Particulate, heterogeneous solid CO2 absorbent composition, method for its preparation and use thereof

Abstract

A particulate, heterogeneous solid CO.sub.2 absorbent composition, comprising decomposition products of Ca.sub.3Al.sub.2O.sub.6 after having been heated to a temperature between 500 C. and 925 C. in the presence of H.sub.2O and CO.sub.2 for a period of time sufficient to allow the Ca.sub.3Al.sub.2O.sub.6 to react and form the particulate, heterogeneous absorbent composition which exhibits a higher concentration of aluminum than calcium in the particle core but a higher concentration of calcium than aluminum at the particle surface. The invention also comprises a method for preparing the particulate, heterogeneous product as well as a method for utilizing the composition for separating CO.sub.2 from a process gas.

Claims

1. A method for preparing a particulate, heterogeneous, solid, CO.sub.2 absorbent composition for use in chemical processing, the method comprising: providing a raw material comprising Ca.sub.3Al.sub.2O.sub.6; and heating the raw material to a temperature between 500 C. and 925 C. in the presence of H.sub.2O and CO.sub.2 for a period of time sufficient to allow the raw material to react and form the particulate, heterogeneous, solid, CO.sub.2 absorbent composition.

2. The method as claimed in claim 1, wherein a ratio of CaO to Ca.sub.12Al.sub.14O.sub.33 in the composition is from 25 to 45% by weight.

3. The method as claimed in claim 1, wherein a ratio of CaO to Ca.sub.12Al.sub.14O.sub.33 in the composition is from 26.7 to 41% by weight.

4. The method as claimed in claim 1, wherein the source of calcium and aluminium is at least partially in the form of a nitrate solution.

5. The method as claimed in claim 1, wherein the raw material comprising Ca.sub.3Al.sub.2O.sub.6 is compacted and optionally agglomerated in order to obtain the particulate, heterogeneous, solid, CO.sub.2 absorbent composition having a desired mechanical stability, crushing strength or resistance to attrition.

6. The method as claimed in claim 1, further comprising adding a catalytically active material so that catalytically active particles are present on the surface of the particulate heterogeneous, solid, CO.sub.2 absorbent composition.

7. The method as claimed in claim 6, wherein the catalytically active material is selected from the group consisting of Ni, Co, Fe, Cr, Cu, Zn, Pt, Pd, Rh, Ru, Ir, and combinations thereof.

8. The method as claimed in claim 6, wherein the catalytically active material is added subsequent to the formation of the particulate, heterogeneous, solid, CO.sub.2 sorbent composition by means of an impregnation technique.

9. The method as claimed in claim 1, wherein the step of providing a raw material comprising Ca.sub.3Al.sub.2O.sub.6, comprises providing a source of calcium and aluminium ions mixed in solution and to dry the solution by heating it to a temperature in the range 100-400 C. for a period of 8-15 hours in the presence of a chelating agent and a polymerizing agent to form a solid precursor and to heat the precursor to a temperature in the range 500 to 850 C. to burn any organic compounds present in the precursor and to heat the resulting powder to a temperature in the range 900-1100 C. for a period of 5 to 30 hours to form an oxide powder comprising Ca.sub.3Al.sub.2O.sub.6.

10. The method as claimed in claim 9, wherein the aluminium and calcium for the source of their ions are extracted from a naturally occurring mineral or rock.

11. The method as claimed in claim 10, wherein the naturally occurring mineral or rock is at least one of anorthosite and calcite.

Description

DETAILED AND EXPERIMENTAL DESCRIPTION OF THE INVENTION WITH REFERENCE TO THE DRAWINGS

(1) The invention will be described in more detail under reference to preferred embodiments and verification tests performed on the preferred embodiment as compared to a conventionally used CO.sub.2 sorbent (dolomite). Throughout this discussion reference will be made to the enclosed drawings, where

(2) FIG. 1 shows the two synthesis routes according to the invention for the synthetic Ca-based CO.sub.2 absorbents: from rocks (Route A) or from calcium or aluminium nitrate precursors (Route B). The figures also shows the two different possibilities to produce powdered or pelletized sorbent as well as the final thermal treatment as described in the invention

(3) FIG. 2 shows the three different alternatives for accommodation of the catalytically active material in the sorbent at various stage of the synthesis of the invention.

(4) FIG. 3 shows X-ray Diffraction patterns of Ca.sub.3Al.sub.2O.sub.6 synthesized according to the invention after calcination at various temperatures.

(5) FIG. 4 shows SEM images of produced Ca.sub.3Al.sub.2O.sub.6 powder according to the invention a) and b): after calcination at 1000 C. for 12 h, c and d): after heat treatment at 800 C. in 50% CO.sub.2/50% H.sub.2O atmosphere for 24 hours.

