The operating profile of combined cycle plants has changed significantly over the last couple of decades. Combined cycle plants were generally designed in the late 1990s and early 2000s for base load operation. These plants were replacing the aging coal fleet and providing base loaded power. However, with the proliferation of renewable energy sources and a growing demand for power, the future will be shaped by an “all of the above” approach towards power generation (nuclear, energy storage, wind, solar, hydro and gas). Under this approach, power generation will no longer be restricted to one or two sources but will rather be comprised of a diverse mix of technologies, with a component of energy storage.
For this generation mix to work seamlessly together, a fast-reacting, dispatchable source of generation is required. Combined cycle plants are now being relied upon to be this quick acting, reliable source of power and hence are being asked to cycle repeatedly. The ability to start quickly and get power on the grid has become very important for combined cycle plants. New plants are being designed with this need in mind and a number of existing plants are contemplating retrofitting their heat recovery steam generators to have this fast start capability.
Conventional start vs fast start
During a conventional start-up, the gas turbine is started up but is sometimes held at a lower load to allow time for components to warm up and for the temperature in the HRSG to increase in a controlled manner. Depending on the configuration of the plant it can take two hours or longer to get to full load. In contrast, in a combined cycle plant optimised for fast start, the gas turbine is started as if it was a simple cycle machine and ramped up without any restriction, to full load. The HRSG and steam turbine are decoupled from the gas turbine and do not place any limitations on the gas turbine start-up. This can mean that the power from the gas turbine is available much more quickly, and this provides flexibility to the grid. However, achieving this fast start capability means that the HRSG components need to be able to accept high exhaust gas flows and rapid changes in exhaust gas temperature and need to be appropriately designed.
Drum thickness considerations
The HP drum is the thickest component in an HRSG and during rapid start-up events, the temperature difference between the outer shell and inner part of the drum is very large. This temperature gradient across the thickness of the drum creates thermal fatigue stresses that over several cycles manifest themselves as cracks and failures.For a fast start application, the thickness of the drums needs to be minimised as much as possible. One way to do this is to use materials with high yield strength, such as SA-299B or SA-302B. Further, the OD of the drum itself, and consequently its thickness, can also be reduced by considering lower drum hold up times (defined as the time taken for the water level in the drum to drop from the normal water level to the low level cutoff) of up to 90 seconds.A growing number of existing plants are planning and implementing drum replacements with the new drums designed for fast start applications (see Figure 1).
HP superheater thickness
HP superheater headers also tend to be very thick components that experience a lot of fatigue stress during fast start operation. Vogt Power International aims to keep header thicknesses below 1.25in for fast start applications. One way to reduce such stress is to reduce the header OD and hence, the header thickness, by adopting a single row, steam side parallel superheater design in lieu of a 2-row superheater. See Figure 2.
The metallurgy of the superheater can also be improved to reduce header thickness and minimise the stress arising from fast start operation. The use of Grade 92 material has become increasingly commonplace especially after the recent reduction of allowable stress values for Grade 91 material.
Attemperators
Final stage attemperators are essential for combined cycle plants considering fast start. Final stage attemperators are needed for steam conditioning before rolling off the turbine or for temperature matching into a common steam header.
The interstage attemperator alone may not be able to control final steam temperature even if it is spraying down close to saturation temperature because with lower steam flows during start-up, the steam may significantly increase temperature in the final stage superheater.
The location of the interstage attemperator is also an important factor to consider. If the attemperator location is biased towards the back end of the superheater modules, it may not effectively control final steam temperature and adversely affect the operating temperatures of the front-end superheater tubes.
A thermal study should be conducted on existing units to determine the suitability of interstage attemperators and the sizing of the final stage attemperators for fast start applications.
Figure 3 shows the damage that can happen to improperly designed attemperator piping.
Pre-warming the HRSG
The pressures in the HRSG during start-up determine whether start should be classified as hot, warm or cold:
HOT is when the HP drum pressure is above 500 psia
WARM is when the HP drum pressure is above 200 psia
COLD is when the HP drum is at ambient conditions
A cold start causes approximately 7 times more life usage than a warm start and a warm start causes 4 times more life usage than a hot start. As such, pre-warming the HRSG or maintaining the HRSG at a hot start condition can prolong the life of the HRSG during frequent fast start events. This can be achieved through the use of auxiliary boilers to create sparging steam that can be used to maintain the pressure in the HP drum. Ensuring that the drain valves in the unit are properly maintained and not leaking can also limit the pressure loss of the unit over time. Some plant owners/operators have also installed stack dampers in their units to retain the heat in the unit.
Emission compliance
Pre-warming the HRSG also has an added benefit of enabling the HRSG to achieve emission compliance quicker during a fast start. Since the catalyst section in an HRSG is typically located downstream of the HP evaporator section, pre-warming the HP drum and evaporators allows the catalyst to get up to operating temperature sooner and achieve emission compliance earlier during start-up. Some plants have also installed startup electric heaters for ammonia vaporisation. Since a hot gas recirculation system, which takes a slipstream of exhaust gas, may not get hot enough during start-up, an electric heater can be used to vaporise the ammonia and achieve emission compliance.
NFPA purge credit
Another factor to consider is purging the HRSG, which can take several minutes and hence delay start-up. One way to mitigate this is to purge the HRSG during shut down and ensure that combustible gases in the fuel lines and ammonia system do not leak into the HRSG. This can be achieved by using a triple block and bleed system in the gas lines along with monitoring pressure and valve position validation (see Figure 4). With this system NFPA (US National Fire Protection Association) allows purging of the HRSG during shutdown, removing the need for a dedicated start-up purge and reducing start-up time.
Other ways to get power on the grid quickly
Power plant owners/operators are also considering other retrofits that allow them to get power on the grid quickly. For instance, a growing number of plants are considering installing a simple cycle bypass stack adjacent to the existing HRSG giving the plant the ability to operate in both simple and combined cycle configurations.
The plant is operated in simple cycle configuration when the priority is getting power on the grid quickly and in combined cycle configuration when the priority is peak generation and efficiency.
Similarly, plants are also installing gas turbine retrofits that allow them to operate at very low loads. The plants use significant less fuel during such operation (25% GT loads) and the benefit is that they are able to ramp up to 100% load much quicker with less thermal stresses on the HRSG components.
Uninterrupted start-up
In summary, when a combined cycle plant is being constructed or modified for fast start capability a good deal of consideration must be given to heat recovery steam generator design. However, HRSGs can be designed and existing HRSGs can be modified to enable uninterrupted gas turbine start-up and quick power delivery to the grid.
