Fast Start Combined Cycles: How Fast is Fast?
http://www.power-eng.com/articles/print/volume-121/issue-3/features/fast-start-combined-cycles-how-fast-is-fast.html03/09/2017,By Mike Eddington
In recent years, the term "fast start" has become commonplace in the power generation industry. Specifically, many new combined cycle units being conceptualized or designed incorporate elements of fast start. However, this term can take on many meanings.
Various solutions exist to address specific requirements, depending on what is driving the need for faster startup times. Is the driver reduced emissions, improving plant dispatch, meeting ancillary services or a combination of factors? This article explores these drivers for fast-start plants and outlines the differences in plant design requirements, initial capital and operations and maintenance (O&M) costs for the various levels of fast-start capability.
Conventional Versus FAST
Generally, conventional start combined cycle units are restricted by heat recovery steam generator (HRSG), steam turbine generator (STG) and interconnecting balance-of-plant (BOP) equipment design. The combustion turbine generator (CTG) is required to follow a restricted load profile so that excessive exhaust energy is not provided to the HRSG and, subsequently, the STG. Fast-start combined cycle plants aim to disconnect (to various extents) the CTG loading from the STG.
In addition, HRSGs must adhere to manufacturer and design-specific temperature gradient limits (how fast the temperature can rise in the evaporator drums). Because of this, the amount of CTG exhaust energy (temperature and mass flow) needs to be controlled to prevent exceedance of this temperature gradient restriction.
In conventional start combined cycle facilities, the interconnecting BOP equipment, including major steam piping and steam turbine bypass systems, may restrict the rate at which steam can be introduced to the steam turbine. During cold start conditions (typically greater than 72 hours following plant shutdown), the major steam lines may reach a cold state. To prevent condensation carry-over to the steam turbine, the major steam lines need to be warmed. This can add significant time to the plant startup sequence.
The steam turbine is typically the most restrictive element of combined cycle startup because of the large thermal inertia. During startup, the steam turbine casing thermally expands at a slower rate than the rotor and blades. To prevent blade rubbing and turbine degradation, the rate at which steam energy is admitted is restricted. In conventional combined cycle plants, this restriction in steam temperature and mass flow is controlled by slowing down the load ramp rate of the combustion turbine.
The fast-start combined cycle unit is designed to remove these bottlenecks and allow the plant to load faster. The figure on this page provides a high-level schematic comparing a CTG load path for a 1x1 conventional combined cycle to that of a fast-start combined cycle....
Details of the differences at the link.
