Potential of carbon capture technologies

Most of the reviewed technologies reach roughly similar techno-economic properties and there are no clear standout technologies that have emerged over the others. Each technology has its specific advantages and challenges, and primarily, capture environment, feed gas composition and end-use application of the captured CO2

determines the most suitable technology for various applications. The applicability is determined by factors like scale, energy requirement, form of energy required, desired capture rate and CO2 purity, waste stream handling, equipment size, and most importantly, cost. It can be expected that future development of carbon capture technologies will include multiple different methods of carbon capture for various applications. If any major technological breakthroughs do not occur, it is unlikely that a single technology would take over the whole carbon capture market.

Post-combustion capture with liquid absorbents is currently the most mature method of carbon capture available, with lots of commercial-scale demonstration to prove its credibility. It is considered as one of the low-risk technologies with the most realistic cost estimates available. More advanced versions of the absorbents with proven feasibility are

constantly being developed, which will expectedly improve the techno-economic properties of these absorbents. Conventional absorbents, such as the previous state-of-the-art technology MEA, suffer from obvious shortcomings such as high energy-intensity and harmful nature to equipment and environment. Despite the high technological maturity, the conventional absorbents are being displaced by more advanced absorbents like advanced amines and multi-phase absorbents.

In addition to absorption-based capture, many other emerging technologies have shown great promise in preliminary and pilot-scale assessments. Many of these technologies have not yet reached large industrial-scale demonstration, which makes it difficult to realistically assess their true potential. For example, fuel cell hybrid cycles have shown very promising techno-economic performance in preliminary assessments but are still considered as high-risk technologies since demonstration in realistic conditions has not yet been conducted. Industrial-scale demonstration is however underway for both technologies, as well as for many other emerging carbon capture technologies, meaning that the true potential of these technologies can be more accurately assessed in the near-future.

In their report on emerging CO2 capture technologies IEAGHG (2019a) ranked the emerging CO2 capture technologies based on the potential of widespread deployment for the near future. The ranking is presented on Figure 4.2. It is based on to a scoring system taking into account each technology’s demonstration level, cost reduction potential, experience from other sectors, component availability, retrofitting compatibility and requirement for further research.

Figure 4.2. Carbon capture technologies ranked by their potential for widespread deployment in 5–10 years from 2019 onwards. Higher score indicates greater potential. (IEAGHG 2019a.)

According to IEAGHG’s technology ranking the highest potential for widespread deployment lies in post-combustion technologies. From these technologies, the highly mature amine-based absorbents rank the highest, closely followed by electrochemical separation (e.g., fuel cells) and adsorption-based capture, which are yet to prove their feasibility in large-scale industrial operation. Membranes and solid sorbents reach good potential in pre-combustion applications, whereas the still immature oxyfuel processes rank notably lower compared to post- and pre-combustion technologies. Based on related literature IEAGHG (2019a) summarize the challenges of widespread carbon capture deployment as following: high cost, investment risk of the initial projects due to weak investment returns, lack of infrastructure regarding transportation and storage of CO2, long duration of the projects, lack of related legislation as well as unclear policymaking regarding carbon capture.

5 DESCRIPTION OF THE PILOT-SCALE CARBON CAPTURE EXPERIMENTS

Experimental part of this thesis is focused on the carbon capture experiments that were conducted as part of VTT’s BECCU project between August and October of 2020. Three absorption-based post-combustion capture technologies were tested at pilot-scale by using synthetic gas mixtures, biogenic flue gases and raw biogas. The tests were conducted by using a 50 kW circulating fluidized bed (CFB) pilot combustor located at VTT’s facilities in Jyväskylä, Finland. Objective was to validate proper function of the technologies in realistic conditions and to gain information about capture performances with various CO2 sources. Additionally, some CO2 was captured, compressed and bottled for further research regarding CO2 utilization via Fischer-Tropsch synthesis. The utilization experiments are not however reviewed in this thesis.

The tested carbon capture technologies were:

• Enhanced soda scrubbing process by VTT,

• Enhanced water scrubbing process by CarbonReUse Finland,

• Kleener liquid – a novel capture absorbent by Kleener Power Solutions.

Compared to more mature amine absorbents these technologies use more eco-friendly capture solvents, which supposedly do not cause any harmful emissions to the environment or to the captured CO2 stream. It is estimated that the solvents are also not as volatile as amines and could require less energy in the capture process, at least when compared to conventional amines like MEA.

Schedule, content and objectives of the experiments are presented in Table 5.1. As seen from the table, not all technologies were tested with all the CO2 sources.

Table 5.1. Schedule, content and objectives of the carbon capture experiments.

Week CO2 source Tested technologies Test objectives

#36 Synthetic gas mixtures

VTT Soda CarbonReUse

• Verifying proper function of the test equipment

• Testing capture performance with modifiable gas compositions

• Post-combustion capture in realistic flue gas conditions

• Post-combustion capture in realistic flue gas conditions

• Technology performance comparison

• Effect of another biomass type on capture performance

• Post-combustion capture in realistic flue gas conditions

• Effect of another biomass type on capture performance

• Performance of VTT’s soda process in biogas purification

The technologies were tested by using capture equipment built inside two shipping containers, which ensure effective mobility of the equipment. CarbonReUse’s technology was tested by using their self-constructed pilot-scale capture container, whereas VTT’s soda solution and the Kleener liquid were both tested in a capture container built by VTT.

The two containers were placed outdoors next to VTT’s 50 kW CFB-pilot facility (Figure 5.1) and the containers were connected to the flue gas line of the CFB.

Figure 5.1. The carbon capture technologies were tested by using capture equipment built inside two shipping containers. The 50 kW CFB-pilot is located in the building behind the containers.

The tested technologies and test arrangements are more specifically reviewed below.

5.1 Enhanced soda scrubbing process by VTT

In the enhanced soda scrubbing process developed by VTT, an aqueous sodium carbonate (Na2CO3) solution is used to capture CO2 via chemical absorption. Originally, the process has been developed for biogas purification applications, but potential has been recognized also in post-combustion carbon capture. The process has been previously experimented in synthetic biogas purification at laboratory-scale by VTT (TRL 4). Via the discussed experiments TRL of the process advanced to 5.

The process includes a novel technical improvement to enhance the slow absorption kinetics of soda-based carbon capture. Instead of using a conventional large-sized absorption column that are typically used in absorbent capture, an ejector is used to mix the flue gas and the solvent flows. Efficient mixing properties of the ejector improve dissolution of CO2 into the solvent, thus enhancing absorption rate. Furthermore, the improved absorption rate decreases size of the absorption equipment, which could

expectedly lower the capital cost of the equipment compared to conventional absorption columns.