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Abstract
Carbon capture and storage (CCS) offers one of the most promising ways for reducing the accumulation of greenhouse gases in the atmosphere. Currently available post-combustion CO2 capture technologies lack the desired energy efficiency, we have developed a CO2 capture technology which converts CO2 to a hydrate under substantially atmospheric temperature and pressure conditions.

Being cooled to a low temperature of 5 degree Celsius and a pressure of 2.2 MPa, CO2 containing water can separate out the only CO2 component as a solid called a hydrate. This technology has long been known to enable separate CO2 from a mixed gaseous stream. However, practical application had been considered difficult due to the high operating cost of high pressure and low temperature conditions. We have discovered a phenomenon in which formation of the hydrate of CO2 is produced under significantly eased conditions of pressure and temperature by using a semi-clathrate hydrate such as tetra-n-butyl
ammonium bromide and other quaternary ammonium compounds. These quaternary ammonium compounds form a semiclathrate hydrate crystal with water molecules under atmospheric pressure.

Our preliminary result of X-ray diffraction shows that there are empty dodecahedral cages. Therefore, semi-clathrate hydrates can be used to separate small gas molecules that fit in these dodecahedral cages. We have found that they could encage CO2 molecules at higher selectivity than nitrogen and oxygen. We have performed a bench scale experiment to encage CO2 under substantially atmospheric pressure and temperature conditions such that pressure of 0.12 MPa and temperature of 18 degree Celsius, confirming the possibility of CO2 capture under those conditions. Our feasibility study has revealed that the operating
costs of carbon capture will be half compared to a conventional chemical absorption process.

We expect to conduct larger scale tests in the future, preconditioned on a CCS plant for CO2 capture of flue gas from thermal power plants and steel works assumed CCS scale from 0.3 million to 1 million tons per year. The improved CO2 capture process with minimized energy demand will play a significant role for the reduction of CO2 emissions.

1.Introduction
Atmosphere carbon dioxide is a focus of attention as one of the greenhouse gases (GHG) which cause global warming. Early implementation of effective measures to prevent global warming is strongly desired. One conceivable measure for preventing global-scale warming is separation and capture of the CO2 contained in flue gas discharged into the atmosphere from thermal power plants, steel works, factories, and other facilities in the course of industrial activity, followed by fixation and effective utilization. This approach, if possible, would make an important contribution to prevention of global warming.

Various methods for separation and capture of CO2 from flue gas have been proposed, including chemical absorption using an amine solution, physical adsorption using an adsorbent, and membrane separation methods, among others. However, in order to realize practical application of these technologies, the cost of CO2 separation and capture must be substantially reduced, as this accounts for a large part of the total cost of carbon dioxide capture and storage (CCS).

This paper presents a hydrate-based CO2 separation and capture method which has the potential for large cost reduction in comparison with conventional techniques in CO2 separation and capture technology.