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Experimental investigation of polycyclic aromatic hydrocarbons growth characteristics of gasoline mixed with methanol, ethanol, or n-butanol in laminar diffusion flames Fushui Liu, Yang Hua, Han Wu, Chia-fon F. Lee, and Yikai Li Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b00693 • Publication Date (Web): 25 May 2018 Downloaded from http://pubs.acs.org on May 25, 2018
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Title: Experimental investigation of polycyclic aromatic hydrocarbons growth
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characteristics of gasoline mixed with methanol, ethanol, or n-butanol in laminar
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diffusion flames
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Authors: Fushui Liua,b, Yang Huaa,*, Han Wua,*, Chia-fon Leea,c, Yikai Lia
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Author affiliations:
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a. School of Mechanical Engineering, Beijing Institute of Technology, Beijing 100081, China
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b. Beijing electric vehicle Collaborative Innovation Center, Beijing 100081, China
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c. Department of Mechanical Science and Engineering, University of Illinois at Urbana-Champaign,
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IL 61801, USA
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*
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Yang Hua, Email:
[email protected]; Tel: +86-18210603556
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School of Mechanical Engineering, Beijing Institute of Technology, Beijing 100081, China
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Han Wu, Email:
[email protected]; Tel: +86-18500028278
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School of Mechanical Engineering, Beijing Institute of Technology, Beijing 100081, China
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ORCID
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Yang Hua: 0000-0001-8058-1527
Corresponding authors:
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Experimental investigation of polycyclic aromatic hydrocarbons growth characteristics
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of gasoline mixed with methanol, ethanol, or n-butanol in laminar diffusion flames
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Fushui Liua,b, Yang Huaa,*, Han Wua,*, Chia-fon Leea,c, Yikai Lia
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a. School of Mechanical Engineering, Beijing Institute of Technology, Beijing 100081, China
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b. Beijing electric vehicle Collaborative Innovation Center, Beijing 100081, China
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c. Department of Mechanical Science and Engineering, University of Illinois at Urbana-Champaign, IL 61801,
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USA
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(*Corresponding authors: E-mail address:
[email protected] (Yang Hua),
[email protected] (Han Wu))
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Abstract:
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Alcohols have been regarded able to reduce particulate emissions from GDI engines, which in turn
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requires a deeper understanding of the effect of fuel chemistry on the formation and growth of PAHs. This
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work focused on combustion and soot forming progress, thus studied the distribution characteristics of
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PAHs with different ring sizes by PLIF and the combustion characteristics by OH and CH luminescence in
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laminar diffusion flames. The effect of methanol, ethanol, and n-butanol addition to gasoline on PAHs was
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evaluated at the same mixing ratio and the same oxygen content, and the effect of alcohol ratio from 0% to
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80% was investigated. The results showed at the same mixing ratio, the ability of alcohols to reduce all size
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PAHs is methanol > ethanol >n-butanol. As the alcohol ratio increases, all size PAHs show a monotonous
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decreasing trend. The ability of methanol or ethanol to reduce large ring aromatics is always far greater
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than that to one ring aromatics (A1), even the percentage reduction of large ring aromatics is always greater
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than the alcohol ratio, while the percentage reduction of all size PAHs by n-butanol is always lower than its
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mixing ratio. At the same oxygen and carbon content, the ability to decrease A1 is n-butanol > ethanol >
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methanol, indicating that dilution plays a key role in A1 reduction. The ability to decrease A4-A5 (450 nm)
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is consistent with the results at the same mixing ratio, inferring that the molecular structure rather than
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oxygen and carbon contents dominates the ability to reduce large PAHs. As the aromatic ring size increases,
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its high-concentration region gradually evolves from the flame center at lower position to the two wings of
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the flame at higher position. The flame lift-off length based on the OH chemiluminescence and the intensity
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of the high temperature reaction near the flame lift-off height shows a monotonous decreasing trend with
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the alcohol chain length and alcohol-mixing ratio. CH mainly locates at the interface between periphery
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OH and inner flame in the middle of the flame, and its luminescence intensities show a monotonous
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decreasing trend with the alcohol ratio.
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Key words: Alcohols; gasoline; polycyclic aromatic hydrocarbons; laser-induced fluorescence; laminar
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diffusion flame
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1. Introduction
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As emissions regulations become more stringent, gasoline direct injection (GDI) engines are suffering
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from soot emissions due to non-homogeneous air-fuel mixing.1-3 It is well-known that alcohols have been
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considered as potential alternative fuels due to their favorable physical and chemical properties, renewable
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characteristics and the ability of soot emission reduction.4,5 Agarwal et al.6, Maricq et al.7, Wang et al.8,
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Karavalakis et al.9 and Tornatore et al.10 studied the effect of methanol, ethanol, and n-butanol addition on
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the combustion and emission performance of gasoline engine respectively, and reported that adding a
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certain percentage of alcohols to gasoline can significantly reduce the particulate emissions without loss of
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engine performance. Thus, adding methanol, ethanol or n-butanol can be a good strategy for gasoline
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engine to reduce soot emissions.
