Gas-phase Combustion Process Research Articles

Heterogeneous and Combined Heterogeneous/Homogeneous Combustion Modeling of SOFC Off-Gases.

The heterogeneous (catalytic) and the hetero-/homogeneous (catalytic and gas-phase) combustion processes of solid oxide fuel cell (SOFC) off-gases with compositions typical of a high cell utilization rate are investigated with high-fidelity 2D simulations in a platinum-coated planar channel using detailed hetero-/homogeneous chemistry. The pressures are 1-8 bar; the reactant streams have volumetric H2 and CO contents 0.7-1.5 and 5.3-9.7%, respectively; H2O and CO2 dilutions are ∼40 and ∼50%, respectively; and the global fuel/air equivalence ratio is 0.90. Water inhibits chemically the catalytic oxidation of H2, as it leads to high H(s) surface coverage that favors the recombinative desorption of H(s) to H2. On the other hand, H2O promotes chemically the catalytic oxidation of CO by creating high OH(s) coverage that in turn accelerates the CO consumption. Strong flames are established at the highest H2 content cases and for pressures p ≥ 3 bar. For all cases with vigorous homogeneous combustion, the catalytic and gas-phase reaction pathways coexist and compete with each other for the consumption of H2 and CO. The large H2O content leads to gas-phase production of H2 via the reaction H2O + H = H2 + OH. However, the gas-phase produced H2 is subsequently consumed by the catalytic pathway, such that nearly complete H2 conversion is attained at the reactor outlet. Gaseous chemistry does not affect the reactor lengths required for complete H2 conversion but substantially reduces the corresponding lengths for CO conversion. The H2 emissions decrease with increasing pressure and are in the range 8-110 ppmv, while the CO emissions increase with rising pressure and span the range 0.3-52 ppmv, thus leading to corrected CO emissions (at 15% O2) of less than 15 ppmv. Finally, the peak wall temperatures are largely acceptable in terms of catalyst thermal stability.

Numerical study on transient regression rate and combustion characteristics of segregated AP-based oxidizer/TAGN-based fuel

In the present study, a two-dimensional combustion model is established to simulate transient flow and combustion characteristics of a segregated oxidizer/fuel solid motor (SOFSM). A detailed gas-phase reaction kinetic mechanism with 35 species and 190-step reactions is developed to describe the gas-phase combustion process between AP-based oxidizer and TAGN-based fuel. The effects of mass flux and initial temperature of inflow oxidizer gas on the transient flow and combustion characteristics of the TAGN-based fuel are numerically investigated. With the mass flux of inflow oxidizer gas increasing from 249 kg·m−2·s−1 to 995 kg·m−2·s−1, the maximum spatially averaged regression rate of the burning surface of TAGN-based fuel increases by almost 200 % and the combustion efficiency first increases and then decreases with a maximum combustion efficiency of 94.3 % at the mass flux of 442 kg·m−2·s−1. With the initial temperature of inflow oxidizer gas increasing from 1000 K to 1750 K, the maximum average regression rate of the burning surface of TAGN-based fuel increases by around 10 %. Meanwhile, the combustion efficiency increases from 88.38 % to 95.8 %. During the combustion process (almost 3 s), the spatially averaged regression rate remains basically stable from 0.5 s to 2 s and increases slightly from 2.0 s to 3.0 s for given mass flux and initial temperature, indicating the combustion process is basically stable under the studied conditions. The results also show that during the transient combustion process, a large vortex first appears and then is split into a major vortex and a small one in the post-combustion chamber. With the increase of the mass flux and initial temperature of inflow oxidizer gas, the size of the vortex increases while the center position of the vortex moves towards the burning surface, which significantly enhances the mixing process and increases combustion efficiency.

