Author: RoCo

Carbon abatement is needed to mitigate the threat of climate change. It is a new area for innovation and investment for future economic development. Scientists, corporations, and governments around the world are stepping up to develop and support technologies which reduce the cost of decarbonizing the economy.

Nature has a carbon cycle. It uses photosynthesis, mineralization, and other natural cycles to balance CO₂. Human activity in the last few hundred years has caused this carbon cycle to be off-balance (Figure 1). The impact has been a change in climate. Therefore, there is an urgent need to engineer a solution and to fix the broken carbon cycle.

Figure 1: Generalized carbon cycle (source: Office of Biological and Environmental Research of the U.S. Department of Energy Office of Science. science.energy.gov/ber/)

Many technologies are being developed to remove CO₂ at the point of source emissions. A large stationary point source of CO₂ is a single localized emitter, such as fossil fuel power plants, oil refineries, industrial process plants and other heavy industrial sources, and accounts for almost 48 percent of total CO₂ emission. Cost effective, point of source capture is an important element to mitigate climate change and to achieve net-zero greenhouse gas emission by 2050.

There are various technologies which can address and remove CO₂ from the point of source emissions. One of the most promising candidates is a solvent-based technology using an amine which can selectively remove CO₂ from flue gas. The chemical absorption process using an amine-based solvent has been employed for CO₂ and H₂S removal—acid gas removal—since 1950 to get natural gas to pipeline quality. The estimated cost for these systems to remove and capture CO₂ from point of source application is around $58 per metric ton, according to the United States Department of Energy (DOE), because of their high capital and energy costs.

To achieve better cost efficiency, the DOE has focused its resources to develop and support technologies which can decrease the cost of capture to $27.26/MT by 2030. RoCo has tackled this challenge, and achieved good results as described below.

Challenge:

• Aqueous amine solvents are expensive as they entail serious economic and environmental problems (e.g., high energy cost needed to regenerate the solvent regeneration (mainly due to high latent heat of vaporization of water); solvent loss by evaporation; thermal and oxidative degradation of the solvent; and equipment corrosion due to high basicity and aqueous nature of the solvents).
• Water-lean solvents are promising materials as they avoid the latent heat of vaporization of water and improve the total capture efficiency. Significant reductions in costs are still not utilized because of the increase in viscosity as well as solvent degradation upon absorption and desorption of CO₂.

Additive approach to decrease viscosity:

The viscosity increase with CO₂ capture in water-lean amine solvents is extremely difficult to avoid. This phenomenon is mainly due to the chemical nature of the products formed upon its reaction with CO₂. Figure 2 illustrates a variety of hydrogen bonds as well as electrostatic charges which are formed upon reacting with CO₂. Water is necessary in aqueous system, but it is not shown for simplification.

Figure 2: Hydrogen bonding and ionic bonding in a monoethanolamine based solvent.

Several studies, assisted by molecular simulations, show that the intramolecular hydrogen bonding and electrostatic charges are the major contribution in increasing viscosity upon adsorption of CO₂ in water-lean solvents solvent systems (2016 Glezakou; 2016 Glezakou; 2008 Maginn; 2007 Chen). To avoid the dramatic increase in viscosity upon CO₂ uptake in water-lean solvents, one strategy is through the formation of intermolecular hydrogen bond to reduce the formation of strong intramolecular hydrogen-bonded networks. Wang and coworkers introduced hydrogen acceptor such as N or O atom into the amino-functionalized ionic liquids (ILs) to stabilize the H of carbamic acid produced from the reaction with CO₂. The ILs with a hydrogen acceptor exhibit a slight increase or even decrease in viscosity after CO₂ capture, while those without a hydrogen acceptor show as much as 132-folds increase in viscosity. Recently, Koech and Glezakou introduced pyridine functionality as a site for internal hydrogen bonding into the water-lean solvents, and the resulting solvents showed the lowest CO₂-rich viscosities of 100% concentrated amines currently reported. These studies also show the importance of hydrogen bonding.

There are two approaches to improve the performance of non-aqueous, water-lean amine-based solvent systems: 1) redesigning the solvent molecules and 2) developing additives. Redesigning solvent molecules is an elegant but expensive approach which requires the design and synthesis of a solvent molecule, application testing to understand the molecular insights, and building a solvent around it. Hence, the additive approach is cost effective allows the use of a number of commercial amines to be a drop-in replacement for the development of non-aqueous amine chemistry.

RoCo Global has partnered with Prof. Hyung Kim’s group at Carnegie Mellon University and engineers from Carbon Capture Scientific, LLC to develop an additive approach on commercially available amines in order to address the issues associated with the rise in viscosity. This work is funded by the DOE. RoCo and its collaborators have developed a viscosity solvent additive package which significantly reduces viscosity of a water-lean, amine-based solvent by breaking the long-range electrostatic and hydrogen bonding into smaller clusters upon the adsorption of CO₂, as illustrated in Figure 3, where segmentation of hydrogen bonding and electrostatic network occurs due to the addition of the additive molecules.

Figure 3: Illustration of fully hydrogen bonding (HB) network (left) and the breakage of the HB network by addition of HB acceptors (right).

Significant Results:

Assisted with molecular simulation insights, numbers of additives are synthesized and screened for their viscosity-reducing performance for water-lean solvents. The water-lean, additive-amine solvent system (RoCo-1) has shown viscosity of less than 5 cP (at 40 °C, >10 wt% CO₂ uptake), which is more than 50% lower than the benchmark solvent (piperazine/MDEA) as shown in Figure 4, left. This decrease in viscosity means significant savings in capture cost when compared to . Case B12B due to lower reboiler duty and decrease capital cost. (Figure 5). The RoCo-1 solvent is currently being tested in a lab-scale capture unit (Figure 4, right) to establish its overall performance, working capacity and ideal operating conditions. Initial engineering analysis shows that 50% decrease in viscosity can result in a net 16% decrease in capital cost (or $80 millions, see Figure 5) and a total saving of xx% (or $3.80 per metric ton CO₂ captured). RoCo continues to explore different conditions and parameters to optimize our system.

Figure 4: Viscosity of RoCo-1 and benchmark solvents as a function of CO₂ concentration (left) and a lab-scale capture unit at RoCo’s lab (right)

Figure 5: Impact of solvent viscosity reduction on the capital cost saving.

Partnering with RoCO Global:

Are you looking for an ideal partner to help you rapidly advance your carbon capture initiatives?  Perhaps you want to design and test custom gas separation membranes for your application?  Contact RoCo Global today to learn more about our Research & Development Services and how we can help you meet, and exceed, your goals.

Acknowledgment:  This material is based upon work supported by the Department of Energy under Award Number DE-FE0031629.

Disclaimer:  This report was prepared as an account of work sponsored by an agency of the United States Government.  Neither the United States Government nor any agency thereof, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights.  Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency thereof.  The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof.

Figure 1: impact of hydrogen bonding on viscosity in simple alcohols

Figure 2: Molecular interactions can decrease viscosity significantly. Top: experimental results showing decrease in viscosity; Bottom: Computer simulation insights  showing increase in intramolecular interactions leads to weaker intermolecular interactions and thus significantly lower viscosity.