Classifications

• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
  • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
  • B01J20/00—Solid sorbent compositions or filter aid compositions; Sorbents for chromatography; Processes for preparing, regenerating or reactivating thereof
  • B01J20/02—Solid sorbent compositions or filter aid compositions; Sorbents for chromatography; Processes for preparing, regenerating or reactivating thereof comprising inorganic material
  • B01J20/04—Solid sorbent compositions or filter aid compositions; Sorbents for chromatography; Processes for preparing, regenerating or reactivating thereof comprising inorganic material comprising compounds of alkali metals, alkaline earth metals or magnesium
  • B01J20/041—Oxides or hydroxides
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
  • B01D—SEPARATION
  • B01D53/00—Separation of gases or vapours; Recovering vapours of volatile solvents from gases; Chemical or biological purification of waste gases, e.g. engine exhaust gases, smoke, fumes, flue gases, aerosols
  • B01D53/34—Chemical or biological purification of waste gases
  • B01D53/46—Removing components of defined structure
  • B01D53/62—Carbon oxides
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
  • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
  • B01J20/00—Solid sorbent compositions or filter aid compositions; Sorbents for chromatography; Processes for preparing, regenerating or reactivating thereof
  • B01J20/02—Solid sorbent compositions or filter aid compositions; Sorbents for chromatography; Processes for preparing, regenerating or reactivating thereof comprising inorganic material
  • B01J20/0203—Solid sorbent compositions or filter aid compositions; Sorbents for chromatography; Processes for preparing, regenerating or reactivating thereof comprising inorganic material comprising compounds of metals not provided for in B01J20/04
  • B01J20/0274—Solid sorbent compositions or filter aid compositions; Sorbents for chromatography; Processes for preparing, regenerating or reactivating thereof comprising inorganic material comprising compounds of metals not provided for in B01J20/04 characterised by the type of anion
  • B01J20/0277—Carbonates of compounds other than those provided for in B01J20/043
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
  • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
  • B01J20/00—Solid sorbent compositions or filter aid compositions; Sorbents for chromatography; Processes for preparing, regenerating or reactivating thereof
  • B01J20/02—Solid sorbent compositions or filter aid compositions; Sorbents for chromatography; Processes for preparing, regenerating or reactivating thereof comprising inorganic material
  • B01J20/06—Solid sorbent compositions or filter aid compositions; Sorbents for chromatography; Processes for preparing, regenerating or reactivating thereof comprising inorganic material comprising oxides or hydroxides of metals not provided for in group B01J20/04
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
  • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
  • B01J20/00—Solid sorbent compositions or filter aid compositions; Sorbents for chromatography; Processes for preparing, regenerating or reactivating thereof
  • B01J20/30—Processes for preparing, regenerating, or reactivating
  • B01J20/34—Regenerating or reactivating
  • B01J20/3433—Regenerating or reactivating of sorbents or filter aids other than those covered by B01J20/3408 - B01J20/3425
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
  • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
  • B01J20/00—Solid sorbent compositions or filter aid compositions; Sorbents for chromatography; Processes for preparing, regenerating or reactivating thereof
  • B01J20/30—Processes for preparing, regenerating, or reactivating
  • B01J20/34—Regenerating or reactivating
  • B01J20/345—Regenerating or reactivating using a particular desorbing compound or mixture
  • B01J20/3458—Regenerating or reactivating using a particular desorbing compound or mixture in the gas phase
  • B01J20/3466—Regenerating or reactivating using a particular desorbing compound or mixture in the gas phase with steam
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
  • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
  • B01J20/00—Solid sorbent compositions or filter aid compositions; Sorbents for chromatography; Processes for preparing, regenerating or reactivating thereof
  • B01J20/30—Processes for preparing, regenerating, or reactivating
  • B01J20/34—Regenerating or reactivating
  • B01J20/3491—Regenerating or reactivating by pressure treatment
• • C—CHEMISTRY; METALLURGY
  • C01—INORGANIC CHEMISTRY
  • C01B—NON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
  • C01B32/00—Carbon; Compounds thereof
  • C01B32/50—Carbon dioxide
• • C—CHEMISTRY; METALLURGY
  • C01—INORGANIC CHEMISTRY
  • C01F—COMPOUNDS OF THE METALS BERYLLIUM, MAGNESIUM, ALUMINIUM, CALCIUM, STRONTIUM, BARIUM, RADIUM, THORIUM, OR OF THE RARE-EARTH METALS
  • C01F11/00—Compounds of calcium, strontium, or barium
  • C01F11/18—Carbonates
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
  • B01D—SEPARATION
  • B01D2251/00—Reactants
  • B01D2251/60—Inorganic bases or salts
  • B01D2251/602—Oxides
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
  • B01D—SEPARATION
  • B01D2257/00—Components to be removed
  • B01D2257/50—Carbon oxides
  • B01D2257/504—Carbon dioxide
• • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
  • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
  • Y02C—CAPTURE, STORAGE, SEQUESTRATION OR DISPOSAL OF GREENHOUSE GASES [GHG]
  • Y02C20/00—Capture or disposal of greenhouse gases
  • Y02C20/40—Capture or disposal of greenhouse gases of CO2
• • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
  • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
  • Y02P—CLIMATE CHANGE MITIGATION TECHNOLOGIES IN THE PRODUCTION OR PROCESSING OF GOODS
  • Y02P20/00—Technologies relating to chemical industry
  • Y02P20/151—Reduction of greenhouse gas [GHG] emissions, e.g. CO2

Definitions

• the present invention relates to the application of chemical sorbents for the separation of CO 2 from gas mixtures.
• the term “supersorbent” shall mean a sorbent as taught in United States Patent No. 5,779,464 entitled “Calcium Carbonate Sorbent and Methods of Making and Using Same", the teachings of which are hereby incorporated by reference.
• the term “microporous” shall mean a pore size distribution of less than 5 nanometers.
• the term “mesoporous” shall mean a pore size distribution of from about 5 nanometers to about 20 nanometers.
• Atmospheric CO 2 concentration has been increasing steadily since the industrial revolution.
• IPCC Intergovernmental Panel on Climate Change 1995 - The Science of climate Change: The Second Assessment Report of the Intergovernmental Panel on Climate Change; Houghton, J. T., Meira Filho, L.G., Callander, B.A., Harris, N., Kattenberg, A., Maskell K, Eds.; Cambridge University Press: Cambridge, U.K., 1996].
• IPCC Intergovernmental Panel on Climate Change
• Adsorption systems capture CO 2 on a bed of adsorbent materials such as molecular sieves and activated carbon [Kikkinides, E.S.; Yang, R.T.; Cho, S. H. Concentration and Recovery of CO 2 from flue gas by pressure swing adsorption. Ind. Eng. Chem. Res. 1993, 32, 2714-2720]. CO 2 can also be separated from the other gases by condensing it out at cryogenic temperatures.
• reaction based processes can be applied to separate CO 2 from gas mixtures.
• This process is based on a heterogeneous gas- solid non-catalytic carbonation reaction where gaseous CO 2 reacts with solid metal oxide (represented by MO) to yield the metal carbonate (MCO 3 ).
• MO solid metal oxide
• MCO 3 metal carbonate
• the calcination reaction can be represented by:
• Figure 1 shows the variation in the free energy of the carbonation reaction as a function of temperature for calcium oxide. From the figure, we can see that the carbonation reaction is thermodynamically favored with a decrease in temperature (Gibbs free energy declines with a decrease in temperature). However, at lower temperatures, the carbonation reaction is kinetically slow. In fact, it takes geological time scales for the formation of CaCO 3 by the reaction between CaO and atmospheric CO 2 (at 280-360 ppm) at ambient temperatures. It should also be noted that the carbonation reaction would be favored as long as the free energy is negative. This creates an upper bound of 890 0 C for carbonation to occur under a CO 2 partial pressure of 1 atm.
• the equilibrium temperature for this reaction is a function of the partial pressure of CO 2 .
