| EP0487102 | 1992-05-27 | |||
| GB2284203A | 1995-05-31 | |||
| 3988422 | 1976-10-26 | |||
| GB819215A | 1959-09-02 | |||
| 4510124 | 1985-04-09 | |||
| 4187279 | 1980-02-05 | |||
| 5584905 | 1996-12-17 | |||
| 3660023 | 1972-05-02 | |||
| GB1305718A | 1973-02-07 | |||
| 3511595 | 1970-05-12 |
CROSS-REFERENCE TO RELATED APPLICATIONS This application claims the benefit of U. S. Provisional Application No. 60/096, 846, filed 8/18/98, and U. S. Application No. 09/314, 220, filed 5/19/1999.
TECHNICAL FIELD The present invention relates generally to a method and apparatus for extracting carbon dioxide (CO2) from a stream or volume of gas, and sequestering said C02 from the atmosphere or other gaseous environment. The invention particularly relates to a method and apparatus that utilize carbonate and water to sequester said CO2 as bicarbonate.
BACKGROUND ART A variety of chemical means exist or have been proposed which consume CO2 contained in emissions from fossil fuel combustion or other gas streams, thus reducing the potential atmospheric CO2 burden (reviews by : H. Herzog and E. Drake,"Carbon Dioxide Recovery and Disposal From Large Energy Systems', Annual Reviews of Energy and Environment Vol. 21, p 145-166, 1996 ; X. Xiaoding and J.A. Moulijn, "Mitigation of CO2 by Chemical Reactions and Promising Products ", Energy and Fuels, Vol. 10, p 305-325,1996).
Among these chemical approaches, the exposure and reaction of such waste CO2 to certain naturally occurring or artificially formed calcium-, magnesium-, sodium-, and/or silica-rich minerals has been explored as reviewed below. The reaction of certain carbonate and silicate minerals with CO2 is a well-known"rock weathering"phenomenon that plays a major role in controlling atmospheric CO2 on geologic time scales (R. A. Berner, A. C. Lasaga, and R. M.
Carrels,"The Carbonate-Silicate Geochemical Cycle and its Effect on Atmospheric Carbon Dioxide Over the Last 100 Million Years ", American Journal of Science, Vol. 283, p 42-50, 1983). Over the very long term such process are expected to eventually consume most of the C02 emitted by man's activities. The problem is that such natural processes occur on the order of >1, 000 year time scales and thus will have little immediate impact on the rapidly increasing
CO2 emissions and atmospheric CO2 burden in the coming centuries. Nevertheless, several researchers have proposed that certain weathering reactions be used to sequester CO2, in particular those reactions which lead to CO2 sequestration or storage in the form of solid carbonates.
For example, fixation and storage of CO2 by artificial weathering of waste concrete in combination with coccolithophorid algae cultures was reported by H. Takano and T.
Matsunaga,"C02 Fixation by Artificial Weathering of Waste Concrete and Coccolithophorid Algae Cultures ", Energy Conversion Management, Vol. 36, No. 6-9. p 697-700,1995. It was shown that C02 can be sequestered into biologically produced carbonate and biomass. Various mechanisms of rock weathering to fix C02 was discussed by T. Kojima,"Evaluation Strategies for Chemical and Biological Fixation/Utilization Processes of Carbon Dioxide", Energy Conversion Management, Vol. 36, No. 6-9, p 881-884,1995. Studies of CO2 fixation by silicate rock weathering were reported by T. Kojima, A. Nagamine, N. Ueno and S. Uemiya, "Absorption and Fixation of Carbon Dioxide by Rock Weathering ", Energy Conversion Management, Vol. 38, Suppl., p S461-S466, 1997. Sequestering of COz as carbonate by reaction with minerals rich in calcium and magnesium oxides was reported by K. S. Lackner, C. H. Wendt, D. P. Butt, E. L. Joyce, D. H. Sharp,"Carbon Disposal in Carbonate Minerals", Energy, Vol. 20, No. 11, p 1153-1170,1995. Reacting flue gas C02 with water and soil to ultimately precipitate and sequester the CO2 as carbonate was explored by T. Chohji, M.
Tabata, and E. Hirai,"C02 Recovery From Flue Gas by an Ecotechnological (Environmentally Friendly) System ", Energy, Vol. 22 No. 2/3, p 151-159,1997. A study by H. Kheshgi ("Sequestering Atmospheric Carbon Dioxide by Increasing Ocean Alkalinity", Energy, Vol. 20, No. 9, p 912-922,1995) looked at the option of adding calcium oxide to the ocean as a means of increasing the C02 absorption capacity of the ocean. The preceding approaches often require elevated temperatures or pressures, significant energy, land, or other resource inputs, and/or have negative environmental impacts. The cost of implementing these technologies is therefore often prohibitive.
As reviewed by H. Herzog and E. Drake, (Annual Reviews, loc. cit. ) several chemical means exist for separating and concentrating CO2 from gas streams. U. S. Patent 4, 376, 101 (Sartori et al) discloses the removal of C02 from a gaseous stream via use of an aqueous solution containing an alkali metal salt or hydroxide and an activator or promoter system comprising an amine compound. While such processes remove or separate C02 from a waste stream, they offer no downstream method of ultimately sequestering the CO2 from the
atmosphere. They also often require elevated temperatures or pressures, exotic chemicals, and/or significant inputs of energy or resources.
Gas/water/calcium carbonate (limestone) reactors have been used in desulfurization of power plants exhaust as reviewed by H. N. Soud and M. Takeshita,"FGD Handbook, IEA Coal Research, London, 438p. , 1994. Such reactors differ from the present invention in three important aspects : 1) The volume of S02 in the gas streams to which desulfurization is applied is vastly smaller than the CO2 content in the same gas stream ; 2) The hydration step in carbonate desulfurization involves combining SO2 with H20 to form the strong acid H2S03. In contrast, the hydration of CO2 envisioned here forms carbonic acid H2CO3, a weak acid which has a slower reaction rate with carbonate than does H2SO3. 3) The reaction of H2SO3 with carbonate (e. g., CaC03) and oxygen forms a solid, CaSO4, and a gas, C02, whereas the H2CO3 with carbonate reaction forms cations and bicarbonate in solution, does not require supplemental oxygen, produces little or no solid waste, and consumes rather than generates gaseous CO2.
