Lean Premixed Flames Research Articles

Investigation of nitric oxide formation in methane, methane/propane, and methane/hydrogen flames under condensing gas boiler conditions

In this study, NOx formation in lean premixed burner-stabilized flames was investigated under condensing gas boiler operating conditions for pure methane, methane/propane, and methane/hydrogen mixtures. Temperature and NOx species profiles were experimentally obtained from thermocouple and wet chemiluminescence detector measurements using in-situ extracted gas samples. Resolved simulations utilizing different NO sub-mechanisms from the GRI 2.11, the GRI 3.0, the CRECK mechanism, and a recent mechanism from Glarborg and co-workers were performed, where the mechanism from Glarborg showed the best agreement with the measured NOx concentrations. While the other mechanisms also agreed fairly well with the experiments, a pathway analysis revealed that this results from error compensation, where the prompt pathway was underestimated, which was balanced by an overestimated contribution of the NNH pathway. Interestingly, the prompt pathway was found to be the major NOx formation route for condensing gas boiler conditions despite the lean mixture. While the temperature level in the post-flame region was too low for the thermal pathway to become dominant, the NNH and N2O pathways reached their chemical equilibrium just downstream of the flame front and therefore did not further contribute to the overall NOx emissions.The partial substitution of methane by hydrogen slightly reduced NOx emissions, which was similarly found in experiment and simulation. The post-flame temperature for methane/hydrogen remained nearly unchanged, because the higher adiabatic flame temperature compared to pure methane was compensated by increased heat losses due to a higher laminar burning velocity. The addition of hydrogen decreased the amount of CH radicals in the flame front, which led to a lower contribution of the most important prompt pathway.Substituting methane by propane increased the temperature level due to a higher adiabatic flame temperature and a slightly smaller laminar burning velocity. While increasing NOx emissions were found in the experiment, the simulation predicted a minimally smaller NOx level compared to pure methane. The higher temperature caused increasing contributions of the thermal, NNH, and N2O pathways. For the prompt pathway, which is again the most important NOx formation route, the temperature trend was superimposed by a decreasing CH concentration in the flame front.

On the spurious entropy generation encountered in hybrid linear thermoacoustic models

This work demonstrates that a hybrid approach for linear thermoacoustic stability analysis that combines the Linearized Navier–Stokes Equations (LNSE) with a global Flame Transfer Function (FTF), generates spurious entropy waves when used to model acoustically forced premixed flames. The inability of the global FTF to account for the effects of flame movement is identified as the root cause of this unphysical behavior. Utilization of a local FTF, which resolves unsteady heat release on scales comparable to the reaction zone of the flame, suppresses the spurious entropy perturbations. This affirms that fine-grained resolution of the spatio-temporal distribution of heat release rate fluctuations in the combustion zone is required to model the movement of the flame front, even for acoustically and convectively compact flames. As an alternative to hybrid models, a Linearized Reactive Flow (LRF) approach is employed, which extends the LNSE by the linearized species transport equations as well as the reaction mechanism. Such a monolithic approach inherently accounts for the locally resolved flame dynamics, including the movement of the flame front, and does not require an external model for the flame-flow interaction. Thus the LRF eliminates the need for the cumbersome identification of a local FTF. Two configurations of lean premixed methane-air flames, i.e. a freely propagating 1D flame and a 2D flame anchored in a duct, are considered for validation. All results obtained with linearized modeling approaches and conclusions deduced thereof are validated against high resolution CFD results with excellent quantitative accuracy.

Effect of hydrogen enrichment on flame broadening of turbulent premixed flames in thin reaction regime

An experimental study to identify the effect of hydrogen enrichment and differential diffusion on the flame broadening is conducted. Turbulent lean premixed flames in the Broadened Preheat–Thin Reaction (BP-TR) regime are obtained. The flames are stabilized on a Bunsen burner and CH4/H2/air mixtures are adopted with three hydrogen fractions of 0, 30% and 60%. The preheat zone and heat release zone are captured with the multi-species Planar Laser-Induced Fluorescence (PLIF) of OH and CH2O radicals. Flame thicknesses of the preheat and heat release layers are measured. Results show broadened preheat zone and thin heat release layers for the flames, as predicted by the BP-TR regime. The preheat zone thickness can be increased to about 3–6 times compared to the laminar preheat thickness. An apparently decreased preheat zone thickness with hydrogen addition is observed. The differential diffusion is anticipated to locally thicken the heat release zone along the flame front. The mean heat release thickness is nearly not affected by the turbulence or hydrogen addition.

