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

Abstract

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 [1]. 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 of CCS have demonstrated its large expense due to substantial energy requirements and capital costs. However, such analyses typically calculate cost indicators using static plant performance parameters that are assumed to be constant over plant lifetime. That is, CO2 capture systems are generally assumed to capture a constant percentage of CO2 from power plant flue gas and consume a particular amount of plant gross generation capacity. Such studies do not consider dynamic plant operation that may result from diurnal and seasonal variations in electricity supply and demand, nor do they capture the economic desire to minimize CO2 emissions costs while maximizing profits by selling electricity at high price times. In this study, CO2 capture systems are analyzed in a grid level dynamic framework by considering the possibility of turning capture systems off during peak system load to regain generation capacity lost to the energy requirement of CO2 capture. This practice eliminates the costs of building additional generation capacity to make up for CO2 capture energy requirements, and it allows plant operators to benefit from selling more electricity during high price time periods. Dynamic CO2 capture operation is particularly suited to post-combustion (PC) CO2 absorption, a leading capture technology that, unlike other capture methods, offers the ability for flexible or on/off operation. This paper presents a case study on the Electric Reliability Council of Texas (ERCOT) electric grid of baseline cost and CO2 emissions estimates associated with different strategies of using on/off CO2 capture operation 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 load hours every day of the year, 2) days of the year of system peak load, and 3) system peak load hours only on seasonal peak load days. The study considers the implications of installing PC CO2 capture on all coal-fired plants in the ERCOT grid to better understand if on/off operation is desirable and which operational strategy may be the most economically viable under a policy of constrained CO2 emissions.