Flameless Combustion Research Articles

이중관식 열교환기가 설치된 역방향 공기 주입 방식 무화염 연소로 내 열유동 특성에 대한 연구

이중관식 열교환기가 설치된 역방향 공기 주입 방식 무화염 연소로 내 열유동 특성에 대한 연구

Effects of the tertiary air injection port on semi-coke flameless combustion with coal self-preheating technology

A novel coal self-preheating combustion technology was adopted to generate high temperature gas-solid two-phase fuel. Experiments on flameless combustion of the two-phase fuel were conducted in a 30-kW coal self-preheating combustion test rig. In flameless mode, effects of the single and the multi levels tertiary air injection ports were discussed, and the flue gas compositions (NOx and CO) and temperatures along the axis of the down-fired combustor were analyzed. To visualize the combustion process, the flame images were captured. It is confirmed that flameless combustion of high temperature gas-solid two-phase fuel can be easily established, and (1) the temperature profile is uniform and it is transparent in the combustor; (2) NOx emission can be reduced about a half compared to other combustion modes; (3) for a single level tertiary air, NOx emission can be minimized by the proper injection port while NOx emission and CO emission might be increased by an inappropriate port; (4) for the multi levels tertiary air, NOx emission can be reduced further by moderate tertiary air injected in the reducing zone, and NOx might be increased while some tertiary air is injected in the burnout zone. It is clear that NOx and CO emissions can be controlled in a low level by rationally arranging two level tertiary air injection ports under flameless combustion.

Flameless burning of wood: parameters of macrokinetics of pyrolysis and thermo-oxidative decomposition

Introduction. Many materials are subject to flameless, smoldering combustion: coal, cotton, peat, carbonizing ­polymers, etc. The fire hazard of smoldering burning of organic materials is that low-calorie ignition sources are sufficient to initiate the combustion process, the process is hidden, making it difficult to detect, and can spontaneously turn into a fiery one. Purpose and objectives. The purpose of this work was to determine the macrokinetics of pyrolysis and thermo­oxidative decomposition of wood of different types of conifers and deciduous species by thermal analysis. Methods. Samples were investigated by thermal analysis in an inert and air environment. For this, we used the automated modular system Du Pont-9900, including the TGA-951 thermobalance, and the DSK-910 diffe­rential scanning calorimeter. Results. It was found that the pyrolysis of the main components of wood (hemicellulose and cellulose) proceeds according to the nucleation and growth mechanism of nuclei according to the case law R ( n = 1) with activa­tion energies close in order of magnitude for different species (98–136 kJ/mol for hemicelluloses and 203–233 kJ/mol for cellulose). At the stages of thermooxidative decomposition of wood components and heterogeneous oxidation of the carbonized product, diffusion of the D3 (D4) type in spherical geometry becomes the mechanism controlling the process. The effective activation energy of the decomposition of hemicelluloses is reduced to 90.9–95.8 kJ/mol, and of cellulose to 138.3–160.9 kJ/mol. The reaction of heterogeneous oxidation of carbonized products makes a significant contribution to the flameless, smoldering combustion of the material. It is diffusion-controlled and is characterized by high values of activation energy (up to 285 kJ/mol). Conclusion. The results of the work make it possible to evaluate the macrokinetic parameters of pyrolysis and thermooxidative decomposition of wood of different species during flameless combustion. The obtained data can be used as the main parameters for modeling the heterogeneous combustion of wood of different species in buildings.