• a reaction based CO 2 separation process offers many advantages. Under ideal conditions, MEA captures 6Og CO 2 /kg, silica gel adsorbs 13.2g CO 2 /kg and activated carbon adsorbs 88g CO 2 /kg.
• the sorption capacity of some metal oxides (such as the modified CaO, presented in this study) is about 70Og CO 2 /kg of CaO. This is about an order of magnitude higher than the capacity of adsorbents/solvents used in other CO 2 separation processes and would significantly reduce the size of the reactors and the material handling associated with CO 2 separation. [0009] Numerous metal oxides exhibit the carbonation and calcination reaction.
• the calcination temperature of a few metal carbonates makes them viable candidates for this process.
• a few metal carbonates CaCO 3 -750 0 C, MgCO 3 ⁇ 385 0 C 1 ZnCO 3 ⁇ 340 0 C, PbCO 3 ⁇ 350 0 C, CuCO 3 -225-290 0 C and MnCO 3 ⁇ 440 0 C
• CaO gas-solid carbonation of other metal oxides
• the carbonation of ZnO to ZnCO 3 at 8-13 0 C was low when exposed to CO 2 and H 2 O for over 100 days (Sawada, Y.; Murakami, M.; Nishide, T. Thermal analysis of basic zinc carbonate. Part 1. Carbonation process of zinc oxide powders at 8 and 13 0 C.
• MnCO 3 undergoes a more complex thermal degradation phenomena. MnCO 3 first decomposes to MnO 2 at 300 0 C, which in turn changes to Mn 2 O 3 at 440 0 C. At higher temperatures (-900 0 C), the final thermal decomposition product was identified as Mn 3 O 4 (Shaheen, W. M.; Selim, M. M. Effect of thermal treatment on physicochemical properties of pure and mixed manganese carbonate and basic copper carbonate. Thermochim. Acta. 1998, 322(2), 117-128.). Different oxides of manganese provide the flexibility of exploiting the carbonation/calcination reaction over a wider temperature range.
• Aqueous phase MgO carbonation has been studied for its suitability for mineral-based CO2 sequestration (Fernandez, A.I.; Chimenos, J. M.; Segarra, M.; Fernandez, M.A.; Espiell, F. Kinetic study of carbonation of MgO slurries. Hydrometallurgy. 1999, 53, 155-167).
• the carbonation extent of Mg(OH) 2 was about 10% between 387-400 0 C and 6% formation between 475-500 0 C (Butt, DP.; Lackner, K.S.; Wendt, C.H.; Conzone, S.D.; Kung, H.; Lu, Y- C; Bremser, J. K.
• the gas-solid CaO-CO 2 reaction proceeds through two rate-controlling regimes.
• the first regime involves a rapid, heterogeneous chemical reaction.
• the reaction slows down due to the formation of an impervious layer of CaCO 3 .
• This product layer prevents the exposure of unreacted CaO in the particle core to CO 2 for further carbonation.
• the kinetics of the second regime is governed by the diffusion of ions through the CaCO 3 product layer.
• the activation energy was estimated to be 21 kcal/mol below 688 K and 43 kcal/mol above it for the product layer diffusion, based on the counter migration of CO 3 2" and O 2" ions through the product layer (Bhatia, S. K.; and Perlmutter, D. D. Effect of the product layer on the kinetics of the CO 2 -Lime Reaction. AIChE J. 1983, 29(1), 79-86).
• Carbon dioxide (CO2) accounts for more than half of the enhanced greenhouse effect, which is responsible for global warming. 1
• the atmospheric concentration of CO2 has increased from 280 ppm before the Industrial Revolution to -365 ppm today.
• 2' 2l3 This is mainly due to the unabated emission of CO2 as a result of increasing consumption of fossil fuels such as coal, oil and natural gas.
• Point sources such as electric utility plants that contribute to about one-third of all anthropogenic CO2 emissions 4 , are ideal candidates for implementing C02 reduction practices due to the relatively high concentration and quantity of CO2 emitted compared to smaller, mobile sources.
• Sulfur present in coal oxidizes to S02 during combustion.
• Calcium based sorbents are widely used for the control of S02 emissions.
• the two principal calcium utilization processes are low temperature wet scrubbing and high temperature furnace sorbent injection (FSI).
• FSI furnace sorbent injection
• SO2 capture occurs through ionic reactions in the aqueous phase.
• high temperature (> 900 C) FSI systems calcium oxide precursors (dolomite, Ca(OH)2 and limestone) and their calcines reacts with SO2 to form CaSO4 via the heterogeneous non-catalytic gas solid reaction.
• PCC achieves a higher extent of sulfation (- 70%) compared to naturally occurring limestone (-30%) at greater than 900 ° C within a residence time of 700 milliseconds.
• the flue gas generated by coal combustion typically contains 10-15% C02, 3-4% 02, 5-7% H2O, 500-3000 ppm SO2 and 150-500 ppm NOx in addition to trace quantities of HCI, arsenic, mercury, and selenium. Separation of CO2 by its absorption in monoethanolamine (MEA) is currently the most viable option for commercial scale deployment.
• MEA forms thermally stable salts with SO2 and NOx, which do not decompose under the regeneration conditions employed in the MEA process. It is necessary to lower SO2 concentration to below 10 ppm to minimize the loss of the costly solvent. Economic analysis of this process, based on a parasitic consumption of MEA of 0.5-2 g MEA/kg C02 separated, show that the cost associated with CO2 separation lies in the $33-73/ton CO2 avoided. 21 A similar hurdle is posed by SO2 for a CaO based CCR process.
• CaO undergoes sulfation with SO2 forming CaSO4, which cannot be thermally decomposed back to CaO within the operating temperature range of the proposed CCR process (400-800 C) as it requires greater than 1100 ° C for its decomposition. Exposure of CaO to higher temperatures leads to a loss in surface area and porosity due to excessive sintering, which drastically reduces its reactivity. Eventually, the CaSO4 buildup in each cycle reduces the regenerative capacity of the CaO sorbent over subsequent cycles ultimately rendering it inactive.
• literature on the sulfation of CaO in the temperature range where CaCO3 is thermodynamically stable is scant. Sulfation of calcium species in this temperature range is crucial for the experiments covered in this paper because this study aims to investigate the effect of SO2 on the carbonation of CaO.
• the gas mixture consisted of 30-80% C02j 3000 ppm SO2 and 3-4% oxygen 28l29
• the initial increase in weight of the sorbent was predominantly due to the carbonation reaction, which occurs to a higher extent than sulfation for the given inlet gas concentration levels.
• Further exposure of the sorbent to the reactant gas mixture results in the direct sulfation of the CaCO3 so formed, and leads to a decrease in the overall extent of carbonation and an increase in sulfation.
• Thermogravimetric Analyzer (TGA). The study demonstrates the effect of solid residence time on the overall extent of simultaneous carbonation and sulfation.
• Water gas a mixture of CO, CO 2 , H 2 O and H 2 , is formed by the gasification of coal by sub-stoichiometric air and/or steam. Irrespective of the initial concentration of these four gases, the reversible water gas shift (WGS) reaction gets initiated until the exact ratio of the concentration of these gases reaches a particular equilibrium constant KWGS that is a function of temperature.
• WGS reaction and its equilibrium constant can be written as:
• the resulting CaO sorbent is recycled to capture CO 2 in the next cycle. This cyclical CCR process can be continued so long as the sorbent provides a satisfactory CO 2 capture. [0025] To obtain high purity H 2 , the WGS reaction is generally carried out in
• dolomite does not entirely contain calcium based material.
• dolomite comprises of nearly 50wt.% calcium, which participates in the reaction to some extent, and the remaining portion of the sorbent (mainly magnesium oxide) stays unreacted. Further, they attribute the incomplete conversions of the calcium material to the concept of pore filling and pluggage at the pore-mouths of these sorbent particles by CaCO 3 product layer, preventing the access of CO 2 in the gas to unreacted CaO surface at the pore interiors.
