Article pubs.acs.org/EF
Smoke Point Measurements of Diesel-Range Hydrocarbon− Oxygenate Blends Using a Novel Approach for Fuel Blend Selection Qi Jiao, James E. Anderson,* Timothy J. Wallington, and Eric M. Kurtz Research & Advanced Engineering, Ford Motor Company, MD RIC-2122, P.O. Box 2053, Dearborn, Michigan 48121-2053, United States S Supporting Information *
ABSTRACT: The use of oxygenated fuels decreases particulate matter (PM) emissions from diesel engines. Studies using engines, experimental flames, and modeling have shown that the decrease in soot emissions depends on the oxygenate molecular structure. To provide a better understanding of the complex processes occurring in engines leading to PM emissions, fundamental and systematic studies of the sooting tendency trends for diesel-range hydrocarbon−oxygenate blends are needed. We present a new approach to selecting fuel blends for sooting tendency measurements that minimizes the confounding effect of dilution of highly sooting components in the base fuel by maintaining constant concentrations of those components in the blends. This novel approach is illustrated by sooting tendency (smoke point) measurements in a diffusion flame for a variety of diesel-range hydrocarbon−oxygenate blends with different molecular structures. The oxygenates included primary alcohols (1butanol, 1-undecanol), diesters (dibutyl succinate, dibutyl maleate), esters (methyl decanoate and methyl oleate), and a glycol triether (tri(propylene glycol) methyl ether). The hydrocarbons included an aromatic (1,2,4-trimethylbenzene), a straight-chain alkane (n-hexadecane), and a highly branched alkane (2,2,4,4,6,8,8-heptamethylnonane). The fuels were investigated as threecomponent blends, with an oxygenate in a hydrocarbon base fuel consisting of a highly sooting hydrocarbon component (1,2,4trimethylbenzene) and a low-sooting hydrocarbon (n-hexadecane). The oxygen-extended sooting index (OESI) provided sooting tendency trends that were generally consistent with expectations for both hydrocarbon-only and oxygenated fuels. The dominant chemical structure factors influencing the sooting tendency of the hydrocarbons were aromaticity and branching. For the oxygenates, the primary alcohols, the saturated monoester, and the glycol triether exhibited the lowest sooting tendency, followed by the shorter-chain diesters, and then the unsaturated monoester, with unsaturation increasing the sooting tendency.
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diesel combustion in a process known as leaner lifted flame combustion (LLFC).14,15 While different oxygen functional groups can have greatly differing impacts on cetane ratings,16,17 high oxygen content generally leads to lower energy content. Better understanding of these relationships are critical for future fuel design. Fuel oxygen content affects engine-out PM emissions by two primary mechanisms: oxygen availability and combustion chemistry. Following the conceptual model of diesel combustion by Dec,18 oxygen downstream of the point of high temperature combustion reduces soot formation in a diesel flame, eventually yielding soot-free combustion with sufficient oxygen.19 Fuel-borne oxygen reduces the degree of air-fuel mixing needed to achieve this condition.14 Within the combusting diffusion flame, the chemical structure of oxygenated molecules also will influence combustion chemistry. To elucidate the influence of oxygenated fuel structure on soot emissions, many experimental, theoretical, and modeling studies have been conducted. Mueller et al.5 investigated the soot-reduction benefits of two potential diesel-fuel oxygenates, dibutyl maleate (DBM) and tri(propylene glycol) methyl ether (TPGME), using experimental and numerical approaches. TPGME was more effective at reducing soot than DBM,
INTRODUCTION Regulations for vehicle emissions are expected to continue to increase in stringency in the future,1 including further lowering of particulate matter (PM) emissions from diesel vehicles. While after-treatment, including a diesel particulate filter (DPF), is a key component to meeting these emission standards,2 engine-out emissions control is critical in a diesel engine. In particular, reductions in engine-out PM can both provide avenues to improve NOx, CO2, and fuel economy through calibration optimization and reduce the frequency with which the DPF must be regenerated, thus minimizing the associated emissions, CO2, and fuel economy penalty. Engineout PM emissions depend on operating conditions including engine load, fuel injection strategies, exhaust gas recirculation (EGR), in-cylinder fuel-air mixture formation, or advanced diesel concepts involving specific controls over combustion and chemical reaction processes.3−7 Fuel properties also affect PM formation in diesel engines, including oxygen content,7 aromatic content,8,9 and cetane rating.8 In general, desirable diesel fuel properties include high cetane rating as well as high energy content and density. Oxygen-containing compounds have been actively studied for PM emissions reduction7,10−12 and because of their potential production from renewable, nonfossil feedstocks. It has been widely observed that engine-out PM emissions decrease as fuel oxygen content increases.7,13 Moreover, increased fuel oxygen content has been suggested as a pathway to achieving soot-free © XXXX American Chemical Society
Received: July 16, 2015 Revised: October 16, 2015
A
DOI: 10.1021/acs.energyfuels.5b01624 Energy Fuels XXXX, XXX, XXX−XXX
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Energy & Fuels Table 1. Test Compounds and Their Chemical Structuresa
a
Astertisk indicates compounds for which sooting tendency measurements are not believed to have been previously reported.
