Oxidation of Ethanol Blended Gasoline Surrogates in a Flow Reactor

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Oxidation of ethanol blended gasoline surrogates in a flow reactor Dongxue Han, Hongming Yin, Enchao Qian, Dajun Liu, and Lili Ye Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.9b00226 • Publication Date (Web): 29 Mar 2019 Downloaded from http://pubs.acs.org on April 1, 2019

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Energy & Fuels

Oxidation of ethanol blended gasoline surrogates in a flow reactor Dongxue Han1, Hongming Yin* 1,2, Enchao Qian2, Dajun Liu2, Lili Ye* 2 1 College

of Environmental Sciences and Engineering, Dalian Maritime University, Dalian 116026, PR China;

2 School

of Science, Dalian Maritime University, Dalian 116026,PR China;

Correspondence to: Hongming Yin, e-mail: [email protected] or Lili Ye,e-mail: [email protected]

Abstract The oxidation of ethanol blended with gasoline surrogates, a mixture of iso-octane and n-heptane, was studied in a flow reactor to evaluate the influence of ethanol content and oxygen concentration on the oxidation behaviors of reactants and the emissions of methanol, formaldehyde and acetaldehyde. The blend with 10% ethanol content accelerated the emergence of the negative temperature coefficient region of iso-octane and n-heptane. The emissions of formaldehyde and acetaldehyde increased with the increasing ethanol content. Increasing the oxygen concentration from 10% to 21% led to increased emissions of methanol, acetaldehyde and formaldehyde. With the oxygen concentration from 21% to 30%, methanol and acetaldehyde emissions decreased. The sources of aldehydes were explored, and found to be formed from different sources in different temperature regions. At low temperature, acetaldehyde was partially derived from ethanol oxidation and partially from n-heptane oxidation; formaldehyde emission was mainly contributed by ethanol 1

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oxidation. At high temperature, acetaldehyde was mainly produced from ethanol oxidation; formaldehyde was partially formed from iso-octane oxidation and partially from the consumption of methanol. Keywords: Ethanol blended gasoline surrogates; Flow reactor; Negative temperature coefficient; Formaldehyde; Acetaldehyde

1. Introduction Gasoline is one of the most common fuels and is typically used in the popular spark-ignition (SI) engines. With the continuously increasing demand, the reserves of gasoline resources, from which gasoline derives, are being rapidly depleted. The lack of energy sources and environmental considerations urgently require us to propose a clean and sustainable energy source supply to support modern transportation. Benefiting from its many favorable physiochemical properties, ethanol is currently playing an important role as the preferred gasoline alternative for SI engines. The use of pure ethanol requires special modifications to vehicle engines, since modern gasoline engines can operate smoothly when fueled by low percentage ethanol blends, the more cost-efficient solution at present is to blend ethanol into gasoline as a fuel additive agent[1-3]. As an oxygenate and octane rating enhancer[4,5], ethanol blended into gasoline increases the completeness of combustion[6-8] and helps to suppress engine knock, which enables its use in high compression ratio SI engines and accordingly improves engine efficiency. With the many benefits of ethanol, practical gasoline fuel in many countries now generally contains a low percentage of ethanol. The experimental 2


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results[9-11] revealed that with increasing ethanol content, the engine brake power slightly increases, the equivalence fuel–air ratio decreases, the heating value decreases, CO and HC emissions decrease, and CO2 emissions increase. Emission is an important indicator of fuel quality. Although ethanol can reduce HC and CO emissions, abnormal aldehydes emissions are a problem for oxygen containing fuels. Formaldehyde, acetaldehyde, and aromatic aldehydes are the most abundant carbonyls in the exhaust of ethanol gasoline; the emissions of aldehydes are higher in ethanol gasoline than in 93# gasoline, and these emissions increasing ethanol content[12-14]. The still bright future of ethanol in the fuel market requires researchers to have an in-depth understanding of the pollutant emission performance of ethanol blended gasoline. Thus, the influence of ethanol content on pollutant emissions (mainly carbonyls here) are systematically investigated in the present work. Gasoline is an extremely complex mixture of hundreds of components, which are mainly C5-C12 hydrocarbons. It is very challenging to examine gasoline directly via the experimental and theoretical mechanisms. Therefore, a representative component or a blend of several representative components is required for use as a gasoline surrogate fuel. Gasoline generally contains linear paraffins, branched paraffins, cyclic paraffins, aromatics and olefins[15]. Iso-octane and n-heptane are the most common and simplest surrogates of gasoline. Iso-octane is a model compound in surrogate fuels for branched paraffin components, and n-heptane is for linear paraffin components. Two-component blends of iso-octane and n-heptane, also called primary reference fuels (PRFs), have been widely used as gasoline surrogates[16-20] for the 3


