Co-based catalyst generally forms methane, at a cost of consuming H.sub.2, as a by-product, resulting in a decrease of overall H.sub.2 production. Preferably, a bimetallic catalyst, e.g., a Ni—Cu/CeO.sub.2 catalyst is utilized in the WGS reaction. Indeed, the Ni—Cu/CeO.sub.2 catalyst is robust, stable, and capable of achieving high production of hydrogen gas during the WGS reaction.

(10) Conversion of steam and CO to produce more H.sub.2 during the WGS process is also driven by reaction equilibrium. Thus, removal of hydrogen through the hollow metal-based membrane can shift the reaction to promote the CO conversion, hence favourably increasing the H.sub.2 production and efficiency of the overall WGS process. Hydrogen permeation rates, in part, depend on the membrane thickness. An ultra-thin layer of a palladium alloy composite membrane serves as a selective membrane with high H.sub.2 selectivity and high permeability. In one embodiment, the hollow metal-based membrane contains a metal such as Pd, Pt, Ni, Ag, Ta, V, Y, Nb, Ce, In, Ho, La, Au, or Ru. A palladium alloy composite membrane is preferred.

(11) The CO.sub.2 produced in the two-reactor system of this invention can be collected separately. Indeed, this system enables effective separation of H.sub.2 and CO.sub.2 for separate collections.

(12) Without further elaboration, it is believed that one skilled in the art can, based on the above description, utilize the present invention to its fullest extent. The following specific examples are, therefore, to be construed as merely illustrative, and not limitative of the remainder of the disclosure in any way whatsoever. All publications cited herein are incorporated by reference.

Example 1

(13) A study was conducted to assess the efficiency of a Ni—La/SBA-15 catalyst combined with hollow fiber membranes in biomass gasification as follows.

(14) Each hollow fiber membrane was prepared by calcining BaBi.sub.0.05Co.sub.0.8Nb.sub.0.15O.sub.3-δ (BBCN) perovskite powders at 1050° C. to form pure perovskite structure, which was further fabricated by a phase inversion and sintering technique. See Wang et al., Journal of Membrane Science, 465, 151-158 (2014); and Wang et al., Journal of Membrane Science 431, 180-186 (2013).

(15) Permeation of O.sub.2 through the BBCN membrane was demonstrated at 600-900° C., a temperature range desirable for biomass gasification. An O.sub.2 permeation flux rate as high as 10 ml cm.sup.−2 min.sup.−1 was achieved in this temperature range. At 950° C., the BBCN membrane unexpectedly showed an oxygen flux rate of 14 ml cm.sup.−2 min.sup.−1, higher than the highest oxygen flux rate of 11.4 ml cm.sup.−2 min.sup.−1 reported in literature for a BaBi.sub.0.05Co.sub.0.8Sc.sub.0.1O.sub.3-δ (BBCS) membrane. See Wang et al., Journal of Membrane Science, 465, 151-158 (2014); and Wang et al., Journal of Membrane Science 431, 180-186 (2013).

(16) A Ni—La/SBA-15 catalyst was prepared by mixing 1.32 g of nickel nitrate hexahydrate and 0.141 g of lanthanum nitrate hexahydrate in 10 mL of de-ionized water, followed by addition of 0.64 g of oleic acid (mol(oleic acid/Ni)=0.5) and 5 g of silica (specific surface area=753 m.sup.2/g). The resulting sample was impregnated at 60° C., dried at 100° C., and calcined at 700° C. to form the nickel-based catalyst. See Sibudjing et al., PCT/SG2014/000108.

(17) The efficiency of the Ni—La/SBA-15 catalyst was assessed in gasification of cellulose (biomass) as follows. The Ni—La/SBA-15 catalyst (200 mg) was packed and placed in the gasification reactor shown in FIG. 1 to form a catalytic bed therein. Prior to the reaction, the Ni—La/SBA-15 catalyst was reduced under pure H.sub.2 at 700° C. for 1 hour. The cellulose (biomass) was then introduced together with the steam to the reactor to react with the O.sub.2 permeating through the BBCN hollow fiber membranes inside the reactor to produce syngas, tar, and ash. The tar thus produced, in the presence of the catalytic bed, reacted with the steam to produce more syngas.

(18) The gasification of cellulose (120 mg/min) was found to result in a H.sub.2 formation rate of 4000˜4500 μmol/min, a CO formation rate of 2500˜3000 μmol/min, a CO.sub.2 formation rate of about 1000 μmol/min, and a CH.sub.4 formation rate of about 500 μmol/min.

Example 2

(19) A study was conducted in the same manner detailed in Example 1 to compare total gas formation yields in biomass gasification using different catalysts and various types of biomass.

(20) The results set forth below indicate that a Ni—La/SBA-15 catalyst outperformed a Ni—PS—Mg catalyst (see Sibudjing et al., PCT/SG2014/000108) or no catalyst.

(21) The gasification of Palm Kernel Shell (biomass, obtained from Palm Plantation, Malaysia) unexpectedly resulted in a total gas formation rate of about 9000 μmol/min using a Ni—La/SBA-15 catalyst and about 7000 μmol/min using a Ni—PS—Mg catalyst, compared with about 3000 μmol/min without using any catalyst.

(22) The gasification of wood (biomass) unexpectedly resulted in a total gas formation rate of about 9000 μmol/min using a Ni—La/SBA-15 catalyst, compared with about 4000 μmol/min without using any catalyst.

(23) The gasification of cellulose (biomass) resulted in a total gas formation rate of about 9000 μmol/min using a Ni—La/SBA-15 catalyst, compared with about 6000 μmol/min using a Ni—PS—Mg catalyst.

Example 3

(24) In a study detailed below, syngas obtained from biomass gasification was subjected to a WGS reactor to react with steam.

(25) A Ni—Cu/CeO.sub.2 catalyst was prepared by mixing nickel nitrate hexahydrate and copper nitrate trishydrate, followed by addition of CeO.sub.2. The resulting catalyst was impregnated and calcined. See Saw et al., Journal of Catalysis, 314, 32-46 (2014); and Sibudjing et al., PCT/SG2014/000108.

(26) A palladium alloy hollow membrane was prepared by a phase-inversion method, followed by coating on an inner surface of the membrane with a palladium-silver alloy film. See Sibudjing et al., WO 2013/133771 A1.

(27) An assay was conducted to assess CO conversion rates during the WGS reaction using the Ni—Cu/CeO.sub.2 catalyst with and without the palladium alloy hollow membrane as follows. The Ni—Cu/CeO.sub.2 catalyst was packed around the palladium alloy hollow membrane. Prior to the reaction, the catalyst was reduced under pure H.sub.2 at 600° C. for 1 hour. The syngas obtained from the gasification reactor was then introduced to the WGS reactor, which was maintained at 2 bar using a back pressure regulator. A sweep gas was introduced into the palladium alloy membrane to carry the H.sub.2 permeating therethrough out of the WGS reactor.

(28) The CO conversion rate was found to be much higher when the membrane was used with the catalyst, compared with that observed when only the catalyst was used, i.e., 60% vs. 40%.

Other Embodiments

(29) All of the features disclosed in this specification may be combined in any combination. Each feature disclosed in this specification may be replaced by an alternative feature serving the same, equivalent, or similar purpose. Thus, unless expressly stated otherwise, each feature disclosed is only an example of a generic series of equivalent or similar features.

(30) Further, from the above description, one skilled in the art can easily ascertain the essential characteristics of the present invention, and without departing from the spirit and scope thereof, can make various changes and modifications of the invention to adapt it to various usages and conditions. Thus, other embodiments are also within the claims.