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In order to accurately predict soot emissions from gasoline engine blended with alcohols, it is
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necessary to establish an accurate detailed soot model. However, the current predictive models are based on
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engine emissions. In fact, it is incomplete to use this macroscopic measure to calibrate the soot model,
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which requires a deeper understanding of the formation and growth of soot and polycyclic aromatic
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hydrocarbons (PAHs) in the blended fuel flames and the role of fuel chemistry in the PAHs formation.11
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PAHs (Polycyclic aromatic hydrocarbons) are typically considered as soot precursors formed in a
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flame,12-14 which bridges the quality gap between soot particles and fuel molecules.15,16 The formation and
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growth of PAHs originate in chemical reactions starting with the fuel pyrolysis lead to the formation of
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incipient aromatic ring such as benzene, and then forming the soot particle inceptions through coagulation
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of grown PAHs.17,18 Therefore, understanding the growth characteristics of PAHs in a well-controlled
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combustion system such as premixed or diffusion flame19 is important and necessary for the development
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of accurate soot models.
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There are already some studies on PAHs in the basic flames. Wu et al.20 investigated the effect of
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ethanol addition on the formation of PAHs and soot in ethylene premixed flames by using LIF and LII
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techniques. They concluded that ethanol addition could reduce the PAHs and soot by reducing the carbon
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content that is available to form precursors. Subsequently, Korobeinichev et al.21 regarded that the decrease
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of carbon content that is available to form PAHs is due to the existence of the reaction path of ethanol
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reducing the formation of benzene and propargyl radicals. Wu et al.22 also reported that C/O ratio plays a
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key role in the formation of soot precursors largely in laminar premixed flames of methanol/methyl
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butanoate blends. In addition, Singh et al.23 observed that butane can form more soot than butanol in
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counter-flow diffusion flame, and regarded that the difference of molecular structure is an important factor
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influencing the PAH formation, and acetylene plays an important role in the PAH formation and growth
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processes. Lemaire et al.24 investigated the PAHs and soot formation characteristics of ethanol-gasoline in
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turbulent spray flames. They reported the reduction of soot and PAHs by ethanol addition was mainly due
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to the effects of both dilution and increased oxygen content.
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However, the addition of alcohol does not always reduce PAHs or soot. It has been reported that in
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some cases, the alcohol addition to the ethylene or methane diffusion flame lead to an increase in PAHs or
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soot. Liu et al.25 studied the PAHs and soot formation in laminar diffusion flames of ethylene mixed with
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dimethyl ether (DME) with PLIF and LII techniques and found that adding a small proportion of DME
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leads to an increase of PAHs and soot. They pointed out the synergistic effect is due to the enhanced methyl
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concentration. The PAHs growth characteristics of DME, methane, and propane in laminar diffusion flames
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were compared by Hayashida et al.17. The results showed the growth of the PAHs in the DME laminar
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diffusion flame is much slower, which is due to the methyl addition/cyclization mechanism that is not
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efficient for the large PAHs growth. Choi et al.26 investigated the PAHs and soot formation characteristics
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in ethanol/ethylene counterflow diffusion flames, and observed adding 0-10% ethanol can lead to higher
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levels of aromatic ring formation. McEnally et al.27 also found that adding 10% ethanol or DME leads to an
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increases of soot in ethylene diffusion flames. They regarded that this mainly because the enhanced
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formation of methyl radical promotes the C1+C2 addition reactions to produce more propargyl radicals,
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whose self-reaction can form more PAHs. The effect of n-butanol addition to methane in laminar coflow
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diffusion flames was studied by Jin et al.28. They observed that the n-butanol addition increased the PAHs
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formation, and pointed out that the increasing PAHs is because that the enhanced formation of C2 species
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promotes the formation of propargyl radical and propyne. However, gasoline with larger alkanes as the
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main component can decompose a larger amount of methyl and C2 species. Therefore, the phenomenon
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when DME or ethanol added to ethylene flame and n-butnaol added to methane flame does not appear in
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the gasoline flame.
