Recently, we have reported on the potential of aerogel sorbents for CO₂ capture and storage (CCS). Despite their favorable properties, the deployed amine functionalized aerogels (AMAs) were found to require optimization to allow for their successful economical implementation. Increasing the activity and capacity of solid sorbents while decreasing their cost, is therefore an issue which is currently under investigation. Researchers from the US and Sri Lanka now report to have found an efficient, cheap and environmentally benign solid CO₂ sorbent: KOH-activated carbon chitin aerogels.

The novel sorbent material was synthesized from commercial chitin powder from shrimp shells, which was dispersed in a sodium-urea-water solution. Repeated freezing/thawing cycles of this solution resulted in the formation of a stable hydrogel, which subsequently was freeze-dried to obtain a chitin aerogel. Thereafter, carbonization of the aerogel was achieved by heating the sample to 800 °C under nitrogen atmosphere. In the last step, the aerogel was again heated to 850 °C (under N₂ atmosphere) in the presence of potassium hydroxide (KOH) to obtain the activated carbon aerogel. Consequently, the inexpensive manufacturing technique, which does not require any costly or toxic chemicals, and the abundance of the precursor materials facilitate the cheap production of chitin-based CO₂ sorbents.

The final activated carbon aerogels were found to exhibit large specific surface areas (> 500 m²/g), more than 35 times larger than that of their parent chitin aerogels. Additionally, the micro pore volume, which is an important parameter for CO₂ capture, increased by the factor of 95 between the chitin aerogel and the carbonized and KOH-activated sample. These two factors explain why the obtained CO₂ sorptivity value of 0.48 mmol/g (1 atm, 0 °C), obtained for the chitin aerogel, could be vastly increased to 5.02 mmol/g by further processing (i.e. carbonization and activation). As shown in the figure below, similar increase in sample sorptivity was also measured at room temperature (0.28 mmol/g and 3.44 mmol/g, respectively). This means that the morphological changes taking place inside the aerogel structure during carbonization and activation have a significant impact on the final sorbent properties.

CO2 adsorption isotherms at 1 atm and 0 °C (a) and 1 atm and 25 °C (b) for chitin aerogels (1), carbonized chitin aerogels (2) and KOH-activated chitin aerogels

The authors conclude that they have found an environmentally benign and very inexpensive way of manufacturing highly active chitin-based sorbents for CO₂ capture. Additionally, the sorbents are synthesized from a biopolymer, making the final material biodegradable and non-toxic.

More details: Dassanayake, R.S., Gunathilake, C., Abidi, N. et al.; Activated carbon derived from chitin aerogels: preparation and CO2 adsorption, Cellulose (2018). https://doi.org/10.1007/s10570-018-1660-3

Coal fired power plants will play a substantial role in energy production for the majority of industrialized nations until at least the middle of this century. This is an issue since the large-scale combustion of carbon-based resources is a major contributor to the rising atmospheric CO₂ levels. In order to achieve the ambitious multilateral goals to lessen the detrimental effects of global warming by stabilizing the global CO₂ levels outlined in the Paris Agreement, the greenhouse gas emissions from coal fired power plants will need to be reduced drastically.

One way to achieve this reduction in emitted greenhouse gases without abandoning coal-based power production is the capturing and sequestration of emitted CO₂ (see graphic below). This can be done by retro-fitting existing power plants with carbon capture and storage (CCS) units, which remove carbon dioxide from flue gas, before storing it in a designated location. Most commonly, temperature-swing adsorption (TSA) processes, utilizing aqueous amine solutions as sorbents, are proposed for CO₂ capturing. However, the regeneration of the amine solution results in an energy penalty which drastically reduces the thermal efficiency of the power plant. Therefore, alternative sorbents requiring less energy for regeneration are under investigation.

Diagram of carbon capture and storage life cycle.
From: Scottish carbon capture and storage

In a joint effort, a project team consisting of Aspen Aerogels, the University of Akron, ADA-ES, and Longtail Consulting have synthesized and tested solid amine functionalized aerogels (AFA) in a bench scale fluidized bed reactor, in order to assess their potential for future application in CCS.

It was found that the AFAs synthesized by Aspen Aerogels showed promising CO₂ adsorption behavior and good stability over numerous adsorption-desorption cycles. On top of that, the novel solid sorbents possess significantly lower heats of reaction with CO₂ than commonly deployed liquid and solid sorbents. In a subsequent step, the AFAs were coated at the University of Akron, yielding pellets possessing a good cyclic stability in the presence of SO₂. The novel AFA pellets were then tested in a bench scale fluidized bed reactor to determine their physical properties (e.g. fluidizing gas velocities, void fraction, etc.) in such a reactor setup.

Based on those findings, Longtail Consulting modeled the hydrodynamic and heat transfer properties of the solid aerogel sorbent in the fluidized bed reactor and finalized the process requirements. Subsequently, a techno-economic analysis (TEA) of the entire CO₂ capturing unit (assuming 90 % capturing efficiency) fitted to a coal fired, supercritical steam cycle power plant producing 550 MW_e was performed. Despite offering a promising behavior from a technological perspective, the TEA revealed that utilizing novel AFA sorbents results in approx. 20 % higher levelized electricity costs. This finding was mainly attributed to the fact that the attrition of the material during fluidization was unknown, yielding high variable costs for the selected scenario. Additionally, a relatively high apparent particle density inside the fluidized be reactor led to a  massive pressure drop, causing a surge in auxiliary electricity input.

In light of these findings, the project team concluded that further testing under practical conditions and simultaneous optimization of the AFAs will be essential to make solid sorbents an economical alternative to state-of-the-art aqueous amines.

As the need for creative solutions to reduce greenhouse gas emissions is only increasing, it is promising to see that the vast potential of aerogels is being explored to address climate change. Whether or not solid sorbents can be successfully utilized in TSA in an economically reasonable way may prove to be one of the key factors determining the success of CCS.

Full Report: https://doi.org/10.2172/1349123

Read more at: https://www.osti.gov/scitech/biblio/1349123