1 Introduction

The main objective of this Section G is to assess the operating flexibility of IGCC power plants, with pre-combustion capture of the CO2 from the shifted syngas.

The considerations shown in this section are based on the assumption that these plant types 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: IGCC plant load operation

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From the above graph, it can be drawn that the IGCC plants are supposed to operate 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 affected by a serious of constraints, mainly related to the inertia of the process units (Gasification, syngas cooling and conditioning line, etc.) and the Air Separation Unit (ASU) to generate and prepare the fuel at the conditions required by the gas turbine. Furthermore, IGCCs require significantly longer time to start up the plant, because of pre-heating requirements related to the gasifier, downstream unit pressurization and because of the deep cool-down sequence of the Air Separation Unit.

However, it is noted that for these plant types there are no specific constraints given by the introduction of the CO2 capture equipment in the AGR, because their normal or transient operation is always in shadow of the other process units.

To investigate these main features, the following cases are presented in this section:

Case 2a: This case considers 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 2b: This case shows how the operating flexibility of the IGCC improves when the plant is designed for the co-production of electricity and hydrogen. As the hydrogen production line can operate independently from the power line, then the gasification, CO2 capture, transport and storage equipment can run continuously at full load, while the power plant follows the variable electricity demand. However, large hydrogen storage is required in this case.
Case 2c: This case shows how the operating flexibility of the IGCC improves when an intermediate storage of de-carbonised fuel gas is considered in the plant design. In this case, the syngas production line can operate constantly at base load, while the power plant follows the variable electricity demand.
Case 2d: This case evaluates the possibility of tuning ON/OFF the CO2 capture in the plant, depending on the possible CO2 allowances cost fluctuations.
Case 2e: 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 to the above, the following cases have been investigated based on a weekly electricity demand curve different from that shown in Figure 1-1:

Case 2f: In this case, the syngas production line is kept constantly at base load (lower than reference case), while the power plant operates similarly to a combined cycle, i.e. at full load during weekday day time and at the lowest load (ideally without exporting power to the grid, i.e. in island mode) during weekend and weekday night time. This case shows how the operating flexibility of the IGCC improves when an intermediate storage of de-carbonised fuel gas is considered in the plant design.
Case 2g: In this case, two hours of peak demand are considered during the day time, while overnight and during the weekend the plant is turned down to 50% output. 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 gasification during the two hours of peak demand, while it is stored overnight when the plant is turned down to 50% output.

It has to be noted that, analogously to the liquid oxygen storage option, in the IGCC plants the storage of CO2-laden solvent from the AGR is technically feasible and, in principle, it improves also the plant operating flexibility as the net power output increases during peak electricity demand period. However, the expected investment cost of this case is higher and the expected power output gain is lower than the oxygen storage solution, so it has been decided of not further investigating this alternative.