(6) FIG. 5 shows X-ray Diffraction patterns of Ca.sub.3Al.sub.2O.sub.6 calcined at various temperatures according to the invention after heat treatment at 800 C. in 50% CO.sub.2/50% H.sub.2O atmosphere for 24 hours.

(7) FIG. 6 shows the thermo gravimetric analysis of Ca.sub.3Al.sub.2O.sub.6 powder according to the invention during heat treatment at 800 C. in 50% CO.sub.2/50% H.sub.2O atmosphere for 24 hours.

(8) FIG. 7 shows the effect of temperature of decomposition of Ca.sub.3Al.sub.2O.sub.6 powder synthesis according to the invention, calcined at 1000 C. for 24 h in controlled atmosphere: 0.5 atm CO.sub.2 and 0.5 atm H.sub.2O (g).

(9) FIG. 8 shows the effect of partial pressure of CO.sub.2 and H.sub.2O (g) at 780 C. on decomposition of Ca.sub.3Al.sub.2O.sub.6 powder calcined at 1000 C. for 24 h according to the invention.

(10) FIG. 9 shows SEM image of a cross section of a sorbent particle after exposure to CO.sub.2 and Steam (a) and elemental composition obtained by EDS at various along the particle (b).

(11) FIG. 10 shows a carbonation/regeneration cycles for Ca.sub.3Al.sub.2O.sub.6 after decomposition produced according to the invention. Powder was carbonated at 780 C. in 50% CO.sub.2/50% H.sub.2O atmosphere for 10 min and regenerated at 870 C. in 50% CO.sub.2 and 50% H.sub.2O for 15 min.

(12) FIG. 11 shows the evolution of the absorption capacity of the Ca.sub.3Al.sub.2O.sub.6 after decomposition produced according to the invention as a function of the number of cycles. Evolution of dolomite in similar conditions is included as a comparison.

(13) FIG. 12 shows X-ray Diffraction patterns of produced CaOCa.sub.3Al.sub.2O.sub.6 powder according to the invention after calcination at 1000 C. for 12 h for different CaO-to-Ca.sub.3Al.sub.2O.sub.6 weight ratios.

(14) FIG. 13 shows the evolution of the absorption capacity of the CaOCa.sub.3Al.sub.2O.sub.6 powder produced according to the invention for different CaO-to-Ca.sub.3Al.sub.2O.sub.6 weight ratios.

(15) FIG. 14 shows SEM image of a pellet of sorbent particle after exposure to 50% CO.sub.2 and steam.

(16) FIG. 15 shows X-ray Diffraction patterns of produced CaOCa.sub.3Al.sub.2O.sub.6NiO powder according to the invention a) after calcination at 1000 C. for 12 h and b) after heat treatment at 800 C. in 50% CO.sub.2/50% H.sub.2O atmosphere for 24 hours.

(17) FIG. 16 shows a SEM image (back scattered electrons) of a pellet of sorbent particle with NiO after decomposition in 50% CO.sub.2 and steam and multi cycle in severe conditions.

(18) FIG. 17 shows the gas composition from the outlet of a fixed bed reactor (dry gas) as a function of time during reforming with methane (Steam/Carbon=3, 650 C., 500 ml/min). Reactor is filled with impregnated CO.sub.2-sorbent according to the invention. Prior the reforming experiment, catalytically active material was reduced in H.sub.2 at 650 C.

(19) Several batches of Ca.sub.3Al.sub.2O.sub.6 mixed CaOCa.sub.3Al.sub.2O.sub.6 and finally mixed NiOCaOCa.sub.3Al.sub.2O.sub.6 were produced according to the inventive method and one sample of natural dolomite conventionally used as CO.sub.2 sorbent were prepared in order to verify the effect of the mixed oxide compound produced according to the invention in a series of comparison tests.

(20) The starting oxide powders were synthesized through the citrate route. Citric acid (CA) (Merck, >99.5%), ethylene glycol (EG) (Merck, >99.5%), aluminium nitrate nonahydrate (Al(NO.sub.3).sub.3.9H.sub.2O, Aldrich >99%) and calcium nitrate tetrahydrate (Ca(NO.sub.3).sub.2.4H.sub.2O, Aldrich >99%) were used as precursors. In addition, nickel nitrate Ni(NO.sub.3).sub.2.6H.sub.2O can be added to the aqueous solution in order to obtain 10 wt % NiO after calcination if needed for the application of the obtained material. Stoichiometric amounts of metal nitrates were dissolved in approximately 100 mL of deionized water in order to obtain the final powder. Citric acid monohydrate (CA) (C.sub.6H.sub.8O.sub.7.H.sub.2O, Merck GR for analysis >99.5%) was added in the molar ratio 2/1 to that of cations to make sure all the cations were complexed in the solution and ethylene glycol (EG) (C.sub.2H.sub.6O.sub.2 pro analysis Merck >99.5%) was further added at a molar ratio EG/CA=3/2.

