Combined Cycle Power Plant

In integrated gasification combined-cycle (IGCC) applications, the clean synthesis gas (syngas) is burned in high efficiency, low emission gas turbines (GT) to generate electricity. The GTs utilized are derivatives of proven natural gas combined-cycle machines that have been specially adapted to burn syngas. Hot exhaust from the GT is cooled through a heat recovery steam generator (HRSG) to generate high pressure superheated steam to make additional power in the steam turbine. The combined cycle plant is supported by a host of utility processing facilities such as steam/condensate collection and distribution, cooling water, boiler feed water and wastewater treatment systems.

Gas Turbines (GT)
Modern GTs have been modified and commercially demonstrated to be able to burn low and medium BTU syngas. The GT rating for syngas firing can be approximately 5-15% higher than that for natural gas firing due to the higher mass flow. For IGCC with carbon capture, the GT must be adapted to burn syngas with essentially no carbon monoxide (CO) (please see Hydrogen Turbines in Gasification Research & Development). Current state-of-the-art GTs are commercially ready to burn syngas with up to approximately 60% hydrogen with the balance being nitrogen or/and water as the diluent. Work is on-going to develop the next generation of high efficiency GT for carbon capture-based IGCC. In some oxygen-blown IGCC designs, the GT also provided a portion of the compressed air to the air separation unit (ASU). This increases the overall IGCC plant efficiency and reduces the capital cost and power consumption of the ASU.

Heat Recovery Steam Generator (HRSG) and Steam Turbine Generator (STG)
Hot exhaust from the GT is cooled through a HRSG to generate high pressure super-heated steam for electrical power production in a steam turbine. Recovery of heat from the GT exhaust (generally about 1,100-1,150°F) provides an effective way of generating additional power through a steam cycle. Heat recovered in the HRSG can produce 1,800 psig, 1,050°F super-heated high pressure (HP) steam, as well as reheat intermediate pressure (IP) steam to 1050 °F without supplemental firing. The HRSG is also used to superheat the HP saturated steam generated in the syngas coolers. Depending on the amount of saturated HP steam generated by the syngas cooler, the HRSG may have to be designed as a superheater rather than a HP steam generator. The HP and IP superheated steam are routed to the STG to generate additional electric power. The STG is an advanced commercially available machine using a 1,800 psig/1,050°F/1,050°F steam cycle.

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Simplified Flow Diagram of a Typical Combined Cycle Plant

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