Atanassova, Paolina (Albuquerque, NM, US)
Shen, Jian-ping (Albuquerque, NM, US)
Brewster, James (Rio Rancho, NM, US)
Napolitano, Paul (Albuquerque, NM, US)
Plaque It!
Sponsored by: Flash of Genius |
2. A method as recited in claim 1, wherein said converting step comprises steam reforming of said carbon-based fuel.
3. A method as recited in claim 3, wherein said first conversion catalyst is a steam reforming catalyst.
4. A method as recited in claim 1, wherein said converting step is selected from the group consisting of auto-thermal reforming, partial oxidation and catalytic partial oxidation of said carbon-based fuel.
5. A method as recited in claim 1, further comprising the step of contacting said H2-rich gas with a water-gas shift catalyst.
6. A method as recited in claim 1, wherein said repeating step comprises repeating steps (a), (b), (c), (d) and (e) at least 50 times.
7. A method as recited in claim 1, wherein said repeating step comprises repeating steps (a), (b), (c), (d) and (e) at least 100 times.
8. A method as recited in claim 1, wherein said repeating step comprises repeating steps (a), (b), (c), (d) and (e) at least 500 times.
9. A method as recited in claim 1, wherein said absorbent material retains at least about 70 mol. % of said theoretical capacity after said repeating step.
10. A method as recited in claim 1, wherein said absorbent material retains at least about 90 mol. % of said theoretical capacity after said repeating step.
11. A method as recited in claim 1, wherein said repeating step comprises repeating steps (a), (b), (c), (d) and (e) at least 200 times and wherein said absorbent material retains at least about 10 mol. % of said theoretical absorption capacity after said repeating step.
12. A method as recited in claim 1, wherein said repeating step comprises repeating steps (a), (b), (c), (d) and (e) at least 200 times and wherein said absorbent material retains at least about 25 mol. % of said theoretical absorption capacity after said repeating step.
13. A method as recited in claim 1, wherein said repeating step comprises repeating steps (a), (b), (c), (d) and (e) at least 200 times and wherein said absorbent material retains at least about 50 mol. % of said theoretical absorption capacity after said repeating step.
14. A method as recited in claim 1, wherein said absorbent material comprises at least one metal oxide selected from the group consisting of Group 1 and Group 2 metal oxides.
15. A method as recited in claim 1, wherein said absorbent material comprises a calcium-containing compound.
16. A method as recited in claim 1, wherein said absorbent material comprises CaO.
17. A method as recited in claim 1, wherein said absorbent material is selected from a group consisting of CaO:MgO, CaO:Al2O3, CaO:TiO2, CaO:ZrO2 and CaO:Al2O3:MgO.
18. A method as recited in claim 1, wherein said absorbent material comprises CaO:Al2O3.
19. A method as recited in claim 1, wherein said absorbent material comprises CaO:TiO2.
20. A method as recited in claim 1, wherein said absorbent material comprises of at least 30 wt. % CaO.
21. A method as recited in claim 1, wherein said absorbent material comprises Li2O.
22. A method as recited in claim 1, wherein said contacting step occurs at a temperature of not greater than about 800° C.
23. A method as recited in claim 1, wherein said carbon-based fuel is a hydrocarbon-based fuel.
24. A method as recited in claim 1, wherein said carbon-based fuel is a gaseous fuel.
25. A method as recited in claim 1, wherein said carbon-based fuel comprises methane.
26. A method as recited in claim 1, wherein said carbon-based fuel comprises a liquid fuel.
27. A method as recited in claim 1, wherein said carbon-based fuel comprises a liquid fuel selected from the group consisting of diesel fuel, JP-8 aviation fuel, kerosene, ethanol and gasoline.
28. A method as recited in claim 1, wherein said H2-rich gas comprises at least about 95 mol. % H2 after each said repeating steps.
29. A method as recited in claim 1, wherein said regenerating step comprises heating said absorbent material to a temperature of at least about 700° C.
30. A method as recited in claim 1, wherein said absorbent material is pelletized.
31. A method as recited in claim 1, wherein said absorbent material is in the form of a monolith.
32. A method as recited in claim 1, wherein said absorbent material is in the form of an extrudate.
33. A method as recited in claim 1, wherein said first conversion catalyst is pelletized.
34. A method as recited in claim 1, wherein said absorbent and said first conversion catalyst are formed into extrudates, at least a portion of said extrudates comprise both of said absorbent and said first conversion catalyst.
35. A method as recited in claim 1, wherein said absorbent material is coated on a support structure.
36. A method as recited in claim 1, wherein said absorbent material has substantially spherical morphology.
37. A method as recited in claim 1, wherein said absorbent material retains at least about 10 grams CO2 per 100 grams unreacted absorbent compound after each of said repeating steps.
38. A method as recited in claim 1, wherein said absorbent material retains at least about 20 grams CO2 per 100 grams unreacted absorbent compound after each of said repeating steps.
39. A method as recited in claim 1, wherein said absorbent material retains at least about 40 grams CO2 per 100 grams unreacted absorbent compound after each of said repeating steps.
40. A method as recited in claim 1, further comprising providing steam with said carbon-based fuel.
41. A method as recited in claim 1, further comprising providing an oxygen-containing gas with said carbon-based fuel.
42. A method as recited in claim 1, wherein said absorbent material is pelletized and wherein said contacting step comprises contacting said intermediate gas product with said pelletized absorbent material having a first bulk density, wherein said repeating steps convert said pelletized absorbent material to a carbonized absorbent material having a second bulk density, and after said repeating steps said carbonized absorbent material has a third bulk density, wherein said third bulk density is greater than said first bulk density.
43. A method as recited in claim 42, wherein said third bulk density is up to about 140% of said first bulk density.
44. A method as recited in claim 1, wherein said carbon-based fuel comprises methane and wherein said first conversion catalyst comprises: (a) a particulate support structure; and (b) a metal dispersed on said support structure, wherein said conversion catalyst is capable of achieving at least about 90% of the theoretical thermodynamic conversion of methane to hydrogen at a temperature of 600° C., a H2O:C ratio of 3:1 and a gas hour space velocity (GHSV) of 5000 h−1 in the absence of an absorbent for CO2.
45. A method as recited in claim 44, wherein said catalyst achieves at least about 95% of the theoretical thermodynamic conversion.
46. A method as recited in claim 44, wherein said catalyst is capable of achieving at least about 90% of the theoretical thermodynamic conversion of methane to hydrogen at a temperature of 600° C., a H2O:C ratio of 3:1 and a gas hour space velocity (GHSV) of 10000 h−1 in the absence of an absorbent for CO2.
47. A method as recited in claim 44, wherein said catalyst is capable of achieving at least about 90% of the theoretical thermodynamic conversion of methane to hydrogen at a temperature of 600° C., a H2O:C ratio of 3:1 and a gas hour space velocity (GHSV) of 12500 h−1 in the absence of an absorbent for CO2.
48. A method as recited in claim 44, wherein said support is selected from the group consisting of the metal oxides of aluminum, cerium, zirconium, lanthanum, silicon, magnesium, zinc and combinations thereof.
49. A method as recited in claim 44, wherein said dispersed metal is selected from the group consisting of Rh, Ni, Ru, Pt, Pd and alloys thereof.
50. A method as recited in claim 44, wherein said dispersed metal comprises Rh.
51. A method as recited in claim 44, wherein said conversion catalyst comprises from about 0.1 wt. % to about 5 wt. % of said metal.
52. A method as recited in claim 44, wherein said support structure comprises Al2O3 and said dispersed metal comprises Rh.
53. A method as recited in claim 44, wherein said conversion catalyst is pelletized.
54. A method as recited in claim 44, wherein said conversion catalyst is coated on a support.
55. A method as recited in claim 44, wherein said reforming catalyst has substantially spherical morphology.
56. A method as recited in claim 44, further comprising the step of contacting said intermediate gas phase with a water-gas shift catalyst.
57. A method as recited in claim 56, wherein said water-gas shift catalyst comprises a metal dispersed on a support phase, said metal being selected from the group consisting of Fe, Co, Cu and Cr.
58. A method as recited in claim 1, wherein said absorbent material and said first conversion catalyst are in the form of a particulate composite material, said particulate composite material comprising a first phase comprising said absorbent material, and a second phase comprising said conversion catalyst.
59. A method as recited in claim 58, wherein the mass ratio of said first phase to said second phase is greater than 1:1.
60. A method as recited in claim 58, wherein the mass ratio of said first phase to said second phase is from about 20:1 to about 3:1.
61. A method as recited in claim 58, wherein the mass ratio of said first phase to said second phase is from about 9:1 to about 5:1.
62. A method as recited in claim 58, wherein said particulate composite material is pelletized.
63. A method as recited in claim 58, wherein said particulate composite material is coated on a support structure.
64. A method as recited in claim 58, wherein said particulate composite material is a monolithic structure.
65. A method as recited in claim 58, wherein said particulate composite material has an average particle size (d50) of from about 1 μm to about 50 μm.
66. A steam-reforming catalyst, comprising: (a) a particulate support structure; and (b) a metal dispersed on said support structure, wherein said steam-reforming catalyst is capable of achieving at least about 90% of the theoretical thermodynamic conversion of methane to hydrogen at a temperature of 600° C., a H2O:C ratio of 3:1 and a gas hour space velocity (GHSV) of 5000 h−1 in the absence of an absorbent for CO2.
67. A reforming catalyst as recited in claim 66, wherein said catalyst achieves at least about 90% of the theoretical thermodynamic conversion.
68. A reforming catalyst as recited in claim 66, wherein said catalyst achieves at least about 90% of the theoretical thermodynamic conversion of methane to hydrogen at a temperature of 600° C., a H2O:C ratio of 3:1 and a gas hour space velocity (GHSV) of 10000 h−1 in the absence of an absorbent for CO2.
69. A reforming catalyst as recited in claim 66, wherein said catalyst achieves at least about 90% of the theoretical thermodynamic conversion of methane to hydrogen at a temperature of 600° C., a H2O:C ratio of 3:1 and a gas hour space velocity (GHSV) of 12500 h−1 in the absence of an absorbent for CO2.
