Fundamental Combustion Properties Research Articles

Evaluation of hydrogen/ammonia substitute fuel mixtures for methane: Effect of differential diffusion

One strategy for utilizing ammonia as an energy vector is to replace conventional hydrocarbons with hydrogen-enriched ammonia in existing or retrofitted combustors. However, the strong differential diffusion effect of hydrogen can significantly alter the combustion properties and cause challenges in combustor operability. Therefore, this numerical study performs a systematic investigation on the effect of differential diffusion on fundamental combustion properties of ammonia/hydrogen fuel blends. The investigated combustion properties span from the initiation of combustion, i.e., ignition, to stretched flame propagation and ultimately to flame stabilization mechanisms. The objective is to evaluate the feasibility of hydrogen/ammonia/air mixtures as substitutes for methane/air particularly considering the implications of differential diffusion effects. To this end, four fuel blends with similar unstretched burning properties are selected: fuel lean ammonia/hydrogen (AH-L), fuel rich ammonia/hydrogen (AH-R), fuel lean methane (M-L) and fuel lean methane/hydrogen (MH-L). First, the forced ignition and stretched flame propagation are evaluated. It is found that the AH-L (AH-R) mixture has a large negative (positive) Markstein length, and thereby among the selected fuel blends, AH-L (AH-R) has the lowest (highest) minimum ignition energy. Then flame stabilization mechanisms are investigated. For a stable flame, AH-L (AH-R) has an intensified (weakened) flame base and a weak (strong) flame tip, which shows a higher flashback (blow-off) propensity. Based on the critical gradient theory, a novel stability regime diagram is proposed. With the regime diagram, the flame stabilization limits (central flashback, boundary layer flashback, stable, and blow-off) and flow conditions can be determined for burners of different sizes.

Impact of Ammonium Perchlorate Content on Electrically Controlled Gel Polymer Electrolyte Monopropellants

A group of five electrically controlled monopropellants were developed, and their fundamental rheological, electrochemical, thermal, and combustion properties were characterized. A baseline monopropellant was composed of lithium perchlorate complexed with polyethylene glycol to form an ionically conductive gel polymer electrolyte. Subsequent candidates were supplemented with varying amounts of ammonium perchlorate at a fixed polymer-to-oxidizer ratio to determine the effects of shifting oxidizer content on the fundamental properties. The ignition of the gel monopropellants using an applied DC voltage potential at atmospheric conditions was observed and determined to be primarily the result of an electrolytic reaction. Time-resolved infrared thermography confirmed initial heating and initiation of the gels at the cathode once temperatures had reached the decomposition temperature of the polymer. Fourier transform infrared analysis of collected residue from experiments halted before ignition revealed lithium deposition on the cathode, supporting electrochemical activity. It was found that the electrolytic ignition delay time was affected by the oxidizer content, the magnitude of the applied voltage, and the distance between the electrodes supplying the voltage.

Laminar burning velocities of rich NH3+N2+O2 flames: Comparing the effects of elevated temperatures and oxygen ratios on model validation

Ammonia (NH3) is a promising carbon-free alternative fuel. One of its fundamental combustion properties, the laminar burning velocity (SL), is widely used to validate the combustion kinetic models. This study investigates the SL across varying unburned mixture temperature Tu and oxygen ratio xO2, particularly regarding their impact on the model validation. The focus is on the fuel-rich conditions that exhibit noticeable discrepancies among different kinetic model predictions. To provide accurate experimental data for the analyses, atmospheric measurements were carried out using the heat flux method, covering equivalence ratios ranging from 1.2 to 1.9, Tu from 298 K to 448 K, and xO2 from 0.30 to 0.38. Notably, many of these conditions have not been thoroughly explored in existing literature. Seven kinetic models were used to compare with the experimental data and assess the simulation discrepancies, where the increment parameters were used to compare the effect of SL vs. Tu and SL vs. xO2 dependences on the model validations. Through these analyses, the α coefficient, describing the SL vs. Tu dependence, is found too difficult to distinguish the performances of the various models and serve as validation targets. Sensitivity analyses reveal that the reactions governing the dependences of SL on both Tu and xO2 are fundamentally the same. Similar flame conditions with nearly identical sensitivity values were identified, suggesting that a model accurately reproducing the SL vs. Tu trend is promised to replicate the SL vs. xO2 trend, and vice versa. Consequently, the SL measurement with elevated Tu, which is difficult in experiments and in distinguishing the performance of different models, is suggested to be substituted by the much easier measurement with elevated xO2. This conclusion, alongside the methodology, holds potential for application in other flame types and help reduce the experimental efforts for model updating.

