(98e) Development and Evaluation of Novel Biphasic Solvents for Post-Combustion Carbon Capture

Amine-based chemical absorption is one of the most developed technologies for post-combustion carbon capture (PCC) from point sources. However, despite its maturity, several challenges still hinder its large-scale deployment. The most significant challenges include high energy use incurred during solvent regeneration and CO₂ compression, high capital costs for equipment, and the cost of solvent loss due to degradation and volatile emissions. To overcome some of these challenges, the CO₂ absorption processes that use biphasic solvents are considered attractive. A key feature of biphasic solvents is the formation of dual liquid phases upon the loading of CO₂ or a temperature swing, with the absorbed CO₂ rich in one phase and lean in the other. Because the CO₂-rich phase for thermal regeneration has a reduced mass of solvent and a concentrated CO₂ loading, the required heat duty can be lowered, and the pressure of CO₂ desorption can be elevated, both of which lead to lower energy use and smaller equipment sizes.

In this study, a new group of biphasic solvents have been developed. These solvents are a blend of chemicals that enhance both capacity and rate in solutions with low water content. The solvent exists as a single phase at low to no CO₂ loading, but it undergoes a phase transition to two liquid phases upon CO₂ loading (see Figure 1a). In the laboratory, approximately 80 biphasic solvents were screened based on multiple criteria, including working capacity, absorption rate, thermal and oxidative stabilities, viscosity, and corrosion tendency (see Figure 1b). Two biphasic solvents emerged as superior to all others. Notably, these two biphasic solvents exhibited high resistance to both oxygen-containing and thermal environments. Thermal incubation measurements revealed that the thermal stability of each biphasic solvent for 2 weeks at temperature as high as 150 °C was similar to that of the benchmark 30 wt.% monoethanolamide (MEA) solvent at 120 °C. Oxidative stability measurements showed that compared to MEA, the biphasic solvents exhibited ~8 times higher resistance to oxygen when bubbled in a gas mixture of 96% O₂ and 4% CO₂ at 50 °C for a period of 2 weeks. Furthermore, under simulated absorption and desorption conditions, the corrosion rates of the two biphasic solvents, as measured with a carbon steel coupon (CS1010), were 2-3 times lower than that of MEA.

To evaluate the performance of the developed biphasic solvents, a 40 kW_e bench-scale capture skid was built and installed at the University of Illinois’ Abbott power plant (see Figure 2). The skid is an integrated system comprising the flue gas polishing, CO₂ absorption, phase separation, and desorption steps. Parametric tests were conducted for the two biphasic solvents and the 30 wt.% MEA solution, with respect to multiple process or operating variables such as liquid-to-gas ratio, CO₂ concentration, stripping pressure/temperature, and CO₂ loading, to validate and compare the performance and operational flexibility of the skid. The results of the parametric tests showed that both biphasic solvents appeared to be more energy efficient than the reference MEA. Under representative operating conditions, the regeneration heat duty required for the two biphasic solvents was reduced by ~40-43%, and the stripping pressures was elevated by ~3 times compared with MEA. After completing the parametric testing, continuous 24/7 testing was performed with a slipstream of actual coal combustion flue gas from the power plant for one selected biphasic solvent over a course of totally four weeks in two test campaigns. Both test campaigns were operated under steady state at pre-selected conditions. The first campaign aimed for a 90% CO₂ removal rate, while the second one targeted a 95% removal rate with the stripping pressure slightly adjusted. During the slipstream testing, continuous, steady-state operations were validated during the cold wintertime. Test results also revealed that the heat duty required for solvent regeneration ranged between 2,180 and 2,450 MJ/tonne of CO₂ captured, and the stripping pressure ranged between 65 and 60 psia, depending on the CO₂ removal rate target (i.e., 90% or 95%).

We further conducted a techno-economic analysis (TEA) study to evaluate the biphasic solvent-enabled absorption process for large-scale PCC applications. The TEA study followed the methodology and assumptions consistent with those used in the US Department of Energy (USDOE)’s cost baseline study (Version of Revision 4). The results of the TEA study revealed that the parasitic power loss for the biphasic solvent-based process installed in a net 650 MWe power plant could be reduced by ~19% and the cost of CO₂ capture by ~20% compared with the State-of-the-Art technology adopted in USDOE’s Base Case B12B. Such technoeconomic advantages resulted from the higher energy efficiency for both CO₂ capture and power generation, smaller power plant for the same net output, and lower capital costs associated with CO₂ absorption, stripping, and compression equipment. This work will continue to investigate and test the performance of the biphasic solvent-based process for CO₂ capture from other industrial point sources such as waste-to-energy facilities.