Ignition and combustion experiments on Mg/AP composite fuels in different reaction environments

Magnesium is considered a viable fuel for underwater propulsion owing to its ignitability and the exothermicity of the Mg-water reaction. In this experimental study, we examine the ignition and combustion of Mg/AP composite fuels in three different reaction environments: air, water vapor, and argon. We consider Mg/AP composite fuels containing different Mg loadings and particle sizes. Using thermogravimetry and time-resolved optical imaging, we analyze the thermal oxidation, ignition delay time, steady-state gas-phase combustion processes, and flame temperature of the composite fuels. Our findings show that a water vapor environment can lower the initial oxidation temperature of Mg by 120 °C relative to a baseline air environment. Laser ignition tests reveal that the combustion intensity of Mg/AP composite fuels rises as the Mg loading increases and as the Mg particle size decreases. The ignition delay time of the Mg/AP composite fuels is found to be longer in a water vapor or argon environment than in an air environment. Owing to the intense reaction of Mg in air, the flame temperature is found to exceed that in argon and water vapor environments. The latent heat of evaporation of water tends to lower the flame temperature in the water vapor environment. From these empirical observations, we propose a phenomenological mechanism for the combustion of Mg/AP composite fuels. Overall, this study shows that the Mg loading, Mg particle size, and environmental composition can have profound effects on the ignition and combustion behavior of Mg/AP composite fuels. These findings could be used to steer the development of water-reactive metallic fuels, with a view to improving the design of underwater propulsion systems.

Evaluation of flamelet-based partially premixed combustion models for simulating the gas phase combustion of a grate firing biomass furnace

Depending on the mass flow rates of fuel, the primary air (injected usually from underneath of the fuel bed) as well as the composition of volatile gases extracted from the bed, it is believed that the volatile gases taking part in the gas-phase combustion undergo a partially premixed process close to the bed section. Therefore, the aim of this study is to evaluate the prediction capability of flamelet-based partially premixed combustion models in simulating the gas-phase combustion process of a grate firing biomass furnace. Additionally, the effects of the adopted premixed models (i.e., C Equation based model and extended coherent flame model) and non-premixed models (i.e., steady diffusion flamelet (SFM) and unsteady diffusion flamelet (UFM)) on the overall prediction of partially premixed model are assessed. The predicted temperature field and species concentrations are compared with published experimental measurements and also with numerical simulations which use other combustion models (i.e., EDC/Flamelet hybrid model, SFM and UFM). The results of this study revealed that except for slow forming and chemically dominated species, partially premixed combustion models (both extended coherent flame model/SFM and C-equation/SFM based partially premixed model) are capable of reproducing the experimental temperature and major species with reasonable accuracy. For instance, the predicted temperature deviates from the experimental counterpart by a maximum of 9.01%, and the major species by 15.44% (CO2) and 21.60% (O2). In addition, C-equation/UFM-based partially premixed model is found to be the most optimum combination of the examined partially premixed models for overcoming the deficiency faced while predicting the slow-forming species.

The control mechanisms of lysine sulfate dust deflagration flame propagation based on flame microstructures and time scale analysis

A series of experiments of the deflagration flame propagation of Lysine sulfate (LS) powder were conducted in a vertical tube with different moisture contents, particle sizes and mass concentrations. The thermal flame model was applied to investigate the dust flame micro-structures combined with the ion probes and the fine thermocouples. The time scales of the dust combustion sub-processes were calculated to reveal the control mechanisms during flame propagation. Results showed that a small amount of water significantly reduced the flame thickness and the heat transfer time, like the SEEP effects (Suppressant Enhanced Explosion Parameter). The practicability of applying the thermal flame model for dust flame was proved. As the mass concentration increased, the effect of the pyrolysis enhanced initially and then decreased during the gas phase combustion process, and the effect of heat transfer weakened during the pyrolysis process.

Investigating Dimensional Effects on Predicting Burning Rates of Heterogeneous Solid Propellants

The combustion of a randomly packed, heterogeneous propellant consisting of ammonium perchlorate and hydroxyl-terminated polybutadiene is investigated in a two-dimensional and three-dimensional numerical model to better understand the dimensional effects on the propellant burning rate. A ammonium perchlorate particle is set in a premixed hydroxyl-terminated polybutadiene binder and investigated using a two-dimensional model with finite-rate chemistry and detailed transport properties. The results from the detailed model are compared to two-dimensional and three-dimensional burning-rate codes with reduced chemistry. The gas-phase combustion chemistry is modeled using a reduced four-step mechanism developed from a detailed microscale model of ammonium perchlorate/hydroxyl-terminated polybutadiene combustion. Simple configurations of a single ammonium perchlorate particle and sandwich propellants at varying pressures are compared to determine leading-order effects of two-dimensional and three-dimensional approaches. The reduced chemistry models are then applied to real-world propellants for which a random packing algorithm is used to construct the ammonium perchlorate/hydroxyl-terminated polybutadiene propellant in a periodic cube for a given size distribution of spherical ammonium perchlorate particles and mass ratio of the oxidizer and binder. The decomposition of the ammonium perchlorate and hydroxyl-terminated polybutadiene on the surface is determined at discrete locations and is used to determine the average burning rate over a period of time. This approach combines a more detailed description of the gas-phase combustion processes, the condensed-phase morphology, and physics in a time-dependent simulation in hopes of developing a more predictive model of solid propellant combustion.

