Flat-flame Burner Research Articles

A Compact Laser Absorption Spectroscopy Tomographic System With Short Spectral Scanning Time and Adjustable Frame Rate

A compact hardware system for laser absorption spectroscopy tomography is introduced to monitor dynamic flame evolution with a short and fixed spectral scanning time and an adjustable frame rate. The system mainly consists of two parts, a pentagon sensor and a reconfigurable laser control and data acquisition hardware. In the sensor, 12 photodetectors are installed in each of the five sides of the pentagon. Fan-shaped laser beams from each vertex illuminate the opposite side and half of its two adjacent sides. Compared with the traditional illuminating scheme from each vertex to only the opposite side, this design reduces the sensor size and improves the spatial resolution. In the hardware, the distributed feedback lasers are integrated with the controller and modulated to generate a programmed waveform and implement spectral scanning. The spectral scanning time is short and fixed, and an adjustable frame rate is achieved by changing the interval, i.e., the waiting time, between two successive scans. The sampling rate of each channel can reach up to 20 MHz and is synchronized to the laser emission. A DDR2 memory is used for temporary data storage for four channels, enabling a full frame rate of up to 2 kHz. In the experiment, water vapor in a flat flame burner was chosen as the target absorbing species. Local integral absorbance and temperature distributions in the case of a moving steel ruler across the burner were reconstructed and agreed well with the observed distributions.

Cellular instabilities of n-butane/air flat flames probing by PLIF-OH and PLIF-CH2O laser diagnosis

The structure and instability characteristics of non-adiabatic fuel-rich n-butane/air cellular flames on McKenna burner were experimentally investigated at atmospheric pressure. Planar Laser Induced Fluorescence (PLIF-OH and PLIF-CH2O) flame diagnosis technology was utilized to probe the flame structure under varied equivalence ratio and inflow mixture velocity respectively. Results show that, the equivalence ratio plays an important role in forming cellular flames. The flat, wrinkled and cellular flames appear in turn when increasing equivalence ratio and inflow velocity. Differed to the separated cells of cellular flames appeared in direct Digital camera images and PLIF-OH images, the PLIF-CH2O images clearly display that all cellular flames are connected. In the PLIF-OH images of cellular flames, the OH radical fluorescence signal intensity is higher in the convex region toward unburned mixture than in the concave region, but in PLIF-CH2O images, the fluorescence signal intensity is nearly unchanged in both convex and concave regions. Quantitative stand-off distance and amplitude data probed by PLIF-OH and PLIF-CH2O diagnosis together reveal that the onset of non-adiabatic n-butane/air cellular flames on flat flame burner is dominantly governed by diffusive-thermal mechanism.

Chemiluminescence-based characterization of tail biogas combustion stability under syngas and oxygen-enriched conditions

After the biogas is upgraded to CH4, a waste stream (tail biogas) is obtained, which contains methane in the order of few to tens vol.%. However, the composition of tail biogases can be unstable, therefore, safety should be maintained during the onsite combustion processes. Accordingly, a process control system with various sensors needs to be introduced. This study aims to analyze the experimental limits of flame stability using the chemiluminescence technique during the premixed combustion of low calorific tail biogas (mixture of CH4 with 15–30 vol% concentration in CO2) by adding oxygen or syngas. For the experimental investigation, a flat flame burner mounted in an enclosed combustion chamber was used. The stability of the flame was analyzed via the lift-off height by capturing the flame chemiluminescence from the excited OH* (282.9 nm), CH* (387.1 nm), and C2* (514.0 nm) radicals.This study reveals that the chemiluminescence spatial intensity of species such as OH*, CH*, and C2* is enhanced with an increase in the addition of syngas and oxygen. The peak position of the emission intensities in the flame shifts depending on the composition of gases, showing the existence of unstable flame that can reach its blow-off limit. When syngas is added, an excess of air leads to an increase in flame lifting; however, an O2 surplus results in more stable flame with minimized lifting. Moreover, it was determined that the lifting height of the flame is higher when syngas is added as compared to O2 enrichment. Finally, the flue gas emissions, such as CO and NOx, were examined and the most influential formation parameter was determined.