• Calcination in a pure CO 2 stream will result in higher operating temperatures due to the thermodynamic limitations of the calcination reaction in presence of the CO 2 product. Higher temperatures and the presence of CO 2 during calcination would cause the sorbent to sinter. This is in agreement with the results of multiple carbonation-calcination cycle tests for dolomite by Harrison and co-workers (Lopez Ortiz and Harrison, 2001) in pure CO 2 stream (800-950 0 C). They observed a decrease in "calcium" conversion from 83 % in the 1 st cycle to about 69 % in the 10 th cycle itself.
• the present invention includes a calcium oxide, its usage for the separation of CO 2 from multicomponent gas mixtures and the optimum process conditions necessary for enhancing the repeatability of the process.
• a preferred method for separating carbon dioxide from a flow of gas comprising carbon dioxide comprises the steps of: (1) directing the flow of gas to a gas-solid contact reactor, the gas-solid contact reactor contains at least one sorbent comprising at least one metal oxide; (2) reacting the carbon dioxide with the at least one sorbent so as to remove the carbon dioxide from said flow of gas, thereby converting the at least one sorbent into spent sorbent; (3) calcining the spent sorbent so as to liberate the carbon dioxide from the spent sorbent, thereby regenerating the sorbent; and (4) repeating the aforementioned steps.
• the at least one metal oxide is selected from the group consisting of: ZnO, MgO, MnO 2 , NiO, CuO, PbO, and CaO.
• the spent sorbent is a metal carbonate.
• the sorbent has a sorption capacity of at least about
• the sorbent has a sorption capacity of at least about 300 grams of carbon dioxide per kilogram of sorbent. Irrespective of the sorption capacity of the sorbent, it is preferred that the sorbent has substantially the same sorption capacity after calcining as the sorbent had prior to adsorbing the carbon dioxide.
• any calcination method may be employed, it is preferred that the calcining is performed under at least partial vacuum. It is also preferred that the calcining is performed by steam.
• a method for separating carbon dioxide from a flow of gas comprising carbon dioxide of the present invention comprises the steps of: (1 ) directing the flow of gas to a first gas-solid contact reactor, the first gas-solid contact reactor containing at least one sorbent, the sorbent comprising at least one metal oxide; (2) reacting the carbon dioxide in the flow of gas on the sorbent in the first gas-solid contact reactor so as to remove the carbon dioxide from the flow of gas; (3) directing the flow of gas to a second gas-solid contact reactor when the sorbent in the first gas-solid contact reactor is spent thereby forming spent sorbent, the second gas-solid contact reactor containing at least one sorbent, the sorbent comprising at least one metal oxide; (4) reacting the carbon dioxide in the flow of gas on the sorbent in the second gas-solid contact reactor so as to remove the carbon dioxide from the flow of gas; (5) calcining the spent sorbent from the first gas-solid contact reactor
• any calcination method may be employed, it is preferred that the calcining is performed under at least partial vacuum. It is also preferred that the calcining is performed by steam. This applies to both gas-solid contact reactors.
• any metal oxide may be utilized, it is preferred that the at least one metal oxide is selected from the group consisting of: ZnO, MgO, Mn ⁇ 2 , NiO, CuO, PbO, and CaO.
• the sorbent has a sorption capacity of at least about
• the sorbent has a sorption capacity of at least about 300 grams of carbon dioxide per kilogram of sorbent. Irrespective of the sorption capacity of the sorbent, it is preferred that the sorbent has substantially the same sorption capacity after calcining as the sorbent had prior to adsorbing the carbon dioxide.
• a method for regenerating a spent sorbent for carbon dioxide of the present invention comprises the steps of: (1) providing a spent sorbent, the spent sorbent comprising metal carbonate; and (2) calcining the spent sorbent so as to liberate carbon dioxide gas and so as to regenerate the spent sorbent thereby forming a sorbent comprising a metal oxide.
• the spent sorbent is calcium carbonate. It is further preferred that the metal oxide is calcium oxide.
• the sorbent has substantially the same sorption capacity after calcining as the sorbent had prior to adsorbing the carbon dioxide.
• any calcination method may be employed, it is preferred that the calcining is performed under at least partial vacuum. It is also preferred that the calcining is performed by steam. This applies to both gas-solid contact reactors.
• the present invention includes facilities practicing the aforementioned method.
• a method for producing a sorbent of the present invention comprises the steps of: (1 ) obtaining a structurally altered high surface area calcium carbonate having a surface area of at least 25.0 m 2 /g, a pore volume of at least 0.05 cm 3 /g, and a mesoporous pore size distribution; and (2) calcining the structurally altered high surface area calcium carbonate so as to produce a sorbent having a surface area of less than 22 m 2 /g, a pore volume of at least 0.005 cm 3 /g, and a mesoporous pore size distribution.
• the present invention includes sorbents made according to the aforementioned method.
• a sorbent according to the present invention comprising calcium oxide having a surface area of at least 12.0 m 2 /g and a pore volume of at least 0.015 cm 3 /g, the calcium carbonate sorbent having sorption capacity of at least about 70 grams of carbon dioxide per kilogram of sorbent.
• Figure 1 depicts the Gibbs Free Energy diagram for the carbonation reaction, CaCO 3 ⁇ CaO + CO2, as a function of temperature.
• Figure 2 illustrates the performance of calcium oxide for the carbonation reaction.
• Figure 3 compares the XRD diffractograms of CaO derived from various precursors.
• Figure 4 is a schematic diagram of a carbonator reactor for the synthesis of precipitated calcium carbonate.
• Figure 5 shows the change in the pH of the slurry as a function of
• PCC Precipitated Calcium Carbonate
• Figure 7 compares the pore size distribution of four CaO precursors.
• Figure 8 compares the conversion of four CaO sorbents under pure CO 2 at 650 0 C.
• Figure 9 illustrates the effect of temperature on the carbonation of
• Figure 10 illustrates the carbonation-calcination cycles on Aldrich
• FIG. 11 shows extended carbonation-calcination cycles on precipitated calcium carbonate (PCC) powder at 700 0 C.
• Figure 12 compares the effect of initial surface area of PCC-CaO to its reactivity towards the carbonation reaction at 700 0 C.
• Figure 13 depicts the effect of vacuum calcination on the reactivity of
• Figure 14 provides a flow sheet depicting the integration of the current process in the overall coal-gasifier electric production facility.
• Figure 15 illustrates thermodynamic data for predicting the temperature zones for hydration and carbonation of CaO.
• Figure 16 illustrates thermodynamic data for predicting the equilibrium
• Figure 17 shows a modified reactor set-up with steam generating unit for investigating WGS and carbonation reactions.
• Figure 18 illustrates the set-up for combined vacuum/sweep gas calcination experiments allowing the use of larger sorbent samples.
• Figure 19 is a pore size distribution of the HTS and LTS obtained from
• Figure 20 shows the pore size distribution of various calcium oxide precursors.
• Figure 22 shows the extent of reaction equilibrium as a function of temperature for the WGS reaction.
• Figure 26 depicts a typical steam generation scenario and use.
• Figure 27 depicts one implementation of one embodiment of the present invention.
• Figure 28 depicts one implementation of one embodiment of the present invention.
• Figure 29 depicts one implementation of one embodiment of the present invention.
• Figure 30 depicts one implementation of one embodiment of the present invention.
• Figure 31 depicts one implementation of one embodiment of the present invention.
• Figure 32 depicts one implementation of one embodiment of the present invention.
• Figure 33 depicts one implementation of one embodiment of the present invention.
• Figure 34 depicts one implementation of one embodiment of the present invention.
• Figure 35 depicts one implementation of one embodiment of the present invention.
• Figure 36 depicts one implementation of one embodiment of the present invention.
• Figure 37 depicts one implementation of one embodiment of the present invention.
• Figure 38 illustrates thermodynamic data for predicting the temperature zones for sulfation of CaO as well as the direct sulfation of CaCO3. (Sulfation was considered at 1 atm total pressure, 4% O2 and 10% CO2)
• Figure 39 illustrates CO2 capture capacity of various high temperature sorbents over multiple carbonation-regeneration cycles.
