(368a) Cryogenic Separation of CO2, SO2, and NOx from Flue Gas

Energy use is the dominant source of U.S. CO₂ emissions, with the share estimated to be about 75%. Addressing climate change requires deep decarbonization of our energy, chemical, and manufacturing sectors. One approach involves carbon dioxide capture and storage (CCS) where CO₂ is captured at point emission sources and then transported to sequestration sites for long-term storage in geologic formations. A key CCS challenge is the high cost of CO₂ capture, which varies significantly depending on source concentrations and volumes [1]. For instance, power plant capture costs exceed $55/tonne, but capture costs can be below $33/tonne in the cement, iron, and steel industries. Although significant advances are underway in developing solvent, sorbent, and membrane capture technologies, their costs remain high.

We have developed a novel cryogenic separation process that can remove CO_2, SO₂, and NO_xfrom power plant flue gas [2]. Cryogenic separation has been substantially ignored by carbon-capture researchers. However, it has great potential because it requires only conventional equipment (e.g., compressors, expanders, heat exchangers, pumps). Furthermore, compared to competing technologies, it does not require expensive materials and chemicals that typically have limited availability (e.g., ionic liquids, metal organic frameworks, graphene).

In the cryogenic process, raw flue gas is compressed and cooled, which causes water vapor to condense. Subsequently, residual water is removed using a drier. Then, the dry flue gas is countercurrently cooled via heat exchange with exhaust gas. The chilled flue gas is refrigerated to condense CO₂ (>90%), SO₂ (>99.99%), and NO_x (>99.999%). The non-condensable nitrogen is warmed by passing countercurrently through the heat exchanger and then exhausted through the expander to capture energy. Then, liquid CO₂, SO₂, and NO_x are pumped to a high pressure (e.g., 150 bar) and are countercurrently warmed and captured. Hence, our cryogenic technology mitigates not only CO₂ emissions, but also SO₂, and NO_x pollutants.

Compared to other processes, the cryogenic process consumes less energy because significant energy integration is utilized. Moreover, appropriate use of waste heat also improves energy efficiency. The flowsheet is further optimized by using our in-house process synthesizer SPICE [3], which further reduces energy consumption and capture costs. Sensitivity analysis on several important decision variables (e.g., rotating equipment efficiency, utility cost) is also performed. Preliminary results indicate that even with the most conservative assumptions, the cryogenic process is very attractive compared to alternative process. Without credits for removing SO₂ and NO_x, current estimated capture and pressurization cost is less than $25/tonne CO₂.

References:

[1] Hasan, M. M. F.; First, E. L.; Boukouvala, F.; Floudas, C. A. A multi-scale framework for CO₂ capture, utilization, and sequestration: CCUS and CCU. Computers & Chemical Engineering 2015, 81, 2–21.

[2] Holtzapple, M.; Monjur, M. S.; Hasan, M. M. F. Cryogenic Separation of Carbon Dioxide and Sulfur Dioxide from Flue Gas. U.S. Patent Application No. 63/132,148, filed Dec. 30 2020.

[3] Demirel, S. E.; Li, J.; Hasan, M. M. F. Systematic Process Intensification using Building Blocks. Computers & Chemical Engineering, 2017, 150, 2–38.