CO2 Dilution Research Articles

Effect of rotating gliding discharges on the lean blow-off limit of biogas flames

This study investigates the effect of a rotating gliding discharge on synthetic biogas combustion at atmospheric pressure. Synthetic biogas was produced by mixing methane and carbon dioxide. Three mixtures were considered: 100%/0%, 70%/30%, and 50%/50% of methane and carbon dioxide, respectively. The plasma effect was investigated in a low-swirl-number burner equipped with a high-voltage electrode to produce gliding discharges. The effect of plasma on the stability limits of the flame is reported for several electrical powers. During plasma-assisted combustion, the lean blow-off limits of biogas-air flames were significantly improved, which agrees with what can be found in the literature for other fuels. The electrical parameters of the discharge and the plasma emissions were measured using electric probes and emission spectroscopy, respectively. The mixture with the CO2 dilution was associated with a higher reduced electric field and higher ion production. A better understanding of the excited-species concentration evolution during plasma is necessary and will be investigated in future work.

Study of peak Laminar Burning Velocity of several syngas compositions at different temperatures

In the context of the current energy transition, the use of biomass-derived syngas (BDS) is often recognized as a fundamental path towards decreasing fossil fuel dependency and greenhouse gas emissions. However, hydrogen-containing BDS are prone to flame instability problems. More efforts are being carried out aiming at efficiently adopting BDS in industrial combustors with CH4 co-firing or inert gas dilutions by exploring accurate knowledge of burning velocity. To do so, a deeper knowledge of the syngas combustion behaviour is strictly necessary. The objective of this study fits in this framework: in particular, a computational study has been carried out to evaluate kinetic models and present fresh insights on the effects of varying syngas mixtures such as CO/H2, CO/H2/CO2 and CO/H2/CH4 on Laminar Burning Velocity (LBV) and peak LBV location (ΦLBV=max). In-detail chemical kinetic simulations of equimolar (CO: H2 = 1:1) forestry waste syngas were systematically carried out taking advantage of the open-source CANTERA solver. Three detailed kinetic models i.e., newly released FFCM-2, USC mech II, and modified GRI mech III were implemented to report accurate flame parameters at 1 bar and different temperature levels (from 300 K up to 450 K). On comparing the results with experiments, FFCM-2 proved to be a good kinetic model for the considered syngas mixtures CO/H2, CO/H2/CO2 and especially for CO/H2/CH4 for mixtures containing a limited share of 30 % methane at normal and moderately elevated temperature at 0.4 ≤ Φ ≤ 2.1. The USC mech II performed very well for CO/H2, and CO/H2/CO2, while the modified GRI mech III model also gave agreeable predictions for CO/H2/CH4 mixture having rich methane content. Additionally, when varying syngas composition analysis was conducted at different temperatures, the progressive CO2 dilution and CH4 addition of up to 30 % reduced the peak LBV and moved the peak LBV locations (ΦLBV=max) towards lean ER conditions with 9 % and 40 % reductions, respectively; however, only the latter effect was enhanced at the elevated initial temperature. Furthermore, sensitivity analysis of respective syngas mixtures is reported at normal and elevated temperatures to explore the most sensitive intermediate reactions relative to LBV. The shift of peak LBV locations and their enhancement at elevated temperatures also open the research path to study the underlying impacts on the flame modes/regimes and structure, especially CO emissions pathways in syngas with 30 % of CH4 and CO2 additions.

Global ocean carbon uptake enhanced by rainfall

Rain alters local sea surface physical and biogeochemical properties but its spatiotemporal variability has led to its overlook in global ocean carbon uptake studies. Different physical and chemical processes in the gaseous and liquid phases control the transfer of carbon dioxide (CO2) between the atmosphere and ocean. Rain impacts the interfacial flux by (1) increasing turbulence in the ocean and (2) modulating the air–sea CO2 concentration gradient. Concurrently, raindrops inject CO2 absorbed during their fall into the ocean through wet deposition. Here this study presents a comprehensive estimate of these effects on the global ocean carbon uptake over 2008–2018 based on observational products (satellite and in situ) and reanalysis. Using various representations of the ocean surface response to rainfall and different rain products, we show that rain increases the ocean carbon sink by +0.14–0.19 PgC yr−1 over 2008–2018, representing an increase of 5–7% of the ocean carbon uptake (2.66 PgC yr−1). Rain-induced interfacial flux and wet deposition have comparable orders of magnitude. The former mainly increases the CO2 sink in the tropics because strong rain rates and weak winds induce noticeable salinity and CO2 dilution. The latter is important in the tropics, storm track regions and the Southern Ocean.