Modeling Temperature Variations in MILD Combustion Using MuSt-FGM

The energy demand in the world is ever increasing, and for some applications combustion is still the only reliable source, and will remain as such in the foreseeable future. To be able to mitigate the environmental effects of combustion, we need to move to cleaner technologies. Moderate or intense low oxygen dilution (MILD) combustion is one of these technologies, which offer less harmful emissions, especially nitric oxide and nitrogen dioxide (NOx). It is achieved by the recirculation of the flue gases into the fresh reactants, reducing the oxygen content, and thereby causing the oxidation reactions to occur at a milder pace, as the acronym suggests. This results in a flameless combustion process and reduces the harmful emissions to negligible amounts. To assist in the design and development of combustors that work in the MILD regime, reliable and efficient models are required. In this study, modeling of the effects of temperature variation in the oxidizer of a MILD combustion case is tackled. The turbulent scales are fully resolved by performing direct numerical simulations (DNS), and chemistry is modeled using multistage flamelet generated manifolds (MuSt-FGM). In order to model the temperature variations, a passive scalar which is created by normalizing the initial temperature in the oxidizer is defined as a new control variable. During flamelet creation, it was observed that not all the compositions are autoigniting. Several approaches are proposed to solve this issue. The results from these cases are compared against the ones performed using detailed chemistry. With the best performing approach, the ignition delay is predicted fairly well, but the average heat release rate is over-predicted. Some possible causes of this mismatch are also given in the discussion.

All-Silicon Zeolite Supported Pt Nanoparticles for Green On-Board Inert Gas Generation System

ABSTRACT Green On-board Inert Gas Generation System (GOBIGGS) is regarded as one of the most promising routes for fuel tank inerting, whose operation efficiency can be remarkably promoted by an excellent catalyst. In the present work, three kinds of all-silicon zeolite-supported Pt nanoparticles, Pt/Si-Beta, Pt/Si-ZSM-5, Pt/SBA-15 were prepared and characterized. The activities of these catalysts for flameless combustion of fuel samples were investigated on a self-designed apparatus. The results show that among these tested catalysts, Pt/Si-Beta has the best catalytic activity, which can catalyze the complete combustion of methylcyclohexane and aviation kerosene RP-3 at 166°C and 241°C, respectively. The superb activity of Pt/Si-Beta can be ascribed to its striking hydrocarbon adsorption capacity. It can also be found that the 1.0 wt% loading amount of Pt is appropriate to meet the actual demand of the GOBIGGS system, considering catalytic effects and costs. Moreover, Pt/Si-Beta tends to exhibit superior activity and stability even in different humidity and space velocity environments for a long time. It can maintain a conversion rate of around 98% even after 30 h of continuous using. All of these show that Pt/Si-Beta has the potential to be applied to the GOBIGGS system.

Effect of catalyst size on flameless combustion's hysteresis characteristics of air-gas mixtures with combustible gas's impurities

Analytical models of heat and mass transfer's hysteresis and combustion kinetics of combustible gas impurities on a metal catalyst particle (wire) in the presence of only one onesided reaction are proposed. The analysis is carried out taking into account thermal diffusion and depending on the catalyst's size and the gas-air mixture's temperature. The dependences of platinum wire's temperature on hydrogen-air mixture's temperature at various wire diameters are presented. Decrease in the wire's diameter under unchanged other conditions leads to a decrease in the hysteresis region and its further degradation. The method used for analytical determination of hysteretic regions and degradation points, which lies in representing the desired dependences of operating parameters in a parametric form (parameter is catalyst's temperature). Analytical dependence (spinodal) gas-air mixture's temperature on catalyst's diameter was found that describes all critical points, including degradation points.The value of the diameter's and temperature's wire catalyst, the mixture's temperature, which correspond to critical condition’s degradation, were obtained in explicit form.

Experimental study on combustion of CH4/NH3 fuel blends in an industrial furnace operated in flameless conditions