Experimental and modeling study of the low to high temperature oxidation of the linear pentanone isomers: 2-pentanone and 3-pentanone

Short-carbon-chain ketones are known for their high octane numbers, their knock resistance, and their low soot emission. However, studies on the combustion behavior of ketones, particularly in the low temperature regime, are sparse, and small ketones as acetone and butanone show limited low temperature chemistry (LTC). Therefore, saturated linear five-carbon ketones are good candidates to better understand the LTC of ketones and the effect of the functional carbonyl group. In this study, the ignition delay times of stoichiometric non-diluted 2- and 3-pentanone-oxygen-inert gas mixtures were measured in a rapid compression machine at pressures between 20 and 40 bar and temperatures ranging between 650 and 950 K. Furthermore, the mole fraction profiles of fuel, oxygen, intermediate species, and products were measured in a laminar flow reactor coupled with molecular-beam mass spectrometry at 0.97 bar, an equivalence ratio of 0.8 and temperatures ranging from 800 to 1050 K, to achieve a better understanding of the oxidation process of linear pentanones. A consistent detailed kinetic model was developed and validated against the newly measured experimental data for 2-pentanone and 3-pentanone to provide insight into the oxidation mechanism of pentanones. The model shows also good agreement with available data from the literature, including ignition delay times measured in a shock tube, speciation in a flat flame burner and laminar burning velocities.

Comparison of pyrolysis of live wildland fuels heated by radiation vs. convection

During wildland fires, which include both planned (prescribed fire) and unplanned (wildfire) fires, live and dead plants may be subject to both radiative and convective heat transfer mechanisms. In this study, the pyrolysis of 14 live plant species native to the forests of the southern United States was investigated using a flat-flame burner (FFB) apparatus under three heating modes in order to mimic pyrolysis of plants during wildland fires. The heating modes were: (1) radiation-only, where the plants were pyrolyzed under a moderate heating rate of 4 °C s−1 (radiative flux of 50 kW m−2); (2) convection-only, where the FFB apparatus was operated at a high heating rate of 180 °C s−1 (convective heat flux of 100 kW m−2); and (3) a combination of convection and radiation, where the plants were exposed to both convective and radiative heat transfer mechanisms. Data were also compared with slow heating experiments (0.5 °C s−1). During the experiments, the pyrolysis products were collected and analyzed using GC–MS for the analysis of tars and GC-TCD for the analysis of light gases. The results indicate that the highest light gas and tar yields were obtained from the combined mode, which was performed at a higher pyrolysis temperature and heating rate. CO, CO2, CH4, and H2 were the major light gas species in all three heating modes. The radiation-only mode led to formation of primary tars and a few secondary tars consisting of aliphatic and 1–2 ring aromatic compounds with 1 to 3 attachments, including alkyl, hydroxyl, and methoxy groups.

Effects of Na and Fe on the formation of coal-derived soot in a two-stage flat-flame burner

Coal-combustion-generated soot can be a source of particulate emissions and significantly affects the temperature profile inside the practical boiler. In this work, we experimentally investigated the effects of sodium (Na) and iron (Fe) on soot formation in pulverized coal combustion. Two kinds of coal samples with similar properties were used, with one rich in Na while the other rich in Fe. The water washed sample for each coal was ultilized to further explore the mineral effects. A novel two-stage flat-flame burner was designed to flexibly simulate the reducing-to-oxidizing atmosphere which a real coal particle stream undergoes in the boiler. The ex-situ sampling indicates the existence of mineral species, including Na and Fe, doped in the mature soot agglomerates. In-situ Laser-Induced Incandescence (LII) diagnostics, incorporated with on-line measurements of the particle size distribution (PSD), gives quantitative concentrations of primary soot particles. The experiment results show a reduced soot yield after water washing for both coal samples, elucidating a promoting effect of Na on soot formation in pulverized coal combustion. A similar effect is inferred for Fe, since the water washed Fe-rich coal produces more soot than the water washed Na-rich coal even though their residual Na contents in ash are close. It is postulated that both Na and Fe form nano-sized particles at the early stage of coal combustion and serve as effective nuclei for the soot precursor, the tar. Besides, the catalytic effect of Na on soot oxidation is confirmed because of the significant soot reduction in the combustion of Na-rich coal when the oxygen mole fraction in the inner region increases.