70. A reforming catalyst as recited in claim 66, wherein said support is selected from the group consisting of the metal oxides of aluminum, cerium, zirconium, lanthanum, silicon, magnesium, zinc and combinations thereof.
71. A reforming catalyst as recited in claim 66, wherein said dispersed metal is selected from the group consisting of Rh, Ni, Ru, Pt, Pd and alloys thereof.
72. A reforming catalyst as recited in claim 66, wherein said dispersed metal comprises Rh.
73. A reforming catalyst as recited in claim 66, wherein said reforming catalyst comprises from about 0.1 wt. % to about 5 wt. % of said metal.
74. A reforming catalyst as recited in claim 66, wherein said support structure comprises Al2O3 and said dispersed metal comprises Rh.
75. A reforming catalyst as recited in claim 66, wherein said reforming catalyst is pelletized.
76. A reforming catalyst as recited in claim 66, wherein said reforming catalyst is coated on a support.
77. A reforming catalyst as recited in claim 66, wherein said reforming catalyst has substantially spherical morphology.
78. A particulate composite material, comprising: (a) a first phase comprising an absorbent material adapted to absorb CO2; and (b) a second phase comprising a conversion catalyst selected from the group consisting of a reforming catalyst and a water-gas shift catalyst.
79. A particulate composite material as recited in claim 78, wherein said absorbent material comprises a calcium compound.
80. A particulate composite material as recited in claim 78, wherein the mass ratio of said absorbent material to said conversion catalyst is greater than 1:1.
81. A particulate composite material as recited in claim 78, wherein the mass ratio of said absorbent material to said conversion catalyst is from about 20:1 to about 3:1.
82. A particulate composite material as recited in claim 78, wherein the mass ratio of said absorbent material to said catalyst is from about 9:1 to about 5:1.
83. A particulate composite material as recited in claim 78, wherein said catalyst comprises a supported metal.
84. A particulate composite material as recited in claim 78, wherein said catalyst comprises a reforming catalyst.
85. A particulate composite material as recited in claims 78, wherein said catalyst is a reforming catalyst comprising a metal selected from Rh, Ni, Ru, Pt, Pd, and alloys thereof dispersed on a support phase selected from the group consisting of metal oxides of aluminum, cerium, zirconium, lanthanum, silicon, magnesium, zinc and combinations thereof.
86. A particulate composite material as recited in claim 78, wherein said catalyst comprises a water-gas shift catalyst.
87. A particulate composite material as recited in claim 78, wherein said catalyst is a water-gas shift catalyst comprising a metal dispersed on a support phase, said metal being selected from the group consisting of Fe, Co, Cu and Cr.
88. A particulate composite material as recited in claim 78, wherein said particulate composite material is pelletized.
89. A particulate composite material as recited in claim 78, wherein said particulate composite material is coated on a support structure.
90. A particulate composite material as recited in claim 78, wherein said absorbent material comprises an absorbent compound having a reaction fraction of at least about 70 mol. %.
91. A particulate composite material as recited in claim 78, wherein said absorbent material comprises an absorbent compound having a reaction fraction of at least about 30 mol. % after 100 cycles.
92. A particulate composite material as recited in claim 78, wherein said absorbent material comprises a reversible absorbent compound.
93. A particulate composite material as recited in claim 78, wherein said absorbent material comprises an absorbent compound that retains at least about 50 mol. % of the theoretical CO2 absorption capacity of the absorbent compound after at least about 10 cycles.
94. A particulate composite material as recited in claim 78, wherein said absorbent material comprises an absorbent compound that retains at least about 70 mol. % of the theoretical CO2 absorption capacity of the absorbent compound after at least about 10 cycles.
95. A particulate composite material as recited in claim 78, wherein said absorbent material comprises an absorbent compound that retains at least about 90 mol. % of the theoretical CO2 absorption capacity of the absorbent compound after at least about 0.10 cycles.
96. A particulate composite material as recited in claim 78, wherein said absorbent material comprises an absorbent compound that retains at least 10 grams CO2 per 100 grams of unreacted absorbent compound after 10 cycles.
97. A particulate composite material as recited in claim 78, wherein said absorbent material comprises an absorbent compound that retains at least 20 grams CO2 per 100 grams unreacted absorbent after 10 cycles.
98. A particulate composite material as recited in claims 78, wherein said absorbent material comprises an absorbent compound that retains at least 30 grams CO2 per 100 grams unreacted absorbent after 10 cycles.
99. A particulate composite material as recited in claim 78, wherein said absorbent material comprises an absorbent compound that retains at least 40 grams CO2 per 100 grams unreacted absorbent after 10 cycles.
100. A particulate composite material as recited in claim 78, wherein said absorbent material comprises an absorbent compound that retains at least 50 grams CO2 per 100 grams unreacted absorbent after 10 cycles.
101. A particulate composite material as recited in claim 78, wherein said composite material has substantially spherical morphology.
102. A method for the fabrication of composite particles including an absorbent material and a catalyst, comprising the steps of: (a) forming a precursor solution, said precursor solution comprising: (i) a liquid; (ii) a precursor to an absorbent material; and (iii) a precursor to a catalyst phase; (b) atomizing said precursor solution to form precursor droplets; and (c) heating said precursor droplets to remove said liquid therefrom and form said composite particles.
103. A method as recited in claim 102, wherein said heating step comprises a first heating step to react said active absorbent precursor to an intermediate precursor compound and a second heating step to convert said intermediate precursor compound to said absorbent material.
104. A method as recited in claim 103, wherein said second heating step is at a temperature that is higher than said first heating step.
105. A method as recited in claim 102 wherein said heating step comprises heating said precursor solution to a temperature sufficient to form said absorbent material in a single step.
106. A method as recited in claim 102, wherein said absorbent material precursor is selected from the group consisting of calcium oxalate, calcium nitrate, calcium acetate, calcium lactate and calcium hydroxide.
107. A method as recited in claim 102, wherein said absorbent material precursor comprises calcium nitrate.
108. A method as recited in claim 102, wherein said precursor solution further comprises a porosity enhancing agent.
109. A method as recited in claim 102, wherein said catalyst phase precursor comprises a precursor to a metal selected from the group consisting of Rh, Ni, Ru, Pt, Pd and alloys thereof.
110. A method as recited in claim 102, wherein said catalyst phase precursor comprises a precursor to Rh.
111. A method as recited in claim 102, wherein said catalyst phase precursor comprises particulate alumina.
112. A method as recited in claim 102, wherein said composite particles have an average particle size (d50) of from about 1 μm to about 30 μm.
113. A method as recited in claim 102, wherein said heating step comprises heating said precursor droplets to a temperature of at least about 200° C.
114. A method as recited in claim 102, wherein said heating step is carried out in a spray dryer.
115. A method as recited in claim 102, wherein said composite particles have substantially spherical morphology.
116. A method for the fabrication of a conversion catalyst, comprising the steps of: (a) forming a precursor solution comprising a metal precursor and a support precursor; and (b) atomizing said precursor solution to form precursor droplets; (c) heating said precursor droplets to convert at least said metal precursor to metal-containing clusters dispersed on said support.
117. A method as recited in claim 116, wherein said metal precursor is selected from group consisting of acetate and nitrate salts of Rh, Ni, Ru, Pt, Pd and mixtures thereof.
118. A method as recited in claim 116, wherein said support precursor is selected from the group consisting of alumina, ceria, zirconia, silica, magnesium oxide, zinc oxide and combinations thereof.
119. A method as recited in claim 116, wherein said support precursor is selected from the group consisting of acetate and nitrate salts of aluminum, cerium, zirconium, silicon, magnesium, zinc and combinations thereof.
120. A method as recited in claim 116, wherein said heating step comprises heating said precursor droplets to a temperature of at least about 200° C.
121. A method as recited in claim 116, wherein said heating step comprises heating said precursor droplets to a temperature of at least about 300° C.
122. A method as recited in claim 116, wherein said heating step is carried out in a spray dryer.
123. A method as recited in claim 116, wherein said heating step is carried out in a spray pyrolysis reactor.
124. A method as recited in claim 116, wherein said the particles have substantially spherical morphology.
CROSS-REFERENCE TO RELATED APPLICATIONS
This application is a continuation-in-part application of U.S. patent application Ser. No. 10/723,424 entitled “FUEL REFORMER CATALYST AND ABSORBENT MATERIALS,” filed Nov. 26, 2003. This application also claims priority to U.S. Provisional Patent Application Ser. No. 60/525,467 entitled “CARBON DIOXIDE ABSORBENT MATERIALS AND METHODS FOR MAKING SAME”, filed on Nov. 26, 2003. This application is also related to U.S. patent application Ser. No. ______, entitled “PARTICULATE ABSORBENT MATERIALS AND METHODS FOR MAKING SAME”, filed on Nov. 24, 2004, and further identified by Attorney File No. 41890-01734. The disclosure of each of the foregoing applications is incorporated herein by reference in its entirety.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to particulate materials that are useful for fuel reformers, such as particulate catalyst materials for hydrogen (H2) production from carbon-based fuels and particulate absorbent materials for the removal of acid gases such as CO2 and H2S from gas streams. The particulate materials can be produced by spray processing of precursors to form a powder batch of the particulate absorbent materials, or intermediate compounds that can be converted to the absorbent material. The present invention is also directed to fuel reformers incorporating the particulate materials and methods for using the materials. The present invention is also directed to the combination of a highly reversible, high-capacity CO2 absorbent material with steam reforming and/or water gas shift catalysts to achieve single step reforming of hydrocarbon fuels to H2 with a high conversion efficiency. The particulate materials can be formed into extrudates, pellets or monoliths, or can be coated onto a substrate.
2. Description of the Related Art
Hydrogen (H2) is an important material in the chemical, petroleum and energy industries. In the chemical and petroleum industries, H2 is used for the manufacture of ammonia (NH4) and methanol (CH3OH), and is used in a variety of petroleum hydrotreating processes. A growing demand for H2 is forecast in the future, particularly for petroleum refining of heavy, high-sulfur crude oil. H2 is also an environmentally clean energy source for the generation of electric power and space heating, and a substantial increase in H2 demand is expected in the near future.