U. S. Patent 5, 100633 (Morrison) describes a process for scrubbing acid-forming gases which include SO2 and C02 from an exhaust gas stream through reactions with alkaline solutions formed from the waste ash from biomass burning. The resulting alkali metal salts are then precipitated or dewatered forming solid, possibly useful waste products. This process does not provide a system for net C02 sequestration, however, considering that the molar ratio of carbon to alkali metals or to alkaline earth metals in the end products is many times lower than that ratio in the original biomass burned to form the alkaline ash. That is, only a very small fractional equivalent of the CO2 released in biomass combustion can be sequestered by this process, and therefore when initial ash and CO2 formation are considered the overall process is a net source rather than a net sink for CO2.
The chemical reactions involving CO2 gas, water, and carbonate minerals (principally calcium carbonate) have been extensively studied as reviewed by J. W. Morse and F. T.
Mackenzie ("Geochemistry of Sedimentary Carbonates ", Cambridge, Amsterdam, 707p., 1990) and by T. Arakaki and A. Mucci ("A Continuous and Mechanistic Representation of Calcite Reaction-Controlled Kinetics in Dilute Solutions at 25°C and 1 Atm Total Pressure", Aquatic Geochemistry, Vol. 1, p 105-130,1995). However, the context of these studies has been to describe the dissolution or precipitation of solid carbonate under various conditions, not the consumption and sequestration of CO2.
DISCLOSURE OF THE INVENTION
An object of this invention is a method and apparatus for extracting carbon dioxide (C02) contained in a stream or volume of gas, and sequestering this CO2 from the stream of volume of gas.
A further object is a method and apparatus which accomplishes said CO2 extraction and sequestration without the requirement of elevated temperatures, pressures, and without significant expenditures of energy or other resources.
A further object is a method and apparatus which utilizes HO and carbonate.
A further object is a method and apparatus in which the reactants are relatively abundant and inexpensive, and the end products and process waste streams are environmentally benign.
A further object is a method and apparatus whose relative simplicity and low cost allow it to be widely employed, therefore impacting CO2 emissions potentially at the global scale.
A further object is a method and apparatus which can utilize a wide range of fresh- and salt-water sources.
A further object is a method and apparatus which is flexible in siting requirements allowing location near CO2 sources, carbonate, and/or water sources.
It is known that carbonic acid reacts with certain metal carbonates to form metal ions and bicarbonate in solution. Such a reaction is employed in the invention to provide a means of extracting and sequestering CO2 from a stream or volume of gas.
One embodiment of the present invention is an integrated apparatus comprising a reactor vessel containing carbonate. A gas stream containing CO2 enters the reactor vessel. In the reactor vessel, CO2 contacts an aqueous solution and becomes hydrated to form carbonic acid, which in turn reacts with the carbonate to form bicarbonate and metal ions. Waste streams exiting the reactor vessel comprise a gas stream now depleted of CO2, and an aqueous solution of metal ions and bicarbonate.
Another embodiment of the present invention is a sequential apparatus comprising a hydration vessel and a carbonate reaction vessel. A gas stream containing CO2 enters the hydration vessel. In the hydration vessel, CO2 contacts an aqueous solution and becomes hydrated to form carbonic acid. The carbonic acid is transported to the carbonate reaction vessel where it reacts with carbonate located therein, to form bicarbonate and metal ions.
Waste streams comprise a C02-depleted gas stream exiting the hydration vessel and an aqueous solution of metal ions and bicarbonate exiting the carbonate reactor vessel.
Due to its relative simplicity, low-cost, and low environmental impact, it is believed that the invention herein disclosed offers distinct advantages over other methods for the
combined process of extracting CO2 from waste gas streams and sequestering this CO2 from the atmosphere.
BRIEF DESCRIPTION OF THE DRAWINGS These and other features, aspects, and advantages of the present invention will become better understood with regard to the following description, appended claims, and accompanying drawings wherein : FIG. 1 illustrates one embodiment of the present invention comprising an integrated configuration that extracts and sequesters CO2 from a gas stream in which CO2 is hydrated and reacted with carbonate in an integrated hydration carbonate reactor vessel ; FIG. 2 illustrates another embodiment of the present invention comprising a sequential configuration that extracts and sequesters CO2 from a gas stream in which CO2 is first hydrated, then the resulting carbonic acid solution is separately reacted with carbonate ; FIG. 3 illustrates further embodiments of the present invention comprising various means to enhance C02 hydration ; FIG. 4 illustrates further embodiments of the present invention comprising various means to enhance the carbonate - carbonic acid reaction ; FIG. 5 illustrates further embodiments of the present invention comprising various means to handle the carbonate.
BEST MODES FOR CARRYING OUT THE INVENTION Definitions alkali metals - elements found in column IA of the periodic table of elements alkaline earth metals - elements found in column IIA of the periodic table of elements carbon dioxide - C02 carbonate - metal carbonate carbonate group - C03 carbonate ion - C03 2- carbonate solution - carbonate particles in suspension or slurry, and/or dissolved in solution C02-depleted gas stream - a gas stream where some or all of its initial CO2 has been removed dissolved carbonate - metal ions and carbonate ions in solution metal carbonate - chemical compound of the form X (C03) m where X is any element or combination of elements that can chemically bond with a carbonate group or its multiple, wherein at least one element is a group IA, IIA, IIIA, IVA, IB, IIB, IIIB, IVB, VB, VIB,
VIIB, or VIIIB element of the periodic table, and m is a stoichiometrically determined positive integer. Examples of X include but are not limited to alkali metals and alkaline earth metals. metal ion - one of the resulting cations formed when metal carbonate X (C03) m has reacted with carbonic acid, wherein the cation is found in solution with bicarbonate, and m is a stoichiometrically determined positive integer. wetted carbonate - static or moving bed, pile, or aerosol composed of carbonate particles wetted by an aqueous solution Abbreviations aq - aqueous Ca - calcium C02 - carbon dioxide C03 - a carbonate group 03 _ carbonate ion HC03- - bicarbonate ion H2C03 - carbonic acid H20 - water Mg - magnesium Na - sodium pCOx - the partial pressure of CO2 gas pH - the negative logarithm of the hydrogen ion concentration 502 - sulfur dioxide <BR> <BR> <BR> <BR> <BR> 503 - sulfite <BR> <BR> <BR> <BR> <BR> <BR> <BR> <BR> S042-- sulfate X - any element or combination of elements that can chemically bond with a carbonate group or its multiple, wherein at least one said element is a group IA, IIA, IIIA, IVA, IB, IIB, IIIB, IVB, VB, VIB, VIIB, or VIIIB element of the periodic table.
