Background

Most assessments undertaken by IEAGHG and others have assumed that power plants with CCS will operate at base load. It is now becoming clear that in many cases CCS plants will need to be able to operate flexibly because of the variability of electricity demand, increased use of variable renewable energy sources such as wind and solar and poor flexibility of some other low-CO₂ generation technologies such as nuclear. However, relatively little work has so far been published on this subject.

IEAGHG has commissioned Foster Wheeler Italiana to carry out a study to review the operating flexibility of the current leading power generation technologies with CCS and to assess performance and costs of some techniques for improving flexibility. This overview of the report was written by IEAGHG.

Conclusions

• CCS may impose additional constraints on the flexible operation of power plants but in general there are ways of overcoming these limitations. A plant with CO₂ capture may even be able to ramp up its net power output more quickly and produce more peak generation than a plant without capture, using the techniques considered in this study.  

• The efficiency penalties for part load operation are expected to be somewhat greater for plants with CO₂ capture than plants without capture, for example around 3 percentage points at 50% load for a pulverised coal plant with post combustion capture compared to around 2 percentage points for a plant without capture.

• Increasing the power output by turning down or turning off the CO₂ capture unit may be an attractive technique for short periods, depending on the peak power price and CO₂ emission cost but preliminary analysis indicates that simple cycle gas turbines may be a lower cost option for peak load generation. Regulations would need to allow the resulting increase in CO₂ emissions, for example by averaging emission performance standards over a long period. Some additional equipment, particularly steam turbine capacity, would have to be installed to obtain the full benefit from turning down or turning off the capture unit, which would increase the capital cost. Turning off capture could increase the net power output by 27% for a pulverised coal fired plant and 16% for a natural gas combined cycle plant.

• Storing CO₂–rich solvent and regenerating it at a later time may be attractive as a way of increasing power plant ramp rates and for increasing the net power output during short term peaks in power demand. However, the large quantity of solvent that would have to be stored would mean that operating at peak output for longer periods of time would not be attractive. Plants could be built with a wide range of storage volumes, solvent regenerator sizes and peak power generation capacities; selecting the optimum would be a difficult commercial decision. Storing solvent could increase the net power output by 22% for a pulverised coal fired plant and 12% for a natural gas combined cycle plant.

• Liquid oxygen and air/nitrogen could be stored in oxy-combustion and IGCC plants to improve flexibility and increase net peak generation by 5-10%. From an economic perspective this is expected to be a relatively attractive option for short term peak power generation.

• Hydrogen produced in IGCC plants with pre-combustion capture could be stored for example in underground salt caverns, which are commercially proven. This would enable the gasification and CCS equipment to operate at continuous full load and only the combined cycle plant would need to operate flexibly to cope with variable power demand. This would be a significant practical and economic advantage for non-base load power generation. Underground hydrogen storage would be suitable for longer-term as well as short term storage, which could be an advantage particularly in electricity systems that include large amounts of variable renewable generation.

• Compressed CO₂ could be stored at capture plants to reduce the variability of flows of CO₂ to transport and storage, if this is found to be necessary. Buffer storage of CO₂ would enable a smaller capacity CO₂ pipeline to be built but this would constrain the ability of the power plant to operate at continuous full load, which may not be commercially attractive.