Modeling and simulation of pollutant generation in the combustion of biodiesel surrogates

Contributions in the study about the generation of pollutants in the combustion of renewable fuels are of great relevance in the current environmental and economic scenario; as it seeks to consolidate the consumption of alternative (bio)fuels to replace non-renewable fuels, such as coal, natural gas and oil. The range of renewable resources, in the short and medium term, used in the production of biofuels has increased significantly, ranging from food crops such as soybeans to biomass from algae. Understanding the combustion process and its effects for these new sources of renewable energy is pertinent to guarantee the quality of the biofuel used and the reduction in emissions of atmospheric pollutants. The objective of this work is to carry out a study on the generation of pollutants, NOx and soot, after combustion of a biodiesel surrogate, methyl decanoate − MD, with chemical formulation C11H22O2, as works in this direction are scarce in the literature. For this, a turbulent diffusion flame of methyl decanoate was considered, modeled by the equations of CFD - Computational Fluid Dynamics for reactive fluids. The LES - Large Eddy Simulation technique was applied to model the flow turbulence and the Smagorinsky model was adopted for the turbulent viscosity. The equations were discretized by the finite difference method and the system of equations was solved numerically by the Rosenbrock method. To validate the implemented numerical procedure, in addition to the numerical results for turbulent diffusive flames of MD, numerical tests were also carried out for turbulent methanol diffusive flames. The results obtained agree with data found in the literature.Contribuições nos estudos acerca da geração de poluentes na combustão de combustíveis renováveis são de grande relevância no cenário ambiental e econômico atual; uma vez que se busca consolidar o consumo de (bio)combustíveis alternativos em substituição aos combustíveis de origem não-renováveis, como o carvão mineral, gás natural e o petróleo. A gama de recursos renováveis, a curto e médio prazos, utilizada na produção de biocombustíveis tem aumentado significativamente, indo de culturas alimentares, como soja, até biomassa proveniente de algas. Compreender o processo de combustão e seus efeitos, para estas novas fontes de energias renováveis é pertinente para que se garanta a qualidade do biocombustível utilizado e a redução nas emissões de poluentes atmosféricos. O objetivo deste trabalho é realizar um estudo sobre a geração de poluentes, o NOx e a fuligem, após a combustão de um substituto do biodiesel, o decanoato de metila − MD, com formulação química C11H22O2, dado que trabalhos nessa direção são escassos na literatura. Para isso, considerou-se uma chama difusiva turbulenta de decanoato de metila, modelada pelas equações da CFD - Dinâmica de Fuidos Computacional para fluidos reativos. Aplicou-se a técnica LES - Large Eddy Simulation para modelar a turbulência do fluxo e o modelo de Smagorinsky foi adotado para a viscosidade turbulenta. As equações foram discretizadas pelo método das diferenças finitas e o sistema de equações foi resolvido numericamente pelo Método de Rosenbrock. Para validação do procedimento numérico implementado, além dos resultados numéricos para chamas difusivas turbulentas de MD, também realizou-se testes numéricos para chamas difusivas turbulentas de metanol. Os resultados obtidos concordam com dados encontrados na literatura

Experimental and Kinetic Modelling Study of Jet A-1/Ethanol Blend Combustion and Oxidation Stability

The increasing demand for air transportation leads to higher jet fuel consumption, and numerous researches have been dedicated to finding a sustainable alternative for fossil fuels, such as bioethanol. Fundamental studies on kerosene/ethanol blend have been reported in the literature; however, they lack a kinetic study and experimental validation. This work aims to provide an accurate kinetic model for simulating jet A-1/ethanol flames and the experimental validation. The chemical structure of the jet A-1/ethanol flames and two jet fuel surrogates at three stoichiometries in a premixed flat flame-burner have been measured by employing a thermocouple, OH, NO PLIF thermometry, and gas analysis. The experimental data from this work and the literature validates the proposed reaction mechanism that comprises of 541 reactions among 85 species for modelling the jet A-1/ethanol flames. This models jet A-1 as 89\% n-decane and 11\% toluene while it has a better accuracy than the previous n-decane/toluene model. The jet A-1 autoxidation characteristics have been evaluated by the PetroOXY fuel thermal stability tester, which showed a decrease with ethanol addition. Nine antioxidants have been tested to improve the oxidation stability of ethanol at 1 g/L. A reaction mechanism generator and a custom PetroOXY model have been employed for modelling jet fuel surrogates and ethanol, which were accurate for predicting the autoxidation of ethanol while optimisation to the mechanisms of jet fuel surrogate is required. This work gives a novel contribution for the experimental database of jet A-1, ethanol, the blend, and jet fuel surrogates in a flat-flame burner as well as the application of a new approach of OH and NO PLIF quantification and thermometry. The simplified kinetic model of ethanol/jet A-1 facilitates further studies, such as CFD modelling. The ethanol addition to the oxidation stability of jet A-1 and the strategy to improve the ethanol stability are reported for the first time

