In this work we discuss a specific approach for the self-sending of CO2. In this concept, slurry balls of liquid and solid CO2 are released at the 200-m depth. The main focus of this work is this concept's thermodynamic issues. We present a general theory for the kinetics of phase transitions and apply this to the calculation of phase transitions for the formation of a thin hydrate layer, and the subsequent growth of ice on the outside of this thin hydrate layer. This kinetic rate of ice growth, and the corresponding heat flux, represent an integral part of the heat transfer dynamics between the sinking balls and the surrounding seawater. The estimates show that the rapid ice growth on balls of dry ice requires balls of approximately 47 cm in order to reach depths where CO2 is heavier than water. The ice thickness at this depth is, however, estimated to be 65 mm. If the balls do not break mechanically by colliding with the ocean floor at these depths, and as a consequence release the CO2, then the overall density of the balls will turn the transport of CO2 upwards again. Similar estimates of sinking slurry balls with solid fractions of 0.3 and 0.5 show more promising trends. Results from simulations using different sizes of these slurry balls indicate a significant potential for adjusting ball size and solid fractions so as to meet the requirements for different sinking depths, according to either sequestration in the ocean or for transport to depths corresponding to the CO2 lake option.
The oceans are undersaturated with respect to CO2, and it is estimated that the storage capacity of the oceans may be in the order of 1000 GtC (Cole et al., 1993; Herzog et al., 1997).
