# Low-Temperature combustion of 3-Pentanol and its blends with Hydrogen: An experimental and modeling study

Marwa Saab, Rana Shebly, Yann Fenard, Guillaume Vanhove

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Marwa Saab, Rana Shebly, Yann Fenard, Guillaume Vanhove. Low-Temperature combustion of 3-Pentanol and its blends with Hydrogen: An experimental and modeling study. Fuel, 2026, 408, pp.137704. 10.1016/j.fuel.2025.137704. hal-05454486

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Full Length Article

# Low-Temperature combustion of 3-Pentanol and its blends with Hydrogen: An experimental and modeling study

Marwa Saab<sup>a</sup>, Rana Shebly<sup>b</sup>, Yann Fenard<sup>b</sup>, Guillaume Vanhove<sup>b,*</sup>

<figure><img src="image_2.png" /></figure>

## ARTICLE INFO

Keywords:

3-Pentanol

Hydrogen

Ignition delay times

Kinetic model

Low temperature combustion

## ABSTRACT

Reducing emissions in the transportation sector has highlighted the potential of oxygenates as alternative fuels, known for lowering soot emissions. Blending these alcohols with hydrogen, a carbon-free fuel, offers further potential for reducing carbon-based emissions. Because the description of the low-temperature combustion pathways of $C_5$ alcohols still needs to be improved, this study investigates the ignition delay times (IDTs) and mole fraction profiles of intermediates of 3-pentanol/hydrogen mixtures using a rapid compression machine over a low-to-intermediate temperature range. Experiments were also conducted for 3-pentanol/$H_2$ blends with mixing ratios of 100/0, 75/25, and 50/50 at pressures of 10, 15, and 20 bar, and equivalence ratios of 1.0 and 0.5. Gas chromatography, flame ionization detection, thermal conductivity detection, and mass spectrometry were employed for the quantification of the intermediates. These results therefore provide a rare detailed database for the validation of 3-pentanol kinetic models at high-pressure, low-temperature combustion conditions.

Furthermore, *ab initio* calculations were performed to determine β-scission and isomerization reaction rates, supporting the development of a novel kinetic sub-mechanism for 3-pentanol, which shows better or equivalent agreement that previous works on the hereby-reported or existing literature data. This mechanism highlights the importance of the branching ratio of the initial H-abstraction step in the prediction of the IDTs as well as the importance of the 3-pentanone relevant pathways. It was used to evaluate the impact of hydrogen addition on 3-pentanol IDTs, revealing the importance of $H_2$ as a sink for hydroxyl radicals, readily converting them to H atoms then hydroperoxyl radicals.

## 1. Introduction

The increasing demand for sustainable transportation fuels has driven significant interest in the research of potential biofuels, such as alcohols. While alcohols have been extensively studied, research has primarily focused on short-chain variants up to $C_4$ [1]. In contrast, higher-chain alcohols, such as 3-pentanol, have received comparatively less attention, particularly in the low-to-intermediate combustion range. Carbonnier et al. [2] conducted the first high-pressure oxidation study of 3-pentanol using a Jet-Stirred Reactor (JSR) and a Shock Tube (ST). They identified and measured species profiles finding that 3-pentanol did not exhibit cool flame behavior and produced aldehydes, ketones, and 2-pentene as key intermediates. A kinetic sub-mechanism was developed, showing good overall performance on the prediction of the experimental data despite some discrepancies. Feng et al. [3] refined the

model by calculating high-pressure rate constants for H-abstraction reactions, leading to an improved version. Chatterjee et al. [4] performed the first low-temperature oxidation study of 2- and 3-pentanol using a Rapid Compression Machine (RCM). They developed detailed kinetic models based on 1-pentanol oxidation, marking the first mechanism for these alcohols. The mechanism was validated against high-pressure ST and RCM data, demonstrating reasonable agreement across various conditions. Köhler et al. [5] conducted the first comparative flame study on 1-, 2-, and 3-pentanol using pentanol-doped hydrogen flames. Through electron ionization molecular-beam mass spectrometry (EI-MBMS), they identified 27 species, including pentanonyl radicals. Their kinetic model accurately captured major species profiles and key intermediates. Distinct decomposition products were observed: pentanal for 1-pentanol, 2-pentanone for 2-pentanol, and 3-pentanone for 3-pentanol. Except from the Chatterjee et al. work, none of these studies have