Combustion characteristics of alternative liquid fuels

Envisaged application of biodiesel in gas turbine engines or furnaces requires extensive tests on the deflagration properties of biodiesel. The laminar flame speeds of Palm Methyl Esters (PME) and blends of PME with conventional fuels are determined using the jet-wall stagnation flame configuration. The same technique is also used to measure the laminar flame speed of diesel, Jet-A1, n-heptane, acetone, methane and methane/acetone. The spray atomization characteristics of a plain-jet airblast atomizer are investigated using a phase Doppler anemometry (PDA) under non-reacting conditions. The droplet size and velocity distribution of biodiesels are compared to conventional fuels. For spray combustion investigations, a generic gas turbine-type combustor is developed to compare the spray flame established from PME, rapeseed methyl esters (RME), diesel, Jet-A1 and biodiesel blends. The spray droplet characteristics in the flame and the flow field in the combustor are investigated. Chemiluminescence imaging of OH* and CH* are applied to capture the global flame structure and heat release region. Flame spectroscopy and long bandpass filtered imaging at > 550 nm are performed to evaluate the tendency of soot formation. In general, biodiesels exhibit flame shapes and spray droplet characteristics that are comparable to conventional fuels. In spite of the higher fuel specific consumption, the emission of NOx is found to be lower for biodiesels compared to conventional fuels. The results show that biodiesels can potentially be used as alternative fuels for gas turbine operation

Temperature dependence of the Laminar burning velocity of methanol flames

To better understand and predict the combustion behavior of methanol in engines, sound knowledge of the effect of the pressure, unburned mixture temperature, and composition on the laminar burning velocity is required. Because many of the existing experimental data for this property are compromised by the effects of flame stretch and instabilities, this study was aimed at obtaining new, accurate data for the laminar burning velocity of methanol–air mixtures. Non-stretched flames were stabilized on a perforated plate burner at 1 atm. The heat flux method was used to determine burning velocities under conditions when the net heat loss from the flame to the burner is zero. Equivalence ratios and initial temperatures of the unburned mixture ranged from 0.7 to 1.5 and from 298 to 358 K, respectively. Uncertainties of the measurements were analyzed and assessed experimentally. The overall accuracy of the burning velocities was estimated to be better than ±1 cm/s. In lean conditions, the correspondence with recent literature data was very good, whereas for rich mixtures, the deviation was larger. The present study supports the higher burning velocities at rich conditions, as predicted by several chemical kinetic mechanisms. The effects of the unburned mixture temperature on the laminar burning velocity of methanol were analyzed using the correlation uL = uL0(Tu/Tu0)α. Several published expressions for the variation of the power exponent α with the equivalence ratio were compared against the present experimental results and calculations using a detailed oxidation kinetic model. Whereas most existing expressions assume a linear decrease of α with an increasing equivalence ratio, the modeling results produce a minimum in α for slightly rich mixtures. Experimental determination of α was only possible for lean to stoichiometric mixtures and a single data point at equivalence ratio= 1.5. For these conditions, the measurement data agree with the modeling results

Propriedades físico-químicas e estudo numérico da combustão de misturas de n-decano - etanol - metil decanoato

