Induction-heated catalytic hydrogen production : a magnetic investigation

Abstract

Due to the rising concerns on global warming, there has been an increasing focus on the necessity of converting to sustainable energies. However, efficient storage of the electricity produced from sustainable energy sources such as wind or solaris still a challenge. One proposed method of storage is to use the excess energy to produce H2. Steam methane reforming (SMR) remains the most cost-optimal technology to produce large amounts of hydrogen. The steam reforming reaction is strongly endothermic (heat consuming), and happens at high temperatures. Electrifying the heating of this process could reduce global CO2 emissions, and using this greener form of technology could be implemented as a transition technology to support global demand for H2 while the world transitions to relying on sustainable energy. This project focused on exploring the possibility of electrifying the heating of the SMR process by exchanging the conventional burner technology with induction heating of magnetic materials (the susceptor) situated inside the steam reforming reactor. In particular, the project revolved around the characterisation of magnetic susceptor materials, focusing on the role of composition of the susceptor particle, particle size, and improving on current magnetic characterisation of the magnetic susceptor materials by controlling the atmosphere around the sample. During this project, induction heated SMR was performed using CoxNi(100–x) nanoparticles on an alumina support, where the nanoparticles act as both catalyst and magnetic susceptor. It was shown that, unlike in a conventional reformer, the CoxNi(100–x)/Al2O3 system was not limited by heat transfer, as the heat is delivered directly to the catalytic sites using the susceptor properties of the system. Instead, the system was shown to be limited by reaction kinetics. Moreover, it was demonstrated that this system had the ability to tune the heating and catalytic capabilities of the material by changing the nanoparticle composition, presenting an opportunity for e.g. layering a reactor with materials of different compositions to optimize reactor performance. Additionally, it was found that the operational temperature in the reactor was limited by the Curie temperature of the material, opening up for applications using the Curie temperature to act as a safety feature to avoid overheating endothermic reactions. This project also studied the optimal size of pure cobalt nanoparticles for maximising hysteresis loss as a function of temperature and applied magnetic field, and estimated the optimal size to be in the range of around 24-31 nm. Through studying the power losses in the induction heated bench scale reformer, the energy transfer efficiency was estimated to be > 80 % when scaling to industrial conditions. This scaling indicates that the system would be competitive with commercially available ways of producing hydrogen via electricity, when considering only energy requirements. Vibrating sample magnetometry (VSM) was instrumental in characterizing the susceptor materials at high temperatures and screening them for which to use in high temperature applications. However, the conventional vibrating sample magnetometers do not allow for control of the atmosphere around the sample. In order to improve on this, a retrofittable in situ holder for commercial vibrating sample magnetometers was conceived of, produced, and tested during this project. This holder was shown to be able to function under temperatures of up to 1000 , while tolerating the vibrations of the magnetometer, and could expose the sample to a well-defined gas mixture chosen by the user. In summary, this work has shown that induction heating of magnetic nanoparticles functions even at high temperatures, that it can heat locally inside a reactor, that these systems are highly tunable, and present unique opportunities to be exploited, e.g. using the Curie temperature to avoid overheating. The retrofittable in situ holder should prove valuable for future studies of magnetic materials at high temperatures. In this project the focus has been on SMR, but showing that induction heating of magnetic nanoparticles can function at the high temperatures, strongly endothermic, and chemically harsh conditions of SMR makes the application of induction heating for other reactions less daunting.