Peak Soot Volume Fraction Research Articles

Assessment of a PAH-based soot production model in laminar coflow methane diffusion flames doped by gasoline surrogate fuels

This article assesses the capability of the PAH-based soot model developed by the authors and validated in ethylene non-premixed flames to predict soot production in flames fueled with gasoline surrogates. The soot model was coupled to a flamelet model and the Rank-Correlated Full-Spectrum k model to simulate laminar coflow nitrogen-diluted methane/air diffusion flames doped with n-heptane/toluene and iso-octane/toluene mixtures. Consistent with our previous studies, the simulation was conducted using the Kaust Mechanism 1, pyrene as soot precursor, and the same set of model parameters. The model reproduced reasonably-well the peak soot volume fraction. However, the soot production onset was predicted much earlier than measurements owing to the early formation of pyrene induced by the presence of toluene. These discrepancies can be partially corrected by selecting a larger PAH than pyrene with a similar level of concentrations as soot precursor. For the present mechanism, anthanthrene was found to be the best candidate. Model results show that different mechanisms dominate the soot mass growth in ethylene and gasoline surrogate flames. While the HACA is more important in the former, PAH condensation largely prevails in the latter. This suggests that ethylene may be not the most relevant reference fuel for developing semi-empirical soot models for fires. Further investigations are required to confirm this conjecture.

Effect of 2-butanone addition to ethylene fuel on soot formation in counterflow diffusion flames using newly proposed soot model

An improved consistent soot model is proposed and applied to evaluate the effect of 2-butanone addition (10 % to 50 % on a volume basis represented as case 1 to case 5) to ethylene fuel on soot formation using a counterflow burner configuration. The predictive capability of the suggested soot model is verified by assessing its performance against existing experimental data on soot formation (SF) configuration-type ethylene counterflow flames at diverse strain rates and various fuel additives. The proposed soot model comprises 55 inception reactions with temperature-dependent collision efficiency and 10 condensation reactions from 10 PAH species (from naphthalene to larger PAHs up to coronene), including modified HACA surface growth and oxidation reactions. 2-butanone is produced as a byproduct during the pyrolysis of biomass and the microbiological fermentation of agricultural waste. It holds various benefits as a prospective biofuel for spark ignition (SI) engines. Limited information exists regarding its sooting characteristics due to a lack of available soot measurements. The simulations are conducted for the soot formation (SF) type counterflow flames with a fixed fuel and oxidizer jet velocity. The proposed soot model can effectively replicate both the qualitative and quantitative aspects of the experimental trends and shows a better agreement than the existing models available in the literature. The soot volume fraction (SVF) and the particle number density (PND) decrease with increasing the 2-butanone concentration in the binary fuel mixture. The PAH concentration decreases with increasing 2-butanone addition in the fuel mixture. The peak SVF and the maximum temperature are reduced by ∼22.7 % and ∼3.6 %, with a 40 % increase in the 2-butanone portion in the fuel mixture from case 1 to case 5. Increasing the 2-butanone content in the fuel mixture decreases the inception rate, HACA rate, and condensation rate while it increases the oxidation rate.

Numerical simulation of soot and NO formation in DME/gasoline laminar co-flow diffusion flames

Dimethyl ether (DME) is regarded as a promising alternative fuel to reduce greenhouse gases and soot emission. However, the application of DME in actual engines is difficult because of its low calorific value and viscosity. DME is commonly utilized as a supplementary component in traditional fuels to enhance their combustion efficiency and emission performance. This study investigated the effect of DME blending on the soot and NO formation in laminar diffusion gasoline flames. The results showed that the peak soot volume fraction (SVF) reduced by 12.3 %, while the peak temperature increased by 19.2 K and the peak concentration of NO increased by 18.2 % when the blending ratio of DME increased from 0 % to 15 %. The primary reason for the reduction in the soot inception rate was the decrease in benzo(ghi)fluoranthene (BGHIF) concentration. The formation of BGHIF was controlled by the C6H5CH3 → A1 → A1-→A1C2H → A2R5 → A2 → A2-→BGHIF pathways. The decrease of initial soot particles led to a reduction in Hydrogen Abstraction Carbon Addition (HACA) surface growth rate. The increase in A4 concentration was due to the increase in the concentration of C4H2 in the reaction A2R5 + C4H2 = A4. Therefore, more benzo[a]pyrene (BAPYR) was formed through A4 + C4H2 = BAPYR, promoted the process of soot condensation. The increase in OH concentration promoted the oxidation of OH, and the decrease in soot inhibited the oxidation of O2. The majority of NO was primarily produced through the NO thermal pathway in the DME/gasoline flame, and N + OH = NO + H was the main reaction to form NO. The increase of O, H and OH radicals caused the increase in NO concentration.