This paper presents the results of an experimental study on the combustion process of methane mixed with NH3 in flameless mode. At a time of striving for CO2-free power, NH3 became a potential energy storage carrier fuel from renewable sources. Flameless combustion features low emissions and is a very efficient technology used in the power sector, as well as steel production, ceramics, etc. Industrial furnaces were tested in the context of pure methane combustion with an addition of NH3, up to 5%. Flameless combustion conditions were achieved with a regenerative gas burner system (high regenerative system). The burner consists of four ceramic regenerators allowing for continuous preheating of air, even up to 50 K lower than the temperature of the combustion chamber wall. Constant power of the introduced fuel was kept at 150 kW and the fuel-air equivalence ratio ranged from 0.75 to 0.95. The results have shown a growth of molar fraction of NO in flue gases when NH3 content in the fuel rose. The increase is more significant for the tests with a higher amount of oxygen in the combustion chamber (a lower fuel-air equivalence ratio). An addition of 5% of NH3 into the fuel caused an emission of NO at the levels of 113 ppmv and 462 ppmv (calculated to O2 = 0%), respectively for low and high fuel-air equivalence ratios.

A novel air injection scheme to achieve MILD combustion in a can-type gas turbine combustor

This article presents the design and development of a can-type gas turbine combustor operating in flameless combustion mode with liquid fuels. The combustor operates with kerosene and thermal intensities varying from 5.1 to 7.5 MW/m3. A novel air-injection scheme is proposed, in which air is supplied from different injection holes namely, swirl air near fuel injection, primary, secondary and dilution air in the downstream. These air injection holes are arranged in a way to help create strong recirculation of hot combustion products leading to increased mixing and dilution of incoming fresh reactants. Direction, orientation and mass fraction of swirl, primary, secondary and dilution air are optimized through a series of reacting flow simulations aimed at maximizing the reactant dilution ratio, a key parameter to achieve flameless combustion. It is observed that if the momentum of the air is increased beyond a critical value, the combustor switches its operation into flameless mode due to increased mixing and dilution of fresh reactants with hot combustion products. Combustion occurs in a well-distributed reaction regime within the combustor volume. Measured NOx emissions are less than 5 ppm and acoustic emissions are significantly reduced during the combustor operation in flameless combustion mode.

Feasibility analysis of flameless combustion in high-parameter gas turbine combustor

Feasibility analysis of flameless combustion in high-parameter gas turbine combustor

Modeling Pollutant Emissions of Flameless Combustion With a Joint CFD and Chemical Reactor Network Approach

The Flameless Combustion (FC) regime has been pointed out as a promising combustion technique to lower the emissions of nitrogen oxides (NOx) while maintaining low CO and soot emissions, as well as high efficiencies. However, its accurate modeling remains a challenge. The prediction of pollutant species, especially NOx, is affected by the usually low total values that require higher precision from computational tools, as well as the incorporation of relevant formation pathways within the overall reaction mechanism that are usually neglected. The present work explores a multiple step modeling approach to tackle these issues. Initially, a CFD solution with simplified chemistry is generated [both the Eddy Dissipation Model (EDM) as well as the Flamelet Generated Manifolds (FGM) approach are employed]. Subsequently, its computational cells are clustered to form ideal reactors by user-defined criteria, and the resulting Chemical Reactor Network (CRN) is subsequently solved with a detailed chemical reaction mechanism. The capabilities of the clustering and CRN solving computational tool (AGNES—Automatic Generation of Networks for Emission Simulation) are explored with a test case related to FC. The test case is non-premixed burner based on jet mixing and fueled with CH4 tested for various equivalence ratios. Results show that the prediction of CO emissions was improved significantly with respect to the CFD solution and are in good agreement with the experimental data. As for the NOx emissions, the CRN results were capable of reproducing the non-monotonic behavior with equivalence ratio, which the CFD simulations could not capture. However, the agreement between experimental values and those predicted by CRN for NOx is not fully satisfactory. The clustering criteria employed to generate the CRNs from the CFD solutions were shown to affect the results to a great extent, pointing to future opportunities in improving the multi-step procedure and its application.