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study of autoignition characteristics[21], laminar flame speeds[22], flame chemistry as well as stable oxidation products and intermediates[23]. PRFs can well describe gasoline behavior and represent gasoline physical and chemical properties. For the study of reaction kinetic mechanism[24-29], the importance of PRFs used as surrogates of linear and branched saturated hydrocarbons is more prominent. The sub-mechanism of PRFs can describe and reproduce the gasoline oxidation reaction. The negative temperature coefficient (NTC) is an important characteristic of hydrocarbon low temperature oxidation, which is related to fuel ignition and pollutant emissions. In this region, the reaction rate is lowered, and low temperature oxidation species are formed. The NTC behavior is indicative of the progress of the reaction and the emission of pollutants. A large number of previous studies have been completed regarding the combustion and oxidation of iso-octane and n-heptane in shock tubes, jet-stirred reactors and flow reactors. The NTC regions of iso-octane and n-heptane oxidation were not observed in shock tubes[31, 32]. The temperature of shock tubes is high and suitable for medium and high temperature regimes. Therefore, it is not feasible to study an NTC in the low temperature region in a shock tube. The low-temperature oxidation of n-heptane and iso-octane has been mostly carried out in jet-stirred reactors[33-37]. Herbinet et al.[33] have investigated the low-temperature oxidation of n-heptane in a jet-stirred reactor, and an NTC region has been observed between approximately 650 and 750 K. Dagaut et al.[34] have used a high-pressure jet-stirred reactor to experimentally investigate the oxidation of mixtures of n-heptane and iso-octane in a wide range from 550 to 1150 K. The NTC region was observed 4


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below 750 K. A small number of experimental studies were performed in flow reactors for the oxidation of n-heptane and iso-octane. Yamamoto et al.[39] investigated the combustion characteristics of an n-heptane/air mixture in a micro flow reactor and did not observe the NTC region in the low temperature region. The flow reactor is a common burner that achieves regional temperature control for both liquid and gas phase reactions. It is necessary to complement experimental studies for the oxidation of iso-octane and n-heptane in flow reactors, especially for NTC behavior and emissions in the NTC region. In the present study, the oxidation of ethanol blended gasoline surrogates comprising iso-octane and n-heptane was investigated in a flow reactor. A gas chromatograph equipping two kinds of columns and detectors was employed to detect the oxidation emissions. The purpose of this paper was to study the influences of ethanol content and oxygen concentration on the oxidation performance and emissions of blended fuels. Significant attention was paid to the NTC behaviors of iso-octane and n-heptane and the emissions of methanol, formaldehyde and acetaldehyde. The sources of aldehydes emissions have also been investigated. In the present study, we aimed to shed further light on the role of ethanol as an additive to practical fuels.

2. Experimental The schematic plot of the experimental setup used to investigate the oxidation of ethanol blended gasoline surrogates is shown in Fig. 1. Uniformly mixed liquid blended fuels were placed in a flask with two necks. One neck was connected to a 5


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burette containing the same blended fuels to keep the volume of the fuel in the flask constant. There was a pipe on the burette for venting from the flask. The other neck of the flask was connected to the gas supply system. The gases used in this study included oxygen (O2, purity > 99.995%), nitrogen (N2, purity > 99.999%) and air (purity > 99.999%), the flow rates of which were controlled by mass flow controllers with 0.20% accuracy. To prevent airflow refluence, check valves were installed in the gas supply system. After being thoroughly mixed, the airflow was divided into two paths by a three-way valve. The path denoted as the cleaning gas path was directly connected to a flow reactor. Before each experiment, N2 was introduced into the reactor though the cleaning gas path for approximately 10 minutes; this introduction was performed on the one hand to remove the residues from the previous experiment and, on the other hand, to remove air from the reactor to ensure the accuracy of oxygen concentration in the next experiment. The other path, denoted the carrier gas path introduced the gas into the bottom of the flask, forming a bubbler. With the saturated vapor pressure, the vapor of the blended fuels was carried into the flow reactor by the carrier gas. The flow reactor comprised a tubular heating furnace, a reaction tube surrounded by the heating furnace body and a temperature control system. The reaction tube was a ceramic tube with an inner diameter of 11 mm, outer diameter of 16 mm and a total length of 500 mm, of which the middle 180 mm was located at the heating zone. A K-type thermocouple was inserted from the middle of the furnace top to measure the temperature of the outer wall of the reaction tube. A temperature controller was connected with the thermocouple to control temperature 6


