Yasuharu Nakajima

Deep ocean experiments with fossil fuel carbon dioxide: Creation and sensing of a controlled plume at 4 km depth

The rapidly rising levels of atmospheric and oceanic CO2 from the burning of fossil fuels has lead to well-established international concerns over dangerous anthropogenic interference with climate. Disposal of captured fossil fuel CO2 either underground, or in the deep ocean, has been suggested as one means of ameliorating this problem. While the basic thermodynamic properties of both CO2 and seawater are well known, the problem of interaction of the two fluids in motion to create a plume of high CO2/low pH seawater has been modeled, but not tested. We describe here a novel experiment designed to initiate study of this problem. We constructed a small flume, which was deployed on the sea floor at 4 km depth by a remotely operated vehicle, and filled with liquid CO2. Seawater flow was forced across the surface by means of a controllable thruster. Obtaining quantitative data on the plume created proved to be challenging. We observed and sensed the interface and boundary layers, the formation of a solid hydrate, and the low pH/high CO2 plume created, with both pH and conductivity sensors placed downstream. Local disequilibrium in the CO2 system components was observed due to the finite hydration reaction rate, so that the pH sensors closest to the source only detected a fraction of the CO2 emitted. The free CO2 molecules were detected through the decrease in conductivity observed, and the disequilibrium was confirmed through trapping a sample in a flow cell and observing an unusually rapid drop in pH to an equilibrium value.

Ocean abyssal carbon experiments at 0.7 and 4 KM depth

Publisher Summary The chapter reviews and compares observations from the two experiments performed in 2003. The main focus is on the way in which CO 2 behaves and is transported away from the site in realistic deep-sea conditions where hydrates form and where CO 2 is exposed to sediments. The synthesis of observations is then used as a basis for discussing and presenting best estimates of the fate of larger quantities of CO 2 placed on the seafloor. The chapter also discusses observations from small-scale CO 2 experiments conducted off the coast of California at 684 m depth and at 3942 m depth. In both experiments, when the seawater velocity was sufficiently strong, parcels of liquid CO 2 were torn off and transported away as discrete units by the turbulent water current. In the deep experiment, newly formed frazil hydrate was observed at the interface, occasionally including sediment particles. Hydrate furthermore collected and created a floating consolidated solid in the downstream end of the trough, dissolving slowly from one day to the next. These observations have important implications for understanding and modeling of larger scale disposal at the seafloor. When CO 2 is released by the interfacial instability mechanism driven by strong currents, the seawater density increase due to dissolution of CO 2 may not have time to act and stabilize the water column before the discrete parcels of liquid phase CO 2 are advected away from the disposal site. The floating solid that formed at the interface is hypothesized to consist of hydrate and additional trapped seawater. Its appearance was not expected in advance and its role in delaying dissolution cannot be determined from the present experimental set-up. A capability for long-term seafloor perturbation experiments is deemed to be crucial both for direct ocean-storage research and for studying effects of invasion of anthropogenic CO 2 from the atmosphere.

Concept of Seafloor Mineral Processing for Development of Seafloor Massive Sulfides

Seafloor Massive Sulfides (SMS), which were formed by deposition of precipitates from hydrothermal fluids vented from seafloor, is one of unconventional mineral resources beneath deep seafloors in the world. The authors have proposed the concept of seafloor mineral processing for development of SMS, where useful minerals included in SMS ores are separated on seafloor to be lifted while the remaining gangue is disposed on seafloor in appropriate ways. To apply column flotation, one of conventional methods in mineral processing, to seafloor mineral processing, the authors carried out simulating experiments of column flotation on deep seafloor using ores including copper, iron, lead and zinc as metallic elements. Prior to the experiments at high pressures, preparatory experiments at the atmospheric pressure were carried out to find out the optimum condition of the properties of pulp, a mixture of feed ore, water and chemical reagents. In flotation experiments at high pressures, formation and overflow of froth layer by bubbling were observed at 1MPa in both of pulps with pure water and artificial seawater. The analytical data showed that the concentration of metallic elements such as copper and zinc in the concentrates recovered from the experiments was higher than that in the feed ores while the concentration of silicon and calcium, which are assigned to gangue, in the concentrates was lower than that in the feed ores. These results suggest that column flotation can be applied to operation on seafloor.Copyright

