Radiation intensity of lignite-fired oxy-fuel flames

The radiative heat transfer in oxy-fuel flames is compared to corresponding conditions in air-fuel flames during combustion of lignite in the Chalmers 100 kW oxy-fuel test facility. In the oxy-fuel cases the flue-gas recycle rate was varied, so that, in principle, the same stoichiometry was kept in all cases, whereas the oxygen fraction in the recycled flue-gas mixture ranged from 25 to 29 vol.%. Radial profiles of gas concentration, temperature and total radiation intensity were measured in the furnace. The temperature, and thereby the total radiation intensity of the oxy-fuel flames, increases with decreasing flue-gas recycle rate. The ratio of gas and total radiation intensities increases under oxy-fuel conditions compared to air-firing. However, when radiation overlap between gas and particles is considered the ratios for air-firing and oxy-fuel conditions become more similar, since the gas-particle overlap is increased in the CO₂-rich atmosphere. A large fraction of the radiation in these lignite flames is emitted by particles whose radiation was not significantly influenced by oxy-fuel operation. Therefore, an increment of gas radiation due to higher CO₂ concentration is not evident because of the background of particle radiation, and, the total radiation intensities are similar during oxy-fuel and air-fuel operation as long as the temperature distributions are similar.     

Measurement of adiabatic burning velocity in natural gas-like mixtures

Experimental measurements of the adiabatic burning velocities were carried out for natural gas-like mixtures burning in air over a range of equivalence ratios at atmospheric pressure. Effect of CO₂ dilution up to 60%, N₂ dilution up to 40% and 25% enrichment of ethane on burning velocity of methane–air flames were studied. Heat flux method with setup similar to that of [K.J. Bosschaart, L.P.H. de Goey, Detailed analysis of the heat flux method for measuring burning velocity, Combustion and Flame 132 (2003) 170–180] was used for measurement of burning velocities. Initially experiments were done for methane–air and ethane–air mixtures at various equivalence ratios and the results were in good agreement with published data in the literature. Computations were performed using PREMIX code with GRI 3.0 reaction mechanism for all the mixtures. Predicted flame structures were used to the explain the effect of N₂ and CO₂ dilution on burning velocity of methane–air flames. Peak burning velocity for CH₄/CO₂–air mixtures occur near to ϕ = 1.0.     

Measurement of propagation speeds in adiabatic cellular premixed flames of CH4 + O2 + CO2

Experimental measurements of the propagation speed of adiabatic flames of methane + oxygen + carbon dioxide are presented. The oxygen content O₂/(O₂ + CO₂) in the artificial air was 31.55% and 35%. Non-stretched flames were stabilized on a perforated plate burner at atmospheric pressure. A heat flux method was used to determine propagation speeds under conditions when the net heat loss of the flame is zero. Under specific experimental conditions the flames become cellular; this leads to significant modification of the flame propagation speed. The onset of cellularity was observed throughout the stoichiometric range of the mixtures studied. Measurements in cellular flames are presented and compared with those for laminar flat flames. Cellularity disappeared when the flames became only slightly sub-adiabatic. Visual and photographic observations of the flames were performed to quantify their cellular structure. Increasing the oxygen content in the artificial air and increasing the temperature of the burner plate led to increase of the number of cells observed.     

Design and operation assessment of an oxyfuel fluidized bed combustor

Oxyfuel combustion is a promising alternative for CO₂ capture. While this has been proven in pulverized fuel (PF) burners, research in fluidized bed (FB) reactors is still scarce. Our work aims to increase the knowledge about this technology. To this purpose, a 95 kW_th FB oxyfuel combustion test rig has been erected. Its main characteristics are described in this paper, giving detailed information on the subsystems: the FB reactor, the fuel and oxidant supplies, and ancillaries. Plant flexibility is emphasized. It allows to operate under different CO₂/O₂ ratios, and to recycle CO₂ from the flue gases. Both the processes design and monitoring are supported by simulations that have been validated against experimental data, regarding fluid dynamics, combustion, and heat transfer. Finally, the performance of the facility has been tested both with coal alone and blended with biomass. CO₂ concentrations over 90% (dry basis) in the flue gases have been obtained. Comparison of air and oxygen combustion tests and operational recommendations are discussed, confirming the feasibility of the FB oxyfuel technology for CO₂ capture purposes.     

