June 15, 2008

Options for reducing a coal-fired plant’s carbon footprint: Part I

Dr. Justin Zachary, Bechtel Power Corp.

Although carbon dioxide (CO₂) emissions are not yet regulated in the U.S., at least one coal-fired plant, proposed for Kansas, has already been denied a construction permit because the project would have produced too much of the greenhouse gas (GHG). Technologies for “carbon capture and sequestration” (CCS) from power generation sources soon will be required in some form or other. Currently, the timing and extent of upcoming regulations are only speculative. The most recent effort in Congress—the Moratorium on Uncontrolled Power Plants Act of 2008, introduced this March—will require plants to permanently sequester 85% of their CO₂ emissions.

 

Any new project that intends to burn coal, petroleum coke, or synthetic gas derived from coal will soon have to meet such a requirement. Although implementation of any legislation for CO₂ may take several years, in the present climate of uncertainty it behooves owners and planners of new plants to consider all options for accommodating CCS in their designs. The variety of CCS technologies currently under development makes that a very challenging task. So far, no existing technology has emerged as the most promising solution, and many new and innovative alternatives are in various stages of research, development, or pilot-scale testing.

For plants currently in the planning or design stage, owners, EPC (engineering, procurement, and construction) contractors, and equipment suppliers are trying to determine which features need to be applied today to minimize the impact of future technologies on plant layout, performance, and operability. Given those circumstances, perhaps the best strategy for reducing a coal plant’s carbon footprint is to consider using several complementary methods simultaneously, rather than wait for a single “silver bullet” technology solution that may never be developed.

Part I of this article addresses the impact that various postcombustion carbon capture techniques have on a coal plant’s design, performance, and complement of equipment. Part II will explore a variety of non-postcombustion carbon capture and reduction methods. Unfortunately, cost comparisons of the various technologies and approaches (Figure 1) can not be included, due to the high volatility of labor, material, and equipment prices.

 

 

What “capture-capable” means

A capture-capable coal plant is defined as one that provides for future incorporation of a CO₂ capture technology. Beyond the technical challenges of carbon capture, the commercial investment in specific features aimed at future CCS must be justified.

There is significant risk in selecting a specific carbon capture technology because it could become obsolete. At this point in time, a pragmatic approach to coal plant design requires evaluating all known factors of existing carbon capture technologies, considering ways to add CCS systems later, and laying out a plant to facilitate the incorporation and/or modification of hardware sometime in the future. Because carbon capture is an energy-intensive process, the discussions below include the impact on plant performance of large steam extractions for CO₂ capture processes and of the use of electric power for CO₂ compression.

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