n-Heptane Blends in a Motored

Article pubs.acs.org/EF

Autoignition of Alcohol/C7-Esters/n‑Heptane Blends in a Motored Engine under HCCI Conditions John R. Agudelo,*,† Magín Lapuerta,‡ Orin Moyer,§ and André L. Boehman∥ †

Departamento de Ingeniería Mecánica, Universidad de Antioquia (UdeA), Calle 70 No. 52-21, Medellín 055420, Colombia Universidad de Castilla-La Mancha, Av. Camilo José Cela s/n, Ciudad Real 13071, Spain § Department of Energy & Mineral Engineering, EMS Energy Institute, The Pennsylvania State University, University Park, Pennsylvania 16802, United States ∥ Department of Mechanical Engineering, The University of Michigan, 2350 Hayward Street, Ann Arbor, Michigan 48109-2125, United States ‡

ABSTRACT: Autoignition characteristics of neat fuels consisting of ethanol, n-butanol, n-heptane (nC7), methyl hexanoate (mhx), and methyl 3-hexenoate (m3h) and their binary and ternary blends were studied in a motored Cooperative Fuels Research (CFR) engine under homogeneous charge compression ignition (HCCI) conditions. The equivalence ratio (ER) and intake temperature were fixed to 0.5 and 155 °C, respectively. Autoignition characteristics were studied through the evolution of CO/CO2 emissions and heat release rate profiles as the compression ratio (CR) was increased. The critical compression ratio (CCR), defined as the CR at which the onset of the autoignition occurs and identified here as the CR where CO2 increases dramatically, varied from 4.8 for neat nC7 to 14.1 for ethanol, which exhibited the lowest reactivity of all fuels. C7 methyl esters exhibited lower CCR than both alcohols, confirming the nonlinear relationship between autoignition trend and cetane number. Results showed that mxh exhibited cool-flame behavior, while m3h and the alcohols did not, and that both alcohols (ethanol to a greater extent than n-butanol) suppressed the low-temperature oxidation reactivity (LTOR) to a higher extent than the C7 methyl esters. Ethanol reduced the autoignition tendency of binary blends with nC7 by twice the extent of m3h and by triple the extent of mhx. The addition of ethanol of up to 20% induced a roughly linear effect on the autoignition characteristics in both binary and ternary blends, which confirms that the suppressing effect of ethanol is stronger than that of m3h addition. For a fixed alcohol content in the blend, the autoignition characteristics are highly nonlinear with the blend composition, and they are less sensitive to the least reactive components in the blend. Finally, it was found that, regardless of the fuel components, blends with similar autoignition characteristics exhibited similar thermal histories (low-temperature heat release, LTHR, rate peaks and onset of LTHR).

1. INTRODUCTION

Some studies have already considered the low-temperature reactivity of fatty acid methyl esters (FAMEs) as surrogates for practical biodiesel fuels.4−16 Several successful approaches including experimental and modeling results have been reported for different FAME molecular compositions, mainly varying their chain length as well as the location and number of the carbon atom double bonds (CC). Earlier research about FAME chemistry focused on the short-chain ester methyl butanoate as a potential biodiesel surrogate mainly due to low computational cost. Gail et al. observed, however, that methyl butanoate does not exhibit any significant cool-flame or negative temperature coefficient (NTC) behavior typical of long-chain biodiesel counterparts, and concluded that it should not be considered a suitable biodiesel surrogate fuel.4 Westbrook et al.5,6 concluded that the CC double bond is an obstacle to the peroxy chemistry, which is predominant in the oxidation pathways in the low-temperature regime. They found that the presence of CC double bonds induced allylic fuel radicals, which decreased the cetane number and quenched most of low-temperature oxidation reactions by inhibiting

Ethanol and n-butanol, when they are produced from lignocellulosic or waste materials, constitute an interesting option to introduce a renewable fraction in diesel fuels, not only for conventional engines, but also for those incorporating new combustion concepts based on low-temperature oxidation strategies. Additionally, they provide some fuel oxygen content, which is helpful to reduce soot formation and enhance soot oxidation, thus decreasing engine-out particulate matter emissions.1−3 Despite the widespread use of diesel particulate filters (DPF), particulate emissions are still an issue due to wear and fuel consumption during regeneration. The renewable energy and oxygen content supplied by alcohols would be additional to those already included in many diesel fuels containing some biodiesel (with methyl esters as major components), which is becoming a common component of diesel fuels in an increasing number of countries. It is foreseen that both alcohol and esters could play a significant role in future engine designs. Among the alcohols, ethanol and nbutanol have the most promising biological production routes. Among methyl esters, the contribution of unsaturated esters is essential to improve cold-flow properties, which are of major concern especially in cold climates. © 2017 American Chemical Society

Received: January 6, 2017 Revised: February 13, 2017 Published: February 14, 2017 2985