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Of course, some studies have also been conducted on the comparison of different alcohols. From the
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perspective of gasoline engine emissions, the difference of different alcohols in soot emissions have been
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evaluated. Maurya et al.29 compared the particulate emissions of HCCI engine after adding methanol and
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ethanol and observed that the concentration of PN after methanol addition was lower. Zhang et al.30
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reported that the PN concentrations and particle size from a GDI engine by ethanol addition were smaller
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than that of n-butanol. However, in order to separate the fuel chemistry from engine operating conditions, 31
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it is necessary to compare the PAHs formation characteristics of different alcohols addition in fundamental
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flames. Inal et al.32 compared the effect of methanol, ethanol, or MTBE (methyl tertiary-butyl ether)
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addition to n-heptane on the PAHs and soot formation in laminar premixed flames, and observed that the
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three oxygenate additives reduced the PAHs and soot by the same extent at the same oxygen content. Xu et
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al.33 investigated the effect of methanol or ethanol addition in n-heptane/toluene laminar premixed flames
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under the same carbon mass flow and the same equivalence ratio, and reported that the reduction of PAHs
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by ethanol was slightly higher than that of methanol due to the enhanced oxidation by producing more OH
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radicals. Esarte et al.34 compared the pyrolysis of acetylene mixed with methanol, ethanol, isopropanol, or
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n-butanol in a flow reactor. They observed that methanol addition decreased the soot most, and pointed out
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this is due to the lowest C/O ratio of methanol.
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The effect of methanol, ethanol, and n-butanol addition to gasoline on soot formation in laminar
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diffusion flames have been investigated in our previous studies.35-37 These researches showed that the soot
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decreasing ability of short chain alcohol is higher than that of long chain alcohol. In order to explain this
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phenomenon, it is necessary to further study the growth process of soot precursors in diffusion flames. The
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growth mechanism of PAHs in diffusion flames of gasoline and alcohol/gasoline blends are not yet known.
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In addition, there is a lack of experiment databases of the PAHs for developing an accurate soot prediction
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model of alcohol-gasoline, especially in comparison between alcohols with different carbon lengths. Thus,
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in this study, the two-dimensional relative concentration distribution of PAHs in laminar diffusion flames of
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alcohols and gasoline blends was measured using the planar laser-induced fluorescence (PLIF) technique.
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The effect of methanol, ethanol, and n-butanol addition to gasoline on PAHs formation characteristic was
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comparatively studied at both the same mixing ratio and the same oxygen content, and the effect of alcohol
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mixing ratio from 0% to 80% was also investigated. The purpose of this study was to compare the effect of
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different alcohols addition to gasoline on the formation and growth characteristics of PAHs in laminar
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diffusion flames, and to provide reference data for the development of accurate soot prediction models for
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alcohols/gasoline blends.
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2. Experimental method and setup
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2.1 Fuels
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This study investigated the pure gasoline (RON: 92, from a commercial gas station in Beijing, China)
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denoted as G100, and mixtures of methanol, ethanol, or n-butanol with gasoline. The gasoline used in this
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study consists of 49.2% saturates, 14.7% olefins, and 36.1% aromatics.38 The fuel mass flow rate was
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maintained at 7 g/h (with an uncertainty of about 0.05 g/h). The physical and chemical properties are shown
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in Table 1. This experiment included three parts: (1) to compare the effect of alcohols with different carbon
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chain lengths on the formation and growth of PAHs at the same dilution level; (2) to study the effect of the
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alcohol mixing ratios on the formation and growth of PAHs; (3) to study the effect of alcohol molecular
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structure at the same oxygen content. In the first part, three kinds of mixtures were investigated respectively,
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where the mixing ratio of methanol, ethanol, or n-butanol was 20 vol%, denoted as M20, E20 and B20. In
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the second part, the blending ratio of methanol, ethanol, or n-butanol was 20, 40, 60, and 80 vol%
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respectively. In the third part, the oxygen content of each alcohol and gasoline mixture was maintained at
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5%, and the blending ratio of methanol, ethanol and n-butanol was respectively 9.2%, 13.2% and 21.2%,
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denoted as M9.2, E13.2 and B21.2.
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Table 1 Physical and chemical properties of gasoline and alcohols Fuel
Gasoline
Methanol
Ethanol
N-Butanol
Carbon content (Weight %)
86.24
37.5
52.2
64.9
Hydrogen content (Weight %)
13.76
12.5
13.0
13.5
Oxygen content (Weight %)
B20 > E20 > M20, which is consistent with the trend of the flame lift-off length. Since
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that burning velocity of alcohols are much faster than that of gasoline, with methanol the fastest49,50 and the
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low temperature oxidation chemical reaction time is shorter, the flame lift-off length is lower. This in turn
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decreases the mutual diffusion of fuel and air and reduces the entrainment of air at the flame lift-off height,
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which leads to a reduction in the intensity of high temperature reaction zone.