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from room temperature to reaction temperature with an accuracy of 0.25%. Compared with reference [39], the flow reactor used in this paper has a different heating method and temperature profile. In reference [39], the external heat source at one end of the tube imparted to the reaction tube a gradually increasing temperature profile along the inner surface of the tube wall. In this paper, since the heating zone was in the middle 180 mm of the reaction tube, when the reaction tube was heated to a stable temperature, its intermediate portion was maintained at that temperature. This design allowed the fuel to react more efficiently at that temperature. The flow reactor was connected to the gas supply system at one end and to a gas chromatograph at the other end. The gas chromatograph was employed to online detect the exhaust gas emitted from the oxidation of blended fuels. The online gas chromatography method provided the advantages of easy separation of different isomers, high sensitivity, and easy and direct quantification of species. In this study, the gas chromatograph was equipped with two sets of chromatographic columns and detectors for the quantification of the reactants and products. The first one, a Porapak Q column combined with a thermal conductivity detector (TCD), was used to quantitatively analyze methanol, formaldehyde and acetaldehyde. The second one was equipped with an SE-54 column collocating a flame ionization detector (FID) for the quantification analysis of ethanol, iso-octane and n-heptane. Calibrations of the species were performed by direct injection of a standard sample into the gas chromatograph. In this study, a mixture of 91% iso-octane and 9% n-heptane (v/v), termed 7


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PRF91, was selected as a gasoline surrogate. Two series of experiments were performed. In the first series, different volumes of ethanol (purity > 99.9%) were blended into PRF91. To explore the influence of ethanol content on oxidation behaviors, the blended fuels were prepared with 5% (v/v), 10% (v/v) and 15% (v/v) ethanol. Here, these blends are referred to as E5, E10 and E15, and their research octane numbers (RONs) were 95.1, 98.7 and 101.2[4], respectively, which are slightly higher than for gasoline fuels (90~95). The flow rates of air and nitrogen were 0.02 and 0.08 L/min, respectively, and a 5% oxygen concentration was therefore acquired to avoid deflagration due to high oxygen concentration. In the second series of experiments, the oxygen concentration was varied to investigate its effects on the oxidation behaviors of blended fuels. The E10 blend, which was composed of 80% iso-octane, 8% n-heptane and 10% ethanol, was chosen as a representative fuel considering that the volume ratio of ethanol added to gasoline was usually approximately 10% in China. As is known, practical engines can operate under oxygen-rich combustion to increase engine efficiency and reduce emissions with turbocharging technology[40]. Engine fuels may also be burned under oxygen-lean conditions when the engine is initiated, especially in diesel engines. As such, the oxidation behaviors of blended fuels under oxygen-lean, natural aspiration and oxygen-rich conditions were investigated. By controlling the flow rates of mixing carrier gas (O2, N2 and air), three oxygen concentrations were obtained, i.e., 10%, 21% and 30%, to represent the above three conditions, respectively. Table 1 summarizes the gas flow conditions. Note that in order to reduce the effect of 8


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residence time on the experimental results, the total flow rate of mixing gas in each set of experiments remained consistent at 0.10 L/min. In the above two series of experiments, the first experiment covered a temperature range from 300 to 950 K, and the second experiment covered a temperature range from 300 to 800 K to avoid deflagration due to the high temperature in the oxygen-rich environment.

3. Results and discussion In the following discussion, the experimental results will be discussed in detail, focusing on the consumption of reactants and the formation of methanol (CH3OH), acetaldehyde (CH3CHO) and formaldehyde (CH2O). Specifically, section 3.1 aims to reveal the influence of ethanol content on the related behaviors of reactants and emissions of products, while section 3.2 concentrates on the effect of oxygen concentration.