Numerical study of a large-scale pulverized coal-fired boiler operation using CFD modeling based on the probability density function method

The paper presents results of numerical modeling of pulverized coal combustion process in the front wall coal-fired boiler. The developed numerical model of the coal combustion process included particle heating, devolatilization, char combustion, as well as turbulent flow and radiative heat transfer. In gas phase combustion process modeling, the mixture fraction approach with PDF was used. Presented modeling results were carried out using the Open Source CFD code – Code_Saturne, and were used to study the pulverized coal combustion in a 225 MWe power plant unit. The input data and boundary conditions were defined based on the collected data about boiler operating conditions and Rosin-Rammler law was used to calculate coal particle distribution. The behavior of pulverized coal and air flow through the burners, as well as the three-dimensional flue gas path through the combustion chamber, was analyzed. Results in form of velocity, temperature, unburned coal and combustion products distribution were presented to show places inside the boiler with high erosion hazard and place where intensive particle deposition may occur, as well as to show the impact of coal particle diameters on Unburned Carbon (UBC) content. Calculation results confirm proper locations of Over-Fire Air (OFA) nozzles and allowing continuing of combustion process at the furnace outlet using only OFA II and OFA III nozzles set. Based on the obtained results is possible to increase the understanding and conduction detailed investigation of boiler operation. The results have been validated with data collected from the power station measurement system and were analyzed in order to find an answer for operational questions regarding optimization of combustion characteristics.

Chain reactions of gas combustion, explosion, and detonation: Chemical methods for process management

It has been shown that gas-phase combustion processes, in contrast to the formed opinion, proceed via a branched-chain mechanism not only under extremely low pressure without temperature change but also at atmospheric pressure and increased pressures in any temperature mode. The features of ignition, flame propagation, explosion, and detonation of gases have been explained on the basis of the theory of nonisothermal chain reactions and experimentally confirmed for pressure and temperature regions important for technology. Chemical methods for the management of these combustion modes have been developed.

Measurement of heat release in laminar diffusion flames fueled by controlled pyrolysis of milligram-sized solid samples: Impact of bromine- and phosphorus-based flame retardants

Brominated flame retardants (BFRs) are widely used in polymers due to their high effectiveness at relatively low cost. Recent studies suggest that certain BFRs may present health and environmental hazards, yet obtaining adequate replacements is an ongoing challenge. To develop new additives, it is of paramount importance to better understand the mechanisms governing the action of flame retardants in polymers, particularly their action in the gas phase, and to develop effective screening techniques for potential candidates. To address this challenge, a novel apparatus, the Milligram-scale Flame Calorimeter (MFC), is proposed. In MFC, the pyrolysis and gas-phase combustion processes are uncoupled. Samples of 35 ± 5 mg are pyrolyzed in an anaerobic atmosphere and their pyrolysis products are burned in a laminar, near axisymmetric diffusion flame under controlled and customizable conditions in a fully enclosed system. Heat release information is obtained through the oxygen consumption calorimetry. The masses of post-combustion pyrolysis residue and solid particulate combustion products (e.g., soot) are measured. In this manuscript, a description of the design, parametric optimization of test conditions, and overall testing methodology is given. Heat release measurements are presented for polystyrene with increasing amounts of brominated polystyrene as well as poly(butylene terephthalate) with increasing concentrations of aluminum diethylphosphinate and results are compared to the Cone Calorimetry (Cone) and Microscale Combustion Calorimetry (MCC) measurements conducted on the same materials. Using combustion efficiency as a metric, the sensitivity to both bromine and phosphorus gas-phase activity is determined. The impact of these flame retardants is pronounced in the Cone experiments and largely undetected by the MCC. The MFC shows trends with respect to the flame retardant content comparable to the Cone. However, the absolute values of MFC combustion efficiencies tend to be notably higher that those observed in the Cone experiments. Overall, the MFC results demonstrate that this novel apparatus can be used to detect gas-phase activity of flame retardants using milligram-sized samples.