<footnote><sup>a</sup> IFP Energies nouvelles, Institut Carnot IFPEN Transports Energie, 1 et 4 avenue de Bois-Préau, 92852 Rueil-Malmaison, France</footnote>

<footnote><sup>b</sup> Univ. Lille, CNRS, UMR 8522 - PC2A, Physicochimie des Processus de Combustion et de l'Atmosphère, F-59000 Lille, France</footnote>

<footnote>* Corresponding author.</footnote>

<footnote>E-mail address: guillaume.vanhove@univ-lille.fr (G. Vanhove).</footnote>
<figure><img src="image_3.png" /><figcaption>**Fig. 1.** Pressure profiles for 3-pentanol/O₂/N₂/Ar stoichiometric mixture, p_c = 20 bar, T_c = 735 K. Red line: Reactive mixture, green line: Non-reactive mixture, blue line: pressure profile of the mixture during sampling at 36.5 ms. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)</figcaption></figure>

been devoted to the low-temperature combustion pathways of 3-pentanol, despite its practical interest as a biofuel. Moreover, the data reported in this study only consisted in ignition delay times. Quantitative measurements of the intermediates formed at low temperature are therefore needed for a more in-depth validation of 3-pentanol models. This study therefore aims at improving the current state-of-the art of the low temperature combustion mechanisms of this fuel. To do so, we investigated the low-temperature oxidation of 3-pentanol using an RCM, by means of IDT as well as species profiles measurements. The obtained experimental data, in addition to literature data, were used to develop a new kinetic mechanism covering both low- and high-temperature oxidation. Fig. 5b.

Additionally, this study builds on previous research that explored hydrogen blending with n-pentane, 1-pentene, and 3-pentanone [6], by extending the analysis to include 3-pentanol. This work is motivated by the need to understand the co-oxidation patterns of hydrogen at temperatures below those at which it would auto-ignite when pure. Except from [6], no prior Rapid Compression Machine studies have examined hydrogen blending with hydrocarbons beyond alkanes, making this the first high-pressure investigation into the impact of hydrogen on the low-temperature oxidation of an alcohol.

## 2. Methods

### 2.1. IDTs and speciation experiments

The ignition delay times (IDTs) of 3-pentanol and its hydrogen blends (25 and 50 %mol) were measured using the ULille (Université de Lille) RCM which is described in detail elsewhere [7,8]. 3-Pentanol (Thermo Scientific, purity > 98 %) was further purified from eventual dissolved gases by several cycles of freezing with liquid nitrogen and pumping. The gases were provided by Air Liquide with purities above 99.9 %. The gas mixtures were prepared inside glass bulbs using the partial pressures method, and left to stir for an hour before use. To prevent piston-rebound and guarantee that a constant volume is maintained after compression, the driving and compressing pistons of the ULille RCM are joined by a cam at a right-angle. The volumetric compression ratio was set to 9.65. Moreover, a creviced piston was employed, and a compression time of 45 ms was used to prevent the

production of piston corner vortices. A thermally shielded piezoelectric pressure transducer (Kistler 601-CA) and a charge amplifier (Kistler 5018) were used to record the dynamic pressure during the experiment. The 3-pentanol/H₂/O₂/inert blends reached core gas temperatures between 690 K and 940 K by varying N₂, Ar, and CO₂ ratios within the inert portion. Uncertainties in T_c (±5 K) and P_c (±0.1 bar) were expected, as determined by the method in [9].

The IDT was defined as the time between End Of Compression (EOC) and (dp/dt)max and was calculated from experimentally acquired pressure profiles as shown in Fig. 1. Each IDT measurement was repeated at least three times to confirm reproducibility, with all data shown in the figures. At such pressures, the occurrence of first-stage ignition is usually deduced from a step-like evolution of the pressure during the ignition delay, which was observed at none of the hereby investigated conditions. Volume profiles were derived from non-reactive pressure traces shown in Fig. 1, obtained by replacing O₂ with N₂, allowing accurate modeling of the compression phase and heat losses.

