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The interaction of coal particles and recycled/recirculated flue gas (RFG) with elevated temperatures and low levels of oxygen occurs in various pulverised coal combustion scenarios. In this work, the effect of oxygen level and temperature on single coal particle combustion characteristics and NO x formation in N 2 diluent is studied by means of fully-resolved particle simulations. Comprehensive gas-phase kinetics are utilised to consider the critical pathways of NO x formation including tar-N. Results show that higher RFG temperatures decrease the time to reach the peaks of temperature and species profiles and increase the corresponding peak values. When decreasing O 2 , irrespective of the RFG temperature, the fuel release period is prolonged, the volatile combustion time increases and the combustion process becomes overall less intense. The reduction of O 2 in RFG results in a significant decrease of NO production, while the reduction of the RFG temperature has a smaller effect. The analysis of the key reactions that contribute to NO production in the region around stoichiometry shows that fuel-NO x is the major contributor. Both NH 3 and HCN in fuel-N play a major role, while tar-N only contributes in the case with the lowest temperature and O 2 concentration. The classical NO x formation pathways are negligible and the initiation reaction of the Zeldovich mechanism is even reversed, i.e. NO ⟶ + N N 2 is dominant and contributes to NO destruction. The destruction of NO mainly occurs in a rich region close to the particle surface where abundant tar species and their derivatives play a major role for NO destruction via the re-burn mechanism. The prompt mechanism is also active in this region and eventually contributes to NO reduction via production of HCN which is the feed to the re-burn mechanism. [ABSTRACT FROM AUTHOR]

Motion and swelling behavior of single bituminous coal particles during volatile combustion are investigated in a laminar flow reactor using a joint experimental and numerical approach. Three different particle samples with mean diameters of 90, 120, and 160 µm are studied in a conventional N 2 /O 2 atmosphere with 20 vol% O 2 mole fraction. Diffuse backlight-illumination (DBI) measurements with high temporal (10 kHz) and spatial (> 19 lp/mm) resolutions, combined with detailed parameter evaluation methods, provide fundamental insights into interactions of particle with flow and flame. The acceleration behavior of different particles is assessed based on the response time following the viscosity drag law. Rotation speed is determined by temporally tracking the orientation angle and shown to strongly correlate with the particle size and the devolatilization process. Simultaneously measured slip velocity and particle diameter enable evaluating time-dependent particle Reynolds numbers R e p. The swelling behavior is temporally synchronized with the devolatilization process and reveals a strong dependency on particle diameters. To better understand experimental observations, detailed simulations are first quantitatively validated against experimental ignition delay times and then applied to predict particle temperature histories. Further, the reduction of particle heating rates with increasing diameters is numerically quantified. The maximum swelling ratio decreases from 1.22 to 1.07 as the heating rate increases from approximately 3 × 10 4 to 8 × 10 4 K/s. [ABSTRACT FROM AUTHOR]

The present work reports an experimental study of particle group combustion of pulverized bituminous coal in laminar flow conditions using advanced multi-parameter optical diagnostics. Simultaneously conducted high-speed scanning OH-LIF, diffuse backlight-illumination (DBI), and Mie scattering measurements enable analyses of three-dimensional volatile flame structures and soot formation in conventional (i.e., N 2 /O 2) and oxy-fuel (i.e., CO 2 /O 2) atmospheres with increasing O 2 enrichment. Particle-flame interaction is assessed by calculating instantaneous particle number density (PND), whose uncertainties are estimated by generating synthetic particles in DBI image simulations. Time-resolved particle sequences allow the evaluation of the particle velocity, which indicates a PND dependency and interactions between particles and volatile flames. 3D flame structure reconstruction and soot formation detection are first demonstrated in single-shot visualizations and then extended to analyze effects of O 2 concentration, PND, and inert gas composition statistically. The increasing O 2 concentration significantly reduces local flame extinction and suppresses soot formation in N 2 and CO 2 atmospheres. Volatile flames reveal higher intensities and lower lift-off heights as O 2 concentration increases. In contrast to that, an increased PND leads to earlier flame extinction and stronger soot formation due to the local gas temperature reduction and oxygen depletion. The lift-off height reduces with increasing PND, which is explained by the complex interaction between particle dynamics, heat transfer, and volatile reactions. Slightly stronger soot formation and delayed ignition are observed in CO 2 atmospheres, whereas CO 2 replacement reveals insignificant influences on the flame extinction behavior. Finally, non-flammability is quantified for particle group combustion at varying PNDs in different atmospheres. [ABSTRACT FROM AUTHOR]