3.1. Influence of Ethanol Content on Oxidation Behaviors of Ethanol Blended Gasoline Surrogates To explore the influence of ethanol content, the oxidation of blended fuels with ethanol contents of 5%, 10% and 15% was investigated from 300 to 950 K under a 5% oxygen concentration. Attention was paid to the consumption of reactants and the formation of CH3OH, CH3CHO and CH2O. 3.1.1. Consumption of reactant components The mole fraction profiles of reactants (i.e., ethanol, iso-octane and n-heptane) are shown in Fig. 2 as a function of reaction temperature. Experimental error bars, 9


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calculated by relative error, are plotted for E10. In particular, panel (a) displays the consumption of ethanol. Comparing the three curves in this panel, it can be seen that ethanol content in blended fuels has little effect on the evolution of ethanol. The reaction temperature ranges for ethanol in E5 and E15 are almost the same: ethanol begins to decompose at 550 K and is completely consumed at 740 K. With respect to E10, the ethanol consumption also starts at 550 K and ends at a slightly higher temperature of 770 K. The mole fraction profiles of iso-octane and n-heptane are also displayed in Figs. 2(b) and 2(c), respectively, as a function of temperature. Apparently, the consumption behaviors of these two alkanes are rather different from that of ethanol. Like many hydrocarbons, the reaction of iso-octane and n-heptane can be classified into two temperature regimes: low temperature and high temperature, and an NTC region exists in the low temperature regime. Different reaction pathways dominate in each regime. At low temperatures, the reaction of O2 with alkyl radicals that are generated from initial fuel molecules proceeds largely to form alkyl-peroxy radicals (QOOH); this is the dominant reaction in the low temperature oxidation. Due to the ease of overcoming the energy barriers to their formation, the chain propagation reactions of QOOH radicals increase as the temperature increases, forming cyclic ethers and olefins. The increase of these propagation channels leads to a lower reactivity of the system, which is observed as the NTC region[41-43]. Take iso-octane in E5 as an example. As shown in Fig. 2(b), iso-octane starts to decompose at approximately 550 K, and the low temperature oxidation chemistry is 10


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mainly dominated by reactions of octane and octanyl-peroxy radicals. As the temperature increases up to 710 K, the oxidation enters the NTC region, in which increasing temperature actually decreases the reaction reactivity. As a result, the consumption of iso-octane slows with the increasing temperature in this region. This can be ascribed to the complex temperature dependence of reactions involving the octanyl-peroxy radical. At temperatures above 830 K, the reactivity is more controlled by high-temperature chemistry and HO2 radical chemistry, containing the decomposition and isomerization of unimolecular fuel and alkyl radical as well as H atom abstraction by radicals[41,42], thus leading to increased reactivity and rapid consumption of iso-octane. From Fig. 2(b), the oxidation behaviors of iso-octane in E15 are very similar to the situation in the E5 case: the oxidation starts at approximately 550 K, and the NTC region locates from 710 to 830 K. In contrast, iso-octane in E10 exhibits different consumption behaviors from E5 and E15. In particular, the initial reactant begins to decompose also at 550 K, and the NTC region is shifted to a lower range, spanning the range from 650 to 800 K. Among the three blends, the oxidation of E10 shows an earlier and wider NTC region than for E5 and E15. The mole fraction profiles of n-heptane are similar to those of iso-octane, as shown in Fig. 2(c). With the E5, E10 and E15 cases, the decomposition of n-heptane all starts at 550 K. The NTC region of n-heptane in E5 and E15 is in the range of 710-830 K, while in E10, the NTC behavior occurs at lower temperature and covers the range of 680-800 K. Dissociation of hydrogen peroxide into two hydroxyl radicals 11


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leads to rapid consumption of the remaining fuel once past the NTC region[43]. Comparing this experiment with reference [39], due to the different heating methods and temperature profiles mentioned above, the detection methods are also different. In reference [39], the function of species concentration versus temperature was obtained by offline detection of the species concentration at each location. In this paper, the reaction tube was heated to a steady temperature, at which the species concentration in exhaust was detected online. By changing this temperature with the temperature controller, the species concentrations at all temperatures were obtained, thus obtaining a function of species concentration and temperature. Since the intermediate portion of the reaction tube was at the detected temperature, this allowed the reactants to have a longer residence time at the detection temperature. The evolution of the reactants with temperature could be observed in more detail, which may be the reason why the NTC of reactants was detected in the present study. Comparisons with the experimental results performed in jet-stirred reactors[33,34], the NTC regions of iso-octane and n-heptane are both obtained in the low and intermediate temperature regime (600-800 K). Due to the elevated pressure and sufficient oxygen, the NTC behavior in references [33,34] exhibited a lower temperature range than that observed in the present study. From the discussion above, it can be derived that the blending of 10% ethanol accelerates the emergence of the NTC region of iso-octane and n-heptane oxidation. The blending of 10% ethanol promotes the chain propagation reactions of QOOH radicals at lower temperatures. Kinetics mechanism research needs to be developed 12