X (aq) - any element or combination of elements in solution that can chemically bond with a carbonate group or its multiple, formed when X (C03) m dissolves in a solution.
X (C03) m - carbonate composed of X bonded to one or more carbonate groups, where m is a stoichiometrically determined positive integer.
The inventive method and apparatus utilize a process comprising two main steps. In
step 1, gaseous CO2 is hydrated to form carbonic acid, as shown in equation 1 : CO2(gas) + H20=> H2CO3 (aq). (1) In step 2, the hydrated CO2 is reacted with a metal carbonate, in solid or dissolved form, to form metal cations and bicarbonate in solution. When the hydrated C02 or carbonic acid is reacted with a metal carbonate in solid form, this reaction may be represented as shown in equation 2a : mH2CO3(aq) + X(CO3)m(solid) => X(aq) + 2mHC03- (aq). (2a) When the hydrated CO2 or carbonic acid is reacted with a metal carbonate in dissolved form, this reaction may be represented as shown in equation 2b : H2CO3 (aq) + CO32-(aq) => 2HCO3-(aq). (2b) Equation 2b implies that the metal carbonate has already undergone a dissolution reaction, which may be represented by equation 3 : X (C03) m (solid) => X (aq) + mC032- (aq). (3) In step 2, X may represent any element or combination of elements that can chemically bond with the C032- or its multiple, and wherein at least one element is a group IA, IIA, IIIA, IVA, IB, IIB, IIIB, IVB, VB, VIB, VIIB. or VIIIB element of the periodic table. Because of their natural abundance and reactivity, X would be preferably represented by a member or members of the group IA and group IIA elements. Carbonates relevant to such a reaction include but are not limited to CaC03, CaMg (C03) 2, MgC03, and Na2C03. For large scale applications X represented by Ca would be preferred because of the relatively high natural abundance and low cost of CaC03 (for example, as contained in limestone). Other sources of carbonates include, but are not limited to, calcite, dolomite and aragonite. X (aq) represents one or more ions in solution containing the elements composing X.
CO2 Hydration In the present invention, CO2 in a gas stream may be hydrated in various ways : by passing the gas stream through an aqueous solution whose surface area is enhanced, preferably by spraying or atomizing, by bubbling the gas stream into an aqueous solution, and/or by passing the gas stream over or through wetted carbonate. The gas stream may encounter the aqueous solution or wetted carbonate vertically, horizontally, or at some other angle. This gas introduction may be assisted by a compressor or other means well known in the art. This may be particularly relevant when the gas stream is bubbled into an aqueous solution, or passed through wetted carbonate that is submerged, where resistance to gas flow from the aqueous solution and/or carbonate particles is expected. Introduction of gas below wetted carbonate may serve to partially or completely fluidize the particle bed, enhancing gas-aqueous solution-
carbonate contact. In this configuration the CO2 hydration occurs in close proximity to the carbonate-carbonic acid reaction, and both reactions are facilitated by the flow of gas and acid solution around the carbonate. The incoming gas is thus exposed to a large surface area of aqueous solution in the form of droplets and wetted carbonate surfaces, facilitating hydration of CO2 to form a carbonic acid solution within the reactor.
Carbonate Forms In the present invention, the carbonate may be presented to the carbonic acid in solid phase form, or in liquid phase form. Preferred carbonate forms include : i) pile or bed of particles (or chunks, slabs or blocks), ii) liquid slurry or suspension of particles, iii) solution of dissolved carbonate, or iv) solution or particle aerosol ; over or through which the carbonic acid solution from step 1 is passed.
If the carbonate used is relatively insoluble in water, e. g., the calcium carbonate contained in limestone, then its reaction with carbonic acid in the aqueous solution will occur largely as a reaction between a liquid (carbonic acid containing solution) and a solid (limestone). Because of the abundance and relative low cost of the latter type of carbonate, the use of this carbonate type seems preferred for large scale applications.
In such applications relatively water-insoluble carbonate will be presented to the gas and aqueous solution in the reaction as a bed, pile, slurry, suspension, or aerosol of carbonate particles. The particulate carbonate may be of homogeneous or heterogeneous size and shape ranging from very fine particles to large chunks. Prior to reaction with the carbonic acid, the original size and shape of the carbonate may be modified by crushing, etching, drilling, sawing, or otherwise forming the carbonate into sizes and shapes advantageous for step 2. Because the overall rate of step 2 will be a function of the surface area of the particles exposed to the aqueous solution, the greatest surface area and hence greatest reaction rate per unit reactor volume will be achieved with the smallest sized carbonate particles. In such cases the particles in contact with the aqueous solution may form a suspension or slurry of particles depending on the size of the particles and the agitation or flow of the solution into which they are immersed.
The size of such particle might be less than 0. 1 mm. At the other extreme would be carbonate particle sizes, e. g., >10 cm whose individual mass would preclude prolonged suspension in air or solution and whose collective mass would then form a static bed, pile, or other configuration of carbonate particles. The aqueous solution, CO2, and carbonic acid solution would then flow in or through the porous carbonate particle mass, facilitating carbonic acid-carbonate contact and possibly gaseous C02-aqueous solution contact. The advantage of such a scheme would be that less carbonate particle size reduction and associated cost by crushing (or other means
common in the art) would be required. It would also preclude the added complexity of handling and pumping solutions containing suspended carbonate particles. With the bed/pile approach, a greater volume of particles and hence a larger reactor vessel size would be needed to attain an equivalent carbonate particle surface area within the reactor. Because it is unlikely and impractical that strict homogeneous particle sizes will be introduced into the reactor and because particle size reduction will occur in the reactor as particles of any size react with carbonic acid, it is likely that some intermediate between a static bed/pile and a dynamic suspension/slurry of carbonate will form in the reactor. The inclusion of carbonate particles in the reactor solution requires attention with regard to solution handling and pumping as will be discussed later.
