Carbon Dioxide Dilution Research Articles

Comparison of non-reactive flow fields in conventional swirl-air combustion and swirl distributed combustion

Flow field distribution in traditional swirl flow and simulated distributed reaction swirl flow configuration was examined at 3 kHz frequency using high-speed Particle Image Velocimetry (PIV). Non-reactive flow field conditions in distributed reaction zones were fostered from the conventional swirl flow case using the designed inlet mixture preparation with CO2 dilution of the inlet airstream that corresponded to the combustion case at a heat release intensity of 5.72 MW/m3-atm. High-speed imaging of flow fields was performed at a framing rate of 3000 frames/s using a Neodymium-doped Yttrium Lithium Fluoride (Nd:YLF) laser beam at 527 nm wavelength to capture the Mie scattering images of the flow field. Mean flow velocity obtained from the post-processing of raw PIV images showed higher momentum flux injection in the swirl distributed flow zone case as compared to the normal swirl flow case. Enhanced velocity fluctuations were also observed in both the axial and radial direction for the distributed swirl flow case resulting in higher turbulence for the examined condition. The distributed swirl flow case showed a prominent corner recirculation zone along with higher flow entrainment within the recirculation zone. Enhanced interaction of average streamlines with vortices was found in the swirl distributed flow case. Increased turbulent fluctuation in the two different spatial directions was responsible for higher Reynolds stress distribution (u′v′) in the distributed flow case, indicating improved mixing behavior at lower global oxygen concentrations from the carbon dioxide (CO2) dilution, and showed different length scales of turbulent eddies. Furthermore, the spatial local Damkӧhler number values for distributed flow case (for its implication in reacting case) were found to be less than unity. The results showed a shorter flow timescale than the corresponding chemical timescales in distributed reaction zone. These results help support substantiating the difference of reaction regimes on the Borghi diagram (plot of velocity fluctuation/ laminar flame speed versus the integral length scale/ flame thickness) between the distributed reaction zone (that possesses thickened reaction zone) and conventional swirl reaction flow regimes having corrugated flamelet zone.

Evaluation of the chemical effect of carbon dioxide dilution to fuel on soot formation in co-flow laminar ethylene and propane diffusion flames

In this study, the effects of carbon dioxide (CO2) addition on soot evolution were isolated systematically by adding 50% N2, 31% Ar+19% CO2, and 50% CO2 to the fuel side, respectively. Two series of diffusion flames, ethylene and propane, were experimentally investigated. Optical diagnostics, including laser-induced fluorescence (LIF), ratio pyrometry, and laser-induced incandescence (LII), were utilized to measure the spatial distributions of polycyclic aromatic hydrocarbons (PAHs), flame temperature, soot volume fraction fv, primary particle size dp, and number density of soot np. It was found that the flame height decreased gradually for the ethylene flame with adding N2/Ar + CO2/CO2. In contrast, the flame height remained essentially unchanged for propane-based flames. The results showed that adding N2/Ar + CO2/CO2 increased the temperature in the upper part of the ethylene flame, while the temperature change in the upper part of the propane flame was subtle and almost negligible. The total PAHs-LIF loading and the spatial distributions of fv/dp/np indicated that the chemical effect of CO2 added to ethylene flames in inhibiting the formation of large-size PAHs and retarding the soot inception stage was insignificant. However, it was not negligible for propane flames. The quantitative results revealed that the inhibition of total soot loading ST in propane flames was dominated by the chemical effect of CO2, while in ethylene flames, it was dominated by the thermal and dilution effect of CO2. The same conclusion was also found in the results of the maximum carbon conversion factor ηmax and the average particle size of soot. The specific growth rate of soot mass was almost unchanged regardless of the mixture on the fuel side. Adding CO2 had a more substantial chemical effect in inhibiting soot formation in propane flames than that in ethylene flames, which could mainly be attributed to a more effective reduction in the time for the increase of soot mass τg during soot evolution.