The introduction of fibrous aspherical biomasses for the substitution of coal as a sustainable solid fuel causes complex particle-turbulence interactions, which need to be captured using minimally invasive laser-based diagnostics. High-resolution two-phase particle image velocimetry (PIV) of high-velocity biomass-laden turbulent flows require laser inter-pulse times of only a few microseconds, which is usually achieved with separate laser systems for low-speed statistical and time-resolved measurements. In this work, we introduce a novel approach to achieve low-speed dual-pulse and ultra-high-speed time-resolved two-phase PIV using a single laser system. The flexible repetition rate laser output is realized by coupling a continuously pulsing fiber laser to an acousto-optic deflector (AOD) for pulse picking. The selective deflection of two adjacent laser pulses into the measurement volume is presented at a camera repetition rate of 25 Hz and laser repetition rate of 312.5 kHz resulting in an inter-pulse time of 3.2 µs. Temporal limitations of the AOD pulse picking system are evaluated beyond the access time limit for partially overlapping pulse pairs. To demonstrate the feasibility of the diagnostic system, low-speed measurements and additional ultra-high-speed measurements at 200 kHz are performed at high spatial resolutions to resolve individual biomass particles and their surrounding flow field in a non-reacting turbulent round jet. Simultaneous diffuse back-illumination measurements using a pulsed LED for particle size measurements of a high-aspect-ratio biomass (miscanthus) are conducted. The diagnostic system offers the capability of determining the slip velocity field and projected particle size, enabling a direct measurement of the local particle Reynolds number under high-velocity turbulent conditions at low and ultra-high repetition rates. [ABSTRACT FROM AUTHOR]

The transition from single to group high-volatile bituminous (hvb) coal particle combustion is experimentally investigated in a laminar flow reactor using state-of-the-art laser diagnostics. Simultaneous volumetric OH-LIF imaging using a novel laser scanning technique is combined with time-resolved diffuse backlight-illumination (DBI) providing fundamental insights into flame topologies. Three-dimensional visualizations of volatile flames are first demonstrated on single particle combustion and then applied to characterize the flame topology associated with group particle combustion. The particle number density (PND) determined by instantaneous DBI images covers a wide range from the individual particle combustion to group combustion enabling the determination of transitional effects. While an enclosed volatile flame is observed with dominant spherical structures at low PND, enveloping flames surrounding non-flammable regions reveal distinct features as PND increases. The corresponding physical process is described by exploring the effect of local gas temperatures. The particle velocity affected by the inter-particle and particle-gas interaction is analyzed along the axial and radial direction. The ignition delay time increases gradually as PND increases. The non-flammable region is quantified by evaluating the non-flammable volume ratio R nf. The non-flammability becomes pronounced if PND exceeds a limit of approximately 0.37 mm − 3 , which corresponds to an inter-particle distance of 4d P in this study. [ABSTRACT FROM AUTHOR]

The ignition and combustion of coal particle groups are investigated numerically in a laminar flow reactor. The Flamelet Generated Manifold method is extended to account for the complex mixture of gases being released during devolatilization, which is calculated with a competing two-step model. A second mixture fraction is introduced to include the mixing with the second methane fuel stream. The interactions of the gas phase with particles are modeled within a fully coupled Euler-Lagrange framework. To investigate the influence of particle groups on ignition and combustion, successively increasing densities of particle streams have been analyzed. The ignition delay time is increased significantly by higher particle densities. This delay is validated successfully with the available measurements. Moreover, the shape of the volatile flame was found to be strongly influenced by the particle number density inside the flame. A transition from spherical flames around single particles to a conical flame around the particle cloud could be found in numerical results as well as in experiments. As the primary mechanism for the substantial ignition delay and the formation of the flame, the increased heat transfer from the gas-phase to the particle group, resulting in lower gas-phase temperatures, was identified. [ABSTRACT FROM AUTHOR]

