Chemical kinetic mechanism study on premixed combustion of ammonia/hydrogen fuels for gas turbine use

To explore the potential of ammonia-based fuel as an alternative fuel for future power generation, studies involving robust mathematical, chemical, thermofluidic analyses are required to progress towards industrial implementation. Thus, the aim of this study is to identify reaction mechanisms that accurately represents ammonia kinetics over a large range of conditions, particularly at industrial conditions. To comprehensively evaluate the performance of the chemical mechanisms, 12 mechanisms are tested in terms of flame speed, NOx emissions and ignition delay against experimental data. Freely propagating flame calculations indicate that Mathieu mechanism yields the best agreement within experimental data range of different ammonia concentrations, equivalence ratios and pressures. Ignition delay times calculations show that Mathieu mechanism and Tian mechanism yield the best agreement with data from shock tube experiments at pressures up to 30 atm. Sensitivity analyses were performed in to identify reactions and ranges of conditions that require optimization in future mechanism development. The present study suggests that the Mathieu mechanism and Tian mechanism are the best suited for the further study on ammonia/hydrogen combustion chemistry under practical industrial conditions. The results obtained in this study also allow gas turbine designers and modelers to choose the most suitable mechanism for combustion studies

Diesel/syngas co-combustion in a swirl-stabilised gas turbine combustor

Multiphase fuel combustion was carried out in a swirl-stabilised combustor with the aim of expanding the fuel flexibility of the gas turbine for, at least, land-based applications. Improved capability of the gas turbine in this regard will not only augur well for energy security but also could be useful in tackling harmful emissions. In the study, varying amounts of syngas was premixed with air and swirled into a burning diesel spray, the flowrate of which was altered to maintain the same heat output at all times. Across the several heat outputs tested, the range of stable flame operation was found to reduce as gas content of fuel mix increased. Moreover, for a combined heat output of 15 kW and a global equivalence ratio of 0.7, a steady increase in flame stability was noted and NOx emissions were found to decrease while CO emissions increased as syngas content in fuel mix increased from 10% to 30%. The increase in flame stability, achieved at the cost of lower heat release rate, was attributed to the changes in reacting flow dynamics evinced by the C2* and CH* species chemiluminescence intensity variation as well as chemical kinetics analysis. The NOX and CO emissions trend was ascribed to increasingly inefficient combustion due to the poorer spray quality obtained from pressure atomiser as liquid flow rate reduces and further worsened by the lower heat release rate and decreasing adiabatic flame temperature as gas ratio of combusted fuel increases

Prediction of novel humified gas turbine cycle parameters for ammonia/hydrogen fuels

Carbon emissions reduction via the increase of sustainable energy sources in need of storage defines chemicals such as ammonia as one of the promising solutions for reliable power decarbonisation. However, the implementation of ammonia for fuelling purposes in gas turbines for industry and energy production is challenging when compared to current gas turbines fuelled with methane. One major concern is the efficiency of such systems, as this has direct implications in the profitability of these power schemes. Previous works performed around parameters prediction of standard gas turbine cycles showed that the implementation of ammonia/hydrogen as a fuel for gas turbines presents very limited overall efficiencies. Therefore, this paper covers a new approach of parameters prediction consisting of series of analytical and numerical studies used to determine emissions and efficiencies of a redesigned Brayton cycle fuelled with humidified ammonia/hydrogen blends. The combustion analysis was done using CHEMKIN-PRO (ANSYS, Canonsburg, PA, USA), and the results were used for determination of the combustion efficiency. Chemical kinetic results denote the production of very low NOx as a consequence of the recombination of species in a post combustion zone, thus delivering atmospheres with 99.2% vol. clean products. Further corrections to the cycle (i.e., compressor and turbine size) followed, indicating that the use of humidified ammonia-hydrogen blends with a total the amount of fuel added of 10.45 MW can produce total plant efficiencies ~34%. Values of the gas turbine cycle inlet parameters were varied and tested in order to determine sensibilities to these modifications, allowing changes of the analysed outlet parameters below 5%. The best results were used as inputs to determine the final efficiency of an improved Brayton cycle fuelled with humidified ammonia/hydrogen, reaching values up to 43.3% efficiency. It was notorious that humidification at the injector was irrelevant due to the high water production (up to 39.9%) at the combustion chamber, whilst further research is recommended to employ the unburned ammonia (0.6% vol. concentration) for the reduction of NOx left in the system (~10 ppm)

Strategies toward experimental assessments of new aviation renewable fuels and blends: The BIOREFLY Project

The reduction of greenhouse gases emissions from the aviation sector is focused on better engine efficiency or optimized flight pathways. However, the most relevant is probably the use of sustainable biofuels. In order to meet the strict jet fuelspecifications for commercial flights, these biofuels(drop-in fuels) must contain only paraffinic hydrocarbons, without heteroatoms. Several renewable aviation fuels have already been certified by ASTM, others are under examination. Anew promising route consists in the thermochemical conversion of lignin, the main co-product from 2nd generation ethanol. The EU FP7 BIOREFLY project will develop a first industrial pre-commercial lignin-to-jet fuel 2000 ty-1demonstration plant. The present work describes strategies, equipment and R&D lines of BIOREFLY, which aims at evaluating the properties of this bio-jet fuel and its blends in view of future ASTM certification. Injection features and the combustion properties of aviation engines will be investigated in an optical combustor rig. Combustion parameters, emissions and chemiluminescence provide fundamental data to understand the combustion behavior for different hydrocarbons species. Tests in micro-gas-turbines (i.e. power generation and APU-derivative units) will assess the effect of fuels in terms of emissions and evaluating their performances