Flame retardance of leather with flame retardant added in retanning process

With the wide application of leather products, higher requirements are put forward for its flame retardancy. In this work, in order to improve the flame retardant performance of leather, phosphorus flame retardant (PFR), nitrogen and phosphorus intumescent flame retardant (NPIFR), nitrogen and phosphorus flame retardant (NPFR) was added to the retanning process, respectively. The flame-retardant mechanism and the interaction relationship between flame retardant and leather was studied. The structure of leather with flame retardant was confirmed by FT-IR. The effects of flame retardant on flame retardant properties of leather were investigated by limiting oxygen index (LOI), vertical burning test, and cone calorimeter test (CCT), respectively. The results showed that three types of flame retardant can enhance the fire retardancy of leather. The leather with flame retardant added in retanning process exhibited a series of improved flame-retardant properties in comparison with blank leather (without flame retardant). Compared with that of blank leather, the highest LOI value of NPIFR leather reached 31.4% from 29.3%. The flame combustion time of all leather with flame retardant reduced, and flameless combustion time was almost 0 s. The time to ignition (TTI) increased and heat release rate (HRR) and fire growth index (FGI) decreased of sample with the increase of flame retardant. Furthermore, this technique might provide a route to synthesize the leather with flame retardant ability.

Investigating the Potential of Different Modeling Setups in Simulating a Flameless Furnace Using Global and Detailed Kinetic Mechanisms

ABSTRACT Attempts were made to reproduce the laboratory measurements of flameless combustion systems using numerical simulation tools, for different chambers designs. In many cases, a significant discrepancy is reported between the experimental measurements and the numerical results. In this work, we have successfully reproduced the reported experimental results (velocity field, temperature field, and the outlet dry NOx emissions) of the “FOUR” MILD combustion facility (proposed by C. Rottier). An acceptable accuracy was achieved in two reference test cases. A concise discussion is presented on the available literature on the subject of numerical modeling for the flameless combustion mode. The use of two turbulence models was explored (standard k-ε, and modified k-ε), along with two kinetic mechanisms (a global 2-step CH4 combustion mechanism, and the more detailed KEE-58 mechanism). The commercial ANSYS FLUENT® software was used in these simulations. The results of seven different modeling setups are presented, comparing the predictions of temperature fields, and NOX emissions. Features such as the onset of premature ignition and the reactants entrainment ratio are also discussed. Our results show that the most accurate model setup employs the following: SKE – KEE-58 – EDC with a reduced volume fraction constant. In regards to reproducing the measurements from the “FOUR” facility, we are not aware of a similar research that runs a concise discussion on the predictions of such broad and different modeling setups.

Combustión sin llama como una alternativa para mejorar la eficiencia de sistemas térmicos: revisión del estado del arte

El aumento de la demanda energética, así como de las emisiones contaminantes ha generado un incremento de la investigación de tecnologías que permitan mitigar ambas problemáticas a nivel mundial. Dentro de las alternativas para mejorar la eficiencia de los procesos térmicos, el régimen de combustión sin llama se presenta como una de las alternativas más promisorias, puesto que permite obtener altos valores del rendimiento térmico mediante el mejoramiento de la transferencia de calor y del proceso de combustión, con la consiguiente reducción de las emisiones contaminantes. Debido a esto, en el presente estudio se realiza una revisión del estado del arte de dicha tecnología, haciendo énfasis en los aspectos fenomenológicos asociados, las principales características del régimen y su estabilidad, pasando por los mecanismos de obtención y la presentación de una serie de estudios, tanto a nivel nacional como internacional, en los que se utilizaron combustibles fósiles y alternativos. La revisión finaliza con la discusión de algunos casos en los cuales se ha implementado el régimen a nivel industrial.

The effect of diluted hot oxidant and fuel on NO formation in oxy-fuel flameless combustion using opposite jet

