Pressure effect on soot formation in turbulent diffusion flames

Soot formation in a methane air turbulent jet diffusion flame is investigated numerically using a semi-empirical model. The temperature, density and species (the soot precursor C2H2) fields are calculated using detailed chemical kinetic mechanism based on the flamelet library approach. The influence of pressure on the soot formation and the behavior of the semi-empirical model in different flame situations are investigated. It is found that the flame shape and the flame temperature can be well predicted by the flamelet library approach. The calculated soot yield is mostly sensitive to the soot surface growth rate and the increase of pressure. The increase of pressure leads to the increase of soot surface growth rate and therefore to the increase of soot volume fraction. By adjusting a model constant in the soot surface growth rate, the soot emissions in both pressure p = 1 atm and p = 3 atm are properly simulated by the current semi-empirical soot model.     

Study on the effect of iron on PM10 formation and design of a particle-generating system using a cocentric diffusion burner flame

Iron pentacarbonyl was added to a cocentric diffusion burner flame burning a mixture of acetylene and ethylene in a co-flowing stream of air. Samples of aerosols and gaseous species were collected within the flames and above the flames with filters and a sampling bottle, and soot volume fraction through the flame was calculated with laser light extinction measurements. Aerosol was isokinetically collected in the inhalation chamber to measure particle concentration and size distribution. Laser extinction measurement showed that iron (Fe) gave an effect on soot formation process and scanning electron microscopy of the aerosol sample showed that soot particle size for the Fe-doped flame was relatively smaller than that of non-Fe-doped flame. Transmission electron microscopy results indicated that Fe species were separated from the soot at the downstream flame. Particles of the soot and Fe mixture could be generated continuously, and the concentration was kept constant under a given experimental condition using the cocentric diffusion flame burner. The mass loading variation for each target concentration (i.e., 100, 200, and 400 microg/m3) in the inhalation chamber was less than +/-5% during 10 hr. This particle-generating burner system could be used effectively for a bioassay test to evaluate risk     

Soot Control During the Combustion of Polystyrene

This paper describes an experimental study on the control of soot formation during the combustion of polystyrene (PS). A stable, twodimensional flame generated by using a Wolfhard-Parker type diffusion flame burner was used to simulate practical combustion situations. The combustion characteristics, effects of operating conditions on soot formation, and the effectiveness of various metallic additives as soot suppressants were investigated. It was found that soot yield could not be significantly reduced by controlling the oxidizer (air) flow rate. Increasing the O₂ mole fraction of the oxidlzer increased soot yield under typical operating conditions. However, soot yield could be greatly reduced by adding small amounts of air into the pyrolysis zone of the flame. The additional air would probably increase the combustion reaction rate while decreasing the soot precursor formation rate, thereby causing the observed effect. It was thus suggested that effective soot control could be accomplished by improving mixing between air and the PS devolatilizatlon products In practical combustion situations. The metallic additives tested in this study were the salts of Na, K, Ca, and Ba. Among these salts, Ca was the least effective in reducing soot, and K and Na were nearly equally effective. Ba was much more effective than all the others, and Its effectiveness was strongly dependent on its addition rate.     

A detailed numerical study of the evolution of soot particle size distributions in laminar premixed flames

In this work, two numerical techniques, viz. the method of moments and a discrete h-p-Galerkin method, have been applied for numerical simulation of soot formation in a laminar premixed acetylene/oxygen/argon flame. From the evolution of the PAH and the soot particle size distributions, new insight into the different processes of soot formation is provided. For this, the single submodels have been examined with respect to their influence on the PAH and the soot particle size distributions. The particle inception step was studied in detail by comparing the simulated PAH size distributions with experimental results. Additionally, an estimation of the interaction energy of layered PAH dimers was performed by quantum chemical calculations. From these results, some evidence for the particle inception model employing coalescence of PAH molecules has been found. The numerical results for the gas phase chemical species, the particle number densities and volume fractions of soot as well as for the soot particle size distributions are compared with experimental data. Thereby, the consistency of the entire model is demonstrated.     