The Structure of the Auto-Ignition Region of Turbulent Dilute Methanol Sprays Issuing in a Vitiated Co-flow

This paper presents planar imaging of laser induced fluorescence (LIF) from key reactive species in the auto-ignition region of dilute turbulent spray flames of methanol. High-speed (5?kHz) LIF-OH imaging as well as low speed (10?Hz) imaging of joint LIF-OH-CH₂O is performed. The product of the OH and CH₂O signals is used as a qualitative indicator of local heat release. The burner is kept intentionally simple to facilitate computations and the spray is formed upstream of the jet exit plane and carried with air or nitrogen into a hot co-flowing stream of vitiated combustion products. The studied flames are all lifted but differ in the shape of their leading edge and heat release zones. Similarities with auto-ignition of gaseous fuels, as well as differences, are noted here. Formaldehyde is detected earlier than OH implying that the former is a key precursor in the initiation of auto-ignition. Growing kernels of OH that are advected from upstream, close in on the jet centreline and ignite the main flame. The existence of double reaction zones in some flames may be due to ignitable mixtures formed subsequent to local evaporation of droplets and subsequent mixing. When air is used as spray carrier, reaction zones broaden with distance, possibly due to increased partial premixing and regions of intense heat release occur near the flame centreline further downstream. With nitrogen as carrier, the flame maintains a nominal diffusion-like structure with reaction zones of uniform width and substantially less concentration of heat release on the flame centreline.     

Radiative heat transfer augmentation of natural gas flames in radiant tube burners with porous ceramic inserts

Experiments were conducted using porous ceramic inserts to enhance the radiative heat transfer from natural gas flames in a straight-through radiant tube burner. The performance of the radiant tube burner with partially stabilized zirconia and silicon carbide inserts is compared to a baseline case of no inserts at three levels of combustion air preheat. Spectral intensities, temperatures within the radiant tube burner, tube wall temperatures, and exhaust temperatures were measured to determine the effectiveness of the enhanced heat transfer due to the inserts. Exhaust emission constituents were also measured to determine the effect that the inserts have on exhaust products. NO_x emissions are reduced by up to 30% with the inserts. The silicon carbide inserts have higher spectral intensities and total radiative energy transfer than partially stabilized zirconia inserts. Both inserts have enhanced radiant heat transfer compared to the no-insert configuration, with the radiative enhancement due to inserts as great as five times that of the no-insert configuration. The net result is increased tube wall temperatures and decreased exhaust temperatures with the ceramic inserts.     

Radiative heat transfer calculations in real gases

A general formulation for radiative heat transfer calculations is presented, based on integrated quantities such as total emissivities and absorptivities. The procedure is intended particularly for combustion chamber applications of varying degree of complexity, the radiative active medium consisting of gases such as H₂O and CO₂ and of soot. First, some preliminary calculations are given for the often treated radiative equilibrium cases of plane parallel plates and infinite concentric cylinders. Then an example of a combustion chamber calculation is studied where the radiative heat transfer calculation is included in a system of partial differential equations describing momentum, heat and mass transfer with combustion.     

Similarity Issues of Kerosene and Methane Confined Flames Stabilized by Swirl in Regard to the Weak Extinction Limit

One of the most promising methods for reducing NO _x emissions of jet engines is the lean combustion process. For realization of this concept the percentage of air flowing through the combustor dome has to be drastically increased, which implies high volume fluxes in the primary zone of the combustion chamber and represents a substantial challenge in regard to the flame stabilization. Swirl motion is thus applied to the air flux by the swirl generator and decisively contributes to the flame stabilization. The current paper reviews an atmospheric investigation of a burner configuration in regard to the weak extinction limit, comprising a confined non-premixed swirl-stabilized flame. The burner can be supplied with either kerosene or after a small adaption with natural gas (methane). Therefore, a comparison of a kerosene-fuelled flame (spray flame) to a natural gas fuelled one (methane flame) can be performed. Both are realized by almost identical burner configuration and at identical conditions. The main idea of this work is to align the stability characteristics of both flames by means of similarity. However, fundamental differences regarding the flame structures of the flames are detected through in-flame measurements. This determines the limits of the current approach and motivates an appropriate choice of flame modeling.     