(21) After achieving complete dissolution, the solution was introduced in an oven preheated at 160 C. for 6 h to evaporate the superfluous water. Upon heating the volume of the solution decreased and the viscosity of the solution increased gradually due to the esterification reaction between CA and EG. No evidence of precipitation was observed during the evaporation process. After evaporation of the water, a brown fluffy gel was obtained which was further pulverized and calcined at 600 C. for 6 h to burn off most of the organic residues. Finally, the powder was calcined at elevated temperature to obtained single phase materials. Prior further analysis, the powder was sieved under 150 m.

(22) Comparison Sample

(23) Sample from Arctic Dolomite were mechanically crushed and sieved in the range 150-300 m for TGA and fixed bed investigations. Before further analysis the samples where calcined in static air at 900 C. for 6 h to decompose the carbonates and remove the organic contents of the minerals.

(24) Techniques of Characterization

(25) The XRD spectra of the sorbents were performed on an Inel XRG 3000 diffractometer with CuK radiation. XRD test results were retrieved and stored using commercial software (Inel Acquisition). The morphology of the powders and element analysis were performed by scanning electron microscopy (SEM, Hitachi S-4800 Field Emission)

(26) Thermal stability, CO.sub.2 sorption capacity and sorption/regeneration kinetics were studied using a thermo gravimetric analyzer (TGA, CI electronics). The TGA test was initiated in a N.sub.2 atmosphere with temperature increased to the desired reaction temperature at a rate of 10 C./min. N.sub.2, CO.sub.2 and/or H.sub.2O were introduced after 5 minutes at the desired temperature. The N.sub.2/CO.sub.2 ratio was controlled by mass flow controllers (Bronkhorst, EL-FLOW Digital series). The steam flow was controlled using a liquid flow controller (Bronkhorst, Liquid-flow). The separate flows were combined using a controlled evaporation mixing system (Bronkhorst, CEM). The effects of CO.sub.2 and steam partial pressures on the carbonation of the Ca-based sorbent were also investigated. Natural Ca-based sorbents like artic dolomite were investigated as a comparison. Similar flow meters and conditions were used to investigate the reforming of methane in the fixed bed reactor. Approx 30 g of the synthesized powder was loaded and mechanically compacted in the reactor prior investigated. Gas composition at the outlet of the reactor was analyzed using a micro GC (Agilent 3000A).

(27) Results and Discussion

(28) 1Decomposition of the Ca.sub.3Al.sub.2O.sub.6

(29) X-ray Diffraction (XRD) patterns of the Ca.sub.3Al.sub.2O.sub.6 powder after calcination at various temperatures are shown in FIG. 3. After calcination at 900 C. for 6 h, CaO did not completely react with the aluminate and traces of Ca.sub.5Al.sub.6O.sub.14 were detected in the powder together with CaO and Ca.sub.3Al.sub.2O.sub.6. The amount of secondary phase decreases with increasing calcination temperature and time and single phase Ca.sub.3Al.sub.2O.sub.6 materials were finally obtained after calcination at 1000 C. for 24 h and 1100 C. for 1 hour. An optimal calcination conditions for the invention was determined to be 12 hours at 1000 C.

(30) The SEM image as the obtained pure Ca.sub.3Al.sub.2O.sub.6 particles is shown in FIGS. 4 a) and b). The typical particle size is 10 to 100 um. The surface of the synthesized Ca.sub.3Al.sub.2O.sub.6 particle is homogeneous and present low apparent porosity.

(31) In order to study the chemical stability of Ca.sub.3Al.sub.2O.sub.6, the powder was exposed to different partial pressure of CO.sub.2 and steam at elevated temperature. After switching atmosphere from N.sub.2 to a CO.sub.2/steam mixture at elevated temperature, a weight increase was observed as illustrated in FIGS. 6, 7 and 8. As evidenced by the XRD pattern (FIG. 5), after exposure to 50% CO.sub.2/steam atmosphere at 800 C. for 24 h, Ca.sub.3Al.sub.2O.sub.6 decomposes into CaO and Ca.sub.12Al.sub.14O.sub.33 according to the reaction (1):

(32) ##STR00001##

(33) The weight increase during decomposition is due to the carbonation of the formed CaO after decomposition of the initial Ca.sub.3Al.sub.2O.sub.6 powder. According to (1) a maximum weight increase of 20,974 g of CO.sub.2 per 100 g Ca.sub.3Al.sub.2O.sub.6 is expected. The measured value of weight increase (FIG. 6) is in accordance with the calculated value. Therefore, a total conversion of more than 90% of the CaO formed during decomposition is evidenced. However powders calcined at 900 C. for 6 h, the secondary phase initially present Ca.sub.5Al.sub.6O.sub.14 is still present in the powder after exposure to a steam and CO.sub.2 at elevated temperature 800 C.