Explosive dynamics of bluff-body-stabilized lean premixed hydrogen flames at blow-off

Two-dimensional direct numerical simulation (DNS) databases of bluff-body-stabilized lean hydrogen flames representative of complicated reactive–diffusive system are analysed using the combined approach of computational singular perturbation (CSP) and tangential stretching rate (TSR) to investigate chemical characteristics in blow-off dynamics. To assess the diagnostic approaches in flame and blow-off dynamics, Damköhler number and TSR variables are applied and compared. Four cases are considered in this study showing different flame dynamics such as the steadily stable mode, local extinction by asymmetric vortex shedding, convective blow-off and lean blow-out. DNS data points in positive explosive eigenvalue conditions were subdivided into four different combinations in TSR and extended TSR space and categorized in four distinct characteristic regions, such as kinetically explosive or dissipative and transport-enhanced or dissipative dynamics. The TSR analysis clearly captures the local extinction point in the complicated vortex shedding and allows an improved understanding of the distinct chemistry-transport interactions occurring in convective blow-off and lean blow-out events.

Simultaneous 10 kHz three-dimensional CH2O and tomographic PIV measurements in a lifted partially-premixed jet flame

High-speed, three-dimensional (3D) scalar-velocity field measurements are demonstrated in a lifted partially-premixed dimethyl-ether/air jet flame using simultaneous laser-induced fluorescence (LIF) of formaldehyde (CH2O) and tomographic particle image velocimetry (TPIV). The 3D LIF measurements are performed by raster scanning the laser beam from a 100 kHz pulse-burst laser across the probe volume using an acousto-optic deflector. The volumetric reconstruction of the LIF signal from ten parallel planes provides quasi-instantaneous 3D LIF measurements that are synchronized with 10 kHz TPIV measurements. The temporally resolved CH2O-LIF and velocity field data are used to analyze Lagrangian particle trajectories and displacement speeds at the base of the lifted flame. The particle trajectories reveal flow structures that are difficult to observe in an Eulerian reference frame. Positive and negative displacement speeds are observed at the CH2O-LIF surfaces at the inner and outer regions of the jet flame with a maximum displacement speed of approximately eight times the laminar flame speed of a stoichiometric dimethyl-ether/air mixture. The observations in present study support formation of lean premixed flames near the flame base whose role in flame stabilization should be investigated in more detail.

Absorption, quenching, and enhancement by tracer in acetone/toluene laser-induced fluorescence* *Project supported by the National Natural Science Foundation of China (Grant Nos. 51536002, 61405048, and 91441130).

To measure the equivalent ratio distribution of the two-stage lean premixed (DLP) flame, we propose using acetone/toluene planar laser-induced fluorescence (PLIF) technology to simultaneously measure the concentrations of the two components. Appropriate excitation laser wavelength and filters are used to assess the influence of acetone and toluene on each other’s fluorescence signal at room temperature. Experimental results show that acetone has a strong absorption effect on toluene’s fluorescence signal, the effective absorption cross-section is 5.77 × 10−20 cm−2. Acetone has an obvious quenching effect on the toluene fluorescence signal, and the Stern–Volmer coefficient is 0.50 kPa−1. The collisions between the molecules of toluene and acetone will lead to the enhancement of the fluorescence signal of acetone, and the enhancement coefficient is exponential with the acetone’s concentration. The quantitative relationship between the fluorescence intensity and the concentrations of the two tracers is obtained by establishing the photophysical model of toluene and acetone’s fluorescence signals.

Strain rate and pressure effects on multi-branched counterflow flames

This study presents methane-air counterflow simulations, in computationally efficient similar form, allowing combustible mixtures to flow from one or both directions in order to learn more about multi-branched propagating flame structures (e.g., a triple flame). These structures with both premixed and non-premixed flames are commonly seen in more practical combustion analyses. A range of realistic mass mixture fractions and asymmetric chemical rate laws are examined while avoiding the commonly forced unreal symmetric behavior with one-step second-order kinetics. Moreover, a survey of critical parameters is performed varying pressure and normal strain rate to define the flame structure and detect different characters. Three flames can co-exist if the strain rate is low enough and the pressure is high enough. However, at higher strain rate and/or lower pressure, only one or two flames might be obtained. Negative regions of heat release rate are observed and linked to potential endothermic reactions. With a rich premixed mixture at low strain rates and pressures, high exothermic reactions producing CO2 and H2O, and consuming CO and H2 cause a heat-release-rate peak. Unexpected character of the lean and rich premixed flames is observed, leading to the conclusion that these flames are diffusion controlled.

