A comprehensive experimental and detailed chemical kinetic modelling study of 2,5-dimethylfuran pyrolysis and oxidation
Introduction
Depletion of fossil energy reserves and increasing concerns over climate change are key incentives for the development of energy technologies which are sustainable from social, economical and environmental perspectives. Biofuels, liquid or gaseous fuels derived from biological sources, are considered as a natural successor to the petroleum derived products which are the dominant energy carriers for the transportation sector.
Ethanol is presently the most abundantly produced biofuel globally, with 86.1 billion litres manufactured in 2011 derived largely via fermentation of sugar cane and corn [1]. Yet despite its status as the leading renewable energy source in the transportation sector, problems with its use are now well founded in the literature.
Production feedstocks are in direct competition with food crops and agricultural land [2], [3], combustion in unmodified direct-injection spark-ignition engines is only possible when blended with fossil fuels, its energy density is much lower than that of gasoline and its volatility and complete miscibility with water make it difficult to transport and store. Nevertheless, its ability to reduce emissions, CO, CO2, NOx and unburnt hydrocarbon (HC), when blended with gasoline and diesel illustrate the environmental benefits which the use of oxygenated biofuels can achieve.
Next-generation production methods have recently been developed [4], [5], [6], [7], [8], [9], [10] capable of converting inedible waste biomass, into the platform chemical 5-hydroxymethyl-2-furaldehyde, with subsequent conversion into the novel biofuel, 2,5-dimethylfuran (25DMF). The literature highlights the energy density of 25DMF (30 MJ L−1) as a notable improvement over that of ethanol (21.3 MJ L−1) with its higher boiling point (366 K) and lower aqueous solubility also making it preferable to the alcohol in terms of transportation and storage [7]. Other considerations such as atmospheric lifetimes and ecological and toxicological aspects are covered in a recent review by Simmie and Würmel [11].
Since the development of these production methods, combustion studies have ensued. 25DMF performed wholly similar to commercial gasoline in research engine tests [12], [13], the authors concluding that no major modifications to engine design would be necessary to achieve equivalent emissions and performance levels to gasoline. Daniel et al. [14] recently found that total carbonyl emissions from a direct-injection spark-ignition engine fuelled with 25DMF were lower than those of methanol, ethanol, n-butanol and gasoline, in particular formaldehyde emissions, which may bear on its suitability as a biofuel. 1,3-cyclopentadiene, methyl vinyl ketone and 2-methylfuran (2MF) were also found in the exhaust gas, with unburnt fuel dominating the characterised emissions.
Wu et al. [15] determined laminar burning velocities of 25DMF–O2–N2/CO2 mixtures as a function of equivalence ratio (ϕ) and dilution ratio at unburnt gas temperatures (Ti) of 393 K and atmospheric pressure. Laminar burning velocity was found to decrease linearly as a function of increased N2/CO2 concentrations, with peak burning velocities occurring for ϕ = 1.1–1.2. They complemented this work with studies on the laminar burning velocities of atmospheric pressure 25DMF-air mixtures over a range of equivalance ratios as a function of Ti (393–473 K) [16] and as a function of pressure (0.1–0.75 MPa) [17] for Ti = 393 K.
Tian et al. [18] determined laminar burning velocities of 25DMF, ethanol and gasoline as a function of ϕ and Ti (323–373 K) under atmospheric pressure in a combustion bomb. The laminar burning velocities of 25DMF were found to be the slowest of all three fuels studied, but were within 10% of gasoline between equivalence ratios of 0.9–1.1. Unfortunately, no measurements were made under directly comparable conditions to those of Wu and co-workers [15], [16], [17]. These laminar premixed flame burning velocity measurements will form a portion of the kinetic mechanism validation described in this work, along with experimental laminar burning velocities measured as part of this work using the heat-flux method.
Experimental work to isolate the chemical pathways of 25DMF combustion have also ensued of late, but other than early works by Grela et al. [20] in 1985 and Lifshitz et al. [21] in 1998 the literature remained sparse until very recently. Grela et al. [20] pyrolysed 25DMF in a heated tubular reactor at very low pressures (1 mTorr) from 1050 to 1270 K, analysing the product mixtures via on-line mass spectrometry. They detected water, CO, C5H6 and C6H6 in their effluent stream and hypothesised their formation from 25DMF by isomerisation to 2,4-dimethylfuran through a cyclopropenylcarbonyl intermediate – 25DMF or 2,4-dimethylfuran could then decompose via biradical intermediates through simple C-O bond fission. Lifshitz et al. [21] studied the thermal decomposition of 25DMF behind reflected shock waves in the temperature range 1070–1370 K, at pressures of 2–3 atm. They analysed the post-shock mixtures via gas chromatography, quantifying the concentrations of 19 intermediate species and reporting a rate constant (pseudo-first order) for the decomposition of 25DMF of 1015.81exp(−75.1 × 103/RT) where R is in units of cal K−1 mol−1. A chemical kinetic mechanism consisting of 50 species and 180 elementary reactions was developed to account for the product distributions.