Prediction of soot for pressurized turbulent kerosene-air diffusion flames using method of moments

ABSTRACT A comprehensive CFD model is developed to predict soot for a turbulent kerosene-air diffusion flame. The method of moments (MOM) soot model has better prediction capability with meaningful soot behavioral predictions than semi-empirical models. The coupled model is primarily validated with experimental measurements from the literature in the form of major species concentration, flame temperature, and soot volume fraction. A surrogate mix comprising 80% n-decane and 20% toluene on a liquid volume basis constitutes kerosene fuel. The 20% aromatic content is able to reproduce the sooting behavior with considerable accuracy. The model can replicate CO, CO2, CH4, C2H2, C2H4, and C6H6 concentrations, flame temperature, soot volume fraction, and soot aggregation parameters for a laminar JetA-1 flame at atmospheric pressure. The reproduction of flame temperature and major species concentrations for a laminar kerosene flame validates the applicability of the gas-phase kinetic mechanism and PAH formation pathway for the reacting flow system. The model also performs appreciably for measurements of a turbulent kerosene flame at higher operating pressures up to 6.44 bar. The peak soot volume fraction matches significantly well with the measurements for all the flames. The predicted peak soot volume fractions are 8.8, 27.3, 68, and 82 ppm compared to measured values of 9.3, 28.3, 63, and 67 ppm at pressures 1, 2.7, 4.81, and 6.44 bar, respectively. However, the location of the peak soot has a slight discrepancy at low pressures till 2.7 bar, showing the predicted peak earlier compared to a later appearance in the measurements. The simulated peak flame temperatures are 1377, 1393, 1412, and 1477 K as operating pressure increases from 1 to 2.7, 4.81, and 6.44 bar. The thermal absorbance from soot and species radiation plays a vital role in predicting the flame temperature. Soot radiation reduces flame temperature by ~ 500 K compared to gaseous radiation (CO2, H2O, CO, CH4, etc.), reducing approximately 100 K. The predictive soot number density, mean diameter, surface area, primary particle count in soot aggregates, and primary particle diameter carry a substantial dependency on the operating pressure.

Hydrogen addition in methane-oxygen laminar inverse diffusion flames: A study focused on free radical chemiluminescence and soot formation

This study investigates the impact of H2 addition on soot formation in inverse diffusion flame (IDF) and explores the underlying mechanism of flame structure on soot formation. Experimental measurements and simulation kinetic calculations were performed to explore the influence of free radical (OH*/CH*/C2*/CO2*) chemiluminescence on soot formation mechanism. The investigation ultimately determines the relationship between free radical chemiluminescence and soot formation. The findings reveal that similar to the simulation results, the peak soot volume fraction (SVF) is reduced by approximately 60 % when the hydrogenation increases to 40 %, and the initial soot production position moves downstream of the flame. The free-radical chemiluminescence distribution is mainly concentrated at the burner exit, where the increase in hydrogen concentration leads to a decrease in CH* and C2* and an increase in OH* content in the flame. The blue region on the fuel side is consistent with the peak CH* and C2* emission region. In the soot formation, the chemiluminescence signal of free radicals is suppressed, indicating that soot formation and free radical chemiluminescence are antithetical. The results of the component yields indicate that soot formation and chemiluminescence are fueled by gas phase components (CH3, C2H3, C2H2, etc.) and there are generated in opposite pathways, which supports the above statement. Furthermore, the increase of hydrogen content inhibits the production of C2H and CH2, resulting in the decrease of C2*, CH* and CO2* radical chemiluminescence by 77.0 %, 75.1 % and 81.2 %, respectively. The decrease of soot concentration is primarily due to the increase of H2, which hinders the formation of C3H3 and C4H5-2.

