- Email: [email protected]
Experiments on the ocean sequestration of fossil fuel CO2: pH measurements and hydrate formation
Marine Chemistry 72 Ž2000. 83–93 www.elsevier.nlrlocatermarchem
Experiments on the ocean sequestration of fossil fuel CO 2 : pH measurements and hydrate formation Peter G. Brewer a,) , Edward T. Peltzer a , Gernot Friederich a , Izuo Aya b, Kenji Yamane b a
Monterey Bay Aquarium Research Institute, 7700 Sandholdt Road, Moss Landing, CA 95039-9644, USA Ship Research Institute, Ministry of Transport, 3-5-10 Amanogahara, Katano, Osaka 576-0034, Japan
b
Received 24 June 1999; received in revised form 9 November 1999; accepted 7 August 2000
Abstract We have carried out a series of in situ experiments to investigate the formation of a CO 2 hydrate ŽCO 2 :5.75 H 2 O. for the purpose of evaluating scenarios for ocean fossil fuel CO 2 disposal with a solid hydrate as the sequestered form. The experiments were carried out with a remotely operated vehicle in Monterey Bay at a depth of 619 m. pH measurements made in close proximity to the hydrate–seawater interface showed a wide range of values, depending upon the method of injection and the surface area of the hydrate formed. Rapid injection of liquid CO 2 into an inverted beaker to form a flocculant mass of hydrate resulted in pH initially as low as 4.5 within a few centimeters of the interface, decaying slowly over 1–2 h towards normal seawater values as dense CO 2 rich brine drained from the hydrate mass. In a second experiment, slower injection of the liquid CO 2 to produce a simple two-layer system with a near planar interface of liquid CO 2 with a thin hydrate film yielded pH values indistinguishable from the in situ ocean background level of 7.6. Both field and laboratory results now show that the dissolution rate of a mass of CO 2 hydrate in seawater is slow but finite. q 2000 Elsevier Science B.V. All rights reserved. Keywords: Ocean sequestration; Fossil fuel CO 2 ; pH measurements; Hydrate formation
1. Introduction Ocean chemists have long explored the remarkable changes in ocean CO 2 chemistry now being forced by the rapid rise in atmospheric CO 2 levels ŽBrewer, 1978; Chen and Millero, 1979; Tsunogai et al., 1993; Goyet et al., 1999.. But few ocean chemists have yet considered the problems that may arise ) Corresponding author. Tel.: q1-831-775-1706; fax: q1-831775-1620. E-mail address: [email protected] ŽP.G. Brewer..
from attempts to accelerate the ocean uptake of fossil fuel CO 2 by direct injection as part of societal schemes to stabilize greenhouse gas concentrations, and thereby climate, early in the 21st century. The first attempt at slowing the growth of greenhouse gas concentrations, the Kyoto Protocol to the United Nations Framework Convention on Climate Change, has as its goal Ato achieve stabilization of greenhouse gas concentrations in the atmosphere at a level which would prevent dangerous anthropogenic interference with the climate systemB. The protocol itself presently limits formal consideration of sinks to
0304-4203r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 4 2 0 3 Ž 0 0 . 0 0 0 7 4 - 8
84
P.G. Brewer et al.r Marine Chemistry 72 (2000) 83–93
forestation initiatives, but encourages research on all forms of carbon sequestration strategies of which ocean disposal is but one. Direct ocean CO 2 disposal was first suggested by Marchetti Ž1977., and a large literature of policy ŽParson and Keith, 1998., engineering ŽKobayashi, 1995., and laboratory pressure vessel studies ŽAya et al., 1997. is now available. The collection of papers on ADirect Ocean Disposal of Carbon DioxideB edited by Handa and Ohsumi Ž1995. provides an excellent introduction. Any assessment of the quantities of CO 2 that must be accommodated in order to stabilize climate yields a very large number. Wigley et al. Ž1996. carefully examined economic and environmental choices in CO 2 stabilization. They compared the widely accepted IPCC IS92a growth scenario with the action necessary to stabilize atmopheric CO 2 levels. For stabilization at 550 ppmv, approxi-
mately double the pre-industrial value, approximately 1 Gt Cryr by 2025, and 4 Gt Cryr by 2050, would have to be accounted for by a combination of efficiency, substitution, or sequestration scenarios. These quantities approach, or may ultimately exceed, those fluxes occurring now by natural processes, and if ocean disposal is acted upon it could profoundly change the chemical signatures we see today. Most deep ocean CO 2 disposal scenarios envisage a hydrate as the sequestered form ŽHarrison et al., 1995., although direct dissolution has also been considered ŽHaugan and Drange, 1992.. CO 2 forms a hydrate with great facility at relatively shallow depths ŽSloan, 1990; Brewer et al. 1998.. Although the density of CO 2 hydrate is greater than seawater Ž1.10–1.12 g cmy3 vs. 1.03 g cmy3 at 600 m, Aya et al., 1992; Ohmura and Mori, 1998., and plans to use this density difference to generate a sinking
Fig. 1. CTD temperature profiles for Ventana dives 1532 Ždashed line. and 1533 Ždotted line. showing the phase boundary for the gas–liquid CO 2 transition ŽPerry, 1950., and the phase boundary for hydrate stability ŽOhgaki et al., 1993. region Žshaded area..
P.G. Brewer et al.r Marine Chemistry 72 (2000) 83–93
plume have been discussed ŽHolder et al., 1995., our experience has been that the ensemble of a liquid CO 2 droplet coated with a thin hydrate skin is buoyant ŽBrewer et al., 1999.. However, liquid CO 2 is far more compressible than seawater, and below about 2600-m depth a sinking plume of pure CO 2 can be generated. The first field experiment to test this was carried out in 1990 by Honda et al. Ž1995., who used the manned submersible AShinkai 6500B. They carried initially solid CO 2 , contained in a transparent acrylic cylinder, to 3073-m depth, and visually observed the changes taking place. Brewer et al. Ž1999. recently reported on the first controlled experiment in which the release of a gravitationally stable mass of liquid CO 2 at 3627-m depth quickly resulted in massive hydrate formation. The injection of large quantities of liquid CO 2 to depths approaching 3000 m is technically difficult, and thus there is great interest in examining fundamental processes at much shallower depths ŽHaugan and Drange, 1992. where injection for sequestration has also been discussed. We report here the results of a field experiment, carried out November 17 and 18, 1998 in Monterey Bay, CA, with controlled release of carbon dioxide at a depth of 619 m in order to test the physical and chemical characteristics of the hydrate formed. In particular, we sought to examine, by means of pH measurements and dye injection, the release rate of CO 2 through dissolution of the hydrate mass under varying conditions of formation by changing injection technology. The phase boundary for the gas–liquid CO 2 transition ŽPerry, 1950., the phase boundary for CO 2 hydrate formation ŽOhgaki et al., 1993., and the local temperature profile at the time of the experiment are shown in Fig. 1.
