Effect of fuel ratio of coal on the turbulent flame speed of ammonia/coal particle cloud co-combustion at atmospheric pressure

This study aims to clarify the effect of fuel ratio of coal on the turbulent flame speed of ammonia/coal particle cloud co-combustion at atmospheric pressure under various turbulence intensities. High-fuel-ratio coals are not usually used in coal-fired thermal power plants because of their low flame stability. The expectation is that ammonia as a hydrogen-energy carrier would improve the ignition capability of coal particles in co-combustion. Experiments on spherical turbulent flame propagation of co-combustion were conducted for various coal types under various turbulence intensities, using the unique experimental apparatus developed for the co-combustion. Experimental results show that the flame speed of co-combustion with a low equivalence ratio of ammonia/oxidizer mixture for bituminous coal case was found to be three times faster than that of pure coal combustion and two times faster than that of pure ammonia combustion. On the other hand, the flame speed of co-combustion for the highest-fuel-ratio coal case is lower than that of the pure ammonia combustion case, although the flame propagation can be sustained due to the ammonia mixing. To explain the difference of tendencies depending on the fuel ratio of coal, a flame propagation mechanism of ammonia/coal particle cloud co-combustion was proposed. Two positive effects are the increases of local equivalence ratio and the increases of radiation heat flux, which increases the flame speed. In opposite, a negative effect is the heat sink effect that decreases the flame speed. The two positive effects on the flame speed of co-combustion overwhelm a negative effect for bituminous coal case, while the negative effect overcomes both positive effects for the highest-fuel-ratio coal case. The findings of the study can contribute to the reduction of solid fuel costs when the ammonia is introduced as CO₂ free energy carrier and can improve the energy security through the utilization of high-fuel-ratio coals.     

Turbulent flame propagation limits of polymethylmethacrylate particle cloud–ammonia–air co-combustion

Ammonia is a highly promising energy carrier for achieving a carbon-neutral society. The co-combustion of solid particle clouds–ammonia, in particular, is considered an efficient and feasible method of reducing carbon dioxide emissions. Understanding turbulent flame stabilization and extinguishment processes during the two-phase hybrid-mixture co-combustion of solid particle clouds–ammonia is essential for the co-combustion technology to be used in combustors. To the best of our knowledge, this is the first study to describe the turbulent flame propagation limits and associated mechanism on the co-combustion of solid particle clouds–ammonia–air. Turbulent flame propagation experiments on silica particle clouds–ammonia–air mixing combustion and polymethylmethacrylate (PMMA) particle cloud–ammonia–air co-combustion were conducted in this work using a novel fan-stirred constant-volume vessel to clarify the turbulent flame propagation limits and associate mechanism of solid particle cloud–ammonia–air co-combustion. Results showed that adding inert silica particles contracted the turbulent flame propagation limits of premixed ammonia–air mixtures. However, adding PMMA particles expanded and then contracted the turbulent flame propagation limits of a premixed ammonia–air mixture as the ammonia equivalence ratio increased from lean to rich. In the solid particle cloud–ammonia–air co-combustion, reactive particles induce two types of effects on the turbulent flame propagation limits of premixed ammonia–air mixtures: The local equivalence ratio increment effect is caused by adding volatile matter from preheated particles in the preheat zone of the flame front, and the heat sink negative effect is induced by the unburned particles.     

Spherical turbulent flame propagation of pulverized coal particle clouds in an O2/N2 atmosphere

The present study aims to clarify the effects of turbulence intensity and coal concentration on the spherical turbulent flame propagation of a pulverized coal particle cloud. A unique experimental apparatus was developed in which coal particles can be dispersed homogeneously in a turbulent flow field generated by two fans. Experiments on spherical turbulent flame propagation of pulverized coal particle clouds in a constant volume spherical chamber in various turbulence intensities and coal concentrations were conducted. A common bituminous coal was used in the present study. The flame propagation velocity was obtained from an analysis of flame propagation images taken using a high-speed camera. It was found that the flame propagation velocity increased with increasing flame radius. The flame propagation velocity increases as the turbulence intensity increases. Similar trends were observed in spherical flames using gaseous fuel. The coal concentration has a weak effect on the flame propagation velocity, which is unique to pulverized coal combustions in a turbulent field. These are the first reports of experimental results for the spherical turbulent flame propagation behavior of pulverized coal particle clouds. The results obtained in the present study are obviously different from those of previous pulverized coal combustion studies and any other results of gaseous fuel combustion research.     