Effects of oxygen addition to the fuel stream on the soot formation in an ethylene coflow partially premixed flame based on the orthogonal decoupling method

In this work, an orthogonal decoupling method based on the orthogonal design and decoupling method was used to simulate the effects of adding oxygen (O2) to the fuel stream on the soot formation characteristics in an ethylene (C2H4) coflow partially premixed flame. The dilution effect, thermal effect and chemical effect of the added O2, as well as the interaction between these effects were decoupled and analyzed. Results show that the peak soot volume fraction (SVF) in the flame decreases first and then increases as the O2 mole fraction gradually increases to 30%, and is lowest at an addition of 20%. The orthogonal decoupling calculations were performed for two O2 concentration ranges of 5%–10% and 25%–30%, and the differences on soot formation at these two ranges were found to be dominated by the chemical effects of O2 through the extremum difference analysis of the calculated peak SVF, number density of primary particle and particle aggregation degree. The chemical effect of O2 inhibits the formation of soot at an O2 addition of 5%–10%, while promotes the formation of soot at an O2 addition of 25%–30%, by affecting the inception, hydrogen-abstraction-carbon-addition (HACA) reaction, and polycyclic aromatic hydrocarbon (PAH) condensation rates of soot formation. When the O2 concentration is 25%–30%, the chemical effect of O2 promotes the soot surface growth rate and therefore increases the soot particle size, which in turn retards the complete oxidation process, leading to an increase in SVF.

A simple method for the quantitative assessment of soot production rate

Sooting tendency has been characterized with various empirical approaches such as smoke height and Threshold Sooting Index (TSI), that can be regarded as qualitative, as well as Yield Sooting Index (YSI), that is semiquantitative since it relies on the measurements of at least the peak soot volume fraction in the flame. All these techniques have the convenience of being easy to implement and of relying on inexpensive equipment. In the present work we present a comparatively easy but quantitative alternative to determine the soot production rate in counterflow flames. The approach is rooted in the use of: a) soot volume fraction measurements by pyrometry, b) the well established one-dimensional computational modeling of such flames for the determination of temperature and velocity profiles and c) the use of the soot governing equation. The technique is applied to several aliphatics, including methane, propane, ethylene, propene and acetylene. Soot production rate per unit flame area for the tested aliphatics ranges between 10−4 and 10−7 g/(cm2s) and, when normalized with respect to the carbon flux, between 10−5 and 10−2. On a logarithmic scale it correlates linealry with the peak temperature for all fuels. Soot yield scales as alkanes< alkenes < alkynes, with acetylene showing the highest sooting tendency even in flames at relatively low temperatures.

Effect of Pressure on Soot Formation and Properties in Laminar RP-3 Kerosene Diffusion Flames

ABSTRACT Obtaining quantitative experimental data on soot properties of aviation kerosene at elevated pressures is a significant concern for the surrogate fuel kinetic model of aviation kerosene. However, there are scarce studies on soot formation and properties of aviation kerosene in tractable flames at elevated pressures in literature. In this study, Chinese RP-3 aviation kerosene is used in relevant experiments and numerical simulations up to 3.5 atm to evaluate the effect of pressure on soot formation and properties. Soot morphology, nanostructure, and concentration are studied by transmission electron microscopy and laser extinction method. Then, an RP-3/PAH kinetic model containing the 5-aromatic rings (A5) growth mechanism is proposed to simulate the soot formation processes at elevated pressures. Quantitative experimental results show that pressure significantly affects the soot morphology, nanostructure parameters. The peak soot volume fractions in the RP-3 flame scale with the pressure as . The numerical results show that the RP-3/PAH model can depict the trends of soot particle size and soot volume fraction with pressure. At 1.0–3.5 atm, the rates of soot growth processes increase with pressure, especially the PAHs condensation. The significantly increased soot surface growth rate may be one of the main factors for the variation of soot properties with pressure. The quantitative experimental and numerical results contribute to the understanding of the kinetic mechanism of soot formation for RP-3 kerosene at elevated pressures.

