Abstract:
A method for making a class of relatively stable porous carbon dioxide absorbents by mixing inert nanoparticles with a CaO precursor followed by high temperature calcination. In the preferred embodiments of this invention this process takes place in the essential absence of nitrates. In some embodiments of the invention the method further includes forming the inert nanoparticles-doped porous CaO material by decomposing a mixture of inert particles and CaO precursor material.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The invention generally relates to absorbents for carbon dioxide capture and particularly to carbon dioxide absorbents containing calcium oxide. 
         [0003]    2. Background Information 
         [0004]    Increasing concentrations of CO 2  in the atmosphere have been identified as a major contributor to global warming. Much of the increase is due to the consumption of fossil fuels which is expected to continue into the foreseeable future. To mitigate these effects, large scale CO 2  capture and sequestration (CCS) technologies and strategies have been proposed and are currently being widely studied. 
         [0005]    The carbonation of calcium oxide materials has been identified as an effective method for removing carbon dioxide from streams of hot gasses. CaO-containing materials provide a variety of advantages due to their high reactivity for CO 2  absorption, high CO 2  capacity, low material cost, and their high carbonation temperature (600-700° C.) which makes it possible to efficiently recover the large amount of energy released during CO 2  capture (178 kJ/mol CO 2 ). 
         [0006]    With proper energy recovery, CaO-based absorbents show great advantage over other absorbents. However, the carbonation and decarbonation reactions of CaO and CaCO 3  are far from complete or reversible. Rapid loss of CO 2  capacity over many carbonation/decarbonation cycles is almost always observed due to absorbent sintering. Absorbent sintering is significantly influenced by several factors including the following: 1) the carbonation process is highly exothermic (CaO+CO 2 =CaCO 3  ΔH° =−178 kJ/mol); 2) there is a large volume increase from CaO to CaCO 3  (16.9 cm 3 /mol to 34.1 cm 3 /mol), which greatly decreases the distance between absorbent particles in the carbonated state; and 3) CaCO 3  has a Tammann temperature (i.e. the highest treatment temperature before the sintering of a material becomes significant) of 533° C., lower than normal carbonation temperatures. 
         [0007]    Various methods have been suggested and researched to attempt to find ways to enhance the durability of CaO-based CO 2  absorbents. So far, three major approaches have been used: 1) incorporation of inert materials, such as MgO, Al 2 O 3 , ZrO 2 , TiO 2 , SiO 2 , La 2 O 3  that physically hinder CaCO 3 —CaCO 3  particle sintering; 2) modification of the pore structure of CaO particles; 3) use stable CaO nanoparticles. However, no materials have been indentified yet which can survive thousands of carbonation-decarbonation cycles required by practical applications with acceptable capacities. The present invention is a significant step toward providing such a material. 
         [0008]    Additional advantages and novel features of the present invention will be set forth as follows and will be readily apparent from the descriptions and demonstrations set forth herein. Accordingly, the following descriptions of the present invention should be seen as illustrative of the invention and not as limiting in any way. 
       SUMMARY 
       [0009]    The present invention is a method for making a class of relatively stable porous carbon dioxide absorbents by mixing inert nanoparticles with a CaO precursor followed by high temperature calcination. In the preferred embodiments of this invention this process takes place in the essential absence of nitrates. In some embodiments, the nanoparticles have a mean particle size less than 20 nanometers. In other embodiments, the inert nanoparticles include MgO. In other embodiments of the invention the method further includes forming the inert nanoparticles-doped porous CaO material by decomposing a mixture of inert particles and CaO precursor material. In some embodiments, this decomposition method is performed thermally. The precursor material in one embodiment is Ca(CH 3 COO) 2 . The step of mixing may be performed by dry mechanical mixing or by other means as deemed necessary and appropriate. The resulting material provides a significant advantage over other materials and methods taught and described in the prior art. 