Flameless combustion has been developed to reduce emissions while improving thermal efficiencies in the combustion system. It is characterized by its distinguished features, such as suppressed pollutant emission, homogeneous temperature distribution, reduced noise and thermal stress. Recently, many studies have been mainly carried out on air-fuel flameless combustion but researches on oxy-fuel flameless combustion are very scarce and in particular, no detailed information of the NO production which should be considered in oxyfuel combustion has not been reported. In view of practical furnace like condition, some air leakage is inevitable but even small amount of N2 leak could generate a large amount of thermal NOX due to the high flame temperature in the case of oxy-fuel combustion. In this work, the effect of the recirculation ratio (KV) on NO formation in the oxy-fuel flameless combustion with 5 % addition of N2 was analyzed using OPPDIF under the condition of diluted oxygen and methane. A series of numerical simulations using OPPDIF code of Ansys Chemkin-pro have been carried out with the GRI 3.0 325-step mechanism. In order to analyze NO formation mechanism in more detail, the influence of recirculation on major NO formation elementary reaction was also clarified through sensitivity analysis as well. The results show that as the KV increases, the temperature and heat release were deceased and the shape of the heat release profile was found to be changed from oxidation with pyrolysis to oxidation only. Through the heat release analysis, the proper oxy-fuel flameless combustion regime for oxygen CH4 flame can be classified in inlet temperature and maximum temperature map.

Thermodynamic Analysis of the Oxidation of Radioactive Graphite in Molten Na2CO3–K2CO3–Sb2O3 in a Carbon Dioxide Atmosphere

More than 100 power reactors, reactors for plutonium production, and research reactors are presently available in the world. In these reactors, graphite is used as a reflector, moderator, and cladding of fuel elements. Flameless combustion is a promising method for reducing the amount of solid radioactive waste. The method is based on the oxidation of solid radioactive waste in oxide–carbonate melts and makes it possible to reduce the radioactive graphite volume considerably. Thermodynamic simulation and analysis of the oxidation of radioactive graphite in molten Na2CO3–K2CO3–Sb2O3 in a carbon dioxide atmosphere are performed using the TERRA software package. The thermodynamic simulation is conducted at a pressure of 1 atm and initial and final temperatures of 273 and 3273 K, respectively. The step of temperature changing is 100 K. Based on these data, the distributions of elements between condensed and gas phases are examined. The simulation results show that carbon disappears at a temperature of 873 K. Heating of the system up to 1073 K leads to the evaporation of condensed antimony compounds. Heating of the system up to 1673 K leads to the evaporation of condensed potassium, sodium, chlorine, uranium, and cesium compounds. Heating of the system up to 2273 K leads to the evaporation of condensed nickel compounds. Heating of the system up to 2573 K leads to the evaporation of condensed calcium, plutonium, beryllium, strontium, americium, and europium compounds. Only a vapor–gas phase is observed at temperatures above 2573 K.

Modelling and simulation of steel reheating processes under oxy-fuel combustion conditions – Technical and environmental perspectives

This paper investigates the impact of flameless oxy-fuel combustion on the thermal performance of a pilot-scale steel reheating furnace. A comprehensive mathematical model, based on the zone method of radiation analysis, was developed, which takes into account the non-grey behaviour of the furnace atmosphere under oxy-fuel combustion conditions. The model was subsequently used to simulate the temperature profile of an instrumented slab used in the experiment. The results showed that the predicted slab temperature profile along the furnace is in good agreement with measurement. However the model over predicted the absolute slab temperatures due to the influence of formation of oxide scales on the slab surface, which was not taken into account in the current model. When compared to air-fuel combustion simulation, the results of oxy-fuel combustion also indicated a marked improvement in the furnace specific fuel consumption (approximately 16%). This was mainly due to the enhanced radiative properties of the furnace atmosphere and reduced exhaust energy losses as the result of less dilution effect from nitrogen. This resulted in reduction in the overall heating time by approximately 14 min. Furthermore, if the economics of carbon capture is taken into consideration, theoretically, the energy consumption per kilogram of CO2 captured can be reduced from 3.5 to 4.2 MJ kg−1 to 0.96 MJ kg−1. In conclusion, the current studies support the view that oxy-fuel combustion retrofitting to reheating furnaces is a promising option, both from a technical and from an environmental point of view.

