Search

Iso-octane is widely recognized as a prominent candidate to represent the oxidation of iso-alkanes within jet fuel and gasoline surrogates. This work evaluated a chemical kinetic mechanism for iso-octane focusing on the model's capability to predict the formation of polycyclic aromatic hydrocarbons (PAHs). As the model is intended to be further coupled with soot models, the chemical kinetic mechanism must supply good predictability of the formation and consumption of PAHs considered as major soot precursors. A first validation of the iso-octane submodel as incorporated within ESTiMatE-Mech, using experimental data from literature, reveals the need to improve the submodel. Considerable deviations were observed in the prediction of the PAHs, although concentration profiles of major species and fundamental combustion properties, here ignition delay time and laminar flame speed, were accurately predicted. Through rate of production and sensitivity analyses of the mechanism, nine reactions were identified to have a strong impact on the (over) prediction of the PAHs. These reactions have been modified based on information gathered from literature resulting in an updated version of the mechanism called ESTiMatE-Mech_mod. Simulation results with this modified mechanism showed that this updated mechanism is now capable of predicting well the targeted PAHs, while retaining the good initial prediction of the major species concentration profiles as well as of laminar flame speeds and ignition delay times. [ABSTRACT FROM AUTHOR]

Iso-octane is widely recognized as a prominent candidate to represent the oxidation of iso-alkanes within jet fuel and gasoline surrogates. This work evaluated a chemical kinetic mechanism for iso-octane focusing on the model's capability to predict the formation of polycyclic aromatic hydrocarbons (PAHs). As the model is intended to be further coupled with soot models, the chemical kinetic mechanism must supply good predictability of the formation and consumption of PAHs considered as major soot precursors. A first validation of the iso-octane submodel as incorporated within ESTiMatE-Mech, using experimental data from literature, reveals the need to improve the submodel. Considerable deviations were observed in the prediction of the PAHs, although concentration profiles of major species and fundamental combustion properties, here ignition delay time and laminar flame speed, were accurately predicted. Through rate of production and sensitivity analyses of the mechanism, nine reactions were identified to have a strong impact on the (over) prediction of the PAHs. These reactions have been modified based on information gathered from literature resulting in an updated version of the mechanism called ESTiMatE-Mech_mod. Simulation results with this modified mechanism showed that this updated mechanism is now capable of predicting well the targeted PAHs, while retaining the good initial prediction of the major species concentration profiles as well as of laminar flame speeds and ignition delay times. [ABSTRACT FROM AUTHOR]

Currently, the aviation sector is seeking for alternatives to kerosene from crude oil, as part of the efforts combating climate change by reducing greenhouse gas (GHG) emissions, in particular carbon dioxide (CO2), and ensuring security of supply at affordable prices. Several synthetic jet fuels have been developed including sustainable biokerosene, a low-carbon fuel. Over the last years, the technical feasibility as well as the compatibility of alternative jet fuels with today's planes has been proven However, when burning a jet fuel, the exhaust gases are a mixture of many species, going beyond CO2 and water (H2O) emissions, with nitrogen oxides (NOx), carbon monoxide (CO), unburned hydrocarbons (UHC) including aromatic species and further precursors of particles and soot among them. These emissions have an impact on the local air quality as well as on the climate (particles, soot, contrails). Therefore, a detailed knowledge and understanding of the emission patterns when burning synthetic aviation fuels are inevitable. In the present paper, these issues are addressed by studying numerically the combustion of four synthetic jet fuels (Fischer-Tropsch fuels). For reference, two types of crude-oil-based kerosene (Jet A-1 and Jet A) are considered, too. Plug flow calculations were performed by using a detailed chemical-kinetic model validated previously. The composition of the multicomponent jet fuels was imaged by using the surrogate approach. Calculations were done for relevant temperatures, pressures, residence times, and fuel equivalence ratios φ. Results are discussed for NOx, CO as well as for benzene and acetylene as major soot precursors. According to the predictions, the NOx and CO emissions are within about ±10% for all fuels considered, within the parameter range studied: T = 1800 K, T = 2200 K; 0.25 ≤ φ ≤ 1.8; p = 40 bar; t = 3 ms. The aromatics free GtL (gas to liquid) fuel displayed higher NOx values compared to Jet A-1/A. In addition, synthetic fuels show slightly lower (better) CO emission data than Jet A-1/A. The antagonist role of CO and NOx is apparent. Major differences were predicted for benzene emissions, depending strongly on the aromatics content in the specific fuel, with lower levels predicted for the synthetic aviation fuels. Acetylene levels show a similar, but less pronounced, effect. [ABSTRACT FROM AUTHOR]

