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• Using plasma to assist ammonia reactivity results in fewer NOx emissions than those in the flames assisted by hydrogen. • Plasma-assisted ammonia combustion is less prone to strain rate than that of hydrogen-assisted ammonia combustion. • High energy plasma discharges broaden the reaction zones by generating radical pools in the flame preheating zone, being more intense under fuel-rich conditions. • High energy plasma discharges activate the DeNOx mechanism in preheating and post-flame zones. • Increasing the mixture pressure deteriorates the impact of plasma on combustion. Such effects are weakly controlled by the reduced electric field. Synergistic effects of nanosecond plasma discharge and hydrogen on the combustion characteristics of ammonia/air are numerically studied under conditions relevant to gas turbine combustion chambers. It is shown that increasing the plasma contribution in assisting the flame results in lower NO X emissions by up to 27% than those in flames assisted by hydrogen for the range of operating conditions considered in this study. Plasma makes the consumption speed of the reactants less prone to the strain rate than that in flames assisted by hydrogen. It is found that discharging plasma with the pulse energy density of 9 mJ/cm3 alongside using 12% hydrogen by volume in the fuel increases the flame speed of ammonia/air to those of conventional fossil fuels such as methane—an improvement that is not achievable by just using hydrogen, even at a high concentration of 30%. Furthermore, raising the pulse energy density beyond a specific value broadens the reaction zones by generating radical pools in the flame preheating zone, which is expedited in fuel-rich conditions with high H 2 fuel fractions. Investigations show that the simultaneous utilization of high-energy plasma and hydrogen reduces the NO X emissions by activating the mechanisms of nitrogen oxide denitrification (DeNO X) in preheating and post-flame zones, being more significant under the lean condition as compared with rich and stoichiometric cases. It is shown that increasing mixture pressure significantly deteriorates the impacts of plasma on combustion. Such unfavorable effects are weakly controlled by changes in the reduced electric field caused by pressure augmentations. [ABSTRACT FROM AUTHOR]

• A novel n-heptane reform gas combustion is investigated using LES and t-PDF model. • The n-heptane reform gas is shown to suppress the ignition and increase the liftoff. • The reforming gas weakens the NTC ignition behavior of n-heptane. • The combustion process involves inner diffusion flame and outer lean premixed flame. • The premixed flame in the surrounding reforming gas/air mixture enhances soot and suppresses NOx emission. This paper presents a large eddy simulation study of n-heptane spray combustion in an n-heptane low temperature reform (LTR) gas environment in a constant volume combustion chamber, under conditions relevant to single-fuel reactivity controlled compression ignition (RCCI) combustion engines. The LTR gas is made up of partially oxidized intermediate species from rich n-heptane/air mixture in an external constant temperature reformer. It is found that a higher reform temperature results in a longer ignition delay time of the n-heptane spray and a higher liftoff length, due to the chemical effect of the LTR gas and the difference in the reaction zone structures. A significantly different spray flame structure is identified in the RCCI case from that of single-fuel spray combustion. After the onset of high temperature ignition, a double-layer flame structure is established in the RCCI case, with a diffusion flame layer and a lean premixed flame layer. The lean premixed flame affects the flow field, which significantly suppresses the mixing around the spray tip. As a result, the RCCI case exhibits a lower NOx formation but a higher soot formation than the single-fuel case. [ABSTRACT FROM AUTHOR]

• CFD simulation of a lab-scale biomass boiler is presented with detailed chemistry. • Multiple tar species are considered to avoid using the global reaction schemes. • Application of selective non-catalytic reduction with ammonia is studied. • Effects of ammonia injection height and flow rate on NO reduction is investigated. In this paper, a numerical study of flow, reactions and NO x emissions from a biomass boiler is presented. Detailed reaction mechanisms for the decomposition of tar species, combustion of hydrocarbons and formation of NO x are employed by adopting appropriate reaction sets from literature. The proposed mechanism is used to perform CFD simulations of a laboratory-scale biomass boiler, and temperature and species concentrations are compared with the experimental data. NO x formation and emissions are studied in detail and the contribution of different pathways for NO x formation are identified. Furthermore, the CFD model is used to study the application of Selective Non-Catalytic Reduction (SNCR) method in this boiler. The effects of height and flow rate of ammonia injection on the performance of SNCR method are studied. It is observed that the SNCR, can reduce up to 63% of NO x emissions. [ABSTRACT FROM AUTHOR]

