Chemical Effects Of CO2 Addition Research Articles

Chemical effects of carbon dioxide in ethylene, ethanol and DME counter-flow diffusion flames: An experimental reference for the fictitious CO2 flame

The chemical effects of carbon dioxide on flame chemistry were investigated numerically and experimentally in ethylene, ethanol, and dimethyl-ether (DME) counter-flow diffusion flames. In this work, a method that could add an experimental reference flame for the fictitious CO2 flame (FCO2) was proposed. The fuel/oxidant carrier gas (FCO2+argon+helium) in the FCO2 flames was replaced by an inert gas mixture (nitrogen+argon+helium) whose composition was optimized to maintain approximately the same thermal and dilution effects as FCO2 addition. The differences in the flame temperatures and the measured species concentration between CO2 flame and the inert gas mixture flame could then be attributed to the pure chemical effects. The new experimental strategy was applied to ethylene, ethanol, and dimethyl-ether (DME) counter-flow diffusion flames. Good agreement was found between the experimental and the computational results. The chemical effects of CO2 addition, indicated by both the numerical FCO2 flame and the experimental inert gas mixture flame, both decreased the peak flame temperatures and the concentration of CH4, H2, and C2H2 except CO. The effects of CO2 addition on the production pathways of CO and C2H2 were very similar in ethylene, ethanol, and DME flames. Decreased CO oxidation rate through CO+OH = CO2+H was a significant feature of the chemical effects identified by both the FCO2 flame and the inert gas mixture flame. Thermal effects of CO2 addition decreased the formation rate of C2H2 through C2H3(+M) = H+C2H2(+M) while the chemical effects on this pathway were weaker.

A numerical investigation of CO2 dilution on the thermochemical characteristics of a swirl stabilized diffusion flame

The turbulent combustion flow modeling is performed to study the effects of CO2 addition to the fuel and oxidizer streams on the thermochemical characteristics of a swirl stabilized diffusion flame. A flamelet approach along with three well-known turbulence models is utilized to model the turbulent combustion flow field. The k-ω shear stress transport (SST) model shows the best agreement with the experimental measurements compared with other models. Therefore, the k-ω SST model is used to study the effects of CO2 dilution on the flame structure and strength, temperature distribution, and CO concentration. To determine the chemical effects of CO2 dilution, a fictitious species is replaced with the regular CO2 in both the fuel stream and the oxidizer stream. The results indicate that the flame temperature decreases when CO2 is added to either the fuel or the oxidizer stream. The flame length reduction is observed at all levels of CO2 dilution. The H radical concentration indicating the flame strength decreases, following by the thermochemical effects of CO2 dilution processes. In comparison with the fictitious species dilution, the chemical effects of CO2 addition enhance the CO mass fraction. The numerical simulations show that when the dilution level is higher, the rate of the flame length reduction is more significant at low swirl numbers.

Numerical investigation on the effect of CO2 and steam for the H2 intermediate formation and NOX emission in laminar premixed methane/air flames

One-dimensional premixed freely-propagating flames for (CH4+CO2/H2O)/air(79%N2+21%O2) mixtures were modeled using ChemkinⅡ/Premix Code with the detailed mechanism GRI-Mech 3.0. The investigation of the effects of CO2 and steam addition on the H2 intermediate formation and NO emission was conducted at the initial conditions of 1 atm and 398 K. Both physical and chemical effects of CO2, H2O on laminar burning velocities and adiabatic flame temperatures were also analyzed. The calculations show that with the increase of αCO2 and αH2O, both physical and chemical effects of CO2 and H2O result in the reduction of laminar burning velocities (LBVs) and adiabatic flame temperatures (AFTs) in which the chemical effects of CO2 addition are more significantly than H2O. Especially, the chemical effects of steam promote the increase of AFTs and the influence in rich BG65 flames are larger than in methane. With a proper amount of H2O addition, the chemical effects of H2O on the peak concentration of H2 are more significantly than physical at Φ = 1.2. Moreover, CO2, steam and their mixture addition have significant reduction on the NO emission. The most sensitive reaction for the formation of H2 and NO emission were determined. The responsible reactions for H2 formation and NO emission are R84 OH + H2 <=> H + H2O and R240 CH + N2 <=> HCN + N (a prompt routine), respectively.

