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Abstract
CO2 capture from CO2/H2 gas mixture is an attractive approach for reducing greenhouse gas emissions. This mixture is associated with power generation processes in power plants and typically contains 40% CO2 and 60% H2. A novel method for capturing CO2 from the above gas mixture is to use gas hydrate formation. This process is based on selective partition of CO2 between hydrate phase and gas phase. Due to their larger diameters, CO2 molecules have higher tendency to go into hydrate clathrates than H2 molecules, which ultimately results in a gas rich in carbon dioxide.

This study demonstrates the concept and presents the results of our experimental investigation for CO2/H2 separation by hydrate formation process. Since hydrate formation from water and gas requires high pressures, tetrabutylammonium bromide (TBAB) is added to the system to reduce the equilibrium pressure. TBAB forms semi-clathrates that can encage small gas molecules. The results shows that in one stage of gas hydrate formation and dissociation, CO2 can be enriched from 40% to 88%. In addition, the concentration of CO2 in equilibrium gas phase is reduced from 40% to 18%. While separation efficiency is comparable with the process without any additive, the presence of TBAB improves the operating conditions significantly. In addition, CO2 could be enriched to 98% by two stages of hydrate formation.

Introduction
Accumulation of CO2 and other greenhouse gases in the atmosphere has led to global warming, climate change and other environmental problems which are threatening the earth and its inhabitants. Coal-based power plants generating electricity are a major contributor to greenhouse emissions producing about one-third of CO2 emissions worldwide.

Carbon capture and storage (CCS) is a technology that holds the promise of trapping up to 90% of the carbon dioxide emissions from power stations and industrial sites. One method used for CCS is pre-combustion capture which is normally applied to IGCC power plants. The coal is gasified to produce a synthetic gas made from carbon monoxide and hydrogen. The former is reacted with water to produce CO2, and more hydrogen (Barchas and Davis, 1992). CO2 is captured from the resultant CO2/H2 mixture, also called fuel gas. The hydrogen can be diverted to a turbine where it can be burned to produce electricity. Alternatively, some of this gas can be bled off to feed hydrogen fuel cells for cars. The captured CO2 can be transported to geological sites for sequestration or to oil-producing fields for the purpose of enhanced oil recovery. In precombustion process, the fuel gas produced contains a high concentration of CO2 (25–40%), and comes out at a total pressure of 2.5–5 MPa (Klara and Srivastava, 2002).

A novel technique for pre-combustion capture of CO2 is to use gas hydrates (Spencer, 1997; Aaron and Tsouris, 2005; Linga et al., 2007a; Kumar et al., 2009; Zhang et al., 2009). This process is based on selective partitioning of the CO2 and H2 gases into different phases during hydrate formation. These gases can both form hydrate with water, but CO2 has higher tendency to go into the hydrate phase at a certain pressure due to its larger molecular diameter and higher thermodynamic stability.