Chemical Effects Of CO Research Articles

Effect of H2/CO addition on soot formation in ethylene diffusion flame

Coupling the San Diego gas phase reaction mechanism and the Moss Brookes soot model using FLUENT14.0 software, the effect of adding H2/CO on the fuel side on soot formation in an ethylene/air laminar diffusion flame was studied. A specific analysis was conducted on the effects of H2, CO and its chemical effects on flame temperature, soot volume fraction, mole fractions of important intermediate products OH, H, and C2H2, as well as rate of soot mass nucleation, surface growth, and oxidation. In the numerical calculation, the virtual substances FH2 and FCO are set to separate the chemical effect of H2 and CO and to analyze the chemical effect of adding H2 and CO on soot formation. The results show that the flame temperature increases slightly, and the soot volume fraction decreases monotonically with adding H2 and CO. The chemical effect of H2 increases the temperature, the mole fraction of C2H2 and H, the soot nucleation rate, and the surface growth rate, and finally, it promotes soot formation. The chemical effect of CO increases the temperature and H mole fraction, reduces the OH mole fraction, and then increases the soot surface growth rate and reduces the soot oxidation rate. The higher soot nucleation, surface growth rate, and lower oxidation rate jointly promote soot formation.

Exploring the Impact of CO2 Atmosphere on Propane Moderate or Intense Low-Oxygen Dilution Combustion: A Numerical Simulation Study

Abstract Moderate or intense low-oxygen dilution (MILD) combustion is a promising combustion technology widely recognized by the international combustion community. In this study, numerical simulation was used to investigate the effects of CO2 atmosphere on MILD combustion of propane in a 20-KW furnace.The results show that the O2/CO2 atmosphere leads to a lower average temperature in the furnace, better temperature uniformity, and more uniform distribution of OH and CH2O compared to MILD combustion in the N2/O2 atmosphere. Propane MILD combustion is established well under the physical and chemical effects of CO2. An analytical approach is proposed to describe the physical and chemical effects of CO2 on MILD combustion. The physical effect of CO2 shortens the ignition delay time and advances the pyrolysis and ignition of propane, which causes a high-temperature zone in the front furnace and reduces the temperature uniformity in MILD combustion. However, the chemical effect of CO2 dominates the establishment of the MILD combustion by increasing the ignition delay time and reducing burning rates, with the help of the physical effects of CO2 by intensifying the entrainment in the furnace. Thus, the overall effects of CO2 lead to enhanced temperature uniformity by enlarging the area and evening the temperature of both the ignition zone and combustion zone. These findings provide valuable insights into the physical and chemical mechanisms of CO2 in MILD combustion and have important implications for optimizing combustion processes for improved efficiency and reduced emissions.

Characteristics of OH formation during single coal particle ignition and volatile combustion in O2/N2 and O2/CO2 atmospheres

