Lean Blowout Limit and NOx-Production of a Premixed Sub-ppm NOx Burner With Periodic Flue Gas Recirculation

The technological objective of this work is the development of a lean-premixed burner for natural gas. Sub-ppm NOx emissions can be accomplished by shifting the lean blowout limit (LBO) to slightly lower adiabatic flame temperatures than the LBO of current standard burners. This can be achieved with a novel burner concept utilizing periodic flue gas recirculation: Hot flue gas is admixed to the injected premixed fresh mixture with a mass flow rate of comparable magnitude, in order to achieve self-ignition. The subsequent combustion of the diluted mixture again delivers flue gas. A fraction of the combustion products is then admixed to the next stream of fresh mixture. This process pattern is to be continued in a cyclically closed topology, in order to achieve stable combustion of e.g. natural gas in a temperature regime of very low NOx production. The principal ignition behavior and NOx production characteristics of one sequence of the periodic process was modeled by an idealized adiabatic system with instantaneous admixture of partially or completely burnt flue gas to one stream of fresh reactants. With the CHEMKIN-II package a reactor network consisting of one perfectly stirred reactor (PSR, providing ignition in the first place) and two plug flow reactors (PFR) has been used. The effect of varying burnout and the influence of the fraction of admixed flue gas have been evaluated. The simulations have been conducted with the reaction mechanism of Miller and Bowman and the GRI-Mech 3.0 mechanism. The results show that the high radical content of partially combusted products leads to a massive decrease of the time required for the formation of the radical pool. As a consequence, self-ignition times of 1 ms are achieved even at adiabatic flame temperatures of 1600 K and less, if the flue gas content is about 50%–60% of the reacting flow after mixing is complete. Interestingly, the effect of radicals on ignition is strong, outweighs the temperature deficiency and thus allows stable operation at very low NOx emissions.

Improvements to an Air Bypass System on a Kawasaki M1A-13X Engine

A new bypass system using an improved design has been fabricated and tested on a Kawasaki M1A-13X gas turbine engine. The engine and catalytic combustor are currently installed at the City of Santa Clara’s Silicon Valley Power municipal electrical generating stations and connected to the utility grid. The use of a bypass system with a catalytic combustor, incorporating the Xonon Cool Combustion™ technology, on an M1A-13X system increases the low emissions load turndown and ambient operating range without impacting engine efficiency. The increased operating range is achieved because the bypass system provides the required adiabatic combustion temperature (Tad) in the combustor’s post-catalyst burn out zone without changing the turbine inlet temperature. A detailed measurement of the pressure drops, in the old bypass system, revealed that there were large flow losses present, particularly in the re-injection spool piece and the extraction plenum. Since it was determined that the spool had the highest pressure loss, this was the component targeted for improvement. The analysis coupled with detailed measurements on the reinjection piece revealed that the effective area actually varied with flow As the flow changed, so did the flow mechanics inside and exiting the spool piece. Therefore, in order to achieve the design target, the flow area of the spool piece had to be optimized at the predicted capacity flow rate. CFD analysis of the spool piece revealed the regions of losses in the re-injection piece. This analysis along with a one-dimensional flow analysis of the entire system enabled the design of new spool re-injection piece. Once the design was completed, the new bypass system was fabricated and tested. Bypass flow capacity was increased by about 22%. This was achieved by alleviating regions of flow losses and also by using a new “scoop” design for the bypass reinjection tubes. As expected, engine turndown capacity and ambient operating range were improved with the new design.

