Flameless Combustion Research Articles

A new low NOx emission technique for NH3/H2 blends in a flameless combustor through offset injection

The application of ammonia (NH3) as a possible future fuel presents a plausible solution for green energy storage. It helps provide a carbon-neutral fuel alternative for industrial power generation and transportation. However, the combustion of NH3 presents a formidable challenge due to its low reactivity, inadequate flame stability, sluggish flame propagation, and high NOx emissions. Consequently, its integration into combustion systems necessitates substantial system and strategy modification to enable its deployment to industrial systems. The current study presents a novel fuel/air injection technique, which emphasizes the high recirculation of hot combustion products and the extended residence time of fuel/air mixtures. A comprehensive experimental and numerical investigation is conducted using a swirl air injection and offset fuel injection to achieve the flameless combustion mode for optimized NH3/H2 fuel blends. A range of mixture conditions (ϕ = 0.5–1.2) and NH3/H2 compositions (50/50–70/30) are experimentally examined. The investigations helped elucidate the effect of residence time and recirculation on NOx emissions through kinetic simulations using a reactor network model. Subsequently, 3-D numerical simulations helped identify regions of high recirculation, quantified through reactant dilution ratios and uniform temperature distribution. These aspects are determined using a new parameter, the temperature uniformity index along the axial direction of the combustor. The emissions of NOx, unburnt NH3, and unburnt H2 are quantified for different equivalence ratios and NH3 mole fractions in the fuel mixture. The investigations reveal that NOx emissions reached their minimum (450–654 ppm) and (344-211 ppm), when the burner operated at lean (ϕ = 0.5–0.8) and rich (ϕ = 1.0–1.2) conditions, respectively, for 70/30 NH3/H2 blend. The emissions of unburnt NH3 and H2 species remain minimal for lean conditions. Both lean and rich operational regimes demonstrated similar or superior emission characteristics in flameless combustion mode when compared to the conventional combustion mode.

Impact of inlet air momentum on flameless combustion

Impact of inlet air momentum on flameless combustion characteristics was investigated numerically. Changing the inlet air momentum was achieved using different inlet air nozzle diameters (12, 10, 8 and 6 mm). The mixture mixing and recirculation ratio were obtained and evaluated. A Three-dimensional model was computed using ANSYS fluent. The computations were carried out using RNG k- ε with standard wall function for turbulence modelling. The GRI-EDC model was used to simulate the interaction between turbulence and chemical kinetic. The predicted results were validated against experimental data of small-scale flameless burner. The results showed that, reducing the air nozzle diameter from 12 to 6 mm increased the inlet air momentum, leading to an increase in the recirculation ration by more than 300% of its value at 12 mm. As a result, the O2 concentration decreased in the mixture at burner tip and supports flameless mode creation. The influence of inlet air momentum on combustion species (OH, HC, CO, and NO) were reported. It was concluded that increasing inlet air momentum reduces the greenhouse gases due to the decrease in reaction temperature and mixture dilution. Moreover, Transient radicals (OH and HC) decreased with increasing the inlet air momentum which supports the flame disappearance.

Predicting extinction limits of concurrent smoldering spread by a reduced analytical model

Smoldering is a flameless combustion mode occurring on the surface of charring fuels, such as wood and cigarettes. Although the smoldering process is slow and has a low temperature compared to flaming, it is easy to be initiated by a weak heat source and persists under poor oxygen conditions. Extensive work has been done for flame extinction to develop scaling models to predict the limiting oxygen concentration (LOC), but limited work is available for the smoldering extinction. This study develops a reduced analytical model to predict the extinction limits of smoldering. The model simultaneously solves smoldering propagation rate, surface temperature, and surface oxygen mass fraction as part of the solutions. The extinction limit is determined as the critical condition where solutions satisfying all governing equations cease to exist. The model provides a qualitative description and captures the essential characteristics of a previous experiment. The smoldering rate decreases with increasing fuel diameter, and a larger-diameter fuel is easier to extinguish. The mechanisms of the extinction process are investigated, showing the dominant role of radiative heat loss in the smothering limit at low airflow velocities and convective heat loss near the blowoff limit at high airflow velocities. Further analysis of the effect of oxygen concentration shows an increasing trend of LOC with fuel diameter, and the smothering branch cannot be predicted without considering the heat loss through radiation from the solid surface. Novelty and Significance StatementThe novelty of this research is the prediction of smoldering propagation rates and extinction limits across various fuel diameters using a reduced analytical model. The model is modified to accommodate a thin fuel configuration and high airflow velocity condition, where convective heat and mass transfer play a more important role. The model's significance is that it not only provides essential insights into the mechanisms but also has the capability to simultaneously determine key parameters such as spread rate, reaction temperatures, and surface oxygen concentration as part of the solutions. Additionally, this study demonstrates the model's ability to reproduce the limit conditions, including smothering/blowoff limits and limiting oxygen concentration (LOC).