In the present study, large eddy simulation is used to investigate combustion recession for the Engine Combustion Network Spray A flame at two ambient temperatures (850 K and 800 K) and two injection pressures (100 MPa and 50 MPa). The present numerical results are able to capture different combustion recession phenomena after the end-of-injection (AEOI). With an injection pressure of 100 MPa , the model predicts a "separated" combustion recession at the ambient temperature of 850 K and no combustion recession at the ambient temperature of 800 K , in which both predictions correspond to the measurements. The combustion recession is mainly controlled by the auto-ignition process at the ambient temperature of 850 K. At the ambient temperature of 800 K , the local temperature within the fuel-rich region is not high enough to promote the high-temperature ignition process. As time progresses, the mixture within the fuel-rich region rapidly transitions to become an overly fuel-lean mixture, which further hinders high-temperature ignition to occur. Nonetheless, it is shown that lowering the injection pressure to 50 MPa causes the combustion recession to occur at the ambient temperature of 800 K. This is likely attributed to the low injection case having a lower air entrainment rate AEOI, which causes the mixtures upstream of the lift-off position to transition slower from fuel-rich to fuel-lean mixtures. • LES model captures the evolution of flame structure during combustion recession. • At high P i n j of 100 MPa, combustion recession occurs at 850 K but not at 800 K. • Lowering P i n j to 50 MPa leads to combustion recession in the 800 K spray case. • Air entrainment after EOI affects the fuel–air mixing and combustion recession. • Lowering P i n j leads to lower entrainment rate after EOI and relatively richer mixture. [ABSTRACT FROM AUTHOR]

Progressively stricter pollutant emission targets in international agreements have shifted the focus of combustion research to low carbon fuels. Ammonia is recognized as one of the promising energy vectors for next-generation power production. Due to the low flame speed and high auto-ignition temperature, ammonia is often burned with a high reactivity pilot fuel (e.g. diesel). However, chemical kinetic mechanisms describing the combustion of ammonia and large hydrocarbon fuels (such as n-heptane, a surrogate of diesel) are less developed. In this work, a skeletal chemical kinetic mechanism for n-heptane/ammonia blend fuels is proposed using a joint decoupling methodology and optimization algorithm. A sensitivity analysis of the ignition delay times of the ammonia/n-heptane mixture is performed to identify the dominant reactions. A genetic algorithm is used to optimize the mechanism further. The final skeletal mechanism is made up of 69 species and 389 reactions. The skeletal ammonia/n-heptane mechanism is validated against the experimental data for combustion of pure ammonia, ammonia/hydrogen and ammonia/n-heptane mixtures, including the global combustion characteristic parameters such as ignition delay times measured in shock tubes or rapid compression machines, laminar burning velocities measured in heat flux burners or spherical flame vessels, and species data measured in jet-stirred reactors. Comparing the results from the skeletal mechanism with those from other mechanisms from the literature is conducted to evaluate the mechanism further. The present skeletal mechanism can well predict the combustion processes for a wide range of conditions, and the mechanism is computationally efficient, showing good potential to model ammonia/n-heptane combustion with good accuracy and efficiency. • A skeletal chemical kinetic mechanism for n-heptane/ammonia blend fuels is proposed. • The skeletal mechanism is widely validated against various experimental datasets. • The mechanism shows a satisfactory prediction for a wide range of conditions. • The final skeletal mechanism is made up of 69 species and 389 reactions. [ABSTRACT FROM AUTHOR]

• Fuel-tracer PLIF is used to quantify the equivalence ratio and temperature. • Misfire of PPC is due to synergistic effect of equivalent ratio and temperature. • In high direct injection pressure, the misfire is due to excessive premixing. • In late direct injection timing, the misfire is due to thermodynamic environment. • Misfire region most likely appears when the equivalence ratio is lower than 0.49. To clarify the misfire mechanism is important for stabilizing combustion in partially premixed combustion (PPC) under low load. Fuel-tracer planar laser-induced fluorescence (PLIF), formaldehyde PLIF, flame and OH* natural luminosity imaging were utilized to qualify the local equivalence ratio, low-temperature reaction and the high-temperature flame features in an optical engine. Results show that in high direct injection (DI) pressure (1000 bar), due to excessive premixing, the local equivalence ratio in the initial timing of the high temperature heat release (HTHR) is low. Although the auto-ignition flame kernels are formed in high DI pressure, they cannot stably develop, resulting in misfire during the flame development process. In late DI timing (-5 crank angle degree after top dead center, °CA ADTC), since the whole heat release process occurs in the expansion stroke, the in-cylinder temperature and pressure continue decreasing. Although the local equivalence ratio in some regions is high enough, the in-cylinder thermodynamic environment does not support the generation of more auto-ignition flame kernels, thus a small amount of auto-ignition flame kernels can only develop through flame propagation. In short, the misfire of PPC occurs in regions where the equivalence ratio is low or the in-cylinder thermodynamic environment does not further support flame development. Therefore, the trade-off relationship between equivalence ratio and temperature determines the formation of auto-ignition kernels. The local equivalence ratio and temperature distribution near the initial timing of HTHR is the key factor to ensure the subsequent stable combustion. Taking the ambient pressure of 18 bar as an example, the boundary condition where the autoignition kernels are most likely formed or the charge is most likely ignited by the nearby flame kernels is in the range of 0.53–0.62 for equivalence ratio and 740–757 K for temperature. The misfire region most likely appears when the equivalence ratio is lower than 0.49. It can be concluded that the misfire of PPC results from the synergistic effect of local equivalent ratio and temperature. The controlling parameters of injection pressure and injection timing are actually optimizing the suitable combinations of equivalence ratio and temperature to stabilize combustion. [ABSTRACT FROM AUTHOR]