Analysis of laser absorption gas sensors employing scanned-wavelength modulation spectroscopy with 1f-phase detection

The recently introduced wavelength-modulation spectroscopy with 1f-phase detection (WMS-$$\theta _{1f}$$) technique showed promising results with potentially improved measurement precision over the popular 1f-normalized WMS-nf (WMS-nf/1f) technique. Like WMS-nf/1f, WMS-$$\theta _{1f}$$ enjoys the typical benefits of WMS methods, including low-frequency noise rejection, correction for non-absorbing losses, and insensitivity to the broadband absorption spectra of interfering species. In this work, we performed a detailed analysis of the spectrally resolved scanned-wavelength WMS-$$\theta _{1f}$$ measurement technique and its direct comparison against the expected performance of scanned-wavelength WMS-nf/1f and scanned-wavelength direct-absorption spectroscopy (SDAS) measurements. This simulation-based analysis identified specific operating regimes in which the performance of WMS-$$\theta _{1f}$$ measurements in terms of accuracy is expected to be greater than the performance of WMS-nf/1f or SDAS. Additionally, improved guidelines for the optimal selection of laser-tuning parameters, including an explicit optimization of the optical scan depth parameter, were developed. Experiments with a CO2 static cell perturbed by a high-speed air jet corroborated the simulation-based findings. Finally, a practical demonstration of a WMS-$$\theta _{1f}$$ sensor for measuring temperature and H2O mole fraction in the exhaust of a CH4/air flat-flame burner was presented, with the results confirming model predictions of the superior precision of WMS-$$\theta _{1f}$$ relative to WMS-nf/1f and SDAS.

Impacts of N2 and CO2 diluent gas composition on flame emission spectroscopy for fuel concentration measurements in flames

Flame emission spectroscopy (FES) is used to determine methane fuel concentration in a premixed flat-flame burner with varying diluent composition. Spectral profiles of the molecular emission bands of OH* (306.4 nm), CH* (431.5 nm), and C2* (520 nm) in the UV-to-visible range are recorded and analyzed for various equivalence ratios (0.72 < ∅ < 1.25), oxygen concentration (0.19 < XO2 < 0.35), and diluent gas species (N2, CO2). The importance of flame radiation, which is strong and reliable in the UV spectrum, is confirmed particularly for quantitative measurement purposes. In air-methane flames, the band intensity ratios of OH*/CH* and C2*/CH* are shown to be highly sensitive to fuel stoichiometry, and can thus serve as accurate indicators of ∅. In addition, the impact of diluent gas composition on flame radiation characteristics and FES utilizing band intensity ratios is investigated for potential applications such as clean combustion utilizing exhaust gas recirculation (EGR), and oxy-combustion in supercritical CO2 power plants. It is shown that the strength of the underlying broadband emission is strongly dependent on the diluent gas composition, and influences the sensitivity and accuracy of the FES-based ∅ measurement.