DOI: 10.1021/acs.energyfuels.7b00059 Energy Fuels 2017, 31, 2985−2995

Article

Energy & Fuels isomerization. Herbinet et al.,7 by comparing the lowtemperature oxidation reactivity (LTOR) of methyl-5-decenoate and methyl-9-decenote in a jet-stirred reactor (JSR), proposed that LTOR was facilitated when the location of the CC double bond was closer to the methyl group (methyl-9decenoate > methyl-5-decenoate) because the saturated proportion of the aliphatic chain favored isomerization reactions. Zhang et al. made similar observations comparing the autoignition behavior of methyl nonenoate, methyl 2nonenoate, and methyl 3-nonenoate, where the presence of the CC bond suppressed LTOR, and the further the CC bond is along the aliphatic chain, the greater is the suppression of LTOR.17 In a recent work, Rodriguez et al.8 provided useful experimental (JSR) and modeled data of the low-temperature reactivity of C18 methyl esters (methyl stearate C18:0 > methyl oleate C18:1 ≫ methyl linoleate C18:2). They confirmed that also for long-chain length FAMEs, the reactivity significantly decreased with the number of CC double bonds. However, it is interesting how at high temperature neat methyl esters containing CC double bonds exhibit oxidation reactivity similar to that of their saturated counterparts,9−11 although the unsaturated methyl esters promote higher sooting tendency, because their oxidation produces more olefins.9 This trend was also observed by Benjumea et al.12 who reported higher particulate matter emissions for highly unsaturated biodiesel when compared to highly saturated biodiesel in an automotive diesel engine. Combustion of model ester compounds and practical biodiesel fuels has been demonstrated to alter the oxidative reactivity of soots formed during diesel and diffusion flame combustion. The first such report was by Boehman et al., who observed more reactive soots and lower regeneration temperatures for diesel particulate filters when operating a diesel engine on soybean-derived biodiesel fuel blends.18 Recently, Barrientos et al. demonstrated that this impact of ester compounds on soot reactivity is amplified when one burns shorter chain esters, demonstrating that as the chain length is shortened the impact of the ester moiety on soot nanostructure and reactivity increases.19 Several researchers have shown that methyl hexanoate (C7H14O2 denoted mhx henceforth), with 6 carbon atoms in the aliphatic chain, might be the simplest ester exhibiting strong NTC behavior.13−15 The opposite has been reported for methyl 3-hexenoate (m3h, C7H12O2), another unsaturated methyl ester with 6 carbon atoms and a double bond in the middle of the aliphatic chain, which does not exhibit any low-temperature heat release (LTHR) due to the inhibition of the isomerization of peroxy radicals over the double bond.7,15,16 In a previous work, Zhang and Boehman15 identified the major oxidation pathways of mhx/nC7 and m3h/nC7 blends at low to intermediate temperatures. Experiments were carried out in the same modified CFR engine used in the present work, at an equivalence ratio of 0.25 and intake temperature of 155 °C. The exhaust gas analysis speciation by using GC−MS and GC− FID led them to confirm that mhx follows typical paraffinic low-temperature reactions, and that m3h undergoes olefinic oxidation reactions. In this research, the selection of mhx and its unsaturated counterpart m3h for a blending study with nC7 and ethanol or n-butanol intends to simulate the autoignition behavior of relevant blend ratios while using surrogates for actual biodiesel and diesel fuels. The relevance and challenges of alcohol combustion at elevated pressure and low temperature were pointed out by Sarathy et al.20 They highlighted the necessity for more

fundamental studies to understand the effects of alcohol blending with multicomponent surrogates and real hydrocarbon fuels. The lack of cool flame behavior, also called twostage heat release, of ethanol in the NTC regime was studied by da Silva et al.,21 who combined computational chemistry, variational transition state theory, and Rice−Ramsperger− Kassel−Marcus (RRKM)/master equation to simulate the reaction of the R-hydroxyethyl radical with O2 (3P) in the autoignition and oxidation of ethanol. They found that the Rhydroxyethyl + O2 reaction produced an adduct with short lifetime (α-hydroxy-ethylperoxy), which proceeds to acetaldehyde + HO2 precluding the oxidation of the alkylhydroperoxide radical, that is crucial for the low-temperature oxidation to take place. This finding has also been extended by other researchers to explain the barely noticeable two-stage heat release at low temperature of n-butanol, who argued that the reaction of O2 with the radical site on the carbon next to the OH group proceeds to the formation of aldehyde + HO2 instead of promoting isomerization reactions.22,23 Yang et al. concluded that for neat n-butanol, H atom abstraction from the γ-carbon was more prone to low-temperature reactivity, while the H atom abstraction from the α-carbon was a major inhibiting reaction.24 For n-butanol/nC7 blends, there is a general agreement that the radical pool generated from the lowtemperature oxidation of nC7 promotes autoignition with a noticeable two-stage heat release behavior, which decreases with increasing n-butanol concentration in the blend.23,25−27 Saisirirat et al.28 proposed that the addition of n-butanol or ethanol to nC7 reduced the production rates of OH radicals, which decreased the cool-flame intensity. They found that ethanol addition reduced the OH production to a higher extent than did n-butanol. Dagaut and Togbé29 observed that the oxidation of the most reactive fuel at low temperature produced radicals that can initiate the oxidation of ethanol, even under conditions where the ethanol alone was not reactive. This conclusion was extended to binary ethanol/methyl octanoate blends30 and n-butanol/methyl octanoate blends as well as to ternary surrogates of real Fischer−Tropsch/biodiesel/ethanol fuel blends.31 The authors also observed that methyl octanoate/ ethanol blends were more prone to emit acetaldehyde (74 ppm) than the corresponding methyl octanoate/n-butanol blends (46 ppm) oxidized in a JSR.32 The autoignition characteristics under diesel-like conditions of both ethanol and n-butanol, neat or blended with nC7 have been studied by several researchers through different experimental and modeling approaches. Experiments have been carried out mainly by using jet-stirred reactors, rapid compression machines (RCM), shock tubes (ST), modified CFR single-cylinder engines, and homogeneous charge compression ignition (HCCI) engines. Although most of them have included intermediate species quantification to provide a useful database for tuning up chemical kinetics models, some others have studied only the heat release characteristics in the low to intermediate temperature regime.33−35 Ternary alcohol/biodiesel/diesel fuel blends can be attractive in the near future because they contribute to increasing the renewable fraction on the diesel fuel, reduce particulate matter emissions, and allow increasing the solubility of the alcohol over a wider range of concentrations than alcohol/diesel fuel blends alone. In this work, a fuel matrix covering ternary blends of alcohol (ethanol or n-butanol)/C7 esters (mhx or m3h)/ nC7, and binary blends of alcohol/nC7 or alcohol/C7 esters, 2986


Article

Energy & Fuels Table 1. Relevant Properties of Pure Fuels formula molecular weight density at 15 °C (g/cm3) lower heating value, LHV (kJ/kg) boiling point, °C air/fuel stoichiometric ratio cetane number

nC7

methyl hexanoate

methyl 3-hexenoate

n-butanol

ethanol

C7H16 100.23 0.695 44566 98 15.18 53a

C7H14O2 130.18 0.885 29912.7 151 10.09 24b

C7H12O2 128.17 0.913 29477.1 169 10.24 11.7c

C4H10O 74.12 0.809 33090 117.7 11.19 16a

C2H6O 46.07 0.789 26830 78.3 9.75 8d

a

Measured. bTaken from the compendium of experimental cetane numbers by Yanowitz et al.42 cTaken from Lapuerta et al.43 dTaken from refs 44 and 45.

