Alcohol combustion chemistry

Alternative transportation fuels, preferably from renewable sources, include alcohols with up to five or even more carbon atoms. They are considered promising because they can be derived from biological matter via established and new processes. In addition, many of their physical-chemical properties are compatible with the requirements of modern engines, which make them attractive either as replacements for fossil fuels or as fuel additives. Indeed, alcohol fuels have been used since the early years of automobile production, particularly in Brazil, where ethanol has a long history of use as an automobile fuel. Recently, increasing attention has been paid to the use of non-petroleum-based fuels made from biological sources, including alcohols (predominantly ethanol), as important liquid biofuels. Today, the ethanol fuel that is offered in the market is mainly made from sugar cane or corn. Its production as a first-generation biofuel, especially in North America, has been associated with publicly discussed drawbacks, such as reduction in the food supply, need for fertilization, extensive water usage, and other ecological concerns. More environmentally friendly processes are being considered to produce alcohols from inedible plants or plant parts on wasteland. While biofuel production and its use (especially ethanol and biodiesel) in internal combustion engines have been the focus of several recent reviews, a dedicated overview and summary of research on alcohol combustion chemistry is still lacking. Besides ethanol, many linear and branched members of the alcohol family, from methanol to hexanols, have been studied, with a particular emphasis on butanols. These fuels and their combustion properties, including their ignition, flame propagation, and extinction characteristics, their pyrolysis and oxidation reactions, and their potential to produce pollutant emissions have been intensively investigated in dedicated experiments on the laboratory and the engine scale, also emphasizing advanced engine concepts. Research results addressing combustion reaction mechanisms have been reported based on results from pyrolysis and oxidation reactors, shock tubes, rapid compression machines, and research engines. This work is complemented by the development of detailed combustion models with the support of chemical kinetics and quantum chemistry. This paper seeks to provide an introduction to and overview of recent results on alcohol combustion by highlighting pertinent aspects of this rich and rapidly increasing body of information. As such, this paper provides an initial source of references and guidance regarding the present status of combustion experiments on alcohols and models of alcohol combustion.     

Ammonia chemistry in oxy-fuel combustion of methane

The oxidation of NH₃ during oxy-fuel combustion of methane, i.e., at high [CO₂], has been studied in a flow reactor. The experiments covered stoichiometries ranging from fuel rich to very fuel lean and temperatures from 973 to 1773 K. The results have been interpreted in terms of an updated detailed chemical kinetic model. A high CO₂ level enhanced formation of NO under reducing conditions while it inhibited NO under stoichiometric and lean conditions. The detailed chemical kinetic model captured fairly well all the experimental trends. According to the present study, the enhanced CO concentrations and alteration in the amount and partitioning of O/H radicals, rather than direct reactions between N-radicals and CO₂, are responsible for the effect of a high CO₂ concentration on ammonia conversion. When CO₂ is present as a bulk gas, formation of NO is facilitated by the increased OH/H ratio. Besides, the high CO levels enhance HNCO formation through NH₂+CO. However, reactions NH₂+O to form HNO and NH₂+H to form NH are inhibited due to the reduced concentration of O and H radicals. Instead reactions of NH₂ with species from the hydrocarbon/methylamine pool preserve reactive nitrogen as reduced species. These reactions reduce the NH₂ availability to form NO by other pathways like via HNO or NH and increase the probability of forming N₂ instead of NO.     

Mechanism and modeling of nitrogen chemistry in combustion

Our current understanding of the mechanisms and rate parameters for the gas-phase reactions of nitrogen compounds that are applicable to combustion-generated air pollution is discussed and illustrated by comparison of results from detailed kinetics calculations with experimental data. In particular, the mechanisms and rate parameters for thermal and prompt NO formation, for fuel nitrogen conversion, for the Thermal De-NO_x and RAPRENO_x processes, and for NO₂ and N₂O formation and removal processes are considered. Sensitivity and rate-of-production analyses are applied in the calculations to determine which elementary reactions are of greatest importance in the nitrogen conversion process. Available information on the rate parameters for these important elementary reactions has been surveyed, and recommendations for the rate coefficients for these reactions are provided. The principal areas of uncertainty in nitrogen reaction mechanisms and rate parameters are outlined.     

