Operability and Performance of Central (Pilot) Stage of an Industrial Prototype Burner

An investigation on the central-pilot stage of a Siemens Industrial Turbomachinery 4^th Generation DLE prototype test burner has been performed to understand the emission performance and operability. The core section, which is defined as RPL (Rich premixed lean) plays an important role for full burner combustion operation by stabilizing the main and pilot flames at different operating condition. Optimal fuel-air flow through the RPL is critical for multiple stages mixing and main flame anchoring. Heat and radical production from the central stage provides the ignition source and required heat for burning the main flame downstream of the RPL section. Surrounding the RPL outside wall cooling air has been blown through an annular passage. The cooling air protects the RPL wall from overheating and provides the oxygen source for the secondary combustion downstream of the RPL. At rich operation unburned hydrocarbon/radicals can pass the RPL and burns by the co-flow air entrainment. To determine the flame stabilization and operability, an atmospheric pressure test has been accomplished using methane as a fuel. Primary flame zone can be identified by a thermocouple placed outside the RPL wall and secondary combustion zone at the exit has been examined by chemiluminescence imaging. Emission measurement and LBO (Lean blow out) limits have been determined for different equivalence ratios from 1.8 to LBO limit. Co-flow air temperature was changed from 303 K to 573 K to evaluate the secondary combustion and RPL wall heat transfer effect on flame stability/emission. It is found that equivalence ratio has strong effect on the RPL flame stabilization (primary/secondary flame). Emissions/radical generation were also influenced by the chemical reaction inside the RPL. It can be noticed that co-flow air temperature has a significant role on emission, LBO and flame stabilization for the central-pilot stage burner due to the heat loss from the flame zone and RPL wall. A chemical kinetic network (Chemkin™) and CFD modelling approaches (Fluent) are employed to understand in detail the chemical kinetics, heat transfer effect and flow field inside the RPL (combustion and heat loss inside and emission capability). Experiment shows that the low CO and NOx levels can be achieved at lean and rich condition due to lower flame temperature. Present experimental results by changing equivalence ratio, residence time and co-flow temperature, creates a complete map for the RPL combustion, which is key input for full 4^th Generation DLE burner design.