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287 288
Figure 5. The comparison of OH luminescence intensity in the G100, M20, E20, and B20 flames: (a) the
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OH distribution; (b) the radial maximum intensity at each height position
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The distribution of CH luminescence intensity in the G100, M20, E20, and B20 flames is compared in
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Fig. 6a. It is found that there is almost no CH generation in the “bell-shaped” area at the bottom of the
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flame (HAB 0~20 mm), the high concentration of CH mainly locates at the interface between periphery OH
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and inner flame in the middle of the flame (HAB 20~30mm). This is because, on the one hand, only after
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heating, pyrolysis (HAB > 20 mm), can the fuel stream generate a large number of small radicals
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containing carbon. On the other hand, OH, O, O2 radicals are widely distributed in the periphery of the
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flame (see Fig. 5a), so a large number of CH radicals are formed at the interface. Specially, the profiles of
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radial maximum intensity at each height position in the G100, M20, E20, and B20 flames is shown in Fig.
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6b. The peak CH intensity is: G100 > B20 > E20 > M20, which is consistent with the trend of OH peak
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intensity (see Fig. 5). Giassi et al.51 pointed out that peak CH concentration can be a sign for the change of
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peak heat release rate. Orain et al.52 reported that OH radical can be also used as an indicator of heat release
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rate. Thus, the trend of the peak intensities of OH and CH also reflect the trend of the heat release rate:
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G100> B20> E20> M20, which can be explained from the heating value of the fuels. From Tab. 1, it can be
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seen that the low heating value is 43.5, 20.26, 25, 33.1 MJ/kg for gasoline, methanol, ethanol, and
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n-butanol.
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In Fig. 6b, there is a slight difference in the location of the CH peak. CH in the M20 flame peaks at the
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lowest height, followed by E20 and B20, and CH in the G100 flame peaks at the highest position. Since
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that M20 has the largest fuel vapor outlet velocity at the same fuel mass flow rate, the time required for
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M20 to complete pyrolysis is shortest compared to E20 and B20. In addition, it is noted that the position at
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which the CH radical reaches a high concentration (HAB about 25~30 mm) almost exactly corresponds to
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the position where the concentration of the larger aromatic rings begins to rapidly decrease (see Fig. 3b).
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(a) 50
G100
M20
E20
B20
3000
2500
Height above burner (mm)
40 2000
30 1500
20 1000
10
0
311
500
-4-2 0 2 4 r (mm)
-4-2 0 2 4 -4-2 0 2 4 r (mm)r(mm)r (mm)
-4-2 0 2 4
0
(b) 1.0 Normalized max. CH intensity
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0.8 0.6 0.4 0.2 0.0
312
G100 M20 E20 B20
0
10
20 30 40 Height above burner (mm)
50
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Figure 6. The comparison of CH luminescence intensity in the G100, M20, E20, and B20 flames: (a) the
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CH distribution; (b) the radial maximum intensity at each height position
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3.2 Effect of alcohol blending ratio
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In order to better understand the effect of alcohols mixing ratio on PAHs formation, the normalized
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maximum LIF signals in the methanol/gasoline, ethanol/gasoline, or n-butanol/gasoline flames with the
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alcohol mixture ratio of 0% to 80% are shown in Fig. 7. It can be seen that in gasoline laminar diffusion
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flames, the addition of alcohols always decreases the formation of PAHs with different ring sizes. The
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concentration of PAHs with different ring sizes shows a monotonous decreasing relationship with the
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alcohol blending ratios. For each alcohol, on the one hand, with the increase of alcohol blending ratio, the
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carbon content in the fuel stream is gradually diluted. Wu et al.20 pointed out that ethanol addition could
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reduce the carbon content that is available to form PAHs. Specifically, Khosousi et al.38 explained that the
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decline in carbon content attributes to a decline in aromatics. Furthermore, Korobeinichev et al.21 pointed
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out that the carbon content that is available to form PAHs reduces is due to the existence of the reaction
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path of ethanol reducing the formation of benzene and propargyl radicals. On the other hand, with the
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increase of alcohol blending ratio, the oxygen content in the fuel stream gradually increases, which can
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enhance the oxidation reaction. Lemaire et al.24 investigated the PAHs formation characteristics in
