Gas Turbine Relevant Conditions Research Articles

Development of Detailed and Reduced Chemical Kinetic Models for Ammonia Gas Turbines

Abstract The power generation sector has been recently moving towards decarbonization and there is an increased interest in replacing conventional fossil fuels with fuels that produce reduced/zero carbon emissions. One such fuel is ammonia (NH3). However, ammonia is hard to ignite, has a low flame speed, and produces a significantly large amount of nitrogen oxide (NOx) emissions. Hence, using 100% ammonia as fuel in gas turbines requires significant modifications and the development of novel combustors. Blending hydrogen with ammonia, however, helps in having better control over the combustion properties. Before utilizing hydrogen-blended ammonia in an actual gas turbine combustor, thorough simulation studies are required to evaluate its performance, possible hazards, and emissions. The literature lacks well-validated chemical kinetic models for the combustion of hydrogen-blended ammonia for undiluted mixtures at gas turbine-relevant conditions (~20 bar). Hence, in this work, we develop a detailed chemical kinetic model for hydrogen blended ammonia combustion and validate it with a wide range of experimental data for both dilute and undiluted mixtures relevant to gas turbine operating conditions. The detailed chemical kinetic mechanism was reduced to a smaller version (32 species mechanism) without significant loss in accuracy using the direct relation graph with error propagation (DRGEP) and full species sensitivity analysis. The resultant mechanism can predict a wide range of experimental results with the least cumulative error and will be a valuable tool in CFD simulations that will enable the development of gas turbines for zero-carbon power generation.

Effect of pressure and H2 content on global consumption-based turbulent flame speed at gas turbine relevant conditions

This study focuses on the effect of pressure level and H2 content in the CH4–H2 fuel mixture on the global consumption-based turbulent flame speed (ST). The data considered in the analysis are collected for a variety of CH4–H2 fuel compositions (from 50% to 100% H2 vol.), pressure levels ranging from 0.5 to 1.5 MPa, and preheating temperatures up to 623.15 K. Different trends of ST/SL0 (SL0 is the adiabatic unstretched laminar flame speed) are observed as a function of the normalized turbulence intensity (u’/SL0), depending on the H2 content in CH4–H2 fuel blends. Yet, for all the CH4–H2 fuel mixtures considered, ST/SL0 consistently increases with H2 content. Rising pressure levels are responsible for larger ST/SL0, in direct proportion with the H2 content. These trends are mostly caused by the decrease in SL0, which causes changes to the characteristics of the flamelet that becomes more sensitive to hydrodynamic wrinkling through the high molecular diffusivity of H2.

Ignition Delay Times and Chemical Kinetic Model Validation for Hydrogen and Ammonia Blending With Natural Gas at Gas Turbine Relevant Conditions

Abstract Ignition delay times from undiluted mixtures of natural gas (NG)/H2/Air and NG/NH3/Air were measured using a high-pressure shock tube at the University of Central Florida. The combustion temperatures were experimentally tested between 1000 and 1500 K near a constant pressure of 25 bar. As mentioned, mixtures were kept undiluted to replicate the same chemistry pathways seen in gas turbine combustion chambers. Recorded combustion pressures exceeded 200 bar due to the large energy release, hence why these were performed at the high-pressure shock tube facility. The data are compared to the predictions of the NUIGMech 1.1 mechanism for chemical kinetic model validation and refinement. An exceptional agreement was shown for stoichiometric conditions in all cases but strayed at lean and rich equivalence ratios, especially in the lower temperature regime of H2 addition and all temperature ranges of the baseline NG mixture. Hydrogen addition also decreased ignition delay times by nearly 90%, while NH3 fuel addition made no noticeable difference in ignition time. NG/NH3 exhibited similar chemistry to pure NG under the same conditions, which is shown in a sensitivity analysis. The reaction CH3 + O2 = CH3O + O is identified and suggested as a possible modification target to improve model performance. Increasing the robustness of chemical kinetic models via experimental validation will directly aid in designing next-generation combustion chambers for use in gas turbines, which in turn will greatly lower global emissions and reduce greenhouse effects.