Steam reforming, including steam-methane reforming (SMR), partial oxidation (POX) and autothermal reforming (ATR) are the major processes for H2 production from fossil-based fuels such as natural gas, and are expected to remain processes of choice for the next several decades. These fuel-processing technologies involve multiple steps and severe operating conditions. For example, SMR involves the endothermic reaction of CH4 (methane, e.g., from natural gas) with water to form H2 and carbon monoxide (CO). The primary reformer operates at a temperature of approximately 800° C. to 850° C. and about 20 atm of pressure, and large quantities of supplemental fuel must be burned to supply the energy necessary to maintain the reformer temperature. The reforming step is followed by at least one water gas shift (WGS) reactor to increase the H2 content and reduce the CO content. This is followed by CO cleanup using selective oxidation, hydrogen membrane separation, pressure swing adsorption (PSA) or methanation. The reactions that occur during SMR of CH4, are illustrated by Equations 1 to 3:
Reforming CH4+H2O→3H2+CO (1)
Shift CO+H2O→H2+CO2 (2)
Cleanup CO+O2→CO2 (3)
Another commercially available method for the production of hydrogen from hydrocarbons is partial oxidation (POX). According to this method, CH4 or a similar hydrocarbon feed stock is oxidized to produce CO and H2 in accordance with the reaction illustrated by Equation 4:
CH4+½O2→2H2+
CO (4)
The efficiency of the POX reactor is relatively high, however, POX systems are typically less energy efficient than SMR because of the utilization of higher temperatures and the problem of heat recovery.
Auto thermal reforming (ATR) combines some of the features of SMR and POX. In ATR, a hydrocarbon feed such as CH4 is reacted with steam and air to produce a H2-rich gas. Both the SMR and POX reactions take place (Equations 1 and 4). With the correct mixture of input fuel, air and steam, the POX reaction supplies all the heat needed to drive the catalytic SMR reaction. However, as with SMR and POX systems, a WGS reactor and a H2 purification stage are required to remove carbon oxides.
Fuel cells provide electricity through chemical oxidation-reduction reactions and have tremendous advantages over other types of power generation devices in terms of energy efficiency and environmental compatibility. For low temperature applications, the most promising type of fuel cell is the proton exchange membrane (PEM) fuel cell, which employs H2 as a fuel in the anode and O2 as an oxidant in the cathode. However, the cost of constructing the distribution infrastructure to safely transport pure H2 gas over long distances presents an economic barrier to the exploitation of fuel cells, particularly for the transportation sector. Therefore, distributed production by smaller reforming systems that convert hydrocarbons to H2 is a more viable option for the near future. However, conventional fuel-processing technologies for H2 production from hydrocarbons are unsatisfactory for providing H2 to PEM fuel cells due to low reforming efficiencies resulting from the multiple steps and severe operating conditions that are required, as is discussed above. Further, the reformate typically has a low H2 content (45 to 50 mol. % on a dry basis) and a high CO and CO2 content. The reformate can also include other gases, such as N2, depending on the reforming method.
The low H2 content in the reformate reduces the fuel cell performance and requires that greater amounts of expensive CO tolerant catalysts (typically Pt-based) and membrane materials be utilized for reasonable system efficiency as compared to fuel cells operating on pure H2. The high levels of CO2 in the reformate also cause two additional problems. The CO2 converts to CO in the PEM stack due to the reverse water gas shift reaction (reverse of Equation 2) and the CO can poison the catalyst. Also, the acidic nature of CO2 and water solutions promotes a number of reactions that can reduce the useful lifetime of the PEM stacks to some extent.
The deficiencies of conventional fuel reforming processes can be overcome to a certain extent by following the WGS step with amine scrubbing, hydrogen membrane separation and/or pressure swing adsorption (PSA). With amine scrubbing it is often necessary to further reduce the concentration of carbon oxides to trace levels by methanation. PSA requires operation at a significant pressure, which lowers system efficiency and produces a tail gas containing 25% to 30% of the H2 produced during column blowdown and purge. While the energy content of the tail gas can be recovered and used in the reforming process, it is often the case that the energy content of the combined fuel cell anode tail gas and the purification tail gas is greater than the energy required by the reforming process.
A variety of approaches have been explored to develop a fuel processing technology that uses simple chemical processes, has low energy consumption and generates high purity H2. These include the application of reaction-separation membranes and the application of absorption materials. One promising approach is absorption enhanced reforming (AER). AER combines a SMR catalyst and a CO2 absorbent (e.g., CaO) in a single reactor so that reforming, shift, and CO2 absorption occur simultaneously.
CO2 absorption CaO+CO2→CaCO3 (5)
Overall CaO+CH4+2H2O→4H2+CaCO3 (6)
Many potential benefits over conventional reforming have been demonstrated using AER. These include: (i) reforming at a significantly lower temperature (about 600° C.), while achieving an increased conversion of CH4 to H2; (ii) lower capital cost as compared to conventional SMR; (iii) producing H2 at feed gas pressure (200 to 400 psig) and at relatively high purity (>95%) directly from the reactor; (iv) reducing or even eliminating downstream purification steps; (v) minimizing side reactions and increasing catalyst lifetime; (vi) reducing the excess steam used in conventional reforming, particularly when treating heavy fuels; and (vii) effective fixing of the CO2.
It should be noted that various terminology has been used in the literature to describe the reaction of CO2 and a solid material such as CaO. Among the terms used are adsorption, absorption, sorption and fixing of CO2. In general, none of these terms precisely describes this complex process, which starts with adsorption of the CO2 onto the surface of a solid, followed by a chemical conversion of the solid and expansion of this process into the bulk of the solid. Therefore, the terms adsorption, absorption and fixing (to describe the process), and adsorbent and absorbent (to describe the solid material) are used interchangeably within the present specification.
The use of AER for H2 production for use in a fuel cell has been disclosed in U.S. Pat. No. 6,682,838 by Stevens. The benefits of this approach for hydrogen production from solid fuels such as biomass and coal have also been demonstrated by S. Lin et al. (Fuel 2002, 81, 2079).
There are a variety of CO2 absorption materials available for AER. Reactive CO2 absorption materials such as CaO-based absorbents are preferred because these types of materials typically have much higher equilibrium capacities than other absorbents. For example, under ideal conditions methylethanolamine captures 6 g/100 g (grams of CO2 per 100 gram of material), silica gel absorbs 1.32 g/100 g, and activated carbon absorbs 8.8 g/100 g. Materials used for PSA such as K2CO3/Hydrotalcite can only remove a small portion of CO2, about 1.98 g/100 g. In contrast, CaO can capture up to 78.57 g/100 g. Even assuming only a 50 wt. % capacity over repeated cycles, the value of 39.3 g/100 g for CaO is 5 to 10 times higher than the above absorbents.
The conversion of hydrocarbons in the presence of steam and a CO2 absorbent can be traced back to as early as 1868. Recently some results for hydrogen production using this concept have been reported by D. P. Harrison et al., Chemical Engineering Science 1999, 54, 3543. The CO2 absorbents typically used for AER in the literature have poor reactivity, low CO2 capacity, and poor recyclability. The key to successfully commercialize AER methods is to develop an absorbent with high activity and capacity, and particularly with high recyclability to maintain sufficient activity and capacity over numerous carbonation and decarbonation cycles.
Natural CaO-based absorbents such as limestone and dolomite are plentiful and inexpensive, but they are soft and friable and do not stand up well to handling and recycle use. To improve the recyclability, some work has been focused on the pelletizing of limestone by using different binders. See, for example, U.S. Pat. No. 4,316,813 by Voss et al. Some work also focused on the modification of natural materials such as dolomite to tailor the physicochemical properties of the material. The synthesis of a CaO-based absorbent through boiling of CaO into Ca(OH)2 or the carbonation of calcium salt solution such as calcium nitrate or Ca(OH)2 into calcium carbonate, then decomposition of the carbonate into CaO has been disclosed by L. S. Fan et al., Ind. Eng. Chem. Res., 1999, 38, 2283. Others have disclosed the preparation of CaO-based materials by aerogel methods.
Another class of sorption materials effective for CO2 removal for both syngas and effluents are lithium-base materials such as mixed oxides of lithium with silicon and/or zirconium. For example lithium zirconate (e.g., LiZrO2) and lithium silicates (having the general formula LixSiyOz) as is described in U.S. Pat. No. 6,387,845 by Masahiro et al., the contents of which are incorporated herein by reference in its entirety, are examples of such materials. It is disclosed that these materials can also incorporate other dopants to enhance their performance, such as Al, K, Fe, Mg and the like, and that the lithium-based materials are reversible upon the application of heat. While the use of lithium zirconate is more widespread at present, the adoption of lithium silicate is increasing due to its lower production costs, lighter weight and more rapid CO2 absorption capabilities. For example, one gram of lithium silicate is capable of absorbing 62 milligrams of CO2, making the material 30 times more efficient than lithium zirconate. Lithium silicate is also 70 percent lighter and about 85 percent less expensive than lithium zirconate, since it uses silicon instead of the more expensive zirconium as a starting material.
The foregoing methods generally result in limited control over the composition and microstructure of the powders. The morphology and surface properties such as surface area, pore volume and pore size are among the characteristics that have a critical impact on the performance of the absorbent. This is due to the nature of the reactions that occur. First, carbonation takes place on the external and internal surfaces of CaO-based absorbent, which forms a carbonate layer. Then, the chemical reaction advances with the diffusion of CO2 through the carbonate layer into the unreacted core CaO active sites. Therefore, higher reactivity and faster kinetics can be expected for small particle size CaO due to the higher surface to bulk ratio of the absorbent species. A more porous structure will also lead to higher reactivity and recyclability, and a lower decarbonation temperature due to the easier CO2 diffusion into or out of the outer carbonate layer.
