Experimental Investigations of Lean Stability Limits of a Prototype Syngas Burner for Low Calorific Value Gases

The lean stability limit of a prototype syngas burner is investigated. The burner is a three sector system, consisting of a separate igniter, stabilizer and Main burner. The ignition sector, Rich-Pilot-Lean (RPL), can be operated with both rich or lean equivalence values, and serves to ignite the Pilot sector which stabilizes the Main combustion sector. The RPL and Main sectors are fully premixed, while the Pilot sector is partially premixed. The complexity of this burner design, especially the ability to vary equivalence ratios in all three sectors, allows for the burner to be adapted to various gases and achieve optimal combustion. The gases examined are methane and a high H2 model syngas (10% CH4, 22.5% CO, 67.5% H2). Both gases are combusted at their original compositions and the syngas was also diluted with N2 to a low calorific value fuel with a Wobbe index of 15 MJ/m3. The syngas is a typical product of gasification of biomass or coal. Gasification of biomass can be considered to be CO2 neutral. The lean stability limit is localized by lowering the equivalence ratio from stable combustion until the limit is reached. To get a comparable blowout definition the CO emissions is measured using a non-dispersive infrared sensor analyzer. The stability limit is defined when the measured CO emissions exceed 200 ppm. The stability limit is measured for the 3 gas mixtures at atmospheric pressure. The RPL equivalence ratio is varied to investigate how this affected the lean blowout limit. A small decrease in stability limit can be observed when increasing the RPL equivalence ratio. The experimental values are compared with values from a perfectly stirred reactor modeled (PSR), under burner conditions, using the GRI 3.0 kinetic mechanism for methane and the San Diego mechanism for the syngas fuels.

Lowering Equivalence Ratio Limit for Stable Combustion in Gas Turbines by Injection of Free Radicals

Lean premixed combustion is one of the widely used methods for NOx reduction in gas turbines (GT). When this method is used combustion takes place under low Equivalence Ratio (ER) and at relatively low combustion temperature. While reducing temperature decreases NOx formation, lowering temperature reduces the reaction rate of the hydrocarbon–oxygen reactions and deteriorates combustion stability. The objective of the present work was to study the possibility to decrease the lower limit of the stable combustion regime by the injection of free radicals into the combustion zone. A lean premixed gaseous combustor was designed to include a circumferential concentric pilot flame. The pilot combustor operates under rich fuel to air ratio, therefore it generates a significant amount of reactive radicals. The experiments as well as CFD and CHEMKIN simulations showed that despite of the high temperatures obtained in the vicinity of the pilot ring, the radicals’ injection from the pilot combustor has the potential to lower the limit of the global ER (and temperatures) while maintaining stable combustion. Spectrometric measurements along the combustor showed that the fuel-rich pilot flame generates free radicals that augment combustion stability. In order to study the relevant mechanisms responsible for combustion stabilization, CHEMKIN simulations were performed. The developed chemical network model took into account some of the basic parameters of the combustion process: ER, residence time, and the distribution of the reactances along the combustor. The CHEMKIN simulations showed satisfactory agreement with experimental results.

Parametric Study of Emissions From Low Calorific Value Syn-Gas Combustion, With Variation of Fuel Distribution, in a Prototype Three Sector Burner

The emission composition is measured for a prototype burner while varying the equivalence ratio in discrete portions of the burner. The burner is a three sector system, consisting of a separate igniter, pilot/stabilizer and main burner. The design allows for discrete control of equivalence ratio in each of the three sectors. The ignition sector, designated RPL (Rich-Pilot-Lean), operates from rich to lean equivalence values, and serves to ignite the pilot sector, which, in turn, stabilizes the main combustion sector. All three burner sections are premixed. The burner is operated at atmospheric pressure with inlet flows heated to 650 K (±8 K). Tests were performed for three gases: methane, a model syngas (10% CH4, 22.5% CO, 67.5% H2), and dilute syngas. The dilute gas includes sufficient nitrogen to lower the heating value to 15 MJ/m3. The model syngas and diluted syngas are representative of fuels produced by gasification process. The burner emissions, specifically, CO, CO2, O2 and NOx, are measured while holding the RPL equivalence value constant and varying the equivalence ratio of the pilot and main sectors. The equivalence ratios for pilot and main sectors are chosen such that the total burner equivalence ratios remain constant during a test sequence. The target total equivalence ratio for each gas is chosen such that all experiments should have the same flame temperature.