Forced convective heat transfer from premixed flames —Part 2: Impingement heat transfer

A study of convective heat transfer from impinging flames is completed with the presentation of heat transfer rates measured in premixed methane-air flames. Unburnt gas equivalence ratios from 0.8 to 1.2 have been examined, with burner exit Reynolds numbers ranging from 2000 to 12 000. Heat fluxes measured at the stagnation point of a body of revolution and a circular cylinder demonstrate that the trends observed in measured heat flux profiles are mainly determined by variations in the mean velocity and temperature within a flame, with peak heat transfer rates occuring within or close to the flame reaction zone. Increases in Reynolds number lead to an increase in the peak heat flux attained within a flame and to a decrease in the axial extent of the flame equilibrium region. Variations in equivalence ratio away from approximately stoichiometric conditions lead to a decrease in the maximum rate of heat transfer from a flame and to a shifting of the position of maximum flux downstream. Theoretical predictions applicable to the equilibrium region of the flames are in reasonable accord with experimental data.     

Experimental study of biogas combustion using a gas turbine configuration

The aim of the present work is to compare stability combustion domains, flame structures and dynamics between CH₄/air flames and a biogas/air flames (issued from waste methanisation) in a lean gas turbine premixed combustion conditions. Velocity profiles are obtained by Laser Doppler Anemometry measurements. CH* chemiluminescence measurements and temporal acquisition of chamber pressure are performed in order to describe flame structure and instabilities. Changes in flame structure and dynamics when fuel composition is varying are found to strongly depend on laminar flame speed. No clear correlation between the unstable flame and the reaction zone penetration in the corner recirculation can be found.     

Influence of the temperature of the surface of an axisymmetrical body and forward radiation on the distribution of a radiant flux on its surface in hypersonic flow-past

A solution is obtained of the flow-past problem for an axisymmetrical body with steady-state hypersonic nonviscous, space-radiating gas flow in a hypersonic approximation. It is shown as illustrated by the example of flow-past of a sphere by an air flow, that the relative distribution of the radiant flux weakly depends on a calculation of surface re-radiation, while the size of the radiant flux substantially depends on body temperature T_W at a critical point. The distributions of radiant flux for sphere flow-past by a CO₂-N₂ gas mixture (at T_W = 0) are calculated using a previously developed method. It is shown that different CO₂ contents in the initial mixture of the incident gas flow weakly affect this distribution. The dependence of the distribution of the radiant flux and departure of the shock wave on the boundary condition for gas enthalpy in the pressure shock, taking into account forward radiation, is investigated. Asymptotic expressions are obtained for sphere flow-past for the case of a strongly radiating gas. Distributions of the radiant flux for different assumptions for the boundary conditions in shocks are calculated.     

Experimental study of the synthesis of fused silica by direct combustion hydrolysis

Oxy-fuel flames for direct combustion hydrolysis of fused silica (DQ) are characterized, using non- intrusive optical measurement techniques only. Flow, temperature, concentrations, development of silica nano-particles in the flame, and surface temperature of the glass in the flame are measured. The setup used for characterization of particle distribution via Rayleigh scattering as well as mandatory improvements of the Raman/Rayleigh technique for temperature and concentration measurements in oxy-fuel flames have been developed in the framework of this study and are presented. The measurement techniques herein demonstrated are not only capable of describing these special extremely hot flames, but are broadly applicable in oxy-fuel flames as well as in chemical vapor deposition (CVD) processes. The presented data evidently shows that if the special character of oxy-fuel flames is taken into account, some of the results drawn from earlier investigations into CVD, concerning particle growth, flame stability, and particle deposition efficiency, are transferable into DQ. From the extensive data given, connections between different information are detected and help to reduce required measurements for further investigation and point to simple techniques that might be used for online process monitoring, at least during research and development of similar flames. Received: 18 December 2000 / Accepted: 14 June 2001     