The behaviors of volatile combustion and ignition of lignite coal were assessed on a concentrating light heating platform using hydroxyl-Planar laser-induced fluorescence (OH-PLIF) over a range of 21–60% O2 in both N2 and CO2 diluent gases. The results indicated that when the O2 concentration was below 40%, substituting CO2 for N2 delayed the ignition delay time (tign). This was evident as no ignition occurred in the 21% O2/CO2 mixture, and a nearly 1.0 s difference in tign emerged when substituting CO2 for N2 in 30% and 40% air atmospheres. Additionally, the weaker flame luminosity and the shorter volatile flame duration time (tvol) were also observed in O2/CO2 compared with that in O2/N2. Correspondingly, a significantly lower OH signal intensity was observed in O2/CO2 compared to O2/N2, which was primarily due to physical and chemical effects of CO2. Firstly, the higher heat capacity and lower diffusion rate of O2 in O2/CO2 leads to a lower volatile release rate and decreasing flame temperature, as evidenced by the closer proximity of the flame front to the coal particle surface. The lower flame temperature and volatile release rate would reduce the number of hydrocarbon molecule (RH) and the reaction rate of RH→hydrocarbon radical (R) + H, leading to a decrease in the concentration of OH radical from the reaction: H + O2→OH + O. Secondly, CO2 participates directly in H + OH + three-body (M)→H2O + M reaction, competing with the primary OH formation pathway, H + O2→OH + O, for H radicals, thus suppressing the OH generation. However, when O2 concentration exceeded 40%, the tign was shorter in O2/CO2 by about 1.2–1.4 s than in O2/N2 with the same O2 concentration. Simultaneously, compared to the O2/N2, similar flame luminosity and tvol were observed in the O2/CO2. An obvious increase in OH intensity was also observed in O2/CO2. With higher O2 concentrations in the O2/CO2, the absorption and emission radiation characteristics of CO2, increased O2 diffusivity and the decreased specific heat capacity in the O2/CO2 gas mixture, synergistically facilitate temperature rise and volatile release, which promotes the generation of RH, and then increases the H concentration due to the reaction RH→R + H. This would lead to a substantial increase in concentration of OH produced by the reaction H + O2→OH + O, which was detected by OH-PLIF.

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.

A shock-tube and modeling study of syngas ignition delay times in rich CO2 environment at elevated pressures

Oxy-fuel combustion techniques are crucial for fossil fuel power plants with substantially reduced greenhouse gas emission. The chemical effect of CO2 as a reactant/product or a third-body is important in modeling oxy-fuel combustion. Many previous kinetic studies focused on evaluating the rate parameters in relevant reactions, but less attention was paid to CO2 collision efficiencies. In this study, ignition delay time data were measured in a shock-tube facility for stoichiometric syngas mixtures with 50%, 70% and 90% CO in fuel at 1 and 5 atm, diluted in 85% CO2 bath gas. Among the chemical effect of CO2, the CO2 collision efficiency in reaction H+O2(+M) = HO2(+M) shows a dominantly high sensitivity value to model prediction over the present experimental conditions. Therefore, the CO2 collision efficiency was evaluated using the current data and a recommended value was reported with uncertainty analysis. Moreover, the CO2 collision efficiencies in reaction H2O2(+M) = OH+OH(+M), HCO(+M) = H+CO(+M), H2+M = H+H+M and O+O+M = O2+M were evaluated using ignition delay time data at higher pressures (40 atm to 200 atm) and recommended CO2 collision efficiency values were reported with uncertainty analysis. The model with recommended CO2 collision efficiency values significantly improved prediction performance at 5 atm and was validated over a comprehensive set of syngas ignition delay time and laminar flame speed data.

Chemical, radiative, and dilutive effects of CO2 addition on soot formation in jet A-1 kerosene co-flow diffusion flame

With the development of aviation industry, it is urgently to investigate the soot formation properties of aviation kerosene to better control the soot emissions. The dilutive, chemical and radiative effects of CO2 on the soot inception, condensation and hydrogen abstraction acetylene addtion (HACA) growth processes in laminar co-flow Jet-A1 kerosene diffusion flames were numerically investigated by employing detailed chemical mechanisms and soot sectional models. The results showed that the addition of CO2 dramatically decreased the maximum temperature (by 92 K) and soot volume fraction (by 41.0%). The dilutive effect of CO2 contributed the most to the decrease of temperature and soot volume fraction. It also was the main factor in the decrease of soot inception, condensation and HACA growth processes. The chemical effect of CO2 had little impact on the decomposition of fuels into light hydrocarbons, but obviously limited the growth of light hydrocarbons to A1. The radiative effect of CO2 decreased the maximum temperature and soot volume fraction by 13 K and 5.2% (from 1.92-1.82 ppm). It had little impact on the soot inception, condensation and HACA growth rates.

Experimental study of the effect of CO2 on temperature and soot volume fraction in C2H4/air co-flow laminar diffusion flame.

