How to Reduce Carbon Capture’s Energy Penalty
Carbon capture from coal-fired power plants takes energy. So more coal has to be used to get the same amount of power from the plant. Thus reducing the energy demand of carbon capture is vital to making it more attractive.
In a report for the International Energy Agency Clean Coal Centre, Power plant CO2 capture heat integration, Dr. Colin Henderson explores various ways to use the waste heat from carbon capture systems in the power plant and so reduce the energy penalty of carbon capture. Addition of CO2 capture systems can result in up to 30 percent loss of electrical efficiency if there are no integration measures installed, IEA says.
In post-combustion capture, CO2 is scrubbed from the flue gases after they emerge from conventional gas cleaning systems. An amine solution, typically monoethanolamine (MEA) at about 40 degrees Celsius in an absorption column is contacted with the cooled flue gas from the boiler. The CO2-rich solvent is then heated in a separate desorber vessel to release the CO2 and regenerate the solvent for reuse.
Large quantities of steam have to be taken from the main plant to provide heat for this duty. This steam extraction and consequent loss of power from the turbine typically accounts for about two thirds of the overall energy penalty of carbon capture. Power to drive fans, compressors and pumps in the capture systems further reduces net output.
If some of the potential sources of heat from the capture plant are used in the water-steam cycle this will reduce the amount of steam extraction needed and the associated drop in gross power. There are large quantities of waste heat available but their temperatures are not high. The low-grade nature of the heat is the greatest challenge to increasing the effectiveness of its use.
Henderson’s report describes the various different approaches such as adding pressure control valves so that steam arrives at the stripper reboiler at the correct temperature (usually 120 degrees Celsius) and to protect the turbines, particularly at times of variable load. Excess energy in the extracted steam would also be exploited, for example, by using a heat exchanger for LP feedwater heating or a let-down turbine.
Before transport as a supercritical fluid to storage, captured CO2 is compressed to around 11 MPa. This compression is carried out in stages, with cooling in-between to control the working temperature and minimise the energy needed. The heat from this could be suitable for LP feedwater heating. If some of the compressor intercoolers are omitted this will raise the temperature of the recovered waste heat, but means higher power consumption by the compressor.
Another possibility is to use compression before liquefaction followed by pumping. This would reduce the need for electrical energy, but it would be offset by the power decrease in the steam turbine as a result of steam extraction to drive the refrigeration cycle. An unconventional CO2 compressor (Ramgen) offers a higher pressure ratio and temperature rise per stage, so higher grade heat could be extracted. The Ramgen compressor could result in an increase in overall net power compared to conventional compression. However, the technology has not yet been commercialized.
Local ambient conditions may affect the opportunities for improvements in efficiency. Heat pumps could aid waste heat utilization. Other methods of optimization have included integration with combined heat and power systems, addition of low temperature bottoming cycles and addition of thermo-electrics.
Henderson says the single most important factor that is holding back deployment of CO2 capture is the high operating cost caused by its negative effect on plant output and efficiency. Incorporating improved heat integration will be valuable in reducing this.
Another carbon capture report, released this week at COP21 by members of the ENGO Network on CCS, found strong carbon capture policies are the “missing ingredient” in faster adoption of carbon capture and storage technology to enable CO2 reductions.
Photo Credit: coal power plant via Shutterstock
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