OCR Text |
Show small orifices at depths of -500 m, satisfies these criteria. In consideration of the high cost associated with the offshore pipeline, liquid carbon dioxide should be discharged at the minimum depth that satisfies criteria (1) and (3). Steinberg, et ale (1984) suggested that one option for effective containment is discharging at 3000 m, where carbon dioxide is negatively buoyant and, thus, would not ascend. However, the present authors' experience with designing deep water pipelines for ocean thermal energy conversion (OTEC) applications suggests that this is not feasible. Given the current state-of-the-art of offshore deployment technology (see, e.g., Brown, 1989), a practical maximum depth of discharge, using a bottom-mounted pipeline, is -1 ()()() m. At depths shallower than 1 ()()() m, liquid carbon dioxide would be buoyant and therefore would rise toward the surface. At some point in its ascent, the temperature and pressure of the surrounding water would induce boiling of the carbon dioxide. The resulting gas bubbles may rise to the surface before going into solution. This type of gross boiling may be avoided by designing the discharge system appropriately so that the liquid carbon dioxide is well-dispersed prior to reaching the boiling depth. Extensive seawater temperature data obtained at a number of potential OTEC sites in the Pacific and elsewhere indicate that, except for those locations within major currents, where accelerated vertical mixing could result in elevated temperatures deep beneath the ocean surface, flashing of liquid carbon dioxide probably would not occur at depths beneath 420 m. Hence, discharging liquid carbon dioxide as turbulent jets at a depth of -500 m generally should be adequate to avoid boiling. Assuming that the pipeline might have to extend up to 100 Ian from the plant to attain the 500 m depth, the frictional pressure drop could be as large as 3 MPa. Heat transfer calculations indicate that under most cases it would be possible to limit the increase in carbon dioxide temperature over the length of the pipeline to about 10° C to 20° C. To enhance spatial dispersion and mixing with seawater, liquid carbon dioxide would be discharged through a series of orifices, spaced one to several meters apart, that spiral around the axis of the pipeline (Mori, 1991a). Orifice diameters would lie between 0.5 cm and 1 em and jet exit velocities would be on the order of 10 mls. Although Golomb, et ale (1989) advocate lowvelocity discharge, enhanced transport associated with turbulent jets appears more desirable for this application. To elucidate the transport dynamics of partially-soluble, non-isothermal, turbulent, liquid carbon dioxide jets in the deep ocean, a high-pressure mixing facility has been constructed at the University of Hawaii's 1.K.K. Look Laboratory in Honolulu. Measurements of scalars and velocities are being performed. Results of this study will be reported in the future. The ultimate fate of the discharged carbon dioxide and the extent of impact on the marine ecosystem would be determined by a combination of physical transport, chemical, and biological processes. Several potential mechanisms exist which could prevent the dissolved carbon from escaping to the atmosphere, including dissolution or precipitation of carbonate salts, nutrient fertilization and increased organic carbon burial, biological consumption and production of calcareous skeletal material, and formation of solid clathrates (crystalline hydrates). The strength of these sinks depend, in part, on the reaction environment, i.e., on local factors such as temperature, chlorosity, density and species of marine fauna and flora, and the degree of saturation of the seawater with calcareous materials (Sabine and MacKenzie, 1990; Sverdrup, et al., 1970). Simulations utilizing experimental data obtained in the aforementioned high-pressure mixing facility are underway to assess the long-term fate of the discharged carbon dioxide and to identify any adverse effects on the marine ecosystem. These studies will supplement an earlier study on the biological impact of deep ocean injection of anthropogenic carbon dioxide performed by Dacey, et ale (1989). The results of such simulations must be coupled with a detailed engineering design study of the offshore pipeline to ensure that the system proposed for the disposal and containment of carbon dioxide extracted from fossil fuels using the pre-combustion removal 6 |