A modeling evaluation of the effect of chlorine on the formation of particulate matter in combustion

The effect of chlorine on the fuel-rich oxidation of hydrocarbons and on the molecular weight growth of aromatics is analyzed by simulating experiments featuring a model chlorinated additive CH3Cl in a jet-stirred/plug-flow reactor and premixed flames. The kinetic model used in this work emphasizes the role of resonantly stabilized radicals in the formation and growth of aromatics, and considers soot inception as the net effect of molecular weight growth and graphitization of aromatic structures. Chlorinated hydrocarbons decompose at temperatures significantly lower than hydrocarbons, producing reactive Cl-atoms, which have a strong tendency to go to HCl. The HCI, tying up the H-atoms, inhibits hydrocarbon oxidation. The model is able to predict not only the levels but the shape of the experiments quite well and also the surprising finding of an increased soot formation associated with lower PAH levels found in rich flames with significant levels of chlorine. Based on reaction kinetic analysis, chlorine addition to the fuel enhances soot formation by promoting the formation of aromatic-ring compounds and accelerating the abstraction of aromatic H-atoms from stable PAH molecules. This process activates the transformation of aromatics to soot.     

Investigation of soot by two-colour four-wave mixing

A novel technique has been used for the temporally resolved investigation of the interaction of laser radiation and soot in a flame. Based on established soot vaporisation models we developed a framework to calculate the two-colour four-wave mixing signal obtained in an acetylene/air diffusion flame at atmospheric pressure. The signal comprises a contribution due to soot and, depending on the wavelength of the probe laser, a contribution because of C2, which is generated by vaporising soot. For both parts of the signal we found a good agreement between measured and calculated power dependency. Furthermore, we measured spatial profiles of the signal in a Wolfhard-Parker slot burner. By analysing the different signal contributions we found that depending on the location within the flame there are particles with different C2 yield.     

Monitoring of fuel consumption and aromatics formation in a kerosene spray flame as characterized by fluorescence spectroscopy

The large presence of aromatic compounds in distillate fossil fuels should allow, in line of principle, to follow the fuel consumption and/or the presence of unburned fuel in a high temperature environment like a burner or the exhaust of combustion systems by exploiting the high fluorescence emission of aromatic fuel components. To this aim an UV-excited fluorescence source has to be used since the aromatic fuel components are strongly fluorescing in the UV region of the emission spectrum.

In this work UV-excited laser induced fluorescence (LIF) diagnostics was applied to spray flames of kerosene in order to follow the fuel consumption and the formation of aromatic species. A strong UV signal was detected in the spray region of the flame that presented a shape similar to that found in the LIF spectra preliminary measured on the cold spray and in the room-temperature fluorescence of fuel solutions.

The decrease of UV signal along the spray flame region was associated to the consumption of the fuel, but more difficult seems to be the attribution of a broad visible emission, that is present downstream of the flame.

The visible emission feature could be assigned to flame-formed PAH species contained in the high molecular weight species, hypothesizing that their fluorescence spectra are shifted toward the visible for effect of the high temperature flame environment.     

Fluorescence spectroscopy of aromatic species produced in rich premixed ethylene flames

The fluorescence spectra of the condensed species (CS) collected in the soot inception region of a rich premixed laminar ethylene/oxygen flame have been measured by excitation in the UV at 266 and 355 nm excitation wavelength. The contribution of the most abundant polycyclic aromatic hydrocarbons (PAH) to the CS fluorescence has been evaluated in order to attribute the CS fluorescence at different emission wavelengths to specific aromatic structures. The fluorescence peaks detected in the UV region of the CS fluorescence spectrum was found to be mainly due to a typical PAH like fluorene, that is, the most fluorescent one among the PAH analyzed in the CS by chromatographic analysis. The CS exhibited the larger emission in the visible where the PAH contribution has been shown to be negligible and other fluorescing aromatic species, not identified by chromatographic analysis of the CS, have to be considered responsible for the visible fluorescence. Laser induced fluorescence (LIF) flame measurements excited at 266 nm and detected at two selected wavelengths (310 and 410 nm) have been performed along the flame axis and compared with the CS fluorescence intensity. The LIF and CS fluorescence signals show quite similar axial trends demonstrating that the LIF signals are related to CS fluorescence. In particular, the LIF fluorescence signals detected in the UV could be attributed to the PAH fluorescence whereas the unidentified species contained in the CS can be followed by LIF detection in the visible region.     

