Search Results (87)

Deep decarbonization of high-temperature industrial furnaces and kilns is essential for reducing greenhouse gas emissions in energy-intensive sectors. Hydrogen and hydrogen-enriched fuels are promising alternatives to conventional fossil fuels; however, their integration is not a straightforward fuel replacement. Owing to hydrogen’s high laminar burning velocity, wide flammability limits, low volumetric heating value, and water-vapor-rich combustion products, hydrogen substitution can substantially alter flame stability, heat transfer pathways, pollutant formation, and material service behavior. This review systematically summarizes the key technologies and application progress of hydrogen-based fuel substitution in high-temperature industrial systems. First, the thermophysical and kinetic differences between hydrogen and hydrocarbon fuels are analyzed. Subsequently, core enabling technologies are discussed, including flashback prevention, low-NOx combustion control, thermal-flow-field regulation, heat transfer optimization, and material compatibility under high-temperature, water-vapor-rich atmospheres. Application progress in representative scenarios—including metallurgy, heat treatment, petrochemical-fired heaters, waste treatment, rotary kilns, and cremation furnaces—is reviewed to identify scenario-specific constraints. The review indicates that successful hydrogen substitution requires a transition from isolated burner optimization toward system-level integration of combustion control, heat transfer management, emission mitigation, and material adaptation. Future research should prioritize integrated furnace design, long-term material service assessment, multi-fuel operating strategies, and data-driven control frameworks. Full article

Ammonia, as a carbonless carrier of energy, presents considerable potential for hydrogen storage and production, as well as for power generation, thanks to its high energy density and relatively easy transportability. However, the practical adoption of ammonia in combustion systems faces major stability challenges—chiefly its low reactivity, slow laminar burning velocity, narrow flammability envelope, and high ignition temperature. These attributes increase the risks of flame instability, misfire, and incomplete combustion, which, in turn, can elevate levels of unburned ammonia and greenhouse gas emissions such as NOx—posing significant health and climate concerns. Stable ammonia combustion demands optimization of several interrelated factors: the air–fuel equivalence ratio, flame temperature, flow regime, and combustor design are critical for maintaining reliable operation. Particularly pivotal is the control of the air–fuel equivalence ratio; excessively lean conditions can trigger flameout. Modern systems utilize real-time monitoring of flame and exhaust properties to diagnose and prevent instabilities. Advanced combustion strategies, such as transitioning to diffusion or flameless (MILD) regimes, substantially expand the stable operating window, especially under lean conditions. Overall, sustaining stable ammonia combustion is essential for maximizing efficiency and emission control, and integrating aftertreatment (deNOx) technologies is crucial for sustainable, clean-energy implementation. Full article

The combustion of candles is known to emit various air pollutants, including particulate matter (PM) and volatile organic compounds (VOCs), into the air. This study characterizes emissions of these pollutants from natural and synthetic candles in a standard, sealed, unventilated residential environment. In addition, computational fluid dynamics (CFD) modeling was used to study the potential effects of inlet air velocity on a paraffin candle flame. A laminar diffusion flame model simulated the distributions of temperature, CO₂, and H₂O. A Testo DiSC mini air sampler was used for ultrafine particles and Lung-Deposited Surface Area (LDSA) data collection, and a CEM DT-9881 sampler was used for recording larger particle number concentrations, temperature, and relative humidity. VOC sorbent tubes were used for the collection of individual and total VOCs. Study findings showed that natural candles produced significantly (p < 0.05) higher LDSA ranges (mean 195.2 µm²/cm³) and ultrafine particle concentrations (mean 8.4 × 10¹¹ No/m³), while paraffin wax synthetic candles exhibited higher 0.3–10 µm PM concentrations (mean 2.0 × 10⁷ No/m³). CFD modeling showed that increasing air velocity produced a shorter, more compact flame and reduced CO₂ and H₂O mass fractions due to enhanced mixing and aerodynamic dilution, highlighting the strong interaction between airflow, temperature, and product formation in laminar paraffin flames. Full article

