Utilizing Solar Thermal Energy for Post-Combustion CO2 Capture

Abstract

There is broad scientific agreement that anthropogenic greenhouse gases are contributing to global climate change and that carbon dioxide (CO2) is the primary contributor. Coal-based electricity generation produces over 30% of U.S. CO2 emissions; however, coal is also an available, secure, and low cost fuel that currently provides roughly half of U.S. electricity. As the world transitions from the existing fossil fuel-based energy infrastructure to a sustainable energy system, carbon dioxide capture and sequestration (CCS) will be a critical technology to allow continued use of coal-based electricity in an environmentally acceptable manner. Post-combustion amine absorption and stripping is one leading CO2 capture technology that is relatively mature, available for retrofit, and amenable to flexible operation. However, standard system designs have high capital costs and can reduce plant output by approximately 30% due to energy requirements for solvent regeneration (stripping) and CO2 compression. A typical design extracts steam from the power cycle to provide CO2 capture energy, reducing net power output by 11–40%. One way to reduce the CO2 capture energy penalty while developing renewable energy technologies is to provide some or all CO2 capture energy with a solar thermal energy system. Doing so would allow greater power plant output when electricity demand and prices are the highest. This study presents an initial review of solar thermal technologies for supplying energy for CO2 capture with a focus on high temperature solar thermal systems. Parabolic trough and central receiver (power tower) technology appear technically able to supply superheated steam for CO2 compression or saturated steam for solvent stripping, but steam requirements depend strongly on power plant and CO2 capture system design. Evacuated tube and compound parabolic collectors could feasibly supply heat for solvent stripping. A parabolic trough system supplying the energy for CO2 compression and solvent stripping at a gross 500 megawatt-electrical coal-fired power plant using 7 molal MEA-based CO2 capture would require a total aperture area on the order of 2 km2 or more if sized for an average direct normal solar insolation of 561 W/m2. The solar system’s capital costs would be roughly half that of the base coal-fired plant with CO2 capture. This analysis finds that irrespective of capital costs, relatively high electricity prices are required for additional electricity sales to offset the operating and maintenance costs of the solar thermal system, and desirable operational periods will be further limited by the availability of sunlight and thermal storage. At CO2 prices near 50 dollars per metric ton of CO2, bypassing CO2 capture yields similar operating economics as using solar energy for CO2 capture with lower capital cost. Even at high CO2 prices, any operating profit improvement from using solar energy for CO2 capture is unlikely to offset system capital costs. For high temperature solar systems such as power towers and parabolic troughs, direct electricity generation is likely a more efficient way to use solar energy to replace output lost to CO2 capture energy. However, low temperature solar systems might integrate more seamlessly with solvent stripping equipment, and more rigorous plant design analysis is required to definitively assess the technical and economic feasibility of using solar energy for CO2 capture.