Experimental study on co-firing characteristics of ammonia with pulverized coal in a staged combustion drop tube furnace

Utilizing ammonia as a co-firing fuel to replace amounts of fossil fuel seems a feasible solution to reduce carbon emissions in existing pulverized coal-fired power plants. However, there are some problems needed to be considered when treating ammonia as a fuel, such as low flame stability, low combustion efficiency, and high NO_x emission. In this study, the co-firing characteristics of ammonia with pulverized coal are studied in a drop tube furnace with staged combustion strategy. Results showed that staged combustion would play a key role in reducing NO_x emissions by reducing the production of char-NO_x and fuel(NH₃)-NO_x simultaneously. Furthermore, the effects of different ammonia co-firing methods on the flue gas properties and unburned carbon contents were compared to achieve both efficient combustion and low NO_x emission. It was found that when ammonia was injected into 300 mm downstream under the condition of 20% co-firing, lower NO_x emission and unburnt carbon content than those of pure coal combustion can be achieved. This is probably caused by a combined effect of a high local equivalence ratio of NH₃/air and the prominent denitration effect of NH₃ in the vicinity of the NH₃ downstream injection location. In addition, NO_x emissions can be kept at approximately the same level as coal combustion when the co-firing ratio is below 30%. And the influence of reaction temperature on NO_x emissions is closely associated with the denitration efficiency of the NH₃. Almost no ammonia slip has been detected for any injection methods and co-firing ratio in the studied conditions. Thus, it can be confirmed that ammonia can be used as an alternative fuel to realize CO₂ reduction without extensive retrofitting works. And the NO_x emission can be reduced by producing a locally NH₃ flame zone with a high equivalence ratio as well as ensuring adequate residence time.     

Turbulent flame propagation limits of ammonia/methane/air premixed mixture in a constant volume vessel

Ammonia is one of promising energy carriers that can be directly used as carbon-neutral fuel for combustion applications. However, because of the low-burning velocity of ammonia, it is challenging to introduce ammonia to practical combustors those are designed for general hydrocarbon fuels. One of ways to enhance the combustibility of ammonia is by mixing it with other hydrocarbon fuels, such as methane, with a burning velocity is much higher than the burning velocity of ammonia. In this study, we conducted flame propagation experiments of ammonia/methane/air using a fan-stirred constant volume vessel to clarify the effect of methane addition to ammonia on the turbulent flame propagation limit. From experimental results, we constructed the flame propagation maps and clarified the flame propagation limits. The results show that the flame propagation limits were extended with an increase in mixing a fraction of methane to ammonia. Additionally, ammonia/methane/air mixtures with the equivalence ration of 0.9 can propagate at the highest turbulent intensity, even though the peak of the laminar burning velocity is the fuel-rich side because of the diffusional-thermal instability of the flame surface. Furthermore, the Markstein number of the mixture obtained in this research successfully expressed the strength of the diffusional-thermal instability effect on the flame propagation capability. The turbulence Karlovitz number at the flame propagation limit monotonically increases with the decreasing Markstein number.     

Effects of radiation heat loss on laminar premixed ammonia/air flames

Ammonia (NH₃) direct combustion is attracting attention for energy utilization without CO₂ emissions, but fundamental knowledge related to ammonia combustion is still insufficient. This study was designed to examine effects of radiation heat loss on laminar ammonia/air premixed flames because of their very low flame speeds. After numerical simulations for 1-D planar flames with and without radiation heat loss modeled by the optically thin model were conducted, effects of radiation heat loss on flame speeds, flame structure and emissions were investigated. Simulations were also conducted for methane/air mixtures as a reference. Effects of radiation heat loss on flame speeds were strong only near the flammability limits for methane, but were strong over widely diverse equivalence ratios for ammonia. The lower radiative flame temperature suppressed the thermal decomposition of unburned ammonia to hydrogen (H₂) at rich conditions. The equivalence ratio for a low emission window of ammonia and nitric oxide (NO) in the radiative condition shifted to a lower value than that in the adiabatic condition.     

