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Pergamon
Energy Com'ers. Mgml Vol. 38. Suppl., pp. $265-$271, 1997
Plh
S0196-8904(96)00280-4
~::~ 1997 Elsevier Science Ltd. All rights reserved
Printed in Great Britain
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OCEAN SYSTEMS FOR MANAGING
THE GLOBAL CARBON CYCLE
DWAiN F. SPENCER, PRINCIPAL, SIMTECHE
WHEELER J. NORTH, PROFESSOR EMERITUS, CALTECH
24 Fairway
Place,
Half Moon Bay,
CA
94019
Kerckhoff Marine Laboratory, Caltech
I01 Dahlia Street, Corona Del Mar, CA
92625
ABSTRACT
Carbon dioxide is formed in all processes utilizing fossil fuels.
C o n t r o l l i n g the emissions of CO 2 from a number of processes by
forming CO 2 hydrates (clathrates), may be an effective approach
for both absorbing CO 2 from m u l t i c o m p o n e n t gas streams (Flue
gases, Anaerobic digester gases, etc.) and sequestering CO 2 in
the deep oceans.
Further, ocean marine farms may be an effective
process for extracting CO 2 from the atmosphere and forming both
v a l u a b l e products and rejecting excess CO 2, in the form of
clathrates, to the deep ocean.
Preliminary engineering analyses
indicates that clathrate formation for controlling both
conventional fossil fuel gaseous CO 2 emissions and those
a s s o c i a t e d with marine farm anaerobic digester gases may provide
a meaningful control strategy for CO 2. © 1997ElsevierScienceLtd
KEYWORDS
Carbon dioxide clathrates; extraction from multicomponent
oceanic sequestration; marine farm systems.
gases;
INTRODUCTION
Over the last 20 years, increasing global emissions of carbon
dioxide from fossil resources, and the potential impacts on the
earths' atmosphere, have become a subject of intense scientific
investigation.
Major emphasis has been placed on assessing the
range of impact of these increased emissions and methods to
reduce or restrict the increasing use of fossil fuels.
Much less emphasis is being placed on the research, development,
and d e m o n s t r a t i o n of engineered systems to manage and control
CO 2 emissions, p a r t i c u l a r i l y from stationary fossil sources,
such as powerplants, chemical plants, petroleum refineries, etc.
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The p u r p o s e of this paper is to summarize key new fundamental
r e s e a r c h on the f o r m a t i o n of CO 2 c l a t h r a t e s from pure CO 2 gas
streams and to discuss the potential for co 2 clathrate formation from multi c o m p o n e n t gas streams.
Typical e m i s s i o n
streams i n c l u d i n g
p o w e r p l a n t stack gases, coal or p e t r o l e u m
coke s y n t h e s i s gases, natural g a s / c a r b o n dioxide mixtures, etc.
are all p o t e n t i a l sources for c a r b o n dioxide e x t r a c t i o n and
fixation, in the form of CO 2 clathrates.
Once formed, these solid c l a t h r a t e s can be injected, in slurry
form, into the o c e a n or deep aquifers for CO 2 sequestration.
Finally, an active CO 2 m a n a g e m e n t and control system will be
d i s c u s s e d which a) extracts CO 2 from the atmosphere, at a
d i s t r i b u t e d level, i.e. non-point source, b) converts the marine
c a r b o n to natural gas and carbon dioxide, c) sequesters the
c a r b o n d i o x i d e in the deep ocean, and d) provides liquified
natural gas, as well as other high v a l u e d products, as articles
of commerce.
F O R M A T I O N OF C A R B O N D I O X I D E C L A T H R A T E S
It is well known that liquid or gaseous carbon dioxide when
m i x e d w i t h water at t e m p e r a t u r e s below 10°C and pressures in the
range of 10-70 bars, form solid c l a t h r a t e s (Takenouchi and
Kennedy, 1965).
These c l a t h r a t e s t y p i c a l l y have 2 to 8 CO 2
molecules, bound into a m a t r i x of 46 water molecules.
If full
lattice o c c u p a n c y is achieved, 8 CO 2 m o l e c u l e s are trapped within
the m a t r i x of 46 water molecules.
This produces a mole fraction
of CO 2 of only 0.148, but a weight fraction of 0.36.
The p u r p o s e of a r e s e a r c h program, u n d e r t a k e n at Caltech and
s p o n s o r e d by the E l e c t r i c Power R e s e a r c h Institute, was to
d e t e r m i n e how such c l a t h r a t e s could be formed under minimal
p r e s s u r e conditions; thus r e d u c i n g c o m p r e s s i o n r e q u i r e m e n t s for
CO 2, or CO 2 c o n t a i n i n g g a s e o u s mixtures.