Effects of CO2 dilution on ignition characteristics in lean premixed H2/Air flames

The research has examined the effects of CO2 dilution on ignition characteristics in H2/air mixtures. An ignition kernel was initiated by a laser-induced plasma in H2/air/CO2 mixtures with a bulk velocity of 6.5 m/s. An infrared camera was used to measure radiation intensity emitted from H2O and ignition probabilities. A high-speed schlieren image system was utilized to measure ignition kernel areas and ignition time. High-speed chemiluminescence was performed to examine OH* intensities. Numerical simulations were conducted using GRI 3.0 mechanisms to calculate reaction rates and laminar flame speeds. The ignition probability is highest for the undiluted mixtures and decreases with increasing the CO2 dilution fraction at constant adiabatic temperatures. A prolonged ignition time is observed at elevated CO2 dilution concentrations. Increasing the CO2 mole fraction decreases the ignition kernel growth, the integrated OH* intensity, and the integrated radiation intensity for each adiabatic temperature. CO2 reacts with H radicals and decreases the production of OH*, OH, and H2O. The CO2 dilution in the H2/air flames significantly contributes to the reduced reaction rate and flame speed, resulting in the low ignition probability. The high ignition probability is found at the large integrated OH* intensity in all cases. The ignition probability is high at the large integrated radiation intensity under the constant adiabatic flame temperatures. The OH* intensity is a critical parameter to estimate the ignition probability in the diluted lean hydrogen flames.

Direct numerical simulation of supercritical carbon dioxide oxy-methane combustion

Supercritical CO2 (sCO2) oxy-methane combustion is a key component of zero-carbon technologies in direct-fired sCO2 power cycles, i.e., the Allam cycle which offers promising solutions for clean and sustainable energy production. The use of sCO2 as both working fluid and diluent to moderate the combustor exit temperature at high pressure and high preheat temperature in the Allam cycle poses a unique combustion behavior. The effects of high sCO2 dilution on sCO2 oxy-methane combustion behavior, flame propagation, and flame stability are not fully resolved due to experimental challenges at such extreme conditions. This study addresses this major challenge by providing an understanding of the effect of sCO2 dilution on supercritical mixing and the combustion behavior in sCO2 oxy-methane combustion. A direct numerical simulation (DNS) integrated with the real-fluid equation of state is developed to provide the first DNS dataset for the realistic operating conditions of sCO2 oxy-methane combustors designed by Southwest Research Institute. The combustion behavior shows that sCO2 dilution has a major impact on mixing, heat release rate, temperature, and flame thickness. A peak in the heat release rate is identified for a given air–fuel ratio and the lowest CO production for 75%–80% CO2 dilution which results in a maximum temperature of 2000 K. By comparing the results obtained from ideal- and real-fluid equation of state, this study shows that real-fluid effects can significantly affect density gradient distribution and heat release rate, impacting supercritical mixing and flame dynamics under high sCO2 dilution. The results provide crucial insight for designing future sCO2 oxy-combustors.

LES of Biomass Syngas Combustion in a Swirl Stabilised Burner: Model Validation and Predictions

Abstract In this work, numerical investigations were performed using large eddy simulations and validated against detailed measurements in the CeCOST swirl stabilised burner. Both cold and reactive flow have been studied and the model has shown a good agreement with experiments. The verification of the model was done using the LES index of quality and a single grid estimator. The cold flow simulations predicted results closely to experiments setting baseline for the reactive simulations. Coherent structures like the vortex rope above the swirler and a precessing vortex core in the combustion chamber were identified. The reactive conditions were modelled with the Flamelet generated manifold and artificially thickened flame models. Simulations were performed for an experimental syngas composition from black liquor gasification at three different CO2 dilution levels. Three different Reynolds numbers were investigated with the model matching closely to experimentally detected 2D flow field and OH for the most CO2 diluted mixture. It was found that the opening angles of the flames differ by a maximum of 13% between experiments and simulations. The most diluted fuel investigated experienced a liftoff distance of 23.5 mm at the Re 25 k. This was also the highest liftoff distance experienced in this cohort of fuels. The same fuel also proved to have the thickest flame annulus at 78.5 mm. Overall, in cases with no experimental data available the predictions made by the model follow the same trends which hints its applicability to higher Re cases.