Pepiot-Desjardins et al.27 used SP measurements to investigate the effect of C1−C8 oxygenates with different functional groups on sooting propensity by blending the target oxygenate with a base fuel containing toluene and n-heptane at a fixed volume ratio (35:65). In blends of the base fuel and oxygenates, decreases of sooting propensity (based on decreasing TSI) were seen with increasing oxygen mass fraction, though at different rates for different oxygenate functional groups. In their experiments, oxygenate addition reduces sooting by two mechanisms: the effect of oxygen in its functional group and a dilution effect in which the more sooting base fuel is replaced by the less sooting oxygenate. The dilution effect confounds the results, making comparisons more difficult. Barrientos et al.28 recently measured SP of fuel blends using the base fuel from ref 27 and 40 different target compounds (hydrocarbons up to C14 and oxygenates up to C12). TSI was shown to be inappropriate for oxygenates due to resulting trends of increasing sooting tendency with oxygen content. It was argued that TSI improperly includes the fuel-borne oxygen from a fuel mass perspective (through its normalization by MW) and does not consider oxygen’s more important effect of reducing the required air entrainment for stoichiometric combustion. A new sooting index was proposed, oxygen extended sooting index (OESI), that includes the effect of fuelborne oxygen by normalizing by the stoichiometric combustion requirement for oxygen from air. The OESI is calculated as follows:
reportedly because the oxygen atoms in TPGME are more evenly spread throughout the molecule than in DBM where the oxygen is present in the ester group is readily converted into CO2. Westbrook et al.20 used detailed chemical kinetic reaction mechanisms for diesel fuel and oxygenates, including DBM and TPGME, and simulated fuel-rich premixed ignition corresponding to conditions of initial soot formation.18 On the basis of the concentrations of known soot precursors, all oxygenates investigated suppressed soot formation. Differences between oxygenates were attributed in part to differences in reaction pathways associated with chemical structures. It was concluded that soot formation is initiated by a fuel-rich premixed ignition of diesel fuel and is later quenched due to lack of oxygen to support the complete burning of the fuel, while subsequent soot burnout takes place in a diffusion flame. A variety of experimental techniques for measuring the sooting tendencies of fuels have been developed, involving both premixed and diffusion flames.21−24 Smoke point (SP) measurement has been found to be a useful and efficient way to explore the sooting tendency of hydrocarbon fuels in a diffusion flame, as reviewed in ref 25. Smoke point is defined as the height in millimeters of the highest flame produced without soot breakthrough when the fuel is burned in a specific wick-fed test lamp.26 With the introduction of the threshold sooting index (TSI) by Calcote and Manos,22 measured SP values from different research groups or different burner types are well correlated and provide a consistent trend of the sooting tendency of individual hydrocarbons. The TSI is defined as follows: TSI = a(MW/SP) + b
OESI = a′(n + [m /4] − [p /2])/SP + b′
(2)
where a′ and b′ are constants for a given experimental setup; and n, m, p are the number of carbon, hydrogen, and oxygen atoms per fuel molecule. OESI was found to be applicable for oxygenates, hydrocarbons, and their blends. In the present study, sooting tendencies of different dieselrange hydrocarbons and oxygenates with different molecular
