CO2 separation by cryogenic and hydrate

In this thesis a new method is developed for cost and energy efficient CO2 sequestration from IGCC sources based on a simple configuration. High feed pressure facilitates bulk removal of CO2 by cryogenic methods, and high energy recovery is achieved through process integration with hydrate formation. Liquid CO2 produced as a result of condensation carries most of the cold energy required for initial refrigeration, and the hydrate unit does not consume any substantial additional energy. Separated CO2 is characterised by high purity sufficient for utilisation in enhanced oil and gas recovery processes. The hydrate can be easily handled and stored. Although the focus is made on IGCC flue gas application, the method can be extended to other sources with high CO2 levels and supplied at high pressure.

On the other hand, high pressure and high CO2 content in the IGCC flue gas provide the ideal conditions for CO2 capture in the form of solid hydrates. This option has been investigated under the guidance of the US Department of Energy by a team of researchers (Los Alamos National Laboratory, Nexant, Inc., and SIMTECHE) since 1999 and at the Chinese Academy of Science. A few proof-of-concept reports can be found stating that the utilisation of the hydrate formation phenomenon for purification of gas streams is less energy intensive than any of the other existing CO2 capture methods. The ability to encapsulate significant amounts of gas in little space and relatively mild conditions of storage make the hydrates an extremely attractive option for easy handling of high rates of GHG emissions. However, this research is still on a laboratory scale.

According to the Intergovernmental Panel on Climate Change Fourth Assessment Report (IPCC AR4), fossil fuels are utilised to produce more than 80% of the world's energy and this is likely to remain unchanged in the nearest future, especially as industrialisation is pursued by such economic giants as China. Without substantial change in energy policies with primary focus on the development of sustainable technologies for power generation, mitigation of associated Green House Gas (GHG) emissions cannot be fully implemented, and will require continual improvement in order to achieve objectives set by the Kyoto protocol. Research and development in the field of Carbon Capture and Sequestration is therefore being thoroughly explored. In this work a new sustainable technology for CO2 capture from IGCC power stations is developed and discussed in detail. This technology is based on cryogenic condensation integrated with gas hydrate formation.

Coal remains the main energy source in the world; for example, in Australia it is providing 40% of total energy and up to 80% of electricity (Cuevas-Cubria et al., 2010). The main advantages of coal over the other fossil energy resources are its abundance, its easy recoverability and lower cost. Massive pollution produced during burning of this fuel forced the creation of new technologies that allow for GHG reduction. Integrated Gasification Combined Cycle (IGCC) is the most favoured advanced option for energy recovery from a variety of sources, particularly coal, the so-called 'clean coal technology'. IGCC generates a high pressure shifted syngas stream composed essentially of Hydrogen and Carbon Dioxide. Historically, the CO2 was separated from rich sources (such as natural gas) via the Ryan-Holmes cryogenic condensation process. However, applied at the gas or oil refinery this method can consume up to 50% of the generated energy to bring the CO2 levels down to pipeline requirements which does not seem attractive in terms of cost of CO2 avoided. High temperatures utilised for coal gasification are also not favourable for the implementation of cryogenic condensation to an IGCC stream.

Additional value is brought to this research by extensive investigation of the phase behaviour of gas mixtures containing CO2. Particular attention is paid to the distinctive features of gas hydrates produced in different systems including mixtures with hydrocarbons and non-hydrocarbons in various concentrations and in the presence of chemicals dissolved in water. This knowledge will contribute to the future development in the field of hydrates and will be useful for both academic research and industrial application.

With the massive global reduction in recoverable oil and the potential size in a few decades time, the accent started to shift towards the other available fossil fuels such as gas and coal. The amount of Natural Gas trapped in the form of solid hydrate sunk in the deep ocean and permafrost areas cannot be estimated precisely, however, the scientific community agrees that values in order of 1015 to 1017 cubic metres are realistic. This has caused overwhelming research into gas hydrates as storage media for different gases. Gas hydrates are highly organized crystalline structures with molecules of light gases encaged in a framework created by water molecules. They can form at any place where free water in intimate contact with hydrate forming gas is exposed to elevated pressures and low temperatures. The ability to store large quantity of gas per unit volume makes gas hydrates an attractive option for any application requiring gas preservation. One of such modern applications for gas hydrates has arisen from the global warming problem and addresses the potential capability to efficiently capture and safely store the CO2.