If the form of the carbonate used is soluble in water, e. g. sodium carbonate, then a large portion if not all of the carbonate will be in ionic, dissolved form in aqueous solution. This means that the carbonic acid - carbonate reaction to form bicarbonate will occur mostly if not entirely in aqueous solution. In addition to the various modes of presenting the carbonic acid to the carbonate described earlier for solid phase carbonate, liquid phase carbonate may be presented to the carbonic acid via means well known in the art, such as spraying, atomizing, blowing, and presentation on wetted structures, or as a pool of liquid into which the gas stream is bubbled.
Introduction, Handling, and Removal of Water and Aqueous Solution As step 2 proceeds, the aqueous mixture in proximity to the carbonate will become increasingly saturated with bicarbonate and the rate of bicarbonate formation will subsequently decline. It may be advantageous to bleed off or remove part of the mixture and replace this removed volume with aqueous solution which is relatively unsaturated with bicarbonate. The amount and timing of such removal will be dictated by the status of the solution chemistry and the desired reaction rates. By means well known in the art, monitoring of one or more solution chemistry parameters such as pH, pC02, conductivity, alkalinity, and/or metal ion concentration, either in the reactor solution or in the recirculating solution, is therefore desired.
Water may be added to, and solution effluent removed from, the reactor by pump, gravity feed, or other means well known in the art for liquid handling. Water addition may occur directly into the reactor or indirectly via addition to and mixture with recirculating aqueous solution prior to this mixture's introduction into the reactor. Also, carbonate solution may be added directly into the reactor or indirectly via addition to and mixture with recirculating aqueous solution prior to this mixture's introduction into the reactor.
The amount of water added to the reactor per unit time relative to the removal of waste
solution effluent from the reactor will determine the solution level within the reactor. In various embodiments reactor solution may be maintained or varied at levels ranging from significantly above to significantly below the top level of the particulate carbonate bed/pile within the reactor. The liquid level will dictate the maximum height above the reactor base where solution can be withdrawn for recirculation or removal. For purposes of allowing carbonate particles to settle and for minimizing particulate load in the recirculated/removed waste solution, it would be advantageous to maintain the solution level and hence the solution outlet or outlets above the carbonate bed/pile level. On the other hand, maintaining liquid levels below the top of the carbonate bed/pile would expose the C02 gas within the reactor to a large wetted surface area formed by the exposed carbonate bed/pile as wetted by the aqueous solution spray, facilitating carbonic acid formation. In either case some carbonate particles may be entrained in the solution outflow which, if deleterious to pumps or other solution handling equipment, could be removed by filtration, settling, or other means well known in the art for liquid/solid separation.
Process Parameters Certain process parameters which affect steps 1 and 2 may be varied to maximize the cost/benefit of a reactor's operation. For example, since high temperatures adversely affect the CO2-hydration and carbonate-carbonic acid reactions, low temperatures are preferred within the range between the freezing and boiling points of water for a given operating pressure.
Cooling means include those well known in the art for cooling liquids and gases ; such may be passive (including radiator fins or heat sinks attached to the reactor vessels or process lines), active (indirect via heat exchanger or direct refrigeration), or a combination of the two. Such cooling means may be used to pre-cool the incoming gas stream, or to cool process particular process components including the recirculated gas stream, the gas contained in the reactor vessel, the aqueous solution, the carbonic acid, and/or such liquid as may be pooled in the reactor vessel. In addition, cooling may be achieved by having water recharge or replenishing carbonate at a lower temperature than the components in the reactor vessels. Higher total process pressures above ambient also benefit the hydration and carbonate reactions, serving to increase CO2 solubility and carbonate reactivity with carbonic acid, and may be cost effective.
Pressurizing means include those well known in the art such as a compressor to increase the pressure of the incoming gas stream or the gas contained within the reactor vessel. It is preferred that the highest possible concentration of carbonic acid solution be presented to the largest possible carbonate surface area, with the pH of the carbonic acid solution being as low as allowed by the operating temperature, the incoming gas stream's pic02, the water volume with which CO2 is hydrated, and the effects of chemical additives (if any). Since the solution
streams in the reactor will likely range from concentrated H2CO3 to concentrated HC03-, pH variation (probably 4 to 8) will need to be considered in designing the reactor bed container, and the solution and gas handling and transport systems. Other parameters to consider include : i) the CO2 concentration, flow rate, and chemical composition of the gas stream entering the reactor, ii) the particle size and total amount of carbonate and thus the total carbonate surface area within the reactor, iii) the rate of physical movement or agitation (if any) of the carbonate and carbonic acid solution, iv) the reactor temperature and/or pressure, and v) the chemical properties, flow rate, and recirculation of solution within the reactor.
