Novel, eco-friendly technology to capture CO2

The captured CO₂ can be either utilized as feedstock for products or processes, or stored permanently underground to avoid releasing the CO₂ into the atmosphere. In future scenarios, utilization of CO₂ as feedstock is presented to be a key element to achieve carbon neutrality, which also sets new criteria for carbon capture. Utilization of CO₂ provides stricter criteria for the captured gas quality with low impurities, flows and feasibility and, thus, require novel technologies.

Today, CO₂ is mostly captured from flue gases of combustion processes or other similar exhaust streams, which is referred to as post-combustion capture. Different pathways for carbon capture from emission point sources can be categorized into four categories: post-combustion capture, pre-combustion capture, oxy-combustion and inherent capture. Post-combustion capture is generally end-of-pipe technology that can be integrated into existing processes without requiring major modifications, making them suitable for retrofitting.

Several techniques for CO₂ capture have been developed such as liquid absorbents, solid adsorbents, membranes, cryogenic separation, chemical looping, and electrochemical separation. With liquid absorbents – that is the most common method – CO₂ is caught into a liquid solution via absorption and released in a separate process phase via regeneration typically by heating the solution. Amine absorbents are commonly used due to high technological maturity and high capture performance. However, amines suffer from disadvantages such as high energy-intensity and harmful amine-based emissions, which has generated interest to develop alternative technologies that are more eco-friendly and consume less energy.

An innovative bicarbonate technology for CO₂ capture, which could offer a more sustainable option for amines, is being developed at VTT. Our technology utilizes an eco-friendly soda solution that is used in an enhanced way with a novel absorber configuration, speeding up dissolution of CO₂ into the liquid. The soda solution can bind up to 30 times more CO₂ than water, while still being almost as safe to handle as water. Furthermore, the process does not generate any harmful pollutants, which is a significant advantage over the commonly used amine absorbents.

Originally, the process was developed for biogas purification, but since potential has been recognized also in post-combustion capture. The process has been previously experimented in projects like BioMet2020 and BECCU, capturing CO₂ from synthetic gas mixtures, flue gases originating from biomass combustion, and raw biogas. In E-Fuel, we aim to develop the soda process towards higher scale via process simulation and practical experiments. VTT’s test bench of TRL 5 is built into container which enables tests also on field circumstances.

In the enhanced soda scrubbing process developed by VTT, an aqueous solution of sodium carbonate (Na₂CO₃), that is commonly known as soda, is used to capture CO₂ via chemical absorption. CO₂ dissolved into the solution reacts with soda and water via chemical absorption to form sodium bicarbonate (NaHCO₃), i.e., baking soda. CO₂ is released from the solution in a regeneration step, where the bicarbonate is converted back to carbonate at sub-atmospheric pressure and temperature of 65-75°C. After regeneration, the capture solution is cycled back to the absorption step, thus creating an efficient CO₂ capture cycle with a low demand of make-up absorbent. Purity of the produced CO₂ stream is over 95 vol-%, which, if needed, can be further purified by drying and nitrogen removal before transportation and/or utilization of the CO₂.  

To improve performance of the process, the gas-liquid mixing is enhanced with a novel microbubble generator that produces very small size bubbles which enable a large gas-liquid reaction area for efficient dissolution of CO₂ into the solvent, thus enhancing CO₂ absorption rate. High absorption rate decreases size of the absorber, which could potentially lower the capital costs compared to conventional absorption columns.

For upscaling the soda capture process, process simulation and optimization of the process parameters are needed. Thus, it was important to collect data on system performance. Focus was set to absorption performance with a target to provide reliable data on mass transfer and kinetics using different flows of liquid and gas. Tests were performed during the summer of 2021 at VTT’s premises in Espoo, Finland. Synthetic gas mixtures were used in the tests as a source of CO₂ and various parameters were measured (e.g., pH, flow rates, CO₂ concentrations, dissolved oxygen and bubble sizes with high speed camera). The tests were made in co-operation with VTT and LUT (Lappeenranta–Lahti University of Technology) and the results were published in a Master’s Thesis.

Efficiency coefficients provide important information for process upscaling. In this case the results showed that the absorption reaction occurs so fast that extra delay time is not needed, which enables lower capital costs for the process. The results show that dissolving of CO₂ is about 10 times faster to soda than to plain water. The higher value of efficiency coefficient indicates better mass transfer and by knowing the related process parameters the optimization can be done. Determination of CO₂ mass transfer efficiencies for the absorption system were performed with air and with 15 vol-% CO₂ gas mixture for water and 8 w-% Na₂CO₃ solution. Tests with air was related to standard dissolved oxygen method which is used for comparing the mass transfer efficiencies of different systems.

In order to be able to define the performance of the microbubble generator parameter’s effect to absorption efficiency, the bubble sizes and bubble velocities were estimated using high-speed camera operated by LUT (see picture below). Image recording was done at three different focal areas in a transparent pipe after micro bubble generator. By measuring an average of 20 bubbles of minimum size and 20 bubbles of maximum size the average bubble size was estimated. In soda solution the average bubble sizes were from 334 µm to 2500 µm, depending on the flow parameters. Respectively, in water the average bubble sizes were from 370 µm to 4140 µm. Together with the absorption, these results show that smaller bubbles indicate higher absorption efficiency.

VTT’s novel soda process for carbon capture can be further developed by improving the regeneration process, optimizing the heat and flow rates, as well as by screening different heat integration options which could be used in industrial plants. Commercializing of the process has begun by Kleener Power Solutions Ltd.

Links to related Master’s thesis

Linjala, Onni. Review on Post-Combustion Carbon Capture Technologies and Capture of Biogenic CO2 Using Pilot-Scale Equipment. Lappeenranta–Lahti University of Technology LUT. 2021. https://urn.fi/URN:NBN:fi-fe202103046520

Narayanasamy, Mohankumar. Mass Transfer Efficiency for CO₂ Capture Using Soda Solutions. Lappeenranta–Lahti University of Technology LUT. 2021. http://urn.fi/URN:NBN:fi-fe2021110954433

In the main photo: Test group of VTT and LUT at the test facilities in VTT Espoo, August 2021. Persons in the picture are Mohankumar Narayanasamy (LUT), Tuula Kajolinna (VTT), Kristian Melin (LUT), Andrey Saren (LUT) and Johannes Roine (VTT). Picture: VTT.