Orientadores: Rogério Gonçalves dos Santos, Dario AlvisoDissertação (mestrado) - Universidade Estadual de Campinas, Faculdade de Engenharia MecânicaResumo: Foram estudados três combustíveis: n-decano (apresentado na literatura como simulacro de querosene, ou como o principal componente de simulacros de diesel), etanol e metil decanoato (apresentado na literatura como um simulacro de biodiesel) e suas misturas (base mássica). Ensaios de solubilidade foram feitos para todas as misturas possíveis, passo 10%, com etanol anidro e etanol hidratado. ...Observação: O resumo, na íntegra, poderá ser visualizado no texto completo da tese digitalAbstract: Three fuels and their blends (weight basis) were studied: n-decane (presented in literature as a kerosene surrogate, or as the main component of diesel surrogates), ethanol and methyl decanoate (presented in literature as a biodiesel surrogate). Solubility tests were made for all possible blends, 10% step, using anhydrous ethanol and hydrated ethanol....Note: The complete abstract is available with the full electronic documentMestradoTermica e FluidosMestra em Engenharia MecânicaCNP

Synthesis gas as a fuel for internal combustion engines in transportation

© 2022 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).The adverse environmental impact of fossil fuel combustion in engines has motivated research towards using alternative low-carbon fuels. In recent years, there has been an increased interest in studying the combustion of fuel mixtures consisting mainly of hydrogen and carbon monoxide, referred to as syngas, which can be considered as a promising fuel toward cleaner combustion technologies for power generation. This paper provides an extensive review of syngas production and application in internal combustion (IC) engines as the primary or secondary fuel. First, a brief overview of syngas as a fuel is presented, introducing the various methods for its production, focusing on its historical use and summarizing the merits and drawbacks of using syngas as a fuel. Then its physicochemical properties relevant to IC engines are reviewed, highlighting studies on the fundamental combustion characteristics, such as ignition delay time and laminar and turbulent flame speeds. The main body of the paper is devoted to reviewing the effect of syngas utilization on performance and emissions characteristics of spark ignition (SI), compression ignition (CI), homogeneous charge compression ignition (HCCI), and advanced dual-fuel engines such as reactivity-controlled compression ignition (RCCI) engines. Finally, various on-board fuel reforming techniques for syngas production and use in vehicles are reviewed as a potential route towards further increases in efficiency and decreases in emissions of IC engines. These are then related to the research reported on the behavior of syngas and its blends in IC engines. It was found that the selection of the syngas production method, choice of the base fuel for reforming, its physicochemical properties, combustion strategy, and engine combustion system and operating conditions play critical roles in dictating the potential advantages of syngas use in IC engines. The discussion of the present review paper provides valuable insights for future research on syngas as a possible fuel for IC engines for transport.Peer reviewe

Exploration of novel fuels for gas turbine (ENV-406) : modeling of T60 test rig with diesel & biodiesel fuels

Dans cette thèse, un modèle numérique a été proposé pour simuler la combustion liquide des carburants conventionnels et non-conventionnels, en particulier le mélange de biodiesel B20. La matrice de test numérique constitue de quatre cas d’écoulement réactifs c.à.d. avec combustion et d’un cinquième avec injection liquide sans combustion (écoulement non-réactif). Les modèles sont calculés à l’aide du logiciel FLUENT™ v.14 en 3D et a l’état stationnaire. Les flammes de diffusion turbulentes sont modélisées en utilisant l’approche de flammelette laminaire stable, avec une fonction de densité de probabilité jointe (PDF). La Validation est effectuée en comparant les mesures expérimentales disponibles avec les résultats obtenus de la CFD. L’aérodynamique de la chambre de combustion, ainsi que les températures de parois extérieures sont captures avec un degré de précision satisfaisant. La validation des principaux produits de combustion, tels que : CO2, H2O et O2, montre des résultats satisfaisants pour tous les cas d'écoulement réactifs, mais certaines incohérences ont été relevées pour les émissions de CO. On pense que le banc d'essai (la géométrie de la chambre de combustion et son état de fonctionnement) n'est pas suffisamment adéquat pour la combustion de combustibles liquides. D’autre part, et d’un point de vue numérique, l’approche de flammelette laminaire stable a été trouvé raisonnablement hors mesure de saisir les effets profonds du non-équilibre chimique qui sont souvent associés au processus de lente formation d’un polluant, comme le CO.In this thesis, a CFD model was proposed to simulate the liquid combustion of conventional and non-conventional biodiesel fuels, in particularly the B20 biodiesel blend. The numerical test matrix consists of four reacting flow cases, and one non-reacting liquid fuel injection case. The models are computed using FLUENT™ v.14 in a 3D steady-state fashion. The turbulent non-premixed diffusion flames are modeled using the steady laminar flamelet approach; with a joint presumed Probability density function (PDF) distribution. Validation is achieved by comparing available experimental measurements with the obtained CFD results. Combustor aerodynamics and the outer wall temperatures are captured with a satisfactory degree of accuracy. Validation of the main combustion products, such as: CO2, H2O, and O2, shows satisfactory results for all the reacting flow cases; however, some inconsistencies were found for the CO emissions. It is believed that the test rig (combustor geometry and operating condition) is not sufficiently adequate for burning liquid fuels. On the other hand, from a numerical combustion point of view, the steady laminar flamelet approach was found not reasonably able to capture the deep non-equilibrium effects associated with the slow formation process of a pollutant, such as CO

