Experimental and modeling study of the high-temperature combustion chemistry of tetrahydrofurfuryl alcohol

Abstract

Lignocellulosic tetrahydrofuranic (THF) biofuels have been identified as promising fuel candidates for spark-ignition (SI) engines. To support the potential use as transportation biofuels, fundamental studies of their combustion and emission behavior are highly important. In the present study, the high-temperature (HT) combustion chemistry of tetrahydrofurfuryl alcohol (THFA), a THF based biofuel, was investigated using a comprehensive experimental and numerical approach. Representative chemical species profiles in a stoichiometric premixed methane flame doped with ~20% (molar) THFA at 5.3 kPa were measured using online gas chromatography. The flame temperature was obtained by NO laser-induced fluorescence (LIF) thermometry. More than 40 chemical products were identified and quantified. Many of them such as ethylene, formaldehyde, acrolein, allyl alcohol, 2,3-dihydrofuran, 3,4-dihydropyran, 4-pentenal, and tetrahydrofuran-2carbaldehyde are fuel-specific decomposition products formed in rather high concentrations. In the numerical part, as a complement to kinetic modeling, high-level theoretical calculations were performed to identify plausible reaction pathways that lead to the observed products. Furthermore, the rate coefficients of important reactions and the thermochemical properties of the related species were calculated. A detailed kinetic model for high-temperature combustion of THFA was developed, which reasonably predicts the experimental data. Subsequent rate analysis showed that THFA is mainly consumed by H-abstraction reactions yielding several fuel radicals that in turn undergo either β-scission reactions or intramolecular radical addition that effectively leads to ring enlargement. The importance of specific reaction channels generally correlates with bond dissociation energies. Along THFA reaction routes, the derived species with cis configuration were found to be thermodynamically more stable than their corresponding trans configuration, which differs from usual observations for hydrocarbons.

Exploring the Flame Chemistry of C5 Tetrahydrofuranic Biofuels: Tetrahydrofurfuryl Alcohol and 2-Methyltetrahydrofuran

“…Compared to the previous model [46], the present model contains only three new species (CCJCCCHO, CJC(OH)CCCHO, CC(=O)COH, see structures in Table S2), which result from new reactions shown either in Fig. 1 or in Table S1.…”

“…For other species, calibration factors were determined relying on the hydrogenation by the methanizer which converts oxygenates to their corresponding alkanes. This method has been applied successfully in previous studies [34,46]. H2O mole fractions were calculated independently from both, the H and the O balances, and the average value is reported.…”

“…A detailed kinetic model for THFA is available from Tran et al [46]. This model was developed based on the MTHF model of De Bruycker et al [30] to which THFA chemistry was added.…”

“…Because the previous THFA model reported in [46] cannot well reproduce the mole fraction profiles of 4-pentenal (4C4H7CHO) and n-butanal (nC3H7CHO), which are two important intermediates measured in the doped THFA flames. The former species was overpredicted, while the latter species was underpredicted.…”

“…The rate coefficients (Table 3) of (i) were calculated in the present study at the CBS-QB3 level of theory using the same procedure as outlined in [46], while those proposed in [60] have been used for (ii) and for the first path of (iii). The first pathway of (ii) and the second pathway of (iii) were already available from [46] but is presented in Fig. 1 to provide an overview of the OH-addition processes.…”