Enhanced operational flexibility scenarios of pilot plants


Power output maximisation by capture plant decoupling

The flow of steam to the reboiler and the flow of flue gas entering the absorber column are rapidly stopped. This represents a by-pass of the capture plant, where steam for solvent regeneration is redirected to the combined cycle, the CO2 compressors are shut down and the flue gas is vented directly to atmosphere after the heat recovery steam generator (HRSG). Solvent flow rate is reduced to 50% to reduce the power consumption of the pumps and the extent to which the lean and rich solvent loading will average, due to mixing effects. This mode of operation increases flow rate of steam to the low-pressure (LP) turbine and hence power plant electricity output. It is a useful capability during instances when the selling price of electricity is high, rapidly increasing the amount of energy available for dispatch to the grid (Haines and Davison, 2014). It should be noted that the viability of this scenario depends on carbon cost, electricity selling price and specific regulations regarding emissions costs. Flue gas venting is uneconomical when CO2 prices are high in comparison to the selling price of electricity.


Power output maximisation by reboiler steam decoupling only

This scenario rapidly stops the flow of steam to the reboiler, while flue gas continues to flow through the absorber. This method of operation is similar to capture plant decoupling, but could also be utilised in power plants, where the ability to bypass the absorber has not been implemented or if some level of capture is still desirable. Again, solvent flow rate is reduced to 50% for the reasons stated above.


Frequency response increase by rapid reboiler steam flow increase

In this operation, the flow of steam to the reboiler is rapidly increased to reach 200% of the baseload value. It could be used in power plants as a complementary method of rapidly decreasing plant electricity output where steam extraction from the combined cycle is ramped up as much as practically possible, in response to a requirement to maintain grid frequency within acceptable limits. A doubling of steam extraction from the combined cycle of a gas power plant is consistent with maintaining a minimum steam flow rate at the inlet of the low pressure turbine for blade cooling. With the increase in steam flow rate there exists a risk of accelerating solvent thermal degradation due to the creation of “hot spots” on the heat exchanger, but this will be plant/solvent specific and could be mitigated via plant design or solvent control methods.

For all data presented, the dynamic scenario is initiated at t = 0 min. To illustrate that the plant is initially operating at steady state, data was logged for a period of several minutes prior to the first dynamic perturbation. Everything before t = 0 is referred to in terms of negative values of time. However, upon analysing the data after the completion of the test campaign, it appears that steady-state operation is not always achieved.

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