U.S. patent application number 13/557701 was filed with the patent office on 2012-11-29 for carbon dioxide capture and mitigation of carbon dioxide emissions.
Invention is credited to Patrick Grimes, Samuel C. Krevor, Klaus S. Lackner, Frank S. Zeman.
Application Number | 20120302469 13/557701 |
Document ID | / |
Family ID | 35320781 |
Filed Date | 2012-11-29 |
United States Patent
Application |
20120302469 |
Kind Code |
A1 |
Lackner; Klaus S. ; et
al. |
November 29, 2012 |
Carbon Dioxide Capture and Mitigation of Carbon Dioxide
Emissions
Abstract
The present invention describes methods and systems for
extracting, capturing, reducing, storing, sequestering, or
disposing of carbon dioxide (CO.sub.2), particularly from the air.
The CO.sub.2 extraction methods and systems involve the use of
chemical processes, mineral sequestration, and solid and liquid
sorbents. Methods are also described for extracting and/or
capturing CO.sub.2 via condensation on solid surfaces at low
temperature.
Inventors: |
Lackner; Klaus S.; (Dobbs
Ferry, NY) ; Grimes; Patrick; (Scotch Plains, NJ)
; Krevor; Samuel C.; (Brooklyn, NY) ; Zeman; Frank
S.; (Brooklyn, NY) |
Family ID: |
35320781 |
Appl. No.: |
13/557701 |
Filed: |
July 25, 2012 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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13114365 |
May 24, 2011 |
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13557701 |
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11579713 |
Oct 25, 2007 |
7947239 |
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PCT/US2005/015453 |
May 4, 2005 |
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13114365 |
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60568091 |
May 4, 2004 |
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Current U.S.
Class: |
507/202 ;
423/437.1 |
Current CPC
Class: |
Y02C 20/40 20200801;
Y02P 20/151 20151101; C01F 11/18 20130101; B01D 53/62 20130101;
B01D 2251/404 20130101; B01D 2257/504 20130101; C01F 5/24 20130101;
Y02C 10/04 20130101; Y02P 20/152 20151101 |
Class at
Publication: |
507/202 ;
423/437.1 |
International
Class: |
C01B 31/20 20060101
C01B031/20; C09K 8/58 20060101 C09K008/58 |
Claims
1. A method for the capture of CO.sub.2 from ambient air, the
method comprising: exposing a solid sorbent material to ambient
air; capturing CO.sub.2 from the ambient air with said solid
sorbent material; recovering said CO.sub.2 from said solid sorbent
material; and subsequently collecting said recovered CO.sub.2.
2. The method of claim 1, wherein said solid sorbent material
comprises a matrix.
3. The method of claim 1, wherein said solid sorbent material
comprises a support material coated with one or more CO.sub.2
sorbents.
4. The method of claim 3, wherein said support material comprises a
matrix.
5. The method of claim 1, wherein said captured CO.sub.2 is
recovered by reducing pressure in a chamber containing said solid
sorbent material.
6. The method of claim 5, wherein pressure in said chamber is
reduced to near vacuum pressure.
7. The method of claim 1, wherein said collected CO.sub.2 is used
in the production of a fuel.
8. The method of claim 1, wherein said collected CO.sub.2 is used
in an enhanced oil recovery process.
9. The method of claim 1, wherein said collected CO.sub.2 is
sequestered underground.
10. The method of claim 1, where said collected CO.sub.2 is
pressurized.
11. The method of claim 1, further comprising using said collected
CO.sub.2 in a secondary process performed proximate to where said
collected CO.sub.2 is captured.
12. The method of claim 11, wherein said secondary process is
enhance oil recovery or sequestration.
13. The method of claim 11, wherein said solid sorbent material
comprises a zeolite.
Description
[0001] This patent disclosure contains material that is subject to
copyright protection. The copyright owner has no objection to the
facsimile reproduction by anyone of the patent document or the
patent disclosure, as it appears in the U.S. Patent and Trademark
Office patent file or records, but otherwise reserves any and all
copyright rights whatsoever.
[0002] All patents, patent applications and publications cited
herein are hereby incorporated by reference in their entirety. The
disclosures of these publications in their entireties are hereby
incorporated by reference into this application in order to more
fully describe the state of the art as known to those skilled
therein as of the date of the invention described and claimed
herein.
BACKGROUND OF THE INVENTION
[0003] The present invention relates generally to methods and
apparatuses for capturing, sequestering, storing, disposing of, or
entraining carbon dioxide (CO2), such as is found in the air and
the environment, as well as for mitigating carbon dioxide
emissions. In some aspects of the invention, the CO2 obtained by
the methods and apparatuses is isolated and stored or disposed of
to keep it from the air.
[0004] A serious environmental problem facing the world today is
global climate change, i.e., global warming, which has been linked
to the increased production of greenhouse gases, namely, carbon
dioxide (CO.sub.2). Growing evidence details the accumulation of
greenhouse gases in the air, the most important of which is
CO.sub.2, as having an associated role in causing global climate
warming. Since 2001, CO.sub.2 accounted for over 82% of all
greenhouse gas emissions in the United States. Nearly 60% of
CO.sub.2 is emitted by utility or industrial power systems, which
are based on fossil fuel combustion. A continuing increase in the
greenhouse gas CO.sub.2 in the air highlights the need to develop
cost effective, reliable and safe methods of CO.sub.2 (or carbon)
sequestration.
[0005] In order for carbon-rich fossil fuels, such as coal and
natural gas, to remain viable and environmentally acceptable energy
sources throughout the 21.sup.st century and beyond, new
technologies that employ capture and sequestration, utilization, or
recycling of CO.sub.2 need to be developed at reasonable costs. The
sequestration of CO.sub.2 would allow the use of carbon-based fuels
to meet the world's increased energy demands far into the future,
without further increasing the atmospheric concentration of
CO.sub.2. Additionally, for fossil fuels to maintain their
predominance in the global energy market, the disposal of CO.sub.2
and the elimination of CO.sub.2 emissions to the air are ultimate
goals for curbing the problem of global warming.
[0006] The present invention addresses the pervasive problems of
the release and presence of excessive amounts of CO.sub.2 in the
air and provides solutions to these problems in the form of methods
and apparatuses for extracting, capturing and sequestering CO.sub.2
and removing excess CO.sub.2 from the air.
SUMMARY OF THE INVENTION
[0007] It is a general aspect of the present invention to provide
new methods or processes for extracting, reducing, capturing,
disposing of, sequestering, or storing CO.sub.2 or removing excess
CO.sub.2 from the air, as well as new methods and processes for
reducing, alleviating, or eliminating CO.sub.2 in the air, and/or
the emissions of CO.sub.2 to the air. Another aspect of the
invention relates to apparatuses, such as wind or air capture
systems, to remove or extract CO.sub.2 from air. As used herein,
the term "air" refers to ambient air, rather than emitted gas, such
as gas that is emitted from a smoke stack or an exhaust pipe. While
the latter may contain air, it is not typically considered ambient
air. In accordance with the present invention, extraction of
CO.sub.2 from air involves source gas, which is at atmospheric
temperature, pressure and ambient concentration of CO.sub.2.
[0008] In accordance with an aspect of this invention, a process
involving acidic pH and elevated temperature for sequestering
CO.sub.2 as solid carbonate materials, e.g., magnesium carbonate,
is provided. Suitable acids for this process are those that
dissolve magnesium bearing silicates, such as serpentine, or
olivine. The acidic solution formed contains dissolved magnesium
salts, as well as some silica and dissolved iron salts. The acidic
solution is neutralized to remove dissolved silica, and the
dissolved iron salts are precipitated out as iron oxides and/or
hydroxides. According to one aspect of the method, the magnesium
salts in the solution are transformed into ammonium salts via
precipitation of magnesium by the addition of ammonia-containing
reagents, such as ammonium hydroxide, ammonium carbonate, or
ammonium bicarbonate. Unless otherwise defined, the terms "method"
and "process" are used interchangeably throughout this
disclosure.
[0009] In an aspect of the present invention, a method of
extracting or sequestering carbon dioxide is provided. The method
comprises (a) dissolving a magnesium bearing silicate in an aqueous
acid to form an acidic solution; (b) increasing the pH of the
solution of step (a) to precipitate one or more magnesium
components; and (c) carbonating the precipitated magnesium
components from step (b) to bind carbon dioxide. In another aspect
the invention provides a method of extracting or sequestering
carbon dioxide, comprising: (a) dissolving a magnesium bearing
silicate in an aqueous acid to form an acidic solution; (b)
increasing the pH of the solution of step (a) to precipitate one or
more magnesium components; (c) carbonating the precipitated
magnesium components from step (b) to bind carbon dioxide; and (d)
recovering ammonia gas and acid by thermal decomposition, e.g.,
heating, or by electrodialysis.
[0010] In another aspect, the present invention provides a method
or process of extracting, sequestering, or capturing carbon
dioxide. The process comprises (a) dissolving a magnesium bearing
silicate in an aqueous acid to form an acidic solution; (b)
neutralizing the acidic solution to remove partially-dissolved
silica and produce a dissolved magnesium component; (c)
precipitating a solid magnesium component from the neutralized
solution with an ammonia containing reagent, thereby producing an
ammonium salt in the solution; (d) precipitating the ammonium salt
from the solution; and (e) carbonating the precipitated magnesium
component to sequester or eliminate carbon dioxide, e.g., from the
air. In addition, ammonia gas and acid (in liquid form) can be
recovered in the method by thermal decomposition, e.g., heating, or
by electrochemical methods.
[0011] In another aspect, the present invention provides a method
of extracting. sequestering, reducing, or eliminating carbon
dioxide involving (a) dissolving a magnesium bearing silicate in an
aqueous acid to form an acidic solution; (b) neutralizing the
acidic solution with a neutralizing agent to precipitate iron and
silicate; (c) precipitating magnesium from the solution with a
base, such as, for example, an ammonia-containing reagent; and (d)
carbonating the precipitated magnesium component from step (c) to
extract, sequester, reduce, or eliminate carbon dioxide. In
addition, thermal decomposition or electrochemical processes can be
used to recover ammonia and acid. As used herein, a base is a
water-soluble compound, or aqueous solution comprised therefrom,
that is capable of reacting with an acid to form a salt.
Illustratively, such compounds comprise molecules, substances, or
ions able to take up a proton from an acid, or able to give up an
unshared electron pair to an acid. Basic solutions comprising the
methods of the invention generally have a pH above about 7.
[0012] In another of its aspects, the present invention provides
processes for extracting carbon dioxide from the air using solid or
liquid sorbents that bind CO.sub.2. Examples of solid sorbents
include, without limitation, activated carbon, zeolites, or
activated alumina. Examples of liquid sorbents include, without
limitation, high pH solutions, such as sodium hydroxide solution,
potassium hydroxide solution, or organic solvents, e.g.,
monoethanolamine (MEA), or SELEXOL.RTM..
[0013] In another aspect, the present invention provides a method
for extracting or capturing carbon dioxide from air using a solid
absorber or sorbent material. The method involves (a) exposing a
carbon dioxide absorber material comprising a large absorption
surface to the air until the absorber material is saturated, or
nearly saturated, with carbon dioxide; (b) removing remnant air
from the saturated absorber material under vacuum or reduced
pressure; (c) condensing the carbon dioxide on a cold surface to
capture the carbon dioxide, e.g., in a solid form; and (d)
releasing the captured carbon dioxide to a system for collection,
storage, or transport. In accordance with this method, the solid
sorbent material can comprise materials, objects, or substances,
such as beads, rods, fabric, or moveable objects comprising rough
surfaces, that move while exposed to air. In one aspect of the
method, the carbon dioxide solid sorbent comprises a material that
absorbs CO.sub.2 throughout its entirety. Such a sorbent is
preferably hydrophobic. Alternatively, the carbon dioxide sorbent
comprises an inert material that is coated or covered with one or
more CO.sub.2 sorbents. Further in accordance with the method of
this aspect of the invention, steps (b) and (c) can be performed in
a first and second vacuum chamber, respectively, as described
further herein. In addition, absorbed CO.sub.2 can be released and
captured in condensed form, i.e., dry ice, in a cold trap serving
as the cold surface.
[0014] In a further aspect, the present invention provides a
cryogenic carbon dioxide capturing or entrapping system comprising
(a) a first chamber, or evacuation chamber, that houses carbon
dioxide sorbent material, which is laden with carbon dioxide, for
example, or on which carbon dioxide is captured; (b) a vacuum
system which connects to the first chamber, partially reduces
pressure therein and removes remnant air from the sorbent material;
and (c) a second chamber which is connected to the first chamber
and which has a temperature suitable for condensation and
collection of carbon dioxide from the first chamber as solid carbon
dioxide onto one or more surfaces in the second chamber. For
example, the temperature of the second chamber can be about
-80.degree. C. or about -100.degree. C. or lower. The second
chamber can comprise a reduced partial pressure relative to the
first chamber.
