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Research Papers

Turning CO2 Capture On and Off in Response to Electric Grid Demand: A Baseline Analysis of Emissions and Economics

[+] Author and Article Information
Stuart M. Cohen

Department of Mechanical Engineering, University of Texas at Austin, Austin, TX 78712stuart.cohen@mail.utexas.edu

Gary T. Rochelle

Department of Chemical Engineering, University of Texas at Austin, Austin, TX 78712gtr@che.utexas.edu

Michael E. Webber

Department of Mechanical Engineering, University of Texas at Austin, Austin, TX 78712webber@mail.utexas.edu

J. Energy Resour. Technol 132(2), 021003 (May 17, 2010) (8 pages) doi:10.1115/1.4001573 History: Received March 01, 2009; Revised March 29, 2010; Published May 17, 2010; Online May 17, 2010

Coal consumption accounted for 36% of America’s CO2 emissions in 2005, yet because coal is a relatively inexpensive, widely available, and politically secure fuel, its use is projected to grow in the coming decades (USEIA, 2007, “World Carbon Dioxide Emissions From the Use of Fossil Fuels,” International Energy Annual 2005, http://www.eia.doe.gov/emeu/iea/carbon.html). In order for coal to contribute to the U.S. energy mix without detriment to an environmentally acceptable future, implementation of carbon capture and sequestration (CCS) technology is critical. Techno-economic studies establish the large expense of CCS due to substantial energy requirements and capital costs. However, such analyses typically ignore operating dynamics in response to diurnal and seasonal variations in electricity demand and pricing, and they assume that CO2 capture systems operate continuously at high CO2 removal and permanently consume a large portion of gross plant generation capacity. In contrast, this study uses an electric grid-level dynamic framework to consider the possibility of turning CO2 capture systems off during peak electricity demands to regain generation capacity lost to CO2 capture energy requirements. This practice eliminates the need to build additional generation capacity to make up for CO2 capture energy requirements, and it might allow plant operators to benefit from selling more electricity during high price time periods. Post-combustion CO2 absorption and stripping is a leading capture technology that, unlike many other capture methods, is particularly suited for flexible or on/off operation. This study presents a case study on the Electric Reliability Council of Texas (ERCOT) electric grid that estimates CO2 capture utilization, system-level costs, and CO2 emissions associated with different strategies of using on/off CO2 capture on all coal-fired plants in the ERCOT grid in order to satisfy peak electricity demand. It compares base cases of no CO2 capture and “always on” capture with scenarios where capture is turned off during: (1) peak demand hours every day of the year, (2) the entire season of peak system demand, and (3) system peak demand hours only on seasonal peak demand days. By eliminating the need for new capacity to replace output lost to CO2 capture energy requirements, flexible CO2 capture could save billions of dollars in capital costs. Since capture systems remain on for most of the year, flexible capture still achieves substantial CO2 emissions reductions.

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Copyright © 2010 by American Society of Mechanical Engineers
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Figures

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Figure 1

A Process diagram shows the key features of a typical CO2 absorption/stripping unit with CO2 compression

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Figure 2

2006 ERCOT installed capacity of each plant type is shown, demonstrating dominance of natural gas (9,21)

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Figure 3

ERCOT load and generation by plant type on August 23, 2005 demonstrates dependence on natural gas on the maximum peak load day in 2005 (10)

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Figure 4

The FLEX Daily load and generation on January 6, 2006 has two CO2 capture off periods when load nears daily peak load

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Figure 5

The seasonal variation in ERCOT daily peak load in 2006 causes a CO2 capture off season for FLEX Season

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Figure 6

The FLEX Hours load and generation on August 17, 2006 (the maximum peak load day in 2006) has a CO2 capture off period when load exceeds the threshold load

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Figure 7

Annual generation by plant type is compared between scenarios, showing relative changes in coal- and natural gas-based electricity generation

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Figure 8

The percent of hours in 2006 that CO2 capture is turned on is compared between scenarios, showing the difference between always on CO2 capture (CCS Base) and the flexible CO2 capture scenarios (FLEX Daily/Season/Hours)

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Figure 9

Annual CO2 emissions are compared between scenarios, showing relative changes in coal- and natural gas-based CO2 emissions

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Figure 10

BAU resource flows include annual fuel input, electricity and CO2 output, and system-wide average generation cost

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Figure 11

FLEX Hours resource flows include annual fuel input, electricity and CO2 output, and system-wide average generation cost

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Figure 12

There is a linear increase of the new capacity required in CCS Base with specified reserve margin, and the horizontal line represents the base case where all capacity lost to CO2 capture is replaced

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Figure 13

The number of hours in a year that CO2 capture is turned off increases the specified reserve margin for FLEX Season and FLEX Hours, which bound the range of system response time

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