Impact of Integrating Flameless Combustion Technology and Sludge–Fly Ash Recirculation on PCDE Emissions in Hazardous Waste Thermal Treatment Systems

Impact of Integrating Flameless Combustion Technology and Sludge–Fly Ash Recirculation on PCDE Emissions in Hazardous Waste Thermal Treatment Systems

Experimental study of CO2/H2/NH3 influence on CH4 flameless combustion process in semi-industrial furnace

A flameless combustion mode was employed in a semi-industrial furnace to investigate the effect of NH3 on CH4, which was diluted by CO2/H2 combustible mixtures, serving as the primary fuel. Emission characteristics, as well as toxic compounds and temperature distribution maps, were presented for selected fuels. The combustion system was tested at a constant firing rate of 150 kWth and a constant fuel bulk velocity of 85 m/s. We investigated the effects of equivalence ratio, fuel composition and volume share of NH3 in the fuel. It was found that the equivalence ratio and NH3 amount have the most significant impact on the emission of fuel nitric oxide in the flameless combustion process. The dilution of CH4 with CO2 affects the stretch rate, resulting in an increase in CO levels as well as promoting NO level growth. The calculated dimensionless Conversion Factor (CF) shows that the dilution gas, CO2, significantly impacts the reduction of NH3 to NO, but only for low-content fuel-bound nitrogen (up to 1%). The lowest values of CF were measured for a high equivalence ratio, corresponding to the typical oxygen content for flameless combustion process.

Sustainability of flameless combustion mode with hydrogen addition to liquid kerosene fuel at high-intensity conditions

The application of hydrogen for fuel enrichment and blending has recently gained importance. The sustainability and performance enhancement through hydrogen addition in flameless combustion mode for liquid fuels needs further investigation to enhance our understanding. Therefore, the present study reports the effect of hydrogen addition on the flameless combustion of a kerosene-fired 30 kW swirl combustor with hydrogen addition. A divergence-shaped conical combustor with tangential air injection and asymmetric fuel injection scheme is adopted. The liquid fuel was injected using a solid-cone swirl nozzle with 34 μm Sauter Mean Diameter. The hydrogen addition was varied from 0 to 20% on a thermal input basis, and the thermal input of liquid fuel was maintained at a 30 kW level. Computational studies helped understand the flow physics behavior within the combustor volume. The hydrogen addition was observed to improve the reactant dilution ratio (Rdil). The maximum values of the Rdil increased from 4.47 to 5.44 with hydrogen addition. Hydrogen addition further helped extend the flameless combustion regime to φ = 0.3 without any air preheating. A uniformly distributed reaction with a reduced-temperature gradient is observed with extended combustion stability limits. A marginal increase in NOx emissions was observed with hydrogen addition, albeit within permissible limits, accompanied by ∼8–20 dB reduction in acoustic emissions.

Mathematical modelling of a slow flameless combustion of a two-dimensional paper

We present a mathematical model for the slow combustion (smoldering) of a two-dimensional sheet of paper. We describe the evolution of the char region, and we investigate the effects of an orthogonal air flow on the shape of the combustion front. The mathematical formulation consists in a set of two nonlinear PDEs for the temperature and the oxygen concentrations coupled with one ODE for the cellulose concentration. The (dimensionless) problem is solved numerically by means of a spectral collocation scheme based on Chebyshev polynomials. Our results show that the Péclet and the Lewis number strongly influence the shape of the ignition front and that the advancement of the combustion front does not occur if advection and diffusion are neglected (zero Péclet and Lewis numbers). In particular we observe that the burning region and the ignition front are strongly influenced by the velocity of the airflow and by the mass and heat transport phenomena due to diffusion and advection. We shall see that the increasing of the ratio between the convective and diffusive characteristic times (Péclet number) and the decreasing of the ratio between the mass and heat diffusive characteristic times (Lewis number) have a “flattening effect” on the combustion front.