[0015] In another aspect, a method of sequestering CO.sub.2 in
ocean waters is described and involves the calcining of a material,
such as limestone, dolomite, or carbonate to capture CO.sub.2 from
the air, for ultimate disposal in the ocean. According to this
method, the alkalinity of the ocean surface is raised by the
introduction (e.g., by injection) of metal oxide and/or metal
hydroxide-containing materials such as, without limitation,
MgO/CaO, Mg(OH).sub.2/Ca(OH).sub.2, MgO/CaCO.sub.3, or
Mg(OH).sub.2/CaCO.sub.3. Such metal oxide materials are obtained by
a calcination process and can lead to the additional capture of two
moles of CO.sub.2 for every mole of CO.sub.2 entered into the
system, as described herein. In this aspect, the CO.sub.2 liberated
in the calcination process plus the CO.sub.2 resulting from the
energy consumption of the calcination process is captured and
disposed of. Accordingly, a method for removing carbon dioxide from
air is provided, involving (a) calcining a metal carbonate- (e.g.,
calcium carbonate and/or magnesium carbonate) containing material
to obtain one or more metal hydroxide calcination products; (b)
introducing the calcination products of step (a) into a body of
water so that the calcination products dissolve at or near the
water surface; and (c) increasing the alkalinity of the water so as
to capture at least two times the amount of carbon dioxide that is
released by the calcining of step (a). After their production, the
calcination products of step (b) can be finely dispersed into ocean
or seawater from one or more vessels that drag behind or between
them a line that drops a fine powder in the water, as described
herein. Alternatively as described, the calcination products are
fashioned into larger pellets that are dropped or ejected into the
water. Such pellets can comprise CaCO.sub.3/MgO mixtures and
disperse and dissolve at the water's surface.
[0016] In another of its aspects, the present invention provides a
method of carbon capture that removes CO.sub.2 from air. The method
also advantageously serves to regenerate the sorbent employed in
the method. The method involves the use of an alkaline liquid
sorbent, e.g., sodium hydroxide (NaOH)-based, to remove CO.sub.2
from ambient air and produce carbonate ions. The resultant sodium
carbonate (Na.sub.2CO.sub.3) solution is mixed or reacted with
calcium hydroxide (Ca(OH).sub.2) to produce sodium hydroxide and
calcium carbonate (CaCO.sub.3) in a causticizing reaction, which
transfers the carbonate anion from the sodium to the calcium
cation. The calcium carbonate precipitates as calcite, leaving
behind a regenerated sodium hydroxide sorbent, thus regenerating
the sorbent. The calcite precipitate is dried, washed and thermally
decomposed to produce lime (CaO) and gaseous CO.sub.2 in a
calcination process. Thereafter, the lime is hydrated (slaked) to
regenerate the calcium hydroxide sorbent. In a related aspect, this
method can be implemented using air capturing systems, for example,
towers or air or wind capture units of various design, which
function as the physical sites where CO.sub.2 is captured and
removed from the air.
[0017] In another aspect, the present invention provides a method
for extracting or capturing carbon dioxide from air, comprising:
(a) exposing air containing carbon dioxide to a solution comprising
a base, resulting in a basic solution which absorbs carbon dioxide
and produces a carbonate solution; (b) causticizing the carbonate
solution with a titanate-containing reagent; (c) increasing the
temperature of the solution generated in step (b) to release carbon
dioxide; and (d) hydrating solid components remaining from step (c)
to regenerate the base comprising step (a).
[0018] In another aspect, the present invention provides a method
for extracting or capturing carbon dioxide from air comprising: (a)
exposing air containing carbon dioxide to a solution comprising a
base, thus resulting in a basic solution which absorbs carbon
dioxide and produces a carbonate solution; (b) causticizing the
carbonate solution with a calcium hydroxide containing reagent; (c)
calcining the resulting calcium carbonate under thermal conditions
in which one or more mixed solid oxide membranes is interposed
between the combustion gases and the input air; and (d) hydrating
the product lime to regenerate the calcium hydroxide involved in
step (b).
[0019] In yet another aspect the present invention provides systems
and apparatuses for extracting, capturing, removing, or entraining
CO.sub.2 from the air. Such capture apparatuses can include wind
and air capture systems or a cooling-type tower for extracting,
capturing, removing, or entraining CO.sub.2 as further described
herein. Fan driven systems are also encompassed.
[0020] Additional aspects, features and advantages afforded by the
present invention will be apparent from the detailed description,
figures, and exemplification hereinbelow.
DESCRIPTION OF THE DRAWINGS
[0021] FIG. 1 presents a schematic depiction of an overview of an
air extraction process. Drying (d) and hydrating (h) are not
specifically shown. In accordance with an embodiment of the present
invention, such a process is functionally integrated into an air
capture system.
[0022] FIGS. 2A and 2B schematically depict a type of CO.sub.2
capture system that is adapted to wind flow, e.g., venturi flows.
In FIGS. 2A and 2B, the thick black lines represent a solid
structure as seen from above. As the air moves through the narrowed
passage, the pressure drops (Venturi Effect). As a result the
higher pressure air inside the enclosures that are open to the back
of the flow have a tendency to stream into the low pressure air
flow.
DETAILED DESCRIPTION OF THE INVENTION
[0023] The present invention generally relates to carbon dioxide
(CO.sub.2) extraction, reduction, capture, disposal, sequestration,
or storage, particularly from the air, and involves new processes
and apparatuses to reduce or eliminate CO.sub.2, e.g., greenhouse
gas CO.sub.2, from the environment. Both air extraction and mineral
sequestration of CO.sub.2 are encompassed by the invention. The
processes and systems described herein are aimed at effective and
efficient carbon management, including cost effectiveness and
efficient heat management resulting from the processes. Such
processes and systems have been developed to extract or remove
CO.sub.2 from air, where, for example, the CO.sub.2 concentration
is approximately 0.037%. Thus, the processes and products of the
invention provide useable and economically viable technologies for
tackling and handling the escalating problem of global warming.
[0024] In one embodiment, the present invention encompasses a
chemical process for extracting, reducing, or sequestering CO.sub.2
by generating solid carbonate minerals, silica and water from basic
rock materials and carbon dioxide. Advantageously, the process can
eliminate the energy penalty that can result from other
implementations of mineral sequestration. The CO.sub.2 disposal
process according to this invention combines an alkaline base, for
example, in the form of magnesium oxide or hydroxide extracted from
peridotite rock, with CO.sub.2 to form stable magnesium carbonate.
The overall chemical process is energy neutral and can achieve the
consumption of only mineral rock and carbon dioxide.
[0025] The process according to this embodiment involves dissolving
or digesting magnesium bearing silicate minerals (e.g., peridotite
rock), such as serpentine, olivine, etc., at an acidic pH to
extract magnesium ions. In acidic solution, magnesium dissolves, as
do some of the silicate and various iron salts; other silicates can
also precipitate out in acidic solution. Suitable acids for this
process are those that dissolve peridotite rock, and can be strong
or weak acids, as conventionally known in the art. Suitable acids
have a pH in the range of about pH 4.5 to pH 5 or less, or molar
concentrations in excess of 0.00001 moles per liter of hydrogen
ions. Illustrative examples of acids for use in the process
include, without limitation, hydrochloric acid, sulfuric acid,
ammonium bisulfate, citric acid, chromic acid, phosphoric acid,
acetic acid and combinations thereof. Also illustratively, a
combination of weaker acids, e.g., orthophosphoric acid and oxalic
acid, can be used. The acidic solution that is formed is separated
from undissolved residues using a counterstream washing operation.
For example, the residues are filtered from the acidic solution and
the filtrate is rinsed with water to remove any acid that is
entrained with the solids.
[0026] The pH of the acidic solution is increased using an alkaline
reagent or base. For example the acidic solution is neutralized
with an amount of base sufficient to raise the solution pH to about
7. As the pH of the solution is increased, silica and iron
hydroxide are precipitated out as iron oxides and/or hydroxides,
thus yielding a "brine" solution of a magnesium salt of the acid.
After neutralization, the dissolved magnesium preferably remains in
solution, although the pH of the solution is close to that at which
the Mg salt precipitates, e.g., Mg precipitates in the form of
magnesium hydroxide when the solution pH exceeds about pH 8. Some
magnesium salts may be even less soluble. The neutralized solution,
e.g., a clear brine, is then titrated with a basic solution, such
as NaOH, KOH, NH.sub.3, NH.sub.4OH, or carbonates, as nonlimiting
examples. The increase in solution pH results in the precipitation
of the magnesium component, e.g., Mg(OH).sub.2. If, for example,
the base used for titration is a hydroxide, e.g., NaOH, KOH,
NH.sub.4OH, the magnesium precipitate constitutes an Mg(OH).sub.2
product, which can be carbonated. Alternatively, if the base is a
carbonate, for example, (NH.sub.4).sub.2CO.sub.3 or
Na.sub.2CO.sub.3, then the precipitate is a carbonate, e.g.,
MgCO.sub.3.
[0027] Prior to carbonation, the precipitated magnesium component
product, e.g., an Mg(OH).sub.2 product, can be washed, e.g., using
a counterstream operation, to remove the salt from the resulting
filter cake. Following the precipitation of the magnesium
component, e.g., Mg(OH).sub.2, a salt (e.g., NaCl) brine remains
and is subsequently processed as described below. More generally,
the salt mixture in this aspect is comprised of the cation
introduced in the titration/precipitation step and the anion
introduced in the above-described first step. The remaining salt
preferably remains in solution, at least until the magnesium
component, e.g., Mg(OH).sub.2, is removed. In one embodiment, the
salt in solution is freed from any dissolved impurities by
crystallizing or precipitating the impurities from the salt, which
preferably stays in solution.
[0028] In a specific embodiment, a Mg(OH).sub.2 or MgCO.sub.3
magnesium component is precipitated with a base comprising an
ammonia-containing reagent. Illustratively, ammonia, ammonium
hydroxide, ammonium carbonate, or ammonium bicarbonate can serve as
ammonia-containing reagents for use in the process to precipitate a
magnesium component, such as Mg(OH).sub.2 or MgCO.sub.3; the
negative ion (anion) from the magnesium salt is then transferred
from the magnesium to the ammonium ion creating ammonium salts,
e.g., ammonium sulfate, via precipitation by the addition of the
ammonia base reagent to increase or raise the pH of the acidic
solution.
[0029] In one embodiment involving the use of an ammonia reagent, a
step of the process involves increasing the temperature, e.g., by
heating, of the ammonium salt to recover the ammonia and the acid.
The temperature of heating depends upon the ammonium salt used. In
general principle, the ammonia salt is heated to relinquish the
ammonia gas, leaving behind an acid or anhydride of an acid. For
example, to transform a mixture of ammonium sulfate and water into
ammonium bisulfate and ammonia, the mixture is heated to about
350.degree. C. Heating serves to carbonate solid magnesium
hydroxide or magnesium carbonate with gaseous carbon dioxide. Thus,
for example, in one embodiment, the ammonium salt can be
precipitated from the solution by reducing the volume of water. The
reduction in water volume can occur by evaporation, membrane
separation, and the like. In another embodiment, the ammonium salt
can be precipitated from the solution by changing the temperature.
For example, lowering the solution temperature causes the
solubility product to be exceeded. This results in the
precipitation of the salt. Lowering of the temperature can be
achieved by allowing the solution to reach room temperature, or by
other means known to those skilled in the art. The precipitated or
solid ammonium salt is then heated to a temperature sufficient to
dissociate the particular salt, for example, about 100.degree. C.
or greater. This releases free ammonia. Thus, having lost its base,
the ammonium salt reverts to an acid and comprises an acid that is
solid in an anhydrous form, e.g., ammonium bisulfate. The acid and
ammonia can be recovered, e.g., by combination with a gas/solid
carbonation cycle as described below, or by absorption into water,
and the magnesium silicate is converted to silica and magnesium
hydroxide.
[0030] Another embodiment of the process involves carbonating the
precipitated magnesium component. Carbonation can occur by bringing
together solid magnesium hydroxide with gaseous carbon dioxide. The
two materials react to form magnesium carbonate and steam in an
exothermic reaction, which can proceed at elevated temperatures. An
advantage of this process is that the above-described acid/ammonia
cycle is combined with a gas/solid carbonation cycle that provides
all or part of the heat for the recovery of the acid and the
ammonia.