Flameless Combustion Characteristics of Producer Gas Premixed Charge in a Cyclone Combustor

Producer gas (PG) flameless combustion in a premixed cyclone combustor was investigated experimentally and numerically. The experiment was carried out over different premixed charges of air/fuel mixture and inlet nozzle diameters under PG fuel input power up to 80 kW. Exhaust temperature was above 1050 K for the whole operating range with CO emission below 0.08 Vol.% and NOx emission of 420 ± 30 ppm. Swirl flow field, internal hot flue gas recirculation, and temperature distributions inside the combustor were numerically studied. The numerical results were validated by the experimental data and it confirmed the achievement of temperature uniformity throughout the combustion volume when operated under flameless combustion mode. The reaction regime of PG flameless combustion was further analyzed with respect to turbulent scale characteristics. The present PG flameless combustions were typified by both moderate Damkohler and Karlovitz numbers, falling into the Flamelets in eddies regime.

Effect of turbulence model on flameless combustion simulation of a regenerative furnace

The high energy consumes have generated the need of developing higher efficiency devices, in this sense flameless combustion is a great alternative. The design of flameless combustion systems by means of Computational Flow Dynamics simulation is an alternative value to reduce costs, however it required an adequate model selection. In the present study a capability evaluation of standard and realizable versions of k-e turbulence model was carried out. Experimental measurements of temperature and chemical species were performed in combustion chamber of a regenerative furnace in order to compare this with numerical data and in this way determinate the performance and the incidence of the selected models. The realizable version shows better results regarding at changes of flow direction and recirculation patterns inside the furnace.

Effects of chemical reaction mechanism and NOx formation pathways on an inter-turbine burner

ABSTRACTOne of the main challenges of future aircraft engines is to achieve low pollutant emissions while maintaining high combustion efficiencies and operability. The Flameless Combustion (FC) regime is pointed as one of the promising solutions due to its well-distributed reaction zones that yield low NOx emissions and oscillations. A dual-combustor configuration potentially facilitates the attainment of FC in the Inter-Turbine Burner (ITB). The development of such burner is dependent on knowledge regarding NOx formation and the parameters affecting it. It is known from the literature that the NOx formation mechanisms are different in FC. Therefore, in an attempt to clarify some of the mechanisms involved in NOx formation at relevant conditions, a chemical reactor network model developed to represent the ITB is explored. The role of prompt NOx was previously shown to be dominant at relatively low inlet temperatures and atmospheric pressure. In order to check these findings, five chemical reaction mechanisms were employed. All of them overpredicted NOx emissions and the overprediction is likely to be caused by the prompt NOx subset implemented in these mechanisms. Higher reactants temperatures and operational pressures were also investigated. Overall NOx emissions increased with temperature and the NOx peak moved to lower equivalence ratios. Operational pressure changed the emissions trend with global equivalence ratio. Leaner conditions had behaviour similar to that of conventional combustors (increase in NOx), while NOx dropped with further increase in equivalence ratio due to suppression of the prompt NOx production, as well as an increase in NO reburning. These trends highlight the differences between the emission behaviour of the ITB with those of a conventional combustion system.

Color Indistinction Method for the Determination of Devolatilization Time of Large Fuel Particles in Chemical Looping Combustion

Chemical looping combustion (CLC) is one of the promising fuel conversion technologies for carbon capture with low energy penalty. Devolatilization is an important physical phenomenon occurring during solid fuel CLC. Devolatilization behavior influences fragmentation, combustion rate, emission, and particulates generation in fluidized bed CLC (FB-CLC), thus a critical input for its design. Existing visual techniques for determining devolatilization time cannot be applied in CLC conditions because of its flameless combustion nature. In the present study, a new, simple, and quick technique called “color indistinction method” (CIM) is proposed for the determination of devolatilization time (τd) in FB-CLC, where the end of devolatilization is inferred from the disappearance of fuel particle in a hot fluidized bed. Single-particle devolatilization studies in FB-CLC are conducted to determine the devolatilization time using CIM for two types of fuels, viz., coal and biomass (Casuarina equisetifolia wood), of si...