Simulating experiments of CO2 ocean storage with a large high-pressure tank

Publisher Summary This chapter discusses the simulating experiments of CO2 ocean storage with a large high-pressure tank to investigate dissolution of CO2 drops with CO2 hydrate films, and compares the experimental results with those predicted with Ranz–Marshalls equation. In the experiments, the CO2 drops were stored in fresh water pressurized at 30MPa in two temperature regions for two days to observe change of drop diameter and to measure pH change in ambient water around the drops. The diameter change in the experiments, where there was no artificial flow, was larger than that predicted under no-flow condition, which implies that the dissolution could be enhanced by slight disturbance around the drops. On the other hand, the pH measurement showed the lowering of pH during the experimental period, and the pH change was depressed at lower temperatures, which agrees with temperature dependency in the dissolution rate predicted.

Progress of COSMOS (CO2 Sending Method for Ocean Storage) and OACE (Ocean Abyssal Carbon Experiment)

The storage of liquid CO2 at an ocean floor, one of promising measures to mitigate the global warming, requires 3500 m depth for the gravitationally stable storage, a breakthrough technology and a reasonable cost to realize, although it has large advantages such as the sequestration term longer than 2000 years. However CO2 can be sent to the ocean floor by shallow release, if we can use the nature that the cold CO2 to be shipped by a CO2 carrier is much denser than the ambient seawater even at shallow depths. The National Maritime Research Institute (NMRI) conducted several joint field CO2 release experiments with the Monterey Bay Aquarium Research Institute (MBARI, USA) since 1999 under the auspices of the NEDO, and proposed the improved COSMOS, CO2 Sending Method for Ocean Storage, in which CO2 is released into 200 m depth as slurry masses (mixture of dry ice and cold liquid CO2 ). Since 2002, under the NEDO Grant, the NMRI started a new international joint research, OACE, Ocean Abyssal Carbon Experiment with the MBARI and the University of Bergen (UoB, Norway), in order to accumulate the basic data on the long-term stability of stored CO2 and its environmental effects around storage site.Copyright

Use of Bio-fluorescent Characteristics for Ecosystem Monitoring on Hydrothermal Deposits

A new technology is necessary for effective monitoring of deep-sea hydrothermal ecosystems towards the environmental impact assessment of marine mining projects. For this purpose, we develop a new deep-sea observation technique using bio-fluorescent characteristics. Ultraviolet (UV)-LED illumination and violet laser illumination are used for fluorescent video recording of a variety of deep-sea organisms. This study reports the bio-fluorescent patterns and colours observed for a variety of deep-sea organisms and their usefulness for monitoring.

A Numerical Model for Environmental Impact on Marine Organisms for Seafloor Resources Development

The exclusive economic zone (EEZ) of Japan has a very wide area due to a lot of islands in the Japanese Archipelago. Recently, the development of power generation facilities, food production facilities, and the natural resources in the Japanese EEZ are planned. As the worldwide supply and demand of mineral resources are being tight, the technology for effective exploitation and the use of mineral resources in the EEZ will become a key for sustainable development in the Japanese industry. Prior to development of marine mineral resources, it is necessary to evaluate its environmental impact on the water column and seafloor of ocean. However, the environmental evaluation method for open ocean has not been established yet. Then, we are developing the environmental impact prediction model for the seafloor mineral resources development in deep sea area with the consideration of benthic organisms. This model can estimate the impacts of excavating seafloor and sedimentation of particles on marine organisms using an ecosystem model. In addition, the influence of an increase in dissolved oxygen on bacteria caused by the discharging of oxygen-rich waste water at seafloor, is considered in this model. The ecosystem in this model includes benthic organisms, zooplankton, bacteria, particulate organic matter and dissolved oxygen as the components. This paper introduces the calculation case using the prototype model.Copyright