(34) The influences of the preparation temperature, absorption temperature and partial pressures of CO.sub.2 and H.sub.2O (g) on kinetics of CO.sub.2-uptake were thoroughly investigated by TGA.

(35) 2Effect of the Calcination Temperature on CO.sub.2-Uptake

(36) The influence of the calcination temperature on the stability of the powder is illustrated in FIG. 6. Materials synthesized at 900 C. for 6 h, 1000 C. for 24 h and 1100 C. for 1 h were successively exposed to low partial pressures of CO.sub.2 and steam at 780 C. In those conditions, a weight increase was measured for the three different powders due to the carbonation of formed CaO during decomposition according to the invention. However, the kinetic of absorption of CO.sub.2 seems to be dependent on the calcination temperature. Powder calcined at 900 C. displays the highest weight increase rate but has a lower maximum weight increase (16 wt %) while powders calcined at higher temperature and for longer time show a slower weight increase but a higher maximum value close to 20 wt %. The lower weight increase for powder calcined at 900 C. is due to the stability of Ca.sub.5Al.sub.6O.sub.14 during thermal treatment (FIG. 5). Thus, in order to obtain a maximum CO.sub.2 absorption capacity after high-temperature treatment in CO.sub.2/H.sub.2O, the synthesized material should be single phase. However, increasing the calcination temperature and the calcination time during synthesis increases the stability of the formed particles and decreases the kinetic of decomposition of Ca.sub.3Al.sub.2O.sub.6 (FIG. 2).

(37) 3Effect of the Treatment Temperature on the Decomposition of Ca.sub.3Al.sub.2O.sub.6

(38) FIG. 7 shows the influence of the temperature on the decomposition rate of the Ca.sub.3Al.sub.2O.sub.6. The experiments were performed in to 0.5 atm CO.sub.2 and 0.5 atm H.sub.2O. A maximum decomposition rate was measured for a decomposition temperature of 780 C. At 700 C. and 780 C., a maximum weight increase of 19.5% was measured while at 600 C., the maximum was not reached because of the slow kinetic. At 850 C., the kinetic of decomposition is decreased as well as the maximum CO.sub.2-uptake (14.5%). Finally the powder was treated at 900 C. in the same atmosphere for 12 hours but no weight increase was observed on the TGA. After exposure at 900 C., the powder was cooled down to 700 C. in N.sub.2 and exposed to 0.5 atm CO.sub.2/0.5 atm H.sub.2O. A swift weight increase of 4 wt % was then measured followed by a steady weight increase similar to the weight increase observed previously for powder decomposed at 700 C.

(39) Both steam and CO.sub.2 must be present in the feed gas mixture to observe the decomposition of Ca.sub.3Al.sub.2O.sub.6. Formation of CaCO.sub.3 at high temperature seems to be a possible driving force for the decomposition of Ca.sub.3Al.sub.2O.sub.6 into CaCO.sub.3 and Ca.sub.12Al.sub.14O.sub.33. The equilibrium partial pressure of CO.sub.2 as a function of calcination temperature calculated from an equation proposed by Baker et al [7]. According to this equation, at P.sub.CO2=0.5 atm, the calcination temperature of CaCO.sub.3 is 853 C. Therefore, when the material was exposed at a CO.sub.2/H.sub.2O mixture at 850 C., the total decomposition of the material was not reached after 20 hours while at 900 C., no decomposition of the Ca.sub.3Al.sub.2O.sub.6 was observed.

(40) 4Effect of Partial Pressure of CO.sub.2 and Steam

(41) The influence of the partial pressure of CO.sub.2 and steam on the decomposition rate of the Ca.sub.3Al.sub.2O.sub.6 was investigated by exposing the materials to different partial pressures of CO.sub.2 and steam at 780 C. (FIG. 8). When the material was exposed to 100% CO.sub.2 or 100% steam, no weight variation could be observed meaning that Ca.sub.3Al.sub.2O.sub.6 is stable. In diluted atmosphere (35% CO.sub.2, 20% H.sub.2O, 45% N.sub.2) the maximum weight increase was 13% after 20 hours. Increasing the partial pressure of H.sub.2O (35% CO.sub.2/65% H.sub.2O) and the partial pressure of CO.sub.2 (80% CO.sub.2/20% H.sub.2O) the decomposition rate of decomposition is increased and the maximum weight uptake of 19.5% was achieved after respectively 9 and 19 hours.