Evidence for the contribution of methane in the formation of aromatics and soot in fuel-rich pre-mixed combustion

Experimental results from an isothermal laminar flow reactor at atmospheric pressure are presented on the chemical composition in the post-oxidative region of two sooting fuel-rich pre-mixed mixtures diluted in nitrogen. A base case composed of n-heptane and O2 in N2 at 1425 K with a C/O of 2.85 was perturbed by substituting 10% of the carbon in n-heptane with carbon as CH4. While these changes would intuitively reduce aromatics and soot formation by increasing H2 and decreasing C2H2 concentrations, we observe the opposite. The concentrations of individual aromatic species are observed to actually increase by up to 50% and the soot yield increases by 80%.     

Mass spectrometry up to 1 million mass units for the simultaneous detection of primary soot and of soot precursors (nanoparticles) in flames

A hybrid setup consisting of low pressure burner, flow reactor and photo-ionization mass spectrometer was used for the simultaneous detection of primary soot and of flame generated nanoparticles precursing soot. The studied flames were low pressure (120-180 mbar) C2H4/O2 flames surrounded by an N2 shield. The flow reactor was not used in this study. Through variation of the burner conditions (stoichiometry, sampling height) it could be shown that nanoparticles and soot are entirely independent species. The former, in particular, are found very early in the flame and their concentration profiles do not vary very much throughout the flame. This renders the possibility that nanoparticles are emitted together with soot and consequently may constitute an additional environmental hazard. Photo-ionization mass spectrometry is well suited for the detection of these particles.     

Suppression of Soot by Metal Additives During the Combustion of Polystyrene

This paper describes an experimental study on the suppression of soot by metal additives during the combustion of polystyrene (PS). A two-dimensional flame generated by using a Wolfhard-Parker type diffusion flame burner was used to simulate practical combustion situations. The PS was continuously fed to the burner and, by controlling the feed rate, the combustion was maintained at a steady state. The additives tested were the salts of Li, Na, K, Mg, Ca, Sr, and Ba, and the combinations of the salts of K and Ca, Sr, or Ba. These additives were added to the flame in the form of small drops of their aqueous solutions generated by an ultrasonic atomizer. Since the flow rate of the carrier gas (air) is very small, this addition causes no noticeable disturbance to the flame. The effectiveness of the alkali metals follows the order of their ease of ionization, i.e., K > Na > Li, and that of the alkaline-earth metals: Ba > Sr > Ca > Mg. At low addition rates, the effectiveness increases with increasing addition rate but becomes unaffected at high addition rates and the maximum percentage of soot suppressed is approximately 50 percent. The combinations of the two metals (i.e., K and Ca, Sr, or Ba) are much more effective than each single metal at the same addition rates and the maximum percentage of soot suppressed reaches approximately 90 percent. It is proposed that the alkaline-earth metals catalyze the ionization of the alkali metals, thus significantly enhancing the effect on soot suppression.     

Comparative analysis of the structure of carbon materials relevant in combustion

The determination of the structure of carbon materials is an analytical problem that join the research scientific communities involved in the chemical characterization of heavy fuel-derived products (heavy fuel oils, coal-derived fuels, shale oil, etc.) and of carbon materials (polycyclic aromatic compounds, tar, soot) produced in many combustion processes.

The knowledge of the structure of these “difficult” fuels and of the carbon materials produced by incomplete combustion is relevant to research for the best low-environmental impact operation of combustion systems; but an array of many analytical and spectroscopic tools are necessary, and often not sufficient, to attempt the characterization of such complex products and in particular to determine the distribution of molecular masses.