Experimental study of CO2 diluted, piloted, turbulent CH4/air premixed flames using high-repetition-rate OH PLIF

High-speed planar laser-induced fluorescence (PLIF) was applied to turbulent premixed flames with CO2 addition. Instantaneous PLIF images capturing emissions from the OH radical via the A2Σ-X2Π (1,0) band were collected. Three flames with varying levels of CO2 addition were established at the same adiabatic flame temperature, Reynolds number, and Lewis number, thereby minimizing the effects of differences in flame temperature and transport. The chemical effects of CO2 addition were investigated through critical combustion parameters revealing turbulent flamelet structure and burning velocity. The flamelet structure analysis suggests that the development of the mean flame brush thickness follows the turbulent diffusion law with a secondary effect introduced by CO2 addition. The flame surface density of these flames is affected by CO2 addition mainly through modification of mean progress variable distributions. The flame length is extended by CO2 addition with an enhancement of unburned pocket formation in the downstream portion of the flame. The apparent size and consumption speed of the fine scale unburned reactant pockets are similar among flames with varying CO2 addition. The global consumption speed of flames with CO2 addition is reduced predominantly by a reduction in the laminar flame speed. The global combustion intensity shows a constant value within uncertainty limits for flames with CO2 addition. The local combustion intensity before x/D = 1.5 is observed to be lower for the flames with CO2 addition due to the attenuation of pocket formation in the flame brush development region. Its effect on global combustion intensity is counterbalanced by the contribution of pocket formation process in the downstream portion of the flame.

A fundamental investigation into chemical effects of carbon dioxide on intermediate temperature oxidation of biodiesel surrogate with laminar flow reactor

The chemical effects of carbon dioxide on the oxidation of biodiesel surrogate, methyl butanoate (MB) were investigated experimentally in the SJTU laminar flow reactor at equivalence ratios of 1.5, 1 and 0.5, at atmospheric pressure. Generally, there are three kinds of effects from carbon dioxide (CO2): thermal, dilution and chemical effects. In this study, argon was replaced by CO2 as the dilute gas as a way to isolate the chemical effects by keeping the factors of temperature profile and dilution ratio constant. Therefore, the differences in concentrations of species at the exit from the flow reactor could be attributed to the chemical effects of CO2. Good agreement was found between the experimental and the computational results. The chemical effects were classified into two types. First, there was the radical factor, by which the reaction CO2 + H = OH + CO played a significant role in the oxidation of MB. In CO2 atmosphere, radical factors led to the reduction in reactivity and suppressed the decomposition of MB. The second effect was the collision factor, which derived from the high frequency of third-body collisions of CO2. This collision factor contributed to the formation of carbon monoxide and ethane. Moreover, in the combustion of biodiesel engine with EGR, pure chemical effects of CO2 addition decreased the concentration of acetylene as well as suppressed formation of larger PAHs.

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.

Physical and Chemical Effects of CO2Addition on CH4/H2Flames on a Jet in Hot Coflow (JHC) Burner

