Producing Negatively Buoyant CO₂ Streams at Intermediate Ocean Depths

A study was carried out to develop a novel method for direct ocean CO₂ injection in which CO₂ can be disposed at ocean depths of 1000-1300 m in the form of a water/CO₂/hydrate composite stream. A negatively buoyant composite stream containing CO₂ hydrate, liquid CO₂, and water at conditions simulating intermediate ocean depths was produced using a novel concept of premixing seawater into a CO₂ stream before injection. The development is significant because it generates a sinking stream at depths < 1.5 km, which will prolong the residence of CO₂ injected into the ocean and improve the overall efficiency of direct CO₂ injections for ocean carbon sequestration. The approach allows CO₂ injections to be carried out with a lower risk of leakage to the atmosphere and without significant increase in operating cost when compared with other proposed injection methods. Because of its low surface-to-volume ratio, the produced composite stream is expected to have a slower dissolution rate than that of a similar volume of liquid CO₂ in the form of a droplet plume. The deeper the CO₂ hydrate stream sinks after injection, the more stable it becomes internally, the deeper it is dissolved, and the more dispersed is the resulting CO₂ plume. This slower rate will reduce the potential for low-pH conditions surrounding the injector, thus minimizing the negative impact of direct CO₂ injections on the ocean environment.