However, it would appear that the mechanistic proposals of Grela et al. [20] and Lifshitz et al. [21] are erroneous, in light of recent theoretical work on the furans, and in particular the quantum chemical calculations by Liu et al. [22], [23] and Sendt et al. [24], which went some way to disproving the long held belief that furan decomposition was routed through biradical intermediates [25], [26], [27]. The authors showed that the unimolecular decomposition of furan was initiated through singlet carbene intermediates formed from hydrogen atom transfer reactions, with Sendt et al. [24] constructing a kinetic scheme capable of reproducing laboratory pyrolysis experiments, thus validating their kinetic and mechanistic proposals.
More recently, Simmie and Curran [28] applied quantum chemical methods (CBS-QB3, CBS-APNO and G3) and developed isodesmic working reactions to calculate enthalpies of formation for a range of substituted furans and their corresponding furfuryl radicals, thus determining bond dissociation energies. They noted that for alkylfurans, the ring–H bonds are extremely strong, in excess of 500 kJ mol−1, but that radicals formed from the alkyl side chains of a range of 2/3-methyl and 2/3/4/5-dimethyl furans, are considerably weak, all in the region of 357–380 kJ mol−1. The important consequence of their findings is that the alkyl side chains of these species are a plausible source of radical initiators within a combustion environment, and that hydrogen atom abstraction by free radicals is likely to occur exclusively at the alkyl side chain.
Simmie and Metcalfe [29] used electronic structure methods and canonical transition state theory to study the initial steps in the thermal decomposition of 25DMF. They provided high pressure limiting kinetics and thermodynamic parameters for the carbene-mediated decomposition of the reactant, reactions of hydrogen atom and hydroxyl radical with the fuel and reactions which open the furan ring once furan-derived radicals are formed. They concluded that hydrogen atom addition to the double bonds of the furan ring is dominant up to temperatures of 2000 K.
Friese et al. [30], [31] used time-resolved resonance absorption spectrometry to detect hydrogen as a product in the thermal decomposition of 25DMF from 1280 to 1520 K, and as a reactant with 25DMF between 980 and 1250 K, at 1.6 and 4.7 bar. Rate coefficients for the reactions + product and 25DMF + products were derived from concentration–time profiles. Statistical rate theory, including a master equation (ME) to describe the thermally and chemically activated processes, was applied to rationalise their results, with accurate prediction of the experimentally derived rate constants found. The total rate constant for the reaction of atom with 25DMF was found to exhibit only a weak dependence on pressure both theoretically and experimentally.
Sirjean and Fournet [32] added to the above work with a full exploration of the unimolecular decomposition pathways of 25DMF, including Rice–Ramsperger–Kassel–Marcus (RRKM) and ME analysis on the carbene and biradical mediated decomposition pathways of the reactant, with rate constants estimated for simple fission processes. They found that a 3–2 hydrogen atom transfer forming 3,4-hexadiene-2-one was the dominant decomposition pathway and direct ring opening reactions to form biradical intermediates are of little significance, as in the case of furan [22], [23], [24]. The same authors carried out an extensive exploration of the potential energy surface (CBS-QB3) upon hydrogen atom addition to the furan ring of 25DMF coupled with RRKM/ME modelling of collisional energy transfer within the chemically activated pathways involved [33]. They found that hydrogen atom addition at carbon atoms remote from the oxygen atom of the furan ring could be neglected based on the endothermicity and barrier heights of the subsequent ring-opening reactions. Hydrogen atom addition at carbon atoms adjacent to the oxygen atom of the furan ring would result in the formation of 2-methylfuran (2MF) predominantly, with lesser yields of 1,3-butadiene and acetyl radical. Only above temperatures of 1300 K would hydrogen atom abstraction by hydrogen atom become dominant. The total rate constant for the reaction of hydrogen atom with 25DMF was found to be nearly pressure independent and within a factor of two of the experiments of Friese et al. [30], [31].