Experimental and chemical kinetic analysis to evaluate CO2 dilution on laminar combustion characteristics of C2H5OH/air blends under normal pressure

Based on a constant volume combustion system, the effect of dilution ratio changes on the laminar combustion characteristics of C2H5OH/air mixture was studied under an initial pressure of 1 bar, initial temperatures of 370 K, 400 K, 450 K, and equivalence ratios of 0.7–1.4. A chemical kinetic mechanism for ethanol-carbon dioxide was established, and the combined physical and chemical effects of carbon dioxide were separated. An independent parameter control method was proposed, introducing virtual species FN2, ICO2, FCO2, and KCO2, to isolate the dilution effect, thermal effect, direct reaction effect, third-body effect, and transport effect. Simulation analysis using the modified chemical kinetic mechanism revealed that various impacts of CO2 reduce the peak progress rate and sensitivity coefficient of key elementary reactions. Furthermore, the dilution effect of CO2 has the greatest influence on the sensitivity coefficients and net progress rates of important elementary reactions. Based on the results of the chemical kinetic simulations, a new global reaction pathway analysis method was created, overcoming the limitations of the Chemkin-pro method. The decomposition pathway of C2H5OH throughout the combustion process was calculated as C2H5OH → SC2H4OH → CH3CHO → CH3CO → CH3 → CH2O → HCO → CO → CO2. Changes in initial temperature and dilution ratio do not alter the main decomposition pathway of C2H5OH. Detailed chemical kinetics and free radical behavior analysis indicate that the LBV of the C2H5OH/CO2/air blends has a linear correlation with [H + O + OH]max.

Experimental Combustion and Flame Characterization of a Chemical Looping-Based Oxidative Dehydrogenation Byproduct Fuel Mixture Containing High CO2 Dilution

Abstract This study investigates the combustion performance of a CO2-rich fuel mixture containing ethane and methane as active species using a constant volume combustion chamber. This fuel is obtained as a byproduct of a chemical looping-based oxidative dehydrogenation (Cl-ODH) process ethylene production. The byproduct gas mixture has 40.79% CO2, 39.49% ethane, and 4.88% methane by weight with other minor compounds. Using this fuel for energy extraction would improve the process efficiency of the ethane to ethylene conversion. After initial combustion modeling, the gas fuel mixture was reduced to just the major species: CO2, ethane, and methane. The mixture was then tested for flammability limits and combustion performance under spark-ignition conditions. Effects of ambient conditions like temperatures between 300 and 400 K with initial pressures from 1 to 10 bar were tested. The effects of stoichiometry were tested to understand flame velocities and heat release. The fuel mixture showed an overall reduced flame velocity compared to gasoline. Instability in combustion was believed to be caused by the dissociation of ethane under elevated conditions. At higher pressures, the flame produces lower cumulative heat release. Simulations were also performed using a model tuned to replicate the operations of the combustion chamber used in the experiments. Heat release and unburnt fuel mass data were calculated to identify the discrepancies in the combustion completeness at elevated pressures. The effects of CO2 quenching the flame coupled with the increased dissociation of the fuel species can lead to up to more than 75% of the fuel mixture being unburnt. Data from this study were used to modify a small-scale spark-ignition engine to use this fuel and produce usable energy.

Effects of H2/CO ratio and CO2 dilution on the explosion behavior and flame evolution of syngas/air mixtures