Various embodiments according to the present invention are described hereunder with reference to FIG. 1. In these embodiments, the H20 hydration and carbonate reactions occur together in one integrated reactor vessel 100 ("integrated configuration "). The reactor vessel 100 comprises two regions : an upper region, and a lower region. An aqueous solution 132 is introduced into the upper region of reactor vessel 100 in the form of mist or droplets via atomizer/sprayer unit 104. The atomizer/sprayer unit 104 comprises at least one atomizer/sprayer positioned wherein the mist/spray is emitted horizontally, vertically upward or downward or other angle. Aqueous solution 132 may also be introduced directly into the lower region of the reactor vessel 100 as shown. Aqueous solution 132 comprises water and may include one or more of the following : dissolved carbon dioxide, carbonic acid, metal ions, bicarbonate, particulate carbonate and dissolved carbonate. A gas stream containing CO2 enters the reactor vessel 100 by one or more entry ways (112a, 112b, and/or 112c). Gas stream 112a enters the upper region of reactor vessel 100. Contact between the gas stream 112a and atomized aqueous solution 132 serves to hydrate CO2, forming carbonic acid 144. Aqueous solution 132 that is unreacted and carbonic acid 144 travel by gravity flow to the lower region of reactor vessel 100 where they impinge and wet carbonate 152 contained therein. The gas stream 112b enters the lower region of reactor vessel 100 and becomes hydrated as it bubbles through a region of unreacted aqueous solution 132 forming carbonic acid 144. Alternatively, gas stream 112b may be bubbled through unreacted aqueous solution 132 in the lower region of reactor 100 to form carbonic acid solution 144. The gas stream 112c enters the lower region of reactor vessel 100 and passes over or through wetted carbonate 152 wherein the CO2 hydration and carbonate - carbonic acid reactions occur in close proximity. The gas stream 112c may be configured to pass over or through carbonate 152 in the vertical, horizontal, or other direction as may be desired. The C02-depleted gas stream 122 exits the reactor vessel 100. Prior to exiting from the reactor vessel 100, the gas stream 122 may be passed through a moisture-eliminator 108 to remove some or all of the water that may be present. The carbonic
acid 144 reacts with carbonate 152 to form metal ion/bicarbonate solution 146. Mixture 142 comprises metal ion/bicarbonate solution 146, unreacted aqueous solution 132, and unreacted carbonic acid 144 which have pooled in the lower region of reactor vessel 100. Mixture 142 is bled from the reactor and disposed of as waste solution 142b and/or recirculated 142a. Water 136b may be added to mixture 142a as it is circulated back to atomizer/sprayer unit 104, or water 136a may be added directly to the lower region of the reactor vessel 100. Carbonate recharge means 134 is provides for adding carbonate to reactor vessel 100. To prevent entrainment of large particulate carbonate, mixture 142 may be passed through a solid/liquid separation means, such as a filter, 106. Portions of the carbonate 152 may extend above the liquid level of mixture 142, thereby exposing the carbonate 152 directly to the atomized aqueous solution 132 and carbonic acid 144. Monitoring means 149 is provided for measuring one or more solution chemistry parameters such as pH, pic02, conductivity, alkalinity, and/or metal ion concentration, in mixture 142 either within the reactor vessel 100 or as the mixture is transported outside of the reactor vessel 100.
Further embodiments according to the present invention are described hereunder with reference to FIG. 2. In these embodiments, ("sequential configuration"), CO2 hydration occurs in a hydration-vessel and the carbonate-carbonic acid reaction occurs in a carbonate reaction vessel, with the hydration vessel being located upstream from the carbonate reaction vessel.
Unless otherwise stated, similarly numbered components are analogous to components in the previous figure. The hydration vessel 200 comprises two regions : an upper region, and a lower region. An aqueous solution 232 is introduced into the upper region of hydration vessel 200 in the form of mist or droplets via atomizer/sprayer unit 204. The atomizer/sprayer unit 204 comprises at least one atomizer/sprayer positioned wherein the mist/spray is emitted horizontally, vertically upward or downward or other angle. Aqueous solution 232 may also be introduced directly into the lower region of hydration vessel 200. Aqueous solution 232 comprises water and may include one or more of the following : dissolved carbon dioxide, carbonic acid, metal ions, bicarbonate, particulate carbonate and dissolved carbonate. A gas stream containing CO2 enters hydration vessel 200 via one or more pathways 212a, b. Gas stream 212a enters the upper region of hydration vessel 200. Contact between the gas stream 212a and aqueous solution 232 serves to hydrate COz forming carbonic acid 244. The gas stream 212b enters the lower region of hydration vessel 200 and becomes hydrated as it bubbles through a region of unreacted aqueous solution 232 and carbonic acid 244. The CO2- depleted gas stream 222 exits hydration vessel 200 as shown. Prior to exiting from the hydration vessel 200, the depleted gas stream 222 may be passed through a moisture eliminator
208. Mixture 242 comprises carbonic acid 244, aqueous solution 232, and may contain mixture 292. Mixture 242 is transported to carbonate reaction vessel 250 as shown in 242b, or recirculated 242a. Water 236b may be added to mixture 242a as it is recirculated back to atomizer/sprayer unit 204, or water 236a may be added directly to the lower region of the hydration vessel 200. Carbonate reaction vessel 250 comprises upper and lower regions.
Mixture 242b may be introduced into the upper region of carbonate reaction vessel 250 in the form of mist or droplets 242c via atomizer/sprayer unit 254, and/or directly 242d into the lower region of carbonate reaction vessel 250. Mixture 242c and/or 242d react with carbonate 252 contained in the lower region of carbonate reactor 250 to form metal ions/bicarbonate solution 294. Mixture 292 comprises metal ion/bicarbonate solution 294 and unreacted mixtures 242c and/or 242d which have pooled in the lower region of carbonate reaction vessel 250. Mixture 292 is bled from the carbonate reaction vessel 250 and disposed of as waste solution 292c, recirculated 292a back to hydration vessel 200 for facilitating further CO2 hydration, and/or recirculated 292b for mixing with mixture 242b. Carbonate recharge means 234 is provided for adding carbonate to the carbonate reactor 250. To prevent entrainment of large particulate carbonate, mixture 292 may be passed through a solid/liquid separation means, such as a filter, 256. Portions of the carbonate 252 may or may not extend above the liquid level of mixture 292, thereby exposing the carbonate 252 directly to the atomized mixture 242c. Carbonate reaction vessel 250 may also contain gas/heat exchange means 272 whereby gas is vented and heat may be exchanged with the ambient atmosphere to moderate interior gas pressure and temperature. Monitoring means 249 and 299 are provided for measuring one or more solution chemistry parameters such as pH, pCO2, conductivity, alkalinity, and/or metal ion concentration, in mixtures 242 and 292 either within reactors 200 and 250, or in the effluent solution.
Hydration Enhancement The object for step 1 is to efficiently obtain the highest possible carbonic acid concentration for the step 2 reaction. The efficiency of CO2 hydration can be enhanced by : i) increasing the surface area of an aqueous solution in contact with a given volume of CO2.
Generally, for a given aqueous solution and CO2 volume this can be accomplished by minimizing the size of the aqueous solution droplets, by maximizing the wetted surface area exposed to the CO2, and/or agitating, mixing, flowing, or stirring the CO2 and aqueous solution. ii) increasing the concentration of pCO2 in the incoming gas stream, achieved by pre- concentrating the CO2 in the gas stream, using means well known in the art, before the gas
stream enters the reactor and/or increasing the total gas pressure within the reactor. iii) increasing C02 solubility by decreasing temperature. iv) adding chemical compounds which catalyze or enhance step 1 (e. g., carbonic anhydrase).
