1 Introduction

Scope of this section is to make a review of the information, available in the public domain, on the flexibility of power plants with carbon capture and storage for three different capture technologies: pre, post and oxy combustion.

In general, it can be stated that the available information focuses on two main different aspects related to the capability of these plants to operate flexibly, as discussed in the following.

The first aspect refers to the possibility to change the power output in response to the variable electricity demand of the grid and the relevant impact of the CO2 capture plant on the operational flexibility of the whole power plant. Particular attention is placed on technical issues, such as the ability of plant to start-up, shut-down and ramp up or down output rapidly, that characterize the suitability of the plant to act as a flexible mid-merit plant.

The second aspect refers to the variability of CO2 emissions costs: until the cost of emitted CO2 is fluctuating around low values, as in the present market conditions, it may be economically convenient not capturing the CO2, rather than limiting the plant flexibility.

Most publications focus on the post-combustion CO2 capture and compression units in pulverised coal boiler power plants and conventional combined cycles. These works mainly assess possible ways, like solvent storage and absorber bypass, for reducing or avoiding the energy penalty related to the operation of the CO2 capture and compression units during peak electricity demand period.

Only a limited amount of information is available on the additional constraints that limit the power plant flexibility with CO2 capture and storage, in terms of cycling rate, start-up and shutdown time and partial load performance.

Strategies for operating flexibly the IGCC plant with pre-combustion capture, identified in available papers and presentations, include oxygen and nitrogen storage, intermediate storage of de-carbonised hydrogen-rich gas and co-production of electricity and hydrogen or other chemicals. Many of these strategies are similar to those identified in Section C of this report, as the addition of the CO2 capture does not represent a major modification of the plant configuration. In fact, minor changes are required in the IGCC in order to make the capture of the produced carbon dioxide.

In the oxy-fuel combustion, many aspects are still under investigation as the technology is relatively recent with respect to pre and post combustion CO2 capture. Significant amount of information is available on the boiler start-up and the changeover from air to oxygen fired mode, as well as on the possibility to switching off the CO2 purification section when additional electricity is required.

A few publications have been made on the dynamics of the CO2 transport pipeline systems. No information is available on the impact of flexible operation of the upstream units on storage systems because, up to now, the commercial applications of CO2 storage (e.g. Weyburn and Sleipner) are operated as a base load, i.e. no specific flexibility is required.