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for further theoretical verification. 3.1.2. Formation of methanol, formaldehyde and acetaldehyde A large number of species are produced and detected during the oxidation experiments of ethanol blended gasoline surrogates. Among them, particular attention is paid to the formation of three pollutant emissions, CH3OH, CH3CHO and CH2O. The mole fraction profiles of CH3OH, CH3CHO and CH2O are described as a function of reaction temperature in Figs. 3(a), 3(b) and 3(c), respectively. Experimental error bars, calculated by relative error, are shown in E10. As seen in Fig. 3(a), CH3OH emissions of the three blended fuels exhibit a great resemblance in their formation. A small amount of CH3OH is observed from 680 to 800 K, whereas as temperature increases above 830 K, its concentration gets rapidly accumulated. As we can see in Fig. 2(a) and Fig. 3(a), ethanol has already been completely consumed when a large quantity of CH3OH starts to emit. In addition, there is no obvious relationship between ethanol content and CH3OH emissions. This is because the formation of CH3OH mainly comes from the oxidation of n-heptane. A small amount of emissions formed from the low-temperature oxidation of n-heptane, and a peak of CH3OH was obtained in the NTC region. CH3OH emissions are mostly formed at the high temperature regime where n-heptane is consumed rapidly. Figs. 3(b) and 3(c) display the mole fraction profiles of the two aldehydes, CH3CHO and CH2O, respectively. Unlike CH3OH, the two aldehydes are more affected by ethanol content in the blended fuels. Comparing Figs. 3(b) and 3(c), it can be deduced that CH3CHO is the more favored aldehydes product throughout the entire 13


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temperature range studied. For all three blended fuels, CH3CHO emissions are able to be detected at the initial stage of oxidation. Unlike the E10 and E15 cases, a small amount of CH3CHO is detected only in the low temperature region in E5 and the formation of CH3CHO is not detected above 680 K. At the low temperature range from 550 K to 650 K, CH3CHO emissions from E10 and E15 are approximately 1.3 and 1.2 times greater than those from E5, respectively, but the difference between E10 and E15 is smaller than that between E5 and E10. At the temperature region between 710 K and 860 K, CH3CHO emissions from E15 are approximately 1.2 times more than those from E10. CH3CHO is an important product in the oxidation of ethanol blended gasoline surrogates, suggesting that emphasis should be placed on those reactions pertinent to the formation of CH3CHO when developing the kinetic mechanisms. As shown in Fig. 3(c), for E15, CH2O is initially detected at 650 K and its concentration rises rapidly with increasing temperature to 710 K. As the temperature increases to 800 K, hardly any CH2O emissions could be observed. It is not until the temperature rises over 880 K that CH2O can be detected again. In E10, CH2O is also obtained initially at 650 K. After that, the concentration undergoes first a rapid increase and then a sharp decrease, obtaining a maximum at 680 K. A large amount of CH2O emissions is detected again above 880 K. In E5, CH2O cannot be detected until above 880 K. The results in the present study indicate that increasing ethanol content leads to higher emissions of CH2O, particularly in low-to-moderate temperature regimes. At the temperature range from 650 to 800 K, increasing the ethanol content 14