One means of maximizing the wetted surface area of an aqueous solution exposed to the CO2 is achieved by use of high surface-area-to-volume-ratio structure (s). Such structures include porous materials, grates, grills, wires, mesh, screens, beads, balls,, plates, fins, vanes, conveyor belts and other structures well known in the art of gas-liquid contacting, which are discussed by J.R. Fair, D. E. Steinmeyer, W. R. Penney and B. B. Crocker."Gas Absorption and Gas-Liquid System Design"and in : R. H. Perry, D. W. Green, and J. O. Maloney (eds. ) Perry's Chemical Engineers'Handbook, 7th Ed., McGraw-Hill, New York, pg. 14-1 to 14-98,1997.
These structures may be stationary or in motion (linear, rotational, unidirectional and/or reciprocating), moved, shaken, vibrated, or otherwise agitated to further facilitate gas-solution contacting.
Various means are well known in the art for agitating the CO2 and aqueous solution to increase contact. Such means may include mixers, mechanical or electromagnetic stirrers, blowers, spinners, shakers, vibrators. or other agitating means well known in the art.
The carbonic acid solution may be recirculated for further CO2 hydration with CO2 to approach CO2 saturation and hence the maximum carbonic acid concentration. The aqueous solution used in the CO2 hydration step need not be pure H20, due either to recirculation of reacted water or the presence of other ions or chemicals in the water prior to use in step 1.
Increasing the solution's ionic strength may adversely affect CO2 solubility to a small extent.
Sources of water, which may be used in step 1 which would contain a significant burden of ions and chemicals prior to reaction, include but are not limited to industrial waste water and seawater.
The solution effluent removed from the reactor will contain significantly elevated bicarbonate, carbonate, and CO2 concentrations. If exposed to ambient air or other low-CO2 gas, the differential in CO2 concentration between air and the solution will cause CO2 to degas from solution, increasing saturation state of the carbonate in solution, which could lead to the precipitation of solid carbonate from the solution. If such precipitation is undesired, it may be reduced or eliminated by copious dilution of effluent solution with water or other solution which is undersaturated with respect to carbonate. Carbonate precipitation may also be hindered by the addition to the effluent of certain ions such as S042- and P042- (as discussed by Morse and Mackenzie, op cit.). Conversely, it may be advantageous to concentrate or solidify
part or all of the bicarbonate/carbonate ion load in the effluent through effluent dewatering by means well known in the art, including drying, distillation, evaporation and membrane separation, described in Mark's Standard Handbook for Mechanical Engineers, eighth edition, pages 6-192 to 6-195. The resulting concentrated bicarbonate/carbonate slurry or solid could then be discarded, or used as chemical feedstock or other useful purpose. Since CO2 dissolved in the effluent may come out of solution during the dewatering process, it may be advantageous to capture this CO2. Further embodiments would include a means to capture and recirculate the water removed in the dewatering process for reuse in the CO2 hydration step or for other uses, and/or a means of recirculating the captured CO2 back to the reactor for hydration to form carbonic acid.
Further embodiments of the present invention comprising means to enhance CO2 hydration are shown in FIG. 3. Although the embodiments are depicted for the integrated configuration, these hydration means are equally applicable to the sequential configuration as well. Unless otherwise stated, similarly numbered components are analogous to components in the previous figures. As stated earlier, CO2 hydration may benefit from use of high surface- area-to volume-ratio structures. Screen 364, in the reactor vessel 300 upper region, partially intercepts and is contacted by the aqueous solution 332 and gas stream 312a thereby enhancing formation of carbonic acid solution 344. The C02-depleted gas stream 322 may be recirculated 324 back into reactor vessel 300 should further CO2 removal be necessary. Monitoring means 329 is provided for monitoring the CO2 concentration in gas stream 322. Prior to exiting from the reactor vessel 300, the gas stream 322 may be passed through a moisture-eliminator 308.
Carbonic acid 344 reacts with carbonate 352 to form metal ion/bicarbonate solution 346.
Mixture 342 comprises unreacted carbonic acid 344, aqueous solution 332 and metal ion/bicarbonate solution 346 which have pooled in the lower region of reactor vessel 300.
Agitating means 374 may be located in the reactor vessel 300 lower region to enhance contact between the gas streams 312b, 312c, carbonate 352 and mixture 342. Agitating means 376 may be located in the reactor vessel 300 upper region to enhance contact between the gas stream 312a and aqueous solution 332. Gas streams 312b and 312c may be passed through gas diffusers 367 and 368 respectively to enhance dispersal of the gas into the collected liquid in the lower region of reactor vessel 300. Chemical additives, such as the C02-hydration-catalyst carbonic anhydrase, may be added to the reactor to enhance carbonic acid formation, 372b shown as being added to aqueous solution 332 and 372a shown as being added directly to reactor vessel 300. The carbonate 352 may be held in a liquid-porous/gas permeable container 373. The container 373 may be in motion (linear, rotational, unidirectional and/or
reciprocating) which serves to enhance contact between the carbonate 352, aqueous solution 332, carbonic acid 344 and gas entering via pathways 312b or 312c. To prevent entrainment of large particulate carbonate, mixture 342 may be passed through a solid/liquid separation means, such as a filter, 306. Water 336b may be added to mixture 342a as it is recirculated back to atomizer/sprayer unit 304, or water 336a may be added directly to the lower region of the reactor vessel 300. Carbonate recharge means 334 is provided for adding carbonate to reactor vessel 300. Waste solution 342b may be passed to water recovery unit 392 which concentrates the metal ions/bicarbonates and partially or completely extracts the water by means well known in the art. Reclaimed water 338 and/or CO2 derived from unit 392 may be added 338 to mixture 342a as shown and recirculated to reactor 300. Concentrated metal ion/bicarbonate 394 exits component 392 as shown. Aqueous solution 332 may be passed through cooling means 395 to enhance CO2 solubility. Mixture 342 may be passed through cooling means 396 to enhance CO2 solubility. Cooling means 397 serves to cool the gas present in the upper region of reactor vessel 300 to enhance CO2 solubility. Passive cooling means 398 serves to remove heat from the interior of reactor vessel 300 to enhance CO2 solubility. Pressure means 399 serves to increase the total pressure in reactor vessel 300 to enhance CO2 solubility.