Turbulent Flame Speed and Flame Characteristics of Lean Premixed H2-CH4 Flames at Moderate Pressure Levels

Abstract Carbon dioxide emissions in gas turbine power generation can be reduced by adding an increasing amount of hydrogen to the existing natural gas-fueled combustion systems. To enable safe operation, more insight on how H2 addition affects turbulent flame speed and other important flame characteristics is needed. In this work, the investigation of hydrogen addition effects on certain flame properties has been carried out in a high-pressure axial-dump combustor at gas turbine relevant conditions. OH planar laser induced fluorescence (PLIF) was applied to retrieve flame front contours and turbulent flame speed. The results show that as the concentration of hydrogen in the fuel mixture increases, turbulent flame speed and flame characteristics change drastically. Two main regimes can be identified: From 0 to 50% vol. Hydrogen, the turbulent flame speed increases weakly in an almost linear fashion, while from 50% vol. to 100% vol. the trend sharply changes and the higher reactivity of hydrogen, combined with a lower Lewis number, cause thermal-diffusive instability and preferential diffusion effects to become increasingly strong, leading to very high burning rates. The presented results help to understand and to define the relevant modifications that are necessary to successfully operate gas turbine combustor systems with high H2 content fuels.

Experimental Investigation of a Multi Tube Burner for Premixed Hydrogen and Natural Gas Low Emission Combustion

Abstract Hydrogen as an essential part of future decarbonization of the energy industry makes it a crucial necessity to replace conventional, natural gas based concepts in gas turbine combustion. This paper presents an experimental study of a multi-tube jet flame burner. The study is carried out with natural gas and pure hydrogen fuel at gas turbine relevant conditions at atmospheric pressure. To identify key differences between hydrogen-air and natural gas–air flames on the overall robustness and flame flashback behavior, air bulk velocity (80–120 m/s), adiabatic flame temperature (1235–2089 K) and air inlet temperature (623–673 K) are varied over a wide range, covering a range of Reynolds numbers of 10,000–20,000. Depending on flame temperature, two different flame shapes are observed for natural gas–air flames. The shape of the hydrogen-air flame changes less over the range of flame temperatures tested, but is generally more compact. The process of fuel-air mixing is further investigated by concentration distribution measurements in a water tunnel setup. Therefore, planar laser-induced fluorescence is utilized for visualization. The measured concentration distributions confirm the overall good mixing quality but also give an explanation on the observed flashback behavior of the different burner designs at reacting tests. The findings of the study are composed in a flashback correlation combining the observed flashback drivers for the burner configurations investigated.

High-fidelity H2–CH4 jet in crossflow modelling with a flame index-controlled artificially thickened flame model

Hydrogen introduction in existing combustion systems can heavily alter the flame morphology and stability limit of the system itself. This makes the numerical prediction of such changes crucial for the development of effective design modifications. In the industrial framework, often it is required to have combustion models capable of handling both the premixed and the diffusive combustion regimes and this represents a real modelling challenge. In this work, a multi-regime CH4–H2 Jet In Crossflow (JICF) flame, at gas turbine relevant conditions, has been investigated with a Flame Index controlled Artificially Thickened Flame Model (ATFM). The numerical prediction shows good agreement with the detailed experimental data from DLR laboratory, and the model has been found to correctly predict the change in the flame anchoring topology due to an increase of H2 content.

Demonstration of Natural Gas and Hydrogen Cocombustion in an Industrial Gas Turbine