This paper presents the numerical results of combustion and soot recession in the Engine Combustion Network (ECN) Spray A and D flames using large eddy simulations (LES). The nominal injector nozzle diameters for the ECN Spray A and D are 90 μ m and 186 μ m. A two-equation soot model is implemented to model the soot formation and oxidation processes. The numerical model is validated by comparing the simulated and measured data in terms of ignition delay time (IDT), lift-off-length (LOL), and temporal evolution of soot mass. The combustion and soot recession processes are analyzed after the end-of-injection (AEOI). The combustion recession processes are first driven by auto-ignition wave propagation followed also by the convective flow in both the Spray A and D flames. A separated high-temperature flame structure is observed in the Spray A flame due to having small favorable mixture regions for the auto-ignition to occur upstream of the quasi-steady lift-off position AEOI. In contrast, a spatially-continuous high-temperature flame structure is formed in the Spray D flame due to having more ignitable mixture regions upstream of the quasi-steady lift-off position and a higher heat release. Soot recession is observed in the Spray D flame but not in the Spray A flame. This is attributed to the mixture upstream of the quasi-steady state soot region becoming favorable to promote soot formation in the Spray D flame, but it becomes fuel-lean AEOI in the Spray A flame. • Effects of nozzle diameters on combustion and soot recession are studied. • ECN Spray A shows separated high-T flame structure after end of injection. • ECN Spray D shows spatially-continuous high-T flame structure after end of injection. • Soot recession is present in the ECN Spray D flame, but not in the ECN Spray A flame. [ABSTRACT FROM AUTHOR]

• Investigation on NO, CO and NH 3 emissions from NH 3 /CH 4 /air premixed flames. • NO x emissions increase with fraction of NH 3 up to 0.5, beyond which they decrease. • NO x emissions decrease as equivalence ratio reduces towards fuel-lean conditions. • CO and NH 3 emissions are rather low regardless of the operating conditions. • Kinetic simulations showed fair agreement with the experimental data. Ammonia (NH 3) is regarded as one of the most viable alternatives to produce carbon-free energy. However, its low flammability and high NO x emissions associated with its combustion inhibit the implementation of pure NH 3 as a fuel. Dual-fuel approaches with NH 3 and more reactive fuels have been proposed; however, information on pollutant emissions is still scarce. The present work focuses on quantifying the gaseous pollutant emissions from the combustion of mixtures of NH 3 and CH 4 as a function of the equivalence ratio and amount of NH 3 in the fuel mixture. A premixed laminar burner was fired with various mixtures of NH 3 /CH 4 in air with equivalence ratios of 0.8, 0.9 and 1, and a fixed thermal input of 300 W. The NH 3 molar fraction in the fuel mixture was varied from 0 (pure CH 4) up to 0.7. Gas temperatures along the burner axis and emissions of NO x , CO and NH 3 were measured for all conditions. The results were compared with kinetic simulations using recent chemical kinetic mechanism models. The experimental results showed that the NO x concentration initially increases as the fraction of NH 3 in the fuel mixture rises up to 0.5, decreasing afterwards. Furthermore, NO x emissions decrease as the equivalence ratio is reduced towards fuel-lean conditions. CO and NH 3 emissions are fairly low indicating complete combustion of CH 4 and NH 3. The chemical kinetic mechanisms considered showed fair agreement with the experimental trends, with the NO x and CO emissions being over-predicted for most conditions. [ABSTRACT FROM AUTHOR]