Carbonic Acid/Carbonate Reaction Enhancement Various means may be used to enhance the reaction of the carbonate with the carbonic acid. Although the carbonate-carbonic acid reaction is not restricted to the use of specific carbonate material sizes (allowable sizes may range from particles of less than 1 micron to greater than 1 meter, e. g., chunks, slabs or blocks), maximizing the surface area of the carbonate presented per volume of carbonic acid solution will maximize the overall rate of the reaction. For a given reactor volume, carbonate surface area is maximized by minimizing the size of the individual carbonate particles used in the reactor.
Use of high surface-area-to-volume-ratio structures to hold, support or contact the carbonate could serve to increase the surface area of carbonate presented per volume of carbonic acid. Such structures include but are not limited to porous materials, grates, grills, plates, wires, mesh, screens, beads, balls, fins, vanes, liquid-porous containers, conveyor belts and other structures well known in the art. Such structures may be stationary or in motion (linear, rotational, unidirectional and/or reciprocating), moved, shaken, vibrated or otherwise agitated to facilitate carbonate/carbonic acid contact.
Although the carbonic acid solution is shown in the Figures as flowing vertically downward over/through the carbonate, the flow configuration is not limited to such. Flow may be vertically upward, horizontal, or at an angle as well. If the carbonate is a slurry, in
suspension, dissolved in solution or in aerosol form, other ways of mixing the carbonate and carbonic acid solution may be used as well, including mixing the two components while airborne, spraying, atomizing, blowing, or as a pool of liquid into which the gas stream is bubbled. The carbonate itself may be static or in motion, as may the structure (s) supporting or containing the carbonate. Having the carbonate in motion (such as a flowing slurry), or moving structure (such as a conveyor belt) may facilitate replenishment of the carbonate for a continuous operation. The direction of movement may be counter, tangential, parallel, or circular to the gas or solution flows or various combinations or these movements according to the desired efficiency of contacting the reactants.
For purposes of enhancing carbonic acid and carbonate contact it may be advantageous to agitate part or all of the reactor or its contents so as to reduce the boundary layer thickness between the unreacted carbonate and carbonic acid solution. Such agitation could be imparted externally to the reactor or to some or all of its contents by one or more shakers, mechanical or electromagnetic stirrers, mixers, spinners, blowers, vibrators, or other agitation means well- known in the art. The carbonate-carbonic acid reaction may also be enhanced by recirculation of unreacted carbonic acid solution within or through the reactor.
Enhanced carbonate-carbonic acid reaction may be achieved by lowering of the reaction temperature. Since the C02-hydration and carbonate-carbonic acid reactions are mildly exothermic, it may be advantageous to modulate reactor heat loss and hence reactor temperature, as well as varying aqueous solution, carbonic acid or gas flow rates or temperatures so as to maximize reaction rates while also preventing excessive H20 vaporization.
Step 2 (carbonate-carbonic acid reaction) is also enhanced by : i) decreasing the pH of the reacting solution, ii) increasing the ionic strength of the reacting solution, and iii) removing or chemically binding chemical impurities (e. g., certain metals, phosphate, sulfite, sulfate, fluoride and organic compounds) which may impede step 2. With regard to iii), if such contaminants deleterious to step 2 are present in the incoming gas or water streams, it would be advantageous to reduce, separate, or remove such contaminants from these streams prior to steps 1 or 2. One way of achieving this would to include chemical additives such as metal or ion chelators or chemical inhibitors of such contaminants (e. g., ethylene-diamine tetraacetate, EDTA) in aqueous solutions within the reactor.
Further embodiments of the present invention comprising various means to enhance the carbonate - carbonic acid reaction are shown in FIG. 4. Although the embodiments are depicted for the integrated configuration, these structures are equally applicable to the sequential
configuration as well. Unless otherwise stated, similarly numbered components are analogous to components in the previous figures. In one embodiment, screen 464 is located in the upper region of reactor vessel 400 upon which carbonate 452a is disposed. Aqueous solution 432, emitted from atomizer/sprayer unit 404, impinges and wets screen 464 and carbonate 452a.
Gas stream 412a passes over or through screen 464. Carbonic acid 444 is formed as C02 from the gas stream becomes hydrated by aqueous solution 432. As the gas stream 412a passes over wetted carbonate 452a, further carbonic acid 444 is formed and a carbonate-carbonic acid reaction occurs. Prior to exiting from the reactor vessel 400, C02-depleted gas stream 422 may be passed through a moisture-eliminator 408. Agitating means 476 may be located in the upper region of reactor vessel 400 to enhance contact between the gas stream 412a and aqueous solution 432. Mixture 442 comprises unreacted carbonic acid 444, aqueous solution 432 and metal ion/bicarbonate solution 446 which have pooled in the lower region of reactor vessel 400.
Agitating means 474 may be located in the reactor vessel 400 lower region to enhance contact between the gas streams 412b and 412c, carbonate 452b, and mixture 442. Chemical additives, such as metal or ion chelators or chemical inhibitors, may be added to reduce contaminants that would interfere with the carbonic acid/carbonate reaction, 472b shown as being added to aqueous solution 432, and 472a shown as being added directly to the lower region of reactor vessel 400. Mixture 442 is bled from the reactor and disposed of as waste solution 442b and/or recirculated 442a. To prevent entrainment of large particulate carbonate, mixture 442 may be passed through a solid/liquid separation means, such as a filter, 406. Water 436b may be added to mixture 442a as it is recirculated back to atomizer/sprayer unit 404, or water 436a may be added directly to the lower region of the reactor vessel 400. Carbonate recharge means 434 is provided for adding carbonate to reactor 400. Carbonate 452b may be contained in a liquid- porous container 466 permeable to carbonic acid that is submerged in carbonic acid 444.
Container 466 may be stationary or in motion relative to said carbonic acid 444. Reactor vessel 400 may further contain agitating means 468 which vibrates or shakes the lower region of reactor vessel 400. Aqueous solution 432 may be passed through cooling means 495 to enhance carbonate reactivity. Mixture 442 may be passed through cooling means 496 to enhance carbonate reactivity. Cooling means 497 serves to cool the gas present in the upper region of reactor vessel 400 to enhance carbonate reactivity. Passive cooling means 498 serves to remove heat from the interior of reactor vessel 400 to enhance carbonate reactivity. Pressure means 499 serves to increase the total pressure in reactor vessel 400 to enhance carbonate reactivity.
