Energy Convers. Mgmt Vol. 34, No. 9-11, pp. 1089-1094, 1993 Printed in Great Britain. All fights reserved
0196-8904/93 $6.00+0.00 Copyright © 1993 Pergamon Press Ltd
CO2 CLATHRATE FORMATION AND ITS PROPERTIES IN A SIMULATED DEEP OCEAN HARUMI KIMURO, FUMIHIKO YAMAGUCHI, KIYOHITO OHTSUBO and TOMOKO KUSAYANAGI Ishikawajima-Harima Heavy Industries, Co., Ltd., Tokyo, Japan MASAO MORISHITA Tokyo Electric Power Company, Tokyo Japan ABSTRACT The following two topics were examined: 1)The conduciveness of conditions for the fmmation of CO2 ciathrate hydrate at ocean depths of 700m and 1,000m with assumption of corresponding deplh-temperature profile of the ocean near Japan. 2)The effect of liquid CO2on an ocean environment at a depth of 3,000m in ambient sea water. Test results concluded that: * While the CO2 clathrate hydrate would form easily at the depths of 700m and 1,000m in the Tsnshima ocean current and Oyashio region, it would be difficult in the Kurnshio region due to a slightly higher water temperature. * The amount of CO2 dissolved into sea water at a depth of 3,000m is proportional to its velocity of injection. * The conditions required for the formation of CO2 clathrate hydrate were clarified. KEYWORDS Greenhouse; CO~; Clathrate; Ocean INTRODUCTION Carbon dioxide (COz) is regarded as one of the gasses contributing to global warming or the "greenhouse effect". Presently, technologies to conlrol CO2 emissions, separate and fixate CO2, and dispose of CO2 are under active development in a variety of fields. Carbon dioxide emissions in Japan amount to approximately 4% of the world tolal(1); of which, roughly 27% of the COz produced by Japan is emitted from utility boilers used for electric power generation(2). It is therefore recognized that certain technologies may be applied to these utility boilers as an effective means of reducing COz emissions into the atmosphere. As a research subject both technology for controlling CO2 emissions and technologies for the separadon and fixation of CO2 have raised considerable attention among the scientific and industrial community. Because of the large amount of CO2 emissions, suitable places for disposal include the deep underground or deep sea. In particular, since Japan is surrounded by the sea with almost all of its fossil fueled power plants located in coastal sites, a suitable site for CO2 disposal around Japan may be in the ocean. Therefore,in terms of Iranspor~tion efficiency, it would be most advantageous to dispose of CO2 in the deep sea. In order to use the ocean as the disposal site of COz, the technology required to inject it into the sea must first be developed. Also, the effects that CO2 will have on the deep sea environment must be evaluated. For the purpose of considering these issues, the Tokyo Eleclric Powar Company(TEPCO) and IsblkRwajima-Harima Heavy Industries Co., Ltd.( IHI ) are jointly conducting experiments of liquid CO2 behavior and the formation of CO2 clathrate hydrate, with the assumption of the ocean being u possible disposal site for CO2. We present the following experimental results: Conditions required for the formation of CO2 clathrate hydrate at ocean depths of 700m and 1,000m. -
1089
1090
KIMURO et
al.:
CO2 CLATHRATE FORMATION
- A m o u n t of C O 2 dissolved and conditions for the formation of C O 2 clathrate hydrate at an ocean
depth of 3,000m. Test procedures. Test facilities. The tests performed for this study were carried out in two types of facilities.Facih'ty specifications are summarized in Table 1. Fig, 1 shows the reactor cell installed in test facility A,and Fig. 2 the flow sheet for test facility B. Table 1. The test facility specifications ' r ~ of f,a~ty
A
B
0.35
16.7
Operating pressure(kg/cm z )
0-120
0-.-320
Operating temperature (~C)
0-n *
0--n"
P~J_~or cell capacity
(8)
n* :Room temperature
Sampling line
A Seawater drain outlet
F~I
The vmtaor cenoftestfad]i~.