High-speed, three-dimensional (3D) scalar-velocity field measurements are demonstrated in a lifted partially-premixed dimethyl-ether/air jet flame using simultaneous laser-induced fluorescence (LIF) of formaldehyde (CH 2 O) and tomographic particle image velocimetry (TPIV). The 3D LIF measurements are performed by raster scanning the laser beam from a 100 kHz pulse-burst laser across the probe volume using an acousto-optic deflector. The volumetric reconstruction of the LIF signal from ten parallel planes provides quasi-instantaneous 3D LIF measurements that are synchronized with 10 kHz TPIV measurements. The temporally resolved CH 2 O-LIF and velocity field data are used to analyze Lagrangian particle trajectories and displacement speeds at the base of the lifted flame. The particle trajectories reveal flow structures that are difficult to observe in an Eulerian reference frame. Positive and negative displacement speeds are observed at the CH 2 O-LIF surfaces at the inner and outer regions of the jet flame with a maximum displacement speed of approximately eight times the laminar flame speed of a stoichiometric dimethyl-ether/air mixture. The observations in present study support formation of lean premixed flames near the flame base whose role in flame stabilization should be investigated in more detail. [ABSTRACT FROM AUTHOR]

Abstract To study volatile combustion processes of single coal particles non-intrusive simultaneous multi-parameter measurements were performed. The experiment was carried out in a fully premixed flat flame burner with well-defined boundary conditions. For flame visualization high-speed luminescence imaging was combined with high-resolution high-speed OH-PLIF. To address particle size and shape a stereoscopic high-resolution backlight-illumination system was set up. Due to simultaneous recording of individual particle events the volatile combustion duration related to particle size, shape and velocity was measured. A comparison of luminescence imaging and OH-PLIF for flame visualization was investigated to define their application areas in coal combustion. The stereoscopic backlight-illumination setup was benchmarked to a well characterized bituminous coal. With a pixel resolution of ∼2.5 µm fine particle contours were resolved. The particle diameter and eccentricity were evaluated by an ellipse approximation. The experimental setup can be used to investigate different coal ranks and biomass in N 2 /O 2 and CO 2 /O 2 atmospheres in future. [ABSTRACT FROM AUTHOR]

Abstract We present a multi-species mole fraction and temperature sensor for in situ exhaust gas diagnostic of internal combustion (IC) engines. The sensor is based on Tunable Diode Laser Absorption Spectroscopy (TDLAS) and incorporates four optical channels - two miniature White cells and two double-traversal cells - with base lengths of 6 cm. It has been demonstrated at a hot air test stand and in the exhaust manifold of a single-cylinder research engine, with measured temperatures of up to 1000 K. Stable operation was achieved with absorption lengths of up to 192 cm (test stand) and 97 cm (engine). Employing time-division multiplexed detection, six species were measured simultaneously in the engine exhaust, at wavelengths ranging from 1.4 µm to 5.2 µm: water vapor (H 2 O), carbon dioxide (CO 2), carbon monoxide (CO), methane (CH 4), nitrogen dioxide (NO 2) and nitric oxide (NO). The effective measurement rate was as high as 1 kHz, and cycle-to-cycle variations were clearly detected. We show the correlation of the air-fuel equivalence ratio with the spectroscopically measured mole fraction of each species. At a cycle-resolved rate, detection limits for the legally regulated species NO and NO 2 were 1 ppm and 4 ppm, respectively. The sensor is intended to help improve the understanding of IC engine emission behavior during fast transients. [ABSTRACT FROM AUTHOR]