• Figure 40 provides a typical curve for combined carbonation and sulfation of PCC-CaO for 3 cycles at 700 0 C for a residence time of 5 minutes (3000 ppm So2, 10% CO2, 4% O2)
• Figure 41 shows the effect of residence time on the extent of carbonation (initial amount of CaO) of PCC-CaO for multiple cycles at 700 C (3000 ppm SO2, 10% CO2, 4% O2)
• Figure 42 shows the effect of residence time on the extent of sulfation
• Figure 43 shows the effect of residence time on the ratio of carbonation to sulfation of PCC-CaO for multiple cycles at 700 C (3000 ppm SO2, 10% CO2, 4%
• Figure 44 illustrates the extent of carbonation of PCC-CaO for multiple cycles at 700 C (10% CO2, 4% O2)
• Figure 45 illustrates the extent of sulfation of PCC-CaO for multiple cycles at 700 C (10%CO2, 4% O2)
• Figure 46 shows the effect of residence time on the ratio of carbonation to sulfation of PCC-CaO for multiple cycles at 700 C for varying SO2 concentrations (3000 - 100 ppm So2, 10% Co2, 4% O2)
• Figure 47 shows the effect of reaction temperature on the ratio of carbonation to sulfation for increasing residence time (10% Co2, 3000 ppm SO2)
• Figure 48 illustrates the effect of reaction temperature on the extent of carbonation of PCC-CaO for increasing residence time (10% CO2, 3000 ppm SO2)
• Figure 49 illustrates the effect of reaction temperature on the extent of sulfation of PCC-CaO for increasing residence time (10% CO2, 3000 ppm SO2)
• Figure 50 provides a flow sheet for the integration of the CCR process in a coal fired utility.
• Figure 51 illustrates the equilibrium partial pressure of CO2 as obtained by thermodynamics (0 - 1 atm)
• Figure 52a illustrates a direct fired calcination configuration in accordance with one embodiment.
• Figure 52b illustrates an indirect fired calciner configuration in accordance with one embodiment.
• Figure 53 illustrates a schematic diagram of one embodiment of a rotary calciner reactor set-up.
• Figure 57 shows the effect of sweep gas flow (FSG) (Sample size: 10g, T: 880 C; P VA c: 28" Hg; F
• Figure 58 shows the effect of diluent gas type (He, N2, Ar) (Sample
• Metal oxides such as ZnO, MgO 1 CuO, MnO 2 , NiO, PbO and CaO that undergo the CCR scheme in the 800-200 0 C temperature range were analyzed for their reactivity in a TGA.
• a powdered sample of these oxides was placed in a quartz pan and pure CO 2 was passed over the sample metal oxide. The temperature was then slowly raised and the weight of the sample was continuously monitored. An increase in the weight of the sample is an indication of the formation of metal carbonate.
• Figure 2 provides experimental data for the carbonation of lime (Ca(OH) 2 ) under flowing pure CO 2 gas. With an increase in temperature, the weight of the sample increases till the temperature reaches about 890 0 C.
• Calcination which is thermodynamically favored above 890 0 C at 1 atm CO 2 partial pressure, causes a rapid decrease in weight until the sorbent converts completely to CaO.
• the weight starts to increase again and the process is repeated once more.
• the data also shows recyclability of the sorbent.
• CaO was identified as a viable candidate for the carbonation- calcination reactions.
• precursors can be calcined to obtain the CaO sorbents necessary for the carbonation reaction.
• Common and economical precursors include calcium carbonate, calcium hydroxide and dolomite.
• the other important source of CaO is via the calcination of synthesized high surface area precipitated calcium carbonate.
• XRD patterns were obtained on all the CaO sorbents. Figure 3 depicts these diffractograms (a. Calcined Aldrich-CaO; b. Dolomite-CaO; c.
• Precipitated Calcium Carbonate (PCC) synthesis Structurally altered high surface area CaO precursors were synthesized based on the procedure outlined elsewhere (Fan, L-S.; Ghosh-Dastidar, A.; Mahuli, S.; Calcium Carbonate Sorbent and Methods of Making the Same. US Patent # 5,779,464 and Agnihotri, R.; Chauk, S.; Mahuli, S.; Fan, L.-S. Influence of Surface Modifiers on Structure of Precipitated Calcium Carbonate. Ind. Eng. Chem. Res. 1999, 38, 2283-2291 ). A schematic diagram of the slurry bubble column used for this purpose is shown in Figure 4.
• the carbonator 40 consists of a 2" OD Pyrex tube 40a.
• a porous frit 4Od at the bottom, disposed over glass beads 4Of, provides good distribution of CO 2 4Og through the slurry 40c.
• a K-type thermocouple 4Oh inserted in the slurry continuously records the slurry temperature.
• a pH probe 40b monitors the pH of the slurry as the reaction medium changes from a basic to an acidic solution as the reaction proceeds. First, 500 ml of distilled water is poured into the carbonator, followed by the addition of 0.0575g of N40V ® . 12.8g of Ca(OH) 2 is added to the solution to provide a loading of 2.56% by weight.
• CaCO 3 has a much lower solubility in water (-0.0012 g/100g water) compared to Ca(OH) 2 and thus precipitates out.
• Ca 2+ ions get depleted, but are continuously replenished by the suspended Ca(OH) 2 .
• the pH remains 12.
• Ca(OH) 2 ultimately gets depleted and the concentration of Ca 2+ ions cannot be maintained at its solubility limit.
• continued dissolution of CO 2 gas leads to the accumulation of H + ions causing the solution to become acidic.
• the pH settles at about 6.0, corresponding to equilibrium solubility of CO 2 in water at ambient temperature. This also signals the end of the carbonation of all Ca(OH) 2 .
• the slurry is then removed from the precipitator, vacuum filtered and stored in a vacuum oven at 90-110 0 C for 20 hours to completely remove the moisture.
• Higher Ca(OH) 2 loading requires more reaction time as evident from Figure 5.
• Precipitated calcium carbonate can be obtained by the reaction between carbonate and calcium ions in solution. It is known that the CaCO 3 nuclei that precipitate out have positive surface charge on them that prevent agglomeration (Agnihotri, R.; Chauk, S.; Mahuli, S.; Fan, L-S. Influence of Surface Modifiers on Structure of Precipitated Calcium Carbonate. Ind. Eng. Chem. Res. 1999, 38, 2283- 2291 ). The resulting structure is also microporous in nature. However, the structural properties of the synthesized PCC can be altered by the use of negatively charged dispersants that neutralize the surface charges.
• CaO sorbents were synthesized by calcining various CaO precursors such as Linwood calcium carbonate (LC), dolomite (DL), Linwood calcium hydroxide (LH), and precipitated calcium carbonate (PCC).
• CaO precursors such as Linwood calcium carbonate (LC), dolomite (DL), Linwood calcium hydroxide (LH), and precipitated calcium carbonate (PCC).
• LC-CaO Linwood calcium carbonate
• DL dolomite
• LH Linwood calcium hydroxide
• PCC precipitated calcium carbonate
• the oxides derived from these sources are termed as LC-CaO, FCD-CaO (for fully calcined dolomite-CaO), LH-CaO, and PCC-CaO, respectively.
• the procedure involved heating the precursor in flowing nitrogen beyond the calcination temperature (800- 950 0 C) for an hour followed by its storage in a desiccator.
• Structural properties such as surface area (SA) and pore volume (PV) of these chemicals are listed in Table 2 and their pore size distributions are shown in Figure 7.
• SA surface area
• PV pore volume
• Table 2 Morphological properties (surface area and pore volume) of various CaO sorbents and their precursors.
• Cyclic calcination and carbonation [00119] One of the possible hurdles in the utilization of metal oxides for the carbonation and calcination reaction scheme is its vulnerability to sintering due to the thermal cycling imposed by the cyclical nature of these reactions. Cyclical studies were carried out to quantify any loss in reactivity of these sorbents upon multiple cycles. The temperature chosen for cyclical studies was 700 0 C. This temperature is sufficient to achieve carbonation in the presence of pure CO2, and also to calcine the CaCO3 so formed after the gas is switched from CO 2 to N 2 . A variety of precursors were first calcined in nitrogen at 700 0 C. The gas was then switched to pure CO 2 and the weight gain continuously tracked.
