Investigations of Gaseous Alternative Fuels at Atmospheric and Elevated Temperature and Pressure Conditions

Increasing interest in alternative fuels for gas turbines stimulates research in gaseous fuels other than natural gas. Various gas mixtures, based on methane as the main component, are considered as possible fuels in the future. In particular, methane enrichment with hydrogen or dilution with carbon dioxide is of considerable interest. Some experiments and numerical calculations have been undertaken to investigate methane-hydrogen and methane-carbon dioxide gas flames, however most of these investigations are limited by particular pressure or temperature conditions. This paper presents the investigation of the combustion of methane–carbon dioxide mixtures at atmospheric and elevated temperature and pressure conditions. Two experimental rigs were used, a Bunsen burner and swirl burner. Bunsen burner experiments were performed in the High Pressure Optical Chamber, which is located within the Gas Turbine Research Centre of Cardiff University — at 3 bara and 7 bara pressure, and 473 K, 573 K and 673 K temperature conditions for lean and rich mixtures. Planar Laser Tomography (PLT) was applied to investigate turbulent burning velocity. Burning velocity of the gas mixture was calculated using two different image processing techniques and the difference in the results obtained using these two techniques is presented and discussed. Laser Doppler anemometry (LDA) was utilised to define turbulence characteristics such as turbulence intensity and integral length scale. Due to the variability of the velocity flow field and turbulence intensity across Bunsen burners, the importance of measuring position and conditions is discussed. The sensitivity of this variance on the flame regime as defined in the Borghi diagram is evaluated. In the second part of the study, a generic swirl burner was used to define the flame flashback limits for methane–carbon dioxide mixtures at atmospheric conditions. The gas mixture stability graphs are plotted, and the effect of CO2 addition are discussed.

Active Control of Combustion Instabilities in a Matrix Burner Using Primary and Pilot Fuel and Acoustic Forcing

Lean premixed flames applied in modern gas turbines leads to reduce NOx emissions, but at the same time they are more susceptible to combustion instabilities than diffusion flames. These oscillations cause pressure fluctuations with high amplitudes and unacceptable noise as well as the risk of component or even engine failure. They can lead to pockets of fuel being formed in the mixing chamber and to bad mixing, which leads to increase in emissions. This paper reports the successful decoupling of the pressure and heat release inside the combustion chamber of a matrix burner using two actuation techniques. This led to the successful attenuation of the dominant instability modes occurring inside the combustor of the matrix burner. In the first case, acoustic forcing was used to decouple the pressure and the heat release inside the combustor. This was achieved by using a loudspeaker to modulate the primary air mass flow. This was followed by using acoustic forcing in CFD to decouple the pressure and heat release inside the combustor. For the action of the loudspeaker, sinusoidal forcing was used to mimic the modulation action of the diaphragm of the loudspeaker. In the second case, a fast gaseous “on-off” injector was used to modulate the primary fuel mass flow. After this, pilot fuel modulation was used to stabilize the flame. The control law governing the primary and pilot fuel modulation is discussed in details. The effect of open loop control on NOx emissions in the burner is also reported and discussed.

Emissions and Noise Pollution Diagnosis of a Turbogas Power Plant During Commissioning Service

This work is focused on the diagnosis of behavior, from the point of view of control emissions and noise level, of a power Turbogas plant during the process of commissioning, to guarantee that its operation complies with national and international standards. The environmental diagnosis of the power plant was developed as part of the performance evaluation of the unit. The conditions of the unit evaluation include operation at base load and partial load, as well as time periods for load changes. The evaluated power plant consists of an aeroderivative gas turbine installed in a simple cycle, operating with a cooling system (chiller) installed in the urban zone of Mexico City. Therefore, it should comply with the legislation and regulations of the city concerning air pollution and allowed noise, besides the international standards established by the manufacturer. The study includes emissions measurements using a Continuous Emissions Monitoring System installed in-situ, previously calibrated and checked during and after the test which was found inside the permissible deviation of 3%. Measurements were recorded at intervals of 5 minutes during test periods of 110 minutes for each load and 45 minutes for load changes. On the other hand, noise pressure evaluation was carried out in near field as well as far field produced by the power plant during operation. Measurements were carried out by using precision instruments installed specifically for it. A temporary system for obtaining data was used to monitoring the environmental conditions every 30 seconds. It was possible to verify that the turbogenerator complies with all noise levels and contaminant emissions requirements and regulations according to the limits established by the manufacturer and national and international standards.