This work was reported
in a n u m b e r of EPRI p r o g r e s s reports (North, et al., 1993; North,
et al., 1995) and in a paper (North,et al., 1993).
Initial e x p e r i m e n t s were c o n d u c t e d in a batch chamber, at
t e m p e r a t u r e s of 4-6°C and p r e s s u r e s of 65 to 200 bars.
Through
a series of experiments, stable CO 2 c l a t h r a t e s with greater than
25% by weight can now be formed at -i to 0°C (salt water) and
p r e s s u r e s of 12 to 14 bars.
These g r e a t l y reduced formation
c o n d i t i o n s have been a c h i e v e d by d e v e l o p i n g an empirical model
of CO 2 c l a t h r a t e formation, which indicates three distinct phases
in the d e v e l o p m e n t of stable c l a t h r a t e formation, namely a) CO 2
d i s s o l v i n g in water, b) h y d r a t e p r e c u r s o r s forming i.e.
s t r u c t u r e d water, and c) final h y d r a t e formation with the
i n t r o d u c t i o n of a d d i t i o n a l CO2.
The research then moved to the use of two i n t e r c o n n e c t e d semi
c o n t i n u o u s flow b u b b l i n g reactors for c l a t h r a t e formation and
f o r m a t i o n p r e s s u r e s were reduced to the 12 to 14 bar levels.
These reduced formation c o n d i t i o n s are a c h i e v e d by stepwise
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f o r m a t i o n / d e c o m p o s i t i o n of the clathrate.
Each subsequent
clathrate formation step occurs at a lower pressure, indicating
that the n u c l e a t e d water has a "memory" from the previous
formation.
Stable formation pressures of 13 bar were achieved,
at reactor temperatures slightly below zero centigrade.
Finally, two continuous flow reactors, designed by D. Johnson, a
project subcontractor, were operated in late 1995.
Our previous
studies had shown that CO 2 hydrate formation at 12-14 bars
p r e s s u r e would most likely require two or three reactor stages,
to ensure that an adequate quantity of precursors occurred in
the stream of CO2-water entering the final stage.
The Johnson Fluidic Venturi consists of two coverging cylindrical
chambers, the eductor tube and plenum, discharging into a common
terminal space, the tail tube.
Two Fluidic Venturis were
constructed.
The second unit produced excellent gaseous CO 2,
water mixing at flow rates of 0.36 liters per second.
Unit 2
could accommodate about 120 grams of CO 2 per second and produce
1.7 metric tons of hydrates per hour.
This represent near full
saturation of the lattice, as the CO 2 weight fraction is 0.33.
Optimal operation occurred when the water pressure in the plenum
was a p p r o x i m a t e l y 6.7 bars greater than the CO 2 gas pressure i.e.
the water educts the CO 2 into the Fluidic Venturi.
Therefore, it
may be possible to further reduce the CO 2 compression
requirements for second stage injection, if the Fluidic Venturi
is used as the clathrate formation reactor.
In a fully
e n g i n e e r e d system, it is likely that a two stage reactor system
would
be employed.
The first to form the clathrate precursors,
and
the second for clathrate production.
A p p r o x i m a t e l y 25% of
the CO 2 is absorbed in the precursor stage, so this portion might
be compressed to 20 bars.
The remaining 75% would be compressed
only to 12-14 bars, and perhaps less, if the eductor tube
provides 6-7 bars of suction.
With these low pressures for
stable clathrate formation, one can consider hydrate formation
from atmospheric pressure CO 2 streams, such as flue gases, as
well as p r e s s u r i z e d streams, such as found in synthesis gas
p r o d u c t i o n from natural gas, coal, petroleum, coke, etc.
CLATHRATE
FORMATION
FROM MULTI
COMPONENT
GAS STREAMS
All of the clathrate formation research performed to date has
been with pure CO 2 gases.
In order to obtain a pure CO 2 stream
from a multi component gas, conventional separaton techniques
subject the multi-component gas to a n u m b e r of energy consuming
steps, namely (1) absorption of the CO 2 from the gaseous stream
by a host solvent e.g. m o n o e t h a n o l a m i n e or Selexol, (2) removal
of CO 2 from the host solvent e.g. by steam stripping, and (3)
c o m p r e s s i o n and cooling of the stripped CO 2 for clathrate
formation or alternatively,
(4) compressing or liquifying the
pure CO 2 gas stream for pipelining or injection into the ocean.
Recently,
clathrate
one of us (Spencer), has conceived an approach for CO 2
formation from multicomponent gas streams, including
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a) Powerplant Flue Gases, b) Turbo Charged Boiler Flue Gases,
c)
Coal G a s i f i c a t i o n Product Gases, d) Shifted Coal G a s i f i c a t i o n
Product Gas, and e) Anaerobic Digester Product Gas.