(42) Introduction of steam in the feed gas was shown to be necessary to decompose the Ca.sub.3Al.sub.2O.sub.6. A higher reactivity was observed at high steam partial pressure. Previous reports have shown the influence of steam on the CO.sub.2 capture but mechanisms are not fully understood. However, steam hydration of CaO increases both pore area and pore volume, consequently improving the long-term conversion to CaCO.sub.3 over multiple cycles [19]. Thus, diffusivity of CO.sub.2 through the product layer is improved and the reaction kinetics enhanced.

(43) 5Morphology of the Sorbent

(44) FIGS. 4 c) and d) shows the SEM image of the Ca.sub.3Al.sub.2O.sub.6 after treatment at 800 C. in 50% CO.sub.2 and 50% H.sub.2O. The morphology of the particles seems not to be affected by the thermal treatment as shown by images (a) and (c). However, the surface of the particles changed after exposure to steam and CO.sub.2 at elevated temperature and small (200-500 nm) spherical particles were formed.

(45) A cross section of the particles after treatment in CO.sub.2/steam at 800 C. is shown in FIG. 9. A homogeneous outer layer of approximately 500 nm to 1 m could be observed around the large particles. The elemental analysis was performed in different points on the particles as shown in FIG. 9(b). An increase of the concentration of Calcium at the surface of the particle is then evident while the concentrations inside the particle correspond to a composition close to Ca.sub.12Al.sub.14O.sub.33.

(46) Those results seem to indicate that those spherical nano particles are mainly constituted of a CaO phase that has formed on the surface of larger calcium aluminate particles. Those nano particles are homogeneous in size and are uniformly distributed on the surface of the aluminate particles forming a thin (500 nm-1 um) CaO-rich layer. The elementary analysis of the cross section of the agglomerates has shown an increased concentration of Ca at the surface of the agglomerates while the Ca-to-Al ratio in the bulk of the particles is close to the atomic ratio corresponding to the formula Ca.sub.12Al.sub.14O.sub.33. The higher concentration of Ca compared to Al in the bulk might be caused by traces of CaCO.sub.3 trapped in the bulk material after regeneration as evidenced by XRD (FIG. 3)

(47) 6Stability During Multi-Cycles Analysis

(48) Because of the available free CaO formed during decomposition of Ca.sub.3Al.sub.2O.sub.6, this material has a potential as high temperature CO.sub.2 acceptor. After decomposition of the Ca.sub.3Al.sub.2O.sub.6 at 800 C. in 50% CO.sub.2 and 50% steam the powder was regenerated in 50% N.sub.2 and 50% H.sub.2O. In order to determine its potential as high temperature CO.sub.2-sorbent, the powder was repeatedly exposed to a mixture of 50% CO.sub.2/50% steam at 780 C. for 10 min, heated to 870 C. for regeneration with a 10K/min heating ramp and finally cooled to 780 C. in 50% N.sub.2/50% H.sub.2O (g) with a 3K/min cooling ramp. The weight variations during the first, 10.sup.th and 70.sup.th cycles are illustrated in FIG. 10 together with the temperature profile. The experiment shows a swift CO.sub.2 uptake at 780 C. followed by a plateau followed by a release of CO.sub.2 at 870 C. The absorption and regeneration kinetics remain unchanged during absorption/calcination multi-cycles. The maximum absorption capacity for each cycle is illustrated in FIG. 10. The CO.sub.2 absorption capacity is increasing from 14 to 18 wt % during the first 20 cycles and is constant around 18.5% for more than 150 cycles afterwards. For comparison the absorption capacity of natural dolomite during multi cycles in the same conditions is illustrated in the same figure. Dolomite shows a large decay of its CO.sub.2-uptake from 44 wt % during the first cycle to 16 wt % at 70.sup.th cycles and finally stabilizes around 10 wt % above 150 cycles as reported previously by various authors [2, 3, 5, 19].

(49) During long term multi-cycling, the material shows a slight increase of the absorption capacity during the 10 first cycles which might be attributed to a completion of the decomposition of the Ca.sub.3Al.sub.2O.sub.6 starting material. After 10 cycles, the absorption and regeneration kinetics are stable whilst the total absorption capacity of the material remains close to 20 g CO.sub.2/100 g sorbent for 150 carbonation/calcination cycles in severe regeneration conditions. This improved stability compared to conventional CO.sub.2 sorbents might be attributed to a limited sintering of the CaO nano particles due to a low agglomeration of the nano particles on the surface and a limited mass transfer between the CaO particles even at elevated temperature. Because of the small size of the CaO particles, the sorbent shows a high reactivity and high a conversion level above 90% of the total CO.sub.2 capacity after 150 cycles.

(50) 7Study of CaOCa.sub.3Al.sub.2O.sub.6 Mixed Oxide Powder

(51) To increase the total CO.sub.2 capacity of powder, mixed powder CaOCa.sub.3Al.sub.2O.sub.6 were synthesized with different CaO-to-Ca.sub.3Al.sub.2O.sub.6 weight ratio, x, (x=m(CaO)/m(Ca.sub.3Al.sub.2O.sub.6)) with x=0.6, 0.4 0.2 and 0.1. As shown in FIG. 11, single phase mixed oxide powder were obtained after calcination at 1000 C. for 12 h.