Isomer-specific combustion chemistry in allene and propyne flames

A combined experimental and modeling study is performed to clarify the isomer-specific combustion chemistry in flames fueled by the C₃H₄ isomers allene and propyne. To this end, mole fraction profiles of several flame species in stoichiometric allene (propyne)/O₂/Ar flames are analyzed by means of a chemical kinetic model. The premixed flames are stabilized on a flat-flame burner under a reduced pressure of 25 Torr (=33.3 mbar). Quantitative species profiles are determined by flame-sampling molecular-beam mass spectrometry, and the isomer-specific flame compositions are unraveled by employing photoionization with tunable vacuum-ultraviolet synchrotron radiation. The temperature profiles are measured by OH laser-induced fluorescence. Experimental and modeled mole fraction profiles of selected flame species are discussed with respect to the isomer-specific combustion chemistry in both flames. The emphasis is put on main reaction pathways of fuel consumption, of allene and propyne isomerization, and of isomer-specific formation of C₆ aromatic species. The present model includes the latest theoretical rate coefficients for reactions on a C₃H₅ potential [J.A. Miller, J.P. Senosiain, S.J. Klippenstein, Y. Georgievskii, J. Phys. Chem. A 112 (2008) 9429-9438] and for the propargyl recombination reactions [Y. Georgievskii, S.J. Klippenstein, J.A. Miller, Phys. Chem. Chem. Phys. 9 (2007) 4259-4268]. Larger peak mole fractions of propargyl, allyl, and benzene are observed in the allene flame than in the propyne flame. In these flames virtually all of the benzene is formed by the propargyl recombination reaction.     

Plasma assisted combustion: Dynamics and chemistry

Plasma assisted combustion is a promising technology to improve engine performance, increase lean burn flame stability, reduce emissions, and enhance low temperature fuel oxidation and processing. Over the last decade, significant progress has been made towards the applications of plasma in engines and the understanding of the fundamental chemistry and dynamic processes in plasma assisted combustion via the synergetic efforts in advanced diagnostics, combustion chemistry, flame theory, and kinetic modeling. New observations of plasma assisted ignition enhancement, ultra-lean combustion, cool flames, flameless combustion, and controllability of plasma discharge have been reported. Advances are made in the understanding of non-thermal and thermal enhancement effects, kinetic pathways of atomic O production, diagnostics of electronically and vibrationally excited species, plasma assisted combustion kinetics of sub-explosion limit ignition, plasma assisted low temperature combustion, flame regime transition of the classical ignition S-curve, dynamics of the minimum ignition energy, and the transport effect by non-equilibrium plasma discharge. These findings and advances have provided new opportunities in the development of efficient plasma discharges for practical applications and predictive, validated kinetic models and modeling tools for plasma assisted combustion at low temperature and high pressure conditions. This article is to provide a comprehensive overview of the progress and the gap in the knowledge of plasma assisted combustion in applications, chemistry, ignition and flame dynamics, experimental methods, diagnostics, kinetic modeling, and discharge control.     

A comprehensive combustion chemistry study of 2,5-dimethylhexane

Iso-paraffinic molecular structures larger than seven carbon atoms in chain length are commonly found in conventional petroleum, Fischer–Tropsch (FT), and other alternative hydrocarbon fuels, but little research has been done on their combustion behavior. Recent studies have focused on either mono-methylated alkanes and/or highly branched compounds (e.g., 2,2,4-trimethylpentane). In order to better understand the combustion characteristics of real fuels, this study presents new experimental data for the oxidation of 2,5-dimethylhexane under a wide variety of temperature, pressure, and equivalence ratio conditions. This new dataset includes jet stirred reactor speciation, shock tube ignition delay, and rapid compression machine ignition delay, which builds upon recently published data for counterflow flame ignition, extinction, and speciation profiles. The low and high temperature oxidation of 2,5-dimethylhexane has been simulated with a comprehensive chemical kinetic model developed using established reaction rate rules. The agreement between the model and data is presented, along with suggestions for improving model predictions. The oxidation behavior of 2,5-dimethylhexane is compared with oxidation of other octane isomers to confirm the effects of branching on low and intermediate temperature fuel reactivity. The model is used to elucidate the structural features and reaction pathways responsible for inhibiting the reactivity of 2,5-dimethylhexane.     