Time-dependent species profiles were obtained during the ignition delay by sampling the reactive mixture at various times between the EOC and total ignition, in a series of experiments under identical initial conditions resulting in T_c = 735 K and p_c = 20 bar. As in our previous studies (see, for example, [7]), the sampled gases were rapidly quenched via expansion cooling into a heated chamber approximately 40 times the volume of the combustion chamber by rupturing a diaphragm at a chosen time after EOC. After gas chromatographic separation, the species were identified using mass spectrometry (MS) and quantified with flame ionization (FID) and thermal conductivity (TCD) detectors. Results were corrected for crevice volume dilution, following the method described in [10]. The uncertainty in measured species mole fractions was ± 10 % for TCD- and FID-calibrated species and ± 20 % for species quantified using the FID and calibrated from the effective carbon number method [11].

### 2.2. Kinetic mechanism development

The base model for C₀-C₄ chemistry is adapted from NUIGmech1.1, a well-validated mechanism across various studies [12–16]. Submechanisms for key 3-pentanol oxidation products, 2-pentene and 3-pentanone, were incorporated based on studies by Dong et al. [17] and Kang et al. [18], respectively. Modifications were made to the 3-pentanone mechanism from Kang et al. [18], including revised H-abstraction rates by hydroxyl radicals, replacing ethyl methyl ketone analogies with values calculated by Serinyel et al. [19]. Recombination reactions were also added based on the experimental detection of the relevant species and estimated at the collision limit, but did not result in significant changes in the predictions of the model. The 3-pentanol submechanism was developed following reaction classes outlined by Pelucchi et al. [20] for n-C₃-C₆ linear alcohols. The final mechanism comprises 1346 species and 6142 reactions and is provided as supplemental material.

The reaction rate parameters for water elimination from 3-pentanol were determined by analogy to 2-butanol, which undergoes a similar dehydration reaction to form 2-butene and water, as reported by Rosado-Reyes et al. in shock tube conditions [21]. Similarly, the kinetic parameters governing C–C and C–H bond dissociations in 3-pentanol were assumed to be comparable to those of 2-butanol and were adopted from the modeling work of Sarathy et al. [1].

Additionally, the reaction rate constants for 3-pentanol undergoing abstraction reactions by radicals (3-pentanol + X → pentanolyl radicals + XH) were selected based on analogies with literature data for 2-butanol, 1-propanol, and 2-propanol, in accordance with their similar bond dissociation energies (BDEs) to 3-pentanol, as provided in S1 in the supplementary material (SM). Overall, the choice of the rate coefficients was made prioritizing rates that resulted from ab initio studies, and that resulted in the best agreement with the experimental dataset. These rates were recently refined using high-level theoretical calculations.
Specifically, the rate for X = OH follows 2-butanol kinetics [22], while X = H is based on analogies with 1- and 2-propanol [23]. For X = HOO, the rate coefficients were also derived from 1- and 2-propanol [24] but were multiplied by a factor of three to better align with the experimental data. The rate for X = $\dot{CH}_3$ was determined using analogies with 1- and 2-propanol [25]. Finally, for hydrogen abstraction reactions by $O_2$ and O-atoms, rate coefficients were chosen based on 2-butanol kinetics [1]. These modifications ensure consistency with established kinetic frameworks while remaining within the estimated uncertainties of theoretical calculations.

The isomerization and β-scission reaction rates of the pentanolyl radicals were computed in this study, as detailed in the following section, and subsequently incorporated into the mechanism. The potential energy surface (PES), shown in S2 in SM, reveals that PENT3OH-3 (R3) is the most stable radical, while PENT3O (R4) is the least stable and exhibits the lowest energy barriers for isomerization into other radicals. Additionally, β-scission reactions involving C–H bonds were found to have higher energy barriers compared to those involving C–C or C–O bonds. A comparison of the hereby calculated rate constants with those from the Chatterjee mechanism is shown in Fig. S3 of the SM.

The rate constants for the first and second $O_2$ addition reactions were taken from Miyoshi [26], who computed these values for alkyl radicals including $C_2H_5$, i-$C_3H_7$, n-$C_4H_9$, s-$C_4H_9$, and t-$C_4H_9$. The reaction rates between alkyl radicals and $RO_2$ were taken from Sarathy et al. [1], based on an analogy with 2-butanol.

For internal hydrogen migration from $RO_2$ to QOOH, the reaction rates were derived from Sharma et al. [27], who calculated rate coefficients for intramolecular hydrogen transfer in alkylperoxy and hydroperoxyalkylperoxy radicals. The elimination of HOO from $RO_2$ and QOOH, as well as cyclic ether formation, followed the work of Villano et al. [28], who determined high-pressure rate coefficients for β-, γ-, and δ-QOOH isomers, including their isomerization, cyclic ether formation, and β-scission pathways for small to intermediate-sized hydroperoxyalkyl radicals.