[0031] After generating a solid magnesium precipitate product,
e.g., a magnesium hydroxide product, this product is carbonated at
elevated pressure and temperature, for example, in an autoclave
system. In such systems, the temperature is typically about
400.degree. C. or 500.degree. C. or above, and the pressure is
typically about 1 atmosphere, (ambient pressure), to about 50
atmospheres or greater. In the process, magnesium hydroxide reacts
with gaseous CO.sub.2 in a carbonation cycle. The reaction is
exothermic; and with appropriate heat management, for example, the
use of suitable heat exchangers and/or the design of a physical
plant to allow efficient heat transfer, this heat can be collected
and applied to the recovery of the ammonia and the acid in the
method. The heat of the reaction is thus harnessed and utilized.
For added efficiency, the system embraces a counterstream heating
of reactants with the sensible heat stored in the reaction
products. To capture and essentially dispose of CO.sub.2, the
ammonia and carbon dioxide cycles can be "tied together" and the
energy in both can be shared. Since the overall reaction is
exothermic, this can provide a substantial reduction in the energy
penalty incurred by currently-performed mineral carbonation
processes.
[0032] In another embodiment, the above-described process can
include the electrochemical recovery of ammonia gas and acid, e.g.,
by the use of electrodialysis. For, example, electrochemical
recovery comprises introducing the salt brine into a dialysis
apparatus, for example, a conventional electrodialysis apparatus
that separates positive and negative ions with an applied current,
thereby effectively re-creating the acid and the base of the above
process. The brine can be diluted prior to dialysis, as a higher
dilution typically requires the use of smaller electromotive
forces. The electromotive force serves to separate the anions and
cations of the salt and to recreate the acid and base from which
the salt was formed. A more concentrated solution will have more
ions per unit volume and will require more electromotive force to
separate the ions. In general, the stronger the acid and the base,
the higher the pH change that needs to be maintained in the
apparatus, which typically is comprised of one or more cells, or
one or more stacks of cells to reduce energy demand. The cells are
comprised of bipolar membranes and cationic and anionic membranes,
as known to and used by skilled practitioners in the art. For
example, in a cell, the bipolar membrane can be situated between
the cationic and anionic membranes. Also, the cationic and anionic
membranes can be alternating.
[0033] Illustratively, the basic dialysis unit contains one
cationic and one anionic membrane with one bipolar membrane on
either end. The bipolar membrane splits water into hydroxide and
hydrogen ions, which allows the acid and base to reform. In
addition, hydrogen and oxygen gas are formed. A stack of cells
decreases the amounts of these gases that are formed per unit
acid/base recovered, and therefore improves the economics of the
process and system. If the acid is a weak acid, e.g., oxalic acid,
then the acid does not completely disassociate; thus, the pH on the
acidic side stays higher, compared with a strong acid, e.g., HCl.
If the positive ion is a weak base, which limits the formation of
free OH.sup.- ions in the brine, then the pH remains lower than for
a strong base, e.g., NaOH.
[0034] When ammonia is involved in the electrodialysis system, the
ammonium (NH.sub.4) ion itself dissociates into NH.sub.3 and
H.sup.+, and the NH.sub.3 leaves the solution as a gas, which can
be re-captured and re-used in the above-described process. As
ammonia is a weak base, the use of ammonia limits the pH of the
solution such that the pH is not raised as high as by using a
stronger base, such as NaOH. In general, the lower the pH
difference between the acid and the base sides of the dialysis
system, the lower the electromotive force that is required.
Therefore, the energy penalty is lowered in the process involving
electrodialysis. Accordingly, ammonia reagents, such as ammonium
oxalate, ammonium citrate, ammonium sulfate, ammonium hydroxide,
ammonium carbonate, or ammonium bisulfate are advantageous in the
method described herein. In addition, at the end of the dialysis,
water can be removed, for example, by the use of osmotic membranes
that reduce the water content of the solution as it leaves the
dialysis unit.
[0035] In another embodiment related to the above process for
extracting or capturing CO.sub.2 from air, the base can be
carbonated prior to its introduction into the brine comprising
magnesium according to the above process. For example, sodium
hydroxide could be exposed to air and absorb CO.sub.2 from the
ambient air. If this sodium hydroxide is then introduced into the
process, carbonates would immediately be formed in the process,
prior to the precipitation of magnesium components.
[0036] In another embodiment, magnesium hydroxide resulting from
the above-described process is washed to remove residual salt,
e.g., sodium chloride, and is exposed to CO.sub.2 above ambient
temperature (e.g., about 25.degree. C.) and pressure (about 1 atm
or 1 bar) to produce magnesium carbonate. In this embodiment, the
carbonation reaction can operate at elevated temperatures, for
example, from about 300.degree. C. to less than about 900.degree.
C., or from about 300.degree. C. to about 500.degree. C., and
pressures of about 1 (i.e., ambient) to 50 atmospheres. Higher
pressures and temperatures are also encompassed, although it is to
be appreciated that the high cost of pressurization may render
higher pressures undesirable. Preferably, the heat content from the
reaction products (e.g., steam and magnesium carbonate) is
transferred to the reactants (e.g., CO.sub.2 and magnesium
hydroxide to avoid the loss of the heat of the reaction into the
heating of the reactants. This is accomplished using commercially
available heat exchangers, which are units built specifically to
allow two materials to transfer heat without physically contacting
each other, such as in a car radiator. The reaction can deliver a
substantial amount of heat energy if the incoming CO.sub.2 is
heated against the outgoing steam. In addition, the gases can be
used to transfer heat between the incoming and outgoing solids. For
example, if the reactants enter a vessel at its operating
temperature, then any heat that is generated can be recovered. The
heat generated by the reaction vessel, e.g., 68 kJ/mole of CO2, can
be used to generate steam and/or electricity for further use. Thus,
the processes as described ideally expend minimal energy to heat
the reactants so that this energy is more easily recovered.
[0037] In another embodiment, the magnesium carbonate is gently
sintered during the cool-down process. Sintering reduces the
reactivity of a solid; thus, if magnesium carbonate, for example,
is sintered, it is less likely to dissolve when exposed or
subjected to water.
[0038] Suitable acids for use in the above-described process for
sequestering, reducing, and/or eliminating CO.sub.2 include those
that can dissolve serpentine, olivine, or similar magnesium bearing
silicates. Illustratively, citric acid, oxalic acid, acetic acid,
chromic acid, sulfuric acid, orthophosphoric acid, oxalic acid,
ammonium bisulfate and combinations thereof, are nonlimiting
examples of suitable acids for use in the process. Dissolution by
the acid should occur in an aqueous system, preferably with a high
concentration of acid. A suitable concentration range for the acids
comprises, for example and without limitation, from about 0.01
mol/L to about 10 mol/L. The reaction should progress rapidly, for
example, in minutes to hours, depending on the strength of the acid
and the fineness of the powder resulting from the dissolution
process. For example, dissolution using HCl, an exemplary strong
acid, can occur in 5 minutes or less for a fine powder. For a
weaker acid, e.g., citric acid, dissolution can take 5 minutes or
longer to several hours. Heat need not be a component of the
reaction; however, the smaller the amount of heat released in the
process, the more advantageous the acid. Illustratively, and
without limitation, acid used for dissolving or digesting in the
methods may be present in an amount that is about 1% to about 20%,
or about 10%, in excess of the stoichiometric amount.
[0039] For the above CO.sub.2 sequestration, reduction, and/or
elimination process, the magnesium and iron salts of the acids are
preferably relatively soluble in water. The solubility of the
ammonia salt is a strong function of the temperature, i.e.,
changing the temperature of the solution is an efficient way to
recover the salt. A suitable precipitate of the ammonium salt is
free of water and the anhydrous form of the acid is a solid. In
addition, the ammonium salt can comprise ammonia and the anhydrous
form of the acid. The lower the temperature of the ammonia release,
the better the salt is for use in the process. Similarly, to
reconstitute the salt, the heat of formation of the ammonia salt is
preferably small. Accordingly, for optimum operating conditions,
lower temperatures and less water in the process result in less
energy expended in carrying out the process.
[0040] In an embodiment, the present invention encompasses a method
of extracting or sequestering carbon dioxide, for example, from
air, comprising (a) dissolving a magnesium bearing silicate in an
aqueous acid to form an acidic solution; (b) increasing the pH of
the solution of step (a) to precipitate one or more magnesium
components; and (c) carbonating the precipitated magnesium
components from step (b) to bind carbon dioxide. In another
embodiment, the method comprises (d) recovering ammonia gas and
acid, e.g., by thermal decomposition or by electrodialysis. In an
embodiment, the method the magnesium bearing silicate of step (a)
comprises peridotite rock, which can be, for example, serpentine or
olivine. In an embodiment, the aqueous acid can be citric acid,
acetic acid, chromic acid, sulfuric acid, orthophosphoric acid,
oxalic acid, ammonium bisulfate, or a combination of two or more
thereof. In an embodiment, the pH of the acidic solution is less
than or equal to about pH 4.5. In another embodiment, the method
can further comprise neutralizing the acidic solution of step (a)
with a neutralizing agent to precipitate iron and silicate. In an
embodiment iron and silicate are precipitated prior to the
precipitation of one or more magnesium components. In an
embodiment, this neutralizing step comprises a neutralizing agent,
which can be ammonia or magnesium hydroxide. In an embodiment, the
pH of the neutralized solution is less than or equal to about pH 8.
In an embodiment, increasing the pH in step (b) involves the use of
an ammonia-containing reagent; the reagent can be selected from
NaOH, KOH, NH.sub.3, NH.sub.4OH, (NH.sub.4).sub.2CO.sub.3,
NH.sub.4HCO.sub.3, Na.sub.2CO.sub.3, or a combination thereof. In
another embodiment, the method can include the use of a reagent
that is carbonated prior to precipitating the one or more magnesium
components in the step of increasing the pH of the acidic solution.
In another embodiment, the ammonia-containing reagent is selected
from ammonia, ammonium hydroxide, ammonium carbonate, or ammonium
bicarbonate. In another embodiment, the reagent that is carbonated
prior to precipitating the one or more magnesium components is an
ammonia-containing reagent. In other embodiments, the one or more
magnesium components of step (b) is Mg(OH).sub.2 or Mg(CO).sub.3.
In an embodiment, the one or more precipitated magnesium components
is carbonated in step (c) by thermal decomposition, e.g., heating,
which can be at a temperature of about 300.degree. C. to less than
about 900.degree. C., or about 300.degree. C. to about 500.degree.
C., or carried out in an autoclave under pressure, e.g., at about 1
to about 50 atmospheres or greater. In an embodiment, the method
can further comprise, following step (c), (i) washing the
precipitated magnesium component to remove residual salt; and (ii)
exposing the precipitated and washed magnesium component to carbon
dioxide at elevated temperature and pressure. Elevated temperature
can be about 300.degree. C. to less than about 900.degree. C., or
about 300.degree. C. to about 500.degree. C., and the elevated
pressure can be about 1 to about 50 atmospheres. In an embodiment,
precipitated magnesium component in the methods is magnesium
hydroxide or magnesium oxide. In another embodiment, the method
further comprises recovering ammonia gas and acid following step
(c) by thermal decomposition, e.g., heating, or by electrodialysis.
In another embodiment of the method, the acid of step (a) is
present in an amount at least about 10% in excess of a
stoichiometric amount for neutralizing magnesium in the magnesium
bearing silicate.
[0041] In another embodiment, the present invention embraces a
method of extracting, sequestering, reducing, or eliminating carbon
dioxide involving (a) dissolving a magnesium bearing silicate in an
aqueous acid to form an acidic solution; (b) neutralizing the
acidic solution with a neutralizing agent to precipitate iron and
silicate; (c) precipitating magnesium components, e.g., Mg salt,
from the solution with a base, such as, for example, an
ammonia-containing reagent; and (d) carbonating the precipitated
magnesium component from step (c) to sequester, reduce, or
eliminate carbon dioxide. In addition, thermal decomposition, e.g.,
heating, or electrochemical processes can be used to recover
ammonia and acid. In this method the magnesium bearing silicate of
step (a) can comprise peridotite rock, such as serpentine or
olivine. Examples of acids suitable for use in the method include,
but are not limited to, citric acid, oxalic acid, orthophosphoric
acid, acetic acid, chromic acid, sulfuric acid, or ammonium
bisulfate, which can provide an aqueous acidic solution with a pH
of about pH -1 to about pH 4.5, or a pH of less than about 4.5.
Suitable yet nonlimiting neutralizing agents for use in the method
include ammonia or magnesium hydroxide, which can provide a
neutralized solution with a pH of near 7 for purposes of
precipitating out impurities like iron and silicates. Bases other
than Mg(OH).sub.2 or Mg salts can be used to precipitate magnesium
from the solution, as they will drive the solution to pH values
from about pH 8-9 (e.g., ammonia) to about pH 14 (e.g., NaOH). In
addition, the base used in the method can be an ammonia-containing
reagent, e.g., without limitation, NH.sub.4OH, NH.sub.3,
(NH.sub.4).sub.2CO.sub.3, or NH.sub.4HCO.sub.3, or a combination
thereof. Other bases can include NaOH, KOH, or Na.sub.2CO.sub.3, or
a combination thereof.