KIMURO et
al.:
CO2 CLATHRATE FORMATION
1091
7 ~ I I[II
i
.
II
~watc~
)
:
N,
to pH measurement system I
s~tem
Fig.2 The flow sheet for test fadlity B.
Test methods. All the experiments reported in this paper were performed with pure CO 2 and artificial sea water. Conditions for the formation of CO~ clathrate hydrate at ocean depths of 700m and 1,000m~ Table 2 lists the temperature profile of sea water at the depth of 700m and 1,000m in the ocean near Japan(3). Table 2. The temperature at deptlm of 700m and 1,000m in the ocean near Japan
water 0epth(m) 700 Ocean region T~ture
(~)
1,000
A
B
C
A
B
C
Feb.
0.23
3.00
7.51
0.15
2.76
4.06
Aug.
0.21
3.07
6.13
0.20
2.72
3.84
A : Tmd~m~ ocean crummt region B : Oymhio region C : Km'eehio region
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KIMURO et
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CO2 CLATHRATE FORMATION
Applying the values shown in Table 2, operating parameters were determined as follows:
P(kg/cm2)= 70 T(C) = 0.2,3.0 and 6.0-7.5 P(kg/cmX)= 100 T(C)
= 0.2,3.o and 4.0 Test facility A is utilized for this experiment. The reactor is first filled with artificial sea water.Then,using a high-pressure pump,the pressure within the reactor cell is increased to the predetermined value.
Dissolution of liquid CO~ and CO~ clathrate hydrate formation at a depth of 3,000m. Test facility B is utilized for this experiment. The reaction cell is first filled with artificial sea water, and then ,using a high-pressure pump, the pressure is increased to a predetermined value. Once the temperature of the water has stabilized, liquid CO~ is injected into the reactor and water is discharged to keep the pressure at a constant level. The behavior of the liquid CO 2 inside of the reactor is monitored via video TV and the concentration of COx dissolved into the water is measured. RESULTS AND DISCUSSION Conditions for the formation of CO, clathrate hydrate at ocean depths of 700m and 1,000m. We realized three types of phenomena through our experiments. 1) It is noted that once liquid CO x was injected into the artificial sea water,CO2 elathrate hydrate was formed instantaneously on the surface of liquid CO2 droplets which rose to the surface and created a layer similar to hunches of grapes. 2) Liquid CO2 droplets injected into the artificial sea water rose up to the surface, without forming CO 2 clathrate hydrate,and broke up to form a liquid CO x layer. a. After liquid CO x remained for a few hours, COx clathrate hydrate was formed at the interface of the liquid CO x and artificial sea water. b. No clathrate hytkate formation was observed within a period of time of a few hours. Respeetively,Oe phenomena of 1) and 2)a. were observed under conditions of pressures of 70kg/crnx and 100kg/cm', and at temperatures below 4 C. The phenomenon of 2)b. was mainly observed under the conditions of pressure of 70kg/cm2 and a temperature above 4 C. The formation of CO x clathrate hydrate was observed with a sudden temperature increase in the reactor. The COx concentration in the artificial sea water after the formation of clathrate hydrate was approximately 22(Nml of COital of sea water). If liquid COx is injected at a depth of 700m in the Oyashio or Tsushima ocean current regions, COx clathrate hydrate can be formed easily. On the other hand,formation of the CO2 clathrate hydrate in the Kuroshio region would be difficult. Furthermore,due to the temperature conditions in the Kuroshio region, it has been confirmed that the formation of CO2 clathrate hydrate would also be difficult if liquid COx were to be injected at a depth of 1,000m. The conclusion was reached that due to the temperature of sea water having a great influence on the formation of CO2 elathrate hydrate , it would be difficult to form COx clathrate hydrate in the shallow waters of the Kuroshio region of the Pacific Ocean near Japan. Our results are summarized in Table 3. Dissolution of liquid CO~ and CO~ clathrate hydrate formation at a depth of 3,000m. If the temperature ,pressure and diameter of the injection nozzle are kept constant,the volume of CO2 dissolved into the artificial sea water increases proportionally with the injection velocity of the liquid CO 2. Figure 3 shows this relationship and lists the experimental conditions. Next, the three types of CO 2 clathrate hydrate formation under the previously mentioned conditions were confirmed. 1) After injection,the liquid CO 2 sunk to the bottom of the reactor. CO 2 clathrate hydrate was then formed at the interface of the liquid CO2 layer and artificial sea water. 2) CO x clathrate hydrate was formed at the interface of the liquid CO x droplet and artificial sea water. a. COx ciathrate hydrate was formed immediately after liquid COx injection. b. CO x clathrate hydrate was formed near the bottom of the reactor on the surface of sinking liquid CO x droplets.