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ethanol-gasoline turbulent spray flames and pointed out the PAHs reduction mainly attributes to the
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combined effects of dilution and increased oxygen content. Thus, the monotonous decreasing trend
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between PAHs concentration and the alcohol blending ratios can attributes to the dilution of carbon content
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that is available to form PAHs and the increase of oxygen content. Normalized max. LIF signals
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1.0
(a) 320 nm
0.8
methanol ethanol butanol
(b) 360 nm
methanol ethanol butanol
(c) 450 nm
methanol ethanol butanol
0.6 0.4 0.2 0.0
0
20 40 60 80 Alcohol volume fraction (%)
0
20 40 60 80 Alcohol volume fraction (%)
0
20 40 60 80 Alcohol volume fraction (%)
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Figure 7. Normalized maximum LIF signals of PAHs with different ring sizes in the flames of methanol,
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ethanol, or n-butanol and gasoline blends
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Furthermore, the reduction percentages of maximum LIF signals for different alcohol-gasoline blends
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at four blending ratios are displayed in Fig. 8. For the methanol, compared with pure gasoline, the peak LIF
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signals of 320 nm (A1) decrease by 25.8%, 43.2%, 58.7%, and 73.8% for the M20, M40, M60, and M80
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flames respectively, and the peak LIF signals of 450 nm (A4-A5) decrease by 34.3%, 58.7%, 74.6%, and
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88.5% for the M20, M40, M60, and M80 flames respectively. For the ethanol, the peak LIF signals of 320
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nm decrease by 23.2%, 40.8%, 51.2%, and 68.7% for the E20, E40, E60, and E80 flames respectively, and
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the peak LIF signals of 450 nm decrease by 25.8%, 53.5%, 69.3%, and 83.7% respectively. It can be seen
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that adding methanol or ethanol to reduce large ring aromatics is always much greater than one ring
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aromatics, and the reduction percentage of large ring aromatics is always greater than the mixing ratio of
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alcohols. This indicates that methanol or ethanol addition not only reduces the formation of A1 by dilution
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effect, but also significantly inhibits the growth of PAHs. Methanol molecule has only one carbon atom
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directly attached to the oxygen atom, and the carbon atom is not easily converted to PAHs and precursors.
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However, n-butanol molecules have four carbon atoms, and three are not connected with the oxygen atom,
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which results in the formation of unsaturated hydrocarbons contributed to the growth of PAHs. Thus, the
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monotonous decreasing of PAHs by methanol and ethanol addition is due to both the dilution of aromatics
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in gasoline and inhibition of the growth of PAHs. Furthermore, the inhibition of the growth of PAHs by
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methanol is more pronounced than that of ethanol. Thus, the lower the carbon number of the normal chain
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alcohol, the more it helps to reduce the formation and growth of PAHs.
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For the n-butanol, compared with pure gasoline, the peak LIF signals of 320 nm for the B20, B40, B60,
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and B80 flames decrease by 17.6%, 28.0%, 35.7%, and 46.1% respectively, and the peak LIF signals of 450
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nm decrease by 10.0%, 28.4%, 54.6%, and 66.9% respectively. Unlike methanol and ethanol, the
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percentage reduction of PAHs with different ring sizes by n-butanol is always lower than its blending ratio.
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This shows that n-butanol has little inhibitory effect on the growth of PAHs, and the monotonous
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decreasing of PAHs by n-butanol addition is mainly due to the dilution of aromatics in gasoline.
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Among different alcohols, the decrease of PAHs are the most significant when methanol is added
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compared to ethanol and n-butanol. As for the differences in the monotonic trends between different
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alcohols addition, it is pointed out in the previous analysis that it may be caused by the oxygen content or
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the molecular structure. Methanol has an oxygen content of 50%, ethanol 34.8%, and n-butanol 21.6%.
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Under the same blending ratio, the oxygen content of methanol/gasoline is the highest, followed by
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ethanol/gasoline, n-butanol/gasoline is the lowest. The factors determining the ability of alcohols to reduce
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PAHs will be further analyzed based on the experiment results at the same oxygen content in the next
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section. Percentage reduction compared to G100 (%)
(a) 320 nm 80
methanol ethanol n-butanol
60
40
20
0
368
20
40 60 Alcohol mixing ratio (%)
80
(b) 450 nm Percentage reduction compared to G100 (%)
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80
methanol ethanol n-butanol
60
40
20
0
20
40 60 Alcohol mixing ratio (%)
80
370