[0042] In an embodiment, the base of the above step (c) is
carbonated prior to precipitating the one or more magnesium
components. In an embodiment, the magnesium component in step (c)
is Mg(OH).sub.2 or Mg(CO).sub.3. In an embodiment, the
precipitating agent is an ammonia-containing reagent, such as,
without limitation, ammonia, ammonium hydroxide, ammonium
carbonate, or ammonium bicarbonate. In another embodiment,
following step (c) of the method, the precipitated magnesium
component is washed, e.g., by flushing the isolated filtrate with
water, to remove residual salt, and the precipitated and washed
magnesium component is exposed to carbon dioxide at an elevated
temperature, e.g., about 300.degree. C. to less than about
900.degree. C., and pressure, e.g., about 1 to about 50
atmospheres, or greater. Exposure of the magnesium component to
carbon dioxide can be performed in an autoclave or in any other
high temperature solids reactor. In another embodiment, the
precipitated magnesium component in the method is carbonated in
step (d) by thermal decomposition, e.g., heating, for example, at a
temperature of about 300.degree. C. to less than about 900.degree.
C., or about 300.degree. C. to about 500.degree. C., at atmospheric
pressure or elevated pressure, such as in an autoclave. In another
embodiment, electrochemical processes are employed to recover
ammonia and acid.
[0043] In another embodiment, a method of CO.sub.2 extraction
comprises (a) dissolving a magnesium bearing silicate in an aqueous
acid to form an acidic solution; (b) neutralizing the acidic
solution to remove dissolved silica and produce a dissolved
magnesium component; (c) precipitating a solid magnesium component
from the neutralized solution with an ammonia containing reagent,
thereby producing an ammonium salt in the solution; (d)
precipitating the ammonium salt from the solution; and (e)
carbonating the precipitated magnesium component to extract,
sequester, eliminate, or reduce the carbon dioxide. In this
embodiment, the magnesium bearing silicate of step (a) can comprise
peridotite rock, such as serpentine or olivine. Examples of acids
suitable for use in the method include, but are not limited to,
citric acid, oxalic acid, orthophosphoric acid, acetic acid,
chromic acid, sulfuric acid, ammonium bisulfate, or a combination
of two or more thereof, which can provide an aqueous acidic
solution with a pH from about pH -1 to about pH 4.5, or less than
pH 4.5. In some embodiments, the acid of step (a) is typically
present in an amount/concentration of about 1% to about 20%, or
about 10%, in excess of stoichiometric need. In the method, the
ammonia-containing reagent of step (c) can be ammonia, ammonium
hydroxide, ammonium sulfate, ammonium oxalate, ammonium citrate,
ammonium carbonate, or ammonium bisulfate. In one embodiment, the
ammonia-containing reagent is carbonated prior to precipitating the
magnesium component, e.g., Mg(OH).sub.2, or Mg(CO).sub.3. In one
embodiment, following step (c) of the method, the precipitated
magnesium component is washed to remove residual salt, and the
precipitated and washed magnesium component is exposed to carbon
dioxide at an elevated temperature, e.g., about 300.degree. C. to
less than about 900.degree. C., or about 300.degree. C. to about
500.degree. C., and pressure, e.g., about 1 to about 50
atmospheres, or greater. In one embodiment, the ammonium salt of
step (d) is precipitated by reducing the volume of the solution,
e.g., by evaporation or membrane separation. In another embodiment,
the ammonium salt of step (d) is precipitated by reducing the
temperature of the solution. In one embodiment, the ammonium salt
precipitate of step (d) is free of liquid water. In one embodiment,
the carbonating step is performed by temperature elevation, e.g.,
heating, for example, to a temperature of about 400.degree. C. to
about 500.degree. C. or above, at atmospheric or elevated pressure.
Elevated pressure, e.g., about 1 to about 50 atmospheres or
greater, can be achieved, for example, by use of an autoclave. In
one embodiment, ammonia and acid can be recovered following step
(e). The recovered acid can be solid and anhydrous. In one
embodiment, the ammonia and acid are recovered by thermal
decomposition. In another embodiment, the ammonia and acid are
recovered through the use of electrochemical techniques.
[0044] In another embodiment, the invention encompasses a method
for extracting, reducing, or sequestering carbon dioxide, which
comprises (a) dissolving a magnesium bearing silicate in an aqueous
acid to form an acidic solution; (b) increasing the pH of the
solution of step (a) to neutralize the acidic solution of step (a);
(c) introducing ammonium carbonate or ammonium bicarbonate into the
solution of step (b) to precipitate magnesium carbonate or related
hydrated forms thereof; and (d) carbonating the ammonia to form
ammonium carbonate or bicarbonate so as to bind carbon dioxide. In
a related embodiment, ammonia gas and acid are recovered following
step (c) and prior to step (d). In an embodiment, the magnesium
bearing silicate of step (a) comprises peridotite rock, which can
be serpentine or olivine, for example. In an embodiment, the
aqueous acid of step (a) is selected from citric acid, acetic acid,
chromic acid, sulfuric acid, orthophosphoric acid, oxalic acid,
ammonium bisulfate, or a combination of two or more thereof. In an
embodiment, the pH of the acidic solution is less than or equal to
about pH 4.5. In an embodiment, the iron and silicate precipitate
in the neutralized solution of step (b) and prior to precipitation
of magnesium carbonate or its related hydrated forms. In an
embodiment of the method, the pH is increased using a neutralizing
agent selected from ammonia or magnesium hydroxide; in an
embodiment, the pH of the neutralized solution is less than or
equal to about pH 8. In an embodiment, increasing the pH in step
(b) comprises an ammonia-containing reagent, which can be one or
more of NaOH, KOH, NH.sub.3, NH.sub.4OH, (NH.sub.4).sub.2CO.sub.3,
NH.sub.4HCO.sub.3, Na.sub.2CO.sub.3, or a combination thereof. In
an embodiment, the ammonia-containing reagent is selected from
ammonia, ammonium hydroxide, ammonium carbonate, or ammonium
bicarbonate.
[0045] In another embodiment, the present invention embraces a
method of extracting, sequestering, reducing, or eliminating
CO.sub.2 from the air using solid sorbents. In this embodiment,
CO.sub.2 can be extracted or captured directly from the air using
the solid sorbents. Thereafter, the sorbent is recycled and the
captured CO.sub.2 is recovered in a pressurized stream of
concentrated, nearly pure CO.sub.2. Solid materials having a high
affinity to CO.sub.2 can absorb CO.sub.2 even at the low partial
pressures as found in air. In accordance with this embodiment, the
solid sorbent material is exposed to air. Suitable absorbers or
sorbents for use in the method include, without limitation,
easily-handled small objects, such as, for example, beads or rods
that move along surfaces exposed to the air. Such objects serve as
collection surfaces and need not be of any particular shape or
size, but are exposed to air that flows over and/or around and/or
through them. Alternatives to beads or rods include absorber
materials that can be fashioned into fabric-like materials that are
exposed to the air and absorb the CO.sub.2, or a fraction thereof,
that is present. Other alternative materials include small boards
with rough surfaces that are attached to wheels rolling down a
track, for example. It will suffice for the suitability of the
absorber (sorbent) that air can reach the absorber surface(s) and
CO.sub.2 can be bound, either loosely or tightly to the CO.sub.2
sorbent.
[0046] Absorber materials can be solid sorbents, i.e., the entire
material is sorbent throughout. Alternatively, the absorber
material can comprise an inert material of which one or more
surfaces are coated with one or more sorbents. In either case, a
large amount of absorption surface is intended. After exposure to a
CO.sub.2 source, the sorbent surfaces become saturated with carbon
dioxide and cease taking up carbon dioxide. A preferred sorbent
material is one that takes up CO.sub.2 under ambient conditions,
and releases a substantial fraction (e.g., in excess of 10%) of the
CO.sub.2 at pressures that are not significantly lower than ambient
pressure. The sorbent material is preferably inert, apart from its
affinity to bind CO.sub.2. In addition, the absorber may or may not
absorb water; it is desirable that the absorber does not absorb
water. Alternatively, at least the CO.sub.2 will quantitatively
displace water from the binding surface of the sorbent material. If
the solid sorbent material absorbs water, it is preferable that it
does not release water in the recycle step of the method described
below, because (i) the material has been displaced by CO.sub.2, and
thus is not present in the recycle loop; (ii) it has bound tightly
enough to remain attached to the sorbent; and/or (iii) the water
vapor pressure in the recovery cycle is sufficiently high to
prevent the freeing of water. Should water absorb onto the sorbent
and displace CO.sub.2, more energy is consumed in the process; this
is not a particularly desirable result. Thus, the sorbent material
should be less attractive to water than it is to CO.sub.2 so that
CO.sub.2 absorption is not reduced, or so that CO.sub.2 is not
released during sublimation.
[0047] Illustrative and nonlimiting examples of solid sorbent
materials for use in the methods of this invention include
hydrophobic carbon compounds that absorb CO.sub.2 from the air (Oak
Ridge National Laboratory, Oak Ridge, Tenn.), activated carbon
(amorphous carbon solids, often from natural biomass sources),
molecular sieve carbon (MSC) (fossil fuel, e.g., coal, petroleum),
carbon fiber composite molecular sieves (CFCMS), formed by joining
petroleum pitch-derived carbon fibers with a phenolic resin-derived
binder to form a monolithic, highly porous carbon filter "cake",
activated alumina, and natural ("mineral") and synthetic zeolites,
or specialized zeolites, such as silicalites. Zeolites can include,
for example, A, X, Y, or mordenite, etc. which tend to possess the
physico-chemical properties that allow them to bind CO.sub.2. Other
zeolites, such as those having a high Si/Al ratio, such as
silicalite, tend to be more stable to hydrothermal treatments and
have less affinity to water. Such materials can be hydrophobic and
could serve as CO.sub.2 capturing agents that are not affected by
the presence of water vapor. Other useful zeolites are those having
a high sodium content combined with a medium Si/Al ratio. In
addition, suitable solid sorbents offer several advantages,
including low binding energy, high stability, selectivity, high
absorption capacity, kinetic advantages, and simplicity of system
design.
[0048] In another embodiment, a cryogenic pressure system is
embraced to perform the CO.sub.2 capture, sequestration, reduction,
or elimination methods of this invention. In accordance with this
embodiment, the CO.sub.2-saturated sorbent material is packed into
a first chamber at room temperature that can be evacuated. The
first chamber can be connected to a low-grade vacuum system that
removes remnant air that is caught in or on the sorbent material.
After a pressure reduction in the first chamber from atmospheric
pressure to near vacuum pressure, CO.sub.2 is allowed to flow from
the sorbent into a second chamber where the CO.sub.2 condenses onto
cold surfaces (e.g., solid substrates) as solid carbon dioxide.
CO.sub.2 flow can be controlled by the opening of a valve or
another means that achieves the desired result. The pressure in the
second chamber is lower than the vapor pressure of CO.sub.2 in the
first chamber, and can depend upon the type of absorbent material
that is utilized in the second chamber. At sufficiently low
temperature, any surface material is suitable for the purpose of
this method.
[0049] Illustratively, the pressure of the second chamber can be
about 100 ppm of an atmosphere, or about 0.001 psi. The temperature
in this second chamber is low enough for the CO.sub.2 to condense
out, e.g., in the form of dry ice. Accordingly, the temperature is
about -80.degree. C. or -100.degree. C. or lower. Liquefied air may
be used as a coolant. Without wishing to be bound by theory,
because the temperature in the second chamber is below the freezing
point of CO.sub.2, the equilibrium partial pressure of CO.sub.2 in
the second chamber is far lower than the pressure of CO.sub.2 over
the saturated (warmer) CO.sub.2 sorbent surfaces in the first
chamber. As a consequence, the system establishes a pressure
gradient between the two chambers and the CO.sub.2 travels from the
chamber having higher pressure to the chamber having lower pressure
chamber until enough CO.sub.2 has been removed from the sorbent so
that the partial pressure in the first sorbent chamber has dropped
as well. When a substantial amount of carbon dioxide has formed as
dry ice on the solid surface(s), which serve as a "cold trap"
within the second chamber, the collected dry ice is confined to a
small volume and brought to ambient temperature. As the dry ice
warms up, it turns into CO.sub.2 gas, which, as it is confined to a
small volume, will be produced at a high pressure. This gas is then
released under pressure from the cryogenic system, e.g., into
containment vessels and the like, for further storage or
collection.
[0050] In this embodiment, the partial pressure of CO.sub.2 is
reduced over the system to the point that a substantial fraction of
the adsorbed CO.sub.2 is released and captured in the cold trap.