Despite the theoretical improvement offered by AER, it has not been widely implemented. One of the major barriers to the implementation of AER has been the need for a CO2 absorbent with high performance (e.g., high CO2 absorption capacity) that does not degrade significantly over the number of cycles (removal of CO2 and generation of H2 followed by regeneration of the CO2 absorbent) that are required for a commercial product. That is, after CO2 absorption, the absorbent must be regenerated, (decarbonized), to remove the absorbed CO2. Currently available absorbent materials start to degrade in performance over just a few cycles and cannot retain a high constant capacity during subsequent cycles, and therefore are not commercially useful for most applications.
It would be advantageous to provide a method for producing absorbent powders that would enable control over the powder characteristics such as particle size, surface area and pore structure, as well as the versatility to accommodate compositions which are either difficult or impossible to produce using existing production methods. It would be particularly advantageous if such powders could be produced in large quantities on a substantially continuous basis. Further value can be derived from these powders if they can be incorporated into structures that can be integrated into reactor beds that enable a suitable combination of high space velocity and high absorption capacity while retaining their performance characteristics.
SUMMARY OF THE INVENTION
The present invention is generally directed to methods and materials that are useful for the conversion of a carbon-based fuel to a H2-rich product gas. These methods are referred to herein as absorption enhanced reforming (AER).
According to one embodiment of the present invention, a method for the conversion of a carbon-based fuel to a H2-rich gas is provided. The method includes the steps of providing a carbon-based fuel, converting the carbon-based fuel to an intermediate gas product by contacting the carbon-based fuel with at least a first conversion catalyst, contacting the intermediate gas product with an absorbent material to absorb CO2 and form a H2-rich gas, where the absorbent material has a theoretical absorption capacity for CO2. The H2-rich gas is extracted from the contacting step and the absorbent is then regenerated. The above steps are repeated at least ten times, wherein the absorbent material retains at least about 50 mol. % of its theoretical absorption capacity after each of the repeating steps. Thus, the absorbent material advantageously maintains a high absorption capacity over a number of cycles. According to one aspect of this embodiment of the present invention, the converting step includes steam reforming of the carbon-based fuel and the first conversion catalyst is a steam reforming catalyst. The converting step can also be selected from the group consisting of auto-thermal reforming, partial oxidation and catalytic partial oxidation of the carbon-based fuel. The H2-rich gas that is extracted from the contacting step can be further contacted with a water-gas shift catalyst.
The absorbent material can also retain a high absorption capacity over many more cycles and in one embodiment can maintain its absorption capacity over at least 50 cycles, over at least 100 cycles and even over 500 cycles. Further, the absorbent material can retain 70 mol. % and even at least 90 mol. % of its theoretical capacity after each of the repeating steps. In one embodiment, the steps are repeated at least 200 times and the absorbent material retains at least about 10 mol. % of its theoretical absorption capacity after each repeating step. The absorbent material can even retain at least about 25 mol. % and even about 50 mol. % of its theoretical absorption capacity after each of the 200 repeating steps.
The absorbent material of the present invention can include a metal oxide selected from the group consisting of Group 1 and Group 2 metal oxides. For example, the absorbent material can be a calcium-containing compound such as calcium oxide. The absorbent material in one aspect is selected from the group consisting of CaO:MgO, CaO:Al2O3, CaO:TiO2, CaO:ZrO2 and CaO:Al2O3:MgO. Particularly preferred among these are CaO:Al2O3, CaO:TiO2. The absorbent material preferably includes at least about 30 weight percent CaO. Other preferred absorbent materials include lithium oxide.
It is preferred that the contacting step occurred at a temperature of not greater than about 800° C. The carbon-based fuel can be a hydrocarbon-based fuel, can be a gaseous fuel such as methane or can be a liquid fuel such as diesel fuel, JP-8 aviation fuel, kerosene, ethanol or gasoline. The fuel can also be LPG. According to one aspect, the H2-rich gas that is extracted from the contacting step includes at least about 95 mol. % H2 after each of the repeating steps.
According to one aspect, the regenerating step includes heating the absorbent material to a temperature of at least about 700° C. The absorbent material can be pelletized, can be in the form of monolith or can be an extrudate. Further, the conversion catalyst can also be pelletized. According to one aspect, the absorbent and the first conversion catalyst are formed into extrudates where the extrudate includes both the absorbent material and the first conversion catalyst. The absorbent material can have a substantially spherical morphology.
The absorbent material is also capable of retaining at least about 10 grams CO2 per 100 grams of unreacted absorbent compound after each of the repeating steps. According to another aspect, the absorbent material can retain at least about 20 grams and even at least about 40 grams of CO2 per 100 grams of unreacted absorbent compound after each of the repeating steps.
According to another embodiment of the present invention, a steam-reforming catalyst is provided. The steam reforming catalyst includes a particulate support structure and a metal dispersed on the support structure. The steam-reforming catalyst has a high conversion efficiency and in one aspect is capable of achieving at least about 90% of the theoretical thermodynamic conversion of methane to hydrogen at a temperature of 600° C., a H2O:C ratio of 3:1 and a gas hour space velocity (GHSV) of 5000 h−1 in the absence of an absorbent for CO2. According to another aspect, the catalyst achieves at least about 90% of the theoretical thermodynamic conversion. According to another aspect, the catalyst achieves at least about 90% of the theoretical thermodynamic conversion of methane to hydrogen at a temperature of 600° C., a H2O:C ratio of 3:1 and a gas hour space velocity (GHSV) of 10000 h−1 in the absence of an absorbent for CO2.
According to another aspect, the catalyst achieves at least about 90% of the theoretical thermodynamic conversion of methane to hydrogen at a temperature of 600° C., a H2O:C ratio of 3:1 and a gas hour space velocity (GHSV) of 12500 h−1 in the absence of an absorbent for CO2.
According to another aspect, the support is selected from the group consisting of the metal oxides of aluminum, cerium, zirconium, lanthanum, silicon, magnesium, zinc and combinations thereof.
According to another aspect, the dispersed metal is selected from the group consisting of Rh, Ni, Ru, Pt, Pd and alloys thereof. According to another aspect, the reforming catalyst comprises from about 0.1 wt. % to about 5 wt. % of the metal. According to another aspect, the support structure comprises Al2O3 and the dispersed metal comprises Rh.
According to another aspect, the reforming catalyst is pelletized. According to another aspect, the reforming catalyst is coated on a support. According to another aspect, the reforming catalyst has substantially spherical morphology.
According to another embodiment of the present invention, a particulate composite material is provided that includes a first phase comprising an absorbent material adapted to absorb CO2 and a second phase comprising a conversion catalyst selected from the group consisting of a reforming catalyst and a water-gas shift catalyst.
According to one aspect the absorbent material comprises a calcium compound.
According to another aspect the mass ratio of the absorbent material to the conversion catalyst is greater than 1:1. According to another aspect the mass ratio of the absorbent material to the conversion catalyst is from about 20:1 to about 3:1. According to another aspect the mass ratio of the absorbent material to the catalyst is from about 9:1 to about 5:1.
According to another aspect, the catalyst is a reforming catalyst, such as one comprising a metal selected from Rh, Ni, Ru, Pt, Pd, and alloys thereof dispersed on a support phase selected from the group consisting of metal oxides of aluminum, cerium, zirconium, lanthanum, silicon, magnesium, zinc and combinations thereof.
According to another aspect the catalyst is a water-gas shift catalyst, such as one comprising a metal dispersed on a support phase, the metal being selected from the group consisting of Fe, Co, Cu and Cr.
According to one aspect the particulate composite material is pelletized. According to another aspect the particulate composite material is coated on a support structure.
According to another aspect the absorbent material comprises an absorbent compound having a reaction fraction of at least about 70 mol. %.
According to another aspect the absorbent material comprises an absorbent compound having a reaction fraction of at least about 30 mol. % after 100 cycles.
According to another aspect the absorbent material comprises a reversible absorbent compound.
According to another aspect the absorbent material comprises an absorbent compound that retains at least about 50 mol. % of the theoretical CO2 absorption capacity of the absorbent compound after at least about 10 cycles.
According to another aspect the absorbent material comprises an absorbent compound that retains at least about 70 mol. % of the theoretical CO2 absorption capacity of the absorbent compound after at least about 10 cycles.
According to another aspect the absorbent material comprises an absorbent compound that retains at least about 90 mol. % of the theoretical CO2 absorption capacity of the absorbent compound after at least about 10 cycles.
According to another aspect the absorbent material comprises an absorbent compound that retains at least 10 grams CO2 per 100 grams of unreacted absorbent compound after 10 cycles.
According to another aspect the absorbent material comprises an absorbent compound that retains at least 20 grams CO2 per 100 grams unreacted absorbent after 10 cycles.
According to another aspect, the absorbent material comprises an absorbent compound that retains at least 30 grams CO2 per 100 grams unreacted absorbent after 10 cycles.
According to another aspect the absorbent material comprises an absorbent compound that retains at least 40 grams CO2 per 100 grams unreacted absorbent after 10 cycles.
According to another aspect the absorbent material comprises an absorbent compound that retains at least 50 grams CO2 per 100 grams unreacted absorbent after 10 cycles.
According to another aspect the composite material has substantially spherical morphology.
According to another embodiment, a method for the fabrication of composite particles including an absorbent material and a catalyst is provided. The method includes the steps of forming a precursor solution, the precursor solution comprising: (i) a liquid; (ii) a precursor to an absorbent material; and (iii) a precursor to a catalyst phase. The precursor solution is atomized to form precursor droplets and which are heated to remove the liquid therefrom and form the composite particles.
According to another aspect the heating step comprises a first heating step to react the active absorbent precursor to an intermediate precursor compound and a second heating step to convert the intermediate precursor compound to the absorbent material. According to another aspect, the second heating step is at a temperature that is higher than the first heating step.
According to another aspect, the heating step comprises heating the precursor solution to a temperature sufficient to form the absorbent material in a single step.
According to another aspect the absorbent material precursor is selected from the group consisting of calcium oxalate, calcium nitrate, calcium acetate, calcium lactate and calcium hydroxide.
According to another aspect the absorbent material precursor comprises calcium nitrate.
According to another aspect the precursor solution further comprises a porosity enhancing agent.
