GT enhancements can facilitate renewables integration

The dozen presentations preceding Bruce Rising’s made it clear that responsibility for dealing with the idiosyncrasies of intermittent renewables rested squarely on the shoulders of the balancing authority—ISO, RTO, or regulated utility. Also, that generating plants powered by gas turbines typically are best suited for filling the gaps in renewables production caused by clouds and changes in wind speed.

Rising’s assignment was to update the group on the capabilities of the latest large frame gas turbines; plus, identify enhancements available to owner/operators for upgrading existing engines to more effectively support grid operations. The ability to provide both ancillary services—such as reactive power and voltage control, load following, etc—and energy is conducive to a stronger balance sheet.

Rising opened his presentation with a passing salute to the “traditional” power market, in which the dispatch order of conventional assets is based on the cost of generation. He then noted the uncertainty surrounding how units will be dispatched in the future in areas with a high penetration of intermittent generation.

The renewables footprint already is becoming evident in system response, Rising said. Ramps up and down are faster and starts appear more frequent. A Siemens’ assessment indicates that a significant amount of installed thermal capacity is approaching a 50-operating-year threshold and that 50,000 to 70,000 MW of old coal-fired capacity could be retired in the near term because they are of marginal value for supporting renewables, as well as other reasons.

Intermittent renewables pose forecasting challenges, Rising continued. Important to remember is that installed renewables capacity does not equate to available capacity. Tools for forecasting wind are subject to increasing error the further out in time the forecasts are made; also, significant variability exists among forecasting methods (Fig 16). Such uncertainty may force backup generating units into rapid ramp events that adversely impact equipment lifetime.

Faster ramps and more starts/stops are not the only challenges facing conventional assets once wind and solar penetration reach a critical threshold. Thinking is that increased turndown of base-load units may be required, with the need to operate units as low as 30% of rated capacity to avoid shutdowns and increasing the number of start/stop cycles.

Fast ramping also may have adverse impact on the efficiency of environmental controls, and could raise permit issues. Trying to balance fuel supply against unpredictable consumption may pose contractual challenges as well.

All gas-turbine OEMs offer or are developing technologies to mitigate the adverse effects of deep cycling and fast ramping. In addition to extended turndown capability, Rising said Siemens can provide the following:

• Fast-start units—Flex-Plant™ 10 and Flex-Plant 30.
• Power augmentation via wet compression and/or higher duct firing to help “fill in the generation gaps” when the wind slows or stops.
• Power Diagnostics® for rigorous monitoring of assets and related instrumentation to advise when maintenance shutdowns are needed to prevent equipment damage.

Note that Flex-Plant 10 refers to a simple-cycle F-class engine capable of injecting 150 MW of firm capacity into the grid within 10 minutes of being called.

Flex-Plant 30 is the product name for an F-class 2 x 1 combined cycle designed to deliver a nominal 500 MW within 72 minutes (hot start condition; 16 hours or less from last shutdown) when conventional drum-type heat-recovery steam generators are installed; five minutes faster with Benson (once-through) HRSGs. The GTs can deliver their rated capacity in 22 minutes with the Benson option; in 32 minutes with a conventional HRSG.

By contrast, a conventional 2 x 1 would require 116 minutes to achieve full output and GT power would not be available for 108 minutes.

For a warm start (up to 64 hours since the last shutdown), a conventional 2 x 1 would require 152 minutes to deliver GT power, 162 minutes to get the entire plant online. Compare these numbers to 37 and 81 minutes for the F-P30 with drum HRSGs and 22 and 75 minutes with once-through boilers. Note that actual startup times may vary slightly because of ambient and other local conditions.

Rising spent several minutes describing Siemens’ solution for expanded turndown with low CO emissions, and its fuel-flexibility option incorporating a combustion dynamics protection system with feed-forward tool to minimize power fluctuations and fast-response Wobbe meter with redundant gas chromatograph (Fig 17).

Subscribers who actively participate in the 501G, F, and D5-D5A user groups and in the CTOTF’s Siemens Roundtable probably are familiar with these offerings. If not, you can get a real-world view of their implementation at www.ccj-online.com/archives.html, click 3Q/2009, click “Klamath gets better with age” on the issue cover. Or you can access Rising’s presentation at www.integrating-renewables.org.

Before wrapping up, Rising allowed attendees to peek into the Siemens development pipeline at a solution currently in field validation to assure that users operating at low GT loads will not exceed CO and NOx emissions limits. Virtually all HRSGs built in the last several years are equipped with SCR (selective catalytic reduction) catalyst to restrict NOx emissions to the ultra-low levels required by law.

Some have CO catalyst as well, but many do not. For those in the latter group requiring expanded turndown capability, addition of CO catalyst is necessary to insure that CO emissions remain below permit limits during transients. The traditional way of doing this would be to add an oxidation catalyst bed ahead of the SCR. But oftentimes the necessary space is not available.

The Siemens answer is to replace the existing SCR catalyst with the company’s Novel catalyst, which destroys NOx and CO simultaneously. It is designed to operate over a wide range of GT loads and, because it occupies less volume than traditional SCR catalyst, system pressure drop is lower.

Pilot test results included the following: less than 1 ppm ammonia slip; less than 2 ppm combined reduction capability for NOx, CO, and volatile organic compounds; lower emissions of CO and VOCs; minimal formation of ammonia sulfates and bisulfates in the cool end of the HRSG.