• Vacuum calcination leads to the formation of a metastable-nanocrystailine calcia structure while calcination in helium atmosphere lead to a stable microcrystalline calcia structure (Dash, S., Kamruddin, M., Ajikumar, P. K., Tyagi, A.K., and Raj, B., "Nanocrystalline and metastable phase formation in vacuum thermal decomposition of calcium carbonate.” Thermochimica acta, 2000, 363, 129-135).
• Beruto et al., [1980] estimated the surface area and pore volume of limestone based CaO to be about 78-89 m 2 /g and 0.269 ml/g respectively.
• Table 3 Structural properties of Calcium based sorbents undergoing vacuum calcination at 750 0 C and carbonation at 700 0 C.
• a variety of chemical processes known to generate syngas include: Steam Gasification: C + H 2 O ⁇ CO + H 2 (X) Steam Methane Reforming: CH 4 + H 2 O ⁇ CO +3H 2 (X)
• the flow sheet shown in Figure 14 integrates the Calcium-based reactive separation process under development in this project with a coal gasifier based electric power/chemical synthesis process plant 140.
• the main coal gasifier 140a consists of a high pressure and high temperature unit that allows contact between coal 140b, steam 14Oe and air/pure oxygen 14Oy in a variety of schemes.
• Boiler feed water 14Od is preheated by passing it through gasifier 140a prior to steam tubine 140c. Waste from the gasifier is collected as slag 14Oz.
• Typical fuel gas compositions from various known coal gasifiers are shown in Table 4.
• the reacted CaCO 3 particles are captured using a high temperature solids separator 14Oh (e.g., a candle filter or a high temperature ESP) and separated fuel gas stream.
• the spent solids are now sent to a rotary calciner 140k to thermally decompose the CaCO 3 14Oj back to CaO 14Of and pure CO 2 140m.
• the high purity CO 2 gas can now be economically compressed 1401, cooled, liquefied and transported for its safe sequestration 140m.
• the rotary calciner allows the calcium particles to remain segregated, which is crucial in maintaining a sorbent structure characterized by a higher porosity.
• the calcination of the sorbent can also be effected under sub-atmospheric conditions that allow the removal of CO 2 as soon as it is formed from the vicinity of the calcining sorbent, thereby aiding further calcination.
• This vacuum can be created by means of ejector systems that are widely used in maintaining vacuum in large vacuum distillation units (VDU) in the petroleum refining industry. Lock and hopper combinations and appropriate seals ensure that the sorbent can be effectively separated from the CO2 stream and re-entrained in the fuel gas duct.
• the hydrogen enriched fuel gas 140i can now be used to generate electric power in a fuel cell 14On or used to make fuels and chemicals 14Oq without any low temperature clean up.
• the fuel cell may receive a supply of air 14Op and discharge steam 140o.
• the hydrogen enriched fuel gas may be sent to gas turbine 14Or used to drive generator 14Ot to produce electricity and air compressor 140s to produce a stream of compressed air.
• the stream of compressed air may be sent to air separator 140x to produce the air/oxygen of 14Oy.
• the discharge from gas turbine 14Ot may be sent through heat exchanger 14Ou prior to being discharged at stack 14Ov.
• the absorbed heat may be collected by steam turbine 14Ow to produce additional electricity.
• CaO calcium oxide
• Table 4 Typical fuel gas compositions obtained from different gasifiers. (Stultz and Kitto, 1992)
• FIG. 15 (b) shows the typical equilibrium CO 2 partial pressures (PCO 2 ) as a function of temperature. From the data in Table 4, it can be inferred that the typical PCO2 in the gasifiers ranges from 0.4-4.3 atm for entrained flow (slurry) and entrained flow (dry) gasifier systems respectively.
• the high and low temperature water gas shift (WGS) reaction catalysts were procured from S ⁇ d-Chemie Inc., Louisville, KY.
• the high temperature shift (HTS) catalysts comprises of iron (III) oxide supported on chromium oxide.
• Precipitated calcium carbonate (PCC) was synthesized by bubbling CO2 through a slurry of hydrated lime.
• the neutralization of the positive surface charges on the CaCO 3 nuclei by negatively charged N40V ® molecules forms CaCO 3 particles characterized by a higher surface area/pore volume and a predominantly mesoporous structure. Details of this synthesis procedure have been reported elsewhere (Agnihotri et al., 1999). Hydrated lime from a naturally occurring limestone (Linwood Hydrate, LH) and a naturally occurring limestone (Linwood Carbonate, LC) was obtained from Linwood Mining and Minerals Co.
• the sorbents and catalyst were analyzed to determine their morphologies using a BET analyzer.
• the BET surface areas, pore volumes, and pore size distributions of the catalysts and sorbents were measured at -196 0 C using nitrogen as the adsorbent in a Nova 2200 Quantachrome BET analyzer. Special care was taken to ensure that all samples were vacuum degassed at 250 0 C for 5 hours prior to BET analysis.
• a reactor setup was designed, underwent several iterations and was assembled to carry out water gas shift reactions in the presence of CaO and catalyst.
• the reactor design assembly used to carry out these experiments is shown in Figure 17.
• This setup enables us to carry out both the water gas shift reaction in the presence of CaO as well as the regeneration of the sorbent in flowing gas such as nitrogen and/or steam.
• the setup 170 consists of a tube furnace 17Op, a steel tube reactor 170a, a steam generating unit 170c, a set of gas analyzers for the online monitoring of CO and CO 2 concentrations 17On, a condenser 170m to remove water from the exit gas stream and a high pressure water syringe pump 170b.
• All the reactant gases (H 2 , CO, CO 2 , and N 2 ) are metered using modified variable area flowmeters 17Oe - h respectively.
• the syringe pump is used to supply very accurate flow-rates of water into the heated zone of the steam- generating unit in the 0.01-0.5 ml/min range.
• the steam generator is also packed with quartz wool 17Od in order to distribute the water drops as they enter into the heating zone.
• the packing is utilized in order to provide greater surface for water evaporation and to dampen out fluctuations in steam formation.
• the main problem with a fluctuating steam supply is that the gas analyzers used to measure the exit CO and CO 2 concentrations are sensitive to gas flow rates. Even though the steam is being condensed out before the gas is sent into the analyzers, surges in the steam supply still affect the overall gas flow rate, causing the CO and CO 2 readings to fluctuate.
• the packing ultimately ensures a more continuous and constant overall gas flow rate into the main reactor and into the analyzers.
• Thermocouple 170k is used to monitor the temperature inside reactor 170a. Any extra gas inlets of reactor 170a are blocked 1701.
• a steel tube reactor is used to hold the Ca-based sorbent and catalyst, and is kept heated using a tube furnace.
• the sorbent loading unit of the reactor is detachable which enables easy removal and loading of the sorbent and therefore minimizes the sorbent loading time between runs. Also, the sorbent can be changed without having to cool down the entire reactor.
• the gas mixture 17Oj entering the reactor is preheated to the reaction temperature before contacting the sorbent/ catalyst particles.
• the gases exiting the reactor first flow through a condenser in order to separate out the moisture and then to a set of gas analyzers.
• Sub Atmospheric Calcination Reactor Setup [00135] Once the Calcium based sorbent has reacted with the CO 2 being produced by the WGSR, the sorbent has to be regenerated for further use in subsequent cycles. During the regeneration of the sorbent, carbon dioxide is released from the sorbent. In order to minimize the necessity for further treatment of this released CO 2 before sending it to sequestration sites, it is necessary to regenerate the sorbent such that a pure stream of CO 2 is released. Vacuum calcination provides one method for ensuring that concentrated streams of CO 2 are release in the regeneration phase. The detailed setup is shown in Figure 18. This setup 180 was assembled to handle the regeneration of large quantities of sorbent ( ⁇ 10-20g per batch).