Abstract In this paper, the spatial and temporal evolution of auto-ignition kernels from methane jets propagating into a NO x -vitiated, high-turbulence, hot air co-flow was studied by means of time-resolved tomographic laser-induced fluorescence of OH (Tomo OH-LIF). Measurements were performed using a burst dye laser system at 10 kHz for volumetric laser illumination and a multi-camera arrangement (8-views) for detection of the fluorescence signal. Auto-ignition kernels were detected three-dimensionally and tracked using a robust algorithm based on the intensity gradient of the volumetrically reconstructed signals. The size and location of the detected kernels were evaluated for operating conditions with different Reynolds numbers of the fuel jet. Results showed that auto-ignition randomly occurred with high probability in a well defined fairly axisymmetric radial region with strong fluctuations in the main direction of the flow. The increase of the Reynolds number of the fuel jet resulted in a radial spread of the location of auto-ignition events. The statistical evaluation of the orientation and growth of auto-ignition kernels with respect to the mean flow field showed that the kernels were oriented tangentially to the flow and temporally evolve towards this preferential direction as the ignition events progressed. [ABSTRACT FROM AUTHOR]

To study devolatilization, ignition, and volatile combustion processes of single coal particles at a fundamental level, a flat flame burner was designed to provide oxygen-enriched exhaust gas environments at elevated temperatures and varying gas compositions. In one set of experiments coal particles were carried by a central jet using a gas mixture of the same composition as the flat flame. By this means a homogenous and connected flame was generated. At the instant when single coal particles transited this stationary flame, start of particle heat-up was well defined. This allows studying start of ignition more accurately compared to the other set of experiments where an inert carrier gas jet was used for transportation of coal particles. Heat-up and ignition were retarded temporally due to precedent mixing between inert gas and ambient flue gases. Coal particles with a high percentage of volatiles (36 wt%) and a particle distribution ranging from 90 to 125 µm were used. Coherent anti-Stokes Raman spectroscopy (CARS) measurements were performed to characterize ambient gas phase temperatures. By high-speed laser-induced fluorescence, imaging of OH-radicals ignition and volatile combustion was visualized for individual coal particles. Reaction zones surrounding each coal particle and the corresponding distances to coal particles were identified. This distance depended strongly on oxygen concentration of the ambient flue gas atmosphere. In addition to an improved phenomenological understanding the data may serve for model validation. [ABSTRACT FROM AUTHOR]

Flame–wall interactions (FWI) of a turbulent side-wall quenching (SWQ) V-flame is characterized by planar laser diagnostics. Velocities and flame front positions are measured simultaneously by two-component particle image velocimetry (PIV) and planar laser induced fluorescence (PLIF) of the OH radical. Flame surface density and reactive flame surface density are derived from experimental data and evaluated in a physical and flame progress variable domain. The impact of the wall on flame structures is clearly observed. Additionally mean reaction rates are derived using both flame surface densities and a wall-influenced local consumption speed. Approaching walls reaction rates strongly decrease. Reaction rates are important for quantifying incomplete combustion near walls and impose a challenge for improved modeling approaches in numerical flame simulations. These experimental results are compared to previous direct numerical simulation (DNS) studies and flame surface density models from literature. Despite different Reynolds-numbers overall a good agreement between experimental data and modeling results is achieved. This is an indication that simplified chemical kinetic modeling in DNS is sufficient for representing kinematics of turbulent flames near walls. [ABSTRACT FROM AUTHOR]

Abstract: High-speed laser induced fluorescence (LIF) thermometry is combined with particle image velocimetry (PIV) to measure the temporal evolution of the spatial unburned gas temperature distribution in a motored spark-ignition (SI) optical engine. Single-line excitation of toluene and subsequent two-color emission detection is employed for LIF thermometry. Precision uncertainty is assessed pixel-wise and found to be ±5K at 295K and ±29K at 550K, but decreases by 34% (i.e., ±19K at 550K) when spatially averaging the LIF signal over a 10×10pixel2 region. The polytropic temperature relation is used to correlate LIF ratio with temperature to calibrate the in-cylinder gas temperature measurements. Instantaneous temperature images and temperature PDFs show homogeneous temperature distribution during compression, while significant temperature inhomogeneities from entrained colder gases are captured during early expansion. Simultaneously acquired PIV and LIF images illustrate the evolution of the cold gas distribution during expansion. The images presented reveal the capability of high-speed toluene-LIF thermometry combined with PIV in an engine to capture the 2D temperature distribution and track structures of colder temperatures. [Copyright &y& Elsevier]