(1)
where a and b are constants used to scale TSI from 0 to 100 for a given apparatus used for the SP measurement, MW is the molecular weight of the fuel, and SP is the measured smoke point. B
DOI: 10.1021/acs.energyfuels.5b01624 Energy Fuels XXXX, XXX, XXX−XXX
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Energy & Fuels Table 2. Test Compound Properties short name
MW (g/mol)
BPa (°C)
densitya (g/cm3)
oxygen (mass %)
H:C molar ratio
O:C molar ratio
NHVa (MJ/kg)
CNb or (DCN)
TMB NHD HMN BuOH UnOH TPGME DBS DBM MD MO
120 226 226 74 172 206 230 228 186 296
169 287 246 119 247 242 275 280 232 346
0.872 0.769 0.782 0.804 0.831 0.966 0.977 0.991 0.868 0.870
0 0 0 22 9 31 28 28 17 11
1.333 2.125 2.125 2.5 2.182 2.2 1.833 1.667 2 1.895
0 0 0 0.25 0.091 0.4 0.333 0.333 0.182 0.105
41.0 43.9 43.9 33.1 39.0 27.7 27.4 27.9 34.1 37.4
(9) 100 15 (8) 53 63 13 29 48 56
Boiling point (BP), density at 25 °C, and net heating values (NHV) are from ref 34 or ref 35. For DBS, BP and density at 20 °C are from ref 36 and NHV is inferred from ref 7. bIf the cetane number (CN) was unavailable in ref 37, then the derived cetane number (DCN) is shown in parentheses.
a
(98 °C for n-heptane and 111 °C for toluene) and higher molecular weight oxygenates such as TPGME (242 °C). This difference may lead to the base fuel vaporizing and burning faster than the oxygenate, even though both the base fuel and oxygenate are drawn up the wick together at the same rate. Instead, base fuels containing two hydrocarbons with more similar (higher) boiling points were used, i.e., a base fuel containing 1,2,4-trimethylbenzene (TMB) and nhexadecane (NHD). This approach completely eliminated the phenomenon of pooling for the fuel blends investigated in this work. In addition, 2,2,4,4,6,8,8-heptamethylnonane (HMN) was included as an example of a highly branched hydrocarbon. Both HMN and NHD are used in primary reference fuels for cetane number determinations. The compositions of the various blends used in the study are provided in the Supporting Information (SI) in Table SI-1. Experimental Approach. A standard smoke point lamp (Koehler Instrument Company, Inc., Bohemia, NY) as specified by ASTM D132226 was used. A black-painted steel enclosure protected the lamp from room air disturbances. Test procedures strictly followed the ASTM method D1322 in which the flame height was controlled by adjusting the length of the wick extending above the wick tube. Triplicate readings were made for each fuel. Data for each fuel and associated statistical information are provided in the SI (Tables SI-1 and SI-2 and associated text). The overall uncertainty in the reported smoke point measurements is approximately 0.6 mm. These were used to calculate 95% confidence intervals for TSI and OESI. The measurement of SP was calibrated according to ASTM D1322 using the two standard reference fuel blends containing 20 vol % and 40 vol % toluene in 2,2,4-trimethylpentane. Ambient temperature and relative humidity were monitored to verify that the environmental conditions did not affect the smoke point measurements. Ambient temperature was 21−26 °C, and relative humidity was 50−58%. No discernible effect of ambient temperature or relative humidity on the measured smoke points was observed. All measurements were performed at atmospheric pressure (approximately 99 kPa).