Assessment of an unstructured exponential scheme discrete ordinates radiation model for non-gray media

For radiative transfer in complex geometries, Sakami and his co-workers have developed a discrete ordinates method (DOM) exponential scheme for unstructured meshes which was mainly applied to gray media. The present study investigates the application of the unstructured exponential scheme to a wider range of non-gray scenarios found in fire and combustion applications, with the goal to implement it in an in-house Computational Fluid Dynamics (CFD) code for fire simulations. The original unstructured gray exponential scheme is adapted to non-gray applications by employing a statistical narrow-band/correlated-k (SNB-CK) gas model and meshes generated using the authors’ own mesh generator. Different non-gray scenarios involving spectral gas absorption by H₂O and CO₂ are investigated and a comparative analysis is carried out between heat flux and radiative source terms predicted and literature data based on ray-tracing and Monte Carlo methods. The maximum discrepancies for total radiative heat flux do not typically exceed 5%.     

Experiments on the instability modes of buoyant diffusion flames and effects of ambient atmosphere on the instabilities

Large scale dynamic behavior of buoyant diffusion flames were studied experimentally. It was found that buoyant diffusion flames originating from circular nozzles exhibit two different modes of flame instabilities. The first mode results in a sinuous meandering of the diffusion flame, characteristic of flames originating from small diameter nozzles. This instability originates at some distance downstream of the nozzle exit in the contraction region of the buoyant flame envelope and develops into a sinuous motion of the flame. The second mode is the varicose mode which develops very close to the nozzle exit as axisymmetric perturbations of a contracting flame surface. In this mode, flame oscillations result in the formation of toroidal vortical structures that convect through the flame and cause periodic burn out at the flame top resulting in the observed flame height fluctuations. The average flame heights are found to be typically shorter for these flames. The oscillation frequencies and their scaling for the two modes are also different with the sinuous mode having higher frequencies than the varicose mode. It was also observed that the instability can switch from one mode to the other and the probability of observing the varicose mode appears to increase with increasing Richardson number. Additionally, the feasibility of altering the behavior of buoyant diffusion flames was explored through variation of the oxidizer medium density. It was found that the flame oscillations can be completely suppressed for flames burning in helium rich helium–oxygen mixtures. At lower helium concentrations, the oscillation frequency can be significantly reduced. In order to enhance the buoyancy effect, CO₂–O₂ mixtures were also studied. However, the density increase and its effects on flame oscillation frequency were found to be small compared to those flames burning in air. These experiments point towards the feasibility of altering buoyant flame behavior under earth gravity and studying the large scale dynamical aspects of buoyant flames without the need of variable gravity environment. Received: 2 March 1999/Accepted: 6 August 1999     

Rich n-heptane and diesel combustion in porous media

Rich n-heptane and diesel flames in two-layer porous media are experimentally investigated in the context of syngas production. The stable operating points of n-heptane reforming have been determined and the mole fractions of H₂, CO, CO₂ and light hydrocarbons have been measured in the exhaust gas at an equivalence ratio of 2 for different thermal input values. The reformer performance has been assessed also from the point of view of the heat losses and the mixture homogeneity. The pre-vapouriser produces an approximately uniform vapour–air mixture upstream of the flame front. The range of flow rates for stable flames decreased with increasing equivalence ratio. Heat losses were about 10% of the thermal input at high firing rates. A 77.2% of the equilibrium H₂ was achieved at a flame speed of 0.82 m/s. The same reactor with a different porous matrix for the reforming stage demonstrates diesel reforming to syngas with a conversion efficiency of 77.3% for a flame speed of 0.65 m/s.     