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from 10% to 15% induces an increase in CH2O emissions of approximately 1.5 times. At the high temperature region above 810 K, CH2O emissions have little change with increasing ethanol content. Aldehydes are important products of the oxidation of blended fuels. Studying the sources of aldehydes emissions can help to reduce their emissions in a targeted way. For the source of these two aldehydes, Liu et al.[12] indicated acetaldehyde definitely comes from ethanol oxidation. For the results of this experiment shown in Fig. 2(a) and Figs. 3(b) and 3(c), ethanol has been completely consumed at 740 K; the maximum of CH3CHO emissions is below 740 K, and CH2O emissions increases again above 880 K. This result indicates that the formation of aldehydes is not completely derived from ethanol. Magnusson et al.[13] suggested the main source for formaldehyde and acetaldehyde was saturated aliphatic hydrocarbons. However, the emissions of aldehydes are affected by ethanol content, as shown in Figs. 3(b) and 3(c). To explore the origin of aldehydes formation for the explanation of their profiles, the oxidation of ethanol, n-heptane and iso-octane was investigated separately. The mole fraction profiles of aldehydes formed from the oxidation of ethanol, n-heptane and iso-octane are shown in Figs. 4(a), 4(b) and 4(c), respectively. As shown in Fig. 4(a), the CH3CHO formation from ethanol oxidation starts at 590 K, reaching a maximum at approximately 710 K. After ethanol is completely consumed, CH3CHO is slowly decomposed; thus, CH3CHO can be detected over a wide range. The CH3CHO formation from n-heptane oxidation is mainly caused by a -decomposition reaction path for hydroperoxy-heptyl radicals[43]. In Fig. 4(b), 15


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CH3CHO is formed as soon as n-heptane begins to be consumed and formed only before n-heptane oxidation enters NTC region. Therefore, CH3CHO formation from n-heptane oxidation occurs at a lower temperature than that from ethanol oxidation and is detected only in the low temperature region from 550 to 680 K. Comparing to Fig. 4(a), the CH3CHO emissions from n-heptane oxidation are much less than those from ethanol oxidation. It can be inferred that the CH3CHO emitted from the blended fuels has different formation channels in different temperature region: at temperatures lower than approximately 680 K, CH3CHO is partially derived from ethanol oxidation and partially from n-heptane oxidation; above 680 K, CH3CHO is mainly derived from the oxidation of ethanol. In Fig. 3(b), CH3CHO emission in E5 is only formed from n-heptane oxidation in the low temperature regime. As shown in Fig. 4(a), the emissions of CH2O from ethanol oxidation can be detected in the temperature range of 650 K to 800 K, and its concentration reaches a maximum at 710 K. As shown in Fig. 4(c), CH2O formation from iso-octane oxidation is detected at the temperatures above 850 K. Therefore, it can be inferred that CH2O emissions in blended fuels come from different formation channels in different temperature regions. From 680 to 800 K in Fig. 3(c), CH2O emissions are mainly contributed by ethanol oxidation. At high temperature, CH2O emissions are partially formed by the iso-octane oxidation. In addition, O-H bond fission of CH2OH ·

from CH3OH is also a main formation channel of CH2O in the high

temperature region[27].

16


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3.2. Effect of Oxygen Concentration on Oxidation Behaviors of Ethanol Blended Gasoline Surrogates This section aims to explore the influence of oxygen concentration on the oxidation behaviors of E10, which contains 10% (v/v) ethanol in the blended fuels. The oxidation of blended fuels was investigated over the temperature range from 300 to 800 K under three oxygen concentrations of 10%, 21% and 30%, representing the oxygen-lean, natural aspiration and oxygen-rich conditions, respectively. The discussion will mainly focus on the consumption of reactants and formation of CH3OH, CH3CHO and CH2O. 3.2.1. Consumption of reactant components Fig. 5 shows the mole fraction profiles of reactants, including ethanol, iso-octane and n-heptane, as a function of reaction temperature with different oxygen concentrations. As mentioned above, the emissions of CH3CHO and CH2O are affected by ethanol oxidation, so the concentration profile of ethanol under different oxygen concentrations is investigated and shown Fig. 5(a). Ethanol in three cases all starts to react at approximately 540 K, and its concentration afterwards gradually decreases with increasing temperature until it gets completely consumed at approximately 750 K. At the temperatures lower than 690 K, ethanol has the largest reaction rate when the blended fuels oxidize under natural aspiration conditions (i.e. 21% oxygen concentration). As shown in Figs. 5(b) and 5(c), the presence of the NTC region leads to remarkably different mole fraction profiles of iso-octane and n-heptane as opposed to 17