Ultra-Low Emission Hydrogen/Syngas Combustion With a 1.3 MW Injector Using a Micro-Mixing Lean-Premix System

This paper discusses the development and testing of a full-scale micro-mixing lean-premix injector for hydrogen and syngas fuels that demonstrated ultra-low emissions and stable operation without flashback for high-hydrogen fuels at representative full-scale operating conditions. The injector was fabricated using Macrolamination technology, which is a process by which injectors are manufactured from bonded layers. The injector utilizes sixteen micro-mixing cups for effective and rapid mixing of fuel and air in a compact package. The full scale injector is rated at 1.3 MWth when operating on natural gas at 12.4 bar (180 psi) combustor pressure. The injector operated without flash back on fuel mixtures ranging from 100% natural gas to 100% hydrogen and emissions were shown to be insensitive to operating pressure. Ultra-low NOx emissions of 3 ppm were achieved at a flame temperature of 1750 K (2690 °F) using a fuel mixture containing 50% hydrogen and 50% natural gas by volume with 40% nitrogen dilution added to the fuel stream. NOx emissions of 1.5 ppm were demonstrated at a flame temperature over 1680 K (2564 °F) using the same fuel mixture with only 10% nitrogen dilution, and NOx emissions of 3.5 ppm were demonstrated at a flame temperature of 1730 K (2650 °F) with only 10% carbon dioxide dilution. Finally, using 100% hydrogen with 30% carbon dioxide dilution, 3.6 ppm NOx emissions were demonstrated at a flame temperature over 1600 K (2420 °F). Superior operability was achieved with the injector operating at temperatures below 1470 K (2186 °F) on a fuel mixture containing 87% hydrogen and 13% natural gas. The tests validated the micro-mixing fuel injector technology and the injectors show great promise for use in future gas turbine engines operating on hydrogen, syngas or other fuel mixtures of various compositions.

Numerical Simulations of Turbulent Mixing and Autoignition of Hydrogen Fuel at Reheat Combustor Operating Conditions

Turbulent mixing and autoignition of H2-rich fuels at relevant reheat combustor operating conditions are investigated in the present numerical study. The flow configuration under consideration is a fuel jet perpendicularly injected into a crossflow of hot flue gas (T > 1000K, p = 15bar). Based on the results of the experimental study for the same flow configuration and operating conditions two different fuel blends are chosen for the numerical simulations. The first fuel blend is a H2/natural gas/N2 mixture at which no autoignition events were observed in the experiments. The second fuel blend is a H2/N2 mixture at which autoignition in the mixing section occurred. First, the non-reacting flow simulations are performed for the H2/natural gas/N2 mixture in order to compare the accuracy of different turbulence modeling methods. Here the steady-state Reynolds-averaged Navier-Stokes (RANS) as well as the unsteady scale-adaptive simulation (SAS) turbulence modeling methods are applied. The velocity fields obtained in both simulations are directly validated against experimental data. The SAS method shows better agreement with the experimental results. In the second part of the present work the autoignition of the H2/N2 mixture is numerically studied using the 9-species 21-steps reaction mechanism of O’Conaire et al. [1]. As in the reference experiments, autoignition can be observed in the simulations. Influences of the turbulence modeling as well as of the hot flue gas temperature are investigated. The onset and the propagation of the ignition kernels are studied based on the SAS modeling results. The obtained numerical results are discussed and compared with data from experimental autoignition studies.

Experimental Investigation of Thermoacoustic Instabilities for a Model Combustor With Varying Fuel Components

In this study, a combustion facility was constructed that includes a flexible fuel supply system to produce synthesis gas using a maximum of three components. The rig with lean premixed burner is able to operate at up to 5 bars. The length of the inlet plenum and the outlet boundary conditions of the combustion chamber are adjustable. Experiments were carried out under a broad range of conditions, with variations in fuel components including hydrogen, methane and carbon monoxide, equivalence ratios, thermal power and boundary conditions. The dynamic processes of self-excited combustion instabilities with variable fuel components were measured. The mechanisms of coupling between the system acoustic waves and unsteady heat release were investigated. The results show that instability modes and flame characteristics were significantly different with variations in fuel components. In addition, the results are expected to provide useful information for the design and operation of stable syngas combustion systems.

Effect of Mixing Quality on NOx Emissions in Reheat Combustion of GT24 and GT26 Engines

Alstoms GT24 and GT26 engines feature a unique sequential combustion system [1, 2]. This system consists of a premixed combustor (called EV), which is followed by a high pressure turbine, a reheat combustor (called SEV) and a low pressure turbine (Figure 1). Recently improvements in NOx performance of the SEV have been demonstrated. Starting with relatively simple methods numerous design variants have been tested and down selected. Further down-selection has been done with methods of increased complexity. Overall a fast and cost effective development process has been assured. During the development process the variation coefficient and unmixedness measured and calculated for mixing only systems (CFD and water channel) has proven to be a reliable indicators for low NOx emissions for the real combustion system on atmospheric and high pressure test rigs. To demonstrate this a comparison of both quantities against NOx emissions is shown. The paper focuses on the NOx results achieved during this development and its relation to mixing quantities. Using this relation, together with a detailed understanding of the flow characteristic in the SEV burner, reductions in NOx emissions for GT24 and GT26 SEV burner and lance hardware can be reached using relatively simple methods.

