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Project: Integration and Operation of Post-combustion Capture System on Coal-fired Power Generation: Load Following and Solvent Storage

Research Team: Robert Brasington, Howard Herzog and Randall Field

Sponsors: Carbon Sequestration Initiative

Year: 2012

Coal-fired power plants with post combustion capture and sequestration (CCS) systems have a variety of challenges to integrate the steam generation, air quality control, cooling water systems and steam turbine with the capture system.  A variety of engineering studies have been completed that cover these aspects when a plant is operating at full load while operating at a 90 percent capture rate.  These studies investigate the basic integration of the these systems, the energy penalty and the effect of capital costs; however, none of these studies comprehensively explore the ability of the capture plant and the balance of the integrated system to respond dynamically to changes in load or capture rate.  These load changes occur due to a change in demand for electricity in the system, generation by variable, intermittent resources, or if the plant is equipped with the ability to store solvent to implement price arbitrage.  The integrated carbon capture system can be broken down into three general modes: full capacity, load following and peak power generation.  Each of these modes presents unique challenges to integration with the CCS system.

The load following mode requires the ability to accommodate different ramp rates that are reflected in flue gas flow and composition.  Operation at partial load will affect the quality of steam sent to the solvent regeneration unit.  Depending on the setup of the steam turbine system, at lower loads multiple extractions points may be necessary or an increase of the amount of extraction steam will be required due to the reduction in steam quality.  Using Aspen Dynamics, a CO­2 capture system using a monoethanolamine (MEA) absorption process is simulated at various plant loads to determine the overall effects on the efficiency of the CCS unit and the balance of the system.  In addition, the dynamic behavior of the CCS unit on power output and emissions is shown to demonstrate that the capability of a coal-fired power plant to load follow is not hindered by the addition of a carbon capture unit. 

The solvent storage mode can be further broken to two operation modes.  The first is peak power production, which occurs when the solvent is capturing CO2 from the flue gas, but is minimizing or delaying regeneration to a future time through storage.  This mode is used to take advantage of peak power prices by maximizing power output of the plant and maintaining a 90 percent capture rate.  The regeneration mode entails the solvent being released from the storage tanks and sent to the reboiler column.  Solvent storage has been shown in previous studies to have the ability to increase operating profits, but these studies have neglected to incorporate the capital costs associated with this type of operation mode and the operational issues and complexity associated with the large swings in quantities of steam required for the solvent regeneration.  By including the capital costs, this study determines that a system with large duration solvent storage is not economically viable given the flexible demands of the system and current electricity price spreads.

This thesis presents a framework for considering the flexible operations of a coal-fired power plant with an integrated carbon capture and sequestration system.  By exploring the operational limitations of the integrated system and the economic costs, an evaluation is made of the viability of different CCS operational schemes.  This study finds that the CCS unit can match the dynamics of the base coal plant and also increase the operational flexibility of the system.  The increased capital expenditure to meet peak demand is viable for larger steam turbine configurations in electricity systems with high peak prices and plants with short duration solvent storage. 

Brasington, R.D., "Integration and operation of post-combustion capture system on coal-fired power," M.I.T. Masters Thesis, June (2012). <PDF>

Brasington, R. and H. Herzog, “Dynamic Response of Monoethanolamine (MEA) CO2 Capture Units,” presented at the Carbon Management Technology Conference, Orlando, Florida, February (2012). <PDF>