(122i) Development of Co2 Injection Method "Cosmos" for Co2 Ocean Storage

Ocean storage of CO2 is one of greenhouse gas control technologies, where CO2 is captured from flue gas of fossil fuels and injected into deep sea below 3500m depth to be sequestered from the atmosphere. The authors have worked for development and evaluation of a CO2 sending method, COSMOS, where cold liquid CO2 is released at 500m depth to descend to deep sea floor below 3500m depth. Such cold CO2 is heavier than ambient seawater even in the shallow water. If a released CO2 drop has a certain volume enough to keep its temperature at low, it reaches 2700m depth, where liquid CO2 at the same temperature to ambient water has the same density to the water. Because liquid CO2 has higher density than ambient water below 2700m depth, CO2 drops reaching the depth spontaneously continue descending to deeper waters. Thus, the size of released CO2 drops should be controlled to keep the temperature at low against heat transferred from ambient water. In the first phase of the COSMOS project, the concept of COSMOS was examined by in situ experiments, which were carried out at 500m depth in Monterey Bay off California, U.S.A. Three models of small-scale CO2 releasing nozzle unit were developed for in situ experiments. The first model of nozzle unit released liquid CO2 as one mass; however, it was immediately broken into small droplets and soon turned to ascend. This result suggested that the thermal insulation of the nozzle unit was insufficient to keep the temperature at low as required and the instability of drop interface caused the rapid break-up. Then, the second and third models were designed to have thermal insulator enough to keep a part of dry ice filled in the units. Both models successfully released liquid CO2 with dry ice, which continued descending for a few minutes. Based on the above results, COSMOS was improved, where injection of a mixture of liquid CO2 and dry ice, CO2 slurry, is expected to enable small CO2 slurry drops to descend to deep sea floor below 3500m depth because under a certain condition the latent heat of dry ice is enough to keep the temperature at low and to form an ice layer around CO2 drops. The releasing depth is expected to be as shallow as 200m if the initial size of slurry drop is more than 40cm under the condition that the ratio of dry ice in the drop is more than 0.5. Then, in the second phase of the COSMOS project, the effect of the releasing conditions on the behavior of released slurry drops has been investigated. In experiments, CO2 slurry was prepared by melting of dry ice in a sealed chamber at the triple point of CO2, 217K and 0.5MPa, and injected from two types of nozzle head. The experimental result implied that the releasing conditions such as releasing rate of CO2 slurry would affect the behavior of the released slurry drops.