Rate constants for the homogeneous gas-phase Al/HCl combustion chemistry

Experimental reaction rate data for the Al/HCl system are very scarce. Such data are needed for the comprehension and for the numerical simulation of the combustion of aluminum particles as encountered in solid segmented motors. Toward this end, we have examined the homogeneous chemistry of this system, computed rate parameters for important reactions using conventional Transition State Theory (TST) and RRKM/master equation simulations, and estimated rate parameters for reactions where rigorous computations are not presently feasible. The reaction mechanism presented in this study consists of 15 species participating in 39 reversible elementary reactions for which rate parameters have been estimated or computed.     

水热提质对木屑燃烧特性和热分解特性的影响

水热处理有效解决了其它热转化技术中湿物料的脱水和干燥问题,避免了复杂的干燥和昂贵的分离程序。文章以松木屑为原料,利用水热技术对其进行提质,研究松木屑在不同水热提质温度提质后的燃烧特性和热分解特性。结果表明:随着水热提质温度的升高,燃烧反应的峰值提前,燃烧温度区间变大,燃烧速率变小,热解残留量增多;水热提质温度为260℃时,木屑的平均燃烧反应速率由0.111 min-1降至0.105 min-1,降幅为38.2%;热解残留量为0.54,是原木屑残留量(0.24)的2.25倍。动力学分析表明:水热提质温度低于240℃时,燃烧反应为二级反应;水热提质温度为260℃时,为一级燃烧反应,木屑的燃烧反应趋于简单,活化能显著降低,燃烧性能得以改善;随水热提质温度的升高,热解反应趋于复杂。水热提质有助于稳定木屑燃烧过程,有利于燃烧过程优化控制。文章的研究结果可为高含水、低能量密度生物质的高效利用提供借鉴。     

A combustion chemistry analysis of carbonate solvents used in Li-ion batteries

Under abusive conditions Li-ion cells can rupture, ejecting electrolyte and other flammable gases. In this paper we consider some of the thermochemical and combustion properties of these gases that determine whether they ignite and how energetically they burn. We find a significant variation among the carbonate solvents in the factors that are important to determining flammability, such as combustion enthalpy and vaporization enthalpy. We also show that flames of carbonate solvents are fundamentally less energetic than those of conventional hydrocarbons. An example of this contrast is given using a recently developed mechanism for dimethyl carbonate (DMC) combustion, where we show that a diffusion flame burning DMC has only half the peak heat release rate of an analogous propane flame. Interestingly, peak temperatures differ by only 25%. We argue that heat release rate is a more useful parameter than temperature when evaluating the likelihood that a flame in one cell will ignite a neighboring cell. Our results suggest that thermochemical and combustion property factors might well be considered when choosing solvent mixtures when flammability is a concern.     

The chemistry of minerals obtained from the combustion of Jordanian oil shale

A characterization study was performed on the spent oil shale (oil shale ash) obtained from the combustion of Jordanian oil shale. This characterization utilized different analytical techniques. These include scanning electron microscope with energy dispersive spectrum analysis, X-ray fluorescence, X-ray diffraction and Qemscan. During the combustion process, minimal fragmentation was encountered since Jordanian oil shale contains large proportions of ash which maintain the original structure of the oil shale particle. Different analytical techniques confirmed that the dominant phase of minerals in the oil shale is calcite, which transforms, in parts, into anhydrite during combustion. Sulphur was found to be mainly of an organic source. This sulphur is combusted to produce SO₂ and then SO₃, which controls the sulphation reaction of the calcite. The dominant phase in the ash was the anhydrite in addition to the calcite, clays and calcium phosphate.     

Analytical O(h2) CFD error annihilation theory: FREE O(h4) upgrade for second-order numerics codes