Experimental investigation on ammonia combustion behavior in a spark-ignition engine by means of laminar and turbulent expanding flames

Ammonia combustion appears as a meaningful way to retrieve stored amounts of excess variable renewable energy, and the spark-ignition (SI) engine has been proposed as a practical conversion system. The present work aims at elucidating the combustion characteristics of ammonia blends in engine-relevant turbulent conditions. To that end, laminar and turbulent flame experiments were conducted in a constant-volume vessel at engine-relevant conditions of 445 K and 0.54 MPa to assess the combustion behavior of ammonia/hydrogen/air, ammonia/methane/air and methane/hydrogen/air mixtures observed in an all-metal single-cylinder SI engine. Results show that the respective accelerating or decelerating effects of hydrogen or methane enrichment observed in the SI engine could not be sufficiently explained by the measured laminar burning velocities of the mixtures. Since the latter are very low, the studied combustion regimes are at the boundary between the thin and broken reaction zones regimes, and thus strongly influenced by flame-turbulence interactions. The quantification of the flame response to turbulence shows much higher effects for ammonia blends, than for methane-based fuels. The aforementioned opposite effects of ammonia enrichment with hydrogen or methane are observed on the turbulent burning velocity during the turbulent flame experiments and correlated to the thermochemical properties of the reactants and the flame sensitivity to stretch. The latter may explain an unexpected bending effect on the turbulent-to-laminar velocity ratio when increasing the hydrogen fraction in the ammonia/hydrogen blend. Nevertheless, a very good correlation of the turbulent velocity was found with the Karlovitz and Damköhler numbers, that suggests that ammonia combustion in SI engines may be described following the usual turbulent combustion models. This encourages further investigations on ammonia combustion for the optimization of practical systems, by means of dedicated experiments and numerical simulations.     

Experimental and numerical study of product gas characteristics of ammonia/air premixed laminar flames stabilized in a stagnation flow

Ammonia has widely attracted interest as a potential candidate not only as a hydrogen energy carrier but also as a carbon free fuel for internal combustion engines, such as gas turbines. Because ammonia contains a nitrogen atom in its molecule, nitrogen oxides (NO_x) and other pollutants may be formed when it burns. Therefore, understanding the fundamental product gas characteristics of ammonia/air laminar flames is important for the design of ammonia-fueled combustors to meet stringent emission regulations. In this study, the product gas characteristics of ammonia/air premixed laminar flames for various equivalence ratios were experimentally and numerically investigated up to elevated pressure conditions. In the experiments, a stagnation flame configuration was employed because an ammonia flame can be stabilized by using such a configuration without a pilot flame. The experimental results showed that the maximum NO mole fraction was about 3,500 ppmv, at an equivalence ratio of 0.9 at 0.1 MPa. The NO mole fraction decreased as the equivalence ratio increased. In addition, the maximum value of the NO mole fraction decreased with an increase in mixture pressure. Furthermore, it was experimentally clarified that the simultaneous reduction of NO and unburnt ammonia can be achieved at an equivalence ratio of about 1.06, which is the target equivalence ratio for emission control in rich-lean two-stage ammonia combustors. Comparison of experimental and numerical results showed that even though the reaction mechanisms employed have been optimized for predicting the laminar burning velocity of ammonia/air flames, they failed to satisfactorily predict the measured species in this study. Sensitivity analysis was used to identify elementary reactions that control the species profiles but have negligible effects on the burning velocity. It is considered that these reaction models need to be updated for accurate prediction of product gas characteristics of ammonia/air flames.     

Towards the development of an efficient low-NOx ammonia combustor for a micro gas turbine

Recent studies have demonstrated stable generation of power from pure ammonia combustion in a micro gas turbine (MGT) with a high combustion efficiency, thus overcoming some of the challenges that discouraged such applications of ammonia in the past. However, achievement of low NOx emission from ammonia combustors remains an important challenge. In this study, combustion techniques and combustor design for efficient combustion and low NOx emission from an ammonia MGT swirl combustor are proposed. The effects of fuel injection angle, combustor inlet temperature, equivalence ratio, and ambient pressure on flame stabilization and emissions were investigated in a laboratory high pressure combustion chamber. An FTIR gas analyser was employed in analysing the exhaust gases. Numerical modeling using OpenFOAM was done to better understand the dependence of NO emissions on the equivalence ratio. The result show that inclined fuel injection as opposed to vertical injection along the combustor central axis resulted to improved flame stability, and lower NH₃ and NOx emissions. Numerical and experimental results showed that a control of the equivalence ratio upstream of the combustor is critical for low NOx emission in a rich-lean ammonia combustor. NO emission had a minimum value at an upstream equivalence ratio of 1.10 in the experiments. Furthermore, NO emission was found to decrease with ambient pressure, especially for premixed combustion. For the rich-lean combustion strategy employed in this study, lower NOx emission was recorded in premixed combustion than in non-premixed combustion indicating the importance of mixture uniformity for low NOx emission from ammonia combustion. A prototype liner developed to enhance the control and uniformity of the equivalence ratio upstream of the combustor further improved ammonia combustion. With the proposed liner design, NOx emission of 42?ppmv and ammonia combustion efficiency of 99.5% were achieved at 0.3?MPa for fuel input power of 31.44?kW.     