Accurate and efficient modeling of laminar premixed flames is essential for chemical mechanism validation and parametric studies in combustion science. For this purpose, CombF was developed—a semi-analytical computational framework for one-dimensional (1D) laminar premixed flames—offering flexible control over nodal distributions and optional incorporation of experimental temperature data. Unlike conventional fully coupled solvers, CombF explicitly separates the initialization and solution stages, enabling structured control over intermediate structure and temperature constraints while preserving physical consistency. The methodology employs linear interpolation between pre- and post-reaction equilibrium states, adaptive grid refinement, and finite-difference solutions of species and energy conservation equations, with radiation heat transfer optionally included. CombF was validated for ethylene–air premixed flames by comparison with experimental data under varying equivalence ratios and inlet velocities using the YARC-AF kinetic mechanism, and for methane–air premixed flames by additional benchmark comparisons with Cantera, employing the DRM22 mechanism. CombF predictions were further validated against methane and propane–air flames under varying inlet compositions and velocities using the Diego mechanism and evaluated using the curve matching (CM) score, L₂ norms, and phase shift alignment via a nonparametric bootstrap approach. The results demonstrate strong agreement for major species (CO₂, H₂O), while intermediate species (CO, CH₂O) show higher sensitivity to temperature profile choice and nodal resolution, providing a more discriminating assessment of model fidelity. Incorporating experimental temperature fields substantially improves species distribution accuracy and structural alignment. Thus, CombF provides a reliable, flexible, and experimentally adaptive framework that is capable of accurately capturing flame structures, offering a practical tool for preliminary analyses, parametric exploration, and instructional applications in combustion research. Full article

Direct-fired supercritical CO₂ cycles are considered a promising way to reduce CO₂ emissions in the energy sector. One of the key elements of such cycles is a combustor, in which natural gas is burned at supercritical pressures up to 300 atm in an O₂/CO₂ environment. Understanding the chemical combustion kinetics of C1–C4 alkanes, the main components of natural gas, in a supercritical CO₂-diluted medium is important for designing such combustors. This article provides an overview of studies on the chemical kinetics of C1–C4 alkanes combustion in CO₂ at ultra-high pressures. It has been established that with increasing pressure, regardless of the diluent, CH₃O₂ and HO₂ chemistries start to significantly influence the combustion of alkanes, but at the moment this influence is not sufficiently understood. Influence of CO₂ dilution on kinetics is mainly thermal, but the chemical effect is also significant. At the same time, the direct chemical effect of CO₂ is more important for the laminar burning velocity, while the indirect third-body effect is more important for the ignition delay time. However, the available literature lacks experimental measurements of the laminar burning velocity in a CO₂ environment at pressures above 70 atm, which limits the current understanding of chemical kinetics at supercritical pressures. Full article

As a zero-carbon fuel, ammonia holds significant potential for achieving the “dual carbon” strategic goals. However, its extremely low laminar burning velocity (LBV) limits its direct application in combustion systems. This work systematically reviews the research progress on the LBV of ammonia flames, focusing on three key aspects: measurement methods, effects of combustion conditions, and reaction kinetic models. In terms of measurement methods, the principles, applicability, and limitations of the spherical outwardly propagating flame method, Bunsen-burner method, counter-flow flame method, and heat flux method are discussed in detail. It is pointed out that the heat flux method and counter-flow flame method are more suitable for the accurate measurement of ammonia flame LBV due to their low stretch rate and high stability. Regarding the effects of combustion conditions, the LBV characteristics of pure ammonia flames under ambient temperature and pressure are summarized. The influence patterns of three factors on LBV are analyzed systematically: blending high-reactivity fuels (e.g., hydrogen and methane), oxygen-enriched conditions, and variations in temperature and pressure. This analysis reveals effective approaches to improve ammonia combustion performance. Furthermore, the promoting effect of high-reactivity fuel blending on liquid ammonia combustion was also summarized. For reaction kinetic models, various chemical reaction mechanisms applicable to pure ammonia and ammonia-blended fuels (ammonia/hydrogen, ammonia/methane, etc.) are sorted out. The performance and discrepancies of each model in predicting LBV are evaluated. It is noted that current models still have significant uncertainties under specific conditions, such as high pressure and moderate blending ratios. This review aims to provide theoretical references and data support for the fundamental research and engineering application of ammonia combustion, promoting the development and application of ammonia as a clean fuel. Full article