In this paper the size exclusion chromatography using N-methyl-pyrrolidinone as eluent has been applied for the characterization of different carbon materials starting from typical carbon species, commercially available like polyacenaphthylene, carbon black, naphthalene pitch up to combustion products like soot and soot extract collected in fuel-rich combustion systems. Two main fractions were detected, separated and molecular weights (MWs) determined by comparison with polystyrene standards: a first fraction consisted of particles with very large molecular masses (>100 000 u); a second fraction consisted of species in a relatively small MW range (200–600 u). The distribution of these fractions changes in dependence on the carbon sample characteristics.

Fluorescence spectroscopy applied on the fractions separated by size-exclusion chromatography has been used and comparatively interpreted giving indications on the differences and similarities in chemical structure of such different materials.     

Reduced combustion mechanism for C1–C4 hydrocarbons and its application in computational fluid dynamics flare modeling

Emissions from flares constitute unburned hydrocarbons, carbon monoxide (CO), soot, and other partially burned and altered hydrocarbons along with carbon dioxide (CO₂) and water. Soot or visible smoke is of particular concern for flare operators/regulatory agencies. The goal of the study is to develop a computational fluid dynamics (CFD) model capable of predicting flare combustion efficiency (CE) and soot emission. Since detailed combustion mechanisms are too complicated for (CFD) application, a 50-species reduced mechanism, LU 3.0.1, was developed. LU 3.0.1 is capable of handling C₄ hydrocarbons and soot precursor species (C₂H₂, C₂H₄, C₆H₆). The new reduced mechanism LU 3.0.1 was first validated against experimental performance indicators: laminar flame speed, adiabatic flame temperature, and ignition delay. Further, CFD simulations using LU 3.0.1 were run to predict soot emission and CE of air-assisted flare tests conducted in 2010 in Tulsa, Oklahoma, using ANSYS Fluent software. Results of non-premixed probability density function (PDF) model and eddy dissipation concept (EDC) model are discussed. It is also noteworthy that when used in conjunction with the EDC turbulence-chemistry model, LU 3.0.1 can reasonably predict volatile organic compound (VOC) emissions as well.

Implications: A reduced combustion mechanism containing 50 C₁–C₄ species and soot precursors has been developed and validated against experimental data. The combustion mechanism is then employed in the computational fluid dynamics (CFD) of modeling of soot emission and combustion efficiency (CE) of controlled flares for which experimental soot and CE data are available. The validated CFD modeling tools are useful for oil, gas, and chemical industries to comply with U.S. Environmental Protection Agency’s (EPA) mandate to achieve smokeless flaring with a high CE.     

Using Radiocarbon to Apportion Sources of Polycyclic Aromatic Hydrocarbons in Household Soot

To determine whether polycyclic aromatic hydrocarbons (PAHs) in household soot were derived from the combustion of scrap wood or creosote that was impregnated in the wood (or some combination of both), the molecular composition and radiocarbon ( 14 C) content of the total carbon and several PAHs in the soot was investigated. The 5730-year half-life of 14 C makes it an ideal marker for identifying creosote-derived PAHs ( 14 C-free) versus those derived from the combustion of wood (contemporary 14 C). The 14 C abundance of phenanthrene, fluoranthene, pyrene, and retene was determined by accelerator mass spectrometry after solvent extraction and purification by preparative capillary gas chromatography. The molecular analysis (presence of retene and 1,7-dimethylphenanthrene) and bulk 14 C content (contemporary) of the soot indicated that wood combustion was a strong source of carbon to the soot. The 14 C of retene in two soot samples was also contemporary, indicating that it was derived from the combustion of the scrap wood. These results are consistent with previous work that has suggested that retene is an excellent marker of wood combustion. However, the 14 C content of phenanthrene, fluoranthene, and pyrene in one soot sample was much lower and revealed that these compounds had a mixed creosote and wood source. Using an isotopic mass balance approach, we estimate that 40 to 70% of phenanthrene, fluoranthene, and pyrene were derived from the combustion of the scrap wood. The results of this study show that molecular marker and bulk 14 C analysis can be potentially misleading in apportioning sources of every PAH, and that molecular-level 14 C analysis of PAHs can be a powerful tool for environmental forensics.     