structure (i.e., branching, unsaturation, aromatic, alcohol, ester, ether) were measured using the smoke point method. Analysis of sooting tendencies were conducted with the two available methods: TSI and OESI. Typically, an oxygenate is blended into a base fuel, which dilutes the sooting contribution of the base fuel.27,28 Here, a novel approach is used in which an oxygenate blend is prepared by replacing the low sooting component in the base fuel while holding the high sooting component constant, thereby reducing the confounding effect of dilution. To our knowledge, this approach has not been used in previous experimental sooting tendency studies.21,22,24,25,27−33
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MATERIALS AND METHODS
Fuels. The oxygenates and hydrocarbons selected for the present study are listed in Table 1 and their properties are given in Table 2. The choice of oxygenates was largely based on relevance to diesel combustion and past experimental studies. A variety of oxygenate types (two alcohols, two monoesters, two diesters, and a glycol triether) were tested. Aldehydes and ketones typically present odor issues and were not tested. Both methyl decanoate (MD) and methyl oleate (MO) are commonly used experimental surrogates for biodiesel fatty acid methyl esters (FAMEs). MO is typically the major monounsaturated component in biodiesel produced from most plant oils, including soy and canola. MD is sometimes used as a surrogate for the saturated esters in biodiesel because methyl stearate (the saturated equivalent of MO) is a solid at room temperature. While 1-butanol (BuOH) has a low boiling point for diesel fuel, it has recently seen interest for experimental diesel applications; 1-decanol (UnOH) was included as it has a diesel-range boiling point. TPGME, a glycol triether, dibutyl maleate (DBM), and dibutyl succinate (DBS), both diethers, have been included in previous studies.5,7,11,20,14 Of these compounds, sooting tendency measurements for UnOH, TPGME, DBS, MD, and MO are not believed to have been previously reported. In this study, attempts to measure the SP of many individual oxygenates resulted in charring of the wick tip (wick tip turning black) probably because the wick could not draw sufficient fuel to match the rate of evaporation and burning, thus allowing the wick tip to burn instead of the fuel. As noted previously by Tran et al.,33 this phenomenon affects the flow, vaporization, and burning of the fuel and thus impacts the smoke point measurement. Tran et al.33 reported this phenomenon for biodiesel blends with greater than 25 vol % biodiesel, speculating that soot on the wick prevented the vaporization of the fuel, which then artificially diminished the flame height as the test progressed. As a result, oxygenates in the present study were mixed with higher sooting propensity fuels to allow valid SP measurements. Initial testing used a base fuel consisting of n-heptane and toluene,27,28 but significant pooling of liquid fuel was observed at the base of the lamp’s wick guide for blends with higher boiling point oxygenates such as TPGME. This phenomenon was likely caused by the significant difference in the boiling temperatures of the base fuel
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RESULTS AND DISCUSSION As discussed earlier, the TSI mixing rule has been the accepted approach for describing and predicting the sooting tendency of hydrocarbon fuel blends. Barrientos et al.28 showed that TSI is inadequate for blends involving oxygenates and instead proposed the OESI. In light of this, and to evaluate these indices for diesel-range fuel compounds, experimental results are presented in terms of SP, TSI, and OESI in the ensuing discussion. For both TSI and OESI, good predictive behavior is expected if the indexed data are approximately linear when plotted versus mole fraction of the components. Model Constants for Sooting Indices. In this study, toluene and isooctane were used as reference fuels to calculate the model constants for TSI and OESI. Measured SP values for toluene and isooctane were 8.3 mm and 42.2 mm, respectively. C
DOI: 10.1021/acs.energyfuels.5b01624 Energy Fuels XXXX, XXX, XXX−XXX
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Energy & Fuels TSI values suggested by Olson et al.29 (44 for toluene and 6.4 for isooctane) were used to determine the model constants (a,b) for the TSI method, while OESI values from ref 28 (46.1 for toluene and 6.8 for isooctane) were used to calculate the model constants (a′,b′) for the OESI method. The derived model constants, a = 4.5 and b = −5.8 for TSI, a′ = 50.16 and b′ = −8.07 for OESI, were used in this study. Sooting Indices for Hydrocarbon and OxygenateHydrocarbon Fuel Blends. Smoke points for two sets of binary blends were obtained, one composed of two hydrocarbons (TMB and NHD) at different ratios, and the other composed of an aromatic hydrocarbon (TMB) and an oxygenate (TPGME). Compositions, measured smoke points with 95% confidence intervals, and calculated sooting indices (TSI and OESI) with 95% confidence intervals are presented in Tables SI-1 and SI-2. In Figure 1 (top panel), TSI values of the two blend series are presented as a function of the mole fraction of NHD and
