Journal of Hydraulic Research Vol. 44, No. 3 (2006), pp. 324–337 © 2006 International Association of Hydraulic Engineering and Research Modeling descending carbon dioxide injections in the ocean Modélisation de la descente des injections de dioxyde de carbonne dans l’océan ERIC J. WANNAMAKER, Envirogroup Limited, Centennial, CO 80112, USA E. ERIC ADAMS, Department of Civil and Environmental Engineering, Massachusetts Institute ofTechnology, Cambridge, MA 02139, USA. E-mail: eeadams@mit.edu ABSTRACT An integral double plume model is used to explore the fate of solid CO 2 hydrate particles released continuously into a quiescent ocean for the purposes of CO 2 sequestration. Such a release is desirable because hydrate particles are negatively buoyant and dissolution of CO 2 enhances this negative buoyancy through the solute density effect. Plume depth and thickness are shown to increase with CO 2 mass loading and initial hydrate particle diameter, and exceed the equivalent rise height and thickness associated with positively buoyant droplet releases. The depths also greatly exceed those associated with a single particle release, highlighting the importance of the “plume” effect. Plumes initially composed of multiple particle sizes produce greater maximum plume depth, but similar average plume depth and dilution compared with plumes composed of homogeneous particles, suggesting that adequate simulations can be made using a single particle size. Although the model is valid only for discharge to quiescent receiving water, analysis shows the effects of an ambient current decrease with increasing mass flow rate or decreasing particle size, making results relevant for most likely release scenarios. RÉSUMÉ Un modèle intégral de panache double est utilisé pour explorer le destin de particules solides d’hydrate de CO 2 déchargées sans interruption dans un océan au repos en vue de la séquestration du CO 2 . Ce type d’émission est souhaitable parce les particules d’hydrate sont négativement flottantes et que la dissolution de CO 2 augmente cette flottabilité négative par l’effet de densité de corps dissous. On montre que la profondeur et l’épaisseur du panache augmentent avec le chargement en masse de CO 2 et le diamètre initial des particules d’hydrate, et excèdent la hauteur d’élévation et l’épaisseur équivalentes associées aux rejets de gouttelettes de flottabilité positive. Les profondeurs également excédent considérablement celles qui sont liées au rejet d’une simple particule, ce qui met en lumière l’importance de l’effet ’“panache”. Les panaches initialement composés de multiples tailles de particules produisent une plus grande profondeur maximum de panache, mais des profondeurs et dilutions de panache moyennes similaires à celles des panaches composés de particules homogènes, suggèrent que des simulations adéquates peuvent être faites en utilisant une seule taille de particules. Bien que le modèle soit valide seulement pour la décharge en eau calme, l’analyse montre que les effets d’un courant ambiant décroissent avec l’augmentation du débit en masse ou la diminution de la taille des particules, rendant des résultats appropriés pour la plupart des scénarios de rejets. Keywords: Ocean carbon sequestration, CO 2 plume, dense plume, CO 2 hydrate. 1 Introduction One suggestion to address global change is to capture CO 2 from stationary power sources and pump it directly into the ocean, by-passing the slower biological and solubility pumps by which approximately 80% of the carbon dioxide that we currently emit to the atmosphere will ultimately invade the ocean (Herzog et al., 2001). The time scale of ocean circulation would limit the ben- efits of such mitigation to a few centuries (Orr et al., 2001), but since this time frame exceeds the expected duration of fossil fuel use, it could serve to reduce both peak atmospheric concentra- tions and their rate of increase (Kheshgi, 2003), both of which have been shown to affect the magnitude of climate response Revision received June 2, 2005/Open for discussion until May 31, 2007. 324 (Stocker and Schmittner, 1997). Eventually (on a time scale of millennia), the concentration in the atmosphere and ocean will reach an equilibrium that is independent of where the CO 2 was released (Herzog et al., 2001). Ocean sequestration can only be considered viable if the resulting impacts to the marine environment are small in com- parison with the avoided impacts associated with climate change. Acute effects—hypercopnia (caused by elevated levels of pco 2 ) and respiratory stress and acidosis (caused by associated lowering of pH)—could be experienced by individual organisms unable to avoid the plume (Knutzen, 1981; Shirayama, 1997; Omori et al., 1998; Tamburri et al., 2000). Additional sub-lethal effects could occur at the community and ecosystem level (Portner and