Overview of Autoignition and Flame Propagation Properties for Ammonia Combustion

With the increasingly stringent CO2 emission, next-generation propulsion systems with chemical reaction combustion involved need to operate with carbon-free fuels, such as ammonia or hydrogen. During the past few decades, intensive research has been conducted on ammonia combustion, which is growingly regarded as a potential alternative fuel to be applied in gas turbines for power generation and aviation propulsion systems to reduce the CO2 footprint and increase carbon-free fuel flexibility. Two major technical challenges with applying such fuel in practical engines are poor ignition and flame propagation behaviors. The present work provides a technical review by presenting state-of-the-art advances in ammonia combustion science and technology by clarifying the fundamental combustion properties and the corresponding enhancement strategies. Experimental techniques applied to measuring the ignition delay time are first introduced and overviewed, along with passive and active means to accelerate them. Then, the laminar burning velocities of ammonia-based dual-fuel combustion at varying operating conditions and their temperature and pressure dependences are described. This is followed by the spin-off applications of ammonia-fueled detonation engines. Finally, we show the prospects and challenges of ammonia combustion and suggest critical topics in aerospace and power generation applications that could benefit from further investigations.

An Upgraded Chemical Kinetic Mechanism for Iso-Octane Oxidation: Prediction of Polyaromatics Formation in Laminar Counterflow Diffusion Flames

Abstract 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.

A Study on Fundamental Combustion Properties of Trimethyl Orthoformate: Experiments and Modeling

Abstract 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.

Impact of Boundary Condition and Kinetic Parameter Uncertainties on NOx Predictions in Methane–Air Stagnation Flame Experiments

Abstract A comprehensive understanding of uncertainty sources in experimental measurements is required to develop robust thermochemical models for use in industrial applications. Due to the complexity of the combustion process in gas turbine engines, simpler flames are generally used to study fundamental combustion properties and measure concentrations of important species to validate and improve modeling. Stable, laminar flames have increasingly been used to study nitrogen oxide (NOx) formation in lean-to-rich compositions in low-to-high pressures to assess model predictions and improve accuracy to help develop future low-emissions systems. They allow for nonintrusive diagnostics to measure sub-ppm concentrations of pollutant molecules, as well as important precursors, and provide well-defined boundary conditions to directly compare experiments with simulations. The uncertainties of experimentally measured boundary conditions and the inherent kinetic uncertainties in the nitrogen chemistry are propagated through one-dimensional stagnation flame simulations to quantify the relative importance of the two sources and estimate their impact on predictions. Measurements in lean, stoichiometric, and rich methane–air flames are used to investigate the production pathways active in those conditions. Various spectral expansions are used to develop surrogate models with different levels of accuracy to perform the uncertainty analysis for 15 important reactions in the nitrogen chemistry and the six boundary conditions (ϕ, Tin, uin, du/dzin, Tsurf, P) simultaneously. After estimating the individual parametric contributions, the uncertainty of the boundary conditions are shown to have a relatively small impact on the prediction of NOx compared to kinetic uncertainties in these laboratory experiments. These results show that properly calibrated laminar flame experiments can, not only, provide validation targets for modeling, but also accurate indirect measurements that can later be used to infer individual kinetic rates to improve thermochemical models.