         [0010]    The purpose of the foregoing abstract is to enable the United States Patent and Trademark Office and the public generally, especially the scientists, engineers, and practitioners in the art who are not familiar with patent or legal terms or phraseology, to determine quickly from a cursory inspection the nature and essence of the technical disclosure of the application. The abstract is neither intended to define the invention of the application, which is measured by the claims, nor is it intended to be limiting as to the scope of the invention in any way. 
         [0011]    Various advantages and novel features of the present invention are described herein and will become further readily apparent to those skilled in this art from the following detailed description. In the preceding and following descriptions I have shown and described only the preferred embodiment of the invention, by way of illustration of the best mode contemplated for carrying out the invention. As will be realized, the invention is capable of modification in various respects without departing from the invention. Accordingly, the drawings and description of the preferred embodiment set forth hereafter are to be regarded as illustrative in nature, and not as restrictive. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]      FIG. 1  shows the performances, Performances of CaO absorbents obtained from different sources. Carbonation at 758° C. in 100% CO 2  for 30 min, decarbonation at 758° C. in 100% He for 30 min. 
           [0013]      FIG. 2  shows SEM images of fresh and used pure CaO absorbents.  FIG. 2   a  shows fresh CaO from Ca(NO 3 ) 2 ;  FIG. 2   b  shows fresh CaO from CaC 2 O 4 ;  FIGS. 2   c  and  2   d  fresh CaO from Ca(CH 3 COO) 2 ;  2   e  and  2   f.  CaO from Ca(CF 3 COO) 2  after 53 cycles of carbonation and decarbonation at 758° C. 
           [0014]      FIG. 3  shows the effect effect of MgO-doping method on the CO 2  capture performances of 42 wt % MgO-58 wt % CaO absorbents. [[Top]]  FIG. 3   a  shows: CO 2  capacity vs. cyde number;  FIG. 3   b  Bottom: CaO utilization vs. cycle number. Carbonation at 758° C. in 100% CO 2  for 30 min, decarbonation at 758° C. in 100% He for 30 min. 
           [0015]      FIG. 4  Carbonation and decarbonation reactions of 42 wt % MgO doped CaO absorbent prepared by physical mixing of Ca(CH 3 COO) 2  and MgO.  FIG. 4   a  [[Top]]: 2 nd  cycle;  FIG. 4   b  Bottom: 50 th  cycle. Carbonation at 758° C. in 100% CO 2  for 30 min, decarbonation at 758° C. in 100% He for 30 min. 
           [0016]      FIGS. 5   a - 5   j  show. Cross-section SEM analysis of fresh and used 26 wt % MgO-doped CaO absorbents prepared by physical mixing of Ca(CH 3 COO) 2  with MgO  FIGS. 5   a - e  show: fresh samples;  FIGS. 5   h - j : show the samples after 100 cycles test.  FIGS. 5   d  and  5   e  show: EDS analysis of selected area in image  FIG. 5   c.    FIGS. 5   i  and  5   j  show: Mg and Ca elemental mapping of image h. 
           [0017]      FIG. 6  shows the effect Effect of calcination temperature of MgC 2 O 4  on the crystallite size of produced MgO particles. Crystal size was estimated from XRD pattern using Scherrer&#39;s equation. Duration of calcination at each temperature: 2 hr. 
           [0018]      FIGS. 7   a  and  7   b  show the effect Effect of MgO doping level on the CO 2  capture performance of three CaO-based absorbents. [[Top]]  FIG. 7   a  shows: CO 2  capacity vs. cycle number; Bottom  FIG. 7   b:  CaO utilization vs. cycle number. Carbonation at 758° C. in 100% CO 2  for 30 min, decarbonation at 758° C. in 100% He for 30 min. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0019]    The following description includes one embodiment of the present invention. It will be dear from this description of the invention that the invention is not limited to these illustrated embodiments but that the invention also includes a variety of modifications and embodiments thereto. Therefore the present description should be seen as illustrative and not limiting. While the invention is susceptible of various modifications and alternative constructions, It should be understood, that there is no intention to limit the invention to the specific form disclosed, but, on the contrary, the invention is to cover all modifications, alternative constructions, and equivalents falling within the spirit and scope of the invention as defined in the claims. 
