In this study, the effects of nanosecond plasma discharges on the combustion characteristics of ammonia are investigated over a wide range of mixture properties and plasma settings. The results reveal that the impacts of the plasma on ammonia combustion change non-monotonically by altering the reduced electric field value. Within the studied range of the reduced electric field, i.e., 100–700 Td, it is shown that plasma is most effective in the medium range, e.g., 250–400 Td. At lower values, the main fraction of the plasma energy is consumed to excite the diluent to higher vibrational levels. At very high reduced electric field values, a substantial portion of the plasma energy is transferred into the ionization reactions of the diluent, which compromises the effective excitations of fuel and oxidizer species. In terms of the pulse energy density, results indicate that an increase in the range of 0–20 mJ/cm3, at a given reduced electric field, decreases the ignition delay time by five orders of magnitude, and increases the laminar flame speed up to an order of magnitude, depending on the mixture composition. The results show that the plasma discharge produces more radicals, electronically excited and charged species when He is used as the diluent in the oxidizer instead of N2, since NH3 and O2 ionization reactions are strengthened in NH3/O2/He. Moreover, plasma discharge is highly effective in assisting the combustion of preheated lean mixtures. The present study also indicates that ammonia flame thickness is minimum at a critical pulse energy density in the range of 12–14 mJ/cm3. Further increases in the pulse energy density can manipulate the inner structure of the flame, altering the pre-heat zone of the flame to include some levels of chemical reactions toward the flameless mode of combustion.
Ammonia has been proposed as a replacement for fossil fuels. Like hydrogen, emissions from the combustion of ammonia are carbon-free. Unlike hydrogen, ammonia is more energy dense, less explosive, and there exists extensive experience in its distribution. However, ammonia has a low flame speed and combustion emits nitrogen oxides. Ammonia is produced via the Haber-Bosch process which consumes large amounts of fossil fuels and requires high temperatures and pressures. A life cycle assessment to determine potential environmental advantages and disadvantages of using ammonia is necessary. In this work, emissions data from experiments with generating heat from tangential swirl burners using ammonia cofired with methane employing currently available technologies were utilized to estimate the environmental impacts that may be expected. Seven ammonia sources were combined with two methane sources to create 14 scenarios. The impacts from these 14 scenarios were compared to those expected from using pure methane. The results show that using ammonia from present-day commercial production methods will result in worse global warming potentials than using methane to generate the same amount of heat. Only two scenarios, methane from biogas combined with ammonia from hydrogen from electricity and nuclear power via electrolysis and subsequent ammonia synthesis using nitrogen from the air, showed reductions in global warming potential. Subsequent analysis of other environmental impacts for these two scenarios showed potentially lower impacts for respiratory organics, terrestrial acidification-nutrification and aquatic acidification depending on how the burner is operated. The other eight environmental impacts were worse than the methane scenario because of activities intrinsic to the generation of electricity via wind power and nuclear fission. The results show that generating heat from a tangential swirl burner using ammonia currently available technologies will not necessarily result in improved environmental benefits in all categories. Improvements in renewable energy technologies could change these results positively. Other means of producing ammonia and improved means of converting ammonia to energy must continue to be explored.
Renewably generated ammonia offers a form of carbon-free chemical energy storage to meet the differences between uncertain supply and fluctuating demand and has the potential to support future energy requirements.The storage/transportation characteristics of NH3 are favorable compared with H2, however there are combustion research challenges to enhance fuel reactivity whilst reducing harmful emissions production.The purpose of this work was to evaluate different fuel delivery concepts for a representative GT combustor.An experimental and numerical comparison was made between swirl-stabilized premixed and diffusion NH3-air flames at elevated inlet temperature (473 K).The exhaust NOx and NH3 emissions generated from each concept were quantified to optimize combustor performance.High-speed OH* and NH2* chemiluminescence was employed to characterize the change in flame topology with variation in fuel-air equivalence ratio, and the resultant influence on measured emission concentrations. Chemiluminescence intensities were shown to elucidate changes in sampled exhaust emissions, enabling detailed analysis of intermediate chemistry. A comparison was made between experimental data and kinetic simulations, demonstrating the sensitivity of NOx emissions to premixed fuel-air equivalence ratio.A comparison was also made between exclusive primary airflow, and the staged introduction of secondary air, to quantify the change in NOx production between each configuration and improve fuel burnout.Secondary air loadings were incrementally increased through the combustor.Finally, reactant humidification was employed as a secondary process for NOx reduction, having shown favorable performance with NH3-H2 mixtures, with the efficacy compared for both premixed and diffusion configurations.
