CO2 removal from air for alkaline fuel cells operating with liquid hydrogen
Thesis DisciplineMechanical Engineering
Degree GrantorUniversity of Canterbury
Degree NameDoctor of Philosophy
The aim of this project was to explore the synergistic use of alkaline fuel cells and liquid hydrogen. For electrochemical power generation, alkaline fuel cells offer the highest efficiency, as well as the possibility of using electro catalysts that do not include platinum. However, since they operate at low temperatures they do not support hydrogen production by reformation of carbonaceous fuels. Further, they are rendered inoperative by the CO2 content of the (reformed) fuel and/or air. Their use has been limited due to this inability to use hydrogen or air which contains CO2, and the added complexity of CO2 removal. For storing hydrogen the most suitable system in terms of low mass and volume, is cryogenic storage of liquid hydrogen. The large temperature difference between the stored liquid and its surroundings means that a substantial thermomechanical exergy component, in addition to the chemical exergy, is available from the fuel. Thermomechanical exergy recovery has the potential to make the use of liquid hydrogen more efficient. The basis of this thesis is the conceptual design, development and testing of a new process for CO2 removal from air for use in alkaline fuel cells operating with hydrogen stored as a liquid, addressing simultaneously: • CO2 removal from atmospheric air, and • thermomechanical exergy recovery from liquid hydrogen. This project was an attempt to address these issues by using the cooling available from vaporisation of liquid hydrogen and/or boil-off vapour, to remove CO2 from the alkaline fuel cell feed air by refrigeration purification, ie. by freezing the CO2 out of the air. A schematic description of the process and an energy balance for refrigeration purification for the CO2 removal are presented, showing that the process relies on high effectiveness heat exchangers and water re-vaporisation. The high effectiveness heat transfer is achieved using perforated plate matrix heat exchangers. Experimental results of heat exchanger effectiveness tests and CO2 removal tests indicate that heat exchangers of the requisite effectiveness were designed and manufactured, and that the proposed process was successful in CO2 removal to the required level. Implicit in this work was: • The development of a new sizing procedure for matrix heat exchangers based on a recently developed approximate analytical solution for their performance. • Experimental testing of matrix heat exchanger performance and correlation with a recently published numerical solution for their performance prediction. • The development of a new method for construction of perforated plate matrix heat exchangers. • The development of a fully instrumented apparatus to test matrix heat exchangers at cryogenic temperatures. • The setting up of a mass spectrometer for gas analysis and continuous process monitoring to monitor the CO2 removal process. • Testing of matrix heat exchangers when used as reversing heat exchangers.