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ethanol. From Fig. 5(b), iso-octane starts to decompose at 510 K under all conditions studied. As temperature increases, the oxidation under oxygen-rich conditions becomes the first to enter the NTC region at approximately 600 K. In this region, the reactivity of iso-octane is decreased with increasing temperature, thus resulting in an increased concentration. For oxidation under oxygen-lean and natural aspiration conditions, the temperatures when the NTC behavior occurs are both at approximately 630 K. Once temperature increases past the NTC region, the reaction rate increases again, leading to a rapid decrease in the concentration of iso-octane. As shown in Fig. 5(c), the mole fraction profiles of n-heptane are quite similar to those of iso-octane. N-heptane starts to decompose at 540 K. The oxidation in an oxygen-rich environment enters the NTC region at a lower temperature than in oxygen-lean and natural aspiration environments. The NTC region of n-heptane oxidation is observed from 630 to 690 K under oxygen concentrations of 10% and 20%, and from 600 to 660 K under an oxygen concentration of 30%. As mentioned in 3.1.1, the increase of the chain propagation reactions of QOOH radicals, formed by the reactions of alkyl radicals with O2, are responsible for the occurrence of NTC behavior. It is speculated that an oxygen-rich condition promotes the chain propagation reactions of QOOH radicals, contributing to the occurrence of NTC at lower temperatures. Therefore, whether for iso-octane oxidation or n-heptane oxidation, the occurrence of NTC is observed first under oxygen-rich conditions. 3.2.2. Formation of methanol, formaldehyde and acetaldehyde The concentration evolution of CH3OH, CH2O and CH3CHO as a function of 18


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reaction temperature is shown in Fig. 6. As shown in Fig. 6(a), the evolution of CH3OH concentration is highly consistent under all oxygen concentrations, which is initially formed at 660 K and decreases after reaching the maximum at 690 K. For oxidation in oxygen-lean environment, increasing oxygen concentration indeed brings about a notable increase in the emission of CH3OH. This result potentially occurs because the decreased oxygen level is insufficient for methanol production. This decrease also corresponds to 680-800 K in Fig. 3(a), and very little CH3OH emissions are observed below an oxygen concentration of 5%. For oxidation under oxygen concentrations of 21% and 30%, the CH3OH emission in the oxygen-rich environment even fails to compete with its emission in natural aspiration environment, especially at the temperatures exceeding 690 K. On the one hand, this result may be because the oxygen-rich environment is conducive to the consumption of CH3OH. On the other hand, this result is possibly observed because n-heptane has a higher consumption rate under natural aspiration than under oxygen-rich conditions, resulting in more CH3OH formation. As shown in Fig. 6(b), CH3CHO is emitted throughout the reactant oxidation temperature range and is measured to have the largest mole fractions among the three emissions. Since CH3CHO formation occurs with the low temperature oxidation of n-heptane, CH3CHO is formed when n-heptane begins to decompose at 510 K. Under three oxygen concentrations, the CH3CHO concentrations all reached their maxima at the end of the n-heptane NTC regions. Moreover, the NTC region of n-heptane 19


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oxidation under oxygen-rich conditions occurs at a lower temperature, so the CH3CHO concentration also reaches its maximum at a lower temperature than under natural aspiration and oxygen-lean conditions. It is worth noting that CH3CHO is consumed with the fastest rate under oxygen-rich conditions, benefiting from abundant oxygen. In contrast, the rate of CH3CHO consumption was the lowest under oxygen-lean conditions. In Fig. 6(c), the evolution of the mole fractions of CH2O is similar in the three oxygen environments, i.e., a peak is obtained first and then a rapid increase occurs again after a slight decrease. CH2O formation is detected at the earliest under the oxygen-rich condition at 660 K, but at 690 K under oxygen-lean and natural aspiration conditions. At the temperatures above 750 K, the increase in CH2O concentration is the greatest under natural aspiration conditions due to the greatest consumption of iso-octane in this temperature region, which is the source of CH2O formation at high temperatures. In future studies, it is recommended that when added into gasoline as blended fuels, the ethanol content should not be too large to maintain low CH2O and CH3CHO emissions, and sufficient oxygen should be provided for accelerating the decomposition of pollutants when possible.

