Transient performance of steam reformers in the context of automotive fuel cell system integration

Proton exchange membrane fuel cells (PEMFCs) and, to a lesser degree, phosphoric acid fuel cells (PAFCs) have been widely studied as possible replacements for transportation internal combustion engines (ICE). These fuel cells consume hydrogen as fuel, which is electrochemically oxidized through an acid electrolyte. Because of the low energy density and scarcity of elemental hydrogen, alternatives such as methanol and natural gas have been investigated as primary fuels for PEMFC and PAFC fuel cell systems. Of these fuels, methanol is the most easily reformed into a hydrogen-rich gas. The most efficient way of doing this is through catalytic steam reforming. Therefore a clear understanding of the performance of steam reformers may lead to better integration of these devices into fuel cell engines.; In this dissertation the results of studies regarding the transient and steady-state performance of methanol steam reformers for automotive fuel cell system integration are provided. To power an automobile, a fuel cell system needs to be capable of changing power output rapidly, to adapt to changing driving loads. Although most fuel cell systems currently use batteries to reduce the required response time associated with the fuel cells and other balance of plant components (including the reformer), the ultimate goal may be to reduce the reliance of the system on batteries (which add cost, weight, and complexity to the system). This means that the fuel cell stack and the reformer must each be capable of changing power outputs quickly and efficiently. Fuel cell stack efficiency is intimately related to reformate gas composition (especially CO concentration). Since the reformer is upstream of the fuel cell stack, it has great influence on the overall power output and efficiency response of the fuel cell system.; The results of this study are based on data obtained from an experimental reformer and numerical models. Non-dimensionalization of the governing equations derived for reformer model development resulted in identification of potential reformer similarity variables. Based on these reformer sizing and scaling theories were developed. Of particular importance was the further demonstration of the insufficiency of space velocity and aspect ratio as sole reformer similarity variables.; Step changes in fuel flow into the experimental reformer produced transient CO concentration spikes. These spikes have also been identified in reformer literature. Through the use of the transient reformer model, potential physical mechanisms that cause these CO concentration spikes were identified.; Studies in conversion efficiency and transient and steady-state reformate hydrogen concentration were also carried out.