increasing oxygen mass fraction. Blending into an aromaticcontaining hydrocarbon base fuel is necessary because the sooting tendency of oxygenates is often too low to be accurately measured using these techniques. However, results by this approach are confounded by the fact that the volume percentage of toluene is reduced as the oxygenate is added, while the volume ratio of n-heptane to toluene is maintained. Of the hydrocarbons present in gasoline and diesel, aromatics such as toluene exhibit the greatest sooting tendency in sooting tendency evaluations.22,27,30 By blending oxygenates into a fixed hydrocarbon base fuel, the sooting tendency of the blend is reduced by at least two factors: (1) reduced volume fraction of the base fuel containing a highly sooting aromatic hydrocarbon; and (2) increased volume fraction of the oxygenate with a different (likely lower) sooting tendency and increased oxygen content. The alternative approach used in this study is to maintain a constant volume fraction of the high sooting tendency component (TMB) in the blend to negate its primary role in influencing sooting tendency. Fuel blend series with eight additional compounds were investigated to explore the controlling factors of sooting tendency of oxygenated fuels. The eight compounds included seven oxygenates (BuOH, UnOH, TPGME, DBS, DBM, MD, and MO) and one hydrocarbon (HMN). In these experiments, starting with a NHD−TMB blend containing 30 vol % TMB, NHD was replaced by the third fuel component to form a threecomponent fuel blend with a constant 30 vol % TMB volume fraction. Also, the series of binary blends with TMB present at higher concentration than 30 vol % (Figure 1) was included, with TMB effectively serving as the third component for comparison with the other compounds. Finally, these data can be directly compared to blends with NHD serving as the third component (constant 70 vol % NHD and 30 vol % TMB, with NHD replacing itself). In the resulting blends, changes in sooting tendency result only from replacement of NHD by the added compound. In doing so, the observed difference in sooting tendency indicates the sooting tendency of the added compound relative to NHD. The resulting smoke point data are provided in Table SI-2 and plotted in Figure SI-2. Figure 2 shows TSI values of the blends of the 10 compounds into the base fuel containing 30 vol % TMB and the balance NHD. The TSI value is constant for the trivial case in which NHD replaces NHD. Substitution of NHD by additional TMB greatly increased TSI. Substitution of NHD by HMN, a highly branched hydrocarbon also containing 16 carbon atoms and having the same molecular weight and carbon content, increased TSI values indicating a greater sooting tendency for HMN than NHD. These trends for hydrocarbons exhibit good agreement with previous studies.38 The two alcohols exhibited the strongest propensity to reduce TSI values as their concentrations increased in the blend, replacing NHD. TPGME, the glycol triether, provided only small reductions in TSI values. DBM and MO, both monounsaturated esters, showed a significant increase in TSI values. Blends with DBS showed lower TSI values than its unsaturated analogue, DBM, as expected.28 However, while MD reduced TSI, DBS appeared to increase TSI, despite the fact that both DBS and MD are saturated esters. Next, these data are interpreted using the OESI, which has been shown to be more suitable for comparing the sooting tendency of oxygenated fuels.
Figure 1. TSI (top) and OESI (bottom) values for TMB−NHD binary blends and TMB−TPGME binary blends versus NHD and TPGME mole fractions. Lines are linear fits to the data.
TPGME. TSI decreases with the increase of NHD and TPGME and associated decrease of TMB in both blend series. The linear correlation was strong (R2 = 0.97) for the hydrocarbononly fuel blend (TMB-NHD) but was somewhat poorer for the oxygenate−hydrocarbon blend (TMB-TPGME). OESI values for the same blend series (Figure 1, bottom) show a strong linear correlation for both types of blends, with TPGME providing a somewhat stronger tendency in reducing soot than NHD. The latter result is consistent with the well-known effect of fuel-borne oxygen reducing PM emissions,5,7,20 while the opposite trend is suggested by TSI. These considerations provide additional confirmation that OESI yields more reasonable sooting tendency trends than TSI for oxygenated fuels.28 Blends of Hydrocarbon Base Fuel and Oxygenates. For measurement of the sooting tendency of oxygenates in liquid fuels, oxygenates are commonly added to a base hydrocarbon fuel with a defined composition. For example, to provide a wide range of fuel oxygen content, prior studies27,28 used blends of various oxygenates in a hydrocarbon base fuel composed of 35 vol % toluene and 65 vol % nheptane. It was shown that TSI always decreased with D