Additionally, the rate constants for the Waddington mechanism in alcohol oxidation were taken from Li et al. [29], based on studies of butanol isomers. The thermodynamic data for the high temperature species in the 3-pentanol sub-model were taken from Carbonnier et al. [2]. The missing thermochemical and transport data were estimated via the group additivity method on the RMG website [30]. The RCM simulations were performed using the ideal gas reactor model from Cantera [31], using the volume profiles derived from non-reactive experiments as input. The input volume profiles are provided as SM.

## 2.3. Ab initio methods

In this study, all electronic structure calculations were performed using the Gaussian 09 software [32]. The molecular geometries and vibrational harmonic frequencies were determined at the B3LYP-D3BJ/Def2-TZVP level of theory, a widely recognized and computationally efficient method that balances accuracy and resource requirements. These optimized geometries were then utilized for single-point energy (SPE) calculations at the CCSD(T) level, employing augmented double- and triple-zeta basis sets (aug-cc-pV(T + D)Z). The reliability of this approach for estimating hydrogen abstraction barriers has been validated with an uncertainty of ± 0.57 kcal/mol [33].

To determine the minimum energy conformer, relaxed one-dimensional (1D) rotor scans were conducted in 30° increments using the previously described computational method. If a new minimum was identified during these scans, the corresponding structure was used for subsequent 1D scans. This iterative process continued until all conformers converged to the same minimum energy state. The resulting minimum energy conformers, along with the 1D hindered rotor potential energy profiles, were then utilized to compute the rate constants. This approach assumes that the lowest-energy structure, combined with the 1D scans, sufficiently captures the relevant conformational space.

<figure><img src="image_4.png" /><figcaption>**Fig. 2.** 3-pentanol IDT measurements at different pressures and equivalence ratios. Solid circles: measured IDT, solid lines: simulated IDTs using the model from this study, dashed lines: simulated IDTs using the model from Chatterjee et al. [4].</figcaption></figure>

The reaction rate constants for hydrogen abstraction were computed using conventional transition state theory (cTST) alongside the zero-curvature Eckart approach for tunneling. Given the high energy barriers and imaginary frequencies associated with these reactions, variational effects are considered negligible. The rate constants were evaluated over a temperature range of 500 K to 2000 K. For unimolecular β-scission and isomerization reactions, temperature- and pressure-dependent rate constants were determined using the Master Equation (ME) approach implemented in the MESS software package [34]. Collisional relaxation was modeled using the Lennard-Jones (LJ) collision frequency model [35] in conjunction with the single-exponential down model. The LJ parameters for 3-pentanol, $σ = 5.921$ Å and $ε = 430.83$ K, were obtained using RMG [30], while for nitrogen as the bath gas, values of $σ = 3.7$ Å and $ε = 85.2$ K were taken from [36]. The single-exponential down model applied an average energy deactivation of $\langle E_{down} \rangle = 200\text{cm}^{-1} \cdot (T/300\text{K})^{0.85}$ ensuring accurate representation of energy transfer processes.
<figure><img src="image_5.png" /></figure>

<figure><img src="image_6.png" /></figure>

<figure><img src="image_7.png" /></figure>

<figure><img src="image_8.png" /><figcaption>**Fig. 3.** Evolution of the mole fraction of the fuel and the intermediates during the ignition delay time of a 3-pentanol/'air mixture' at Φ = 1, p<sub>c</sub> = 20 bar and T<sub>c</sub> = 735 K. Solid lines present simulation done using the model from this study, and dashed lines present simulations with the Chatterjee mechanism [4].</figcaption></figure>

**3. Results and discussion**

**3.1. 3-pentanol/'air' mixtures**

The IDTs of 3-pentanol were measured using the RCM under experimental conditions of p<sub>c</sub> = 20 and 15 bar, with equivalence ratios (E.R.) of 1.0 and 0.5, over a temperature range of 650–950 K, and are depicted in Fig. 2. No first-stage ignition was detected during the experiments. The IDTs do not exhibit a distinct NTC behavior, although a deviation from Arrhenius behavior is observed. No ignition was observed at 10 bar, nor at the lowest temperatures at 15 bar under fuel-lean conditions.