The threat of global warming caused by greenhouse gases such as CO2 to the environment is one of the most intractable challenges. The capture and utilization of CO2 are essential to reduce its emission and achieve the goal of being carbon neutral, in which CO2-diluted combustion is an efficient carbon capture technology. In this research, the effects of CO2 addition in the fuel side (CO2-F), oxidizer side (CO2-O) and both sides (CO2-F/O) on temperature and soot formation in C2H4/air laminar co-flow diffusion flames were researched. The flame images were measured by a complementary metal-oxide-semiconductor (CMOS) imaging equipment. The two-dimensional distributions of temperature and soot volume fraction in C2H4/air laminar co-flow diffusion flames were measured employing the inverse Abel transform. The results demonstrated that the effect of amount variation of CO2-F on the decrease of flame temperature was enhanced by the CO2-O. The reduction in peak flame temperature was 4 K in the CO2-F cases, while the reduction in peak flame temperature was 83 K in the CO2-F/O cases. The soot formation was suppressed significantly by the effects of CO2-F/O. Compared with the CO2-F cases, the reductions in peak soot volume fraction were 22.5% and 23.5% in the CO2-F/O cases. The suppression effect of amount variation of the CO2-F on soot formation became more significant with the increase of flame height. The reductions in peak soot volume fractions were 0.3%, 3.07% and 6.38% at the flame heights of 20 mm, 30 mm and 40 mm in the CO2-F cases, and the corresponding reductions were 4.92%, 5.2% and 16% in the CO2-F/O cases, respectively.

Experimental and modeling study of the low to high-temperature oxidation of the methyl isopropyl ketone in O2/N2/Ar and O2/CO2/Ar atmospheres

Few studies on the low-temperature combustion behavior of MIPK, being a promising fuel additive, have been conducted. In this work, ignition delay times (IDTs) of MIPK were measured in the temperatures ranging between 780–910 K and pressures of 20 and 25 bar using a rapid compression machine (RCM). Oxy-fuel combustion combined with biofuel could remove CO2 from the atmosphere. The IDTs of MIPK were measured in the temperatures ranging between 1125–1600 K under the O2/CO2 atmosphere at the pressures of 1 and 10 bar using a shock tube. A low to high-temperature MIPK kinetic model (HUST-MIPK model) was proposed, in which the low-temperature sub-model consists of 19 low-temperature reaction classes and was constructed by analogy-based method, the high-temperature sub-model was adapted from the works of Cheng et al.. The predictions of HUST-MIPK model are in good agreement with the present low-temperature IDTs, high-temperature O2/CO2 atmosphere IDTs, and the literature experimental data. The negative temperature coefficient (NTC) behavior was not observed in the temperature range from 790 to 910 K in the present RCM experiments, but was observed for methyl propyl ketone (MPK) and diethyl ketone (DEK) under similar conditions. The low-temperature chemistry of three pentanone isomers (MIPK, MPK, and DEK) was compared using the flux and sensitivity analysis. The comparison of the experimental high-temperature IDTs between O2/CO2 and O2/Ar atmospheres indicates the IDTs of MIPK under O2/CO2 atmosphere are longer than those under O2/Ar atmosphere at 1 bar, and the effects of CO2 are almost independent of the pressure. The physical and chemical effects of CO2 on the ignition were studied in detail.