Numerical study of interaction of coal dust with premixed fuel-lean methane-air flames

Interaction of combustible particles with flames occurs in fire scenarios. In this study, numerical investigation of interaction of coal dust or micron-sized particles with lean premixed methane-air flames is presented. A two-dimensional axisymmetric domain is employed to simulate conical premixed flames from lab-scale Bunsen burner. A chemical kinetic mechanism having 25 species and 121 elementary reactions, temperature dependent thermo-physical properties, multi-component diffusion with Soret effect and radiation model accounting for gas and soot radiation are used. Discrete Phase Model (DPM) is used to simulate the transport of coal particles. Coal particles in varies size ranges and concentrations are injected into the premixed reactant mixture at equivalence ratio between 0.75 and 0.85. Multiple species from devolatilization of coal particles are considered to enter the gas-phase. Laminar flame speeds are predicted using numerical shadowgraphs and validated against the experimental data from literature. Injection of coal particles affects the laminar burning velocity and flame structure. The numerical model is able to predict the variation trends in the laminar flame speed data quite reasonably. A detailed analysis of injection of coal particles on the resultant flame dynamics are presented using the fields of temperature, flow, species, net reaction rate, heat release rate and DPM.

Enhancement of the lean stability and blow-off limits of methane-air swirl flames at elevated pressures by nanosecond repetitively pulsed discharges

This paper examines the ability of nanosecond repetitively pulsed (NRP) plasma discharges to improve stabilization and extend the blow-off limit of lean premixed methane-air swirl flames at pressures up to 5 bar. The effect of two discharge regimes, NRP glows and NRP sparks, was investigated. The electrical characterization of the discharges was performed and direct images at 60 Hz of the flames, with and without NRP discharges, were collected to assess the effect of the discharges on flame stabilization. Results showed that NRP discharges efficiently extended the lean blow-off and stability limits of premixed methane-air swirl flames, at pressures up to 5 bar. These results were obtained for a ratio of NRP discharge power to flame thermal power of 0.7% or less. Moreover, the peak voltage necessary to maintain constant this power ratio did not increase linearly with increased pressure, even though the reduced electric field should linearly decrease with the pressure. It was also observed that the relative effectiveness of the NRP glows and NRP sparks changed by increasing the pressure. Based on discharge physics and current knowledge of the effect of pressure on the electrical properties of flames, explanations for these results are proposed.

Response of lean premixed swirl tubular flame to acoustic perturbations

This study experimentally examined the response of a lean premixed methane/air swirl tubular flame to the longitudinal acoustic perturbations. Experiments were conducted by varying the acoustic frequency (f0), acoustic power (P0) and inlet air flow rate (QA) to systematically study the acoustic-excited flame dynamics, including the flame stability, flame structure, heat release rate and acoustic pressure fluctuations. The results demonstrated that the lean tubular flame was much sensitive to longitudinal perturbations below 210 Hz. Even at low power of P0, the flame was forced to oscillate intensively when the perturbation was operated in two discontinuous frequency ranges of f0 = 150–200 and f0 = 50–100 Hz, while a weakened oscillation regime was observed in the middle range of f0 = 100–150 Hz; a lower frequency (less than 50 Hz) would extinguish the flame. An increase in P0 intensified the flame oscillation under the perturbations of 50–100 Hz, and even resulted in flame lift-off and blow off with f0 between 150 and 200 Hz. In comparison to f0 and P0, the effects of air flow rate (QA) seemed to be weaker. The proper orthogonal decomposition (POD) analysis was performed to interpret the contributions from the mean mode and fluctuating axial and circumferential modes that led to combustion instability. It was found that as the energy fraction of the axial mode exceeded 5.0%, the flame lift-off appeared in the lean swirl tubular configuration.