Entropy Generation Analysis in Turbulent Reacting Flows and Near Wall: A Review

This paper provides a review of different contributions dedicated thus far to entropy generation analysis (EGA) in turbulent combustion systems. We account for various parametric studies that include wall boundedness, flow operating conditions, combustion regimes, fuels/alternative fuels and application geometries. Special attention is paid to experimental and numerical modeling works along with selected applications. First, the difficulties of performing comprehensive experiments that may support the understanding of entropy generation phenomena are outlined. Together with practical applications, the lumped approach to calculate the total entropy generation rate is presented. Apart from direct numerical simulation, numerical modeling approaches are described within the continuum formulation in the framework of non-equilibrium thermodynamics. Considering the entropy transport equations in both Reynolds-averaged Navier–Stokes and large eddy simulation modeling, different modeling degrees of the entropy production terms are presented and discussed. Finally, exemplary investigations and validation cases going from generic or/and canonical configurations to practical configurations, such as internal combustion engines, gas turbines and power plants, are reported. Thereby, the areas for future research in the development of EGA for enabling efficient combustion systems are highlighted. Since EGA is known as a promising tool for optimization of combustion systems, this aspect is highlighted in this work

Analyzing the Effect of Fuel Nitrogen on Soot Formation

In modern-day life, energy is primarily supplied by the combustion of carbon-containing fuels. As global economic and population growth occurs, the demand for energy, and consequently the amount of energy supplied by carbon-containing fuels, is predicted to increase. Despite overwhelming use of carbon-containing fuels, there are various human health, climate, and environmental issues related to combustion emissions like carbon dioxide (CO2) and soot. To alleviate our dependence on fossil fuels while concomitantly minimizing the negative impacts of these pollutants, research into renewable fuels, engine geometries, and burning strategies is needed alongside advancements in renewable energy technologies. One strategy to reduce emissions is to change the fuel-type, which requires understanding the relationship between fuel structure and pollutant formation pathways. The dominance of combustion in providing energy in the modern age, coupled with the drive to control harmful emissions from these systems, motivates the work presented in this dissertation. This thesis aims to elucidate the effect of fuel-nitrogen on soot formation, a subject receiving comparatively less attention than regular and oxygenated fuels. The effect of nitrogen on soot formation becomes relevant for diesel fuels with nitrogen-containing additives, as well as biomass or biomass-derived fuels, which can contain up to 30% nitrogen-containing compounds by dry weight. In addition, nations such as Korea, Japan, and Australia are exploring ammonia (NH3) as a CO2-neutral fuel. Due to issues in stabilizing NH3-combustion, initial efforts look to enhance the stability of NH3-combustion by co-firing it with hydrocarbons. This doesn’t completely eliminate CO2 emissions, but it reduces them and serves as a stepping stone to burning pure NH3/hydrogen. In these scenarios, soot formation can occur, and the influence of NH3 on soot emissions becomes relevant. In this work, various experimental and computational techniques were employed to study the influence of fuel-nitrogen on soot formation. To understand the chemical influence of fuel-nitrogen on soot formation, the sooting tendencies of 14 amines were measured. Sooting tendencies were quantified by re-scaling relative soot concentrations measured in fuel-doped methane flames into Yield Sooting Indices (YSIs). All amines had lower sooting tendencies than structurally analogous hydrocarbons, and the sooting tendencies of amines with the same chemical formula varied significantly. Calculations were performed to analyze decomposition pathways for three of the amines, revealing that trends in sooting tendency correlate with predicted primary decomposition products. The results suggest the suppressive effect of amines on soot formation may be due to carbon-nitrogen interactions which interfere with aromatic growth pathways. While 2D simulations have been implemented to understand NH3 oxidation and NOx emissions from NH3-seeded hydrocarbon mixtures, few studies have analyzed the ability of chemical mechanisms to capture flame characteristics and soot formation in 2D atmospheric nonpremixed NH3-CH4 flames with large ratios of NH3. To fill this gap, experiments were performed in nonpremixed NH3-CH4 and N2-CH4 co-flow flames with varying ratios of NH3/N2 to CH4, and compared to simulations. Experimentally, NH3 had a strong chemical effect on suppressing soot formation, which is attributed to NH3-hydrocarbon interactions which reduce the formation of aromatics. While the model was able to capture the physical flame characteristics, it was unable to capture the inhibitive effect of NH3 on soot. This highlights the need to identify and include nitrogen-hydrocarbon reactions relevant to soot formation in the underlying chemical mechanism. For the first time, synchrotron X-ray fluorescence (XRF) and X-ray scattering (XRS) were employed to measure spatially-resolved temperatures and mixture fractions in sooting methane/air flames. Both techniques provide evidence that the flame physics are well-captured by the model, and suggest that issues in capturing the suppressive effect of NH3 on soot is related to deficiencies in the kinetic mechanism. The XRF technique was shown to be insensitive to soot and compositional variations in the flame, and measured temperatures displayed excellent agreement with simulated temperatures. Simulated mixture fractions showed satisfactory agreement with mixture fractions determined by XRS, demonstrating the potential of this technique for probing mixing characteristics in sooting flames. Lastly, YSIs were measured in partially-premixed flames, demonstrating that sooting tendency trends hold across a range of temperatures and air-to-fuel ratios relevant to soot formation. This suggests that the sooting tendency trends reported for the amines may also hold across these conditions. While a wide range of studies are reported in this thesis, they overlap, and help to strengthen our understanding of fuel-chemistry and soot formation. The work presented here is expected to aid in the development of models which describe nitrogen-hydrocarbon interactions, ultimately enabling the rational design of fuel-types and combustion geometries that mitigate pollutant formation