Study on Grinding Technology for Seafloor Mineral Processing

Seafloor Massive Sulfides (SMSs), which are formed by precipitates from hydrothermal fluids vented from seafloor, have been expected as one of mineral resources to be developed. The authors have proposed the concept of seafloor mineral processing for SMS mining, where valuable minerals contained in SMS ores are separated on seafloor. To apply a ball mill to the grinding unit for seafloor mineral processing, grinding experiments were carried out using a small-scale ball mill applicable to high-pressure condition. In the experiments, wet grinding and water-filled grinding of size-classified silica sands were carried out at three rotation rates to compare the grinding performance in both cases. In both cases, the silica sands were finely ground. The measurement of particle size of samples from the experiments showed that water-filled grinding had comparable grinding performance to wet grinding while the suitable rotation rate for water-filled grinding shifted to higher than that for wet grinding. This result suggests the possibility of water-filled grinding for seafloor mineral processing. If water-filled grinding can be employed for the grinding unit, the structure of the grinding unit would be simplified in comparison with wet grinding that leads to the saving of grinding costs.Copyright

Study on Seafloor Mineral Processing for Mining of Seafloor Massive Sulfides

Seafloor Massive Sulfides (SMSs), which were formed by deposition of precipitates from hydrothermal fluids vented from seafloor, has been expected as one of unconventional mineral resources on deep seafloors in the oceans. The authors have proposed the concept of seafloor mineral processing for SMS mining, where valuable minerals contained in SMS ores are separated on seafloor while gangue minerals are disposed on seafloor in appropriate ways. To confirm the applicability of column flotation, which is one of conventional mineral processing methods, to seafloor mineral processing, the authors carried out experiments simulating column flotation under the pressure conditions corresponding to the water depths down to 1000m in maximum using ore samples containing copper, iron, zinc and lead. In the experiments, formation of fine bubbles suitable to flotation and overflow of froth layer were observed at high pressures. The contents of copper and zinc in the concentrates recovered in the experiments at 1MPa were higher than those in the feed ores while the contents of silicon and calcium in the concentrates were lower than those in the feed ores. These results suggest that column flotation would be applicable to seafloor mineral processing.Copyright

Analysis Tool for Environmental Impact of Seafloor Resources Development

The exclusive economic zone (EEZ) of Japan has a very wide area due to a lot of islands in Japanese Archipelago. As the worldwide supply and demand of natural resources and foods are being tight, the technology for the effective use of the EEZ will become the key for the sustainable development of Japan. From this background, the Ministry of Land, Infrastructure, Transport and Tourism of Japan (MLIT) started an R&D project of floating offshore platform technologies in 2007[1]. In this project an integrated design support tool is developed for evaluating the economical and safety aspects of the offshore platforms. This tool contains the function to estimate an environmental impact of seafloor resource development. We postulated the process of seafloor resource development as follows. The ores mined on seabed are sent up to the platforms as a mixture of the ore particles and water, then the waste water containing inorganic suspended particles is discharged into sea. The particles exhausted in the sea are transported by the advection and diffusion, and sink gradually on seafloor. We developed an analytical code simulating the distribution of discharged particle in both sea and sediments by a simple technique. Mass Consistent Flow Model is used to save the time in the calculation of ocean flow. In addition, the ecosystem model in sediment is developed to estimate the ecological impact of seafloor resource development. It includes benthic organism, zooplankton, bacteria, and particulate organic matter as the components. The ecological impact was assessed based on the recovery period of the biomass. In this paper, the outline of this environmental impact analysis tool and results of the trial calculation for seafloor resource mining are shown.Copyright