Table 2. Test Fuels Matrixa

a

pure fuels

binary alcohol blends

mhx blends

m3h blends

n-heptane (nC7) ethanol (E) n-butanol (Bu) methyl hexanoate (mhx) methyl 3-hexenoate (m3h)

5E-95nC7 10E-90nC7 15E-85nC7 20E-80nC7 20Bu-80nC7

30mhx-70nC7 70mhx-30nC7 5E (30mhx-70nC7) 10E (30mhx-70nC7) 15E (30mhx-70nC7) 20E (30mhx-70nC7) 20E (70mhx-30nC7) 20Bu (30mhx-70nC7) 20Bu (70mhx-30nC7)

30m3h-70nC7 70m3h-30nC7 5E (30m3h-70nC7) 10E (30m3h-70nC7) 15E (30m3h-70nC7) 20E (30m3h-70nC7) 20E (70m3h-30nC7) 20Bu (30m3h-70nC7) 20Bu (70m3h-30nC7)

All fuel blends were prepared on a volume basis (% v/v). pronounced cool-flame behavior.15 Among the contents tested for methyl esters, 30% by volume represents a realistic future scenario of biodiesel penetration (beyond 2020 targets)39 and a currently typical scenario of biodiesel use in captive fleets for public and commercial transport. In fact, blends from 20% to 30% are subjected to standard EN 16709 in Europe.40 Blending at 70% by volume does not represent any foreseeable scenario, but is useful for a better observation of the trends when the methyl ester content is varied. Ethanol and n-butanol contents were chosen to observe their suppressing effect on the coolflame behavior of the biodiesel/diesel surrogates, and also because of recent interest in blending both alcohols with diesel to incorporate further renewable components. Finally, alcohol contents were limited to no more than 20% by volume because higher contents of any of these alcohols in diesel blends would not be feasible or would require modifications in the engine design due to weak miscibility, increase in flash point, and significant loss in heating value and cetane number.41 2.2. Engine Setup. A modified motored CFR octane rating engine with a compression ratio (CR) range of 3.8−15 was used in this work. A gasoline direct injection (GDI) injector was installed in the upstream part of the intake manifold at a height of about 1.5 m above the intake valve. Both the injector and the cooling jacket temperatures were kept constant at 90 °C using chiller-circulators. The equivalence ratio (ER) = 0.5 was obtained by a calibration of the fuel supply system (injector and pulse counter) for each fuel tested and by drying the air entering the engine with a chiller (set point temperature of 5 °C). The intake manifold of the CFR engine was heated to a constant temperature of 155 °C via electric heaters to allow liquid fuels to be fully vaporized and well-mixed with intake air before entering the engine cylinder. Liquid fuels were injected at a pressure of 4.83 MPa and a frequency of 10 Hz with the CFR engine operating at 600 rpm. The instantaneous in-cylinder pressure was recorded via a Kistler 6052B piezoelectric pressure transducer installed in the cylinder head and a Kistler 5011B charge amplifier. Figure 1 shows schematically the engine test setup. Fourty pressure curves were registered in each test to guarantee confidence in the heat release analysis. That number of pressure curves led to a coefficient of variation (COV) in indicated mean effective pressure of under 3% for all of the tests performed. The instantaneous piston position was determined using an Accu-Coder angular encoder with a resolution of 3600 pulses/revolution (resolution of 0.1°) coupled to the crankshaft. High and low speed

was tested in a modified CFR single-cylinder Octane Rating engine under gaseous-phase premixed charge conditions at an equivalence ratio of 0.5 and an intake temperature of 155 °C. Although most of the combustion chemistry community has moved toward experiments and modeling of higher molecular weight esters, which more closely resemble practical biodiesel fuels, this study uses the same C7 esters as Zhang and Boehman15 to observe how the autoignition characteristics were affected by modifying both the equivalence ratio and the use of alcohols in the blends. In addition, the low volatility of C7 esters favored the intake of a gas-phase premixed charge, which was adopted in the experiments. The CCR, determined through the variation of the CO/CO2 molar concentration, and apparent heat release rate were compared to determine the combined impact of alcohol type and the CC double bond on autoignition characteristics of complex fuel blends in the low-temperature regime. Although intermediate species were not analyzed, this work explores the autoignition process of alcohols and methyl esters blended with diesel fuel under premixed engine conditions. Such premixed conditions might have practical interest in novel engine technologies such as HCCI engines, or early or late injection conditions.

2. EXPERIMENTAL SECTION 2.1. Test Fuels. In the present study, five pure fuels were examined: ethanol, n-butanol, n-heptane (nC7), and two C7 methyl ester biodiesel surrogates, methyl hexanoate (mhx) and methyl 3hexenoate (m3h) (Table 1). The pure fuels were blended on a volume basis (% v/v) into 23 formulations (9 binary and 14 ternary blends) and are represented in Table 2. The nC7 was chosen as a surrogate for paraffins (major components of diesel fuels) because it has betterknown chemistry than other larger paraffins, and because it has a cetane number similar to that of petroleum-derived diesel of around 53. Similar tests with other important diesel components such as naphthenes36,37 or aromatics38 would be helpful to complete this study. The two esters were chosen to study the effect of unsaturation on cool-flame behavior. Specifically, mhx was selected because it has a 2987


Article

Energy & Fuels

Table 4. Repeatability Study for Critical Compression Ratio with nC7 ER

intake temp (°C)

CCR

average

SD

0.5

120

5.23 5.27 5.24 4.8 4.82 4.8 4.81 6.39 6.36 6.4 6.38

5.25

0.02

4.81

0.01

6.38

0.02

155

0.25

120

Figure 1. Schematic of the motored engine test setup. data were acquired using a LabView-based acquisition system. On the basis of the obtained cylinder pressure data, apparent heat release rate and bulk in-cylinder temperature calculations were performed using the zero-dimensional single-zone model developed by Heywood.46 Table 3 shows a summary of engine operating conditions, and detailed engine setup information can be found in refs 15, 37, and 47.