Turbulent flame speed and morphology of pure ammonia flames and blends with methane or hydrogen

Ammonia appears a promising hydrogen-energy carrier as well as a carbon-free fuel. However, there remain limited studies for ammonia combustion especially under turbulent conditions. To that end, using the spherically expanding flame configuration, the turbulent flame speeds of stoichiometric ammonia/air, ammonia/methane and ammonia/hydrogen were examined. The composition of blends studied are currently being investigated for gas turbine application and are evaluated at various turbulent intensities, covering different kinds of turbulent combustion regimes. Mie-scattering tomography was employed facilitating flame structure analysis. Results show that the flame propagation speed of ammonia/air increases exponentially with increasing hydrogen amount. It is less pronounced with increasing methane addition, analogous to the behavior displayed in the laminar regime. The turbulent to laminar flame speed ratio increases with turbulence intensity. However, smallest gains were observed at highest hydrogen content, presumably due to differences in the combustion regime, with the mixture located within the corrugated flamelet zone, with all other mixtures positioned within the thin reaction zone. A good correlation of the turbulent velocity based on the Karlovitz and Damköhler numbers is observable with the present dataset, as well as previous experimental measurements available in literature, suggesting that ammonia-based fuels may potentially be described following the usual turbulent combustion models. Flame morphology and stretch sensitivity analysis were conducted, revealing that flame curvature remains relatively similar for pure ammonia and ammonia-based mixtures. The wrinkling ratio is found to increase with both increasing ammonia fraction and turbulent intensity, in good agreement with measured increases in turbulent flame speed. On the other hand, in most cases, the flame stretch effect does not change significantly with increasing turbulence, whilst following a similar trend to that of the laminar Markstein length.     

Experimental characterization of spark ignited ammonia combustion under elevated oxygen concentrations

Due to its nature as a carbon free fuel and carrying hydrogen energy ammonia has received a lot of attention recently to be used as an alternative to fossil fuel in gas turbine and internal combustion engines. However, several barriers such as long ignition delay, slow flame speed, and low reactivity need to be overcome before its practical applications in engines. One potential approach to improve the ignition can be achieved by using oxygen enriched combustion. In this study, oxygen-enriched combustion of ammonia is tested in a constant volume combustion chamber to understand its combustion characteristics like flame velocity and heat release rates. With the help of high speed Schlieren imaging, an ammonia-oxygen flame is studied inside the combustion chamber. The influence of a wide range of oxygen concentrations from 15 to 40% are tested along with equivalence ratios ranging from 0.9 to 1.15. Ammonia when ignited at an oxygen concentration of 40% with an equivalence ratio of $\varphi =$ 1.1 at 10 bar has a maximum flame velocity of 112.7 cm/s. Reduced oxygen concentration also negatively affects the flame velocity, introducing instabilities and causing the flame to develop asymmetrically due to buoyancy effects inside the combustion chamber. Heat release rate (HRR) curves show that increasing the oxygen concentration from 21 to 35% of the mixture can help reduce the ignition delays. Peak HRR data shows increased sensitivity to air fuel ratios with increased oxygen concentrations in the ambient gas. HRR also shows an overall positive dependence on the oxygen concentration in the ambient gas.     

Optical diagnostics on coal ignition and gas-phase combustion in co-firing ammonia with pulverized coal on a two-stage flat flame burner