         [0020]    The method of the present invention provides a solution to the well-established sintering effect that takes place with CaO-based materials. A class of CaO-based absorbents with improved durability and CO 2  reactivity were prepared by physical mixing of Ca(CH 3 COO) 2  with small MgO particles followed by high temperature calcination. With 26 wt % MgO doping a CaO—MgO mixture prepared by this method gives as high as 53 wt % CO 2  capacity after 50 carbonation-decarbonation cycles at 758° C. Without MgO doping, the CO 2  capacity of pure CaO obtained from same source decreases from 66 wt % for the 1 st  cycle to 26 wt % for the 50 th  cycle under the same test conditions. 
         [0021]    Four pure CaO absorbents were prepared by thermal decomposition of Ca(OH) 2 , Ca(CH 3 COO) 2 , Ca(NO 3 ) 2 , and CaC 2 O 4 . Their CO 2  capture performances, along with those of two commercial CaO samples (10 μm particles, and 160 nm particles), were evaluated in a TGA unit at 758° C. The results of this testing is shown in  FIG. 1 . As this figure shows, CaO absorbents obtained from different sources exhibit quite different CO 2  capture performances. Among all the samples tested, CaO prepared by direct decomposition of Ca(CH 3 COO) 2  gives the best performance, even better than the commercial 160 nm CaO.  FIG. 2  gives SEM images of three fresh CaO samples (decomposition of nitrate, oxalate, and acetate) and one used sample (CaO from acetate, after 52 cycles of carbonation-decarbonation at 758° C.). Decomposition of Ca(NO 3 ) 2  gives very dense CaO sample. As a result, very poor CO 2  capture performance was observed. Decomposition of Ca(CH 3 COO) 2  produces small CaO crystals with unique porous structure. This special structure contributes to its good long-term CO 2  capture performance. Based on this observation, Ca(CH 3 COO) 2  was used as major CaO source throughout the rest of this study. 
         [0022]    To improve the long-term performance of these CaO-based absorbent, MgO-doping effect was extensively studied. While MgO was used in these examples, it is fully anticipated that a variety of other materials might be alternatively embodied and used in these situations. These include but are not limited to: Al 2 O 3 , SiO 2 , TiO 2 , ZrO 2 , and lanthanide oxides. The results of the present invention show that best performance was obtained with materials, precursors, substrates that are essentially free of nitrates. 
         [0023]    In one set of experiments, three samples with 42 wt % MgO and 58 wt % CaO were prepared using three different methods: co-precipitation, solution mixing, and dry physical mixing of Ca(CH 3 COO) 2  with MgO (from decomposition of MgC 2 O 4  at 600° C.). The following outlines the materials, methods and testing involved. 
         [0024]    Materials and Preparation Methods. Reagent-grade chemicals CaO (160 nm powder), Ca(OH) 2 , Ca(NO 3 ) 2 -4H 2 O, MgO (325 μm), Mg(OH) 2 , Mg(CH 3 COO) 2 -4H 2 O and Na 2 CO 3  were purchased from Sigma-Aldrich Co. Reagent-grade chemicals CaO (10 μm), Ca(CH 3 COO) 2 -0.4H 2 O, calcium oxalate CaC 2 O 4 , dolimite natural mineral CaMg(CO 3 ) 2 , magnesium oxalate MgCO 4 -2H 2 O and Mg(CH 3 COO) 2 -4H 2 O were purchased from Alfa Aesar. Two nano-sized MgO samples, NanoActive® Magnesium Oxide (crystallite size ˜8 nm, volume weighted mean aggregate size ˜16 μm) and NanoActive® Magnesium Oxide Plus (crystallite size ˜4 nm, volume weighted mean aggregate size ˜16 μm), were ordered from NanoScale Corporation (Manhattan, Kans., US). 
         [0025]    Pure CaO samples were prepared by direct thermal decomposition of CaO-containing sources at 800° C. for 2 hr in air. Four different methods were used to prepare MgO-doped CaO absorbents. Table 1 briefly summarizes these methods. 
         [0000]    
       