The kinetic model developed in this study demonstrates very good prediction across all tested conditions. The simulated IDTs generally fall within a 20 % uncertainty range of the experimental results, except at 15 bar under stoichiometric conditions, where the model underpredicts the IDTs at the lowest temperatures by approximately 30 %. In contrast, the Chatterjee et al. [4] mechanism exhibits discrepancies exceeding 50 % in most cases and incorrectly predicts a pronounced NTC behavior, which was not observed in the experiments. The overestimation of the NTC behavior, which was also observable in the initial comparison plots [4], could indicate that the low-temperature reactivity promoting pathways may have been overestimated in the Chatterjee model.

A separate set of sampling experiments was carried out at EOC conditions of 735 K and 20 bar, where the IDT is 71 ms. Throughout the oxidation process, 11 different intermediate species were identified. The measured and simulated mole fraction profiles of 3-pentanol, and its

intermediates, are illustrated in Fig. 3. The fuel shows minimal conversion before 40 ms of its total IDT, with most species beginning to form around this time, and being consumed at the time of ignition. 3-pentanone and 2-pentene, however, were observed as soon as the EOC. A prepared mixture of 3-pentanol/'air' was also analyzed, which confirmed that no reactivity occurred prior to injection into the RCM and that the detected alkenes were not impurities in the original fuel.

Three species originating from 3-pentanone were also identified: 1-pentene-3-one, 2-methyltetrahydrofuran-3-one, a cyclic ether produced by 3-pentanone, and 2-methyl-3-pentanone, a recombination product of the 3-pentanon-2-yl radical with a methyl radical. The last two were not included in the Chatterjee et al. mechanism. 2-methyl-3-pentanone and methyl-vinyl-ketone mole fractions were below the quantification limit, however potential routes to their formation were still incorporated into the mechanism.

The graphs in Fig. 3 are presented without normalization of the time with regards to the IDT. The mechanism developed in this study accurately predicts both the IDT and the evolution of the fuel's mole fraction, unlike the Chatterjee mechanism, which overestimates the IDT by 43 % and misrepresents the evolution of the mole fractions qualitatively as well as quantitatively, as also demonstrated by the plot as a function of the normalized time displayed in Fig. S4 of the SM. The Chatterjee mechanism predicts that fuel conversion primarily occurs at the point of ignition, which consequently results in the delayed formation of species. Another notable difference in the predictions of both models is the branching between products originating from different pentanolyl
<figure><img src="image_9.png" /><figcaption>Fig. 4. Influence of hydrogen addition on the IDTs of 3-pentanol at different pressures and E.R. = 1. Solid circles: measured IDT, lines: simulated IDTs.</figcaption></figure>

radicals. This could originate from an improved prediction of the branching ratio in the initial H abstraction reactions in the current model. This demonstrates the added value of RCM sampling experiments for the validation of kinetic models dedicated to high-pressure, close dilution to air, conditions.

The present mechanism successfully captures the overall trends for 3-pentanone, propanal, and 2-butanone, though it overestimates their peak production by a factor of 2 for propanal and 3-pentanone and 1.6 for 2-butanone. Additionally, it provides an accurate prediction of the evolution of acetaldehyde, 1-pentene-3-ol, 1-pentene-3-one, and 2-methyl-tetrahydrofuran-3-one, demonstrating strong agreement with the experimental data. However, the mole fraction of 2-pentene remains nearly constant throughout the ignition delay, a trend that the model does not fully capture, indicating the possibility of missing reaction pathways. Additionally, the model predicts a peak in ethylene production just before ignition, which was not observed experimentally, likely because of limitations in the experimental sampling time resolution.

Additional validation plots of the current model on the experimental

data from Carbonnier et al. can be found in Figs. S7 of the SM. They demonstrate that while the agreement of the current model is superior on the hereby reported experimental database, the agreement is also at least comparable, and sometimes superior than for the Chatterjee model on other experimental data.

## 3.2. 3-pentanol/hydrogen blends

The IDTs of 3-pentanol were also measured in blends with hydrogen in different proportions: 25 % and 50 %mol, at different pressures: $p_c$ = 20 and 15 bar, and at equivalence ratios of 1 and 0.5, and are presented in Fig. 4 and S5 of the SM, respectively.