Abstract Hydrogen cofiring in a gas turbine is believed to cover an energy transition pathway with green hydrogen as a driver to lower the carbon footprint of existing thermal power generation or cogeneration plants through the gradual increase of hydrogen injection in the existing natural gas grid. Today there is limited operational experience on cocombustion of hydrogen and natural gas in an existing gas turbine in an industrial environment. The ENGIE owned Siemens SGT-600 (Alstom legacy GT10B) 24 MW industrial gas turbine in the port of Antwerp (Belgium) was selected as a demonstrator for cofiring natural gas with hydrogen as it enables ENGIE to perform tests at higher H2 contents (up to 25 vol. %) on a representative turbine with limited hydrogen volume flow (one truck load at a time). Several challenges like increasing risk of flame flashback due to the enhanced turbulent flame speed, avoiding higher NOx emissions due to an increase of local flame temperature, supply and homogeneous mixing of hydrogen with natural gas as well as safety aspects have to be addressed when dealing with hydrogen fuel blends. In order to limit the risks of the industrial gas turbine testing a dual step approach was taken. ENGIE teamed up with the German Aerospace Center (DLR), Institute of Combustion Technology in Stuttgart to perform in a first step scaled-burner tests at gas turbine relevant operating conditions in their high-pressure combustor rig. In these tests the onset of flashback as well as the combustor characteristics with respect to burner wall temperatures, emissions and combustion dynamics were investigated for base load and part load conditions, both for pure natural gas and various natural gas and hydrogen blends. For H2 < 30 vol. % only minor effects on flame position and flame shape (analyzed based on OH* chemiluminescence images) and NOx emission were found. For higher hydrogen contents the flame position moved upstream and a more compact shape was observed. For the investigated H2 contents no flashback event was observed. However, thermo acoustics are strongly affected by hydrogen addition. In general, the scaled-burner tests were encouraging and enabled the second step, the exploration of hydrogen limits of the second generation dry low emissions burner installed in the engine in Antwerp. To inject the hydrogen into the industrial gas turbine, a hydrogen supply line was developed and installed next to the gas turbine. All tests were performed on the existing gas turbine hardware without any modification. A test campaign of several operational tests at base and part load with hydrogen variation up to 25 vol. % has been successfully performed where the gas composition, emissions, combustion dynamics and operational parameters are actively monitored in order to assess the impact of hydrogen on performance. Moreover, the impact of the hydrogen addition on the flame stability has been further assessed through the combustion tuning. The whole test campaign has been executed while the gas turbine stayed online, with no impact to the industrial steam customer. It has been proved that cofiring of up to 10% could be achieved with no adverse effects on the performance of the machine. Stable operation has been observed up to 25 vol. % hydrogen cocombustion, but with trespassing the local emission limits.

Direct numerical simulation of flame-wall interaction at gas turbine relevant conditions

A direct numerical simulation (DNS) with finite rate chemistry was performed to evaluate the main influences on carbon monoxide (CO) emissions in gas turbine combustion. A lean methane/air mixture is burned in fully turbulent jet flames in a domain enclosed by isothermal walls. The formation of CO is found to be affected by the mean strain rate of the turbulent flow, the flame-wall interaction (FWI), and the interactions of the flame with the recirculation zones of the flow. The CO production and consumption in the turbulent flame differ strongly from the reaction rates in a freely propagating flame. In the upstream part of the domain, the mean strain rate of the turbulent flow mainly affects the CO formation, while wall heat loss influences the CO oxidation process towards the end of the domain, where the strain rate decreases. In an optimal estimator analysis, the relevant parameters that dominate the formation and consumption of CO are identified as the local CO mass fraction YCO, the wall heat loss, described by the enthalpy defect Δh, and the mass fraction of the OH radical YOH. The heat loss is particularly influential close to the wall while the effects far from the wall are negligible. Using the local CO mass fraction as parameter describes the late-stage oxidation of CO well in the entire domain. In particular, YCO should not be neglected at the wall. YOH is well suited to describe the processes involved in CO oxidation, as it both parameterizes the turbulent strain and is the main reaction partner for CO oxidation. The combination of YCO and Δh was able to improve the domain-averaged irreducible error by almost half compared to only a progress variable. Adding YOH to the parameter set further reduced the error to 25% of the original error.

Plasma reforming for enhanced ammonia-air ignition: A numerical study

Using an in-house 0D plasma chemical solver, this paper investigates the species involved in plasma-assisted reforming of both pure ammonia and stoichiometric ammonia-air mixtures. A nanosecond repetitively pulsed plasma is simulated for dielectric barrier discharge conditions, with reduced electric fields of 180 and 360 Td, energies per pulse of 0.5 and 1 mJ/cm3, and pulse repetition frequencies up to 500 kHz. To show the effect of reforming on combustion performance, the reformates are fed into a neutral species combustion chemistry solver to calculate the ignition delay time at gas-turbine relevant conditions. For a reformed stoichiometric mixture, it is possible to achieve a reduction of two orders of magnitude in ignition delay time. This reduction, however, comes at the cost of lost enthalpy, as ammonia reacts with oxygen to create water. Path flux and sensitivity analyses were performed, and it as found that the two most crucial species in the reformate were H2 and NH2. The presence of NH2 in high concentration also resulted in lower concentrations of NO after ignition, compared to the unreformed mixture. When reforming pure ammonia, the same number of pulses and energy as in the stoichiometric case reduce ignition by one order of magnitude. A higher reduction is possible with more pulses, unlike the stoichiometric reforming case in which ignition is reached during the reforming process, and with no loss of enthalpy due to oxidation. At 200 kHz, a reduction of two orders of magnitude is possible after 1500 pulses. These results support the feasibility of plasma-assisted reforming for the improvement of ammonia combustion characteristics at relevant conditions.