Effects of Fuel Nozzle Condition on Gas Turbine Combustion Chamber Exit Temperature Distributions

The effects of fuel nozzle condition on the temperature distributions experienced by the nozzle guide vanes have been investigated using an optical patternator. Average spray cone angle, symmetry, and fuel streaks were quantified. An ambient pressure and temperature combustion chamber test rig was used to capture exit temperature distributions and to determine the pattern factor. The rig tests matched representative engine operating conditions by matching Mach number, equivalence ratio, and fuel droplet size. It was observed that very small deviations (± 10° in spray cone angle) from a nominal distribution in the fuel nozzle spray pattern correlated to increases in pattern factor, apparently due to a degradation of mixing processes, which created larger regions of very high temperature core flow and smaller regions of cooler temperatures within the combustion chamber exit plane. The spray cone angle had the most measureable influence while the effects of spray roundness and streak intensity had slightly less influence. Comparisons were made with published studies conducted on the combustion chamber geometry, and recommendations were made for fuel nozzle inspections.

Numerical Simulation of Combustion Induced Noise Using LES and Computational Aeroacoustics

Many technical combustion devices are susceptible to thermoacoustic instabilities. In this work, the noise emission by a turbulent jet flame is analyzed by means of a hybrid LES/CAA (Large Eddy Simulation/Computational Aero Acoustics) approach as a first step towards a numerical investigation of combustion instability. The hybrid LES/CAA approach is based on a LES of the reactive flow utilizing a low Mach number formulation. Within the CAA part of the simulations, linearized Euler equations (LEE) are solved. A simplified formulation to describe the thermoacoustic sound sources is extracted from the reactive LES. For the present study, the CFD code FASTEST is coupled with the aeroacoustic simulation tool PIANO. The two solvers are combined to a single tool for the description of the acoustics of reacting flows. Both codes make use of geometry flexible grids enabling the simulation of complex geometries commonly used within technical combustion systems.

Experiments on Flame Flashback in a Quasi-2D Turbulent Wall Boundary Layer for Premixed Methane-Hydrogen-Air Mixtures

Premixed combustion of hydrogen-rich mixtures involves the risk of flame flashback through wall boundary layers. For laminar flow conditions, the flashback mechanism is well understood and is usually correlated by a critical velocity gradient at the wall. Turbulent transport inside the boundary layer considerably increases the flashback propensity. Only tube burner setups have been investigated in the past and thus turbulent flashback limits were only derived for a fully-developed Blasius wall friction profile. For turbulent flows, details of the flame propagation in proximity to the wall remain unclear. This paper presents results from a new experimental combustion rig, apt for detailed optical investigations of flame flashbacks in a turbulent wall boundary layer developing on a flat plate and being subject to an adjustable pressure gradient. Turbulent flashback limits are derived from the observed flame position inside the measurement section. The fuels investigated cover mixtures of methane, hydrogen and air at various mixing ratios. The associated wall friction distributions are determined by RANS computations of the flow inside the measurement section with fully resolved boundary layers. Consequently, the interaction between flame back pressure and incoming flow is not taken into account explicitly, in accordance with the evaluation procedure used for tube burner experiments. The results are compared to literature values and the critical gradient concept is reviewed in light of the new data.