Syngas, which is used as both a fuel and a raw material, presents numerous safety risks during its utilization. In this paper, the effects of H2/CO ratio (R = 0%, 30%, 50%, 70% and 100%) and CO2 dilution ([CO2] = 0%–20%) on the explosion behaviors of syngas/air mixtures with different equivalence ratios (φ = 0.8, 1.0, 1.6, and 2.5) are examined via experiments and numerical calculations. The results show that the peaks of explosion pressure and OH* spectral intensity are significantly influenced by the H2 percentage in the syngas. The introduction of CO2 notably decreases these parameters in oxygen-rich and severe oxygen-poor states. Additionally, the flame propagation speed noticeably increases with increasing H2 proportion in the syngas. The influence of CO2 on the buoyancy instability of the spherical flame of syngas is considered negligible when compared to that of other combustible gases, such as CH4. Nevertheless, the presence of CO2 decreases the density ratio between the unburned gas and burned gas and increases the flame thickness of syngas/air mixtures. This trend reduces the cellular instability of the spherical flame, particularly in severe oxygen-poor states. Notably, there is a definite correlation between the formation of cellular flames and the rise rate of explosion pressure. The cellular structure on the flame surface becomes more pronounced when the rise rate of explosion pressure increases. Conversely, it develops later or is noticeably inhibited. Furthermore, the main reaction pathways are H2 → ∙OH → H2O2 → H2O and H2 → ∙OH → CO → CO2 during the syngas explosion process. ∙OH is critical in the entire chain reaction because it promotes the conversion of H2 to H2O and acts as a vital connection between the reactants H2 and CO in the explosion process.

CO2, Ar, and He dilution effects on combustion dynamics and characteristics in a laboratory-scale combustor

This study aimed to investigate the impact of CO2, Ar, and He addition on combustion instability under varying acoustic frequencies, dynamic pressure amplitudes, and light emission intensities. Experiments involved using pure methane as fuel at a 1.0 equivalence ratio, with three distinct acoustic frequencies. The burner operated at 5 kW, and the combustion chamber was excited at frequencies of 90 Hz, 160 Hz, and 320 Hz. It was observed that with dilution, the temperatures decreased more from the burner exit temperature to the exhaust section. As the oxygen concentration in the oxidizer decreases in Ar, He, and CO2 dilution, CO emissions increase, but CO2 emissions decrease. The addition of Ar, He, and CO2 causes an effect that reduces the flame temperature and brightness while causing the flame length to extend due to the diluting effect. While a decrease in instability was observed in He-Ar-CO2 dilution in 20 % oxygen, instability increased again in the three 19 % cases. It is observed that mixtures under the diluent effect approach the blowoff limit due to the increase in ignition delay. It has been determined that the addition of diluent increases the tendency for the flame to go out, especially due to the thermal effect. The methane fuel with an 18 % oxygen concentration in the oxidizer of Ar remained below lean extinction limits, undergoing blowoff. The temperature was measured as 1156 K at 21 % oxygen concentration, 584 K at 19 % oxygen concentration for carbon dioxide, 877 K for Helium, and finally 777 K for argon. In the case of pure air for the combustion of methane, CO emissions were measured at 140 ppm. Average CO levels at 20 % oxygen concentration in the oxidizer are 1254 ppm, 356 ppm, and 308 ppm with carbon dioxide, argon, and helium, respectively. The study also shows that an increase in O2 in oxidizing air leads to a decrease in CO emissions, while an increase in He/Ar/CO2 gases leads to an increase in CO emissions. Regarding NOX emissions, undiluted conditions measured a value of 14 ppm. NOX emission levels approached 0 ppm in all dilution cases and achieved its goal.

Experimental investigation of the effect of CO2 dilution on the performance and hydrogen knock limit of a hydrogen–gasoline dual fuel spark ignition engine 1

This study investigates the effect of CO2 dilution on the performance and hydrogen knock limit of a spark ignition engine fueled with hydrogen–gasoline mixtures. The experimental runs were performed on a Ricardo single-cylinder engine with direct gasoline injection and hydrogen port induction. The hydrogen flow rate was increased at a step size of 2 L per minute (LPM) until the onset of knock, and the CO2 gas was introduced to the intake manifold with flow rates of 2, 4, and 6 LPM. The results revealed that CO2 dilution deteriorated the engine performance regarding the brake mean effective pressure and brake thermal efficiency. The hydrogen enrichment shortens the combustion duration, enhancing the performance and combustion characteristics. Increasing CO2 dilution levels decreased NOx emissions and increased CO emissions. Moreover, CO2 dilution extended the hydrogen knock limit to 10 and 16 LPM for spark timings of 12° and 4° crank angles before top dead center, respectively.

Stability and Emission Characteristics of a Stratified Hydrogen-Enriched Oxy-Methane Flame on a Multihole Burner: An Experimental Study.