(52) During thermal treatment at 800 C. for 24 h in a CO.sub.2/steam gas mixture, a first swift weight increase due to the carbonation of the CaO introduced in the powder during synthesis was observed. A second weight increase with a slower rate was also observed which can be attributed to the carbonation of CaO formed during decomposition of the Ca.sub.3Al.sub.2O.sub.6 as illustrated in (1).

(53) To determine the optimal CaO content in the mixed powder, multi cycles experiments similar to those described previously were carried out on mixed powder after decomposition of the Ca.sub.3Al.sub.2O.sub.6. The powders were studied in severe calcination conditions (carbonation at 800 C. and calcination at 925 C. in 85% CO.sub.2 and 15% steam) to enhance the sintering process of CaO. The evolution of the absorption capacity for those powders during multi cycling is illustrated in FIG. 13.

(54) Increasing the CaO content in the powder, the total absorption capacity is increased but the stability during multi cycles is also decreased. At high Ca-content (x=0.6 and 0.4, the powder shows a large decay of absorption after respectively 10 and 20 cycles. However for x=0.1 and x=0.2, no decrease of absorption capacity was observed even after 200 cycles in those severe conditions. The kinetics of absorption and calcinations remained unchanged during multi cycles indicating a stable reactivity of the CaO particles and a stable CO.sub.2 capacity of respectively of approximately 21 and 29 g CO.sub.2/100 g sorbent.

(55) 8Formation of Pellet by Compaction of the Powder

(56) Before heat treatment powders were compacted using a uniaxial press in order to form pellets of the synthesized. Pressure between 20 and 250 MPa were applied to compact the sorbent powders causing the green apparent density of the ceramic pellets to increase from 35% to a maximum of 50%. The formed pellets were further exposed to CO.sub.2 and Steam for heat treatment at 800 C. and cycled as described previously.

(57) FIG. 14 shows a SEM image of the 20 wt % CaOCa.sub.3Al.sub.2O.sub.6 powder after heat treatment and 200 carbonation/regeneration cycles. A partial sintering of the Ca.sub.12Al.sub.14O.sub.33 can be observed. Necks between the particles were formed, creating a large porous matrix where smaller CaO particles were formed after decomposition. The partly-sintered matrix of Ca.sub.12Al.sub.14O.sub.33 is believed to contribute to the good mechanical stability of the obtained pellet whilst the porous structure allows a good diffusion of the reactant gases to the CaO nano particles homogeneously distributed in the pores. No further sintering of the pellets were observed during multi-cycling.

(58) The particle strength was measured using a uniaxial strength gauge. A maximum strength of 29.41 N (+/4.5) was measured for sorbent particles with a size distribution between 0.5 and 1 mm, compacted with a 250 MPa and heat treated 12 hours at 800 C. in steam and CO.sub.2. As a comparison, natural dolomite with the same particle size has a crushing strength of 11.2 (+/2.8) before calcination, 5.8 N (+/2.5) after calcination at 900 C. for 6 h and 23.9 (+/1.5N) after 200 carbonation/regeneration cycles (fully sintered particles).

(59) 9Catalytic Material in the Synthesis of the Sorbent Material

(60) In order to accommodate a catalytically active material in the sorbent particles, three different alternatives were found as illustrated in FIG. 2.

(61) The first alternative is to introduce the catalytically active material among the sorbent precursors within the synthesis precursors. The final composition is obtained after calcination at high temperature according to the invention. The catalytically active material is homogeneously distributed in the powder. FIG. 15 shows the XRD pattern of the powder obtained after calcination at 1000 C. for 12 h when stoichiometric amount of nickel nitrate (to obtain 10 wt % NiO in the final powder) is introduced in the precursor solution. The three oxide phase CaO, Ca.sub.3Al.sub.2O.sub.6 and NiO can be detected without secondary phases.

(62) The obtained powder was further compacted in pellet and heat treated to decompose the Ca.sub.3Al.sub.2O.sub.6. FIG. 16 shows the SEM image of the calcined powder observed with back scattered electron diffraction. FIG. 12 shows brighter spots homogeneously distributed on the surface and in the porous sorbent. Those can be attributed to NiO nano particles (5-15 nm) formed during the synthesis. Those particles were shown to be stable and no secondary reactions with Ca.sub.12Al.sub.14O.sub.33 was observed after multi-cycling in the operative conditions.

(63) Finally the material was reduced in 50% H2/50% steam at 500 C. for 4 h. FIG. 15, shows the XRD pattern of the material after reduction. The NiO particles were successfully reduced to metal particle without affecting the calcium-based sorbent particle.