Effect of swirl intensity on the flow and combustion of a turbulent non-premixed flat flame

An experiment in a turbulent non-premixed flat flame was carried out in order to investigate the effect of swirl intensity on the flow and combustion characteristics. First, stream lines and velocity distribution in the flow field were obtained using PIV (Particle Image Velocimetry) method in a model burner. In contrast with the axial flow without swirl, highly swirled air induced streamlines going along the burner tile, and its backward flow was generated by recirculation in the center zone of the flow field. In the combustion, the flame shape with swirled air also became flat and stable along the burner tile with increment of the swirl number. Flame structure was examined by measuring OH and CH radicals intensity and by calculating Damkohler number (Da) and turbulence Reynolds number (Re _T ). It appeared that luminescence intensity decreased at higher swirl number due to the recirculated flue gas, and the flat flames were comprised in the wrinkled laminar-flame regime. Backward flow by recirculation of the flue gas widely contacted on the flame front, and decreased the flame temperature and emissions concentration as thermal NO. The homogeneous temperature field due to the widely flat flame was obtained, and the RMS in the high temperature region was rather lower at higher swirl number. Consequently, the stable flat flame with low NO concentration was achieved.     

Effect of turbulence on flame radiative emission

A collection of radiative emissions by several flames is analyzed. Such flames have been obtained in a number of burners, either premixed or not, fed with different fuels, while radiative emission is collected by means of a photo-diode, whose sampled signal is able to carry a large quantity of information about chemistry of flames and its interaction with turbulence. All the spectra computed show a decaying trend towards high frequencies with a slope of −5/3, that is known to be the inertial scaling of kinetic energy for homogeneous, isotropic, nonreacting turbulent flows. The aim of this work is to propose a physical model for the interpretation of the radiant energy scaling law coming from this simple instrument. To this purpose radiative emission of the flame has been splitted into two contributions, and its dynamics analyzed. The first contribution is the chemiluminescence effect, whereas the other is to be accounted for thermal emission of a gray/black body, ruled by Planck radiative equation. The time fluctuation of radiative emission is proposed to be linked to a thin reacting surface fluctuating in time and space under the constraint of turbulent fluid dynamic field. The thin reacting surface should be considered as the local flame front in premixed combustion, and as the local stoichiometric mixture fraction front in nonpremixed case. Taking into account such emission mechanisms, a physical interpretation of the −5/3 slope is proposed.     

Structures and stabilization of low calorific value gas turbulent partially premixed flames in a conical burner

Experiments are carried out on partially premixed turbulent flames stabilized in a conical burner. The investigated gaseous fuels are methane, methane diluted with nitrogen, and mixtures of CH₄, CO, CO₂, H₂ and N₂, simulating typical products from gasification of biomass, and co-firing of gasification gas with methane. The fuel and air are partially premixed in concentric tubes. Flame stabilization behavior is investigated and significantly different stabilization characteristics are observed in flames with and without the cone. Planar laser induced fluorescence (LIF) imaging of a fuel-tracer species, acetone, and OH radicals is carried out to characterize the flame structures. Large eddy simulations of the conical flames are carried out to gain further understanding of the flame/flow interaction in the cone. The data show that the flames with the cone are more stable than those without the cone. Without the cone (i.e. jet burner) the critical jet velocities for blowoff and liftoff of biomass derived gases are higher than that for methane/nitrogen mixture with the same heating values, indicating the enhanced flame stabilization by hydrogen in the mixture. With the cone the stability of flames is not sensitive to the compositions of the fuels, owing to the different flame stabilization mechanism in the conical flames than that in the jet flames. From the PLIF images it is shown that in the conical burner, the flame is stabilized by the cone at nearly the same position for different fuels. From large eddy simulations, the flames are shown to be controlled by the recirculation flows inside cone, which depends on the cone angle, but less sensitive to the fuel compositions and flow speed. The flames tend to be hold in the recirculation zones even at very high flow speed. Flame blowoff occurs when significant local extinction in the main body of the flame appears at high turbulence intensities.