Hydrates are crystalline materials made up of water and one or more hydrate forming substances such as CO2, nitrogen, methane etc. Within the hydrates, the hydrate forming molecules are held by Van der Waals forces in a metastable crystal lattice made up of water molecules (Makogon, 1985). The water molecules form a three dimensional shell, into whose voids, molecules with low molecular weight and small rotational diameter enter and become stable. At high pressures and/or at low temperatures, thermodynamically stable hydrates will form.

Disposal of CO2 in form of CO2 hydrate in the ocean may be a possibility of reducing the atmospheric concentrations of this greenhouse gas and assist in mitigating global warming. CO2 emitted from burning fossil fuels is believed to be a major contributor to the amount of CO2 levels in the atmosphere. CO2 hydrate is denser than seawater (Adam, 1995), and therefore CO2 hydrate deposited in the ocean will sink to the seabed as long as the disposal site is within the hydrate formation envelope.

CO2 Hydrate Potential

PVTSim(Calsep, 2008) was used to analyze the CO2 hydrate potential for CO2 disposal. We estimated hydrate equilibrium lines for different samples and salinity environments without making an attempt to calibrate the model with experimental data.

Fig.1 compares the equilibrium curves of a dry gas sample (99% methane), 100% CO2 in fresh water, and 100% CO2 sample in seawater. This figure indicates that salinity has a pronounced inhibitor effect on the hydrate equilibrium formation pressure of CO2. Above 37 oF, the pressure for 100% CO2 sample in seawater to form hydrates increases drastically. Therefore it is important to form hydrate in seawater below < 37 oF. The seawater composition use in the analysis was 96.5% water and 3.5% NaCl.

CO2 Disposal Options

The options for carbon dioxide disposal using hydrate technology are listed below.

1. The first option is to use pure CO2, form the hydrate at surface and dispose it at sea. This option requires a hydrate formation plant and large amounts of water. In some countries water is available from produced water or through desalination plants.

2. The second option is to obtain CO2 (pure or with impurities) and to dispose it using hydrate technology at sea.

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source: Feature Articles, Mar  04  2012 (Carbon Capture Journal)