Worldwide, computational fluid dynamics (CFD) codes for Navier–Stokes (NS), Reynolds-averaged Navier–Stokes (RaNS), and/or large eddy simulation NS (LES) partial differential equation (PDE) systems are invariably based on second-order discrete numerics. Resulting nonlinear convection term discretizations inject an O(h²) dispersive error mechanism, h the mesh measure, inducing code algebraic destabilization for practical Reynolds numbers (Re). Code universal resolution is PDE discretization augmentation with a (usually) difference algebra derived numerical diffusion scheme to render O(h²) dispersion error destabilization nonpathological. The penalty of such schemes is artificial diffusion compromising of sharp fronts and/or discontinuities and generation of nonmonotone CFD approximations. Such legacy practices are now rendered obsolete by a totally analytical theory that rigorously identifies, in the continuum (!), the O(h²) truncation error terms resident but unspecified in NS/RaNS/LES PDE system second-order CFD spatial discretizations. The theory removes identified O(h²) error terms by alteration of the continuum appearance of NS/RaNS/LES PDE systems with nonlinear vector differential calculus operators. Theory is amenable to any second-order “tri-diagonal stencil” equivalent CFD discretization and, upon implementation, elevates the original second-order numerics code to O(h⁴) with no further action. This Taylor series error estimate is weak form theory formalized to a regular solution adapted nonuniform mesh refinement O(h⁴) asymptotic error estimate. Theory implementation in a linear basis optimal Galerkin criterion weak form algorithm CFD code enables a posteriori data generation validating annihilation of O(h²) dispersive error mechanisms for reduced NS, full NS, and RaNS PDE systems. In every instance, theory implementation leads to CFD monotone solution distributions free from artificial diffusion influence on sufficiently refined meshes. Differential definition Galerkin weak forms, code post-processed, quantify theory annihilated O(h²) dispersion error spectra, RaNS state variable member specific.     

Computationally-efficient and scalable parallel implementation of chemistry in simulations of turbulent combustion

Large scale combined Large-Eddy Simulation (LES)/Probability Density Function (PDF) parallel computations of reactive flows with detailed chemistry involving large numbers of species and reactions are computationally expensive. Among the various techniques used to reduce the computational cost of representing chemistry, the three approaches in widest use are: (1) mechanism reduction, (2) dimension reduction, and (3) tabulation. In addition to these approaches, in large scale parallel LES/PDF computations, we need strategies to distribute the chemistry workload among the participating cores to reduce the overall wall clock time of the computations. Here we present computationally-efficient strategies for implementing chemistry in parallel LES/PDF computations using in situ adaptive tabulation (ISAT) and x2f_mpi – a Fortran library for parallel vector-valued function evaluation (used with ISAT in this context). To test the strategies, we perform LES/PDF computations of the Sandia Flame D with chemistry represented using (a) a 16-species augmented reduced mechanism; and (b) a 38-species C₁–C₄ skeletal mechanism. We present three parallel strategies for redistributing the chemistry workload, namely (a) PLP, purely local processing; (b) URAN, the uniform random distribution of chemistry computations among all cores following an early stage of PLP; and (c) P-URAN, a Partitioned URAN strategy that redistributes the workload only among partitions or subsets of the cores. We show that among these three strategies, the P-URAN strategy (i) yields the lowest wall clock time, which is within a factor of 1.5 and 1.7 of estimates for the lowest theoretically achievable wall clock time for the 16-species and 38-species mechanisms, respectively; and (ii) for reaction, achieves a relative weak scaling efficiency of about 85% when scaling from 2304 to 9216 cores and a relative strong scaling efficiency of over 60% when scaling from 1152 to 6144 cores.     

A filtered tabulated chemistry model for LES of premixed combustion

A new modeling strategy called F-TACLES (Filtered Tabulated Chemistry for Large Eddy Simulation) is developed to introduce tabulated chemistry methods in Large Eddy Simulation (LES) of turbulent premixed combustion. The objective is to recover the correct laminar flame propagation speed of the filtered flame front when subgrid scale turbulence vanishes as LES should tend toward Direct Numerical Simulation (DNS). The filtered flame structure is mapped using 1-D filtered laminar premixed flames. Closure of the filtered progress variable and the energy balance equations are carefully addressed in a fully compressible formulation. The methodology is first applied to 1-D filtered laminar flames, showing the ability of the model to recover the laminar flame speed and the correct chemical structure when the flame wrinkling is completely resolved. The model is then extended to turbulent combustion regimes by including subgrid scale wrinkling effects in the flame front propagation. Finally, preliminary tests of LES in a 3-D turbulent premixed flame are performed.     

