Flame sensor design tailored to Frame 5 DLN1 turbines
Every gas turbine exhaust produces NOx emissions, the amount of which depends on combustion temperature. With a stochiometric fuel-to-air ratio (when the amount of air is precisely enough to consume all of the fuel in a combustion reaction), a large amount of NOx is produced. A turbine with dry low NOx combustion (DLN1), however, uses a fuel-lean ratio to minimize NOx. That requires precise premixing of fuel and air before ignition.
Each turbine has 10 combustion chambers, each having a plane of primary nozzles that are lit first for start-up, and a central secondary nozzle that's lit next. Four of the chambers are equipped with flame sensors—one for the primary nozzles, and one for the secondary nozzle. So each turbine has eight flame sensors in all.
During start-up, the DLN1 system runs through multiple modes to get to premix mode. The transition to premix mode requires stopping fuel supply to the primary nozzles so those flames extinguish while the secondary flame continues burning. When the primary fuel is restarted, the fuel and air are premixed for combustion at the secondary nozzle.
For most gas turbine installations, a standard sensor like the Reuter-Stokes Flame Tracker Dry 325 (FTD 325) is used to help control this complex process. It's a robust device with a hot-end assembly that operates at an impressive temperature of 325°C. Because of this, it requires no water or air cooling, which mitigates risks of compressor blade rubs caused by water leaks, and eliminates troublesome sensor failures due to inadequate cooling. The sensor is also easy to remove and reinstall during outages, so it saves the customer labor costs.
The FTD 325 senses ultraviolet (UV) wavelengths to signal if a flame is burning or not. The challenge with DLN1 turbines, however, is that light intensity can be too low for detection by standard sensors, as was the case recently at a customer's LNG plant. When the two turbines were at transition to premix, very low levels of light reached their secondary sensors, even though the secondary flames were lit. So the turbines could not transition to premix mode—NOx emissions remained high, and the turbines couldn't successfully complete the start-up process.
Solution
Our Reuter-Stokes team created a new version of the FTD 325, increasing the sensor’s gain so it can react to smaller UV signals like those occurring during the transition to-premix mode. They ensured that this change didn't affect its sensitivity to bright light—so the new model has the same high reliability for baseload operation as the standard FTD 325 detectors.
As a test, two of the new FTD 325 ILG (increased low gain) sensors were installed at secondary flame locations in each of these Frame 5 DLN1 gas turbines. The customer was able to easily transition from lean-lean to premix combustion without interruption—enabling successful start-up of both units.
Benefits
With output levels twice as high as the standard sensors, the new ILG sensors performed as expected, and are an ideal fit for this application. Customers currently using the non-ILG version can easily upgrade if required by their turbine conditions. The new product’s success also gives customers yet another option, especially to replace older Geiger Muller-type sensors. The new sensor will work on any turbine model, not only DLN1, with no adverse effect or invalidation of specifications.