At stoichiometric conditions, as the hydrogen proportion in the blend increases from 0 to 50 %, the IDTs increase. The model predicts this trend qualitatively; however, with 50 % hydrogen, it underestimates the IDTs by up to 50 % at the lowest temperatures. In these conditions, the measured IDTs are above 200 ms, which is the commonly accepted validity limit of the adiabatic core gas hypothesis. While this can explain part of the observed discrepancy, this also suggests that improvements in the description of the hydrogen kinetics at the lowest temperatures may be needed.

Under fuel-lean conditions, below 800 K, the addition of hydrogen leads to the increase of the IDTs at 20 bar, while no ignition was observed at 15 bar. Above 800 K, the IDTs are minimally affected by hydrogen addition, as captured by the model. However, as the hydrogen content increases, the discrepancy between simulated and experimental IDTs reaches 50 % at lower temperatures, where the measured IDTs are very long (~415 ms) and above the commonly accepted limit of the adiabatic core hypothesis. The qualitative trend is however captured in all cases, allowing the use of the kinetic model for further analysis.

## 3.3. Pathway and sensitivity analyses

Reaction pathway and brute-force sensitivity analyses are presented with and without hydrogen in Figs. 5 and 6, in conditions relevant to sampling experiments. The detected species relevant to 3-pentanol are illustrated in green in Fig. 5a. The detected products originating from 3-pentanone are highlighted in a dedicated reaction pathway analysis in Fig. S6 of the SM.

Hydrogen abstraction from 3-pentanol generates four distinct radicals: primary, secondary, tertiary, and alkoxy radicals. Due to the molecular structure of 3-pentanol, its decomposition predominantly yields secondary radicals, as the secondary carbon site has four hydrogen atoms available for abstraction, whereas the tertiary site only has one. Although primary carbon sites contain the highest number of hydrogen atoms (six), primary radicals are not preferentially formed due to the higher BDE of primary C-H bonds. Among the four radicals, the alkoxy radical formation is the least favorable because the O-H bond has the highest BDE. Of the radicals produced, the secondary and primary radicals play a crucial role in chain-branching, leading to the regeneration of a radical pool. They also take part in chain-propagation by yielding pentenols PENT3OH-D1 and PENT3OH-D2 along with hydroperoxyl radicals. The OOOH radical formed during branching can also decompose to give alkoxy and hydroxyl radicals. The tertiary radical 3-pentanol-3-yl forms 3-pentanone and a hydroperoxyl radical, and 3-pentanone then follows the reaction pathways shown in Fig. S6 of the SM. The 3-pentanoxyl radical decomposes into propanal and an ethyl radical.

Fig. 5b. highlights the modification of the relative importance of the reaction pathways associated with the $H_2$ submechanism in the case of blending with pentanol. Notable points include the reversal of the conversion of H into $H_2$ in the case where hydrogen is the fuel, where the abstraction of an H atom by a hydroxyl radical results in the formation of H. This atom is readily converted into a hydroperoxyl radical, shedding light in the reduction of the reactivity towards ignition in the case where
<figure><img src="image_10.png" /><figcaption>**Fig. 5.** Reaction Pathway analysis at φ = 1, pc = 20 bar, T~C~ = 735 K and 10 % fuel consumption (a) performed for 3-pentanol oxidation, following 3-pentanol. The quantified species in the sampling experiments are drawn in green. (b) following H~2~ in the pure pentanol (black numbers) and pentanol/H~2~ blend (blue italic numbers) cases. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)</figcaption></figure>

H₂ is added. The sources of ÓH, in this case, primarily lie in the pentanol submechanism, with the exception of a small but significant contribution by the decomposition of H₂O₂.