In a recent study, Sirjean and Fournet [34] also investigated the thermal reactions of the 5-methyl-2-furanylmethyl radical, formed from C–H fission or hydrogen atom abstraction from 25DMF. Through CBS-QB3 calculations and RRKM/ME modelling of the detailed, and complex, potential energy surfaces they found that the resonantly stabilised radical predominantly undergoes ring opening, followed by a hydrogen atom transfer reaction and ring enlargement to cyclohexenone radicals. Linear and cyclic unsaturated C5 species could also be produced with CO in lesser quantities. The cyclohexenone radicals could decompose to form hydrogen atom and stable cyclohexadienone isomers which could undergo keto-enol tautomerisation to form phenol, through a well established mechanism [35], [36], [37]. Pressure and temperature dependent rate constants were provided from 0.01–10 bar and 1000–2000 K.
Phenol is therefore a likely intermediate in the combustion of 25DMF, with plausible pathways to its formation now recognised. It was recently detected in a low pressure premixed laminar 25DMF–O2–Ar flame by Xu et al. [38] on the basis of its ionisation energy, along with 2MF, furan and 1,3-butadiene for which credible formation channels are now well characterised based on the above work.
Djokic et al. [39] recently used a 1.475 m long heated flow reactor to pyrolyse 25DMF using GC × GC-FID (TOF-MS) to quantify the decomposition of the fuel and formation of intermediates from 873 to 1098 K, at 1.7 bar and at heating times of 300–400 ms. They were able to quantify species which are important indicators of the fuel decomposition such as phenol, 2MF, 1,3-butadiene and 1,3-cyclopentadiene, along with a host of small hydrocarbon species and mono- and poly-aromatic species up to C17.
Recently, Sirjean and co-workers [40] described a chemical kinetic model for the high temperature combustion of 25DMF. They validated their mechanism against ignition delay time measurements from 1300 to 1831 K, at 1 and 4 bar, for mixtures of 25DMF in argon at ϕ = 0.5, 1.0 and 1.5, and the experimental data of Lifshitz et al. [21]. They identified reactions important in predicting their experimental targets, although no comparison was made with literature flame speeds. Together with the kinetic model described by Somers et al. [41] for 2MF which could accurately describe ignition delay times and laminar burning velocities, only two detailed mechanisms to describe the combustion of alkylfurans currently exist in the literature.
Here we aim to remedy this deficit by providing a detailed chemical mechanism to describe the combustion of 25DMF based on the theoretical and experimental works described above, along with experiments detailed in the following sections, thus providing the most comprehensive experimental and modelling analysis of its combustion to date.
Section snippets
Experimental
Experiments have been carried out in five separate facilities in order to gather data relevant to both practical combustors and to the validation of a detailed chemical kinetic mechanism, including:
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A single pulse shock tube to investigate the pyrolysis of 25DMF.
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A low pressure shock tube to measure ignition delay times of dilute 25DMF/O2/Ar mixtures.
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A high pressure shock tube to measure ignition delay times of non-dilute 25DMF/O2/N2 mixtures, representing “fuel in air”.
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A jet-stirred reactor to
Unimolecular decomposition
The unimolecular decomposition pathways of 25DMF have been studied by Simmie and Metcalfe [29] previously, and more recently in detail by Sirjean and Fournet [32]. Both works concluded that the thermal decomposition of 25DMF should proceed with competition between simple C–H bond fission from the alkyl side chain of the fuel, and carbene formation and consumption via hydrogen atom transfer reactions. Simmie and Metcalfe [29] calculated a high pressure limit rate constant of 9.48 × 1013
Pyrolysis
The pyrolysis of 25DMF has been studied previously by Lifshitz et al. [21] for mixtures of 0.5% fuel in argon bath gas from 1070 to 1370 K, at residence times ≈2 ms and at pressures of ≈2–3 atm. Small quantities (0.1%) of 1,1,1-trifluoroethane (1,1,1-TFE) were added to the reaction mixtures to act as a chemical thermometer and the reflected shock temperatures determined from the relationship:where τ is the reaction dwell time, and χ is defined as:
Conclusions
This paper presents novel experiments on the pyrolysis and oxidation of 2,5-dimethylfuran. We also describe the development of a detailed kinetic mechanism which is based on a combination of literature theoretical studies, newly presented ab initio calculations and by analogy with similar chemical systems. The mechanism is compared with our new experiments and relevant literature data with rate of production and sensitivity analyses used to identify important reaction pathways and kinetic
Acknowledgments
This work was partially initiated by interactions arising out of COST Action CM0901, Detailed Chemical Kinetic Models for Cleaner Combustion. It was partially funded by the Région Lorraine and the European Research Council through the “Clean ICE” Advanced Research Grant. FG thanks CM0901 for the award of a Short Term Scientific Mission scholarship. KPS and FG would like to acknowledge the support of Science Foundation Ireland under Grant No. [08/IN1./I2055] as part of their Principal
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