The Impact of Fuel Flexible Gas Turbine Control Systems on Integrated Gasification Combined Cycle Performance

Interest in Integrated Gasification Combined Cycle (IGCC) is developing from a need for fuel diversification as a hedge for natural gas price and availability. In IGCC, the gas turbine combustion system is critical to meeting this need. The combustion system also needs to achieve superior environmental performance. This paper discusses specific requirements for IGCC combustion systems that derive from characteristics of gasification fuels and integration with the gasification process. Tradeoffs between system physical design parameters and control strategies must be evaluated in terms of overall functionality of the IGGC process. The key metrics for evaluating “goodness” of design are reliability, availability, maintainability (RAM), robustness to process variability, response to upsets and trips, time to synchronization and startup and shutdown automation. For IGCC, high availability is achieved from the capability of the turbine to robustly co-fire low-calorific synthesis gas with supplementary fuels. Co-firing compensates for shortfalls in gasifier output and maintains continuity of power service during servicing of the gasification plant. Controls need to provide seamless transfers between varying levels of syngas and supplementary fuel, and over the widest range of fuel mixes and power levels. Low calorific fuels provide special challenges to control system design. Variability in syngas composition, temperature and pressure will impact the minimum and maximum nozzle pressure drops and controllability. The effect of fuel constituents on controllability is captured in the modified Wobbe index. Stability and margin against flameout is captured in the upper-to-lower flammability ratio. The paper discusses the restrictions on these parameters for IGCC combustion systems. Control hardware and manifolding necessary with low calorific fuel can potentially conflict with accessibility to the gas turbine. Safe transfers from natural gas to syngas and shutdowns require purge strategies that account for residual energy in ductwork. Finally, the design of the Exxon Singapore IGCC control system is described which provides an extended range of cofiring and load control.