Numerical Simulation of Diffusion of CO2 Released by Thirty Ships in Mesoscale Deep Ocean

Among possible carbon capture and storage methods to mitigate the global warming, the direct injection of CO2 into the deep ocean by the moving ship method, is considered to be a feasible way and expected to minimize its environmental impacts on marine organisms in the vicinity of the injection points. In this study, a simple but effective numerical model for the given practical scenario of very large system was developed with adopting moving and nesting grid technique and low-wavenumber forcing technique. The calculated results show that the maximum change of additional PCO2 is lower than a non-observed effect concentration, +5,000 μatm, in the both small and mesoscale domains. This indicates that the scenario of 30 ships with different length of injection pipes injecting total CO2 of 50 million t/yr and moving in the 110 × 330 km operation area is efficient and effective. The developed techniques demonstrated its efficiencies and applicability to give an outline for the optimization of the CO2 ocean sequestration system, by which biological impacts should be minimized and insignificant.

A Lagrangian Two-Phase Model for Assessing the Near-Field Impacts of CO2 Ocean Sequestration

A new version of a two-phase numerical model is developed to simulate CO2 droplet dissolution and the plume dynamics of CO2 enriched seawater produced by direct release of liquid CO2 into deep ocean from a towed pipe. This Lagrangian framework model consists of three sub-models. They are the CO2 droplet moving and dissolving sub-model, the turbulent dispersion of CO2 enriched seawater sub-model, and the biological impact sub-model. We performed simulations of direct injection of liquid CO2 from a release platform towed by a moving-ship into mid-depth seawater to examine the roles of injection parameters, including release platform type and initial CO2 droplet size. Results from the simulations show that a horizontal release platform can create a plume with less physical and biological impacts than a plume created by a vertical release platform. With an injection rate of 100kg/s, simulations predict that injection of small droplets (5mm in diameter) would produce up to 1.5 times the pH reduction of larger droplets (15 mm in diameter). This large pH reduction may significantly affect ambient zooplankton.

Effects of CO2 Ocean Sequestration on Marine Fish

Ocean sequestration of CO2 has been proposed as a possible measure to retard the increasing rate of the atmospheric CO2 concentration. Since some negative impacts on marine animals and ecosystems are likely to ensue, we must carefully investigate biological effects of ocean CO2 sequestration before embarking on this mitigation practice. Considering the expected depths for CO2 ocean sequestration (> 1,000 m), it is desirable to use deep-sea animals for the experimental assessment of CO2 ocean sequestration. In addition, experimental protocols preferably mimic environmental conditions at the releasing site: CO2 concentrations vary due to mixing with surrounding seawater at low temperatures (0–2 °C) and under high pressures. This paper describes our recent experiments to elucidate the effects of high CO2 on marine fishes. A deep-sea fish Careproctus trachysoma (habitat depth 400–800 m) can be captured alive and be used for in vivo CO2 exposure experiments. 100% mortality occurred when the fish was exposed to seawater equilibrated with a gas mixture containing 3% CO2 conditions at 2 °C within 48 h, whereas mortality was never observed when shallow-water fishes (Mustelus manazo, Paralichthys olivaceus and Seriola quinqueradiata) were tested under the same CO2 conditions but at higher temperatures (17–20 °C). It is currently not clear whether this difference in mortality is due to often presumed high susceptibility of deep-sea organisms to environmental perturbations. Subsequent experiments demonstrated that low water temperature accelerates mortality by CO2 exposure. Thus, half lethal time decreased from 105h to only 5 h when water temperature was decreased from 26 °C to 20 °C (CO2 8.5%, Sillago parvisquamis). Therefore, the high CO2 susceptibility of C. trachysoma could be solely due to low water temperature. Temporally varying CO2 conditions resulted in markedly different mortality patterns when compare with mortality recorded under constant CO2 conditions. Step-wise increases in ambient CO2 resulted in much lower mortalities than under one-step increases to the same CO2 levels. Further, a sudden drop of CO2 from 9–10% CO2 to air level (0.038%) killed all the surviving fish within a few minutes.

Development of Environmental Assessment Technique for CO2 Ocean Sequestration

Increasing atmospheric concentrations of greenhouse gases are suspected of causing a gradual warming of the Earth’s surface and potentially disastrous changes to global climate. Because CO2 is a major greenhouse gas, CO2 ocean sequestration is being explored as one possible option to limit the accumulation of greenhouse gases in the atmosphere. In the case of CO2 ocean sequestration, an assessment of environmental impacts to the ocean is the most important issue. Sequestration research requires a development of new cost-effective observation techniques to monitor dilution and diffusion of the sequestrated CO2. We developed an in-situ pH/pCO2 sensor, a tracking neutral buoy system and a towing multi-layer monitoring system to observe the pH change, water movement and diffusion at a mid-depth of the ocean as an environmental assessment technique for CO2 ocean sequestration.