Conference Agenda

Due to the increasing momentum for the development of green processes, hydrogen may play a significant role in the energy sector as it can be thought of as a potential futuristic replacement for fossil fuel. Industrially, hydrogen can be produced by several processes such as steam reforming of hydrocarbons and water gas shift reaction. However, both reactions suffer from thermodynamics limitation leading to low conversion and selectivity. Consequently, several improvements and modifications such as installing hydrogen membranes to conversional chemical reactors are implemented which led to improving the performance of the catalytic membrane reactors compared to the conventional reactors. However, these hydrogen membrane reactors have not yet been optimized with respect to the hydrogen flux which makes the membrane reactors economically issuable.

In this work, it is intended to consider the optimization of the performance of a catalytic membrane water gas shift reactor using the hydrogen flux profile as a control variable. To achieve this aim, a one-dimensional homogeneous reactor model is developed considering the variation of the component molar flowrates, temperature within the reaction side and the permeate side, and the pressure drop in the reaction side. The performance of the reactor is assessed using the achievable conversion of carbon monoxide to hydrogen, hydrogen recovery, reactor resistance time and temperature control on the reaction side.

As the climate crisis continues to grow as an existential threat, significant efforts are underway to establish reliable energy resources that can be both renewable and zero-carbon emitting. Off the back of these efforts, hydrogen has emerged as critical player for future energy resource purposes due to its high gravimetric energy density and near-abundant availability. However, hydrogen has its own disadvantages too. Foremost among these are its low volumetric energy density and its challenges associated with storage and transport. An identified solution to this problem is the metal hydride, which is a solid-state storage method that provides a viable solution to the current limitations. Storage is achieved through the chemical absorption of hydrogen into a porous metal alloy’s sublattice. However, despite the biding promise that metal hydrides bring, their challenging thermodynamics leaves a gap between the ideal storage capacity that current industry requires and the limited storage capacity that reusable metal hydrides currently provide.

This work, therefore, uses mathematical modelling to determine optimum operating conditions for the metal hydride in order to maximise hydrogen storage capacity. Computational fluid dynamics is used to simulate the coupled heat and mass transfer that occurs during the hydrogen absorption process into the metal alloy. An exploration of numerical methods to complete this work is conducted - namely the simple explicit method, Crank-Nicolson method and alternating-direction implicit (ADI) method. The ADI method provides the most stable platform to conduct analyses on the variables affecting storage. The hydride bed thickness, heat transfer coefficient, supply pressure and cooling fluid temperature are the variables of focus. It is demonstrated that bed thickness and supply pressure possess the greatest influence on storage capacity and rate of absorption, respectively. Another non-physical variable that bears significant influence is mesh grid size used during the simulation. The alloy MmNi_4.6Al_0.4 is used in the investigation.

Hydrogen is one of the most significant tools in the energy transition to reach a high share of decarbonization. A wide range of applications has been proposed as energy storage or energy carrier systems. However, some limitations in terms of storage and transportation limited its implementation. Therefore, the use of other chemicals derived from hydrogen emerges. Among them, two liquid options stand out: methanol and ammonia. These chemicals can be used as green fuels or as hydrogen carriers. In the first approach, a direct transformation into power is performed (Salmon & Bañares-Alcántara, 2021). In the second alternative, methanol or ammonia are transformed into hydrogen which are used for power generation (Nemmour et al., 2023). At this point, a trade-off emerges between these two routes. On the one hand, direct fuel cell systems for methanol and ammonia present a simpler scheme, however, efficiencies are currently low. On the other hand, hydrogen fuel cells present a higher performance, however, the process is more complex.

In this work, a systematic comparison of the two alternative routes to transform methanol and ammonia into power using fuel cell technology is performed. The first one is based on the direct use of these chemicals into a direct ammonia (DAFC) or direct methanol (DMFC) fuel cell. In the case of methanol, a PEM fuel cell is selected followed by a CO₂ separation system. For ammonia, an alkaline membrane fuel cell is introduced followed by a SCR treatment to control de NO_x emissions. The second alternative to generate electricity from these chemicals is based on a first stage to produce H₂ followed by a SOFC hydrogen fuel cell. In this work, methanol reforming and ammonia decomposition are evaluated. After the SOFC, an organic Rankine cycle is introduced to improve the energy efficiency of the integrated system.

All the cases have been analyzed for a fuel cell capacity equal to 1000 kW. In the direct use of methanol/ammonia as green fuels in DMFC or DAFC, the energy efficiency is 26.5% and 16.8% respectively. Related to the use as hydrogen carriers, the final conversion of methanol/ammonia is around 97% reaching a hydrogen yield of 0.11 kg H₂/kg methanol and 0.12 kg H₂/kg ammonia. The final efficiencies of these integrated systems rise to 35.8% (methanol) and 42.3% (ammonia) due to the better energy performance of hydrogen fuel cells. In terms of economics, the cost of electricity for the use as green fuels is around 1200€/MWh and, for the use as hydrogen carriers, about 700 €/MWh. A comparison including transportation and production is also included. The use of methanol and ammonia emerges as a competitive option for distances above 3000 km considering the current efficiency values.

References

Nemmour, A., Inayat, A., Janajreh, I., & Ghenai, C. (2023). Green hydrogen-based E-fuels (E-methane, E-methanol, E-ammonia) to support clean energy transition: A literature review. International Journal of Hydrogen Energy, 48(75), 29011-29033.

Salmon, N., & Bañares-Alcántara, R. (2021). Green ammonia as a spatial energy vector: a review. Sustainable Energy & Fuels, 5(11), 2814-2839.