Full-Scale Demonstration of Surface-Stabilized Fuel Injectors for Sub-Three ppm NOx Emissions

ALZETA Corporation has developed surface-stabilized fuel injectors for use with lean premixed combustors which provide extended turndown and ultra-low NOx emission performance. These injectors use a patented technique to form interacting high-flow and low-flow flame zones immediately above a selectively-perforated porous metal surface. This allows stable operation at low reaction temperatures. This technology has been given the product name nanoSTAR™. Previous work involved the development of nanoSTAR technology from the proof-of-concept stage to prototype testing. Rig testing of single injectors and of two injectors simulating a sector of an annular combustion liner have been completed for pressure ratios up to 17 and combustion air inlet temperatures up to 700 K (800°F). This paper presents results from the first ever full-scale demonstration of surface-stabilized fuel injectors. An annular combustion liner, fitted with twelve nanoSTAR injectors was successfully tested up to a pressure ratio of 12 and combustion air inlet temperature of 700 K (800°F). NOx emissions were 2 ppm with CO emissions of 3 ppm both corrected to 15% O2. The combustion system exhibited excellent temperature uniformity around the annular combustor outlet with a maximum pattern factor of 0.16 and engine-appropriate radial profiles.

Gas Turbine Industrial Fellowship Program

Under the Gas Turbine Industrial Fellowship Program, students in Bachelor’s, Master’s and Ph. D. programs studying gas turbine-related technology spend 10 to 12 weeks employed at the facilities of turbine manufacturers or users of gas turbine equipment. The program is funded by the U.S. Department of Energy. This paper describes the Fellowship program, its relationship to the DOE Turbine Program, the University Turbine Systems Research (UTSR) program, and plans for future Fellowship development.

Simulation of Fuel/Air Premixing Under the Effects of Acoustic Forcing

To achieve ultra-low NOx emissions in practical lean premixed combustors, a uniform spatial and temporal fuel/air distribution is required. In addition the response of the fuel/air premixer to acoustic perturbations plays a central role in the coupling of the combustion and acoustic processes that potentially lead to thermoacoustic instabilities. In this paper, results from the simulation of a fuel/air premixer with and without acoustic forcing are reported. An unsteady Reynolds-Averaged Navier-Stokes (RANS) computational fluid dynamics (CFD) code is used in the simulations and the computed results are compared with data obtained using a synchronous PLIF technique. Planar phase-locked images of the fuel/air distribution during the forcing cycle are measured from which the planar averaged (bulk) fuel/air ratio “signal” is obtained. The fuel/air ratio to velocity transfer function (time delay and magnitude) at a specific forcing frequency is computed corresponding to a given jet-to-air momentum flux ratio. The comparisons between the computed and measured time delays are quite good, while the magnitude comparisons are currently inconclusive.

Local, Near-Field Mixing Augmentation Using Square Coaxial Jets

Results from un-forced experiments in flows ensuing from circular and equivalent square coaxial nozzles with parallel sides are presented in this paper. The nozzles are contoured and are designed so that the hydraulic diameters of the internal flow passages are identical for both geometries. The flow experiments were conducted at a co-flow-jet Reynolds number of Re = 16,000 and inner-to-outer jet nominal velocity ratios of λ = 0, 0.5, 1.5. Axis switching, a phenomenon readily observed in single non-axisymmetric nozzles, is shown for the first time to occur in the square coaxial nozzles as well. Comparisons of the mixing regions of the flows from both geometries are made to examine mixing advantages when using square nozzle configurations. Comparisons of stream wise mean velocity fields measured on a center plane parallel to the square nozzle sides, on a diagonal plane of the square nozzle and the center plane of the corresponding circular nozzle, are presented and discussed. Axis switching is shown to be evident in the near-field shear regions for all velocity ratios, resulting in considerable mixing advantages. The spreading rates (and therefore mixing rates) of the outer mixing region of the square nozzles clearly exceed the spreading rate observed in the circular case on the central plane. Axis switching and improved mixing is also observed in the inner mixing region of the square nozzle. This work is relevant to coaxial nozzles for gas turbine combustor applications, although the study has been carried out in a scaled up geometry with respect to this application.