According to another aspect the catalyst phase precursor comprises a precursor to a metal selected from the group consisting of Rh, Ni, Ru, Pt, Pd and alloys thereof.
According to another aspect the catalyst phase precursor comprises a precursor to Rh.
According to another aspect the catalyst phase precursor comprises particulate alumina.
According to another aspect the composite particles have an average particle size (d50) of from about 1 μm to about 30 μm.
According to another aspect the heating step comprises heating the precursor droplets to a temperature of at least about 200° C.
According to another aspect the heating step is carried out in a spray dryer.
According to another aspect the composite particles have substantially spherical morphology.
According to another embodiment of the present invention, a method for the fabrication of a conversion catalyst is provided. The method includes the steps of forming a precursor solution comprising a metal precursor and a support precursor and atomizing the precursor solution to form precursor droplets, and heating the precursor droplets to convert at least the metal precursor to metal-containing clusters dispersed on the support.
According to one aspect the metal precursor is selected from group consisting of acetate and nitrate salts of Rh, Ni, Ru, Pt, Pd and mixtures thereof.
According to another aspect the support precursor is selected from the group consisting of alumina, ceria, zirconia, silica, magnesium oxide, zinc oxide and combinations thereof.
According to another aspect the support precursor is selected from the group consisting of acetate and nitrate salts of aluminum, cerium, zirconium, silicon, magnesium, zinc and combinations thereof.
According to another aspect the heating step comprises heating the precursor droplets to a temperature of at least about 200° C.
According to another aspect the heating step comprises heating the precursor droplets to a temperature of at least about 300° C.
According to another aspect the heating step is carried out in a spray dryer.
According to another aspect the heating step is carried out in a spray pyrolysis reactor.
According to another aspect the particles have substantially spherical morphology.
According to another embodiment of the present invention, a method for the fabrication of a particulate absorbent material is provided. The method can include the steps of atomizing a liquid-containing precursor solution to form precursor droplets, the precursor solution comprising at least a first precursor to an absorbent compound, heating the precursor droplets to form dried precursor droplets, and converting the dried precursor droplets to an absorbent material comprising an absorbent compound.
According to one aspect, the heating step and the converting step occur sequentially in a spray pyrolysis operation. According to another aspect, the heating step forms an intermediate compound capable of being post-processed to form a particulate absorbent material, and the converting step comprises heating the intermediate compound to form the particulate absorbent material.
According to another aspect, the first precursor is at least partially soluble in the precursor solution. According to another aspect, the first precursor is selected from the group consisting of metal oxalates and metal hydroxides. According to another aspect, the first precursor is selected from the group consisting of calcium nitrate, calcium acetate, calcium oxalate and calcium hydroxide. According to another aspect, the first precursor comprises calcium oxalate.
According to another aspect, the heating step comprises heating the droplets in the presence of an oxygen-containing gas.
According to another aspect, the precursor solution further comprises a morphology-enhancing agent, such as a morphology-enhancing agent selected from the group consisting of lactic acid, glycine, alcohols, ammonium nitrate, polymers and carbohydrazide.
According to another aspect, the precursor solution further comprises a second precursor, the second precursor being selected to form a compound selected from the group consisting of aluminum oxides, magnesium oxides, silicon oxides and titanium oxides. According to another aspect, the precursor solution further comprises a second precursor, the second precursor being selected to form magnesium oxide.
According to another aspect, the precursor solution further comprises a second precursor comprising magnesium nitrate. According to another aspect, the precursor solution further comprises a second precursor, the second precursor being selected to form alumina. According to another aspect, the precursor solution further comprises a second precursor comprising particulate alumina.
According to another aspect, the precursor solution further comprises a second precursor, the second precursor being selected to form a metal selected from the group consisting of Mg, Ni, Zn and Cu.
According to another aspect, the heating step comprises heating the precursor droplets to a temperature of at least about 300° C.
According to another aspect, the converting step comprised heating the intermediate compound.
According to another aspect, the atomizing step comprises atomizing the precursor solution using a spray nozzle. According to another aspect, the atomizing step comprises atomizing the precursor solution using ultrasonic transducers.
According to another aspect, the particles have an average size of from about 1 μm to about 50 μm. According to another aspect, the particulates have substantially spherical morphology.
According to one aspect, the absorbent material comprises CaO. According to another aspect, the absorbent material comprises ZnO. According to another aspect, the absorbent material comprises Li2O.
These and other aspects of the present invention are described in more detail herein below.
DESCRIPTION OF THE DRAWINGS
FIG. 1 schematically illustrates a spray pyrolysis method for the fabrication of particles according to an embodiment of the present invention.
FIG. 2 schematically illustrates a spray conversion method for the fabrication of particles according to an embodiment of the present invention.
FIG. 3 schematically illustrates a post-processing method for the fabrication of particles according to an embodiment of the present invention.
FIG. 4 illustrates the calculated equilibrium gas composition for steam reforming of methane in the presence of CaO as a CO2 absorbent.
FIG. 5 illustrates the calculated equilibrium gas composition for steam reforming of methane without a CO2 absorbent.
FIG. 6 illustrates the calculated equilibrium gas composition for steam reforming of methane in the presence of Li2O as a CO2 absorbent.
FIG. 7 illustrates a process of the absorption of a contaminant from a gas stream with a regenerating step.
FIG. 8 illustrates the aggregate particle morphology of a particulate absorbent material according to an embodiment of the present invention.
FIG. 9 illustrates the aggregate particle morphology of a particulate absorbent material according to an embodiment of the present invention.
FIG. 10 illustrates the aggregate particle morphology of a supported absorbent material according to an embodiment of the present invention.
FIG. 11 illustrates the aggregate particle morphology of an absorbent material according to an embodiment of the present invention.
FIG. 12 illustrates the structure of a supported composite particle including a water gas shift catalyst and reforming catalyst according to an embodiment of the present invention.
FIG. 13 illustrates the structure of a composite absorbent/catalyst particle according to an embodiment of the present invention.
FIG. 14 illustrates the particle size distribution of absorbent powders according to the present invention compared to the prior art.
FIG. 15 illustrates the BET surface area of absorbent powders according to the present invention compared to the prior art.
FIG. 16 illustrates the pore volume of absorbent powders according to the present invention compared to the prior art.
FIG. 17 illustrates the carbonation and decarbonation kinetics of a commercial CaO powder according to the prior art.
FIG. 18 illustrates the absorption capacity of a commercial CaO absorbent powder over 26 cycles.
FIG. 19 illustrates the particle size distribution of a commercial CaO absorbent powder before and after 27 cycles.
FIG. 20 illustrates the pore volume, BET surface area and average pore size of a commercial CaO absorbent powder before and after 27 cycles.
FIG. 21 illustrates the absorption capacity in terms of CaO reaction fraction of CaO-based absorbents over multiple cycles according to the present invention compared to the prior art.
FIG. 22 illustrates the carbonation and decarbonation kinetics of CaO-based absorbents according to the present invention compared to the prior art.
FIG. 23 illustrates the particle size distribution of absorbent powders according to the present invention.
FIG. 24 illustrates the pore volume, BET surface area and average pore diameter for absorbent powders according to the present invention.
FIG. 25 illustrates the absorption capacity in terms of CaO reaction fraction of absorbent powders according to the present invention over multiple cycles.
FIGS. 26(a) and 26(b) illustrate SEM photomicrographs of a commercial CaO powder before and after 27 cycles.
FIGS. 27(a) and 27(b) illustrate SEM photomicrographs of an absorbent powder according to the present invention before and after 12 cycles.
FIGS. 28(a) and 28(b) illustrate SEM photomicrographs of an absorbent powder according to the present invention before and after 12 cycles.
FIG. 29 illustrates the particle size distribution of absorbent powders according to the present invention before and after 12 cycles.
FIG. 30 illustrates the absorption capacity in terms of CaO reaction fraction of absorbent powders according to the present invention over 12 cycles.
FIG. 31 illustrates the carbonation and decarbonation kinetics of several absorbent powders according to the present invention.
FIG. 32 illustrates the carbonation and decarbonation kinetics of a composite absorbent powder according to the present invention.
FIG. 33 illustrates the adsorption capacity over 12 cycles for 2 absorbent materials according to the present invention.
FIG. 34 illustrates the carbonation and decarbonation kinetics of several absorbent powders according to the present invention.
FIG. 35 illustrates the particle size distribution of absorbent powders according to the present invention before and after 12 cycles.
FIG. 36 illustrates the pore volume, BET surface area and average pore diameter of absorbent powders according to the present invention before and after 12 regeneration cycles.
FIG. 37 illustrates the methane conversion over steam reforming catalysts according to the present invention compared to a prior art catalyst.
FIG. 38 illustrates the methane conversion over steam reforming catalysts according to the present invention compared to a prior art catalyst.
FIG. 39 illustrates the methane conversion over steam reforming catalysts according to the present invention compared to a prior art catalyst.
FIG. 40 illustrates the methane conversion over steam reforming catalysts according to the present invention compared to a prior art catalyst.
FIG. 41 illustrates the absorption capacity in terms of CaO reaction fraction of multi-functional composite powders according to the present invention.
FIG. 42 illustrates the carbonation and decarbonation kinetics of multi-functional composite powders according to the present invention.
FIG. 43 illustrates the absorption capacity over 92 cycles in terms of CaO reaction fraction of a multi-functional composite powder according to the present invention.
FIG. 44 illustrates the absorption capacity over 92 cycles of a multi-functional composite powder according to the present invention.
FIG. 45 illustrates the carbonation and decarbonation kinetics for a multi-functional composite powder according to the present invention.
FIG. 46 illustrates the absorption capacity in terms of CaO reaction fraction of pelletized absorbent powders according to the present invention.
FIG. 47 illustrates the absorption capacity of pelletized absorbent powders according to the present invention.
FIG. 48 illustrates the carbonation and decarbonation kinetics of pelletized absorbent powders according to the present invention.
FIG. 49 illustrates the absorption capacity in terms of CaO reaction fraction of pelletized absorbent powders according to the present invention over 116 cycles.