2. Methods 2.1. ROV We have used the ROV AVentanaB operated by the Monterey Bay Aquarium Research Institute from the research vessel APoint LobosB, to carry out our experiments. The basic vehicle has a depth rating of 1850 m and is powered by a 40-hp electrorhydraulic power pack. The vehicle is linked to the surface by a Kevlar armored tether with five copper conductors
85
and an optical fiber core of 10 elements that carry all control and telemetry signals. Imaging is provided by a Sony DXC-3000 three-chip color video camera with a Fujinon 5.5 to 40 mm zoom lens through which we observed and recorded the experiments. A Conductivity–Temperature–Pressure sensor ŽCTD; Sea Bird Instruments. is mounted on the vehicle and data is telemetered in real time to the control room. 2.2. CO2 release system Liquid CO 2 was expelled into an inverted 4-l beaker attached to a short anchored mooring line and maintained about 1.5 m above the sea floor ŽFig. 2a. for each experiment described here. However, the apparatus used for dispensing the CO 2 differed slightly for each dive. For both dives, the liquid carbon dioxide was contained in a Parker Hydraulic Piston Accumulator ŽPart No. A4N0578D3K. equipped with ALo TempeB nitrile o-rings. This cylinder has an internal volume of approximately 9 l; by applying sufficient pressure to one side of the piston, the liquid contained on the other side can be slowly dispensed. For Dive 1532, we used a hydraulic piston on the ROV Žnormally used to extend the tool-sled drawer. to deliver seawater to the cylinder and drive-out the liquid CO 2 . On dive 1533, we used a paired hydraulic piston pump. One piston Žof the pair. could be driven in both directions hydraulically by the ROV. It was mechanically coupled to the second piston of the pair. With each stroke of the paired piston pump, seawater was expelled from one side of the second piston and simultaneously filled the other side. In this way, multiple 127-ml aliquots of seawater could be delivered to the piston accumulator driving out equivalent volumes of liquid CO 2 . By counting strokes, a measured volume of CO 2 was delivered to the experimental apparatus. A quarterturn on–off valve was used on the outlet of the piston accumulator cylinder to control release of the liquid CO 2 . For dive 1532, 1r8Y o.d. polypropylene tubing was used to connect between the on–off valve and the 4-l beaker. For dive 1533, we desired a slower delivery, so a 1r4Y i.d. tygon tube was used to dispense the liquid CO 2 . 2.3. pH sensor We used a glass combination pH electrode ŽAgrAgClrref electrode, Sea Bird Instruments
86
P.G. Brewer et al.r Marine Chemistry 72 (2000) 83–93
Fig. 2. Ža. Wide-angle image of the inverted beaker containing the flocculant hydrate mass of large surface area generated on dive 1532 with Žb. the pH electrode placed in close proximity to the hydrate–seawater boundary by the vehicle arm. The electrode is inverted in comparison to normal laboratory procedure in order to place the sensing tip close to the buoyant hydrate mass. Dye injected into the beaker showed a sinking plume of dense fluid falling, with an estimated residence time of water in the beaker of tens of seconds.
Model SBE-18. modified by the manufacturer to be deployed at depth. The output from the pH electrode
is internally buffered and offset to yield an analog output signal voltage between 0 and 5 V. For the
P.G. Brewer et al.r Marine Chemistry 72 (2000) 83–93
purpose of this test we used the data channel on the ROV that is normally used for the transmissometer signal. The analog output signal from the pH probe was digitized at the ROV, telemetered in real time via the ROV tether to the control room aboard the RrV Point Lobos, and the 12-bit signal was recorded using the on-board computer data acquisition system. Conversion of the recorded voltages to pH was done in a post-dive processing mode. The factory calibration was used to convert the output voltage of the probe to pH: pH s 7 q Ž VOU T y offset. r Ž K = T = slope. , where, VOU T is the pH probe analog output voltage, K s Ž RrF . = ln10 s 1.98416 = 10y4 V Ky1 , R is the gas constant Ž8.31434 J Ky1 moly1 ., F is the Faraday constant Ž9.64867 = 10 4 C moly1 ., and T is temperature in degrees Kelvin. This equation is a modification of the standard Nernst equation to accommodate the internal signal offset and amplification. Electrode calibration Ždetermination of the slope and offset variables. was performed using standard NBS buffers. Although these buffers are commonly used in most laboratory work, they are less than ideal for seawater applications ŽMillero, 1986; Dickson, 1993a.. If desired, these values can be converted to the seawater pH scale ŽMillero et al., 1988. with an uncertainty of less than 0.01 pH unit ŽBrewer et al., 1995.. Since this was our first attempt to deploy a pH electrode from the ROV, we deemed this calibration suitable for preliminary test purposes. For future work, calibration using TRIS seawater buffers will be required to obtain data comparable to the seawater pH scale ŽUNESCO, 1987; Dickson, 1993b.. The absence of a pressure term in the equation used to convert output voltage to pH reflects the fact that pressure has little effect on the output of the pH probe in this depth range. The primary effect of pressure on pH electrodes is to change the asymmetry potential ŽDisteche, 1959. but this effect is small, ` about 0.01 pH units at 600-m depth ŽPark, 1966.. Of greater importance is the effect of pressure on the dissociation constant of carbonic acid which makes the interpretation of the pH data more complex as the stability constants which are well known for one atmosphere of pressure cannot be used at higher pressures Ž60 atm. without some error. However, for
87
the experiments described here, the relative changes in pH near the hydrate mass rather than the absolute seawater values are the topic of interest.
3. Results The results obtained are from a combination of video imagery of the experiment, which enabled the physical behavior of the system to be observed, and pH data obtained in close proximity to the hydrate mass ŽFig. 2b.. In the following report, all times are given in UTC to correspond to the time sequence laid down on the videotape and the automated ROV data archive. The local temperature profile is shown in Fig. 1, and background oxygen levels were low. The ocean pH profiles as recorded by the vehicle held electrode are shown in Fig. 3. DiÕe 1532. 11r17r98. 619-m depth. T s 4.778C, S s 34.296‰. The experiment was initiated at 21:53 h with the energetic injection of liquid CO 2 which rose to the top of the beaker, instantly forming a
Fig. 3. pH profiles for the water column obtained from the ROV mounted pH electrode from the down portion of the dive only. At present, we do not have an explanation for the small offset in pH between the two dives. One possibility is that this is due to a change in the zero offset of the sensor input circuit on the ROV. Alternatively, this may be due to an improper correction for temperature or hysteresis of the pH probe. Further investigation is required to discern the cause of the offset between dives.
88
P.G. Brewer et al.r Marine Chemistry 72 (2000) 83–93
bright white frothy mass due to the strong turbulent mixing of water and CO 2 ŽFig. 3.. Several pulses of CO 2 followed, with injection terminated at 21:55 h with the inverted beaker about half full of the hydrate mass. The vehicle arm holding the pH electrode, upside down so as to place the sensor tip close to the hydrate, was then maneuvered carefully into position. As the electrode first approached the base of the beaker a marked drop in pH from the ambient seawater value of 7.6 to about 4.5 was recorded ŽFig. 4, event A.. After a few minutes observation the electrode was removed at 22:08 h ŽFig. 4, event B. and a small amount of a fluorescein dye solution, made up to closely approximate the density of seawater, was injected at 22:12 h to observe mixing within the beaker. The dye was observed to mix downwards from the hydrate surface quite rapidly, and could be imaged as a plume falling towards the sea floor. The dye residence time within the beaker was less than 2 min. The electrode was re-inserted at 22:18 h ŽFig. 4, event C., and held in place to 23:45 h. The hydrate
Fig. 4. Plot of pH versus time for the experiment carried out on dive 1532. Initially, the electrode was outside of the beaker. When the electrode is placed within the beaker and close to the hydrate mass ŽA and C., the data reveal a stream of low pH, CO 2 rich water, descending within the beaker. When the electrode is removed from the beaker ŽB and D., the pH returns to the initial ambient reading. The amount of CO 2 transferred to the aqueous phase declines with time and hence the pH rises Ždata points C thru D. throughout the experiment.
mass slowly consolidated as the fine-grained flocculant material originally formed consolidated into larger granules, with the surrounding fluid becoming apparently ApureB liquid CO 2 . A gradual increase in pH in the seawater below this interface, from about 5.0 to 6.0, was observed during this period. On removing the electrode at the end of this time, it quickly recorded normal ocean background levels ŽFig. 4, event D.. DiÕe 1533. 11r18r98. 617-m depth. T s 4.748C, S s 34.310‰. In an attempt to provide a contrast with the experimental results obtained on dive 1532, we elected to change the injection strategy while keeping other experimental components near constant Žpressure, temperature, pH electrode, beaker etc... The mechanics of the CO 2 delivery system were changed Žsee Section 2. so as to permit more careful control of dispensed amounts. The release of CO 2 into the inverted beaker then was contrived to yield a simpler more plane liquid CO 2 –water interface by adding CO 2 until the level moved below the tip of the injection tube. No further water was then entrained into the entering liquid CO 2 stream, and the hydrate mass initially formed quickly became flooded with liquid CO 2 , which then formed the water contact surface. A hydrate layer could be observed at the interface ŽFig. 5., and the liquid CO 2 mass contained many hydrate crystals. The results obtained from the pH measurements were dramatically different from those obtained in the previous experiment, and they are shown in Fig. 6. While the beaker was being filled with liquid CO 2 , the pH electrode recorded the ambient seawater pH ŽFig. 6, event A.. At about 22:10 h, the pH electrode was carefully placed, by the vehicle arm, so that the sensing tip was about 1–1.5 cm from the interface ŽFig. 6, event B.. pH levels indistinguishable from the background ocean signal were observed; at about 22:14 the electrode was carefully moved so as to very briefly touch the interface, and a sharp drop in pH was recorded ŽFig. 6, event C., proving that the sensor was working well, and that very large chemical gradients were present. These gradients were however on a much smaller length scale than in our previous experiment, with the hydrate interface now providing a strong barrier against CO 2 diffusion into the aqueous phase. In addition, the interface was now much closer to the mouth of the beaker than in
P.G. Brewer et al.r Marine Chemistry 72 (2000) 83–93
89
Fig. 5. Image of the hydrate mass generated on dive 1533. In contrast to the experiment shown in Fig. 3, the manner of CO 2 injection was carefully planned so as to flood the inverted beaker with liquid, resulting in a far smaller hydrate–seawater interface. The pH electrode is again shown Ža. close to the interface and Žb. in direct contact with the interface. Dye injection similarly revealed a short water residence time due to water mass motions past the mouth of the beaker, but the dye plume did not show as strong a density contrast as in Fig. 3.
