New Amine-Containing Membranes for CO2 Capture

This presentation covers new advances in amine-containing membranes for carbon dioxide capture from flue gas in coal- and/or natural gas-fired power plants and from <1% CO₂ concentration sources, e.g., the residual flue gas after the primary CO₂ capture system and coal gas streams. The post-combustion carbon capture from flue gas requires a high CO₂/N₂ selectivity of 140 along with a very high CO₂ permeance of about 700 GPU (1 GPU = 10^-6 cm³ (STP)/(cm² · s · cmHg)) or higher in order to use a stand-alone membrane process. To achieve the high membrane performance, we have synthesized new membranes comprising high-molecular-weight polyvinylamine (PVAm) as the fixed-site carrier and aminoacid salt (e.g., piperazine glycinate (PG)) as the mobile carrier for facilitated transport of CO₂ via reversible CO₂ reactions with the amine carriers. PVAm samples with different molecular weights were synthesized through free radical polymerization by adjusting the monomer concentration and initiator amount. The synthesized PVAm showed both a higher molecular weight and a higher solution viscosity than commercially available PVAm. The high viscosity of the PVAm solution at a low concentration allowed for the preparation of much thinner membranes. It could also help reducing penetration of the polymer solution into the pores of the substrate, further minimizing the mass transfer resistance and hence increasing the CO₂ permeance. The membranes synthesized showed a high CO₂/N₂ selectivity of greater than 140 and a high CO₂ permeance of about 800 GPU. The membranes were scaled up to 14 inches in width by using a continuous roll-to-roll fabrication machine. Aided by a material balance equation, three variables, namely the coating-knife gap setting, substrate rolling speed, and coating solution concentration, were identified as the critical factors to control the membrane selective layer thickness. This resulted in thin membranes achieved with a selective layer of < 200 ± 20 nm. From gas transport measurements, the flat-sheet samples of the scale-up membranes exhibited similar performances compared to the membranes synthesized in lab scale. The scale-up membrane was fabricated into spiral-wound membrane modules for testing with simulated flue gas (saturated with water vapor) containing about 20% CO₂, 77% N₂, 3% O₂ and 1 – 3 ppm SO₂ (on dry basis), showing similar results as the flat-sheet membrane tested in the lab. Techno-economic analysis has shown that the post-combustion capture process using the membrane is promising for meeting DOE’s capture cost target set for 2025. For carbon capture from <1% CO₂ concentration sources, we have elucidated the carrier saturation phenomenon. With reducing the CO₂ concentration in the feed gas, both CO₂ permeance and CO₂/N₂ selectivity increased. These were mainly due to more available amine carriers for CO₂ molecular transport at lower CO₂ concentration conditions. For the membrane showing a CO₂ permeance of 806 GPU and a CO₂/N₂ selectivity of 173 with 20% CO₂ concentration feed gas, the same membrane exhibited a CO₂ permeance of 982 GPU and a selectivity of 211 for 1% CO₂ concentration feed gas. Based on these results, a high-level techno-economic analysis for ³90% CO₂ capture with ³95% CO₂ purity will be presented.