Influence of spray/combustion interactions on auto-ignition of methanol spray flames

Auto-ignition of sprays plays a key role in diesel engines, high-altitude relight of aircraft engines, and augmenters. The ignition process occurs exclusively at the small scales and represents the interaction of evaporation, turbulent mixing, and combustion. Here, a large eddy simulation (LES) methodology combined with a probability density function (PDF) approach is used to simulate a series of methanol spray flames. To account for the small-scale interaction between spray droplets and the gas-phase combustion process, a stochastic model is proposed. Detailed chemical kinetics is used along with in situ adaptive tabulation (ISAT) to accelerate the computations. The simulations produce very good agreement with experimental data. From the LES computations, it is postulated that the ignition process happens through two different mechanisms: an entrainment process that traps droplets in high-temperature oxidizer pockets, and a stratified premixed process. It is shown that without the explicit model for the spray/combustion coupling, a more premixed propagation is observed due to the artificial mixing induced by the algorithm. Finally, it is shown that increased subfilter mixing leads to a reduction in peak temperatures, where ignition kernels are dissipated quickly.

Detailed transient numerical simulation of H2/air hetero-/homogeneous combustion in platinum-coated channels with conjugate heat transfer

Transient two-dimensional simulations of fuel-lean H2/air combustion were performed in a 2-mm-height planar channel coated with platinum, using detailed hetero-/homogeneous chemistry and transport as well as heat conduction in the solid wall. The developed model resolved, for the first time, all relevant spatiotemporal scales in a practical channel-flow reactor configuration. A parametric study was carried out to investigate the effects of wall material, inlet velocity, and inlet temperature on the fundamental catalytic and gas-phase combustion processes. Computational singular perturbation (CSP) analysis identified the key catalytic reactions affecting light-off and homogeneous ignition. Homogeneous ignition crucially depended on the OH desorbing fluxes from the catalyst, while flame propagation and stabilization involved time scales of a few milliseconds. During the short duration of the light-off event, the ensuing Stefan velocity appreciably altered the flow field. Predictions of time accurate numerical simulations were further compared against those of a code relying on the quasisteady state assumption, and the specific conditions under which the latter was invalidated were identified. Finally, CSP analysis unraveled the reasons for the high computational cost of the fully transient 2-D simulations. The surface reaction mechanism exhibited a high stiffness with fastest time scales of the order of 10-12s, pertaining to the hydrogen adsorption and to the H(s)+O(s)=OH(s)+Pt(s) reactions. These time scales were in turn six orders of magnitude shorter than the ones associated with gas-phase chemistry or with a simplified single-step catalytic reaction.

Gas-phase combustion processes in light of the theory of non-isothermal chain reactions

The presented experimental and theoretical results demonstrate, in contrast to the concept commonly accepted until recently, the key role of the branching-chain reaction mechanism in gas-phase combustion processes, not only at pressures much lower than atmospheric but at atmospheric and increased pressure as well. Features of combustion, explosion, and detonation, along with critical conditions and the dependence of the process rate on temperature and active impurities, are explained and described by a theory that considers the chain character of reactions. New laws important for theory and practice are revealed and predicted.

A Model for Combustion of Firebrands of Various Shapes

The lifetime of a firebrand before burning out controls the maximum distance a firebrand can travel to cause spotting. Thus, combustion of firebrands of various shapes and sizes and their burnout time during transport is studied. The analysis assumes “quasi-steady” burning. In the present context, “quasi-steady” means that the rate processes controlling the gas phase fuel consumption and energy release are much faster than the particle fuel depletion time or the gas phase transport times. The Reynolds number based on the overall particle dimension and velocity relative to the ambient flow is assumed to be small. The gas phase combustion processes are represented by the evolution of a mixture fraction variable. It is shown that the velocity field near the particle can be described by a potential flow whose functional form is determined by the mass conservation equation and that this flow satisfies the particle surface boundary conditions. Gas phase solutions are obtained for two-parameter family of firebrand shapes composed of oblate and prolate ellipsoids of revolution. Prolate ellipsoids range from a thin needle to a sphere and oblate ellipsoids range from a sphere to a thin disc. Thus, they cover all possible firebrand shapes. The ambient velocity field does not need to be aligned with the firebrand axis of symmetry, so that the composite velocity and mixture fraction fields are three-dimensional. While a variety of steady-state condensed phase models are compatible with this picture, results are first presented for an ablating solid describable by the Spalding B number. B-numbers representative of flaming combustion of wood firebrands and glowing combustion of remaining char are used. All quantities are calculated as a function of ellipsoidal aspect ratio, B number, and the Reynolds number. Surprisingly, it is found that the firebrand burnout time is shape independent. All possible shapes were considered by using oblate and prolate ellipsoids of different sizes and aspect ratios. The burnout time depends only on the firebrand mass under the assumptions used.