Table 3. Engine Operating Conditions engine speed (rpm) equivalence ratio intake temperature (°C) intake pressure (MPa) fuel injection pressure (MPa) cooling jacket temperature (°C) injector temperature (°C)

Figure 2. Heat release profile of two separate runs for nC7 to demonstrate repeatability and sensitivity.

600 0.5 155 0.1 4.8 90 90

CR (0.2 units) over the heat release rate curves is more noticeable than the difference induced by measuring at different days. 2.4. Determination of the Critical Compression Ratio (CCR). For consistency in the determination of the critical compression ratio (i.e., CR at which onset of autoignition occurs), CO and CO2 emissions were monitored via an AVL CEB II emissions analysis system. Just after the onset of autoignition, CO decreased drastically, while CO2 dramatically increased, which means that at this equivalence ratio, CO is fully oxidized to CO2. Online monitoring of CO emissions has shown to be a good indicator of the global oxidation reactivity of a fuel during the compression ratio sweep in a motored engine under premixed homogeneous charge, because CO is largely formed in localized regions of the combustion chamber in which low-temperature reaction pathways are active.48 CO is mainly formed through a consecutive hydrogen abstraction from formaldehyde (CH2O), which is mostly formed during low-temperature heat release.49 Furthermore, the global oxidation reactivity of a fuel under HCCI-like conditions can be identified by the CCR where the CO emission begins to decline, also resulting in a dramatic increase of CO2 emission. Figure 3 shows CCR for nC7 at ER = 0.5 and an intake temperature of 155 °C. The autoignition event was chosen at the compression ratio where the first exponential increase in CO2 was detected as indicated by the dashed vertical line. An alternative method has been proposed by Kang et al.,48 who identify the CCR where the CO emissions begin to decline. A small difference of less than 0.1 CR is observed in Figure 3 when comparing both criteria.

The compression ratio (CR) of the modified CFR engine was gradually increased in a stepwise manner from the lowest point (CR = 3.8) to the point where the onset of high-temperature heat release (HTHR) occurred for each fuel tested. The CR was increased in 0.01 steps from the vicinity of the onset of autoignition. The pressure history, and hence the average in-cylinder bulk temperature and heat release rate, were registered for each fuel at every single step from the lowest CR to the CCR. 2.3. Repeatability and Sensitivity Analysis of Engine Test Rig. A repeatability study was carried out with nC7. Eleven tests were conducted on different days (in some cases different months) and at different environmental conditions for two equivalence ratios (ER) of 0.5 and 0.25 and two different air intake temperatures of 120 and 155 °C. Experimental data at ER = 0.25 were included in the repeatability study to validate the feasibility of the modified CFR engine to carry out autoignition studies under HCCI conditions at equivalence ratios different from those at which the experiments were conducted. Repeatability results are presented in Table 4. The uncertainty of the CCR was found to be ±0.02 units. The heat release profiles for two different tests for nC7 at an ER = 0.5 and intake temperature of 155 °C are shown in Figure 2 to demonstrate repeatability. The dashed black lines correspond to the heat release rate profiles at CR of 4.4, 4.6, and 4.8, named run #1. The solid black lines correspond to the heat release rate profiles for the same compression ratios, but for a separate test carried out on different days (run #2). As can be observed, the effect of a small change in the

3. RESULTS AND DISCUSSION 3.1. Autoignition of Neat Fuels. CO emissions are a good indicator of low-temperature oxidation, because CO is produced mainly by decomposition of the aldehydes formed via O2 addition to the alkyl chain.50−52 The higher are the CO emissions, the higher is the oxidation reactivity. Figure 4 2988


Article

Energy & Fuels

Figure 3. Criterion to determine the critical compression ratio.

Figure 5. Heat release rate profiles for all neat fuels at their respective CCR.

the carbon next to the OH group favoring the reaction of aldehyde + HO2 over the isomerization reactions.21−23 The reactivity in the low-temperature regime from both nC7 and mhx can also be analyzed from the evolution of LTHR (cool-flames) for different compression ratios. As expected, both fuels exhibited cool-flame behavior, as inferred from the two-stage ignition process shown in Figure 6a and b.15 The maximum peaks of heat release rate from nC7 were more than double those from mhx, due to the high heating value of the former. This was quantified by comparing the ratio between the integral of the LTHR rate (also called cumulative LTHR) with respect to the integral of the total heat release rate (cumulative HRR) at the CCR, as proposed by Szybist et al.47 This fraction was 14.3% for neat nC7 and 6.1% for mhx. Both fractions were higher than those reported by Zhang and Boehman15 for the same fuels due to differences in the equivalence ratios (0.25 in their work against 0.5 here). As both in-cylinder pressure and temperature increase, the heat release peak is advanced with respect to top dead center until the high temperature heat release (HTHR) takes place, and in the case of nC7, the magnitude of the LTHR peak increased. The higher reactivity in the low-temperature regime of paraffins versus saturated methyl esters containing the same number of carbon atoms has been thoroughly discussed elsewhere.15 3.2. Impact of Alcohols and C7 Methyl Esters on the Autoignition of Binary Blends. LTOR, quantified by CCR (Figure 7), CO emissions, and LTHR, of either ethanol or nbutanol blended with nC7 or with C7 methyl esters decreased as the content of alcohol increased (Figure 8a−c). Although peak CO emissions of alcohol blends in nC7 barely decreased, the onset of CO shifted toward higher values of CR (Figure 8a). These results are in agreement with Saisirirat et al.28 who found via a JSR that the addition of ethanol or n-butanol to nC7 reduced the production rates of OH radicals, and that ethanol addition reduced the OH production to a higher extent than did n-butanol. The oxidation of nC7, which is the most reactive of the fuels tested, forms radicals that initiate the LTOR of blends with both alcohols and C7 methyl esters (Figure 8a and b).29,30 It was confirmed that n-butanol was more reactive than ethanol, because at the same alcohol blend of 20% in either nC7, mhx, and m3h, the CCR was lower for n-butanol (Figure 7), and the onset of CO emissions took place to a lower CR for n-butanol than for ethanol (Figure 8a and c). With regard to LTHR, Figure 8d confirms the suppressing effect on coolflames as alcohol concentration was increased in the blend with