TDLAS-based in situ diagnostics for the combustion of preheated ultra–lean dimethyl ether/air mixtures

A combined experimental and modelling study of the structure of a laminar premixed ultra–lean (ϕ=0.33) dimethyl ether/air flame at atmospheric pressure and an elevated temperature was carried out. The work aimed to apply tunable diode laser absorption spectroscopy to the identification of various flame regimes that are relevant to the oxidation of dimethyl ether. One-dimensional calculations employing burner-stabilized flame assumptions confirmed the significance of low-temperature combustion chemistry. A stable double-flame structure was predicted using state-of-the-art chemical kinetic schemes and was revealed by the experimental observations. The feasibility of the novel experimental strategy based on the preheated flat-flame burner and scanned wavelength modulation spectroscopy was investigated in this context. The absorption features of hot water and the hydroxyl radical near 1572 nm were selected as appropriate targets for distinguishing the transition from the cool flame regime to the hot flame regime. Discrepancies in the water line position and intensities were found within the 1509 nm region.

Ignition and combustion characterization of single nitromethane and isopropyl nitrate monopropellant droplets under high-temperature and quasi-steady conditions

The dynamics of reacting droplets of the monopropellants nitromethane (NM) and isopropyl nitrate (IPN) are compared to those of methanol. Single droplets of these three fuels were burned in steady flow high-temperature conditions, produced by a McKenna flat-flame burner. Coherent anti-Stokes Raman scattering (CARS) thermometry was employed to characterize the temperature of the flow-field experienced by the droplets. Ignition delay times and droplet burning rates were obtained for droplets with initial diameters ranging between 0.4 and 1.4 mm. Droplet dynamics such as deformation, puffing, stripping and micro-explosions are qualitatively discussed. Lastly, the D2 law and the hybrid combustion model are applied to IPN and NM droplets, and the experimental mass burning rates are compared to theoretical predictions.

Computational fluid dynamics simulation to predict the effect of the equivalence ratio on the temperature distribution in a pyrolysis furnace

This research investigated numerically the effect of the equivalence ratio on the temperature distribution in an industrial non-premixed flat flame burner with methane as the fuel. The burner configuration is a wall-fired one to produce a flat flame with a large cross-sectional area to effectively radiate heat to tubes contained in a pyrolysis furnace. The temperature distribution was predicted by using a phenomenological model consisting of mass balance, energy balance and momentum balance with computational fluid dynamics approach. Fluid flow was explained by the k-ε turbulent momentum balance, and the reaction rate was approached to eddy dissipation model. Simulation results show that the change in the equivalence ratios affects the temperature distribution so that this variable needs to be adjusted with certain considerations for pyrolysis process.

Characterizing early stage sub-micron particle formation during pulverized coal combustion in a flat flame burner

Mineral matter evolution, soot formation, and soot oxidation are the governing processes for fine particle evolution during the early stages of pulverized coal combustion. Soot and ash can be simultaneously present in PM 0.1, and experimental results obtained to understand early stage particle formation can be ambiguous if the two are not independently evaluated. A sampling system proposed in this work differentiates between soot and mineral matter by employing a high temperature furnace downstream of the sampling system, supplied with sufficient oxygen to oxidize the soot in the sampled aerosol stream. The stream can be routed either through the furnace or bypassing it. In the bypass configuration, the SMPS measures the total PM size distribution. In the flow-through configuration, the SMPS measures the ash PM size distribution, and the difference identifies the contributions from the soot aerosols. This study has shown that there is a high probability of confounding effects in experimental measurements of early stage ultra-fine particulate matter evolution, and the effects were shown to exist at different conditions and with different coals. The results show that for PRB coal combustion, as residence time increases from 9 ms to 33 ms, the volume of total PM 0.1 decreases by 68%, and the ash PM 0.1 increases by 600%, whereas for Hongshayuan lignite coal, the volume of total PM 0.1 increases by 275%, and ash PM 0.1 increases by 939%. These results indicated that soot constitutes a major part of PM 0.1 during the early stages of fine particle evolution.

