CO 2 removal from air for alkaline fuel cells operating with liquid hydrogen — Heat exchanger development

Abstract

Two independent problems when using hydrogen as an energy source in vehicles are: (i) gaseous hydrogen boil-off from stored liquid hydrogen, its associated pressure rise and hence necessary venting; and (ii) CO 2 removal from air for hydrogen-air alkaline fuel cells. An experimental program has been undertaken to see if in a system using an alkaline fuel cell running on hydrogen stored as liquid, both these problems may be solved concurrently. For vehicular hydrogen storage the most suitable system in terms of low mass and volume, is cryogenic storage of liquid hydrogen. Unfortunately, with liquid hydrogen there is always a small amount of boil-off, which necessitates venting of gaseous hydrogen from the storage vessel. Vented boil-off is hazardous and wasteful, and so it is essential that it be rendered safe, and preferably utilized efficiently. For electrochemical power generation, alkaline fuel cells offer the highest efficiency, as well as the possibility of using electrocatalysts 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 CO 2 content of the (reformed) fuel and or air. Their application to vehicular use is impaired by this inability to use other than pure hydrogen and complexity of on-board CO 2 removal. This paper describes a novel and simple process whereby H 2 boil-off can be used to continuously sublime and remove CO 2 from air, prior to the purified air and hydrogen being supplied to an alkaline fuel cell. It includes a schematic description of the process and an energy balance for refrigeration purification for the CO 2 removal. It is shown that the process relies on high effectiveness heat exchangers and water re-vaporization. An overview of thermal analysis of matrix heat exchangers is presented along with experimental results of heat exchanger effectiveness tests, which indicate that heat exchangers of the required effectiveness are easily designed and manufactured.