Dynamic Methanol Reforming in Fixed Bed Reactors

Abstract

This study concerns the dynamic operation of methanol steam reforming in fixed bed reactors for shipboard hydrogen production. The work is motivated by the need to reduce greenhouse gas emissions and harmful exhaust pollutants from the maritime sector, while also addressing the practical difficulty of storing hydrogen directly on board ships. Methanol is considered a promising alternative fuel and hydrogen carrier because it is liquid under ambient conditions, has relatively high energy density, is already handled at industrial scale, and can be used both in combustion-based transition technologies and in future fuel cell systems. In this context, methanol reforming offers a route to produce hydrogen on demand, avoiding some of the storage challenges associated with compressed or cryogenic hydrogen.

The dissertation focuses specifically on methanol steam reforming for polymer electrolyte membrane fuel cell systems. This reforming route is attractive because it provides a high hydrogen yield and comparatively low carbon monoxide selectivity. This is important because PEM fuel cells require pure hydrogen feed. The selected reactor type is a fixed randomly packed catalytic bed. This a configuration is widely used and commercially relevant for reforming applications.

The shipboard operation introduces requirements that are not prioritized in conventional land-based chemical plants. A ship power system must respond to changing loads, and therefore the reformer must be understood in transient operation, in addition to steady state operation. The central question of this dissertation is how fixed-bed methanol reformers behave dynamically, what limits their response time, and what modelling or experimental tools are needed for their reliable analysis.

The second chapter examines the broader problem of dynamic operation in fixed bed reactors. It shows that dynamic behaviour can arise from several motivations, including process flexibility, periodic operation, and on-demand production. These different cases require different performance metrics, modelling approaches, and measurement strategies. The dissertation points how dynamic reactor analysis cannot simply reuse steady state design priorities. Moreover, the objective of the transient analysis must first be clearly defined in each case. This chapter also reviews available numerical modelling approaches and analytical techniques for measuring changing gas compositions. A key point is that models of higher fidelity than 1D are often necessary for dynamic analysis, but not always readily available or computationally convenient. Likewise, transient experiments require instruments with suitable sampling rates, and often a combination of analytical techniques is more likely than using a single method.

To demonstrate the importance of model fidelity, this study develops and applies a dynamic 2D fixed bed reactor model for methanol steam reforming. A comparison with a 1D plug flow model shows that models may agree reasonably well at steady state but differ during transients, especially when predicting local temperature fields and species selectivity. This is important because transient CO formation and hydrogen production are directly relevant to operation and control in fuel cell systems. The 2D model therefore becomes the central tool for the later chapters, which focus on origins of reformer response time.

The second major part of the dissertation, starting with chapter 3, investigates the internal heat transfer dynamics of the fixed bed reactor. Methanol steam reforming is endothermic, so reactor performance depends strongly on heat transport from the reactor wall into the catalyst bed. For this work, the model only deals with the internal packed bed behaviour by imposing changes directly in wall temperature and reactant throughput. Thus the limitations of the external heating system influence is excluded. This allows for an estimation of a theoretical lower limit for the reactor transient time. The results show that, for typical tube diameters of 2-3 cm, internal heat and mass transfer lead to transient times of up to about one minute. This suggests that internal radial heat transfer alone is unlikely to explain the much slower response times often associated with practical methanol reformer systems. The packed bed may only amplify slow external heat supply changes, but it is probably not the dominant source of long load-change durations.

Chapter 4 combines experiments and modelling to study external heat transfer effects and validate the dynamic reactor model. A laboratory fixed bed methanol steam reformer is used for gathering steady state and transient data, mainly consisting of internal bed temperatures and reformate carbon monoxide content. The model is adapted to include all detected experimental non-uniformities and heat transfer imperfections. This enables a complete comparison between simulated and measured reactor behaviour. The results show that the model can capture the main dynamic trends, but they also reveal a strong influence of practical heating details during transient operation. Important external factors include the heat capacity of the reactor and supporting structure, axial heat loss profiles, non-uniform axial heat supply, and instability in inlet reactant temperature. Among these, the heat capacity of the reactor assembly is especially influential because it stores and releases heat during load changes, thereby prolonging the transient response even in a relatively small and well-insulated laboratory reactor.

Overall, the conclusions of this study is that dynamic shipboard methanol reformer design requires a broader approach than conventional steady state reactor sizing. A comprehensive analysis includes definition of the cause and purpose of dynamic operation, selection of a model with sufficient spatial and temporal fidelity, and finally the use analytical instruments fast enough to validate transient predictions. From this study it is also concluded that the intrinsic internal dynamics of typical fixed bed reformer tubes are relatively fast compared with the load change times reported for practical systems. Therefore, future improvements in transient performance should focus strongly on the external heating arrangement, reactor thermal mass, heat integration strategy, and system level control. These findings are relevant not only for methanol reforming but also for other shipboard chemical reactors that may become part of future low emission maritime energy systems.