Fuel-lean ammonia/biogas combustion characteristics under the reacting swirl flow conditions

Ammonia has been identified as a viable energy vector for power generation. Using dual-fuel operation that mixes the ammonia with higher reactivity gaseous fuel can be vital in enhancing ammonia combustion. This study examined the fundamental swirl combustion characteristics of fuel-lean premixed ammonia/biogas via a numerical approach. The flame was established at an input thermal power of 7 kW and a global equivalence ratio of 0.8. The numerical model was validated with biogas emissions data acquired through experimental work. At 20 mm downstream the burner, increased carbon dioxide mass fraction in the biogas lowered the peak flame temperature by around 300 K. Moreover, the deformation of flame temperature radial profiles was also found aggravated as carbon dioxide concentration in the biogas elevated from 0 % to 40 %. The reduction in premixed reactant mixture reactivity not only initiated flame temperature profile deformation but also reduced the peak Damköhler number significantly. The peak Damköhler number was lowered by a factor of ∼ 1.5 when carbon dioxide dilution in the biogas elevated by 40 %. The premixed combustion was directed into the thin reaction flamelets zone with elevated carbon dioxide mass fraction, owing to the intensified flow fluctuation. This, in turn, elevates the normalised flame turbulent propagating speed, gas flow average velocities and turbulent kinetic energy, notwithstanding that heat release rate and averaged laminar flame speed declined. In all, the presence of carbon dioxide has been shown to lower the ammonia/methane mixture reactivity whilst escalating the reacting flow fluctuation.

Effects of hydrogen and carbon dioxide on the laminar burning velocities of methane–air mixtures

The effects of different mole fractions of hydrogen and carbon dioxide on the combustion characteristics of a premixed methane–air mixture are experimentally and numerically investigated. The laminar burning velocity of hydrogen-methane-carbon dioxide-air mixture was measured using the spherically expanding flame method at the initial temperature and pressure of 283 K and 0.1 MPa, respectively. Additionally, numerical analysis is conducted under steady 1D laminar flow conditions to investigate the adiabatic flame temperature, dominant elementary reactions, and NO formation. The measured velocities correspond with those estimated numerically. The results show that increasing the carbon dioxide mole fraction decreases the laminar burning velocity, attributed to the carbon dioxide dilution, which decreases the thermal diffusivity and flame temperature. Conversely, the velocity increases with the thermal diffusivity as the hydrogen mole fraction increases. Moreover, the hydrogen addition leads to chain-branching reactions that produce active H, O, and OH radicals via the oxidation of hydrocarbons, which is the rate-determining reaction. Furthermore, an increase in the mole fractions of hydrogen and carbon dioxide decreases the NO production amount.

OH-Planar Laser-Induced Fluorescence Measurements in Laminar Diffusion Flames of n-Heptane with Coflow Air Diluted by N2 and CO2

The distribution of hydroxyl (OH) radicals in the laminar diffusion flame of n-heptane was studied by planar laser-induced fluorescence (PLIF). The influences of nitrogen (N2) and carbon dioxide (CO2) dilution on the formation and distribution of OH radicals were analyzed. The corresponding dilution ratios (volume fractions) of both N2 and CO2 vary from 0 to 5%. The results show that for the n-heptane flame, the OH radical is mainly concentrated in the two wings of the flame, presenting a radially approximate symmetrical distribution. Both N2 and CO2 dilutions decrease the intensity of the maximum OH radical fluorescence and the total OH radical fluorescence. Moreover, the flame temperature decreases more significantly with the CO2 dilution ratio due to the combination of the dilution effect and thermal effect.

