No abstract

The low-swirl injector (LSI) is a simple and cost-effective lean premixed combustion method for natural-gas turbines to achieve ultra-low emissions (< 5 ppm NOx and CO) without invoking tight control of mixture stoichiometry, elaborate active tip cooling or costly materials and catalysis. To gain an understanding of how this flame stabilization mechanism remains robust throughout a large range of Reynolds numbers, laboratory experiments were performed to characterize the flowfield of natural gas flames at simulated partial load conditions. Also studied was a flame using simulated landfill gas of 50% natural gas and 50% CO2. Using Particle Image Velocimetry (PIV), the non-reacting and reacting flowfields were measured at five bulk flow velocities. The results show that the LSI flowfield exhibits similarity features. From the velocity data an analytical expression for the flame position as function of the flowfield characteristics and turbulent flame speed have been deduced. It shows that the similarity feature coupled with a linear dependency of the turbulent flame speed with bulk flow velocity enable the flame to remain relatively stationary throughout the load range. This expression can be the basis for an analytical model for designing LSIs that operate on alternate gaseous fuels such as slower burning biomass gases or faster burning coal-based syngases.

Solar Turbines Incorporated (Solar) under U.S. government sponsored programs has been evaluating ceramic matrix composite (CMC) combustor liners in test rigs and Solar Centaur® 50S engines since 1992. The objective was to evaluate and improve the performance and durability of CMCs as high temperature materials for advanced low emissions combustors. Field testing of CMC combustor liners started in May 1997 and by the end of 2004, over 67,000 operating hours have been accumulated on SiC/SiC and oxide/oxide CMC liners. NOx and CO emissions measured were < 15 ppmv and < 10 ppmv, respectively. Long test durations of 15,144 hrs and 13,937 hrs have been logged for SiC/SiC liners with protective environmental barrier coatings (EBCs). An oxide/oxide CMC liner with a Friable Graded Insulation (FGI) coating has been tested for 12,582 hrs. It was observed that EBCs significantly improve SiC/SiC CMC liner life. The basic three-layer EBC consists of consecutive layers of Si, mullite, and barium strontium aluminum silicate (BSAS). The durability of the baseline EBC can be improved by mixing in BSAS with mullite in the intermediate coating layer. The efficacy of replacing BSAS with SAS has not been demonstrated yet. Heavy degradation was observed for two-layer Si/BSAS and Si/SAS EBCs, indicating that the elimination of the intermediate layer is detrimental to EBC durability. Equivalent performance was observed when the Hi-Nicalon fiber reinforcement was replaced with Tyranno ZM or ZMI fiber. Melt infiltrated (MI) SiC/SiC CMCs have improved durability compared to SiC/SiC CMCs fabricated by Chemical Vapor Infiltration (CVI) of the matrix, in the absence of an EBC. However, the presence of an EBC results in roughly equivalent service life for MI and CVI CMCs. Early results indicate that oxide/oxide CMCs with protective FGI show relatively minor degradation under Centaur 50S engine operating conditions. The results of and lessons learned from CMC combustor liner engine field testing, conducted through 2004, have been summarized.

The continued development of a low swirl injector for ultra-low NOx gas turbine applications is described. An injector prototype for natural gas operation has been designed, fabricated and tested. The target application is an annular gas turbine combustion system requiring twelve injectors. High pressure rig test results for a single injector prototype are presented. On natural gas, ultra-low NOx emissions were achieved along with low CO. A turndown of approximately 100°F in flame temperature was possible before CO emissions increased significantly. Subsequently, a set of injectors was evaluated at atmospheric pressure using a production annular combustor. Rig testing again demonstrated the ultra-low NOx capability of the injectors on natural gas. An engine test of the injectors will be required to establish the transient performance of the combustion system and to assess any combustor pressure oscillation issues.

The low-swirl injector (LSI) is a simple and cost-effective lean premixed combustion method for natural-gas turbines to achieve ultralow emissions (<5 ppm NOx and CO) without invoking tight control of mixture stoichiometry, elaborate active tip cooling, or costly materials and catalysts. To gain an understanding of how this flame stabilization mechanism remains robust throughout a large range of Reynolds numbers, laboratory experiments were performed to characterize the flowfield of natural-gas flames at simulated partial load conditions. Also studied was a flame using simulated landfill gas of 50% natural gas and 50% CO2. Using particle image velocimetry, the nonreacting and reacting flowfields were measured at five bulk flow velocities. The results show that the LSI flowfield exhibits similarity features. From the velocity data, an analytical expression for the flame position as function of the flowfield characteristics and turbulent flame speed has been deduced. It shows that the similarity feature coupled with a linear dependency of the turbulent flame speed with bulk flow velocity enables the flame to remain relatively stationary throughout the load range. This expression can be the basis for an analytical model for designing LSIs that operate on alternate gaseous fuels such as slower burning biomass gases or faster burning coal-based syngases.