Effect of hydrogen addition on the combustion and emission characteristics of methane under gas turbine relevant operating condition

The influence of hydrogen addition (0–100%) on the combustion and emission characteristics (NO and CO formation) of laminar-premixed methane flame under gas turbine relevant operating conditions was numerically studied. The PREMIX code of ANSYS CHEMKIN-PRO and the GRI-Mech 3.0 were utilized. The study of the effect of increasing hydrogen content in the methane-hydrogen mixtures on combustion and emissions at T = 298 K, P = 1 atm can provide a reference for our subsequent research on gas turbine conditions (T = 723 K, P = 16.5 atm). The LBV (laminar burning velocity) and AFT (adiabatic flame temperature) with different hydrogen contents (XH2=0–100%) and equivalence ratio (Φ = 0.6–1.4) were investigated. In addition, under gas turbine relevant lean burning conditions (Φ = 0.6, 0.8), the effect of hydrogen addition on the NO and CO emissions were investigated. The rate and sensitivity coefficients of NO/CO production were also examined. The results show that the LBV and AFT increase with the hydrogen content. Both the mole fraction of H, O and OH radicals and the flame temperature were observed to increase with the hydrogen content. Furthermore, the hydrogen addition can significantly increase the NO emissions and on the other hand suppress the CO formation. Reducing the equivalence ratio (from 1.2 to 0.6) can significantly reduce the NO emission. The addition of hydrogen can also increase the combustion stability of methane fuel under fuel-lean conditions. This is a good hint for gas turbine device to reduce NO emissions at fuel-lean side (Φ < 1).

Ignition Studies of C1–C7 Natural Gas Blends at Gas-Turbine-Relevant Conditions

Abstract New ignition delay time measurements of natural gas mixtures enriched with small amounts of n-hexane and n-heptane were performed in a rapid compression machine to interpret the sensitization effect of heavier hydrocarbons on auto-ignition at gas-turbine relevant conditions. The experimental data of natural gas mixtures containing alkanes from methane to n-heptane were carried out over a wide range of temperatures (840–1050 K), pressures (20–30 bar), and equivalence ratios (φ = 0.5 and 1.5). The experiments were complimented with numerical simulations using a detailed kinetic model developed to investigate the effect of n-hexane and n-heptane additions. Model predictions show that the addition of even small amounts (1–2%) of n-hexane and n-heptane can lead to an increase in reactivity by ∼40–60 ms at compressed temperature (TC) = 700 K. The ignition delay time (IDT) of these mixtures decreases rapidly with an increase in concentration of up to 7.5% but becomes almost independent of the C6/C7 concentration beyond 10%. This sensitization effect of C6 and C7 is also found to be more pronounced in the temperature range 700–900 K compared to that at higher temperatures (&amp;gt;900 K). The reason is attributed to the dependence of IDT primarily on H2O2(+M) ↔ 2ȮH(+M) at higher temperatures while the fuel-dependent reactions such as H-atom abstraction, RȮ2 dissociation, or Q˙OOH + O2 reactions are less important compared to 700–900 K, where they are very important.