Direct Numerical Simulations of Ignition Effects on Low Temperature Combustion (LTC) Systems

Department of Mechanical Engineeringsystem configurations. First, two-dimensional direct numerical simulations (DNSs) of ignition of lean primary reference fuel (PRF)/air mixtures at high pressure and intermediate temperature near the negative temperature coefficient (NTC) regime were performed with a 116 species-reduced mechanism to elucidate the effects of fuel composition, thermal stratification, and turbulence on PRF homogeneous charge compression-ignition (HCCI) combustion. Second, the characteristics of autoignited laminar lifted methane/hydrogen jet flames in heated coflow air are numerically investigated with a 57-species detailed methane/air chemical kinetic mechanism. The autoignited laminar lifted jet flames can be categorized into three regimes of combustion mode: the tribrachial edge flame regime, the Moderate or Intense Low-oxygen Dilution (MILD) combustion regime, and the transition regime in between. In certain condition, an unusual decreasing liftoff height behavior with increasing $U_0$ is observed. Additional simulations with modified hydrogen mass diffusivity were performed to deeply understand unusual decreasing liftoff height behaviors. Third, similar to methane/hydrogen jet flame, $n$-heptane diluted with N$_2$ lifted laminar lifted jet flames are simulated with the 68-species skeletal mechanism. Near autoignition condition, $n$-heptane laminar lifted flame exhibits a stiff increase in its liftoff height at a certain $U_0$. Schmidt number analysis is used to elucidate this abnormal phenomenon. At the end of the thesis, turbulent lifted hydrogen jet flames with different coflow temperatures (i.e. $T_c$ = 750, 850, and 950 K) near the auto-ignition limit of hydrogen/air mixture are investigated using 3-D DNSs with a detailed hydrogen/air chemical mechanism. The DNSs are performed at a jet Reynolds number of 8,000 with over 1.28 billion grid points. Overall combustion characteristics and stabilization mechanisms of lifted flames were studied with comparing previous high-temperature coflow. Several physical parameters and chemical explosive mode analysis (CEMA) used to identify the fundamentals of LTC in various combustion system configurations.clos