UV-visible spectroscopy of organic carbon particulate sampled from ethylene/air flames

A systematic comparison of spectra obtained with extra and in situ diagnostics in the soot preinception region of rich, premixed ethylene air flames suggests that combustion generated organic carbon (OC) particulate can be extracted from flames and isolated from other flame material for further chemical analysis. Both the trend with height above the burner and the form of UV fluorescence and absorption spectra from extra situ sampled material captured in water agree with those measured in situ. These results show that the OC particulate formed in flames is partially water soluble. However, the collection efficiency can be increased using less polar solvents, like acetonitrile and dichloromethane. The fluorescence spectra from the water samples are comprised both a naphthalene-like component and a broad band UV fluorescence component similar to that observed in situ which is attributed to flame generated OC particulate. The broad band UV fluorescence centered around 320 nm is also observed very early in flames and does not change considerably with increasing flame residence time. These results support previous hypotheses that the UV broad band fluorescence is from carbonaceous material comprised two-ring aromatics, formed earlier than soot in the flame, and is still present along with soot at higher heights or flame residence times.     

Analysis of exhausts emitted by i.c. engines and stationary burners, by means of u.v. extinction and fluorescence spectroscopy

Optical investigations of the exhausts emitted by internal combustion (i.c.) engines and a stationary burner were performed, in order to assess their relative role as sources of organic matter to the atmosphere. Extinction spectra of air-diluted exhausts in the 200-400 nm u.v. band reveal the expected existence of gaseous trace-species (NO, NO2 and SO2) and carbonaceous particulate matter (soot). In addition, after subtracting the absorption contribution from known species, a strong residual absorption band remains below 250 nm, which is attributed to organic aromatic matter, involving no more than two aromatic rings. A set of ex situ extinction and laser induced fluorescence (LIF) experiments were carried out on condensed combustion-water samples. Extinction measurements from the water samples show absorption spectra similar to those observed from air-diluted samples, which are attributed to low volatility organic compounds, as they are trapped in the condensed phase. Combining the indications of extinction data for both air-diluted and condensed samples, it is suggested that the absorbing species might be molecular clusters of one/two aromatic rings. LIF spectra from condensed samples evidence two fluorescence bands, centered above 300 and 400 nm, respectively, whose intensities correlate with the combustion regimes. Analogous optical analysis on rain samples, collected in an urban area, showed that rain absorption and fluorescence spectra are similar to those found in condensed exhaust samples, which is consistent with the prevailing contribution of i.c. engines to the urban air pollution. The combined experimental data suggest that the absorbing and fluorescent species trapped in the condensed samples are organic (aromatic) compounds, involving mostly one two aromatic rings structural units, since they do not absorb above 250 nm. The overall molecular weight of the trapped material is likely heavy as they show low volatility.     

Combustion aerosols: factors governing their size and composition and implications to human health

Particulate matter (PM) emissions from stationary combustion sources burning coal, fuel oil, biomass, and waste, and PM from internal combustion (IC) engines burning gasoline and diesel, are a significant source of primary particles smaller than 2.5 microns (PM2.5) in urban areas. Combustion-generated particles are generally smaller than geologically produced dust and have unique chemical composition and morphology. The fundamental processes affecting formation of combustion PM and the emission characteristics of important applications are reviewed. Particles containing transition metals, ultrafine particles, and soot are emphasized because these types of particles have been studied extensively, and their emissions are controlled by the fuel composition and the oxidant-temperature-mixing history from the flame to the stack. There is a need for better integration of the combustion, air pollution control, atmospheric chemistry, and inhalation health research communities. Epidemiology has demonstrated that susceptible individuals are being harmed by ambient PM. Particle surface area, number of ultrafine particles, bioavailable transition metals, polycyclic aromatic hydrocarbons (PAH), and other particle-bound organic compounds are suspected to be more important than particle mass in determining the effects of air pollution. Time- and size-resolved PM measurements are needed for testing mechanistic toxicological hypotheses, for characterizing the relationship between combustion operating conditions and transient emissions, and for source apportionment studies to develop air quality plans. Citations are provided to more specialized reviews, and the concluding comments make suggestions for further research.     