The dry ice that forms in the cold trap is collected over time,
e.g., from about 15 minutes to several hours, or from about 20
minutes to one hour. For example, the rate of heat transfer between
the cold trap and the solid sorbent in the second chamber can be as
fast as about 50 g/m.sup.2/sec. Thus, a system containing 1 ton of
sorbent containing about 50 kg of CO.sub.2 could release its
CO.sub.2 as dry ice in about 20 minutes. When sufficient amounts of
dry ice are available, the dry ice is confined to a small volume,
e.g., by scraping it from the cold trap and moving it to a suitably
sized vessel. The size of the vessel is such that when the solid
CO.sub.2 is allowed to warm or heat to ambient conditions, it is
then at the desired pressure. The CO.sub.2 is then released under
pressure, e.g., about 60 to about 200 bar pressure, to a suitable
or desired CO.sub.2 containment vessel, or handling, storage, or
transportation system. Advantageously, such a vacuum system
effectively requires no pumps to pressurize gaseous CO.sub.2. By
keeping the CO.sub.2 confined, pressure is obtained from the energy
input that was provided in the refrigeration system that maintained
a low temperature in the cold trap.
[0051] In another embodiment, the present invention embraces a
system comprising a solid sorbent for CO.sub.2 capture comprising
impregnated support matrices or substrates, e.g., cloth materials,
to transport solid particles into and out of an air stream. This
system removes carbon dioxide from the atmosphere in a manner akin
to that of a wind capture device. The support matrix provides a
means of moving the solid sorbent into and out of the air stream,
similar to the operation of a conveyor belt. The air stream can be
air, or a flue gas, or essentially any substance, material, or
matter that comprises CO.sub.2 In an embodiment, the solid sorbents
are mounted or made to adhere to a solid matrix or substrate, such
as a cloth material, which is then placed on a moving system, e.g.,
a clothesline-type system, to move the matrix or substrate into the
air stream, e.g., wind, and back again.
[0052] In another embodiment, particulates are used inside a
chamber (or container) followed by air filtration systems that
recapture particulates from the stream. The system according to
this embodiment is similar to a fluidized bed. One or more solid
sorbents can be placed into a moving gas stream, air or flue gas,
where the sorbent "floats" around while absorbing CO.sub.2. After a
specified period of time the sorbent is removed and the absorbed
CO.sub.2 is stripped therefrom, for example, by diverting the gas
stream through a second chamber (or container). Accordingly, one
chamber (or container) absorbs while the second chamber (or
container) is being recycled without release of the particulates to
the environment.
[0053] In another embodiment, the present invention embraces
methods and systems for extracting CO.sub.2 from the air using
liquid sorbents. Accordingly, the invention provides a method of
carbon capture that removes CO.sub.2 from air using solid oxide
membrane and liquid sorbents. Suitable sorbents include basic
solutions, such as sodium hydroxide (NaOH) or potassium hydroxide
(KOH), and other often viscous fluids, which are typically caustic.
More specifically, the method involves the use of a
hydroxide-based, alkaline liquid sorbent, e.g., NaOH solutions and
the like, to absorb and remove CO.sub.2 from ambient air and
produce carbonate ions. The resultant sodium carbonate
(Na.sub.2CO.sub.3) solution is mixed or reacted with calcium
hydroxide (Ca(OH).sub.2) to produce sodium hydroxide and calcium
carbonate (CaCO.sub.3) in a causticizing reaction, which transfers
the carbonate anion from the sodium to the calcium cation. The
calcium carbonate precipitates as calcite, leaving behind a
regenerated sodium hydroxide sorbent, thus, regenerating the
sorbent. The calcite precipitate is dried, washed and thermally
decomposed to produce lime (CaO) in a calcination process. The
thermal decomposition is preferably performed to avoid mixing the
CO.sub.2 resulting form the combustion process, providing the heat
with ambient air. This embodiment uses solid oxide membranes to
separate input air from the combustion process. Oxygen at elevated
temperatures can pass through these membranes. After calcination,
the lime is hydrated in a slaking process. In a related embodiment,
this method can be implemented using air capturing systems, for
example, towers or air capture units of various design, which
function as the physical sites where CO.sub.2 is captured and
removed from the air.
[0054] In yet another embodiment, the resultant NaOH is recycled
using sodium tri-titanate rather than calcium hydroxide. In this
embodiment, the reaction occurs in a molten rather than in an
aqueous state. As a result, the absorption solution is highly
caustic in order to minimize the amount of water evaporation
required. In another embodiment, the invention encompasses a method
for capturing carbon dioxide from air, comprising (a) exposing air
containing carbon dioxide to a solution comprising a base resulting
in a basic solution which absorbs carbon dioxide and produces a
carbonate solution; (b) causticizing the carbonate solution with a
titanate containing reagent; (c) increasing the temperature of the
solution generated in step (b) to release carbon dioxide; and (d)
hydrating solid components remaining from step (c) to regenerate
the base comprising step (a). In one embodiment of this method, the
base of step (a) is selected from sodium hydroxide, calcium
hydroxide, or potassium hydroxide. In another embodiment, the
carbonate solution of step (a) of the method is a sodium carbonate
(Na.sub.2CO.sub.3) solution. In another embodiment of the method,
the solution of step (a) is causticized with sodium
tri-titanate.
[0055] In related embodiments, the present invention provides
systems and apparatuses for extracting, capturing, or entraining
CO.sub.2 from the air or wind. Such capture apparatuses can include
a wind capture system or a cooling-type tower for extracting,
sequestering, or capturing CO.sub.2 as further described herein.
Fan driven systems are encompassed. Wind capture systems refer to
freestanding objects similar in scale to a wind energy turbine. For
example, such devices contain a pivot that ensures that contacting
surface can follow the wind directions. The device can operate with
either liquid or solid CO.sub.2 sorbents. A liquid based system
operates using pumps at the base, which pump sorbent to the top of
the device. Once at the top, the sorbent flows under gravity back
to the bottom via a circulation system. The circulation system can
encompass troughs or other flow channels that expose the sorbent to
air. Alternatively, the system could be vertical wires on which
sorbent flows from top to bottom. The device is sized such that the
sorbent is saturated in one pass. A solid system contains moving
components on which one or more solid sorbent is bound. These
components are mechanically raised into the wind so as to absorb
CO.sub.2. Once saturated, the components are removed from the wind
stream, isolated and stripped of CO.sub.2. In another embodiment, a
cooling tower contains a CO.sub.2 removal zone in the air inlet at
the base, which may contain either solid or liquid sorbents in a
manner described above.
[0056] In another embodiment, a CO.sub.2 capture system according
to this invention can comprise filter systems wetted by a flow of
sodium hydroxide that readily absorbs CO.sub.2 from the air, and in
the process, converts it to sodium carbonate. Without wishing to be
bound by theory, if the pressure drop across the system due to
viscous drag is comparable to the kinetic energy density in the
air, then the fraction of CO.sub.2 removed from the flow stream
becomes significant, so long as the sorbent materials are strong
absorbers. This is because the momentum transfer to the wall
follows essentially the same physical laws of diffusion as the
carbon dioxide transfer. In a cooling tower type of system, intake
air is pulled through a filter system that is continuously wetted
with sodium hydroxide. Another type of system can involve a slight
pressure drop generated by other means. In yet another system, air
contacts sorbent surfaces simply by the wind (or moving air)
passing through the device or system. It will be appreciated that
in the design of a contact system, the rate of absorption should be
considered. In this regard, the volume of sorbent per unit output
of CO.sub.2 is independent of the specific details of the air
contacting design.
[0057] Advantageously, air extraction of CO.sub.2 and systems for
this purpose can be sited based on site-specific conditions, which
can include temperature, wind, renewable energy potential,
proximity to natural gas, proximity to sequestration site(s) and
proximity to enhanced oil recovery site(s). The system should be
designed for ease of relocation. For example, the extractor may be
sited at an oil field in order to minimize transport. In such a
case, oil could be reformed and used in the calcination system.
[0058] In other embodiments, chemical processes, e.g., calcinations
and calcining carbonate, are encompassed for the recovery of
CO.sub.2. One process involves oxygen blown calcinations of
limestone with internal CO.sub.2 capture. Such calcinations are
carried out in a calcining furnace that uses oxygen rather than
air. The use of oxygen results in the product stream including only
CO.sub.2 and H.sub.2O, which can be easily separated. In addition,
power plant and air capture sorbent recovery can be integrated into
one facility. Another process involves electrically heated
calcinations. Yet another process involves solid oxide ionic
membranes and solid oxide fuel cell (SOFC)-based separation
processes (e.g., Example 2). Another chemical process involves the
electrochemical separation of CO.sub.2 from Na.sub.2CO.sub.3, for
example, using a three-chamber electrolytic cell containing one
cationic membrane and an anionic membrane. The cationic membrane is
located between the central cell and the negative electrode while
the anionic membrane is located between the center and the cathode.
A current is applied to the cell and then sodium carbonate is
introduced into the center cell. The ions move toward the opposite
electrode. Hydrogen is evolved at the anode and oxygen gas is
evolved at the cathode, resulting in the formation of NaOH at the
anode and carbon dioxide gas at the cathode. Several cells can be
stacked together by placing a bipolar membrane at the electrode
locations of the single cell. This serves to reduce the amount of
gas evolved per unit reagent regenerated.
[0059] The present invention embraces remote CO.sub.2 sequestration
sites via air capture. Such remote sequestration following the
capture of CO.sub.2 from air can include ocean disposal from
floating platforms or mineral sequestration in territories or
environments having the appropriate mineral sites and deposits. The
capture of CO.sub.2 from air allows CO.sub.2 to be disposed of in
remote areas that otherwise would be inaccessible to CO.sub.2
disposal due to the prohibitively high cost of transporting
CO.sub.2 to remote locations.
[0060] The present invention further encompasses CO.sub.2
extraction from the ocean using limestone and dolomite as sources
of alkalinity. If provided with sufficient alkalinity, the ocean
can remove carbon dioxide from the air. According to this
embodiment, ocean disposal can be improved by calcining limestone
or dolomite to capture CO.sub.2 from the air. During this process,
CO.sub.2 is released to the air, but the resulting CO.sub.2 uptake
is nearly twice as large as the initial CO.sub.2 emission. Thus, a
metal hydroxide, e.g., magnesium or calcium hydroxide, dissolved
into the surface of the ocean will raise the alkalinity of the
water leading to the additional capture of two moles of CO.sub.2
for every mole of CO.sub.2 entered into the system. Illustratively,
and without limitation, in solid form, an ion, such as a calcium
ion, Ca.sup.+2, can trap one CO.sub.2 molecule in the form of
CaCO.sub.3. However, in dissolved form, the same ion can trap two
CO.sub.2 molecules as bicarbonate ions (HCO.sub.3.sup.-).
Therefore, limestone that is heated (calcined) as described herein
releases one CO.sub.2 molecule, but when it is dissolved in the
ocean, two bicarbonate ions are trapped. In this embodiment, the
CO.sub.2 that is dissolved in the mixed layer at the top of the
ocean is kept in solution by the addition of calcium or magnesium
ions. The mixed layer typically, but not necessarily, reaches a
depth of approximately 100 m. Suitable sources of metal hydroxides
include, without limitation, limestone, dolomite, or smaller
deposits of magnesium carbonates. Although calcium carbonate is
supersaturated in sea water and is thus difficult to dissolve, sea
water is still far below the point at which calcium carbonate
spontaneously precipitates out, thereby allowing for some increase
in carbonate and/or calcium in the surface waters of the ocean.
Further, the total dissolved calcium in the ocean is a quite large
amount; therefore, the ocean is generally insensitive to additions
that could allow for substantial increases in stored CO.sub.2.
Magnesium carbonate also dissolves in sea water, but at a slower
rate than does calcium carbonate. Also, the slow dissolution of
magnesium carbonate can raise the carbonate ion concentration of
sea water, which may be counterproductive to dissolving additional
carbonate. Because added calcium ions disperse relatively rapidly
upon exposure to the ocean surface, this can prevent a risk of
precipitation of calcium carbonate into the ocean waters.
[0061] More specifically regarding this embodiment, a method is
provided to introduce alkalinity into the water as one or more
metal hydroxides, e.g., without limitation, MgO/CaO;
Mg(OH).sub.2/Ca(OH).sub.2; MgO/CaCO.sub.3; or
Mg(OH).sub.2/CaCO.sub.3, or a combination thereof. These metal
hydroxides are calcination products obtained by calcining a
suitable starting calcium carbonate- or magnesium
carbonate-containing material, e.g., dolomite, limestone, or
magnesite, at a temperature above about 400.degree. C., or above
about 900.degree. C. The resulting carbon dioxide is captured and
sequestered at the calcination site. For dolomite at a temperature
above about 400.degree. C., the CaO component is not calcined,
while MgO is calcined at this relatively lower temperature.
