Combustion Engine vs Gas Turbine: Startup Time
Power plants that can be dispatched within minutes are important assets for balancing electric system loads and maintaining grid reliability. The generating technology affects the time required for a power plant to startup and reach full load. While combined cycle gas turbines can take over 30 minutes to start, combustion engine power plants can start and reach full load in less than 10 minutes – providing flexible, quick-start capability.
Increasing penetration of renewable energy sources presents challenges for transmission grid operators to maintain electric reliability despite the intermittency of wind and solar power. This variability is managed with redundant generating capacity that can quickly respond to fluctuations in demand, and has predominately been served by coal and gas-fired units that are synchronized to the grid but operating at part load. Flexible power generation that can be rapidly brought online reduces the inefficiency of relying on part load operation. System operators, such as PJM, California ISO and ERCOT define such “quick-start” or “non-spinning” reserve as generation capacity that can be synchronized to the grid and ramped to capacity within 10 minutes.
Where as conventional steam cycle generators (based on the Rankine cycle) can take more than 12 hours to reach full load, internal combustion engines can be dispatched in minutes. The two primary internal combustion engine technologies utilized for power generation are combustion engines and gas turbines. The differences between the two technologies affect startup time and their suitability to provide flexible power.
Startup time is a significant metric for flexibility, but comparison of different technologies and designs is complicated by the way startup time is measured by different manufacturers. The startup time quoted can be from push of the start command or from ignition. In the case of gas turbines, this difference in “start” definition can be as much as 20 minutes. Further, it is important to differentiate between time to achieve full load versus partial load.
Gas Turbine Power Plant Startup
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.
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.
Combustion Engine Power Plant Startup
A combustion engine power plant can start and ramp to full load very quickly due to rapid ignition of fuel within the cylinders and the coordinated starting of multiple generating sets. Wärtsilä combustion engine power plants employ high efficiency lean-burn technology that can reach full load in as little as two (2) minutes under “hot start” conditions. To meet “hot start” conditions, cooling water is preheated and maintained above 70ºC, engine bearings are continuously prelubricated, a jack up pump supplies prelubrication to the generator bearings, and the engine is slow turning (cycling). The Wärtsilä 34SG power plant requires only 30 seconds to complete startup preparations, speed acceleration, and synchronization to the grid. Loading to full power occurs rapidly in just 90 seconds. Startup time is not affected by the amount of time the unit had been previously shut down. The 50SG power plant takes seven (7) minutes to reach full load. Under cold startup conditions, the 34SG power plant can reach full load in 10 minutes and the 50SG in 12 minutes. Combustion engine power plants also have combined cycle advantages as sufficient steam pressure can be generated with only a subset of the engines operating.
Rapid Startup for Flexible Power Generation
Figure 1 shows a startup time comparison of the Wärtsilä 34SG and 50SG power plants with simple cycle and combined cycle gas turbine plants from manufacturers GE, Alstom, and Siemens. All startup times are measured from operator initiation of the start sequence. As can be seen from the graph, Wärtsilä power plants provide quick start ability under 10 minutes, which meets system operator requirements. Unlike CCGTs, hot start conditions in a Wärtsilä power plant can be maintained regardless of how long the engines had previously been inactive.