DOI: 10.1021/acs.energyfuels.5b01624 Energy Fuels XXXX, XXX, XXX−XXX
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Energy & Fuels
consistent with the unsaturation of MO likely leading to a higher propensity to form the soot precursor acetylene.5 A similar relationship was seen for the saturated and unsaturated diesters (DBS and DBM). The sooting tendency of MO was highest of the oxygenates, was greater than NHD, and was similar to HMN. Comparing MO to NHD, any beneficial effect of oxygen in the MO ester group was more than offset by the unsaturation. Also, the combined effects of ester oxygen, unsaturation, and somewhat greater carbon number of MO had a similar sooting tendency effect as the highly branched nature of HMN. The high boiling point of MO is another possible factor, which was highest of the compounds tested. As shown in Figure SI-3, higher boiling points correlate with smaller OESI reductions for the seven oxygenates, though this is confounded by other differences such as oxygen functional group and unsaturation. High boiling point has not been shown to be as significant a factor for diesel engine PM emissions39,40 as it has for gasoline.41 MO has the largest carbon number and molecular weight of all the compounds tested, which contributes to its high boiling point and also contributes to differences in the OESI value (i.e., greater carbon number tends to give a greater OESI for a given SP value). While the TMB volume fraction was held constant in ternary blends in this study, it is also reasonable to consider an approach that maintains a constant TMB mole fraction, consistent with the linear blending by mole fraction seen for OESI and TSI. For the present study using 30 vol % TMB, the TMB fraction was 48 mol % in the base fuel (with 70 vol % NHD). Blends with MO and HMN, the highest molecular weight compounds, were similar to the base fuel with 47 mol % and 51 mol % TMB for the cases with 70 vol % HMN and 70 vol % MO, respectively. The TMB mole fraction was as little as 22 mol % for the 70 vol % BuOH case. The other five oxygenates were fairly consistent, with the TMB mole fraction reaching 39−42 mol % for the cases with TPGME, UnOH, DBS, DBM, and MD at 70 vol %. From this perspective, BuOH is a case where the TMB mole fraction is considerably lower than the base fuel, while MO and HMN have TMB mole fractions similar to the base fuel. In making the comparisons above, this difference could conceivably contribute to the comparatively low OESI values for 1-butanol and higher values for MO and HMN as seen in Figure 3. Further study of this approach is warranted. Molar fractions are used in Figures 1, 2, and 3 as it has been shown to provide linear relationships with OESI and TSI for blends,28 which these data tend to confirm. Figure 4 shows the same data in terms of volume fraction of the added compound. On this basis, the trends for blends were not linear, but there was no great change in relative order between compounds. However, because BuOH has a much lower molar volume than the other compounds, the relative attractiveness of BuOH appears greater on this basis. The data can also be interpreted in terms of oxygen mass fraction as shown in Figure 5. The oxygen content is strongly indicative of the energy content of a fuel and has also been correlated with PM emissions reductions in diesel engines.10 For different oxygenate additions, significantly different levels of OESI reduction were observed at a given oxygen mass fraction, confirming that oxygen content is not the only factor influencing the sooting tendency of oxygenates as expressed by OESI. For each line, the maximum oxygen mass fraction data point represents the fuel blend in which the NHD is
Figure 2. TSI values for blends of TMB, NHD, and a third component (hydrocarbon or oxygenate) versus mole fraction of the added component. TMB maintained at 30 vol % in all blends with balance as NHD.
In Figure 3, trends of OESI values for hydrocarbon blends (NHD, HMN, TMB) follow their TSI trends, consistent with
Figure 3. OESI values for blends of TMB, NHD, and a third component (hydrocarbon or oxygenate) versus mole fraction of the added component. TMB maintained at 30 vol % in all blends with balance as NHD.