Moderate or intense low-oxygen dilution (MILD) combustion is a promising technology for energy savings and pollutants emission reduction, and it has been experimentally shown that the MILD regime can be achieved more easily when using CO2 as diluent with respect to N2 [Li, P.; Combust. Flame 2013, 160 (5), 933−946]. Since CO2 is different from N2 in both physical and chemical properties, the present study aims at distinguishing the physical and chemical effects of CO2 addition on the establishment of MILD combustion. A jet in hot coflow (JHC) burner firing CH4/H2 blended fuel is numerically modeled coupled with a detailed chemical kinetics mechanism. Following the examination of grid independency and model validation, the differences in combustion temperature, minor and major species formations as well as the CH4 oxidation pathway are compared by, respectively, replacing N2 with CO2 or X (artificial CO2) in low-oxygen coflow. An interesting phenomenon is presented that the chemical effects of CO2 play a comparable role in the suppression of temperature rise to the physical effects. However, with the replacement of N2 by CO2, the contribution of chemical effects to temperature reduction is gradually weakened. Moreover, the chemical effects of CO2 are responsible for the ignition delay as well as the enhanced CO emission when is CO2 replacing N2, which is due to the inhibition of CH4 oxidation through Routes I (CH4 → CH3 → CH2(S)/CH2 → (CH → CH2O →) HCO → CO → CO2) and II (CH4 → CH3 → CH2O → HCO → CO → CO2) together with the enhanced CO2 dissociation by R99 (OH + CO ↔ H + CO2) and R153 (CO2 + CH2(S) ↔ CO + CH2O).

Effects of CO2 addition on the evolution of particle size distribution functions in premixed ethylene flame

The effect of CO2 addition on sooting behavior in premixed ethylene/oxygen/argon flat flames at atmospheric pressure was investigated both experimentally and computationally. The evolution of soot particle size distribution functions (PSDFs) was examined using Burner-Stabilized Stagnation (BSS) probe/Scanning Mobility Particle Sizer (SMPS) technique. To explain the experimental observations, numerical simulations of the flames were carried out to analyze the thermal and chemical effects of CO2 addition on the precursor chemistry. The evolution of PSDFs of nascent soot showed that the CO2 addition gave rise to a decline in soot nucleation rate and a slower mass growth rate, hence significantly lowering soot yield. The inhibition of soot formation by CO2 addition is found to be mainly through chemical effect, while the thermal effect only plays a minor role.

Chemical Effects of Carbon Dioxide Addition on Dimethyl Ether and Ethanol Flames: A Comparative Study

The chemical effects of CO2 addition on premixed laminar low-pressure dimethyl ether and ethanol flames were studied by comprehensive numerical analysis from fuel-lean to fuel-rich conditions. Added CO2 is assumed as normal reactive CO2 and fictitious inert CO2 to assess the chemical effects of CO2. The dilution and thermal effects of CO2 addition decrease C2H2 mole fractions in ethanol flames instead of DME flames, but the chemical effects can reduce C2H2 mole fractions in both DME and ethanol flames at all equivalence ratios, which reveals that C2H2 formation can be suppressed chemically by CO2 addition. The chemical effects have a weak influence on formaldehyde formation in both DME and ethanol flames. The CO2 chemical effects only result in a slight decrease of acetaldehyde peak mole fractions in DME flames but not in ethanol flames at all equivalence ratios. Mole fractions of the H radical decrease because of the chemical effects of CO2 addition by shifting the equilibrium of CO + OH = CO2 + H in bot...

Chemical effects of CO2 addition to oxidizer and fuel streams on flame structure in H2-O2 counterflow diffusion flames

Numerical simulation of CO2 addition effects to fuel and oxidizer streams on flame structure has been conducted with detailed chemistry in H2–O2 diffusion flames of a counterflow configuration. An artificial species, which displaces added CO2 in the fuel- and oxidizer-sides and has the same thermochemical, transport, and radiation properties to that of added CO2, is introduced to extract pure chemical effects in flame structure. Chemical effects due to thermal dissociation of added CO2 causes the reduction flame temperature in addition to some thermal effects. The reason why flame temperature due to chemical effects is larger in cases of CO2 addition to oxidizer stream is well explained though a defined characteristic strain rate. The produced CO is responsible for the reaction, CO2+H=CO+OH and takes its origin from chemical effects due to thermal dissociation. It is also found that the behavior of produced CO mole fraction is closely related to added CO2 mole fraction, maximum H mole fraction and its position, and maximum flame temperature and its position. Copyright © 2003 John Wiley & Sons, Ltd.