Experimental and Numerical Characterization of a Novel Natural Gas Low NOx Burner in Gas Turbine Realistic Environment

Abstract A fundamental milestone in the development of a low NOx burner technology is the demonstration of its capabilities in realistic environment. This is especially true for the novel burner subject of this paper, which has been extensively characterized throughout single burner scale experiments. An exhaustive description of the early development phases of the novel burner has been provided by authors in recently published works. The most promising geometry was selected for the assessment in real combustor arrangement, consisting of a full-scale annular combustor test rig. This paper reports the main results of such an assessment. Pollutant emissions and pressure pulsations have been measured at gas turbine relevant operating conditions. Moreover, dedicated blow-out tests have been performed to obtain the extinction equivalence ratio at both ambient and pressurized conditions, as done during the past single burner rig campaign. Basically, an adequate set of data has been gathered, allowing a direct comparison between full-annular and reduced-scale tests. A general alignment of behavior has been observed, as both low NOx capability and blow-out characteristics of full-annular arrangement turned out to be substantially unchanged with respect to the single burner. Nevertheless, some discrepancies in magnitude have been highlighted and discussed. Details have been given involving deeper numerical analysis by means of a dedicated model developed by the authors in previous works. Indeed, improvement to the model has been introduced in the context of this paper to overcome some limitations arisen in predicting emissions. Finally, a preliminary stability analysis has been carried out, with the aim to describe the onset of thermoacoustic instability tendency as observed in the full-annular tests.

Boundary Layer Flashback Limits of Hydrogen-Methane-Air Flames in a Generic Swirl Burner at Gas Turbine-Relevant Conditions

AbstractOperating stationary gas turbines on hydrogen-rich fuels offers a pathway to significantly reduce greenhouse gas emissions in the power generation sector. A key challenge in the design of lean-premixed burners, which are flexible in terms of the amount of hydrogen in the fuel across a wide range and still adhere to the required emission levels, is to prevent flame flashback. However, systematic investigations on flashback at gas turbine relevant conditions to support combustor development are sparse. The current work addresses the need for an improved understanding with an experimental study on boundary layer flashback in a generic swirl burner up to 7.5 bar and 300 °C preheat temperature. Methane-hydrogen-air flames with 50 to 85% hydrogen by volume were investigated. High-speed imaging was applied to reveal the flame propagation pathway during flashback events. Flashback limits are reported in terms of the equivalence ratio for a given pressure, preheat temperature, bulk flow velocity, and hydrogen content. The wall temperature of the center body along which the flame propagated during flashback events has been controlled by an oil heating/cooling system. This way, the effect any of the control parameters, e.g., pressure, had on the flashback limit was decoupled from the otherwise inherently associated change in heat load on the wall and thus change in wall temperature. The results show that the preheat temperature has a weaker effect on the flashback propensity than expected. Increasing the pressure from atmospheric conditions to 2.5 bar strongly increases the flashback risk, but hardly affects the flashback limit beyond 2.5 bar.

Partial Premixing Effects on the Reacting Jet of a High-Pressure Axially Staged Combustor

Abstract The effects of partial premixing on a reacting jet-in-crossflow is investigated in a five atmosphere axially staged combustor at stationary gas turbine relevant conditions. The facility consists of a dump style headend burner that provides a crossflow with a quasi-uniform velocity and temperature profile to the axial stage to isolate the effects of the jet-in-crossflow. The headend burner is run with methane and air at a lean equivalence ratio to match industry emission standards. For this work, the total air to the headend and axial stage is kept constant, and fuel is split between the headend and axial stage to represent different gas turbine loading conditions. For the cases analyzed, the fuel split to the axial stage went up to 25%. The axial stage consists of an optically accessible test section with a coaxial injector that provides variability to how long the methane and air can mix before entering the facility. Three different premixed levels are studied: fully premixed, nonpremixed, and partially premixed. The flow-field characteristics of the reacting jet-in-crossflow are analyzed using particle image velocimetry (PIV), and flame behavior is quantified by employing CH* chemiluminescence. NO measurements are made at the exit of the facility using a Horiba emissions analyzer. Two different flames are observed: flames that burn in the leeward recirculation region and flames that burn at the core of the jet.