The impact of gasoline formulation on turbulent jet ignition

Turbulent jet ignition (TJI) is a promising technology for burning ultra-lean mixtures; the process is comprised of hot reactive jets issuing from a pre-chamber (PC) and initiating combustion in the main chamber (MC). The present study employs a simplified zero-dimensional (0D) partially stirred reactor (PaSR) model to describe the complex mixing and reaction progress within the PC and its subsequent impact on the MC combustion in terms of combustion efficiency and pollutant formation characteristics. Full three-dimensional (3D) computational fluid dynamics (CFD) data are used to calibrate the PC model, which is subsequently linked to predict the MC combustion behavior. We propose a model to predict the effects of the fuel formulations with varying research octane number (RON) and octane sensitivities (OS) on the TJI performance. After a careful parametric study, a dedicated merit function for identifying the optimal TJI operating conditions was proposed to assess multiple fuel properties and their influence on MC combustion. The model properly accounts for micro-mixing effects in the early jet expansion phase, and represents the effects of a PC jet on enhancing flammability and pollutant mitigation. It was demonstrated that aromatic content affects not only the progress of the thermokinetic runaway, but also the importance of NO formation paths in MC (N2O vs NNH routes), and the effect of the PC jet on MC flammability limits. Among the jet active species, OH and NO exhibited the greatest chemical impact on MC reactivity, while the chemical effects of CO2 and H2O remained limited. The overall fuel TJI merit function showed optimum performance for fuels with 2 < OS < 6 and high RON, similar to the requirements for spark-ignited engine operation beyond motor octane number (MON) conditions, fuel lean advanced compression ignition operation, and spark-induced compression ignition.

Effects of CO2 on the laminar burning velocities of toluene reference fuel (TRF) with increasing initial temperatures and pressures

Spherically expanding flames of TRF/air mixtures diluted with different levels of CO2 content at elevated temperatures and pressures were experimentally studied in this work to clarify the effects of exhaust gas recirculation (EGR), especially the effects of CO2 in EGR gases on the laminar flame characteristics of gasoline fuel. The experiments have been performed using a constant volume combustion bomb with schlieren method. The laminar burning velocities (LBV) and markstein lengths of the TRF flame with different dilution level of CO2 (0–20%) at the elevated temperature of 373 K, 443 K and pressures of 1 bar, 3 bar and 5 bar have been studied. Numerical decoupling simulations were then applied to investigate the individual dilution, thermal and chemical effects of CO2, respectively. The CO2 addition has a suppressing effect on the LBV of the TRF fuel at all the tested conditions. The LBV of the TRF mixture diluted with 20% CO2 reduced to about 60% of the non-diluted flame at 373 K and atmospheric pressure. The LBV were promoted by the elevation of initial temperature, while inhibited by the promotion of initial pressure. The increasing of initial temperature extended both the lean and rich limits, while the elevation of initial pressure would facilitate the combustion on the leaner flame. The Markstein length had a slight increase, indicating a small increase of flame instability, with the increase of the of CO2 dilution ratio for the rich flames and decreased for the lean flames. The crossover equivalence ratio, φ*, was witnessed to shift slightly towards the leaner side, while the critical equivalence ratio, φc, moved to the richer side. Among the three factors, the dilution effect played the dominant role and the chemical effect came up the next. The thermal effect took up the least portion, yet non-negligible. In addition, the chemical effect showed a more sensitive respond to the temperature variation.

Separate physicochemical effects of CO2 on the coal char combustion: An experimental and kinetic study

The overall impact of CO2 on the coal char combustion is complicated due to the interplay between the CO2 thermal effect and chemical effect (including the gasification endothermicity, and its direct additional carbon consumption). To assess the impact of these factors on the carbon consumption process, we conduct a quantitative study on the separate physicochemical effects of CO2 through experimental and kinetic research. Two oxygen concentrations (15% and 21% O2), representing industrial boiler and air combustion environments, were selected. The experiment research on its thermal effect (evaluated by carbon conversion ratios) is performed at 1773 K in a high-temperature drop tube furnace. The chemical effect of CO2 is predicted for a 100 μm coal char particle using a self-developed char burning kinetics model. Results indicate that the carbon conversion ratios show a V-type distribution with higher CO2 concentrations, as a result of its complex physicochemical effects, with minimal carbon conversion points around the CO2 concentrations of 5–10 vol.%. The thermal effect initially increases with higher CO2 concentrations from 0 to 10 vol.%, but it declines with a further increase in CO2 concentrations, while the endothermicity effect of the gasification reaction increases with higher CO2 concentrations. The relative contribution of the endothermicity effect on the char consumption is 20.4% in the 21%O2/10CO2/N2 environment, lower than the thermal effect (29.9%), but it continuously increases and becomes the most influential inhibitory factor at higher CO2 concentrations. The strongest suppression effect of CO2 corresponds to the CO2 concentrations of 10 vol.%.