our earlier experiment, where it was exposed to better mixing and faster exchange with passing sea-
water. The electrode was held in place until about 23:29 h ŽFig. 6, event D.. The signal was extraordi-
90
P.G. Brewer et al.r Marine Chemistry 72 (2000) 83–93
We hypothesize that rapid dissolution occurred at this depth.
4. Discussion
Fig. 6. Plot of pH versus time for the experiment carried out on dive 1533. ŽA. The pH probe is outside the beaker; ŽB. the pH probe is placed inside the beaker; and ŽD. the pH probe is withdrawn from inside the beaker. In contrast to the signal obtained in the earlier dive, the pH recorded is identical to that of the open ocean, and shows no trend with time. We interpret this as resulting from the generation of a simple two-layer liquid CO 2 – water system, with a thin hydrate film between the two. The relatively small surface area permits only a very small loss of CO 2 to the aqueous phase. The spike of low pH at approximately 22:10 h ŽC. results from deliberately moving the electrode into very brief contact with the liquid CO 2 surface.
narily stable during this time; sampling of several data sections within this 1.5-h period showed a standard deviation of only "0.001 pH units. Free release of CO2 into the water column. At the end of dive 1533, at 23:30 h, we rotated the beaker containing the liquid CO 2 and hydrate, and observed the rising stream of liquid globules by flying the vehicle upwards while attempting to keep the material in view. In this way, we were able to obtain an approximate ascent rate for the droplets. The mass of CO 2 quickly broke up into small droplets of about 0.5–1 cm diameter which rose through the water column. Some evidence of either a small hydrate AtailB on individual globules, or simply an attached marine snow fragment, was seen as the material rose, but this did not seem to materially affect the rise rate. Visual contact was lost as the liquid–gas phase boundary ŽFig. 1. at about 380 m, and the limit of hydrate stability, was approached.
The choice of a controlled field experiment to investigate these issues offers a bridge between laboratory studies ŽHanda and Ohsumi, 1995., and larger-scale less controlled releases. It permits realistic evaluation of fluid motions, possible ecological consequences Žsee paper by Tamburri et al., 2000, this volume., and the possible effects of inclusion of natural particles and trace substances. However, the techniques of instrumenting such an experiment are still in their infancy, and not all of the desired data to obtain a full description has been obtained at present. In each experiment, the mass of CO 2 hydrate formed was strongly buoyant, in spite of the great difference in manner of injection. This has been observed by others, and it arises from the fact that although CO 2 hydrate itself is denser than seawater ŽOhmura and Mori, 1998., liquid CO 2 is not until pressures equivalent to about 2600-m water depth are reached. The ensemble density of a small droplet of CO 2 , wrapped with a thin hydrate skin, is such that the unit is buoyant. Holder et al. Ž1995. have given the following equation for the ensemble density of a particle of hydrate forming liquid CO 2 Ž rp . as a weighted average of the densities of the liquid CO 2 Ž rc . and solid hydrate Ž r h .:
rp s Ž 1 y x h . q Ž 1 q n Ž 18r44. . x h r Ž 1 y x h . rrc q Ž n Ž 18r44. . x hrr h , where n is the hydrate number and x h is the mass fraction of carbon dioxide converted to hydrate. For the conditions of our experiment at 620 m depth, T s 4.748C and salinitys 34.310‰, seawater has an in situ density of 1.030 g cmy3 , and with a hydrate density of about 1.116 g cmy3 ŽAya et al., 1992., hydrate must comprise approximately 54% Žby volume. of the particle to achieve neutral buoyancy. This translates into a hydrate skin thickness of about 0.2281 of the radius of the particle or about 2.3 mm for a 2-cm-diameter droplet. In spite of many efforts to model or predict sinking behavior in this depth region ŽHolder et al.,
P.G. Brewer et al.r Marine Chemistry 72 (2000) 83–93
1995., our field experience so far has been that this has not yet been achieved with a CO 2 –water system alone. This simple fact presents a major barrier to plans for CO 2 disposal as a hydrate at shallow to intermediate depths. This problem has been recognized, and a number of strategies to overcome it have been proposed. For instance, Hirai et al. Ž1999. have described a nozzle technology for creating small water droplets within the released CO 2 droplets, thus creating hydrate growth within the released CO 2 . We may expect a range of innovative approaches to be tried as experience is gained, and field experiments become more practical. We plan to field test new approaches to the creation of a sinking mass in the near future. It is clear that the manner of injection, and the vigor of CO 2 –water mixing, has a profound effect on the pattern of hydrate formation and the effectiveness of mass transfer of CO 2 to the ocean Žcompare Figs. 4 and 6.. Rapid injection of liquid or gaseous CO 2 results in a flocculant hydrate mass with a relatively large surface area-to-volume ratio that dissolves relatively quickly. Slow injection yields a homogenous mass of liquid or gaseous CO 2 with little water entrainment and little hydrate formation with a small surface area per unit volume. This homogenous mass tends to dissolve relatively slowly. Thus, nozzle technology and injection velocity have important effects. The formation of a hydrate ŽCO 2 :6 H 2 O. is an exothermic process, and for CO 2 the heat released is quite large. Ohgaki et al. Ž1993. report that the enthalpy change of reaction, D H Žreact.., defined as the difference between the overall D H and the heat of dissolution, was close to 50 kJ moly1 , and 60% larger than the enthalpy change of forming 6 mol of water ice. We did not have a temperature sensor in the hydrate mass during the formation process, but we may assume that heat was generated within the white flocculant mass, and that this may have contributed in some positive way to the overall buoyancy. The process of hydrate formation also rejects salt in a manner very similar to the formation of water ice. We could not place a conductivity head in the same location as the pH probe, but again we may assume that significant brine rejection occurred during hydrate formation. As the heat of reaction dissi-
91
pated the density of the brine must have increased, and the downward flow of a CO 2 rich fluid, draining out of the hydrate mass, and observed by our pH electrode Žas shown in Fig. 4. must have occurred. The partial molal volume of CO 2 in seawater is about 31 cm3 moly1 ŽOhsumi et al, 1992., the solubility of CO 2 in seawater at high pressure and low temperature is high, and thus CO 2 saturated water shows a strong density increase over the natural background. Ohsumi et al. Ž1992. calculated the effect of the increased density on the diffusion of a deep-sea plume surrounding a CO 2 release. The sinking pH 5.5 fluid draining from our hydrate mass, and falling below the beaker as seen in the dye stream, is entirely consistent with this effect. It is possible from this first experiment to make only very approximate calculations on the pH data set. Given normal alkalinities of the water in the CO 2 rich plume and a pH of about 5.5, we would estimate more than 10 mM TCO 2 in the fluid; we could not estimate the pH or CO 2 content within the frothy white hydraterwaterrCO2 mass. The undetectable D pH in the fluid just below a near planar CO 2rhydraterseawater interface Žsee Figs. 5B and 6. may be roughly assessed as follows: taking the D pH to be - 0.005, the volume sensed by the electrode at the mouth of the beaker to be about 0.2 l, and the flushing time to be about 10 s, and the surface area of hydrate exposed to be 180 cm2 , then the maximum rate of release of CO 2 into solution from the hydrate surface must be less than 3.4 = 10y1 0 mol cmy2 sy1 . Such a low release rate has important consequences for estimating the survival time of a hydrate mass in the deep sea, and for enhancing the sequestration of fossil fuel CO 2 . It is desirable to compare these estimates from a field experiment with laboratory studies, but the conditions of these two approaches have not yet been well matched. Aya et al. Ž1997. measured the dissolution rate of a CO 2 droplet, in a flow of varying velocity, at 30 MPa. These data were used by Mori and Mochizuki Ž1998. in a study of the mechanism controlling the rate of release of CO 2 into the aqueous phase adjacent to a hydrate film. They then calculated the change in size, due to dissolution, of an ascending stream of 1-cm diameter CO 2 droplets released in the ocean at 1500-m depth, and suggested that dissolution would be complete by about 1200-m
92
P.G. Brewer et al.r Marine Chemistry 72 (2000) 83–93
depth. Our experiments were at shallower depth than this, but nonetheless covered an approximately 300-m depth interval for the released droplets. We observed much slower rates of reduction in droplet size than was calculated by Mori and Mochizuki Ž1998.. And our rough estimate, based on the pH data shown in Fig. 6, is of a slower dissolution rate through the hydrate film than has previously been reported. The hydrate phenomenon is of critical importance for ocean CO 2 sequestration scenarios. At great depth ŽBrewer et al., 1999., it can create a dense solid mass that greatly enhances sequestration times and inhibits transfer to the seawater phase. At shallower depths this inhibition of CO 2 dissolution works against effective sequestration if it results in a plume of droplets rising to the depth of the hydrate phase boundary with little mass transfer to the ocean.