Distributed combustion mode in a can-type gas turbine combustor – A numerical and experimental study

A novel combustor design assisted with increased hot air jet momentum is used to enhance the internal recirculation. Geometric modifications are incorporated in terms of altered air injection scheme to improve the mixing and internal recirculation of hot combustion products. The combustor operates with liquefied petroleum gas in a distributed combustion mode, and thermal intensities vary in the range of 20–40 MWm−3atm−1. Numerical calculations of the reacting flow field show the presence of large recirculation zones, and they are quantified using reactant dilution ratio, Rdil. It is greater than 2.5 for the most part of the combustor, thereby creating a sufficiently heated and diluted environment for sustaining the distributed combustion at high thermal intensities. OH* chemiluminescence studies are carried out for the first time to observe the combustion zone structure for the conventional and distributed combustion regime in a gas turbine combustor. The distribution of OH* becomes increasingly uniform with reduced maximum OH* intensity in this combustion mode. The measured NOx is less than 8 ppm, and CO emission is below 25 ppm for all the operating conditions (ϕ = 0.2–1). Acoustic emissions drop significantly, as the combustor switches its operation to a distributed combustion mode. The novelty of this study lies in the application of this combustion mode in a gas turbine engine. This article also sheds light on the importance of the internal recirculation of combustion products, to achieve a distributed combustion mode in a generic can combustor at gas turbine relevant conditions.

High-Momentum Jet Flames at Elevated Pressure, C: Statistical Distribution of Thermochemical States Obtained From Laser-Raman Measurements

Abstract A detailed investigation on flame structures and stabilization mechanisms of confined high momentum jet flames by one-dimensional (1D)-laser Raman measurements is presented. The flames were operated with natural gas (NG) at gas turbine relevant conditions in an optically accessible high-pressure test rig. The generic burner represents a full scale single nozzle of a high temperature FLOX® gas turbine combustor including a pilot stage. 1D-laser Raman measurements were performed on both an unpiloted and a piloted flame and evaluated on a single shot basis revealing the thermochemical states from unburned inflow conditions to burned hot gas in terms of average and statistical values of the major species concentrations, the mixture fraction and the temperature. The results show a distinct difference in the flame stabilization mechanism between the unpiloted and the piloted case. The former is apparently driven by strong mixing of fresh unburned gas and recirculated hot burned gas that eventually causes autoignition. The piloted flame is stabilized by the pilot stage followed by turbulent flame propagation. The findings help to understand the underlying combustion mechanisms and to further develop gas turbine burners following the FLOX concept.

Measurements of the reactivity of premixed, stagnation, methane-air flames at gas turbine relevant pressures

The adiabatic, unstrained, laminar flame speed, SL, is a fundamental combustion property, and a premier target for the development and validation of thermochemical mechanisms. It is one of the leading parameters determining the turbulent flame speed, the flame position in burners and combustors, and the occurrence of transient phenomena, such as flashback and blowout. At pressures relevant to gas turbine engines, SL is generally extracted from the continuous expansion of a spherical reaction front in a combustion bomb. However, independent measurements obtained in different types of apparatuses are required to fully constrain thermochemical mechanisms. Here, a jet-wall, stagnation burner designed for operation at gas turbine relevant conditions is presented, and used to assess the reactivity of premixed, lean-to-rich, methane–air flames at pressures up to 16 atm. One-dimensional (1D) profiles of axial velocity are obtained on the centerline axis of the burner using particle tracking velocimetry, and compared to quasi-1D flame simulations performed with a selection of thermochemical mechanisms available in the literature. Significant discrepancies are observed between the numerical and experimental data, and among the predictions of the mechanisms. This motivates further chemical modeling efforts, and implies that designers in industry must carefully select the mechanisms employed for the development of gas turbine combustors.

Analysis of air-staged combustion of NH3/CH4 mixture with low NOx emission at gas turbine conditions in model combustors

The NOx formation characteristics have been numerically investigated in the NH3/CH4 fired model combustor consisting of perfectly stirred reactors (PSR) and plug flow reactors (PFR) with 84-species Tian mechanism at gas-turbine relevant conditions of 23 atm and 1873 K at the combustor outlet. Specifically, the impacts of NH3 dilution on NOx formation were first studied in a non-staged model combustor consisting of one PSR representing main combustion zone and one PFR representing post-combustion zone over a wide range of the volumetric ratio of NH3 in the NH3/CH4 fuel. It was found that the NOx emission increases sharply with the NH3 addition mainly through the enhanced HNO pathway in the main reaction zone. Recognizing that the NOx formation via the HNO pathway is limited by the availability of oxygen, the effects of fuel/air equivalence ratio on NOx formation were then quantified under different levels of NH3 dilution. It was revealed that at fuel-rich conditions, the NOx formation via HNO pathway can be offset by the reduction via other pathways, such as pathways related to NHi, leading to the overall low NOx emission. Finally, an air-staged model combustion system was proposed, in which the equivalence ratio in the primary stage was chosen to be 1.5 to take advantage of the low NOx emission under fuel-rich conditions. The air split among the two stages was determined by the fixed outlet temperature of 1873 K. A parametric study demonstrated that less than 30 ppm NOx emission can be achieved with simple two-stage combustion even when the volumetric ratio of the NH3 reaches 40%, which is dramatically lower than the one of thousands ppm in the non-staged combustion. The results were further verified with the 47-species Konnov mechanism, demonstrating the great potential of air-staged combustion for low NOx emission in the NH3/CH4 fired combustion.