Gradual substitution of coal with green ammonia is a practical approach for the coal power phasedown at a minimal cost of modification, but the ignition and gas-phase reaction during co-firing NH₃ with coal remain largely unclear. In this work, we investigate the co-combustion behaviors of NH₃ and a high-volatile coal on a two-stage flat flame burner. Remarkably, the post-flame oxygen mole fraction X_i,O2 of the inner stage can be manipulated to reproduce a proper reducing-to-oxidizing environment that coal particles experience in the practical combustor. We first reveal that, under certain values of X_i,O2 and NH₃ co-firing energy ratios E_NH3, the reaction intensity (manifested by OH-PLIF signals) in the NH₃-coal flame is stronger than burning either pure coal or NH₃. This synergetic effect originates from an NH₃-combustion-induced enhancement of volatile release. We then propose a characteristic time scale τ_OH from the OH signals for the initiation of overall reactions in the system. In the case of X_i,O2=0, τ_OH monotonically increases with E_NH3, while for X_i,O2=0.2, the trend transitions to a decreasing one. It can be interpreted by comparing τ_OH with the characteristic O₂ diffusion time, coal particle heating time, and the coal pyrolysis time under different X_i,O2. Furthermore, the coal particle ignition in coal-NH₃ flames can no longer be determined by visual images. Instead, we apply CH* chemiluminescence to identify the stages of coal particle ignition and volatile combustion in the NH₃-coal flame. While NH₃ addition has both positive (elevating temperatures & diluting coal particles) and negative (consuming O₂) effects on coal ignition, the combined influence of E_NH3 is marginal on coal ignition delay time. On the other hand, the volatile combustion time decreases linearly with E_NH3, suggesting a pure effect of reduced coal feed rate.     

Effects of equivalence ratio variation on lean,stratified methane–air laminar counterflow flames

The effects of equivalence ratio variations on flame structure and propagation have been studied computationally. Equivalence ratio stratification is a key technology for advanced low emission combustors. Laminar counterflow simulations of lean methane–air combustion have been presented which show the effect of strain variations on flames stabilized in an equivalence ratio gradient, and the response of flames propagating into a mixture with a time-varying equivalence ratio. ‘Back supported’ lean flames, whose products are closer to stoichiometry than their reactants, display increased propagation velocities and reduced thickness compared with flames where the reactants are richer than the products. The radical concentrations in the vicinity of the flame are modified by the effect of an equivalence ratio gradient on the temperature profile and thermal dissociation. Analysis of steady flames stabilized in an equivalence ratio gradient demonstrates that the radical flux through the flame, and the modified radical concentrations in the reaction zone, contribute to the modified propagation speed and thickness of stratified flames. The modified concentrations of radical species in stratified flames mean that, in general, the reaction rate is not accurately parametrized by progress variable and equivalence ratio alone. A definition of stratified flame propagation based upon the displacement speed of a mixture fraction dependent progress variable was seen to be suitable for stratified combustion. The response times of the reaction, diffusion, and cross-dissipation components which contribute to this displacement speed have been used to explain flame response to stratification and unsteady fluid dynamic strain.     

Burning rates of turbulent iso-octane aerosol mixtures in spherical flame explosions

Experimental studies of aerosol combustion under quiescent and turbulence conditions have been conducted to quantify the differences in the flame structure and burning rates between aerosol and gaseous mixtures. Turbulence was generated by variable speed fans to yield rms turbulence velocities between 0.5 and 4.0 m/s and this was uniform and isotropic. Homogeneously distributed and near monodispersed iso-octane-air aerosol clouds were generated using a thermodynamic condensation method. Spherically expanding flames, following central ignition, at near atmospheric pressures were employed to quantify the flame structure and propagation rate. The effects of the diameter of fine fuel droplets on flame propagation were investigated. It is suggested that the inertia of fuel droplets is an important cause of flame enhancement during early flame development. During later stages, cellular flame instability and the effective, gaseous phase, equivalence ratio becomes important. The latter effect leads has increases the flame speed of rich mixtures, but decreases that of lean ones. Droplet enhancement of burning velocity can be significant at low turbulence but is negligible at high turbulence.     

Effects of fuel stratification on ignition kernel development and minimum ignition energy of n-decane/air mixtures

Fuel-stratified combustion has broad application due to its promising advantages in extension of lean flammability limit, improvement of flame stabilization, enhancement of lean combustion, etc. In the literature, there are many studies on flame propagation in fuel-stratified mixtures. However, there is little attention on ignition in fuel-stratified mixtures. In this study, one-dimensional numerical simulation is conducted to investigate the ignition and spherical flame kernel propagation in fuel-stratified n-decane/air mixtures. The emphasis is placed on assessing the effects of fuel stratification on the ignition kernel propagation and critical ignition condition. First, ignition and flame kernel propagation in homogeneous n-decane/air mixture are studied and different flame regimes are identified. The minimum ignition energy (MIE) of the homogeneous n-decane/air mixture is obtained and it is found to be very sensitive to the equivalence ratio under fuel-lean conditions. Then, ignition and flame kernel propagation in fuel-stratified n-decane/air mixture are investigated. The inner equivalence ratio and stratification radius are found to have great impact on ignition kernel propagation. The MIEs at different fuel-stratification conditions are calculated. The results indicate that for fuel-lean n-decane/air mixture, fuel stratification can greatly promote ignition and reduce the MIE. Six distinct flame regimes are observed for successful ignition in fuel-stratified mixture. It is shown that the ignition kernel propagation can be induced by not only the ignition energy deposition but also the fuel-stratification. Moreover, it is found that to achieve effective ignition enhancement though fuel stratification, one needs properly choose the values of stratification radius and inner equivalence ratio.     