FIG. 50 illustrates the absorption capacity of pelletized absorbent powders according to the present invention over 116 cycles.
FIG. 51 illustrates the absorption capacity in terms of CaO reaction fraction of pelletized absorbent powders according to the present invention over multiple cycles.
FIG. 52 illustrates the absorption capacity of pelletized absorbent powders according to the present invention over multiple cycles.
FIG. 53 illustrates the absorption capacity in terms of CaO reaction fraction of pelletized absorbent powders according to the present invention over multiple cycles.
FIG. 54 illustrates the absorption capacity of pelletized absorbent powders according to the present invention over multiple cycles.
FIG. 55 illustrates the absorption capacity in terms of CaO reaction fraction of pelletized absorbent powders according to the present invention over multiple cycles.
FIG. 56 illustrates the absorption capacity over 66 cycles of pelletized absorbent powders according to the present invention.
FIG. 57 illustrates the absorption capacity over 66 cycles in terms of CaO reaction fraction of pelletized absorbent powders according to the present invention. FIG. 58 illustrates the absorption capacity over 66 cycles of pelletized absorbent powders according to the present invention.
FIG. 59 illustrates the absorption capacity over 66 cycles in terms of CaO reaction fraction of pelletized absorbent powders according to the present invention over multiple cycles.
FIG. 60 illustrates the absorption capacity over 66 cycles of pelletized absorbent powders according to the present invention.
FIG. 61 illustrates the absorption capacity over 64 cycles in terms of CaO reaction fraction of pelletized absorbent powders according to the present invention.
FIG. 62 illustrates the absorption capacity over 64 cycles of pelletized absorbent powders according to the present invention.
FIG. 63 illustrates the absorption capacity over 64 cycles in terms of CaO reaction fraction of pelletized absorbent powders according to the present invention.
FIG. 64 illustrates the conversion efficiency of a composite absorbent/catalyst material according to an embodiment of the present invention.
FIG. 65 illustrates the conversion efficiency of a composite absorbent/catalyst material according to an embodiment of the present invention.
DESCRIPTION OF THE INVENTION
The present invention is directed to improved materials, and methods for making the materials, that are particularly useful in absorption-enhanced reforming (AER) processes for the production of a hydrogen-rich gas from carbon-based fuels such as hydrocarbons. The materials can include, but are not limited to, absorbent materials, such as CaO-based reactive absorbent materials for CO2 absorption, and catalytic materials, such as supported Rh/Al2O3 catalysts for steam reforming and the water-gas shift reaction. Absorbent materials and catalytic materials are collectively referred to herein as active materials.
The “absorption” of one species by an absorbent material can occur by a variety of different mechanisms and is often described using a number of different terms, which can often lead to confusion. In the present application, the word absorption and the process of absorption is used in the broadest sense to include at least physisorption, chemical absorption and absorption with chemical reaction.
The absorbent materials according to the present invention include at least a first absorbent compound. Preferably the absorbent compound is a reactive absorbent compound. Preferred reactive absorbent compounds can be selected from metal oxides, and in particular can be selected from Group 1 and Group 2 metal oxides. Examples of Group 1 metal oxides include lithium oxide (Li2O), sodium oxide (Na2O) and potassium oxide (K2O). Examples of Group 2 metal oxides include magnesium oxide (MgO), calcium oxide (CaO), strontium oxide (SrO) and barium oxide (BaO). Particularly preferred among these for CO2 absorption are the Group 2 metal oxides, particularly calcium oxide. Lithium oxide is also preferred for some applications. It will be appreciated that other metal oxide compounds (e.g., silver oxide) may be useful for the absorption of a particular chemical species, and such metal oxide compounds are also within the scope of the present invention. It will be appreciated that combinations of two or more absorbent compounds can also be utilized in the absorbent material.
Preferred steam reforming catalysts include rhodium (Rh) metal dispersed on a support such as alumina (Al2O3). The active metal species can also include metals such as nickel (Ni), platinum (Pt), palladium (Pd) and combinations of these metals. These and other active materials are discussed in more detail below.
Overview of Spray Processing
The materials of the present invention are preferably fabricated by spray drying, spray conversion or spray pyrolysis methods, which are collectively referred to herein as spray processing methods. The spray processing methods of the present invention are capable of producing a wide variety of active materials and microstructures. The major attribute of spray processing is the ability to fabricate compositions and microstructures that cannot be fabricated by other powder manufacturing methods, combined with the ability to economically produce high volumes of material. The flexibility to fabricate unique combinations of compositions and microstructures comes from the fact that spray processing combines aspects of both liquid phase and solid state processing.
Spray processing generally includes the step of providing a precursor composition, typically in a flowable liquid form. The precursor composition typically includes at least a precursor to the desired active material. In the case of supported active materials, the precursor composition can also include a precursor to or a suspension of the support phase. The precursor composition is atomized to form a suspension of liquid precursor droplets and the liquid is removed from the liquid precursor droplets, such as by heating, to form the desired powder, or a dry precursor to the desired powder. Typically, at least one component of the liquid precursor is chemically converted into a desired component of the powder.
Group 1 and Group 2 metal oxides are preferred absorbent materials according to one embodiment of the present invention. Spray processing precursors for Group 1 and Group 2 metal oxides can be selected from Group 1 metal salts and Group 2 metal salts, such as nitrate, oxalate, acetate and hydroxide metal salts, with oxalates, nitrates and hydroxides being particularly preferred for some applications. Thus, particularly preferred precursors include Ca-oxalate, Ca-nitrate, Ca-hydroxide, Li-oxalate, Li-nitrate and Li-hydroxide.
According to one embodiment, precursors such as oxalates may be formed in a liquid precursor solution from which droplets are subsequently formed by spray processing. For example, a nitrate salt (M(NO3)x) where M is a metal, can be reacted with ammonium oxalate to precipitate M-oxalate in the liquid.
It is often desirable to form the metal oxide absorbent with a high level of porosity or crystallinity and therefore it may be advantageous to include a morphology enhancing agent in the liquid precursor composition, where the morphology enhancing agent is adapted to enhance the porosity or crystallinity of the final powder. Morphology enhancing agents can preferably be selected from lactic acid, glycine, alcohols, ammonium nitrate, polymers and carbohydrazide. The amount of morphology enhancing agent in the precursor solution can be varied, and in one embodiment is from about 5 vol. % to about 40 vol. % of the total amount of precursors in the precursor solution.
Other components can be included in the absorbent materials according to the present invention, as is discussed in more detail below. Included among these are inert materials such as aluminum oxides (e.g., Al2O3), magnesium oxides (e.g., MgO), silicon oxides (e.g., SiO2), titanium oxides (e.g., TiO2) and mixtures of two or more oxides. As used herein, inert materials are those that are substantially non-reactive with the chemical species such as CO2 within the temperature range that the reactive absorbent compound is reactive with the chemical species. For example, MgO is reactive with CO2 at relatively low temperatures, but is considered inert with respect to a CaO absorbent compound since MgO does not react with CO2 at the high temperatures utilized with CaO. Precursors for such materials can include metal oxides and metal salts, particularly nitrate salts such as Al-nitrate or Mg-nitrate. In addition, the absorbent material can include other metals such as iron (Fe), magnesium (Mg), zinc (Zn) and copper (Cu). Precursors for these metals can be selected from the metal nitrates, sulfates, carbonates, acetates, oxalates, hydroxides and metal oxide nanoparticles, including fumed metal oxides.
For the production of catalyst materials according to the present invention, a precursor to the active metal species is provided. For example, Rh precursor can preferably be selected from rhodium metal salts such as rhodium acetate Rh(OAc)3 and rhodium nitrate Rh(NO3)3. The active metal species can also include metals such as nickel (Ni), platinum (Pt) and palladium (Pd) and precursors for these metals can be selected from nitrates, ammines, hydroxides, chlorides and the like.
The spray processing methods can combine the drying of the precursors and the conversion to the active material in one step in the same reactor, where both the removal of the solvent and the conversion of a precursor occur essentially simultaneously. Although the following discussion is directed primarily to the production and use of CaO as an absorbent material, it will be appreciated that the methods are also applicable to other metal oxide absorbent compounds disclosed herein. This method is referred to as spray pyrolysis, and is schematically illustrated in FIG. 1 for the production of CaO from Ca(NO3)2. In another embodiment, the spray processing method dries the precursors and partially converts the precursors to an intermediate compound that can subsequently be converted to the absorbent material. This method is schematically illustrated in FIG. 2 for the production of a CaC2O4 intermediate compound. The complete conversion to the active material and/or the crystallization of the active material can occur in a second step, such as the step that is schematically illustrated in FIG. 3 for the conversion of CaC2O4 to CaO or CaCO3. This second step is referred to herein as post-processing. Thus, spray processing can be followed by heating or calcination, e.g., a method illustrated in FIG. 2 followed by a method such as that illustrated in FIG. 3. By varying reaction time, temperature and type of precursors, the spray processing methods can produce powder morphologies and active material structures that yield improved performance.
When the active material phase is dispersed on an inert support in the form of active material clusters, the precursor composition can include particulates that form the inert support phase. According to the present invention, such particulates can include particulate carbon or particulate aluminum oxide, such as particulate boehmite.
Preferably, the supported active material phase is formed while the precursor to the active material phase is in intimate contact with the surface of the support phase particles and the active material precursor is rapidly reacted on the surface of the support phase particles. The reaction and formation of the supported active material preferably occurs over a very short period of time such that the growth of large active material clusters is limited. Preferably, the active material precursor is exposed to the elevated reaction temperature to form the active material for not more than about 600 seconds, more preferably not more than about 100 seconds and even more preferably not more than about 10 seconds. The means by which the active material precursor is reacted is discussed in detail below.
Preferably, the spray processing methods are capable of forming a spherical aggregate particle structure. As used herein, an aggregate particle structure is a cohesive particulate that is comprised of many smaller primary particles. The spherical aggregate particles form as a result of the formation and drying of the precursor solution droplets during spray processing.