Acknowledgements This work was supported by a grant to MBARI from the David and Lucile Packard Foundation, and for I.A. and K.Y. by support of the Ship Research Institute. These experiments could not have been carried out without the exceptional support provided by the captain and crew of the RV APoint LobosB, and by the skilled pilots and technicians of the ROV AVentanaB. We would also like to thank Drs. S. Tsunogai, K.A. Hunter and an anonymous reviewer for their most helpful comments and suggested revisions to an earlier version of this manuscript.
References Aya, I., Yamane, K., Yamada, N., 1992. Stability of clathrate–hydrate of carbon dioxide in highly pressurized water. Fundamentals of Phase Change: Freezing, Melting, and Sublimation. In: Kroeger, P.G., Bayazitoglu, Y. ŽEds.., Amer. Soc. Mech. Eng. HTD vol. 215, pp. 17–22, New York, NY. Aya, I., Yamane, K., Nariai, H., 1997. Solubility of CO 2 and density of CO 2 hydrate at 30 MPa. Energy 22, 263–271. Brewer, P.G., 1978. Direct observation of the oceanic CO 2 increase. Geophys. Res. Lett. 5, 997–1000. Brewer, P.G., Glover, D.M., Goyet, C., Shafer, D.K., 1995. The pH of the North Atlantic Ocean: improvements to the global model for sound absorption in seawater. J. Geophys. Res. 100, 8761–8776. Brewer, P.G., Orr, F.M. Jr., Friederich, G., Kvenvolden, K.A.,
Orange, D.L., 1998. Gas hydrate formation in the deep sea: in situ experiments with controlled release of methane, natural gas, and carbon dioxide. Energy Fuels 12, 183–188. Brewer, P.G., Friederich, G., Peltzer, E.T., Orr, F.M. Jr., 1999. Direct experiments on the ocean disposal of fossil fuel CO 2 . Science 284, 943–945. Chen, C.-T.A., Millero, F.J., 1979. Gradual increase of oceanic CO 2 . Nature 277, 205–206. Dickson, A.G., 1993a. The measurement of seawater pH. Mar. Chem. 44, 131–142. Dickson, A.G., 1993b. pH buffers for seawater media based on the total hydrogen ion concentration scale. Deep-Sea Res. 40, 107–118. Disteche, A., 1959. pH measurements with a glass electrode ` withstanding 1500 kgrcm2 hydrostatic pressure. Rev. Sci. Instrum. 30, 474–478. Goyet, C., Coatanoan, C., Eischeid, G., Amakoa, T., Okuda, K., Healy, R., Tsunogai, S., 1999. Spatial variation of total CO 2 and total alkalinity in the northern Indian Ocean: a novel approach for the quantification of anthropogenic CO 2 in seawater. J. Mar. Res. 57, 135–163. Handa, N., Ohsumi, T., 1995. Direct Ocean Disposal of Carbon Dioxide. Terra Scientific Publishing, Tokyo, 274 pp. Harrison, W.J., Wendtland, R.F., Sloan, E.D., 1995. Geochemical interactions resulting from carbon dioxide disposal on the sea floor. Appl. Geochem. 10, 461–475. Haugan, P.M., Drange, H., 1992. Sequestration of CO 2 in the deep ocean by shallow injection. Nature 357, 318–320. Hirai, S., Tabe, Y., Tanaka, G., Okazaki, K., 1999. Advanced CO 2 sequestration technologies in intermediate depth of ocean. Proceedings 5th ASMErJSME Thermal Engineering Joint Conference, San Diego. Holder, G.D., Cugini, A.V., Warzinski, R.P., 1995. Modeling clathrate hydrate formation during carbon dioxide injection into the ocean. Environ. Sci. Technol. 28, 276–278. Honda, M., Hashimoto, J., Naka, J., Hotta, H., 1995. CO 2 hydrate formation and inversion of density between liquid CO 2 and H 2 O in deep sea: experimental study using submersible AShinkai 6500B. In: Handa, N., Ohsumi, T. ŽEds.., Direct Ocean Disposal of Carbon Dioxide. Terrapub, Tokyo, pp. 35–43. Kobayashi, Y., 1995. Physical behavior of liquid CO 2 in the ocean. In: Handa, N., Ohsumi, T. ŽEds.., Direct Ocean Disposal of Carbon Dioxide. Terrapub, Tokyo, pp. 165–181. Marchetti, C., 1977. On geoengineering and the CO 2 problem. Clim. Change 1, 59–68. Millero, F.J., 1986. The pH of estuarine waters. Limnol. Oceanogr. 31, 839–847. Millero, F.J., Plese, T., Fernandez, M., 1988. The dissociation of hydrogen sulfide in seawater. Limnol. Oceanogr. 33, 269–274. Mori, Y.H., Mochizuki, T., 1998. Dissolution of liquid CO 2 into water at high pressures: a search for the mechanism of dissolution being retarded through hydrate-film formation. Energy Convers. Manage. 39, 567–578. Ohgaki, K., Makihara, Y., Takano, K., 1993. Formation of CO 2 hydrate in pure and seawaters. J. Chem. Eng. Jpn. 26, 558–564. Ohmura, R., Mori, Y.H., 1998. Critical conditions for CO 2 hy-
P.G. Brewer et al.r Marine Chemistry 72 (2000) 83–93 drate films to rest on submarine CO 2 pond surfaces: a mechanistic study. Environ. Sci. Technol. 32, 1120–1127. Ohsumi, T., Nakashiki, N., Shitashima, K., Hirama, K., 1992. Density change of water due to dissolution of carbon dioxide and near-field behavior of CO 2 from a source on deep-sea floor. Energy Convers. Manage. 33, 685–690. Park, K., 1966. Deep-Sea pH. Science 154, 1540–1542. Parson, E.A., Keith, D.W., 1998. Fossil fuels without CO 2 emissions. Science 282, 1053–1054. Perry, J.H. ŽEd.., Chemical Engineers’ Handbook. 3rd edn. McGraw-Hill, New York, p. 254, Table 208. Sloan, E.D., 1990. Clathrate Hydrates of Natural Gases. Marcel Dekker, New York, p. 641.
93
Tamburri, M.N., Peltzer, E.T., Friederich, G.E., Aya, I., Yamane, K., Brewer, P.G., 2000. A field study of the effects of CO 2 ocean disposal on mobile deep-sea animals. Mar. Chem. 72, 95–101. Tsunogai, S., Ono, T., Watanabe, S., 1993. Increase in total carbonate in the western North Pacific water and a hypothesis on the missing sink of anthropogenic carbon. J. Oceanogr. 49, 305–315. UNESCO, 1987. Thermodynamics of carbon dioxide systems in seawater. UNESCO Tech. Pap. Mar. Sci. 51. Paris, 55 pp. Wigley, T.M.L., Richels, R., Edmonds, J.A., 1996. Economic and environmental choices in the stabilization of atmospheric CO 2 concentrations. Nature 379, 240–243.
