1 Introduction

The main objective of this Section I is to assess the operating flexibility of oxy-combustion PC power plants, with cryogenic purification of the flue gases for the capture of the CO2.

Similarly to the conventional air-fired boiler plants evaluated in Section H, the considerations shown in this section are based on the assumption that the oxy-combustion plants will be requested to operate in the mid merit market, thus participating to the first step of the variable electricity and generally following a weekly demand curve as shown in Figure 1-1.

Figure 1-1: Oxy-combustion PC plant load operation

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From the above graph, it can be drawn that the oxy-combustion plants will be maintained at base load for 80 hours per week, while 50% of their overall net power production capacity shall be generated during the remaining 88 hours.

The capability of these plant types for a flexible operation is mainly affected by the constraints related to the ASU, the CO2 purification unit and transportation pipeline. To investigate these main features, the following cases are presented in this section:

Case 4a: This case assesses the constraints given by the ASU in relation to the normal load change capacity of conventional PC-based power plants, investigating the use of a oxygen storage system to overcome this limitation.
Case 4b: This case considers the liquid oxygen (LOX) storage, in conjunction with either ASU partial load operation or reduced ASU design capacity, in order to minimize the plant power consumption and increase the overall power production during peak load demand period.
Case 4c: This case assesses the introduction in the power plant of a CO2 storage system, which allows to maintain a constant CO2 flowrate in the pipeline, despite the cycling operation of the plant, thus avoiding a two-phase flow or a significant change of the physical properties.

In addition, the following case has been investigated using an alternative weekly demand curve, based on the assumption that the plant will need to provide two hours of peak operation per each working day, while it is turned down to 50% output during night and weekend (off-peak):

Case 4d: This case considers liquid oxygen (LOX) storage, in conjunction with ASU partial load operation, in order to minimize the plant power consumption and increase the overall power production during peak load demand period. Stored oxygen is supplied to the boiler for two hours of peak demand during the day and is stored overnight, during off-peak demand.