Study of temperature, apparent spectral emissivity, and soot loading of a single burning coal particle using hyper-spectral imaging technique

A hyper-spectral imaging device that can simultaneously capture three-dimensional data information of spectra, space, and time is employed to experimentally investigate the combustion behavior of a single coal particle. Three types of single pulverized coal particles are injected and burned on a Hencken flat-flame burner, and their flames are captured by a high-speed camera and the hyper-spectral imaging device. The camera observations demonstrate that the three single coal particles undergo particle heat-up, volatile release and combustion, and volatile and char oxidation during the combustion process. Furthermore, the image captured by the hyper-spectral imaging device records the spectral and spatial history of the single burning coal particles throughout the combustion process. Then, the time–space temperature and apparent spectral emissivity of the single burning char particles are calculated from the time–space spectra radiative intensity obtained by the hyper-spectral imaging device. The gray characteristic exhibited by the spectral emissivity of the three single burning char particles in the longer wavelength range demonstrates the feasibility of calculating the temperature using the two-color method. Finally, the soot temperature and volume fraction distribution in the volatile flames are obtained to investigate the formation of soot during the combustion of a single coal particle. The results show that an elongated tail-like soot cloud is formed around the two single bituminous coal particles during the combustion process, but a nearly spherical soot cloud is formed around the single lignite coal particle. Moreover, the range of soot volume fractions in the three envelope volatile flames is comparable to the order of magnitude of the values measured by other researchers.

Machine learning applied to retrieval of temperature and concentration distributions from infrared emission measurements

Inversion of temperature and species concentration distributions from radiometric measurements involves solving nonlinear, ill-posed and high-dimensional problems. Machine Learning approaches allow solving such highly nonlinear problems, offering an alternative way to deal with complex and dynamic systems with good flexibility. In this study, we present a machine learning approach for retrieving temperatures and species concentrations from spectral infrared emission measurements in combustion systems. The training spectra for the machine learning model were synthesized through calculations from HITEMP 2010 for gas mixtures of CO2, H2O, and CO. The method was tested for different line-of-sight temperature and concentration distributions, different gas path lengths and different spectral intervals. Experimental validation was carried out by measuring spectral emission from a Hencken flat flame burner with a Fourier-transform infrared spectrometer with different spectral resolutions. The temperature fields above the burner for combustion with equivalence ratios of ϕ = 1, ϕ = 0.8, and ϕ = 1.4 were retrieved and were in excellent agreement with temperatures deduced from Rayleigh scattering thermometry.

Development of novel ultrasonic temperature measurement technology for combustion gas as a potential indicator of combustion instability diagnostics

In this study, a high-speed, fast responsive, non-intrusive ultrasonic thermometry system was proposed as a potential candidate for overcoming the disadvantages of thermocouples. The principle of this system is based on the thermal dependence of the speed of sound, and the temperature is measured by detecting the flight time of ultrasonic wave (USW) between the transmitter and the receiver. For a fast and exact measurement, the algorithm was developed as simple as possible, and the exact values of the physical properties, such as specific heat ratio and molecular weight of combustion gas, were taken from the computational calculation of CHEMKIN-Pro with GRI 3.0 mechanism. The performance of the system was verified by two experiments. First, the system was applied to measure the temperature of heated air. Results showed high precision with a 0.3% error when incorporating a modification equation and fast responsive dynamic performance, which directly reflect the rapid temperature change. Second, the combustion gas temperature above a multi-jet burner, which provides a horizontally uniform temperature distribution similar to a flat flame burner, was measured. Five different flame temperatures were measured using a thermocouple and an optic-based measurement method based on two-line atomic fluorescence (TLAF) as well as ultrasonic thermometry to compare their performances. Ultrasonic thermometry showed a slightly lower accuracy than those of TLAF and the thermocouple. This condition could be overcome by correcting the results using linear fitting, as the temperature measured by USW showed the best linearity among them. The USW technique showed excellent performance in terms of the measurement speed of 1000 samples/s and uncertainty under 0.73%. This USW thermometry of combustion gas can be applied to many combustion systems, including boilers and gas turbines. Specifically, fast temperature measurement at a speed of around 1 kHz enables the diagnosis of the combustion instability phenomenon, which is a difficult task when using conventional methods of temperature measurement. Furthermore, both dynamic pressure and temperature can be simultaneously measured at a high rate, thus synergistically increasing the accuracy of combustion instability diagnosis with sufficient information, such as the Rayleigh index or the flame transfer function.

