An experimental investigation of furfural oxidation and the development of a comprehensive combustion model

Abstract

The oxidation of furfural has been studied experimentally in a jet-stirred reactor (JSR) under fuel-lean (Φ = 0.4) and fuel-rich conditions (Φ = 2.0) in the temperature range of 650–950 K; in addition, laminar burning velocity data have been measured at T = 473 K and p = 1 bar within a wide fuel-air range. From the JSR experiments, 13 species profiles have been identified and quantified by GC–MS and GC. A detailed kinetic reaction model involving 382 species and 2262 reactions was developed by exploiting the experimental data base provided within the present work as well as experimental data reported in literature. The rate coefficients of reactions of H abstraction, H addition as well as of decomposition of furfural were calculated by quantum chemical methods at CBS-QB3 level. A general agreement was achieved when simulating the experimental data. Rate of production analysis as well as sensitivity analysis were performed to get a deeper insight into the combustion of furfural, e.g. for the jet-stirred reactor data at around 50% fuel conversion, as well as sensitivity analysis of laminar flame speeds conducted for a fuel-air ratio Φ = 0.9, 1.2, and 1.6. According to the analysis, the main consumption pathways of furfural oxidation were identified as H abstraction reactions of the R-CHO (aldehyde) group by H, OH, O, and HO₂ to produce a furfural radical (furfural-6). At pyrolysis condition, the dominant pathways within the furfural decay were found to occur via ring opening by splitting the CO bond followed by isomerization to form α-pyrone (C₅H₄O₂). Even more, the measured laminar flame speed data are well reproduced by the reaction model developed within the present work. The experimental data base as well as the developed reaction model will assist and contribute to a more detailed understanding of the combustion behavior of furfural and of furan derivatives as well.

Introduction

Nowadays, new energy sources are urgently needed because of the worldwide energy shortage and environmental pollution caused by the use of fossil fuels. Biofuels in general, such as ethanol or biodiesel [1,2], have attracted much attention due to their renewability (enabling a lower CO₂ footprint) and large storage capacity, besides further benefits to the environment by a reduced emission pattern, e.g. lower amounts of soot particles (in the case of oxygenated fuels). In this context, furan derivatives (see Fig. 1), produced from non-edible feedstocks, especially 2-methylfuran (MF) and 2,5-dimethylfuran (DMF), were demonstrated to have additional benefits as next generation alternative fuels due to their significant advantages over traditional biofuels [3], [4], [5], [6], [7]. For instance, DMF-diesel blends produce less soot compared to blends of diesel and n-butanol [3,5]. Furthermore, the octane number of DMF is higher than that of ethanol [8]. Also, the heating values of furan, MF, and DMF are larger than that of ethanol [4] and thus, similar to that of gasoline. Sudholt et al. [9] investigated derived cetane numbers (DCN) of several furan derivatives, with furan, MF, DMF, among other, to characterize their ignition behavior. According to DCN and bond dissociation energies (BDE) at CBS-QB3 level, they found that the ignition behavior of the furan derivatives is governed by the ring structure, while the chain structure has a negligible effect.

Furfural is generally produced from C5 sugars (such as arabinose and xylose) being constituents of hemicellulose derivative [10]. Furfural is a feedstock of several furan derivatives [4,8] and thus, an important intermediate within furan derivatives oxidation [11]. Moreover, furfural, as a non-petroleum based chemical feedstock, is also a precursor for the production of valerate esters, pentanols, and 1-octanol as well as various C₁₀–C₁₅ coupling products [12], [13], [14]. Furfural also shows some interesting combustion properties. Within furfural combustion, a reduced production of soot was reported compared to hydrocarbon fuels [15]. Blending furfural in the tri-propylene glycol mono-methyl ether (C₁₀H₂₂O₄), being a possible fuel additive, was shown to enhance the ignition reactivity compared to blending with furan or methyl substituted furans [16]. Therefore, studies focusing on the oxidation process at lean conditions are necessary for studying the combustion properties of furfural as an additive to diesel. Furthermore, furfural is also an important compound within the thermal conversion process of biomass [17], [18], [19], [20], [21]. Pyrolysis experiments as well as those at fuel-rich conditions are needed to investigate its conversion pathways. Hence, research on furfural combustion is meaningful for its efficient usage as well as to understand more comprehensively the combustion of furan derivatives and to assess better biomass pyrolysis.

Some earlier work has been performed to identify the major furfural consumption pathways, through theoretical calculation and experiments, as summarized in Table 1. Zhao et al. [22] reported that the furfural oxidation which they studied at ambient pressure was initiated via hydroxyl radicals reactions by using quantum chemistry and kinetic calculations. The furfural oxidation was also investigated by Thorton et al. [23] in a JSR combined with gas chromatography (GC), within temperatures between 1000 and 1300 K at atmospheric pressure. They identified as the main fuel consumption pathway the H abstraction of furfural forming a furfural radical which reacts further decomposing into a furan radical and CO. Due to the lack of high-precision experimental measurements and quantum chemistry calculations, Thorton et al. [23] proposed a preliminary mechanism for furfural oxidation; however, no detailed kinetic model was provided. Vasiliou et al. [24] investigated the thermal decomposition of furfural in a heated micro tubular flow reactor within the temperature range of 1200–1800 K, at 75–150 torr. They found that the ring-opening pathway has a lower energy barrier compared to the formation pathway of carbenic intermediates and proposed the initial fuel decomposition process to be an unimolecular decomposition reaction step: furfural (+ M) = furan + CO (+ M). Vermeire et al. [25] studied the furfural pyrolysis at atmospheric pressure in a jet-stirred reactor (JSR) combined with GC and GC-mass spectrometry (GC–MS). The important intermediates α-pyrone and furan were observed and quantified, besides others. Based on experimental results and the calculated potential energy surface of furfural initiation reactions, they concluded that furfural decomposes mainly through a ring-opening isomerization step forming formylvinylketene (cis-O=C=CH–CH=CH–CH=O). Li et al. [26] investigated the thermal decomposition of furfural in a flow tube reactor at 30 torr by synchrotron vacuum ultraviolet photoionization and mass spectrometry. Also, they calculated the H addition and H abstraction reactions via H radical at the CBS-QB3 level.

Although there are several studies available on furfural combustion as discussed above, none of them was dedicated to studying the detailed decomposition pathways of furfural. Thus, additional investigation on furfural is desirable to be able to predict more comprehensively its combustion behavior and to reveal its dominant reaction pathways by the construction of a comprehensive chemical-kinetic reaction model being validated by experimental data.

In the present work, furfural oxidation was investigated in a JSR at atmospheric pressure in the temperature range of 650–950 K, at two equivalence ratios Φ: 0.4 (fuel-lean) and 2.0 (fuel-rich). Furthermore, the laminar burning velocity of furfural in air was measured over a Φ-range from 0.6 to 1.8 at a preheat temperature of T = 473 K and at ambient pressure, p = 1 bar. Based on the experimental data, a comprehensive detailed chemical kinetic reaction model was developed to simulate the furfural oxidation (present work) and its pyrolysis (experimental data taken from Vermeire et al. [25]) as well as the laminar flame speed data (present work). Also, rate of production (ROP) and sensitivity analyses were used to identify the dominant consumption pathways of furfural. Furthermore, reaction rate expressions of major initiation reactions within furfural consumption have been determined by applying quantum chemical calculations, at CBS-QB3 level found to be appropriate for these types of fuels [29], [30], [31], [32].