During startup, the gas turbine (GT) undergoes a sequence of increasing compressor spin to reach firing speed, ignition, turbine acceleration to self-sustaining speed, synchronization, and loading. There are numerous thermo-mechanical constraints during startup of the GT, including limits on airflow velocity through the compressor blades to prevent stall, vibrational limits, and combustion temperature limits to prevent turbine blade fatigue, with the significant parameter being the turbine inlet temperature. Aeroderivative gas turbine technology is better suited for frequent start ups and for power on-demand -operation. Modern aeroderivative gas turbines are capable of fast start-up time of less than 10 minutes. However, frequent fast start ups may incur a maintenance penalty.
In combined cycle operation, the heat recovery steam generator (HRSG) imposes additional thermal limitations on the gas turbine power plant, as the high temperature environment subjects HRSG components to thermal stress. The HRSG is directly coupled to the gas turbine, so changes in turbine exhaust gases induce flow, temperature, and pressure gradients within the HRSG. These gradients must be carefully controlled to prevent adverse impacts such as material fatigue, creep (damage caused by high temperatures) and corrosion. In order to avoid impacts, it takes longer to start the HRSG from cold conditions than from hot conditions. The definition of “hot” conditions varies by manufacturer, but is generally defined as within eight (8) to 16 hours of HRSG shutdown. As a result, the amount of time elapsed since last shutdown greatly influences startup time. Once-through HRSGs are used by some manufacturers to overcome the startup thermal and pressure limitations that exist with steam drums.
CCGTs are also subject to purge requirements to prevent auto-ignition from possible accumulation of combustible gases in the gas turbine, HRSG and exhaust systems. The purge is required before the unit is restarted. Purge times depend on the boiler volume and air flow through the HRSG, and are typically set to about 15 minutes. This purge time adds to the overall start time. In addition, the steam turbine can restrict the GT loading rate if the steam temperature leaving the HRSG exceeds steam turbine limits. To avoid this, temperature matching using GT holds as the load is increased may be necessary.
In order to enable faster startup, CCGT manufacturers have attempted to decouple the gas turbine startup from the HRSG and steam turbine warm-up. Process- and equipment-enhanced start options have been developed that can be used under hot start conditions. A “purge credit” allows the system purge to be completed at shutdown, eliminating the requirement for a redundant purge at next startup. The purge credit can only be used in some HRSGs that have no duct burners and where the GT is fired on natural gas only. Bypass dampers can be used to restrict the exhaust gas flow to the HRSG. However, pollution control equipment for nitrogen oxides (NOx) and carbon monoxide (CO) are typically integrated within the HRSG and environmental regulations for these emissions may prohibit the startup of the GT without the HRSG. Another method for decoupling the HRSG and steam turbine from the GT exhaust gas uses spray water attemperators or air attemperators to control the steam temperature so that gas turbine loading is not limited for temperature matching. This enables parallel loading of the gas turbine and steam turbine.
Although hot start conditions for CCGTs vary somewhat by manufacturer, maintaining energized electrical systems, purge credit, and steam temperature control enable CCGT startup times of about 30 to 35 minutes from initiation of the start sequence. This is about half the time for conventional hot start that would require purge and gas turbine holds. In simple cycle, published start times for gas turbines are about 10 to 15 minutes.