Assessing the predictions of a NOx kinetic mechanism on recent hydrogen and syngas experimental data

A detailed chemical kinetic mechanism has been developed to describe the pyrolysis and oxidation of the hydrogen/NOx and syngas/NOx systems. The thermodynamic data of nitrogenous compounds have been updated based on the study of Bugler et al. (2016). The rate constants of individual elementary reactions associated with the Zeldovich mechanism, the N/O sub-mechanism (NO2, N2O and NO3), the H/N/O sub-mechanism (HNO/HON, HNO2/HONO and HONO2) and the NH3 mechanism (NNH and NH2OH) have been selected through a synthetic comparison of the data available in the literature and the adoption of the latest available published rate constant data. The proposed mechanism has been validated against a large number of experimental data including pyrolysis histories, ignition delay time data, species profile versus time and temperature and flame speed measurements over a wide range of initial combustion conditions and various experimental devices including shock tubes, flow reactors, jet-stirred reactors and spherical combustion bombs.The simulations of the proposed model have also been compared to those from five recently published kinetic models available in the literature. It was found that although these mechanisms generally reproduced well the data for which they were validated, they did not globally capture the combustion characteristics of all of the hydrogen/NOx and syngas/NOx systems.Finally, the proposed model has been used to simulate the formation of NO at practical gas-turbine relevant conditions. A detailed flux analysis has been performed to kinetically explore the NO formation mechanism under various combustion conditions.

Effects of nitrogen dilution on the NOx and CO emission of H2/CO/CH4 syngases in a partially-premixed gas turbine model combustor

In this study, the nitrogen dilution effect in H2/CO/CH4 syngas flames on the characteristics of NOx and CO emissions were investigated in a partially premixed gas turbine model combustor. Nitrogen was diluted into the fuel side and combustor liner temperature, and the flame structures were observed in order to find information that was critical for analyzing the results of the NOx and CO emissions. The addition of nonflammable N2 was effective in decreasing the combustion temperature, which was attributed to the reduction in the reaction rate of syngas oxidation as well as the increase in the overall heat capacity of the air-fuel mixture at a constant heat input. NOx suppression was successfully achieved up to the one digit ppmv level for all heat input and all fuel composition by the 60%-fuel side dilution of nitrogen. However, increases in nitrogen dilution increased the possibility of CO emission; therefore, extremely high CO was emitted in a 100%-CO flame at a low load with 60%-nitrogen dilution reaching 10,000 ppmv. The dilution of nitrogen in syngas considerably changed the flame structure, such as flame length, thickness, angle, location, and intensity. These flame structures provided valuable clues for the analysis of the NOx and CO emission results in relation to the flame temperature results. Based on the integration of the test results, a cause-and-effect diagram of NOx and CO emissions was plotted, and the root causes of NOx suppression were determined to be the decreases in both temperature and residence time in the hot combustion zone, and the critical path of these root causes was clarified. Although the nitrogen dilution technique has been investigated in detail, and the effects are fully understood, the careful monitoring of CO and flame detachment from the fuel nozzle is recommended when applying the nitrogen dilution technique to reduce NOx emission, especially in cases of low load operation. A practical dataset was collected from the combustion test at gas turbine relevant conditions for a wide range of fuel compositions. These results and the knowledge gained from this study could be usefully applied for the environmentally friendly and safe operation of integrated gasification combined cycle and synthetic natural gas firing gas turbine power plants.