Carbonate Handling
The carbonate is introduced/replenished in the reactor either as dry particles, as wetted or immersed particles, or dissolved in ionic form in water or other appropriate aqueous solution using methods well known in the art of solids or solution transport including but not restricted to conveyor belts, pumps, blowers, and sprayers (e. g., D. A Dahlstrom and others,"Liquid- Solid Operations and Equipment", In : R. H. Perry, D. W. Green, and J. O. Maloney (eds.) Perry's Chemical Engineers'Handbook, 7th Ed. , McGraw-Hill, New York, pg. 18-1 to 18-133, 1997. ). The introduction of the carbonate may occur vertically, horizontally, or at some other angle relative to the reactor bed. Carbonate may be added to the reactor by various means, including a passive gravity feed or stream, or by active pumping, spraying, blowing, or conveying of dry, wet, or immersed carbonate particles or carbonate solution into the headspace or into the lower region of the reactor, in batch mode or continuous feed mode. In other embodiments, particulate carbonate within the reactor may be held by or fastened onto structures such as static or moving porous materials, grates, grills, plates, screens, wires, mesh, beads, balls, fins, vanes, conveyor belts, or held in liquid-porous containers, whose purpose is to facilitate carbonate addition to the reactor. As was stated earlier, lower temperatures facilitate the carbonate-carbonic acid reaction. Therefore it may be advantageous to replenish the carbonate with carbonate at a lower temperature than that in the carbonate reaction vessel.
Further embodiments of the present invention comprising carbonate handling means, including means to add carbonate to the reactor vessel are shown in FIG. 5. Although the embodiments are depicted for the integrated configuration, these carbonate replenishment means are equally applicable to the sequential configuration as well. Unless otherwise stated, similarly numbered components are analogous to components in the previous figures. Aqueous solution 532 is introduced into the upper region of reactor vessel 500 via atomizer/sprayer unit 504. Aqueous solution 532 may be introduced directly into the lower region of the reactor vessel 500 as shown. Gas stream 512a enters the upper region of reactor vessel 500 where it contacts aqueous solution 532, which serves to hydrate CO2, forming carbonic acid 544.
Aqueous solution 532 that is unreacted and carbonic acid 544 travel by gravity flow to the lower region of reactor vessel 500 where they impinge and wet carbonate 552 contained therein. The gas stream 512b enters the lower region of reactor vessel 500 and becomes hydrated as it bubbles through a region of unreacted aqueous solution 532 forming carbonic acid 544. Alternatively, gas stream 512b may be bubbled into a region of carbonate solution 552 whereby the CO2 hydration and carbonate - carbonic acid reactions occur in solution. The gas stream 512c enters the lower region of reactor vessel 500 and passes over or through wetted carbonate 552 wherein the CO2 hydration and carbonate - carbonic acid reactions occur
in close proximity. Prior to exiting from the reactor vessel 500, the CO2-depleted gas stream 522 may be passed through a moisture-eliminator 508. The carbonic acid solution 544 reacts with carbonate 552 to form metal ion/bicarbonate solution 546. Mixture 542 comprises metal ion/bicarbonate solution 546, unreacted aqueous solution 532, and unreacted carbonic acid 544. Mixture 542 is bled from the reactor and disposed of as waste solution 542b and/or recirculated 542a. Water 536b may be added to mixture 542a as it is circulated back to atomizer/sprayer unit 504, or water 536a may be added directly to the lower region of the reactor vessel 500. Carbonate recharge means 534b is provided for adding carbonate to mixture 542a as it is recirculated. Carbonate recharge means 534a is provided for adding carbonate to the lower region of reactor vessel 500. Carbonate recharge means 534c is provided for blowing or spraying fine carbonate particles (dry, wetted or dissolved) into the reactor vessel 500. Carbonate recharge means 534d is provided for pouring carbonate into reactor 500. Carbonate 534e may be introduced into reactor vessel 500 via a moving structure such as conveyor belt 514, entering via port 510. To prevent entrainment of large particulate carbonate, mixture 542 may be passed through a solid/liquid separation means, such as a filter, 506. A liquid-porous container 556 may be used to hold particulate carbonate 552 which would facilitate replenishment as the carbonate becomes depleted. In the present invention, the reactor vessel 500 may have a tapered bottom section 573 in which solid residue 576 resulting from the carbonic acid/carbonate reaction may collect. The bottom section 573 has a closeable port 575 which allows removal of the solid residue 576.
INDUSTRIAL APPLICABILITY The apparatus may be used in any application where the extraction and sequestration of at least part of the CO2 contained in a gas stream or volume is desired. Such applications include but are not limited to C02 extraction from waste gas associated with : i) the combustion or processing of coal, petroleum, natural gas, or other fossil fuel, or organic compounds derived from the preceding, ii) the combustion, processing, or metabolism of biomass such as wood, peat, plant products, or organic compounds derived thereof, iii) the decarbonation of limestone in the production of lime, cement, and gypsum. The process may also be used to reduce the CO2 burden in less concentrated sources such as bulk air.
The apparatus (or portions thereof) may be stationary or located on a ship, barge, train, truck, automobile or any other means of conveyance. Locating the apparatus (or portions thereof) on or near a body of water (e. g., navigable river, lake, sea, or ocean) would be advantageous because it could allow the relatively inexpensive over-water transport of
carbonate, provide a ready source of water for the reactor, and provide a place to dispose of the reactor's relatively benign liquid waste stream.
Applications include but are not limited to extracting and sequestering CO2from exhaust gas from stationary sources (power plants, process plants and factories) and various modes of transportation, including but not limited to seacraft, aircraft, and land based vehicles such as trains, trucks, buses, and automobiles. In addition, the present invention could be located downstream from an SO2 removal apparatus such as are well known in the art, to remove C02 from the gas stream after SO2 has been removed.
Obviously numerous modifications and variations of the present invention are possible in the light of the above teachings. It is therefore to be understood that within the scope of the appended claims the invention may be practiced otherwise than as specifically described therein.