Thermoacoustic analysis of lean premixed hydrogen flames in narrow vertical channels

Thermoacoustic instabilities arise for lean hydrogen-air flames propagating in narrow channels. We provide here a detailed experimental analysis of such phenomena in a semi-confined vessel, analyzing the effect of the mixture composition, geometry and gravity on the onset of acoustic-driven flame vibrations.Downward-propagating flames leaner than a critical value vibrate smoothly and transit to the secondary oscillating instability, which develops strong variations of pressure that couple with the propagation dynamics. The transition threshold changes during the propagation along very narrow channels, where heat losses are no longer negligible. The parametric region of equivalence ratio for the secondary thermoacoustic instability diminishes, showing an additional transition for very lean flames. There, the front breaks into several structures and the flame-wave feedback becomes weaker.The influence of gravity is studied by comparing upward and downward propagating flames, where the Rayleigh–Taylor instability arises for sufficiently small values of the Froude number in slow-propagating lean flames. For a constant mixture, buoyancy-driven upward-propagating flames develop less wrinkled fronts than those propagating downwards, and remain unresponsive to acoustic-front interaction. We show here a direct relation between front shape and thermoacoustics.In agreement with previous studies [1–3], curvature and strain effects on conduction and diffusion characterize the response of the flame to pressure perturbations, with the Markstein number controlling the aforementioned transition. Nevertheless, the theoretical analyses found in the literature can only be used on nearly equidiffusional mixtures, and are not accurate enough to describe the highly diffusive fuel mixtures (i.e. lean hydrogen-air flames) considered in our experiments.

A numerical study of combustion and NOX emission characteristics of a lean premixed model gas turbine combustor

The combustion characteristics of a model gas-turbine combustor are investigated by performing 3-D Reynolds-averaged Navier-Stokes equations (RANS) simulations of swril-stabilized lean-premixed flames of methane/air for different equivalence ratio and swirler type. Their NOX emission characteristics and thermal fatigue life are also elucidated with chemical reactor network (CRN) analysis and finite element analysis (FEA). To validate numerical models, the RANS simulations results are compared to experimental data, which shows reasonable agreement between them. The CRN analysis reveals that the amount of NOX can be reduced by low temperature and short residence time. The FEA identifies areas vulnerable to thermal fatigue. The largest strain amplitude is observed at a single nozzle exit because of large temperature variation and complicated structure. The cycles to failure at the vulnerable regions are estimated using the e-N curve of AISI type 304 stainless steel.

Effect of differential diffusion on turbulent lean premixed hydrogen enriched flames through structure analysis

This study presents the flame structure influenced by the differential diffusion effects and evaluates the structural modifications induced by the turbulence, thus to understand the coupling effects of the diffusively unstable flame fronts and the turbulence distortion. Lean premixed CH4/H2/air flames were conducted using a piloted Bunsen burner. Three hydrogen fractions of 0, 30% and 60% were adopted and the laminar flame speed was kept constant. The turbulence was generated with a single-layer perforated plate, which was combined with different bulk velocities to obtain varied turbulence intensities. Quasi-laminar flames without the plate were also performed. Explicit flame morphology was obtained using the OH-PLIF. The curvature, flame surface density and turbulent burning velocity were measured. Results show that the preferential transport of hydrogen produces negatively curved cusps flanked with positively curved bulges, which are featured by skewed curvature pdfs and consistent with the typical structure caused by the Darrieus-Landau instability. Prevalent bulge-cusp like wrinkles remain with relatively weak turbulence. However, stronger turbulence can break the bulges to be finer, and induce random positively curved cusps, therefore to destroy the bulge-cusp structures. Evident positive curvatures are generated in this process modifying the skewed curvature pdfs to be more symmetric, while the negative curvatures are not affected seriously. From low to high turbulence intensities, the hydrogen addition always strengthens the flame wrinkling. The augmentation of flame surface density and turbulent burning velocity with hydrogen is even more obvious at higher turbulence intensity. It is suggested that the differential diffusion can persist and even be strengthened with strong turbulence.