Figure 4. Critical compression ratio of neat fuels. Solid lines indicate CO emissions (left). Dashed lines indicate CO2 emissions (right).

compares the low-temperature oxidation of the five neat fuels through the evolution of CO and CO2 emissions along the compression ratio. The oxidation reactivity of neat fuels, from the highest to the lowest, followed the order: nC7 ≫ mhx > m3h > n-butanol ≫ ethanol, which is in agreement with the decrease in cetane number shown in Table 1. As mentioned above, another parameter used here to analyze the autoignition process is the CCR. The decrease in reactivity increases the CCR and delays the onset of CO emissions (they become noticeable for higher values of the CR), as shown in Figure 4. The CCR varied from 4.8 for neat nC7 to 14.1 for ethanol, which exhibited the lowest reactivity of all fuels. CO emissions from nC7 and mhx exhibited a plateau behavior prior to autoignition, which can be representative of a negative temperature coefficient (NTC) or cool-flame behavior, as a manifestation of increased low-temperature reactivity. On the other side, m3h, n-butanol, and ethanol did not exhibit this CO plateau region. This can also be seen in the heat release rate profiles for each fuel at its respective CCR shown in Figure 5. The low-temperature heat release (LTHR), which is indicative of cool-flames, was only appreciable for nC7 and mhx, but it was not noticeable for the other neat fuels. Similar reactivity results for these neat fuels have been reported elsewhere with jet-stirred reactors,16,28 rapid compression machines,23,24,26,53 or homogeneous charge compression ignition engines (HCCI)28,33,54 at equivalence ratios and temperatures different from those used in this work. In the case of m3h, it is apparent that the double carbon bond suppresses the LTHR, leading to a delay in onset of autoignition due to inhibition of isomerization reactions of peroxy radicals.5−7,15,16 In the case of both alcohols, the inhibiting effect on LTHR has been explained by the preferential reaction of oxygen with the radical site on 2989


Article

Energy & Fuels

Figure 6. Heat release rate (HRR) profiles at different compression ratios for nC7 (a) and mhx (b).

characteristics, the thermal history (peaks intensity and onset of LTHR rate) corresponding to CR previous to CCR are coincident as shown in Figure 9a−c for 30m3h/70nC7, 15ethanol/85nC7, and 20n-butanol/80nC7, respectively. A similar comparison can be made if nC7 is replaced with 30% of saturated methyl ester, mhx. In this case, because it is more reactive than m3h, only 10% of ethanol would be required instead to approximate their autoignition characteristics (Table 5, last two entries). This means that ethanol reduces the autoignition tendency twice as much as unsaturated C7 methyl ester and three times as much as the saturated methyl ester. If one replaces the C7 methyl esters with alcohols in blends with nC7, a proportionally lower content of the alcohol would be required to maintain the same autoignition tendency. The corresponding heat release rate profiles for 30mhx/70nC7 and 10ethanol/90nC7 are shown in Figure 10a and b, respectively. As with the previous binary blends, it was observed that the thermal histories were similar along the CR sweep up to CCR. 3.3. Impact of Alcohols on the Autoignition Characteristics of Ternary Blends. Results confirmed that ternary blends of alcohols/C7 methyl esters/nC7 followed the autoignition trends previously observed for neat fuels and binary blends: the addition of both alcohols suppressed lowtemperature reactivity, with ethanol being less reactive than nbutanol; mhx was more reactive than m3h, and both alcohols were less reactive than C7 methyl esters. However, the autoignition characteristics of the ternary blends were dramatically affected by differences in concentrations among the parent fuels. In Figure 11a and b, it can be observed that at 20% (v/v) alcohol content, the CCR of ethanol blends was always higher than that of butanol blends, regardless of the type of methyl ester used in the blend. However, this relative increase varied from 20% (when alcohols were blended with 70mhx/30nC7) to only 6% (when blended with 70mhx/ 30nC7) and to 11% (when blended with 70m3h/30nC7). This confirms that any change in the blend composition is much more noticeable when the blend has low reactivity (when methyl esters are the major components) than when it is very reactive (nC7 is the major component), consistent with the well-known nonlinearity of the relation between autoignition time and cetane number.55 Moreover, when the least reactive component (ethanol) is added to the blends, the reactivitysuppressing effect of ethanol dominates over the reactivity of all other components, to such an extent that when 20% ethanol is added, the CCR increases by around 20% over the initial CCR,

Figure 7. Critical compression ratio for binary blends with nC7.

nC7. The 20n-butanol/80nC7 blend exhibited a fraction of cumulative LTHR = 12.5%, which was higher, although very similar, than that of 20ethanol/80nC7 = 12.4%. No LTHR was observed for 20% alcohol (n-butanol or ethanol) blended with mhx, nor with m3h. The decrease in LTHR was expected with increasing alcohol content because alcohols, similar to m3h, suppressed cool-flame behavior, as previously shown in Figure 5, and the differences of the impact of the alcohols are consistent with their octane numbers. The C7 methyl esters/nC7 results, which are an extension of those previously presented by Zhang and Boehman,15 exhibited lower CO emissions, and at the same time their onset of CO shifted toward higher CR values, as both methyl ester contents in nC7 increased. This confirms that mhx was more reactive than m3h and that both C7 methyl esters were less reactive than nC7. This can also be seen through the higher CCR (Figure 7), the lower CO emissions, and the higher CR for the onset of CO emissions (Figure 8b) of m3h blends when compared to mhx blends. Results shown in Figure 7 additionally confirmed that both alcohols were less reactive at low temperature than both C7 methyl esters, even if they were used neat or blended with nC7, corresponding with their lower cetane number. From the results presented, five binary blends were selected for comparison (Table 5). In a first blend, nC7 was partially replaced with 30% of m3h. If an alcohol was used as replacing fuel instead, while keeping the autoignition characteristics, then only 15% in the case of using ethanol or 20% in the case of using n-butanol would be required (Table 5, first three entries). It can also be observed that under similar autoignition 2990