Measurements of Laminar Burning Velocities of Alternative Fuels: Application in Transport and Aerospace

The laminar burning velocity belongs to the fundamental combustion properties of fuels being a measure for their heat release, flame length, as well as reactivity and combustion stability, and thus, may impact the design of burners and combustion chambers. Also, these experimental data are needed for the validation and optimization within the construction and development of detailed chemical kinetic reaction mechanisms. Within this study, an overview of the different applications of fuel characterization regarding the specific area of interest (road transport, aviation, and aerospace) will be given. Depending on the application, effects of different molecular characteristics on the laminar burning velocity are evaluated: the presence of oxygen atoms and the grade of branching in a specific fuel molecule as well as the difference in the type of a chemical bond, here, single and double bonds. Examples of alternative fuels being discussed in the present study are: (I) oxymethylene ether (OMEn) in the field of road transport; (II) a paraffinic Alcohol-to-Jet fuel as sustainable aviation fuel; and (III) mixtures of ethane or ethene with nitrous oxide as green propellants for rocket propulsion applications.

Automatically generated model for light alkene combustion

Light alkenes are common combustion intermediates for a variety of fuels. Therefore, understanding their oxidation and pyrolysis chemistry is key to building detailed mechanisms for heavier fuels. This work was focused on the development and evaluation of a detailed kinetic mechanism suitable for the combustion of light alkenes up to C4 without the use of tuned parameters, instead, the parameter values come from first principles or direct measurements. The generated mechanism accurately estimates the laminar burning velocity (Su) and ignition delay time (IDT) of light alkenes available in the literature, which represent fundamental combustion properties at a wide range of conditions. Because each parameter is thought to have a physically realistic value, not tuned to these measurements, the new model could be used as a sub-mechanism in models for other applications.The reaction network was generated with the open-source Reaction Mechanism Generator (RMG) software. Sensitivity analyses were performed under wide ranges of temperatures and pressures, allowing for the identification of the most impactful species and reactions. Based on these, a comprehensive thermochemistry database, including calculations on 550 molecules performed in this work at the CBS-QB3 level of theory, and a kinetic library, including theoretically-derived reaction rates retrieved from the literature, were built and used in the mechanism generation. The developed mechanism was compared against several existing detailed kinetic mechanisms for ethene, propene, 1-butene, 2-butene, and isobutene. The newly generated model is the most accurate among the ones analyzed, in terms of fractional bias and normalized mean square error. Hence, this new model was used to analyze the chemistry of alkene combustion. Key rate coefficients were compared, to identify the cause of deviations between the models and possible areas for further improvements.

A combined experimental and modeling study of combustion properties of an isoparaffinic alcohol-to-jet fuel

This work presents an investigation of fundamental combustion properties, specifically laminar burning velocity and ignition delay time, of an Alcohol-to-Jet Synthetic Paraffinic Kerosene (AtJ-SPK). Used in blends, this fuel is a sustainable aviation fuel that consists mostly of two long-chained, highly branched alkanes. Laminar burning velocities were measured at a preheat temperature of 473 K and pressures of 1 and 3 bar using the cone angle method. Ignition delay times of fuel-air mixtures diluted in nitrogen (N2) were experimentally determined behind reflected shock waves at two fuel-air equivalence ratios, 1.0 and 2.0, at a pressure of 16 bar. In addition to these experiments, a modeling study was conducted using a new chemical kinetic reaction mechanism developed to describe the combustion behavior of the investigated AtJ-SPK. The simulations show that the new detailed mechanism is able to predict sufficiently the laminar flame speed at ambient pressure as well as the ignition delay time at elevated pressure. Sensitivity analyses for laminar flame speed and ignition delay time were performed as well.

