Abstract
Retrofitting a significant fraction of existing coal-fired power plants is likely to be an important part of a global rollout of carbon capture and storage. For plants suited for a retrofit, the energy penalty for post-combustion carbon capture can be minimised by effective integration of the capture system with the power cycle. Previous work on effective integration options has typically been focused on either steam extraction from the power cycle with a reduction of the site power output, or the supply of heat and electricity to the capture system via the combustion of natural gas, with little consideration for the associated carbon emissions.This article proposes an advanced integration concept between the gas turbine, the existing coal plant and post-combustion capture processes with capture of carbon emissions from both fuels. The exhaust gas of the gas turbine enters the existing coal boiler via the windbox for sequential combustion to allow capture in a single dedicated capture plant, with a lower flow rate and a higher CO2 concentration of the resulting flue gas. With effective integration of the heat recovery steam generator with the boiler, the existing steam cycle and the carbon capture process, the reference subcritical unit used in this study can be repowered with an electricity output penalty of 295kWh/tCO2 – 5% lower than a conventional steam extraction retrofit of the same unit – and marginal thermal efficiency of natural gas combustion of 50% LHV – 5% point higher than in a configuration where the gas turbine has a dedicated capture unit.
Highlights
Unlike in conventional repowering projects, we propose to achieve effective integration with the capture plant with the addition of a heat recovery steam generator (HRSG) after the gas turbine, using thermal energy in the flue gas to generate steam for power generation and heat for the capture plant
It consists of a CCGT where the HRSG is a triple pressure system and the steam cycle comprises a High Pressure and an Intermediate Pressure turbine, but not a Low Pressure turbine, and the intermediate Pressure (IP) steam turbine exits into the solvent reboiler and, supplemented by saturated steam generated in the low pressure part of the HRSG, supplies a fraction of the thermal energy for solvent regeneration
The necessary reduction by 10% of coal flow rate combined with the effective integration of the heat recovery steam generator with the existing steam cycle and the capture process presents several advantageous features: - Number and sizing of absorber columns
Summary
The contribution of post-combustion capture (PCC) technology to retrofit existing coal plants could play an important role in the deployment of CO2 capture and storage (CCS) for a fast-track emission mitigation strategy (IEA, 2012). This article discards gas ancillary boilers retrofits on the basis of low thermal efficiency and high electricity output penalties with capture, as shown for example in (Lucquiaud and Gibbins, 2012) It focuses instead on the retrofit of exiting coal plants with Gas Turbine (GT) CHP units. - The transmission capacity of the existing site - The integration between the existing coal plant and the gas turbine cycle with a fraction, possibly all, of the thermal energy for solvent regeneration supplied by the gas Combined Heat and Power (CHP) plant.
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