Effect of hydrogen enrichment on swirl/bluff-body lean premixed flame stabilization

This paper reports the mechanism of hydrogen enrichment in stabilizing swirl/bluff-body CH4/air lean premixed flame. Large Eddy Simulation (LES) coupled with Thickened Flame (TF) model was performed to resolve the turbulent reacting flow. A detailed chemistry was used to describe the oxidization of CH4/H2/air mixtures. Particle Image Velocimetry (PIV) and Planar Laser-Induced Fluorescence of OH (OH-PLIF) simultaneous measurements were conducted to obtain the velocity fields and flame structures respectively. The numerical methods were validated by experimental data and showing good agreements. Both the experimental and numerical results show that, the flame brush attachment tends to leave the inner shear layer with increasing hydrogen addition, which will reduce the risk of flame lift-off. The chemical analyses prove that the attachment of CH4/air flame is inherently weak. On the one hand, the CH4/air flame is stabilized by the hot products inside the recirculation. On the other hand, the burnt gas suppresses the oxidation of H2 and CO through H2 + OH = H + H2O and CO + OH = CO2 + H, respectively. Although the proportion of CH4 decomposition through CH4 + OH = CH3 + H2O will be reduced by hydrogen addition, the path of CH4 + H = CH3 + H2 will be enhanced significantly. Hydrogen addition will not only increase the overall reaction rate, but also change the combustion intensity at the nozzle exit from relatively weak to strong, which is also important for flame stabilization. The robust flame attachment obtained by hydrogen addition can attributed to the enhanced reactions of H2 + OH = H + H2O and CH4 + H = CH3 + H2.

Effect of waste cooking oil methyl ester – Jet A-1 fuel blends on emissions and combustion characteristics of a swirl-stabilized lean pre-vaporized premixed flame

In the current work, Lean Pre-vaporized Premixed (LPP) combustion concept was used to investigate the flame characteristics of blended fuels using 5, 10, 15, and 20% blending ratios of Waste Cooking Oil Methyl Ester (WCOME) with Jet A-1 (symbolized as B5, B10, B15, and B20). Here, WCOME was produced from raw waste cooking oil by transesterification process with the help of ultrasonicator to reduce the production time. The flame was stabilized using a swirl burner characterized by the formation of central recirculation zone with Swirl Number (SN) equal 0.55; commonly called High Swirl Burner (HSB). The incoming air was preheated to a constant temperature of 250 °C and mixed with air at equivalence ratio φ of 0.75 for all tested fuel blends. Comparing to neat Jet A-1 fuel flame, flames of WCOME/Jet A-1 fuel blend revealed great effect of adding biodiesel to Jet A-1 on local and global emission characteristics of LPP combustion with comparable temperature distributions. Results showed a significant reduction in CO and NOx emissions indices for B20 at combustor exit by 41% and 30%, respectively, compared to those for pure Jet A-1 fuel. Aiming to increase the share of biofuel in fuel combustion, it can be concluded that, Jet A-1 fuel can be replaced by B20 without any modifications in the combustor design as they have similar temperature distribution with potential to remarkably reduce emissions.

Mutual synchronization of two flame-driven thermoacoustic oscillators: Dissipative and time-delayed coupling effects.

Low-emissions can-annular gas turbines are prone to develop low-frequency self-excited thermoacoustic oscillations. Such oscillations arise from the coupling between adjacent combustors and can increase wear and thermal stresses. In this experimental study, we explore the mutual synchronization of two thermoacoustic oscillators (i.e., two model combustors) interacting via dissipative and time-delayed coupling, as introduced via a cross-talk section. Unlike most previous studies, our study makes use of a turbulent lean-premixed flame in each combustor, bringing the system configuration closer to that of practical gas turbines. Using stationary and transient measurements, we examine the effect of the cross-talk diameter and length so as to gain insight into the effect of dissipative and time-delayed coupling. We find that strengthening the dissipative coupling promotes mutual synchronization, but that weakening the dissipative coupling leads to weakly coupled or desynchronized oscillations. On operating the two combustors at different conditions, we find a significant reduction in their overall oscillation amplitude for some coupling conditions. On varying the combustor length and examining the transient response, we find elaborate changes in the pressure-heat-release-rate coupling, spontaneous mode transitions between coupled thermoacoustic modes, and the emergence of a rhomboid structure in the phase plane owing to the coexistence of in-phase and out-of-phase synchronization. In the combustion community, these two types of synchronization are known to be associated with push-push modes and push-pull modes. These findings offer new insight into the mutual synchronization of low-frequency, self-excited thermoacoustic oscillations in can-annular gas turbines, paving the way for the development of improved control strategies.

