In the words of Maite Nkoana-Mashabane*, President of the Durban UN Climate Change Conference, "we should not let the perfect become the enemy of the good and the possible." Given the thousands of fossil fuel-fired power plants around the world (including about 3,000 in North American alone), this “good and possible” likely means a future that includes coal, natural gas, and oil as primary energy resources. So, how can we use these fossil fuels in a more environmentally responsible way within practical constraints? One option could be found in the flexible operation of carbon capture and sequestration technology on the world’s coal-fired power plants.
Carbon capture and sequestration (CCS) technology is designed to prevent carbon dioxide (CO2) from being released into the air by fossil fuel-fired power plants. In most cases, the “capture” portion of CCS refer to separating carbon dioxide from the power plant’s exhaust fumes using scrubbers and solvents. The gas is then compressed and stored – a.k.a. “sequestered” - in either underground caverns (called “geological storage”) or in man-made facilities (i.e. tanks). In some cases, this gas can be piped to local oil fields to help increase production through enhanced oil recovery.
The process of capturing and sequestering carbon dioxide is not free. It not only comes with a hefty initial price tag for equipment additions and retrofits, but the operation of CCS equipment itself requires power. And, this power comes from the power plant itself, meaning that more fossil fuels must be burned to produce the same net amount of electricity out of the plant. Cost considerations have led many researchers to consider their options for reducing both the initial capital investment and the daily energy requirements of a CCS-equipped power plant1.
According to a paper by Cohen, et al. in the Journal of Energy Resources Technology (JERT), turning CO2 capture on and off in response to electricity demand could allow regions to realize significant environmental gains without the types of extreme economic impact that CCS critics fear. In this paper, the authors present an analysis of a post-combustion carbon capture and sequestration system, where this greenhouse gas is captured using a series of scrubbers placed after the boiler and initial high-pressure turbine.
But, unlike most CCS technology, where the system is either “on” or “off” at all times when the power plant is generating electricity, the analysis presented in this paper focuses on a flexible system. In the author’s analysis, carbon dioxide is captured and released in order to maximize the profit of the power plant itself in the face of a range of carbon prices and market conditions2.
According to the authors of this paper, this process could be important because:
“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 decades3. 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.“
Translation: Coal is cheap and abundant, but it is also responsible for a lot of greenhouse gas emissions. CCS technology might be able to help reduce the environmental impact of coal. But, CCS is expensive and it requires a lot of energy. The cost of CCS might be lowered if the system was allowed to respond to electricity market signals.
“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…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.”
Translation: If power plants were to turn CCS technology on or off in response to the market price of electricity, they could realize the bulk of the potential environmental gain while saving BILLIONS of dollars.
Perhaps the “good and possible” alternative to 1) closing the world’s fossil-fuel based power plants or 2) continuing to pump large amounts of carbon dioxide into the air could be found in the flexible use of carbon capture and storage technology.
- Photo of W.A. Parish Power Plant and a coal-filled train © Copyright roy luck and licensed for reuse under this Creative Commons Licence
- Presentation by and interview with Stuart M. Cohen, a Ph.D. candidate at The University of Texas at Austin who studies the potential impacts of flexible CO2 capture systems in U.S. and international markets
- Turning CO2 Capture On and Off in Response to Electric Grid Demand: A Baseline Analysis of Emissions and Economics. Stuart M. Cohen, Gary T. Rochelle, and Michael E. Webber, J. Energy Resource. Technol. 132, 021003 (2010), DOI:10.1115/1.400157
- 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
*Presumably referring to Voltaire’s famous quote “Le mieux est l'ennemi du bien" or "The best is the enemy of good.”