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How to reduce the energy penalty of capturing CO2?

World Coal,

Carbon dioxide capture from coal-fired power plant requires using energy. So more coal has to be used to get the same amount of power from the plant. Reducing the energy demand of carbon capture is vital to making it more attractive. In his new report for the IEA Clean Coal Centre, Power plant CO2 capture heat integration, Dr Colin Henderson explores various ways to use the ‘waste’ heat from 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% loss of electrical efficiency if there are no integration measures installed.

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°C 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.

Dr Colin 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°C) 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 commercialised.

Local ambient conditions may affect the opportunities for improvements in efficiency. Heat pumps could aid waste heat utilisation. Other methods of optimisation have included integration with combined heat and power systems, addition of low temperature bottoming cycles and addition of thermo-electrics.There are various methods being developed to reduce the energy penalty of carbon capture and Dr Henderson explains and summarises these in his report.

Commenting on his report, Dr Henderson said that 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.

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