A study on fundamental combustion properties of oxymethylene ether-1, the primary reference fuel 90, and their blend: Experiments and modeling

Oxymethylene ethers (OMEn) are discussed as attractive carbon-reduced alternative fuels because they offer the potential to inhibit the formation of soot particles. In the present study, the effect of oxymethylene ether-1 (OME1) addition to a gasoline surrogate, here the primary reference fuel 90 (PRF90), on ignition delay times and laminar burning velocities was investigated. Ignition delay times of OME1, PRF90, and their blend with OME1 blending fraction of 70% (by liquid vol.) were measured in a shock tube over a wide range of conditions: fuel-air ratio φ = 1.0, dilution of 1:5 with N2, T = 950 K-2000 K, and initial pressures of 1, 4, and 16 bar. Laminar burning velocities of OME1, PRF90, and their blend with OME1 blending fraction of 30% (w/w) were measured by using the cone angle method at a preheat temperature of 473 K, φ = 0.6–2.0, and at pressures of 1, 3, and 6 bar. The experimental data sets have been compared to predictions made using the in-house DLR reaction mechanism, which was developed for surrogate modeling of gasoline, diesel, and jet fuels as well as oxygenated species such as alcohols (C1C4) and oxymethylene ether (OMEn, n = 1–5). The results revealed that the DLR mechanism satisfactorily predicts the experimental data for ignition delay times and laminar burning velocities of both the individual fuels and the blend. Both the experimental and calculation results show that ignition delay times of OME1 / PRF90 are shorter than those of PRF90 for 1, 4, and 16 bar. The values of the measured burning velocities also agree with the corresponding predicted laminar flame speed data, except at high pressure (6 bar) and at high φ values. The effects of increasing OME1 fraction in the blend was tested, with the results showing a corresponding non-linear decrease of ignition delay times. Sensitivity analyses and radical mole fractions analysis calculations were performed to interpret the effect of adding OME1 to PRF90 on ignition delay times and laminar flame speeds.

Natural gas versus methane: Ignition kinetics and detonation limit behavior in small tubes

Detonation properties of methane and natural gas were studied using ZND simulations and detonation limit experiments. The experiments were performed in two tube sizes, 32 mm and 6.4 mm in inner diameter over a range of initial pressures (between 3.4 kPa and 35 kPa) and with stoichiometric fuel-oxygen compositions. The fuels considered are high purity methane, high purity methane with dopant ozone, a real natural gas, and a family of natural gas surrogates. The natural gas surrogates were developed based on North American natural gas composition variations and designed to capture the expected mean and variance of fundamental combustion properties of natural gases. All natural gases tested in this work show a substantially smaller detonation induction length (about 40%) and lower detonation pressure limit (30% in terms of limiting pressure) than high purity methane. The ozonated methane at 3000 PPMv of ozone doping performed similarly to the natural gases. Overall, the results suggest that in methane-based mixtures, a smaller induction length correlates with a more predictable detonation behavior as evidenced by a lower detonation pressure limit. As such, natural gases are expected to have a wider operating range when used as a fuel for detonation-based engines. As importantly, induction length calculation results reveal that the variability in detonation and combustion behaviors resultant from composition variability is expected to be similar between natural gases and commercial methane. Finally, the results suggest that for safety-related studies, neat methane is a poor surrogate for studying natural gas explosions.

Laminar burning velocities, CO, and NOx emissions of premixed polyoxymethylene dimethyl ether flames