(64) A second alternative to incorporate the catalyst in the sorbent particle is to homogeneously mix the sorbent particle with a pre-reduced catalyst powder for reforming of hydrocarbon containing gas (FIG. 2).

(65) Finally a third alternative to incorporate the catalyst in the sorbent is to impregnate the pellets formed after decomposition and partial sintering of the Ca.sub.12Al.sub.14O.sub.33 with a liquid precursor of the catalytically active material. The agglomerates are impregnated under vacuum to obtain diffusion of the liquid within the pores of the agglomerates. The agglomerates are then dried at a temperature between 100 and 300 degrees to evaporate the liquid solvent and further heated at a temperature between 500 and 800 C. and finally, of necessary reduced under H.sub.2 gas to activate the catalyst.

(66) This method was used to impregnate the sorbent synthesized according to the invention. The liquid used was a Ni-nitrate. Powder was then calcined at 700 C. for 6 h and reduced in H.sub.2 for 30 min at 650 C.

(67) The impregnated powder was then arranged in a fixed bed reactor and exposed to a mixture of methane and steam in reforming conditions. FIG. 17 shows the gas composition at the outlet of the fixed bed reactor. A yield of H.sub.2>95% was obtained during 25 min while CO.sub.2, CO and CH.sub.4 concentrations remain low. After 25 min, when the sorbent is fully converted into CaCO.sub.3, H.sub.2 concentration rapidly drops down to approximately 75 mole % while the concentrations of CO, CO.sub.2 and CH.sub.4 increase in the outlet gas mixture. Those concentrations correspond to normal reforming concentrations in such conditions. These results show that the catalyst incorporated in the particle is active and can efficiently reform methane and possibly other hydrocarbon containing gases. This also shows that the sorbent synthesized according to the invention efficiently capture CO.sub.2 produced during the reforming and the water gas shift reaction in the reactor.

ADVANTAGES OF THE INVENTION

(68) As shown above a new CaO/Ca.sub.12Al.sub.14O.sub.33 type of high temperature CO.sub.2 sorbent has been synthesized via decomposition of Ca.sub.3Al.sub.2O.sub.6 during thermal treatment in a steam/CO.sub.2 gas mixture. This new sorbent shows improved CO.sub.2-acceptor property compared to conventionally used natural dolomite material. The new sorbent synthesized according to the invention shows a high reactivity and good CO.sub.2 absorption capacity. A total conversion level of 90% during more than 150 carbonation/calcination cycles at 870 C. with 50% steam in CO.sub.2 was evidenced for the sorbent obtained from decomposition of single phase Ca.sub.3Al.sub.2O.sub.6 material.

(69) Based on equation 1 the relative amount of CaO: Ca.sub.12Al.sub.14O.sub.33 is 26.7:73.3 (by weight). It is experimentally found that an increase of CaO is beneficial within a certain range, but that amounts in excess of 45% by weight is detrimental to the absorption properties and therefore not desired. A relative amount of CaO in the range 25-45% is preferred, and more preferred an amount in the range 26.7-41% by weight (of the total CaO+Ca.sub.12Al.sub.14O.sub.33 amount)

(70) Thus, by increasing the CaO content in the starting composition, the total CO.sub.2 absorption capacity is increased, however, the stability during multi-cycling is decreased. An optimal CaO content of 20 wt % CaO-80 wt % Ca.sub.3Al.sub.2O.sub.6 was shown to be the most appropriate starting composition. The reason for the improved properties of the composite material is the formation of nano particles of CaO at the surface of larger calcium aluminate particles. Experimental studies evidenced limited grain growth and absence of sintering of the CaO particles even under severe calcination conditions. Improved stability during absorption/regeneration cycling show that CaO/Ca.sub.12Al.sub.14O.sub.33 material obtained via decomposition of Ca.sub.3Al.sub.2O.sub.6 has a great potential for applications with high temperature CO.sub.2 sorption.

(71) The developed synthesis route offers the possibility to tailor the final composition of the material by adjusting the stoichiometry in the precursor's solution. Introduction of 20 wt % excess CaO in relation to the Ca.sub.3Al.sub.2O.sub.6 phase improved the total CO.sub.2 capacity without modifying the chemical stability of the formed sorbent. Introduction of nickel nitrate salt in the solution was shown to form homogeneously dispersed nano particles of NiO on the surface of the sorbent that could be easily reduced to active Ni metal in H2 rich gas flux at high temperature without modifying the structure of the CO.sub.2 sorbent.