4. Conclusions The influence of ethanol content and oxygen concentration on the oxidation behaviors of ethanol blended gasoline surrogates, a mixed fuel composed of 91% iso-octane and 9% n-heptane (v/v), was investigated in a flow reactor. Significant 20


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attention was paid to the NTC behaviors of iso-octane and n-heptane as well as to the emissions of CH3OH, CH2O and CH3CHO. The sources of CH3OH, CH2O and CH3CHO in the exhaust have also been explored. The NTC regions of iso-octane and n-heptane were observed below 800 K, and the blend with 10% ethanol content accelerated their emergence. The emissions of CH2O and CH3CHO increased with the increasing ethanol content. Increasing the ethanol content from 10% to 15% increased the CH2O emissions by approximately 1.5 times. Between 550-650 K, CH3CHO emissions from E10 and E15 were approximately 1.3 and 1.2 times greater than those from E5, respectively. Between 710 - 860 K, CH3CHO emission from E15 was approximately 1.2 times greater than that from E10. Increasing the oxygen concentration from oxygen-lean to natural aspiration indeed increased CH3OH, CH3CHO and CH2O emissions, whereas when transitioning from natural aspiration to oxygen-rich, their emissions were reduced. The oxygen-rich conditions accelerated CH3CHO consumption at high temperatures. The aldehydes formed from different sources at different temperature regions: at low temperatures, CH3CHO was partially derived from the ethanol oxidation and partially from the n-heptane oxidation; while at high temperatures, CH3CHO derived mainly from ethanol oxidation; at low temperatures, CH2O emission was mainly contributed by ethanol oxidation, while at high temperatures, CH2O was partially formed from the iso-octane oxidation and partially from the consumption of CH3OH.

Acknowledgments This work was supported by National Natural Science Foundation of China 21


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(U1232135 and 51606122).

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List of Captions for Tables and Figures Table 1. Gas flow conditions for the oxidation of blended fuels with various oxygen concentrations. Figure 1. Schematic of the experimental setup. Figure 2. The mole fraction profiles of ethanol (a), iso-octane (b) and n-heptane (c) from the oxidation of blended fuels with ethanol contents of 5%, 10% and 15% under 5% oxygen concentration. The experimental error bars are shown for E10. Figure 3. The mole fraction profiles of methanol (a), acetaldehyde (b) and formaldehyde (c) emitted from the oxidation of blended fuels with ethanol contents of 5%, 10% and 15% under 5% oxygen concentration. The experimental error bars are shown for E10. Figure 4. The mole fraction profiles of aldehydes formed from the oxidation of ethanol (a), n-heptane (b) and iso-octane (c) under an oxygen concentration of 5%. Figure 5. The mole fraction profiles of ethanol (a), iso-octane (b) and n-heptane (c) from E10 oxidation under oxygen concentrations of 10%, 21% and 30%. The experimental error bars are shown for 21%. Figure 6. The mole fraction profiles of methanol (a), acetaldehyde (b) and formaldehyde (c) emitted from E10 oxidation under oxygen concentrations of 10%, 21% and 30%. The experimental error bars are shown for 21%.

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Table 1 Gas flow conditions for the oxidation of blended fuels with various oxygen concentrations. O2 (L/min)

N2 (L/min)

Air (L/min)

Oxygen Concentration

0.00

0.05

0.05

10%

0.00

0.00

0.10

21%

0.03

0.07

0.00

30%

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Figure 1

Fig. 1. Schematic of the experimental setup.

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Figure 2

Fig. 2. The mole fraction profiles of ethanol (a), iso-octane (b) and n-heptane (c) from the oxidation of blended fuels with ethanol contents of 5%, 10% and 15% under an oxygen concentration of 5%. The experimental error bars are shown for E10.

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Figure 3

Fig. 3. The mole fraction profiles of methanol (a), acetaldehyde (b) and formaldehyde (c) emitted from the oxidation of blended fuels with ethanol contents of 5%, 10% and 15% under 5% oxygen concentration. The experimental error bars are shown for E10.

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Figure 4

Fig. 4. The mole fraction profiles of aldehydes formed from the oxidation of ethanol (a), n-heptane (b) and iso-octane (c) under an oxygen concentration of 5%.

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Figure 5

Fig. 5. The mole fraction profiles of ethanol (a), iso-octane (b) and n-heptane (c) from E10 oxidation under oxygen concentrations of 10%, 21% and 30%. The experimental error bars are shown for 21%.

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Figure 6

Fig. 6. The mole fraction profiles of methanol (a), acetaldehyde (b) and formaldehyde (c) emitted from E10 oxidation under oxygen concentrations of 10%, 21% and 30%. The experimental error bars are shown for 21%.

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Oxidation of Ethanol Blended Gasoline Surrogates in a Flow Reactor

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