Industrial Trent Combustor — Combustion Noise Characteristics

Thermoacoustic resonance is a difficult technical problem that is experienced by almost all lean-premixed combustors. The Industrial Trent combustor is a novel dry-low-emissions (DLE) combustor design, which incorporates three stages of lean premixed fuel injection in series. The three stages in series allow independent control of two stages — the third stage receives the balance of fuel to maintain the desired power level — at all power conditions. Thus, primary zone and secondary zone temperatures can be independently controlled. This paper examines how the flexibility offered by a 3-stage lean premixed combustion system permits the implementation of a successful combustion noise avoidance strategy at all power conditions and at all ambient conditions. This is because at a given engine condition (power level and day temperature) a characteristic “noise map” can be generated on the engine, independently of the engine running condition. The variable distribution of heat release along the length of the combustor provides an effective mechanism to control the amplitude of longitudinal resonance modes of the combustor. This approach has allowed the Industrial Trent combustion engineers to thoroughly “map out” all longitudinal combustor acoustic modes and design a fuel schedule that can navigate around regions of combustor thermoacoustic resonance. Noise mapping results are presented in detail, together with the development of noise prediction methods (frequency and amplitude) that have allowed the noise characteristics of the engine to be established over the entire operating envelope of the engine.

Large-Eddy Simulation of Combustion Oscillation in Premixed Combustor

To analyze combustion oscillation in the premixed combustor, a large-eddy simulation program for premixed combustion flow was developed. The subgrid scale (SGS) model of eddy viscosity type for compressible turbulence (Speziale et al., 1988) was adopted to treat the SGS fluxes. The fractal flamelet model, which utilizes the fractal properties of the turbulent premixed flame to obtain the reaction rate, was developed. Premixed combustion without oscillation was analyzed to verify the present method. The computational results showed good accordance with experimental data (Rydén et al., 1993). The combustion oscillation of an “established buzz” type in the premixed combustor (Langhorne, 1988) was also analyzed. The present method succeeded in capturing the oscillation accurately. The detailed mechanism was investigated. The appearance of the non-heat release region, which is generated because the supply of the unburnt gas into the combustion zone stagnates, and its disappearance play an important role.

Detailed Analysis of the Acoustic Mode Shapes of an Annular Combustion Chamber

To get a better understanding of the formation of thermoacoustic oscillations in an annular gasturbine combustor, an analysis of the acoustic eigenmodes has been conducted using the Finite Element (FE) method. The influence of different boundary conditions and a space dependent velocity of sound has been investigated. The boundary conditions actually define the eigenfrequency spectrum. Hence, it is crucial to know e.g. the burner impedance. In case of the combustion system without significant mixing air addition considered in this paper, the space dependence of the velocity of sound is of minor importance for the eigenfrequency spectrum leading to a maximum deviation of only 5% in the eigenvalues. It is demonstrated that the efficiency of the numerical eigenvalue analysis can be improved by making use of symmetry, by splitting the problem into several steps with alternate boundaries conditions, and by choosing the shift frequency ωs in the range of frequencies one is interested in.

Nitrogen Oxide Reduction by Staged Combustion of Biomass Gas in Gas Turbines — A Modeling Study of the Effect of Mixing

Detailed chemical kinetic modeling has been used to study the reduction of nitrogen oxides at gas turbine (GT) combustor conditions. A gas from gasification of wood with air has been used as the fuel. An air-staged combustion technique has been adapted. In our previous study a simple plug flow model was used to study the effects of pressure and temperature among others process variables. The air-fuel mixing was assumed perfect and instantaneous. Results showed the NOx reduction mainly affected by both pressure and temperature. The aim of the present work is to establish the effect of air-fuel mixing delay on NOx predictions and to extrapolate indications options for GT. To model the mixing delay, a varying number of air sub-streams are mixed with the fuel gas during different time periods. Alternatively, a combination of a perfectly mixed zone followed by a plug flow zone is illustrated. Results by any air-fuel mixing model show similar affect of process variables on NOx reduction. When a mixing delay is assumed instead of the instantaneous mixing the NOx reduction is enhanced, and only with delayed mixing NOx are affected by CH4. Lower temperature and higher pressure in the GT-combustor can enhance the NOx reduction. Also air staging is an effective option: a 3 stages combustor designed for low mixing speed appear competitive compared to more complicate combustors. The fewer hydrocarbons in the gasification gas the high NOx reduction.

The Effect of Discrete Pilot Hydrogen Dopant Injection on the Lean Blowout Performance of a Model Gas Turbine Combustor

In view of increasingly stringent NOx emissions regulations on stationary gas turbines, lean combustion offers an attractive option to reduce reaction temperatures and thereby decrease NOx production. Under lean operation, however, the reaction is vulnerable to blowout. It is herein postulated that pilot hydrogen dopant injection, discretely located, can enhance the lean blowout performance without sacrificing overall performance. The present study addresses this hypothesis in a research combustor assembly, operated at atmospheric pressure, and fired on natural gas using rapid mixing injection, typical of commercial units. Five hydrogen injector scenarios are investigated. The results show that (1) pilot hydrogen dopant injection, discretely located, leads to improved lean blowout performance and (2) the location of discrete injection has a significant impact on the effectiveness of the doping strategy.