Trimethyl orthoformate (TMOF: HC(OCH3)3) has recently been examined as a viable biofuel. TMOF is a branched isomer of oxymethylene ether-2 (OME2) that, due to its high oxygen content and lack of direct carbon-carbon bonds, considerably reduces the formation of soot particles. To meet the challenges of a more flexible and sustainable power generation, a detailed understanding of its combustion properties is essential for its safe and efficient utilization, neat or in blends. In this work, two fundamental combustion properties of TMOF were studied: (i) Auto-ignition of TMOF/synthetic air mixtures (φ = 1.0; diluted 1:5 with N2) using the shock tube method at pressures of 1, 4, and 16 bar, and (ii) Laminar burning velocities of TMOF/air mixtures using the cone angle method at ambient and elevated pressures of 3 and 6 bar. Furthermore, the impact of TMOF addition to a gasoline surrogate (PRF90) on ignition delay times was studied using the shock tube method at φ = 1.0, 1:5 dilution with N2, T = 900-2000 K, and at 4 bar. The experimental data sets have been compared with predictions of the in-house chemical kinetic reaction mechanism (DLR concise mechanism) developed for interpreting the high-temperature combustion of a broad spectrum of different hydrocarbon fuels as well as oxygenated fuels, including TMOF. The results demonstrate that the ignition delay times of TMOF and OME2 are nearly identical for all pressures studied in the moderate-to high-temperature region. The results obtained for the blend indicate that ignition delay times of the TMOF/PRF90 blend are shorter than those of the primary reference fuel 90 (PRF90) at 4 bar. In the lean-to stoichiometric region, the results obtained for laminar burning velocities of TMOF and OME2 are similar. However, in the fuel-rich domain (φ > 1.0), laminar burning velocities for TMOF are noticeably lower, indicating a decreased reactivity. The model predictions based on the in-house model reveal a good agreement compared to the measured data within the experimental uncertainty ranges. In addition, sensitivity analyses regarding ignition delay times and laminar flame speeds were performed to better understand TMOF oxidation. [ABSTRACT FROM AUTHOR]

The development of advanced reaction models to predict pollutant emissions in aero-engine combustors usually relies on surrogate formulations of a specific jet fuel for mimicking its chemical composition. 1,3,5-trimethylbenzene is one of the suitable components to represent aromatics species in those surrogates. However, a comprehensive reaction model for 1,3,5-trimethylbenzene combustion requires a mechanism to describe the m-xylene oxidation. In this work, the development of a chemical kinetic mechanism for describing the m-xylene combustion in a wide parameter range (i.e., temperature, pressure, and fuel equivalence ratios) is presented. The m-xylene reaction submodel was developed based on existing reaction mechanisms of similar species such as toluene and reaction pathways adapted from literature. The submodel was integrated into an existing detailed mechanism that contains the kinetics of a wide range of n-paraffins, isoparaffins, cycloparaffins, and aromatics. Simulation results for m-xylene were validated against experimental data available in literature. Results show that the presented m-xylene mechanism correctly predicts ignition delay times at different pressures and temperatures as well as laminar burning velocities at atmospheric pressure and various fuel equivalence ratios. At high pressure, some deviations of the calculated laminar burning velocity and the measured values are obtained at stoichiometric to rich equivalence ratios. Additionally, the model predicts reasonably well concentration profiles of major and intermediate species at different temperatures and atmospheric pressure. [ABSTRACT FROM AUTHOR]