Polyoxymethylene dimethyl ethers (OMEn) with the chemical structure of CH3O(CH2O)nCH3 are a promising synthetic e-fuel class that can be produced from renewable electricity and improves the emission characteristics of internal combustion engines. However, knowledge of their fundamental combustion properties, such as the laminar burning velocity, is limited in the literature, preventing the full assessment of their application potential in combustion devices. Thus, a heat flux burner is applied in this study to measure the laminar burning velocities of OME1 and OME2 over a wide range of equivalence ratios of 0.6–1.9 and at initial temperatures of 383–401 K under atmospheric pressure for the first time. The obtained data are compared with computed results using a recently developed reaction mechanism for OME1–4. Good agreement is observed. The experimental results are also discussed in the context of a comparison to literature data of dimethyl ether and OME3, demonstrating the minor impact of chain length of (OMEn) on their burning velocities, which is further analyzed in terms of sensitivity analysis on reaction kinetics. Furthermore, the exhaust gas composition of NOx and CO is determined experimentally for flames of OME1 and OME2 using a probe method and compared to the numerical results, observing mostly good agreement, where the remaining differences could be explained on the basis of combined experimental and model uncertainties. This provides unique knowledge on the emission performance of the combustion of (OMEn).

Experimental study on combustion stability characteristics in liquid-fueled gas turbine model combustor: Fuel sensitivities and flame/flow dynamics

In this paper, combustion stability characteristics of a laboratory-scaled lean premixed prevaporized gas turbine model combustor operated with different liquid fuels were experimentally examined. Test fuels involved one linear alkane (n-dodecane), one branched alkane (iso-octane), one cyclic alkane (methylcyclohexane (MCH)) and practical multi-component RP-3 jet fuel. Tests conducted under a wide range of equivalence ratios (∅) and inlet air velocities suggested that n-dodecane flame featured the smallest (∅=0.34) lean blow out (LBO) limit, while iso-octane flame was characterized by the highest (∅=0.39). On the other hand, n-dodecane flame shifted from stable to unstable combustion at the highest equivalence ratio (∅=0.52), which is ~18% higher than the lowest case RP-3 (∅=0.44). Additional flame and flow dynamics were characterized by simultaneous high-speed OH* chemiluminescence (CL) imaging and planar particle image velocimetry (PIV) measurements. Results suggested that the instability frequency and amplitude were dependent on the fuel type and could be well related to their fundamental combustion properties. Moreover, both phase-averaged and time-resolved OH* CL imaging and PIV results demonstrated that the global unsteady flame and flow dynamics were similar for different fuels. However, a time-lag behavior was observed when iso-octane and MCH flames were subject to similar acoustic pressure oscillations in the combustor. This was speculated to be dominantly caused by their distinct ignition delay times (IDTs), which resulted in different phase lags between heat release rate and acoustic pressure oscillations. Such findings were further supported by proper orthogonal decomposition (POD) analysis performed on the simultaneously recorded OH* CL and PIV measurements.

An updated short chemical‐kinetic nitrogen mechanism for carbon‐free combustion applications

Chemical-kinetic combustion mechanisms for hydrogen-oxygen-nitrogen systems, motivated originally by concerns about NOx emissions during hydrogen burning, have recently acquired renewed interest as a result of the possibility of employing ammonia-hydrogen mixtures in gas turbines and reciprocating engines as drop-in fuel to replace the use of natural gas. Specifically, this is of relevance to the implementation of engineering approaches for economical power generation with carbon sequestration or to large-scale energy-storage schemes, based on hydrogen or efficient hydrogen carriers such as ammonia. Because computational investigations are facilitated by short mechanisms (since the use of large mechanisms is often prohibitively expensive in reactive flow simulations), in response to the original concerns, a short nitrogen mechanism was developed in San Diego in the 1990s (not updated since 2004), without consideration of ammonia combustion. In view of the renewed interest in this topic, that mechanism has now been expanded to encompass 60 elementary steps among 19 reactive chemical species, including ammonia burning and NOx production, as reported herein, greatly improving predictions. With particular attention to high reactant temperatures and high-pressure conditions, relevant to industrial applications, it is shown that the present short mechanism retains satisfactory accuracy, exhibiting deviations that in most cases are within acceptable bounds (±20%). The revisions maintain the shortness of the original mechanism, adding only one more reactive species and six more elementary steps (while updating values of rate parameters of nine other steps, on the basis of newly available information). In addition, the short mechanism is applied herein to the analysis of fundamental combustion properties of ammonia/hydrogen/nitrogen-air laminar premixed flames, at unstrained and strained conditions, for comparison with methane-air flames as a reference gas-turbine fuel. It is found by comparing carbon-free and hydrocarbon laminar flames that these reactive mixtures, even if characterized by nearly identical adiabatic flame temperature and laminar flame speeds, nevertheless exhibit substantially different resistance to strain, with the ammonia/hydrogen flames exceeding the strain limit of methane flames by a factor of 5.