ref 28, with HMN and TMB both increasing OESI relative to NHD. For the oxygenated fuels, significantly different trends were obtained for OESI and TSI. All but one of the oxygenates tested reduced OESI relative to NHD, including both diesters. As with TSI, BuOH (a C4 alcohol) showed the greatest reduction in OESI of the oxygenates tested, greater than UnOH (a C11 alcohol). TPGME (a C10 glycol triether) and MD (a saturated C10 methyl ester) showed reductions in OESI similar to UnOH. These three compounds have a similar carbon number, molecular weight, and boiling point, but greatly differing oxygen content (a factor which is incorporated in the OESI scaling in eq 2). Of the four ester compounds tested, the two diesters (DBS and DBM) exhibited less of an OESI reduction than the saturated monoester (MD) despite the fact that the diesters (each containing two C4 chains) have greater oxygen content and lesser carbon content than MD (C10 chain). Of the two monoesters, MD exhibited a much lower OESI than MO, E
DOI: 10.1021/acs.energyfuels.5b01624 Energy Fuels XXXX, XXX, XXX−XXX
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Energy & Fuels
low oxygen content in their neat forms. As with the earlier comparisons based on mole fraction, the alcohols give the greatest OESI reduction with greater reduction for the C4 than the C11 alcohol. MD, the saturated C10 methyl ester is next most effective, followed by TPGME and the two diesters (DBS and DBM), while again MO appears to increase soot relative to NHD. TPGME has a sooting tendency similar to MD, but also has the advantage of a much higher cetane rating (Table 2). While long-chain alcohols appear to exhibit a high OESI reduction and have relatively high cetane ratings, they contribute comparatively little oxygen to blends even when blended at high concentrations. Conversely, the short-chain alcohols exhibit high OESI reduction and could contribute considerable oxygen, but have relatively low cetane ratings. While oxygen content in fuel is typically measured as a mass fraction or related to the carbon content in terms of the oxygen-to-carbon (O:C) ratio, Mueller42 introduced a potentially more relevant metric for combustion in engines, the fueloxygen ratio (Ωf). This parameter describes the fuel oxygen content relative to the amount required to completely oxidize the fuel to standard products (i.e., CO2 and water for organic compounds containing only C, H, and O), somewhat consistent with the approach in the OESI. Because oxygen mass fraction is highly correlated with fuel oxygen ratio for species that contain only carbon, hydrogen, and oxygen (the case here), the trends of OESI versus fuel oxygen ratio (Figure SI-4) were nearly identical to those seen versus oxygen mass fraction (Figure 5) for all of the tested three-component fuel blends including different oxygenates. Influence of Aromatic Hydrocarbons. The use of a fixed 30 vol % TMB in TMB−NHD−oxygenate blends allowed exploration of the influencing factors of various oxygenates while avoiding the confounding effect of diluting out the high sooting propensity of TMB. To investigate possible effects of aromatic content, smoke points were measured for TMB− NHD−TPGME blends that contained a fixed TMB fraction of either 30 vol % or 50 vol %. In Figure 6, the influence of aromatics on sooting tendency is paramount: the increase from 30 vol % to 50 vol % TMB significantly increased OESI values. Similar extents of OESI reduction with increasing TPGME content were seen using either TMB concentration, using either a TPGME mole fraction basis or an oxygen mass fraction
Figure 4. OESI values for blends of TMB, NHD, and a third component (hydrocarbon or oxygenate) versus volume fraction of the added component. TMB maintained at 30 vol % in all blends with balance as NHD.
Figure 5. OESI values versus oxygen mass fraction for blends of TMB, NHD, and a third component (hydrocarbon or oxygenate). TMB maintained at 30 vol % in all blends with balance as NHD.
completely substituted by an oxygenate. The relatively low oxygen mass fractions shown for UnOH and MO reflect their
Figure 6. OESI values versus TPGME mole fraction (left) and oxygen mass fraction (right) for blends of TMB−NHD−TPGME with two different fixed TMB volume fractions. F
DOI: 10.1021/acs.energyfuels.5b01624 Energy Fuels XXXX, XXX, XXX−XXX
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Energy & Fuels basis, confirming that replacement of NHD by TPGME significantly reduced the sooting tendency. Relevance to Diesel Engine Emissions. In these diffusion flame experiments using a smoke point lamp, dilution of the high-sooting aromatic hydrocarbon by the oxygenate had a primary effect on soot reduction, regardless of oxygenate type. Relative to a n-hexadecane reference, differences in sooting tendency reduction between oxygenates were of the same magnitude as the increases seen from different types of hydrocarbons. Similar results have been reported previously for other oxygenates tested in diffusion flames, both in smoke point lamps and premixed diffusion flames.27,28,30,32,33,38,43,44 However, these trends differ substantially from those reported for emissions from diesel engines. In engines, oxygenates provide substantial PM emission reductions, considerably greater than from reduction of aromatic hydrocarbon content and more closely related to the fuel oxygen content.3,5,7,11,45−49 For example, Kurtz et al.7 measured PM emissions for three hydrocarbon diesel fuels and two oxygenated fuels. As compared to the reference diesel fuel containing 29 vol % aromatics, a 91% reduction in PM emissions was seen using canola biodiesel and a 97% reduction was obtained using a blend of canola biodiesel and DBS (with greater oxygen content). In comparison, a low-aromatics (
