Pergamon Energy Vol. 21, No. 9, pp. 765-714. 19% PII: SO360-5442(%)00021-7 Copyright 0 1996 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0360-5442/96 $15.00 + 0.00 THE FATE OF LIQUID CO2 DISPOSED OF IN THE OCEAN H. TENG,t A. YAMASAKI and Y. SHINDO National Institute of Materials and Chemical Research, Agency of Industrial Science and Technology, MITI, l-l Higashi, Tsukuba, Ibaraki 305, Japan zyxwvutsrqponmlkjihgfedcbaZYXWVUT (Received 8 December 1995) Abstract-The behavior of CO2 droplets in intermediate and deep ocean waters has been studied. Because of CO2 hydrate formation on surfaces, CO2 droplets in the ocean cannot, as predicted previously, either be converted into hydrate particles or form a continuous liquid phase. CO2 mol- ecules in the hydrate are found to diffuse readily. As the result, hydrate layers formed on droplets do not stop mass transfer of CO2 from droplets to seawater and, therefore, the CO2 droplets will be dissolved. This predicted fate for CO2 droplets in the ocean agrees with laboratory simulations reported in the literature. Copyright 0 1996 Elsevier Science Ltd. 1. INTRODUCTION Increasing demand for energy required to supply the needs of industrial developments-and burgeoning human populations has led to accelerated mining and combustion of fossil fuels. These world-wide activities cause a continuing increase of the rates of anthropogenic CO, emissions. Owing to the long lifetime of CO2 in the atmosphere (ca 250 years) and slow uptake by the ocean, the levels of atmospheric CO, have increased steadily. The build-up of this primary greenhouse species in the atmosphere is believed to be causing significant climatological changes through perturbations of the earth’s radiative balance. At present rates of emissions, it is predicted that without actions to reduce emissions of anthro- pogenic COZ, the concentration of atmospheric CO* will increase from the present level of 360 ppm to 1500 ppm by the end of the 21st century, which may induce global warming of 2°C (low estimate) to 5°C (high estimate) over the next century.’ Since the primary removal process for atmospheric CO* is uptake by the ocean, the disposal and sequestration of anthropogenic CO2 in the ocean has been considered as a potential strategy for counter- acting atmospheric CO:! build-up. The majority of disposal scenarios proposed to date call for discharge of liquefied CO2 through submerged pipelines at depths below the mixed ocean layer, either at inter- mediate water levels (typically from 500 to 1500 m) or in deep waters (depths 2 3000 m). Since CO, is only slightly soluble with seawater,2 the COJseawater system is hydrodynamically unstable and liquid CO* effluent issuing from such pipelines (i.e. liquid CO2 jets in seawater) will break up into droplets.3 If liquid CO* is disposed of in seawater at a pressure p greater than 44.5 bar and a temperature T less than 283 K, a thin solid hydrate layer will form rapidly on the CO* droplets.4 Compared with seawater (the density of seawater is 1030 kg/m3), liquid CO* is much more compressible (e.g. at T = 280 K, the density of liquid CO* is 893 kg/m3 at p = 50 bar and becomes 1031 kg/m3 at p = 300 bar). Thus, the motion of CO* droplets in the ocean depends strongly on the disposal depth. The fate of liquid CO2 in the ocean has been discussed by many investigators.5-12 Golomb et al predicted that a CO2 droplet in the ocean would be converted rapidly into a hydrate particle which is denser than seawater; thus, the CO* would form a negatively-buoyant particle plume in the ocean and would eventually be captured in the ocean in the form of CO2 hydrate.6 Holder et al assumed that phase conversion from liquid CO2 to the solid hydrate is a finite-time process and thus the bulk density of a CO2 droplet depends on the degree of phase conversion; therefore, the buoyancy of a CO2 droplet at the intermediate water depths would be positive during the early stage of the phase conversion and become negative at later stages? In order for CO* to be captured in the ocean, the droplet size and disposal depth must be appropriately selected. If CO2 is disposed of in deep waters, then it is denser than seawater. In this case, the buoyancy of the CO2 is always negative. According to Golomb et al tTo whom all correspondence should be addressed. 765