Calcination can be performed by conventional methods (e.g.,
Boynton, R. S., 1966, Chemistry and Technology of Lime and
Limestone, Interscience Publishers, New York, pp. 3, 255, 258), or
by using another energy source, such as solar energy, wind energy,
electrical energy, nuclear energy, remote sites with unusable
methane, etc. According to this method, the calcination product is
finely dispersed into ocean or sea water by various procedures. For
example, introduction into the water can occur from one or more
ships or vessels that drag behind or between them a line that drops
fine powder in the water. The size of the line is long enough so
that local concentrations of the material are not driven very high.
Alternatively, the calcination products can be fashioned into
larger pellets, as conventionally known in the art. The pellets are
dropped or ejected into the water, dissolve slowly and distribute
the material relatively uniformly over a larger area as they drift
along. Pellets should contain sufficient amounts of air, e.g., have
sufficient air pockets, to float. Such pellets can advantageously
be added to the water in larger quantities versus a fine
dispersion. Of particular interest are pellets comprising
CaCO.sub.3/MgO mixtures. By the practice of this method, the net
CO.sub.2 balance is positive, even if the CO.sub.2 from the
calcination is not captured. Notwithstanding, for every CO.sub.2
molecule released by this method, nearly two CO.sub.2 molecules are
absorbed into the ocean, which takes up CO.sub.2 from distributed
sources.
[0062] In another embodiment, the present invention relates to
methods of transitioning from today's energy system comprising
unsequestered CO.sub.2 resulting from the use of fossil fuels to
the capture and disposal of CO.sub.2, and ultimately, to renewable
energy with recycling of CO.sub.2. Such transitioning methods
comprise combining CO.sub.2 capture with magnesium silicate
disposal. In this embodiment, CO.sub.2 can be removed from the air,
but rather than disposing of the removed CO.sub.2, it is used as a
feedstock for making new fuel. The energy for the fuel derives from
a renewable energy source or any other suitable source of energy
that does not involve fossil fuels, such as hydroelectricity,
nuclear energy. For example, CO.sub.2 is initially collected and
disposed of or sequestered in underground deposits (such as in
enhanced oil recovery, (EOR)) or in mineral sequestration. In this
way, the source of the energy is fossil fuel that can be extracted
from the ground. To maintain an environmentally acceptable material
balance, the carbon must be re-sequestered or disposed of.
Alternatively, carbon can be recycled as an energy carrier.
Hydrocarbon, i.e., reduced carbon, contains energy that is removed
by the consumer by oxidizing the carbon and the hydrogen, resulting
in CO.sub.2 and water. The capture of CO.sub.2 from air allows the
CO.sub.2 to be recovered; thereafter, renewable energy can be used
to convert the CO.sub.2 (and water) back into a new hydrocarbon.
The production of hydrocarbon can include a number of processes.
Illustratively, Fischer Tropsch reactions are conventionally used
to convert carbon monoxide and hydrogen to liquid fuels, such as
diesel and gasoline (e.g., Horvath I. T., Encyclopedia of
Catalysis, Vol. 2, Wiley Interscience, p. 42). Similar methods
using CO.sub.2 and hydrogen are also established. Hydrocarbon can
be produced from CO.sub.2 and hydrogen. Hydrocarbon production
typically involves the use of energy, e.g., electric energy, to
convert water into hydrogen and oxygen, or CO.sub.2 into CO and
oxygen. Thereafter, fuels such as methanol, diesel, gasoline,
dimethyl-ether (DME), etc. can be made.
[0063] In other embodiments of this invention, CO.sub.2 capture
apparatuses and systems are encompassed, especially for use in
connection with the described processes. In one embodiment, a wind
capture system comprises a CO.sub.2 capture apparatus in which the
air delivery system relies on natural wind flow. Such a CO.sub.2
capture apparatus can be situated in the same or similar areas to
those in which wind turbines are employed. In another embodiment,
the invention embraces a water spray tower CO.sub.2 capture
apparatus comprising a cylindrical tower, e.g., approximately 100
feet in height, which is open to the air at its top and contains
ground level exit vents. A vertical pipe comprises the center of
the tower through which water can be pumped; the pipe can be capped
with a nozzle that sprays water horizontally. Water is pumped to
the top and sprayed into the air. The resultant evaporation creates
a pocket of air that is colder and denser than the air below it.
This leads to a down draft which forces air through the exit vents.
The exit vents contain a solid or liquid sorbent for CO.sub.2
capture. In another embodiment, the invention embraces an air
convection tower CO.sub.2 capture apparatus comprising a vertical
cylindrical tower that is attached to a glass skirt situated
approximately 1 foot above the ground level. The glass insulates
the air between the ground and itself, which raises the air
temperature. The hot air then exits through the central tower. A
solid or liquid CO.sub.2 capture device is contained in the tower.
In another embodiment, the invention encompasses a CO.sub.2 capture
apparatus comprising a glass covered slope, which comprises a glass
sheet situated some distance above ground level, e.g., between 0.3
m and 30 m, depending on the size of the overall apparatus. The
glass acts as an insulator that causes the air to heat in the
sunshine and this results in a draft up the hill. The resulting
flow is guided over CO.sub.2 absorber surfaces, which removes
CO.sub.2 from the air passing through it. In another embodiment,
the invention encompasses cooling towers to replace a conventional
water cooling liquid with a liquid sorbent. The liquid sorbent
evaporates water; in addition, the liquid sorbent collects CO.sub.2
in concentrated form. In all cases, the saturated sorbent is
stripped of its CO.sub.2 as described herein.
[0064] In another embodiment, wind funneling devices are optimized
for throughput rather than air speed, thereby leading to
optimization for CO.sub.2 capture and sequestration. For example,
air convection towers employed for CO.sub.2 capture can be shorter
than towers designed for electricity production, since increased
height to promote air speed is not a requisite for CO.sub.2
sequestration. Further, in such CO.sub.2 capture apparatuses,
textile membranes are used to separate alkaline fluids from the
open air. Such membranes comprise cloth-type fabrics that allow air
passage while limiting sorbent loss through spray. An illustrative,
yet nonlimiting, fabric is Amoco 2019. Other CO.sub.2 capture
systems include those that are adapted to wind flow, e.g., venturi
flows that create suction on a set of filters that are balanced by
adjusting the size of the openings so as to maintain constant flow
speed through the filtration system. As an example, FIG. 2 shows a
solid structure (black lines) seen from above. As the air moves
through the narrowed passage, the pressure drops (Venturi Effect).
As a result, the higher pressure air inside the enclosures that are
open to the back of the flow will have a tendency to stream into
the low pressure air flow. Openings are preferably large in a high
speed wind and small in a low speed wind in order to maintain a
constant pressure drop across the filter system and thus optimize
the efficiency of the collector even in the face of variable wind
speeds. By adjusting the size of the opening, e.g., using shutters,
baffles, etc., one can control the pressure drop across the filter
and can control the amount of air that emerges for optimized flow
rates.
[0065] It will be appreciated that fans can comprise fan driven
CO.sub.2 capture apparatuses and systems, e.g., in CO.sub.2 capture
systems at the site of an oil well to perform EOR. The use of a fan
or forced air system ensures a specified air throughput, rather
than having to rely on the fluctuations of natural wind. By
creating a constant air flow, a specified production can be
achieved which may be desirable for production schemes that require
constant carbon dioxide output rates. The price for such an
arrangement is higher energy cost and capital cost in the
installation and operation of fans.
[0066] The examples described below are provided to illustrate
aspects of the present invention and are not intended to limit the
invention.
Example 1
Capturing CO.sub.2 Directly from the Atmosphere
[0067] Alkaline Sodium Sorbents: Removal of a gaseous component
through contact with a liquid is known as wet scrubbing. Wet
scrubbing can be divided into processes where there is a chemical
reaction between the sorbate and the sorbent and where the sorbate
is physically dissolved into the sorbent solution. For the air
extraction process, an alkaline sodium solvent is embraced which
reacts chemically with the entrained CO.sub.2. The chemical
reaction for this process is shown below as reaction (1):
2NaOH.sub.(aq)+CO.sub.2(g).fwdarw.Na.sub.2CO.sub.3(aq)+H.sub.2O;
(1)
[0068] The aqueous carbonate reaction can be simplified by omitting
the cation, resulting in the following ionic reaction:
2OH.sup.-.sub.(aq)+CO.sub.2(g).fwdarw.CO.sub.3.sup.2.sub.(aq)+H.sub.2O.s-
ub.(l)
.DELTA.G.degree.=-56.1 kJ/mol (.DELTA.H.degree.=-109.4 kJ/mol)
(2)
[0069] It is noted that the enthalpy and free energy of the
reaction are for a nominal 1 molar solution. The thermodynamic
data, given at 298K and a pressure of 1 bar, was obtained from the
available literature (Lide D. R., editor in chief, CRC Handbook of
Chemistry and Physics 81.sup.st Ed., CRC Press LLC, Boca Raton Fla.
(2000)). As a comparison, the free energy of mixing CO.sub.2 with
nitrogen and oxygen to form ambient air is given by
.DELTA.G=RT ln(P.sub.atm/P.sub.CO2).about.20 kJ/mol. (3)
[0070] Accordingly, sodium hydroxide provides a sufficient driving
force to effectively collect CO.sub.2 from ambient air. Even though
a lower binding energy might be desirable, the high binding energy
of chemical sorbents proves useful in absorbing CO.sub.2 from
streams with low partial pressures of CO.sub.2. As an alternative
with a weaker binding energy, sodium or potassium carbonate buffer
solutions can be used as sorbents. In this case the absorption can
be described by:
CO.sub.2(g)+CO.sub.3.sup.2-+H.sub.2O.sub.(l).fwdarw.2HCO.sub.3.sup.-
.DELTA.G.degree.=-14.3 kJ/mol (.DELTA.H.degree.=-27.6 kJ/mol)
(4)
[0071] Even in this case a sufficient thermodynamic driving force
is available to remove CO.sub.2 from the air. For a two molar
solution of bicarbonate ions, the free energy of the reaction from
ambient air is negative if the bicarbonate concentration stays
below 0.15 molar. A similar result can be obtained by calculating
the mass action equilibrium using empirical values for the
equilibrium constants. Reaction (4) is effectively trimolecular and
is the result of a sequence of reactions which have fast kinetics
at high temperatures or very high carbonate to bi-carbonate ratios.
Otherwise the process occurs in the diffusion regime, which is much
slower, making this reaction kinetically limited for air
extraction.
[0072] According to the present method, chemical decomposition of
the resulting sodium carbonate is achieved using calcium hydroxide
as an intermediary. A sodium hydroxide solution provides a liquid
sorbent that is far more easily cycled through a piping system than
a calcium hydroxide suspension. Its binding energy is strong enough
and its reaction kinetics fast enough to obviate the need for
heating, cooling, or pressurizing the air. Because CO.sub.2 is so
dilute, any such action would result in an excessive energy
penalty. The hydroxide solution avoids all such complications.
Since sodium hydroxide is cheaper than potassium hydroxide, the
starting point for the air extraction design will be based on
sodium hydroxide.
[0073] Generally in wet scrubbing, transport resistance is
considered to comprise two distinct components, air side resistance
and liquid side resistance. The air side resistance is dominated by
the diffusion barrier in the laminar boundary layer. Typically,
such a boundary layer also exists on the liquid side. In the bulk
fluid, dissolved CO.sub.2 reacts with water, or hydroxide ions to
form carbonate or bicarbonate ions. In contrast to the reactions of
CO.sub.2 with water, the reactions with hydroxide reactions are
very fast and their reaction time can be ignored. However, since
diffusion coefficients of CO.sub.2 in air are roughly four orders
of magnitude larger than ionic diffusion coefficients in water, it
is easy to become rate limited on the liquid side. Avoidance of
liquid side rate limitations is the goal of a good design.
[0074] For a one molar carbonate ion concentration in the liquid,
the concentration ratio between carbonate ions in the fluid and
CO.sub.2 molecules in the gas is 66,000:1. Thus, it will take time
to fill up a boundary layer on the liquid side. This suggests that
at sufficiently low partial pressures of CO.sub.2 the extraction
process will be limited by air-side resistance. For ambient air in
a packed column-type system (Tepe, J. B. and Dodge, B. F.,
Absorption of Carbon Dioxide by Sodium Hydroxide Solutions in a
Packed Column, Trans. Am. Inst. Chem. Engrs., 39, 255 (1943)),
CO.sub.2 absorption rates have been determined to be proportional
to G.sup..alpha., where G is the air flow rate and the coefficient
.alpha. varies from 0.35 at low flow rates to 0.15 at high flow
rates. (Spector, N. A. and Dodge, B. F., Removal of carbon dioxide
from atmospheric air, Trans. Am. Inst. Chem. Engrs., 42, 827-48
(1946)). This suggests that at low CO.sub.2 concentrations, such as
0.031%, the liquid side resistance to transport ceases to be
dominant. It is likely that in these experiments, fluid surface
regeneration was sufficiently fast to prevent a built up of
liquid-side flow resistance.