Spray processing methods for the production of the active materials can be grouped by reference to several different attributes of the apparatus used to carry out the spray processing method. These attributes include: the main gas flow direction (e.g., vertical or horizontal); the type of atomizer (e.g., submerged ultrasonic, ultrasonic nozzle, two-fluid nozzle, single nozzle pressurized fluid); the type of gas flow (e.g., laminar with no mixing, turbulent with no mixing, co-current of droplets and hot gas, countercurrent of droplets and gas or mixed flow); the type of heating (e.g., hot wall system, hot gas introduction, combined hot gas and hot wall, plasma or flame); and the type of powder collection system (e.g., cyclone, bag house, electrostatic or settling).
The absorbent and catalytic powders of the present invention can be prepared by starting with an aqueous-based precursor liquid consisting of a dissolved metal salt. The processing temperature can be controlled so the metal precursor decomposes to form the active material, or an intermediate compound that can be converted to the active material, such as by heating in a post-processing step.
The first step in the process is the evaporation of the solvent (typically water) as the droplet is heated resulting in a particle of dried solids and/or metal salts. A number of methods to deliver heat to the droplet/particle are possible: horizontal hot-wall tubular reactors, spray dryer and vertical tubular reactors can be used, as well as plasma, flame and laser reactors. Horizontal hot-wall tubular reactors are disclosed in U.S. Pat. No. 6,103,393 by Kodas et al. Spray dryers are disclosed, for example, in U.S. Pat. No. 5,615,493 by Funder and U.S. Pat. No. 5,100,509 by Pisecky et al. A plasma reactor is disclosed in U.S. Pat. No. 6,689,192 by Phillips et al. and a flame reactor is disclosed in U.S. Pat. No. 5,958,361 by Laine et al. Laser reactors are disclosed in U.S. Pat. No. 6,248,216 by Bi et al. Each of the foregoing U.S. patents is incorporated herein by reference in its entirety.
As the particles experience either higher temperature or longer time at a specific temperature, the metal precursor decomposes. Preferably, the time that the droplets/particles experience a given temperature is controlled and therefore the degree of porosity, crystallinity, the microstructure and other properties can be controlled.
The atomization technique for generating the precursor droplets has a significant influence over the characteristics of the final active material powder, such as the particle surface area, porosity, size, the spread of the particle size distribution (PSD), as well as the production rate of the powder. Some techniques cannot atomize fluids with even moderate particle loadings or high viscosities. Several methods exist for the atomization of precursor compositions, including those that contain suspended particulates. These methods include but are not limited to: ultrasonic transducers (usually at a frequency of 1-3 MHz); ultrasonic nozzles (usually at a frequency of 10-150 KHz); rotary atomizers; two-fluid nozzles; and pressure atomizers.
Ultrasonic transducers are generally submerged in a liquid and the ultrasonic energy produces atomized droplets on the surface of the liquid. Two basic ultrasonic transducer disc configurations, planar and point source can be used. Deeper fluid levels can be atomized using a point source configuration since the energy is focused at a point that is some distance above the surface of the transducer. The scale-up of submerged ultrasonic transducers can be accomplished by placing a large number of ultrasonic transducers in an array. Such a system is illustrated in U.S. Pat. No. 6,338,809 by Hampden-Smith et al., the disclosure of which is incorporated herein by reference in its entirety.
Scale-up of nozzle systems can be accomplished by either selecting a nozzle with a larger capacity or by increasing the number of nozzles used in parallel. Typically, the droplets produced by nozzles are larger than those produced by ultrasonic transducers. Particle size is also dependent on the gas flow rate. For a fixed liquid flow rate, an increased airflow decreases the average droplet size and a decreased airflow increases the average droplet size. It is difficult to change droplet size without varying the liquid or airflow rates. However, two-fluid nozzles have the ability to process larger volumes of liquid per unit time than ultrasonic transducers.
Ultrasonic spray nozzles also use high frequency energy to atomize a fluid. Ultrasonic spray nozzles have some advantages over single or two-fluid nozzles such as the low velocity of the spray leaving the nozzle and lack of associated gas flow. The nozzles are available with various orifice sizes and orifice diameters that allow the system to be scaled for the desired production capacity. In general, higher frequency nozzles are physically smaller, produce smaller droplets, and have a lower flow capacity than nozzles that operate at lower frequencies. A drawback of ultrasonic nozzle systems is that scaling up the process by increasing the nozzle size increases the average particle size. If a particular particle size is required, then the maximum production rate per nozzle is set. If the desired production rate exceeds the maximum production rate of the nozzle, additional nozzles or additional production units are required to achieve the desired production rate.
The shape of the atomizing surface determines the shape and spread of the spray pattern. Conical, microspray and flat atomizing surface shapes are available. The conical atomizing surface provides the greatest atomizing capability and has a large spray envelope. The flat atomizing surface provides almost as much flow as the conical surface but limits the overall diameter of the spray. The microspray atomizing surface is for very low flow rates where narrow spray patterns are needed. These nozzles are preferred for configurations where minimal gas flow is required in association with the droplets.
Particulate suspensions in the precursor solution (e.g., for the production of a supported active material) may present several problems with respect to atomization. For example, submerged ultrasonic atomizers re-circulate the suspension through the generation chamber and the suspension concentrates over time. Further, some fraction of the liquid atomizes without carrying the suspended particulates. Other problems encountered when using submerged ultrasonic transducers is that the transducer discs can become coated with the particles and or precursor over time. Further, the generation rate of particulate suspensions can be very low using submerged ultrasonic transducer discs. This is due in part to energy being absorbed or reflected by the suspended particles.
For spray drying, the aerosol can be generated using three basic methods. These methods differ in the type of energy used to break the liquid masses into small droplets. Rotary atomizers (utilization of centrifugal energy) make use of spinning liquid droplets off of a rotating wheel or disc. Rotary atomizers are useful for co-current production of droplets in the range of 20 μm to 150 μm in diameter. Pressure nozzles (utilization of pressure energy) generate droplets by passing a fluid under high pressure through an orifice. These can be used for both co-current and mixed-flow reactor configurations and typically produce droplets in the range of 50 μm to 300 μm. Multiple fluid nozzles such as a two-fluid nozzle (utilization of kinetic energy) produce droplets by passing a relatively slow moving fluid through an orifice while shearing the fluid stream with a relatively fast moving gas stream. As with pressure nozzles, multiple fluid nozzles can be used with both co-current and mixed-flow spray dryer configurations. This type of nozzle can typically produce droplets in the size range of 5 μm to 200 μm.
For example, two-fluid nozzles are used to produce aerosol sprays in many commercial applications, typically in conjunction with spray drying processes. In a two-fluid nozzle, a low-velocity liquid stream encounters a high-velocity gas stream that generates high shear forces to accomplish atomization of the liquid. A direct result of this interaction is that the droplet size characteristics of the aerosol are dependent on the relative mass flow rates of the liquid precursor and nozzle gas stream. The velocity of the droplets as they leave the generation zone can be quite large which may lead to unacceptable losses due to impaction. The aerosol also leaves the nozzle in a characteristic pattern, typically a flat fan, and this may require that the dimensions of the reactor be sufficiently large to prevent unwanted losses on the walls of the system.
Thus, numerous atomization techniques for spray processing are possible for the production of active material powders and different versions are preferred for different feed streams and products.
The atomized precursor composition must be heated to remove solvents and react precursor components. For example, a horizontal, tubular hot-wall reactor can be used to heat a gas stream to a desired temperature. Energy is delivered to the system by maintaining a fixed boundary temperature at the wall of the reactor and the maximum temperature of the gas is the wall temperature. Heat transfer within a hot wall reactor occurs through the bulk of the gas. Buoyant forces that occur naturally in horizontal hot wall reactors aid this transfer. The mixing also helps to improve the radial homogeneity of the gas stream. Passive or active mixing of the gas can also increase the heat transfer rate. The maximum temperature and the heating rate can be controlled independent of the inlet stream with small changes in residence time. The heating rate of the inlet stream can also be controlled using a multi-zone furnace.
The use of a horizontal hot-wall reactor according to the present invention is generally preferred to produce particles with a size of not greater than about 5 μm. Above about 5 μm, settling of particles can cause significant material losses. One disadvantage of such reactors is the poor ability to atomize particulates when using submerged ultrasonics for atomization.
Alternatively, the horizontal hot-wall reactor can be used with a two-fluid nozzle to atomize the droplets. This approach is preferred for precursor feed streams containing higher levels of particulate materials, such as a particulate support precursor. A horizontal hot-wall reactor can also be used with ultrasonic nozzle atomization techniques. This allows atomization of precursor containing particulates, however the large droplet size leads to losses of materials on reactor walls and other surfaces making this an expensive method for powder production.
While horizontal hot-wall reactors are specifically useful for some particle morphologies and compositions according to the present invention, particularly for the spray pyrolysis method illustrated in FIG. 1, spray processing systems in the configuration of a spray dryer are the preferred production method for large quantities of absorbent and catalytic powders in accordance with some applications of the present invention. Such spray processing systems are particularly useful for the general method schematically illustrated in FIG. 2.
Spray drying is a process wherein powders are produced by atomizing a precursor to produce droplets and evaporating the liquid to produce a dry aerosol, wherein thermal decomposition of one or more precursors may take place to produce the powder. The residence time in the spray dryer is the average time the process gas spends in the drying vessel as calculated by the vessel volume divided by the process gas flow using the outlet gas conditions. The peak excursion temperature (i.e., the reaction temperature) in the spray dryer is the maximum temperature of a particle, averaged throughout its diameter, while the particle is being processed and/or dried. The droplets are heated by supplying a pre-heated carrier gas.
Three types of spray dryer systems are useful for the spray drying of the active material powders according to the present invention. An open system is useful for spray drying of powders using air as an aerosol carrier gas and an aqueous feed solution as a precursor. A closed system is useful for spray drying of powders using an aerosol carrier gas other than air. A closed system is also useful when using a non-aqueous or a semi-non-aqueous solution as a precursor. A semi-closed system, including a self-inertizing system, is useful for spray drying of powders that require an inert atmosphere and/or precursors that are potentially flammable.
