by: Michael McManus, David Boyce, Raymond Baumgartner, Siemens Power Generation POWER-GEN International 2007
The US electric power industry has changed dramatically since the downturn of the gas-turbine-based market in the early 2000’s. In the years prior to this, nearly all combined-cycle plants were planned, permitted and built with the expectation to be operated predominantly in base load. With the addition of excess generating capacity in the industry coupled with rising natural gas prices, many of the new plants had to address a new set of challenges posed by nightly and weekend shutdowns and subsequent fast start-up requirements to remain economically viable. These same challenges will be imposed on new natural gas-fired units in the foreseeable future. To enhance the value of the new power generation assets, the US market will require these new power plants to provide a much higher level of operating flexibility, including the following needs:
- Built to span from continuous- to intermediate-duty operation
- High efficiency to maximize generation opportunities
- Rapid start to react to market opportunities
- Lower start-up emissions
- Lower operating costs (high start efficiency)
- Lower demineralized water consumption
Siemens has developed highly integrated power plant solutions that offer reliable rapid start and cycling flexibility without compromising efficiency. This paper discusses proven technologies that have been integrated to permit F-class gas turbines in combined-cycle plants to be brought to 150 MW load in as short as 10 minutes and bottoming cycles that can be brought to full load in less than half the times of their predecessors. It will also address results of testing in operating power plants that incorporate these integrated components and systems.
Development of the gas-turbine based market in the U.S. illustrates the classic boom-bust cycle in the electric power generation industry. As shown in Figure 1, the Public Utilities Regulation Policy Act (PURPA) passed in late 1978 consequently led to a small boom of natural gas-fired generation capacity additions from the mid-1980’s through the mid-1990’s. During this 10-year period, gas-turbine based additions accounted for roughly half of the total power generation installed in the U.S. During the same time period however, total power generation additions significantly dropped to a fraction of the historical level of 20 to 30 GW per annum. This trend bottomed in 1998, when only 3 GW of total capacity were added (only 0.8 GW of which was gas-turbine based). This prolonged lull in building power generation set the stage for what occurred from 1999 through 2005, when more than 200 GW of gas-turbine-based capacity were added. Low reserve margins caused widespread concern in numerous areas throughout the U.S. to add generation as quickly as possible. With natural gas pricing of approx. $2 per mm BTU during the timeframe that this capacity was being planned and ordered, gas-turbine based technology best satisfied this need, as either simple-cycle or combined-cycle power plants.
Figure 2 shows gas-turbine-based capacity additions in the U.S. since 1998 . There is a good mix of simple-cycle and combined-cycle applications with a shift toward the latter in the later years of the boom, which to some degree reflects project lead time differences between the two applications. Implicit in the chart is the traditional application of simple-cycle units for peaking/cycling duty and combined-cycle units for base-load/continuous operation duty.
With the sharp increase of natural gas prices to $5- $8 per mm BTU in 2004, owners and operators of gas-turbine-based generation needed to revaluate their fleet to optimize the utilization of these assets. As evidenced by a recent analysis conducted by Strategic Power Systems , there has been noticeable change in service hours/start for F-class gas turbine units in base-load/continuous duty (i.e combined-cycle applications) from the 5-year time period of 1997 to 2001 to that from 2002 to 2006. The trend has been toward a significantly lower average from 108 to just under 59 service hours/start. At the same time, the average service factor (service hours/period hours) for these units decreased only 1.5% from 64.5% to just under 63%. This signifies that on-average the F-class combined-cycle fleet is presently being shut down and started almost twice as often as before. The implication is that many F-class plants are now being started on a daily basis.
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