Hot Gas Filters for Coal and Biomass Power Systems

Several advanced, coal- and biomass-based combustion turbine power generation technologies are currently under development and demonstration. A key technology component in these power generation systems is the hot gas filter. These power generation technologies must utilize highly reliable and efficient hot gas filter systems to protect the turbine and to meet environmental constraints if their full thermal efficiency and cost potential is to be realized. Siemens Westinghouse Power Corporation (SWPC) has developed a hot gas filter system to near-commercial status for large-scale power generation applications. This paper reviews recent progress made by SWPC in hot gas filter test development programs and in major demonstration programs. Two advanced hot gas filter concepts, the “Inverted Candle” and the “Sheet Filter”, having the potential for superior reliability are also described.

Fuel Droplet Evaporation in a Supercritical Environment

This paper reports a numerical investigation of the transcritical and supercritical droplet vaporization phenomena. The simulation is based on the time-dependent conservation equations for liquid and gas phases, pressure-dependent variable thermophysical properties, and a detailed treatment of liquid-vapor phase equilibrium at the droplet surface. The numerical solution of the two-phase equations employs an arbitrary Eulerian-Lagrangian, explicit-implicit method with a dynamically adaptive mesh. Three different equations of state (EOS), namely the Redlich-Kwong (RK), the Peng-Robinson (PR) and Soave-Redlich-Kwong (SRK) EOS, are employed to represent phase equilibrium at the droplet surface. In addition, two different methods are used to determine the liquid density. Results indicate that the predictions of RK-EOS are significantly different from those obtained by using the RK-EOS and SRK-EOS. For the phase-equilibrium of n-heptane-nitrogen system, the RK-EOS predicts higher liquid-phase solubility of nitrogen, higher fuel vapor concentration, lower critical-mixing-state temperature, and lower enthalpy of vaporization. As a consequence, it significantly overpredicts droplet vaporization rates, and underpredicts droplet lifetimes compared to those predicted by PR- and SRK-EOS. In contrast, predictions using the PR-EOS and SRK-EOS show excellent agreement with each other and with experimental data over a wide range of conditions. A detailed investigation of the transcritical droplet vaporization phenomena indicates that at low to moderate ambient temperatures, the droplet lifetime first increases and then decreases as the ambient pressure is increased. At high ambient temperatures, however, the droplet lifetime decreases monotonically with pressure. This behavior is in accord with the reported experimental data.

Gas Turbine Combustor Design Parameter Analysis for Soot Reduction Using CFD

AlliedSignal’s F124 combustor is analyzed using CFD as part of an effort to improve the design of the combustor. A reduction of soot emissions, without negative impact on other performance features such as liner life and lean stability, was the primary objective. The existing F124 combustor (TFE1042) was modeled using the commercial CFD-ACE+ software package to validate the CFD results and provide a basis for comparison for the modified design. Two design of experiment (DOE) matrices of the redesigned combustor were analyzed using CFD modeling. The results of the CFD solutions led to the selection of two configurations for combustor rig experimental testing. The test configurations were selected based on CFD predicted trends for smoke, ignition, lean stability and pattern factor. Engine tests demonstrated a smoke number reduction from more than 40 to less than 10. Lean stability was degraded as a result of a leaner primary zone, but adequate lean stability margin was maintained.

An Advanced Spray Model for Application to the Prediction of Gas Turbine Combustor Flowfields

It is well known that fuel preparation, its method of injection into a combustor and its atomization characteristics have a significant impact on emissions. A simple dilute spray model which assumes that droplet heating and vaporization occur in sequence has been implemented in the past within computational fluid dynamics (CFD) codes at GE and has been used extensively for combustion applications. This spray model coupled with an appropriate combustion model makes reasonable predictions of the combustor pattern factor and emissions. In order to improve upon this predictive ability, a more advanced quasi-steady droplet vaporization model has been considered. This paper describes the evaluation of this advanced model. In this new approach, droplet heating and vaporization take place simultaneously (which is more realistic). In addition, the transport properties of both the liquid and vapor phases are allowed to vary as a function of pressure, gas phase temperature and droplet temperature. These transport properties which are most up to date have been compiled from various sources and appropriately curve-fit in the form of polynomials. Validation of this new approach for a single droplet was initially performed. Subsequently, calculations of the flow and temperature field were conducted and emissions (NOx, CO and UHC) were predicted for a modern single annular turbofan engine combustor using both the standard spray model and the advanced spray model. The effect of the number of droplet size ranges as well as the effect of stochastic treatment of the droplets were both investigated.

Test Validation of Flamelet Model Predictions of NOx

Calculations of NOx emissions were made for the original high pressure combustor and for the original and a modified design of the low pressure combustor used in a Compressed Air Energy Storage (CAES) plant. All were typical diffusion flame combustors. Since a CAES plant has an independent air supply, the relationship between combustor inlet temperature and pressure is not typical for gas turbines, and the pressure level for the HP combustor is unusually high (up to 4.5 MPa). Vitiated air from HP combustor exhaust is used as combustion air in the LP combustor. The NOx emissions prediction method, which was used, for calculations is based on a flamelet model which takes detailed kinetic schemes for fuel oxidation, NOx generation and turbulence/chemistry interaction into account. Site measurements over the entire load curve confirmed the numerical predictions for both the original combustors and the newly developed LP combustor design.