Formation and inhibition of chloroaromatic micropollutants formed in incineration processes

The formation pathways for chlorinated aliphatic and chlorinated aromatic compounds in technical incineration processes are reviewed. It is shown that acetylene is converted to chloroaromatic compounds including PCDD/F in a special flow reactor by catalytic activity of CuCl2 in the temperature regime of a post-combustion zone of technical incinerators. Mechanistic pathways begin with chlorination of acetylene. Dichloroacetylene is further condensed to C-4 and C-6 units. Hexachlorobenzene is the dominant aromatic compound and a likely precursor to chlorinated phenols and PCDD/F. Two specific mechanisms of formation of chlorinated aromatic compounds including PCDD/F have been advanced. Both mechanisms begin with the formation of dichloroacetylene from flame pyrolysis products like acetylene. Condensation of dichloroacetylene is mediated by copper species via metallacyclic intermediates and/or a catalytic cycle involving copper stabilized trichlorovinyl radicals. The final pathways of conversion of chlorinated benzenes to PCDD/F via chlorophenols are under active investigation.     

Combustion Aerosols: Factors Governing Their Size and Composition and Implications to Human Health

ABSTRACT

Particulate matter (PM) emissions from stationary combustion sources burning coal, fuel oil, biomass, and waste, and PM from internal combustion (IC) engines burning gasoline and diesel, are a significant source of primary particles smaller than 2.5 μm (PM_2.5) in urban areas. Combustion-generated particles are generally smaller than geologically produced dust and have unique chemical composition and morphology. The fundamental processes affecting formation of combustion PM and the emission characteristics of important applications are reviewed. Particles containing transition metals, ultrafine particles, and soot are emphasized because these types of particles have been studied extensively, and their emissions are controlled by the fuel composition and the oxidant-tem-perature-mixing history from the flame to the stack. There is a need for better integration of the combustion, air pollution control, atmospheric chemistry, and inhalation health research communities. Epidemiology has demonstrated that susceptible individuals are being harmed by ambient PM. Particle surface area, number of ultrafine particles, bioavailable transition metals, polycyclic aromatic hydrocarbons (PAH), and other particle-bound organic compounds are suspected to be more important than particle mass in determining the effects of air pollution. Time- and size-resolved PM measurements are needed for testing mechanistic toxicological hypotheses, for characterizing the relationship between combustion operating conditions and transient emissions, and for source apportionment studies to develop air quality plans. Citations are provided to more specialized reviews, and the concluding comments make suggestions for further research.     

Flamelet modeling of NO formation in laminar and turbulent diffusion flames

The applicability of the laminar flamelet concept for the formation and destruction of nitric oxides in laminar and turbulent diffusion flames has been studied. In a first step, temperatures and species concentrations in an axisymmetric laminar diffusion flame have been calculated (i) by solving the detailed conservation equations and (ii) by applying the laminar flamelet concept. The main purpose of this step was the identification of differences between results from both approaches. It turned out that for highly temperature sensitive or relatively slow chemical processes, the inclusion of the full range of the prevailing scalar dissipation rates plays a major role for the calculated species concentrations. This behavior is obvious from the concept of the laminar flamelet model, where the scalar dissipation rate can be discussed in terms of the reciprocal of a residence time for attaining chemical equilibrium. In a second step, flamelet modeling of NOx formation was extended to a turbulent hydrogen diffusion flame. In both the steps, the flow fields of the flames were calculated by solving the Navier-Stokes equations in axisymmetric formulation using the SIMPLER algorithm. For the turbulent flow, Favre-averaged equations have been used and turbulence was modeled with the standard k-epsilon model including a correction term for axisymmetric systems. The averaging of the species concentrations was accomplished with presumed shape probability density functions (pdfs). The pdf of the mixture fraction was described with a beta-function whereas that of the scalar dissipation rate was assumed to be log-normal. Buoyancy effects have been taken into account. The calculated temperatures and concentrations were compared with data from different experiments.