Figure 8. Reduction percentages of maximum LIF signals for different alcohol-gasoline blends at four
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blending ratios
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In order to understand the effect of alcohols mixing ratio on PAHs distribution characteristics, the
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concentration distribution of PAHs with different aromatic ring numbers in the G100, E20, E40, E60, and
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E80 flames is shown in Fig.9. It can be seen that with the increase of ethanol ratio, the overall PAHs with
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different ring sizes in the flame decreases gradually. When the ethanol ratio increases up to 80%, the signal
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of LIF-450 in the flame almost disappears. In addition, as the ethanol mixing ratio increases, the visual
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flame height decreases, as well as the distribution area of PAHs with different aromatic ring numbers
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gradually decrease, and the most significant decrease is large aromatic rings (450nm). (a) 320nm 50
(c) 450nm
(b) 360nm 4000
G100
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r (mm)
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Figure 9. The concentration distribution of different aromatic rings in the ethanol/gasoline flames with
381
different mixing ratios
382
In order to understand the effect of alcohol mixing ratio on flame structure, the distribution of OH
383
luminescence intensity in the G100, E20, E40, E60, and E80 flames is shown in Fig. 10a. It is observed that
384
the distribution characteristics of OH are similar for ethanol/gasoline laminar diffusion flames with
385
different blending ratios, that is, OH mainly distributes at the periphery of the flame, whose luminescence
386
intensity peaks near the nozzle outlet. Specially, the radial maximum intensity at each height position and
387
the flame lift-off length in the G100, E20, E40, E60, and E80 flames are shown in Fig. 10b. Compared with
388
pure gasoline, the OH peak intensity in the E20, E40, E60, and E80 flames decrease by 24.7%, 29.2%,
389
46.3%, and 50.5% respectively. The intensity of the high temperature reaction at the flame lift-off height
390
shows a monotonous decreasing trend with the alcohol-blending ratio, which is consistent with the trend of
391
flame lift-off length. As shown in Fig. 10b, the flame lift-off lengths are 5.08, 4.00, 3.81, 3.08, and 3.00
392
mm for the G100, E20, E40, E60, and E80 flames. Moreover, according to the flame lift-off length and the
393
calculated fuel outlet velocity (see Fig. 11), the time it takes for the fuel stream leaving the nozzle outlet to
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Energy & Fuels
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the high temperature reaction zone can be calculated, as shown in Fig 11. At the same fuel mass flow rate,
395
the fuel outlet velocity increases linearly with the alcohol-mixing ratio. The time for the fuel stream to the
396
high temperature reaction zone shows a monotonous decreasing trend with the ethanol-mixing ratio. The
397
shorter low temperature chemical reaction time reduces the air entrainment volume, then reducing the
398
content of H2O2 produced via the oxidation reactions at flame lift-off height, and ultimately inhibit the OH
399
radicals mainly produced by H2O2 + M = 2OH + M.53 Furthermore, it can be inferred that the alcohol
400
addition reduces the temperature of the flame due to the weakened “Hot flame” reaction (CO + OH = CO2
401
+ H).53
(a) G100
Height burner (mm) Heightabove above burner (mm)
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Flame lift-off height (mm)
(b) 1.0 Normalized max. OH intensity
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0.8 0.6 0.4
404
4 3 2 1 0
0
20 40 60 80 Ethanol mixing ratio (%)
G100 E20 E40 E60 E80
0.2 0.0
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5
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5
10 15 20 25 Height above burner (mm)
30
35
Figure 10. The comparison of OH luminescence intensity in ethanol/gasoline flames with different mixing
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ratios: (a) the OH distribution; (b) the radial maximum intensity at each height position 0.06
9.8
0.05 9.6 0.04 9.4 0.03 0.02
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0.01
Fuel outlet velocity (cm/s)
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OH peak time (s)
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Energy & Fuels
9.0 0.00
406
0
20 40 60 Ethanol mixing ratio (%)
80
407
Figure 11. Relationship between OH intensity peak time and ethanol mixing ratio after the fuel stream
408
leaves the nozzle
409
Fig. 12a shows the distribution of CH luminescence intensity in the G100, E20, E40, E60, and E80
410
flames. It is found that as the ethanol mixing ratio increases, the flame height and CH luminescence
411
intensity decrease gradually, and the high concentration area of CH radicals gradually move toward the top
412
of the flame. Besides, the “bell-shaped” area where no CH generation at the lower part of the flame
413
increases with the increasing ethanol mixing ratio. Specially, the profiles of radial maximum intensity along
414
the axial direction in the G100, E20, E40, E60, and E80 flames are displayed in Fig. 12b. Compared with
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pure gasoline, the CH peak intensities in the E20, E40, E60, and E80 flames decreased by 6.6%, 19.9%,
416
30.7%, and 57.4% respectively. The CH luminescence intensity shows an enhanced monotonous decreasing
417
trend with ethanol blending ratio, inferring that the heat release rate also decreases gradually with the
418
increasing ethanol content.51 In addition, it can be seen CH luminescence intensity peak at a height of
419
around 26 mm for the G100, E20, E40, E60, and E80 flames. Under the same fuel mass flow rate, the exit
420
velocity of the fuel vapor increases as the ethanol blending ratio increases (see Fig. 11). Therefore, as the
421
ethanol content increases, the shorter the time required for pyrolysis of the fuel stream.