Investigating the Mineralogical and Chemical Effects of CO2 Injection on Shale Wettability at Different Reservoir Temperatures and Pressures

Investigating the Mineralogical and Chemical Effects of CO₂ Injection on Shale Wettability at Different Reservoir Temperatures and Pressures

EGR thermal and chemical effects on combustion and emission of diesel/natural gas dual-fuel engine

In this paper, by coupling a D19 diesel engine three-dimensional and one-dimensional models with a diesel/NG dual-fuel chemical dynamics mechanism, respectively, the thermal and chemical effects of the EGR major components (CO2 and H2O) on in-cylinder combustion and emissions were studied. The results show that the EGR combined effect is the sum of the thermal and chemical effect, and the thermal effect is greater than the chemical effect. In the EGR major components, the CO2 thermal effect is greater than that H2O, and the chemical effect is smaller than that H2O. At the 50% EGR rate, CO2 and H2O thermal and chemical effects accounted for 65.3% and 36.7% of the combined effects in diesel mode, respectively. At the 50% EGR rate, the thermal and chemical effects of CO2 and H2O accounted for 62.2% and 39.2% of the combined effects in dual-fuel mode, respectively. The effects of CO2 and H2O can be superimposed with each other. CO2 and H2O have a greater influence in dual-fuel mode, that is, the dual-fuel is more sensitive to EGR.

The Third Body Effect of Carbon Dioxide on N-heptane Ignition Delay Characteristics under O2/CO2 Conditions

ABSTRACT In order to study the impact of CO2 during n-heptane ignition delay under O2/CO2 conditions, this study presents a numerical and experimental study in four cases (40 mol.% O2/60 mol.% CO2, 50 mol.% O2/50 mol.% CO2, 57 mol.% O2/43 mol.% CO2 and 65 mol.% O2/35 mol.% CO2). Firstly, a third body effect (TBE) model is presented, which considers the third body efficiency of CO2. Secondly, the TBE model adds the third body efficiency into a one-step global mechanism and obtains an ignition delay time formula according to the auto-ignition theory. Thirdly, experiments are carried out in a constant volume combustion chamber (CVCC) test system. Furthermore, the experiment and simulation results are compared and discussed. In addition, two parameters (α and β) are defined to study the third body effect and the chemical effect of CO2 respectively. Finally, chemical kinetic analyses are used to study the CO2 effect. The results show that the TBE model is in agreement with the experiment and the maximum difference is 8.94% under the 57 mol.% O2/43 mol.% CO2 condition. The third body effect is the main effect of CO2 and it can decrease small molecule intermediates and radicals. Besides, C3H6O is the most affected species by the third body effect of CO2 among four prominent intermediate species. Furthermore, the third body effect of CO2 inhibits most paths of OH production. H+ O2→O+ OH (R11) is the most affected pathway whose peak rate of production (ROP) decreases by 51.5%.

Numerical investigation of physicochemical effects of CO2 on coal particle ignition in O2/CO2 ambience

