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Separation of carbon dioxide from fuel gas ("pre-combustion capture") via hydrate crystallization Kumar, Rajnish

Abstract

Conventional coal-fired power plants that rely on the combustion of the coal are the largest anthropogenic point sources of atmospheric carbon dioxide (CO₂). An alternative approach of producing electricity with CO₂ capture is pre-combustion decarbonisation whereby the coal is used to produce an intermediate hydrogen-rich gas (CO₂/H₂ mixture). This route is known as integrated gasification combined cycle (IGCC). The CO₂/H₂ mixture is called fuel gas and is separated into a CO₂-rich and a H₂-rich stream. The H₂ may be burnt to produce electricity or used in fuel cells. The assumption is that the CO₂ can possibly be stored safely in a suitable geological formation. It is noted that other fossil fuels may be used in a similar manner as coal. This thesis examines the prospect of employing a novel method for the separation of carbon dioxide (CO₂) from CO₂/H₂ mixture (fuel gas mixture) via clathrate hydrate crystal formation. Experiments and theory are employed at the engineering (macroscopic) and molecular level to achieve the objectives. The focus is on the study of the thermodynamic and kinetic properties of CO₂/H₂ and CO₂/H₂/C₃H₈ hydrates. The basis for separation of the CO₂ is the fact that when a CO₂/H₂ or CO₂/H₂/C₃H₈ mixture is allowed to form hydrate, CO₂ preferentially gets incorporated into the hydrate phase. The addition of 2.5 mol % C₃H₈ in the fuel gas mixture was found to reduce the hydrate formation pressure and thus improve the process economics. Based on the data obtained a conceptual separation process was developed. It involves two hydrate stages coupled with a membrane-based gas separation stage. The two hydrate stages operate at ~3.5 MPa and 273.7 K. The power penalty for a 500 MW power plant is estimated to be about 2.5% of the power output. Crystal structures and cage occupancies for the CO₂/H₂ and the CO₂/H₂/C₃H₈ hydrate were determined using several spectroscopic techniques. This enabled an understanding of the separation efficiency values obtained for the process.

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