Clean ramping: the next challenge for quick start combined cycle operation
Thanks to flexible quick-start design, today’s CCGT plants can start fast and produce low start-up emissions. But now there is a new challenge: how do you keep emissions in compliance while changing load.
Siemens has developed what they call “clean ramp” technology which uses patent pending features to integrate the combined cycle plant’s and emissions reduction system controls so that NOx, CO and ammonia slip (from an SCR) remain at base load emission levels even when frequently ramping up or down.
In June 2013, with the cooperation of NRG, the concept was demonstrated in the field with convincing results at the El Segundo combined cycle facility which is powered by two “Flex-Plant 10” 1x1 SGT6-5000F units.
Demo test. Gas turbine ramped up and down at 30 MW/min between 115MW and 180MW base load during two 1-hour test runs.
Control logic. Load change emissions predicted by control logic system used to adjust SCR ammonia injection flow rate to maintain stack emissions.
Test results. Stack NOx and CO emissions maintained at less than 2 ppmvd (at 15% O2) throughout the test with ammonia slip also controlled to under 2 ppmvd.
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Most environmental permits in the U.S. regulate on a time-averaged requirement, typically over a 1-hour rolling average. When plant load changes only a few times a day, short-lived transients and emissions spikes during startup and ramping are seldom a problem.
But with frequent load changing there is no time to average out the peak and there is a risk of going out of emissions compliance.
Today’s quick-start design features enable the gas turbine side of a combined cycle plant to smoothly ramp up to base load output like a simple cycle plant. And do so without holding the gas turbine at low load while the steam turbine is loaded.
This puts MWs on the grid faster than before and, importantly, it results in much lower startup emissions.
Earlier combined cycle plants require that the gas turbine startup cycle include part-load hold points to enable the bottoming cycle to gradually warm up prior to loading.
However, as pointed out by Ramesh Kagolanu, Manager of Environmental Engineering for Siemens Energy, Orlando, low-load gas turbine operation produces much higher levels of NOx and CO than when operating at full load.
Low-load GT emissions for combined cycles fitted with selective catalytic reduction are especially critical because a cold SCR (located within the HRSG) is not very effective at removing NOx and CO produced during startup.
Typically, conventional F-class CCGT plants produce about 180 lbs of NOx and 1,340 lbs of CO (per GT unit) during a cold startup – compared to about 13 lbs NOx and 340 lbs CO for a quick-start plant.
Clean Ramp does it better
The need for “clean ramp” technology is to further reduce those already low emission levels during startup and also to handle variations in gas turbine emissions during load changes, Kagolanu emphasizes.
The objective is to maintain compliance over the full spectrum of combined cycle operation and without having to restrict the amount of ramping requested by the system operator.
Without clean ramp, he says, handling of transient emission spikes would basically depend solely on the gas turbine emissions controls. This could mean placing limits on ramp rates and frequency in order to keep emissions (on a 1-hour rolling average) in compliance.
DLN flame an issue
This issue is due mainly to an inherent limitation of part-load performance of today’s dry low NOx (DLN) combustion systems, and regulations that don’t always recognize the limitations of the hardware.
A fact of life with advanced gas turbines is they generally produce more NOx and CO when they are changing load, points out Kagolanu.
The reason has to do with the complexity and sensitivity of today’s DLN combustion systems, where the fuel supply is typically divided into multiple injection points, or stages.
In addition to two (or sometimes three) lean-premixed flame zones needed for low NOx, there is an upstream diffusion flame pilot zone needed to help maintain combustion stability.
But there is also a less desirable characteristic of the high-temperature diffusion flame pilot zone -- it is the largest contributor to NOx emissions. During steady state operation, fuel flow to the pilot is set at a minimum, as needed to maintain stability in the downstream lean premixed flame zones, and to avoid unwanted combustor dynamics.
During transients, however, the stable nature of the diffusion flame becomes even more critical, so fuel to the pilot is increased. But this causes the engine NOx to increase above the 9 ppm steady state design value and requires a change in SCR operation to maintain control over plant NOx emissions.
A multivariable challenge
“Increasing NOx concentration (ppm) is not the only problem to contend with during load reduction,” says Kagolanu.
“It’s actually a multivariable challenge since the mass flow through the engine is also changing during load change, as variable inlet guide vanes come into play, so the ammonia injection rate must be adjusted to account for both of these changes happening at the same time.”
The key and main technical challenge, he says, is integrating the gas turbine controls with the ammonia injection system in a way that assures reliable and consistent results.
By having access to a large amount of field operating data and, most importantly, being able to change the way the gas turbine itself is controlled, Siemens was able to develop such an integrated control scheme (dubbed “clean ramp”) to keep stack emission low during transients.
With growing use of intermittent renewables the expectation is that plants will be required to change load more frequently. As a result, says Kagolanu, it puts a premium on being able to operate with unrestricted ramping and without increasing emissions over the permit limit.
As already noted, trying to accomplish this simply by addressing this problem of part-load engine NOx was not going to offer the desired result.
Solution: link SCR and DLN
According to Siemens, their “Clean Ramp” solution provides an innovative control scheme that enables a direct link between SCR (selective catalytic reduction) and DLN combustion systems when operating under transient conditions.
Power plants in the US commonly use an ammonia-based SCR, located within the HRSG, to control stack emissions to levels substantially below those in the GT exhaust.
For example, if the gas turbine exhaust NOx is 9 ppmvd (current state of the art for modern DLN burners), meeting a permit stack limit of 2 ppmvd requires that the exhaust be further treated to achieve a nearly 80% reduction.
This is done by passing the exhaust through the catalytically supported reaction zone where ammonia (NH3) is injected to react with the NOx, “reducing” it to nitrogen and water vapor.
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