Oxymethylene ethers (OMEn, n = 1-5) are a promising class of synthetic fuels that have the potential to be used as additives or substitutes to diesel in compression ignition engines. A comprehensive understanding of their combustion properties is required for their safe and efficient utilization. In this study, the results of a combined experimental and modeling work on oxidation of OME2 are reported: (i) Ignition delay time measurements of stoichiometric OME2/synthetic air mixtures diluted 1:5 with nitrogen using the shock tube method at pressures of 1, 4, and 16 bar, and (ii) laminar flame speeds of OME2/air mixtures using the cone angle method at atmospheric and elevated pressures of 3 and 6 bar. The experimental data sets obtained have been used for validation of three detailed reaction mechanisms of OME2 obtained from literature. The results of ignition delay time measurements showed that ignition of OME2 is characterized by pre-ignition activity at the low temperature side of the measurements regardless of the pressure. Regarding the performance of the different reaction mechanisms, the model from Cai et al. (2020, "Auto-Ignition of Oxymethylene Ethers (OMEn, n = 2-4) as Promising Synthetic e-Fuels From Renewable Electricity: Shock Tube Experiments and Automatic Mechanism Generation," Fuel, 264, p. 116711) best predicted the temperature and pressure dependence of ignition delay times. For laminar flame speeds, the experimental data were well matched by the mechanism from Ren et al. (2019) at p = 1, 3, and 6 bar and for all equivalence ratios considered. From sensitivity analyses calculations, it was observed that chain reactions involving small radicals, i.e., H, O, OH, HO2, and CH3 control the oxidation of OME2. The comparison of the results of this work and our previous work (Ngugi et al. (2021)) on OME1 show that these two fuels have similar oxidation pathways. The results obtained in this work will contribute to a better understanding of the combustion of oxymethylene ethers, and thus, to the design and optimization of burners and engines as well. [ABSTRACT FROM AUTHOR]

Over the last years, global concerns about energy security and climate change have resulted in many efforts focusing on the potential utilization of nonpetroleum-based, i.e., bioderived, fuels. In this context, n-butanol has recently received high attention because it can be produced sustainably. A comprehensive knowledge about its combustion properties is inevitable to ensure an efficient and smart use of n-butanol if selected as a future energy carrier. In the present work, two major combustion characteristics, here laminar flame speeds applying the cone-angle method and ignition delay times applying the shock tube technique, have been studied, experimentally, and by modeling exploiting detailed chemical kinetic reaction models, at ambient and elevated pressures. The in-house reaction model was constructed applying the reaction model generation (RMG)-method. A linear transformation method recently developed, linTM, was exploited to generate a reduced reaction model needed for an efficient, comprehensive parametric study of the combustion behavior of n-butanol-hydrocarbon mixtures. All experimental data were found to agree with the model predictions of the in-house reaction model, for all temperatures, pressures, and fuel-air ratios. On the other hand, calculations using reaction models from the open literature mostly overpredict the measured ignition delay times by about a factor of two. The results are compared to those of ethanol, with ignition delay times very similar and laminar flame speeds of n-butanol slightly lower, at atmospheric pressure. [ABSTRACT FROM AUTHOR]