• the setup includes an alumina tube reactor 180b, which would hold the sorbent samples in a split tube furnace 180c that provides the heat necessary to calcine the sorbent 18Od, two Non Dispersive Infra Red (NDIR) analyzers 180k -I to monitor the CO 2 concentration (ranges 0-2500ppm and 0-20%) and two vacuum pumps 18Of and 18Oi.
• 1Og of sorbent yields about 2.4L of CO 2 at atmospheric pressure and temperature over the entire decomposition process. This gas needs to be diluted with air in order to ensure that the CO 2 concentration lies in the detection range of the CO 2 analyzers.
• Vacuum Pump 18Of is a dry vacuum pump procured from BOC Edwards capable of achieving vacuum levels as low as 50mtorr and gas flowrates of 6m 3 /hr.
• the CO2 analyzers have their own inbuilt pumps and are capable of drawing up to 2LPM from the header for online CO2 analysis.
• the second pump 18Oi is a smaller dry pump and is put in place to ensure that there is no pressure buildup in the %" lines connecting the vacuum pump to the analyzers. Pump 18Oi discharges to vent 18Oj.
• the temperature of the furnace is controlled with a thermocouple inserted into the central zone of the furnace.
• the temperature of the reactor was also monitored using a second thermocouple inserted into the center of the alumina tube.
• the setup is also capable of combining vacuum calcination with flow of sweep gas 180a. As it may not be feasible to supply very low vacuum levels for the calcination of the sorbent in industrial settings, it may be necessary to study the calcination process in combination with the addition of various sweep gases such as N2/ steam. Pressure gauges 18Oe, h and volumetric flow meters are included to monitor the vacuum pressure in the reactor, the pressure in the 14" lines and the flows of the sweep gases into the calciner and the flow of the air 18Og used in the dilution of the exhaust CO 2 before sending it to the analyzers. The analyzers are also connected to a data acquisition system 180m that can record analyzer readings every second. [00136] INTRODUCTION:
• Calcination Configurations [00143] The CCR scheme can be carried out in two modes of operation viz. temperature and pressure swing and any combination thereof. Calcination can be induced by either increasing the temperature of the carbonated product or by reducing the PCO2 in the calciner such that the process conditions fall below the thermodynamic equilibrium curve.
• Figure 2 shows various configurations of the calciner operation which detail the mode of heat input to the calciner.
• make-up calcium would be necessary to replace the sorbent consumed by SO2 in the calciner (Iyer et al., 2004) [00144]
• the heat of calcination can be supplied indirectly as shown in Figure 2(b). The addition of heat will induce calcination, which leads to CO2 buildup in the calciner.
• the flow of CO2 out of the reactor is possible only if the PCO2 becomes greater than I bar. Thermodynamically, P ⁇ 2 becomes greater than 1 bar only above 89O 0 C. It is well known that high temperatures cause sorbent sintering, which reduces its porosity, thereby leading to a drastic reduction in reactivity.
• Pressure swing mode of operation enables lowering of the calcination temperature to circumvent the sintering problem.
• a lower PCO2 required by pressure swing operation, is achieved by either dilution of evolved CO2 or by an overall reduction in pressure of the calciner. For example, a reduction in PCO2 below 0.0272 bar would lower the calcination temperature to below 700 ° C. Lowering PCO2 can be accomplished by flowing diluent gas through the calciner. However, only steam is an acceptable diluent gas since any other gas such as air, nitrogen, etc. will mix with the evolved CO2 defeating the overall objective of isolating a pure CO2 stream.
• the reduction in overall calciner pressure, while maintaining 100% pure C02, can be achieved using a vacuum pump which removes CO2 as it evolves from calcination.
• Rao et al. (1989) used thermo-gravimetric reaction data aloni with a grain model to arrive at the reaction rate constant expression of 1.18x10 3 exp(- 1.85x10 /RT).
• Samtani et al. (2002) investigated the kinetics of calcite decomposition under an atmosphere of N2 and determined an activation energy of 192.5kJ/mol and an 1 nA of 20.73 (where A is the pre-exponential value of the Arrhenius rate law). Calcite was determined to undergo a zero-order decomposition mechanism, and further investigation into the effect of flow rate, heating rate and sample size did not yield any deviation in the kinetic parameters and mechanism of the process.
• the reactor setup as shown in Figure 3 was assembled to handle a wide range of calcination conditions such as the calcination of 0.5-2Og sorbent samples under a variety of vacuum, vacuum + sweep gas conditions and calcination temperatures of up to 950 C. Since early experiments had shown that sorbent heaping affects both the calcination kinetics as well as sorbent morphology, the reactor was designed to allow the calcination to take place under rotary motion which disperses the sorbent thereby minimizing sorbent heaping.
• a quartz tube reactor was used to carry out calcination kinetic studies.
• the reactor tube was designed to have a conical shaped tapered central zone in order to keep the particles from dispersing axially away from the heated zone. Baffles were incorporated to ensure particle dispersion.
• the reactor was placed on two sets of rotary rollers, and was attached to a motor via a rubber belt mechanism.
• An electric split tube furnace was used to provide the necessary heat of calcination.
• Rotary seals enabled the rotation to take place while maintaining the desired level of vacuum level in the reactor tube. A vacuum level of -28 in Hg was achieved in this configuration.
• the gas exiting the calciner was further diluted with air in order to ensure that the CO2 concentration fell in the detection range of the CO2 analyzers.
• a dry vacuum pump procured from BOC Edwards capable of achieving vacuum levels as low as 50 millitorr and gas flowrates of 6m7hr was used to supply the necessary reactor vacuum level.
• the second pump in the setup is a smaller dry pump and was put in place to ensure that there was no pressure buildup in the '/+" lines connecting the vacuum pump to the analyzers.
• Two Non Dispersive Infra Red (NDIR) analyzers were used to monitor the CO2 concentration (ranges 0- 2500ppm and 020%). These C02 analyzers have their own inbuilt pumps and are capable of drawing upto 2LPM from the header for online CO2 analysis.
• the temperature of the furnace was controlled with a thermocouple inserted into the central zone of the furnace.
• the temperature of the reactor was monitored using a second thermocouple inserted into the center of the quartz / alumina tube. Pressure gauges and volumetric flow meters are included to monitor the vacuum pressure in the reactor, the pressure in the 7," lines and the flows of the sweep gases into the calciner and the flow of the air used in the dilution of the exhaust CO2 before sending it to the analyzers.
• the analyzers are also connected to a data acquisition system that can record analyzer readings every second.
• the calcination studies were performed to investigate the role of calcination temperature, level of vacuum, thermal properties of sweep gas and effect of gas flow on the kinetics of calcination and the morphology of the resultant CaO sorbent.
• Sweep gas is necessary to aid calcination.
• Prior experiments carried out under high vacuum conditions in the absence of sweep gas revealed a longer time for calcination.
• Experiments were performed to determine the possibility of performing sub-atmospheric calcination in combination with the flow of gas.
• PCC and LC samples of 0.5g were calcined with a sweep N2 gas flow of 50m1/min under 25"Hg vacuum.
• Figures 4 and 5 show the conversion plots for LC and PCC at temperature ranges of 700 ° C to 750 ° C. The resulting plots show that the calcination time for PCC is much lower than that for the naturally occurring LC.
• the low temperature shift (LTS) catalyst has a BET surface area of 85 m /g and a total pore volume of about 0.3 cc/g. The majority of the pores were found to occur around 180 A as evident from the maximum in its pore size distribution plot shown in Figure 19.
• the low temperature shift (LTS) catalyst has a BET surface
• SA surface area
• PV pore volume
• Figure 20 shows the pore size distribution (PSD) of these precursor fines. It can be seen that LC fines do not have high SA/PV. However, upon calcination and subsequent hydration, the SA/PV of the calcium hydroxide (LH) fines increase as can be observed for the LH sample. The porosity is maximized in the microporous range (30-50 A range). In contrast, the SA/PV of the morphologically altered PCC are much higher. Further, most of the porosity lies in the 100-300 A range.