Article

Energy & Fuels

Figure 8. Autoignition characteristics of binary blends. (a) Alcohol in nC7, (b) C7 methyl ester in nC7, (c) 20% alcohol in binary blends, and (d) cumulative LTHR (%).

ethanol leads to a roughly linear decrease in the peak of CO emissions. The slope of this line is slightly higher for saturated methyl ester blends than for unsaturated methyl ester blends, and is considerably higher when the concentration of methyl esters is increased to 70% (as discussed below). From Figure 11a and b, it can be inferred a nonlinear tendency of the autoignition when alcohols are added in concentrations above 20% with both C7 methyl esters/nC7 blends. These nonlinear effects have been described as the combination of an energy effect (associated with the reduced heating value of alcohols), a chemical effect (associated with their reduced oxygen requirement), and a dilution effect (associated with their enhanced dilution derived from their higher autoignition time).56 The observation of similar thermal histories (LTHR rate peaks and the onset of LTHR) for binary blends with similar autoignition characteristics can be extended to ternary blends. Two blends with similar autoignition characteristics (similar CCR, CO emissions peak values, and CR of the onset of CO) were selected for comparison of their LTHR rates profiles: 15ethanol/(30mhx/70nC7) versus 20n-butanol/(30mhx/ 70nC7), as shown in Figure 12a and b. Although not shown here, similar results were obtained for 10ethanol/(30m3h/ 70nC7) versus 20n-butanol/(30m3h/70nC7) blends. This similarity could be explained by the reduction in the production rates of OH radicals when ethanol or n-butanol was added to the C7 methyl ester/nC7 blend, as observed by Saisirirat et al.28 for ethanol or n-butanol blended with nC7. As with binary blends, nC7, which is the most reactive of all components in the blend, still forms radicals that initiate the low-temperature reactions as can be seen in the LTHR rate profiles exhibited by ethanol/m3h/nC7 blends. This is interesting because, as

Table 5. Comparison of Binary Fuels with Similar Autoignition Characteristics blend

CCR

30m3h/70 nC7 15ethanol/85nC7 20n-butanol/80nC7 30mhx/70nC7 10ethanol/90nC7

5.76 5.53 5.46 5.3 5.3

peak CO emissions CR of the onset of CO 5270 5300 5200 5400 5300

4.5 4.5 4.6 4.3 4.4

regardless of the rest of the mixture components. This indicates that the alteration of autoignition characteristics induced by adding ethanol in these ternary blends may not be affected by the CC double bond of the m3h. However, if butanol (slightly more reactive than ethanol) is added, the increase in CCR becomes somewhat dependent on the reactivity of the rest of the components. Figure 11 shows the CO emissions for ternary blends in 30mhx/70nC7 (Figure 11a) and 30m3h/70nC7 (Figure 11b). The similar peak values of CO emissions at the CCR of these blends indicate that at high temperature, ternary blends containing 30% of C7 methyl ester with a CC double bond exhibit oxidation reactivity similar to that of the saturated counterpart. This result was previously observed elsewhere, but for neat methyl esters.9−11 However, in the low-temperature regime, it was clear that the CC double bond retarded autoignition, because the onset of CO emissions started at a higher CR for all m3h blends (Figure 11d) in comparison with the respective mhx blends (Figure 11c). For example, the CO onset of 30m3h-70nC7 occurred at a CR = 5.7, while the CO onset of 30m3h-70nC7 occurred at CR = 5.2. The addition of 2991


Article

Energy & Fuels

Figure 13 shows the effect of blending 20% of alcohol, either ethanol or n-butanol, with a binary blend containing 70% of mhx or m3h with 30% nC7 on CO emissions. Results confirmed the reactivity trend previously observed for ternary blends containing 30% of mhx or m3h with 70% nC7: also in this case, n-butanol contributed to an increase in the reactivity of the blend more than ethanol, and mhx contributed more than m3h in the low-temperature regime. However, in contrast with the previous results shown in Figure 11c and d, CO emissions decreased significantly when 70% of methyl ester was used. For the three fuel blends containing ethanol, the lowtemperature reactivity from high to low followed the order: ethanol/nC7 ≫ ethanol/(30mhx/70nC7) > ethanol/(30m3h/ 70nC7). The same observation is valid for the ternary blends including n-butanol. Again, for a fixed alcohol concentration in the ternary blend, it was confirmed that n-butanol contributed more to the reactivity of the blend than ethanol. Another trend observed is that for a fixed CR, as the ethanol concentration increased in the mhx/nC7 ternary blend (from 0% to 20%), the LTHR rate was lower, and the onset of heat release occurred further after top dead center (Figure 14). This confirms that blends with increasing ethanol content were less reactive in the low-temperature regime. The impact of increasing ethanol to the 30mhx/70nC7 or to the 30m3h/70nC7 blends on the fraction of cumulative LTHR with respect to the total cumulative heat release at the respective CCR is shown in Figure 15. This result shows that there is an almost linear decrease in low-temperature reactivity with the addition of ethanol in ternary blends. This demonstrates that the least reactive component in the ternary blend (ethanol in this case) affects to a greater extent the autoignition characteristics independent of the unsaturation of the methyl ester.

4. CONCLUSIONS In this experimental work, the autoignition characteristics in the low-temperature regime of five pure fuels, nC7, methyl hexanoate (mhx), methyl 3-hexenoate (m3h), ethanol, and nbutanol, and their binary and ternary blends, were investigated in a modified CFR motored-engine operating with a gaseous premixed charge at an equivalence ratio of 0.5 and intake temperature of 155 °C. The autoignition was studied by increasing the compression ratio in a stepwise manner from the

Figure 9. LTHR profiles for binary blends with similar autoignition characteristics (with m3h as ester): (a) 30m3h/70n-hept; (b) 15E/ 85n-hept; and (c) 20Bu/80n-hept.

previously shown in Figure 5, neither ethanol nor m3h exhibited cool-flame behavior.

Figure 10. LTHR profiles for binary blends with similar autoignition characteristics (with mhx as ester): (a) 30mhx/70n-hept and (b) 10E/90nhept. 2992


Article

Energy & Fuels

Figure 11. Autoignition characteristics of ternary blends.

