Carbon-neutral synthetic fuel production via CO2 methanation over supported Ni catalysts

Omoregbe, Osaze ORCID: 0000-0002-1956-9155 (2024). Carbon-neutral synthetic fuel production via CO2 methanation over supported Ni catalysts. University of Birmingham. Ph.D.

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Abstract

The innovative Power-to-Gas concept is a promising solution for converting surplus energy into synthetic natural gas (SNG). Through catalytic CO2 methanation, renewable H2 and captured CO2 are converted to methane (CH4) or SNG, thereby offering a potent energy storage medium characterised by its elevated heating value. Intrinsic to the global push towards sustainable energy solutions, CO2 methanation stands as a mature reaction, perpetually demanding high-performing catalyst design and synthesis to enhance industrial process efficiency.

The main thesis goals come together along three crucial trajectories. First, it involves the synthesis of high-performance Ni-based catalysts, including YSZ, CeO2, Al2O3, and SBA-15, followed by their performance evaluation for CO2 methanation. Second, the investigation of the influence of distinct reactor conditions, encompassing temperature, H2/CO2 ratio, catalyst positioning, CH4 presence in feed, gas hourly space velocity (GHSV), pressure, and the effect of La-promoter on the yield and selectivity of the optimal catalyst. Finally, the third trajectory delves into the design and modification of monolithic and split reactors for CO2 methanation. This chapter encompasses an investigation into dual catalyst beds and the dynamic interplay of catalyst ratios within reactor subsections.

The thesis started with a background on the recent trend in energy demand, the impact of high fossil fuel dependency on climate change, and the transition from fossil fuel to renewable energy. The discussion also covered an overview of carbon capture technologies and the potential of CO2 methanation as a strategy for CO2 mitigation. The thermodynamics, reactor considerations, and intricate reaction mechanisms of CO2 methanation are discussed.

The catalysts were synthesised using the wetness impregnation technique, followed by comprehensive characterisation, including BET surface measurements, thermogravimetric analysis, scanning electron microscopy, and Raman spectroscopy, for the understanding of their physicochemical properties. The performance of the synthesised catalysts was studied in a continuous flow quartz tube fixed-bed reactor with an internal diameter of 5.5 mm and wall thickness of 2 mm at a temperature range of 473 to 823 K and pressure range of 0.5 to 3 bar depending on the test setup. The gaseous feed stream employed contains N2 (60 mL min-1), H2 (60 mL min-1) and CO2 (15 mL min-1) in the ratio of 4:4:1. Mears' and Weisz-Prater criteria obtained indicated negligible transport limitations.

Activity tests revealed Ni/YSZ as the optimal catalyst with a CO2 conversion rate of approximately 83%. The superior performance of Ni/YSZ is attributed to its distinctive properties, including high oxygen vacancies, augmented basic sites, and an intrinsic resilience to metal sintering, underscored by a low activation energy. The optimum temperature was between 633 and 643 K beyond which the catalyst activity declined. While an increase in the H2:CO2 ratio favoured both the CH4 yield and CO2 conversion, CH4 yield was found to decline at an H2:CO2 ratio beyond 4.

For the investigation of the effect of La-promoter on the performance of Ni/YSZ, a slight stability enhancement was observed, but the catalytic activity was largely unaltered. Notably, there was a structural change in Ni/YSZ as observed from the characterisation results upon La-promoter addition indicating its potential to positively influence catalyst performance in other reforming reactions.

For the reactor design, three distinctive configurations—the single reactor with one catalyst bed, the single reactor with dual catalyst beds, and a two-reactor series, were considered. In the latter configuration, the catalyst ratio within the reactors was varied from 20 to 80%. The outcome revealed that the series-connected two-reactor system with an installed water-removing device and catalyst of ratio of 30%:70% is the best, providing a CO2 conversion of approximately 99%.

In a broader perspective, the findings from this work can help in advancing the research on the seamless and efficient integration of a methanation reactor with a solid oxide electrolyser, considering that the best performance was achieved at reaction conditions such as relatively elevated temperatures and low pressures, which are similar to the operating conditions of a solid oxide electrolyser.

Type of Work:	Thesis (Doctorates > Ph.D.)
Award Type:	Doctorates > Ph.D.
Supervisor(s):	Supervisor(s) Email ORCID Steinberger-Wilckens, Robert UNSPECIFIED UNSPECIFIED Majewski, Artur UNSPECIFIED UNSPECIFIED El-Kharouf, Ahmad UNSPECIFIED UNSPECIFIED
Licence:	All rights reserved
College/Faculty:	Colleges > College of Engineering & Physical Sciences
School or Department:	School of Chemical Engineering
Funders:	Other
Other Funders:	Petroleum Technology Development Fund (PTDF), Institution of Chemical Engineers (IChemE), GilChrist Educational Trust, Student Support Fund, University of Birmingham
Subjects:	T Technology > TP Chemical technology
URI:	http://etheses.bham.ac.uk/id/eprint/14918

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