Minimum oxygen supply rate for smouldering propagation: Effect of fuel bulk density and particle size

Smouldering is a low-temperature, flameless, and persistent combustion process driven by heterogeneous oxidations. Oxygen supply is a key parameter of smouldering and is sensitive to fuel density and particle size, but our understanding is still limited. Herein, we explore the oxygen threshold for smouldering propagation under upward internal airflow velocities up to 5 mm/s. Pine needles with different bulk densities (55–120 kg/m3) and wood samples with different particle sizes (1–50 mm) are tested. We found that the minimum airflow velocity for sustaining smouldering propagation increases with the decrease of the bulk density or the increase of the particle size. By increasing the airflow velocity, the smouldering front first propagates unidirectionally (opposed) and then bidirectionally (opposed + forward). Nevertheless, when the pore size is large (the fuel particle size is large or the fuel bulk density is small), bidirectional propagation always occurs, because the oxygen can leak through the opposed smouldering front. A simplified thermochemical analysis is proposed to reveal the influence of interparticle heat transfer on the minimum oxygen supply rate of smouldering propagation. This work advances the fundamental understanding of smouldering on solid fuel particles and their smouldering fire risks and helps optimize the efficiency and safety of smouldering processes.

Enhancing pure NH3 combustion: Impacts of O2 enrichment under MILD conditions in a 20-kW semi-industrial scale furnace

This experimental work provides insights into the effects of oxygen (O2) enrichment on pure ammonia (NH3) combustion in a 20-kW semi-industrial scale furnace featuring an industrial-grade burner operated under MILD/Flameless conditions. The impact of O2 enrichment on the NH3 combustion stabilization, the flame and furnace temperature, the exhaust emissions, and the OH* chemiluminescence were explored. The tests were conducted by varying O2 concentration in the oxidizer stream from 21 % to 50 % and using two different oxidizer injectors to achieve average inlet velocities of 80 m/s and 150 m/s while keeping the inlet power and the total oxidizer mass flow rate constant. Stable flameless combustion with a distributed reaction zone and a consistent and uniform temperature profile throughout the combustion chamber was achieved with O2 enrichments higher than 34 %. Nevertheless, high levels of NH3 slips were observed, which were drastically reduced to negligible concentrations at higher O2 enrichments. On the contrary, NO emissions increased with the O2 fraction with an opposite non-linear trend compared to that of NH3 slip. An optimum trade-off between NOx emissions and NH3 slip was achieved by co-firing NH3 with O2=37 % with ID16 and 36 % with ID20 injector. This study also compares the behavior of pure O2-enriched NH3 combustion under flame and MILD combustion conditions to highlight the advantages of oxygen-enhanced NH3 flameless combustion. While operating in flame mode ensured a stable ammonia oxidation process already at low O2 enrichment=30 %, NO emissions and NH3 slips were significantly higher than those measured under MILD conditions. Finally, the impact of varying industrial load and the related heat balance on the NH3 slip was assessed at different levels of O2 enrichments at a fixed equivalence ratio. The overall results show that O2-enriched flameless ammonia combustion can simultaneously expand the range of stable combustion and reduce NO and NH3 emissions.

Simulation study on effect of nozzle geometry on flameless combustion of non-preheated methane gas

In this paper, the effects of different burner configurations on the characteristics of flameless combustion were evaluated by comparing the temperature field, NOx, OH, and H2CO at different burner inlet velocity and angles through a combination of experimental and numerical simulations. The results show that increasing the burner inlet angle and gas velocity is imperative in achieving the flameless combustion, increasing the re-circulation rate in the furnace, making the temperature distribution in the furnace uniform, and reducing the emission of NOx at the end of the furnace. During the simulation of flameless combustion, it was found that OH radicals and H2CO radicals were well correlated with the reaction exothermic zone, and the Reynolds number was positively correlated with the re-circulation rate in the furnace. With the increase of Reynolds number, the entrainment rate of flue gas increases, and the combustion state is closer to flameless combustion. When re-circulation rate Kv >2, combustion becomes flameless. Through the summary analysis of the data, it can be found that there is a critical Reynolds number for the burner to achieve flameless combustion, and flameless combustion occurs only when the Reynolds number is greater than 1.0?104.