Exploring the combustion chemistry of a novel lignocellulose-derived biofuel: cyclopentanol. Part II: experiment, model validation, and functional group analysis

In part I of this paper, the reaction kinetics and thermochemistry of cyclopentanol have been studied numerically. In this part, the ignition and combustion behavior of cyclopentanol are studied experimentally in a shock tube, a rapid compression machine, a combustion vessel, and a counterflow burner. Fundamental combustion properties, such as ignition delay times, laminar flame speeds, and extinction strain rates, are reported. All measurements probe a variety of initial conditions and provide an initial evaluation of the performance of cyclopentanol under different combustion modes. The experimental results are compared with those computed using the chemical mechanism presented in Part I. Reasonable agreement is observed. The controlling kinetics of cyclopentanol oxidation is explored considering various combustion modes. Moreover, cyclopentanol is compared to a variety of C5 fuels including cyclopentane, n-pentanol, and n-pentane with respect to their combustion characteristics in order to evaluate the impact of functional groups. The auto-ignition propensity of cyclopentanol is found to be lower than that of its paraffinic and linear counterparts, while the premixed flames of all these fuels propagate with approximately identical velocities at the investigated conditions.

Paths to alternative fuels for aviation

Almost the complete amount of jet fuel available on the global market is produced from fossil crude oil being an exhaustible raw material. Furthermore its use is inherently connected with emissions of the greenhouse gas CO2. To cope with this, several processes for the production of alternative aviation fuels were developed including the use of biomass as a renewable feedstock. Since biomass from cultivation farming is in competition with food and fodder production, the preferred raw material would be residues from agriculture and forestry or municipal waste, also microalgae can be used. Independent of the raw material, the conversion of biogenic feedstock into alternative jet fuel is based on microbial, thermal and/or chemical breakdown of larger (bio)-molecules into smaller ones, followed by the catalytic formation of fuel molecules and hydrogenation. An overview on the production of different alternative bio-based jet fuels is given including a survey about producers and capacities, focusing on already certificated bio-based jet fuels. In addition to that, a comparison of fundamental combustion properties between Jet A-1 and different alternative jet fuels is presented: Laminar burning velocities and ignition delay times, each measured for two synthetic jet fuels based on fossil resources (coal-to-liquid—CtL and gas-to-liquid—GtL) as well as for two biofuels (farnesane and an Alcohol-to-Jet fuel—AtJ). Measurements of the burning velocities were performed at a preheat temperature of 473 K and pressures of 1 and 3 bar by variation of the fuel–air-equivalence ratios φ. Ignition delay times were determined for φ-values of 0.5, 1.0, and 2.0, at an initial pressure of about 16 bar and temperatures ranging between 800 and 1700 K. It turns out that with respect to the characteristic combustion properties tested the considered alternative fuels have a combustion behavior similar to Jet A-1.