         
               
             
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Preparation methods for MgO-doped CaO absorbents 
               
             
          
           
               
                 Method 
                 Details 
               
               
                   
               
               
                 Co- 
                 Precipitation of aqueous solution containing Ca and Mg 
               
               
                 precipitation 
                 acetates with 1 M Na 2 CO 3 , followed by filtration, dryness, 
               
               
                   
                 and calcination in air at 800° C. for 2 hr. 
               
               
                 Solution 
                 Direct dryness of aqueous solution containing Ca and Mg 
               
               
                 mixing 
                 acetates, followed by calcination in air at 800° C. for 2 hr. 
               
               
                 Dry physical 
                 Overnight ball-milling of solid CaO and MgO sources, 
               
               
                 mixing 
                 followed by calcination in air at 800° C. for 2 hr. 
               
               
                   
               
             
          
         
       
     
         [0026]    High Temperature Carbonation-decarbonation Performance Measurement. A Netzsch 409C thermogravimetric analyzer (TGA) was used to screen the performance of absorbents. Typical measurements employed ˜20 mg powder, and the carbonation-decarbonation test was carried out at a fixed temperature, 758° C. During each test, 30 ml/min 100% CO 2  (for carbonation) and 60 ml/min pure He (for decarbonation) were introduced into the system alternatively via an automated switch valve every 30 minutes. The cyclic number varied according to the performance of each absorbent. In order to compare the performances of stable absorbents, 50-100 carbonation-decarbonation cycles were normally carried out. CO 2  absorption capacity was calculated using the total weight gain during each carbonation cycle divided by the total weight of absorbent in the oxide form. CaO utilization was calculated as the percentage of CaO converted to CaCO 3 , based on CO 2  capacity and CaO concentration in the absorbent. 
         [0027]    Characterization. Scanning electron microscopy (SEM) analysis was carried out with a JEOL JSM-5900LV microscope. Selected area energy dispersive X-ray spectroscopy (EDS) was performed on regions of interest using a Links EDS system equipped on the microscope. Powder X-ray Diffraction (XRD) measurement and analysis were conducted with a Philips PW3050 diffractometer using Cu Kαradiation and JADE, a commercial software package. The nitrogen BET surface area was measured with a QUANTACHROME AUTOSORB 6-B gas sorption system with degassed samples. 
         [0028]      FIG. 3  shows the CO 2  capture performances of these various materials as well as those of a natural mineral with similar Ca—Mg ratio, dolomite. Although all these four samples have same quantities of base materials, their long-term performances are surprisingly different. The absorbent obtained from co-precipitation of Ca(CH 3 COO) 2  and Mg(CH 3 COO) 2  with Na 2 CO 3  gives the worst performance, with less than 10 wt % CO 2  capacity after 30 cycles. Ca(CH 3 COO) 2  and Mg(CH 3 COO) 2  solution mixing followed by calcination produced an absorbent with a similar performance as that of natural dolomite, indicating molecular level mixing of CaO and MgO can be achieved with this method. The absorbent obtained from dry physical mixing of Ca(CH 3 COO) 2  with MgO shows the best stability and the highest CO 2  capacity (&gt;43 wt %), as well as the highest CaO utilization (&gt;95%) after the 50 cycles carbonation-decarbonation test. 
         [0029]      FIG. 4  shows the carbonation and decarbonation reactions of this absorbent at the 2 nd  and the 50 th  cycle. During the carbonation steps of both the 2 nd  cycle and the 50 th  cycle, more than 80% of total CO 2  capture happens within the first 4 minutes, indicating this absorbent has good CO 2  capture kinetics. On the other hand, it takes about 17 minutes (the 2 nd  cycle) and about 20 minutes (the 50 th  cycle) to completely regenerate the CO 2 -loaded absorbent after changing the TGA flow gas from CO 2  to He. Based on thermodynamic calculation, temperatures higher than 758° C. are preferred to decompose CaCO 3 . To accelerate the decarbonation step and to get high concentration CO 2  streams, a combined CO 2  pressure swing-calcination temperature swing process should be used in practical applications. 
         [0030]    By physical mixing of Ca(CH 3 COO) 2  with MgO obtained from different precursors, the effect of MgO source was evaluated. In order to accelerate the screening process, 26 wt % MgO-doped absorbents were used. Table 2 compares their performances. In general, the effect of MgO source is not as large as that of CaO source. Except one absorbent with MgO from decomposition of Mg(OH) 2 , all the MgO-doped samples show much better long-term performances than pure CaO absorbents. Among all the tested samples, MgO obtained from thermal decomposition of MgC 2 O 4  at 700° C. shows the best performance. Even after 100 cycles, the CaO absorbent containing 26 wt % of this MgO still has 45 wt % CO 2  capacity, corresponding to 77% total CaO utilization. However, SEM analysis shows there are dramatic morphology change and CaO and MgO particles re-distribution after the 100 cycles carbonation and decarbonation test ( FIG. 5 ). Although some particle sintering was observed after the cycling test, the absorbent still maintained a porous structure. We believe this is the major contributor to the observed good CO 2  capture performance. Uniform mixing of MgO and CaO particles was developed after the 100 cycles test. 
         [0000]    
       