Effects of Radiation Model on Soot Modeling in Laminar Coflow Diffusion Flames at Elevated Pressure

ABSTRACT Detailed numerical simulations of C2H4/Air coflow laminar sooting flames at atmospheric and elevated pressures are performed with fully coupled flow, gas-phase reactions, soot dynamics, and nongray radiative heat transfer. The soot dynamics is modeled using a hybrid method of moments combined with a polycyclic aromatic hydrocarbon (PAH) mechanism. Nongray radiative heat transfer by CO2, H2O, and soot is calculated using different methods to study the effect of radiation model on the predictions. The discrete ordinates radiation model (DOM) and P1 radiation model are compared. Three radiative property models with different accuracy and efficiency are included: a) full-spectrum correlated- distribution (FSCK) model, b) weighted sum of gray gas model with original parameters (WSGG-Smith), and c) WSGG model with optimized parameters for variable mole ratios and elevated pressure (WSGG-SK). The optically thin approximation (OTA) model is also compared to a simple baseline case. The results show that the temperature and soot volume fraction predicted by the DOM combined with the WSGG-SK model are consistent with the FSCK model, with errors less than 2%. When the pressure increases to 8 bar, errors of P1 and OTA models increase significantly, the maximum temperatures predicted by the P1 and OTA models have errors of 36 K and 63 K, and the relative errors of the peak soot volume fraction are 18% and 24%, respectively.

Effect of pressure and CO2 dilution on soot formation in laminar inverse coflow flame at conditions close to autothermal reforming

Autothermal reforming (ATR) is an important technology for hydrogen production from natural gas, where soot formation deserves particular attention as it may cause catalyst poisoning. The typical configuration of ATR reactors is that of an inverse diffusion flame (IDF). In this study, soot formation near ATR conditions is investigated experimentally and numerically, focusing on the effects of pressure and CO2 dilution in IDFs with pressure reaching 5 bar. The fuel stream consists of methane diluted with CO2 or N2. The mole fraction of O2 in the oxidant stream is 70%. Polycyclic aromatic hydrocarbons (PAHs) and soot volume fraction are measured by the planar laser-induced fluorescence (PLIF) and planar laser-induced incandesce (PLII) methods. High-fidelity simulations with a detailed soot aerosol model are also performed, and an empirical reactive soot inception model is proposed. A comparison of the results shows that the soot behavior is well predicted by the empirical reactive inception model but not fully captured by the physical inception model based on irreversible dimerization, demonstrating the importance of radical involvement in the soot inception process. Both measurements and predictions show a linear relationship between peak soot volume fraction and pressure, which can be explained by the linear increase of PAH molar concentration with pressure. Simulation analysis indicates that the dimer adsorption and HACA mechanism have a similar quantitative contribution to the soot growth. The CO2 shows stronger suppression effects in soot formation at the investigated pressures compared to N2. The contributions of chemical and thermal effects to soot suppression are numerically analyzed and the results indicate that the chemical effect of CO2 is mainly due to the removal of H radicals through the reaction CO2 + H→CO + OH, providing a soot suppression contribution of two times larger compared to the thermal one.

Effects of intramolecular oxygen and partial premixing on soot formation in ethane/ethanol flames