Study on premixed combustion characteristics of co-firing ammonia/methane fuels

Ammonia is believed eventually play an important role in substituting conventional fossil fuels for future energy systems. In this study, to gain a deep insight into the combustion properties of co-firing ammonia/methane fuel blends for the power and steel industry, a detailed chemical-kinetics mechanism model was developed for comprehensively modelling ammonia/methane fuels combustion. Characteristics of ignition delay time, unstretched laminar burning velocity and NO, CO2 and CO emissions in the exhaust gas were obtained over a wide range of equivalence ratios and ammonia fractions. High NO emissions will be a main problem as CO and CO2 emissions tend to drop when adding ammonia into methane. To gain a further understanding of the effect of ammonia substituting methane for combustion use, analyses of laminar premixed flame structures were performed. The impact of ammonia substitution was illustrated by analysing relevant specific radicals. Furthermore, to study the combustion characteristics of ammonia/methane under more practical conditions, effects of engine relevant conditions (elevated pressure and initial temperature) were also studied. Results indicate that pressure has a more prominent effect than initial temperature and there is a good potential that unwanted emissions can be reduced significantly under industrial conditions

Visualisation of turbulent flows in a swirl burner under the effects of axial air jets

Meeting emission regulations represents a real challenge in the power generation sector. Swirl combustors and their operation under lean premixed (LP) conditions are a step towards attaining low emissions, especially NOx formation, while ensuring high efficiency. However, performing modifications on combustors and reaching the requirements of efficient combustion systems is difficult due to many combustion problems such as extinction, low reaction rates, mild heat release, instabilities, and mixing issues. Thus, giving careful attention to the hydrodynamics design of the swirl burners with extensive testing methods in both experimental and numerical approaches is crucial to stabilise the combustion phenomena in gas turbines. As a result, this study employed the implementation of CFD simulations in the design of a 150 kW tangential swirl burner and considered the consequences of 50 LPM diffusive air injection at different positions on three-dimensional isothermal flow field characterizations, especially the turbulence, downstream the burner nozzle. Various mass flow rates from 600 to 1000 l/min were used at atmospheric conditions with a geometrical swirl number of 0.913. Experimental work was conducted with good correlation. It was found that using the air injection system could increase the flashback resistance by affecting the velocity defect downstream the burner nozzle. Moreover, the axial air jet reduces the flow field turbulence at the central recirculation zone (CRZ) tip and hence minimises the flow fluctuations and affect its size and position. CFD results show a very good agreement with Laser Doppler Anemometry (LDA) data acquired from the experimental work

Modeling combustion of ammonia/hydrogen fuel blends under gas turbine conditions

To utilize ammonia as an alternative fuel for future power generation, it is essential to develop combustion chemical kinetic mechanisms which can describe in some detail the reaction characteristics and combustion properties. In the present study, a detailed chemical-kinetics mechanism is developed to validate premixed combustion characteristics of ammonia and hydrogen fuel blends comprehensively. In order to obtain a useful model for gas turbine applications, the proposed kinetic mechanism is verified in terms of NOx emission, laminar burning velocity, and ignition delay times, focusing particularly on elevated conditions which are encountered during gas turbine operation. Results have shown that the proposed kinetic model performs with satisfactory accuracy under different practical equivalence ratio conditions. The comparison with other mechanisms from the literature also demonstrates that the model can comprehensively describe the reaction process of ammonia/hydrogen fuels in terms of different combustion properties especially under gas turbine conditions. Finally, to develop the kinetic model for more practical applications, the proposed mechanism is reduced and appraised in a 2D large-eddy-simulation representing turbulent combustion for ammonia/hydrogen fuels under gas turbine conditions. The reduced mechanism shows good agreement with the parent model, while offering considerably greater computationally efficiency, hence providing optimizm for the application of detailed ammonia chemistry for future CFD analysis under gas turbine combustion conditions

Editorial

No abstract

Coherent structures and their effects on processes occurring in swirl combustors

Access to energy is fundamental to civilisation, both as economic and social yTL development. This is fuelling a growing demand for reliable, affordable and clean energies. The current problems related to climate change have made imperative the search of technologies that can produce higher amounts of energy at lower emission rates. Therefore, technologies such as swirling flows with premixed lean injection have been characterized as one of the most reliable to achieve this objective. However, the use of this technology implicates the appearance of phenomena that have been barely studied such as the manifestation of coherent structures that are crucial for the stability and high efficiency of the combustion process, and which have been assumed from indirect measurements. Moreover, these structures have been recognised as major players in the generation of instabilities such as pressure and heat transfer variations, internal vibrations and flashback into the mixing chambers. Therefore, a better understanding of these structures will allow the design of better burners and a greater control over the former, permitting a more efficient process. This project is intended to reveal some of the characteristics of these structures, showing their high 3 dimensionality and high dependence on geometrical parameters, equivalence ratio, Swirl and Reynolds numbers, amongst other factors. It is recognised how under isothermal conditions the system produces strong Precessing Vortices that are fundamental in the final shape of the flow field, while the Central Recirculation Zones are dependent on the pressure decay ratio inside of the combustion chamber. Combustion conditions showed the high dependence on the method of fuel injection used, with the appearance of stronger structures at lower equivalence ratios when high amounts of premixed gas were pumped into the system and the change in shape of the recirculation zones by using different injectors. Flashback demonstrated to be a factor highly related to the strength of the Central Recirculation Zone for those cases where a Combustion Induced Vortex Breakdown was allowed to enter the swirl chamber, whilst cases where a bluff body impeded its passage showed a considerable improvement to the resistance of the phenomenon. The use of nozzle constrictions also reduced flashback at high Re. All these results were aimed to contribute to better designs of future combustors.EThOS - Electronic Theses Online ServiceGBUnited Kingdo