Modeling and Performance of an Expander Cryogenic Carbon Capture System

Abstract An expander cryogenic carbon capture (EC3) is a novel cryogenic carbon technology with the potential to improve the techno-economics of carbon capture and sequestration (CCS) from flue gas. The expansion process in a multi-phase condensing turbine cools the flue gas to approximately −150°C causing the CO2 to condense as solid dry ice. This solid CO2 can be mechanically separated from the remaining uncondensed flue gas and sent for sequestration. The innovative cooling process through turbine expansion overcomes a key challenge with previous cryogenic CO2 capture solutions in that it does not require condensation within a heat exchanger, thus avoiding critical challenges with the buildup of dry ice. This study modeled the EC3 system using industry-standard process software. Since the process uses the formation of solid phase CO2 condensing out of the gas to drive the separation of CO2 from the flue gas, the process software was required to model gas-to-solid phase equilibrium calculations to accurately calculate the temperature at which the solid forms and the energy required for the phase change. Aspen Plus supports gas-to-solid phase equilibrium directly within the process fluids, providing the most accurate results. For the process model, common property methods include Peng-Robinson (PR), Soave-Redlich-Kwong (SRK), and GERG2008. When testing these property methods against empirical data, PR provided a better match than SRK and will have good compatibility when used with hydrocarbons in other parts of the EC3 process. In addition, PR did not produce simulation errors with some process blocks, but GERG2008 generated simulation errors when passing solids through some process blocks. A comparison of the solid-vapor equilibrium predictions for different methods showed that PR was the preferred property model, giving a 95% confidence in results when checked against empirical data. The system process included multiple steps of water removal and cooling of the flue gas to cryogenic temperatures of about −90°C. The system is then modeled to expand the gas to a colder temperature, given a turbine isentropic efficiency, and the carbon dioxide desublimates into a solid. This solid is mechanically separated and liquified into a high purity CO2 product. The cold potential from various process streams is recuperated into other parts of the cycle to improve efficiency. Flue gas inlet properties are selected. The first cases are for a natural gas combined cycle (NGCC) system with CO2 at 4.08% concentration by mol. The next case is a fluidized catalytic cracker (FCC) system where the flue gas has CO2 at 17.8% concentration by mol. The overall system power balance is estimated and the EC3 with NGCC inlet conditions is shown to require 437.4 kWh/tonneCO2 and 438.9 kWh/tonneCO2 at 90% capture and 95% capture efficiency. The EC3 with FCC inlet conditions and 90% capture efficiency is shown to require 385.2 kWh/tonneCO2.