         
               
             
               
               
             
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                 TABLE 2 
               
             
             
               
                   
               
               
                 Effect of MgO source on the performance of 24 wt % MgO-doped CaO 
               
               
                 absorbent a,b,c   
               
             
          
           
               
                   
                 CO 2  capacity at different carbonation- 
               
               
                   
                 decarbonation cycles, wt % 
               
             
          
           
               
                   
                 5 th   
                   
                   
                   
               
               
                 MgO source 
                 cycle 
                 25 th  cycle 
                 50 th  cycle 
                 100 th  cycle 
               
               
                   
               
             
          
           
               
                 No MgO doping 
                 60.8 
                 36.2 
                 27.0 
                 N.M. 
               
               
                 MgO (~325 μm) 
                 47.9 
                 45 
                 42.2 
                 N.M. 
               
               
                 NanoActive ® MgO (~8 nm) 
                 46.2 
                 44.3 
                 41.1 
                 N.M. 
               
               
                 NanoActive ® MgO Plus 
                 47.9 
                 48.0 
                 47.9 
                 N.M. 
               
               
                 (~4 nm) 
               
               
                 MgO from decomposition 
                 49.1 
                 46.5 
                 34.1 
                 N.M. 
               
               
                 of Mg(OH) 2  at 800° C., 2 hr 
               
               
                 MgO from decomposition 
                 49.0 
                 44.3 
                 N.M. 
                 N.M. 
               
               
                 of Mg(CH 3 COO) 2  at 700° C., 
               
               
                 2 hr 
               
               
                 MgC 2 O 4   
                 50.5 
                 49.6 
                 44.7 
                 N.M. 
               
               
                 MgO from decomposition 
                 55.4 
                 53.7 
                 51.5 
                 43.6 
               
               
                 of MgC 2 O 4  at 500° C., 2 hr 
               
               
                 MgO from decomposition 
                 55.9 
                 55.0 
                 52.9 
                 44.1 
               
               
                 of MgC 2 O 4  at 600° C., 2 hr 
               
               
                 MgO from decomposition 
                 57.0 
                 55.0 
                 52.9 
                 45.2 
               
               
                 of MgC 2 O 4  at 700° C., 2 hr 
               
               
                 MgO from decomposition 
                 53.9 
                 52.3 
                 50.7 
                 45.2 
               
               
                 of MgC 2 O 4  at 800° C., 2 hr 
               
               
                 MgO from decomposition 
                 52.2 
                 53.4 
                 49.6 
                 42.6 
               
               
                 of MgC 2 O 4  at 900° C., 2 hr 
               
               
                 MgO from decomposition 
                 50.3 
                 48.9 
                 N.M. 
                 N.M. 
               
