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Carbon-Based Energy Systems > CO2 Capture

Carbon Capture Systems Analysis: Comparing Exergy Efficiency and Electricity Cost of Various Technology Options

Start Date: September 2012
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Investigators

Chris Edwards, Department of Mechanical Engineering, and Adam Brandt, Department of Energy Resources Engineering, Stanford University

Objective

This project will compare carbon dioxide (CO2) capture technologies based on their technical performance, lifecycle environmental impact and estimated cost.  The analysis will focus on CO2 capture using four technologies: an amine solvent, a solid sorbent, and an ionic-liquid system.

Background

Because carbon-based fuels will likely continue to be used in the coming decades for base-load power generation and firming of intermittent renewable power sources, CO2 capture and storage (CCS) is a necessary part of any comprehensive strategy for achieving required reductions in carbon emissions to the atmosphere.  Recent studies conclude that existing CCS technologies consume up to 30% of power-plant output, which could increase the cost of electricity by up to 80% compared to the same power plant operating without CCS.

However, thermodynamic analysis indicates that much lower energy costs for CCS are likely achievable. The theoretical exergy requirement[1] for separating and compressing CO2 in a natural gas-fueled power plant is only 3.5% of the exergy contained in the fuel.  Since a typical power plant has an exergy efficiency of 50%, this suggests that the theoretical energy cost for CCS is 7% of the plant’s output – much lower than for existing technologies.

Identifying which advanced technologies might approach the underlying thermodynamic limits for the energy cost of CO2 capture can provide an important guide for future CCS research.  Related analysis of the materials requirements and the costs for various technology options can help assess the practical feasibility of deployment.

Approach

The overall analysis consists of four thrusts: 1) exergetic systems analysis; 2) exergetic lifecycle analysis; 3) technoeconomic analysis; and 4) a synthesis framework for CCS technology comparison.

Exergetic systems analysis quantifies the distribution of exergy within a system, and the transformations that destroy exergy.  This type of analysis highlights which process steps should be targeted for improvement, and provides an unambiguous measure of process quality based on fundamental thermodynamics to facilitate comparison.  This analysis will be developed in Aspen Plus, augmented where necessary by custom code (e.g. Matlab) when Aspen Plus capability is absent or insufficient for specific system components. Special care will be taken to develop a well-optimized, credible model for the integration of CCS into the overall power-plant process.  Special attention will also be given to accurate representations of working fluids, since commonly used but imprecise methods (such as cubic equations of state) can compromise an analysis.

The results of the exergetic systems analysis will be integrated into a life-cycle analysis (LCA) to determine all of the environmental impacts associated with implementing a CCS technology.  This exergetic LCA will analyze the material and energy flows required for the assembly and decommissioning of a CCS unit, as well as for its operation.  A key benefit of LCAs is the ability to identify the system-wide impacts of various process changes, which are often not intuitive. While LCAs are a powerful tool for comprehensively assessing the impact of a technology, they often lack a rigorous, fundamental understanding of underlying technologies of interest. In contrast, the exergetic LCA developed in this project will draw on the rigorous, original exergetic systems analysis data described above to provide input values.

To provide cost estimates for different advanced CCS technologies, technoeconomic analyses will be performed to determine cost metrics such as capital cost per kilowatt and levelized cost of electricity (LCOE) using standard methods. The costing methodologies will follow federal guidelines so as to be directly comparable to other CCS technologies analyzed by the U.S. Department of Energy.

Finally, a unique comparison tool will be developed to facilitate informed judgments about different combinations of electricity generation and CCS technologies.  The tool will visualize the exergy efficiency and emissions intensity of different technology

Figure 1

Figure 1: Work-specific exergy consumption vs. carbon emission from a range of technologies without carbon capture and storage. Values are as determined from published data (not a detailed systems analysis). The straight, sloped line for each energy carrier (fuel) is indicative of its exergy-to-carbon ratio.

configurations, providing an intuitive reference for evaluating tradeoffs.  The framework for the tool is a coordinate system that plots the work-specific exergy consumption of an individual technology against its work-specific carbon emisisons to the atmosphere (Figure 1). The data portrayed in the plots will include not only the construction and operation of power plants and CCS units, but the full lifecycle impacts of individual CCS technologies.  This resource will integrate the thermodynamic and lifecycle data developed in the project to provide a unique, data-rich decision tool.


[1] Exergy is the useful energy in a system that is available to do work.