Detailed soot concentration and temperature fields were measured in coflow laminar diffusion flames of neat ethane and ethane/ethanol mixtures, and partially premixed flames of ethane/ethanol mixtures. Two-dimensional diffuse-light line-of-sight attenuation and spectral soot emission techniques were employed for soot volume fraction and temperature measurements, respectively. The choice of flow rates prioritized tractability and comparability between flames. To this end, the total carbon mass flow rate and the C/H ratio were kept constant for all ethane/ethanol mixture flames, i.e., oxygen was introduced into the fuel stream either as intramolecular oxygen in ethanol or both as intermolecular and molecular oxygen. Soot concentration was observed to decrease monotonically with an increasing ethanol content in ethane/ethanol mixtures without synergistic effects. Soot concentration was also measured in neat ethane flames with flow rates matching those of ethane in ethane/ethanol mixtures. A comparison of these flames with ethane/ethanol flames revealed a striking soot suppression effect by ethanol addition. Although the carbon flow rate was less in neat ethane flames than the total carbon flow rate of their corresponding ethane/ethanol flames, the peak soot volume fractions were significantly higher in neat ethane flames. This points to a chemical soot suppression effect beyond that can be explained by the lower sooting propensity of ethanol fuel. These results were discussed in light of the synergistic effect studies in the literature. Lastly, measurements from partially-premixed ethane/ethanol mixtures are reported. The increased oxygen content significantly increased the peak soot volume fraction in these flames. These results support that intermolecular oxygen in ethanol and molecular oxygen in the fuel tube behave drastically differently in an ethane base flame. Ethanol addition does not exhibit synergistic effects on soot formation, whereas molecular oxygen addition increases maximum soot concentration, indicating the activation of unique soot production pathways.

Blending Effects of Nitrogenous Bio-Fuel on Soot and NOx Formation during Gasoline Combustion

Bio-fuel plays an important role in addressing the issue of CO2 and pollutant emission during cocombustion with fossil fuels. In this work, the effects of nitrogenous bio-fuel on soot and NOx formation during gasoline combustion were numerically investigated. Three bio-fuels, modeled by mixtures of pyridine, ethyl acetate, and ethanol with different volume fractions of pyridine and named SIM1 (0%), SIM2 (0.5%), and SIM3 (1%), were adopted to investigate the formation of soot and NOx when coburning with gasoline. Two mechanisms, i.e., the Glarborg mechanism and the Okafor mechanism, were employed to study the difference in soot and NOx formation. Results showed that pyridine exhibited different effects on soot formation in different regions. Based on the Okafor mechanism, soot formation was suppressed at the height of z = 3–4 cm, while it was promoted at the height of z = 4.5–5.5 cm. However, a reverse trend was predicted by the Glarborg mechanism. Moreover, the peak soot volume fraction (SVF) calculated by the Okafor mechanism was obviously higher than that calculated by the Glarborg mechanism; the major reason for the inconsistency of SVF in the two mechanisms was that more C6H5CH3 reacted with OH radicals to form C6H5CH2 in the Glarborg mechanism, resulting in less C6H5CH3 forming benzene (A1). Along with the increase of the pyridine proportion in the bio-fuel, the concentrations of NO increased by 92.1 and 91.4% in both mechanisms, attributed to the pyrolysis of pyridine into HCN, which promoted the formation of NO precursors (N, NH, NCO, and HNO) and thus NO. The difference in NO formation between the two mechanisms was because the reaction rate of NO converted from NO precursors in the Glarborg mechanism was higher than that in the Okafor mechanism. The NO2 was converted from NO via the reaction of NO + HO2 = NO2 + OH, and it distributed in the region with a lower NO concentration.

The effects of oxygen index on soot formation of ethylene, propane and their mixtures in coflow diffusion flames

Oxygen index (OI) effects on soot formation and oxidation of ethylene, propane and their mixtures were investigated in coflow diffusion flames. Experimental results show that the peak soot volume fraction (SVF) firstly increases and then reaches the plateau region as OI increases from 0.21 to 0.42 in ethylene and 90%ethylene/10%propane mixture flames. However, the peak SVF firstly increases and then decreases with the increase of OI in propane flame. All these three cases present a consistent trend in terms of maximum soot yield which increases first and then decreases as OI increases. Synergistic effect was captured in ethylene/propane mixtures while it was firstly enhanced and then mitigated when OI increases in the investigated OI range. Numerical simulations were also conducted. A significant relationship was found among flame temperature, O2 mole fraction and SVF in terms of their spatial gradients. The non-linear trends of SVF and soot yield with OI can be attributed to the competition between soot formation and oxidation. Such competition was also indicated from polycyclic aromatic hydrocarbons (PAHs) growth. Numerical analyses revealed that small radicals including O, CH2 and C6H5 contribute to the non-linear changes of PAHs in the synergistic effect.