A Global Kinetic Model for the Combustion of the Evolved Gases in Wildland Fires

The analysis of combustion kinetics in the gas-phase is decisive for wild land fire behavior modeling. However, the use of detailed reaction mechanisms, which involves a large number of species and reactions, is impractical due to large computational time requirements. The present work proposes a five-step chemical kinetic mechanism to simulate the gas phase combustion processes taking place in wildland fires. Both experimental data and data from simulations run using the PSR code from the CHEMKIN-II package with a detailed kinetic mechanism (GDF-kin 3.0) have been used to calibrate and evaluate the global model under typical wild land fire conditions in terms of the inlet mixture composition, equivalence ratio, and range of temperatures.

Homogeneous combustion of fuel-lean H 2/O 2/N 2 mixtures over platinum at elevated pressures and preheats

The gas-phase combustion of H 2/O 2/N 2 mixtures over platinum was investigated experimentally and numerically at fuel-lean equivalence ratios up to 0.30, pressures up to 15 bar and preheats up to 790 K. In situ 1-D spontaneous Raman measurements of major species concentrations and 2-D laser induced fluorescence (LIF) of the OH radical were applied in an optically accessible channel-flow catalytic reactor, leading to the assessment of the underlying heterogeneous (catalytic) and homogeneous (gas-phase) combustion processes. Simulations were carried out with a 2-D elliptic code that included elementary hetero-/homogeneous chemical reaction schemes and detailed transport. Measurements and predictions have shown that as pressure increased above 10 bar the preheat requirements for significant gas-phase hydrogen conversion raised appreciably, and for p = 15 bar (a pressure relevant for gas turbines) even the highest investigated preheats were inadequate to initiate considerable gas-phase conversion. Simulations in channels with practical geometrical confinements of 1 mm indicated that gas-phase combustion was altogether suppressed at atmospheric pressure, wall temperatures as high as 1350 K and preheats up to 773 K. While homogeneous ignition chemistry controlled gaseous combustion at atmospheric pressure, flame propagation characteristics dictated the strength of homogeneous combustion at the highest investigated pressures. The decrease in laminar burning rates for p ⩾ 8 bar led to a push of the gaseous reaction zone close to the channel wall, to a subsequent leakage of hydrogen through the gaseous reaction zone, and finally to catalytic conversion of the escaped fuel at the channel walls. Parametric studies delineated the operating conditions and geometrical confinements under which gas-phase conversion of hydrogen could not be ignored in numerical modeling of catalytic combustion.

Direct detection of pyridine formation by the reaction of CH (CD) with pyrrole: a ring expansion reaction

The reaction of the ground state methylidyne radical CH (X(2)Pi) with pyrrole (C(4)H(5)N) has been studied in a slow flow tube reactor using Multiplexed Photoionization Mass Spectrometry coupled to quasi-continuous tunable VUV synchrotron radiation at room temperature (295 K) and 363 K, at 4 Torr (533 Pa). Laser photolysis of bromoform (CHBr(3)) at 248 nm (KrF excimer laser) is used to produce CH radicals that are free to react with pyrrole molecules in the gaseous mixture. A signal at m/z = 79 (C(5)H(5)N) is identified as the product of the reaction and resolved from (79)Br atoms, and the result is consistent with CH addition to pyrrole followed by H-elimination. The photoionization efficiency curve unambiguously identifies m/z = 79 as pyridine. With deuterated methylidyne radicals (CD), the product mass peak is shifted by +1 mass unit, consistent with the formation of C(5)H(4)DN and identified as deuterated pyridine (d-pyridine). Within detection limits, there is no evidence that the addition intermediate complex undergoes hydrogen scrambling. The results are consistent with a reaction mechanism that proceeds via the direct CH (CD) cycloaddition or insertion into the five-member pyrrole ring, giving rise to ring expansion, followed by H atom elimination from the nitrogen atom in the intermediate to form the resonance stabilized pyridine (d-pyridine) molecule. Implications to interstellar chemistry and planetary atmospheres, in particular Titan, as well as gas-phase combustion processes, are discussed.