(72) Finally, the compaction of the powder synthesized according to the invention was shown to form a partly sintering ceramic material with improved mechanical properties. Because of the decomposition of the Ca.sub.3Al.sub.2O.sub.6 phase and a partial sintering of the formed Ca.sub.12Al.sub.14O.sub.33, a porous ceramic matrix was obtained. Because of the high porosity of the particles, excess CaO, CaO formed during decomposition of Ca.sub.3Al.sub.2O.sub.6 and NiO are easily accessible to gas for gas-solid reactions such as reforming of hydrocarbon rich gases and CO.sub.2 absorption.

REFERENCES

(73) [1] Harrison, D. P. The role of solids in CO.sub.2 capture: A mini review, Greenhouse Gas Control Technologies 7, 2005, 1101-1106. [2] Abanades, J. C., Anthony, E. J., Lu, D. Y., Salvador, C. and Alvarez, D. Capture of CO.sub.2 from combustion gases in a fluidized bed of CaO. AlChE Journal, 50:1614-1622, 2004a. [3] Abanades, J. C., Rubin, E. S. and Anthony, E. J. Sorbent cost and performance in CO2 capture systems. Industrial & Engineering Chemistry Research, 43:3462-3466, 2004b. [4] Abanades, J. C. and Alvarez, D. Conversion limits in the reaction of CO2 with lime. Energy & Fuels, 17:308-315, 2003. [5] Grasa, G.; Abanades, J. C. CO2 Capture Capacity of CaO in Long Series of Carbonation/Calcination Cycles. Industrial and Engineering Chemistry Research 2006, 45, 8846-51 [6] Green, D. A., Turk, B. S., Gupta, R. P., Portzer, J. W., McMichael, W. J., Harrison, D. P. Capture of carbon dioxide from flue gas using solid regenerable sorbents. Int. Journal of Environmental Technology and Management, 4:53-67, 2004. [7] Baker, E. H. The calcium oxide-carbon dioxide system in the pressure range 1-300 atmospheres. J. Chem. Soc. 1962, 464. [8] Alvarez, D., Abanades, J. C., 2005b. Pore-size and shape effects on the recarbonation performance of calcium oxide submitted to repeated calcinations/recarbonation cycles. Energy & Fuels 19, 270-278. [9] J. S. Dennis and R. Pacciani, The rate and extent of uptake of CO.sub.2 by a synthetic, CaO-containing sorbent, Chemical Engineering Science, Volume 64, Issue 9, 1 May 2009, Pages 2147-2157 [10] Lu H., Smirniotis P. G., Ernst F. O. and Pratsinis S. E., Nanostructured Ca-based sorbents with high CO.sub.2 uptake efficiency. Chemical Engineering Science, Volume 64, Issue 9, 1 May 2009, Pages 1936-1943. [11] Florin N. H., Harris A. T., Reactivity of CaO derived from nano-sized CaCO.sub.3 particles through multiple CO.sub.2 capture-and-release cycles. Chemical Engineering Science, Volume 64, Issue 2, January 2009, Pages 187-191. [12] Lu H., Reddy E. P., and Smirniotis P. G., Calcium Oxide Based Sorbents for Capture of Carbon Dioxide at High Temperatures. Ind. Eng. Chem. Res. 2006, 45, 3944-3949. [13] Muoz-Guillena M. J., Macas-Prez M. C., Linares-Solano A., Salinas-Martnez de Lecea C., CaO dispersed on carbon as an SO.sub.2 sorbent. Fuel, Volume 76, Issue 6, May 1997, Pages 527-532 [14] Feng B., Liu W., Li X., and An H. Overcoming the Problem of Loss-in-Capacity of Calcium Oxide in CO2 Capture Energy & Fuels 2006, 20, 2417-2420 [15] Li Z.-S., Cai N.-S., Huang Y.-Y, and Han H-J. Synthesis, Experimental Studies, and Analysis of a New Calcium-Based Carbon Dioxide Absorbent. Energy & Fuels, 2005, 19, 1447-1452. [16] Wu S. F., Li Q. H., Kim J. N., and Yi K. B. Yi\ Properties of a Nano CaO/Al2O3CO2 Sorbent Ind. Eng. Chem. Res. 2008, 47, 180-184 [17] Martavaltzi C. S., Lemionidou A. A., Parametric Study of the CaOCa12Al14O33. Synthesis with respect to high CO2 sorption capacity and stability on multicycle operation. Industrial And Engineering Chemistry Research, 2008, 47, 23, 9537-9543. [18] Martavaltzi C. S., Lemionidou A. A., Development of new CaO based sorbent materials for CO.sub.2 removal at high temperature. Microporous and Mesoporous Materials Volume 110, Issue 1, 1 Apr. 2008, Pages 119-127. [19] Hughes R. W., Lu D., Anthony E. J. and Wu Y., Improved Long-Term Conversion of Limestone-Derived Sorbents for In Situ Capture of CO2 in a Fluidized Bed Combustor. Ind. Eng. Chem. Res. 2004, 43, 5529-5539.