Optoelectronic and Structural Properties of Plasma Deposited Nanocrystalline Hydrogenated Silicon Oxide Thin Films

To develop wide bandgap materials for solar cells and other optoelectronic devices, undoped hydrogenated silicon oxide (SiOx:H) thin films are prepared by conventional radio frequency plasma enhanced chemical vapor deposition (RF PECVD) method. The variation of carbon dioxide dilution ([Formula: see text]) on optoelectronic and structural properties are studied thoroughly by keeping silane and hydrogen gas flow fixed. Surface morphology of the SiOx:H films have been studied by Field Emission Scanning Electron Microscopy (FESEM) and Atomic Force Microscopy (AFM). Distinct silicon nanocrystallites of average diameter [Formula: see text] 3–6[Formula: see text]nm embedded uniformly in amorphous SiOx network have been observed in high resolution Transmission Electron Microscopy (HRTEM). From Fourier Transform Infrared spectra (FTIR), it is observed that oxygen content ([Formula: see text]) increases initially with [Formula: see text] and afterwards it decreases. Strong room temperature photoluminescence (PL) peak is obtained for the as-deposited films having lower oxygen content ([Formula: see text]). The origin of room temperature PL spectra and its correlation with [Formula: see text] can be explained by quantum confinement effect (QCE) theory.

Study of the influence of water vapour and carbon dioxide dilution on pollutants emitted by swirled methane/oxygen-enriched air flames

The effects of Exhaust Gas Recirculation (EGR) through the dilution by carbon dioxide and water vapour on methane flames/oxygen-enriched air in the case of the pressurized premixed combustion encountered in gas turbines are studied. The chosen configuration is a confined swirled premixed flame with a constant swirl number (Sn = 0.90). The input parameters are the initial temperature, adiabatic flame temperature, pressure (0.1 MPa), and the flow rates and concentrations of CH4, O2, CO2, H2O and N2. The pollutant emissions of NO and CO were measured at the exit of the combustion chamber and expressed as a volume fraction on dry gas. The gaseous emissions of NOx, CO, CO2 and O2 were measured by a HORIBA PG250 analyser, and in the case of high CO2 dilutions, the CO2 concentration measurements were made using a HORIBA VA-3000 analyser. The pollutant emissions were measured for the two initial temperatures T0 = 373 and 473 K and for the four adiabatic temperatures (1773, 1873, 1973 and 2073 K). Finally, the role of parameters (enrichment, diluent, equivalence ratio) likely to play a part in the production of CO and NO pollutant emissions was analyzed. The influence of the diluent on the emissions is highlighted. Dilution with CO2 decreases NO emissions but leads to an increase in CO. Dilution with water vapour is more attractive since at a constant adiabatic temperature, it leads to lower emissions of CO2, CO and NO.

A novel developed weighted exponential sum model for char cloud conversion behavior under air- and oxy-combustion: Experiments and kinetics modeling analysis

Comprehension of the fossil fuel chemical reaction conversion behavior benefits to insight micro physico-chemistry mechanism of burning chemical kinetics especially for conventional air-fired combustion and oxyfuel combustion. Detailed combustion reaction experiments and a novel developed kinetics model were proposed to investigate coal char conversion behavior under enriched-oxygen and high carbon dioxide dilution environment compared with air environment in present work. Primary characteristic parameters, i.e., combustion conversion rate, conversion ratio, burnout time, etc., mainly affected by limited chemical interactions between char oxidation reaction and gasification reaction, were studied under typical environments, i.e., O2/CO2, O2/Ar, CO2/Ar, O2/N2, using detailed experiments and a novel developed kinetics model, i.e., the weighted exponential sum model (WESM). Present kinetics predictions were also compared with previous typical kinetics models, i.e., the homogeneous reaction model (HRM), the shrinking core model (SCM), the modified volumetric reaction model (MVRM). Results showed that present kinetics model gave relatively more coherent predictions of the char conversion rate, conversion ratio, the maximum conversion rate and burnout time compared with those predicted by HRM, SCM and MVRM models, respectively. Meanwhile, the contribution ratio from endothermal gasification chemical reaction, i.e., C+CO2→2CO, on char maximum conversion rate was 41.1% and 45.2% in 1473 K and 1663 K, respectively. Correspondingly, the maximum and average interaction contribution ratios (χinteramax,χinteraave) decreased as 9.0% and 1.7%. The maximum rate ratio for oxyfuel combustion increased from 52.7% to 61.7%, and the average rate ratio slightly increased from 52.5% to 54.2% in these combustion reactions.