Experiments on the ocean sequestration of fossil fuel CO 2 : pH measurements and hydrate formation Peter G. Brewer a,) , Edward T. Peltzer a , Gernot Friederich a , Izuo Aya b, Kenji Yamane b a
Monterey Bay Aquarium Research Institute, 7700 Sandholdt Road, Moss Landing, CA 95039-9644, USA Ship Research Institute, Ministry of Transport, 3-5-10 Amanogahara, Katano, Osaka 576-0034, Japan
b
Received 24 June 1999; received in revised form 9 November 1999; accepted 7 August 2000
Abstract We have carried out a series of in situ experiments to investigate the formation of a CO 2 hydrate ŽCO 2 :5.75 H 2 O. for the purpose of evaluating scenarios for ocean fossil fuel CO 2 disposal with a solid hydrate as the sequestered form. The experiments were carried out with a remotely operated vehicle in Monterey Bay at a depth of 619 m. pH measurements made in close proximity to the hydrate–seawater interface showed a wide range of values, depending upon the method of injection and the surface area of the hydrate formed. Rapid injection of liquid CO 2 into an inverted beaker to form a flocculant mass of hydrate resulted in pH initially as low as 4.5 within a few centimeters of the interface, decaying slowly over 1–2 h towards normal seawater values as dense CO 2 rich brine drained from the hydrate mass. In a second experiment, slower injection of the liquid CO 2 to produce a simple two-layer system with a near planar interface of liquid CO 2 with a thin hydrate film yielded pH values indistinguishable from the in situ ocean background level of 7.6. Both field and laboratory results now show that the dissolution rate of a mass of CO 2 hydrate in seawater is slow but finite. q 2000 Elsevier Science B.V. All rights reserved. Keywords: Ocean sequestration; Fossil fuel CO 2 ; pH measurements; Hydrate formation
1. Introduction Ocean chemists have long explored the remarkable changes in ocean CO 2 chemistry now being forced by the rapid rise in atmospheric CO 2 levels ŽBrewer, 1978; Chen and Millero, 1979; Tsunogai et al., 1993; Goyet et al., 1999.. But few ocean chemists have yet considered the problems that may arise ) Corresponding author. Tel.: q1-831-775-1706; fax: q1-831775-1620. E-mail address: [email protected] ŽP.G. Brewer..
from attempts to accelerate the ocean uptake of fossil fuel CO 2 by direct injection as part of societal schemes to stabilize greenhouse gas concentrations, and thereby climate, early in the 21st century. The first attempt at slowing the growth of greenhouse gas concentrations, the Kyoto Protocol to the United Nations Framework Convention on Climate Change, has as its goal Ato achieve stabilization of greenhouse gas concentrations in the atmosphere at a level which would prevent dangerous anthropogenic interference with the climate systemB. The protocol itself presently limits formal consideration of sinks to
0304-4203r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 4 2 0 3 Ž 0 0 . 0 0 0 7 4 - 8
84
P.G. Brewer et al.r Marine Chemistry 72 (2000) 83–93
forestation initiatives, but encourages research on all forms of carbon sequestration strategies of which ocean disposal is but one. Direct ocean CO 2 disposal was first suggested by Marchetti Ž1977., and a large literature of policy ŽParson and Keith, 1998., engineering ŽKobayashi, 1995., and laboratory pressure vessel studies ŽAya et al., 1997. is now available. The collection of papers on ADirect Ocean Disposal of Carbon DioxideB edited by Handa and Ohsumi Ž1995. provides an excellent introduction. Any assessment of the quantities of CO 2 that must be accommodated in order to stabilize climate yields a very large number. Wigley et al. Ž1996. carefully examined economic and environmental choices in CO 2 stabilization. They compared the widely accepted IPCC IS92a growth scenario with the action necessary to stabilize atmopheric CO 2 levels. For stabilization at 550 ppmv, approxi-
mately double the pre-industrial value, approximately 1 Gt Cryr by 2025, and 4 Gt Cryr by 2050, would have to be accounted for by a combination of efficiency, substitution, or sequestration scenarios. These quantities approach, or may ultimately exceed, those fluxes occurring now by natural processes, and if ocean disposal is acted upon it could profoundly change the chemical signatures we see today. Most deep ocean CO 2 disposal scenarios envisage a hydrate as the sequestered form ŽHarrison et al., 1995., although direct dissolution has also been considered ŽHaugan and Drange, 1992.. CO 2 forms a hydrate with great facility at relatively shallow depths ŽSloan, 1990; Brewer et al. 1998.. Although the density of CO 2 hydrate is greater than seawater Ž1.10–1.12 g cmy3 vs. 1.03 g cmy3 at 600 m, Aya et al., 1992; Ohmura and Mori, 1998., and plans to use this density difference to generate a sinking
Fig. 1. CTD temperature profiles for Ventana dives 1532 Ždashed line. and 1533 Ždotted line. showing the phase boundary for the gas–liquid CO 2 transition ŽPerry, 1950., and the phase boundary for hydrate stability ŽOhgaki et al., 1993. region Žshaded area..
P.G. Brewer et al.r Marine Chemistry 72 (2000) 83–93
plume have been discussed ŽHolder et al., 1995., our experience has been that the ensemble of a liquid CO 2 droplet coated with a thin hydrate skin is buoyant ŽBrewer et al., 1999.. However, liquid CO 2 is far more compressible than seawater, and below about 2600-m depth a sinking plume of pure CO 2 can be generated. The first field experiment to test this was carried out in 1990 by Honda et al. Ž1995., who used the manned submersible AShinkai 6500B. They carried initially solid CO 2 , contained in a transparent acrylic cylinder, to 3073-m depth, and visually observed the changes taking place. Brewer et al. Ž1999. recently reported on the first controlled experiment in which the release of a gravitationally stable mass of liquid CO 2 at 3627-m depth quickly resulted in massive hydrate formation. The injection of large quantities of liquid CO 2 to depths approaching 3000 m is technically difficult, and thus there is great interest in examining fundamental processes at much shallower depths ŽHaugan and Drange, 1992. where injection for sequestration has also been discussed. We report here the results of a field experiment, carried out November 17 and 18, 1998 in Monterey Bay, CA, with controlled release of carbon dioxide at a depth of 619 m in order to test the physical and chemical characteristics of the hydrate formed. In particular, we sought to examine, by means of pH measurements and dye injection, the release rate of CO 2 through dissolution of the hydrate mass under varying conditions of formation by changing injection technology. The phase boundary for the gas–liquid CO 2 transition ŽPerry, 1950., the phase boundary for CO 2 hydrate formation ŽOhgaki et al., 1993., and the local temperature profile at the time of the experiment are shown in Fig. 1.
2. Methods 2.1. ROV We have used the ROV AVentanaB operated by the Monterey Bay Aquarium Research Institute from the research vessel APoint LobosB, to carry out our experiments. The basic vehicle has a depth rating of 1850 m and is powered by a 40-hp electrorhydraulic power pack. The vehicle is linked to the surface by a Kevlar armored tether with five copper conductors
85
and an optical fiber core of 10 elements that carry all control and telemetry signals. Imaging is provided by a Sony DXC-3000 three-chip color video camera with a Fujinon 5.5 to 40 mm zoom lens through which we observed and recorded the experiments. A Conductivity–Temperature–Pressure sensor ŽCTD; Sea Bird Instruments. is mounted on the vehicle and data is telemetered in real time to the control room. 2.2. CO2 release system Liquid CO 2 was expelled into an inverted 4-l beaker attached to a short anchored mooring line and maintained about 1.5 m above the sea floor ŽFig. 2a. for each experiment described here. However, the apparatus used for dispensing the CO 2 differed slightly for each dive. For both dives, the liquid carbon dioxide was contained in a Parker Hydraulic Piston Accumulator ŽPart No. A4N0578D3K. equipped with ALo TempeB nitrile o-rings. This cylinder has an internal volume of approximately 9 l; by applying sufficient pressure to one side of the piston, the liquid contained on the other side can be slowly dispensed. For Dive 1532, we used a hydraulic piston on the ROV Žnormally used to extend the tool-sled drawer. to deliver seawater to the cylinder and drive-out the liquid CO 2 . On dive 1533, we used a paired hydraulic piston pump. One piston Žof the pair. could be driven in both directions hydraulically by the ROV. It was mechanically coupled to the second piston of the pair. With each stroke of the paired piston pump, seawater was expelled from one side of the second piston and simultaneously filled the other side. In this way, multiple 127-ml aliquots of seawater could be delivered to the piston accumulator driving out equivalent volumes of liquid CO 2 . By counting strokes, a measured volume of CO 2 was delivered to the experimental apparatus. A quarterturn on–off valve was used on the outlet of the piston accumulator cylinder to control release of the liquid CO 2 . For dive 1532, 1r8Y o.d. polypropylene tubing was used to connect between the on–off valve and the 4-l beaker. For dive 1533, we desired a slower delivery, so a 1r4Y i.d. tygon tube was used to dispense the liquid CO 2 . 2.3. pH sensor We used a glass combination pH electrode ŽAgrAgClrref electrode, Sea Bird Instruments
86
P.G. Brewer et al.r Marine Chemistry 72 (2000) 83–93
Fig. 2. Ža. Wide-angle image of the inverted beaker containing the flocculant hydrate mass of large surface area generated on dive 1532 with Žb. the pH electrode placed in close proximity to the hydrate–seawater boundary by the vehicle arm. The electrode is inverted in comparison to normal laboratory procedure in order to place the sensing tip close to the buoyant hydrate mass. Dye injected into the beaker showed a sinking plume of dense fluid falling, with an estimated residence time of water in the beaker of tens of seconds.