• Table 5 Morphological properties of the natural and synthesized CaO precursors and the HTS catalyst obtained from BET analyses.
• thermodynamic equilibrium constant (K) for any temperature for this reaction was computed using the software "HSC Chemistry v 5.0" (Outokumpu Research Oy, Finland).
• the observed ratio was computed from the experimental data by obtaining the ratio of the partial pressures of the products and the reactants as per the eqn (9) below:
• Figure 22 illustrates the effect of temperature on the ratio of partial pressures (Kobs) obtained from the experimental data. This is compared with the thermodynamic equilibrium values (K eq ). From the figure it is evident that we are operating in the region that is below the thermodynamic equilibrium. At 500 0 C the K Obs is 0.028 while the corresponding K eq is 4.77. K eq monotonically decreases with increasing temperature. In contrast, K obs increases with temperature for our operating conditions. Thus, at 600 0 C the K obs increases to 1.4 while the K eq moves down to 2.5. This trend continues and it is clearly evident from the figure that the system moves closer to equilibrium as we progressively increase the temperature from 500 to 700 0 C.
• the combined carbonation and WGS reaction was tested in the reactor assembly used for the catalyst testing.
• the experimental conditions were exactly identical to that used for testing the catalyst.
• the runs were performed in a reaction mixture comprising of 3% CO and 9% H 2 O, the balance being 5.0 grade N 2 .
• the total gas flow-rate was maintained at about 1.5 slpm and the steam/CO ratio was set at ⁇ 3.
• Different calcium oxide precursors were tested.
• Naturally occurring limestone, Linwood Carbonate (LC) and the corresponding hydrated lime, Linwood Hydroxide (LH) were obtained from Linwood Mining and Minerals Co.
• the structurally modified calcium carbonate (PCC) was prepared in-house and the details are outlined below.
• the sorbent gradually achieves its maximum loading capacity with time and finally at around 2500 seconds (42 min) the sorbent reaches its breakthrough loading. Beyond this the CO conversion of 81% corresponds to that obtained with only the catalyst at 600 0 C. This can be validated from Figure 21.
• Figure 25 compares the CO conversion breakthrough curves for the PCC and LH sorbent-catalyst systems. The curves are for the 1 st reaction cycle. The CO conversion at any given time for PCC-CaO is always higher than that of LH-CaO. The PCC system gives almost 100% conversion for first 240 seconds (4 min) while the LH sorbent system sustains this conversion only in the initial few seconds. Subsequently, the PCC system gives about 90% CO conversion at 1000 seconds (16.5 min) followed by 85% in 1600 seconds (27 min). In contrast, the LH system gradually gives about 90% CO conversion at 900 seconds (15 min) and followed by
• Figure 26 illustrates the generation 1 MWe of steam.
• Figure 27 illustrates one embodiment of the present invention providing
• Figure 28 illustrates a second embodiment of the present invention providing 1 MWe total capacity.
• Figure 29 illustrates another embodiment of the present invention providing 1.33 MWe total capacity.
• Figure 30 illustrates yet another embodiment of the present invention providing 1.33 MWe total capacity
• Figure 31 illustrates an alternative embodiment of the present invention providing 1.54 MWe total capacity.
• Figure 32 illustrates yet another alternative embodiment of the present invention providing 1.07 MWe total capacity.
• Figure 33 illustrates an alternative embodiment of the present invention providing 1 MWe total capacity.
• Figure 34 illustrates an alternative embodiment of the present invention providing 1 MWe total capacity.
• Figure 35 illustrates yet another embodiment of the present invention providing 1.54 MWe total capacity.
• Figure 36 illustrates an alternative embodiment of the present invention providing 1 MWe total capacity at 80% CO 2 capture.
• Figure 37 illustrates another embodiment of the present invention providing 300 MWe total capacity at 90 CO 2 capture.
• PCC yields a predominantly mesoporous structure in the 5-20 nm range with a surface area (SA) of 49.2 m /g and a pore volume (PV) of 0.17 cc/g obtained by
• N2 and C02, obtained from Praxair, lnc were 99.999% and 99.9% pure, respectively.
• Mixtures of 02 and SO2 in N2 were also supplied by Praxair, Inc.
• the BET SA, PV, and pore size distribution (PSD) were measured at -196 ° C using nitrogen by a NOVA 2200 analyzer (Quantachrome Company).
• a small sample of the sorbent (about 10-12 mg) was placed in a quartz sample holder and brought to 700 ° C under nitrogen flow. The temperature of the TGA was then maintained at 700 C throughout the experiment to effect the calcination of PCC. After the calcination step, the valve was switched to allow the flow of reactant gas mixture over the calcined sorbent (PCC-CaO).
• An automated multi-position valve (VICI Corporation, Model # EMTMA-CE) actuated by a programmable electronic timer (VICI corporation, Model # DVSP4) was used to switch between pure nitrogen stream and the reaction gas mixture at programmed time intervals in order to effect the cyclical calcination and carbonation and sulfation of the sorbent.
• the alternating flows are adjusted to minimize any variations in weight of the pan/sorbent system due to buoyancy changes.
• the reactant gas mixture enters the TGA from the side port and gets diluted by the TGA-N2 stream coming from the balance dome.
• the flow of the reactant gas mixture causes an immediate increase in the weight of the sorbent due to the formation of higher molecular weight products such as CaCO3 and CaSO4.
• the automated valve toggles the flow back to the "calcination nitrogen".
• the sorbent weight starts dropping immediately due to the calcination of the CaCO3 product that is formed in the previous reaction step.
• the raw data is then analyzed to obtain the conversion plots. [00191] RESULTS AND DISCUSSION
• Thermodynamic analysis was carried out to understand the effect of reaction temperature and gas concentration on the spontaneity of the various reactions.
• Thermodynamic analysis [00193] Primarily four gas-solid reactions can occur when calcium oxide is exposed to flue gas from coal combustion. CaO can undergo hydration, carbonation and sulfation reactions with H2O, CO2 and SO2, respectively. In addition, SO2 can react with the CaCO3 formed due to the carbonation reaction, thereby causing direct sulfation of the carbonate. These can be stoichiometrically represented as:
• the equilibrium temperature for CaO-CaCO3 system is 760 ° C. Therefore, the temperature of the carbonator needs to be kept below 760 ° C in order to effect the carbonation of CaO in a 10% CO2 stream.
• a temperature of 700 ° C offers a reasonable rate of carbonation and calcination reactions and enabled us to carry out multiple CCR cycles under isothermal conditions.
• Thermodynamic data for the equilibrium temperature versus SO2 concentration for the sulfation of CaO and direct sulfation of CaCO3 are shown in Figure 1(b).
• the S02 concentration for the sulfation of CaO system is depicted in terms of ppmv for a total system pressure of 1 bar at 4% 02.
• the equilibrium partial pressure of SO2 is 1.84 and 5.72 ppt (parts per trillion) for the sulfation of CaO and the direct sulfation of CaCO3. Since SO2 concentration in the inlet flue gas is in the 500-3000 ppm range, sulfation of CaO and the CaCO3 will definitely occur until virtually all SO2 is consumed. Table 1 summarizes the temperature below which the three reactions are thermodynamically favored at the typical flue gas concentrations at 1 bar total pressure.
• the sorption capacity of the sorbent is quantified as wt% CO2 captured by the calcined sorbent.
• the wt% capacity of the LC based sorbent towards CO2 capture reduces from 58% in the first cycle to 20% at the end of the 50 cycle.
• the microporous structure of LC being susceptible to pore pluggage and pore mouth closure, does not attain high conversion.
• Figure 2 depicts graphically the wt% CO2 capture attained by LC, PCC and a host of other high temperature sorbents reported in the literature for multiple CCR cycles. 30 While a variety of sorbents have been screened for this CCR process, a candidate sorbent that shows consistently high reactivity and sorption capacity over multiple cycles remains to be identified.