Hydrogen (H2) enrichment is a viable solution to deter the early blowout of oxy-fuel flames at high CO2 dilution levels and to enhance combustor turndown in the supercritical-CO2 cycles of zero-emission power plants. This was already implemented successfully in a previous study by the author, where H2 was premixed with the oxy-reactants upstream of the burner headend in a fully premixed flame stabilized on a micromixer-like burner. The micromixer technology is known for its superior flame stability and inherent fuel/oxidizer flexibility. To avoid the higher risk of flashback when H2 is premixed with the primary reactants, the present study introduces the enrichment H2 in a nonpremixed manner from the enhanced-blowout fuel circuit of the micromixer burner, which comprises circumferential jets that surround the core oxy-flame. This approach aims to enhance combustor turndown even further by benefiting from inducing stratification in the flame. It was found that the blowout limit of the fully premixed (CH4/H2/O2/CO2) flame did not improve by stratification (H2-CH4/O2/CO2); it deteriorated slightly. The present study thus analyzes the stability, morphology, and CO emissions of the stratified flame, in order to explain this behavior and provide recommendations for how hydrogen enrichment should be introduced in oxy-fuel micromixer combustors.

The effect of CH4 addition on high-H2 syngas combustion characteristics under N2/CO2 dilution

The composition of biomass gas is complex, in order to promote the more extensive and efficient utilization of low-calorific syngas, it is necessary to master the basic combustion characteristics of low-calorific syngas under different compositions. In this paper, the effects of addition of CH4 (0% – 20%) on the combustion characteristics of laminar flow flame of high-H2 syngas (H2:CO = 3:1) were studied by experiments and numerical simulations under the dilution of N2 and CO2 (40%). The laminar burning velocity (LBV) and Markstein length are obtained by experiments. CHEMKIN-PRO software package was used to compare the prediction results of the four mechanisms with the measured data. Finally, GRI3.0 mechanism was selected as the simulation mechanism in this paper. The experimental results indicate that the LBV of N2 dilution is higher than that of CO2 dilution, and the flame is more stable, with adding CH4, the LBV of syngas mixtures is decreased, and the LBV of rich-fuel mixtures is decreased faster, but the stability of premixed flames of syngas is improved. Numerical simulation shows that the dominant branch chain reaction changes from R99 to R38 after CH4 is added, and the mole fraction of H, O decreases while the mole fraction of OH, CH3 increases. The dilution of CO2 results in a lower concentration of active radicals than N2 because CO2 inhibits the important reaction R99. The GRI 3.0 mechanism was used to simulate the formation of NO: the addition of CH4 promoted the formation of hot NO. The NO production of NNH and N2O intermediate mechanism is decreased, and a new way of N2 and CH reaction to HCN is increased. The dilution of CO2 can reduce NO emission more than that of N2. The insights gained from this study have practical implications in a variety of production applications. For example, optimizing mixture equivalence ratios can improve combustion efficiency, thereby increasing energy efficiency and reducing environmental emissions.

Experimental and numerical research on the effects of pressure and CO2 dilution on soot formation in laminar co-flow methane/air diffusion flames.

An experimental and numerical investigation was conducted to examine the formation of soot in methane/air laminar diffusion flames under varying CO2 dilution ratios, ranging from 0% to 40%, and pressures between 5 and 10 atm. The experimental methodology incorporated diffuse-light two-dimensional line-of-sight attenuation (diffuse 2D-LOSA) to ascertain the volume fraction and peak temperature distribution of soot within the flames. For the numerical methodology, CoFlame-an open-source computational code-was utilized to calculate the detailed flame temperature, soot volume fraction, and the mole fractions of key intermediate species pivotal to soot generation. The study reveals that an increased dilution ratio of CO2 can reduce flame temperature and the molar fraction of hydrogen (H), while simultaneously increasing the molar fraction of hydroxyl (OH). This shift in chemical composition results in a reduced rate of soot nucleation and an intensified oxidation process during the later stages of soot development, thereby diminishing the overall soot volume fraction. An increase in pressure significantly boosts the processes of soot nucleation, HACA surface growth, and PAH condensation, thereby promoting the formation of soot. Elevated pressure corresponds to an increase in flame temperature and a narrower soot formation region. Additionally, the inhibitory effect of CO2 dilution on soot formation is mitigated under increased pressure. The findings from this research are expected to provide valuable insights and strategic guidance for the management and control of pollutants in the context of hydrocarbon fuel combustion, particularly when CO2 dilution is employed.