In these
applications, the exhaust (flue) gas from processes a and e are
at a t m o s p h e r i c pressure, and processes b,c, and d at pressures
from I0 to 60 b a r s .
Mole fractions of CO 2 in the multi component gas streams vary from 0.02 to 0.50 over the range of these
applications.
Due to the e x t r e m e l y high p r e f e r r e d solubility of carbon dioxide
in water, and even higher p r e f e r r e d CO 2 solubility in nucleated
water, a single step process for extracting CO 2 from these
gaseous streams, forming CO 2 clathrates, and clathrate slurries,
has been designed.
The CO 2 is now ready for injection as a CO 2water slurry into either the ocean or a deep aquifer system.
A
U.S. patent a p p l i c a t i o n has been filed (Spencer,1996), earlier
this year.
At this point, only theoretical separation efficiencies have been
developed.
Comparisons of the energy efficiency for separating
and p i p e l i n i n g CO 2 using conventional amine s c r u b b i n g / s t r i p p i n g
techniques e . g . m o n o e t h a n o l a m i n e ,
with the new process of forming
CO 2 clathrates directly, have been d e v e l o p e d for both a 500 Mwe
conventional coal fired powerplant and utilizing Selexol for CO 2
separation in an integrated coal g a s i f i c a t i o n combined cycle
(IGCC) powerplant.
The base energy losses for both of these
systems are taken from Smelser (1991).
The energy loss
a s s o c i a t e d with s e p a r a t i o n and pipelining of CO 2 using
c o n v e n t i o n a l t e c h n o l o g y from a conventional coal fired powerplant
is e s t i m a t e d to consume 35 percent of the power output.
The
similar loss for an integrated
coal g a s i f i c a t i o n combined cycle
powerplant is 12 percent.
Theoretical estimates of these losses, if the new clathrate
formation process is u t i l i z e d for both separation and slurry
pipelining, are 9.0 and 4.4 percent respectively for the
conventional coal plant or the IGCC powerplant.
Thus, the energy
losses, as well as process simplification, are much more
attractive u t i l i z i n g the new clathrate separation process.
Hidy and Spencer have submitted a grant a p p l i c a t i o n to the U.S.
Environmental Protection Agency for a 3 year research program
entitled, "Exploratory D e s i g n and E v a l u a t i o n of a New CO 2 Removal
Process" (Hidy, et al., 1996).
This work is planned to p e r f o r m e d
at the U n i v e r s i t y of R i v e r s i d e Center for Environmental Research
and Technology, in c o n j u n c t i o n with Dr. North of Caltech.
At
least, two f u n d a m e n t a l l y different reactor configurations will be
tested, one a con-current flow reactor, the second, a
c o u n t e r c u r r e n t flow reactor.
CLATHRATE
SEQUESTRATION
IN THE OCEAN
As d i s c u s s e d above, it is e n v i s a g e d that slurries of CO 2
clathrate will be formed for injection into the ocean or deep
aquifers.
With a slurry c o n c e n t r a t i o n of 50 percent by weight
clathrate, the CO 2 content of the slurries is approximately 16-
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17 percent. The specific gravity of the clathrates are
a p p r o x i m a t e l y 1.16, so that the hydrate should slowly settle
through sea water after injection and will either redisolve or
sink to the ocean floor. (Cole,et al., 1993) have shown that
the deep ocean can be a repository for at least 1200 gigatons of
CO 2, with minimal change in oceanic acidity.
This amount is
equivalent to a p p r o x i m a t e l y 2 pre-industrial atmospheric CO 2
contents.
If the clathrates reach the sediments, it is estimated that an
additional 2800 gigatons of carbon could be sequestered.
Of
course, these estimates assume uniform dispersal over the sea
floor or injection in a deep advection zone such as the North
Atlantic Deep Water flow.
In any case, there is clearly a large
repository potential in the deep oceans.
Pumping clathrate
slurries to depth of 700 meters or so and discharging the
slurries in a d o w n w a r d flow direction should allow complete
s e q u e s t r a t i o n of the CO 2 for many centuries, if not permanently.
MARINE
FARM SYSTEMS
FOR GLOBAL
CARBON MANAGEMENT
One of us, (Spencer,1991), has proposed large scale floating
macroalgal systems for CO 2 fixation from the atmosphere.
These "so called" marine farms, each some i00,000 acres in
extent, would not only fix airborne CO2, but also, produce
valuable byproducts. By harvesting and processing species such as
M a c r o c y s t i s Pyrifera, e.g. by anaerobically digesting
these
species to form a p p r o x i m a t e l y a 50:50 mixture of methane and
carbon dioxide, a portion of the carbon is used to produce clean
fuels and the remainder of the carbon can be sequestered.
