Electrification of adsorptive carbon capture: adsorbent regeneration via joule and microwave heating

Onderzoeksoutput: PhD Thesis

Samenvatting

The increasing amount of carbon dioxide (CO2) in the Earth’s atmosphere is causing global warming,which will lead to catastrophic consequences. Therefore, it is critical to limit anthropogenic emissions of CO2. These emissions mainly originate from flue gases of fossil fuel-powered factories, so capturing CO2 before it is emitted is essential. In the meantime, switching to alternative energy sources such as biogas will also be necessary. Biogas upgrading is necessary to obtain a gas with a high energy contentthat could be used for all purposes. Also in this process, the separation of CO2 (in this case from CH4instead of N2) will be crucial. Ideally, the captured CO2 could then be used as a primary material allowing the transition to a carbon-neutral circular economy. However, the high energy requirement of the CO2 capture step is currently a major limitation. Therefore, developing new, less energyintensive technologies is vital for the viability of the process.
The main objective of this thesis is to investigate energy-efficient adsorption-based processes for separating CO2 as an alternative to the currently used CO2 absorption-technique. Specifically, the aim is to develop an adsorption technique for small to mid-scale applications such as biogas production. In the context of the electrification of the chemical industry, where processes will be powered by green electricity, the direct use of the electricity in order to regenerate the adsorbents is (further) explored. The electricity is then converted into heat, resulting in the desorption of CO2.
The first part of this PhD thesis focuses on electrical swing adsorption (ESA). In this technique, a current is passed through the adsorbent which heats up due to its resistance. The advantage of this method is the direct and fast heating of the adsorbent, rather than heating it indirectly through the gas phase, which is a slower process and causes heat losses. However, the configuration of the adsorbent and electrodes appears to be crucial to achieving a high energy conversion efficiency into heat. Previous studies have shown that the use of a structured adsorbent, in this case a monolith, is necessary to minimize the contact resistances that occur in a packed bed. In Chapter 3, we investigated the impact of the material and placement of the electrodes. These showed to have a significant effect on heating efficiency. First, the importance of minimizing contact resistances at the interface between the electrodes and adsorbent was demonstrated. This can be achieved either by mechanical pressing or by using conductive paint, necessary for the homogeneous heating of the monolith. Next, the resistance of the electrodes (Rel) was found to be very important. The higher the resistance, the more energy was lost to the atmosphere. By reducing Rel from 0.54 to 0.16 Ω (with Rmonolith = 4.67 Ω), the heating efficiency increased from 53.3 to 73.7%. The placement of the electrodes (top-bottom or by the sides of the monolith) also leads to interesting results, where side heating could be the preferred method in case of scale-up.
Chapter 4 aimed to identify a suitable adsorbent for cyclic ESA experiments by optimizing both the Joule heating and the adsorptive properties. Therefore, several composite materials were produced. Two different activated carbon (AC)-based monoliths were 3D-printed using the same AC but a different binder. The organic binder was converted by a thermal process into an active phase, resulting in a monolith with a higher capacity (1.55 mmol/g at 0.3 bar) but a too high conductivity, leading to important contact resistances. On the other hand, the potassium silicate binder, although contributing slightly through a chemisorption process, resultsin more homogenous heating but lower capacity (1.48 mmol/g). Next, due to the limited adsorption capacity that can be obtained with ACs, the use of zeolite 13X was explored. A 3D-printed monolith combining 13X with the phenolic binder resulted in a sufficiently conductive adsorbent. This led to a high capacity (2.95 mmol/g at 0.3 bar) as well as a very high heating efficiency (>80%). Unfortunately, this sample degraded in the presence of O2 at higher temperatures. Another way to use the 13X was to extrude monolith using a bentonite binder and addresistive wires into the structure to be able to heat it up through Joule heating. This led to samples with a very high capacity (4.01 mmol/g at 0.3 bar), but the energy losses to the surroundings were important due to part of the wires being in contact with the atmosphere (heating efficiency <28%).
The AC-monolith with the glue binder was used in cyclic ESA experiments for the separation of a typical biogas mixture (30% CO2-70% CH4) (Chapter 5). The main parameters, such as the power, electrification time, purge flow rate and purging time were analyzed: 1) Higher voltages are preferred both for productivity (shorter heating steps) and to reduce heat losses; 2) The electrification time leads to a trade-off between purity and regeneration efficiency on the one side, and energy consumption onthe other; 3) Analysis of the purge step revealed that it is preferable to use a higher purge flow rate for a shorter time (compared to a lower purge flow rate during a longer time). Next, the addition of a rinse step was found to be beneficial for the purity of the CO2 coming out of the monolith during the regeneration.
In Chapter 6, instead of using the electricity for Joule heating, microwave (MW) heating of the adsorbent was evaluated for activated carbon pellets. First, the inhomogeneous heating inside a MW cavity was assessed. Therefore, to homogenize the temperature inside the adsorbent bed, the effect of a sweep flow was evaluated. The higher the flow, the better the temperature distribution over the bed, but the higher the dilution of the product, requiring a trade-off. Next, heating experiments were done to show the rapid heating rates (up to 400°C/min) that could be obtained. Later, in cyclic runs, it was shown that higher, continuous powers are favored as it leads to a higher energy efficiency and concentration factor. It was found that MW heating led to complete regeneration of the bed, while not degrading the bed over 11 consecutive cycles, showing the limited impact of local hotspots in the regeneration.
Originele taal-2English
Toekennende instantie
  • Vrije Universiteit Brussel
Begeleider(s)/adviseur
  • Denayer, Joeri, Promotor
Datum van toekenning15 mrt 2023
Plaats van publicatieBrussel
Uitgever
Gedrukte ISBN's9789464443554
StatusPublished - 2023

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