Figure 12. LTHR rate profiles for 15ethanol/(30mhx/70nC7) (a) and 20n-butanol/(30mhx/70nC7) (b).

Figure 14. Autoignition characteristics of ethanol in 30mhx/70nC7 at a fixed CR = 5.

Figure 13. CO emissions concentration of alcohols in 70 C7 methyl ester/30 nC7.

2993


Article

Energy & Fuels

Magín Lapuerta: 0000-0001-7418-1412 Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS We gratefully acknowledge the Spanish Ministry of Education for the financial support to M.L. for his stay at the EMS Energy Institute, Pennsylvania State University (PR2010-0419). J.R.A. wishes to thank the Universidad de Antioquia (UdeA) for the financial support through the Sostenibilidad program and the Universidad de Castilla-La Mancha for the financial support through the program for visiting researchers 2016.

■

Figure 15. Fraction of cumulative LTHR at the respective CCR for ethanol in 30 C7 methyl ester/70 nC7.

(1) Agarwal, A. K. Prog. Energy Combust. Sci. 2007, 33 (3), 233−271. (2) López, A. F.; Cadrazco, M.; Agudelo, A. F.; Corredor, L. A.; Vélez, J. A.; Agudelo, J. R. Fuel 2015, 153, 483−491. (3) Ruiz, F. A.; Cadrazco, M.; López, A. F.; Valdepeñas, J.; Agudelo, J. R. Fuel 2015, 161, 18−25. (4) Gail, S.; Thomson, M. J.; Sarathy, S. M.; Syed, S. A.; Dagaut, P.; Diévart, P.; Marchese, A. J.; Dryer, F. L. Proc. Combust. Inst. 2007, 31 I, 305−311. (5) Westbrook, C. K.; Pitz, W. J.; Sarathy, S. M.; Mehl, M. Proc. Combust. Inst. 2013, 34 (2), 3049−3056. (6) Westbrook, C. K. Annu. Rev. Phys. Chem. 2013, 64 (1), 201−219. (7) Herbinet, O.; Pitz, W. J.; Westbrook, C. K. Combust. Flame 2010, 157 (5), 893−908. (8) Rodriguez, A.; Herbinet, O.; Battin-Leclerc, F.; Frassoldati, A.; Faravelli, T.; Ranzi, E. Combust. Flame 2016, 164, 346−362. (9) Sarathy, S. M.; Gail, S.; Syed, S. A.; Thomson, M. J.; Dagaut, P. Proc. Combust. Inst. 2007, 31 I, 1015−1022. (10) Hakka, M. H.; Glaude, P. A.; Herbinet, O.; Battin-Leclerc, F. Combust. Flame 2009, 156 (11), 2129−2144. (11) Wang, W.; Gowdagiri, S.; Oehlschlaeger, M. A. Combust. Flame 2014, 161 (12), 3014−3021. (12) Benjumea, P.; Agudelo, J. R.; Agudelo, A. F. Energy Fuels 2011, 25 (1), 77−85. (13) Dayma, G.; Gail, S.; Dagaut, P. Energy Fuels 2008, 22 (3), 1469−1479. (14) Glaude, P. A.; Herbinet, O.; Bax, S.; Biet, J.; Warth, V.; BattinLeclerc, F. Combust. Flame 2010, 157 (11), 2035−2050. (15) Zhang, Y.; Boehman, A. L. Combust. Flame 2012, 159 (4), 1619−1630. (16) Zhang, K.; Togbé, C.; Dayma, G.; Dagaut, P. Combust. Flame 2014, 161 (3), 818−825. (17) Zhang, Y.; Boehman, A. L. Combust. Flame 2009, 156 (6), 1202−1213. (18) Boehman, A. L.; Song, J.; Alam, M. Energy Fuels 2005, 19 (5), 1857−1864. (19) Barrientos, E.; Matti Maricq, M.; Boehman, A. L.; Anderson, J. SAE Technical Paper 2015-01-1080, 2015. (20) Sarathy, S. M.; Oßwald, P.; Hansen, N.; Kohse-Höinghaus, K. Prog. Energy Combust. Sci. 2014, 44, 40−102. (21) da Silva, G.; Bozelli, J.; Liang, L.; Farrell, J. J. Phys. Chem. A 2009, 113, 8923−8933. (22) Sarathy, S. M.; Vranckx, S.; Yasunaga, K.; Mehl, M.; Oßwald, P.; Metcalfe, W. K.; Westbrook, C. K.; Pitz, W. J.; Kohse-Höinghaus, K.; Fernandes, R. X.; Curran, H. J. Combust. Flame 2012, 159 (6), 2028− 2055. (23) Kumar, K.; Zhang, Y.; Sung, C. J.; Pitz, W. J. Combust. Flame 2015, 162 (6), 2466−2479. (24) Yang, Z.; Qian, Y.; Yang, X.; Wang, Y.; Wang, Y.; Huang, Z.; Lu, X. Energy Fuels 2013, 27 (12), 7800−7808. (25) Zhang, Y.; Boehman, A. L. Combust. Flame 2010, 157 (10), 1816−1824. (26) Karwat, D. M. A.; Wagnon, S. W.; Wooldridge, M. S.; Westbrook, C. K. J. Phys. Chem. A 2012, 116, 12406−12421.