Currently, new concepts for power generation are discussed, as a response to combat global warming due to CO2 emissions stemming from the combustion of fossil fuels. These concepts include new, low-carbon fuels as well as centralized and decentralized solutions. Thus, a more diverse range of fuel supplies will be used, with (biogenic) low-caloric gases such as syngas and coke oven gas (COG) among them. Typical for theses low-caloric gases is the amount of hydrogen, with a share of 50% and even higher. However, hydrogen mixtures have a higher reactivity than natural gas (NG) mixtures, burned mostly in today's gas turbine combustors. Therefore, in the present work, a combined experimental and modeling study of nitrogen-enriched hydrogen-air mixtures, some of them with a share of methane, to be representative for COG, will be discussed focusing on laminar flame speed data as one of the major combustion properties. Measurements were performed in a burner test rig at ambient pressure and at a preheat temperature T0 of 373 K. Flames were stabilized at fuel-air ratios between about φ = 0.5-2.0 depending on the specific fuel-air mixture. This database was used for the validation of four chemical kinetic reaction models, including an in-house one, and by referring to hydrogen-enriched NG mixtures. The measured laminar flame speed data of nitrogen-enriched methane-hydrogen-air mixtures are much smaller than the ones of nitrogen-enriched hydrogen-air mixtures. The grade of agreement between measured and predicted data depends on the type of flames and the type of reaction model as well as of the fuel-air ratio: a good agreement was found in the fuel lean and slightly fuel-rich regime; a large underprediction of the measured data exists at very fuel-rich ratios (φ > 1.4). From the results of the present work, it is obvious that further investigations should focus on highly nitrogen-enriched methane-air mixtures, in particular for very high fuel-air ratio (φ > 1.4). This knowledge will contribute to a more efficient and a more reliable use of low-caloric gases for power generation. [ABSTRACT FROM AUTHOR]

In response to the limited resources of fossil fuels as well as to their combustion contributing to global warming through CO2 emissions, it is currently discussed to which extent future energy demands can be satisfied by using biomass and biogenic by-products, e.g., by cofiring. However, new concepts and new unconventional fuels for electric power generation require a re-investigation of at least the gas turbine burner if not the gas turbine itself to ensure a safe operation and a maximum range in tolerating fuel variations and combustion conditions. Within this context, alcohols, in particular, ethanol, are of high interest as alternative fuel. Presently, the use of ethanol for power generation-in decentralized (microgas turbines) or centralized gas turbine units, neat, or cofired with gaseous fuels like natural gas (NG) and biogas-is discussed. Chemical kinetic modeling has become an important tool for interpreting and understanding the combustion phenomena observed, for example, focusing on heat release (burning velocities) and reactivity (ignition delay times). Furthermore, a chemical kinetic reaction model validated by relevant experiments performed within a large parameter range allows a more sophisticated computer assisted design of burners as well as of combustion chambers, when used within computational fluid dynamics (CFD) codes. Therefore, a detailed experimental and modeling study of ethanol cofiring to NG will be presented focusing on two major combustion properties within a relevant parameter range: (i) ignition delay times measured in a shock tube device, at ambient (p=1 bar) and elevated (p=4 bar) pressures, for lean (φ=0.5) and stoichiometric fuel-air mixtures, and (ii) laminar flame speed data at several preheat temperatures, alsofor ambient and elevated pressure, gathered from literature. Chemical kinetic modeling will be used for an in-depth characterization of ignition delays and flame speeds at technical relevant conditions. An extensive database will be presented identifying the characteristic differences of the combustion properties of NG, ethanol, and ethanol cofired to NG. [ABSTRACT FROM AUTHOR]

Fuels from low quality feedstock such as biomass and biomass residues are currently discussed with respect to their potential to contribute to a more sustainable electrical power supply. In the present work, we report on the study of generic representative gas mixtures stemming from the gasification of different feedstock, from wood and algae. Two major combustion properties--burning velocities and ignition delay times--were measured for different parameters: (i) for two pressures--1 bar and 3 bar--at a constant preheat temperature T0 = 373K, to determine burning velocities by applying the cone angle method; and (ii) for elevated pressures--up to 16 bar--in the temperature range between about 1000 and 2000 K, at fuel--equivalence ratios φ of 0.5 and 1.0, to obtain ignition delay times by applying the shock tube method. Additional studies performed in our group on gas mixtures of natural gas, methane, and hydrogen were also taken into account as major components of biogenic gas mixtures. It was found that the reaction behavior of the wood gasification product (N2, CO, H2, CO2, CH4) is mainly determined by its H2 content, besides CH4; methane determines the kinetic behavior of the algae fermentation product (CH4, CO2, N2) due to its relatively high amount. Detailed chemical kinetic reaction models were used to predict the measured data. The trends and main features were captured by predictions applying different reaction models. The agreement of the experiments and the predictions is dependent on the pressure range. [ABSTRACT FROM AUTHOR]