The operational optimization of industrial boilers utilizing hydrogen-enriched natural gas is constrained by two critical gaps: insufficient understanding of the coupled effects of hydrogen blending ratio, equivalence ratio, and boiler load on combustion performance—compounded by unresolved challenges of combustion instability, flashback, and elevated NO_x emissions—and a lack of systematic investigations combining these parameters in industrial-scale systems (prior studies often focus on single variables like hydrogen fraction). To address this, a comprehensive computational fluid dynamics (CFD) analysis was conducted on a 2.1 MW industrial boiler, employing the Steady Laminar Flamelet Model (SLFM) with a modified k-ε turbulence model and the GRI-Mech 3.0 mechanism. Simulations covered hydrogen fractions (f(H₂) = 0–25%), equivalence ratios (Φ = 0.8–1.2), and load conditions (15–100%). All NO_x emissions reported herein are normalized to 3.5% O₂ (mg/Nm³) for regulatory comparison. Results show that increasing the hydrogen content raises the flame temperature and NO_x emissions while reducing CO and unburned hydrocarbons; a higher equivalence ratio elevates temperature and NO_x, with Φ = 0.8 balancing efficiency and emission control; and reducing load significantly lowers furnace temperature and NO emissions. Notably, the boiler’s unique staged-combustion configuration (81% fuel supply to the central rich-combustion nozzle, 19% to the concentric lean-combustion nozzle) was found to mitigate NO_x formation by 15–20% compared to single-inlet burner designs, and its integrated cyclone blades (generating maximum swirling velocity of 14.2 m/s at full load) enhanced fuel–air mixing, which became particularly critical for maintaining combustion stability at low loads (≤20%) and high hydrogen blending ratios (≥20%). This study provides quantitative trade-off insights between combustion efficiency and pollutant formation, offering actionable guidance for the safe, efficient operation of hydrogen-enriched natural gas in industrial boilers. Full article

To mitigate explosion hazards arising from hydrogen leakage and subsequent mixing with air, the injection of inert gases can substantially diminish explosion risk. However, prevailing research has predominantly characterized inert gas dilution effects on explosion behavior under quiescent conditions, largely neglecting the turbulence-mediated explosion enhancement inherent to dynamic mixing scenarios. A comprehensive investigation was conducted on the combustion behavior of 30%, 50%, and 70% H₂-air mixtures subjected to jet-induced (CO₂, N₂, He) turbulent flow, incorporating quantitative characterization of both the evolving turbulent flow field and flame front dynamics. Research has demonstrated that both an increased H₂ concentration and a higher jet medium molecular weight increase the turbulence intensity: the former reduces the mixture molecular weight to accelerate diffusion, whereas the latter results in more pronounced disturbances from heavier molecules. In addition, when CO₂ serves as the jet medium, a critical flame radius threshold emerges where the flame propagation velocity decreases below this threshold because CO₂ dilution effects suppress combustion, whereas exceeding it leads to enhanced propagation as initial disturbances become the dominant factor. Furthermore, at reduced H₂ concentrations (30–50%), flow disturbances induce flame front wrinkling while preserving the spherical geometry; conversely, at 70% H₂, substantial flame deformation occurs because of the inverse correlation between the laminar burning velocity and flame instability governing this transition. Through systematic quantitative analysis, this study elucidates the evolutionary patterns of both turbulent fields and flame fronts, offering groundbreaking perspectives on H₂ combustion and explosion propagation in turbulent environments. Full article

Ammonia, as a promising carbon-neutral fuel, has attracted growing attention for blended combustion applications from academia to industry. Low-NO_x-combustion strategies such as staged combustion, oxygen-enriched combustion, and exhaust gas recirculation may lead to ammonia combustion in CO₂-rich and NO-rich environments. In this work, the laminar burning velocities (S_L) in NH₃/CH₄/O₂/NO/CO₂ flames with various ammonia blended ratios under atmospheric pressure were investigated using the heat flux method. The addition of NO to the oxidizer significantly enhances S_L, with the enhancement factor ξ proportional to the NO fraction in the oxidizer and strongly dependent on the fuel composition. Chemical effects rather than thermal-diffusion effects dominate the enhancement of S_L. Kinetic analysis shows that NO actively participates in the reaction network during the early flame stage, promoting the formation of key radicals such as H and OH through pathways like NH₂ + NO = NNH + OH and NNH = N₂ + H, thereby accelerating chain-branching and sustaining flame propagation. Full article