Investigating particle and volatile evolution during pulverized coal combustion using high-speed digital in-line holography

Devolatilization is an important process in pulverized coal combustion because it affects the ignition, volatile combustion, and subsequent char burning and ash formation. In this study, high-speed digital in-line holography is employed to visualize and quantify the particle and volatile evolution during pulverized coal combustion. China Shanxi bituminous coal particles sieved in the range of 105–154 µm are entrained into a flat flame burner through a central tube for the study. Time-resolved observations show the volatile ejection, accumulation, and detachment in the early stage of coal combustion. Three-dimensional imaging and automatic particle extraction algorithm allow for the size and velocity statistics of the particle and stringy volatile tail. The results demonstrate the smaller particle generation and coal particle swelling in the devolatilization. It is found that the coal particles and volatiles accelerate due to the thermal buoyancy and the volatiles move faster than the coal particles. On average, smaller particles move faster than the larger ones while some can move much slower possibly because of the fragmentation.     

Experimental and numerical investigations on the interactions of volatile flame and char combustion of a coal particle

The model that takes chemical reactions, heat and mass transfers in the boundary layer of the particle into account simultaneously, is developed for simulating the combustion of a pulverized coal particle. The FTIR in situ temperature-measurements and the comparison between numerical simulations for the pulverized coal and the devolatilized char show that the volatile flame induces the combustion of the primary product of surface oxidation CO. Due to the influence of volatile flame, the char particle can be ignited at temperature lower than its heterogeneous ignition temperature, which elucidates the physical essence of joint hetero-homogeneous ignition mode discovered by Jüntgen.     

单只水平浓缩煤粉燃烧器在1MW燃烧试验台上的试验研究

单只水平浓缩煤粉燃烧器在１ＭＷ燃烧试验台上的试验研究孙绍增，吴少华，李争起，杨明新，王新雷，陈力哲，庞丽君，邢春礼，朱彤，孙恩召，秦裕琨（哈尔滨工业大学动力工程系哈尔滨１５０００１）关键词：水平浓淡燃烧，煤粉燃烧器，稳燃，低ＮＯ_ｘ一、引言电力工业对?..     

稀甲烷/氢气预混湍流传播火焰实验研究

本文采用定容湍流燃烧弹获取了稀甲烷/氢气/空气在强湍流条件下的火焰发展历程,研究了湍流火焰在负马克斯坦数条件下的传播特性.结果表明,湍流火焰呈现自相似传播特性,即使在强湍流条件下,湍流传播火焰仍然会受到不稳定性的影响.并且随着马克斯坦数的减小,不稳定性对湍流传播火焰的影响增强。同时,本文获得一种新的湍流燃烧速度拟合公式,包含了负马克斯坦数条件下不稳定性对湍流燃烧速度的影响。     

Experimental investigation of 40 kWth methane-assisted and self-sustained pulverized biomass flames

This paper presents an assessment of the effects of methane assistance on pulverized biomass swirl flames specifically regarding flow fields and flame structure. Experiments are carried out using a pilot-scale down-fired cylindrical combustion chamber equipped with a swirl burner and biomass/methane fuel mixtures. Studied conditions have an identical thermal output of 40 kW, with the thermal output share of methane gradually decreasing from 50% to 0% while the biomass share (walnut shells) increased from 50% to 100% (self-sustained condition). A detailed flow field characterization of the respective flames is conducted by in-flame, two-dimensional laser Doppler velocimetry measurements. These measurements are complemented by narrow-band flame imaging conducted at two different wavelengths (OH* and CH* radical band heads). Results show that the methane flames have a significant influence on the ignition and the determination of the flame flow field structure, including higher peak and overall velocities, as well as major changes in the ratio of tangential over axial velocity component. Further on, flame attachment of the self-sustained flame can be permanently achieved by the initial, short-term assistance of a methane flame with comparatively low thermal output. These observations are analysed and discussed, where higher measured velocities and overall changes in the flame structure between the self-sustained and the methane-assisted flames are attributed to important local expansion and momentum changes of the combustion gases introduced by the combustion of methane.