Effects of mechanically-induced oscillations on soot formation and flame temperature in laminar diffusion flames of ethylene

An experimental study was performed to measure the effects of flame oscillations on soot concentration and flame temperature in co-flow laminar diffusion flames of ethylene. One novel aspect of this study is that flame oscillations were mechanically induced by moving the burner assembly in a reciprocating motion with an industrial computer-controlled linear stage. Motor speed was used to control the oscillation frequency. The peak-to-peak oscillation amplitude was defined as stroke, which was then used to calculate the Strouhal number. Oscillation frequencies of 5‒20 Hz and peak-to-peak amplitudes of 0.5‒3 mm were studied. At 1 mm peak-to-peak oscillation amplitude, flame oscillations switched from tip flickering type to tip-clipping type oscillations above 10 Hz. Time-averaged soot concentration increased from its value in the steady-state flame when the flame was perturbed by small oscillation frequencies and amplitudes but then decreased as the oscillation intensity was increased. The peak time-averaged soot volume fraction was 9.5 ppm in the steady-state flame and 4.5 ppm at a peak-to-peak oscillation amplitude and an oscillation frequency of 1 mm and 20 Hz, respectively. Similarly, the peak soot volume fraction was 5.8 ppm at a peak-to-peak oscillation amplitude and an oscillation frequency of 3 mm and 10 Hz, respectively. The time-averaged flame temperatures were in the range of 1500‒2000 K. The low-temperature zone was observed at the flame core in the steady-state flame and for low-intensity oscillations, and at the flame tip for high-intensity oscillations. The literature has discrepancies on how flame oscillations affect the total soot loading in ethylene flames. Our results show that the total soot loading increased by about 10% from steady-state to low-intensity oscillations, then slightly decreased as either oscillation frequency or amplitude was increased. The effects of increasing the oscillation frequency or amplitude were similar and well captured by the revised Strouhal number.

Impact of ammonia addition on soot and NO/N2O formation in methane/air co-flow diffusion flames

Ammonia (NH3) is receiving considerable attention as a potential substitute for carbon-containing fuels to reduce CO2 and soot emissions. Co-burning with hydrocarbons is a feasible solution to improve the combustion performance of NH3, which however, brings new challenges such as NO emissions. Hence, it is essential to investigate the detailed suppression/formation mechanisms of soot and NO during the co-burning of NH3 and hydrocarbons. In this study, the effects of NH3 addition in the fuel stream on soot and NO formations in a CH4/air co-flow atmospheric-pressure diffusion flame were numerically investigated using a 2D code and a combined chemical mechanism comprised of a model of polycyclic aromatic hydrocarbons (PAHs) up to five-rings and detailed nitrogen-containing reactions. The integral of reaction rates over the whole flame were obtained in both CH4 and CH4NH3 co-flow flames to quantitatively investigate how the NH3 addition affected the soot and NO formation pathways. The results showed that the addition of 10% NH3 in the fuel stream of a CH4/air diffusion flame had a strong suppression on soot formation, decreasing the peak soot volume fractions by 38.9%. It was found that the NH3 addition led to decreases in CH4 and H mole fractions close to the burner exit, and hence lowered the integral of reaction rate of R67 (CH4+H=CH3+H2) by 1.07×10−5 mol/s, which in turn reduced the A1 production mainly through the pathway of CH4→CH3→C2H6→C2H5→C2H4→C2H3→C2H2→C3H3→A1. NO and NH played an important role in the consumption of C1∼C2 species involved in A1 formation pathways. The NH3 addition led to an increase in the integrated rates of CH3+NH=CH2NH+H and C2H+NO=HCN+CO by 2.45×10−6 and 2.59×10−6 mol/s, respectively. The decrease of A1 mole fraction reduced the inception, condensation and HACA surface growth rates. The peak NO mole fraction was increased by two orders of magnitude, from 28 ppm in the CH4 flame to 3060 ppm in the CH4NH3 flame. A promoting effect of NH3 addition on N2O was observed in CH4NH3 mixtures, however, through different channels than that of CH4 flames. In addition, more CN species, such as CH2NH, H2CH and CH2CN, were produced in the CH4NH3 flame compared to the CH4 flame.