The ammonia-n-heptane reaction mechanism is essential for simulation of the in-cylinder process for diesel-ignited ammonia engines. To gain insight into the differences in predictive performance among various ammonia-n-heptane reaction mechanisms, four mechanisms were comprehensively evaluated and analyzed based on the modeling of ignition, oxidation, laminar flame propagation and in-cylinder combustion processes. The result shows that only under high ammonia blending ratios and elevated temperatures are discrepancies in predicted ignition delay times observed among the studied reaction mechanisms. Regarding the oxidation process, on the whole, the concerned mechanisms can reasonably predict concentrations of reactants and complete combustion products. However, significant discrepancies exist among the mechanisms in predicting concentrations of intermediate species and other products. For laminar burning velocity, the modeled values from the studied mechanisms are consistent with experimental results under both fuel-lean and -rich conditions. The Wang mechanism exhibits significant deviations from the other three mechanisms in predicting reaction pathways of ammonia and n-heptane. From the perspective of reaction class, the studied mechanisms are similar to each other, to some extent, in the key reactions governing consumption of ammonia and n-heptane. For the engine simulation, the predicted in-cylinder pressure and temperature profiles show minimal variations across different reaction mechanisms. In conclusion, the Fang mechanism can be selected to understand more accurately ignition, oxidation and flame characteristics of ammonia-n-heptane mixtures, while to reduce the engineering computational cost of the engine simulation, the Wang mechanism tends to be a good choice. Full article

The utilization of ammonia as a fuel for gas turbines involves practical challenges due to its low reactivity, narrow flammability limits, and slow laminar flame propagation. One of the potential solutions to enhance the combustion reactivity of ammonia is co-firing with syngas. This paper presents an experimental and numerical investigation of the laminar burning velocity (LBV) of ammonia/syngas/air mixtures under elevated pressures (up to 10 bar) and temperatures (up to 473 K). Experiments were conducted in a constant-volume combustion chamber with a total volume of 11 L equipped with a dual-electrode capacitive discharge ignition system. A systematic sensitivity analysis was conducted to experimentally evaluate the system performance under various syngas compositions and equivalence ratios from 0.7 to 1.6 and ultimately identify the factors with the most impact on the system. As a complement to the experiments, a detailed numerical simulation was carried out integrating available kinetic mechanisms—chemical reaction sets and their rates—to support advancements in the understanding and optimization of ammonia/syngas co-firing dynamics. The sensitivity analysis results reveal that LBV is significantly enhanced by increasing the hydrogen content (>50%). Furthermore, the LBV of the gas mixture is found to increase with the use of a rich flame, higher mole fractions of syngas, and higher initial temperatures. The results indicate that higher pressure reduces LBV by 40% but at the same time enhances the adiabatic flame temperature (by 100 K) due to an equilibrium shift. The analysis was also extended to quantify the impact of syngas mole fractions and elevated initial temperatures. The kinetics of the reactions are analyzed through the reaction pathways, and the results reveal how the preferred pathways vary under lean and rich flame conditions. These findings provide valid quantitative design data for optimizing the combustion kinetics of ammonia/syngas blends, offering valuable design data for ammonia-based combustion systems in industrial gas turbines and power generation applications, reducing NOₓ emissions by up to 30%, and guiding future research directions toward kinetic models and emission control strategies. Full article

This study presents a detailed analysis of the combustion dynamics of stoichiometric H₂–air and CH₄–air mixtures in a 20 L closed vessel over an initial temperature range of 298–423 K. We integrate experimental pressure–time P(t) measurements with numerical analysis to extract laminar burning velocity (LBV) and deflagration index (K_G) values, and we assess three independent kinetic mechanisms (KiBo_MU, University of San Diego, Lund University) via simulations. For H₂–air, LBV increases from 0.50 m/s at 298 K to 0.94 m/s at 423 K (temperature exponent α ≈ 1.79), while for CH₄–air, LBV rises from 0.36 m/s to 0.96 m/s (α ≈ 2.82). In contrast, the deflagration index K_G decreases by ca. 20% for H₂–air and ca. 30% for CH₄–air over the same temperature span. The maximum explosion pressure (P_max) and peak pressure rise rate ((dP/dt)_max) also exhibit systematic increases with temperature. A comparison with model predictions shows agreement within experiments, providing data for safety modeling and kinetic mechanism validation in H₂- and CH₄-based energy systems. Full article