Ignition Delay Times of Oxy-Syngas and Oxy-Methane in Supercritical CO2 Mixtures for Direct-Fired Cycles

Abstract The direct-fired supercritical CO2 (sCO2) cycles promise high efficiency and reduced emissions while enabling complete carbon capture. However, there is a severe lack of fundamental combustion kinetics knowledge required for the development and operation of these cycles, which operate at high pressures and with high CO2 dilution. Experiments at these conditions are very challenging and costly. In this study, a shock tube was used to investigate the auto-ignition tendencies of several mixtures under high carbon dioxide dilution and high fuel loading. Individual mixtures of oxy-syngas and oxy-methane fuels were added to CO2 bath gas environments and ignition delay time data were recorded. Reflected shock pressures neared 100 atm, above the critical pressure of carbon dioxide into the supercritical regime. In total, five mixtures were investigated with a pressure range of 70–100 atm and a temperature range of 1050–1350 K. Measured ignition delay times of all mixtures were compared with two leading chemical kinetic mechanisms for their predictive accuracy. The mixtures included four oxy-syngas and one oxy-methane compositions. The literature mechanisms tended to show good agreement with the data for the methane mixture, while these models were not able to accurately capture all behavior for syngas mixtures tested in this study. For this reason, there is a need to further investigate the discrepancies. To the best of our knowledge, we report the first ignition data for the selected mixtures at these conditions. Current work also highlights the need for further work at high pressures to fully understand the chemical kinetic behavior of these mixtures to enable the sCO2 power cycle development.

Study of the influence of water vapour and carbon dioxide dilution on flame structure of swirled methane/oxygen-enriched air flames

In this work, the effect of dilution with water vapour H2O and carbon dioxide CO2 on the structure and stability of a methane/enriched air premix flame, confined and swirled at atmospheric pressure was studied. Measurements were carried out at constant adiabatic temperature (1773, 1873, 1973 and 2073 K), from air to oxycombustion (enrichment OI ranging from 21 to 100%), for two inlet temperatures (T0 = 373 K and 473 K), at constant equivalence ratio maintained at 0.91 and inlet aerodynamic conditions maintained constant: average inlet velocity at 30 m·s−1 and geometric swirl number at Sn = 0.90. The experimental setup is a burner used consisting of a swirled stainless steel single injector, mounted in a combustion chamber operating at atmospheric pressure. The flames were visualized by CH* chemiluminescence. From instantaneous images, the flame contour, detected with a Matlab® processing, was determined. From this flame contour, flame height (Hf) and lift-off height (Hlo) were extracted by measuring respectively the maximum height of the flame top and the minimum height of the bottom flame. The evolution of the mean macrostructure of the flame was then studied. The results of the steam dilution were detailed and compared with those obtained with carbon dioxide. Links between the laminar flame speed and the flame structure and stability were clearly established.

Swirl assisted distributed combustion behavior using hydrogen-rich gaseous fuels

Controlled entrainment of hot reactive gases into the fresh oxidizer mixture prior to ignition is critical for achieving colorless distributed combustion (CDC) condition that results in a uniform thermal field, ultra-low emissions, enhanced flame stability, low noise and mitigation of combustion instability. This paper examines the fuel flexibility of colorless distributed combustion in a swirl-stabilized burner using hydrogen enriched methane gas. Distributed combustion condition was achieved by reducing the oxygen concentration in the oxidizer through addition of either N2 or CO2. Three different gas fuel compositions were investigated. Hydrogen enriched methane in various concentrations in the presence of CO2 and N2 represented coke oven gas. Results on the effect of fuel properties on global flame structure and emissions are reported here. The effect of equivalence ratio on flame stability and emissions was also investigated to determine the impact of air dilution with no change in oxygen concentration. The results showed that for the different gas compositions investigated, transition to distributed combustion conditions occurred at oxygen concentrations of 10–12% with N2 and 13–15% with CO2 as the diluent gas. Increase in hydrogen concentration (or decrease in methane concentration) in the gas mixture resulted in transition to distributed condition at reduced oxygen concentration. The NO emission decreased considerably (to less than 1 ppm) as compared to normal air combustion under the distributed combustion condition for all the gas compositions examined using the different diluents. The emission of CO decreased gradually when approaching distributed conditions, and then increased slightly when the condition moved toward the lower flammability limit. Single digit ppm CO levels were achieved with CDC under nitrogen dilution while carbon dioxide dilution resulted in slightly higher CO emission that was attributed to the dissociation of CO2 at high temperatures. The propensity for flashback with high hydrogen content gas was eliminated under distributed condition and provide wider flame stability limits. These results demonstrate fuel flexibility of diluted hydrogen rich methane gas (akin to coke oven gas) under distributed combustion that provided enhanced stability and ultra-low emissions.