Impact of burner plenum acoustics on the sound emission of a turbulent lean premixed open flame

The acoustic field of a turbulent lean cpremixed open flame is numerically investigated by a hybrid method solving the Navier-Stokes equations in a large-eddy simulation formulation and the acoustic perturbation equations. The interaction of acoustic modes of a burner plenum and the turbulent flame is analyzed with respect to the sound emission of the flame. It is investigated if a simplified computation yields a good broadband agreement of the sound pressure spectrum with experimental measurements. The results of two numerical setups, i.e., the first configuration consists of the burner plus the plenum geometry while in the second configuration the plenum is neglected, which is often done in technical applications due to computational efficiency reasons, are compared with experimental findings. It can be concluded that the plenum has a pronounced impact on the dynamics and combustion noise of the open flame. To be more precise, the comparative juxtaposition of the numerical and experimental results shows a good agreement only for the full burner-plenum computation since the interaction of the acoustic quarter-wave modes of the burner plenum with the jet flow has to be captured. The interaction of these quarter-wave modes with the flow is analyzed and the acoustic response to heat release fluctuations of the flame of the full burner-plenum computation is compared to that of the simplified burner computation, in which the plenum acoustics is neglected. Due to the excitation by the plenum acoustics, the jet flow of the full burner plenum contains higher turbulent kinetic energy and the flame is excited at several additional frequencies which result in distinct peaks in the acoustic spectrum and a higher overall sound pressure level.

Unstable behavior of hydrogen-air lean premixed flames (Effects of unburned-gas temperature, heat loss and scale)

Unstable behavior of hydrogen-air lean premixed flames (Effects of unburned-gas temperature, heat loss and scale)

Fuel and Equivalence Ratio Effects on Transfer Functions of Premixed Swirl Flames

This paper reports on the effects of fuel and equivalence ratio on the response of lean premixed swirl flames to acoustic perturbations of the flow, at atmospheric pressure. The response is analyzed using flame transfer functions, which relate the relative heat release rate fluctuations from the flame to the relative velocity fluctuations of the incoming flow. Two fuels, propane and methane, and five equivalence ratios are considered. The 10 flames investigated are selected to exhibit the local maximum of the transfer function gain around the same frequency, 176 Hz. The results show that changing fuel and equivalence ratio influences both the gain and the phase of the transfer function. The changes observed at 176 Hz, where the dynamics of the flame is mainly controlled by the flame vortex roll-up mechanism, are discussed. Based on the analysis of the flowfields and the flame wrinkling, the laminar burning velocity and the flame temperature are identified as the main parameters controlling the gain. They have two competing effects: first, by enhancing the flame vortex roll-up and, second, by affecting the strength of the vortex generated by the acoustic forcing due to changes in the height of the flame stabilization location.

Effects of non-thermal plasma on the lean blowout limits and CO/NOx emissions in swirl-stabilized turbulent lean-premixed flames of methane/air

This study investigates experimentally the effects of non-thermal plasma (NTP) induced by a dielectric barrier discharge (DBD) reactor on the characteristics of swirl-stabilized turbulent lean-premixed methane/air flames in a laboratory scale combustor by systematically varying the applied AC voltage, VAC, and frequency, fAC. Especially, it is elucidated how the NTP influences the lean blowout (LBO) limits and the characteristics of CO/NOx emissions depending on flame configuration. Without applying the NTP as the mixture equivalence ratio, ϕ, decreases from the stoichiometry to an LBO limit, the flame configuration changes from an M-flame (Regime I) to a conical flame (Regime II) and to a columnar flame (Regime III) for the whole range of the mixture nozzle exit velocity, U0, (4–10 m/s). With the NTP, however, it exhibits only Regimes I and II at relatively-low U0 range (4–6 m/s), while all three regimes at relatively-high U0 range (7–10 m/s). For both velocity ranges, the LBO limits are significantly extended by the NTP enhancing the flame stability. Under the relatively-low U0 range, streamers induced by the DBD reactor play a critical role in stabilizing the flames such that the degree of extension of the LBO limit depends linearly on VAC and fAC. Under the relatively-high U0 range, however, ozone generated by the DBD reactor in Regime III is found to be a major reason in extending the LBO limit, which is substantiated by another flame regime diagram with ozone addition only, and hence, the extension of LBO limit minimally depends on fAC. Simultaneously, the NTP considerably reduces CO emission, while slightly increases NOx emission near the LBO limits due to the enhanced combustion by ozone.