KIMURO et
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CO2 CLATHRATE FORMATION
1093
Table 3. The test r e ~ e t for CO 2 clathrate hydrate formation Depth Se....aa Temp. C O , injection Artificial sea C O , dathrate region .volume water volume hydrate formation (m) (mR) (mr) 700 A
0.2
150
220
formed
700 B
2.5 -4.0
110
260
formed
700 C
6.0 -7.5
110
260
not formed
1,000 A
0.1
110
260
formed
1,000 B
2.3 -3.2
110
260
formed
1,000 C
4.0
140
230
not formed
A : Ts~hlma ocean current region
B : Ora~o region C : Kuroshio region
Nozzle d/a. (ram) : 1.0 Temperature (°C) : 1 - 3_ Pressure(kg/cm 2 ) : 320
5O
/. .'/
i
•
/
140
!
•
•
|
10
I 0
1
I 2
3
4
5
Injection velocity o f l i q ~ CO,(m/sec) F~.3
ECM 34/9-11--AA
The relatio~hlp between liquid CO, injection velocity and dissolved CO2
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KIMURO et al.: CO2 CLATHRATE FORMATION
The formation of CO2 clathrate hydrate depends on the concentration of the CO2 dissolved in the artificial sea water. If the CO2 concentration is high,CO2 clathrate hydrate forms immediately after liquid CO2 injection. However, if the CO2 concentration is low, COz clathrate hydrate formes at the interface of the liquid CO2 layer and artificial sea water at the bottom of the reactor. The CO2 concentration in artificial sea water just after the formation of CO2 clathrate hydrate was 20-22(Nml of CO2/ml of sea water). CONCLUSION We have performed experiments on the behavior of CO~ in artificial sea water to see preliminarily whethe~ or not the ocean is suitable as a disposal site for CO2. A summary of our test results are as follows: 1. CO2 ciathrate hydrate is formed under the conditions of temperatures of 4 C or below in shallow seas of 700m or 1,0(0)o in depth. However,formation of COz clathrate hydrate is difficult at a temperature above 4C. 2. In a deep ocean situation,such as 3,000re,the quantity of CO2 dissolved in sea water is proportional to the CO 2 injection velocity. 3. The CO2 concentration in the sea water affects the formation of CO2 ciathrate hydrate. Lastly,we will next explore the difference, if any, between the formation of CO2 clathrate hydrate using pure CO2 and that using CO2 emitted from an electric power generation plant(after separation from other gasses). REFERENCES (1)Worldwatch Instituteestimates based on data from United Nations and from World Resources Institute/international Institute for Environment and Development,World Resources 1986(New York:Basic Books,1986) (2)NIKKEI MECHANICAL ,Nikkei Business publications Inc.Japan, October,1990(in Japanese) (3)National Astronomical Observatory,ed.,Rika Nenpyo(Chronological Scientific Tables),Maruzen Co.,Ltd.,Japan, 199 l(in Japanese)