Evaluation of high-pressure syngas ignition under high-CO2 dilution in shock tubes

Due to its importance as a candidate fuel for power cycles for future energy production, there have been active efforts to improve our understanding of the chemical kinetics of the Allam-Fetvedt cycle that utilizes syngas highly diluted with CO2. Shock-tube ignition delay times (IDT) of syngas-CO2 mixtures over a range of pressures, syngas blends, and equivalence ratios are now available in the literature, but their unique behavior deserves further study before the resulting data can be used to modify existing syngas kinetics models. In this study, an analysis of CO2-diluted syngas ignition behind reflected shock waves was conducted at the High-Pressure Shock Tube facility at Texas A&M University using an endwall imaging system. A syngas blend of 1:1 H2:CO at an equivalence ratio of 1 was studied at pressures around 12 atm for CO2 dilutions of 85, 88, and 94 % by volume. The combined results of the high-speed OH* imaging through the endwall and OH* time histories from sidewall windows indicate that the initial ignition event appears to be homogeneous and occurs first near the endwall, with no apparent premature ignition or localized flame kernels. An important finding of this investigation is that the existing syngas-CO2 IDT data in the literature are likely free from shock-tube ignition inhomogeneities. Therefore, they can be used to improve existing chemical kinetics mechanisms, which currently have trouble modeling the existing literature data over the range of temperatures of the shock-tube studies (∼1000 – 1400 K). Additionally, the lack of a measured post-combustion pressure rise makes it necessary to use a constant-pressure-enthalpy constraint when modeling the shock-tube IDT event.

Exploration of NH3 and NH3/DME laminar flame propagation in O2/CO2 atmosphere: Insights into NH3/CO2 interactions

Cofiring with reactive hydrocarbon and oxygenated fuels is an effective approach to enhance ammonia (NH3) combustion, which raises growing interest in understanding CN interactions. Among these fuels, dimethyl ether (DME, CH3OCH3) has attracted extensive attention due to its renewable nature and potential for large-scale synthesis from CO2 feedstock. This work reports an experimental and modeling investigation on the laminar flame propagation of NH3 and NH3/DME in O2/CO2 atmosphere, with an emphasis on evaluating the enhanced NH3/CO2 interactions under flame conditions. Laminar burning velocities (LBVs) were measured at elevated pressures up to 5 atm in a constant-volume combustion vessel. A kinetic model was developed and can well reproduce the measured LBVs in this work, as well as the LBVs and speciation data in literature. Modeling analyses and the modified fictitious diluent gas method were used to explore the effects of DME cofiring, pressure and CO2 dilution on LBVs. The results reveal that the chemical effect plays a crucial role in promoting the laminar flame propagation of NH3 with DME cofiring, while the reduction of LBVs with CO2 dilution is predominantly influenced by the thermal effect. LBVs of NH3/DME present a stronger pressure dependency than that of pure NH3, which is attributed to the enhanced pressure-sensitive three-body reactions, including CH3 + H (+M) = CH4 (+M) and H + O2 (+M) = HO2 (+M). Phenomenological analysis of LBVs was conducted by varying the diluent gas from pure N2 to pure CO2, complemented by modeling analyses with adjusted rate constants, to provide insights into NH3/CO2 interactions. The results show that CO2 reduction reactions, including NH + CO2 = HNO + CO and CO + OH = CO2 + H, compete for reactive radicals like H and NH with chain-branching reactions H + O2 = OH + H and consequently prohibit the laminar flame propagation.

Effect of CO2 dilution on laminar burning velocities, combustion characteristics and NOx emissions of CH4/air mixtures

AbstractThe laminar combustion characteristics of CH4/air premixed flames with CO2 addition are systemically studied. Experimental measurements and numerical simulations of the laminar burning velocity (LBV) are performed in CH4/CO2/Air flames with various CO2 doping ratio under equivalence ratios of 1.0–1.4. GRI 3.0 mech and Aramco mech are employed for predicting LBV, adiabatic flame temperature (AFT), important intermediate radicals (CH3, H, OH, O) and NOx emissions (NO, NO2, N2O), as well as the sensitivity analysis is also conducted. The detail analysis of experiment and simulation reveals that as the CO2 addition increases from 0% to 40%, the LBVs and AFTs decrease monotonously. Under the same CO2 doping ratio, the LBVs and AFTs increase first and then decrease with the increase of equivalence ratio, and the maximum of LBV is reached at equivalence ratio of 1.05. The mole fraction tendency of important intermediates and NOx with equivalence ratio and CO2 doping ratio are similar to the LBVs and AFTs. Reaction H + O2 ⇔ O + OH is found to be responsible for the promotion of the generation of important intermediates and NOx under the equivalence ratios and CO2 addition through sensitivity analysis. The sensitivity coefficients of elementary reactions that the increasing of CO2 doping ratio promotes or inhibits formation of intermediate radicals and NOx decreases. Graphical abstract