Flameless Catalytic Combustion of Ethanol on Copper, Chromium, and Nickel Oxides

The possibility of using 3d-metal oxides as catalysts of self-sustaining catalytic flameless combustion of ethanol was examined. Monolithic catalysts CuO and Cr2O3, supported catalysts 25% CuO/SiO2 and 25% CuO/Al2O3, and ultrasonically treated catalyst 25% NiO/SiO2 can operate for a long time without changes in properties, ensuring 100% ethanol conversion. The wide range of the temperatures reached (300–590°C) allows choosing a catalyst meeting the process tasks.

UiO-66-NH2-PEI combined with a hand-held catalytic combustion sensor for trace acetone detection in exhaled breath

The trace acetone determination in breath can be regarded as a noninvasive method for diagnostic of diabetes. There is an urgent demand to develop a simple method for acetone detection which can be easily performed at home. Here, UiO-66-NH2@PEI with the appropriate aperture size combined with the hand-held sensor was used to construct the adsorption–desorption sensing platform for separation and enrichment of acetone in exhaled breath (EB). UiO-66-NH2@PEI can realize the selective adsorption, desorption and separation for acetone in EB. The flameless combustion hand-held sensor can sensitively respond to acetone in the range of 0.2–12.0 ppm and it can achieve the accurate determination of acetone content in EB of diabetic patients. The hand-held sensor is inexpensive, lightweight and easy to operate, which has great application prospects for the widespread use for diabetic patients in family life.

Classification and characteristics of ammonia combustion in well stirred reactor

The present work investigates the regime classification and chemical characteristics of the premixed ammonia combustion in a well stirred reactor (WSR). Three combustion regimes can be classified based on the lowest autoignition and highest extinction temperatures and the reacting temperature increment, which are: (i) flameless combustion (FLC); (ii) high temperature combustion (HTC); (iii) traditional combustion (TC). When introducing the upper limit of NOx emission into the classification, MILD combustion can be defined as a sub-regime of FLC. Relative to the methane counterpart, the ammonia combustion is found to exhibit larger FLC and smaller HTC regimes delimited by the inlet temperature and oxygen fraction. It is also revealed that the MILD combustion of ammonia differs significantly from its burning under other regimes in heat production and NO formation mechanisms. In the MILD combustion, multiple reactions dominate the heat production while the DeNOx route greatly reduces the NO formation. Contrarily, in other combustion regimes, heat is controlled by one or two specific reactions while notably the Extended Zeldovich route dominates the NO destruction. Moreover, the working temperature controls the top reactions for both heat and NOx productions, regardless of different combustion regimes.

CFD study of flameless combustion in a real industrial reheating furnace considering different H[formula omitted]/NG blends as fuel

On the path towards decarbonisation of the steel industry, the use of H2/NG blends in furnaces, where high temperatures are needed, is one of the alternatives that needs to be carefully studied. The present paper shows the CFD study carried out for a full-scale reheating furnace burner case. The real operating conditions as well as experimental measurements provided by the furnace operator were used to validate the results and reduce simulation uncertainties. The burner under consideration (2.5 MW) works in flameless mode with natural gas and preheated air (813 K). Starting from this point, another three fuel blends with volumetric percentages of 23% (also known as G222), 50% and 75% of H2 in natural gas were considered. For this purpose, the open source CFD code OpenFOAM was used, where the novel NE-EDC turbulence-chemistry interaction model was implemented, which has already been successfully validated specifically for flameless combustion in a furnace. The implementation incorporated an enhanced approach for calculating the chemical time-scale, coupled with a specific post-processing solver to predict NOx emissions. The study analyses the relative impact of the considered fuel blends on NOx formation and flameless regime. The modelling results demonstrated the burner’s capability to operate efficiently with high concentrations of hydrogen, maintaining flameless regime in all cases. This condition ensured uniform temperature distributions and low levels of NOx emissions, reaching a maximum value of 86 mg/m3. These results indicated the proper functionality of the existing natural gas-based burner with H2/NG blends, which was the primary requirement for the conversion process.