Coal combustion is a complex phenomenon involving multiscale chemical and physical processes. In this paper, the physical and chemical effects of CO2 on the ignition behaviors of dispersed coal particles are investigated based on a self-developed transient ignition model. Attention has been paid on the separate effects of CO2, namely the thermal conductivity, the species diffusion rate, the heat capacity, and the gasification reaction, on the ignition behaviors in oxy-fuel ambience. Differences of coal ignition delay time and ignition modes in air and oxy-fuel ambience are compared. The heterogeneous ignition (HI) is significantly delayed in CO2 ambience, with the most significant contribution from the lower thermal conductivity of CO2. The negative effects of slower species diffusion rates and the endothermic C-CO2 gasification reaction in CO2 are less important; however, they both become prominent with increasing ambient temperature up to 1800 K. By contrast, the gas phase homogeneous ignition (GI) is less sensitive to CO2 enrichment. The positive effects of slower species diffusion rates, together with the negligible effects of higher heat capacity and extra gasification reaction, impair the negative effect from lower thermal conductivity. In summary, the CO2 substitution makes the ignition mode move from the hetero-homogeneous joint regime towards the homogeneous dominant regime. Thus, coal particle ignition under high temperature (1500–1800 K) and low oxygen fraction (~0.1) transits to the homogeneous dominant case, yielding a reduced difference of ignition delay between air and oxy-fuel combustion. This feature has been observed for coal samples of different ranks from several independent macroscopic experiments, and can be well interpreted by the transition mechanism of ignition mode.

Coupling of the Chemical Effects of CO and CO2 on Soot Formation in C2H4 Diffusion Flame in O2-CO2 Atmosphere

Previous studies found that the chemical effect of CO (CE-CO) enhances the formation of soot in laminar coflow C2H4/air diffusion flames, while the chemical effect of CO2 (CE-CO2) inhibits soot formation. The coupled CE-CO and CE-CO2 on soot formation characteristics is investigated numerically in O2-CO2 atmosphere through blending fuel flow rate with 10%, 30% and 50% CO and virtual CO (FCO) in a laminar C2H4 diffusion flame. The results show that CE-CO increases the flame temperature, while CE-CO2 is opposite; the CE-CO promotes soot formation attributed to soot oxidation process which decreases the mole fraction of OH by OH+CO↔H+CO2, while the CE-CO2 inhibits the formation of soot ascribed to the increase of OH and O2 concentration promoting soot oxidation rate and the decrease of flame temperature, mole fractions of C2H2 and H inhibiting surface growth rate. Moreover, the chemical promotion effect of CO decreases in O2-CO2 atmosphere, and the coupled CE-CO and CE-CO2 suppresses soot formation.

Auto-ignition characteristics of methane/n-heptane mixtures under carbon dioxide and water dilution conditions

To further optimize the combustion performance of natural-gas/diesel engine under exhaust gas recirculation (EGR) condition, influence of physicochemical impacts of H2O and CO2 on the ignition characteristics of n-heptane/methane mixture (Φ = 0.5/1.0) under various thermodynamic conditions (p = 2 bar/1199 K < T < 1568 K, p = 60 bar/700 K < T < 1200 K) were investigated by shock tube and NUI mechanism. Experiments indicated at p = 2 bar/1199 K < T < 1568 K, CO2 and H2O additions increased and shortened ignition delay times (IDT) respectively. Mixture of CO2 and H2O slightly accelerated ignition. Original and minor modified NUI mechanism well captured the inhibition and acceleration effect of CO2 and H2O on ignition respectively. Both thermal and chemical effects of CO2 (R36:CO + OH = CO2 + H) were responsible for its ignition inhibition impacts at higher temperatures. Whereas thermal effect of CO2 became the dominant factor at lower temperatures. Chemical effect of H2O (H2 + OH<=>H + H2O&O + H2O<=>2OH) promoted OH formation and enhanced whole system reactivity, which suppressed its thermal effect and accelerated ignition process. At p = 60 bar/700 K < T < 1200 K, CO2 addition significantly retarded ignition due to its thermal effect. At 900 K < T < 1200 K, the higher third-body efficiency of H2O promoted ignition (R21:H2O2(+M) = 2OH(+M)&R34:H + O2(+M) = HO2(+M)). At 700 K < T < 900 K, thermal effect of H2O, which suppressed its third-body effect, inhibited ignition progress. These observations implied that the impact of exhaust gases on the ignition of n-heptane/methane depended on the coupled influence mechanisms of physicochemical effects, the composition of exhaust gases and located thermodynamic conditions. At intermediate-temperature high-pressure conditions (typical thermodynamic conditions of dual-fuel engine), raising the concentration of H2O or CO2 in exhaust gases accelerated or delayed the combustion progress of natural-gas/diesel engine at EGR condition by enhanced third-body effect or thermal effect respectively.