[0075] The advantage of using a strong hydroxide for CO.sub.2
capture is a high load capacity and a fast reaction time. The
removal of CO.sub.2 from air can be accomplished by a system that
will be limited by transport resistance in the air side of the
air-liquid contact surface. In a regime where the dominant
transport resistance is on the air side, it is possible to estimate
the size of the CO.sub.2 extractor by the air drag the extractor
causes on the flow. Apart from the pressure gradient driven
momentum flow, momentum transfer to the wetted surface follows a
similar transport equation as the CO.sub.2 diffusion. As a
consequence, a system that incurs a pressure drop roughly equal to
.rho.v.sup.2, which extracted virtually all of the initial
momentum, will be able to extract a substantial fraction of the
CO.sub.2 from the flow. To set the scale of the operation, at 10
m/s, the air flow through an opening of 1 m.sup.2 carries a
CO.sub.2 load that equals the CO.sub.2 produced by generating 70 kW
of heat from coal (Lackner K. S. et al., Carbon dioxide extraction
from air: is it an option, Proceedings of the 24.sup.th Annual
Technical Conference on Coal Utilization and Fuel Systems, (1999)).
A 100 MW power plant operating at 33% efficiency would require
9,000 m.sup.2 of wind cross section, if CO.sub.2 collection
efficiency is to be about 50%.
[0076] Causticization: Causticization refers to the transformation
of sodium carbonate into sodium hydroxide. It is generally
performed by adding solid calcium hydroxide to the sodium carbonate
solution. The solubility of calcium hydroxide is such that an
emulsion is formed according to reaction (5):
Na.sub.2CO.sub.3(aq)+Ca(OH).sub.2(s).fwdarw.2NaOH.sub.(aq)+CaCO.sub.3(s)
(5)
[0077] This reaction can also be written in its ionic form as
follows:
CO.sub.3.sup.2-+Ca(OH).sub.2(s).fwdarw.2OH.sup.-+CaCO.sub.3(s)
.DELTA.G.degree.=-18.2 kJ/mol (.DELTA.H.degree.=-5.3 kJ/mol)
(6)
This process step regenerates the sodium sorbent. The CO.sub.2 is
removed as a solid through a filtration process. Lime, which would
slake immediately in the aqueous solution, could be used as a
starting material; however, in air extraction, it is important
recover the heat of the slaking reaction at elevated temperatures.
Thus, the slaking step has been separated from causticization.
[0078] Causticization rate has been shown to increase with
temperature. The initial sodium carbonate concentration for those
experiments was .about.2.0 mol/l and the samples were subjected to
constant stiffing. Reaction (5) eventually approaches equilibrium
and causticizing efficiency is generally in the range of 80 to 90%.
Causticizing efficiency refers to the amount of sodium carbonate
converted to sodium hydroxide. It has been determined that the rate
constant for reaction (5) increased by a factor of 3 as the
operating temperature was raised from 353 to 393K. (Dotson B. E.
and Krishnagopalan A., Causticizing Reaction Kinetics, Tappi
Proceedings 1990 Pulping Conference, 234-244 (1990)). The rate
constant dropped when the feed solution contained sodium hydroxide.
The experimental causticizing efficiency for pure sodium carbonate
and a mixture of sodium hydroxide and sodium carbonate were
.about.94% and .about.85%, respectively. The rate constant for the
causticization is driven by the concentration of free Ca ions in
solution. Highly alkaline solutions will limit the availability of
dissolved Ca.sup.++ at any time and consequently reduce the rate of
conversion. Elevated temperatures and active stirring reduce
diffusional resistance and thus will increase the rate of
reactions.
[0079] The concentrations of all the calcium species have been
found to remain essentially constant throughout the reactions due
to their low solubility, thus suggesting that the efficiency and
rate constants may change if insufficient calcium is present. This
occurrence has been prevented by using a 10% stoichiometric excess
of lime. However, such an excess results in solid calcium hydroxide
being entrained with the filtrate, which can produce higher energy
consumption in the lime kiln due to the dehydration reaction.
[0080] The concentrations of the various species, both sodium and
calcium, have a profound effect on the quality of the resultant
filtrate. It has been observed that the solid phase of calcite is
unstable in pure NaOH solution greater than 2 mol/l and easily
converts to Ca(OH).sub.2 (Konno H. et al., Powder Technology, 123,
33-39 (2002)). These solids become stable in a 1 mol/l NaOH
solution containing at least 0.02 mol/l Na.sub.2CO.sub.3. This
latter solution mixture suggests that for an initial sorbent
concentration of 1 mol/L, 96% of the hydroxide ions were converted
to carbonate according to reaction (2). The presence of
Na.sub.2CO.sub.3 also reduces the solubility of calcite. The
solubility of Ca(OH).sub.2 is strongly dependent on the NaOH
concentration and drops by a factor 4, to 5.times.10.sup.-4 mol/l
as the NaOH increases from 0 to 0.5 mol/l. Observations of the
concentrations of Ca.sup.2+ and NaOH during the reaction have shown
that the Ca.sup.2+ concentration dropped and the NaOH concentration
increased as the reaction progressed. The initial Ca.sup.2+
concentration was .about.1.times.10.sup.-3 mol/l. It was also noted
that the Ca(OH).sub.2 super-saturation ratio is the driving force
for nucleation. In effect, these processes balance the solubility
of calcium hydroxide against the solubility of calcium carbonate.
The values for the dissociation constants are available in the
literature (Snoeyink V. L. and Jenkins D., Water Chemistry, p. 295,
John Wiley and Sons, New York (1980)).
[Ca.sup.++][OH.sup.-].sup.2<K.sub.OH=10.sup.-1.49 mol.sup.3/l
(8)
[Ca.sup.++][CO.sub.3.sup.2-]<K.sub.CO3=10.sup.-3.22 mol.sup.2/l
(9)
Given that the calcium concentration is the same in both (8) and
(9), the carbonate concentration is solved as follows:
[CO.sub.3.sup.2-]=(K.sub.CO3/K.sub.OH).times.[OH.sup.-].sup.2
(10)
[0081] Assuming a 1 molar sodium solution, the effect of calcium on
the charge balance can be neglected, and the sodium concentration
must therefore balance all the negative ions. If causticizing
efficiency (.epsilon.) is defined as the ratio of hydroxide ions
over sodium ions, the following relationship is obtained:
= ( 2 K CO 3 K OH [ OH - 1 ] + 1 ) - 1 ( 11 ) ##EQU00001##
Thus, the stable solution suggested above (Konno et al.) would
contain approximately 1 mol/l hydroxide ions, suggesting a
theoretical causticizing efficiency of 96%, which is slightly
higher than the experimental value that has been obtained (Dotson
B. E. and Krishnagopalan A., Tappi Proceedings 1990 Pulping
Conference, 234-244 (1990)). The difference is likely due to the
omission of ionic activity in the calculations.
[0082] The experimental work discussed above provides a pathway for
recovering sodium hydroxide from sodium carbonate. In the process,
the carbon dioxide has been transferred into a solid form of
calcium carbonate, which can be readily removed from the liquid.
After washing and drying, it can be thermally decomposed. The
causticization takes place in an emulsion of calcium hydroxide.
Calcination of Limestone: The final stage of the air extraction
process is the recycling of the calcite precipitate. This is
accomplished through thermal regeneration or calcination. Lime and
limestone are among the oldest materials used, with the first
recorded use in the Egyptian pyramids. There are three essential
factors in the kinetics of dissociation: the dissociation
temperature, the duration of calcination, and the CO.sub.2 in the
surrounding atmosphere. The reaction is shown below:
CaCO.sub.3(s).fwdarw.CaO.sub.(s)+CO.sub.2(g);
.DELTA.H.degree.=+179.2 kJ/mol (12)
The first quantification of the results of thermal decomposition
involved a decomposition temperature of 1171K in a 100% CO.sub.2
atmosphere at atmospheric pressure (Johnston J., J. Am. Chem. Soc.,
32, p. 938 (1910)). Current practices use lime kilns to dissociate
the calcite; these kilns vary greatly in their performance. A very
important performance metric for air extraction is the thermal
efficiency, which is the product of the theoretical heat
requirement and the available oxide content divided by the total
heat requirement. The thermal efficiency refers to the proximity to
the theoretical minimum heat requirement as defined by reaction
(12), available lime refers to the amount of inert material
present, in this case 7%. This translates into a total heat
requirement of 3.03 MMBtu per ton of lime, or 4.5 GJ per tonne of
CO.sub.2. The thermodynamic minimum heat requirement of 4.1
GJ/tonne CO.sub.2 can be calculated from the enthalpy value in
reaction (12). The potential cost of air extraction will be
dominated by reaction (12); any improvement regarding the
above-mentioned kinetic factors (sorbents, causticization and
calcination) will directly affect the cost of the project. A lower
dissociation temperature will require less heat input, as will a
shorter duration of calcination and a lower CO.sub.2 content in the
surroundings. Air Extraction as Carbon Capture: This example
relates to the capture of CO.sub.2 directly from the atmosphere in
a cost effective manner. As such, a brief comparison with the
industry standards provides a benchmark for future work. Sterically
hindered amines (SHA) and MEA are considered to be potential
CO.sub.2 capture technologies. They are regenerated using steam and
their thermal energy requirements are 700 and 900 kcal/kg CO.sub.2
for KS-2 and MEA, respectively. These values can be converted to
2.9 and 3.8 GJ/tonne CO.sub.2. In one instance 90% of the CO.sub.2
generated by the power plants was captured (Mimura T. et al.,
Development of energy saving technology for flue gas carbon dioxide
recovery in power plant by chemical absorption method and steam
system, Energy Conyers. Mgmt., 38, S57-62 (1997)); thus, the
remainder has to be mitigated by other means. An economic analysis
of CO.sub.2 capture using MEA obtained a cost of $50 per tonne of
CO.sub.2 avoided. The durability of the sorbent is also an
important cost factor.
[0083] The air extraction process encompasses hydrating or slaking
the resulting lime. Generally lime is regenerated in the hydration
process, thus a great reduction in capture efficiency from one
cycle to the next is not expected. The process of hydration is
believed to proceed via the migration of water into the pores of
the lime particle. The hydration reaction, shown below, then takes
place.
CaO.sub.(s)+H.sub.2O.sub.(l).fwdarw.Ca(OH).sub.2(s).DELTA.H.degree.=-64.-
5 kJ/mol (13)
[0084] The hydration causes both expansion and the liberation of
heat, which, in turn, causes the particle to split, exposing fresh
surfaces and thereby reducing the effects of sintering. The
inclusion of this reaction in the carbonation process will likely
alter the performance and durability of the lime cycle, as will the
lower temperatures of reaction. Slaked lime undergoes dehydration
at temperatures above .about.700K under ambient conditions. This
defines the range of possible operating temperatures for the
hydration process. Hydration is highly exothermic and can provide
useful heat energy if it is performed efficiently at high
temperatures.
[0085] The feasibility of air extraction will depend on the overall
cost compared with alternative removal technologies. The cost per
unit removal will further depend on the energy requirements, the
durability of the sorbents, and costs external to the process.
These external costs could include excessive water losses from the
wet scrubbing. As this part of the process will be in contact with
the open atmosphere, evaporation can be expected. This can be
minimized by adjusting the sorbent concentration, which varies the
vapor pressure until it matches that of the ambient air. The
thermophysical properties of sodium hydroxide solution are known
for a wide range of concentrations. The calcination reaction is
likely the most energy intensive for the stated process. This
highlights the need for efficient heat management within the
system. Additionally, any significant lime degradation will rapidly
raise costs and CO.sub.2 management issues. Lime make up will have
to be generated through the calcination process, thereby releasing
CO.sub.2. This additional CO.sub.2 will raise the cost of the
process either through lowering the net amount avoided or
increasing the total amount sequestered.
[0086] Although the air contactors in the systems described herein
can be much larger than the equivalent contact surfaces in a flue
stack involving MEA, their contribution to the total cost may be
very small. The recovery MEA sorbents require similar amounts of
energy; in contrast, in the present procedures and systems, because
the air is clean and hydroxides are not subject to oxidative
losses, the make-up costs are low in a hydroxide system.
Accompanying the air extraction of CO.sub.2 is the efficient
management of heat generation. The processes of this invention will
maximize CO.sub.2 capture while minimizing energy consumption.
Example 2
[0087] A second example of a CO.sub.2 capture process and system of
the present invention is presented herein below. As described, the
process and system involve the formation of sodium carbonate from
sodium hydroxide and CO.sub.2 from the air and the conversion of
calcium hydroxide into calcium carbonate. The calcium hydroxide is
recovered by calcining the limestone precipitate and then slaking
it with water. Individual reactions related to the process and
system, along with free energy or enthalpy values, are presented
below; thermodynamic values are based on those as conventionally
known in the pertinent art.