Influence of steam dilution on ignition delay time of dimethyl ether/air mixtures at high pressure measured behind reflected shock waves

This work provides the first experimental data on ignition delay times of steam-diluted dimethyl ether/air mixtures at high pressure obtained behind reflected shock waves. Mixtures are prepared with 20 and 40vol% steam dilution and at equivalence ratios of 0.5, 1.0 and 2.0. The measurements comprise temperatures between 731 and 1314K and are recorded at nominal pressures of 15 and 35bar. The obtained experimental data is used for comparison and evaluation of predictions of common chemical-kinetic models. The effect of dilution on ignition delay time is furthermore studied numerically utilizing zero-dimensional reactor simulations with the chemical-kinetic model AramcoMech 3.0. Thermal and chemical effects of steam-dilution are analyzed in comparison to CO2 and N2 dilution. Steam dilution features a smaller heat capacity than CO2 at the investigated temperatures, which leads to a lower thermal effect and therefore shorter ignition delay time. The chemical effect of steam decreases ignition delay time at intermediate and high temperatures below the ignition delay time of a mixture with the same amount of N2 dilution. The chemical effect is mainly due to the high collision efficiency of H2O in the third body reactions H2O2 + H2O ↔ 2OH + H2O, H + O2 + H2O ↔ HO2 + H2O and CH3OCH3 + H2O ↔ CH3 + CH3O + H2O. At high temperatures the direct participation of H2O in reactions and its chemical conversion additionally exerts a further decreasing effect on ignition delay time.

Experimental and modeling study of ignition delay times of natural gas with CO2 dilution

Pressurized oxy-fuel combustion has attracted considerable attention as an efficient technology with lower emissions. Natural gas, which primarily consists of methane, ethane, and propane, is a widely used fossil fuel owing to its safety and low cost. The auto-ignition of 90 %methane/9 %ethane/1 %propane mixture diluted in CO2 or N2 were investigated using a shock tube at temperatures of 1280–1645 K, pressures of 2 and 10 bar, and equivalence ratios of 0.5–2.0 in this study. An updated model named OXY-NG can accurately predict the ignition delay times, species yields, and laminar flame speeds of natural gas mixtures in oxy-fuel and air combustion modes. The ignition delay times increased as the equivalence ratios increased related to the recombination of CH3 radicals at 2 bar but decreased as the equivalence ratios increased owing to reactions involving HO2 at 250 bar. The diluent CO2 has inhibitory effects on ignition compared with N2, and these effects were minor under high pressure and fuel-rich conditions when diluted in CO2. The effects on natural gas mixture ignition owing to chemical and physical properties of CO2 are discussed in detail.

Optimization of a two-step CH4/air reaction mechanism in a CO2-enriched environment for high-fidelity combustion simulations

In the gas turbine framework, the adoption of carbon capture and storage (CCS) systems coupled with strategies to improve the exhaust CO2 content is a promising technology to abate the carbon footprint of such machines. However, any departure of the oxidant from the air can compromise the accuracy of the conventional models to represent the combustion process. In this work, the effect of the CO2 enrichment of the mixture on an atmospheric premixed swirled flame is investigated by means of large eddy simulation (LES), comparing the numerical predictions with the experimental results. The high-fidelity numerical model features a dedicated global reaction mechanism derived through an in-house optimization procedure presented in this study. The chemical scheme is obtained by optimizing a widely used CH4–air two-step mechanism to improve key flame parameters such as the laminar flame speed and thickness and the resistance of the flame to the stretch with moderate CO2 dilution. The numerical results are analyzed in terms of flame shape, heat losses, and pressure fluctuations, showing a promising agreement with the experimental measurements and demonstrating the capabilities of the numerical model for CO2-diluted combustion.