Differences in Formation and Oxidation of Colombian Coal Chars in Air and Oxy-fuel Atmospheres

The increasing interest towards more efficient and clean technologies, specially paying attention to CO2 neutral processes is encouraging the investigation of coal thermochemical conversion under oxy-fuel atmospheres (i.e. without N2). The process offers many advantages such as easy separation of the CO2 produced and low NOX/SOX emissions. While coal conversion in air is already well understood, full understanding of the influences of CO2-rich atmospheres is still required. A series of experiments in thermogravimetric analyser, drop-tube reactor and flat-flame burner were performed using a mid-range bituminous coal (Colombian coal) to understand the differences in the chars obtained after the pyrolysis step. Comparing the pyrolysis in N2 and CO2 atmospheres, significant differences were observed in the resulting chemical (composition) and physical properties of the chars, whereas mass loss was very similar for short residence times (< 130 ms). Afterwards, the chars obtained were submitted to oxidation and gasification, under several different operating conditions, in order to evaluate the difference in reactivity of these chars. Chars obtained under CO2 atmospheres revealed a lower reactivity, despite their higher surface area. These aspects cannot be explained and captured by a model which does not focus on some important details. In this paper, the results obtained in these experiments are summarized and discussed on the point of view of the POLIMI modelling approach for thermochemical conversion of solids. This model offers several advantages, such as being flexible to improvements, requiring simple experimental data of the fuel and offers an all-in-one solution for describing the kinetics of the whole process. The developments accounted for a wide range of experimental data, which allowed its calibration for several fuels, mostly in air combustion. It was first developed to describe the pyrolysis step, and later char oxidation/gasification was included in a simplified approach. The detailed mechanism of homogeneous gas phase reactions of the volatiles is also coupled. In order to extend the predictive capabilities of the model for oxy-fuel conditions, dedicated experiments must be considered for future improvements. In this work these missing effects are discussed, identifying the main necessary improvements, allowing this model to be extended and applied also for the designing of reactors that use oxy-fuel technologies.

Combustion characteristics of magnesium borides and their agglomerated particles

Boron is an attractive fuel for propellants because of its high energy density. However, its combustion is inhibited by the oxide layer that covers the particles. The use of magnesium borides instead of boron was shown to promote the combustion of boron-based propellant. The combustion behavior of magnesium borides and their agglomerated particles in CH4/O2/N2 premixed flames were studied using flat-flame burner, high-speed camera and spectrometer. The experimental results showed that the combustion of magnesium dominated the early stage of magnesium borides combustion process. As the combustion reaction proceeding, the combustion of boron gradually replaced magnesium to dominate the combustion of magnesium borides. When oxygen mole fraction was 0.13, MgB2 and MgB4 agglomerated particles would extinguish after ignition within the temperature range explored (1364K–1541 K). When oxygen mole fraction increased to 0.22 and 0.31, the burning time of MgB2 and MgB4 agglomerated particles were much shorter than that of amorphous boron agglomerated particles between 1400 K and 1600 K, which could improve the combustion performance of boron-based propellant. When temperature increased to 1600 K, the burning time of MgB4 agglomerated particles was close to that of amorphous boron agglomerated particles, while the burning time of MgB2 agglomerated particles was still shorter than amorphous boron agglomerated particles.