Model SBE-18. modified by the manufacturer to be deployed at depth. The output from the pH electrode
is internally buffered and offset to yield an analog output signal voltage between 0 and 5 V. For the
P.G. Brewer et al.r Marine Chemistry 72 (2000) 83–93
purpose of this test we used the data channel on the ROV that is normally used for the transmissometer signal. The analog output signal from the pH probe was digitized at the ROV, telemetered in real time via the ROV tether to the control room aboard the RrV Point Lobos, and the 12-bit signal was recorded using the on-board computer data acquisition system. Conversion of the recorded voltages to pH was done in a post-dive processing mode. The factory calibration was used to convert the output voltage of the probe to pH: pH s 7 q Ž VOU T y offset. r Ž K = T = slope. , where, VOU T is the pH probe analog output voltage, K s Ž RrF . = ln10 s 1.98416 = 10y4 V Ky1 , R is the gas constant Ž8.31434 J Ky1 moly1 ., F is the Faraday constant Ž9.64867 = 10 4 C moly1 ., and T is temperature in degrees Kelvin. This equation is a modification of the standard Nernst equation to accommodate the internal signal offset and amplification. Electrode calibration Ždetermination of the slope and offset variables. was performed using standard NBS buffers. Although these buffers are commonly used in most laboratory work, they are less than ideal for seawater applications ŽMillero, 1986; Dickson, 1993a.. If desired, these values can be converted to the seawater pH scale ŽMillero et al., 1988. with an uncertainty of less than 0.01 pH unit ŽBrewer et al., 1995.. Since this was our first attempt to deploy a pH electrode from the ROV, we deemed this calibration suitable for preliminary test purposes. For future work, calibration using TRIS seawater buffers will be required to obtain data comparable to the seawater pH scale ŽUNESCO, 1987; Dickson, 1993b.. The absence of a pressure term in the equation used to convert output voltage to pH reflects the fact that pressure has little effect on the output of the pH probe in this depth range. The primary effect of pressure on pH electrodes is to change the asymmetry potential ŽDisteche, 1959. but this effect is small, ` about 0.01 pH units at 600-m depth ŽPark, 1966.. Of greater importance is the effect of pressure on the dissociation constant of carbonic acid which makes the interpretation of the pH data more complex as the stability constants which are well known for one atmosphere of pressure cannot be used at higher pressures Ž60 atm. without some error. However, for
87
the experiments described here, the relative changes in pH near the hydrate mass rather than the absolute seawater values are the topic of interest.
3. Results The results obtained are from a combination of video imagery of the experiment, which enabled the physical behavior of the system to be observed, and pH data obtained in close proximity to the hydrate mass ŽFig. 2b.. In the following report, all times are given in UTC to correspond to the time sequence laid down on the videotape and the automated ROV data archive. The local temperature profile is shown in Fig. 1, and background oxygen levels were low. The ocean pH profiles as recorded by the vehicle held electrode are shown in Fig. 3. DiÕe 1532. 11r17r98. 619-m depth. T s 4.778C, S s 34.296‰. The experiment was initiated at 21:53 h with the energetic injection of liquid CO 2 which rose to the top of the beaker, instantly forming a
Fig. 3. pH profiles for the water column obtained from the ROV mounted pH electrode from the down portion of the dive only. At present, we do not have an explanation for the small offset in pH between the two dives. One possibility is that this is due to a change in the zero offset of the sensor input circuit on the ROV. Alternatively, this may be due to an improper correction for temperature or hysteresis of the pH probe. Further investigation is required to discern the cause of the offset between dives.
88
P.G. Brewer et al.r Marine Chemistry 72 (2000) 83–93
bright white frothy mass due to the strong turbulent mixing of water and CO 2 ŽFig. 3.. Several pulses of CO 2 followed, with injection terminated at 21:55 h with the inverted beaker about half full of the hydrate mass. The vehicle arm holding the pH electrode, upside down so as to place the sensor tip close to the hydrate, was then maneuvered carefully into position. As the electrode first approached the base of the beaker a marked drop in pH from the ambient seawater value of 7.6 to about 4.5 was recorded ŽFig. 4, event A.. After a few minutes observation the electrode was removed at 22:08 h ŽFig. 4, event B. and a small amount of a fluorescein dye solution, made up to closely approximate the density of seawater, was injected at 22:12 h to observe mixing within the beaker. The dye was observed to mix downwards from the hydrate surface quite rapidly, and could be imaged as a plume falling towards the sea floor. The dye residence time within the beaker was less than 2 min. The electrode was re-inserted at 22:18 h ŽFig. 4, event C., and held in place to 23:45 h. The hydrate
Fig. 4. Plot of pH versus time for the experiment carried out on dive 1532. Initially, the electrode was outside of the beaker. When the electrode is placed within the beaker and close to the hydrate mass ŽA and C., the data reveal a stream of low pH, CO 2 rich water, descending within the beaker. When the electrode is removed from the beaker ŽB and D., the pH returns to the initial ambient reading. The amount of CO 2 transferred to the aqueous phase declines with time and hence the pH rises Ždata points C thru D. throughout the experiment.
mass slowly consolidated as the fine-grained flocculant material originally formed consolidated into larger granules, with the surrounding fluid becoming apparently ApureB liquid CO 2 . A gradual increase in pH in the seawater below this interface, from about 5.0 to 6.0, was observed during this period. On removing the electrode at the end of this time, it quickly recorded normal ocean background levels ŽFig. 4, event D.. DiÕe 1533. 11r18r98. 617-m depth. T s 4.748C, S s 34.310‰. In an attempt to provide a contrast with the experimental results obtained on dive 1532, we elected to change the injection strategy while keeping other experimental components near constant Žpressure, temperature, pH electrode, beaker etc... The mechanics of the CO 2 delivery system were changed Žsee Section 2. so as to permit more careful control of dispensed amounts. The release of CO 2 into the inverted beaker then was contrived to yield a simpler more plane liquid CO 2 –water interface by adding CO 2 until the level moved below the tip of the injection tube. No further water was then entrained into the entering liquid CO 2 stream, and the hydrate mass initially formed quickly became flooded with liquid CO 2 , which then formed the water contact surface. A hydrate layer could be observed at the interface ŽFig. 5., and the liquid CO 2 mass contained many hydrate crystals. The results obtained from the pH measurements were dramatically different from those obtained in the previous experiment, and they are shown in Fig. 6. While the beaker was being filled with liquid CO 2 , the pH electrode recorded the ambient seawater pH ŽFig. 6, event A.. At about 22:10 h, the pH electrode was carefully placed, by the vehicle arm, so that the sensing tip was about 1–1.5 cm from the interface ŽFig. 6, event B.. pH levels indistinguishable from the background ocean signal were observed; at about 22:14 the electrode was carefully moved so as to very briefly touch the interface, and a sharp drop in pH was recorded ŽFig. 6, event C., proving that the sensor was working well, and that very large chemical gradients were present. These gradients were however on a much smaller length scale than in our previous experiment, with the hydrate interface now providing a strong barrier against CO 2 diffusion into the aqueous phase. In addition, the interface was now much closer to the mouth of the beaker than in
P.G. Brewer et al.r Marine Chemistry 72 (2000) 83–93
89
Fig. 5. Image of the hydrate mass generated on dive 1533. In contrast to the experiment shown in Fig. 3, the manner of CO 2 injection was carefully planned so as to flood the inverted beaker with liquid, resulting in a far smaller hydrate–seawater interface. The pH electrode is again shown Ža. close to the interface and Žb. in direct contact with the interface. Dye injection similarly revealed a short water residence time due to water mass motions past the mouth of the beaker, but the dye plume did not show as strong a density contrast as in Fig. 3.
our earlier experiment, where it was exposed to better mixing and faster exchange with passing sea-
water. The electrode was held in place until about 23:29 h ŽFig. 6, event D.. The signal was extraordi-
90
P.G. Brewer et al.r Marine Chemistry 72 (2000) 83–93
We hypothesize that rapid dissolution occurred at this depth.
