Mathematical Modelling and Simulation of Catalyst Deactivation for the Continuous Thermo-Catalytic Decomposition of Methane

  • A clean energy future based on a hydrogen economy has been proposed as a feasible alternative to the current combustion of fossil fuels to reduce CO2 and other greenhouse gas (GHG) emissions. However, hydrogen’s “clean” reputation is questionable due to its main production method of steam methane reforming (SMR), which produces large quantities of CO2 emissions. To abide by recent international regulations, the production of hydrogen needs an immediate transition to low-emission production methods. A promising solution is the thermo-catalytic decomposition of methane (TCMD), which thermally decomposes methane into hydrogen gas and solid carbon without any direct GHG emissions. The problem with this process is that the catalyst deactivates quickly and therefore must be replaced periodically for sustained hydrogen yields. This results in high catalyst turnover costs, which is the main bottleneck in the successful commercialisation of this process. By developing a simplified model using the most commercially viable parameters and linking the turnover costs with the deactivation of the catalyst, this study aimed to accelerate the adoption of the TCMD process by better enabling companies to analyse the feasibility of their potential low-emission TCMD solutions. The most commercially viable solution featured the use of a fluidised bed reactor (FBR) for continuous operation with an iron-based catalyst due to their low-cost. Catalyst regeneration was found to be ineffective and the best method for mitigating catalyst deactivation was the optimisation of the process conditions. A simplified mathematical model was then constructed to enable this adjustment to maximise the production of hydrogen and minimise the turnover costs. This model was based on the ideal continuous stirred tank reactor but incorporated the fluidising behaviour through several variables including the development of the novel “Fluidisation Factor”. An optimisation ratio was also developed to quantify the simulation results and obtain the optimal conditions. The results showed that the ideal conditions for this process was at the highest temperature before the catalyst starts to sinter (≈950ºC) and at the maximum pressure. The largest catalyst particle size of 150 𝜇m and the maximum amount of catalyst was found to allow for higher fluidisation velocities and to delay the deactivation time respectively. The inlet gas velocity and the catalyst activity limit were found to be highly dependent on the hydrogen production rate and therefore due to the conditions used in this study the optimum inlet gas velocity was 10% of the fluidisation velocity range (and composed of pure methane), whilst the optimum activity limit was at 22% activity. Lastly, a comparison of the heating method found that controlled heating was more suitable than constant heating due to the stable temperatures during deactivation, which prevent catalyst sintering. Comparing the optimised results from this study with the SMR production method found that the estimated catalyst turnover costs were 10¢/kg H2 and 18¢/kg H2 respectively. This demonstrates that the simplified model developed in this work can better enable companies to optimise and assess the feasibility of their proposed TCMD solutions and help transition their hydrogen production processes to cleaner alternatives.

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Author:Brock Lumbers
Examiner:Frank Platte, Joachim Gebel
Document Type:Bachelor Thesis
Date of Publication (online):2021/04/29
Year of first Publication:2020
Publishing Institution:Hochschule Rhein-Waal
Degree-granting institution:Hochschule Rhein-Waal
Release Date:2021/09/27
Tag:Catalyst deactivation; Low-Emission hydrogen; Methane pyrolysis; Modelling and simulation; Thermo-Catalytic Methane Decomposition
Faculties and Institutes:Fakultät Technologie und Bionik
Licence (German):License LogoCC BY 4.0 International - Namensnennung