Turbomachine-based Cryogenic CO2 Capture,18-R6350

Background

Mitigation of climate change effects is among the greatest challenges of our day. One of the promising tools for managing greenhouse gas emissions is the post-combustion capture and sequestration of CO2 from the flue gas of power plants and industrial processes. This project is investigating a novel technology for implementing a cryogenic CO2 capture system utilizing a multi-phase condensing turbine expansion to cool the flue gas to approximately -120°C causing the CO2 to condense as solid dry ice (Figure 1). The CO2 condenses as solid dry ice because the partial pressure of CO2 in the mixture is below the triple point causing the phase change to be a de-sublimation directly from gas to solid (Figure 2). This solid CO2 can be mechanically separated from the remaining uncondensed flue gas and sent for sequestration. The innovative cooling process through the expander cryogenic carbon capture (ECCC) 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.

Approach

The project began with a literature and IP search which included the entire spectrum of potential CO₂ removal technologies. This survey was used to construct a techno-economic analysis framework. The evaluation of the system proceeded through the following steps:

• Evaluation of the baseline system performance, component sizing, and preferred system architecture applicable to a natural gas combined cycle and compared with Department of Energy baseline cases from the open literature.

• Completion of a 1-D real gas turbine design for a condensing cryogenic turbine to inform the system performance estimates and identify technology gaps and include the development of general-purpose axial turbine design tools applicable to future work.

• Estimation of capital and operating costs for the selected system architectures and comparison of performance and cost against state-of-the-art CO₂ capture systems using the techno-economic framework.

• Creation of a technology development roadmap that accounts for technology gaps and risks identified during the project.

Accomplishments

An initial literature review revealed high quality reference cases for natural gas combined cycle (NGCC) power plants. One of these cases was added to the project technoeconomic analysis scope to allow direct comparison to the reference. Fluid property models to handle real gas effects (Figure 3) in an axial turbine design tool and a method to account for gas to solid transition within a turbine for system modelling have been developed. Real gas tabular fluid properties interpolation methods were paired with traditional loss model curves for axial turbines to enable a real gas turbine design tool to calculate the meridional flow path design and estimate turbine performance to input into the system model. System modelling in Aspen included solid CO₂ formation and a major refinement to the system drying processes, as shown in Figure 4. System performance results were fed into the techno-economic framework to assess costs compared to competing technologies.

The technical feasibility and economic potential of the ECCC System has been assessed by comparing the following metrics to competing post combustion capture technologies:

1. Effectiveness of removing CO₂ from the flue gas stream. The system design was completed for 95% capture, however, a 90% capture case was used for the full system design for comparison with the DOE baseline case which was at 90% capture.

2. Power requirements, material consumption, operating costs, and capital cost. The ECCC applied to a NGCC resulted in an LCOE of \$58.3/MWh and \$44/tonne CO₂, which are competitive with and better than the DOE baseline, respectively.