4. Discussion
Fig. 6. Plot of pH versus time for the experiment carried out on dive 1533. ŽA. The pH probe is outside the beaker; ŽB. the pH probe is placed inside the beaker; and ŽD. the pH probe is withdrawn from inside the beaker. In contrast to the signal obtained in the earlier dive, the pH recorded is identical to that of the open ocean, and shows no trend with time. We interpret this as resulting from the generation of a simple two-layer liquid CO 2 – water system, with a thin hydrate film between the two. The relatively small surface area permits only a very small loss of CO 2 to the aqueous phase. The spike of low pH at approximately 22:10 h ŽC. results from deliberately moving the electrode into very brief contact with the liquid CO 2 surface.
narily stable during this time; sampling of several data sections within this 1.5-h period showed a standard deviation of only "0.001 pH units. Free release of CO2 into the water column. At the end of dive 1533, at 23:30 h, we rotated the beaker containing the liquid CO 2 and hydrate, and observed the rising stream of liquid globules by flying the vehicle upwards while attempting to keep the material in view. In this way, we were able to obtain an approximate ascent rate for the droplets. The mass of CO 2 quickly broke up into small droplets of about 0.5–1 cm diameter which rose through the water column. Some evidence of either a small hydrate AtailB on individual globules, or simply an attached marine snow fragment, was seen as the material rose, but this did not seem to materially affect the rise rate. Visual contact was lost as the liquid–gas phase boundary ŽFig. 1. at about 380 m, and the limit of hydrate stability, was approached.
The choice of a controlled field experiment to investigate these issues offers a bridge between laboratory studies ŽHanda and Ohsumi, 1995., and larger-scale less controlled releases. It permits realistic evaluation of fluid motions, possible ecological consequences Žsee paper by Tamburri et al., 2000, this volume., and the possible effects of inclusion of natural particles and trace substances. However, the techniques of instrumenting such an experiment are still in their infancy, and not all of the desired data to obtain a full description has been obtained at present. In each experiment, the mass of CO 2 hydrate formed was strongly buoyant, in spite of the great difference in manner of injection. This has been observed by others, and it arises from the fact that although CO 2 hydrate itself is denser than seawater ŽOhmura and Mori, 1998., liquid CO 2 is not until pressures equivalent to about 2600-m water depth are reached. The ensemble density of a small droplet of CO 2 , wrapped with a thin hydrate skin, is such that the unit is buoyant. Holder et al. Ž1995. have given the following equation for the ensemble density of a particle of hydrate forming liquid CO 2 Ž rp . as a weighted average of the densities of the liquid CO 2 Ž rc . and solid hydrate Ž r h .:
rp s Ž 1 y x h . q Ž 1 q n Ž 18r44. . x h r Ž 1 y x h . rrc q Ž n Ž 18r44. . x hrr h , where n is the hydrate number and x h is the mass fraction of carbon dioxide converted to hydrate. For the conditions of our experiment at 620 m depth, T s 4.748C and salinitys 34.310‰, seawater has an in situ density of 1.030 g cmy3 , and with a hydrate density of about 1.116 g cmy3 ŽAya et al., 1992., hydrate must comprise approximately 54% Žby volume. of the particle to achieve neutral buoyancy. This translates into a hydrate skin thickness of about 0.2281 of the radius of the particle or about 2.3 mm for a 2-cm-diameter droplet. In spite of many efforts to model or predict sinking behavior in this depth region ŽHolder et al.,
P.G. Brewer et al.r Marine Chemistry 72 (2000) 83–93
1995., our field experience so far has been that this has not yet been achieved with a CO 2 –water system alone. This simple fact presents a major barrier to plans for CO 2 disposal as a hydrate at shallow to intermediate depths. This problem has been recognized, and a number of strategies to overcome it have been proposed. For instance, Hirai et al. Ž1999. have described a nozzle technology for creating small water droplets within the released CO 2 droplets, thus creating hydrate growth within the released CO 2 . We may expect a range of innovative approaches to be tried as experience is gained, and field experiments become more practical. We plan to field test new approaches to the creation of a sinking mass in the near future. It is clear that the manner of injection, and the vigor of CO 2 –water mixing, has a profound effect on the pattern of hydrate formation and the effectiveness of mass transfer of CO 2 to the ocean Žcompare Figs. 4 and 6.. Rapid injection of liquid or gaseous CO 2 results in a flocculant hydrate mass with a relatively large surface area-to-volume ratio that dissolves relatively quickly. Slow injection yields a homogenous mass of liquid or gaseous CO 2 with little water entrainment and little hydrate formation with a small surface area per unit volume. This homogenous mass tends to dissolve relatively slowly. Thus, nozzle technology and injection velocity have important effects. The formation of a hydrate ŽCO 2 :6 H 2 O. is an exothermic process, and for CO 2 the heat released is quite large. Ohgaki et al. Ž1993. report that the enthalpy change of reaction, D H Žreact.., defined as the difference between the overall D H and the heat of dissolution, was close to 50 kJ moly1 , and 60% larger than the enthalpy change of forming 6 mol of water ice. We did not have a temperature sensor in the hydrate mass during the formation process, but we may assume that heat was generated within the white flocculant mass, and that this may have contributed in some positive way to the overall buoyancy. The process of hydrate formation also rejects salt in a manner very similar to the formation of water ice. We could not place a conductivity head in the same location as the pH probe, but again we may assume that significant brine rejection occurred during hydrate formation. As the heat of reaction dissi-
91
pated the density of the brine must have increased, and the downward flow of a CO 2 rich fluid, draining out of the hydrate mass, and observed by our pH electrode Žas shown in Fig. 4. must have occurred. The partial molal volume of CO 2 in seawater is about 31 cm3 moly1 ŽOhsumi et al, 1992., the solubility of CO 2 in seawater at high pressure and low temperature is high, and thus CO 2 saturated water shows a strong density increase over the natural background. Ohsumi et al. Ž1992. calculated the effect of the increased density on the diffusion of a deep-sea plume surrounding a CO 2 release. The sinking pH 5.5 fluid draining from our hydrate mass, and falling below the beaker as seen in the dye stream, is entirely consistent with this effect. It is possible from this first experiment to make only very approximate calculations on the pH data set. Given normal alkalinities of the water in the CO 2 rich plume and a pH of about 5.5, we would estimate more than 10 mM TCO 2 in the fluid; we could not estimate the pH or CO 2 content within the frothy white hydraterwaterrCO2 mass. The undetectable D pH in the fluid just below a near planar CO 2rhydraterseawater interface Žsee Figs. 5B and 6. may be roughly assessed as follows: taking the D pH to be - 0.005, the volume sensed by the electrode at the mouth of the beaker to be about 0.2 l, and the flushing time to be about 10 s, and the surface area of hydrate exposed to be 180 cm2 , then the maximum rate of release of CO 2 into solution from the hydrate surface must be less than 3.4 = 10y1 0 mol cmy2 sy1 . Such a low release rate has important consequences for estimating the survival time of a hydrate mass in the deep sea, and for enhancing the sequestration of fossil fuel CO 2 . It is desirable to compare these estimates from a field experiment with laboratory studies, but the conditions of these two approaches have not yet been well matched. Aya et al. Ž1997. measured the dissolution rate of a CO 2 droplet, in a flow of varying velocity, at 30 MPa. These data were used by Mori and Mochizuki Ž1998. in a study of the mechanism controlling the rate of release of CO 2 into the aqueous phase adjacent to a hydrate film. They then calculated the change in size, due to dissolution, of an ascending stream of 1-cm diameter CO 2 droplets released in the ocean at 1500-m depth, and suggested that dissolution would be complete by about 1200-m
92
P.G. Brewer et al.r Marine Chemistry 72 (2000) 83–93
depth. Our experiments were at shallower depth than this, but nonetheless covered an approximately 300-m depth interval for the released droplets. We observed much slower rates of reduction in droplet size than was calculated by Mori and Mochizuki Ž1998.. And our rough estimate, based on the pH data shown in Fig. 6, is of a slower dissolution rate through the hydrate film than has previously been reported. The hydrate phenomenon is of critical importance for ocean CO 2 sequestration scenarios. At great depth ŽBrewer et al., 1999., it can create a dense solid mass that greatly enhances sequestration times and inhibits transfer to the seawater phase. At shallower depths this inhibition of CO 2 dissolution works against effective sequestration if it results in a plume of droplets rising to the depth of the hydrate phase boundary with little mass transfer to the ocean.