Effects of carbon dioxide dilution and composition on tip opening and OH radical distribution in laminar non-premixed Bunsen flame

Effects of CO2 dilution and composition on tip opening and flame OH radical distribution in laminar non-premixed Bunsen flame were investigated by using planar laser induced fluorescence (PLIF) technique. Experiments were conducted at different component fractions diluted with CO2 at 20%, 25%, and 30%. The syngas were produced by biomass catalytic gasifying. Stretch rates were calculated and combined with OH-PLIF images and simulation. We conducted kinetic simulation to analyze the production of OH radical. Results show that as VH2 increases and hydrogen slightly increases, the effect of mass diffusion is enhanced, in addition, the value of stretch rate decreases, and the tip opening becomes further obvious. The profile of OH radical distribution also becomes broad. Finally, the total production rates of OH radicals increases with a rate less than 7.5%. Hence, the maximum value of OH radical counts is reduced by 33.33%, 27.92%, and 23.09%. Nonetheless, with the increasing addition of CO2, the effect of mass diffusion is weakened; besides, chemical reaction residence time increases, and the value of stretch rate increases; the tip opening is less obvious. Although the distribution area of OH was narrowed, its production decreased and the relative concentration of all OH radical decreased.

Experimental study on explosion characteristics of hydrogen–propane mixtures

The explosion characteristics of a hydrogen–propane mixture under different initial temperatures and at different hydrogen addition, equivalence, and dilution ratios were studied using a 20-L apparatus. First, the effects of hydrogen addition and equivalence ratio were studied. The results showed that the maximum explosion pressure and the maximum rate of pressure rise first increased, then decreased from lean to an equivalence ratio of 1.6, and reached the maximum value at the equivalence ratio of 1.2. The maximum explosion pressure and the maximum rate of pressure rise increased with the increasing hydrogen proportion of the mixtures, and the time to the maximum explosion pressure decreased. Furthermore, the mixed fuel with the equivalence and hydrogen addition ratios of 1.0 and 0.4, respectively, was explored under nitrogen or carbon dioxide dilution. Carbon dioxide exhibited a stronger dilution effect than nitrogen. For the mixed fuel with the equivalence and hydrogen addition ratios of 1.0 and 0.4, respectively, the maximum explosion pressure saliently decreased with the increasing initial temperature in the range of 25 °C-120 °C. The variation trend of experimental results was consistent with theoretical predictions.

Numerical and experimental study of the influence of CO2 dilution on burning characteristics of syngas/air flame

In this study, effect of carbon dioxide dilution on explosive behavior of syngas/air mixture was investigated numerically and experimentally. Explosion in a 3-D cylindrical geometry model with dimensions identical to the chamber used in the experiment was simulated using ANSYS Fluent. The simulated results showed that after ignition, the flame front propagated outward spherically until it touched the wall, like the propagating flame observed in the experiment. Both experimental and simulated results presented a same trend of decreasing the maximum explosion pressure and prolonging the explosion time with CO2 dilution. The results showed that for CO2 additions, the maximum explosion pressure decreased linearly and the explosion time increased linearly, while the maximum rate of pressure rise decreased nonlinearly, which can be correlated to an exponential equation. In addition, both results showed a good agreement for syngas/air flame with CO2 addition up to 20% in volume. However, larger discrepancies were observed for higher levels of CO2 dilutions. Of the three diluents tested, carbon dioxide displayed the strongest effect in reducing explosion hazard of syngas/air flame compared to helium and nitrogen. Chemical kinetic analysis results showed that maximum concentration of major radicals and net reaction rates of important reactions drastically decreased with CO2 addition, causing a reduction of laminar flame speed.