Blends of green hydrogen-ammonia for Gas Turbines (GT) are gaining more and more interest, as their carbon neutrality can abate CO2 emissions. Such fuel mixture could be particularly effective since the low reactivity of ammonia can compensate the aggressive properties of hydrogen in terms of both flame speed and low heating value, reducing the re-design effort of the traditional dry-low emission combustors. From the environmental perspective, the main drawback is represented by the NOx emissions, mostly related to the fuel-bound pathway activated by the cracking of NH3: its mitigation requires the proper control of the equivalence ratio of the blend along the selected ratio of ammonia and hydrogen in the fuel mixture. Additionally, at relevant GT conditions, a moderate benefit for NOx reduction can be played by the pressure rise. In this work, the effect of the operating pressure onto the NOx emission is investigated numerically in the range 1.1–2 bar along a perfectly premixed mixture of 25 % NH3 – 75 % vol. H2 at constant equivalence ratio, leveraging the corresponding experimental data. The tests employ a radial swirler whose performances are measured not only in terms of NOx but also through detailed flame imaging. The latter is used to evaluate the accuracy of a Thickened Flame Model (TFM) in predicting the flame shape and position. The TFM is based on a skeletal mechanism consisting of 27 species that embeds the OH* to directly compare the numerical line of sight with the images from the experiment and the NOx chemistry as well. Regarding the NOx, the numerical results show a reasonable accuracy: not only can the overall flame length be captured but also a quantitative estimation can be retrieved from the numerical model. Lastly, the impact onto NOx of the hydrogen preferential diffusion related to the flame curvature is discussed.
Exhaust gas recirculation (EGR) is one of the main techniques studied over the years to enable the use of oxyfuel combustion for carbon capture and storage (CCS). However, the use of recirculated streams with elevated carbon dioxide poses different challenges from the control of the flow rates and flue stream characteristics to the suppression of unwanted instabilities during the combustion process. Therefore, this study evaluates the use of various CO2 enriched methane blends and their response towards the formation of a great variety of structures that appear in swirling flows, which are the main mechanism for combustion control in current gas turbines systems. The study uses a 100kW acoustically excited swirlstabilised burner to investigate the flow field response. The results showed improved thermal efficiency of the system with high swirl and forcing while the blend of CO2 with methane balanced the heat release fluctuation with a corresponding reduction in the acoustic amplitudes of the system for a smooth running, suggesting that certain CO2 concentrations in the fuel can provide more stable flames at a certain carbon dioxide concentration.
The use of gas for power generation is likely to increase in the medium term. Also, the introduction of new fuels will ensure a higher generation with lower emissions under continuous operation. These scenarios lead to the conclusion that there will be a considerably more diverse range of fuel supply. However, the use of these new fuels contrasts with recent experiences of global operators who report increasing emissions and difficult combustion dynamics with even moderate variations in their fuel characteristics. Clearly there are significant challenges for fuel flexible gas turbines, particularly emission control, combustor dynamics and flame stability. Trials using a power derivative gas turbine combustor and a high hydrogen content fuel produced unusual flashback events, in that flashback was induced by either leaning of the fuel mixture by the increase of combustion air, or by a change in composition through the reduction of methane pilot fuel. The introduction of CO2 through the combustors pilot injector prevented flashback from occurring under these circumstances. The resulting reduction of temperature in the combustion zone, indicated by lower burner tip temperatures causes a reduction in the emissions of nitrous oxides, whilst there is minimal effect on the effective turbine inlet temperature, only a 2.3% reduction. Investigations using a ‘generic’, radial swirl burner and stereo PIV demonstrated how the flashback depended on a combination of flow structure augmentation and changes in mixture burning rate. The injection of methane or CO2 had differing effect on these parameters of the combustion zone, but both produced combinations that facilitated stability.