Soot formation from n-heptane counterflow diffusion flames: Two-dimensional and oxygen effects

Soot formation of n-heptane is experimentally and numerically studied in low-strain rate (K = 50 s−1) counterflow soot formation (SF) flames. Flame temperatures and soot volume fractions at different oxygen mole fractions on the oxidizer stream (xO2=0.3–0.45) are measured by using thermocouple-calibrated OH two colors laser-induced fluorescence (2C-PLIF) and laser-induced incandescence (LII) calibrated by light extinction methods, respectively. Mono (MAHs) and polycyclic aromatic hydrocarbons (PAHs) are also qualitatively measured by using the PAHs-LIF technique. Good agreement between measured and predicted maximum flame temperature (Tmax) and peak soot volume fraction (fv,peak) is obtained. However, discrepancies in the shape of the entire soot volume fraction profiles along the axial centerline are observed between 1D and 2D simulations. This result is found to be primarily related to the thermophoretic and radial effects in the low strain rate flames investigated, which hamper neglecting the underlying 2D effects. MAH/PAH peak mole fractions and fv,peak increase from xO2=0.3 to xO2=0.45 due to the related increase of flame temperature, which fosters n-heptane pyrolysis near the fuel nozzle leading to higher concentrations of C1C4 hydrocarbons. The latter are found to grow to MAHs/PAHs and finally to soot particles through analogous pathways independently of xO2. However, the higher flame temperature in the sooting region at xO2=0.45 leads to soot particles and aggregates characterized by larger sizes and more dehydrogenated than those produced at xO2=0.3.

Temperature, soot, and OH*/CH* chemiluminescence measurements of partially-premixed CO2-diluted propane flames on a linear Hencken burner

Recent studies highlight the need for further examination of partially-premixed diffusion flames with applications in modern burner configurations that increasingly rely on technologies such as flue gas recirculation and oxy-fuel combustion, which increase the CO 2 content in the combustion environment. The partially-premixed nature of the linear Hencken burner (LHB) offers a unique platform with many advantages over comparable slot-type or radially-symmetric burner configurations. In this work, partially-premixed laminar propane flames were examined under increasing CO 2 percentages (0 to 50% v ). Prosumer grade digital CMOS cameras were calibrated and used to determine two-dimensional, quantitative measurements of soot, temperature, and OH* and CH* chemiluminescence. Soot values were additionally supported via laser extinction measurements using a 532 nm continuous wavelength laser. The peak soot volume fraction was found to decrease from 318 ppbv for the pure propane case down to 37 ppbv for the 50% v CO 2 case, while the corresponding temperature decreased from 1876 K down to 1742 K. The data also suggested that soot maturity decreased in conjunction with overall concentration. The regions of peak OH* and CH* concentrations maintained their transverse positions as well as their relative positions with respect to one another for all flame conditions, though moved axial further away from the flame base with increasing CO 2 . Additionally, the peak OH* region remained closer to the central flame axis across all conditions, a result that is counter to other studies that examined CO 2 dilution in purely non-premixed flames. • 2D quantitative soot and temperature measurements of CO2 diluted partially-premixed propane flames. • Laser extinction measurements bolster 2D soot values. • Fuel dilution over 40%v CO2 starts to significantly lower soot maturity. • 2D quantitative chemiluminescence measurements of OH* and CH* in CO2 diluted partially-premixed propane flame. • Movement of peak chemiluminescent regions show significant differences versus premixed and dilution flames.