Evaluation of safety parameters of light alkenes by means of detailed kinetic models

Abstract The oxidation of light alkenes is the core of the modern chemical industry and a pivotal point for several environmental and safety considerations. However, few works have dealt with the prediction of fire and explosion parameters for the definition of process conditions, for the design and safe handling of such reactive substances. In this work, flammability parameters for light alkenes (ethylene, propylene and the three butylene isomers) have been calculated by adopting the detailed kinetic mechanisms of the University of California, San Diego, integrated with C4 reactions by the Lawrence Livermore National Laboratory mechanism. The new model has been adopted for the definition of flammability limits, adiabatic flame pressure and temperature, maximum rate of pressure rise, gas deflagration index (KG), auto-ignition temperature and minimum oxygen concentration. Flammability regions for nitrogen and carbon dioxide dilution have also been reported.

Flame fluctuations in Oxy-CO2-methane mixtures in swirl assisted distributed combustion

Swirl assisted oxy-methane combustion investigated here has focused on the impact of carbon dioxide dilution. Carbon dioxide dilution is essential as it lowers the flame adiabatic temperature and flame speed of oxy-fuel combustion leading to a more stable operation. However, carbon dioxide dilution may result in flame fluctuations, under fuel lean conditions. Experiments using a swirl burner examined the flame fluctuations with different amounts of CO2 dilution, with the goal to achieve stable oxy-CO2-methane combustion via colorless distributed combustion (CDC). This novel CDC technique has shown significant performance gains in gaseous fuel-air flames on emission reduction, enhanced thermal field uniformity, noise reduction, and improved flame stability. Results have shown that for oxy-CO2-fuel combustion, most of the heat release fluctuations had frequencies below 60Hz. The results obtained revealed that increase in CO2 dilution increased the magnitude of flame fluctuation till a maximum of 28% O2 concentration in the oxidizer. Further increase in dilution gases and reduction in O2 concentration led to a more stable flame, along with more favorable conditions for colorless distributed combustion. Data analysis of flame images revealed the source of oscillations under different conditions. At maximum fluctuations with 28% O2, the flame oscillated between two modes leading to unstable operation and large fluctuation in the heat release signal. Decrease in oxygen concentration to below 27%, promoted a more stable flame behavior, occupying a larger flame volume, which is characteristic of distributed combustion. Further reduction in O2 led to flame blow off at around 21% O2. This work outlines the importance CO2 dilution in oxy-fuel flames for achieving stable distributed combustion and mitigate the increased oscillations normally encountered in higher oxygen concentration flames.

Effects of carbon dioxide and nitrogen addition on soot processes in laminar diffusion flames of ethylene-air at high pressures

An experimental assessment of the influence of carbon dioxide and nitrogen dilution on sooting characteristics of laminar ethylene diffusion flames at pressures up to 20atm is presented. Two dilution rates, defined as the ratio of mass flow of the fuel to that of the diluent gas, of 1:2 and 1:3 were used at all pressures with a fixed ethylene mass flow rate. A wider range of nitrogen dilution, from 1:1 to 1:4, was investigated at 10atm. In the pressure range of interest and with the mass flow rates of fuel and diluents, resulting flames were stable and nonsmoking. Spectrally-resolved line-of-sight soot radiation measurements were obtained to infer the radial soot and temperature distributions within the flame envelope through an Abel inversion process. The sooting propensity, in terms of maximum soot yield, was found to be significantly lower with carbon-dioxide dilution in the pressure range of 1–15atm but approached to comparable values to those with nitrogen-dilution at 20atm. The implication of this finding is that the chemical suppression effect of carbon dioxide dilution, which was proven at atmospheric pressure previously, exists also at elevated pressures up to 15atm and becomes relatively small at higher pressures. Variation of the maximum soot yields with pressure indicated that carbon dioxide-diluted flames show a relatively stronger dependence to pressure as compared to nitrogen-diluted flames. Temperatures decreased with increasing pressure as expected due to increasing radiative heat loss, and the peak temperatures were observed near the flame tips as a result of the heat release from soot oxidation.