               
                 of MgC 2 O 4  at 1000° C., 2 hr 
               
               
                   
               
               
                   a All the absorbents were prepared using dry physical mixing of Ca(CH 3 COO) 2  with MgO source, followed by calcination at 800° C. in air for 2 hours. 
               
               
                   b Carbonation at 758° C. in 100% CO 2  for 30 min, decarbonation at 758° C. in 100% He for 30 min. 
               
               
                   c N.M. = not measured. 
               
             
          
         
       
     
         [0031]    All the MgO samples obtained from decomposition of MgC 2   4  at different temperatures were characterized using XRD analysis and the MgO crystallite size was roughly estimated using Jade Software based on Scherrer&#39;s equation.  FIG. 6  shows very small MgO crystallites can be obtained by direct thermal decomposition of MgC 2   4  below 800° C. The surface area of the MgO sample prepared by calcination of MgC 2 O 4  at 600° C., as measured by BET method, is 181 m 2 /g. If all the small MgO crystallites are considered to be spherical and theoretical MgO density (3.58 g/cm 3 ) is used, the calculated MgO crystallite size is about 9 nm, which is close to that estimated from XRD analysis (˜8 nm). As shown in Table 2, these small MgO particles, even with only 26 wt % loading, can effectively improve the CO 2  capture performances of CaO-based absorbents. Table 2 also shows two commercial nano MgO samples are not as effective, probably due to MgO particles agglomeration (manufacture-reported volume weighted mean aggregate size is ˜16 μm) in these two products. 
         [0032]    To optimize the absorbent&#39;s composition, the effect of MgO concentration in CaO absorbent was studied. The absorbents were prepared by dry physical mixing of Ca(CH 3 COO) 2  with MgO obtained from calcination of Mg 2 C 2 O 4  at 700° C.  FIG. 7  gives the performances of absorbents with 19, 24, and 42 wt % MgO doping. Higher MgO doping gives better stability and higher CaO utilization, but relatively lower CO 2  capacity. In practical applications, the optimized MgO doping level will be largely decided by the operation cost, especially by the cost of the absorbent and the absorbent replacement ratio during each cycle. An economic evaluation needs to be carried out before a meaningful recommendation of absorbent composition can be given. 
         [0033]    In summary, we discovered the sintering effect of CaO-based CO 2  absorbents can be effectively mitigated by doping MgO nanoparticles into porous-structured CaO materials. Doping method plays an important role in producing stable absorbents. Among the three preparation methods used in this work, i.e. solution mixing, co-precipitation, and physical mixing, physical mixing produces the most durable absorbents with high CO 2  capacity. The source of MgO also has some effect on the performance of MgO—CaO mixture, although this effect is not as big as that of doping method. CaO doped with MgO nano particles prepared by thermal decomposition of MgC 2 O 4  at 700° C. shows the best performance. With 26 wt % doping of MgO prepared by this method, a CaO—MgO physical mixture gives more than 50 wt % CO 2  capacity (more than 77% total CaO utilization) for 50 cycles of carbonation-decarbonation at 758° C. Without MgO doping, the CO 2  capacity of pure CaO obtained from same source decreases from 66 wt % for the 1 st  cycle to 26 wt % for the 50 th  cycle under the same test conditions. 
         [0034]    While various preferred embodiments of the invention are shown and described, it is to be distinctly understood that this invention is not limited thereto but may be variously embodied to practice within the scope of the following claims. From the foregoing description, it will be apparent that various changes may be made without departing from the spirit and scope of the invention as defined by the following claims.