Non-gray gas and particle radiation in a pulverized coal jet flame

In pulverized coal combustion, the participating radiative media includes gas, soot, and coal/char particles; a fundamental understanding of their radiation behavior is essential for the prediction of radiation heat transfer and control of radiation in the furnace. In this study, the individual contributions of gas, soot, and coal particles to the total radiation were examined based on the large eddy simulation of the CRIEPI pulverized coal jet flame. The radiation transfer equation was solved using the discrete ordinate method with state-of-the-art non-gray radiative property models of multi-phase media. The full spectrum correlated k-distributions (FSCK) model was used to calculate the radiative property of the gas-soot mixture. A burnout-based particle radiative property model was applied for coal/char particles. The results showed that the predicted spatial distributions of coal particles, tar, and soot were in good agreement with the experimental data. The gray gas radiation model predicted temperatures approximately 50 K lower in this flame, and the peak soot volume fraction was predicted at approximately 22% lower. For the laboratory scale burner under examination, the individual radiative contributions of gas, soot, and coal/char were found to be 35%, 40%, and 25%, respectively, so that it was necessary to consider all components for accurate prediction. Moreover, these contribution ratios varied with the flame position. Because of the high particle concentration, the individual contribution of coal/char particles accounted for up to 65% in the devolatilization region. Soot radiation played an important role in the pulverized coal flame. In the region where the ensemble-averaged soot volume fraction reached the maximum of 2.7 ppm, the individual contribution of soot was 50% of the total radiation, which accounted for 70% of the total particles (soot and char) radiation.

The effects of CO2/CH4 ratio on soot formation for autothermal reforming of methane at elevated pressure

Autothermal reforming of methane (ATR) technique is a promising technology for H2 production. Soot formed in the combustion region of ATR may deactivate the downstream catalyst used to enhance the reforming reactions. In this work, the effects of CO2/CH4 ratio on soot production and main gas-phase components involved in reforming reactions were investigated by experiment and numerical simulations in laminar inverse coflow diffusion flames, at conditions close to ATR. The CO2/CH4 ratio ranges from 0.27 to 4.90, and the pressure ranges from 1 atm to 10 atm. The oxidant consists 70% O2 and 30% N2 to mimic the oxy-combustion in the ATR process. The soot, polycyclic aromatic hydrocarbon (PAHs), OH* distribution, and flame structure were measured using planar laser-induced incandesce, planar laser-induced fluorescence, OH* luminescence, and flame luminescence, respectively. High fidelity simulations with a reduced gas-phase kinetic mechanism and soot aerosol models were also undertaken to supplement the findings. The results show that soot formation is suppressed by a higher CO2/CH4 ratio, while the level of soot reduction is attenuated as the pressure increases. The peak soot volume fraction increases with the flame height, and then levels off. PAHs concentration monotonically increases with flame height. The spatial distributions of PAH and soot along flame height were not well captured by numerical simulations, and the used physical based soot inception model is expected to account for the discrepancy. The peak soot volume fraction correlates with CO2/CH4 ratio by an exponentially decaying function, with the exponent decreasing with increasing pressure. A similar trend of PAHs concentration along with CO2/CH4 ratio was also observed. Non-sooting flame is achievable at elevated pressure if the CO2/CH4 ratio is higher than 4.0, at the expense of lowering the syngas yield in the combustion region. The simulations also predicted that the flame temperature and concentration of H2 and CO decrease with CO2/CH4 ratio.

Experimental investigation of soot formation in inverse diffusion flames of CO2 and N2 addition: Isolation of dilution and thermal effects

ABSTRACT This experimental study compares the flame structure, temperature, and soot formation characteristics of normal (NDFs) and inverse diffusion flames (IDFs), evaluates the effect of CO2 addition, and isolates the thermal and dilution effects of CO2 and N2 addition on soot formation in IDFs. A hyperspectral imager using the TR-GSVD algorithm measures IDFs with different CO2 addition (XD) in the oxidant, with N2 addition as the comparison. In contrast to NDFs, IDFs have an opened tip, and the visible flame height and flame width decrease as XD grows. IDFs have a higher peak temperature than NDFs, but their peak soot volume fraction is substantially lower. Compared with N2 addition at the same XD, CO2 addition narrows IDFs and makes the reaction zone visible, and the peak temperature of CO2 addition is 350 K lower than that of N2 addition. The suppression effect of CO2 on soot formation is more potent than N2, with a soot formation rate of 30% that of N2 addition and a soot loading of 20–60%. The activation energy of soot formation reaction with CO2 addition is higher than N2 addition. The isolating results reveal that the thermal effect contributes to the main suppression effect of CO2 addition on soot formation, while the dilution effect of N2 addition is stronger than its thermal effect.