The
carbon dioxide, representing approximately 50 percent of the
anaerobic digester gas, would be stripped from the methane in the
form of CO 2 clathrates and injected as a slurry into the ocean.
The methane would be liquified and become an article of commerce.
Other coproducts can also be produced from these marine farms.
A I00,000 acre marine farm fixes approximately 1 million tons of
carbon per year, of which 50 percent is recycled as fuel
(methane) and 50 percent is sequestered as CO 2 clathrates in the
ocean.
Thus; I000 such marine farms w o u l d be necessary for
managing 1 gigaton of carbon annually.
By utilizing anaerobic digestion of the harvested macro algal
species, such as Macrocystis Pyrifera, 90-95 percent of the
nutrients (nitrogen and phosphorus) are retained in the digester
liquors and can be recycled to the farm to provide the necessary
nutrients.
Upwelled seawater would provide the "makeup"
nutrients necessary for e q u i l i b r i u m macro algal production.
Overall system and process designs have been developed (SIMTECHE
P r o p r i e t a r y Information); however a full p r e l i m i n a r y design of
the system has not yet been conducted.
Very preliminary
p e r f o r m a n c e and costs indicate that such a system could be
e c o m i c a l l y viable, if a broad product slate of high value
products can be established.
Such a
multi-component
product
slate could
include
a) Food-
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algae, fish and invertebrate polycultures, b) Biopolymers, such
as agar, algin, carogens, c) Organics, such as acetone, and
organic acids, d) Fuels, such as alcohols, in addition to
methane, etc.
This type
of floating, tended farm system could
serve many useful purposes, and assist in the distribution of
CO 2 clathrates being injected over large ocean areas.
SUMMARY
This paper summarized some recent experimental work focused
c o n t r o l l i n g CO 2 emissions from stationary sources.
It has
that systematic study of the properties of CO2-H20 mixtures
conditions, can greatly reduce the energy necessary to fix
sequester CO 2.
on
shown
and
and
Further, recent advances have shown that a) stable, high mole
fraction CO 2 clathrates can be formed in continuous reactors, b)
CO 2 clathrate formation from a range of multi component gaseous
streams is feasible and for some gases, very easily achieved,
and c) system concepts w h i c h include global CO 2 management can
be integrated with p r o d u c t i v e marine farm systems for fuels,
chemicals, etc. and clathrate injection of the CO 2 waste stream.
A l t h o u g h this is only the b e g i n n i n g of the search for meaningful
engineered systems to manage C02, the efforts to date, are both
promising and exciting.
A l t h o u g h the cost effectiveness of
these systems must yet be determined, the fundamental reductions
of energy losses a s s o c i a t e d with these systems, compared to
conventional techniques, indicate that this cost effectiveness
should be achieved, as e n g i n e e r i n g and operational data are
obtained.
REFERENCES
Cole, K.H., Stegen, G.R., and Spencer, D.F., The capacity of the
deep oceans to absorb carbon dioxide, Presented at IEA
International Conference on Climate Change, Oxford, U.K. (1993).
Hidy, G.M., and Spencer, D.F., Grant a p p l i c a t i o n for Exploratory
design and e v a l u a t i o n of a new CO 2 removal process, Research
proposal submitted to U.S. Environment Protection Agency, (1996).
North, W.J., Morgan, J.J., Johnson, D.E., Spencer, D.F., Use of
hydrate for s e q u e s t e r i n g CO 2 in the deep ocean , Second U.S./
Japan w o r k s h o p on m i t i g a t i o n and a d a p t a t i o n technologies related
to climate change. Honolulu, Hawaii (1993).
North, W.J. and Morgan, J.J., Investigation of CO 2 hydrate
formation and dissolution, EPRI Progress Report for I October
1991 to 30 June, 1993.(1993)
North, W.J. and Morgan, J.J., Investigations of CO 2 hydrate
formation and dissolution, EPRI Progress Report for 1 July 1993
to 31 December, 1995 (In Publication).(1995)
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Smelser, S.C., Stock, R.M., and McCleary, G.J., Engineering and
Economic Evaluation of CO 2 Removal from Fossil-Fuel-Fired Power
Plants-Vols 1,2, EPRI IE-7365, J u n e 1991.
Spencer, D.F., Open ocean macroalgal farms for CO 2 mitigation and
energy production, Proceedings of IEA International Conference on
Technology Responses to Global Environmental Challenges, Kyoto,
Japan (1991).
Spencer, D.F., Methods of selectively separating CO 2 from a
multicomponent gaseous stream, U.S. Patent Application No.
081643,151, 30 April 1996.(1996)
Takenouchi, S. and Kennedy, G.C., Dissociation pressures of the
phase C02-5 3/4 H20, J. Geol., 73, 383-390, (1965).