Two spray dryer designs are particularly useful for the production of the active material powders of the present invention. A co-current spray dryer is useful for the production of materials that are sensitive to high temperature excursions (e.g., greater than about 350° C.) or that require a rotary atomizer to generate the aerosol. Mixed-flow spray dryers are useful for producing materials that require relatively high temperature excursions (e.g., greater than about 350° C.) or require turbulent mixing forces.
In a co-current spray dryer, the hot gas is introduced at the top of the unit where the droplets are generated with any of the atomization techniques mentioned above. The maximum temperature that a droplet/particle is exposed to in a co-current spray dryer is the temperature of the outlet. Typically, the outlet temperature is limited to about 200° C., although some designs allow for higher temperatures. In addition, since the particles experience the lowest temperature in the beginning of the time-temperature curve and the highest temperature at the end, the possibility of precursor surface diffusion and agglomeration is high.
A preferred spray processing system according to the present invention is based on a mixed-flow spray dryer. A mixed-flow spray dryer introduces the hot gas at the top of the unit and the precursor droplets are generated near the bottom and are directed upwardly. The droplets/particles are forced towards the top of the unit then fall and flow back down with the gas, increasing the residence time in the spray dryer. The temperature the particles experience is also higher as compared to a co-current spray dryer. This is important, as most spray dryers are not capable of reaching the higher temperatures that are required for the conversion of some metal salts.
For mixed flow spray dryers the reaction temperatures can be high enough for the decomposition of metal precursors such as Rh precursors (e.g., between 250° C. and 350° C.) useful for preparation of reforming catalysts such as a Rh/Al2O3 supported catalyst. The highest temperature in these spray dryers is the inlet temperature (e.g., 600° C. and higher), and the outlet temperature can be as low as 90° C. Therefore, the particles reach the highest temperature for a relatively short time, which advantageously reduces precursor migration or surface diffusion. This spike of high temperature quickly converts the precursor and is followed by a mild quench since the spray dryer temperature quickly decreases after the maximum temperature is achieved. The spike-like temperature profile is advantageous for the generation of highly dispersed metal or metal oxide active material clusters on the surface of a support phase. Mixed flow spray dryers are particularly preferred for the spray processing of a precursor solution that includes particulate metal compound precursors, such as Ca-oxalate particulates.
The range of useful residence times for producing the active material powders depends on the spray dryer design type, atmosphere used, nozzle configuration, feed liquid inlet temperature and the residual moisture content. In general, residence times for the production of the active material powders can range from 5 seconds up to 5 minutes. According to one embodiment, the residence time is from about 15 seconds to about 45 seconds.
The range of inlet temperatures for producing the active material powders depends on the spray dryer design type, atmosphere used, nozzle configuration, feed liquid, and energy required to perform drying and/or decomposition functions.
In general, the outlet temperature of the spray dryer determines the residual moisture content of the powder. For the production of the absorbent and catalytic powders according to the present invention, the range of useful outlet temperatures depends on the spray dryer design type, atmosphere used, nozzle configuration, feed liquid, inlet temperature, and residual moisture content. For example, a useful outlet temperature according to one embodiment of the present invention ranges from about 200° C. to about 350° C. According to another embodiment, a useful outlet temperature is at least about 450° C., more preferably at least about 600° C. Other equipment that is desirable for producing the active material powders using a spray dryer includes a heater for the gas and a collection system. Either direct heating or indirect heating, including burning fuel, heating electrically, liquid-phase heating or steam heating, can accomplish heating of the gas. The most useful type of heating for the production of the powders processed with an inlet temperature greater than 350° C. is direct fuel burning.
Many collection methods are useful for collecting powders produced on a spray dryer. These methods include, but are not limited to those using cyclone, bag/cartridge filter, electrostatic precipitator, and various wet collection techniques.
The active material powders may also need to be post-processed by conventional calcination methods to convert them into another chemical composition prior to pelletization. For example, it may be advantageous to convert a metal oxalate with a particular structure into a metal carbonate and/or further to a metal oxide through a thermal post-processing step (FIG. 3). Likewise, it may be advantageous to convert a metal carbonate to a metal oxide while retaining the beneficial attributes of the pore structure. The post processing ideally needs to be carried out under conditions that are not detrimental to the structure and performance of the material. Where the post processing is a thermally-induced transformation, the temperature needs to be carefully chosen to effect the chemical change, without inducing significant sintering or significantly altering the performance of the material. A number of methods can be used to effect this thermal transformation including heat treatment in a static bed or a moving bed.
According to one preferred embodiment, the post-processing occurs in a moving bed such as a rotary calciner in which the powder is delivered to a furnace which contains a rotating reactor tube such that the bed of particles is constantly moving to avoid particle to particle agglomeration and also provide a fresh exposure of the surface of the particle bed to allow out gassing of the material (CO2 in the case of the examples described above). This continual “agitation” of the powder bed avoids depth-dependant variations that can occur with a fixed bed reactor. Preferred heating rates for such post-processing are between about 1° C./min and 100° C./min, more preferably between 1° C. and 10° C./min.
In accordance with the foregoing, the present invention is directed to the fabrication of an absorbent material that is useful for AER by spray processing. The spray processing includes the formation of a precursor solution and the conversion of the precursor solution into a plurality of droplets. The droplets are then heated to a reaction temperature to convert the droplets to an absorbent material and/or an intermediate compound that can be subsequently converted to an absorbent material. Preferred precursors for the absorbent materials and catalyst materials according to the present invention are discussed above.
For the fabrication of metal oxide absorbent materials by spray processing according to the present invention, including Group 1 and Group 2 metal oxides such as calcium oxide, it is preferred that the reaction temperature be at least about 300° C. and more preferably is at least about 600° C. Further, the reaction temperature preferably does not exceed about 1150° C. and more preferably does not exceed about 900° C. The reaction time (i.e., approximate residence time in the spray reactor) can be at least about 10 seconds and preferably does not exceed about 500 seconds.
As is discussed above, the spray processing method can be used to form an intermediate compound that is capable of being converted to an absorbent material. It will also be appreciated that spray processing can produce a mixture of both the absorbent material or absorbent compound, and an intermediate compound. The conversion of the intermediate compound to an absorbent material is referred to herein as post-processing. Preferably, the post—processing includes heating the intermediate compound, such as to a temperature of at least about 250° C. and preferably at least about 750° C. The post-processing heating temperature preferably is not so high that the particles sinter or agglomerate, and in one embodiment is not greater than about 900° C. Preferably, the post-processing time is at least about 10 minutes and preferably does not exceed about 6 hours, and more preferably does not exceed about 3 hours.
AER
The particulate materials of the present invention are particularly applicable to AER processes. As is discussed above, AER has significant advantages over conventional reforming technologies. The fuels that can be used in AER processes include carbon-based fuels, such as natural gas, other gases such as propane, liquids such as alcohols, gasoline, diesel and jet fuel, and solid fuels such as biomass, coal and other forms of carbon. A general AER method for hydrogen production using various fuels is illustrated by the reactions of Equations 11 to 14:
Reforming CxHyOz+(x−z)H
2O→xCO+(x−z+y/2)H2 (11)
Water gas shift xCO+xH2O→xH2
+xCO2 (12)
CO2 fixing xCaO+xCO2→xCaCO3 (13)
Overall xCaO+CxHyOz+(2x
z)H2O→(2x+y/2−z)H2+xCaCO3 (14)
FIGS. 4 and 5 illustrate the calculated thermodynamic gas composition (dry basis) obtained by steam methane reforming (SMR) as a function temperature at atmospheric pressure with and without a CaO absorbent. Specifically, FIG. 4 illustrates that at 600° C., AER can achieve at least 98% conversion to H2, as compared to only 75% conversion under normal conditions of SMR, as illustrated in FIG. 5. Further, FIG. 4 illustrates that the CO concentration can be decreased to less than 1% in an AER reformer as compared to the normal of 6 to 10% in conventional SMR, and the total amount of carbon oxides including CO and CO2 is less than 2%, as compared to 25% in conventional SMR (FIG. 5). Similar conclusions can be reached by comparison of SMR and AER thermodynamics for reforming of other fuels such as LPG, gasoline, kerosene, jet fuel, bio-oil from biomass and. Therefore, for a variety of fuels, it is desirable to remove the CO2 during the reforming step as opposed to post WGS CO2 removal.
According to one embodiment of the present invention, lithium oxide (Li2O) is utilized as the reactive absorbent compound. Li2O is particularly useful for absorption temperatures in the range of from about 300 to about 600° C. and desorption temperatures in the range of from about 750 to about 1100° C. FIG. 6 illustrates the calculated thermodynamic gas composition for lithium oxide.
In addition to AER, absorbents such as CaO are used in a variety of other applications to remove poisons, contaminants or reaction by-products. The compounds to be removed are normally gases under the operating conditions of the system and the absorbents are usually heated to elevated temperatures in order to initiate a chemical reaction between the gas to be removed and the surface of the absorbent. The chemical reactions between the absorbent and the gas to be removed are normally stoichiometric, so once the active material in the absorbent has either been consumed or rendered passive it is either physically replaced or the material is isolated and the reaction is reversed to re-generate the original material.
For example, H2S can be removed from a gas stream using CaO as the absorbent to react with the H2S to form calcium sulfide (CaS). The CaO absorbent can be regenerated from the CaS bed by oxidation-reduction reactions.
A simple illustration of one cycle of carbonation/decarbonation of CaO with CO2 is illustrated in FIG. 7. Referring to FIG. 7, an activated absorbent bed comprising CaO (as loose powder, pellets, extrudates or a monolith) is packed into a reactor. The absorbent bed can include loose CaO powder, pelletized CaO powder, a surface coated with CaO powder or a monolith of the CaO. A gas stream including CO2 is admitted to the reactor and the CaO reacts selectively with the CO2 to form CaCO3. Further reaction between the CaO and CO2 eventually consumes all the available CaO and converts it into CaCO3, which is non-reactive with CO2. Further use of this reactor under these conditions will not remove additional CO2 and the contaminated gas will pass through the reactor. The feed is then sto