Sustainable aviation fuels (SAFs) are vitally important for aviation decarbonization. The laminar burning velocity (LBV), a key parameter reflecting the combustion behavior of fuel/oxidizer mixtures, serves as a fundamental metric for evaluating SAF performance. This paper systematically reviews and evaluates the LBV experiment method and the performance of traditional aviation fuel, SAFs produced via different pathways, and individual components (n-alkanes, iso-alkanes, cycloalkanes, and aromatic hydrocarbons, as well as the impacts of isomers and homologues) in aviation fuels. It is found that LBV values of different SAFs exhibit significant fluctuations, approaching or slightly deviating from those of conventional aviation fuels. Carbon number, branching degree, substituent types, and testing methods in the components all affect LBV performance. Specifically, increased branching in iso-alkanes reduces LBV, cyclohexane and benzene show higher LBV than their methylated counterparts (methylcyclohexane and toluene), and n-alkylcyclohexanes/benzenes with short (C1–C3) side chains demonstrate minimal LBV variation. Spherical flame methods yield more consistent (and generally lower) LBV values than stagnation flame techniques. These findings provide insights for optimizing SAF–conventional fuel blends and enhancing drop-in compatibility while ensuring operational safety and usability. Full article

Combustion caused by hydrogen-dominated combustible gas mixtures presents critical threats to nuclear safety during severe accidents in nuclear power plants, primarily due to their propensity for flame acceleration, deflagration, and subsequent detonation. Although the direct initiation of detonation from localized hydrogen accumulation at critical concentrations remains challenging, flame acceleration can induce rapid pressure escalation and lead to deflagration-to-detonation transition under suitable conditions. The ultra-high-pressure loads generated almost instantaneously will pose serious risks to containment integrity and equipment or instrument functionality within nuclear facilities. This paper investigates the flame acceleration mechanism and criterion, which is crucial for precise hydrogen risk assessment. A hydrogen combustion flame acceleration model is developed, integrating both laminar and turbulent flame propagation across multiple control volumes. Validated against the RUT test, the model demonstrates high fidelity with a maximum 3.17% deviation in flame propagation velocity and successfully captures the pressure discontinuity. The developed model enables comprehensive simulation with improved predictive accuracy of the flame acceleration process, making it an essential tool for advancing fundamental understanding of hydrogen behavior and severe accident analysis. This model’s development marks a paradigm in nuclear safety research by providing an analytical instrument for integrated severe accident programs in nuclear power plants, contributing to improving the potential hydrogen risks assessment and management in next-generation reactor safety. Full article

Methane–hydrogen–air mixtures present a viable alternative to conventional fuels, reducing CO₂ emissions while maintaining high energy density. This study numerically investigates their combustion characteristics in millimeter-scale reactors, focusing on flame stabilisation and combustion dynamics in confined spaces. A species transport model with volumetric reactions incorporated a detailed kinetic mechanism with 16 species and 41 reactions. The simulations employed a laminar flow model, second-order upwind discretisation, and SIMPLE algorithm for pressure–velocity coupling. The key parameters analysed include equivalence ratio, hydrogen volume fraction, inlet velocity, and gas pressure and their impact on fuel conversion efficiency and heat release was evaluated. The results indicate that hydrogen enrichment enhances flame stability and combustion efficiency, with optimal performance over 40% hydrogen content. Additionally, increased outlet pressure raises flame temperature by 15%, while larger reactor diameters reduce heat losses, improving combustion efficiency by 20%. Emissions of CO decrease significantly at higher hydrogen fractions, demonstrating the potential for cleaner combustion. These findings support the integration of methane–hydrogen mixtures into sustainable energy systems, providing insights for designing efficient, low-emission micro-combustors. Full article