On the occurrence of two-stage combustion in alkali metals

Pool combustion experiments have been conducted for three alkali metals, namely, lithium (Li), sodium (Na) and potassium (K). Lithium and sodium are found to show a two-stage combustion behaviour which has been reported for a number of other metals. Here, the combustion is characterized by a sporadic rise in the flame temperature accompanied by a bright glow. Potassium is found to burn in vapour phase combustion in all cases without sporadic temperature excursions. In the present study, this different burning behaviour is attributed to the formation of thick oxide agglomerates in the case of Li and Na through the pores of which oxygen/metal vapour has to diffuse for combustion to occur. In such cases, a second stage of vapour phase combustion occurs when the oxide agglomerate is heated sufficiently so that the vapour of the liquid metal trapped in the pores breaks through to the surface. In the case of potassium, a self-cleaning mechanism, attributable to the high solubility of the metal oxides in liquid potassium and the relatively low melting point of the potassium oxides, enables a clear liquid surface to be exposed throughout for vapour phase combustion to prevail always. Recorded temperature profiles, SEM analysis of the oxide agglomerates as well as calculations of the metal–oxygen equilibrium thermo-chemistry for the three metals confirm this scenario.     

The use of dynamic adaptive chemistry in combustion simulation of gasoline surrogate fuels

A computationally efficient dynamic adaptive chemistry (DAC) scheme is described that permits on-the-fly mechanism reduction during reactive flow calculations. The scheme reduces a globally valid full mechanism to a locally, instantaneously applicable smaller mechanism. Previously we demonstrated its applicability to homogeneous charge compression ignition (HCCI) problems with n-heptane [L. Liang, J.G. Stevens, J.T. Farrell, Proc. Combust. Inst. 32 (2009) 527-534]. In this work we demonstrate the broader utility of the DAC scheme through the simulation of HCCI and shock tube ignition delay times (IDT) for three gasoline surrogates, including two- and three-component blends of primary reference fuels (PRF) and toluene reference fuels (TRF). Both a detailed 1099-species mechanism and a skeletal 150-species mechanism are investigated as the full mechanism to explore the impact of fuel complexity on the DAC scheme. For all conditions studied, pressure and key species profiles calculated using the DAC scheme are in excellent agreement with the results obtained using the full mechanisms. For the HCCI calculations using the 1099- and 150-species mechanisms, the DAC scheme achieves 70- and 15-fold CPU time reductions, respectively. For the IDT problems, corresponding speed-up factors of 10 and two are obtained. Practical guidance is provided for choosing the search-initiating species set, selecting the threshold, and implementing the DAC scheme in a computational fluid dynamics (CFD) framework.     

A wide range modeling study of formation and nitrogen chemistry in hydrogen combustion

The chemistry of nitrogen species and the formation of NO_x in hydrogen combustion are analyzed here on the basis of a large set of experimental measurements.The detailed kinetic scheme of H₂/O₂ combustion was updated and upgraded using new kinetic and thermodynamic measurements, and was validated over a wide range of temperatures, pressures and equivalence ratios. The mechanism's performance at high pressures was greatly improved in particular by adopting higher rate parameters for the H+OH+M=H₂O reaction.The NO_x sub-mechanism was further validated and updated. The kinetic parameters of the NO₂+H₂=HONO+H and N₂H₂+NO=N₂O+NH₂ reactions were updated in order to improve model predictions in specific conditions.Sensitivity analyses were carried out to determine which reactions dominate the H₂/O₂ and H₂/NO_x systems at particular operating conditions.Good overall agreement was observed between the model and the wide range of experiments simulated.     

Finite rate chemistry and presumed PDF models for premixed turbulent combustion

The sensitivity of the prediction of mean reaction rates in turbulent premixed flames to presumed PDF shape is studied. Three different presumed PDF shapes are considered: (i) a beta function PDF, (ii) a twin delta function PDF, and (iii) a PDF based on unstrained laminar flame properties. The unstrained laminar flame has the same thermochemistry as the turbulent flame. Emphasis is placed on capturing the finite rate chemistry effects and obtaining a simple expression for the mean reaction rate. It is shown that, as the PDFs approach their bimodal limit, the mean reaction rate expressions obtained using the above three PDFs reduce to a common form. These expressions differ only in the numerical value of a multiplying factor. Predictions are compared with DNS data. Under the conditions of this comparison, the beta function and twin delta function PDFs lead to significant errors, while the PDF based on properties of an unstrained laminar flame gives good agreement with the DNS.