Evolution of volatile cloud in pulverized coal combustion with high-speed digital inline holographic visualization

The coal devolatilization plays a significant role in the combustion of pulverized coal particles. The evolution of volatile cloud during devolatilization of pulverized coal particles (105–125 μm) is studied in a high-temperature flat-flame burner by combining high-speed photography with high-speed digital inline holography (DIH). By the high-speed holographic visualization, the evolution of volatile cloud of pulverized coal from volatile release to soot aggregation generation can be divided into four stages. Effects of coal type on volatile cloud evolution are investigated using three different coals, i.e., Shanxi bituminous coal, Ximeng lignite and Yinni lignite. The results show that both the Shanxi bituminous coal and Ximeng lignite produce soot aggregation during devolatilization, which is rarely observed for Yinni lignite. Moreover, Shanxi bituminous coal has a higher potential in soot cluster formation for its higher coal rank than Ximeng lignite. The high-speed reconstructed image sequences are analyzed to measure the velocity slip between the parent particle and volatile cloud. Compared with Shanxi bituminous coal, Ximeng lignite exhibits a larger slip velocity. This work also demonstrates that high-speed DIH has the powerful capacity of directly observing the evolution of volatile cloud, and helps to gain a deep understanding of pulverized coal combustion.

Research of the impact of the method of heating of heat units on the qualitative characteristics of treated materials

An analytical method for calculating the chemical potentials of the components of the gas-solid system based on thermodynamic calculations of carbon potentials of the С–О–Н–N gas mixture (combustion products of the methane-air mixture) and the solid phase (alloyed steel) is developed. Dependences describing the influence of the main parameters of heating the medium composition, flow rate, as well as their interaction, on metal losses associated with decarburization are obtained. Thermodynamic calculations of carbon potentials of alloyed steel and natural gas combustion products of different composition (α=0.2÷1.2), metal and combustion products temperatures of 1,100÷1,500 K are performed. Based on the analysis of the structure of the thermal and diffusion boundary layers, it is proved that the decrease in the temperature of the layer of combustion product flowing around the solid product and surface flow rate reduces the diffusion flow of carbon in the boundary layer. This effect reduces the decarburization of steel. It is found that when heating the heat unit according to the principle of indirect radiant heating (IRH) during the operation of the flat-flame burner, the main gas volume, localized at the metal surface, has a temperature significantly lower than the layer adjacent to the lining. This reduces the metal loss with decarburization compared with furnaces of the traditional heating system. Combustion of gas in flat-flame burners with an intense circulation of combustion products in the working space of the heat unit ensures that the heated products have a uniform composition of combustion products corresponding to a practically equilibrium one. This allows recommending flat-flame burners for widespread use in modern heat-power units in the industry

Assessment of combustion rates of coal chars for oxy-combustion applications

A drop tube reactor with high heating rates typical of pulverized boilers (>104 K/s) has been used to carry out experiments with coal in different atmospheres: N2, CO2, O2/N2 and O2/CO2. The reactor wall temperature was set at 1573 K and the particles’ residence time was kept below 130 ms.In O2/N2 and O2/CO2 atmospheres coal pyrolysis was complete and additional char conversion occurred. The degree of char conversion increased with oxygen concentration values but was further enhanced by the presence of carbon dioxide, suggesting a positive contribution of CO2 to the overall rate of conversion. Chemico-physical and structural analysis of chars revealed internal burning under regime II conditions and highlighted that the presence of CO2 favors the formation of lactones in the chars.In N2 and CO2 atmospheres the pyrolysis stage was completed, but char conversion was negligible. The combustion stage of the N2 and CO2 chars was investigated in a second stage by thermogravimetric (TG) analysis (in regime I conditions) and in a flat flame burner (in regime II conditions) to separate atmospheric effects on char formation from those on char combustion. In TG, the CO2 chars resulted to be less reactive then the N2 chars, but in the flat flame burner, the experimental rate of carbon conversion of the N2 char and the CO2 char were similar.The TG results were worked out to estimate the intrinsic kinetics of the N2 and CO2 chars towards oxygen, carbon dioxide and O2/CO2 mixtures. Kinetic rate expressions were extrapolated to regime II conditions after consideration of mass transfer limitations. Notably, the kinetic model developed for the CO2-char matched the observed rate of char (oxy-) combustion well, whereas the kinetic model of the N2-char overpredicted the reaction rate.