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This paper introduces the research advances on replacement of CH 4 in Natural Gas Hydrates (NGHs) by use of CO 2 and discusses the advantages and disadvantages of the method on the natural gas production from such hydrates. Firstly, the feasibility of replacement is proven from the points of view of kinetics and thermodynamics, and confirmed by experiments. Then, the latest progress in CH 4 replacement experiments with gaseous CO 2 , liquid CO 2 and CO 2 emulsion are presented Moreover, the superiority of CO 2 emulsion for replacement of CH 4 is emphasized. The latest experiment progress on preparation of CO 2 emulsions are introduced. Finally, the advancements in simulation research on replacement is introduced, and the deficiencies of the simulations are pointed. The factors influencing on the replacement with different forms of CO 2 are analyzed and the optimum conditions for the replacement of CH 4 in hydrated with different forms of CO 2 is suggested. Keywords: gas hydrate; replacement; carbon dioxide; feasibility; emulsion; simulation OPEN ACCESSEnergies 2012, 5 400 Nomenclature: t = time, s X CH 4 /X CO 2 = ratio of CH 4 and CO 2 in the vapor phase (X CH 4 /X CO 2 ) 0 = initial ratio of CH 4 and CO 2 in the vapor phase α = fitting parameter related to the a condensation rate of CH 4 molecules from the vapor phase n CH 4 .H = remaining amount of CH 4 in the hydrate phase, mol n CO 2 .H = amount of CO 2 in the hydrate phase, mol f = fugacity, MPa k Dec = overall rate constant of the decomposition, mol/s·m·MPa k Dec.R = reaction rate constant of decomposition, mol/s·m·MPa k Dec.D = decomposition rate constant of mass transfer in the hydrate phase, mol/s·m·MPa k Form = overall rate constant of the formation, mol/s·m·MPa k Form.R = reaction rate constant of formation, mol/s·m·MPa k Form.D = formation rate constant of mass transfer in the hydrate phase, mol/s·m·MPa A = surface area between the gas and the hydrate phase, m 2 H = hydrate phase G = gas phase

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This paper presents developments in numerical simulations of a cross-flow vertical-axis marine current turbine (straight-bladed Darrieus type) with particular emphasis on rotor-performance prediction and hydrodynamic loads for structural design calculations. This study initially used theoretical double-multiple-streamtube models, followed by physical testing on a scaled-down model turbine and primarily numerical simulations. Numerical investigations of a proposed full-scale turbine (power coefficient, blade loads and flow behaviour) were undertaken using the developed computational models. The turbine design was studied using a time-accurate Reynolds-averaged Navier–Stokes (RANS) commercial solver. A transient-rotor-stator model with a moving mesh technique was used to capture the change in flow field at a particular time step. A shear stress-transport [Formula: see text] turbulence model was used to model turbulent features of the flow. The numerical results show good agreement with experimental measurements and the theoretical double-multiple-streamtube model. Turbine sensitivity to parametric variations was also demonstrated in the full-scale numerical study. This work concludes that the developed model can effectively predict hydrodynamic performance and structural design blade loads of a vertical-axis marine current turbine.