The sensitivity analysis shown in Fig. 6 suggests that the formation of the 3-pentanol-3-yl tertiary radical inhibits the reactivity towards ignition. This is due to the chain-propagation leading to the formation of 3-pentanone and HO'O radicals, rather than causing chain-branching of the radicals through the O₂ addition – internal isomerization pathways. The low-temperature reactivity is primarily driven by the primary and secondary 3-pentanolyl radicals (PENT3OH-1 and PENT3OH-2, respectively). Their initial formation exhibits a negative sensitivity coefficient, highlighting their critical role in promoting reactivity at such conditions. In particular, the reactions with the highest impact on increasing reactivity are those involved in the low-temperature chain-branching of the secondary 3-pentanolyl radicals. Conversely, the reaction that most inhibits reactivity is the decomposition of 2-peroxy-3-pentanolyl (TC5H10OH-2O2) into HO'O radicals and 2-pentene-3-ol (PENT3OH-D2). This pathway competes with the chain-branching pathway that primarily drives reactivity, ultimately reducing the overall reactivity. The better performance of the current model over the Chatterjee et al. model could therefore be attributed to incorrect branching ratio favoring the H-abstraction on the primary carbon, consequently overestimating the low-temperature branching pathways. This could be caused by the use of analogies with alkanes for the rate parameters of the

H abstractions by ÓH and HO'O on the primary carbon sites.

To understand the impact of hydrogen on IDTs, the sensitivity analysis was also conducted for a 50/50 3-pentanol/H₂ blend mixture at the same conditions. As already hinted by the reaction pathway analysis, the results confirm that hydrogen has an inhibiting effect on reactivity, primarily through the reaction H₂ + ÓH → Ḣ + H₂O, which has a negligible sensitivity coefficient in the absence of hydrogen but becomes significant when hydrogen is present, as observed for other species [6]. At these pressures, the Ḣ atoms produced by this reaction predominantly contribute to the formation of HO'O radicals via Ḣ + O₂ + M → HO'O + M, a reaction that exhibits twice the positive sensitivity coefficient compared to the hydrogen-free case. ÓH radicals play a crucial role in the initial fuel conversion during low-temperature combustion, while HO'O radicals are likely to readily yield H₂O₂, which will not yield back ÓH radicals sufficiently to cause ignition at these temperatures. This explains the observed reduction in reactivity when hydrogen is added to the fuel mixture.

## 4. Conclusions

This study presents a comprehensive experimental and modeling investigation into the low-temperature combustion of 3-pentanol and its blends with hydrogen using a rapid compression machine (RCM). Ignition delay times (IDTs) were measured over a wide range of conditions,
<figure><img src="image_11.png" /><figcaption>**Fig. 6.** Brute-force sensitivity analysis carried out on the ignition delay time of 3-pentanol/"air" and 50/50 mol 3-pentanol/H₂/"air" mixtures at φ = 1, p_c = 20 bar, T_c = 735 K.</figcaption></figure>

and time-resolved speciation experiments were conducted to capture key intermediate species, providing an in-depth validation database for future models. A new kinetic mechanism was developed by means of *ab initio* calculations and analogies. It demonstrates strong agreement with the original and existing experimental data and improves upon existing models.

The results highlight the importance of the correct description of the branching ratio for the initial H atom abstraction reactions from the fuel, as well as of the 3-pentanone formation and decomposition pathways, on the ignition delay. The addition of hydrogen is shown increase the ignition delay, particularly at lower temperatures. With help from reaction pathway and sensitivity analyses, this effect is shown to originate from the preferential reaction of H₂ with OH, leading to H atoms, then hydroperoxyl radicals, thereby slowing the reactivity towards ignition.

### CRediT authorship contribution statement

**Marwa Saab:** Writing – original draft, Methodology, Investigation, Formal analysis. **Rana Shebly:** Investigation. **Yann Fenard:** Methodology, Conceptualization. **Guillaume Vanhove:** Writing – review & editing, Supervision, Resources, Project administration, Funding acquisition, Data curation, Conceptualization.

### Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

### Acknowledgements

The CaPPA project (Chemical and Physical Properties of the Atmosphere) is funded by the French National Research Agency (ANR) through the PIA (Programme d'Investissement d'Avenir) under contract "ANR-11-LABX-0005-01" and by the Regional Council "Hauts-de-France" and the "European Funds for Regional Economical Development" (FEDER). The authors thank the Région Hauts-de-France, the Ministère de l'Enseignement Supérieur et de la Recherche and the

European Fund for Regional Economic Development for their financial support to the CPER ECRIN program. The authors finally acknowledge the use of the RWTH Aachen University High-Performance Computing (HPC) Cluster, which provided the computational resources necessary for conducting the *ab initio* calculations. The support from the HPC team is greatly appreciated.

## Appendix A. Supplementary data

Supplementary data to this article can be found online at https://doi.org/10.1016/j.fuel.2025.137704.

### Data availability

All the data has been provided as SM

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