Recent Trends in the Production, Combustion and Modeling of Furan-Based Fuels

There is growing interest in the use of furans, a class of alternative fuels derived from biomass, as transportation fuels. This paper reviews recent progress in the characterization of its combustion properties. It reviews their production processes, theoretical kinetic explorations and fundamental combustion properties. The theoretical efforts are focused on the mechanistic pathways for furan decomposition and oxidation, as well as the development of detailed chemical kinetic models. The experiments reviewed are mostly concerned with the temporal evolutions of homogeneous reactors and the propagation of laminar flames. The main thrust in homogeneous reactors is to determine global chemical time scales such as ignition delay times. Some studies have adopted a comparative approach to bring out reactivity differences. Chemical kinetic models with varying degrees of predictive success have been established. Experiments have revealed the relative behavior of their combustion. The growing body of literature in this area of combustion chemistry of alternative fuels shows a great potential for these fuels in terms of sustainable production and engine performance. However, these studies raise further questions regarding the chemical interactions of furans with other hydrocarbons. There are also open questions about the toxicity of the byproducts of combustion.

Investigation of Methane Oxy-Fuel Combustion in a Swirl-Stabilised Gas Turbine Model Combustor

CO2 has a strong impact on both operability and emission behaviours in gas turbine combustors. In the present study, an atmospheric, preheated, swirl-stabilised optical gas turbine model combustor rig was employed. The primary objectives were to analyse the influence of CO2 on the fundamental characteristics of combustion, lean blowout (LBO) limits, CO emission and flame structures. CO2 dilution effects were examined with three preheating temperatures (396.15, 431.15, and 466.15 K). The fundamental combustion characteristics were studied utilising chemical kinetic simulations. To study the influence of CO2 on the operational range of the combustor, equivalence ratio (Ф) was varied from stoichiometric conditions to the LBO limits. CO emissions were measured at the exit of the combustor using a water-cooled probe over the entire operational range. The flame structures and locations were characterised by performing CH chemiluminescence imaging. The inverse Abel transformation was used to analyse the CH distribution on the axisymmetric plane of the combustor. Chemical kinetic modelling indicated that the CO2 resulted in a lower reaction rate compared with the CH4/air flame. Fundamental combustion properties such as laminar flame speed, ignition delay time and blowout residence time were found to be affected by CO2. The experimental results revealed that CO2 dilution resulted in a narrower operational range for the equivalence ratio. It was also found that CO2 had a strong inhibiting effect on CO burnout, which led to a higher concentration of CO in the combustion exhaust. CH chemiluminescence showed that the CO2 dilution did not have a significant impact on the flame structure.

Compositional effects on PAH and soot formation in counterflow diffusion flames of gasoline surrogate fuels

Gasoline surrogate fuels are widely used to understand the fundamental combustion properties of complex refinery gasoline fuels. In this study, the compositional effects on polycyclic aromatic hydrocarbons (PAHs) and soot formation were investigated experimentally for gasoline surrogate mixtures comprising n-heptane, iso-octane, and toluene in counterflow diffusion flames. A comprehensive kinetic model for the gasoline surrogate mixtures was developed to accurately predict the fuel oxidation along with the formation of PAHs and soot in flames. This combined model was first tested against ignition delay times and laminar burning velocities data. The proposed model for the formation and growth of PAHs up to coronene (C24H12) was based on previous studies and was tested against existing and present new experimental data. Additionally, in the accompanied soot model, PAHs with sizes larger than (including) pyrene were used for the inception of soot particles, followed by particle coagulations and PAH condensation/chemical reactions on soot surfaces. The major pathways for the formation of PAHs were also identified for the surrogate mixtures. The model accurately captures the synergistic PAH formation characteristics observed experimentally for n-heptane/toluene and iso-octane/toluene binary mixtures. Furthermore, the present experimental and modeling results also elucidated different trends in the formation of larger PAHs and soot between binary n-heptane/iso-octane and ternary n-heptane/iso-octane/toluene mixtures. Propargyl radicals (C3H3) were shown to be important in the formation and growth of PAHs for n-heptane/iso-octane mixtures when the iso-octane concentration increased; however, reactions involving benzyl radicals (C6H5CH2) played a significant role in the formation of PAHs for n-heptane/iso-octane/toluene mixtures. These results indicated that the formation of PAHs and subsequently soot was strongly affected by the composition of gasoline surrogate mixtures.