Performance of small spark ignition engine fueled with biogas at different compression ratio and various carbon dioxide dilution

In order to cope with power shortage in rural areas of developing countries and to reduce greenhouse gas emissions, we study the performance of small biogas-fueled internal combustion engines of which power is less than 5kW. In particular, the influence of compression ratio on the engine is primarily examined. By decreasing the combustion chamber volume (from 19.3cc to 16.6cc), compression ratio (from 8.01:1 to 9.22:1) is increased to enhance brake power output (from 2.2kW to 2.68kW), brake thermal efficiency (from 22.0% to 29.8%) and brake specific fuel consumption of the engine (from 290.6gh×PS to 218.6 gh×PS). In addition, as means to examine the effect of the biogas composition on engine performances, various carbon dioxide dilution ratios (from 0% to 50%) are tested. At lowest carbon dioxide dilution, maximum engine performance is obtained.

Formation of Soot in Counterflow Diffusion Flames with Carbon Dioxide Dilution

ABSTRACTExperimental and numerical modeling studies have been performed to investigate the effect of CO2 dilution on soot formation in ethylene counterflow diffusion flames. Thermal and chemical effects of CO2 addition on soot growth was numerically identified by using a fictitious CO2 species, which was treated as inert in terms of chemical reactions. The results showed that CO2 addition reduces soot formation both thermodynamically and chemically. In terms of chemical effect, the addition of CO2 decreases soot formation through various pathways, including: (1) reduced soot precursor (PAH) formation leading to lower inception rates and soot number density, which in turn results in lower surface area for soot mass addition; (2) reduced H, CH3, and C3H3 concentrations causing lower H abstraction rate and therefore less active site per surface area for soot growth; and (3) reduced C2H2 mole fraction and thus a slower C2H2 mass addition rate. In addition, the sooting limits were also measured for ethylene counterflow flames in both N2 and CO2 atmosphere and the results showed that sooting region was significantly reduced in the CO2 case compared to the N2 case.

Effects of simultaneous hydrogen enrichment and carbon dioxide dilution of fuel on soot formation in an axisymmetric coflow laminar ethylene/air diffusion flame

Effects of simultaneous hydrogen enrichment and carbon dioxide dilution to hydrocarbon fuels on soot formation are of fundamental and practical interest. Previous studies found that addition of either hydrogen or carbon dioxide to fuel reduces soot chemically in addition to the dilution effect in laminar coflow ethylene/air diffusion flames. A numerical study was carried out in this work to investigate the effects of adding hydrogen and carbon dioxide simultaneously to fuel on soot formation in an axisymmetric laminar coflow ethylene/air diffusion flame at atmospheric pressure. Numerical calculations were conducted using detailed gas-phase chemistry and thermal and transport properties. Soot inception is assumed to be the result of collision of two pyrene molecules. The subsequent particle surface growth, soot oxidation, and particle interactions are modeled by the hydrogen abstraction C2H2 addition (HACA) mechanism and a sectional model. Soot surface growth through condensation of pyrene was also taken into account. The flame model is able to reproduce fairly well the chemical effects of adding either hydrogen or carbon dioxide to ethylene observed experimentally in the literature. Addition of hydrogen is more effective on soot inception suppression and addition of carbon dioxide is more effective on soot surface growth suppression. The simultaneous hydrogen enrichment and carbon dioxide dilution to ethylene retains the individual soot suppression benefits of hydrogen enrichment and carbon dioxide dilution. These results suggest that the chemical interactions between hydrogen and carbon dioxide on soot formation are weak.