Development and Experimental Testing of a Pilot Burner for DLN Combustors

Different geometrical modifications have been investigated and experimentally tested to improve a pilot burner for low emission industrial gas turbine combustors. Results of the ongoing collaboration between the DE of Florence and the Italian electric company ENEL are reported. The activity is dedicated to the improvement of the pilot burner to extend the operable margin of the engine and to reduce, at the same time, the emissions. The study has been performed mainly by means of experimental investigations both on isothermal flow as on combustion test rig. Results of the activity were employed both to obtain design information about the swirler and injection fuel holes for the pilot burner under investigation. Moreover the post-processing of the experimental data permitted the improvement of the correlation implemented into the 1-D model for the prediction of the injected fuel path. These results were implemented in the routine DoFHIS (Design of Fuel Holes Injection Systems) developed for the analysis/design of injection fuel systems.

Fuel Injection and Emissions Characteristics of a Commercial Microturbine Generator

Microturbine generators (MTGs) offer an attractive alternative for addressing future demand for electrical power. However, increasingly stringent emission regulations such as those found in California pose a major technical challenge that these devices must overcome if they are to make significant market penetrations. In the context of these regulations, the present study characterizes the exhaust emissions and mixing capability of a commercial MTG and assesses (1) the ability of this device to meet future emissions regulatory requirement and (2) the extent to which mixing can be used to reduce emissions. The results establish that, for this MTG, both NOx and CO are minimized for 80–100% load. Kinetics and CFD analysis illustrate how NOx emissions are affected by local equivalence ratios and how fuel staging and local quenching impact CO emissions. Measured injector premixing levels indicate a standard deviation of less than 4% relative to the mean. Subsequent analysis using a well-stirred reactor approach suggests a maximum of a 10% reduction in NOx could be achieved with further improved premixing.

Design and Evaluation of the AIMS Combustor

This paper describes a reduced NOx and CO, partially premixed flame combustor that has been developed for the 175 kW Advanced Integrated Microturbine System (AIMS) recuperated cycle gas micro-turbine. The AIMS turbine is equipped with a recuperated silo combustor. The new, reduced emissions combustor retains key features of the conventional Dry Low NOx (DLN) combustors; the differences are the arrangement of the premixers, the novel head-end assembly design, and the liner cooling and dilution features. The combustion system was designed and tested at the GE Global Research facilities in Niskayuna, NY and leverages technology developed by GE Power Systems (GEPS) and GE Aircraft Engines (GEAE). Laboratory tests show that when firing with natural gas, without water or steam injection, NOx and CO emissions from the new combustor are in single digits at full-speed, full-load conditions. CO emissions show a strong pressure effect, increasing at base load (when compared to similar conditions in commercial combustors running at higher pressures). The standard combustor on the AIMS gas turbine is a reversed flow cylindrical can. An array of 4 fuel nozzles is located at the head end of the can and produces a swirl stabilized premixed flame. The liner contains an array of cooling and dilution holes that provide the air needed to dilute the burned gas to the desired turbine inlet temperature.

Micro Gas Turbine Combustion Chamber Design and CFD Analysis

In the framework of the non-standard fuel combustion research in micro-small turbomachinery, a newly designed micro gas turbine combustor for a 100-kWe power plant in CHP configuration is under development at the Ansaldo Ricerche facilities. Combustor design starts from a single silo chamber shape with two fuel lines, and is associated with a radial swirler flame stabiliser. Lean premix technique is adopted to control both flame temperature and NOx production. Combustor design process envisages two major steps, i.e. diagnostics-focussed design for methane only and experimentally validated design optimisation with suitable burner adaptation to non-standard fuels. The former step is over, as the first prototype design is ready for experimental testing. Step two is now beginning with a preliminary analysis of the burner adaptation to non-standard fuels. The present paper focuses on the first step of the combustor development. In particular, main design criteria for both burner and liner cooling system development are presented. Besides, design process control invoked both 2D and 3D CFD analysis. Two turbulence models, FLUENT standard k-ε model and Reynolds Stress Model (RSM), are refereed and the results compared. Here both a detailed analysis of CFD results and a preliminary analysis of main chemical kinetic phenomena are discussed.