Analysis of NOX Formation in an Axially Staged Combustion System at Elevated Pressure Conditions

The objective of this investigation was to study the effect of axially staged injection of methane in the vitiated air cross flow in a two stage combustion chamber on the formation of NOX for different momentum flux ratios. The primary cylindrical combustor equipped with a low swirl air blast nozzle operating with Jet-A liquid fuel generates vitiated air in the temperature range of 1473–1673 K at pressures of 5–8 bar. A methane injector was flush mounted to the inner surface of the secondary combustor at an angle of 30°. Oil cooled movable and static gas probes were used to collect the gas samples. The mole fractions of NO, NO2, CO, CO2 and O2 in the collected exhaust gas samples were measured using gas analyzers. For all the investigated operating conditions, the change in the mole fraction of NOX due to the injection of methane (ΔNOX) corrected to 15% O2 and measured in dry mode was less than 15 ppm. The mole fraction of ΔNOX increased with an increase in mass flow rate of methane and it was not affected by a change in the momentum flux ratio. The penetration depth of the methane jet was estimated from the profiles of mole fraction of O2 obtained from the samples collected using the movable gas probe. For the investigated momentum flux ratios, the penetration depth observed was 15 mm at 5 bar and 5 mm at 6.5 and 8 bar. The results obtained from the simulations of the secondary combustor using a RANS turbulence model were also presented. Reaction modeling of the jet flame present in a vitiated air cross flow posed a significant challenge as it was embedded in a high turbulent flow and burns in partial premixed mode. The applicability of two different reaction models has been investigated. The first approach employed a combination of the eddy dissipation and the finite rate chemistry models to determine the reaction rate, while the presumed JPDF model was used in the further investigations. Predictions were in closer agreement to the measurements while employing the presumed JPDF model; this model was also able to predict some key features of the flow as the change of penetration depth with the pressure.

Comparison Between Self-Excited and Forced Flame Response of an Industrial Lean Premixed Gas Turbine Injector

An experimental study was conducted to compare the relationship between self-excited and forced flame response in a variable-length lean premixed gas turbine (LPGT) research combustor with a single industrial injector. The variable-length combustor was used to determine the range of preferred instability frequencies for a given operating condition. Flame stability was classified based on combustor dynamic pressure measurements. Particle velocity perturbations in the injector barrel were calculated from additional dynamic pressure measurements using the two-microphone technique. Global CH* chemiluminescence emission was used as a marker for heat release. The flame’s response (i.e. normalized heat release fluctuation divided by normalized velocity fluctuation) was characterized during self-excited instabilities. The variable-length combustor was then used to tune the system to produce a stable flame at the same operating condition and velocity perturbations of varying magnitudes were generated using an upstream air-fuel mixture siren. Heat release perturbations were measured and the flame transfer function was calculated as a function of inlet velocity perturbation magnitude. For cases in this study, the gain and phase between velocity and heat release perturbations agreed for both self-excited and forced measurements in the linear and nonlinear flame response regimes, validating the use of forcing measurements to measure flame response to velocity perturbations. Analysis of the self-excited flame response indicates the saturation mechanism responsible for finite limit amplitude perturbations may result from nonlinear driving or damping processes in the combustor.

An Experimental and Modeling Study of Laminar Flame Speeds of Alternative Aviation Fuels

The present work reports on measurements of burning velocities of synthetic fuel air mixtures exploiting the cone-angle method, as part of the EU project ALFA-BIRD. The GtL (Gas-to-Liquid)-air mixtures — (i) 100% GtL and (ii) GtL+20% hexanol, respectively — were studied at atmospheric pressure, with values of the equivalence ratio φ ranging between φ ∼ 1.0 and φ ∼ 1.3, at preheat temperatures To = 423 K (GtL+20% hexanol) as well as To = 473 K (for 100% GtL and GtL+20% hexanol). A comparison between experimentally obtained burning velocities and predicted values of laminar flame speed is presented, too. In general, good agreement was found between predicted and measured data for the range of conditions considered in the present study. The predictive capability of the detailed reaction model consisting of 3479 reactions involving 490 species will be discussed focusing on the laminar flame speed and the combustion of the components (n-decane, iso-octane, and 1-hexanol) of the surrogate used.