[0088] (1) 2NaOH+CO.sub.2.fwdarw.Na.sub.2CO.sub.3+H.sub.2O;
.DELTA.H.degree.=-171 kJ/mol
[0089] (2) Na.sub.2CO.sub.3+Ca(OH).sub.2.fwdarw.2NaOH+CaCO.sub.3;
.DELTA.H.degree.=57.1 kJ/mol
[0090] (3) CaCO.sub.3.fwdarw.CaO+CO.sub.2; .DELTA.H.degree.=179.2
kJ/mol
[0091] (4) CaO+H.sub.2O.fwdarw.Ca(OH).sub.2; .DELTA.H.degree.=-64.5
kJ/mol
[0092] (5) CH.sub.4+2O.sub.2.fwdarw.CO.sub.2+2H.sub.2O;
.DELTA.H.degree.=-890.5 kJ/mol
[0093] (6) H.sub.2O.sub.(t)=H.sub.20.sub.(g); .DELTA.H.sub.vap=41
kJ/mol@373K, 0.1 Mpa
[0094] In the system, sodium hydroxide, a caustic soda, is used as
a sorbent. An aqueous solution can drip over internal surfaces in
the filter system and thus allow for the capture of CO.sub.2 from
the air that passes through the system. Once the sorbent solution
has reached the bottom of the system, it is either re-circulated or
removed from the system for recycling. The purpose of a recycling
plant is to strip the CO.sub.2 from the spent sorbent and return
fresh sorbent to the capture device. After the CO.sub.2 has been
stripped, it can be compressed and sent to a disposal site.
Overall Energy Balance:
[0095] In considering this system in terms of its energy balance,
it can be compared with the basic reaction of forming carbon
dioxide:
C+O.sub.2.fwdarw.CO.sub.2; .DELTA.G.degree.=-394.4 kJ/mol or 9
GH/tonne
The heat of this reaction is given per tonne (metric ton) of
CO.sub.2. The combustion processes that lead to the production of
CO.sub.2 typically generate more heat, since nearly all of these
processes involve not only the oxidation of carbon, but to a
smaller or larger extent the oxidation of hydrogen. Heat of
combustion values for carbonaceous fuels range from about 500
kJ/mole of C for coal to 890 kJ per mole of carbon for natural gas
(or 11.4 GJ/tonne of CO.sub.2 for coal and 20.2 GJ/tonne of
CO.sub.2 for methane).
[0096] The regeneration of sodium hydroxide from sodium carbonate
requires an input of energy. The primary chemical reaction in this
process is the thermal decomposition of calcium carbonate, reaction
(3). The enthalpy of reaction (3) is 179.2 kJ/mol or 4.1 GJ/ tonne
of carbon dioxide. This is the theoretical minimum energy penalty
required to recycle the sorbent. It would be equally possible to
express numbers in terms of tonnes of limestone or tonnes of lime.
For every tonne of carbon dioxide that is freed from the sorbent,
2.3 tonnes of limestone enter the calcination process and 1.3
tonnes of lime leave the kiln. The value of 4.1 GJ/tonne of carbon
dioxide is approximately 45% of the free energy released during the
combustion of pure carbon. For natural gas this number would drop
to 21%.
[0097] A method involving sodium hydroxide with lime washing as a
viable solution to air capture provides a base system in which to
identify and estimate cost. The viability of the sorbent recovery
process will be judged on the net carbon production and the cost
per tonne of carbon dioxide. In this Example, a preliminary
evaluation has been performed in which various assumptions for
losses and parasitic energy requirements were made. For example, it
would be advantageous to have the pumping requirements met through
renewable sources. Although a conventional calciner system is not
ideal for calcining calcium carbonate in an air capture system due
to the large amount of carbon dioxide that would be emitted to the
atmosphere, several different approaches can obtain heat without
carbon dioxide emissions. This Example embraces a membrane process
that uses a mixed conductor membrane (MCM) to keep unused air
separate from the combustion products. The advantage to this system
is that the combustion occurs in an O.sub.2/CO.sub.2 environment,
thereby producing a pure stream of CO.sub.2 rather than a mixture
with N.sub.2 and impurities. In principle, transporting oxygen
across a membrane in the present system is analogous to the solid
oxide fuel cell (SOFC). However, a solid oxide based membrane
system (SOMS) is expected to be substantially less expensive than a
solid oxide fuel cell, which uses similar types of membranes as
electrolytes. Fuel cells, in contrast to membrane separators, must
provide charge electrodes on the surfaces of these devices in order
to carry the return current. In the membrane separators, the
electronic back current is in effect short circuiting the cell.
[0098] Without wishing to be bound by theory, in a SOMS system as
described herein, the economic data for fuel cells will be used in
determining a cost estimate. If treated as simple heat generators,
these fuel cells could reach efficiencies of 80-85%; waste heat
will be very valuable in this process. A commercially available
product is the PC25.TM. system manufactured by International Fuel
Cells, LLC. This is a 200 kW fuel cell system that consumes 2100
cft/hr or 86 lbs/hr (39 kg/hr) of natural gas. The rated efficiency
is 87% with 37% being electrical and 50% being thermal. The
estimated installed cost is approximately $4500/kW with a start up
cost of $15,000. Federal and state funding may be available and
could amount to $10001 kW15. These specifications are used for the
calculations as the first generation SOFC's are currently
unavailable and the DOE's performance target is for second
generation SOFC's. The most important difference with the
commercial PC25.TM. is that the MCM or SOFC would operate at
temperatures in the 1200-1300K range, which is suitable for
calcinations and better suited for heat exchange. Operating costs
also depend greatly on the price of natural gas. Current NYMEX
prices are approximately $5.501 million BTU 16, but more typical
long time averages have been around $3.00 per million BTU.
[0099] The specifications listed above can be scaled down to a
separation plant that has a thermal throughput of 100 kW, equal to
a power plant of 80% percent efficiency that would provide
electricity and waste heat. The plant would have the
characteristics listed in the Table 1. The primary energy is based
on the free energy of methane combustion shown in reaction (5)
above in this Example.
TABLE-US-00001 TABLE 1 Specifications for Feasibility SOFC Power
Station Specification PC25TM Feasibility SOFC Rating 200 kW 100 kW
Methane Throughput 39 kg/hr 16 kg/hr'7 Primary Energy 17,140 GJ/yr
7,020 GJ/yr Installed Cost $4,500/kW.sub.e $4,500/kW.sub.e Total
Cost $900,000 $450,000
The size of the capture devices and their design will not impact
the cost of recycling the sorbent because the volume of sorbent is
independent of the partial pressure. Thus, it was appropriate to
cost the system beginning at the downstream end, or the calciner.
This is a reasonable procedure as the calciner station is the most
expensive item. To maximize efficiency, it would be highly
desirable to keep it operating continuously. In view of this,
overall values for efficiency and availability, as well as
parasitic energy requirements, have been assumed. Parasitic energy
refers to the energy required to operate all of the equipment aside
from the calciner. For these calculations, the overall efficiency
was 80%, the availability was 99% and the parasitic requirement was
1%. Given those values, the total available energy for calcining is
approximately 5,500 GJ/year. If the aforementioned 4.8 GJ/tonne for
the calcining is assumed, an annual CO.sub.2 extraction of 1,150
tonnes is arrived at. To check the assumption regarding the
parasitic requirement, a simple calculation was performed to
estimate the energy involved in lifting the sorbent material
through an air contacter system. If it is assumed that the
calcining occurs once per day and the sorbent solution
concentration is 0.5 mol/l, then about 140 m.sup.3 of solution is
required to be circulated. An additional assumption of a 30 m
pumping height produces a daily consumption of 0.04 GJ. Given a
factor 2 for inefficiencies, an annual consumption of 30 GJ or 0.5%
is obtained.
[0100] As expected, variations in the fuel and fuel cell costs will
affect the cost per tonne. Table 2 contains a simple sensitivity
analysis in which fuel and fuel cell costs are varied. The values
in the table are cost per tonne extracted.
TABLE-US-00002 TABLE 2 Sensitivity Analysis Variables Cost per kW
Installed Fuel Cost Overall Efficiency $4500 $1500 $500 $5.50/MMBtu
80% 68 46 39 90% 61 41 35 $3.00/MMBtu 80% 52 30 23 90% 46 27 20
These values do not include the cost of sorbent replacement,
roughly $2 per tonne of CO.sub.2 extracted. This could be compared
with the estimate costs using monoethanolamine (MEA) of
approximately C37-50/tonne $, roughly $40-55 USD/tonne.
[0101] For sizing purposes, a daily material balance, in tonnes per
day, has been prepared. The balance assumes that the incoming
limestone contains 20% water, by volume. The fuel cell is assumed
to absorb 50% of the oxygen from the airflow. There are no figures
for the sodium hydroxide as it is recycled within the system. In
order to maximize the energy efficiency of the process it will be
necessary to minimize heat loss. This will require an energy
inventory in order to determine the optimal use of the heat
produced by the process. The lime will be reacted with water to
form slaked lime, an exothermic process that occurs at 500.degree.
C. according to reaction (4).
[0102] For the initial calculations it is assumed that the input
temperature is 300K and the fuel cell temperature is 1200K. The
theoretical calcinations temperature for calcium carbonate is
approximately 1173K. The values for sensible heat reflect the
energy consumed or released by changing the temperature of the
various material streams. These numbers were calculated by
integrating formulae for the specific heat, CP, of each component.
The specific heat data was obtained from the literature 8.20. Four
reactions occur in the process; the evaporation of water from
limestone, the calcination of limestone, the combustion of methane,
the hydroxylation of calcium oxide. A daily material and energy
balance is presented in Table 3. Energy consumed is positive and
energy released is represented as a negative number, in
parenthesis.
TABLE-US-00003 TABLE 3 Daily Material and Energy Balance Mass
Temperature Energy Sensible Reaction (tonne) Start Finish (kJ/mol)
(GJ) (GJ) Input CaCO.sub.3 (s) 7.13 300 1200 169.79 12.10 12.77
H.sub.2O (l) 1.28 300 374 5.60 0.40 2.90 N.sub.2 (g) 9.67 300 1200
3.72 1.28 O.sub.2 (g) 3.10 300 1200 38.30 3.30 CH.sub.4 (g) 0.38
300 1200 56.18 1.35 (21.37) Output CaO (s) 3.99 1200 800 (21.27)
(1.52) CO.sub.2 (g) 4.19 1200 374 (41.41) (3.95) N.sub.2 (g) 9.67
1200 374 (3.57) (1.23) O.sub.2 (g) 1.55 1200 374 (35.97) (1.55)
H.sub.2O (g) 0.86 1200 374 (32.08) (1.54) (1.95) Calcium Hydroxide
H.sub.2O (g) 1.28 374 800 15.53 1.11 Ca(OH).sub.2 5.27 800 374
(45.28) (3.23) (0.53) Total 6.53 (8.18) Grand Total (1.65)
The first observation to be made is that the entire process is a
net heat producer. However, there are no explicit losses built into
the calculations. If the absolute value of all the energy changes
is summed, -72 GJ is obtained, meaning the system can tolerate
loses of up to 2%. The input water is the water contained in the
limestone plus the amount required for the hydroxylation of the
lime.
[0103] Based on initial analyses, the cost of air contacting in an
air contact system is expected to be small compared to the sorbent
recycle system, which has been discussed herein above. The goal is
to ensure the adequate supply of adsorbed carbon dioxide while
keeping the infrastructure as small as possible. Given that the
density of carbon dioxide is approximately 0.015 mol/m.sup.3, an
exposed area of .about.20 m.sup.2 with a wind speed of 6 m/s and an
efficiency of 50% would be suitable for the system.
[0104] Despite the low concentration of carbon dioxide in air and
system constraints, there is no fundamental reason why carbon
capture from air is not possible. The system presented in this
Example will generate extra carbon dioxide, but has been designed
such that this carbon dioxide is not released to the environment.
For the design presented in this Example, the cost depends on the
availability of solid dense membranes through which oxygen ions can
diffuse. It is also affected by the design of the fluidized bed. It
is advantageous that the process is a net heat producer. It is also
noteworthy that this design sequesters 1,100 tonnes of carbon
dioxide a year.
[0105] All patent applications, published patent applications,
issued and granted patents, texts, and literature references cited
in this specification are hereby incorporated herein by reference
in their entirety to more fully describe the state of the art to
which the present invention pertains.
[0106] As various changes can be made in the above methods and
compositions without departing from the scope and spirit of the
invention as described, it is intended that all subject matter
contained in the above description, shown in the accompanying
drawings, or defined in the appended claims be interpreted as
illustrative, and not in a limiting sense.
* * * * *