Acknowledgements This work was supported by a grant to MBARI from the David and Lucile Packard Foundation, and for I.A. and K.Y. by support of the Ship Research Institute. These experiments could not have been carried out without the exceptional support provided by the captain and crew of the RV APoint LobosB, and by the skilled pilots and technicians of the ROV AVentanaB. We would also like to thank Drs. S. Tsunogai, K.A. Hunter and an anonymous reviewer for their most helpful comments and suggested revisions to an earlier version of this manuscript.
References Aya, I., Yamane, K., Yamada, N., 1992. Stability of clathrate–hydrate of carbon dioxide in highly pressurized water. Fundamentals of Phase Change: Freezing, Melting, and Sublimation. In: Kroeger, P.G., Bayazitoglu, Y. ŽEds.., Amer. Soc. Mech. Eng. HTD vol. 215, pp. 17–22, New York, NY. Aya, I., Yamane, K., Nariai, H., 1997. Solubility of CO 2 and density of CO 2 hydrate at 30 MPa. Energy 22, 263–271. Brewer, P.G., 1978. Direct observation of the oceanic CO 2 increase. Geophys. Res. Lett. 5, 997–1000. Brewer, P.G., Glover, D.M., Goyet, C., Shafer, D.K., 1995. The pH of the North Atlantic Ocean: improvements to the global model for sound absorption in seawater. J. Geophys. Res. 100, 8761–8776. Brewer, P.G., Orr, F.M. Jr., Friederich, G., Kvenvolden, K.A.,
Orange, D.L., 1998. Gas hydrate formation in the deep sea: in situ experiments with controlled release of methane, natural gas, and carbon dioxide. Energy Fuels 12, 183–188. Brewer, P.G., Friederich, G., Peltzer, E.T., Orr, F.M. Jr., 1999. Direct experiments on the ocean disposal of fossil fuel CO 2 . Science 284, 943–945. Chen, C.-T.A., Millero, F.J., 1979. Gradual increase of oceanic CO 2 . Nature 277, 205–206. Dickson, A.G., 1993a. The measurement of seawater pH. Mar. Chem. 44, 131–142. Dickson, A.G., 1993b. pH buffers for seawater media based on the total hydrogen ion concentration scale. Deep-Sea Res. 40, 107–118. Disteche, A., 1959. pH measurements with a glass electrode ` withstanding 1500 kgrcm2 hydrostatic pressure. Rev. Sci. Instrum. 30, 474–478. Goyet, C., Coatanoan, C., Eischeid, G., Amakoa, T., Okuda, K., Healy, R., Tsunogai, S., 1999. Spatial variation of total CO 2 and total alkalinity in the northern Indian Ocean: a novel approach for the quantification of anthropogenic CO 2 in seawater. J. Mar. Res. 57, 135–163. Handa, N., Ohsumi, T., 1995. Direct Ocean Disposal of Carbon Dioxide. Terra Scientific Publishing, Tokyo, 274 pp. Harrison, W.J., Wendtland, R.F., Sloan, E.D., 1995. Geochemical interactions resulting from carbon dioxide disposal on the sea floor. Appl. Geochem. 10, 461–475. Haugan, P.M., Drange, H., 1992. Sequestration of CO 2 in the deep ocean by shallow injection. Nature 357, 318–320. Hirai, S., Tabe, Y., Tanaka, G., Okazaki, K., 1999. Advanced CO 2 sequestration technologies in intermediate depth of ocean. Proceedings 5th ASMErJSME Thermal Engineering Joint Conference, San Diego. Holder, G.D., Cugini, A.V., Warzinski, R.P., 1995. Modeling clathrate hydrate formation during carbon dioxide injection into the ocean. Environ. Sci. Technol. 28, 276–278. Honda, M., Hashimoto, J., Naka, J., Hotta, H., 1995. CO 2 hydrate formation and inversion of density between liquid CO 2 and H 2 O in deep sea: experimental study using submersible AShinkai 6500B. In: Handa, N., Ohsumi, T. ŽEds.., Direct Ocean Disposal of Carbon Dioxide. Terrapub, Tokyo, pp. 35–43. Kobayashi, Y., 1995. Physical behavior of liquid CO 2 in the ocean. In: Handa, N., Ohsumi, T. ŽEds.., Direct Ocean Disposal of Carbon Dioxide. Terrapub, Tokyo, pp. 165–181. Marchetti, C., 1977. On geoengineering and the CO 2 problem. Clim. Change 1, 59–68. Millero, F.J., 1986. The pH of estuarine waters. Limnol. Oceanogr. 31, 839–847. Millero, F.J., Plese, T., Fernandez, M., 1988. The dissociation of hydrogen sulfide in seawater. Limnol. Oceanogr. 33, 269–274. Mori, Y.H., Mochizuki, T., 1998. Dissolution of liquid CO 2 into water at high pressures: a search for the mechanism of dissolution being retarded through hydrate-film formation. Energy Convers. Manage. 39, 567–578. Ohgaki, K., Makihara, Y., Takano, K., 1993. Formation of CO 2 hydrate in pure and seawaters. J. Chem. Eng. Jpn. 26, 558–564. Ohmura, R., Mori, Y.H., 1998. Critical conditions for CO 2 hy-
P.G. Brewer et al.r Marine Chemistry 72 (2000) 83–93 drate films to rest on submarine CO 2 pond surfaces: a mechanistic study. Environ. Sci. Technol. 32, 1120–1127. Ohsumi, T., Nakashiki, N., Shitashima, K., Hirama, K., 1992. Density change of water due to dissolution of carbon dioxide and near-field behavior of CO 2 from a source on deep-sea floor. Energy Convers. Manage. 33, 685–690. Park, K., 1966. Deep-Sea pH. Science 154, 1540–1542. Parson, E.A., Keith, D.W., 1998. Fossil fuels without CO 2 emissions. Science 282, 1053–1054. Perry, J.H. ŽEd.., Chemical Engineers’ Handbook. 3rd edn. McGraw-Hill, New York, p. 254, Table 208. Sloan, E.D., 1990. Clathrate Hydrates of Natural Gases. Marcel Dekker, New York, p. 641.
93
Tamburri, M.N., Peltzer, E.T., Friederich, G.E., Aya, I., Yamane, K., Brewer, P.G., 2000. A field study of the effects of CO 2 ocean disposal on mobile deep-sea animals. Mar. Chem. 72, 95–101. Tsunogai, S., Ono, T., Watanabe, S., 1993. Increase in total carbonate in the western North Pacific water and a hypothesis on the missing sink of anthropogenic carbon. J. Oceanogr. 49, 305–315. UNESCO, 1987. Thermodynamics of carbon dioxide systems in seawater. UNESCO Tech. Pap. Mar. Sci. 51. Paris, 55 pp. Wigley, T.M.L., Richels, R., Edmonds, J.A., 1996. Economic and environmental choices in the stabilization of atmospheric CO 2 concentrations. Nature 379, 240–243.