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Energy & Fuels
(a) G100
Height above (mm) Height above burner burner (mm)
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(b) G100 E20 E40 E60 E80
1.0 Normalized max. CH intensity
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0.8 0.6 0.4 0.2 0.0
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0
10
20 30 40 Height above burner (mm)
50
424
Figure 12. The comparison of CH luminescence intensity in ethanol/gasoline flames with different mixing
425
ratios: (a) the CH distribution; (b) the radial maximum intensity at each height position
426
3.3 Effect of different alcohols at the same oxygen and carbon content
427
In order to understand the role of oxygen content of normal alcohols in its ability to reduce PAHs, the
428
effect of methanol, ethanol, and n-butanol on PAHs formation are further investigated at the same oxygen
429
content. At the same oxygen content (5%), the mixing ratio of methanol is the smallest (9.2%), followed by
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ethanol (13.2%), n-butanol is the largest (21.2%). In this case, the carbon content of M9.2, E13.2, and
431
B21.2 is 81.39%, 81.35%, and 81.31%, thus the carbon content is also similar. Fig. 13 shows the
432
concentration distribution of PAHs with different ring sizes in M9.2, E13.2, and B21.2 flames at the same
433
oxygen content. It is observed that compared to pure gasoline, the LIF signals at different wavelengths in
434
M9.2, E13.2 and B21.2 flames all decrease, with the most significant decrease being at 450 nm. The
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435
alcohol addition reduces the distribution area of large PAHs (450 nm) with methanol being the most
436
significant, and has a little effect on the distribution area of small aromatics (320 nm). For M9.2, E13.2,
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and B21.2 flames, the difference of distribution characteristics at 320 and 360 nm is not significant,
438
whereas at 450 nm there was a significant difference in their high concentration areas and peaks. (a) nm (a) 320 320 nm 50
G100
M9.2
E13.2
(b) (b)360 360 nm nm B21.2
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r (mm)
0
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r (mm)
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Figure 13. The concentration distribution of different aromatic rings in the G100, M9.2, E13.2, and B21.2
441
flames
442
Specifically, the normalized maximum LIF signals around 320, 360, and 450 nm in the G100, M9.2,
443
E13.2, and B21.2 flames are displayed in Fig. 14. At 320 nm (A1), compared with pure gasoline, the peak
444
LIF signals in the M9.2, E13.2, and B21.2 flames decrease by 9.2%, 12.0%, and 17.7%. The difference
445
between the three alcohols is large. Thus, under the same oxygen and carbon content condition, the ability
446
of n-butanol to decrease A1 (320 nm) is the strongest, followed by the ethanol, and the methanol is the
447
weakest, which is different from the result at the same mixing ratio (see Fig.4). This is mainly because that
448
the gasoline contains about 36% aromatics (mainly one and two ring), the one ring aromatics can be better
449
diluted by adding more alcohol. This also indicates that in gasoline laminar diffusion flame, the mixing
450
ratio of alcohol (that is dilution level) plays a key role in A1 formation. However, at 360 nm (A2-A3),
451
compared with pure gasoline, the peak LIF signals in the M9.2, E13.2, and B21.2 flames decrease by
452
17.0%, 17.5%, and 18.6%. Although B21.2 has the strongest effect of diluting two ring aromatics in
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453
gasoline, finally the effects of M9.2, E13.2 and B21.2 on the reduction of A2-A3 are almost the same. This
454
is due to the effect of surface growth inhibition. M9.2 had the strongest inhibitory effect on Al growth to
455
A2-A3, which counteracted the difference in dilution levels between it and B21.2.
456
At 450 nm (A4-A5), compared with pure gasoline, the peak LIF signals in the M9.2, E13.2, and B21.2
457
flames decrease by 25.7%, 18.4%, and 10.2%. The ability of methanol to decrease A4-A5 (450 nm) is the
458
strongest, followed by the ethanol, and the n-butanol is the weakest, which is consistent with the results at
459
the same mixing ratio (see Fig. 4). This indicates that at the same dilution level, the ability of alcohols to
460
reduce PAHs (see Fig. 4) is not due to its oxygen content but mainly due to the differences in molecular
461
structure. Methanol molecule has only one carbon atom directly attached to the oxygen atom, Westbrook et
462
al. pointed out that once the carbon atom form a stable C-O bond with oxygen atoms in the oxygenated fuel,
463
the carbon atom would not be converted into PAHs. In fact, the reaction path [(45), (49)] of methanol is:
464
CH3OH → CH2OH → CH2O → HCO → CO → CO2, in which unsaturated hydrocarbons are not formed.