Predictive Emissions Monitoring Using a Continuously Updating Neural Network

In the European Union, power plants of more than 50 MW (thermal energy) need to comply with the Large Combustion Plant Directive (LCPD, 2001) implying that flue gas emissions need to be measured continuously. Traditionally, emissions from power plants are measured using Automated Measuring Systems (AMS). The LCPD states that no more than 10 days of emission data may be lost within one year including days needed for maintenance. This is the reason why more and more power plants are currently installing a second, back-up AMS since they have problems with the availability of their AMS. Since early 1990’s, Predictive Emissions Monitoring Systems (PEMS) are being developed and accepted by some local authorities within Europe and the United States. PEMS are in contrast to AMS based on the prediction of gaseous emissions (most commonly NOx and CO) using plant operational data (eg. fuel properties, pressure, temperature, excess air, …) rather than the actual measurement of these emissions. The goal of this study is to develop a robust PEMS that can accurately predict the NOx and CO emissions across the entire normal working range of a gas turbine. Furthermore, the PEMS should require as little maintenance as possible. The study does not intend to replace the AMS by a PEMS but rather to use the PEMS as a backup for the AMS. Operational data of a gas turbine, acquired over a long period, was used to identify inputs with a high influence on the NOx and CO formation. Consequently, simulations were done testing different model structures and calibration methodologies. The study shows that a static model failed to predict the emissions accurately over long time periods. In contrast, a dynamic or self-adapting algorithm proved to be most efficient in predicting the emissions over a long time period with a minimum of required intervention and maintenance. The self-adapting algorithm uses measured AMS data to continuously update the neural network. Since the PEMS is developed as a backup for the AMS, these data are readily available. The study shows that in case of a failing AMS, the developed model could accurately predict the NOx emissions for a duration of several weeks. Although not discussed in detail in this study, a quality assurance system of the PEMS is also developed since the PEMS needs to comply to the EN14181 (as does any AMS). The PEMS as a backup of the AMS instead of a second AMS is cost and time saving. Not only is the purchase of a second AMS avoided (between 40 and 100 k€) but equally important and of the same order of magnitude are the cost and time savings with respect to the Quality Assurance of the second AMS.

Flashback Propensity of Syngas Flames at High Pressure: Diagnostic and Control

Nowadays, the establishment of IGCC (integrated gasification combined cycle) plants, prompts a growing interest in synthetic fuels for gas turbine based power generation. This interest has as direct consequence the need for understanding of flashback phenomena for premixed systems operated with H2-rich gases. This is due to the different properties of H2 (e.g. reactivity and diffusivity) with respect to CH4 which lead to higher flame speeds in the case of syngases (mixtures of H2-CO). This paper presents the results of experiments at gas turbine like conditions (pressure up to 15 bar, 0.2 < Φ < 0.7, 577K < T0 < 674K) aimed to determine flashback limits and their dependence on the combustion parameters (pressure, inlet temperature and inlet velocity). For the experimental facility used for this work the back propagation of the flame is believed to happen into the boundary layer of the fuel/air duct. Flashback propensity was found to have an appreciable dependence on pressure and inlet temperature while the effects of inlet velocity variations are weak. Explanations for the dependence on these three parameters, based on consideration on laminar and turbulent flame speed data (from modeling and experiments), are proposed. Within the frame of this work, in order to avoid major damages, the experimental facility was equipped with an automatic control system for flashback described in the paper. The control system is able to detect flame propagation into the fuel/air supply, arrest it and restore safe operating conditions by moving the flame out of the fuel/air section without blowing it out. This avoids destruction of components (burner/mixing) and time consuming shut downs of the test rig.

OH* Chemiluminescence and OH-PLIF Measurements in a Micro Gas Turbine Combustor

In this work the combustion behavior of the Turbec T100 natural gas/air combustor was analyzed experimentally. For the visualization of the flame structures at various stationary load points OH* chemiluminescence and OH-PLIF measurements were performed in a micro gas turbine test rig equipped with an optically accessible combustion chamber. The OH* chemiluminescence measurements are used to get an impression of the shape and the location of the heat release zones. In addition the OH-PLIF measurements enabled spatially and temporarily resolved information of the reaction zones. Depending on the load point the shape of the flame was seen to vary from cylindrical to conical. With increasing thermal power load the maximum heat release zones shift to a lifted flame. Moreover, the effect of the optically accessible combustion chamber on the performance of the micro gas turbine is evaluated.

An Experimental Ignition Delay Study of Alkane Mixtures in Turbulent Flows at Elevated Pressure and Intermediate Temperatures

Autoignition delay times of mixtures of alkanes and natural gas were studied experimentally in a high pressure and intermediate temperature turbulent flow reactor. Measurements were made at pressures between 7 and 15 atm and temperatures from 785 to 935K. The blends include binary and ternary mixtures of methane, ethane and propane; along with various natural gas blends. Based on these data, the effect of higher hydrocarbons on the ignition delay time of natural gas type fuels at actual gas turbine engine conditions has been quantified. While the addition of higher hydrocarbons in quantities of up to 30% were found to reduce the ignition delay by up to a factor of four, the delay times were still found to be greater than 60 milliseconds in all cases which is well above the residence times of most engine premixers. The data were used to develop simple Arrhenius type correlations as a function of temperature, pressure and fuel composition for design use.