lowest level (CR = 3.8) to the point where the high temperature heat release rate was reached. The parameters used to characterize the autoignition for all fuels tested were the critical compression ratio (CCR), which was identified by the exponential increase of CO2 emissions concentration, the CO emissions, the CR of the onset of CO emissions, and the fraction of cumulative low-temperature heat release rate, which was used as an indication of how cool-flames are being affected. This work extends the current experimental data on autoignition of these neat fuels, binary blends (ethanol or n-butanol blended with nC7), and ternary blends of alcohols/C7 methyl esters/nC7. It was found that for binary nC7 blends with similar autoignition characteristics, ethanol reduces the autoignition tendency by twice the extent of unsaturated C7 methyl ester and by triple the extent of the saturated methyl ester. The addition of ethanol up to 20% in nC7, 30mhx/70nC7, and 30m3h/70nC7 promoted a roughly linear effect on the CCR, on the delay of the onset of HTHR, and on the fraction of cumulative LTHR. This suggests that the effect of ethanol addition on autoignition characteristics of ternary blends is stronger than the effect of unsaturation of the methyl ester. Results confirmed that unsaturated methyl esters, ethanol, and n-butanol (ethanol at a higher extent) contributed to decreasing the low-temperature oxidation reactivity, not only in binary blends but also in ternary blends. Such a decrease was slightly higher when two reactivity-suppressing components (alcohols and unsaturated esters) coexist in the blend. For a fixed alcohol content, the autoignition characteristics are highly nonlinear with the blend composition, and they are less sensitive to the least reactive components in the blend. Finally, it was found that regardless of the components, blends with similar autoignition characteristics exhibited similar thermal histories (CCR and LTHR rate) in the low-temperature regime. These results could provide some guidelines for the design (compression ratio, injection timings) of future conventional diesel engines or low-temperature combustion engines, which may need to be compatible with the penetration of oxygenated biofuels, such as methyl esters and alcohols, in increasing concentration in petroleum-derived base fuels.

■

REFERENCES

AUTHOR INFORMATION

Corresponding Author

*Tel.: +574 2198549. E-mail: [email protected]. ORCID

John R. Agudelo: 0000-0003-1304-9375 2994


Article

Energy & Fuels (27) Dagaut, P.; Togbé, C. Energy Fuels 2009, 23 (7), 3527−3535. (28) Saisirirat, P.; Togbé, C.; Chanchaona, S.; Foucher, F.; MounaimRousselle, C.; Dagaut, P. Proc. Combust. Inst. 2011, 33 (2), 3007− 3014. (29) Dagaut, P.; Togbé, C. Fuel 2010, 89 (2), 280−286. (30) Togbé, C.; May-Carle, J. B.; Dayma, G.; Dagaut, P. J. Phys. Chem. A 2010, 114, 3896−3908. (31) May-Carle, J. B.; Pidol, L.; Nicolle, A.; Anderlohr, J. M.; Togbé, C.; Dagaut, P. Combust. Sci. Technol. 2012, 184, 901−915. (32) Togbé, C.; Dayma, G.; Mzé-Ahmed, A.; Dagaut, P. Energy Fuels 2010, 24 (7), 3906−3916. (33) Saisirirat, P.; Foucher, F.; Chanchaona, S.; Mounaim-Rousselle, C. SAE Technical Paper 2009-24-0094, 2009. (34) Lu, X.; Yang, Z.; Zhou, X.; Huang, Z. Fuel 2013, 112, 1−7. (35) Baumgardner, M. E.; Sarathy, S. M.; Marchese, A. J. Energy Fuels 2013, 27 (12), 7778−7789. (36) Kang, D.; Lilik, G.; Dillstrom, V.; Agudelo, J. R.; Lapuerta, M.; Al-Qurashi, K.; Boehman, A. L. Combust. Flame 2015, 162 (4), 877− 892. (37) Yang, Y.; Boehman, A. L. Proc. Combust. Inst. 2009, 32, 419− 426. (38) Battin-Leclerc, F. Prog. Energy Combust. Sci. 2008, 34 (4), 440− 498. (39) Landolina, S.; Maltsoglou, I. How2Guide for Bioenergy Roadmap Development and Implementation, 2017. (40) European Standard EN 16709: Automotive f uels - High FAME diesel fuel (B20 or B30) - Requirements and test methods, 2016. (41) Lapuerta, M.; García-Contreras, R.; Campos-Fernández, J.; Dorado, M. P. Energy Fuels 2010, 24 (8), 4497−4502. (42) Yanowitz, J.; Ratcliff, M.; Murphy, M. J.; Taylor, J. D.; Mccormick, R. L. Compendium of Experimental Cetane Number Data, 2004. (43) Lapuerta, M.; Rodríguez-Fernández, J.; Armas, O. Chem. Phys. Lipids 2010, 163 (7), 720−727. (44) Zelenka, P.; Kapus, P.; Milkulic, L. A. SAE Technical Paper 910851, 1991. (45) Li, X.; Qiao, X.; Zhang, L.; Fang, J.; Huang, Z.; Xia, H. Renewable Energy 2005, 30 (13), 2075−2084. (46) Heywood, J. B. Internal Combustion Engine Fundamentals, 1st ed.; Mac-Graw Hill: New York, 1988. (47) Szybist, J. P.; Boehman, A. L.; Haworth, D. C.; Koga, H. Combust. Flame 2007, 149 (1−2), 112−128. (48) Kang, D.; Kalaskar, V.; Kim, D.; Martz, J.; Violi, A.; Boehman, A. L. Fuel 2016, 184, 565−580. (49) Glassman, I.; Yetter, R. Combustion, 4th ed.; Elsevier: New York, 2008. (50) Leppard, W. R. SAE Technical Paper 892081, 1989. (51) Curran, H. J.; Simmie, J. M.; Pitz, W. J.; Westbrook, C. K. Int. J. Chem. Kinet. 2004, 36 (11), 603−622. (52) Westbrook, C. K. Proc. Combust. Inst. 2000, 28 (2), 1563−1577. (53) Wagnon, S. W.; Karwat, D. M. A.; Wooldridge, M. S.; Westbrook, C. K. Spring Technical Meeting of the Central States Section of the Combustion Institute; Dayton, OH, 2012. (54) Vuilleumier, D.; Kozarac, D.; Mehl, M.; Saxena, S.; Pitz, W. J.; Dibble, R. W.; Chen, J. Y.; Mani Sarathy, S. Combust. Flame 2014, 161 (3), 680−695. (55) Lapuerta, M.; Hernández, J. J.; Sarathy, S. M. Combust. Flame 2016, 164, 259−269. (56) Lapuerta, M.; Hernández, J. J.; Fernández-Rodríguez, D.; CovaBonillo, A. Energy 2017, 118, 613−621.

2995


n-Heptane Blends in a Motored

Recommend Documents