465
However, ethanol or n-butanol molecules have two or four carbon atoms, respectively, and one or three are
466
not connected with the oxygen atom, which leads to the formation of unsaturated hydrocarbons contributed
467
to the growth of PAHs.54,55 Therefore, it can be inferred that the carbon chain length of the normal alcohols
468
plays a key role in reducing the formation of large aromatic rings. The shorter the carbon chain length, the
469
more conducive to reduce the formation of large aromatic rings in the gasoline diffusion flames. Normalized max. LIF signals
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470 471
(a) 320 nm
1.0
(b) 360 nm
1.0
0.8
0.8
0.8
0.6
0.6
0.6
0.4
0.4
0.4
0.2
0.2
0.2
0.0
G100
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B21.2
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G100
M9.2
E13.2
(c) 450 nm
1.0
B21.2
0.0
G100
M9.2
E13.2
B21.2
Figure 14. The comparison of normalized maximum LIF signals in the G100, M9.2, E13.2, and B21.2
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472 473
flames 4. Conclusions
474
Based on laminar diffusion flames, the study measured the relative concentrations of PAHs with
475
different ring sizes by PLIF technique, and recorded the OH and CH luminescence intensities by
476
application of band-pass filter. The effect of methanol, ethanol, and n-butanol addition to gasoline on the
477
formation and growth of PAHs was comparatively studied at both the same mixing ratio and the same
478
oxygen content, and the effect of alcohol mixing ratio from 0% to 80% was investigated.
479
At the same mixing ratio, the PAHs reducing ability decreases with the chain length, that is methanol >
480
ethanol >n-butanol. As the alcohol ratio increases, all size PAHs show a monotonous decreasing trend. The
481
ability of methanol or ethanol to reduce large ring aromatics is always far greater than that to one ring
482
aromatics, even the percentage reduction of large ring aromatics is always greater than the alcohol mixing
483
ratio, while the percentage reduction of all size PAHs by n-butanol is always lower than its blending ratio.
484
Methanol has the most significant inhibition on the growth process of large aromatic rings, and the
485
n-butanol is very small.
486
At the same oxygen and carbon content, the ability to decrease A1 (320 nm) is n-butanol (B21.2) >
487
ethanol (E13.2)> methanol (M9.2), indicating that the dilution level plays a key role in A1 formation. The
488
ability of alcohols to decrease A4-A5 (450 nm) is consistent with the results at the same mixing ratio,
489
inferring that the molecular structure rather than oxygen and carbon contents dominates the ability to
490
reduce large PAHs. The shorter the carbon chain length, the more conducive to reduce the formation of
491
large PAHs.
492
As the aromatic ring number increases, the high-concentration region of aromatics gradually evolves
493
from the flame center to the two wings of the flame. The flame lift-off length and the intensity of the high
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Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
494
temperature reaction at the flame lift-off height based on the OH chemiluminescence shows a monotonous
495
decreasing trend with the alcohol chain length and alcohol-mixing ratio. CH mainly locates at the interface
496
between periphery OH and inner flame in the middle of the flame, and its luminescence intensities show a
497
monotonous decreasing trend with the alcohol ratio.
498
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499
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Figure Captions
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Figure 1: The schematic of liquid burner and PLIF systems
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Figure 2: The distribution of PAHs-LIF signals detected at 320, 360, and 450 nm in the G100, M20, E20, and
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B20 flames
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Figure 3: The profiles of normalized PAHs-LIF signals along the axial centerline of the G100, M20, E20,
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and B20 flames
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Figure 4: The comparison of maximum LIF signals in the G100, M20, E20, and B20 flames
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Figure 5: The comparison of OH luminescence intensity in the G100, M20, E20, and B20 flames: (a) the OH
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distribution, (b) the radial maximum intensity at each height position
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Figure 6: The comparison of CH luminescence intensity in the G100, M20, E20, and B20 flames: (a) the CH
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distribution, (b) the radial maximum intensity at each height position
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Figure 7: Normalized maximum LIF signals of PAHs with different ring sizes in the flames of methanol,
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ethanol, or n-butanol and gasoline blends
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Figure 8: Reduction percentages of maximum LIF signals for different alcohol-gasoline blends at four
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blending ratios
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Figure 9: The concentration distribution of different aromatic rings in the ethanol/gasoline flames with
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different mixing ratios
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Figure 10: The comparison of OH luminescence intensity in ethanol/gasoline flames with different mixing
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ratios: (a) the OH distribution, (b) the radial maximum intensity at each height position
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Figure 11: Relationship between OH intensity peak time and ethanol mixing ratio after the fuel stream leaves
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the nozzle
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Figure 12: The comparison of CH luminescence intensity in ethanol/gasoline flames with different mixing
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ratios: (a) the CH distribution, (b) the radial maximum intensity at each height position
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Figure 13: The concentration distribution of different aromatic rings in the G100, M9.2, E13.2, and B21.2
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flames
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Figure 14: The comparison of normalized maximum LIF signals in the G100, M9.2, E13.2, and B21.2
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flames
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