Effects of carbon dioxide on the upper flammability limits of methane in O2/CO2 atmosphere

In this paper, the effects of carbon dioxide on the upper flammability limits of oxy-methane were investigated. Firstly, the upper flammability limits of oxy-methane were measured under normal pressure and temperature. Then, the chemical effect of CO2 on the upper flammability was studied using the kinetic simulation. Finally, the thermal and radiative effects of CO2 on the upper flammability limits were discussed using energy balance analysis. The experimental upper flammability limits of CH4/O2/CO2 are from 9.5% to 33.25% in the condition of increasing oxygen concentrations from 15% to 50%. The analysis shows that the upper flammability limits of methane are mainly determined by the thermal property of CO2 and not significantly changed by the chemical and radiative effects.

Chemical Effects of CO2 and H2O Addition on Aromatic Species in Ethanol/Air Diffusion Flame

ABSTRACT Exhaust gas recirculation (EGR) is an effective technique used widely to meet current emission regulations. In this work, the mechanism of H2O and CO2 addition (a main component of exhaust gas) on polycyclic aromatic hydrocarbons (PAHs, the precursor of soot) formation was investigated numerically in ethanol diffusion flames, motivated by current concerns on particulate matter (PM) emissions when ethanol is applied to modern gasoline engines, especially under high load and temperature. To understanding the underlying mechanisms of H2O and CO2 on ethanol flame, a comprehensive chemical kinetic analysis was performed by modeling gas-phase chemistry with PAHs formation up to pyrene. In order to distinguish the important chemical role of H2O and CO2 addition through partial replacement of N2 from their thermal effects, additional calculations were performed with two fictitious species. The numerical results show that the thermal effect of H2O and CO2 reduces the peak flame temperature, but their chemical effect influences the peak flame temperature differently. The results also showed that the addition of either H2O or CO2 led to a significant reduction of PAHs via thermal and chemical effects. The chemical suppressing effect of H2O and CO2 addition on PAHs formation was identified and attributed to reduced PAHs precursor species and slow of PAH growth process. However, reaction pathway analysis showed that the chemical effects of H2O and CO2 influence the PAHs chemistry differently.

The impact of EGR components on ozone decomposition under engine relevant conditions in a rapid compression machine

In this work, with the addition of 90 ppm ozone, the influence of the main EGR components, namely nitrogen, carbon dioxide, steam, and methane, along with the gasoline surrogate iso-octane, on ozone oxidation were investigated at 30 bar, 800 K using a rapid compression machine (RCM). Taking the case diluted with nitrogen as a reference, it was found that the chemical effects of CO2 on ozone decomposition accelerated with the increase in the carbon dioxide concentration. The promoting effect of steam on ozone decomposition remained constant regardless of the steam mole fraction added. The mutual effect of methane and ozone was detectable, but the enhancement of ozone decomposition was lower than when iso-octane was added. A comprehensive detailed kinetic model was employed to interpret the experimental data. The model performance was improved by replacing the rate coefficients of O3 + O = 2O2. Analysis of the rate of production showed that the IC8H18 + O = CC8H17 + OH reaction predominated over the chain terminating reaction O3 + O = 2O2, which can explain the significant promoting effect in the presence of iso-octane. When steam is added, two OH radicals are derived from the chain branching reaction H2O + O = 2OH. When methane is added, only one OH radical, originating from the O radical attacking methane, is formed. Consequently, the promoting effect of methane addition is the lowest.