Accelerated diffusion of chain carriers and kinetic features of heterogeneous processes in gas-phase chain reactions

In gas-phase combustion processes, the regeneration of free atoms and radicals in chain propagation reactions enhances the diffusion flux of these species from the flame zone. In flame propagation in tubular reactors and in filtration combustion, this effect facilitates the access of chain carriers to the surface even at atmospheric pressure, increases the role of heterogeneous reactions (primarily chain termination), and enhances heat removal due to heterogeneous recombination.

Grate Furnace Combustion: A Submodel for the Solid Fuel Layer

The reduction of NOx formation in biomass-fired grate furnaces requires the development of numerical models. An essential part of such a model is the submodel for the conversion of the solid fuel on the grate. The submodel presented is able to predict the velocity of the conversion front as well as spatial profiles of porosity, species mass fraction and temperature. The possibility to make use of detailed kinetic mechanisms previously applied only for gas-phase combustion processes is a new feature of our model. Application of these mechanisms is essential step for future toward future NOx predictions.

Crystallogenesis of Particles Formed in Hydrocarbon Combustion

The formation of crystallinity in combustion generated soots is explored by three traditional methods that are based on their sensitivity to the Bragg diffraction produced by multilayer atomic structures. These techniques are high resolution electron microscopy (HRTEM), X-Ray diffraction (XRD), and dark field transmission electron microscopy )DFTEM). These methods provide complementary information on the nature of the crystallinity in combustion generated soots. The lattice parameters of both flame generated soots and diesel soots are consistent with the structure of disordered carbons with graphitic basal planes. The initial evidence of crystallinity in flame generated particles is detected by XRD in the precursor particles that previously have been found to contain multiring polycyclic aromatic hydrocarbons (PAHs). These results are consistent with the hypothesis of Oberlin (1984) that PAHs in carbonizing hydrocarbon pitches are assembled in parallel layers to produce diffraction peaks. The more intense display of diffraction maxima is evident in DFTEM when the particles sampled from flames undergo the transformation from isolated precursor spheroids to clustered aggregates during the carbonization process. These results support the view that the PAHs initially formed in the gas phase combustion processes undergo a series of transformations in which the hexagonal geometry is preserved and form the basal planes of the crystallites found carbonaceous soot particles. This description evidently applies to the formation of soot from a wide variety of hydrocarbon fuels burned in various combustion devices ranging from gaseous-fueled laboratory burners to diesel engines.

Combustion of methane in catalytic honeycomb monolith burners

The heat transfer efficiency, stability, and pollutant emissions characteristics of ultra-lean methane-air combustion in some precious metal-based honeycomb monoliths were investigated. The interpretation of the experimental results was assisted using numerical modelling of the gas-phase combustion process. The thermal radiation output of the monoliths varied between 27 and 30 per cent of the thermal input, and this compared favourably with equivalent porous inert media burners. The minimum fuel concentrations for very-low emission stable combustion were found to be significantly lower than for conventional gas-phase combustion and were shown to vary with the nature and loading of the catalyst, as well as with flow rates. The palladium catalyst was found to have a larger window of mixture strengths and flow rates for stable operation than the platinum one. During all the runs under stable combustion conditions, only extremely small amounts of CO, NO x and unburnt hydrocarbons were detected. Thus, the operating conditions verified 'near-zero' pollutant emissions that only a catalytic combustion process can achieve at present. Temperature profiles inside the monoliths channels proved that the catalyst's role was not only to enable the ignition of fuel mixtures below flammability limits, but also to ensure the complete oxidation of the fuel to CO 2 via surface reactions in the steady state. The reaction zone inside the catalysts was found to end at about 10 mm from the monolith's entrance. The effect of monolith length was investigated and a reduction of 70 per cent in the original length was found possible.