• the experimental conditions used in the studies referred to in Figure 5 are detailed in Table 2. This table highlights important process conditions such as carbonation and calcination temperatures, solid residence times, number of cycles, sorption capacities (wt%), and the CO2 concentration in the gas mixture during the reaction and regeneration steps.
• PCC- CaO attains 68 wt% increase in 30 minutes and 71.5 wt% after 120 min at the end of the first cycle.
• earlier studies have shown a sorption capacity of about 71 wt% (90% conversion) in a pure C02 stream after 60 min on stream at 650 ° C.
• factors like CO2 concentration, temperature and cycle time play a significant role in determining the sorption capacity for the same sorbent.
• Barker on 10 micron CaO powder demonstrate a drop in the sorption capacity from 59 wt% in the first carbonation cycle to 8 wt% at the end of 25* cycle.
• a lithium zirconate (U2ZrO3) based sorbent provided 20 wt% capacity over two cycles.
• 32 In another study, researchers at Toshiba Corp. observed that the reactivity of lithium orthosilicate (Li4SiO4) was better than that of lithium zirconate.
• Extended cyclical studies performed on lithium orthosilicate samples attained 26.5 wt% sorption capacity over 50 cycles without any change in the reactivity.
• 34 Harrison and co-workers developed an enhanced hydrogen production process from the water gas shift reaction by removing CO2 from the gas mixture through the carbonation of CaO.
• FIG. 3 shows a sample plot of raw data typical for all the experiments conducted in this section. The x-axis represents the residence time and the y-axis shows the actual weight of the sorbent at any given instant. In all the experiments, calcination of PCC was carried out typically for 20-30 minutes. The residence time for the reaction step was maintained for 5 minutes in each of the three cycles for this specific run.
• Carbonation and sulfation occur as heterogeneous non-catalytic gas solid reactions. Higher concentration of CO2 (10% or 100,000 ppm) compared to SO2 (3000 ppm) could result in a higher conversion towards the carbonation reaction. However, the higher free energy change associated with the sulfation reaction thermodynamically favors it over the carbonation reaction. The process conditions employed can have a significant impact on the relative rates of these two reactions.
• ⁇ so2 attained under simultaneous exposure to CO2 and SO2 is higher than the X502 obtained by either the pure sulfation of CaO or the direct sulfation of CaCO3 reaction, which are the only possible routes for sulfation.
• the nascent CaCO3, formed due to the parallel carbonation reaction has a higher reactivity for SO2 than the CaCO3, which forms a part of the stable crystal structure that characterizes the original PCC.
• ⁇ co2 starts dropping, but it continues to be higher than ⁇ s02 until it reaches 40 minutes. Beyond 40 minutes, XC o 2 starts dropping even below XSO2 due to continued direct sulfation.
• Figures 4 and 5 depict the effect of residence time over three CCR cycles on ⁇ co2 and Xso2, respectively.
• PCC-CaO attains a maximum XC o2 of -50 wt% at 10 minute residence time.
• the data in Figures 4 and 5 show that ⁇ co2 a ⁇ d xso2 decrease with increasing number of cycles for any residence time due to the formation of CaSO4 which reduces the availability of CaO in the subsequent cycle.
• the primary reason for this observation is the fact that there is a loss in the free CaO due to the formation of non-regenerable CaSO4.
• Figure 5 shows that X502 remains virtually the same in each of the three cycles until a residence time of 10 minutes.
• ⁇ ⁇ 2 shows a significant loss in reactivity over each subsequent cycle in the same duration.
• XCO2 which was only 22.5% in the first cycle, reduced to almost zero in the second cycle, indicating a high extent of the direct sulfation reaction.
• the sorbent is completely spent at the end of the second cycle that it shows no reactivity to either gas in the third cycle.
• the overall xso 2 for PCC-CaO at the end of three cycles was 88.2%
• Figure 6 illustrates the ratio R obtained from XCO2 and ⁇ so 2 attained during simultaneous carbonation and sulfation. From Figure 6, we can observe that the magnitude of R is smaller than that derived from the "individual" reactions and it shifts to 5 minutes instead of 8 minutes seen earlier. This is probably due to the fact that the rate of sulfation is enhanced due to the simultaneous sulfation of CaO and the higher reactivity of nascent CaCO3 as explained earlier. From Figure 6, it is evident that the maximum in the ratio occurs at a reaction time of about 5 min for all the three cycles. The magnitude of the ratio falls with each subsequent cycle and longer residence time.
• Figures 7 and 8 below show the extent of carbonation and sulfation respectively on PCC-CaO at 700 ° C with varying SO 2 concentration from 100 - 3000 ppm over multiple cycles. It is evident from the plots that the carbonation conversions decrease with increasing cycles and SO2 concentrations. The effect of sulfation is very drastic for 3000 ppm and not so severe with 100-300 ppm range. The extent of sulfation is also low in this range as can be observed from Figure 8.
• Figure 9 shows the ratio "R" for increasing CCR cycles with SO2 concentrations varying from 3000 to 100 ppm. It is evident from the plots that for each SO2 concentration curve there exists a maximum in the ratio, which depends on the residence time in the system. The ratio is maximum at 160 for a SO2 concentration of 100 ppm while it monotonically decreases and reaches a meager value of 5 for 3000 ppm SO2 as seen earlier. [00215] Effect of Temperature
• the simultaneous experiments were conducted for a 3000 ppm SO2, 10% CO2 and 4% 02 stream.
• the ratio (R) decreases with increasing residence time for all reaction temperatures (except for 700 C). This is due to the onset of direct sulfation of CaCO3 product.
• 650 ° C is the highest, which is followed by 600 ° C and subsequently by the values at 700 C (except at 2 min).
• the R values at 700 C are lower than that obtained at temperatures of both 600 and 650 ° C as sulfation starts to dominate at higher temperatures and the kinetics between these two competing carbonation and sulfation reactions start to play a significant role in determining their ratios.
• 650 C seems to be the optimal temperature to operate with minimal sulfation effects for a 3000 ppm SO2 and 10% CO2 stream.
• the optimal temperature for streams with varying SO2 concentrations (3000-100 ppm) needs to be identified.
• the ratio starts with a value of 15 for a residence time (RT) of 2 min and subsequently starts to monotonically decrease to about 9 for 5 min, 5 for 10 min and finally 2 for 30 min.
• RT residence time
• the corresponding XC o 2 is 34% for 2 min, which peaks to 45% at 20 min with a ratio of only 3. It is evident that the extent of carbonation is the highest for the temperature of 650 ° C for any residence time. The only exception is the ⁇ co 2 ⁇ f 52% at 700 C for a 10 min residence time. However the R corresponding to this point is around 3.
• the flue gas is then lowered in its S02 content by the injection of PCC.
• This FSI mode of sulfur capture has been investigated in an earlier OCDO-OSU sponsored OSCAR project. Results from that project will be factored in to use an optimal Ca/S ratio for SO2 control.
• the optimum temperature for PCC injection is about 800100O C.
• the entrained fly ash and partially sulfated solids (containing CaSO4 and unreacted CaO) are then physically removed from the flue gas through the use of a cyclone. The use of a single cyclone effectively separates >99% of all solids. This solid mixture is then safely disposed. [00219]
• the solids depleted flue gas is then subjected to CO2 removal.
• the carbonated sorbent is removed from the flue gas by an identical cyclone, downstream of the carbonation reactor.
• the carbonated hot solids are then sent to the calciner, which provides the heat required to raise the temperature of the solids to 770-830 C and calcine the carbonated portion of the solids.
• Prior laboratory data indicates that the calcination under sub-atmospheric conditions aids in maintaining a higher porosity CaO sorbent, which also exhibits higher reactivity.
• a water ejector will generate vacuum for the calcination.
• the water flowing through the throat of the eductor causes the absolute pressure to fall, allowing the suction of C02 out of the calciner.
• the two-phase fluid is then sent to a knockout drum, where water is separated from the "separated" C02.
• the water stored in the knockout drum is continuously recycled through the water pump for continuous vacuum building.

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