Effects of Flameless Catalytic Combustion of Hydrogen on the Parameters of Catalytic Sensors with Platinum-group Catalysts

Effects of Flameless Catalytic Combustion of Hydrogen on the Parameters of Catalytic Sensors with Platinum-group Catalysts

Significantly different inhibition mechanisms of graphene oxide in laser-ignited combustion of micro and nano CL-20

Because of its high energy density, hexanitrohexaazaisowurtzitane(CL-20) has been widely used in propellants in place of conventional oxidizers. The adjustable burning rate of the CL-20 is a key factor in adapting the propellant to various environments. In this work, the effects of graphene oxide(GO) content on the thermal release, ignition and combustion characteristics of micro and nano CL-20 were thoroughly conducted based on the differential scanning calorimetry measurements (DSC), laser ignition, burning rate test and condensed combustion products collection. The results show GO has a positive effect on micro and nano CL-20 in ignition performance and a negative effect on combustion rate. Owing to the addition of GO, the original flameless combustion of CL-20 changes to flaming combustion. With increasing GO content, the burning rates of micro and nano CL-20 show completely different descending laws. Furthermore, the combustion mechanism of nano and micro CL-20/GO has been studied in detail. The burning rate of micro CL-20/GO is primarily determined by the condensed phase, and that of nano composite is controlled by both of the condensed and gas phases. This work provides a deeply insight into the influence mechanism of GO on the properties of nano and micro CL-20.

Numerical study of inlet air conditions on methane flameless combustion characteristics

The current study investigates the influence of inlet air momentum and temperature for flameless characteristics. Three-dimensional computations were implemented by ANSYS FLUENT. A small-scale experimental combustor geometry was used for validation study. Turbulence modelling was performed by RNG k- ε model and the GRI-Eddy Dissipation Concept (GRI-EDC) model was used for the combustion modelling. The influence of inlet air velocity was investigated through using different air jet diameters at constant flow rate. Five different air jet diameters (4,6,8,10 and 12 mm) were studied. It was found that decreasing air jet diameter leads to an increase in the exhaust gas recirculation which reserve in mixture dilution (low O2 content mixture) subsequently, the reaction rate decreases which support the flameless mode creation. Also, this recirculated gas participates in heating up of fresh reactants which make the mixture tends to complete combustion. The influence of combustion air preheating temperature on NO and CO emissions were studied. CO and NO concentration were decreased with reducing the temperature of inlet preheated combustion air.

Study on Flow and Heat Transfer Characteristics of 25 kW Flameless Combustion in a Cylindrical Heat Exchanger for a Reforming Processor

Flameless combustion has advantages such as low pollution and uniform temperature in the combustion chamber, making it an excellent option for heat exchangers. Previous studies have focused solely on the flameless combustion phenomenon, without considering its interaction with the target being heated. In this study, we conducted experimental and computational fluid analyses on a cylindrical reformer for reverse air injection flameless combustion. Typically, small-scale reformers of 10 kW or less are coaxial triple-tube cylindrical reformers. In contrast, multitubular reformers are used for larger-scale applications, since the heat transfer rate in single-burner cylindrical reformers decreases sharply as the scale increases. Flameless combustion, with high heat transfer efficiency, helps overcome the limitation of premixed burner. Compared with conventional premixed burners, flameless burner decreases the combustion gas outlet temperature by 30% at 25 kW while reducing energy consumption by 24% (owing to the high heat transfer rate) for a given cooling fluid outlet temperature. Furthermore, it is shown that introducing a ring at the combustion chamber exit can enhance combustion gas recirculation. The experimental result was confirmed through computational fluid analysis. It is concluded that for reverse air injection flameless combustion, the combustion gas recirculation rate in the combustion chamber is strongly related to the heat transfer.

Conversion of Copper and Zinc Compounds in the Flameless Combustion Wave of RDX

Conversion of Copper and Zinc Compounds in the Flameless Combustion Wave of RDX

ЭФФЕКТИВНОСТЬ ОГНЕЗАЩИТНЫХ ПРОПИТОЧНЫХ СОСТАВОВ ДЛЯ ДЕРЕВЯННЫХ КОНСТРУКЦИЙ

The article considers the results of evaluating the flame retardant effectiveness of various flame retardants by the method of fire exposure according to GOST 53292–2009 and by methods of thermal analysis (thermogravimetry, differential thermogravimetry, derivative thermal analysis). According to the results of fire of surface application of flame retardants from 350 to 550 kg/m2, group I or II of flame retardant
 efficiency is provided (mass loss from 8,67 to 25 %). It is shown that the group of flame-retardant effectiveness does not actually reflect the nature and degree of realization of the mechanism of flame retardant action of flame retardants, their ability to influence the features of thermal transformations of wood. It has been established that the type of flame retardant, its chemical component composition and the mechanism of flame retardant action to varying degrees affect the main stages of thermal oxidative decomposition of wood. The results obtained can be used to evaluate the effectiveness of fire protection in reducing the intensity of the charring process and predicting the intensity of smoldering (flameless) combustion of wood. tests, it was found that with the consumption