A novel fuel converter for the production of hydrogen from natural gas

Nowadays, hydrogen potentially plays a crucial role in clean, secure, affordable energy scenarios. There is a significant interest in residential, small and medium-scale hydrogen generation because of the high cost of compression and transportation of hydrogen[1-4]. Thermal methane cracking (TMC) is an alternative process for high-purity hydrogen manufacturing along with traditional commercial processes such as steam reforming, coal gasification, partial oxidation, and water electrolysis. Employing the TMC process for very high-purity hydrogen production on a small or medium-scale plant with the minimum requirement of separation units is the main incentive of this thesis. Using catalysts for TMC can decrease the working temperature to below 800 °C. However, it could create some significant issues, especially catalyst deactivation (a low catalyst life due to carbon deposition), so it is not still a viable method for production plants. On the other hand, supplying the reaction heat and reactor blockages are two fundamental challenges for a non-catalytic reaction. A regenerative reactor could be a part of a solution to overcome these obstacles. The working principle of the proposed process was quite simple, relying as it does on the cyclic conversion of methane. The first cycle is for hydrogen generation via contacting a storage medium (e.g., a bed of ceramic packing), which supplies the necessary heat for the reaction and collects carbon particles and deposits. In the second cycle, carbon will be removed from the storage medium, and the bed will be reheated for the conduction of the generation cycle. A laboratory-scale experimental setup was established to examine the roles and relative importance of parameters such as the overall reaction rate, characterise the types of carbon deposited in the reactor, and investigate the possible auto-catalytic effect of carbon formation inside the reactor. The results reveal that the storage medium is a bed for carbon deposition and successfully supplies the heat of the reaction, and 99.7 % hydrogen yield was obtained at higher temperatures (more than 1150 ˚C). Results showed the cracking reaction divided into two steps, non-isothermally and isothermally, at 850 to 1170 ˚C. It means that the reaction occurs when the methane is heating up, and after reaching the reactor temperature, the reaction continues in an isothermal condition. The thesis tried calculating the general reaction rate based on isothermal and non-isothermal conditions and generating an experimental reaction rate to cover both stages at 850 to 1170 ˚C. Results showed that carbon black had been mostly formed but in different sizes from 100 nm to 2000 nm. Increasing the reactor temperature decreased the size of the generated carbon. Pre-produced carbon in the reactor did not affect the production rate and is almost negligible at more than 850 ˚C. A CFD modelling was conducted to have a more in-depth study on the effect of other variables in a regenerative reactor design with the help of COMSOL Multiphysics software. The model was validated using the reaction rate obtained via experimental data and other conditions governing the reactor, so the average error was less than 3%. In this study, for the first time, the heat transfer effect on the reaction rate, the storage medium voidage on the reactor performance, and changes in the reactor voidage as a function of time on a stable heat source and an unstable heat source have been investigated. Also, the simultaneous impact of voidage changes caused by carbon decomposition in the reactor, temperature and gas flow, and conversion rate were studied under varying temperature conditions.